Diversity Combining Technique for Soft Handoff in

OFDMA Cellular Systems

Xiu-Sheng Li and Yuh-Ren Tsai

Presented by Xiu-Sheng Li ( 李修聖 ) 2007 05 18

Wireless communication system LAB. NTHU

Outline

Introduction System Model Diversity Combining Techniques System Performance and Simulation Results Conclusion

Introduction

OFDMA cellular system – Ex : WiMAX Mobility Handoff problem

Soft handoff against hard handoff Reduces the “ping-pong” effect Communication link is always active Enhanced performance by diversity combining

Goal : to propose a novel diversity combining technique so that soft handoff can be implemented in OFDMA cellular systems

Introduction

Reuse factor : To reduce co-channel interference

D

A

B

C

D

B B

C

A

B

A B C D

Cellular systemwith 4

Each with bandwidth subcarrier number

N

Total bandwidth subcarrier number

N

Conventional Strategies

Strategy I conventional

Strategy II conventional

Strategy III to be proposed in this work

When soft handoff is activated, all bandwidth is collected, DFT with size is operated. Diversity combining is performed.

Sampling rate : DFT size :

Strategy I

N

N

D

C

A

BMS

A B C D

Diversity combining (ex:MRC)

Strategy II

When soft handoff is activated, two receiving front ends are necessary. The signal from each BS is processed independently, and finally diversity combining is performed.

Two receivers Sampling rate : DFT size :

D

C

A

BMS

N

A C

Diversity combining (ex:MRC)

Strategy III

Strategy III key points changing sampling rate and DFT size Alamouti space time code (STC) is applied

Compared to Strategy I smaller sampling rate and smaller DFT size

Compared to Strategy II one DFT block despite of a little higher sampling

rate and DFT size

One subchannel occupies subcarriers Total subchannels

Subcarrier assignment schemes Subband based schemes

Interleaved schemes

System Models

( / )K N MM

8, 2, 4N M K

1

122 ( / 2) /

0

1( ) , 0

( )

( ) ( ) , 0

Nj f tj n N t T

d nn

cp d cp

x t X e e t TNx t

x t x t T T t

2

122 ( / 2) /

0

1( ) , 0

( )

( ) ( ) , 0

Nj f tj n N t T

d nn

cp d cp

y t Y e e t TNy t

y t y t T T t

2 1

1 12 ( )2 ( / 2) / 2 ( / 2) /

, ,0 0

( ) ( ).N N

j f f tj n N t T j n N t Tx n X n y n Y nn n

r t E X H e E Y H e e w t

Down-converted by

1f

1fcarrier 2fcarrier

System Models

bandwidth bandwidth

BS1

BS2

BackboneNetwork

MS( )x t ( )y t

( )r t

Synchronouscellular system

A B C D

1f 2f

Diversity Combining Technique

1 12 ( / 2) 2 ( / 2) 2

, ,0 0

[ ]

[ ], 0 1

k k CNkN Nj n N j n N jN N N N N N

s x n X n y n Y nn n

s

r kT E X H e E Y H e e

w kT k N N

By sampling rate fNNf s )(

)( NN - point DFT is applied

, 0 ~ 1m m m mR A B W m N N

2 1f f C N f C With the assumption

, for 0 ~ 1

0 for ~ 1x m X m

mE X H m NA

m N N N

( )mod( ) ,( )mod( ) for ( ) mod( ) 1

0 for ( ) mod( ) 1

y m CN N N Y m CN N Nm

E Y H m CN N N NB

m CN N N N

Filtering

Diversity Combining Technique

By sampling rate fNNf s )(

2 1 8f f f DCf f

10 f

10 f

8N 2N Repeat in every( )N N f

)( NN - point DFT to extract valuesBy

DCf

Diversity Combining Technique

By changing appropriate sampling rate, desired signals can be gathered

and interference can be avoided

Alamouti space time code (STC) is required for desired signals

After sampling by ff s 10

2 1 8f f f DCf

, ,0 7X n nH X n , ,0 7Y n nH Y n

f

0 1 2 3

:desired signals

( , , , )X X Y Y

DCf f

1 20, 1k k

: subchannel index for BS1

1k 2k: subchannel index for BS2

After sampling by 12sf f

6 7 2 3

:desired signals

( , , , )X X Y Y

Diversity Combining Technique

2 1 8f f f DCf

, ,0 7X n nH X n , ,0 7Y n nH Y n

f

DC f

Another example for subband based schemes

1 23, 1k k

Appropriate ΔN

Appropriate is determined by subcarrier assignment schemes : subchannel index for BS1 : subchannel index for BS2 : subchannel number : subcarrier number of a subchannel : carrier spacing parameter

N

1k

2kKMC

For subband based schemes with

Appropriate ΔN

1 2 ( 1)k k C K

0 1 2 3

:desired signals

( , , , )X X Y Y

1 20, 1k k 2 1 8f f f DCf

, ,0 7X n nH X n , ,0 7Y n nH Y n

f

DCf

10sf f

2N

2 1 16f f f DC f

DCf

18sf f 10N

2 1 16f f f DC f

DCf

9sf f 1N

C=1

C=2 (a) C=2 (b)

For subband based schemes with Criterion

Overlapped desired signals

Interference avoidance

DFT size restriction

Final result

Appropriate ΔN

1 2 ( 1)k k C K

2 1( ) ( ): , is any positive integer

minsuch that is a positive integer.

C n N k k Mx x n

N nx

2 1( ( 1) ) / , positive integersf k C K M k M f n n

positive integerN

sf N f

For subband based schemes with Criterion

To guarantee that desired signals from BS1 do not interfered by signals from BS2

To guarantee that desired signals from BS2 do not interfered by signals from BS1

Final result

Appropriate ΔN

1 2 ( 1)k k C K

1 2max ( ) , ( 1)N K k M k M DCf f

DC f

4N

1 2sk M f f N f

2( 1)sf N k M f

Similar results can be obtained for interleaved schemes

Appropriate ΔN

1 2 ( 1)k k C K

1 2 ( 1)k k C K

2 1N K k k

2 1( ): , is any positive integer

minsuch that is positive integer.

C n N k kx x n

N nx

Alamouti STC Alamouti STC with 2TX&1RX

Information source 1 2x x

*1 2x x

*2 1x x

1h

2h

1 2r r

1 1 1 2 2 1r h x h x n * *

2 1 2 2 1 2r h x h x n

BS1

BS2

InformationSource

Modulator

MS

Backbone Network

At l-th symbol time

At (l+1)-th symbol time

Frequency Domain

1kon -th subchannel

At l-th symbol time

At (l+1)-th symbol time

Frequency Domain

2kon -th subchannel

Alamouti STC

(1) (1) (1) (1),0 ,1 , 1[ ]l l l l MX X X X

(2) (2) (2) (2),0 ,1 , 1[ ]l l l l MX X X X

(1)lX

(1)*lX

(2)lX

(2)*l X

Received Signals For subband based schemes with 1 2 ( 1)k k C K

1 1 2 1

(1) (2), 1 , 1, 2 , 2, ,l k M m l m k M m l m k M m l k M mR E X H E X H W

1 1 2 1

(2)* (1) *1, 1 , 1, 2 , 2, 1,l k M m l m k M m l m k M m l k M mR E X H E X H W

1~0 Mm

1~0 Mm

f

DCf

Standard Alamouti decoding is applied & diversity order is 2

l -th symbol

(l+1) -th symbol

Received Signals For subband based schemes with

f

f

1 2 ( 1)k k C K

1 1 1

(1), 1 , 1, ,l k M m l m k M m l k M mR E X H W

2 2 2

(2), 2 , 2, ,l N k M m l m k M m l N k M mR E X H W

1 1 1

(2)*1, 1 , 1, 1,l k M m l m k M m l k M mR E X H W

2 2 2

(1) *1, 2 , 2, 1,l N k M m l m k M m l N k M mR E X H W

1~0 Mm

1~0 Mm

l -th symbol

(l+1) -th symbol

Maximal ratio combining (MRC) is applied & diversity order is 2

Received Signals For interleaved schemes with1 2 ( 1)k k C K

1 1 2 1

(1) (2), 1 , 1, 2 , 2, ,l k mK l m k mK l m k mK l k mKR E X H E X H W

1~0 Mml -th symbol

(l+1) -th symbol 1 1 2 1

(2)* (1) *1, 1 , 1, 2 , 2, 1,l k mK l m k mK l m k mK l k mKR E X H E X H W

1~0 Mm

DC f

DC f

2N

Standard Alamouti decoding is applied & diversity order is 2

For interleaved schemes with

Received Signals

1 2 ( 1)k k C K

1 1 2 1

(1) (2), 1 , 1, 2 , 1 2, ( 1) ,l k mK l m k mK l m k m K l k mKR E X H E X H W

2~0 Mm

1 1 1

(1), ( 1) 1 , 1 1, ( 1) , ( 1)l k M K l M k M K l k M KR E X H W

2 2 2

(2), 2 ,0 2, ,l N k l k l N kR E X H W

l -th symbol

(l+1) -th symbol 1 1 2 1

(2)* (1) *1, 1 , 1, 2 , 1 2, ( 1) 1,l k mK l m k mK l m k m K l k mKR E X H E X H W

2~0 Mm

1 1 1

(2) *1, ( 1) 1 , 1 1, ( 1) 1, ( 1)l k M K l M k M K l k M KR E X H W

2 2 2

(1)*1, 2 ,0 2, 1,l N k l k l N kR E X H W

DC f

DC f

2N Standard Alamouti decoding and MRCare applied & diversity order is 2

System Performance

Uncoded BPSK BER over Rayleigh fading channel

BPSK Alamouti BER with 2TX&1RX over Rayleigh fading channel

BPSK BER with 2-branch MRC

1( ) 1

2 1nhoP

2

1 1 1( ) 1 2

4 1 2 / 1 2 /AlaP

2

1( ) 1 2

4 1 1MRCP

20[ ]bE E N

20[ ]bE E N

bE overall bit energy

20[ ]bE E N

bEbit energy from one branch

System Performance

Strategy III BER at non handoff state

Strategy III BER at soft handoff state

: the proportion that Alamouti decoding is applied

: the proportion that MRC is adopted

_ _

1( ) ( ) 1

2 1S III nho nhoP P

20[ ]bE E N bEbit energy from one BS

_ _ 1 _ _ 2 _ _( ) ( ) ( )S III ho S III Ala S III MRCP P P P P

1P

2P

System Performance

Strategy III BER at handoff state with Alamouti decoding

Strategy III BER at handoff state with MRC

_ _ ( ) (2 / 2) ( )S III Ala Ala AlaP P P

_ _ ( ) ( )S III MRC MRCP P

20[ ]bE E N bEbit energy from one BS

noise enhancement

definition : overall bit energy is 2 bE

Noise Enhancement

Noise enhancement for Alamouti decoding Twice noise power

No noise enhancement for MRC

f

DCf

f

f

Bit

Err

or

Pro

bab

ility

(dB)0 5 10 1510-4

10-3

10-2

10-1

100

_ _

_ _

nho

S III Ala

S III MRC

P

P

P

System Performance

Subband Based Schemes BER

Simulation parameters OFDM symbol length (without CP) : CP length : ITU-5 channel model

1 2 ( 1)k k C K

_1 _ _ ( ) ( )Sub S III Ala AlaP P P 0,1 21 PP

1 20, 1P P _ 2 _ _ ( ) ( )Sub S III MRC MRCP P P 1 2 ( 1)k k C K

1024N 1C

91.5 s11.4 s

64M

Bit

Err

or

Pro

bab

ility

(dB)0 5 10 1510

-4

10-3

10-2

10-1

100

Non-handoff state Analytical “ ” case Analytical “ ” case Analytical “ ” case Simulation “ ” case Simulation

1 2k k

1 2k k

1 25, 10k k

1 210, 5k k

Subband Based Schemes BER

Interleaved Schemes BER

Simulation parameters OFDM symbol length : , CP length : ITU-5 channel model, Case I : Case II : Case III : Case IV :

1 2 ( 1)k k C K

1 2 ( 1)k k C K

1C 91.5 s 11.4 s

0,1 21 PP _1 ( )Inter AlaP P

_ 2

1 1( ) ( )Inter Ala MRC

MP P P

M M

1 2

1 1,

MP P

M M

1024N 128M 1 23& 6k k 1024N 128M 1 26 & 3k k

1 26 & 3k k 1 26 & 3k k

128N 16M 16N 2M

Interleaved Schemes BERB

it E

rror

Pro

bab

ility

(dB)0 5 10 15

10-4

10-3

10-2

10-1

100

Analytical Simulation : Case I : Case II : Case III : Case IV : Analytical non-handoff state

Strategy I, II, III Comparison

BPSK BER for Strategy I Non handoff state

Soft handoff state

BPSK BER for Strategy II Non handoff state

Soft handoff state

Simulation parameters

_ _ _ _

1( ) ( ) 1

2 1S I nho S III nhoP P

_ _ ( ) ( )S I ho MRCP P

_ _ _ _

1( ) ( ) 1

2 1S II nho S III nhoP P

_ _ ( ) ( )S II ho MRCP P

1 2

1 2

1 2

1 2

Strategy I: 4096, 4, 64, 5, 10

Strategy II: 1024, 4, 64, 5, 10

Strategy III case (a): 1024, 4, 64, 5, 10, Subband Based Scheme

Strategy III case (b): 1024, 4, 64, 10, 5, Subband Based Schem

N M k k

N M k k

N M k k

N M k k

e

Strategy I, II, III Comparison

0 5 10 1510

-4

10-3

10-2

10-1

100

Non-handoff state Analytical Subband based “ ” Simulation Subband based “ ” Simulation Strategy I = Strategy II Analytical Strategy I Simulation Strategy II Simulation

1 2k k1 2k k

(dB)

Bit

Err

or

Pro

bab

ility

Strategy I, II, III Comparison

Strategy I Sampling rate : DFT size :

Strategy II Two receivers Sampling rate : DFT size :

Strategy III Sampling rate : can be restricted by DFT size : can be restricted by Channel re-allocation is sometimes required.

N

N

22N

Conclusion

The proposed diversity combining consists of Changing sampling rate Appropriate DFT size Alamouti STC

By the proposed technique, diversity combining can be realized for OFDMA cellular systems

The proposed scheme may have some implementation advantages against conventional ones

Conclusion

The proposed scheme can be considered as a generalized scheme of conventional ones.

The proposed diversity combining technique may have applications other than soft handoff.