11

Comparative Evaluation of the Comparative Evaluation of the FreeBits PHY:FreeBits PHY:

OFDM vs. EqualizationOFDM vs. Equalization

Shrenik Patel & Narayan MandayamShrenik Patel & Narayan Mandayam

May 20, 2002

22

Objective

To design the PHY layer for the FreeBits/ Infostations project

WHAT’S SO SPECIAL ABOUT INFOSTATIONS ?

• Cover geographically disconnected areas

• Proposed for close-range wireless systems (< 20m)

• Target very high data rates (100s of Mbps)

• Targeted to be a system with low cost (complexity)

=> Essentially, we want bits for free !

33

OverviewOverview

Need for channel modeling, channel models 1Need for channel modeling, channel models 1--33

Link budget, System parameters, Frame structure Link budget, System parameters, Frame structure

Performance analysis of matched filter, SC + Performance analysis of matched filter, SC + Equalization, OFDM + CE in channels 1Equalization, OFDM + CE in channels 1--33

BER, WERBER, WERGood throughput (goodput)Good throughput (goodput)

Conclusions and future workConclusions and future work

44

Need for Channel ModelingNeed for Channel Modeling

PROBLEM in using an existing channel modelPROBLEM in using an existing channel model

Existing models valid for micro or macro cellular coverage areasExisting models valid for micro or macro cellular coverage areas

We are interested in channel behavior for distances less than 20We are interested in channel behavior for distances less than 20m m between the transmitter and the receiver between the transmitter and the receiver

SOLUTION => Measurements and Channel ModelingSOLUTION => Measurements and Channel Modeling

(Andrej Domazetovic, Larry Greenstein, (Andrej Domazetovic, Larry Greenstein, Narayan Mandayam, Ivan Seskar)Narayan Mandayam, Ivan Seskar)

55

• Two-ray deterministic part

• Ricean : high K factor > 15 dB

• δ < 7 ns < Ts = 10 ns

Channel Model 1 : Along ramp in front of Channel Model 1 : Along ramp in front of WINLABWINLAB

deterministic

+

stochastic

+0γ

1Lγ

α +δ−zβ

1Lz−

66

Channel Model 2 : Along path between ECE & Channel Model 2 : Along path between ECE & CORECORE

• Four-ray deterministic part

• Ricean : high K factor > 15 dB

• 2 additional paths insignificant

• Performance in Channel 2 very similar to that in Channel 1

deterministic

+

stochastic+0γ

1Lγ

α +1δ−z1β

2δ−z2β

3δ−z3β

2Lz−

77

Channel Model 3 : In WINLAB parking lot , Channel Model 3 : In WINLAB parking lot , generic LOS generic LOS

Known characteristics

• LOS

• K factor ≈ 10 dB

• RMS delay spread ≈ 5-10 ns

Assumed Model

• ‘Spike’ + ‘EXP’ delay profile

• 11 paths at 5 ns intervals

• K factor, Trms ind. of distance

deterministic

+

stochastic

+0γ

1Lγ

α

3Lz−

88

Link BudgetLink BudgetReceived power,

PRx(dBm) = 20log (λ/4π) + PTx(dBm) + GT (dBm) + GR (dBm) – 20log(d)

Noise variance,

N = KTB + NF = -83 dBm

At d = 10 m, SNR = 22 dB (at d = 10 m, PTx = 0 dBm)

Tx RxPTx = 1 mW = 0 dBm

NF1 = 3 dBGR = 3 dB

NF2 = 3 dBGT = 3 dB

NF3 = 5 dB

d = 10 mLoss = 67 dB

• Sufficiently high SNR even at a low transmit power of 1 mW

99

System parameters and frame structureSystem parameters and frame structureCarrier frequency = 5.3 GHz, available BW =100 MHzCarrier frequency = 5.3 GHz, available BW =100 MHz

SuperSuper--frame length = 7.2 µs (< 5 % of coherence time, assuming frame length = 7.2 µs (< 5 % of coherence time, assuming vehicular speeds)vehicular speeds)

OFDM systemOFDM system32 subchannels, 12.5 % cyclic prefix, 3 virtual subcarriers 32 subchannels, 12.5 % cyclic prefix, 3 virtual subcarriers => Efficiency ≈ 80%=> Efficiency ≈ 80%

2 out of 20 OFDM symbols used as a training preamble (simple cha2 out of 20 OFDM symbols used as a training preamble (simple channel nnel estimation) estimation) => Efficiency = 90 %=> Efficiency = 90 %

Single carrier systemSingle carrier system

Raised cosine filtering with 25% bandwidth expansionRaised cosine filtering with 25% bandwidth expansion=> Efficiency = 80%=> Efficiency = 80%

10 % of super10 % of super--frame used for equalizer training (adaptive decision feedback frame used for equalizer training (adaptive decision feedback equalizer based on LMS algorithm)equalizer based on LMS algorithm)=> Efficiency = 90 %=> Efficiency = 90 %

1010

Performance of Matched Filter for MQAMPerformance of Matched Filter for MQAM

• PTx = 1 mW = 0 dBm

256-QAM

64-QAM

16-QAM

4-QAM

Channel 1

LOS only

• Small M => Low complexity system : Simple matched filter (at small distances)

• Large M => High throughput system : OFDM or SC+Equalization ?

1111

Channel Model 1 : OFDM without codingChannel Model 1 : OFDM without coding

64-QAM

• PTx = 1 mW = 0 dBm

• Insignificant gain with increase in the training overhead beyond 10%

Max throughput (10% overhead) ≈ (0.8)(0.9)(0.9)(600) = 388 Mbps in 100 MHz CRC overheadCyclic prefix, null subcarriers overhead

Training Preamble overhead

1212

Channel Model 1 : SCChannel Model 1 : SC--Equalization without Equalization without codingcoding

64-QAM

• PTx = 1 mW = 0 dBm

• Error floor reached due to insufficient training for the equalizer

Max throughput (10% overhead) ≈ (0.8)(0.9)(0.9)(600) = 388 Mbps in 100 MHz CRC overheadBandwidth expansion due to raised-

cosine pulse shapingEqualizer training overhead

1313

Channel Models 1 & 3 : OFDM without codingChannel Models 1 & 3 : OFDM without coding

64-QAM

• PTx = 1 mW = 0 dBm

• Performance in Channel 3slightly worse than in Channel 1

Max throughput (10% overhead) ≈ (0.8)(0.9)(0.9)(600) = 388 Mbps in 100 MHz CRC overheadCyclic prefix, null subcarriers overhead

Training preamble overhead

1414

Channel Model 3 : SC+Equalization and OFDM Channel Model 3 : SC+Equalization and OFDM without codingwithout coding

64-QAM

• PTx = 1 mW = 0 dBm

• OFDM performs better than SC+Equalization

Max throughput (10% overhead) ≈ (0.8)(0.9)(0.9)(600) = 388 Mbps in 100 MHz CRC overheadBandwidth expansion/Overhead

Equalizer training overhead

1515

Channel Model 3 : SC+Equalization and OFDM Channel Model 3 : SC+Equalization and OFDM without codingwithout coding

256-QAM

• PTx = 1 mW = 0 dBm

• OFDM performs better than SC+Equalization

Max throughput (10% overhead) ≈ (0.8)(0.9)(0.9)(800) = 518 Mbps in 100 MHz CRC overheadBandwidth expansion / Overhead

Equalizer training overhead

1616

Goodput for OFDM Goodput for OFDM (with Lang Lin, Roy Yates)

Single mode schemes

Coded 64-QAM265.71 Mbps

Coded 256-QAM 221.35 Mbps

Variable mode scheme

321.33 Mbps

• Average goodput of ≈ 321 Mbps achievable using a variable mode scheme• At a speed of 0.29 m/s, total bits transferable= 321*40 /(0.29) Mbits

≈ 4.7 Gigabytes! A DVD as one ambles by an Infostation

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ConclusionsConclusions

A simple matched filter is sufficient for lower order modulationA simple matched filter is sufficient for lower order modulationschemes at close distancesschemes at close distances

The performance of SC+Equalization and OFDM systems compares The performance of SC+Equalization and OFDM systems compares well for 64well for 64--QAM (and lower order MQAM); OFDM preferable for QAM (and lower order MQAM); OFDM preferable for 256256--QAMQAM

Large files (several CDs, single DVD) can be downloaded as a useLarge files (several CDs, single DVD) can be downloaded as a userrslowly walks through an Infostations coverage areaslowly walks through an Infostations coverage area

Future WorkFuture WorkTo analyze the potential of improving the goodput by using rate To analyze the potential of improving the goodput by using rate and and power adaptationpower adaptation

To investigate the effect of synchronization errorsTo investigate the effect of synchronization errors in OFDM and in OFDM and SC systems, in order to make a fairer comparisonSC systems, in order to make a fairer comparison