◆ LTE and HSPA�: Revolutionary andEvolutionary Solutions for Global MobileBroadbandAnil M. Rao, Andreas Weber, Sridhar Gollamudi, and Robert Soni
Universal Mobile Telecommunications System (UMTS) with its high speedpacket access (HSPA) enhancements is currently being deployed as theprimary mobile broadband solution by operators worldwide. To ensurecontinued competitiveness of the Global System for Mobile Communications(GSM) family of technologies in the world market, the 3rd GenerationPartnership Project (3GPP) is rapidly standardizing the long term evolution(LTE) of UMTS, with significant performance improvement targets comparedto HSPA. The fact that LTE is not backwards compatible with HSPA spurnedthe introduction of the HSPA evolution (HSPA�) effort in 3GPP to protectcurrent operator investments in HSPA. HSPA� provides a framework forHSPA enhancements with the goal of providing performance similar to LTE ina 5 MHz carrier, while at the same time offering the advantage of backwardscompatibility with earlier releases. In this paper, we identify the keytechnology features of LTE which allow it to meet the desired performanceimprovements compared to HSPA, and then describe the key features ofHSPA� which allow it to remain competitive with LTE. We examine featuresat the physical layer, medium access control (MAC) layer, and networkarchitecture layer, as well as provide detailed air interface performancestudies. © 2009 Alcatel-Lucent.
packet access (HSDPA) in March 2002, which marked
a significant evolution of UMTS into the so-called 3.5G
space, providing not only significant improvements in
spectral efficiencies, but greatly enhancing the end-
user experience. HSDPA has already become widely
available with over 180 operators in service in almost
80 countries [12]. Operators deploying UMTS today
are typically doing so with HSDPA. To complement
the downlink improvements offered by HSDPA, the
IntroductionBuilding on the tremendous success of Global
System for Mobile Communications* (GSM*) for sec-
ond generation (2G) deployments, 3rd Generation
Partnership Project (3GPP*) Release 99 introduced
Universal Mobile Telecommunications System (UMTS)
in March 2000, and it has become the dominant third
generation (3G) technology in the world with over
200 operators in service in almost 90 countries [12].
3GPP Release 5 introduced high speed downlink
Bell Labs Technical Journal 13(4), 7–34 (2009) © 2009 Alcatel-Lucent. Published by Wiley Periodicals, Inc. Publishedonline in Wiley InterScience (www.interscience.wiley.com) • DOI: 10.1002/bltj.20334
8 Bell Labs Technical Journal DOI: 10.1002/bltj
Panel 1. Abbreviations, Acronyms, and Terms
2G—Second generation3G—Third generation3GPP—3rd Generation Partnership Project4G—Fourth generationACK—AcknowledgementAMR—Adaptive multi rateBPSK—Binary phase shift keyingCDF—Cumulative distribution functionCDMA—Code division multiple accessCLTD—Closed-loop transmit diversityCP—Cyclic prefixCPC—Continuous packet connectivityCQI—Channel quality indicatorCS-RS—Channel sounding reference signaldB—DecibelDCH—Dedicated channelDFT—Discrete Fourier transformDL—DownlinkDM-RS—Demodulation reference signalDPCCH—Dedicated physical control channelDSCH—Downlink shared channelDRX—Discontinuous receiveEDGE—Enhanced data rates for GSM evolutionE-DPCCH—Enhanced DPCCHeNB—Enhanced node BEPC—Evolved packet coreEPS—Evolved packet systemE-UTRAN—Evolved UTRANEV-DO—Evolution data optimizedFACH—Forward access channelFDD—Frequency division duplexFDMA—Frequency division multiple accessFFR—Fractional frequency reuseFTP—File Transfer ProtocolGERAN—GSM/EDGE radio access networkGGSN—Gateway GPRS support nodeGPRS—General packet radio serviceGSM—Global System for Mobile CommunicationsGW—GatewayHARQ—Hybrid automatic repeat requesths—High speedHSDPA—High speed downlink packet accessHSPA—High speed packet accessHSPA� —HSPA evolutionHSUPA—High speed uplink packet accessID—IdentificationIDFT—Inverse discrete Fourier transformIFFT—Inverse fast Fourier transformIoT—Interference over thermalIP—Internet ProtocolIPv4—IP version 4ISI—Inter-symbol interferenceIST—Information Society Technologieskm/h—Kilometers per hourLMMSE—Linear minimum mean square errorLTE—Long Term EvolutionMAC—Medium access controlMC—Multi-carrierMCS—Modulation and coding scheme
MIMO—Multiple input-multiple outputMME—Mobility management entityMMSE—Minimum mean square errorMRC—Maximum ratio combiningms—MillisecondsNACK—Negative acknowledgementNGMN—Next-generation mobile networkOFDM—Orthogonal frequency division multiplexOFDMA—Orthogonal frequency division multiple
accessPAPR—Peak to average power ratioPARC—Per antenna rate controlPCH—Paging channelPDN—Packet data networkPMI—Precoding matrix indicatorPS—Packet switchedPSD—Power spectral densityPSTN—Public switched telephone networkPUCCH—Physical uplink control channelQAM—Quadrature amplitude modulationQoS—Quality of serviceQPSK—Quadrature phase shift keyingRACH—Random access channelRAN—Radio access networkRLC—Release completeRNC—Radio network controllerRoHC—Robust header compressionRoT—Rise over thermalRRC—Radio resource controlRTP—Real Time Transport ProtocolRx—ReceiveSA—System architectureSAE—System architecture evolutionSC—Single carrierSCCH—Shared control channelS-CCPCH—Secondary common control physical
channelSDMA—Spatial division multiple accessSFBC—Space frequency block codingSGSN—Serving GPRS support nodeSIC—Successive interference cancellationSINR—Signal-to-interference-plus-noise ratioSMS—Short message serviceSRS—Sounding reference signalTA—Timing advanceTS—Technical specificationTTI—Transport time intervalTU—Typical urbanTx—TransmitUDP—User Datagram ProtocolUE—User equipmentUL—UplinkUMTS—Universal Mobile Telecommunications
SystemUTRAN—UMTS terrestrial radio access networkURA—UTRAN registration areaVoIP—Voice over Internet ProtocolWINNER—Wireless World Initiative New Radio
DOI: 10.1002/bltj Bell Labs Technical Journal 9
high speed uplink packet access (HSUPA) technology
was introduced in 3GPP Release 6 in March 2005. The
combination of HSDPA and HSUPA is called simply
high speed packet access (HSPA), and is strongly posi-
tioned to become the dominant high speed wireless
data technology for many years.
Even before the standardization of HSUPA had
been completed, in December 2004, 3GPP initiated a
feasibility study regarding the long term evolution
of UMTS, in order to ensure that the GSM family of
technologies maintained a competitive position in the
world market. The introduction of LTE was seen as a
way to provide a smooth migration to the yet-to-be-
defined fourth generation (4G), and to take advantage
of new spectrum allocations with wider bandwidths
that would become available (e.g., in the 2.6 GHz 3G
extension band). During the LTE feasibly study, an
aggressive set of performance targets and require-
ments were agreed upon to form the basis for LTE
standardization work. To justify the introduction of a
new technology, LTE would be required to provide
very large performance gains compared to HSPA in
3GPP Release 6 and to fully take advantage of new
spectrum allocations as wide as 20 MHz. In order to
satisfy these requirements, it was clear that LTE would
have to be built from the ground up, and could not
offer backwards compatibility with UMTS/HSPA.
The fact that LTE would not be backwards com-
patible with HSPA was not necessarily received well
by the large number of operators who had already
made significant investments in UMTS/HSPA and had
not yet begun to realize the benefits of those invest-
ments. This spurned the introduction of the HSPA
evolution (HSPA�) effort in 3GPP in March 2006.
While 3GPP had already started working on Release 7
enhancements as early as 2005, there was no general
framework to these enhancements. HSPA� formally
defined a broad framework and a set of requirements
for the evolution of HSPA; the primary goal being to
provide performance similar to LTE in a 5 MHz carrier,
while offering backwards compatibility with Release
99 through Release 6. HSPA� would then provide a
compelling alternative to LTE for operators who were
already deploying UMTS/HSPA, allowing them the
flexibility to introduce LTE in new spectrum while
enjoying enhancements which would protect their
existing investments in UMTS/HSPA.
In this paper, we will describe the requirements set
forth in standardization for both LTE and HSPA�, and
then give an overview of the key features of each tech-
nology which allow them to achieve their performance
requirements. We will see that many of the features
introduced in HSPA� closely parallel the innovations
developed in LTE. Where applicable, we provide
detailed system performance studies which illustrate
how close the technologies come to meeting the desired
performance targets. The remainder of the paper is
organized as follows: we start with the performance
requirements set forth in standards, give an overview of
the system architecture enhancements, describe the key
features in the downlink, describe the key features in
the uplink, describe the features in LTE and HSPA� that
enable efficient transmission of Voice over Internet
Protocol (VoIP), and offer our conclusions.
Requirements and Performance TargetsDuring the initial study item phase for both LTE
and HSPA�, 3GPP agreed upon a set of requirements
and performance targets to form the basis of the stan-
dardization work, and to determine what key features
or enhancements should be included as part of the new
technology. In this section, we discuss the requirements
and performance targets for both LTE and HSPA�.
LTE Requirements and Performance TargetsWhile 3GPP understood in early 2005 that the
HSPA technology—the uplink component of which
had just been standardized—would provide a highly
competitive mobile broadband solution for several
years, potential threats from other technologies cre-
ated a desire to ensure competitiveness in an even
longer time frame (i.e., for the next 10 years and
beyond). This formed the justification for opening the
LTE study item in 3GPP very quickly.
Important considerations for the long term evo-
lution of 3GPP included reduced latency, higher user
data rates, improved system capacity and coverage,
and reduced cost for operators. In order to achieve
this, it was seen that both an evolution of the air
interface as well as the network architecture would
need to be taken into account. Looking to the future,
10 Bell Labs Technical Journal DOI: 10.1002/bltj
the desire for even higher data rates also needed to
factor-in future additional 3G spectrum allocations,
and hence, LTE would need to include support for
transmission bandwidths greater than 5 MHz. At the
same time, support for transmission bandwidths of
5 MHz and less than 5 MHz would be needed to allow
for flexibility in whichever frequency band the system
might be deployed in. The requirements and per-
formance targets for LTE were agreed upon in [2],
and it should be noted that these performance targets
were decided when HSPA Release 6 was still being
finalized. Hence, the targeted improvements are in
many cases set relative to HSPA Release 6. The points
relevant to this paper are summarized here:
• Packet-only technology. LTE would support only the
packet switched (PS) domain of the core network,
and would be optimized to provide a high data
rate, low-latency, packet-optimized radio access
technology.
• Scaleable bandwidth. Support for bandwidths of
1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and
20 MHz. Originally, the smallest bandwidth allo-
cation was going to be 1.25 MHz to fit existing
CDMA2000*/evolution data optimized (EV-DO)
spectrum, but this was later changed to 1.4 MHz
which would fit an integral multiple of GSM
carriers.
• Improved peak rates. 100 Mbps peak rate in the
downlink in 20 MHz (5 bps/Hz) and 50 Mbps
peak rate in the uplink in 20MHz (2. 5bps/Hz). Peak
rates should scale linearly with the spectrum allo-
cation.
• Improved spectrum efficiency targets.
– Three to four times improvement in the
downlink spectral efficiency compared to
Release 6 HSDPA. This assumes a 1�2
antenna configuration for HSDPA but a 2�2
antenna configuration for LTE.
– Two to three times improvement in the uplink
spectral efficiency compared to HSUPA
Release 6. This improvement assumes a 1�2
antenna configuration for both HSUPA Release
6 and LTE.
• Improved user throughput. Target improvements are
placed on both average user throughput as well as
user throughput at the edge of the cell. The cell
edge user throughput is defined as the fifth per-
centile of the user throughput cumulative distri-
bution function (CDF); this quantity is important to
ensure broadband rates can be achieved through-
out most of the cell coverage area. The target
improvements below use the same assumptions
described above for the improved spectrum effi-
ciency:
– Three to four times improved average user
throughput per MHz in the downlink and two
to three times improved user throughput per
MHz in the uplink compared to Release 6.
– Two to three times improved cell edge user
throughput per MHz compared to Release 6.
• Improved latency. LTE targets significantly improved
control plane and user plane latency, including a
– Control plane supporting transition time of less
than 100 milliseconds (ms) from a camped state
(i.e., idle) to an active state (i.e., CELL_DCH),
and a transition time of less than 50ms from a
dormant state (i.e., URA/CELL_PCH) to an
active state.
– User plane supporting latency of less than
5 ms should be possible on the user plane in
an unloaded condition. The user plane
latency is defined as the one way transit time
between the user equipment (UE) and the
radio access network (RAN) edge node.
• Co-existence and inter-working with UMTS and GSM.
Given that LTE would co-exist with both
UMTS/HSPA terrestrial radio access network
(UTRAN) and GSM/EDGE radio access network
(GERAN), requirements were placed on inter-
working with these legacy systems. An interrup-
tion time of less than 500 ms is targeted for a
handover of a non-real time service between LTE
and either UTRAN or GERAN, while an interrup-
tion time of less than 300 ms is targeted for real-
time services.
• Given the scope of these requirements for evolu-
tion work, 3GPP agreed upon a work split—the
evolution of the radio access network would take
place in the 3GPP RAN working groups, and in
parallel, work on an evolved packet core (EPC)
DOI: 10.1002/bltj Bell Labs Technical Journal 11
would take place in the system architecture (SA)
working groups. At this point, it is useful to clarify
some terminology: the radio access network
enhancements are referred to as either evolved
UMTS terrestrial radio access network (E-UTRAN)
or LTE; the names are used interchangeably. The
evolution work for the EPC is referred to as sys-
tem architecture evolution (SAE). For some time,
the combination of these enhancements was
referred to as LTE/SAE, but more recently it has
become known as the evolved packet system
(EPS).
HSPA� Requirements and Performance TargetsIn order to protect operator investments in HSPA
and provide a smooth evolution path towards LTE,
which would not offer backwards compatibility with
earlier releases of UMTS/HSPA, a study item on HSPA
evolution was opened in 3GPP in March 2006. While
development of HSPA Release 7 enhancements was
already underway in 2005 with open work items
regarding HSDPA multiple input-multiple output
(MIMO), continuous packet connectivity (CPC), and
the “one tunnel” solution for optimization of packet
data traffic, there was no general framework in place
to guide the evolution of HSPA. The HSPA� effort
provided a broad framework for HSPA evolution with
a clear set of requirements and performance targets,
with the intent of identifying what performance bene-
fits could be achieved with the existing Release 7
work items and what gaps still existed.
The goal of HSPA� is not to replace LTE, but
rather to enhance HSPA by providing an incremental
evolution path for both the RAN and core network
which will enhance performance while leveraging
existing infrastructure. In addition, HSPA� aims to
enable co-existence with the EPS since it will be part
of future 3G systems. As described in [3, 8], the guid-
ing principles behind HSPA� are as follows:
• HSPA spectrum efficiency, peak data rates, and
latency should be comparable to LTE in a 5 MHz
bandwidth.
• The inter-working between HSPA� and LTE
should be as smooth as possible and facilitate joint
technology operation; the possibility of reusing
the evolved packet core defined as part of the sys-
tem architecture evolution should be analyzed.
• HSPA� should be able to operate as a packet-only
network, based on the utilization of shared chan-
nels only (i.e., HSDPA and HSUPA).
• HSPA� shall be backwards compatible in the sense
that legacy terminals compatible with Release 99
through Release 6 are able to share the same
carrier with terminals implementing the latest
HSPA� features, without any performance degra-
dation.
• Ideally, existing infrastructure should only need a
simple upgrade to support the features defined as
part of HSPA�.
As we will see in later sections, the framework
provided by the HSPA� effort initiated the develop-
ment of several new HSPA enhancements beyond
what was already being considered in early 2006, in
Release 7.
Network Architecture ImprovementsGiven the requirements and performance targets
described in the previous section, it was clear that not
only were enhancements to the radio interface
required, but in addition, the network architecture itself
needed enhancements. In the next section, we describe
the network architecture enhancements for both LTE
and HSPA�.
Evolved Packet System Architecture for LTEThe goal of the system architecture evolution
effort in 3GPP is not just to define an efficient packet
core network and RAN architecture for LTE to meet
the requirements described in [2], but rather to
develop a framework for an evolution and migration
of current systems to a high data rate, low latency,
packet-optimized system that supports mobility and
service continuity across heterogeneous access net-
works, since it is envisioned that Internet Protocol
(IP)-based services would be provided through vari-
ous access technologies.
In its simplest form, the EPS architecture consists
of two basic nodes in the user plane: a single node
called the enhanced node B (eNB) comprises all radio
access functions and a single node called the EPS
gateway comprises the entire bearer plane (i.e., user
12 Bell Labs Technical Journal DOI: 10.1002/bltj
plane) in the core network. In the control plane, the
mobility management entity (MME) node is logically
separated from the user plane EPS gateway with an
open interface between them. Figure 1 provides a
comparison of the EPS network architecture and the
UMTS network architecture. EPS offers a flatter net-
work architecture than UMTS, especially as far as the
user plane is concerned, which reduces latency.
The clean separation of the user plane and control plane
is a key feature of the EPS architecture, as it allows for
independent scaling of control plane functionality and
user plane functionality. This is very important from
a technical viewpoint because the scaling of the two
depends on different factors: the capacity of the con-
trol plane functionality typically depends on the num-
ber of mobile devices and their mobility patterns,
whereas the capacity of the user plane depends on
aggregate data throughput required to be supported.
Drawing a parallel between the EPS architecture and
UTRAN, the enhanced node B absorbs all radio access
functions that were contained in the node B and radio
network controller (RNC) elements in UTRAN. Note
that the eNBs are directly connected to each other via
an interface called X2; this facilitates seamless mobility
and interference management.
Figure 2 presents a more detailed view of the EPS
architecture, with the interfaces that exist to support
mobility between 3GPP and non-3GPP networks. The
EPS gateway may be split into two separate logical
nodes with the optional S5 interface: the serving gate-
way (GW), and the packet data network (PDN)
gateway. The serving GW terminates the core network
interface towards 3GPP radio access networks and
serves as the local mobility anchor point for inter-eNB
handover within the EPS, as well as mobility anchor-
ing for inter-3GPP mobility (i.e., between EPS and
UTRAN/GERAN). Note the direct control plane inter-
face (S3) and user plane interface (S4) between the
EPS network and the serving GPRS support node
(SGSN) in the UMTS/GSM networks; such an inter-
face allows for a packet session to be maintained in a
way that is seamless to the user of a multimode ter-
minal that migrates across LTE, UMTS/HSPA, and
GSM/EDGE coverage areas. This meets the requirements
Node BNode B Node BNode B
Packet corenetwork
Radio accessnetwork
eNode B eNode B
Internet Internet
UMTS EPSU-planeC-planeGGSN
SGSN
RNCRNC
MME
EPSgateway
EPS—Evolved packet systemGGSN—Gateway GPRS support nodeGPRS—General packet radio serviceHSPA—High speed packet access
MME—Mobility management entityRNC—Radio network controllerSGSN—Serving GPRS support nodeUMTS—Universal Mobile Telecommunications System
Figure 1.Comparison of UMTS/HSPA network architecture and EPS network architecture.
DOI: 10.1002/bltj Bell Labs Technical Journal 13
for co-existence/inter-working and gives the operator
the flexibility to roll out LTE gradually, starting with
the areas of highest demand first. The PDN gateway
provides access to the packet data network through
the control of IP data services and allocation of IP
addresses; it also serves as an anchor for mobility
between 3GPP and non-3GPP access systems, which is
sometimes referred to as the SAE anchor function.
Note that the specification of logical nodes does not
mandate a mapping to physical entities. For example,
the serving GW, PDN GW, and MME may be imple-
mented in the same physical entity, or the MME may be
integrated into the eNB. The mapping of logical nodes
to physical entities may follow a highly integrated
approach or a more distributed approach, based on ven-
dor implementations and deployment scenarios.
HSPA� Network ArchitectureFor the HSPA� network architecture, we begin
with a description of the one tunnel enhancement
(also referred to as “direct tunnel”) that was already
being discussed as part of Release 7 prior to the
HSPA� initiative. 3GPP recognized that the amount of
user plane data would significantly increase in the
near future because of the introduction of HSPA. With
the existing system illustrated in Figure 1, packet data
traffic must traverse both the gateway GPRS support
node (GGSN) and serving GPRS support node in the
UMTS core network (through the use of two tunnels),
independently of how the data traverses the UTRAN.
A more scalable architecture is possible with the one
tunnel solution, which permits direct tunneling of the
user plane data between the GGSN and the RNC, as
illustrated in Figure 3. In this way, a cleaner separa-
tion between the control plane and the user plane is
achieved, the advantages of which were discussed
previously for LTE. The new SGSN controller performs
all the control functions of the SGSN, and the
enhanced GGSN takes over all data transport func-
tionality that resided in the previous GGSN and SGSN.
To further flatten the network architecture,
HSPA� introduced the option of integrating the RNC
serving GW PDN GW
EPS gateway
E-UTRAN
UTRAN
GERAN
Non-3GPP† access
S1-MME
S1-U
S11
S4S3
S5Internet
SGSN
MME
3GPP—3rd Generation Partnership Project EDGE—Enhanced data rates for GSM†† EvolutionEPS—Evolved packet systemE-UTRAN—Evolved UTRANGERAN—GSM/EDGE radio access networkGPRS—General packet radio serviceGSM—Global System for Mobile Communications††
†Trademark of the European Telecommunications Standards Institute. ††Registered trademarks of the GSM Association.
GW—GatewayMME—Mobility management entityPDN—Packet data networkSGSN—Serving GPRS support nodeUMTS—Universal Mobile Telecommunications SystemUTRAN—UMTS terrestrial radio access network
Figure 2.Detailed view of the EPS network architecture, with interfaces to support mobility across 3GPP and non-3GPP access.
14 Bell Labs Technical Journal DOI: 10.1002/bltj
and node B functionality into a single node for packet
switched services (denoted here as node B� ); this is
also shown combined with the one tunnel solution
in Figure 3. The flatter architecture reduces latency
and could be useful in deployments in which a high
level of integration is desirable (i.e., HSPA femtocells).
From Figure 3, it quickly becomes apparent how simi-
lar the HSPA� network architecture is to the EPS net-
work architecture shown in Figure 1, which allows
easy integration of HSPA� and EPS networks.
Key Features in the Downlink of LTE and HSPA�
The downlink (DL) of a RAN bears a higher
amount of data traffic compared to the uplink (UL)
due to the increasing demand for unbalanced data
services like, for example, FTP download or video
streaming. A number of features have been intro-
duced in order to support increasing data rates.
LTE Downlink Key FeaturesIn the following subsections, we will discuss key
features of the LTE DL such as OFDM transmission,
MIMO, the possibility of using higher order modulation
schemes, time and frequency selective scheduling,
and fractional frequency reuse. Details about the
LTE DL channel structure can be found in [7]
and [12].
Orthogonal Frequency Division MultiplexThe LTE DL air interface is based on orthogonal
frequency division multiplexing (OFDM) which is a
technique that avoids inter-symbol interference,
exploits the scarce frequency resource nearly opti-
mally, combines the advantages of broadband and
narrowband transmission, and, at the same time,
avoids their disadvantages. OFDM is further described
below.
In conventional high bit rate air interfaces, the
data symbols are transmitted sequentially over the air
interface. According to the Nyquist theorem, the mini-
mum required bandwidth B is related to the symbol
duration Tsym with B � 1/Tsym. In real systems,
guard bands are required at both ends of the used
spectrum due to the application of non-ideal filters. In
multipath environments, broadband channels show
Node-B
Packet corenetwork
Radio accessnetwork
Internet
HSPA�one tunnel
HSPA�one tunnel
withintegrated
RNC/node B
GGSN
SGSN
RNC
Internet
GGSN
SGSN
Node-B�
GGSN—Gateway GPRS support nodeGPRS—General packet radio serviceHSPA�—High speed packet access evolution
U-planeC-plane
RNC—Radio network controllerSGSN—Serving GPRS support node
Figure 3.HSPA� network architecture.
DOI: 10.1002/bltj Bell Labs Technical Journal 15
a frequency selective behavior with several deep fades
in the frequency domain. In the time domain, this
behavior corresponds to an overlapping of symbols,
which causes the so called inter-symbol interference
(ISI), illustrated in Figure 4. The smaller the symbol
duration, i.e., the higher the symbol rate, the more
symbols experience ISI. In broadband transmissions,
an inversion of the channel transmission function is
required which corresponds to an equalization of the
received signal in order to cope with inter-symbol
interference caused by the relatively short symbol
duration (compared to the delay spread of the chan-
nel echoes).
One means to reduce inter-symbol interference
is to extend the symbol duration so that it is longer
than the difference between the delays of the earliest
and latest channel echo. A further improvement is to
extend the symbol duration by a guard time, during
which transmission of the new symbol has already
started but which is discarded by the receiver. This
feature is called cyclic prefix (CP). The total symbol
duration in this case is the sum of the original sym-
bol plus the CP duration, which should be longer than
the difference between the delays of earliest and lat-
est echo in the multipath channel.
The reason for the low ISI in the time domain is
because of a flat channel in the frequency domain. In
a multipath environment, this corresponds to a nar-
row bandwidth used for the transmission of the sym-
bol. Many parallel narrowband transmissions are
required to obtain a high bit rate channel. OFDM
avoids the guard band between the so-called subcar-
riers by a modulation of these subcarriers with rec-
tangular pulses, using the rect(t) function. In the
frequency domain, the spectrum of a pulse with dura-
tion Tsym corresponds to the sinc(x) � sin(x)/x func-
tion with zero crossings at k/Tsym, k � . . . � 2, � 1,
1, 2, . . . . Consequently, if these pulses modulate a
number of subcarriers, the inter-subcarrier interfer-
ence is zero with a subcarrier spacing of 1/Tsym which
is, according to the Nyquist theorem, the optimal
value, as shown in Figure 5. Furthermore, this opti-
mal value is reached without any filter.
The modulation of the equally-spaced subcarri-
ers with rect pulses corresponds to an inverse discrete
Fourier transform (IDFT) in the time domain. At the
receiver, the original symbols are reconstructed using
the opposite function, namely a discrete Fourier trans-
form (DFT). Figure 6 shows a schematic view of the
OFDM transmission chain.
Transmitted signal
Echo 1
Echo 2
Echo 3
Received signal
Symbol n Symbol n�1ISI
ISI—Inter-symbol interference
Figure 4.Inter-symbol interference caused by channel echoes.
16 Bell Labs Technical Journal DOI: 10.1002/bltj
In OFDM the bandwidth can be easily adapted to
the needs of the network operator. LTE FDD, for
example, offers bandwidths of 1.4 MHz, 3 MHz ,
5 MHz , 10 MHz , 15 MHz , and 20 MHz with a sub-
carrier spacing of 15 kHz. The total bandwidth
includes guard bands at both ends of the spectrum,
so that 72, 180, 300, 600, 900, and 1200 subcarriers
are conveyed in the respective bandwidth, as outlined
in [10].
The inverse fast Fourier transform (IFFT) and
corresponding FFT enable a very efficient calcula-
tion of the transmitted signal and the correspon-
ding reconstruction of the symbols. FFT and IFFT
require that the number of subcarriers N is N � 2n
with an integer value of n, although not all of these
subcarriers have to be transmitted. The granularity
in which the transmitter has to calculate the IFFT
and in which the receiver has to sample the
1
0.8
0.4
0.6
0
0.2
�0.2
Am
plit
ud
e/m
axim
um
am
plit
ud
e
�0.4�10 �5 0 5 10
Frequency (1/Tsym)
OFDM—Orthogonal frequency division multiplexing
Figure 5.Frequency domain of seven subcarriers of an OFDM signal.
Mapper IDFT DFT DemapperChannel
Bits Symbols s(t) s’(t) Symbols Bits
DFT—Discrete Fourier transform IDFT—Inverse discrete Fourier transformOFDM—Orthogonal frequency division multiplexing
Figure 6.OFDM transmission chain.
DOI: 10.1002/bltj Bell Labs Technical Journal 17
channel is Tsym/N and, consequently, depends on
the FFT size.
Multiple Antenna AlgorithmsMIMO, a synonym for a technique that uses at
least two antennas at the transmitter and at least two
antennas at the receiver for the transmission of signals
over the air interface, is depicted in Figure 7. The
antennas can be used to obtain an array gain, i.e., a
diversity gain, to reduce co-channel interference, or to
enable multiplexing of several data streams to the
same or to different receivers. Consequently, MIMO is
able to increase quality of service (QoS), coverage,
spectral efficiency, and peak data rate.
One or several data streams are, after channel
coding and modulation, multiplied with a precoding
vector and mapped on the different transmission
antennas. The precoding vector describes the phase
shifts of the data symbols and the mapping of these
data symbols on the antenna ports. The transmit (Tx)
and receive (Rx) antennas, respectively, are either
widely-spaced (or alternatively cross polarized) or
closely spaced in a linear array. In the first case, the
channel state at the antennas is uncorrelated. For
widely-spaced antennas, the antenna pattern gener-
ated is frequency dependent and has an irregular
shape. In the case of a linear array, the channel state
at the antennas is correlated; the generated antenna
pattern is frequency independent over the used band-
width and shows a regular shape with main and side
lobes. A special case for the uplink is virtual MIMO,
shown in Figure 8. In this case, two or more widely-
spaced mobile terminals are multiplexed on the same
resources. In contrast to the base stations, these
devices do not require multiple antennas. In general,
the maximum number of data streams corresponds
to the minimum of Tx and Rx antennas. In the case of
virtual uplink MIMO, the number of antennas in the
base station is the limiting factor.
In a closed loop transmission procedure, the pre-
coding vector is chosen so that a constructive super-
position of the signals is obtained at the receiver, or
that different data streams conveyed over the same
resources can be easily separated. In the LTE closed
loop mode, the receiver chooses the precoding vector
out of a limited set of possible precoding vectors in
order to reduce feedback signaling load. In this case,
the precoding feedback is reduced to an index in a
table of predefined precoding vectors, known as the
precoding matrix indicator (PMI). In the open loop
mode of LTE DL, space frequency block coding (SFBC)
is used under bad to medium channel conditions,
while per antenna rate control (PARC) is used under
good channel conditions and enables two stream
transmissions. Feedback from the receiver is still
required in order to signal the supportable rank, i.e.,
the number of supportable streams and the channel
quality.
The LTE DL works with orthogonal pilots for the
different transmission antennas in order to enable
the receiver to calculate the channel transmission
function, i.e., the transmission function from every
CodingModulationWeightingMapping
Channel
Data stream(s)
WeightingDemappingDemodulationDecoding
Data stream(s)
MIMO—Multiple input-multiple output
Figure 7.MIMO transmission and reception.
18 Bell Labs Technical Journal DOI: 10.1002/bltj
transmission to every receive antenna. The following
antenna algorithms can be applied with multiple
antenna systems:
• Transmit and receive diversity (with widely-
spaced or cross polarized antennas),
• Beamforming, or beamswitching (with closely-
spaced antennas),
• Spatial multiplexing (with widely-spaced or cross
polarized antennas), and
• Combinations of the previous algorithms.
Transmit diversity generates a rich number of
channel echoes at the receiver, which arrive from vari-
ous directions. By leveraging maximum ratio com-
bining (MRC), the receiver may use this receive
diversity in order to combine the signal of the differ-
ent receive antennas so that the received signal level
is optimized.
For beamforming and beamswitching, the anten-
nas have to be closely spaced, with a spacing of half
the wavelength of the carrier frequency. In an open
loop algorithm, the antenna spacing has to be cali-
brated. For a closed loop algorithm with channel feed-
back from the receiver, calibration is not mandatory
since the receiver chooses the optimal beam. Several
data streams can be conveyed over the same resources
to different users, if the beams have sufficient angular
separation, possible via spatial division multiple access
(SDMA).
Spatial multiplexing is an antenna algorithm
based on widely-spaced or cross polarized antennas.
Several data streams are mapped on the transmit
antennas. The receiver, e.g., a minimum mean square
error (MMSE) receiver, repeats to combine the sig-
nals of the different receive antennas so that the
signal-to-noise-ratio of one data stream is optimized
while the other data streams are suppressed, until all
data streams are reconstructed. Successive interfer-
ence cancellation (SIC) is an enhanced receiver algo-
rithm that subtracts the interference of successfully
received data streams from other data streams with
low signal-to-interference-plus-noise ratio (SINR).
A combination of spatial multiplexing and beam
forming or beam switching is enabled by a number
of closely spaced, cross polarized antennas at the
transmitter. This antenna allows the transmission of
up to two data streams and, at the same time, it allows
a beam to form in order to optimize the signal level at
the receiver.
CodingModulationWeightingMapping
Channel
Data stream(s)WeightingDemappingDemodulationDecoding
Data stream(s)
Data stream(s)
WeightingDemappingDemodulationDecoding
MIMO—Multiple input-multiple output
Figure 8.Virtual MIMO in the uplink.
DOI: 10.1002/bltj Bell Labs Technical Journal 19
Other Performance Enhancing TechniquesLink adaptation is a state of the art technique that
allows the adjustment of channel protection to chan-
nel quality by choosing the best suited modulation
and coding scheme (MCS). LTE allows coding rates,
i.e., the ratio of data bits and transmitted bits, close to
1 for excellent channel quality. Quadrature phase shift
keying (QPSK), 16 quadrature amplitude modulation
(QAM), and 64 QAM are possible modulation
schemes and can be combined with any code rate.
QPSK conveys 2 bits in every data symbol (resource
element), 4 bits in 16 QAM, and 6 bits in 64 QAM.
Consequently, in good channel conditions, three times
more bits can be conveyed using 64 QAM than under
bad channel conditions when QPSK is applied.
However, only those MCS that have the best throughput
performance for a given channel quality will be
applied, i.e., those which are part of the hull curve of
all MCS as shown in Figure 9.
Frequency selective scheduling improves spectral
efficiency and cell border throughput for OFDM by
choosing only the best set of physical resource blocks
for a transmission. In a frequency division duplex
(FDD) system, for frequency selective scheduling, the
mobile terminal has to feed back the channel quality
for either all resources or for a subset of resources
with the best channel qualities, i.e., channel quality
indicator (CQI). The scheduler evaluates the individ-
ual sets of CQI values in combination with the indi-
vidual throughput of the mobile devices and
SISO AWGN, 630 resource elements per transport block
0
1
2
3
4
5
6
�10 �5 0 5 10 15 20 25
SNR (dB)
Thro
ug
hp
ut
(bit
s p
er s
ymb
ol)
MCS 1MCS 2MCS 3MCS 4MCS 5MCS 6MCS 7MCS 8MCS 9MCS 10MCS 11MCS 12MCS 13MCS 14MCS 15MCS 16MCS 17MCS 18MCS 19MCS 20MCS 21MCS 22MCS 23MCS 24MCS 25MCS 26MCS 27
AWGN—Additive white Guassian noise dB—DecibelMCS—Modulation and coding scheme
OFDM—Orthogonal frequency division multiplexingSISO—Single input-single outputSNR—Signal-to-noise ratio
Figure 9.Throughput versus SNR for different MCS for the OFDM downlink.
20 Bell Labs Technical Journal DOI: 10.1002/bltj
calculates a priority for every resource of every device.
The target of the scheduler is to work as close as pos-
sible at a predefined operating point which corre-
sponds to a compromise between fairness and sector
throughput. Measured over a certain period of time,
a proportional fair scheduler gives every mobile
device approximately the same number of resources.
These are the best resources possible compared to an
average channel quality in frequency and in time.
However, the scheduling strategy can also be modified
gradually towards higher cell throughput or fair
mobile throughput.
Fractional frequency reuse (FFR) is a technique
that uses most of the downlink resources for every
sector without any restriction. A small portion of these
resources, e.g., one third or one seventh, are either
never used, or are used with reduced power by the
base station. Network planning is necessary in order to
assign a different set of these resources to different
base stations in such a way that every pair of neighbor
sectors either do not use, or reduce the power of a dif-
ferent set of resources. In the latter case, due to power
reduction, these resources can still be used close to the
base station, i.e., by mobile terminals which have good
channel conditions. For a device at the cell border
towards a neighbor sector, some resources are avail-
able that have an artificially increased quality, namely
resources that are either not used or are used with
reduced power by the neighbor sector.
HSPA� Downlink Key FeaturesThe code division multiple access (CDMA)-based
HSDPA in 3GPP Release 5 and Release 6 already pro-
vided an efficient high speed downlink air interface
through the use of a short subframe length (2 ms),
hybrid automatic repeat request (HARQ), and fast,
channel-sensitive scheduling on a shared channel,
facilitated by the use of channel quality feedback and
the addition of a new advanced scheduling entity,
MAC high speed (MAC-hs) located in the base sta-
tion. HSDPA, in Release 5 and Release 6, supports
QPSK and 16 QAM modulation, and offers a peak of
14. 4 Mbps. Several enhancements have been intro-
duced for HSDPA in Release 7 as part of HSPA� in
order to improve spectral efficiency and cell border
throughput [9].
Higher Order ModulationHSPA� allows up to 64 QAM modulation in the
downlink, which conveys 6 bits per symbol instead of 4
bits in the case of 16 QAM and consequently increases
the peak data rate by 50 percent to 21.6Mbps. 64 QAM
can be applied under good channel conditions. Due to
this fact, the possibility of using 64 QAM will enhance
spectral efficiency but will not have a high impact on
cell border throughput. For backward compatibility,
new terminal types have been defined that support
64 QAM.
MIMOHSPA� allows closed loop 2x2 MIMO with two
transmit antennas and two receive antennas. Under
good channel conditions, dual stream transmissions
are possible that can double the peak bit rate to
28.8 Mbps. As already described for LTE MIMO, the
mobile terminal chooses the best precoding vector out
of a set of predefined precoding vectors together with
CQI values for one or two streams. In order to enable
the device to measure the signal quality separately for
both antennas, the antennas carry orthogonal pilot sig-
nals. In case of dual stream transmission, both streams
can have different modulation and coding schemes
according to their channel quality. In case of low chan-
nel quality, the scheduler can decide to switch back to
single stream transmission, which then takes place on
the two antennas via closed-loop transmit diversity
(CLTD). The MIMO scheme, precoding vector, and
MCS signal the mobile device via the high speed
shared control channel (HS-SCCH). Dual stream
MIMO in HSPA� supports improved system capacity
rather than improved cell border throughput.
However, the fallback mode of CLTD for single stream
transmission will increase cell border throughput com-
pared to the case of transmitting with a single antenna
only. The combination of MIMO with 64 QAM is not
foreseen for Release 7, but will be part of Release 8 of
HSPA�, increasing the peak rate to 43. 2 Mbps.
Enhanced Receiver TypesOne common means to increase downlink sys-
tem capacity and cell border throughput is to enhance
the requirements for the mobile receiver. For Release 5,
requirements are based on a single antenna rake
receiver. Release 6 defined requirements based on a
DOI: 10.1002/bltj Bell Labs Technical Journal 21
rake receiver with dual antenna receive diversity
(enhanced receiver type 1) and on a single antenna
receiver with equalization, e.g., an MMSE receiver
(enhanced receiver type 2). Release 7 defines require-
ments for the combination of dual antenna receive
diversity and equalization (enhanced receiver type 3).
The introduction of so-called interference aware
receivers further improves performance. Using this
feature, the receiver reduces the interference from
neighbor cells, which works to enhance cell border
throughput. This feature is used in conjunction with
equalization for single antenna (enhanced receiver
type 2i) or dual antenna Rx diversity (enhanced
receiver type 3i).
Downlink Performance ComparisonTable I and Table II show the performance com-
parison of HSDPA Release 6 with 5 MHz bandwidth
and LTE DL with 5 MHz and 10 MHz bandwidth,
respectively. For the basic assumptions we used the
HSDPA Improvement Release 6 LTE LTE compared(5 UEs/cell (5 UEs/cell (10 UEs/cell to HSDPAin 5 MHz) in 5 MHz) in 10 MHz) 1 � 2
Case 1 (1 � 2) 0.47 1.33 1.52 2.8 – 3.2x
Case 1 (2 � 2) NA 1.47 1.60 3.1 – 3.4x
Case 1 (4 � 2) NA 1.73 1.85 3.7 – 3.9x
Case 3 (1 � 2) 0.44 1.24 1.40 2.8 – 3.2x
Case 3 (2 � 2) NA 1.37 1.50 3.1 – 3.5x
Case 3 (4 � 2) NA 1.60 1.70 3.6 – 3.9x
Table I. Average downlink spectral efficiency (bps/Hz/cell) with NGMN assumptions.
HSDPA—High speed downlink packet accessLTE—Long term evolutionNA—Not applicableNGMN—Next-generation mobile networkUE—User equipment
HSDPARelease 6 LTE LTE Improvement
(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in compared to5 MHz) 5 MHz) 10 MHz) HSDPA 1 � 2
Case 1 (1 � 2) 195 223 321 1.1 – 1.6x
Case 1 (2 � 2) NA 257 345 1.3 – 1.8x
Case 1 (4 � 2) NA 337 462 1.7 – 2.4x
Case 3 (1 � 2) 170 140 209 0.8 – 1.2x
Case 3 (2 � 2) NA 186 262 1.1 – 1.5x
Case 3 (4 � 2) NA 257 323 1.5 – 1.9x
Table II. Five percent CDF downlink user throughput (kbps) with NGMN assumptions.
CDF—Cumulative distribution functionHSDPA—High speed downlink packet accessLTE—Long term evolutionNA—Not applicableNGMN—Next-generation mobile networkUE—User equipment
22 Bell Labs Technical Journal DOI: 10.1002/bltj
3GPP performance verification framework [5], which
is based on TS 25.814 [4]. Contrary to this frame-
work, we scaled the average number of users per sec-
tor according to the bandwidth in order to have a fair
comparison across the different systems. For HSDPA
mobile terminals we used enhanced receiver type 1,
i.e., a rake receiver with two antenna receive diver-
sity. For LTE, we used a maximum ratio combining
receiver in the case of single stream transmission and
a linear minimum mean square error (LMMSE)
receiver in the case of dual stream transmission.
Switching between single and dual stream in the case
of 2�2 and 4�2 transmission was performed accord-
ing to the precoding matrices presented in 3GPP TS
36.211 [7]. For all DL cases, we used the spatial channel
model WiM C2 “macro urban” from the Information
Society Technologies Wireless World Initiative New
Radio (IST WINNER) project. We performed the
simulations for case 1 with an inter site distance of
500 meters (m) and for case 3 with an inter site dis-
tance of 1732 m; both cases use a penetration loss
of 20 decibels (dB) and are at a carrier frequency of
2 GHz. The 3GPP performance verification framework
requests an improvement factor between 3 and 4 for
DL spectral efficiency and between 2 and 3 for DL cell
border throughput, which is defined as the fifth per-
centile of the mobile terminal’s cumulative through-
put distribution function. However, for the 3GPP
performance verification framework, HSDPA with
5 MHz bandwidth is compared to LTE with 10 MHz
bandwidth, and with 10 users in an average per sec-
tor for both systems, which penalizes the average
mobile device and cell border throughput of HSPA by
a factor of two. Consequently, in our comparison, as
we scale the number of users with the bandwidth,
the required improvement factor for the cell border
throughput of 3GPP has to be divided by a factor of 2
and, hence, shall be between 1 and 1.5. The required
improvement factor for spectral efficiency remains the
same. For all simulations, a standard proportional fair
scheduler has been used which, in the long term,
assigns approximately the same number of resources
to the mobile devices. For HSDPA, the proportional
fair algorithm has been performed in time, for LTE DL
in time and frequency. It is interesting that for case 3,
i.e., for an inter-site distance of 1732 m, the most
comparable case, namely 1�2 with 5MHz bandwidth,
the LTE cell border throughput is 18 percent smaller
but the spectral efficiency is 180 percent higher
compared to HSDPA. However, the LTE DL propor-
tional fair scheduler could be easily tuned towards
higher cell border throughput on the cost of spectral
efficiency. For case 1, a 500 meter inter-site distance,
cell border throughput is superior to HSDPA Release
6 while we still obtain a 180 percent gain in spectral
efficiency. Due to a higher channel diversity, spectral
efficiency and cell border throughput increases with
increasing bandwidth. With increasing numbers of
transmit antennas, we also obtain a gain in both spec-
tral efficiency and cell border throughput due to better
exploitation of channel diversity. As a consequence, if
we combine both effects, the spectral efficiency gains
for increasing bandwidth decreases with increasing
numbers of transmit antennas due to the fact that
both exploit channel diversity.
Further improvements are possible for HSDPA and
LTE. HSDPA, according to Release 7, introduces 64
QAM and MIMO in the DL as well as new enhanced
receiver types with equalizers and with intra-cell inter-
ference cancellation. 64 QAM will lead to an improve-
ment of spectral efficiency. Dual stream MIMO will
not lead to an improvement of cell border through-
put. However, transmit diversity, the fallback mode
for 2�2, may enhance cell border throughput as well
as the new enhanced receiver types. All these features
will reduce the gap between HSDPA and LTE DL per-
formance. However, all results presented for LTE are
without interference rejection combining, which will
reduce neighbor cell interference and, consequently,
will increase LTE DL cell border throughput.
Key Features in the Uplink of LTE and HSPA�
The uplink is most often the limiting link of
mobile broadband technologies, due in large part to
the limited transmit power available at the terminal as
well as the complexity and battery life constraints
imposed by practical handheld and portable devices.
In this section, we describe the key features of the
LTE uplink followed by the uplink enhancements in
HSPA�.
DOI: 10.1002/bltj Bell Labs Technical Journal 23
LTE Uplink Key FeaturesThe key features of the LTE uplink include a sin-
gle carrier multiple access technique which provides
in-cell orthogonality, the ability to obtain scheduling
gains based on both the time and frequency varia-
tions of the radio channel, and the ability to provide
the always-on connectivity experience for the
end user.
Multiple Access TechniqueUnlike OFDM used for the LTE downlink, which
is a multi-carrier OFDM transmission technique,
single carrier frequency division multiple access
(SC-FDMA) was chosen for the LTE uplink due to its
more favorable peak to average power ratio (PAPR)
characteristics. Consideration of PAPR is crucial in the
uplink where power efficient amplifiers are required
in the mobile device. In 2005, there was significant
discussion in 3GPP on whether single carrier fre-
quency division multiple access (SC-FDMA), orthogo-
nal frequency division multiple access (OFDMA),
and even multi-carrier CDMA (MC-CDMA) should
be chosen as the uplink multiple access method. In
the end, a majority of companies took the position
that SC-FDMA was the best uplink multiple access
method for LTE [1]. While SC-FDMA does have bet-
ter PAPR than OFDMA, unlike OFDMA the
SC-FDMA method suffers from inter-symbol inter-
ference when the assigned bandwidth is comparable
to or larger than the coherence bandwidth of the
channel (i.e., when the channel is frequency selec-
tive within the assigned bandwidth); OFDMA does
not have this drawback. Therefore, equalization is
required in the SC-FDMA receiver. To facilitate a sim-
ple one-tap frequency domain equalizer, the chosen
SC-FDMA technique uses a cyclic prefix as done in
the OFDMA downlink. Further, the basic numerol-
ogy in the uplink and downlink is the same; they
share the same resource block size of 180kHz (12 sub-
carriers), the maximum bandwidth utilization is the
same, and the SC-FDMA symbol time is the same as
the OFDMA symbol time for the downlink. In fact,
one way to implement the SC-FDMA transmitter is to
simply first take a DFT of the modulation symbols
prior to mapping the IFFT in a conventional OFDMA
transmitter. The only restriction is that the subcarriers
which are utilized for a particular user must be
contiguous in order to maintain the single carrier
property. While the single carrier property can also
be obtained using a distributed allocation with uni-
formly spaced subcarriers and inserting zeros in
between, this option was rejected by 3GPP due to
poor channel estimation performance and the
increased susceptibility to small frequency offsets from
the individual mobile devices.
In-Cell OrthogonalityOne important characteristic of the LTE uplink is
that the users remain orthogonal even in the pres-
ence of multipath, which is very different from the
HSUPA uplink based on asynchronous CDMA.
Orthogonality in the LTE uplink is maintained in two
ways: first, by time synchronizing the users to within
a small fraction of the CP through the use of timing
advance (TA) signaling; and second by the base station
scheduler ensuring that different users are assigned
different subcarriers. Loss of orthogonality between
users only occurs due to non-idealities in the system
such as a residual frequency offset between users or in
the case of very high Doppler where there is appre-
ciable variability in the channel during the SC-FDMA
symbol time. The fact that the LTE uplink is orthogo-
nal means that, ideally, users do not see intra-cell (i.e.,
same cell) interference as in the case of CDMA, and
hence the rise over thermal (RoT) is no longer the
factor which determines performance, rather it is
the interference over thermal (IoT), which is defined
as the total interference power (other cell interfer-
ence plus thermal noise) divided by the thermal noise.
Techniques to manage and control the IoT are still
being discussed in 3GPP, and require communication
between eNBs, which is made possible through the
X2 interface; this is sometimes also referred to as inter-
cell power control. As intra-cell interference is signifi-
cantly reduced in the LTE uplink, the use of fast
intra-cell power control is no longer necessary as in
the case of a CDMA uplink; rather, slow power con-
trol to compensate the path loss and shadowing is the
baseline power control technique for the LTE uplink.
Practically, such power control is needed to ensure that
the received power level from different mobile terminals
stays within a prescribed range due to the non-idealities
24 Bell Labs Technical Journal DOI: 10.1002/bltj
which lead to some degree of intra-cell interference,
as well as dynamic range and bit-width considera-
tions in the base station receiver.
Frequency Selective SchedulingAnother important feature of the LTE uplink
which differentiates it from HSUPA is the availability
of a channel sounding reference signal (SRS). The
SRS is a known sequence which is transmitted by the
mobile device, possibly over a wide bandwidth, in
order to allow the base station scheduler to obtain
channel state information. In this way, channel sen-
sitive scheduling in both time and frequency, referred
to generally as frequency selective scheduling,
becomes possible. This enhances the spectral effi-
ciency of the LTE uplink compared to HSUPA, espe-
cially for latency insensitive traffic, i.e., best-effort
traffic such as File Transfer Protocol (FTP) uploads or
e-mail attachments, in low Doppler conditions.
In order to capitalize on the improved SINR
offered by the orthogonal uplink and frequency
selective scheduling, the LTE uplink allows not only
QPSK modulation as in HSUPA Release 6, but also for
16 QAM and optionally 64 QAM.
Always-On ConnectivityFinally, a key feature of next-generation mobile
broadband technologies is to provide the end user
with the feeling of so called “always-on” data con-
nectivity; that is, the end user of a mobile broadband
device should experience connectivity similar to the
always-on wired broadband connections used in the
home or office. The simple solution is to keep users
fully connected to the wireless network as long as
the device is powered on; however this will pose
significant problems not only with device battery
lifetime, but also may result in inefficiency in terms
of air interface and base station channel card
resources.
In UMTS/HSPA, several levels of connectivity are
defined through the radio resource control (RRC)
states illustrated in Figure 10, which allow efficient
Idle mode
RRC connected mode
Establish RRCconnection
Release RRCconnection
Triggered by data activity
DCH—Dedicated channel FACH—Forward access channelPCH—Paging channelRRC—Radio resource control
Triggered by data inactivityURA_PCH or
CELL_PCH
CELL_FACH CELL_DCH
UMTS—Universal Mobile Telecommunications SystemURA—UTRAN registration areaUTRAN—UMTS terrestrial radio network
Figure 10.Radio resource control states for UMTS.
DOI: 10.1002/bltj Bell Labs Technical Journal 25
management of terminal battery lifetime as well as
base station channel card resources. The RRC con-
nected states are characterized as follows:
• URA_PCH/CELL_PCH allows the terminal to be
kept in a dormant (or standby) mode while
retaining an RRC connection with the RAN, as
well as a signaling and bearer plane connection to
the core network (i.e., the terminal retains its IP
address). This is very efficient in terms of terminal
battery life, as the device transceiver powers-on
periodically only to listen for paging messages, or
when it needs to send signaling messages.
• CELL_FACH allows for connectionless packet data
transfer that involves the use of the shared for-
ward access channel (FACH) in the downlink, and
the contention-based random access channel
(RACH) in the uplink. This state is used when only
small amounts of data need to be exchanged, i.e.,
via short message service (SMS), or as a transi-
tional state in which signaling messages are
exchanged between the terminal and the network,
i.e., to move from URA/CELL_PCH to CELL_DCH.
• CELL_DCH is a fully connected state in which
large amounts of data can be exchanged efficiently
between the terminal and the network; note that
both HSDPA and HSUPA are only applicable to
the CELL_DCH state in UMTS Release 6. In
Release 6, this state is characterized in the uplink
by the use of an always-on dedicated physical
control channel (DPCCH) which continuously
transmits a pilot, even in the absence of data, so
that power control can track the fading channel.
While the number of RRC connected sub-states
offers flexibility and efficiency in terms of terminal
battery life, air interface usage, and base station chan-
nel card resource utilization, it also results in relatively
long latencies when the terminal needs to transition
between dormant states (i.e., URA_PCH/CELL_PCH)
and fully active states (i.e., CELL_DCH), which makes
it difficult to give the end user the always-on data con-
nectivity experience.
In LTE, the states and state transitions are simpli-
fied significantly, as illustrated in Figure 11. Only an
RRC_IDLE and RRC_CONNECTED state are defined,
and in the RRC_CONNECTED state data can be quickly
exchanged between the terminal and the network
while terminal battery life can be managed via highly
flexible discontinuous receive (DRX) periods which
are under the control of the eNB. As the EPS archi-
tecture contains only a single network element in the
RAN (the eNB), the terminal can quickly move from
being dormant to being active as signaling only needs
to be exchanged between the terminal and the eNB.
In addition, there is no continuously transmitted pilot
channel in the LTE uplink as there is in HSUPA
Release 6; a data demodulation reference signal
(DM-RS) is transmitted by the terminal only when
there is data transmitted on the uplink, which is more
RRC_IDLE
Establish RRCconnection
Release RRCconnection
RRC_CONNECTED
LTE—Long term evolutionRRC—Radio resource control
Figure 11.Radio resource control states for LTE.
26 Bell Labs Technical Journal DOI: 10.1002/bltj
efficient from both an air interface point of view and
a terminal battery lifetime point of view.
HSPA� Uplink EnhancementsThe main enhancements for the HSPA� uplink
include the addition of 16 QAM in order to improve
peak user data rates as well as the continuous packet
connectivity feature which allows for an improved
always-on connectivity experience. Neither of these
features results in significant improvements in uplink
spectral efficiency, particularly in the typical macro-
cellular deployment conditions; hence HSPA� uplink
data capacity still falls short of LTE.
16 QAM ModulationHSPA� extends the uplink peak rate of HSUPA
from 5.76Mbps to 11.52Mbps through the addition of
16 QAM modulation. HSUPA Release 6 only allowed
QPSK modulation, more specifically, multi-code
binary phase shift keying (BPSK). 16 QAM is only
available with the 2 ms transport time interval (TTI)
length. As significantly higher SINRs are required to
support the extended rates offered by 16 QAM, an
advanced receiver at the node B, such as an LMMSE
sub-chip equalizer, is needed in order to prevent
SINR saturation due to self-noise in highly dispersive
channels. In addition, 16 QAM requires a stronger
phase reference than QPSK, so 3GPP has introduced
the concept of boosting the enhanced dedicated physi-
cal control channel (E-DPCCH) power level when 16
QAM modulation is used, with the intent that the
E-DPCCH be used as additional pilot in the demodu-
lation. Due to the range of SINRs experienced in typi-
cal macro-cellular deployments, 16 QAM offers little
gain in terms of average spectral efficiency; however
environments which offer a higher degree of cell
isolation (e.g., femtocells) may benefit more from
16 QAM.
It should be pointed out that if we consider an
LTE system in 5 MHz and consider that the minimum
amount of bandwidth is reserved for the physical
uplink control channel (PUCCH) is two resource blocks,
and also account for standards-based restrictions on the
number of subcarriers that can be allocated to a single
user, we find with 16 QAM (the highest mandatory
modulation in LTE) the peak user data rate in 5 MHz
of exactly 11.52 Mbps, which is precisely the uplink
peak rate offered by HSPA�.
Continuous Packet ConnectivityThe importance of the always-on connectivity
experience has already been discussed. It was recog-
nized in 2005 that HSPA would require enhance-
ments to efficiently support this feature, and a work
item called continuous packet connectivity was intro-
duced in 3GPP. Recall from Figure 10 that a user
needs to be in the CELL_DCH state in order to effi-
ciently exchange large amounts of data between the
user and the network; however, in the CELL_DCH
state the terminal is continuously transmitting the
DPCCH (consisting mainly of pilot and power control
bits). The DPCCH always transmits in the CELL_DCH
state even when the terminal has no data to transmit
in the uplink. In HSPA,it is common to move the user
into a CELL_PCH or URA_PCH state when there has
been a sufficiently long period of data inactivity from
the terminal; doing so not only reduces the uplink
interference level, but also extends terminal battery
life and saves base station channel card resources.
When data arrives in the terminal’s buffer, signaling
messages must be exchanged between the terminal
and the RNC in the CELL_FACH state in order to
move the terminal back into the CELL_DCH state.
This process involves a random access procedure and
establishment of radio bearers, which can take 700 ms
to 1 second depending on radio conditions. The latency
involved in moving a user between CELL_DCH and
CELL/URA_PCH repeatedly during a packet session
immediately detracts from the feeling of being always
connected. Hence, HSPA� introduced the CPC fea-
ture in order to make it more efficient to keep a user
in the CELL_DCH state. As illustrated in Figure 12,
the DPCCH is gated off when there is no data to trans-
mit on the uplink, which acts as a very quick way to
get the benefits offered by the CELL_PCH and
URA_PCH states; i.e., it reduces the level of interfer-
ence generated in the uplink as well as preserves the
terminal battery life. A DPCCH transmission cycle is
defined so that the DPCCH is transmitted periodically
even during data inactivity, in order to loosely main-
tain the power control state and synchronization with
the network. This allows the terminal to remain in
DOI: 10.1002/bltj Bell Labs Technical Journal 27
an efficient semi-dormant sub-state, and the transi-
tion back to a fully active sub-state can occur in less
than 50 ms; essentially this is the time required to
fully regain the power control state.
Use of HSDPA and HSUPA in CELL_FACHThe use of CPC in CELL_DCH state does not
eliminate the need for the URA_PCH or CELL_PCH
states, as it is still more efficient from a terminal bat-
tery lifetime point of view to be in the URA_PCH or
CELL_PCH states. In addition, when in CELL_DCH
state, the user mobility between cells is controlled
by the network, which incurs a higher signaling
overhead compared to the user-controlled mobility
through cell reselection in the URA_PCH and
CELL_PCH states. CPC allows for the expiration
timer to be set at much longer intervals before the
network decides to move the terminal into the
URA_PCH or CELL_PCH state, so that the user can
experience the feeling of always-on connectivity
during an extended data session. To further improve
the always-on experience, HSPA� introduced
enhancements known as enhanced CELL_FACH and
enhanced uplink in CELL_FACH which allows the
use of HSDPA and HSUPA in the CELL_FACH state,
respectively. HSDPA is used in lieu of the secondary
common control physical channel (S-CCPCH) to
carry both FACH and paging information and HSUPA
is used in lieu of sending RACH messages. In addi-
tion, it has been agreed that HSUPA can be used in
lieu of RACH messages even in the transition from
idle mode, i.e., for transmission of the common con-
trol channel. The intent here is to allow for faster
exchange of signaling messages to move the user
more quickly from idle, URA_PCH, or CELL_PCH
states to the CELL_DCH state. CPC, combined with
the use of HSDPA and HSUPA in the CELL_FACH
state, is the HSPA� solution to provide the user
experience of always-on data connectivity. Note that
the CPC feature does not directly translate into sig-
nificant improvement in spectral efficiency for best
effort data applications.
Uplink Performance ComparisonGiven that the HSPA� uplink features do not
result in significant spectral efficiency improvement
for best effort data applications, we focus here on
comparing the performance of LTE with HSUPA
Release 6 to check if the desired performance require-
ments of LTE were met. As in the case of the downlink,
the simulation assumptions from the next-generation
mobile networks (NGMNs) forum in [5] have been
Without CPC(Release 6)
With CPC(Release 7)
CPC—Continuous packet connectivityDPCCH—Dedicated physical control channel
DPCCH Data burst
Figure 12.Comparison of DPCCH transmission with and without CPC.
28 Bell Labs Technical Journal DOI: 10.1002/bltj
used; detailed assumptions can be found in [6]. We
have provided simulation results for the average cell
spectral efficiency in Table III for HSUPA and LTE
using the NGMN simulation assumptions. Results for
LTE have been given for a 5 MHz carrier for direct
comparison with HSUPA, as well as with a 10 MHz
carrier which is a more likely deployment scenario
for LTE. Unlike the NGMN simulation assumptions,
which simulate ten UEs per cell for both HSUPA in
5 MHz and LTE in 10 MHz, we have taken a more fair
approach and assumed five UEs per cell for HSUPA
as well as LTE in 5 MHz, and assumed ten UEs per cell
for LTE in 10 MHz. This ensures that the number of
UEs scales proportionally with the bandwidth, allow-
ing for a fair comparison of user throughputs. TableIV provides the cell edge user data rates for the
NGMN simulation assumptions. Note from Table III
and Table IV that LTE is able to provide the targeted
two to three times improvement in spectral efficiency
and cell edge user data rate compared to HSUPA in
Release 6.
While the NGMN set of simulation assumptions
described in [5] do provide a framework in which
results can be compared across different companies,
we feel that it is not necessarily a very realistic set of
assumptions even for the purposes of evaluating rela-
tive performance gains. One point in particular we
would like to highlight is that the NGMN assumption
set uses a typical urban (TU) channel model with a 3
kilometer/hour (km/hr) velocity. This channel type
has significant frequency selectivity (i.e., significant
multi-path delay spread) which hinders the performance
HSUPA Release 6 LTE LTE
(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in5 MHz) 5 MHz) 10 MHz) Improvement
Case 1 (1 � 2) 0.26 0.72 0.75 2.8–2.9x
Case 1 (1 � 4) 0.36 0.96 0.97 2.6–2.7x
Case 3 (1 � 2) 0.27 0.61 0.67 2.3–2.5x
Case 3 (1 � 4) 0.33 0.83 0.93 2.5–2.8x
Table III. Average uplink spectral efficiency (bps/Hz/cell) with NGMN assumptions.
HSUPA—High speed uplink packet accessLTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipment
HSUPA Release 6 LTE LTE
(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in5 MHz) 5 MHz) 10 MHz) Improvement
Case 1 (1 � 2) 125 245 306 2.0–2.4x
Case 1 (1 � 4) 168 368 430 2.2–2.6x
Case 3 (1 � 2) 35 50 60 1.4–1.7x
Case 3 (1 � 4) 42 85 125 2.0–3.0x
Table IV. Five percent CDF uplink user throughput (kbps) with NGMN assumptions.
CDF—Cumulative distribution functionHSUPA—High speed uplink packet accessLTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipment
DOI: 10.1002/bltj Bell Labs Technical Journal 29
of the HSUPA Release 6 system using a rake receiver
at the base station, while providing an advantage to
the LTE system, which capitalizes on the frequency
selectivity and low Doppler with its frequency selec-
tive scheduling feature. For additional insight on the
comparison between HSUPA Release 6 and LTE per-
formance, Table V and Table VI provide the per-
formance figures using all the NGMN assumptions
with the exception that the channel model is changed
to a mixture of ITU channel types given by the fol-
lowing: 30 percent Pedestrian A 3 km/hr, 30 percent
Pedestrian B 10 km/hr, 20 percent Vehicular A
30 km/hr, 10 percent Pedestrian A 120 km/hr, and 10
percent Ricean with K factor of 10 dB. This channel
mix uses channel types with both low and high fre-
quency selectivity as well as low and high Doppler.
We see from Tables V and VI that with this channel
mixture, LTE now improves performance only by a
factor of one to two over HSUPA Release 6. An
overview of LTE performance, with a more compre-
hensive set of realistic assumptions compared to those
used by NGMN, is given in [11].
Voice Over IP TransmissionWhile there is an increasing demand for high speed
data transmission over cellular networks, voice
still remains the dominant application today. Compared
to the traditional circuit-switched transmission, Voice
over IP offers a great deal of flexibility in providing
value-added services to the consumer, such as
enhanced caller identification (ID), call waiting and
voice mail features. In addition, VoIP is much easier to
integrate into multimedia applications which make use
of voice features (i.e., video calling, push-to-talk, and
HSUPA Release 6 LTE LTE
(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in5 MHz) 5 MHz) 10 MHz) Improvement
Case 1 (1 � 2) 0.42 0.74 0.75 1.8x
Case 1 (1 � 4) 0.64 1.01 1.03 1.6x
Case 3 (1 � 2) 0.38 0.58 0.63 1.5–1.7x
Case 3 (1 � 4) 0.58 0.83 0.94 1.4–1.6x
Table V. Average uplink spectral efficiency (bps/Hz/cell) using channel mixture.
HSUPA—High speed uplink packet accessLTE—Long term evolutionUE—User equipment
HSUPA Release 6 LTE LTE
(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in5 MHz) 5 MHz) 10 MHz) Improvement
Case 1 (1 � 2) 200 216 238 1.1–1.2x
Case 1 (1 � 4) 308 380 435 1.2–1.4x
Case 3 (1 � 2) 42 42 45 1.1x
Case 3 (1 � 4) 50 65 100 1.3–2.0x
Table VI. Five percent CDF uplink user throughput (kbps) using channel mixture.
CDF—Cumulative distribution functionHSUPA—High speed uplink packet accessLTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipment
30 Bell Labs Technical Journal DOI: 10.1002/bltj
interactive gaming). Another inherent advantage of
VoIP, especially in an end-to-end VoIP call, is that the
speech packets no longer need to traverse the public
switched telephone network (PSTN), which has a lim-
ited bandwidth and hence only supports an 8 KHz
(narrowband) sampling frequency. Significant
improvements in voice quality are possible by moving
to wideband vocoders which utilize a 16kHz sampling
rate, and this sampling rate can be maintained as the
packets traverse an IP network as opposed to
the PSTN.
In a VoIP system, the speech signal, after being dig-
itized and compressed by a vocoder, is packetized into
voice frames of a fixed duration in the application layer
called the Real Time Transport Protocol (RTP). A voice
frame duration of 20 ms is used by adaptive multi rate
(AMR) vocoders that are used in LTE and HSPA net-
works. The output bit-rate of AMR vocoders can be
adjusted between 12.2 kbps (best quality, highest bit-
rate) to 7.95 kbps to 5.9 kbps (lowest bit-rate, at the
expense of some voice quality). The RTP packets are
then transported in the network using a transport pro-
tocol such as User Datagram Protocol (UDP), and
routed using the Internet Protocol. Since a large over-
head of 40 bytes (more than 100 percent overhead) is
added by the RTP/UDP/IP version 4 (IPv4) layers, a
technique to compress the header information, called
robust header compression (RoHC), can be used to sig-
nificantly reduce the overhead. For example, in the
absence of RoHC, the 244 source bits that comprise a
20 ms AMR 12.2 kbps speech packet increases to 576
bits with the addition of headers and other overhead
before it reaches the LTE packet core. RoHC compresses
the 40 byte RTP/UDP/IPv4 header down to just 4 bytes.
Then we have two bytes of radio link control (RLC)
and MAC header that get added in the RAN.
VoIP Transmission Over LTEBoth LTE and HSPA� have incorporated air inter-
face enhancements in the MAC and physical layers
to enable efficient transmission of VoIP packets. In
this section we describe these features and provide
results of performance analysis for LTE and HSPA�.
SchedulingIn LTE, there is a choice of semi-persistent or
dynamic scheduling for VoIP packets. Semi-persistent
scheduling refers to the mode of scheduler operation
where a set of dedicated resources in time and fre-
quency are pre-allocated for the initial HARQ trans-
mission of every MAC packet. This means that the
network can allocate to a user a set of resource units
at specific intervals of time (e.g., once every 20 ms) on
the downlink and/or uplink, which will be used for the
transmission of initial HARQ transmissions of VoIP
packets. Retransmissions for these packets will have to
be scheduled dynamically, which on the uplink is
dynamic only in the frequency domain due to syn-
chronous HARQ operation. The benefit of semi-
persistent scheduling is in the reduction of MAC
control signaling that results from not having to trans-
mit dynamic scheduling grants on the uplink and data
associated signaling on the downlink for initial HARQ
transmissions.
A VoIP user may also be scheduled in a purely
dynamic scheduling mode, similar to any other data
user. Dynamic scheduling provides the flexibility
of scheduling the user’s transmissions at any time
and frequency, at the expense of higher control
signaling.
Frequency HoppingIn many instances, it may not be possible or effi-
cient to implement channel-selective scheduling for
VoIP users in LTE. Channel sensitive scheduling
cannot be used for persistently scheduled uplink
transmissions as the resources are pre-allocated. For
uplink transmissions that are dynamically scheduled,
channel-sensitive scheduling requires the uplink
channel sounding reference signal to be transmitted
over multiple resource blocks, which consumes
uplink bandwidth, and becomes infeasible when the
number of users is large. On the downlink, channel-
sensitive scheduling requires the mobile terminal to
provide CQI feedback for different frequency
resource blocks, which again becomes infeasible
when the number of voice users in the cell is large.
In situations where channel quality information can-
not be used for scheduling in the frequency domain,
frequency hopping can provide a significant diversity
gain by ensuring that a slow-moving user is not
“stuck” with a bad channel for a long time. Physical
layer frequency hopping is allowed in LTE.
DOI: 10.1002/bltj Bell Labs Technical Journal 31
Uplink Power ControlThe light load and low delay tolerance of VoIP
traffic imply that we cannot rely on a large number of
HARQ transmissions over a long period of time to
average out channel variations. Some form of power
control can hence be very useful to ensure that the
required signal-to-interference-plus-noise ratio is
consistently maintained and no more interference
than necessary is generated. In LTE, uplink power
control is applied on the transmit power spectral den-
sity (PSD) at the mobile device, and defined as the
transmitted power per physical resource block of
180 kHz. The PSD is computed using an open-loop
portion that depends on the long-term path loss
experienced at the mobile device, the average inter-
ference level seen at the base station receiver, and a
closed-loop portion that adjusts the open-loop set-
point using power control commands that may be
issued by the base station.
Coverage Enhancement TechniquesAs discussed in the next subsection on capacity
analysis, semi-persistently scheduled VoIP users on
LTE uplink face a coverage limitation in macro-cell
deployments with large building penetration losses.
Two coverage enhancement techniques are currently
being discussed in 3GPP to overcome this problem:
one is to let a single HARQ transmission span several
subframes before receiving an acknowledgement/
negative acknowledgement (ACK/NACK); the other is
to segment voice frames within the MAC layer before
transmission and use multiple HARQ processes to trans-
mit the different segments. Both techniques aim to
increase the amount of energy received from a cell-
edge mobile device per unit of time, thereby enhanc-
ing the cumulative received SINR for a voice frame.
Capacity AnalysisSimilarly to most other air interfaces, VoIP capac-
ity in LTE turns out to be uplink-limited. Using
semi-persistent scheduling, frequency-hopping and
closed-loop power control, we simulated uplink VoIP
transmissions at the system level under the NGMN
set of assumptions described in [5]. Capacity is defined
as the largest value of the average number of users per
cell for which no more than 5 percent of the users
each experience larger than 2 percent voice frame
outage. A voice frame is declared to be in outage if it
is not received successfully at the receiver, or if it is
received after the maximum tolerable one-way air
interface delay of 50 ms. The results are summarized
in Table VII for case 1 using an AMR 12.2 kbps
vocoder. We see that LTE provides approximately a
200 percent capacity improvement over Release 6
HSPA for case 1, which corresponds to a micro-cell
deployment. For case 3, which corresponds to a large
macro-cell deployment with 20 dB in-building pene-
tration loss, coverage was found to be inadequate for
the LTE uplink. The coverage enhancement tech-
niques outlined in the previous sub-section would
HSUPA Release 6 HSPA� LTE(5 MHz) (5 MHz) (5 MHz) Improvement
Case 1 (1 � 2) 73 100 220 3x HSUPA
Release 6
2.2x HSPA�
Table VII. VoIP capacity for uplink LTE and uplink HSPA for case 1 with NGMNassumptions, AMR 12.2 kbps vocoder.
AMR—Adaptive multi rateHSPA—High speed packet accessHSPA� —HSPA evolutionHSUPA—High speed uplink packet access
LTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipmentVoIP—Voice over Internet Protocol
32 Bell Labs Technical Journal DOI: 10.1002/bltj
have to be used before evaluating capacity for this
case.
VoIP Transmission Over HSPA�
HSPA� inherits all the key features of Release 6
HSPA that are useful for VoIP transmission, namely,
the scheduled nature of transmission, HARQ retrans-
missions, and link adaptation. The following are some
new features in HSPA� that further enhance VoIP
performance.
Uplink CPCAs described above in the section on HSPA�
uplink enhancements, uplink CPC decreases the frac-
tion of time that the mobile device is transmitting the
pilot channel (DPCCH). This is useful not only during
voice inactivity on the uplink, but even during a talk
burst since not all HARQ processes are utilized when
the 2 ms TTI length is chosen for HSUPA. The reduc-
tion in the DPCCH transmission, both during a talk
spurt and also during voice inactivity, results in a
reduction of the total interference level seen at the
base station receiver, which allows a larger number of
VoIP users to be supported for a given target loading.
In Table VII, we have included the HSPA� VoIP
capacity which utilizes the CPC feature, and we see a
37 percent improvement over HSUPA Release 6 VoIP
capacity. However, LTE still offers a 120 percent
improvement in VoIP capacity over HSPA�.
HS-SCCH Less OperationIf a system is loaded with a high number of low bit
rate users, the HS-SCCH would use a significant
amount of spreading codes and power on the HSDPA
downlink. A means to avoid this is the introduction of
the HS-SCCH less operation. A mobile device that
takes part in this operation mode has to blindly decode
all transport blocks sent over one spreading code of
the high speed downlink shared channel (HS-DSCH).
The corresponding transport blocks have only a limited
set of code rates and sizes, which are configurable per
mobile device, and sent with QPSK modulation only.
ConclusionIn this paper, we have described the key features
of the two technology paths in 3GPP for global mobile
broadband: the evolutionary HSPA� approach and
the revolutionary LTE approach. HSPA� offers the
advantage of backwards compatibility with earlier
releases, allowing operators an easier upgrade while
exploiting their current HSPA investment. On the
other hand, LTE offers significant improvements in
performance, especially in larger spectrum allocations,
but does not offer backwards compatibility to HSPA.
To deploy LTE, operators will need to consider new
spectrum which is becoming available, and eventu-
ally swap-out existing GSM/EDGE/UMTS/HSPA spec-
trum as the LTE technology matures.
The comparison provided in this paper illustrates
that there are many feature similarities between
HSPA� and LTE: the flatter network architecture with
clean separation of the user plane and control plane,
the availability of higher order modulations such as 64
QAM on the downlink and 16 QAM on the uplink,
MIMO techniques, and the ability to provide the
always-on data connectivity experience. While these
similarities exist, there are fundamental differences
between LTE and HSPA�; namely, the use of orthogo-
nal multiple access in LTE (OFDMA in the downlink,
SC-FDMA in the uplink) and the ability to exploit the
frequency-selective nature of the channel in both the
downlink and uplink. We have seen that LTE offers a
considerable advantage in spectral efficiency for best
effort data in the downlink and especially the uplink.
We have also seen that while both LTE and HSPA� pro-
vide features that enhance VoIP performance, LTE is
able to offer twice the VoIP capacity compared to
HSPA� for micocell deployments.
AcknowledgementsWe gratefully acknowledge Lutz Schönerstedt for
the LTE DL performance data and Michael Wilhelm
for the HSDPA performance data.
*Trademarks3GPP is a trademark of the European Telecommuni-
cations Standards Institute.CDMA2000 is a trademark of the Telecommunications
Industry Association.GSM and Global System for Mobile Communications are
registered trademarks of the GSM Association.
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[2] 3rd Generation Partnership Project,“Requirements for Evolved UTRA (E-UTRA) andEvolved UTRAN (E-UTRAN) (Release 7),” 3GPPTR 25.913, v7.3.0, Mar. 2006, �http://www.3gpp. org/ftp/Specs/html-info/25913.htm�.
[3] 3rd Generation Partnership Project, “Scope ofFuture FDD HSPA Evolution,” 3GPP RANPlenary #31, RP-060217, Mar. 2006, �http://www.3gpp.org�.
[4] 3rd Generation Partnership Project, “PhysicalLayer Aspects for Evolved Universal TerrestrialRadio Access (UTRA) (Release 7),” 3GPP TR25.814, v7.1.0, Sept. 2006, �http://www.3gpp.org/ftp/Specs/html-info/25814.htm�.
[5] 3rd Generation Partnership Project, “LTEPhysical Layer Framework for PerformanceVerification,” 3GPP TSG-RAN1 #48, R1-070674,Orange, China Mobile, KPN, NTT DoCoMo,Sprint, T-Mobile, Vodafone, Telecom Italia, Feb.2007, �http://www.3gpp.org�.
[6] 3rd Generation Partnership Project, “Uplink E-UTRA Performance Checkpoint,” 3GPP TSG-RAN WG1, R1-071989, Alcatel-Lucent, Apr.2007, �http://www.3gpp.org�.
[7] 3rd Generation Partnership Project, “EvolvedUniversal Terrestrial Radio Access (E-UTRA),Physical Channels and Modulation (Release 8),”3GPP TS 36.211, v8.1.0, Nov. 2007, �http://www.3gpp.org/ftp/Specs/html-info/36211.htm�.
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[11] K. Balachandran, Q. Bi, A. Rudrapatna, J. Seymour, R. Soni, and A. Weber, “PerformanceAssessment of Next-Generation Wireless MobileSystems,” Bell Labs Tech. J., 13:4 (2009), 35–58.
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(Manuscript approved October 2008)
ANIL M. RAO is a member of technical staff in Alcatel-Lucent’s wireless research anddevelopment (R&D) organization inNaperville, Illinois. He received a B.S. inapplied mathematics from the University ofAlaska, Fairbanks, and M.S. and Ph.D.
degrees in electrical engineering from the Universityof Illinois at Urbana Champaign where he held aNational Science Foundation graduate researchfellowship. Dr. Rao joined Alcatel-Lucent afterassignments with NASA’s Jet Propulsion Laboratoryand TRW. His work at Alcatel-Lucent has involvedvarious aspects of system design, performance analysis,and algorithm development for UMTS, HSPA/HSPA�,and LTE. He has actively contributed to both thestandardization and product realization of thesetechnologies. His interests include intelligentantennas, scheduling and resource allocationalgorithms, and optimizing the end-to-endperformance of mobile broadband wireless systems.
ANDREAS WEBER is team leader of the mobile systemperformance evaluation group in Bell Labs’Radio Access domain in Stuttgart, Germany.He received Dipl.-Ing. and Dr.-Ing. degreesin electrical engineering from the Universityof Stuttgart, Germany. Prior to joining
Alcatel-Lucent, Dr. Weber worked in the field ofsatellite communications as a member of scientific staffat the Institute of Communications Switching and DataTechnics, University of Stuttgart. During his tenure atAlcatel Research & Innovation and later at Bell Labs, heworked on the performance evaluation andoptimization of 2G, 3G, and beyond 3G mobilecommunication systems. Currently, he and his teamwork on LTE Advanced and WiMAX.
SRIDHAR GOLLAMUDI is a member of technical staffwith Alcatel-Lucent’s Wireless research anddevelopment (R&D) organization inWhippany, New Jersey. He received his Ph.D.in electrical engineering from the Universityof Notre Dame, Indiana.
Dr. Gollamudi worked at Motorola Inc. beforebeginning his career at Alcatel-Lucent. His researchinterests include statistical signal processing, resourceallocation in wireless systems, physical and MAC layeralgorithm design, and performance analysis ofcommunications systems.
34 Bell Labs Technical Journal DOI: 10.1002/bltj
ROBERT SONI is a technical manager in Alcatel-Lucent’sWireless business group in Whippany, NewJersey. He supervises a group which isinvestigating and developing new advancedantenna, physical layer and MAC layertechnologies for 3G/4G cellular systems. He
received a Ph.D. and MSEE in electrical engineeringfrom the University of Illinois at Urbana-Champaign,and received his BSEE, summa cum laude, from theUniversity of Cincinnati in Ohio. Dr. Soni began hiscareer as a member of technical staff at Alcatel-Lucentten years ago. He also teaches part-time at ColumbiaUniversity in New York City, and the New JerseyInstitute of Technology in Newark, New Jersey. ◆
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