Event: CUBAN/WIP Workshop - Tunisia Date: 23-25 May 2005 Slide 1 Adaptive modulation and multiuser scheduling gains in adaptive TDMA/OFDMA systems in the

Event: CUBAN/WIP Workshop - Tunisia

Date: 23-25 May 2005

Slide 1

Adaptive modulation and multiuser scheduling gains in

adaptive TDMA/OFDMA systems in the WINNER framework

Adaptive modulation and multiuser scheduling gains in

adaptive TDMA/OFDMA systems in the WINNER framework

Sorour Falahati , Mikael Sternad, Tommy Svensson, Daniel Aronsson

Uppsala UniversityChalmers University of Technology


Date: 23-25 May 2005

Slide 2

OutlineOutline

• Introduction• FDD downlink and uplink structure• Timing events in DL/UL transmission• Key techniques

– Channel prediction– Scheduling– Link adaptation– Compression of feedback information

• Simulation results• Summary


Date: 23-25 May 2005

Slide 3

IntroductionIntroduction

• Predictive adaptive resource scheduling using TDMA/OFDMA:– Providing fast link adaptation in an OFDM

system based on the predicted channel state information of time-frequency chunks

– Providing multi-user scheduling gain by allocating the resources to the flows with the potential of improving the throughput based on their channel status.


Date: 23-25 May 2005

Slide 4

FDD downlink and uplink structureFDD downlink and uplink structure

Chunk

Chunk

BW

T chunk

8 s

ub-c

arr

iers

6 TOFDM

time

time

freq

freq

FDD downlink

FDD uplinkO

O

O

O

O

O

O

O

D D P U U P

D D P U U P

C C

C C

C C

C C

O: overlapping pilots

C: DL control feedback

P: pilots symbols

D: DL control symbols

U: UL control symbols


Date: 23-25 May 2005

Slide 5

Timing events in DL/UL transmissionTiming events in DL/UL transmission

DL UL DL

O

O

O

O

O

O

O

O

C C

C C

C C

C C

D D P U U P

D D P U U P

D D P U U P

D D P U U P

1. DL control symbols:

Report which present chunks

belong to which flows

2. DL pilot symbols:

Used for channel prediction

Used for channel estimation

3. UL control symbols:

Report which next UL chunks

appointed to which uplink flows

4. Pilot symbols:

Used for coherent detection

And updating predictor states

UL

O

O

O

O

O

O

O

O

C C

C C

C C

C C

6. UL overlapping pilot symbols:

Used for prediction

5. DL control feedback symbols:

Carry DL channel prediction report

Dl prediction horizon: 2.5X0.3372ms=0.843ms

UL prediction horizon: 2.5X0.3372ms=0.843ms


Date: 23-25 May 2005

Slide 6

Channel predictionChannel prediction

• Prediction in frequency domain• A set of linear predictors, one for each sub-band• Kalman predictor:

– Predict the complex channel and its power– Using pilots in parallel sub-carriers:

• Utilizing correlation in frequency and time domain

• Generalized Constant Gain (GCG) algorithm:• No need to update a sate-space Riccati difference eq.• Moderate complexity and negligible performance loss as

compared to Kalman algorithm


Date: 23-25 May 2005

Slide 7

Channel prediction …Channel prediction …

• SINR and prediction horizon limit at 5 GHz downlink:30 km/h 50 km/h 70 km/h

<0 dB, 0.117 6 dB, 0.195 12.5 dB, 0.273


Date: 23-25 May 2005

Slide 8

Channel prediction …Channel prediction …

• SINR and prediction horizon limits at 5 GHz uplink:

30 km/h 50 km/h 70 km/h

<0 dB, 0.117 7 dB, 0.195 15 dB, 0.273 30 km/h 50 km/h 70 km/h

3.5 dB, 0.117 11dB, 0.195 20 dB, 0.273

2 users 8 users


Date: 23-25 May 2005

Slide 9

SchedulingScheduling

• Resource scheduling– Proportional fair strategy:

• Allocating resources (chunks) to the user with the highest SINR relative to its average

• For users with the same average SINR, this strategy reduces to Max. Throughput strategy.

– Allocating chunks to users with the highest MC rate.

• Due to curvature within the chunk, MC scheme is determined based on:

[dB])1([dB][dB] wav SINRbSINRbSINR

Chunk Average SINR Chunk minimum SINR


Date: 23-25 May 2005

Slide 10

Link adaptationLink adaptation

• Each user selects a modulation and coding (MC) scheme for each chunk in competition based on the prediction SINR:

– The rate limit for a set of MC schemes are adjusted based on the TBER, average SNR and prediction error variance

– Based on the predicted chunk SINR, a MC scheme which fulfills the BER requirement and maximized the throughput is selected.


Date: 23-25 May 2005

Slide 11

Link adaptation … Link adaptation …

• BER performance of MC schemes for perfect and imperfect prediction (NMSE=0.1)


Date: 23-25 May 2005

Slide 12

Link adaptation … Link adaptation …

• Variation of rate limits of MC schemes with prediction quality:

• SNR=10 dB and TBER=0.001


Date: 23-25 May 2005

Slide 13

Compression of feedback informationCompression of feedback information

• Tricks or tools to reduce downlink overhead:– Use implicit signaling of utilized

modulation rate whenever possible

– Contention-band: The active terminals are in competition for only a part of the total BW

– Use short-hand addresses to indicate identities of active users whenever possible.


Date: 23-25 May 2005

Slide 14

Compression of feedback information …Compression of feedback information …

• Tricks or tools to reduce uplink overhead:– Contention-band– Compression of feedback information

• Discrete cosine transform: utilizing correlation in frequency

• Sub-sampling of transform coefficients in the time domain

FF

TRx

Cal

cula

tech

unk

SN

R

DC

T

Sca

lar

Qua

ntiz

ers

104 104 104

Sub

-sa

mpl

ing

Tx feedback

FF

TRx

Cal

cula

tech

unk

SN

R

DC

T

Sca

lar

Qua

ntiz

ers

104 104 104

Sub

-sa

mpl

ing

Tx feedback

Q-1

Receivedfeedback

Ups

ampl

ing

IDC

T

104 104104

ChunkSNR

Sch

edul

ing

Q-1

Receivedfeedback

Ups

ampl

ing

IDC

T

104 104104

ChunkSNR

Sch

edul

ing


Date: 23-25 May 2005

Slide 15

Compression of feedback information …Compression of feedback information …

• THP as a function of feedback rate:• ITU VA channels, v=50 km/h, sub-sampling factor of 2

1 user

5 users

10 users


Date: 23-25 May 2005

Slide 16

Simulation resultsSimulation results

• Simulation set-up– Wide-area full-duplex FDD downlink– WINNER Urban Macro channel model– Single cell (sector) and SISO– Users with equal velocities and average SINRs

Center frequency 5.0+/-0.384 GHz

Number of OFDM sub-carriers 1024

FFT BW 20 MHz

Signal BW 16.25 MHz paired

Number of used sub-carriers 832

Sub-carrier spacing 19531 Hz

OFDM symbol length (exc. CP) 51.20 microseconds

Cyclic prefix (CP) length 5.00 microseconds

Physical chunk size 156.24kHz x 337.2 microseconds

Chunk size in symbols 8 x 6=48


Date: 23-25 May 2005

Slide 17

Simulation results …Simulation results …

• Multi-user diversity, channel variations: • THP versus SNR for 2 and 8 users


Date: 23-25 May 2005

Slide 18


• Multi-user diversity, channel variations: • BER versus SNR for 2 and 8 users


Date: 23-25 May 2005

Slide 19


• Prediction quality, multi-user diversity, channel variation: • THP versus number of users (19 dB)


Date: 23-25 May 2005

Slide 20


• Prediction quality, multi-user diversity, channel variation: • BER versus number of users (19 dB)


Date: 23-25 May 2005

Slide 21


• Prediction quality, multi-user diversity, channel variation: • THP versus number of users (10 dB)


Date: 23-25 May 2005

Slide 22


• Prediction quality, multi-user diversity, channel variation: • BER versus number of users (10 dB)


Date: 23-25 May 2005

Slide 23


• TDMA/OFDMA versus use of TDMA: • THP versus number of users (19dB)


Date: 23-25 May 2005

Slide 24

SummarySummary

• An adaptive transmission based on TDMA/OFDMA using multiuser scheduling is investigated.

– Predictive adaptation to the short-term fading and frequency-domain channel variability leads to significant multi-user diversity gain.

– With TDMA instead of TDMA/OFDMA, only half of these gains are realized for channels with Urban Macro scenarios.

– Predictive adaptation can use MC rate boundaries adjusted so that BER constraints are fulfilled in the presence of SINR prediction uncertainty.


Date: 23-25 May 2005

Slide 25

Summary…Summary…

– Feasibility of adaptive transmission is limited by prediction accuracy.• Prediction accuracy is determined by SINR and

terminal velocity.• For realistic SINR values, transmission at 50 km/h is

feasible at 5 GHZ in FDD DL.

– A solution to reduce the required feedback rate:• To feed back the required SINR and source code it by

a combination of transform coding in the frequency direction and sub-sampling in the time direction.

Documents

Event: CUBAN/WIP Workshop - Tunisia Date: 23-25 May 2005 Slide 1 Adaptive modulation and multiuser scheduling gains in adaptive TDMA/OFDMA systems in the