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1WiMAX Worldwide Interoperability for Microwave Access
WiMAXWorldwide Interoperability for Microwave Access
White Paper
WiMAX is a wireless access technology for building networks with large coverage areas and high data rates,so-called Metropolitan Area Networks (MANs). It focuses on various usage scenarios for serving fixed, no-madic and mobile subscribers and incorporates a broad range of transmission and access technologies, whichcan be dynamically applied for serving these different types of subscribers. In addition, it provides mechanismsfor giving Quality-of-Service (QoS) guarantees, and thus it is predestined for enabling real-time services likeVoice over IP (VoIP), video on demand or multiplayer gaming.
1 Introduction
Broadband access is the main prerequisite for delivering
highly sophisticated IT services to the end user, for example,
video on demand, video conferencing, Voice over IP (VoIP)
or interactive gaming. After the Internet and mobile com-
munications reached the mass markets in the mid-1990s,
it turned out very soon that existing network technologies
such as the analog Plain Old Telephone System (POTS),
the Integrated Services Digital Network(ISDN) or the Glo-
bal System for Mobile Communications (GSM) could not
fulfil the requirements imposed by these applications. The
main reason for this lack was simply the fact that these sys-
tems were initially designed for speech telephony, which tra-
ditionally is a circuit-switched service of comparatively low
bandwidth. As a result, standardization and manufacturers
created enhancements and auxiliary technologies for bridg-
ing the gap between the capabilities of existing networks
and the requirements of emerging applications. Examples
are the Digital Subscriber Line (DSL) for delivering pack-
switched data at high rates over the telephone wire to the
end user or the General Packet Radio Service(GPRS) for
introducing packet-switched services in GSM networks.
In the recent years, DSL has become the standard solu-
tion for fixed broadband access in the consumer market.However, it requires complex modifications on the infra-
structure of telephony networks and is therefore often not
available in rural environments with a low population density.
In the mobile area, on the other hand, the breakthrough of
data services is still missing. GPRS has been introduced by
all GSM operators in the meantime, but it suffers from low
data rates and high delays. Even the Universal Mobile Te-
lecommunications System (UMTS), which was introduced
a few years ago as the successor of GSM and which actu-
ally targets also at packet-switched data, could not initiate
a turn-around towards a broad acceptance of mobile data
services so far.
A new technology that specifically focuses on broadband
access is called WiMAX (Worldwide Interoperability for Mi-
crowave Access). It is a wireless technology that does not
necessarily replace the systems mentioned before, but
that at least acts as an extension, for example, in regions
where other broadband technologies are not available or
do not provide sufficient capacity or bandwidth. WiMAX has
been designed for operation in a broad range of licensed
and unlicensed frequency bands, thereby being much more
flexible than cellular networks like GSM and UMTS, which
are confined to operation in dedicated, licensed frequency
bands being subject to regulation. WiMAX covers different
usage scenarios, ranging from supporting mobile users to
connecting LANs (Local Area Networks) to the Internet. To
fulfil the heterogeneous requirements on data transmission
imposed by these scenarios, WiMAX incorporates severalphysical layers with different modulation schemes, antenna
designs and other features. Furthermore, WiMAX provides
sophisticated functions for guaranteeing a certain quality-
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2 WiMAX Worldwide Interoperability for Microwave Access
of-service(QoS) during transmission, which is of particular
importance for real-time or near real-time applications like
VoIP or video streaming.
WiMAX is standardized by the Institute of Electrical and
Electronics Engineers (IEEE), the same institution that is
also responsible for standardization of other wired and wire-
less access technologies, for example, Ethernet and WLAN
(Wireless Local Area Network). The group within IEEE
consigned with the specification of WiMAX is known under
the identifier 802.16 and denoted as Broadband Wireless
Access Working Group. Strictly speaking, the official term
for WiMAX is actually Wireless Metropolitan Area Network
(WirelessMAN). The term WiMAX stems from the WiMAX
forum, which is an organization of more than 400 opera-
tors and manufacturers being concerned with promot-
ing and certifying the compatibility and interoperability of
broadband wireless access equipment that conforms to the
IEEE 802.16 standards [1]. However, in the recent years,
the term WiMAX has prevailed against WirelessMAN or802.16, and that is why this term is also used throughout
this paper.
The following sections give an overview of WiMAX and
introduce its usages scenarios, transmission technologies
and basic services.
2 WiMAX Usage Scenarios
The WiMAX usage scenarios are commonly referred to as
fixed, nomadicand mobile access, and they are covered by
different documents of the IEEE 802.16 standards family.The scenarios impose very different requirements on the
used frequency bands, modulation schemes, medium ac-
cess, and mobility mechanisms, and hence WiMAX today
incorporates a number of variants of these technologies.
2.1 Fixed WiMAX
Initially, WiMAX was designed only for fixed access. The first
in a series of standards was released in December 2001 by
IEEE and was called IEEE 802.16. It defines a system for
the wireless transmission between stationary senders and
receivers in outdoor environments. The main components
of the system are base stations, which are located at thecell sites of the WiMAX operator, and subscriber stations,
which are usually installed at the roofs of buildings at the
WiMAX customers, see Figure 1. The subscriber stations
have antennas with dimensions comparable to those of sat-
ellite dishes. They are connected typically to a local network
of the subscriber, for example, a WLAN or Ethernet instal-
lation inside the building. The base stations, on the other
hand, may be interconnected to public networks like the
Internet or to private ones. In an alternative scenario, Fixed
WiMAX might also be used by a cellular network operator
for realizing connectivity between the cell sites and the core
network.Fixed WiMAX has been designed for operation in a very
broad frequency range between 10 and 66 GHz with band-
widths of 20, 25 or 28 MHz per radio channel. Under opti-
mal conditions, it may achieve transmission ranges of up to
70 km and data rates of up to 134 Mbps. As radio signals
above 10 GHz can hardly penetrate obstacles like buildings
or hills, an important prerequisite for successful transmis-sion is that a line-of-sight(LoS) path exists between sub-
scriber and base station that is not obstructed by obstacles.
Thus, Fixed WiMAX represents an interesting alternative to
older or proprietary LoS radio systems of less bandwidth,
for example, Wireless Local Loop(WLL).
2.2 Nomadic WiMAX
The major drawback of Fixed WiMAX is the need for out-
door antennas at the subscriber, which requires a cumber-
some wiring inside buildings and fixed antenna installations
at roofs of considerable height for guaranteeing LoS condi-tions to the next base station. In order to address these is-
sues, the IEEE has released another standard in April 2003,
which is called IEEE 802.16a and which focuses on the no-
madic WiMAX access. Radio channels of Nomadic WiMAX
occupy frequency bands in the range between 2 and 11
GHz, which in contrast to higher frequencies allow for non-
light-of-sight(NLoS) transmissions between subscriber and
base stations and vice versa. As a result, it becomes pos-
sible to built WiMAX transceivers with integrated antennas,
which can be connected directly to a PC or included into
handheld devices or laptops, for example, in form of PCM-
CIA cards. A fixed-installed outdoor antenna is not neces-
sary any longer, and the WiMAX customer can enter intocontact from everywhere within the coverage area of a base
station, even from the inside of buildings. This is illustrated
in Figure 2.
A radio channel of nomadic WiMAX occupies a band-
width between 1.75 and 20 MHz. The bandwidth has been
kept variable, because frequency allocation and licensing
are managed very irregular in different countries of the
world and significantly vary in the size of frequency bands
assigned to the operators.
However, the flexibility of nomadic access must be paid
by a significant decrease in the transmission range and data
rates when compared to Fixed WiMAX. The coverage areaof a base station is limited to a radius of 5 km. The maximum
data rate, which only has been achieved in field tests so
far, is about 70 Mbps, but is expected to be much lower for
Figure 1. Fixed WiMAX
((((((
Base station
Subscriber station
Indoor network installation
LoS
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networks operating under real conditions.
Both Fixed and Nomadic WiMAX can be operated in
two modes, which are referred to as point-to-point (PTP)
and point-to-multipoint(PMP) modes. In the former, a basestation serves only a single subscriber station, which can
exclusively use the entire bandwidth of the radio channel.
In PMP, on the other hand, a base station supplies several
subscriber stations at once, and hence the available band-
width must be shared among all subscribers residing in the
particular cell. The PTP mode is primarily intended for Fixed
WiMAX, while PMP is the preferred choice for nomadic ac-
cess.
In June 2004, standardization activities for Fixed and
Nomadic WiMAX were merged. The resulting standard
document is called IEEE 802.16-2004 [2] and replaces
the former versions IEEE 802.16 and 802.16a. Frequencylicensing and first commercial trials for WiMAX in many
countries started in 2005, while related products and serv-
ices for the mass market have been announced to become
available in 2007. Starting from this time, it is expected that
Fixed and Nomadic WiMAX will be requested especially by
customers residing in rural areas, which often suffer from
the unavailability of wired broadband technologies like DSL,
cable modem or T1 access.
2.3 Mobile WiMAX
A drawback of Nomadic WiMAX is that a service session
can only be maintained as long as the subscriber residesin the coverage area of the base station where this session
has been initiated. If the subscriber moves from one cover-
age area to that of another base station, the session is ter-
minated and must be re-initiated at the new base station. An
automatic transfer of the session from the serving to another
target base station, a process which is called handover, is
not possible in Fixed or Nomadic WiMAX systems.
The missing support of mobile subscribers has led to
initiatives for creating Mobile WiMAX, which, besides vari-
ous handover mechanisms, also incorporates other mobil-
ity functions (see also Figure 3). Mobile WiMAX envisages
two access modes, which are called portable and mobileaccess. The portable access mode serves customers trav-
elling at pedestrian speeds. When changing the cell, the
service session is transferred to the target base station by
3WiMAX Worldwide Interoperability for Microwave Access
a so-called hard handover. This type of handover is charac-
terized by the fact that the connection to the serving base
station is terminated before a new one to another target
base station is initialized (break-before-make). As a result,the customer experiences a short degradation in the quality
of service, that is, an interruption of the data transfer, until
the handover is completed. The mobile access mode, on
the other hand, has been designed for supporting custom-
ers travelling at velocities of up to 125 km/h. It implements
a soft-handover, where the connection to the target base
station is established before the old connection is released
(make-before-break). A soft handover happens seamless-
ly from the point of view of the customer and has a much
lower latency than a hard handover. However, this reduced
latency must be paid by a much higher complexity in the
hardware.Besides these handover mechanisms, Mobile WiMAX
includes location management functions, which enable to
determine from the set of all base stations a WiMAX net-
work is made up of the base station the target subscriber
is currently attached to and which are necessary whenever
network-initiated data, for example, incoming Emails, needs
to be pushed to a subscriber. Furthermore, Mobile WiMAX
defines different power-saving modes to which the device
changes if there is no data transmission in progress and
which thus contribute to a significant reduction of battery
consumption when compared to devices used for fixed or
nomadic access. Finally, as data transmission in mobile net-
works is always exposed to varying radio propagation con-ditions, Mobile WiMAX comes up with improved modulation
and error correction schemes.
The specification for Mobile WiMAX has been released
as an amendment to the 802.16-2004 standard, and is
called IEEE 802.16e [3]. It emerged from the Korean WiBro
(Wireless Broadband) technology, which is being developed
since the beginning of the millennium by the Korean tele-
communications industry under significant participation of
Samsung Electronics. Since 2004, WiBro is being standard-
ized by the Korean Telecommunications Technology Associ-
ation(TTA), and first WiBro networks went into operation in
2005. In November 2004, it was decided to adopt the WiBrotechnologies for Mobile WiMAX and to keep both systems
compatible to each other.
Figure 3. Mobile WiMAXFigure 2. Nomadic WiMAX
Base station
NLoS
Subscriber station
Base station
Handover
Subscriber
station
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2.4 Mobile WiMAX - Difference to other Systems
The emergence of Mobile WiMAX networks expected for
the next years imposes the question of how this system
relates to classical wireless technologies like WLAN, GSM
and UMTS. The answer to this question is not clear yet and
has led to controversies among experts whether WiMAX is
rather a competing or complementary technology. In order
to get an idea about the role of Mobile WiMAX in the orches-
tra of wireless consumer technologies it might be helpful
to consider these systems regarding their data rates and
mobility support capabilities, see Figure 4.
Similar to WiMAX, WLANs according to the IEEE stand-
ards family 802.11 offer network access, which, however,
in contrast to WiMAX is limited to local environments, pre-
dominantly inside buildings. A WLAN access point has a
typical range of a few hundreds of meters (rather only a few
dozens of meters indoors), and a couple of them may be in-
terconnected to a so-called extended service setfor provid-ing larger areas with seamless coverage. A cell change is
supported by a handover function, which, however, causes
noticeable interruptions during transmission and only works
at very low speeds. The data rate supported by most WLAN
installations today is about 54 Mbps and is thus beyond of
what the typical Mobile WiMAX subscriber can expect. Due
to its limited mobility support, WLAN is the preferred choice
whenever an expensive wiring inside buildings should be
avoided, for example, when a PC or notebook needs to be
connected to a DSL modem, or for nomadic customers,
which require high data rates on the spot, but do not move
considerably. For mobile customers, however, WLAN is lesssuited, as it is very difficult and expensive to build a WLAN
that seamlessly cover larger outdoor areas.
Other than WLAN and WiMAX, which only provide net-
work access capabilities, traditional cellular systems like
GSM and UMTS realize several high-level services such as
speech and video telephony, transfer of short messages, or
browsing the Internet via the Wireless Application Protocol
(WAP). A single network usually spans an entire country,
and it consists of many locally operating access networks
that are interconnected via a common core network. GSM
and UMTS provide full mobility support, including hando-
ver, localization and roaming capabilities. Roaming enables
customers to request and use services in foreign networks,
4 WiMAX Worldwide Interoperability for Microwave Access
and was one of the main driving forces behind the success
of GSM. As GSM and UMTS in the meanwhile are offered
by over 700 network operators in 214 countries and territo-
ries, customers on the move experience a nearly seamless
world-wide mobility support that no other network technol-
ogy can provide today.
On the other hand, GSM and UMTS only support
moderate data rates when compared to those that can be
achieved with Mobile WiMAX. GSM was initially designed
for circuit-switched speech telephony only and data rates
of the packet-switched GPRS are limited to about 60 kbps
(depending on the capabilities of the used terminal and the
configuration of the serving network). Data rates in UMTS
are considerably higher. In the first network expansion
stage, these networks provide services with a maximum of
384 kbps, which may be extended to up to 14 Mbps if UMTS
is combined with a new technology known as Highspeed
Downlink Packet Access (HSDPA) and Highspeed Uplink
Packet Access(HSUPA) respectively.To draw a conclusion, from a today's perspective, Mobile
WiMAX may be classified as a technology that bridges the
gap between traditional cellular networks (seamless mobil-
ity support and comparatively low data rates) on the one
hand and local wireless technologies like WLAN (high data
rates, but only rudimentary mobility functions) on the other.
3 WiMAX Protocol Stack
The WiMAX specifications do not define an entire network
infrastructure or high-level services as known from telecom-munications systems like GSM or UMTS. They only fix an
access technology for connecting subscriber stations over
the so-called last mile to a base station, comparable to DSL
in the wired domain. This base station then provides inter-
connectivity with a fixed network, however, the related pro-
tocols and mechanisms used for this are out of scope of the
IEEE specifications for WiMAX. In terms of the seven layers
of the OSI protocol stack, WiMAX covers only the physi-
cal(PHY) and medium access(MAC) layers and is thus in
close compliance to other IEEE specifications like WLAN
802.11 or Ethernet 802.3. The resulting protocol stack is de-
picted in Figure 5.
The physical layer primarily deals with the representa-tion of data bits by radio signals, for which different modula-
tion schemes are envisaged, as well as with related aspects
like antenna technologies and power control. Furthermore,
it manages the separation of uplink and downlink transmis-
sion, which is called duplexing, and incorporates methods
for error correction and detection. For the different variants
of WiMAX several physical layers are envisaged, which are
called WirelessMAN-SC, WirelessMAN-SCa, WirelessMAN-
OFDMand WirelessMAN-OFDMA. Some characteristic fea-
tures of them are highlighted in the following sections.
As suggested by its name, the medium access layer pro-
vides mechanisms that define how a radio channel providedby the physical layer is shared between different subscriber
stations. The primary goal of medium access is to avoid
collisions that would occur when two or more subscriberFigure 4. Mobile WiMAX - Difference to other systems
WLAN(IEEE 802.11)
UMTS
HSDPA/
HSUPA
GSM/
GPRS
Mobile
WiMAX(IEEE 802.16e)
Mobility
Data rates
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Figure 5. WiMAX Protocol Stack
stations use the same radio resources at the same time.
Another important focus is on control mechanisms for guar-
anteeing a certain performance of data transmission, which
is referred to as Quality of Service(QoS). This performance
can be described by several parameters, among them data
rate, delay, jitter (variation in delay) and error rates. Differentapplications, for example, multimedia streaming, VoIP and
web browsing, impose different requirements on QoS, and
WiMAX provides adequate mechanisms to fulfil them.
As can be derived from Figure 5, the common part of
the medium access layer is supplemented by two sub-lay-
ers, referred to as MAC privacy sub-layer and MAC con-
vergence sub-layer. The former provides the usual security
mechanisms needed for the authentication of subscribers,
the exchange of key and the ciphering of messages. The
convergence sub-layer acts as an interface between exter-
nal non-WiMAX protocols and the WiMAX medium access
layer. Its main task is the encapsulation and decapsulationof external Protocol Data Units (PDUs) into and from so-
called Service Delivery Units(SDUs), which are exchanged
between subscriber and base station. The convergence
sub-layer is also responsible for bandwidth allocation and
the adherence of negotiated QoS parameters. Two specific
convergence sub-layers so far exist, one for carrying data of
packet-switched networks like IPv4 or IPv6 and another one
for connecting to networks being operated according to the
Asynchronous Transfer Mode(ATM).
4 WiMAX Physical Layer
This section highlights the physical layer and gives an over-
view of modulation schemes, antennas, error correction
schemes and frame formats used for WiMAX.
4.1 Modulation Schemes
Modulation is a process to represent data by changing the
parameters of a periodic sinusoidal electromagnetic wave,
which is known as carrier. WiMAX incorporates a multitude
of modulation schemes, which can be dynamically deployed
under consideration of the error characteristics of the radio
channel and the required data rates.
Single Carrier Modulation
In a single carrier modulation scheme, the transmitter gen-
5WiMAX Worldwide Interoperability for Microwave Access
erates a single carrier of a certain amplitude, frequency and
phase. For data transmission, one or several of these pa-
rameters are changed depending on the data to be trans-
mitted, which, as mentioned before, is called modulationor,
using an alternative term, shift keying. The resulting signal
is then emitted by the antenna connected to the transmit-
ter, propagates in the environment, and is finally caught by
another antenna, which is connected to a receiver. This re-
ceiver then interprets the incoming signal and recovers the
data bits originally sent, which is called demodulation.
In each modulation scheme, data bits are represented in
form of symbols, and each symbol is given by a certain con-
stellation of the carriers amplitude, frequency, and phase,
the so-called signal state. WiMAX envisages different vari-
ants of phase shift keying. The simplest variant is Binary
Phase Shift Keying(BPSK) and modulates data by shifting
the carrier phase between two signal states, one represent-
ing the binary 1 and the other the binary 0. Thus, each
symbol only carries a single bit. For transferring more bitsper symbol, one needs a modulation scheme that defines
more signal states. Quadrature Phase Shift Keying(QPSK)
fixes four signal states and thus represents two bits by one
symbol. The modulation of a carrier with QPSK is demon-
strated in Figure 6. The four symbols 00, 01, 11, and
10 are assigned to the carrier phases 45, 135, 225,
and 315. The number of bits per symbol can be further
increased by changing the signals amplitude in addition,
which is called Quadrature Amplitude Modulation (QAM).
WiMAX supports 16, 64 and 256-ary QAM (16-QAM, 64-
QAM, 256-QAM), which represent 4, 6 and 8 bits by one
symbol. Figure 7 shows the signal states of 64-QAM.Of particular concern is the symbol rate, which denotes
the number of symbols transmitted per second. The symbol
rate is an important measure for the bandwidth the signal
adopts in the frequency domain. The higher the symbol
rate, the more bandwidth is required and vice versa. The
data rate is the product of symbol rate and bits carried per
symbol. For increasing the data rate, either the symbol rate
must be increased or the number of bits per symbol must be
increased by using another modulation scheme. The former
spreads the bandwidth of the radio channel, while the latter
makes the signal more susceptible to interferences. This is
due to the fact that with an increasing number of symbols the
signal states need to be spaced closer and closer together,and hence even small interferences during the propagation
may result in misinterpretations of the incoming signal at
the receiver.
Figure 6. Modulation of a carrier with QPSK
Physical layer (PHY)
Medium access layer (MAC)
MAC common part
MAC convergence sub-layer
MAC privacy sub-layer
Network layer (e.g., IP)
{WiMAX
00(45 shift)
10(135 shift)
11(225 shift)
01(315 shift)
Symbolduration T
10 00
11 01
Q
I
Unmodulated
Carrier
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Single carrier modulation is used for Fixed and NomadicWiMAX and is part of the physical layers WirelessMAN-
SC and WirelessMAN-SCa. As mentioned before, Fixed
WiMAX operates in the frequency ranges between 10 and
66 GHz, and thus it is only suitable for LoS transmission. A
radio channel has a bandwidth of 20, 25 or 28 MHz, and the
supported modulation schemes are QPSK, 16-QAM and
64-QAM, which can be deployed depending on the error
characteristics of the radio link and the desired data rates.
For Nomadic WiMAX, single carrier modulation is only
optional. Nomadic WiMAX focuses on NLoS transmission
and has therefore been developed for operation in frequen-
cy ranges between 2 and 11 GHz. The channel bandwidthis scalable and may vary between 1,75 and 20 MHz. Single
carrier modulation works similar as in Fixed WiMAX, but has
been extended by BPSK and 256-QAM.
Multi Carrier Modulation
There are multiple error sources a radio signal is exposed
to during transmission, for example, multipath propagation,
attenuation, noise, shadowing by buildings and, in the case
of Mobile WiMAX, frequency deviations, which are called
Doppler shifts and which are caused by movements of the
mobile subscriber station during transmission.
Of particular concern in WiMAX as well as in all other
wireless networks with large data rates and long transmis-sion ranges is multipath propagation. As depicted in Figure
8, this phenomenon arises if a signal is reflected, scattered
and diffracted from and at obstacles like buildings, trees or
hills. As a result, the signal is copied during transmission,
and the receiver not only receives the primary impulse of a
signal, but also several delayed secondary impulses of it as
shown in Figure 9a. The travelling time of a signal impulse
corresponds to the length of the path at which it propagates
from the transmitter to the receiver. The delay between ar-
rival of a signals primary impulse and the arrival of its last
secondary impulse is called delay spread, and its size sig-
nificantly depends on the range of the transmitter and thedensity of obstacles in the close surrounding. The longer
the ranges and the higher the density of obstacles at which
the signal is reflected and scattered, the larger is the delay
6 WiMAX Worldwide Interoperability for Microwave Access
spread.
Multipath propagation may cause heavy interferences if
the symbol duration Tused during transmission is smaller
than the delay spread. The symbol duration denotes the
length of time a single symbol is transmitted, and thus it cor-
responds to the reciprocal of the symbol rate. As depicted
in Figure 9b, the delayed secondary impulses of a symbol
may destruct the impulses of subsequent impulses if the
symbol duration is much smaller than the delay spread. This
phenomenon is called intersymbol interferenceand is one
of the main sources for transmission errors.
Intersymbol interference does not represent a serious
problem for Fixed WiMAX, as these networks operate above
10 GHz, where effects of multipath propagation hardly ap-
pear and where radio signals are increasingly radiated in a
directional fashion from the emitting antenna. As a conse-
quence, the antennas of subscriber and base stations must
be adjusted for LoS transmission, and a significant delay
spread does not occur.However, one of the main motivations behind the devel-
opment of Nomadic and Mobile WiMAX was to enable NLoS
transmission. As consequence, radio signals in these sys-
tems are reflected and scattered for several times until they
reach the receiver. In order to cope with the resulting inter-
symbol interferences, Nomadic and Mobile WiMAX apply a
technique known as multi carrier modulation. As suggested
by its name, in multi carrier modulation a single radio chan-
nel of a certain bandwidth is subdivided into Nsub-carriers,
and the data stream to be sent is distributed over these sub-
carriers. The total symbol rate of the radio channel remains
the same, but because each sub-carrier transmits only theN-th part of the entire data, the symbol duration at each
sub-carrier is Ntimes larger compared to the symbol dura-
tion of a conventional single carrier modulation. Accordingly,
each sub-carrier occupies the N-th part of bandwidth of the
entire radio channel. Following this approach, intersymbol
interferences are avoided, because the symbol duration on
each sub-carrier is larger than the expected delay spread,
assuming Nis chosen sufficiently large
However, multi carrier modulation may suffer from so-
called side lobesin the frequency domain, which result from
out-of-band radiation in the frequency bands below and
Figure 8. Multipath propagation
110100 110110 111110 111100
110101 110111 111111 111101
110001 110011 111011 111101
110000 110010 111010 111000
100000 100010 101010 101000
100001 100011 101011 101001
100101 100111 101111 101101
100100 100110 101110 101100
110100 110110 111110 111100
110101 110111 111111 111101
110001 110011 111011 111101
110000 110010 111010 111000
001000 001010 000010 000000
001001 001011 000011 000001
001101 001111 000111 000101
001100 001110 000110 000100
011100 011110 010110 010100
011101 011111 010111 010101
011001 011011 010011 010001
011000 011010 010010 010000
Q
I
Figure 7. 64-QAM signal states
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above each sub-carrier. These side lobes do not carry any
useful information that is needed for interpreting the incom-
ing signal at the receiver, but they can distort the transmis-
sion in neighbouring sub-carriers. An important concernwhen using multi carrier modulation is therefore to select an
appropriate frequency space between the sub-carriers. For
this purpose, the sub-carriers are placed orthogonal to each
other in the frequency domain, a technique that is called Or-
thogonal Frequency Division Multiplexing(OFDM). A pair of
sub-carriers is said to be orthogonal if the frequency space
between them is given by 1/TsHz, where Tsrepresents the
symbol duration on each sub-carrier. As depicted in Figure
10, the advantage of orthogonality is that the peak of a sub-
carriers main lobe corresponds to the zero crossings of the
neighbouring sub-carriers. In this way, out-of-band radiation
in the side lobes neutralize each other, and the transmissionin a sub-carrier have no negative impacts on its neighbour-
ing sub-carriers. Furthermore, OFDM allows the overlap-
ping of the main lobes of neighbouring sub-carriers, and
hence they can be arranged very close together, which is
very bandwidth efficient when compared to a non-orthogo-
nal multi carrier modulation.
In WiMAX, OFDM has been extended with a feature
called sub-channelization, see Figure 11. The OFDM radio
channel is subdivided into several sub-channels, and each
sub-channel, in turn, is composed of several sub-carriers.
Instead of using all sub-carriers the radio channel consists
of, a transmitter may send on only one or several select-
ed sub-channels. In this way, multiple users can share thesame OFDM channel simultaneously. Therefore, sub-chan-
nelization in OFDM is basically a multiple access scheme,
and therefore this variant of OFDM is called Orthogonal
Frequency Division Multiple Access (OFDMA). OFDMA
Figure 9. Delay spread and intersymbol interference
7WiMAX Worldwide Interoperability for Microwave Access
has also advantages regarding power control and battery
consumption. For example, base stations can increase the
transmit power on sub-channels serving indoor subscriber
stations, and decrease it for subscriber stations stayingoutdoors or in the close surrounding of the base station.
Subscriber stations, on the other hand, may concentrate
transmit power in a few sub-carriers by OFDMA, thereby
saving valuable battery resources, which is especially of ad-
vantage for small, mobile devices with integrated subscriber
station as intended for Mobile WiMAX.
The physical layers envisaged for Nomadic and Mobile
WiMAX incorporate different variants of OFDM and OFD-
MA respectively. In WirelessMAN-OFDM, the radio channel
is subdivided into 256 sub-carriers, each of which can be
modulated with QPSK, 16-QAM or 64-QAM. The channel
can adopt different bandwidths between 1,75 and 20 MHz.From the 256 sub-carriers, only 192 carry user data. The re-
maining ones are needed for frequency synchronization (pi-
lot sub-carriers) or as guard bands (NULL sub-carriers) for
avoiding neighbour channel interferences that result from
side lobes of adjacent radio channels. Sub-channelization
is only applied on an optional basis for transmissions in the
uplink. WirelessMAN-OFDMA, on the other hand, subdi-
vides the radio channel into 2048 sub-carriers. Thus, the
symbol duration on each sub-carrier is much longer here
than in WirelessMAN-OFDM, and hence the signals are
less susceptible to intersymbol interferences. In contrast to
WirelessMAN-OFDM, sub-channelization is mandatory for
both directions. It can be used in different configurationsthat differ from each other regarding the fragmentation of
the OFDM radio channel into sub-channels.
Mobile WiMAX adopts the WirelessMAN-OFDMA physi-
cal layer, but introduces a new feature that is called Scal-
Primary impulse ofsymbol n
Primary impulse ofsymbol +1n
Delay spread
Symbol duration T
Secondary impulses Secondary impulses
Power
Time
(a) Delay spread without intersymbol interferences (b) Delay spread with intersymbol interference
Primary impulse ofsymbol n
Primary impulse ofsymbol +1n
Primary impulse ofsymbol +2n
Delay spread
Symbolduration T
Power
Time
fn
fn+1
fn+2
fn+3
fn+4
1/T
Main lobes
Side lobes
Figure 10. OFDM
Sub-
channel 1
Sub-
channel 2
Sub-
channel 3
Sub-
channel 4
OFDM channel
Guard bands Guard bands
Frequency
Figure 11. OFDMA
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able-OFDMA (SOFDMA). While in Nomadic WiMAX, the
number of sub-carriers remains constant irrespective of
the channel bandwidth, which can vary between 1,25 and
20 MHz, in SOFDMA the number of sub-carriers is scaled
in dependence on the channel bandwidth. As a result, the
spacing between sub-carriers and the symbol durations
remain constant for varying channel bandwidths, which re-
duces the system complexity needed for smaller channels
and improves the performance of wider ones.
4.2 Antennas
Besides multi carrier modulation, another key factor for
making the transmission more robust and for achieving high
data rates is the choice of an appropriate antenna technol-
ogy. Most wireless systems today follow a single-antenna
approach, where each base station is connected to a single
antenna, which either radiates power in all directions equally
(omnidirectional antenna) or which concentrates power in abeam of a certain direction and width (directional antenna)
for serving only the sector of a radio cell.
For WiMAX, the usage of intelligent multiple-antenna
architectures is envisaged, where base stations and sub-
scriber stations are equipped with several highly directional
antennas (arranged in a so-called multi-antenna array),
each of it connected to a dedicated transmitter and receiver
respectively. The antennas can be dynamically adjusted to
radiate power in a certain direction under consideration of
the subscribers positions within the coverage area and the
current conditions of multipath propagation. Because the
power is concentrated into a beam of small width, the cov-erage area can be increased and interferences eliminated.
Furthermore, the different transmitters connected to a mul-
ti-antenna array can independently transmit different data
streams on the same radio channel, assuming that their
beams are sufficiently separated in space. This technique is
known as Space Division Multiplexing(SDM) and increases
the capacity within a radio cell linearly with the number of
antennas deployed.
WiMAX incorporates two different multiple-antenna tech-
nologies, which are called Adaptive Antenna System(AAP)
and Multiple Input Multiple Output (MIMO). Both of them
are compared in Figure 12. The former technique is based
8 WiMAX Worldwide Interoperability for Microwave Access
Figure 12. Comparison of AAS and MIMO
on beamforming and generates a beam that is directed to-
wards a subscriber or a group of subscribers staying close
by. MIMO, on the other hand, utilizes the effects of multipath
propagation and is the preferred choice in cluttered environ-
ments. Signals from the different antennas are radiated in
a way that they travel at different paths from the sender to
the receiver. The different paths may either carry the same,
redundant copies of the data stream or they might be used
to transfer different data streams to the receiver. The former
approach makes the transmission more robust, because in-
terferences on a certain path may be compensated by the
transmissions received from another path, or transmissions
from different paths may be combined at the receiver to get
a useful signal. This option is the preferred choice for serv-
ing mobile subscribers, which suffer from rapidly changing
radio conditions. The transfer of different data streams, on
the other hand, increases the capacity, but is less robust. It
is primarily intended for fixed and nomadic subscribers.
4.3 Channel Coding
The goal of channel coding is to prepare the data stream to
be transmitted in a manner that errors that may occur dur-
ing transmission can be reliably detected and corrected at
the receiver. This is accomplished by calculating redundant
data from the data stream. WiMAX applies different error
coding schemes, and their deployment and parameters de-
pend on the physical layer used.
In general, it can be distinguished between block and
convolutional codes. Block coding subdivides the data
stream into blocks of nbits, and generates a parity word foreach block that is attached to it, resulting in a block of size m
bits (m>n) that is then further processed. The type of block
code used in WiMAX is called Reed-Solomon code. Con-
volutional coding takes nbits from a continuous input data
stream and maps them onto mbits of an output stream. The
generation of output bits is realized by combining (convolv-
ing) the outputs of several linear feedback shift registers in
a certain manner.
The quality of error coding can be measured by the
maximum number of errors that can be corrected in a data
block or stream of fixed size and whether error bursts or
only single-bit errors can be corrected. These capabilities
mainly depend on the algorithms used for error correctionas well as on the code rate r=n/m, which expresses the
number of output bits per input bit. The lower the code rate
is, the higher is the probability that errors can be corrected,
but the lower is the net data rate that can be achieved at the
radio channel. Therefore, the WiMAX standards envisage to
dynamically fix an appropriate code rate under considera-
tion of the expected degree of interferences.
Error correction mechanisms reliably detect and correct
errors. Unfortunately, each radio transmission suffers from
the appearance of error bursts, which are characterized by
a large number of errors occurring in consecutive bits. Be-
cause it is difficult or even impossible in many cases to cor-rect such errors, the output bits generated by error coding
can be mixed prior to transmission, a process that is known
as interleaving. For this purpose, the data stream is subdi-
(a) Adaptive Antenna System
(b) Multiple Input Muliple Output
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vided into code words of fixed length, and the consecutive
bits of a code word are exchanged with the bits of previous
and subsequent words according to a certain algorithm. At
the receiver, the original bit sequence is then reassembled
by a de-interleaving process. Thus, error bursts occurring
during transmission are distributed over several code words,
that is, they are subdivided into single bit errors that can be
corrected in most cases.
As stated before, WiMAX supports different options
for error coding. Figure 13 depicts a two-step error coding
process that applies both block and convolutional coding. In
the first step, which is also referred to as outer coding, the
transmitter encodes the data stream with a Reed-Solomon
code. The resulting blocks together with their parity words
are then interleaved. In order to improve robustness, a con-
volutional coding process is then applied in the last step,
which is also called inner coding. The decoding steps at the
receiver are then executed in reverse order.
4.4 Duplexing
Another task of the physical layer is the separation of uplink
and downlink transmissions, which is commonly referred to
as duplexing. Two fundamental approaches exist, which are
called Frequency Division Duplex(FDD) and Time Division
Duplex (TDD) and which are both supported in all WiMAX
variants.
In FDD mode, uplink and downlink are separated in the
frequency domain, that is, there exists a dedicated radio
channel for each direction, which is demonstrated in Figure
14a. Both uplink and downlink channels are subdivided intoframes of a certain duration, and each frame, in turn, con-
sists of several data bursts. Each subscriber station is as-
signed two data bursts, one on the downlink channel for re-
ceiving data from the base station and another on the uplink
channel for transferring data to the base station. In addition,
there is a dedicated data burst for broadcast transmissions,
which is used by the base station to supply all subscriber
stations with control information. This will be explained in
subsequent sections.
FDD can be operated in full duplex(FD) or half duplex
(HD) mode. In full duplex, the stations can send and receive
simultaneously. However, a major problem of FDD is that
the transmission power of an outgoing signal is much higherthan the received power of an incoming signal, and there-
fore the side lobes of the outgoing signal may drown out
the incoming signal. To cope with this problem, it is recom-
mended to arrange uplink and downlink far away from each
other in the frequency domain. Nevertheless, there often
remain interferences, which can only be avoided by using
frequency filters, which, however, makes mobile devices
complex and expensive. Another solution is therefore to use
half duplex, where subscriber stations do not receive and
transmit at the same time.
9WiMAX Worldwide Interoperability for Microwave Access
Figure 14b demonstrates full and half duplex modes for
different subscriber stations. SS #1 is a full duplex station,
and can thus send and receive simultaneously. SS #2 and
#3, on the other hand, are half duplex stations. Their uplinkand downlink bursts must be arranged in a way that they
do not overlap. It must also be considered that their uplink
bursts are not in conflict with broadcast transmissions from
the base station.
Using TDD, downlink and uplink share a common radio
channel and are separated in the time domain as demon-
strated in Figure 14b. A transmission frame is subdivided into
downlink and uplink subframes, each of it consisting again
of a number of data bursts assigned to different subscriber
stations. A challenge of TDD is to avoid an overlapping be-
tween downlink and uplink subframes. The overlapping may
result from the fact that different subscriber stations are lo-cated at different distances to the base station, and hence
do not receive the end of a downlink subframe simultane-
ously. Therefore, uplink and downlink subframes must be
separated by guard periodsduring which no transmission
is allowed.
Both FDD and TDD are available for all physical layers
of WiMAX. FDD is the preferred solution for regulated op-
eration in licensed frequency bands, while TDD is primarily
deployed in unlicensed bands, which require less regulatory
and organizational constraints. The following section gives
a more detailed overview of the structure of transmission
frames used for FDD and TDD.
4.5 Frame Format
The physical layers of WiMAX come along with different
frame formats, which, however, only slightly differ from each
other. Therefore, only their common elements are explained
here. Figure 15 shows a simplified version of the frame
structure as used for the TDD mode. A frame consists of
a downlink and uplink subframe and lasts 5, 10 or 20 ms.
Consecutive downlink and uplink subframes are separated
by guard periods (as explained in the last section), which
are called Transmit/Receive Transition Gap(TTG) and Re-
ceive/Transmit Transition Gap (RTG) and during which nodata transmission is allowed. For FDD, basically the same
format is used: the downlink and uplink subframes shown
in Figure 15 are assigned to different radio channels for
ConvolutionalCoding(Inner coding)
InterleavingBlock coding(Outer coding)
Figure 13. Channel coding in WiMAX
BCSS#1(VD)
SS#2(HD)
SS#3(HD)
BCSS#1(VD)
SS#2(HD)
SS#3(HD)
SS #1(VD)
SS #3(VD)
SS #2(VD)
SS #1(VD)
SS #3(VD)
SS #2(VD)
Downlink frame n Downlink frame n+1
Uplink frame n Uplink frame n+1
xx MHz
yy MHz
BCSS#1(VD)
SS#2(HD)
SS#3(HD)
SS #1(VD)
SS #3(VD)
SS #2(VD)
Downlink subframe n Uplink subframe n
Frame n
xx MHz
(a) Frequency Divsion Duplexing (FDD)
(b) Time Divsion Duplexing (TDD)
Figure 14. Channel coding in WiMAX
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Figure 15. TDD frame format
parallel transmission and constitute an entire frame with a
maximum length of 20 ms. Furthermore, there is no need in
the FDD mode to separate consecutive frames by TTG and
RTG respectively. The following descriptions refer to both
TDD and FDD.Because the WiMAX physical layers provide several
options, for example, regarding modulation schemes or
error coding rates, it is necessary to inform all subscriber
stations in a radio cell about the configuration of the radio
channel. For this purpose, the serving base station broad-
casts control information at the beginning of each frame,
which is received by all subscriber stations connected to
that base station. The broadcast is constituted by a pream-
ble, a so-called frame control header (FCH) and the first
data burst, see Figure 15. Modulation and coding of these
fields are standardized in order to make them interpretable
for all subscriber stations being in the process of networkentry, which will be explained below.
The preamble indicates the beginning of a frame and
enables the synchronization of subscriber stations to the
transmissions of the base station. It always has a length of
two OFDM symbols of a fixed radio pattern and is modulat-
ed with QPSK. The preamble is followed by the FCH field,
which carries the so-called burst profileof the first downlink
burst. This burst profile indicates the modulation scheme
and code rate used in the first burst. The FCH field consists
of only one OFDM symbol and is modulated with BPSK.
The first burst then carries a so-called broadcast control
field, which is composed of further fields denoted as DL-
MAP, UL-MAP, Downlink Channel Descriptor (DCD) andUplink Channel Descriptor (UCD). DL-MAP and UL-MAP
indicate the positions of all downlink and uplink bursts within
the corresponding subframes as well as their burst profiles.
DCD and UCD are complex descriptions of the configura-
tion of downlink and uplink, and contain information like the
identifier of the serving base station, the frame length, the
length of various fields within a frame, the frame number
and information for power adjustment and initial ranging, to
name only a few. Each subscriber station that wants to get
access to a base station has to receive these descriptions
and adjust to it.
An uplink subframe starts with two fields denoted as ini-tial rangingand bandwidth request. The former is accessed
by subscriber stations in order to determine the range to
the base station. This process is performed during the net-
work entry and will be described below. Using the band-
width request field, a subscriber station can announce its
bandwidth requirements to the base station, which will also
be explained later.
The broadcast fields (in the downlink) and the fields
for initial ranging and bandwidth request (in the uplink) are
followed by data bursts for individual transmissions to and
from subscriber stations. A data burst is of variable length
and carriers the protocol data units of the medium access
layer (MAC PDU). The assignment of data bursts to sub-
scriber stations is part of the medium access layer and is
executed by the base station under consideration of QoS
requirements.
For each data burst, another configuration of modulation
scheme and error coding rate can be used, which is speci-
fied in the DL-MAP and UL-MAP fields of the frame. The
configuration can be dynamically selected under considera-
tion of the capabilities of the subscriber station, the required
data rates and the expected robustness of transmission. Forexample, subscriber stations located close-by to the base
station may be served by 64-QAM, which provides high
data rates but which is very susceptible to interferences,
while for subscriber stations located farther away the more
robust QPSK modulation may be preferred. This is demon-
strated for the downlink in Figure 16. However, the usage of
different modulation schemes imposes certain constraints
regarding the ordering of bursts within a frame. A particular
concern is that a subscriber station that wants to transmit
in (or receive) a burst has to detect the end of the previ-
ous burst assigned to another subscriber station. This can
only be guaranteed if for the previous burst either the samemodulation scheme is used or another one that is more ro-
bust against interferences. In Figure 15, SS#1 is located far-
thest away from the base station and is served with a QPSK
modulation in the first burst. SS#2, on the other hand, is
closer by and receives in the second burst modulated with
the less robust 16-QAM. It can easily detect the end of the
first burst, because QPSK is more robust than 16-QAM. If,
however, the order of the two bursts would be exchanged,
SS#1 could hardly detect the end of the first burst, because
it is located out of the range where 16-QAM modulated sig-
nals can be reliably received. Therefore, the bursts within
a frame must always be arranged in decreasing order with
Broad-cast
MACPDUs
MACPDU #1
MACPDU #n
DLMAP
ULMAP
DCD UCDMAC
HeaderMAC
PayloadCRC
Frame n-1 Frame n Frame n+1Time
DLBurst
#1
DLBurst
#2
DLBurst
#nPream
ble
FCH
Downlink sub-frame Uplink sub-frame
ULBurst
#1
ULBurst
#2
ULBurst
#nTTG
Initial
rang
ing
Ban
dw
idth
reques
t
RTG
Figure 16. Modulation of data bursts
10 WiMAX Worldwide Interoperability for Microwave Access
SS#4
SS#2
SS#3
SS#1
SS #1(QPSK)
SS#2(16-QAM)
SS#3(16-QAM)
SS#4(64-QAM)
Pream
ble
FCHDownlink
subframe
Increasing interferences
BS
Decreasing robustness of modulation
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regard to the robustness of the used modulation schemes.
Finally, Figure 17 shows a possible appearance of down-link and uplink frames for the case that OFDMA is used.
The different data bursts are not only separated in the time
domain here. They can also be transmitted simultaneously
assuming that they adopt different sub-channels, which are
composed of the several sub-carriers built by the multi car-
rier modulation.
5 WiMAX Medium Access Layer
If a base station operates in the point-to-multipoint mode
(see Section 2.2), subscriber stations located within its cov-
erage area compete against each other for access to the ra-dio channel. This access is coordinated by the base station
and belongs to the main tasks of the medium access layer.
The primary focus of medium access on the one hand is to
avoid collisions, which would occur if two or more subscrib-
er stations would enter the same radio channel (or some of
its sub-carriers if OFDMA is applied) simultaneously and,
on the other, to guarantee the access in a way that QoS re-
quirements are met. Besides this, the medium access layer
also provides related functions, for example, authentication
and ciphering as well as error correction and radio link con-
trol. The following sub-sections provide a short overview of
the most important procedures of medium access.
5.1 MAC Protocol Data Units
Data is transferred via protocol data units (PDUs) of the
MAC layer, which, in turn, are included into the data bursts
provided by the physical layer. There may be several MAC
PDUs per data burst. The PDUs carry user data, control and
management information as well as bandwidth requests
issued by the subscriber stations to announce their band-
width requirements for uplink transmission. Apart from the
bandwidth request, which only consists of a single header,
a PDU contains a header field, a payload field and another
field for error detection, see also Figure 15.The header is of fixed length and carries control infor-
mation, for example, the identifier of the connection (see
description below), whether or not encryption and error de-
11WiMAX Worldwide Interoperability for Microwave Access
tection are activated as well as the length of the entire PDU.
The bandwidth request header additionally contains the
number of bytes the subscriber station intends to transmit
in the uplink. The payload field carries the actual user data
as well as control and management information and is of
variable length. For example, the payload field may carry IP
data packets, which are filled into the payload field by the
convergence sub-layer.
Finally, the Cyclic Redundancy Check(CRC) field con-
tains a checksum that the transmitter calculates from the
header and payload fields. The term CRC denotes a special
mechanism of error detection, where the checksum is given
by the remainder of a polynomial division. The checksum is
analyzed by the receiver in order to detect those errors that
could not be corrected by the channel decoding process of
the physical layer (see also Section 4.3).
5.2 Service Flows and MAC Connections
The medium access layer of WiMAX organizes the ex-
change of data between subscriber and base station by
the concept of service flows. A service flow is always uni-
directional, that is, it is defined either for uplink or downlink
direction. It is represented by a unique Service Flow Identi-
fier(SFID) and characterized by a set of QoS parameters,
for example, data rate, latency and jitter. The requirements
of different applications on these parameters are very het-
erogeneous. For example, VoIP without silence suppression
demands for a constant bit rate and a guaranteed maximum
latency and jitter, while a simple file transfer only requires
a minimum data rate, but no guarantees regarding otherQoS parameters. Each service flow is realized by a MAC
connection, which is referenced by a Connection Identifier
(CID) and which is constituted by a series of data bursts
allocated by the base station in the different transmission
frames. This allocation has to be organized in a way that the
QoS requirements of the service flow the connection carries
are fulfilled. This process represents the core mechanism
of medium access. It is called schedulingand is based on
sophisticated algorithms.
The allocation of data bursts has to be considered for
downlink and uplink direction differently. For the downlink,
the allocation is comparatively simple, because the base
station is the only sender in this direction. The data of anexternal network, for example, the Internet, arrives at the
base station and is there assigned to the service flow that
is maintained between the base station and the subscriber
station the data is intended for. The scheduling algorithm of
the base station then identifies one or several bursts within
one or several frames for data transmission.
In the uplink, the medium access is much more com-
plicated, because it has to be coordinated among all sub-
scriber stations within a cell. In classical mobile networks,
for example, GSM, the problem of assigning transmission
capacity to mobile stations is often solved by reserving a
burst of fixed length in each frame and for each active sta-tion. In other wireless system, for example, WLAN, access
to the radio channel is not centrally coordinated. Instead,
the stations enter the channel whenever they have data to
Preamble
DL-MAP
UL-MAP
DL-MAP
FCH
DL Burst #1
UL Burst #1
UL Burst #2
UL Burst #1DL Burst #2
DL Burst #3
DL Burst #4
DLBurst#5
(Multicast/Broad
castburst)
DLBurst#5
IR
BW
TTG RTGTime domain - OFDMA symbols
Frequencydomain-O
FDMAsub-carriers
DL sub-frame UL sub-frame
Figure 17. Example of frame structure when using
OFDMA
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12 WiMAX Worldwide Interoperability for Microwave Access
send. Collisions between the transmissions of different sta-
tions are avoided in that the channel has to be sensed free
prior to transmission. The former approach is suitable for the
adherence of QoS guarantees, but it suffers from an inef-
ficient utilization of the channel if the stations do not use the
full capacity of a burst, for example, during periods of silencein a VoIP session. The latter approach, on the other hand,
performs much better with regard to channel utilization, but
it is not suitable for providing a negotiated QoS, for example,
when too many stations contend for channel access. There-
fore, in order to cope with the antagonism of efficiency and
quality, WiMAX provides different access mechanisms for
the uplink that can be dynamically deployed under consid-
eration of QoS requirements.
One of these mechanisms is polling, which is depicted in
Figure 18. The base station here explicitly invites a subscrib-
er station to announce its uplink bandwidth demand for a
particular connection. The polling request specifies the CIDof the connection the polling refers to, and it is encoded as a
special element of the uplink map. Upon arrival of a poll, the
subscriber station determines the number of bytes it wants
to transmit in the uplink and returns this number to the base
station by sending a bandwidth request header (see Section
5.1). Besides the byte number, this header also specifies the
connection the request refers to and whether the bandwidth
request is incrementalor aggregated. Using an incremental
request, the subscriber station indicates a change of band-
width demand with regard to previous requests, while an
aggregated request specifies the total amount of bytes that
needs to be sent. The bandwidth request header is included
in the bandwidth request field of the uplink frame, see Fig-ure 15. After the base station has received the bandwidth
request header, it reserves a burst of appropriate size for
the next uplink frame. The parameters for uplink transmis-
sion, for example, the burst number and length, are then
indicated to the subscriber station in the next UL-MAP sent
on the downlink.
The polling of a subscriber station may be performed
regularly or irregularly, which depends on the base sta-
tions scheduling algorithm and the QoS parameters that
have been negotiated for the respective service flow. Fur-
thermore, it is distinguished between unicastand multicast/
broadcast polling. In the former category, the polling refersto only a single subscriber station, while multicast/broad-
cast polling addresses several or all subscriber stations lo-
cated in a cell.
Another mechanism for requesting bandwidth is piggy-
backing. The name is derived from the fact that bandwidth
requests are piggybacked (or attached) to the regular uplink
transmissions of a subscriber station, instead of sending
a dedicated bandwidth requests header. Piggybacking is
performed independently from polling, that is, a subscriber
station does not have to wait until it is polled, but can imme-
diately inform the base station about changing bandwidth
demands if necessary. The bandwidth request is included
into the header of a conventional MAC PDU and always re-
fers to the connection this PDU is part of.
Finally, a subscriber station can get assigned uplink
bursts of fixed length at regular intervals without the need
to explicitly request them. This mechanism is called unsolic-
ited schedulingand is the preferred choice for applications
that require a constant bit rate during the entire session.
The reservation may hold for the entire duration of the serv-
ice session, but it may be temporarily cancelled in the case
of inactive time periods. Furthermore, a subscriber stationcan indicate additional bandwidth demand for an unsolic-
ited connection if it turns out that the amount of unsent data
exceeds a pre-defined value. In this case, the subscriber
station sets a so-called slip indicator bit in the MAC PDU
header, whereupon the base station allocates more band-
width for the respective connection.
5.3 Service Classes
The different mechanisms of bandwidth request presented
previously are used for realizing different service classes,
which differ from each other in the QoS they provide. Theseservice classes are basically descriptions of service flows
with a pre-configured set of QoS parameters and are sup-
ported by associated scheduling algorithms in the base sta-
tion. The following services classes have been defined for
WiMAX:
Unsolicited Grant Service (UGS). This service class
has the strongest requirements on QoS mechanisms
and has been designed for supporting real-time appli-
cations of constant bit rate, that is, for applications that
periodically create a certain amount of data for realtime
transfer over the network. A typical example is VoIP
without silence suppression, where both periods of con-versation and silence are encoded and transferred with
a constant bit rate. The bandwidth request mechanism
in the uplink used for this service class is unsolicited
scheduling.
Real-time Polling Service (rtPS). Another real-time
service class is the Real-time Polling Service, which in
contrast to UGS supports the periodic transfer of data
packets of variable size. A typical example is MPEG-
compressed video, where the single frames of a video
stream are encoded depending on the data of previous
and following frames and therefore differ in size. Thisservice class is based on the polling and piggybacking
mechanisms for bandwidth request.
Figure 18. Polling
Polling[UL-MAP]
Bandwidth grant[UL-MAP]
Bandwidth request[Bandwidth request field]
UL transmission[assigned data burst]
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Non-real-time Polling Service (nrtPS). This service
class supports typical non-real-time applications suchas file transfer or Internet browsing. This service class
does not necessarily need periodic transmission oppor-
tunities or a guaranteed end-to-end latency. However,
in order to provide an acceptable QoS, the base station
issues unicast polls on a regular basis.
Best Effort Service (BE). Services of this class do not
receive any QoS guarantees at all. They are only served
if sufficient capacity is available. Examples for such low-
priority applications are those which create a low amount
of data that may be delivered with a considerable delay,
that is, Email, instant messaging or chat applications.
If required, it is possible to modify the QoS parameters
of a services class or to determine its own set of QoS pa-
13WiMAX Worldwide Interoperability for Microwave Access
rameters for a service flow, which, however, increases the
complexity of configuration.
5.4 Procedures of the MAC Layer
This section gives an overview of the procedures taking
place between subscriber station and base station in order
to register with the network and to get assigned resources.
The different steps are depicted in Figure 19.
Network Entry
Before a subscriber station can use any services, it must
first introduce to the base station, a process that is known
as network entry. An important goal when designing WiMAX
was to avoid complex and cumbersome manual configura-
tions to be made by the subscriber, as often required, for
example, in order to get access to a WLAN system. Instead,
the subscriber should enter into contact with a WiMAX net-
work in a plug-and-play fashion, that is, in a similar manneras mobile phones register with a GSM network, for example.
Therefore, the different steps of network entry are hidden
from the subscriber as far as possible, and may only require
a pre-configuration of subscriber stations by the respective
operator (to be made before they are delivered to the sub-
scriber).
The first step a subscriber station has to perform for net-
work entry is called downlink channel synchronization(see
Step (1) in Figure 19). The subscriber station scans the fre-
quency range for detecting the downlink channel of a base
station and then listens to the preamble periodically broad-
cast in each downlink frame. If the subscriber station is syn-chronized, it derives information about the organization of
uplink and downlink, that is, about the type of physical layer
and the used modulation and error correction schemes,
from the broadcast control field of the first burst.
In the next step, which is denoted as initial ranging, the
range between subscriber and base station is determined
in order to fix a suitable transmission power and timing cor-
rections. For this purpose, the subscriber station enters the
initial ranging field of an uplink frame and sends a ranging
request message (2) with the minimum transmission power.
If no response is received from the base station within a
certain timeout period, this message is resent with an in-
creased transmission power. This process is repeated untilthe subscriber station receives a ranging response mes-
sage (3), which either contains corrections for transmission
power and timing or which indicates success.
The last step of network entry is capability negotiation,
and it is used to inform the base station about the modula-
tion schemes, error correction schemes and rates as well
as duplexing methods supported by the subscriber station.
Upon arrival and checking of the capability request mes-
sage from the subscriber station (4), the base station can
accept or deny network entry in a capability response mes-
sage (5).
Authentication and Key Exchange
After network entry is completed, the subscriber station must
authenticate towards the network, which is necessary in or-
Figure 19. MAC procedures
Regular DL transmissions
Downlink channelsynchronization
Authorization
Ranging response
Ranging request[Initial ranging field]
Authentication request
Registration request
Registration response
Authentication response
Capability request
Capability response
Network entry
Authentication andkey exchange
Registration
Connection Setup
DHCP/Internet Time Protocol/TFTP
Dynamic service addition request
Dynamic service addition ack.
DS received
Dynamic service addition response
10
14
15
13
12
11
9
8
7
6
5
4
3
2
1
Subscriberstation
Basestation
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der to validate the subscribers identity. The authentication is
based on X.509 certificates, which are issued by the manu-
facturer of the subscriber station and which are encrypted
with the subscribers secret key. The certificate is passed to
the network (6) and is decrypted there with the subscribers
public key. If the validation is successful, the base station
sends an authentication response (7), which contains an
authorization key for ciphering subsequent messages. This
authorization key is encrypted with the subscribers public
key and can only be decrypted with her secret key at the
subscriber station.
Registration and IP Connectivity
The subscriber station can now register with the network
and be configured for IP operation. Upon sending a registra-
tion request message (8), it receives information about the
used IP version, supported protocols for retransmission of
erroneous data (Automatic Repeat Request, ARQ) and oth-
er capabilities needed for medium access (9). Finally, fur-ther operations are executed for IP connectivity (10), among
them the allocation of an IP address by using the Dynamic
Host Configuration Protocol(DHCP), the exchange of cur-
rent date and time via the Internet Time Protocoland the
download of operational parameters by using the Trivial File
Transfer Protocol(TFTP).
Connection Setup
After a subscriber station is known to the network, service
flows can be established in both directions, for which a se-
ries of management messages is exchanged. The service
flows may be initiated by the subscriber station or by thebase station. In the former case, which is shown in Figure
19, the subscriber station sends a request message to the
base station (11), which addresses the convergence sub-
layer the service flow refers to (that is, IP or ATM) and which
contains the desired QoS parameters. After reception of this
message, the base station acknowledges its reception (12)
and checks whether the requesting subscriber is allowed at
all to request service flows with the specified QoS configu-
ration (13). If this check is successful, this is indicated to the
subscriber station by sending another message (14). The
connection setup is completed if the subscriber station then
acknowledges this message (15).
In addition to this setup procedure, an existing serviceflow can be reconfigured (regarding its QoS parameters) or
deleted, for which similar procedures exist. Also, it is pos-
sible to establish several service flows in parallel.
6 Mobility Support
This section gives an overview of functions for mobility sup-
port in Mobile WiMAX systems. The focus is on the different
types of handover, modes for power saving, procedures of
location management and a reference model for a WiMAX
network architecture.
6.1 Handover
Basically, the handover process as performed in most mo-
bile networks can be subdivided into the three following
phases: measurements, decisionand execution. In Mobile
WiMAX, all of them are initialized by the subscriber station,
but supported by the base stations involved in the handover
procedure, that is, the serving base stationand possible tar-
get base stations.
As stated in Section 2.1, Mobile WiMAX supports hard
as well as soft handover. The three handover phases for
both types are explained in the following.
Hard Handover
A hard handover is characterized by the fact that the con-
nection to the serving base station is released before an-
other one is established to the new base station ("break-
before-make").
Measurements are made by the subscriber station andrefer to observing the signal-to-noise ratio(SNR) of down-
link transmissions from the serving base station as well as
from possible target base stations. The SNR expresses the
ratio between the reception power of the intended signal
and that of other interferences. If the SNR of the serving
base station gets low, the error rates increase, and a hando-
ver to another base station should be performed.
In order to measure the SNR of possible target base
stations, the measuring subscriber station must be aware of
their existence and the configuration of the associated radio
channels. Therefore, the serving base station periodically
Figure 20. Hard handover
14 WiMAX Worldwide Interoperability for Microwave Access
Network topology advertisement
Regular downlink transmissions
Regular downlink transmissions
Scanning request
Handover request
Handover response
Scanninginterval
Scanning response
Capacity & QoSchecks
7
5
4
3
2
1
Subscriberstation
Servingbase station
Target #1Target #2
6
8
9 Network entry, registration, ...
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broadcasts a so-called network-topology-advertisement
message, which contains a list of all neighbouring base sta-
tions together with their configuration, see Step (1) in Figure
20. Basically, this configuration is an aggregation of the DCD
and UCD fields broadcast by the respective base stations in
their downlink frames. After analyzing the network-topology-
advertisement message, the subscriber can switch between
different neighbouring base stations and obtain the SNRs of
their downlink transmissions.
However, for listening to the transmissions of neighbour-
ing base stations, the subscriber station must interrupt the
reception of the serving base station. For this purpose, it
requests the serving base station for the assignment of a
so-called scanning interval (2). During this interval, trans-
missions to and from the requesting subscriber station are
interrupted. The beginning and length of the scanning in-
terval are returned in a scanning-response message to the
subscriber station (3), whereupon it starts the scanning of
neighbouring base stations (4).After the scanning is complete, the subscriber station
compares the measured SNRs with the SNR of the serving
base station and decides whether or not a handover is nec-
essary. This decision process also includes the identification
of potential target base stations, which are also selected
under consideration of the transmission quality experienced
during the scanning interval. However, this decision cannot
be made by the subscriber station solely. The list of potential
target base stations must first be sent to the serving base
station (5), which then checks whether the identified base
stations have enough capacity at all to serve the subscriber
station and to maintain the QoS parameters of its serviceflows (6). After the results of this check are available, the
serving base station returns the list of remaining target base
stations or proposes new ones (7). If the subscriber station
does not accept one of the chosen base stations, it returns
a negative acknowledgement. This negotiation process can
then be repeated for several times until a suitable target
base station is determined (8).
If the subscriber station does not return a negative ac-
knowledgement within a specified time period, the serving
base station acts on the assumption that the subscriber sta-
tion has switched to one of the proposed target base sta-
tions and releases all connections. The subscriber station,
on the other hand, registers with the new target base station
Figure 21. Multiple connections to different base stations
during soft handover
then (9) and for this purpose executes network entry, regis-
tration and all following steps as explained in Section 5.4.
Soft Handover
During a soft handover, the subscriber station maintains
several connections to different base stations simultane-
ously ("make-before-break"), see Figure 21. Measurements
are performed in the same manner as for the hard hando-
ver during scanning intervals, see Step (1)-(4) in Figure 20.
However, handover decision and execution are handled dif-
ferently.
The base stations a subscriber station is connected to
are managed in its active set. At the beginning, the active
set only contains the base station the subscriber station has
initially registered with, which is called anchor base station.
The active set can be extended if the subscriber station
measures an SNR from another base station that exceeds a
pre-defined threshold value. If this happens, the subscriber
station requests the anchor base station for updating theactive set, which can be accepted or denied depending on
similar capacity checks as performed for the hard handover,
see Step (6) in Figure 20. Analogously, a base station can
be removed from the active set if its SNR falls below an-
other threshold value. Furthermore, if the SNR of the anchor
base station is lower than that of another base station for a
certain period of time, the subscriber station can request to
change the anchor.
Maintaining connections to several base stations simul-
taneously means that the subscriber station receives the
same data for several times over different paths. In order to
improve the reception quality, all signals are combined toan aggregated signal, which usually shows a much better
SNR when compared to that of a single signal. This feature
is called macro diversityand is also implemented in UMTS
networks. In the uplink, the signals of the subscriber station
are received by all base stations of the active set. Instead
of summing up the signals, only that with the best quality
is selected and further processed, which is called selective
diversity.
Making soft handovers possible is a difficult task and
requires a careful design and planning of WiMAX networks
as well as complex and sophisticated coordinations during
their operation. The realization of macro and selective diver-
sity makes it necessary that neighbouring base stations op-erate at the same frequencies and follow the same structure
of data bursts within the transmission frames. Furthermore,
in order to avoid interferences, the frames must be exactly
synchronized in time, for which GPS receivers mounted at
15WiMAX Worldwide Interoperability for Microwave Access
Activemode
Sleepmode
Idlemode
Ready for reception/transmission
Handover
StandbyHandover
SuspendedPaging + Location Update
Figure 22. Power saving modes
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the base stations deliver a common time basis. Finally, there
is additional management overhead, for example, the coor-
dination of uplink and downlink maps and the assignment of
common CIDs and SFIDs, to name only a few.
6.2 Power Saving
A typical problem of mobile devices is the lack of sufficient
battery resources, and that is why for Mobile WiMAX new
operational modes are defined, see Figure 22. These are
called sleepand idle modeand consume considerably less
power than the conventional active mode.
A subscriber station turns from the active into the sleep
mode if no data is to be sent in the various service flows it
maintains with the base station. This may happen, for exam-
ple, if a service flow is used for transferring web pages, and
the subscriber remains on a certain web page over a longer
period of time before requesting the next one. The sleep
mode is characterized by alternating listeningand sleep pe-riods. In a sleep period, the subscriber station is deactivated
and does neither monitor the downlink transmission frames
from the base station nor does it transmit in the uplink. From
time to time, however, the subscriber station changes into
a listening period in order to check whether data from the
network has arrived. If so, it then returns to the active mode.
If data arrive from the network during the sleep periods, the
base station has to buffer it until the next listening period
occurs. The start and length of sleep and listening periods
are negotiated between subscriber station and base station
before starting into the sleep mode.
In the idle mode, the subscriber station is suspendedfrom the network, but remains available for the case that
network-initiated data is to be delivered, for example, an
incoming VoIP session or push email. The subscriber sta-
tion does neither transmit nor receive, similar to the sleep
periods in the sleep mode, and hence saves its power re-
sources. It only awakens for listening to so-called paging in-
tervals, during which it is informed about incoming data and
other procedures of the location management, which will
be described in the next section. The idle periods between
two paging intervals can alternatively be used for scanning
neighbouring base stations if the transmissions in the pag-
ing intervals from the serving base station get too weak.
6.3 Location Management
A subscriber station being in active or sleep mode always
performs a handover when it leaves the coverage area of