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Understanding WiMAX PHY

BP Tiwari

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WIMAX Series WIMAX Series –– Understanding PHY Understanding PHY

Agenda

OFDMA WiMAX Frame Sub channels and Permutations Smart Antenna Systems Summary of WiMAX PHY features

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Lets understand fading in wireless communications

Delay Spread)

Frequency selective fading ( Delay Spread)

Multiple copies of signal arrive at the receiver, offset in time

Time Selective Fading ( Doppler )

MU

LT

IPA

TH

FA

DIN

G C

HA

NN

EL

S

Fading (Delay Spread)

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WiMAX Addresses Fading

Multiple copies of signal arrive at the receiver, offset in time

Fading (Delay Spread)Frequency selective fading ( Delay Spread)

OFDMA

Diversity Schemes

Multiple Antenna Systems

Cyclic Prefix ( ISI )

Forward Error Correction

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Concept of OFDMSubcarrier

OFDM is robust to fading

High spectral efficiency Eliminate Inter-symbol

Interference (ISI) by the introduction of the cyclic prefix (CP)

Sensitivity to frequency offset

Higher Peak to average power ratio(PAPR)

In OFDM available bandwidth is divided into N (large number of) orthogonal sub-carriers

Each sub-carrier is individually modulated (QAM) at 1/N total data rate

FFT implementation (fast and efficient)

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OFDM vs. OFDMA Subcarrier

– Type: Data, Pilot, null (guard and DC)•

– Number: 128, 512, 1024, 2048 (at least one)

Subchannel– A set of sub carriers forms a

subchannel– The sub carriers may and

may not be adjacent– Distributed allocation

(diversity)– Contiguous allocation

(selectivity)

OFDM

OFDMA

SC

Subcarrier

Subchannels

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OFDMA Advantages

• Users/bursts are scheduled across both frequency (subchannels)and time (symbols)

• Per burst modulation, coding and TX power

• Higher granularity in resource allocation

• More degrees of freedom in scheduling

• Improved fairness and QoS

D D

C C

D D

C C

B B

A A

B B

E E

TIME

SU

BC

HA

NN

EL

•Orthogonality of users are maintained using large symbol duration and cyclic prefix

•Users concentrate transmit power over partial bandwidth

•High granularity in resource allocation & power control

•Inherent resistance to multi-path (OFDM)

•Flexible Sub-channelization

•Distributed (pseudo-random) permutations –frequency diversity

•Contiguous (clustered) permutations -> selectivity

•Scalability of Channel Sizes

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OFDM Symbols and Cyclic Prefix

Use Cycle Prefix (CP) as guard interval

Prepend segment of OFDM symbol –> overhead

CP duration > max delay spread

Eliminates ISI

Retains orthogonally between sub-carriers (ICI)

Pilots used for channel estimation and tracking

Interleaving (bits distributed across tones) and error correcting facilitates recovery under frequency selective fading or narrow-band interference

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OFDMA Frame Structure

PREAMBLE

DL-MAP

UL-MAP

DL BURST 2

DL BURST 1

DL BURST 3

DL BURST 4

SUBCHANNELS

S1 S2 SnDOWNLINK

CDMARANGING

PeriodiCRang.

ACKCH’s

CQICHs

UL BURST 1

UL BURST 2

UL BURST 3

UPLINKTTG RTG

Uplink and Downlink is divided into time zones

Different Permutation for downlink and uplink– DL PUSC,AMC etc – UL PUSC,AMC– Activation STC/MIMO

CDMA Codes– Special allocation in

uplink for ranging and BW Request

Fast Feedback/CQICHs– SS measured CINR

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Localized vs Distributed Sub channels

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Subchannels in 16e

UL PUSC– 1 tile = 4 subcarriers x 3 OFDM symbols– 1 subchannel = 6 tiles (frequency domain)– 1 slot =1 subchannel x 3 OFDM symbols– 1 slot = 72 subcarriers

= 48 data subcarriers + 24 pilots

DL/UL AMC2X3– 1 bin = 9 subcarriers x 1 OFDM symbols– 1 subchannel = 2 bins (frequency domain)– 1 slot = 1 subchannel x 3 OFDM symbols– 1 slot = 54 subcarriers

= 48 data subcarriers + 6 pilots

DL PUSC– 1 cluster = 14 adjacent subcarriers

x 2 OFDM symbolsx 2 OFDM symbols– 1 subchannel = 28 subcarriers– 1 slot = 1 subchannel x 2 OFDM symbols– 1 slot = 56 subcarriers

= 48 data subcarriers + 8 pilots

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Permutation schemes in DL : PUSC

2 S

ym

2 S

ym2

Sym

2 S

ym2

Sym

Cluster

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Permutation schemes in UL : PUSC

3 SY

MB

OLS

3

Sym

3

Sym

3

Sym

3

Sym

Pilot

Tile Structure

• 4 Sub carrier X 3 Symbol

• 4 Pilot + 8 Data

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PHY Data Rates

Without MIMO and overheads

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Advanced Antenna Systems : STBC

STC gains vary from 5-10 db depending on channels condition

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MIMO B : Spatial Multiplexing

SM can double throughput in theory by require very high CINR conditions of uncorrelated signal to achieve full advantages. 30% sector improvement in loaded conditions.

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Beam forming

Beamforming can provide additional 5-6 db gains in downlink improving overall network quality. Improves sector capacity by 20 to 25% 4Rx in uplink provides additional 3 db gain

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UL Collaborative MIMO

Collaborative Spatial Multiplexing for two MSs with single transmit antenna Two MS transmit on the same

resource (sub channels) For orthogonal zing

– Orthogonal pilot patterns (pattern A, pattern B)

Multiple data detection– Using multiple receive antennas– Interference suppression: MMSE,

ZF– Nonlinear interference

cancellation: SIC, PIC

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MIMO Summary

BF helps in improving capacity and improve network quality.

20 to 25% over MIMO

5-6 db in Downlink

DL-Beam Forming

MIMO A in low SINR condition improves channel fading. Switching between MIMO A and B is automatic and controlled by BTS.

5-10*MIMO A

Improves cell radius by 3db in uplink.

Nil 3 db over 2nd

order diversity

4th order Diversity

Average improvement of 30% in full load , customers higher CINR or uncelebrated signals will take advantage of MIMO B

30% in average

Nil MIMO B

CommentsGain in Capacity

Gain in Link Budget

Parameters

*STC + 2 Rx in receiver with possibility MRC combining

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Mobile WiMAX PHY Summary

Downlink : 2x2 , STC , SM Uplink : 1X2 , Virtual SM

MIMO

AMC or PUSC UL channel sounding

BeamForming

Downlink :QPSK,16QAM,64QAM Uplink : QPSK, 16QAM

Modulation

Ranging , CQICH’s , ACKCHUplink Control channels

PUSC,AMC,FUSCSub-carrier allocations

CC, CTC with HARQ Error correction and HARQ

5 ms , 1/8Frame size and cyclic prefix

TDD , 5 and 10 MHz , 20 (optional) Scalable Channel Bandwidth

Support Feature

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Thanks