Feedback for Interference Mitigation

David TseWireless Foundations

Dept. of EECS U.C. Berkeley

CWIT May 20, 2011

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Drive for Wireless Spectral Efficiency

• Lots of PHY layer advances in the past 1.5 decade.

• Focus on point-to-point and single-cell performance.

• Recent research has shifted to interference.

The Interference BarrierInterference gets worse as:• density of wireless nodes increases.• wireless architectures become more heterogeneous and

decentralized

Heterogeneous networks

Macro-cell

pico-cell

Tx1

Rx1

Peer-to-peer networks

Tx2

Rx2

Tx3

Rx3

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What I’ll Talk About

• Discuss several interference mitigation approaches.

• Focus on a common key enabler: feedback.

Interference Mitigation Approaches

• Multiuser MIMO

• Network MIMO

• Interference alignment

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Downlink Multiuser MIMOmitigates intra-cell interference

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Can transmit K symbols/s/Hz using K transmit antennas.(degrees of freedom = K)

Inter-Cell Interference Mitigation

Multiuser MIMO can also mitigate inter-cell interference

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Network MIMOmitigates inter-cell interference

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Central Unit

Inter-cell Interference Mitigation

With multiuser MIMO, each out-of-cell user to be nulled out costs one dimension.

Network MIMO removes that cost at the expense of additional infrastructure.

Can one reduce that cost without infrastructure cooperation?

Example: Single Tx Antenna at BS

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• Cannot beamform with one antenna.• D.o.f. per cell = ½ sym/s/Hz (frequency sharing)• Turns out that with more users per cell and frequency

diversity, one can do better.

XX

Downlink Interference Alignment

Interference alignment between out-of-cell and intra-cell interference

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Fix 2 dim. reference plane in 3-dim signal space, independent of channel gains

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1

1

2

2

transmission over 3 sub-carriers

! 1; K ! 1 :

(Suh et al 10)

Key Enabler For All Techniques

Timely channel knowledge at the transmitters:

• Multiuser MIMO

• Network MIMO

• Interference alignment.

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Central Unit

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1

1

2

2

Channel State FeedbackSuch CSI is typically obtained via feedback.• Feedback overhead:

f = 2 GHz, v = 30m/s, delay spread 5 m s

coherence time ¼ fv/c = 5ms

coherence bandwidth ¼ 200 kHz

coherence block ¼ 1000 sym

• Feedback delay

several milliseconds

can be of order of channel coherence time

Delay is a critical issue in hi-mobility.

Coherence block

Conventional Approach

• Transmitter predicts current channel state based on fed back information.

• Predicted channel state is used in place of true state in interference mitigation schemes.

Prediction is very difficult, especially if it’s about the future.

Niels Bohrs

(or was it Yogi Berra?)

Completely Outdated FeedbackWe ask:

What if current channel is completely independent from the fed back information?

Conventional wisdom:

Feedback is totally useless.

Is conventional wisdom correct?

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Downlink MIMOPerfect channel knowledge: 2 symbols per time slot

A

B

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Downlink MIMOPerfect channel knowledge: 2 symbols/s/Hz

No channel knowledge: 1 symbol/s/Hz

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Downlink MIMOOutdated channel knowledge? 4/3 symbol/s/Hz

reconstruct

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K by K Downlink MIMO

symbols/s/Hz

......

K antennas

K receivers(Maddah-Ali & T. 2010)

Perfect CSI: K symbols/s/Hz

No feedback: 1 symbol/s/Hz

Outdated feedback:

3 x 3 Case

Phase I

Transmit:

Symbols for individual usersSymbols wanted by 2 users

Phase IIPhase III

Symbols wanted by 3 users

Details on Phase II

Phase III: Transmit any two linear combinations of the three

Information Theoretic OptimalityTheorem:

The d.o.f. of the K by K MIMO broadcast channel with i.i.d. Rayleigh fading under feedback is:

Outer bound:

Physically-degraded BC:

Feedback does not help (El Gamal 78)

X ! (Y1;Y2; : : :;YK ) ! (Y2; : : : ;YK ) : : : ! YK...

...X

Y1

Y2

YK

Extension to Interference Channels

reconstruct

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2-user MIMO IC with arbitrary # of antennas: (Vaze and Varanasi 11)3-user IC (Maleki et al 10)

Feedback in Information Theory

Shannon 1956:

Feedback does not increase capacity of memoryless channels.

Xmemoryless channel

encoder decodermemoryless channel

encoder

delay

decoder

Feedback in Practice• Hybrid ARQ/ rateless codes:

Tries to achieve the capacity without knowing h.• Prediction

Tries to exploit memory in channel process {h[m]}.

encoder decoderencoder

delay

decoderh[m]

encoder decoderencoder

delay

decoderunknown h

Feedback in Memoryless Networks

encoder

dec 1

dec 2

memorylessbroadcastchannel

delay

delay

• Dueck provided an example of a BC where feedback helpsbut the noises at the two receivers are dependent.• Our result: feedback increases not only capacity but d.o.f. even when noises at receivers are independent.• What is this new role of feedback?

Side Information

reconstruct

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side info.

side info.

Role of FeedbackOld:• Predict channel.

New:• Learn about side information at receivers.

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Learning about the past is often easier than predicting the future.

Conclusions• Interference is a central barrier to scalability of wireless

systems.

• Multiple approaches to mitigate interference are emerging.

• New use of feedback is a key enabler for these approaches.

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Reference

M. Maddah-Ali and D. Tse, “Completely Stale Transmitter Channel State Information is Still Very Useful”, Allerton Conference, 2010.

http://arxiv.org/abs/1010.1499

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