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HUAWEI TECHNOLOGIES CO., LTD.
www.huawei.com
Algorithmic Aspects of Massive MIMO
And Application to the 5G PHY Layer
Maxime Guillaud
Mathematical and Alorithmic Sciences Lab, Huawei Technologies France
IRACON (COST Action 15104) Technical Meeting
Lille, France, May 30, 2016
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Covered Topics
Use cases in 5G (IoT / eMBB…) / KPIs
Introduction to Massive MIMO
CSI acquisition
Multi-User precoding / Resource allocation
Multiple access issues
Hardware impairments
Array geometries and channel models
Full duplex / relays
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5G Standardization Objectives (3GPP TR38.913)
Usage scenarios, requirements and deployment scenarios
Enhanced mobile broadband (eMBB)
Massive-to-machine (M2M) communications
Ultra reliable and low latency communications (V2V, traffic safety…)
(Some) key performance indicators targets
Peak data rate (20Gbps DL/10 Gbps UL for 1 user)
Peak spectral efficiency (30bps/Hz DL / 15bps/Hz UL)
Control plane latency (<10ms energy saving mode -> active transmission)
User plane latency (1ms over the radio link)
Reliability (99.999% packet delivery within 1ms)
UE battery life (10 years for 200 bytes/day UL + 20 bytes/day DL with 5Wh stored energy)
Area traffic capacity (in Mbit/s/m2)
User experienced data rate (5%-percentile of the user throughput)
Connection density (1 000 000 device/km2)
New!
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What is Massive MIMO?
In theory, like standard MIMO with some key differences
More BTS antennas (M) than users (K): 𝑀
𝐾≫ 1
Simplified multi-user processing, link adaptation, scheduling
Revisited CSI acquisition
M antennas
Consider the downlink of a multi-user channel. BTS has M antennas. Each user has 1 antenna.
Channel of user i: 𝒉𝑖 is a M-dimensional vector
Consider jointly the (downlink) channels to all users: 𝐇 = 𝒉𝟏 , … , 𝒉𝐾𝑇.
Downlink transmission:
𝑦1⋮𝑦𝐾
= 𝐇 𝐱 (+ noise). 𝐱 is the M-dimensional transmitted signal
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Intuition: DL linear precoding consisting in superposition coding with simple matched precoder
𝐱 =𝟏
𝑴 𝒊=𝟏…𝑲𝒉𝒊𝑠𝑖 where 𝑠𝑖 is the data symbols for user i
At user j: 𝑦𝑗= 𝒉𝒋𝑻 𝟏
𝑴 𝒊=𝟏…𝑲𝒉𝒊𝑠𝑖 =
𝟏
𝑴𝒉𝒋
𝑻𝒉𝒋𝑠𝑗 +
𝟏
𝑴 𝒊≠𝒋𝒉𝒋
𝑻𝒉𝒊𝑠𝑖 (+ noise)
With iid unit-variance fading: lim𝑀→∞
𝟏
𝑴 𝒊≠𝒋𝒉𝒋
𝑻𝒉𝒊 = 0 while lim𝑀→∞
𝟏
𝑴𝒉𝒋
𝑻𝒉𝒋 = 1
Massive MIMO:
How did it all start?
Signal of interest Interference
SINR 𝑀→∞
∞
(without Tx power increase)
No fadingon the effective channel:
channel hardening
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Certain “truths” about MIMO (single-user) need to be revisited:
In multi-user Massive MIMO, spatial separability of the users (from the BTS point of
view) is the key:
LoS is acceptable (as long as two users are not perfectly aligned)
Fading is not needed (exploit multi-user diversity rather than single-user MIMO diversity)
Array aperture ultimately governs the multiplexing gain (# of users served simultaneously)
What is a good Massive MIMO Channel?
LoS is detrimental to capacityi.i.d. fading channels are
preferable
𝜆
2antenna spacing
is necessary
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Downlink transmission:
𝑦1⋮𝑦𝐾
= 𝐇 𝐱 (+ noise). 𝐱 is the M-dimensional transmitted signal
MRC Precoding: matched precoder: 𝐱 =𝟏
𝑴𝐇𝑻 𝐬 where 𝐬 =
𝑠1⋮𝑠𝐾
are user symbols
CSI in Massive MIMO Downlink Transmission
lim𝑀→∞
𝟏
𝑴𝐇𝐇𝑻 = 𝑰𝑲
𝑦1⋮𝑦𝐾
=
𝑠1⋮𝑠𝐾
Downlink Channel
Precoder:must be estimated
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CSI Acquisition Strategies
Time Division Duplex (TDD) Frequency Division Duplex (FDD)
UL
DL
time
freq
uen
cy
Same time and different frequency
time
freq
uen
cy
Different time and same frequency
UL DL
UL channel estimation at the BTS
based on pilot sequences
DL channel obtained by reciprocity
(same as UL channel)
DL channel estimation at the UE based
on pilot sequences
Feedback (UE -> BTS) of estimated CSI
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Why FDD Matters
Depending on the region, 65 to 94% of 4G spectrum is FDD
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Uplink Multi-User Channel Estimation
Uplink Channel estimation: users transmit pilot sequences simultaneously:
𝑝𝑖(𝑡) is the (known) pilot symbol for user i at time t,
𝐲(t) = 𝒊=𝟏…𝑲 𝒉𝒊𝑝𝑖 𝑡 = 𝐇(𝒖𝒍) 𝐩 𝑡
Length-T training phase in matrix form: 𝐘 = 𝐲 1 ,… , 𝐲 𝐿 , 𝐇(𝒖𝒍) = 𝒉𝟏 , … , 𝒉𝐾 ,
𝐏 = 𝐩 1 ,… , 𝐩 𝐿
CSI acquisition for all K users:
𝐘 = 𝐇 𝐏 (+noise)
Intuition: linear estimation problem (observed 𝐘 is a linear combination of the etimee 𝐇).
Trivial solution if P is invertible: 𝐇 = 𝐘𝐏−𝟏
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Uplink Multi-User Channel Estimation
CSI acquisition for all K users treated jointly:
𝐘 = 𝐇 𝐏 (+noise…)
Pilot design:
𝐏 = 𝐈𝑲 : round-robin CSI estimation across the users
𝐏𝐏𝐻 = 𝐈𝑲 : orthogonal pilots across the users (requires 𝐿 ≥ 𝐾)
Non-orthogonal pilots: not a problem as long as rank(P)=K
rank(𝐏) <K: 𝐇 can not be identified (under-determined linear system)
Pilot reuse across cells: 𝐏 =𝐏1𝐏1
(pilot contamination)
The properties of the pilot matrix Pgovern CSI estimation
M×L M×K K×L
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Channel Covariance Information in Massive MIMO
θ
d
Tx correlation model: hi= Ri
1/2 .wi for user i
• Ri : the BTS-side channel covariance, assumed known (for now)• wi dimension d ≤ 𝑀, captures the fast fading
Motivation:• The contribution from a single scatterer (seen at angle 𝜃) to the channel is a fast-fading coefficient
times the rank-1 array response 𝐯𝜃 =
1
𝑒𝑗2𝜋𝑑
𝜆cos 𝜃
⋮
𝑒𝑗2𝜋(𝑀−1)𝑑
𝜆cos 𝜃
.
• In Massive MIMO, M ≫ #significant scatterers:
we expect low-rank Ri
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Exploiting Channel Covariance Information:
Uplink Multi-User CSI Estimation
𝐘 = 𝐇 𝐏 ⇔ vec(𝐘) = 𝐏𝑇⨂𝐈𝑀 .
𝐑1 1 2
⋱
𝐑𝐾 1 2
.
𝒘𝟏
⋮𝒘𝑲
• Linear estimation of the the fast fading coefficients with known 𝐑𝑘
• Extreme case: users with 𝐑𝐾 1 2 spanning orthogonal linear subspaces
→ Users fully separated in spatial domain, do not require orthogonal pilots
• Pilots sequence length L can be reduced!
• This model captures pilot contamination
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Exploiting Channel Covariance Information:
Uplink Multi-User CSI Estimation
vec(𝐘) = 𝐏𝑇⨂𝐈𝑀 .
𝐑1
1 2
⋱
𝐑𝐾
1 2
.
𝒘𝟏
⋮𝒘𝑲
How to choose the pilots (𝐏) as a function of 𝐑1…𝐑𝐾?
• Classical approach: reuse of orthogonal pilots across cells
• Reuse across groups of user (clustering based on 𝐑𝑘)
• “Unconstrained” optimization of 𝐏 based on covariance knowledge:
Reuse 𝑓 = 4“Universal” Reuse 𝑓 = 1
Reuse 𝑓 = 3
B. Tomasi, M. Guillaud , Pilot Length Optimization for Spatially Correlated Multi-User MIMO Channel Estimation, Asilomar 2015. http://arxiv.org/abs/1602.05480
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Exploiting Channel Covariance Information:
Downlink Multi-User CSI Estimation
• Prior knowledge of covariance can similarly be leveraged to improve DL
channel estimation:
• Normally, pilots of length at least M should be required to be able to
discriminate the BTS antennas
• With prior knowledge of 𝐑𝑖(𝐷𝐿)
, DL pilot sequence length and the rate of
UE → BTS feedback can be reduced (JSDM - Joint Spatial Division and
Multiplexing, Adhikary, Nam, Ahn, Caire 2012)
• Unconstrained pilot design based on DL covariance
Hot topic: Estimating and tracking uplink 𝐑𝑖
and downlink 𝐑𝑖(𝐷𝐿) covariance matrices
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The Massive number of antennas enables averaging over space:
Resource allocation, scheduling, link adaptation… can be done based on channel
statistics (channel hardening effect)
Multi-user Resource Allocation
lim𝑀→∞
𝟏
𝑴𝐇𝐇𝑻 = 𝑰𝑲
DL Channel Precoder
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Large Arrays with many RF chains: need to consider using
cheap hardware
Typical impairments:
Imperfect Tx/Rx Reciprocity
Over-the-Air calibration being proposed (ARGOS…)
Clocks Phase noise
Classically mitigated and ignored – now
signal processing-based compensation approaches
Low-accuracy ADCs
Partly analog (hybrid) beamforming, other digital approaches
Hardware Impairments in Massive MIMO
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Summary
Massive MIMO is like standard MIMO with some key differences:
More BTS antennas (M) than users (K): 𝑀
𝐾≫ 1
Simplified many aspects of the signal processing:
multi-user precoding
Scheduling and link adaptation (due to channel hardening)
CSI acquisition becomes the performance bottleneck
Revisited CSI estimation and feedback strategies
Pilot design (to minimize contamination)
TDD operation is preferred, FDD needed for legacy frequency allocation
M antennas