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Limited Feedback based Double-SidedFull-Dimension MIMO for Mobile Backhauling
Asilomar Conference on Signals, Systems and Computers
Stefan Schwarz and Markus RuppNovember 7, 2016
Dependable Wireless Connectivity for the Society in Motion
Motivation – Mobile Backhauling
base station
I Increasing usage of mobile devices in public/private transportation
I Frequently unsatisfactory QoS at high mobility due to PHY/MAC inefficiency
I Promising solution: vehicle-mounted small cells for traffic aggregation
⇒ High-performance wireless backhaul required
⇒ Directive beamforming utilizing full-dimension (FD) MIMO arrays
Slide 2 / 23
Dependable Wireless Connectivity for the Society in Motion
Contents
System Model
Geometric vs. Algebraic Beamforming
Simulations
Conclusion
Slide 3 / 23
Dependable Wireless Connectivity for the Society in Motion
System Model (I)
scatterer
U mobile users
scatterer
base station
LOS component
NLO
S comp
onen
t
clusters
rays ;ÒD;u " )
(c;r )(¶D;u
(c;r)
;ÒA ";u )(c;r )
(¶A;u
(c;r)
I Double-directional channel of user u between RX antenna i and TX antenna j
h(u)i,j (t , τ, φR , θR , φT , θT ) =
Nc∑c=1
g(c)u
Nr∑r=1
α(c,r)u a(R)
i,u (φR , θR) a(T )j (φT , θT )
δ(τ − τ (c)u ) δ(φR − φ
(c,r)A,u ) δ(θR − θ
(c,r)A,u ) δ(φT − φ
(c,r)D,u )δ(θT − θ
(c,r)D,u )
I Time-variant channel impulse response by integrating over (φR , θR), (φT , θT )
Slide 4 / 23
Dependable Wireless Connectivity for the Society in Motion
System Model (II)
RF/BB
processing
base station
RF/BB
process.
user 1
RF/BB
process.
user U
channel
DoD
CSI feedback
CSI feedback
I Effective subcarrier-wise base-band channel after OFDM processing
yu [n, k ] = gu [n, k ]HHu [n, k ]∑j∈S
fj [n, k ]sj [n, k ] + gu [n, k ]Hnu [n, k ]
I CSI at TX obtained via dedicated limited feedback from users
⇒ CSI pilots inserted every mTs TTIs on pilot subcarriers P ⊆ {1, . . . ,K}
Slide 5 / 23
Dependable Wireless Connectivity for the Society in Motion
Contents
System Model
Geometric vs. Algebraic Beamforming
Simulations
Conclusion
Slide 6 / 23
Dependable Wireless Connectivity for the Society in Motion
Transceiver Strategies
I Single layer transmission per user + multi-user multiplexing
I Users estimate channel matrices Hu [n, k ], ∀k ∈ P and n = mTs
I Geometric approach
I Non-frequency-selective/wide-band processing⇒ potentially in RF
I Max-angular-gain/max-angular SLNR beamforming at TX(updated every mTs TTIs)
I Max-angular-gain antenna combining at RX (updated every TTI)
I Algebraic approach
I Frequency-selective⇒ base-band processing
I Zero-forcing beamforming at TX(updated every mTs TTIs, nearest-neighbor interpolation)
I MET or blind residual interference cancellation at RX(updated every TTI on each subcarrier)
Slide 7 / 23
Dependable Wireless Connectivity for the Society in Motion
Transceiver Strategies
I Single layer transmission per user + multi-user multiplexing
I Users estimate channel matrices Hu [n, k ], ∀k ∈ P and n = mTs
I Geometric approach
I Non-frequency-selective/wide-band processing⇒ potentially in RF
I Max-angular-gain/max-angular SLNR beamforming at TX(updated every mTs TTIs)
I Max-angular-gain antenna combining at RX (updated every TTI)
I Algebraic approach
I Frequency-selective⇒ base-band processing
I Zero-forcing beamforming at TX(updated every mTs TTIs, nearest-neighbor interpolation)
I MET or blind residual interference cancellation at RX(updated every TTI on each subcarrier)
Slide 7 / 23
Dependable Wireless Connectivity for the Society in Motion
Transceiver Strategies
I Single layer transmission per user + multi-user multiplexing
I Users estimate channel matrices Hu [n, k ], ∀k ∈ P and n = mTs
I Geometric approach
I Non-frequency-selective/wide-band processing⇒ potentially in RF
I Max-angular-gain/max-angular SLNR beamforming at TX(updated every mTs TTIs)
I Max-angular-gain antenna combining at RX (updated every TTI)
I Algebraic approach
I Frequency-selective⇒ base-band processing
I Zero-forcing beamforming at TX(updated every mTs TTIs, nearest-neighbor interpolation)
I MET or blind residual interference cancellation at RX(updated every TTI on each subcarrier)
Slide 7 / 23
Dependable Wireless Connectivity for the Society in Motion
Geometric Approach – RX Beamforming
I Max-angular-gain receive beamforming1
gu [n] = argmax∑k∈P
∥∥gHHu [n, k ]∥∥2,
subject to:
g = a(R)u (φ, θ) , (φ, θ) ∈ Φq ×Θq
I Effective channel-vector
hu [n, k ] = Hu [n, k ]Hgu [n]
1For concreteness we consider UPAs
[a(φ, θ)](`−1)Nh+k =1√
Nrexp
(j2π(δv (`− 1) cos θ) + δh(k − 1) sinφ sin θ
),
` ∈ {1, . . . ,Nv}, k ∈ {1, . . . ,Nh}
Slide 8 / 23
Dependable Wireless Connectivity for the Society in Motion
Geometric Approach – CSI FeedbackFeedback of Ne strongest angular directions and corresponding gains
I Consider the angular energy-distribution
A1(φ, θ) =∑k∈P
∣∣a(φ, θ)Hhu [n, k ]∣∣2 , (φ, θ) ∈ Φq ×Θq
I First direction:
(φ(1)T ,u , θ
(1)T ,u) = argmax
(φ,θ)∈Φq×Θq
A1(φ, θ),
g1,u = quant(
A1(φ(1)T ,u , θ
(1)T ,u)
)I Subtract energy-contribution corresponding to this direction
A2(φ, θ) = max(A1(φ, θ)− g1,uP(φ, θ), 0
),
P(φ, θ) =∣∣∣a(φ, θ)Ha(φ
(1)T ,u , θ
(1)T ,u)
∣∣∣2I Iterate to find further directionsI Incoherent energy-based channel decomposition
Slide 9 / 23
Dependable Wireless Connectivity for the Society in Motion
Geometric Approach – CSI FeedbackFeedback of Ne strongest angular directions and corresponding gains
I Consider the angular energy-distribution
A1(φ, θ) =∑k∈P
∣∣a(φ, θ)Hhu [n, k ]∣∣2 , (φ, θ) ∈ Φq ×Θq
I First direction:
(φ(1)T ,u , θ
(1)T ,u) = argmax
(φ,θ)∈Φq×Θq
A1(φ, θ),
g1,u = quant(
A1(φ(1)T ,u , θ
(1)T ,u)
)I Subtract energy-contribution corresponding to this direction
A2(φ, θ) = max(A1(φ, θ)− g1,uP(φ, θ), 0
),
P(φ, θ) =∣∣∣a(φ, θ)Ha(φ
(1)T ,u , θ
(1)T ,u)
∣∣∣2I Iterate to find further directionsI Incoherent energy-based channel decomposition
Slide 9 / 23
Dependable Wireless Connectivity for the Society in Motion
Geometric Approach – TX Processing
I Max-angular-gain transmit beamforming
fj [n] = a(φ(1)T ,j , θ
(1)T ,j )
I Max-angular-SLNR transmit beamforming(incoherent addition of leakage energy)
fj [n] =f̃(L)j [n]∥∥∥̃f(L)j [n]
∥∥∥ , f̃j [n] =(σ2
n INt + L)−1
a(φ(1)T ,j , θ
(1)T ,j ),
L =∑
`∈S, 6̀=j
Ne∑i=1
gi,` a(φ(i)T ,`, θ
(i)T ,`)a(φ
(i)T ,`, θ
(i)T ,`)
H
I Greedy multi-user scheduling based on incoherent SINR estimate
Slide 10 / 23
Dependable Wireless Connectivity for the Society in Motion
Geometric Approach – TX Processing
I Max-angular-gain transmit beamforming
fj [n] = a(φ(1)T ,j , θ
(1)T ,j )
I Max-angular-SLNR transmit beamforming(incoherent addition of leakage energy)
fj [n] =f̃(L)j [n]∥∥∥̃f(L)j [n]
∥∥∥ , f̃j [n] =(σ2
n INt + L)−1
a(φ(1)T ,j , θ
(1)T ,j ),
L =∑
`∈S, 6̀=j
Ne∑i=1
gi,` a(φ(i)T ,`, θ
(i)T ,`)a(φ
(i)T ,`, θ
(i)T ,`)
H
I Greedy multi-user scheduling based on incoherent SINR estimate
Slide 10 / 23
Dependable Wireless Connectivity for the Society in Motion
Geometric Approach – TX Processing
I Max-angular-gain transmit beamforming
fj [n] = a(φ(1)T ,j , θ
(1)T ,j )
I Max-angular-SLNR transmit beamforming(incoherent addition of leakage energy)
fj [n] =f̃(L)j [n]∥∥∥̃f(L)j [n]
∥∥∥ , f̃j [n] =(σ2
n INt + L)−1
a(φ(1)T ,j , θ
(1)T ,j ),
L =∑
`∈S, 6̀=j
Ne∑i=1
gi,` a(φ(i)T ,`, θ
(i)T ,`)a(φ
(i)T ,`, θ
(i)T ,`)
H
I Greedy multi-user scheduling based on incoherent SINR estimate
Slide 10 / 23
Dependable Wireless Connectivity for the Society in Motion
Algebraic Approach – RX Beamforming
I Maximum eigenmode transmission (MET)
gu [n, k ] = u(1)u [n, k ],
hu [n, k ] = σ(1)u [n, k ]v(1)
u [n, k ],
Hu [n, k ] = Uu [n, k ]Σu [n, k ]Vu [n, k ]H
I Maximum expected achievable rate combining (MERC) 2
⇒ Blind residual interference cancellation based on quantized CSI
2S. Schwarz and M. Rupp, Maximum expected achievable rate combining for limited feedback
block-diagonalization, ICASSP 2015
Slide 11 / 23
Dependable Wireless Connectivity for the Society in Motion
Algebraic Approach – RX Beamforming
I Maximum eigenmode transmission (MET)
gu [n, k ] = u(1)u [n, k ],
hu [n, k ] = σ(1)u [n, k ]v(1)
u [n, k ],
Hu [n, k ] = Uu [n, k ]Σu [n, k ]Vu [n, k ]H
I Maximum expected achievable rate combining (MERC) 2
⇒ Blind residual interference cancellation based on quantized CSI
2S. Schwarz and M. Rupp, Maximum expected achievable rate combining for limited feedback
block-diagonalization, ICASSP 2015
Slide 11 / 23
Dependable Wireless Connectivity for the Society in Motion
Algebraic Approach – CSI Feedback
I Decompose the channel into a sum of plane-waves using an over-completebasis of quantized directions Φq ×Θq
{gi,u [k ], φ
(i)T ,u [k ], θ
(i)T ,u [k ]
}= argmin
gi ,(φi ,θi )
∥∥∥∥∥∥hu [n, k ]−Ne∑i=1
gi aP(φi , θi )
∥∥∥∥∥∥2
,
subject to:
gi ∈ C, (φi , θi ) ∈ Φq ×Θq
I Iterative solution using orthogonal matching pursuit (OMP) – perfectreconstruction with at most Nt coefficients
I Coherent channel decomposition
I Frequency-selective feedback on CSI pilot positions
Slide 12 / 23
Dependable Wireless Connectivity for the Society in Motion
Algebraic Approach – TX Processing
I Frequency-selective greedy multi-user scheduling 3
⇒ Constant schedule in-between CSI feedback positions in time and frequency
I Zero-forcing (ZF) beamforming based on quantized CSI
⇒ increasing CSI mismatch over time is neglected
3S. Schwarz and M. Rupp, Evaluation of distributed multi-user MIMO-OFDM with limited feedback, IEEE
Transactions on Wireless Communications, vol. 13, no. 11, 2014
Slide 13 / 23
Dependable Wireless Connectivity for the Society in Motion
Contents
System Model
Geometric vs. Algebraic Beamforming
Simulations
Conclusion
Slide 14 / 23
Dependable Wireless Connectivity for the Society in Motion
Simulation Setup
120°
550m
I One base station serving ten UEs at 2.1 GHz
I UEs traveling at approx. 50 km/h (100 Hz Doppler)
I UPA of dimension 10× 10 at TX
I UPA of dimension {1× 1, 2× 2, 4× 4} at RX
I Microscopic fading only at SNR = 20 dB
I CSI feedback every mTs = 10 TTI (delayless)
I Non-line of sight propagation (3GPP 3D model)
I Normalized TTI: τs fd , τs ∈ [0,mTs − 1]
I Feedback of Ne = 4 angular directions
Slide 15 / 23
Dependable Wireless Connectivity for the Society in Motion
Angular CSI Feedback
Azimuth [°]
Ele
va
tio
n [
°]
Re
lati
ve
po
we
r
-80 -60 -40 -20
40
60
80
100
120
140 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
806040200 -80 -60 -40 -20Azimuth [°]
40
60
80
100
120
140
Ele
va
tio
n [
°]
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Re
lati
ve
po
we
r
806040200
Fig. 1: Angular CSI feedback of the geometric method (left) and the algebraic method (right).
I Geometric method: Ne peaks of signal-energy distribution
I Algebraic method: phase-coherent decomposition into Ne multipath-components
Slide 16 / 23
Dependable Wireless Connectivity for the Society in Motion
Algebraic Beampatterns
-40
-20
0
20
40
60
-6030 60 90 120 150
-40
-20
0
20
40
60
-6030 60 90 120 150
20
0
-20
-40
-60
-80
-100
Pow
er [d
BW]
Fig. 2: Max gain transmit beamforming (left) and max SLNR transmit beamforming (right).
I Max gain: largest signal gain but potentially strong interference
I Max SLNR: decreased interference for the cost of reduced intended signal
⇒ Smart multi-user scheduling important in both cases
Slide 17 / 23
Dependable Wireless Connectivity for the Society in Motion
Achievable Rate Comparison 1 × 1
0.90.80.70.60.50.40.30.20.10
50
40
30
20
10
0
normalized TTI
Ach
ieva
ble
rate
[bits
/s/H
z]
10x10_1x1_20dB_100Hz_NLOS
BD perfect CSI
BD Fs=1
BD Fs=12
max Gain Fs=72max SLNR Fs=72
BD Fs=72
single user perfect CSI
I Large achievable rate of BD (algebraic method) with accurate CSI
I More robust behavior of geometric beamforming⇔ angles change slowly
Slide 18 / 23
Dependable Wireless Connectivity for the Society in Motion
Achievable Rate Comparison 2 × 2
0.90.80.70.60.50.40.30.20.10
50
40
30
20
10
0
normalized TTI
Ach
ieva
ble
rate
[bits
/s/H
z]
10x10_2x2_20dB_100Hz_NLOS
BD perfect CSI
BD-MET Fs=1
BD-MET Fs=12
single user perfect CSI
BD-MET Fs=72
max SLNR Fs=72
max Gain Fs=72
I Similar behavior with multiple RX and interference-ignorant receivers
I Significant performance gain through RX-side interference-suppression
Slide 19 / 23
Dependable Wireless Connectivity for the Society in Motion
Achievable Rate Comparison 2 × 2
0.90.80.70.60.50.40.30.20.10
50
40
30
20
10
0
normalized TTI
Ach
ieva
ble
rate
[bits
/s/H
z]
10x10_2x2_20dB_100Hz_NLOS
BD perfect CSI
BD-MERC Fs=1
BD-MET Fs=1
BD-MERC Fs=12
BD-MET Fs=12
single user perfect CSI
BD-MET Fs=72
BD-MERC Fs=72max SLNR Fs=72
max Gain Fs=72
I Similar behavior with multiple RX and interference-ignorant receivers
I Significant performance gain through RX-side interference-suppression
Slide 19 / 23
Dependable Wireless Connectivity for the Society in Motion
Achievable Rate Comparison 4 × 4
0.90.80.70.60.50.40.30.20.10
50
40
30
20
10
0
normalized TTI
Ach
ieva
ble
rate
[bits
/s/H
z]
10x10_4x4_20dB_100Hz_NLOS
single user perfect CSIBD-MET Fs=72
BD perfect CSI
BD-MET Fs=12
BD-MET Fs=1
BD-MERC Fs=72
BD-MERC Fs=12
BD-MERC Fs=1
max SLNR Fs=72max Gain Fs=72
I Sufficient spatial degrees of freedom to virtually suppress all residualinterference (even blindly)
Slide 20 / 23
Dependable Wireless Connectivity for the Society in Motion
Contents
System Model
Geometric vs. Algebraic Beamforming
Simulations
Conclusion
Slide 21 / 23
Dependable Wireless Connectivity for the Society in Motion
Summary and Outlook
I Geometric beamforming performs more robustly
I Algebraic approach has much higher spatial multiplexing potential/capabilities
I Hardware-complexity advantages of geometric approach
I Without RX-side interference-suppression geometric beamforming with imperfectCSI outperforms algebraic approach
I RX-side interference-suppression can safe the day (even blindly)
I Selection of appropriate method depending on CSI accuracy
I Combination of geometric TX beamforming (robustness) and algebraic RXbeamforming (interference-suppression)
I Hybrid combination of both methods
Slide 22 / 23
Dependable Wireless Connectivity for the Society in Motion
Summary and Outlook
I Geometric beamforming performs more robustly
I Algebraic approach has much higher spatial multiplexing potential/capabilities
I Hardware-complexity advantages of geometric approach
I Without RX-side interference-suppression geometric beamforming with imperfectCSI outperforms algebraic approach
I RX-side interference-suppression can safe the day (even blindly)
I Selection of appropriate method depending on CSI accuracy
I Combination of geometric TX beamforming (robustness) and algebraic RXbeamforming (interference-suppression)
I Hybrid combination of both methods
Slide 22 / 23
Limited Feedback based Double-SidedFull-Dimension MIMO for Mobile Backhauling
Asilomar Conference on Signals, Systems and Computers
Stefan Schwarz and Markus RuppNovember 7, 2016
Dependable Wireless Connectivity for the Society in Motion
References I
Slide 1 / 1