YYYY-MM-DD IEEE C802.20-03/19grouper.ieee.org/groups/802/20/Contribs/C802.20-03-19.pdf ·...

YYYY-MM-DD IEEE C802.20-03/19

1

Project IEEE 802.20 Working Group on Mobile Broadband Wireless Access

<http://grouper.ieee.org/groups/802/20 >

Title Frequency-Domain-Oriented Approaches for MBWA: Overview and Field Experiments

Date Submitted

2003-03-06

Source(s) Kevin Baum Motorola Labs 1301 E. Algonquin Rd., Rm 2928 Schaumburg, IL 60196

Email: [email protected]

Re: MBWA ECSG Call for Contributions

Abstract This presentation provides an overview of frequency-domain-oriented approaches for mobile broadband air interfaces, and presents some related results from recent field experiments.

Purpose For informational use only.

Notice This document has been prepared to assist the IEEE 802.20 Working Group. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

Release The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.20.

Patent Policy

The contributor is familiar with IEEE patent policy, as outlined in Section 6.3 of the IEEE-SA Standards Board Operations Manual <http://standards.ieee.org/guides/opman/sect6.html#6.3> and in Understanding Patent Issues During IEEE Standards Development <http://standards.ieee.org/board/pat/guide.html >.

Frequency-Domain-Oriented Approaches for MBWA: Overview and Field Experiments

IEEE 802.20March 10-14, 2003

March 2003 2 of 16 IEEE 802.20 MBWA

MotivationMotivationMotivation

• The bandwidth of wireless systems continues to increase over time• The impact of a constant delay-spread increases with bandwidth

• Frequency domain approaches, based on efficient FFT processing, can be investigated to reduce the implementation complexity of broadband systems (Nlog2N complexity vs. N2 or N3)

Millions of Complex Multiplies per second for Computing the Output of a 20 µµµµs Time Domain Equalizer

0.01

0.1

1

10

100

1000

10000

IS-54 GSM 1.25 MHz 5 MHz 20 MHz

0.1

1

10

100

1000

10 100 1000 10000 100000Sys te m Symbol or Chip Rate (kHz)

Spa

n of

a 1

0 us

Del

ay S

prea

d P

rofi

le in

Sy

mbo

ls o

r C

hips

IS -136

GS M

1.25 MHz MBWA

5 MHz MBWA

20 MHz


Complexity of single carrier time domain equalization and OFDM for 10 us delay spread

0.1

1

10

100

1000

10000

0.1 1 10

Channel Bandwidth (MHz)

Co

mp

lex

Mu

ltip

lies

(MO

PS

)

Single Carrier with TimeDomain Processing

OFDM

Ratio of Single Carrier toOFDM

Complexity of Time Domain Equalization and OFDM “Equalization”Complexity of Time Domain Equalization Complexity of Time Domain Equalization and OFDM “Equalization”and OFDM “Equalization”

• The complexity advantage of frequency-domain approaches becomes compelling as the bandwidth increases

Assumptions• Includes only equalizer

output computation (at the symbol rate) and equalizer tap computation

• Equalizer taps computed from known channel impulse response every 1/(10Fd) sec

• Fd =200 Hz Doppler• Time domain equalizer length

= 2x channel length• Complexity model of matrix

inverse: ( L3)/6• Complexity model of FFT:

(N/2)log2(N)


Frequency-Domain-Oriented ApproachesFrequencyFrequency--DomainDomain--Oriented ApproachesOriented Approaches

– High performance with low complexity for broadband channels – Well suited for advanced multiple antenna methods (MIMO,

space-time coding, SDMA, adaptive antennas)

Frequency domain implementation of conventional linear filtering (receive-only)

– Overlap-add, overlap-save filtering techniques

– Useful for “retrofit” applications– Does not change the transmit signal

format

– Still has a high computational load for determining tap values

– Not discussed in this presentation

Frequency domain oriented transmission and reception

– Transmission format specifically designed to support low complexity frequency domain processing

– Focus of this presentation


Transmission Transmission Transmission

• The main frequency domain oriented transmission methods:– Multicarrier (regular OFDM and spread OFDM/MC-CDMA)– Cyclic-prefix (CP) single carrier with frequency domain

equalization– Others also exist– For brevity, this presentation will focus on OFDM and CP single

carrier

Pul

sesh

ape

filte

r

Dat

asy

mbo

ls o

rch

ips

add

cycl

ic p

refix

para

llel t

o se

rial

seria

l to

para

llel

add

cycl

ic p

refix

para

llel t

o se

rial

IFF

T

data

sym

bols

or

chip

s

Data (and spreading code, if used) in time domain

Data (and spreading code, if used) in frequency domain

Basic Tx structure for OFDM Basic Tx structure for CP-single carrier


Tx Time Format and ReceiverTxTx Time Format and ReceiverTime Format and Receiver

• Design Guidelines– Make CP longer than channel delay spread– Make data portion large enough that CP overhead is small– Make data portion short enough that channel does not change over the block

Cyclic Prefix

Time

Data Portion(Length N)

. . .. . .

FFT Interval of Receiver(Length N)

Output of IFFT (OFDM)Or block of symbols (CP-single carrier)

Copy of the last Npsamples of the data portion

Cyclic prefix makes the linear convolution with the channel equivalent to a circular convolution (within the data portion)

FFT’s are very efficient for processing circular signals! Frequency domain implementation of channel estimation, equalization, combining, …

N-point FFT

N-point IFFT (for CP-single

carrier)

Equalization W

eightingRemove

prefix

Basic Receiver Structure

To channel decoder

To channel decoder (for OFDM)


Simulation Example 1: Frequency Domain vs. Conventional Time DomainSimulation Example 1: Frequency Domain Simulation Example 1: Frequency Domain vs. Conventional Time Domainvs. Conventional Time Domain•• 5 MHz channel bandwidth, Vehicular A and GSM TU channels5 MHz channel bandwidth, Vehicular A and GSM TU channels

–– Ideal channel knowledge, block fading Ideal channel knowledge, block fading

•• BlueBlue – Conventional single-carrier (without cyclic prefix) with time domain MMSE linear transversal equalizer (2x the channel length)

• Black – Cyclic-Prefix single-carrier with block size N = 384, frequency domain MMSE equalization

GSM TU: 5 µµµµs span, 1 µµµµs RMSVehicular A: 2.5 µµµµs span, 370 ns RMS

Larger Delay Spread, Higher-order ModulationLow Delay Spread, Low-order Modulation

9 10 11 12 13 14 15

10−3

10−2

C/I (dB)

Dec

oded

BE

R

QPSK, r=1/2 Turbo code, VehA

CP−SCMMSE−TDE

12 14 16 18 20 22 2410

−4

10−3

10−2

10−1

C/I (dB)

Dec

oded

BE

R

16−QAM, r=1/2 Turbo code, GSMTU

CP−SCMMSE−TDE


Example 2: Link Simulation of Different Frequency-Domain ApproachesExample 2: Link Simulation of Different Example 2: Link Simulation of Different FrequencyFrequency--Domain ApproachesDomain Approaches

• Cyclic-prefix single carrier (CP-SC) and OFDM performance for R= ½ turbo coded QPSK, 16-QAM, 64-QAM modulation/coding schemes (MCS)

– Assumptions:• 5 MHz channel bandwidth, block-faded GSM TU channel (5 µµµµs span, 1 µµµµs RMS delay spread)• Frequency Domain MMSE equalizer, ideal channel knowledge• In practice, the MCS would be adaptively selected based on link quality (and additional MCS levels

may be included)

Red – OFDM

Blue – CP-single carrier with frequency domain equalization

4 6 8 10 12 14 16 18 20 22 24

10−3

10−2

C/I (dB)

Dec

oded

BE

R

OFDM v/s CP−SC, r=1/2 Turbo code, GSMTU

OFDMCP−SC

QPSK

16−QAM

64−QAM


Tradeoffs between OFDM and CP-SCTradeoffs between OFDM and CPTradeoffs between OFDM and CP--SCSC

• CP single carrier benefits– Low peak-to-average power ratio

• A significant benefit for the uplink

– Obtains frequency diversity regardless of coding rate

• Leads to a performance benefit for QPSK with R > 2/3 coding

• OFDM benefits– Orthogonality between symbols in

delay-spread channels• No noise enhancement • Better performance when MCS set

is carefully chosen (e.g., use R = 3/8 16-QAM for 1.5 b/symbol rather than R = ¾ QPSK)

– Full access to the “time-frequency grid”

• Frequency selective transmission techniques can be considered

– See analysis of frequency selective AMC and scheduling in Classon et al., ICC’03

SINR

Frequency

Inst. SINR

Avg. SINR

MCS 0

MCS 2

MCS 1

Frequency-selective AMC

1 2 3 4 5 6 7 8 9

10−3

10−2

10−1

PAPo (dB)

Pr

(PA

P >

PA

Po)

Complementary CDF of Peak−to−Avg Power (dB)

QPSK

16−QAM

64−QAM

Single Carrier

OFDM, 750 carriers

Multicode CDMASF=16, 16 codes

Field Experiments


Mobile Broadband Field Data CollectionMobile Broadband Field Data CollectionMobile Broadband Field Data Collection

Test TruckBase Site Antennas

3.675 GHz carrier20 MHz channel BW

Two identical & independent Rx5 dBi omni antennas, spaced ~9.3 λλλλSynchronized to GPS and received signalTime & Frequency domain data720 snapshots of 9 MBytes per hour, 6.4GB/h

6 sectors, 2 antennas/sectorLocated on top of 6-story building


Field Data Collection Drive RoutesField Data Collection Drive RoutesField Data Collection Drive Routes

• Test area contains a mixture of single and multistory residential and commercial buildings with some undeveloped areas

• Several different modulation formats and MCS levels are transmitted and captured– OFDM, SOFDM– CP single-carrier– CDMA– Plus various forms

of Tx/Rx diversity and MIMO

• Ten drive routes• Vehicle speed varies

from 0 to 60 mph• Most of the data

captured within 2 miles from the base


Understanding the Mobile Broadband ChannelUnderstanding the Mobile Broadband ChannelUnderstanding the Mobile Broadband Channel

RMS delay spread0 5 10

-50

-40

-30

-20

-10

0

ma

gn

itud

e (

dB

)

Example 1

delay (µµµµs)

RMS delay spread = 0.81 µµµµs

• Variation across Time, Frequency & Space– Delay spread

• Low delay spread still causes significant frequency selectivity on the broadband channel

• Larger observed delay spreads occurred when a strong line-of-sight ray was absent

– Path Loss– Spatial conditioning


Example of Identified ScatterersExample of Identified Example of Identified ScatterersScatterers

Identified sources of specific delayed rays in the power delay profile


Experimental System Modulation StudyExperimental System Modulation StudyExperimental System Modulation Study

• OFDM and CP single-carrier with MMSE equalization– 1 Tx and 1 Rx antenna– 20 MHz bandwidth, various drive routes at various speeds– Comparison of different constellation sizes (QPSK, 16-QAM, 64-QAM)

Decoded BER with Rate=½ convolutional coding

Red – OFDM

Blue – CP single-carrier

Trends appear consistent with earlier simulation results 2 4 6 8 10 12 14 16

10−4

10−3

10−2

Eb/No (dB)

De

cod

ed

BE

R

OFDM v/s CP−SC, r=1/2 Conv Code, K=7

OFDMCP−SC

QPSK 16−QAM

64−QAM


SummarySummarySummary

• Frequency-domain-oriented approaches appear promising for future mobile broadband wireless systems– As the channel bandwidth increases, their benefits become

more compelling• This presentation focused mainly on the larger bandwidths (i.e.,

5 to 20 MHz)• Further investigation for the “narrow” channel case (1.25 MHz)

would be useful

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