GLOBECOM 2014 paper

1. Beamforming for Multiuser Massive MIMO Systems:
Digital versus Hybrid Analog-Digital
Tadilo Endeshaw Bogale and Long Bao Le
Institute National de la Recherche Scientifique (INRS), Canada
Email: {tadilo.bogale, long.le}@emt.inrs.ca
OBJECTIVES
System Model: Multiuser Massive MIMO (mmWave)
Problem: Weighted Sum Rate Maximization
• Design Hybrid Analog-Digital Beamforming
• Compare Hybrid and Digital Beamformings
• Examine effects of # RF chains and ADCs
DIGITAL BEAMFORMING (Summary)
Settings: The kth UE symbol dk ∈ CSk×1
BS and kth UE # ant, N and Mk
⇒Received signal : rk = HH
k
K
i=1
Bidi + nk
Estimated signal : ˆdD
k = WH
k rk
Digital Precoding:
K
i=1 Bidi ∈ CN×1
Digital Estimation: WH
k rk ∈ CSk×1
∴ Needs N and Mk RF chains at BS and kth UE
⇒ Too expensive for Massive MIMO (N,Mk large)
HYBRID BEAMFORMING
Goals:
• Use limited RF chains at BS and UEs (low cost)
• Employ PSs only for analog beamformer (low cost)
• Apply hybrid precoding and estimation
• Achieve same(closer) performance as digital one
Main idea:
• Maintain A
K
i=1
˜Bidi ≈
K
i=1 Bidi
• Maintain ˆdHy
k = ˜WH
k FH
k rk ≈ WH
k rk = ˆdD
k
• Design ˜Bk ∈ CPt×Sk
( ˜Wk ∈ CPrk×Sk
) in digital
• Design A ∈ CN×Pt
(Fk ∈ CMk×Prk
) in analog
Key Challenges ?
• Constraints: rank( ˜WH
k FH
k ) ≤ Prk, |Fk(ij)|2
= 1
rank(A[˜B1, · · · , ˜BK]) ≤ Pt, |Aij|2
= 1
PROPOSED HYBRID BEAMFORMING
Motivation:
• Good hybrid solution approaches the digital one
∴ Choose HB matrix closer to that of DB one
⇒ Minimize MSE between ˆdD
k and ˆdHy
k
Incorporate weight to ensure fairness
Problem Formulation
• Step 1: Choose a reference DB
Block diagonalization DB (simple)
⇒ ˆdD
k = Zk
√
Qkdk + ˜UH
hknk (no interference)
where Zk, Qk, ˜Uhk depend on H (see paper)
• Step 2: Design HB to solve WSMSE
min
A,˜Bk, ˜Wk,Fk
K
k=1
tr{(Zk Qk)−1
ξk(Zk Qk)−1
}
s.t ξk = E{(ˆdHy
k − ˆdD
k )(ˆdHy
k − ˆdD
k )H
}
K
k=1
tr{A˜Bk
˜BH
k AH
} = Pmax
|A(i,j)|2
= 1, |Fk(i,j)|2
= 1
Non Convex
Objective
Constraints
PROPOSED ALGORITHM
Given: Zk, Qk, Pmax
Tool: OMP
Optimize ˜Wk, Fk
for all UEs
For fixed ˜Wk, Fk, ∀k,
optimize A, ˜Bk, ∀k jointly
FINISH
SIMULATION RESULTS
INTERESTING PROBLEM:
Given an arbitrary reference DB, how many RF
chains and PSs do we need ensuring HB=DB ?
Simulation Parameter Settings:
ULA channel with Lk = 16 scatterers and K = 4
BS and MSk # ant (RF chain): 128(KPrk) and 32 (Prk)
−7.5 −5 −2.5 0 2.5 5 7.5 10 12.5 15 17.517.5
50
100
150
200
250
300
SNR (dB)
SumRate(b/s/hz)
Digital vs Hybrid: Sk
=8, Prk
=16
Digital
Hybrid (P
rk
=L
k
=16)
8 10 12 14 16 18 20
60
80
100
120
140
160
180
Prk
SumRate(b/s/hz)
Effect of Prk
Digital (SNR = −4 dB)
Hybrid (SNR = −4 dB)
Digital (SNR = 6 dB)
Hybrid (SNR = 6 dB)
Huge Gap
2 4 6 8 10 12
20
40
60
80
100
120
140
S
k
SumRate(b/s/hz)
Effect of S
k
Digital
Hybrid (Prk
= 8)
Hybrid (Prk
= 16)
Big Gap
Call for Papers for
Wireless Communications Symposium
Scope and Motivation:
The Wireless Communications Symposium covers all aspects related to wireless
communications and its applications, with a focus on topics related to physical layer (PHY),
MAC layer, cross-layer, and physical layer-related network analysis and design. High quality
papers reporting on novel and practical solutions to PHY, MAC, and cross-layer design in
wireless communication systems are encouraged. In addition, papers on field tests and
measurements, field trials and applications from both industries and academia are of special
interest.
Main Topics of Interest:
To ensure complete coverage of the advances in wireless communications technologies for
current and future wireless systems, the Wireless Communications Symposium cordially invites
original contributions in, but not limited to, the following topical areas:
 Advanced equalization, channel estimation and synchronization techniques
 Broadband wireless access techniques and systems
 Channel and network interference characterization and modeling
 Channel state information feedback techniques
 Coexistence in unlicensed spectra
 Cross-layer design and physical-layer based network issues
 Device-to-device and machine-to-machine communications
 Digital video broadcasting (DVB) and digital audio broadcasting (DAB) techniques
 Distributed multipoint, relay assisted, and cooperative communications
 Field tests and measurements
 Heterogeneous and femtocell networks
 Hybrid wireless communication systems (e.g. satellite/terrestrial hybrids)
 Interference management, alignment and cancellation, inter-cell interference coordination
(ICIC), and coordinated multi-point transmission (CoMP)
 Localization techniques
 MIMO, multi-user MIMO, and massive MIMO

2. Beamforming for Multiuser Massive MIMO Systems:
Digital versus Hybrid Analog-Digital
Tadilo Endeshaw Bogale and Long Bao Le
Institute National de la Recherche Scientifique (INRS), Canada
Email: {tadilo.bogale, long.le}@emt.inrs.ca
HYBRID BEAMFORMING (Block Diagram) [1]
Source Tx (Digital part) RF Chain Tx (Analog part)
Freq. Dom.
data source
d1
d2
dK
•••
Freq. Dom.
digital BF
1
2
Pt
•••
IFFT (row)
& add CP
1
2
Pt
•••
RF1
analog
RF2
analog
RFPt
analog
•••
Analog
BF
Analog
BF
Analog
BF
•••
N
N
N
•••
2
2
2
1
1
1
1
2
N
•••
•••
•••
•••
•••
•••
H
1 Discard CP
& take FFT
ˆd1 Decode
ˆd1
2 Discard CP
& take FFT
ˆd2 Decode
ˆd2
K Discard CP
& take FFT
ˆdK Decode
ˆdK
•••
d ˜B A
rk = hH
k A˜Bd + nk
If OFDMA
If OFDMA
HYBRID BEAMFORMING [1]
Settings: K single antenna UEs and N >> K
⇒ rk = hH
k Bd + nk [Digital]
• For any B, rank(B) ≤ K
• UQ = svd(B), with UH
U = IK, Q ∈ CK×K
⇒ rk = hH
k UQd + nk
Goal
• Step 1: Design Q in Digital (K RF Chains OKEY)
• Step 2: Design U in Analog with PSs only (HOW ?)
since Umn = amnejθmn
, amn ≤ 1, |Umn| = 1 if amn = 1
Key Result:
• Theorem 1 of [1]: amn = ej cos−1
( amn
2 )
+ e−j cos−1
( amn
2 )
⇒ Umn = ej(cos−1
( amn
2 )+θmn)
+ e−j(cos−1
( amn
2 )−θmn)
⇒ Umn ≡ 2PSs (U can be implemented with 2NK PSs)
∴ ˜B = Q and A = U
Given an arbitrary reference DB, how many RF chains and
PSs do we need ensuring HB=DB ?
MISO System: Maximum of K RF chains and 2NK PSs
PSs can be reduced (see [1] for details)
RF chains can be reduced if B is low rank [1]
MIMO System: Can be extended like in [1]
REREFENCES
1. T. E. Bogale, L. Le, and A. Haghighat, Hybrid
analog-digital beamforming: How many RF chains and
phase shifters do we need?, IEEE Trans. (Submitted),
http://arxiv.org/abs/1410.2609.
2. T. E. Bogale and L. B. Le, Beamforming for multiuser
massive MIMO systems: Digital versus hybrid
analog-digital, in Proc. IEEE Global Communications
Conference (GLOBECOM), Austin, Tx, USA, 10-12 Dec.
2014.
3. O. E. Ayach, S. Rajagopal, S. Abu-Surra, Z. Pi, and R.
W. Heath, Spatially sparse precoding in millimeter wave
MIMO systems, IEEE Tran. Wirel. Com., Jan. 2014.
4. S. Hur, T. Kim, D. J. Love, J. V. Krogmeier, T. A.
Thomas, and A. Ghosh, Millimeter wave beamforming for
wireless backhaul and access in small cell networks, IEEE
Trans. Commun., vol. 61, no. 10, Oct. 2013.
5. X. Zhang, A. F. Molisch, and S-Y. Kung,
Variable-phase-shift-based RF-Baseband codesign for
MIMO antenna selection, IEEE Trans. Signal Process.,
vol. 53, no. 11, pp. 4091 4103, Nov. 2005.
6. J. Tropp and A. Gilbert, Signal recovery from random
measurements via orthogonal matching pursuit, IEEE
Tran. Info. Theory Dec. 2007.
7. T. Yoo and A. Goldsmith, On the optimality of
multiantenna broadcast scheduling using zero-forcing
beamforming, IEEE Trans. Sel. Area. Commun., vol. 24,
no. 3, pp. 528 541, Mar. 2006.
SOME SIMULATION RESULTS [1]
OPEN PROBLEM
For any H and reference DB, how to reduce RF
chains < K while ensuring HB=DB ?
Simulation Parameter Settings:
Downlink MISO and Flat fading ULA channel
ZF reference DB, N = 64 and K = 16
NP S denotes # PSs and SNR = 10dB
2 4 6 8 10 12
0
10
20
30
40
50
60
70
80
Averagesumrate(b/s/hz)
Pt
=16 RF chains
Number of scatterers (L
k
)
DB
HB in [1] (NPS
=98)
HB in [1] (NPS
=40)
HB in [1] (NPS
=20)
HB in [2] (NPS
=64)
HB in [3] (N
PS
=64)
Antenna selection DB
2 4 6 8 10 12
30
35
40
45
50
55
60
65
70
75
80
Number of scatterers (Lk
)
Averagesumrate(b/s/hz)
Pt
=24 RF chains
DB
HB in [1] (NPS
=98)
HB in [1] (NPS
=40)
HB in [1] (N
PS
=20)
HB in [2] (N
PS
=64)
HB in [3] (NPS
=64)
Antenna selection DB
CONCLUSIONS
• DB achieves the best performance
• Significant performance loss is incurred in HB
approaches of [2] and [3] when Lk is large (e.g.,
Rayleigh fading)
• HB approach of [1] uses the lowest RF chains and
PSs, and achieves same performance as DB
• Number of PSs can be reduced with negligible
performance loss [1] (see also the above plot)
Call for Papers for
Wireless Communications Symposium
Scope and Motivation:
The Wireless Communications Symposium covers all aspects related to wireless
communications and its applications, with a focus on topics related to physical layer (PHY),
MAC layer, cross-layer, and physical layer-related network analysis and design. High quality
papers reporting on novel and practical solutions to PHY, MAC, and cross-layer design in
wireless communication systems are encouraged. In addition, papers on field tests and
measurements, field trials and applications from both industries and academia are of special
interest.
Main Topics of Interest:
To ensure complete coverage of the advances in wireless communications technologies for
current and future wireless systems, the Wireless Communications Symposium cordially invites
original contributions in, but not limited to, the following topical areas:
 Advanced equalization, channel estimation and synchronization techniques
 Broadband wireless access techniques and systems
 Channel and network interference characterization and modeling
 Channel state information feedback techniques
 Coexistence in unlicensed spectra
 Cross-layer design and physical-layer based network issues
 Device-to-device and machine-to-machine communications
 Digital video broadcasting (DVB) and digital audio broadcasting (DAB) techniques
 Distributed multipoint, relay assisted, and cooperative communications
 Field tests and measurements
 Heterogeneous and femtocell networks
 Hybrid wireless communication systems (e.g. satellite/terrestrial hybrids)
 Interference management, alignment and cancellation, inter-cell interference coordination
(ICIC), and coordinated multi-point transmission (CoMP)
 Localization techniques
 MIMO, multi-user MIMO, and massive MIMO