Feedback for Interference Mitigation David Tse Wireless Foundations Dept. of EECS U.C. Berkeley CWIT May 20, 2011 TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAAA

2 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 Barrier Interference 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 3

What Ill Talk About Discuss several interference mitigation approaches. Focus on a common key enabler: feedback.

Interference Mitigation Approaches Multiuser MIMO Network MIMO Interference alignment 5

Downlink Multiuser MIMO mitigates intra-cell interference 6 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 7

Network MIMO mitigates inter-cell interference 8 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 10 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. X X

Downlink Interference Alignment Interference alignment between out-of-cell and intra-cell interference 11 Fix 2 dim. reference plane in 3-dim signal space, independent of channel gains transmission over 3 sub-carriers (Suh et al 10)

Key Enabler For All Techniques Timely channel knowledge at the transmitters: Multiuser MIMO Network MIMO Interference alignment. 12 Central Unit

Channel State Feedback Such CSI is typically obtained via feedback. Feedback overhead: f = 2 GHz, v = 30m/s, delay spread 5 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 its about the future. Niels Bohrs ( or was it Yogi Berra? ) 15

Completely Outdated Feedback We ask: What if current channel is completely independent from the fed back information? Conventional wisdom: Feedback is totally useless. Is conventional wisdom correct? 16

Downlink MIMO Perfect channel knowledge: 2 symbols per time slot A B 17

Downlink MIMO Perfect channel knowledge: 2 symbols/s/Hz No channel knowledge: 1 symbol/s/Hz 18

Downlink MIMO Outdated channel knowledge?4/3 symbol/s/Hz reconstruct 19

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 users Symbols 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 Optimality Theorem: 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)

Extension to Interference Channels reconstruct 24 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. X memoryless channel encoder decoder memoryless 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 decoder encoder delay decoder encoder decoder encoder delay decoder unknown h

Feedback in Memoryless Networks encoder dec 1 dec 2 memoryless broadcast channel delay Dueck provided an example of a BC where feedback helps but 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 28 side info.

Role of Feedback Old: Predict channel. New: Learn about side information at receivers. 29 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. 30

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