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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


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,


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-


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


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


7/18

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


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



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


8/18

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


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


9/18

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.


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


10/18

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


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


11/18

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-


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


12/18


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]


13/18

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-


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


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, ...


15/18

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


Activemode

Sleepmode

Idlemode

Ready for reception/transmission

Handover

StandbyHandover

SuspendedPaging + Location Update

Figure 22. Power saving modes


16/18

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

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