Designing Multi-User MIMO for Energy Efficiency

1. Designing Multi-User MIMO for Energy Efficiency When is Massive MIMO the Answer? Emil Björnson‡* Joint work with: Luca Sanguinetti‡§, Jakob Hoydis†, and MérouaneDebbah‡ ‡Alcatel-Lucent Chair on Flexible Radio, Supélec, France *Dept. Signal Processing, KTH Royal Institute of Technology, Sweden §Dip. Ingegneria dell’Informazione, University of Pisa, Pisa, Italy †Bell Laboratories, Alcatel-Lucent, Stuttgart, Germany Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

2. Outline • Presentation is based on E. Björnson, L. Sanguinetti, J. Hoydis, M. Debbah, “Designing Multi-User MIMO for Energy Efficiency: When is Massive MIMO the Answer?,” Submitted IEEE WCNC 2014 Preprint available on arXiv: http://arxiv.org/abs/1310.3843. • Main Question • How should a single-cell downlink system be designed to maximize energy efficiency? • Optimization variables: Number of base station antennas Number of active user equipments Data rate guaranteed per user • Conclusions • Result depends strongly on physical layer precoding scheme • Unconventionally many users and antennas can be optimal! Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

3. Introduction Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

4. What are the Expectations? • Tons of Plenary Talks and Overview Articles • Fulfilling dream of ubiquitous wireless connectivity • Expectation: Many Metrics Should Be Improved in 5G • Higher user data rates • Higher area throughput • Great scalability in number of connected devices • Higher reliability and lower latency • Better coverage with more uniform user rates • Improved energy efficiency • These are Conflicting Metrics! • Impossible to maximize all metrics simultaneously • Our goal: High energy efficiency (EE) with uniform user rates Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

5. Multi-User MIMO System • Multi-User Multiple-Input Multiple-Output (MIMO) • One base station (BS) with array of antennas • single-antenna user equipments (UEs) • Downlink: Transmission from BS to UEs • Share a flat-fading carrier • Multi-Antenna Precoding • Spatially directed signals • Signal improved by array gain • Adaptive control of interference • Serve multiple usersin parallel Space-division multiple access(SDMA) Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

6. Multi-User MIMO System (2) • Cell: Area with user location and pathloss distribution • Scheduling: Pick users randomly, with random location • Some UE • Distribution Clean-Slate Design Select and to maximize EE! Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

7. How to Measure Energy Efficiency? • Energy Efficiency in bits/Joule • Conventional Academic Approaches • Maximize throughput with fixed power • Minimize transmit power for fixed throughput • New Problem: Balance throughput and power consumption • Crucial: Account for overhead signaling • Crucial: Use reasonable power consumption model Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

8. System Model:Average Sum Throughput Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

9. Time-Division Duplex (TDD) Protocol • Coherence Period: [channel uses] • Assumption: • Perfect estimation Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

10. Average Sum Throughput • System Model • Precoding vector of User : • Channel vector of User : • Random User Selection, • Channel variances Independent random variables, • Achievable Rate of User : • Signal-to-interference+noise ratio • (SINR) • Average over channels and user selection • Cost of estimation Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

11. Impact of Precoding • What Determines User Rates? • Precoding (vector directions and power allocations) • “Optimal” precoding: Extensive computations – Not efficient • Notation • Matrix form: , • Total radiated power: ) • Heuristic Closed-Form Precoding • Maximum ratio transmission (MRT): • Zero-forcing (ZF) precoding: • Regularized ZF (RZF) precoding: • Maximize • signal • Minimizeinterference • Balance signal and interference Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

12. Uniform User Performance • Assumption: Uniform user performance • Same rate at every user: • Scaling parameter can be optimized • Consequence: • We use ZF in analysis and other precoding for simulation Lemma 1 Consider ZF precoding and the user rates above, the average radiated power is • where depends on UE distribution, propagation, etc. Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

13. System Model:Power Consumption Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

14. Reasonable Power Consumption Model • What Consumes Power? • Examples will motivate our model • Transmit Power: • = Average radiated transmit power • = Efficiency of power amplifier at BS • Transceiver Chains: • = Circuit power / BS antenna (converters, mixers, filters) • = Power of common oscillator at BS • = Circuit power / UE (oscillator, converters, mixer, filters) • Coding and Decoding: • = Power for coding at BS / user • = Power for decoding at each user Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

15. Reasonable Power Consumption Model (2) • Computational Efficiency: operations per Joule • Uplink Channel Estimation: • Only once per coherence period • channel components per user, processed separately • Precoding: • Only once per coherence period • Depends on precoding: • Architectural Costs: • Control signaling, backhaul infrastructure, load-independent processing, etc. Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

16. Reasonable Power Consumption Model (3) • Summary • General model of power consumption: for some parameters and Energy Efficiency for ZF • User rate: • Radiated power: • Design parameters: , , and Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

17. Optimize System Parametersfor Energy Efficiency Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

18. Preliminaries • Our Goal: • Optimize number of antennas • Optimize the (normalized) transmit power • Optimize number of active UEs • Definition • Lambert function, , solves equation • The function is increasing and satisfies • behaves almost as linear: Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

19. Optimal Number of BS Antennas • Find that maximizes EE with ZF precoding: • Observations • Increases sublinearly with power but linearly at high • Increases with circuit power coefficients independent of • Decreases with circuit power coefficients multiplied with Theorem 1 (Optimal ) Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

20. Optimal Transmit Power • Find 𝜌 that maximizes EE with ZF precoding: • Observations • Increases power with number of antennas as • Opposite to recent claim: Power should decrease as • Intuition: Higher circuit power  Use more transmit power • Theorem 2 (Optimal 𝜌) Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

21. Optimal Number of Active UEs • Find that maximizes EE with ZF precoding: where and are fixed. • Observations • Decreases with circuit power coefficients multiplied with or • Increases with the static hardware power • Increases with the propagation parameter • Theorem 3 (Optimal ) • Solution is a root to a quartic polynomial: • Closed-form but very large expressions Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

22. Numerical Illustrations Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

23. Simulation Scenario • Main Characteristics • Circular cell with radius 250 m • Uniform user distribution with 35 m minimum distance • Uncorrelated Rayleigh fading, typical 3GPP pathloss model Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

24. Optimal System Design: ZF Precoding Optimum User rates: as 256-QAM Massive MIMO! Very many antennas, Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

25. Optimal System Design: MRT Optimum User rates: as 64-QAM Single-user transmission! Only exploitprecoding gain Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

26. Why This Huge Difference? • Interference is the Limiting Factor • ZF: Suppress interference actively • MRT: Only indirect suppression by making • More results: RZFZF, same trends under imperfect CSI • 100x • difference • in throughput • Only 2x • difference • in EE Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

27. Energy Efficient to Use More Power? • Recall Theorem 2: Transmit power increases with • Figure shows EE-maximizing power for different • Intuition: More Circuit Power  Use More Transmit Power • Essentially • linear • growth Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

28. Conclusions Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

29. Conclusions • What if a Single-Cell System Designed for High EE? • Need: Reasonable throughput model • Need: Reasonable power consumption model • Contributions • General power consumption model • Closed-form results for ZF: Optimal number of antennas Optimal number of active UEs Optimal transmit power • Observations: More circuit power  Use more transmit power • Numerical Example • ZF/RZF precoding: Massive MIMO system is optimal • MRT precoding: Single-user transmission is optimal • Small difference in EE, but huge difference in throughput! Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)

30. Thank You for Listening! • Questions? • Main reference: • E. Björnson, L. Sanguinetti, J. Hoydis, M. Debbah, “Designing Multi-User MIMO for Energy Efficiency: When is Massive MIMO the Answer?” • Submitted IEEE WCNC 2014Preprint available on arXiv: http://arxiv.org/abs/1310.3843 Greentouch Open Forum, Designing Multi-User MIMO for Energy Efficiency, E. Björnson (Supélec & KTH)