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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
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Outline
Introduction System Model Diversity Combining Techniques System Performance and Simulation Results Conclusion
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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
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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
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Conventional Strategies
Strategy I conventional
Strategy II conventional
Strategy III to be proposed in this work
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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)
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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)
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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
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One subchannel occupies subcarriers Total subchannels
Subcarrier assignment schemes Subband based schemes
Interleaved schemes
System Models
( / )K N MM
8, 2, 4N M K
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Noise Enhancement
Noise enhancement for Alamouti decoding Twice noise power
No noise enhancement for MRC
f
DCf
f
f
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.