WR2600 S3 WiMAX Air Interface

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SECTION 3

WIMAX AIR INTERFACE

WiMAX Engineering Overview

I© Wray Castle Limited




II © Wray Castle Limited



Single Carrier WiMAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.1

Multi-carrier WiMAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2

WirelessHUMAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3

Radio Carrier Orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.4

Spectral Efficiency in OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5

Resilience to Time Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6

Resilience to Multipath Fading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.7

The OFDM Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.8

The OFDM Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.9

Subcarrier Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.10

OFDMA Resource Allocation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.11

Channel Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.12

MIMO Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.13

The Benefits of MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.14

WiMAX Error Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.15

CONTENTS

WiMAX Air Interface

III© Wray Castle Limited




IV © Wray Castle Limited



At the end of this section you will be able to:

describe the air interface options available within the WiMAX standard

outline the difference between the WirelessMAN and WirelessHUMAN concepts

describe the basic concept that underlies OFDM

explain the meaning of the term ‘orthogonality’ with reference to OFDM systems

outline the functionality and use of fast Fourier transforms in OFDM

demonstrate an understanding of the concept of orthogonal carriers and the relationship

between carrier spacing and bandwidth

outline the differences between OFDM and OFDMA and the benefits associated with each

system

describe the channel coding options available in the OFDM and OFDMA variants

OBJECTIVES

WiMAX Air Interface

V© Wray Castle Limited




VI © Wray Castle Limited



WirelessMAN-SC and WirelessMAN-SCa

Single carrier per link

LOS and NLOS versions

FDD and TDD supported

Licensed and Unlicensed bands

10–66 GHz and 2–11 GHz

TDM/TDMA modes

SCa supports more advancedfeatures

SC does not includemobility functions

WR2600/v3 3.1© Wray Castle Limited

Single Carrier WiMAX

WirelessMAN-SC variant was designed to operate in licensed frequency bands between 10 and 66 GHz.

The short wavelength signals employed at these frequencies require a line of sight service and cannot

make use of multipath propagation effects. Modulation takes place on a single carrier in Time Division

Duplex (TDD) mode or on dual carriers in Frequency Division Duplex (FDD).

WirelessMAN-SC uses ‘burst mode’ transmission and can support both PtP and PMP services. It sharescapacity among users on a TDM basis where each user is assigned a specific frame in which to transmit

or receive a burst of data – the frame size and modulation technique employed can be varied on a frame

by frame basis to accommodate users requiring differing data rates.

SC links use a limited subset of 802.16 MAC functions and do not support advanced features such as

Automatic Retransmission Request (ARQ) or adaptive antenna systems.

The WirelessMAN-SCa variant operates in licensed bands lower than 11 GHz where the lower

frequencies and longer wavelengths make NLOS services possible. This means that WiMAX services

can be offered in denser urban areas without customers needing to have a fixed antenna installed with a

clear LOS to the base station. SCa operates in a similar way to the SC variant, but has added support for

multipath equalization, adaptive antenna systems and enhanced error correction techniques such as ARQ.

The multiplexing method is still based on TDM principles, but also supports Time Division Multiple

Access (TDMA), which allows frames to be assigned to users on a dynamic and flexible basis, thus

increasing the number of users that can be served over a given period of time.

WiMAX Air Interface



WirelessMAN-OFDM and WirelessMAN-OFDMA

Multiple subscribers per link

OFDM supports 256 carriers

OFDMA supports up to 2048 carriers

Both are NLOS versions

FDD and TDD supported

Licensed and Unlicensed bands

2–11 GHz only

TDM/TDMA and OFDMA modes

OFDM supports Mesh modeOFDM and OFDMA include mobility functions

WR2600/v33.2 © Wray Castle Limited

Multi-carrier WiMAX

The WirelessMAN-OFDM system employs multiple subscribers (256) to allow the uplink or downlink

WiMAX signal to be spread over a wide bandwidth channel. This has benefits in terms of the

connection’s ability to handle noise and also serves to increase the effective data rate of the link.

This WiMAX variant is designed to operate in bands below 11 GHz and is therefore capable of making

use of multipath propagation effects to extend cell size and improve network coverage.

The bandwidth allocation method is based on TDMA principles with each user sharing a link being

assigned uplink and downlink capacity on a frame by frame basis. Usage ‘maps’ inserted at the start of

each frame show the intended recipient of each downlink frame and the assigned user for the next

relevant uplink frame. This allows for fast and dynamic allocation of capacity.

The OFDM variant also supports Mesh mode transmission, where a subscriber station has the option of

communicating directly with a base station or with the BS via an intermediate SS – this can significantly

extend the cell size and service penetration of WiMAX systems.

WirelessMAN-OFDMA takes the functions of the OFDM variant and adds an extra level of complexity. To

increase potential data rates, OFDMA links use up to 2048 subcarrier channels. Optionally, groups of subcarriers (known as subchannels) can be assigned to particular users during a transmission period,

meaning that several users can be served by the same frame on both uplink and downlink.

Scalable OFDMA systems can select to use smaller numbers of subcarriers in lower bandwidth channels

– with 1024, 512 and 128 subcarrier options being available.




WirelessMAN-SCaTDD Mode

WirelessMAN-OFDM

TDD Mode

WirelessMAN-OFDMA

TDD Mode

WirelessHUMAN

Unlicensed

Spectrum

Different frequency bands

Dynamic Frequency Selection (DFS)

Power limitations


WirelessHUMAN

The High-speed Unlicensed Metropolitan Area Network variant was developed to allow network

operators the freedom to establish WiMAX networks using the unlicensed bands in the microwave

frequencies below 11 GHz.

WirelessHUMAN systems can use the basic air interface functions of the TDD versions of the SCa,

OFDM and OFDMA variants of WiMAX but have added support for frequency selection and interferenceavoidance. WirelessHUMAN only operates using the TDD versions

A basic feature of HUMAN operation is Dynamic Frequency Selection (DFS), which allows a BS to select

and reselect the channel to be used for combined uplink and downlink operation based on

measurements taken of the local radio environment.

As unlicensed bands are shared with many other applications, including Bluetooth™ and WiFi systems,

the added ability to ‘hop’ around the available spectrum allows WirelessHUMAN to select the best

channel to use on a dynamic basis.

WiMAX Air Interface



5 kHz

Spacing to next allocated

carrier needs to be large

f 1

f 1 f 2


Radio Carrier Orthogonality

Consider a radio carrier being modulated by a 10 kbit/s bit steam using QPSK (Quadrature Phase Shift

Keying). It could be expected to see a spectral envelope following a (sin x)/ x function, as shown in the

diagram, with the first null located 5 kHz from the centre frequency.

In a classic FDM (Frequency Division Multiplexing) system, other radio carriers would be allocated and

spaced far enough away from the first to ensure minimal adjacent channel interference. The size of theguard band required would depend on the transmitter and receiver characteristics as well as the relative

powers.

However, in such a system it is assumed that there is no synchronization between the potential

interferers. It is this that leads to the need for large frequency spacing between adjacent carriers. In fact,

if there was synchronization between adjacent channels, a much smaller frequency spacing could be

used. The key is to be able to make use of the complex nature of the instantaneously transmitted

spectrum. The modulation envelope is only an artificial way of indicating all possibilities over time; a

snapshot at an instant in time would look different.

Consider a second radio carrier allocated such that its centre frequency coincides exactly with the null in

the first carrier’s envelope. It is using the same modulation scheme and carrying the same data rate. Theresult is as shown. Note that the carrier spacing of 5 kHz is the same magnitude as the symbol rate of 5

ksps. The spectra of the two carriers now overlaps, but as long as the carrier frequencies and the

baseband data remain accurately synchronized, both can be demodulated successfully. The reason is

that this relationship between centre frequency offset and symbol rate maintains a high level of

orthogonality between the two radio carriers.




f 1 f 2

Centre

frequency

2 QPSK sub carriers

10 kbit/s per subcarrier

15 kHz total bandwidth

15 kHz

f 1

Centre

frequency

1 QPSK carrier

20 kbit/s

20 kHz total bandwidth

20 kHz


Spectral Efficiency in OFDM

Considering again the two overlapping QPSK radio carriers, it can be seen that there is a relatively large

spectral efficiency gain. If the effective bandwidth of the transmitted signal is considered to be the

frequency separation of the first nulls then a single QPSK carrier modulated with 10 kbit/s would have a

null-to-null bandwidth of 10 kHz.

However, here there are two sub-carriers, each of which is carrying 10 kbit/s using QPSK. Their respective null-to-null spectra overlap by 5 kHz. This gives a collective null-to-null bandwidth for the pair

of sub-carriers of 15 kHz. Thus QPSK is being used to carry 20 kbit/s in a radio bandwidth of 15 kHz.

Note that a single QPSK modulated carrier carrying 20 kbit/s would result in a null-to-null bandwidth of 20

kHz.

The principle of independent reception of orthogonal radio carriers with overlapping spectrum can be

extended by using a large number of narrowband radio carriers within one wideband channel allocation.

This results in a very spectrally efficient channel that can carry high bit rates.

For example, if 1000 orthogonal radio carriers were modulated using QPSK, each carrying 10 kbit/s, the

net throughput for the channel would be 10 Mbit/s. This would require a total channel bandwidth of

slightly more than 5 MHz. Carrying the same bit rate with QPSK modulation onto a single radio carrier would require a null-to-null bandwidth of 10 MHz. Thus OFDM (Orthogonal Frequency Division

Multiplexing) almost doubles the spectral efficiency. Moreover, the resulting OFDM transmission is more

resilient to multipath effects in the channel.

WiMAX Air Interface



High-bit-rate serial stream

S to P

Low-bit-rate parallel streams

Multipath 1

Multipath 1

Multipath 1

Guard

periodUseful symbol period


Resilience to Time Dispersion

Spectral efficiency is not the only benefit associated with using OFDM. It also exhibits good tolerance to

the effects of multipath propagation in the channel; both fading and time dispersion.

Because the data rate on individual subcarriers with the channel is very low, the symbol period is

correspondingly long. The resulting symbol period is typically significantly longer than the time dispersion

that occurs in the channel. This means that relatively simple equalization can be used to counteractmultipath even though the net rate in the whole channel is very high.

Furthermore, a guard period can be inserted in every symbol period that covers the expected time

dispersion for the channel. This removes most of the time dispersion distortion from the useful symbol

period.

This guard period is usually created by repeating a copy of the last part of the symbol at the start. In this

case it is referred to as the cyclic prefix.




F r e q u e n c y

A m p l i t u d e

T i m e F r e q u e n c y

A m p l i t u d e

T i m e


Resilience to Multipath Fading

Tolerance to multipath fading effects comes from the overall wideband characteristic in the channel. A

narrowband channel tends to exhibit flat fading characteristics; that is to say, the fading characteristics

are coherent across the whole channel bandwidth. The effects of this can be seen in the diagram.

OFDM channels, on the other hand, are usually used to carry very high data rates and therefore require

many subcarriers occupying a relatively large bandwidth. In most cases the bandwidth will exceed thecoherence bandwidth by a large factor, so differing fading characteristics will be seen in different parts of

the channel. In effect, the wide channel provides a degree of frequency diversity with a resulting

improvement in performance.

However, it would be wrong to assume that this benefit for OFDM results solely because the channel

bandwidth is wide. A single carrier system with the same bit rate would also result in a wide radio

channel. Therefore, a single carrier system also benefits from this form of frequency diversity to some

extent.

In the single channel system, energy from each symbol will be spread across the whole radio channel

and each symbol will therefore suffer some distortion from any fading that may occur in any one part of

the channel. In an OFDM system only those symbols transmitted on subcarriers in the part of the channelaffected by a fade will be distorted. Symbols transmitted on other subcarriers will remain unaffected. It is

then possible to adapt the subcarriers in use according to the varying fading characteristics. This means

than only non-fading carriers will be used.

WiMAX Air Interface



Serial dataS P

M-ary

symbol bit

grouping

{b0, b1, b2…bn}

M-ary

symbol

mapping

N

parallel

streams

N

complex

symbols

N -point

IFFT

I (real)

Q (imaginary)

N complex

samples in one

symbol period

sine

cosine

Up-conversion

D/A

f c

OFDM signal with

N subcarriers


The OFDM Transmitter

The diagram shows a block representation of the transmitter that brings together the elements of symbol

mapping for QAM and the application of the IFFT in order to produce an OFDM signal.

The serial data to be carried on the radio link is first passed through a serial-to-parallel conversion

process. The number of parallel streams will be equivalent to the number of data-carrying subcarriers in

the system. This number will usually be a power of two since this makes best use of the efficienciesoffered by the IFFT.

Bits on the parallel data streams will also be grouped as appropriate for the symbol constellation of

M-ary QAM scheme in use. For example, for QPSK bits are grouped in pairs; for 16QAM they are

grouped in fours and for 64QAM they are grouped in sixes.

The next process is symbol point mapping for the bit groups on each parallel data stream. The resulting

complex number symbols then form the input to an N-point IFFT where N will be a power of two

equivalent to the number of subcarriers in use.

The output of the IFFT will be a series of complex number digital samples representing the OFDM signal

during each symbol period. At this point the cyclic prefix is added by copying the last samples onto thebeginning of the symbol period. These complex real and imaginary sample values are used to form the I

and Q symbol streams. Next, the I and Q branches are subsequently multiplied onto sine and cosine

representations of the radio carrier. This generates a digital representation of the required multicarrier M-

ary QAM modulated transmit signal.

After analogue-to-digital conversion the resulting signal can be up-converted to the required channel

centre frequency before amplification and transmission.




OFDM signal with

N subcarriers

Serial dataP S

{b0, b1, b2…bn}

Integration

and

symboldecisions

N

parallel

streams

N

complex

symbols

N -point

FFT

I (real)

Q (imaginary)

N complexsamples in one

symbol period

Down-conversion

A/D

f c

sine

cosine


The OFDM Receiver

The filtered OFDM signal is down-converted and then sampled for analogue to digital conversion. The

sampling rate at this point will be factored to allow for the inclusion of the cyclic prefix.

The cyclic prefix is removed and the sampled signal is separated into I and Q components. The result is

a series of complex samples that are used as the input to the FFT.

The FFT deconstructs the complex waveform in the symbol period to N complex values, each

representing a modulation symbol on one of the subcarriers. M-ary demodulation by integration and

reverse symbol mapping is performed to recover groups of bits represented by each of the received M-

ary modulation symbols.

Finally, parallel-to-serial conversion reconstructs that original serial bit stream.

WiMAX Air Interface



Lower

unused/guardsubcarriers

Upper

unused/guardsubcarriers

DC

subcarrier

Reference/pilot

subcarriersData

subcarriers


Subcarrier Assignment

Different subcarriers from across the population of subcarriers created by an OFDM channel are

assigned to different functions. Most subcarriers will be assigned to carry modulated user data signals.

Each data subcarrier will be modulated to carry one part of the entire parallel signal being transmitted

across the multi-tone channel. The data rate of each data subcarrier is determined by a combination of

the symbol rate and the modulation scheme employed.

In some variants of OFDM (such as that employed by WiMAX), entire subcarriers are given over to

carrying ‘pilot signals’. Pilot subcarriers allow channel quality and signal strength estimates to be made

and allow other control functions, such as frequency calibration, to operate. Pilots are generally

transmitted at a higher power level than data subcarriers - typically 2.5 dB higher - which serves to make

them more easily acquired by receiving stations.

In LTE and other systems, including DVB (Digital Video Broadcasting), the same function is performed by

‘reference signals’. A reference signal, like a pilot, allows a receiving station to recalibrate its receiver and

make channel estimates, but instead of occupying an entire subcarrier it is periodically embedded in the

stream of data being carried on a ‘normal’ subcarrier.

There are also two types of ‘null’ subcarrier - guards and the DC carrier. Nothing is transmitted on nullsubcarriers.

Guard subcarriers separate the top and bottom data subcarriers from any adjacent channel interference

that may be leaking in from neighbouring channels and, in turn, serves to limit the amount of interference

caused by the OFDM channel. The more guard subcarriers that are assigned, the lower the amount of

adjacent channel interference that will be caused or detected, but this also has an impact on the data

throughput of the channel.

The centre subcarrier of each OFDM channel - the one that has a 0 Hz offset from the channel’s centre

frequency - is known as the ‘DC carrier’ and is also null.




Symbol periods (time)

S u b

c a r r i e r s ( f r e q u e n c y )

0 1 2 3 4 5 6 7 8 9

User 1 User 2

U s e r 3

User 4

OFDM with

time

multiplexing

OFDM with time

and frequency

multiplexing

(OFDMA)

Symbol periods (time)

S u b c a r r i e r s ( f r e q u e n c y )

0 1 2 3 4 5 6 7 8 9

User 2

User 4 U s e r 5

User 8

User 1

User 3

U s e r 6

User 9

User 7


OFDMA Resource Allocation Strategies

The simplest option for multiple access in an OFDM system is to use a form of time multiplexing on the

OFDM radio bearer. This is illustrated in the top part of the diagram. Each user is allocated the full

channel bandwidth and all data subcarriers exclusively for a defined number of symbol periods.

The greatest efficiency can be achieved if dynamic time allocation is applied so that users with higher bit

rate requirements are allocated a greater proportion of time. However, in such a system the minimumresource allocation is one OFDM symbol. Even with dynamic time allocation, such an arrangement can

still become very inefficient when there is strong demand for multiple lower bit rate connections, for

example when multiple voice circuits are active. Consider an OFDM system operating in a 10 MHz

bandwidth, with a 512-point FFT and using 16QAM. Allowing for null and reference subcarriers, such a

system could transfer in the order of 1,600 bits in a single OFDM symbol period. This may seem a

modest resource unit, but delay requirements must also be accounted for. For a real-time service such

as voice it is essential to avoid excessive round-trip delay. To meet the delay requirement for a voice

service, resources may need to be allocated, for example once every 20 ms. This would mean in a

minimum bandwidth allocation to one user of 80 kbit/s (or 120 kbit/s if 64QAM is in use). Even allowing

for the error protection overhead this minimum resource will significantly reduce system efficiency and its

ability to benefit from optimal techniques such as discontinuous transmission and channel adaptation.

Greater efficiency in resource allocation can be gained from the use of subchannelization. This involves

division of resource by time and by frequency. Thus a user may be allocated a subset of the subcarriers

available in the system, as illustrated in the lower part of the diagram. This approach allows much finer

granulation in resource allocation and therefore greater efficiency. OFDM systems that support this are

usually described as OFDMA (Orthogonal Frequency Division Multiple Access) systems.

WiMAX Air Interface



Modulation (QPSK/16QAM/64QAM)

Error protection coding rate

Adaptive fast scheduling

Poor radio path

Interference


Channel Adaptation

The quality of the radio link is affected by many factors including fading, interference and time dispersion.

Terrestrial mobile radio channels, which are usually assumed to be non-line of site, can be very poor.

Therefore most terrestrial cellular radio systems are designed with robust modulation schemes and large

error protection overheads.

However, close examination of real channel conditions shows them to be very variable in short timeframes, and much of the time any given channel will show good performance. Thus the standard

approach engineers the channel to deal with the worst case, which only occurs for a small amount of

time.

It is clear that if the channel could be adapted at a rate fast enough to track changing channel conditions

then the average performance of a channel could be significantly improved. This is the principle of

channel adaptation. Channel adaptation is a common approach in many broadband radio systems and in

most cases involves the adaptation of the modulation scheme and the error protection overhead applied.

Adaptive scheduling can also be very effective, enabling the cell to make the best use of the pool of

channels allocated to different mobiles, each of which will be varying independently.




Data

stream

mapping

Pre-

coding

matrix

Signal

generation

MIMO

decoding

and channel

estimation

Stream 1

Stream 2

Layer 1

Layer 2

2x2 MIMO or Rank 2 4x4 MIMO or Rank 4

Power weightings and

beamforming

Feedback


MIMO Concept

MIMO (Multiple Input Multiple Output) antenna arrays offer significant performance improvements over

conventional single antenna configurations.

The technique involves placing several uncorrelated antennas at both the receiving and transmitting ends

of the communication link. If there are four uncorrelated antennas at the transmitter and a further four

uncorrelated antennas at the receiver, then there will be 16 possible direct radio paths between thetransmitter and the receiver. Each of these is open to multipath effects, creating even more radio paths

between the transmitter and the receiver. These radio paths can then be constructively combined, thus

producing micro diversity gain at the receiver.

Since the receiver can distinguish between the various uncorrelated antennas, it is possible to transmit

different data streams in different paths. The stream applied to each antenna can be referred to as a

‘layer’ and the number of antennas available at the transmitter and receiver can be referred to as ‘rank’.

For example, a system operating with a 4x4 MIMO antenna array can be described as having four layers

and being of rank four. The way in which data streams are mapped to layers will change the specific

benefits offered by a particular MIMO implementation, and the specification of this is an important part of

system design. Pre-coding may also be used to improve the MIMO system performance. Pre-coding may

be adaptive and as such would be based on some source of channel estimation. This could be derived atthe transmission or the reception end of the link.

It is relatively easy to mount antennas on the base station in an uncorrelated manner. For a 2x2 MIMO

array a single cross-polar panel could be used. A 4x4 MIMO array would require two cross-polar panels

with suitable special separation. This is harder to achieve in a mobile. However, as for the base station,

2x2 MIMO could be achieved with cross polarization, but this could result in some undesirable directivity

in the antenna.

WiMAX Air Interface



MIMO brings

Decorrelates fading

through different

transmission paths

Enables multiple data

streams to be transmitted on

the same frequency/time

resource

Provides a beamforming

effect that focuses radiated

energy in the direction of

the receiver

Diversity gain Array gain Spatial multiplexing gain


The Benefits of MIMO

MIMO is potentially a complex technology but it can provide very significant benefits in system capability.

There are three key ways in which MIMO improves system performance. Any given MIMO

implementation may make use of all these benefits or may be configured to take particular advantage of

one of them. Ideally, a system should be designed with sufficient flexibility in MIMO implementation to

allow a system operator to choose the most suitable implementation for different environments or system

goals.

Diversity gain arises out of the provision of multiple antennas at the transmitting and/or receiving end of

the radio link. This creates multiple transmission paths with decorrelated fading characteristics. The

result is an overall improvement in channel signal-to-noise ratio leading to increased channel throughput

and reliability.

Array gain refers to the beamforming capability of a multiple antenna array. With suitable signalling of

feedback from the receiver, or with measurements made on a return link, it is possible to direct radiated

energy toward the receiver in a steered beam. The result is improved channel performance and

increased throughput.

Spatial multiplexing gain arises out of the orthogonality between the multiple transmission paths createdby the multiple antenna array. Since the receiver can resolve independent transmission paths it is

possible to map different information streams into the transmission paths, identifiable by their spatial

signature. This results in a direct increase in the channel throughput in proportion to the number of

separate transmission streams used.




Forward Error

Correction

(FEC)

Convolutional Coding

(CC)

Block Turbo Coding

(BTC)

Convolutional

Turbo Coding(CTC)

Optional

Optional

Mandatory

WirelessMAN-OFDMA Options


WiMAX Error Protection

There are three Forward Error Correction (FEC) options specified for WirelessMAN-OFDM:

Concatenated Reed-Solomon/Convolutional Coding (RS-CC)

Block Turbo Coding (BTC)

Convolutional Turbo Coding (CTC)

Support for BTC and CTC are optional, RS-CC is mandatory. The RS-CC coding scheme combines an

outer coding stage that employs Reed-Solomon coding and an inner coding stage that uses a rate-

compatible convolutional code. WirelessMAN-OFDM employs a set of fixed error coding rates that are

tied to the modulation schemes employed.

WirelessMAN-OFDMA also supports three FEC options:

Convolutional Coding (CC)

Block Turbo Coding (BTC)

Convolutional Turbo Coding (CTC)

Only support for CC is mandatory. The OFDMA channel coding scheme also supports optional repetition.

For channels perceived to be performing badly, data blocks may be copied and sent via parallel adjacent

subchannels. If one copy of the block suffers unrecoverable errors, there is a chance that the second

block may be received accurately. Repetition increases the reliability of the service at the expense of

throughput.

WiMAX Air Interface

WR2600 S3 WiMAX Air Interface

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