5 HSPA Evolution

Ericsson Review No. 1, 200832

HSPA Evolution – Boosting the performance of mobile broadband accessJohan Bergman, Mårten Ericson, Dirk Gerstenberger, Bo Göransson, Janne Peisa and Stefan

Wager

Introduction

Today, 84% of all reported global cellular subscribers make calls via radio access tech-nology specifi ed by the Third Generation Partnership Project (3GPP). This signifi es that WCDMA is the predominant third-generation (3G) radio network technology, and as such, represents a substantial economy of scale.

In addition, the recent introduction of high-speed packet access (HSPA, 3GPP Re-lease 6) has

signifi cantly improved the performance of packet data traffi c in Ericsson products – on the downlink (HSDPA) 14Mbps, and uplink (E-UL), 1.4Mbps, giving users a mobile broadband experience that is on par with ADSL; and given rise to large increases in packet data traffi c – packet data is now the principal type of traffi c in 3G networks.1-2

•

•

Ericsson continues to build on this success by enabling operators who have deployed WCDMA to boost their system performance through software upgrades. The HSPA Evo-lution specifi cation (3GPP Releases 7 and 8) introduces new features that support higher data bit rates, lower latency, greater capacity, better support for VoIP, and improved sup-port for multicast services.1-2 Release 7 of the specifi cation introduces:

higher-order modulation (HOM);multiple input, multiple output (MIMO); continuous packet connectivity (CPC); layer-2 enhancements;enhanced CELL_FACH; multicast/broadcast single-frequency net-work (MBSFN); andadvanced receivers.

In addition, 3GPP is considering multi-carrier operation, downlink-optimized broadcast (DOB) and more advanced receiv-ers for future releases of the specifi cation.

••••••

•

These new concepts will yield substantial-ly higher peak data bit rates as well as greater spectral effi ciency and voice-over-IP (VoIP) capacity. In Release 8, for example, the peak data bit rates will reach up to 42Mbps in the downlink and 11Mbps in the uplink (per 5MHz carrier).

This article deals primarily with the fea-tures 3GPP has added to HSPA in Releases 7 and 8 of the WCDMA specifi cation.1-2 It also touches on candidate techniques for further evolving the specifi cation in future releases.

HSPA features and performance

Higher-order modulationThe digital modulation scheme determines how bits are mapped to the phase and am-plitude of transmitted signals. Figure 1 de-picts the constellation diagrams of a variety of modulation schemes. Each consecutive bit sequence is mapped to a modulation symbol whose phase and amplitude cor-respond to one of the constellation points. The number of bits conveyed per modula-tion symbol is as follows: 1 for BPSK, 2 for QPSK, 4 for 16QAM, and 6 for 64QAM. Therefore, higher modulation order means greater achievable peak data bit rate for a given symbol rate.

HSPA (3GPP Release 6) supports the QPSK and 16QAM modulation schemes in the downlink and the BPSK and QPSK modulation schemes in the uplink. Release 7 introduces higher-order modulations that increase the data bit rate. In the down-link, it introduces 64QAM, increasing the peak data bit rate by 50% from 14Mbps to 21Mbps. In the uplink, 16QAM doubles the peak data bit rate from 5.7Mbps to 11Mbps.

Furthermore, the physical control channels (HS-SCCH, HS-DPCCH, E-AGCH and E-DPCCH) have been modifi ed to support

signaling of the new modulation schemes; larger transport block sizes; andlarger range for the channel quality indica-tor (CQI).

Higher-order modulation enables users to experience signifi cantly higher data bit rates under favorable radio-propagation condi-tions. Careful cell planning is a prerequisite for achieving these conditions. Table 1 sum-marizes a number of HSPA features.

•••

This article presents the main concepts of HSPA Evolution (high-speed packet access evolution), which is currently being standardized by 3GPP in Releases 7 and 8 of the WCDMA specifi cation. The aim of HSPA Evolu-tion is to further improve the performance of WCDMA systems through higher peak rates, lower latency, greater capacity, increased battery times, better support for VoIP, and improved multicast/broadcast capabili-ties.

The authors briefl y cover MIMO, higher-order modulation, protocol optimizations, optimizations for VoIP, improved multicast/broadcast, and advanced receivers. They also describe a variety of features (such as multicarrier operation) that will boost performance even further in subse-quent releases of the WCDMA specifi cation.

TERMS AND ABBREVIATIONS

3GPP Third Generation Partnership

Project

ADSL Asymmetric digital subscriber line

BPSK Binary phase-shift keying

CPC Continuous packet connectivity

CQI Channel quality indicator

DOB Downlink-optimized broadcast

E-UL Enhanced uplink

FDD Frequency-division multiplexing

HARQ Hybrid automatic repeat request

HOM Higher-order modulation

HSPA High-speed packet access

(HSPDA + E-UL)

HSPDA High-speed packet downlink

access

MAC Media access control

MBSFN Multicast/broadcast single-

frequency network

MIMO Multiple input, multiple output

MMSE Minimum mean-squared error

PDU Packet data unit

QAM Quadrature amplitude modulation

QPSK Quadrature phase-shift keying

RLC Radio link control

RRC Radio resource control

RTT Roundtrip time

SDU Service data unit

TDD Time-division multiplexing

UE User equipment (terminal)

VoIP Voice over IP

WCDMA Wideband code-division multiple

access

Review108.indd 32Review108.indd 32 08-01-21 16.12.0908-01-21 16.12.09

Ericsson Review No. 1, 2008 33

Multiple input, multiple outputOne can increase the data bit rate by trans-mitting multiple transport blocks in paral-lel using multiple antennas to a single user. This technique is often termed MIMO with spatial multiplexing (as opposed to trans-mit/receive diversity, where a single trans-port block is sent or received by multiple antennas). The receiver uses channel proper-ties and knowledge of the coding scheme to separate the data streams. Note: A prerequi-site for MIMO is the standardization of the multilayer transmission scheme.

For HSDPA, 3GPP chose a MIMO scheme that is based on precoded and rank-adaptive multi-codeword transmission. This means that

each layer (substream) carries separate transport blocks; and the number of parallel streams can be adapted to the current channel conditions (rank adaptation).

Separate encoding facilitates the use of suc-cessive interference cancellation receivers, which are anticipated to boost performance compared with linear receivers (for example, MMSE-based receivers).

Before data is transmitted, the modulated and spread signal is spatially weighted (pre-coded) or, in other words, the data streams are transmitted over separate transmit an-tennas using different transmission weights. The preferred weights are estimated by the user equipment (UE) and fed back to the network together with the channel quality indicator. The main benefi t of precoding is that both power amplifi ers are loaded even when a single transport block is transmitted. When two streams are transmitted, each stream contains the same channelization codes. Figure 2 shows the MIMO transmit-ter chain.

To incorporate MIMO into Release 7, 3GPP has updated the uplink and downlink physical control channels (HS-SCCH and HS-DPCCH) to accommodate information about precoder weights, the transport for-mat, and hybrid automatic repeat request (HARQ) parameters per stream.

In Release 7, MIMO is defi ned for trans-mitting up to two streams. In this case, each stream can use QPSK or 16QAM, extending the peak data bit rate of HSDPA to approxi-mately 28Mbps. In Release 8, each stream can use 64QAM, which extends the peak data bit rate to 42Mbps (Figure 3).

HARQ acknowledges each stream sepa-rately and independently. Undetected blocks

•

•

TABLE 1. HSPA FEATURES

Feature Explanation

Higher-order modulation Using more bits per symbol raises the peak

data bit rate by 50% in the downlink and 100%

in the uplink.

Multiple input, multiple output Employing two transmitter antennas at Node-B

and at least two receiver antennas at the UE

has the potential to increase the peak data bit

rate by 100%.

Multicarrier By using N 5MHz carriers, one can increase

the data rate can N-fold.

Continuous packet connectivity (CPC) CPC improves the physical layer signaling,

resulting in lower latency, greater capacity, and

better battery time.

Layer-2 enhancements To match the higher bit rates provided by the

physical layer, layer 2 has been optimized to

support higher peak data bit rates and greater

coverage.

Enhanced CELL_FACH Improved support for background traffi c and

faster switching to continuous transmission

state.

MBMS single-frequency network, downlink-

optimized broadcast

Transmitting the same waveform from mul-

tiple cells puts the broadcast capabilities of

WCDMA on par with, for example, DVB-H.

Figure 1Constellation diagrams of modulation schemes.



can thus be retransmitted independently be-tween the two streams.

Multicarrier operationMulticarrier operation is a candidate for a future 3GPP release of the WCDMA speci-fi cation. Operators with access to multiple adjacent paired frequency bands can make effi cient use of spectrum by operating HSPA on multiple adjacent 5MHz carriers in a co-ordinated way. Control channel structures, for instance, need not be fully replicated in all carriers. By having only one anchor car-rier, for example, one can have more power for HSPA in the other carriers. Therefore, if it is impractical to deploy 2x2 MIMO, opera-tors might consider multicarrier operation on two carriers as an alternative way of reach-ing downlink peak data bit rates of 42Mbps. On the other hand, however, the operation of two carriers in combination with 2x2 MIMO and 64QAM will yield peak data bit rates of up to 84Mbps without requiring 4x4 MIMO antenna systems. What is more, four carriers can support 4x42Mbps.

Layer-2 enhancementsThe downlink peak data bit rate using ac-knowledged-mode radio link control (RLC) is limited by the size of the RLC protocol data unit (PDU), RLC roundtrip time (RTT), and the size of the RLC window.

A large RLC PDU size is needed to sustain the peak data bit rates obtained in the down-link with MIMO and 64QAM modulation. Therefore, to make effi cient use of large PDU sizes, and to generally enhance the performance of the layer-2 protocol, Release 7 adopts fl exible RLC sizes, media access control (MAC) segmentation, and improved MAC multiplexing for downlink transmis-sion. As a consequence, the transmitter may freely select the size of the RLC PDU.

The ability to segment the RLC has been preserved, and in general the network can segment RLC service data units (SDU) into PDUs, which facilitates more effi cient trans-mission and retransmission over the air inter-face. The ability of the transmitter to fl exibly select the size of RLC PDUs helps reduce level-2 protocol overhead by reducing RLC header overhead and padding. In addition, the use of larger PDUs means that UEs need not process as many of them.

In the downlink, the RLC protocol is originated in the radio network controller (RNC), whereas the MAC-hs is terminated in the Node-B. If, due to changing radio con-

Figure 3Ninetieth percentile throughput in Pedestrian A-channel for higher-order modulation and multiple input/multiple output (MIMO).

Figure 2The MIMO transmitter chain.



ditions, the RLC PDUs are too large to be transmitted over the air interface with a rea-sonable number of HARQ retransmissions, then they must be segmented. Accordingly, Release 7 introduces a new MAC protocol, MAC-ehs, which supports fl exible RLC PDU sizes and the segmentation of RLC PDUs. In addition, the MAC multiplexing capabilities have been improved so that RLC PDUs that carry signaling or data from different radio access bearers can now be multiplexed into a single MAC-ehs PDU.

In Release 8, the enhancements made to the downlink protocol will be applied to the uplink protocol. Support for fl exible RLC PDU sizes improves uplink coverage and helps reduce processing and level-2 over-head.

Continuous packet connectivityThe activity level of packet data users var-ies considerably over time. Even so, from an end-user point of view, in order to avoid the delays associated with state transitions, it might be advantageous to remain in a state with a dedicated connection (CELL_DCH) even when temporarily inactive.

3GPP has worked to make the dedicated connection state for packet data users more

effi cient in Release 7. The result of these ef-forts is commonly referred to as continuous packet connectivity (CPC). CPC consists of two main features called UE DTX/DRX and HS-SCCH-less operation.

UE DTX/DRX

UE DTX (discontinuous transmission from the UE) allows UEs to switch off continuous transmission of the dedicated physical control channel (DPCCH) when there is no informa-tion to transmit in the uplink. When this is the case, only a minimum of transmission is needed to maintain synchronization and control power. Two immediate benefi ts of switching off transmission are reduced bat-tery consumption and reduced interference, which increases uplink capacity (in terms of number of users).

By the same token, UE DRX (discontinu-ous reception at the UE) allows UEs to switch off their receivers when there is nothing to be received in the downlink. This technique can be exploited to further reduce battery con-sumption. The UEs need only periodically check to see if they should to wake up from their “micro sleep” mode.

In Release 7, UE DTX/DRX can be de-ployed even during very short periods of in-

activity between the packets of a VoIP call.HS-SCCH-less operation

When many small packets (typically VoIP packets) are transported in the downlink, the overhead from the downlink control channel, HS-SCCH, becomes signifi cant. Release 7 introduces HS-SCCH-less operation, which reduces this overhead by removing the HS-SCCH transmission completely for the fi rst HARQ transmission. HS-SCCH-less opera-tion relies on blind decoding of up to four different formats of the downlink data chan-nel, HS-DSCH, in the UE to eliminate the need for HS-SCCH transmission. In sum-mary, the concept increases capacity in the downlink by reducing code usage as well as interference from control signaling. Figure 4 shows the relative increase in VoIP capacity compared with 3GPP Release 6.

Simulations show that the CPC concept in Release 7 boosts capacity for VoIP by around 40% in the uplink and 10% in the downlink. The addition of advanced receivers boosts ca-pacity even further.7

Enhanced CELL_FACHHSPA is becoming a replacement to ADSL for connecting PCs to the internet. This change in behavior has an impact on traf-

Figure 4Relative increase in VoIP capacity.



fi c load and network characteristics. PCs run a range of applications that communi-cate in the background without the need for end-user interaction. Among other things, background traffi c consists of keep-alive messages, probes for software updates, and presence signaling. To effi ciently support this traffi c, the 3GPP has worked to enhance the CELL_FACH state in Releases 7 and 8.

In Release 7, HSDPA has been activated for users in CELL_FACH. In the downlink, UEs monitor the HSDPA control channels to detect scheduling information for their own specifi c identities (H-RNTI). The lack of a dedicated uplink channel means that the specifi cation (Release 7) does not support continuous transmission of the channel qual-ity indicator (CQI) or HARQ feedback. As a consequence, 3GPP has modifi ed link ad-aptation and HARQ. The solution has been to employ HARQ repetitions and to base link adaptation on measurements of radio resource control (RRC).

In Release 8, the uplink is improved by activating E-DCH in CELL_FACH. Trans-mission begins by ramping up power on a random preamble as is done in Rel-99. After having detected the preamble, the Node-B assigns the UE to a common E-DCH con-fi guration (managed by Node-B). Contention on the common E-DCH is resolved by means of UE identities in the E-DCH transmis-sion. A given UE can effi ciently be moved to CELL_DCH for continuous transmission.

The enhanced CELL_FACH attempts to have the same layer-2 header format as that of the CELL_DCH described above. This way, data transmissions can continue with-out interruption, even when switching be-tween CELL_FACH and CELL_DCH. This enhancement signifi cantly improves user perception of performance compared with Release 6, which suspends data transmission for channel switching.

MBSFN, downlink-optimized broadcast Release 7 has further optimized MBMS to boost transmission effi ciency beyond what could be achieved with multicell MBMS transmission in Release 6.8 The solution, multicast/broadcast single-frequency net-work (MBSFN) operation, calls for simulta-neous transmission of the exact same wave-form from multiple cells. This way, the UE receiver perceives the multiple MBSFN cells as one large cell (Figure 5). Also, instead of inter-cell interference from neighboring cell transmissions, the UE experiences construc-tive superposition of the signals transmitted from multiple MBSFN cells. What is more, advanced UE receiver techniques such as G-RAKE eliminate intra-cell interference by resolving the time difference of multipath propagation. The result is highly effi cient radio broadcast transmission derived from WCDMA technology.

A critical enhancement that eliminates inter-cell interference is the use of a common scrambling code on downlink carriers re-served for MBSFN transmission. In this case, 3GPP did not have to modify the standard because synchronized network operation for FDD was included in Release 99.

Broadcast data is transmitted using the same logical and physical channel structures as for MBMS – that is, MTCH, FACH and S-CCPCH, along with control channels such as MICH and MSCH. MBSFN improves power effi ciency so much that the limiting factor in the radio downlink is no longer power but rather codes. For this reason, to make full use of all available radio resources, 16QAM modulation has been introduced for MBSFN FACH. And to further enhance channel estimation in the mobile receiver, Release 7 introduces support for a time-multiplexed pilot that uses the synchroniza-tion channel (SCH). To signifi cantly reduce battery consumption in UEs, one may even

multiplex services per transmission time in-terval (TTI).

3GPP proposes to take MBSFN operation one step further, by introducing the down-link-optimized broadcast (DOB) concept as a special mode of 3.84Mbps time-division duplex (TDD) operation in unpaired bands of spectrum.

The principles and radio solutions for DOB are identical to those for MBSFN FDD, which means radio performance is identical. It also means that WCDMA opera-tors with access to unpaired spectrum have highly attractive options for migrating their networks. The impact of DOB on UEs and Node-Bs is minor thanks to maximum com-monality between WCDMA MBMS and MBSFN. DOB is thus a promising option for deploying MBSFN in unpaired bands of spectrum.

Advanced receiversReceiver structures in UEs and Node-Bs are constantly being improved as products evolve and more complex features are added to HSPA. The result is improved system performance and higher user data bit rates. This trend is refl ected in constantly chang-ing UE receiver requirements in 3GPP: advanced UE receiver requirements were introduced in Releases 6 and 7, refl ecting the use of

UE receive antenna diversity (UE receiver type-1);linear equalizers, such as G-RAKE (UE receiver type-2); andlinear equalizers in combination with UE receive antenna diversity, such as G-RAKE2 (UE receiver type-3, suitable for example, for MIMO).

Release 8 will introduce requirements for even advanced receivers (type-3 G-RAKE2) with additional support for interference can-cellation (UE receiver type-3i).

•

•

•

Figure 5Mobile TV broadcast with MBSFN.



Conclusion

HSPA Evolution (3GPP Releases 7 and 8) enables operators to prolong the life of past investments by further improving the performance of WCDMA systems. In par-ticular, HSPA Evolution introduces several new features that support higher data bit rates, reduce latency, increase capacity, and improve support for VoIP and multicast ser-vices:

Higher-order modulation. In the down-link, the introduction of 64QAM in-creases the peak data bit rate to as much as 21Mbps. Likewise, the introduction of 16QAM in the uplink increases the peak data bit rate to 11Mbps.Multiple input, multiple output (MIMO). In Release 7, MIMO is defi ned for trans-mitting up to two streams. In this case, each stream can use QPSK or 16QAM, ex-tending the peak data bit rate of HSDPA to approximately 28Mbps. In Release 8, each stream can use 64QAM, which ex-tends the peak data bit rate to 42Mbps.Continuous packet connectivity (CPC). Simulations show that the CPC concept in Release 7 boosts capacity for VoIP by around 40% in the uplink and 10% in the downlink.Layer-2 enhancements. Release 7 introduc-es a new MAC protocol, MAC-ehs, which supports fl exible RLC PDU sizes and the segmentation of RLC PDUs. In addition, the MAC multiplexing capabilities have been improved so that RLC PDUs which carry signaling or data from different ra-

•

•

•

•

dio access bearers can now be multiplexed into a single MAC-ehs PDU. In Release 8, the enhancements made to the downlink protocol will be applied to the uplink pro-tocol. Support for fl exible RLC PDU sizes improves uplink coverage and helps reduce processing and level-2 overhead.Enhanced CELL_FACH. In Release 7, HSDPA has been activated for users in CELL_FACH. In Release 8, the uplink is improved by activating E-DCH in CELL_FACH. This enhancement signifi -cantly improves user perception of perfor-mance compared with Release 6, which must suspend data transmission for chan-nel switching.Multicast/broadcast single-frequency net-work (MBSFN). MBSFN calls for simul-taneous transmission of the exact same waveform from multiple cells. This way, the UE receiver perceives the multiple MBSFN cells as one large cell.Downlink-optimized broadcast (DOB). 3GPP proposes to take MBSFN opera-tion one step further, by introducing DOB as a special mode of 3.84Mbps time-division duplex (TDD) operation in un-paired bands of spectrum.Advanced receivers. Receiver structures in UEs and Node-Bs are constantly be-ing improved as products evolve and more complex features are added to HSPA. The result is improved system performance and higher user data bit rates.

For future releases of the specifi cation, 3GPP is considering multicarrier operation and even more advanced receivers.

•

•

•

•

REFERENCES

Third Generation Partnership Project, http://www.3gpp.org

Dahlman, E., Parkvall, S., Sköld, J. and Beming, P.: 3G Evolution, HSPA and LTE for Mobile

Broadband. Academic Press

Peisa, J., Wager, S., Sågfors, M., Torsner, J., Göransson, B., Fulghum, T., Cozzo, C. and Grant,

S.: High Speed Packet Access Evolution – Concept and Technologies. Proceedings of the

IEEE VTC Spring 2007

Göransson, B., Cairns, D., Wang, Y.-P. E., Cozzo, C., Fulghum, T. andGrant, S.: Evolution of

WCDMA High Speed Access and Broadcast Services. SPAWC 2007, Helsinki, Finland

Derksen, J., Jansen, R., Maijala, M. and Westerberg, E.: HSDPA performance and evolution.

Ericsson Review, Vol 83(2006):3, pp 117-120

Peisa, J., Ekström, H., Hannu, H. and Parkvall, S.: End-to-End Performance of WCDMA

Enhanced Uplink. VTC Spring 2005

Murai, H. et. al.: System Performance for WCDMA Up-Link Interference Cancellation –

Simulated Results and Field Measurements Properties. WPMC’03 Osaka, Japan

Bakhuizen, M. and Horn, U.: Mobile Broadcast/Multicast in Mobile Networks. Ericsson Review,

Vol. 82(2005): 1 pp. 6-13

1.

2.

3.

4.

5.

6.

7.

8.


5 HSPA Evolution

Documents

Transcript of 5 HSPA Evolution