M. Stemick, S. Olonbayar, H. Rohling

Hamburg University of Technology

Institute of Telecommunications

PHY-Mode Selection and PHY-Mode Selection and

Multi User Diversity in Multi User Diversity in

OFDM based Transmission SystemsOFDM based Transmission Systems

Institute of Telecommunications 2

OFDM-FDMA System (Single Cell)

TimeF

requ

ency

0 4 8 12 16 20-40

-30

-20

-10

0

10

Bandwidth [MHz]

|H|[d

B]

user1user2user3

|H|2

[dB

]

Bandwidth [MHz]

|H|2

[dB

]

1) Frequency Selectivity 2) Additional Path Loss + Shadowing

Bandwidth [MHz]

Institute of Telecommunications 3

Define User Capacity

User Capacity: Number of users per cell @ fixed data rate

Goal: maximize user capacity

maximize number of users at a fixed data rate

Institute of Telecommunications 4

Cell Models

• Same data rate for all users

• Perfect channel knowledge

and synchronisation

• Downlink situation

• Time-invariant channel

Single cell with N users at the

same distance from base station

This scenario is based on a frequency selective channel model

Institute of Telecommunications 5

Channel Model

Maximum delay 3,2 µs

Power delay profile Exp.

Number of multipathes 30

OFDM symbol duration 16 µs

Bandwidth 20 MHz

Subcarriers 2560 4 8 12 16 20

-40

-30

-20

-10

0

10

Bandwidth [MHz]|H

|[dB

]

user1user2user3

Fading of subcarriers varies strongly between users

Channel variation over bandwidth:WSSUS channel parameters:

Frequency [MHz]|H

|2 [d

B]

Institute of Telecommunications 6

Subcarrier Selection Algorithm

,1

1 for all uN

i ji

x j

Selection criteria: maximize Z

2

, ,1 1

uN K

i j i ji j

Z H x

user subc.

Selection parameter

,

1

0i jx

user

subc

arrie

r

allocation

no allocation

under the following constraints:

Solution of optimization problem by Hungarian Algorithm

Institute of Telecommunications 7

Simulation Results

Subcarrierwise selection, QPSK, R=1/2

→ Adaptive subcarrier allocation yields a high diversity gain

4 users 8 users 16 users

random select.

best subc. select.

adapt. select.

Increasing number of users

Institute of Telecommunications 8

Path loss and shadowing cause high variation in average

receive power between users

• Users are uniformly distributed

• Path loss:

• Shadowing: Log normal distribution ( )

• Cell radius: R = 100m

Additional Path Loss and Shadowing

2.6PLG d

New situation in the cell:

Consequence:

4SH dB

Institute of Telecommunications 9

Frequency [MHz]

Received SNR For Different Users

High variance of receive power:

• Different number of subcarriers

per user to fulfil QoS

• Additional PHY-Mode selection

Higher complexity of subcarrier allocation

SN

R [

dB

]

Received SNR for different users

Institute of Telecommunications 10

Modified Allocation Algorithm

1) Select one subcarrier for each user by

Hungarian Algorithm

2) Determine SNR for each selected subcarrier

and choose a suitable PHY-Mode

3) Repeat steps 1) and 2) for every user,

until demands for data rate are satisfied

Choose PHYfor selectedsubcarriers

start

end

Rate achieved?

Select onesubcarrier for

every user

yes

no

Institute of Telecommunications 11

Allocation Example

1 2 3

1 11 7 5

2 8 12 7

3 4 6 9

4 9 6 7

5 5 7 11

6 12 8 11

7 5 11 13

8 6 9 10

9 9 7 10

10 12 6 8

1 2 3

1 11 7 5

3 4 6 9

4 9 6 7

5 5 7 11

6 12 8 11

8 6 9 10

9 9 7 10

1 2 3

1 11 7 5

3 4 6 9

4 9 6 7

9 9 7 10

1st iteration

2nd iteration

3rd iteration

Selecting subcarriers based on

subcarriers

users

2

,i j

i

H

H

Institute of Telecommunications 12

Subcarrier Allocation & Data Rate Development

256 QAM

128 QAM

64 QAM

32 QAM

16 QAM

QPSK

Target Data Rate: 3MbpsU

ser

Inde

x

1

2

3

4

5

6

7

8

9

10

Number of Subcarriers

1 2 3 4 5 6 7 8 9 10

Institute of Telecommunications 13

Simulation Results

Percentage of satisfied users for different data rates

• Subcarriers allocated by Hungarian Algorithm

• Subcarrierwise PHY-Mode selection

10 20 users 38 users

Institute of Telecommunications 14

Comparison Between Considered Models

Comparison between uniform und circular user distribution

data rate: 3.0 Mbps cell radius: 100m

Institute of Telecommunications 15

Conclusions

• Adaption to frequency selectivity achieves a high diversity

gain of maximum 7 dB

• Simultaneous adaption to path loss and frequency

selectivity achieves high user capacity

• OFDM-FDMA + PHY-Mode selection allows flexible

adaption to various QoS demands and system loads

Institute of Telecommunications 16

Thank you for your attentionThank you for your attention