Performance evaluation of adaptive sub-carrier allocation scheme for OFDMA

Performance evaluation of adaptive sub-carrier allocation scheme for OFDMA

Thesis presentation 16th Jan 2007Author: Li XiaoSupervisor: Professor Riku JänttiInstructor: Lic.Sc Boris MakarevitchPlace: Communications Laboratory

HELSINKI UNIVERSITY OF TECHNOLOGYCommunications Laboratory

Agenda Introduction Overview of OFDM OFDM based multiple access schemes Adaptive sub-carrier allocation algorithm Simulation Conclusions


IntroductionBackground Multi-carrier transmission methods attract much focus to support high speed

and reliable wireless communications A good OFDMA sub-carrier allocation scheme should use spectral as

efficiently as possible and achieve minimum cost of service based upon user’s QoS requirement

Objectives Transmission power minimization as cost of service in Downlink and Uplink Performance evaluation of adaptive sub-carrier allocation for OFDMA

Methodology Adaptive OFDMA sub-carrier allocation algorithm implementation in Matlab Performance comparison among adaptive OFDMA sub-carrier allocation

scheme and other static schemes


OFDM

Dividing the total bandwidth into a number of sub-carriers

OFDM realization

Intersymbol interference

Intercarrier interference

Cyclic prefix


OFDM Based Multiple Access SchemesAdvantage Disadvantage

OFDM-TDMA Easiest implementationSimple resource allocationNo intra-cell MAILow processing requirementPower savingLow signaling overhead

High latencyLowest flexibilityHigh peak to average power ratio

OFDM-CDMA Spectral efficiencyFrequency diversityMAI and inter-cell interference resistanceHighest flexibilitySimple resource allocationLow signaling overhead

Implementation complexityRequirement of power controlOnly coherent modulation possibleIntra-cell interferenceHigh peak to average power ratio

OFDMA Simple implementationResource allocation flexibilityAdaptation to channel characteristics (adaptive scheme)Better BER performance (adaptive scheme)

Inter-cell interferenceLow spectral efficiencyHigh peak to average power ratioSignaling overhead (adaptive scheme)


OFDMA

Each user transmits on a certain number of OFDM sub-carriers during all time slots

Static sub-carriers assignment and dynamic sub-carriers assignment Multirate system Multiuser diversity Adaptive modulation (bit rate, transmission power, channel coding rate or

scheme)


Mobile WiMAX

Extension of WiMAX for fixed access

Scalable OFDMA High data rate Quality of Service Scalability Security Mobility

Parameters Values

System Channel Bandwidth (MHz)

1.25 5 10 20

Sampling Frequency (MHz) 1.4 5.6 11.2 22.4

FFT Size 128 512 1024 2048

Number of Sub-Channels 2 8 16 32

Sub-Carrier Frequency Spacing

10.94kHz

Useful Symbol Time (Tb = 1/f)

91.4 us

Guard Time (Tg = Tb/8) 11.4 us

OFDMA symbol Duration (Ts = Tb+Tg)

102.9 us

Number of OFDMA Symbols in 5ms Frame

48


Adaptive sub-carrier allocation algorithm

Adaptive means Number of sub-carriers each user needs is adaptive Sub-carriers allocation among users is adaptive Bit loading to sub-carriers is adaptive Adaptive modulation scheme for each sub-carrier

Users’ QoS requirement Minimum Reserved Rate Bit Error Rate


Downlink system structure of OFDMA

BS has the perfect knowledge of instantaneous channel information for all users

Bandwidth of each sub-carrier is smaller than channel coherence bandwidth

Each sub-carrier can only be occupied by one user

No free sub-carrier left

Sub-carrier allocation and

bit loadingIFFT P/S

Add cyclic prefix

Remove cyclic prefix

S/PFFTSub-carrier selector and

P/S

Sub-carrier power based allocation

algorithm

Channel state information

User 1

User 2

User K

.

.

.

f1

f2

fN

.

.

.

.

.

.

.

.

.

.

.

.

User k

BS transmitter

Receiver of user k


Adaptive sub-carrier allocation algorithm

Objective function Transmission power minimization Downlink: Minimize the interference from BS in question to the MSs

in other cells Uplink: MS battery saving

Constraints Bit rate (bit/symbol) BER requirement

Three sub-algorithms Number of sub-carriers determination Sub-carriers allocation Bit loading


Number of sub-carriers determination Inputs: Each user’s bit rate constraint and

average channel gain for each user Output: Number of sub-carriers each user gets

assigned

Two types of sub-carriers: Minimum required sub-carrier and Extra sub-carrier

Minimum required sub-carriers are to fulfill the user’s bit rate constraint in the case that maximum amount of bits will be transmitted in each sub-carrier

Extra sub-carriers will share bits with minimum required sub-carriers so that the loaded bits in each sub-carrier can be reduced and with an adaptive modulation scheme transmission power to all user can decrease

No free sub-carrier left

Start

For k = 1,…, K

max

min

R

Rm

k

k

Is

?

K

kk Nm

1

kKk

mk

1

* minarg

0* km

Is

?

K

kk Nm

1

)/())1/(( minmin1

kk

k

kk

k

k

kk mRf

G

mmRf

G

m

kKk

l 1

minarg

1 ll mm

Y

N

Y

Kk ,...,1

exit

N


Sub-carrier allocation Inputs: Channel State Information for

each user and number of sub-carriers each user gets assigned

Output: sub-carriers allocation

Phase 1: Constructive initial allocation

1. List the sub-carriers for each user in descend order according to channel gain

2. Check sub-carriers user by user if the number of sub-carrier each user gets is achieved or the sub-carrier has already been assigned to some users

3. If both are NO, assign the sub-carrier to this user, otherwise skip this user to next user

Start

Is

?

k

N

nnk m

1,

Kk ,...,2,1

Is

k>K

?

k=1

nkSnGn ,

* maxarg

Are

and

?

k

N

nnk m

1,

0

1, *

K

knk

k=k+1

*nSS

*nSS kk 1*,

nk

exit

Y

N

Y

Y

N


Sub-carrier allocation

Phase 1 may achieve only a local minimum but not total minimum transmission power

Phase 2: Iterative improvement For every iteration, swap a pair of sub-

carriers allocated to two users such that the result power can be reduced further

Power reduction factor is the cost function in order to select the pair of users and pair of sub-carriers which can reduce power most

Iteration is over when the maximum possible power reduction is less than zero

Start

ji

jiKjKiPji ,

11maxarg),(

Is

?

0max , jiP

),( jiij nnswap

Update sub-carrier allocation list for each user

ji

jiKjKiPji ,

11maxarg),(

Y

exit


Bit loading Inputs: Sub-carriers allocation, channel gain

and bit rate constraint Output: Bits loaded to achieve each user’s bit

rate constraint

Levin-Campello algorithm

1. Each time selecting the sub-carrier that requires the least additional power to add one more bit

2. Check if the maximum amount of bits loaded in this sub-carrier has already been achieved and if this user’s bit rate constraint has been fulfilled

3. If both are NO, loading one more bit to this sub-carrier, otherwise selecting the sub-carrier which requires second least additional power and repeat 2

Start

Kk

Sn

c

k

nk

1

,0,

Evaluate

),( ,, nknk cP

Kk

Sn k

1

)(minarg ,,

1

*nknk

KkSn

cPnk

Find

Which makes

*k

**

kSn

1**** ,,

nknkcc

If

?

nnkMc ** ,

evaluate

)( **** ,, nknkcP

)( **** ,, nknkcP

1RR

Is

?

TRR

N

N

Y

Exit

Y

0R


Simulation

1 2 3 4 5 6 7 80

20

40

60

Users

Requ

ired

bit p

er s

ymbo

l

1 2 3 4 5 6 7 80

0.002

0.004

0.006

0.008

0.01

Users

requ

ired

BER

Bandwidth (MHz) 5

Sampling Frequency (MHz)

5.6

FFT size (NFFT) 128

Number of users K 2-10

Symbol time (us) 25.81

Channel Sets 200


Simulation results:Number of sub-carriers determination

1 2 3 4 5 6 7 80

5

10

15

20

25

Users

Num

ber o

f Sub

-car

riers

Minimum Required Sub-carriers

Extra Sub-carriersUser Allocated

sub-carriers

Minimum required sub-carriers

Extra sub-carriers

1 19 7 12

2 13 7 6

3 17 8 9

4 18 8 10

5 9 4 5

6 9 5 4

7 24 7 17

8 19 8 11


Simulation results:Sub-carriers allocation

20 40 60 80 100 12010

-3

10-2

10-1

100

101

Subcarrier index

Chan

nel G

ain

Channel Set = 50

User1 ChannelUser2 ChannelUser3 ChannelUser4 ChannelUser5 ChannelUser6 ChannelUser7 ChannelUser8 ChannelUser1 Sub-carriersUser2 Sub-carriersUser3 Sub-carriersUser4 Sub-carriersUser5 Sub-carriersUser6 Sub-carriersUser7 Sub-carriersUser8 Sub-carriers

20 40 60 80 100 12010

-3

10-2

10-1

100

101

Subcarrier index

Chan

nel G

ain

Channel Set = 100


Simulation results:Bit loading

0 50 100

100

Sub-carrier allocation for user 1

Subcarrier index

Channel G

ain

20 40 60 80 100 1200

2

4

6

Bit Loading for user 1

Subcarrier index

Num

ber

of

bits

0 50 100

100


20 40 60 80 100 1200

2

4

6


0 50 100

100


20 40 60 80 100 1200

2

4

6


0 50 100

100


20 40 60 80 100 1200

2

4

6


0 50 100

100


Subcarrier index

Channel G

ain

20 40 60 80 100 1200

2

4

6


Subcarrier index

Num

ber

of

bits

0 50 100

100


20 40 60 80 100 1200

2

4

6


0 50 100

100


20 40 60 80 100 1200

2

4

6


0 50 100

100


20 40 60 80 100 1200

2

4

6



Simulation results:BER performance

-10 -5 0 5 10 15 20 25 30 3510

-5

10-4

10-3

10-2

10-1

100

Average Bit SNR (dB)

Bit E

rror R

ate

Adaptiveallocationwith ExtraSub-carriers

Adaptiveallocationwithout ExtraSub-carriers

OFDM-TDMAOFDM-FDMA

OFDMInterleave-FDMA

Minimum 11.33dB gain in SNR using Adaptive allocation OFDM without extra sub-carriers over OFDM Interleave-FDMA

11.84dB gain over OFDM-TDMA 14.35dB gain over OFDM-FDMA 6.91dB gain from extra sub-carrier

presence compared with no extra sub-carrier case


Simulation results:Convergence of algorithm

5 10 15 20 25 30 3527

28

29

30

31

32

33

Iteration

bit S

NR

1 2 3 4 5 6 7 8 9 10 115

10

15

20

25

30

Number of users

Num

ber o

f ite

ratio

ns


Conclusions

Adaptive sub-carriers allocation algorithm can enhance the BER performance compared with static schemes

The use of extra sub-carriers can improve the BER performance and decrease the total transmission power further

Speed of the algorithm (convergence speed) is fast to meet the real time application requirements

The speed of algorithm is not affected by the number of users much which guaranttes it perform well in high load system

BS could use algorithm to increase the total number of users that can be accommodated for a given power budget


Future study

Minimization of transmission power in Uplink Scalable OFDMA


Thank you!Thank you!

Documents

Performance evaluation of adaptive sub-carrier allocation scheme for OFDMA