Noncoherent Communications with Large Antenna Arrays

Transcription

1 Noncoherent Communications with Large Antenna Arrays Mainak Chowdhury Joint work with: Alexandros Manolakos, Andrea Goldsmith, Felipe Gomez-Cuba and Elza Erkip Stanford University September 29, 2016

2 Wireless propagation

3 The promise of large antenna arrays What is an antenna array? Group of antennas receiving/transmitting signals simultaneously wavelength/2 wavefront

4 The promise of large antenna arrays What is an antenna array? Group of antennas receiving/transmitting signals simultaneously wavelength/2 wavefront Benefits Signal becomes stronger and less uncertain Fading and noise vanish [Marzetta, 2010] Beams are directed and steerable Used for precise imaging and tracking Note large gain along a particular direction

5 The promise with large antenna arrays Better cellular network with heterogenous demand (objects and people)

6 Challenges with large arrays in communication systems antenna array fronthaul baseband unit Space constraints Number of radio frequency (RF) front ends is large Channel state information (CSI) needs to be acquired fast CSI needs to go through rate limited (fronthaul) links

7 Challenges with large arrays in communication systems antenna array fronthaul baseband unit Space constraints Number of radio frequency (RF) front ends is large Channel state information (CSI) needs to be acquired fast CSI needs to go through rate limited (fronthaul) links Number of antennas doesn t scale easily!

8 Our contributions In large antenna arrays: Fading channel models need to be rethought Coherence time and antenna correlation are different

9 Our contributions In large antenna arrays: Fading channel models need to be rethought Coherence time and antenna correlation are different Many benefits also possible with noncoherent schemes Schemes with slowly changing statistics of the channel

10 Our contributions In large antenna arrays: Fading channel models need to be rethought Coherence time and antenna correlation are different Many benefits also possible with noncoherent schemes Schemes with slowly changing statistics of the channel Simple transmission and detection works well ON-OFF keying, one-shot detectors

11 Plan Why large antenna arrays? Channel models for MIMO large antenna arrays Noncoherent schemes Transmission and detection schemes Conclusions

12 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m )

13 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models

14 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models

15 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models Models for multi antenna systems Independent and identically distributed (IID) fading models

16 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models Models for multi antenna systems Independent and identically distributed (IID) fading models Actual antennas not IID modeling antenna correlation

17 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models Models for multi antenna systems Independent and identically distributed (IID) fading models Actual antennas not IID modeling antenna correlation

18 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models Models for multi antenna systems Independent and identically distributed (IID) fading models Actual antennas not IID modeling antenna correlation Models for a single time varying channel Characterize joint distribution of h m (t), h m (t + τ)

19 Fading models Channel from user equipment (UE) to m th antenna element is h m Models for single antenna systems Statistical fading models (distribution on h m ) Ray tracing models Models for multi antenna systems Independent and identically distributed (IID) fading models Actual antennas not IID modeling antenna correlation Models for a single time varying channel Characterize joint distribution of h m (t), h m (t + τ) Coherence time T c implies h m (t) h m (t + τ) for τ < T c

20 Our analysis: starting point Main assumptions Number of antennas N colocated UE b leads to L rays (or paths) Each ray has gain G b,p = 1 L and Base station. angle of arrival θ b,p h m = 1 L L p=1 Beam Path e jφ b,p+j2πm sin(θ b,p ) λ User 1 User 2

21 Our analysis: starting point Main assumptions Number of antennas N colocated UE b leads to L rays (or paths) Each ray has gain G b,p = 1 L and Base station. angle of arrival θ b,p h m = 1 L L p=1 Beam Path e jφ b,p+j2πm sin(θ b,p ) λ User 1 User 2 Implications For a finite L, there exists antenna correlations As L, h CN (0, I)

22 Our analysis: starting point Main assumptions Number of antennas N colocated UE b leads to L rays (or paths) Each ray has gain G b,p = 1 L and Base station. angle of arrival θ b,p h m = 1 L L p=1 Beam Path e jφ b,p+j2πm sin(θ b,p ) λ User 1 User 2 Implications For a finite L, there exists antenna correlations As L, h CN (0, I) IID Rayleigh fading

23 Coherence time as a function of N Fact φ b,p [t] changes much faster than G b,p [t] or θ b,p [t] (10 to 1000 )

24 Coherence time as a function of N Fact φ b,p [t] changes much faster than G b,p [t] or θ b,p [t] (10 to 1000 ) Angle of arrival Instantaneous phase Time (s) θ 1,1 variation with time Time (s) φ 1,1 variation with time

25 Coherence time as a function of N Fact φ b,p [t] changes much faster than G b,p [t] or θ b,p [t] (10 to 1000 ) 5 Channel coefficient amplitude evolution in time Element 1 Element Channel coefficient phase evolution in time Element 1 Element 2 4 Amplitude of channel coefficient Phase of channel coefficient Time (s) h 1 and h 2 variation Time (s) Phase variation

26 Coherence time as a function of N Fact φ b,p [t] changes much faster than G b,p [t] or θ b,p [t] (10 to 1000 ) 5 Channel coefficient amplitude evolution in time Element 1 Element Channel coefficient phase evolution in time Element 1 Element 2 4 Amplitude of channel coefficient Phase of channel coefficient Time (s) h 1 and h 2 variation Time (s) Phase variation h m changes much faster than spatial parameters θ or G

27 Quantifying coherence time Idea When N, becomes possible to track θ, G 35 Time 0, 8 antennas 400 Time 0, 256 antennas Amplitude Amplitude Angle in the beamspace Angle in the beamspace DFT(h) for N = 8 DFT(h) for N = 256 { } N 1 N 1 1 DFT(h) = h n e j2πkn/n N n=0 k=0 Coherence time of r = n h nω n 2 can be much larger

28 Quantifying coherence time Idea When N, becomes possible to track θ, G 35 Time 0, 8 antennas 400 Time 0, 256 antennas Amplitude Amplitude Angle in the beamspace Angle in the beamspace DFT(h) for N = 8 DFT(h) for N = 256 { } N 1 N 1 1 DFT(h) = h n e j2πkn/n N n=0 Coherence time of r = i hn 2 N k=0 can be much larger

29 Quantifying coherence time T c Idea For any time series r(t), look at A r (t, τ) = E[r(t)r(t + τ)] A r (0, τ) 1 3 db 0 T c τ

30 Autocorrelation function plots for different values of N Ar(0, τ)/ar(0, 0) N = 1 N = 2 N = 4 N = τ Note that coherence time increases for increasing N

31 Thus... Coherence times can be large for large antenna arrays

32 Thus... Coherence times can be large for large antenna arrays Rest of the talk... How do we exploit that?

33 Plan Why large antenna arrays? Channel models for MIMO large antenna arrays Noncoherent schemes Transmission and detection schemes Conclusions

34 Main assumptions Fast changing channel unknown, slow fading channel known Slow changing information depends on the number of multipath components L When L large, statistics of the channel known energy σh 2 for Rayleigh fading line of sight (LOS) and non-los energy for Rician fading For small L, the spatial parameters (θ and G) are known Beam Path User 1 Base station. User 2

35 Related (prior) work Noncoherent capacity Capacity when the channel realization is unknown at the transmitter and the receiver Capacity/bounds with knowledge only of channel statistics Characterization of noncoherent capacity achieving distributions Error-exponent optimal distributions for channels

36 Related (prior) work Noncoherent capacity Capacity when the channel realization is unknown at the transmitter and the receiver Capacity/bounds with knowledge only of channel statistics Characterization of noncoherent capacity achieving distributions Error-exponent optimal distributions for channels We focus instead on the one shot communication problem

37 The one-shot problem Base station has an N element linear antenna array User equipments (UEs) have 1 antenna each Uplink (single antenna UEs to base station) y = h i x i + ν i Downlink (base station to UEs). User 1 User 1 User 2 y = h T i x + ν. User 2 Objective How do we choose transmissions, and how do we detect them?

38 The one-shot problem Base station has an N element linear antenna array User equipments (UEs) have 1 antenna each Uplink (single antenna UEs to base station) y = h i x i + ν i Downlink (base station to UEs). User 1 User 1 User 2 y = h T i x + ν. User 2 Short answer Depends heavily on channel characteristics and N

39 Channel models considered in the next few slides Narrowband IID Rayleigh fading models Uplink Downlink Wideband IID fading model Uplink Downlink Ray tracing models (uplink and downlink)

40 Observation In the uplink, Discussions Narrowband IID Rayleigh fading uplink y f( y 2, x) One-shot detection performance depends only on the received energy y n = y 2 N con- almost As N increases, verges to x 2 + σ 2 surely At detector use threshold energy detection

41 Optimizing transmit constellations Idea The width of the histogram can be derived from the rate function I x (d) which satisfies ( y 2 Prob N x 2 σ 2 < d =. ) e NIx(d) Design criterion Choose {x} and thresholds to minimize probability of error or maximize min x I x (detection threshold width for x)

42 Optimizing transmit constellations Idea The width of the histogram can be derived from the rate function I x (d) which satisfies ( y 2 Prob N x 2 σ 2 < d =. ) e NIx(d) Design criterion Choose {x} and thresholds to minimize probability of error or maximize min x I x (detection threshold width for x) Capacity scaling result For a large N, and a fixed number of users, the rate scales as Θ(log(N)) with a vanishing probability of error

43 Optimizing transmit constellations Idea The width of the histogram can be derived from the rate function I x (d) which satisfies ( y 2 Prob N x 2 σ 2 < d =. ) e NIx(d) Design criterion Choose {x} and thresholds to minimize probability of error or maximize min x I x (detection threshold width for x) Capacity scaling result For a large N, and a fixed number of users, the rate scales as Θ(log(N)) with a vanishing probability of error Knowing the channel h does not improve capacity scaling

44 Narrowband IID Rayleigh fading downlink Observation y = h T x + ν CN (0, σ 2 + P ), independent of N Result Under an average transmit power constraint x 2 P, increasing N does not improve detection performance

45 Narrowband IID Rayleigh fading downlink Observation y = h T x + ν CN (0, σ 2 + P ), independent of N Result Under an average transmit power constraint x 2 P, increasing N does not improve detection performance With IID fading, adding antennas does not help in the downlink

46 Wideband IID Rayleigh fading uplink Setting B parallel narrowband channels (frequency bins) under total power constraint P bin 0 bin 1 bin 3... bin B Results If B is fixed and N, rates Θ(log 2 (N)) achievable If B and N fixed, rates proportional to Θ(1) achievable Joint scaling : if B = N ɛ, for 0 < ɛ < 0.5, rates close to Θ(B log 2 (N)) achievable for ɛ > 0.5, rates cannot be better than Θ(N 0.5 )

47 Wideband IID Rayleigh fading uplink Setting B parallel narrowband channels (frequency bins) under total power constraint P bin 0 bin 1 bin 3... bin B Results If B is fixed and N, rates Θ(log 2 (N)) achievable If B and N fixed, rates proportional to Θ(1) achievable Joint scaling : if B = N ɛ, for 0 < ɛ < 0.5, rates close to Θ(B log 2 (N)) achievable for ɛ > 0.5, rates cannot be better than Θ(N 0.5 ) Too much channel uncertainty for ɛ > 0.5, bandwidth limited below ɛ < 0.5

48 Beyond IID: ray tracing models Ray tracing with finite number of multipath components For a large enough N, no noise or interuser interference For a finite N, may experience significant interference 400 Time 0, 256 antennas 300 Amplitude Angle in the beamspace

49 Beyond IID: ray tracing models Ray tracing with finite number of multipath components For a large enough N, no noise or interuser interference For a finite N, may experience significant interference Reasons for performance degradation Outage: Two rays closer than from each other cause destructive or constructive interference Inter-path interference (IPI): Rays or paths not in outage also interfere Additive receiver noise Objective Study number of users B supported with a vanishing outage and good detection error performance

50 Our result Theorem For both uplink and downlink, if the number of users B = o( N) then one can choose with a vanishing outage probability and with a non-outage diversity gain same as the one with a single user Diversity gain definition lim N log(probability of error) N

51 Our result Theorem For both uplink and downlink, if the number of users B = o( N) then one can choose with a vanishing outage probability and with a non-outage diversity gain same as the one with a single user Diversity gain definition log(probability of error) lim N N With a slowly growing number of users, multiuser interference vanishes even if fast fading not known

52 Plan Why large antenna arrays? Channel models for MIMO large antenna arrays Noncoherent schemes Transmission and detection schemes Conclusions

53 Noncoherent transceivers Why noncoherent? Do not have to track instantaneous phase/fast fading Simpler design Energy efficient Lower channel feedback rate requirements

54 Noncoherent transceivers Why noncoherent? Do not have to track instantaneous phase/fast fading Simpler design Energy efficient Lower channel feedback rate requirements Examples of noncoherent architectures

55 Noncoherent transceivers Why noncoherent? Do not have to track instantaneous phase/fast fading Simpler design Energy efficient Lower channel feedback rate requirements Examples of noncoherent architectures Energy detectors

56 Noncoherent transceivers Why noncoherent? Do not have to track instantaneous phase/fast fading Simpler design Energy efficient Lower channel feedback rate requirements Examples of noncoherent architectures Energy detectors Fourier transform

57 Noncoherent transceivers Why noncoherent? Do not have to track instantaneous phase/fast fading Simpler design Energy efficient Lower channel feedback rate requirements Examples of noncoherent architectures Energy detectors Fourier transform Other unitary precoders/receiver shaping transforms not dependent on fast fading

58 Benefits from noncoherent transceiver antenna array fronthaul (9 Gbps) baseband unit Bit rate requirements Fronthaul limits the number of antennas that can be supported N 9 Gbps 2 bits per sample bandwidth in hertz no. of sectors 5 Noncoherent architectures are independent of the fronthaul rate limits Energy consumption RF front ends for each antenna infeasible Energy consumption for analog-to-digital converters high Efficient architectures for energy detection or fixed precoders

59 Plan Why large antenna arrays? Channel models for MIMO large antenna arrays Noncoherent schemes Transmission and detection schemes Conclusions

60 Conclusions Noncoherent architectures promise benefits for large antenna arrays. We discussed how: channel fading is different noncoherent schemes are optimal according to various metrics noncoherent transceiver architectures are practical

61 Conclusions Noncoherent architectures promise benefits for large antenna arrays. We discussed how: channel fading is different noncoherent schemes are optimal according to various metrics noncoherent transceiver architectures are practical TODO... Test theory on real implementations

62 The end Thank you for your attention!