Cooperative Relaying Networks A. Wittneben Communication Technology Laboratory Wireless Communication Group Outline • Pervasive Wireless Access • Fundamental Performance Limits • Cooperative Signaling Schemes • Joint Cooperative Diversity and Scheduling • The Rich Array/Poor Scattering Regime • The RACooN Laboratory
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Cooperative Relaying Networks · 1 Cooperative Relaying Networks A. Wittneben Communication Technology Laboratory Wireless Communication Group Outline • Pervasive Wireless Access
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1
Cooperative Relaying Networks
A. Wittneben
Communication Technology LaboratoryWireless Communication Group
Outline
• Pervasive Wireless Access• Fundamental Performance Limits• Cooperative Signaling Schemes• Joint Cooperative Diversity and Scheduling• The Rich Array/Poor Scattering Regime• The RACooN Laboratory
infrastructureDesign objectives• data rate, QoS• range• position location• low cost• low EM exposure
Existing systems are insufficient
Outline
• Pervasive Wireless Access• Fundamental Performance Limits• Cooperative Signaling Schemes• Joint Cooperative Diversity and Scheduling• The Rich Array/Poor Scattering Regime• The RACooN Laboratory
4
Capacity of Wireless Networks(Gupta/Kumar, Trans. On IT, 2000)
• n nodes optimally placed• Each node can transmit at W bits/sec• Traffic patterns, ranges, powers are
optimally assignes• Point-to-point coding• Gaussian interference model (no joint
decoding)• Main Result: Order of the aggregate
throughput capacity is
( )( ) = bit/secWn n n Wn
= Θ ⋅ Θ ⋅
λ
Capacity of Wireless Relay Networks(Gastpar/Vetterli, Infocom 2002)
• n Nodes randomly distributed over a disk• Source and destination randomly chosen• Ever node can hear every other node• Source transmits only half the time• Relay traffic pattern with one active
source-destination pair• Gaussian channels• Arbitrary complex network coding is used
• Main result:
logC n∞ =
source
destination
• average per-node power constraint• coherent combining on downlink
5
Maximizing Degrees of Freedom in Wireless Networks(Borade et al., Allerton 2003)
Degrees of Freedom in Wireless Networks(Borade et al., Allerton 2003)
• Amplify-and-forward relays
• Establishes a distributed point-to-point MIMO channel
• Source uses same codebook as for a MIMO system (Gaussiancodebooks)
• For fixed k and n the systemachieves for high SNR a rate
Multi-hop network
(SNR) log(SNR)R n≈
2
PSNR =σ
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Degrees of Freedom in Wireless Networks(Borade et al., Allerton 2003)
• No communication is possible when k ∞ (number of hops) and SNR is fixed
• Full degrees of freedom are achieved when k is fixed and SNR ∞
• Question: For which functions kn(SNR) full n degrees of freedom can beachieved?
• Answer:
SNR
(SNR)lim 0log(SNR)nk
→∞=
4.3 /ke SNR dB hop≤ ⇒
Outline
• Pervasive Wireless Access• Fundamental Performance Limits• Cooperative Signaling Schemes• Joint Cooperative Diversity and Scheduling• The Rich Array/Poor Scattering Regime• The RACooN Laboratory
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Cooperative Diversity
analog
diskret
MAC
LLC
Physical
Data Link
amplify, forward
decode, forwardfilter, amplify, forward
basestation
user 1
user 2
Distributed Antenna Uplink Scenario
centralprocessor
user 1 1R
2Ruser 2
1P
2P
*10k
*20k
0Z
+
0Y
basestation
0Z
+
0Y
Hk
X
kk
X
P
XX
2
1 2 2Z
P kR R R C
σ
⋅ = + <
1R
2R
• achievable rate region
• perfect CSI at transmitter• beamforming (coherent combining)
1 2P P P= +
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Multi-Access Uplink Scenario
user 1 1 1;W R
2 2;W Ruser 2
1P *10k
*20k
0Z
+
0Y
basestation
2
1 2 22 Z
P kR R R C
σ
⋅ = + < ⋅
1R
2R
• achievable rate region for
• coherent combining not possibledue to independent codebooks
• with CSI: power loading
1X
2X
X
2P
X
1 2 / 2P P P= =
2
101 22 Z
P kR C
σ
⋅ < ⋅
2
202 22 Z
P kR C
σ
⋅ < ⋅
User Cooperation Diversity
1W
2W
10P
*10k
*20k
0Z
+
0Y
basestation
1R
2R
• achievable rate region for
• perfect CSI• users share a part of their data• this part is transmitted coherently with
the same codebook
10X X
1 2 / 2P P P= =10
12
W
W
U
+1 10 10/UP k k⋅
X
20P
20X X
21
20
W
W
+
U
2 20 20/UP k k⋅
X
W∆
1 2W W W∆ = +
0W∆ =
[Sendonaris et al 98/02]
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Outline
• Pervasive Wireless Access• Fundamental Performance Limits• Cooperative Signaling Schemes• Joint Cooperative Diversity and Scheduling• The Rich Array/Poor Scattering Regime• The RACooN Laboratory
Multiuser diversity vs. low mobility
• In a large wireless network the probability is high that the base station can serve one high-data rate user -> multiuser diversity
• aggregate throughput (system throughput) can be maximized by always serving the user with the strongest channel
• disadvantage: in low mobility environments channel variations are not sufficiently large enough -> high delays at some user nodes
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Fairness and delay
• in asymmetric channels (near-far situation) adaptivescheduling is unfair => high delays even in high mobility environment
• challenge is to achieve multiuser diversity gains while providing certain amount of fairness
0 100 200 300 400 500 600 700 800-10
-5
0
5
10
15
20
25
30
35
time slots
SNR
[dB
]
SNR of asymmetric user channels
User 1User 2
Proportional Fairness
• Question: How to achieve fairness among users with different fading statistics?
• Solution: Serve user who has best SNR compared to its average SNR (within a given latency time-scale tc)
• Comments:– proportional fair scheduler normalizes the SNR of each user to a
similar average value– scheduler operates away from aggregate throughput optimum
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Proportional Fairness and Low Mobility Environments
• in low mobility environments channel variations are not sufficient to achieve multiuser diversity gains with fairness
• introduce channel fluctuations artificially?
0 100 200 300 400 500 600 700 800-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
time slots
norm
aliz
ed S
NR
[dB
]
SNR of asymmetric user channels
User 1User 2
note different y-scale
Opportunistic Beamforming using dumb antennas(Viswanath, Tse, and Laroia, Trans. Inf. Theory 2002)
• basestation with multiple antennas• time-variant weights at each antenna• SNR feedback from all users• randomly swept beam and opportunistically send data to
best user
12
Slow Fading Environment: Before and after
• artificially introduced high mobility (time-variance)
Before After
Performance
• for large number of users performance of truebeamforming is achieved
• less feedback and channel measurements required
13
Distributed Relay networks
• amplify-and-forward relays introduce channel fluctuations by time-variant amplification gain at relays
Joint Cooperative Diversity and Scheduling(Wittneben/Hammerström Globecom 2004)
• 1% aggregate outage throughput is improved by a factor of nine if six active source/destination pairs are considered
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Outline
• Pervasive Wireless Access• Fundamental Performance Limits• Cooperative Signaling Schemes• Joint Cooperative Diversity and Scheduling• The Rich Array/Poor Scattering Regime• The RACooN Laboratory
Array and Propagation Model
( ) ( )2 2 2a a ,0 0 a ,0 N a,0 0N N f / f N f with N 16 A / ≈ ⋅ ≡ ⋅ = ⋅ π ⋅ λ
( )( ) ( )2PL 0 TX RX PL,0 0x / 4 d G G x d / d
−γ= λ π ⋅ ⋅ ⋅ ⋅ ⋅
( )( )2 2 2PL 0 TX RX PL,0 Nx / 4 d G G x b / f= λ π ⋅ ⋅ ⋅ ⋅ ≡
/ 2λ
A
0dd
fixed distance
Number of antenna elements
Power path loss
in the sequel b=1 without loss of generality
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Some Capacity Considerations
Ergodic capacity: ( )SDSD H SD SDC E C H =
Instantaneous capacity: ( ) ( ) ( )( )( )aN2 k 2
SD SD 2 s a SD wk 1
C H log 1 P / N /=
= + ⋅ σ σ∑• no power loading• per complex dimension
SD SD,N N SD,N NH H b / f H / f= ⋅ ≡Channel matrix:• singular values ( ){ }k
SDσ
No scattering: ( )2SD 2 s a,0 wC log 1 P N /= + ⋅ σ( ) ( )1 1
SD,N a SD a NN N / fσ = ⇒ σ =[ ]SD,NH m,n 1=
( ) ( ) ( )( )N
2 2 2 2SD a N a,0 2 s N N w aC N f N E log 1 P / f / N
σ = ⋅ ⋅ + ⋅ σ σ
Rich scattering:[ ] ( )SD,NH m,n CN 0,1=
: one-dimensional pdf of singular values of SD,N aH 1/ N⋅( )Np σ
essentially independent of foraN aN 4≥
( ) ( ) ( )2 2SD a,0 s w s wC N / ln 2 P / A P /∞ = ⋅ σ ⋅ σ∼Asymptotic value :N af ; N → ∞