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Transmission strategies for cell-free massive mimo with limited capacity fronthaul links

Hamed Masoumi, Mohammad Javad Emadi

TL;DR

The results indicate that at high SNR regime and for large enough capacity of FHLs, estimating channels at the CU rather than APs result in smaller estimation error, and geometric programming power allocations are developed for CFE and ECF to maximize sum rates.

Abstract

We study an uplink scenario of a cell-free massive multiple-input multiple-output (CF-mMIMO) system with limited capacity fronthaul links (LC-FHLs) connecting each access point (AP) to the central unit (CU), where user equipments and APs are subject to hardware impairments. Therefore, to efficiently use the capacity of FHLs to maximize the achievable rate, we analyze three strategies for performing compression and forwarding of channel state information (CSI) and data signals over the LC-FHLs to the CU; Compress-forward-estimate (CFE), estimate-compress-forward (ECF), and estimate-multiply-compress-forward (EMCF). For CFE and EMCF achievable rates are derived, and for ECF one upper and lower bounds are presented which are tight for ideal hardware and FHLs. Also for forwarding the quantized version of CSI and data signals of each user, low-complexity fronthaul capacity allocations are proposed for ECF and EMCF strategies, which considerably improve the performance of the system, especially for limited capacity FHLs. Our results indicate that at high SNR regime and for large enough capacity of FHLs, estimating channels at the CU rather than APs result in smaller estimation error. Then, geometric programming power allocations are developed for CFE and ECF to maximize sum rates. Finally, to highlight the performance characteristics of the system numerical results are presented.

Transmission strategies for cell-free massive mimo with limited capacity fronthaul links

TL;DR

The results indicate that at high SNR regime and for large enough capacity of FHLs, estimating channels at the CU rather than APs result in smaller estimation error, and geometric programming power allocations are developed for CFE and ECF to maximize sum rates.

Abstract

We study an uplink scenario of a cell-free massive multiple-input multiple-output (CF-mMIMO) system with limited capacity fronthaul links (LC-FHLs) connecting each access point (AP) to the central unit (CU), where user equipments and APs are subject to hardware impairments. Therefore, to efficiently use the capacity of FHLs to maximize the achievable rate, we analyze three strategies for performing compression and forwarding of channel state information (CSI) and data signals over the LC-FHLs to the CU; Compress-forward-estimate (CFE), estimate-compress-forward (ECF), and estimate-multiply-compress-forward (EMCF). For CFE and EMCF achievable rates are derived, and for ECF one upper and lower bounds are presented which are tight for ideal hardware and FHLs. Also for forwarding the quantized version of CSI and data signals of each user, low-complexity fronthaul capacity allocations are proposed for ECF and EMCF strategies, which considerably improve the performance of the system, especially for limited capacity FHLs. Our results indicate that at high SNR regime and for large enough capacity of FHLs, estimating channels at the CU rather than APs result in smaller estimation error. Then, geometric programming power allocations are developed for CFE and ECF to maximize sum rates. Finally, to highlight the performance characteristics of the system numerical results are presented.

Paper Structure

This paper contains 19 sections, 7 theorems, 60 equations, 13 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Hardware Impairment Model
  4. Pilot Transmission
  5. Uplink Data Transmission
  6. Rate-Distortion Theory
  7. Performance Analysis
  8. Compress-Forward-Estimate Strategy
  9. Channel Estimation
  10. Uplink Achievable Rate
  11. Estimate-Compress-Forward Strategy
  12. Channel Estimation
  13. Uplink Achievable Rate
  14. Estimate-Multiply-Compress-Forward Strategy
  15. Processing at APs
  16. ...and 4 more sections

Key Result

Theorem 1

For the CFE strategy, UE$k$ has the following signal to noise and interference ratio

Figures (13)

  • Figure 1: Cell-free mMIMO with limited capacity fronthaul links.
  • Figure 2: Non-ideal hardware model at the transmitter/receiver.
  • Figure 3: Rate-distortion theoretic test channel.
  • Figure 4: Sum spectral efficiency versus fronthaul capacity for $K=20$ and $M=200$. Solid, dash and dash-dot lines represent perfect hardware, $\{\xi_{r} = 0.8,\ \xi_{t} = 1\}$ and $\{\xi_{r} = 1,\ \xi_{t} = 0.8\}$, respectively.
  • Figure 5: Difference of lower and upper bounds of SSE for ECF with $C=1$ [bits/s/Hz]
  • ...and 8 more figures

Theorems & Definitions (18)

  • Theorem 1
  • proof
  • Remark 1
  • Theorem 2
  • proof
  • Theorem 3
  • proof : Sketch of Proof
  • Remark 2
  • Remark 3
  • Proposition 1
  • ...and 8 more