Abstract
Cell-free (CF) massive multiple-input-multiple-output (mMIMO) deployments are usually investigated with half-duplex nodes and high-capacity fronthaul links. To leverage the possible gains in throughput and energy efficiency (EE) of full-duplex (FD) communications, we consider a FD CF mMIMO system with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">practical limited-capacity fronthaul links</i> . We derive closed-form spectral efficiency (SE) lower bounds for this system with maximum-ratio combining/maximum-ratio transmission processing and optimal uniform quantization. We then optimize the weighted sum EE (WSEE) via downlink and uplink power control by using a two-layered approach: the first layer formulates the optimization as a generalized convex program, while the second layer solves the optimization decentrally using the alternating direction method of multipliers. We analytically show that the proposed two-layered formulation yields a Karush-Kuhn-Tucker point of the original WSEE optimization. We numerically show the influence of weights on the individual EE of the users, which demonstrates the utility of the WSEE metric to incorporate heterogeneous EE requirements of users. We show that low fronthaul capacity reduces the number of users each AP can support, and the cell-free system, consequently, becomes user-centric.
Highlights
Massive multiple-input-multiple-output wireless systems employ a large number of antennas at the base stations (BSs), and achieve higher spectral efficiency (SE) and energy efficiency (EE) with relatively simple signal processing [1], [2]
Two distinct massive multiple-input-multiple-output (mMIMO) variants are being investigated in the literature: i) co-located, wherein all antennas are located at one place [1]; and ii) distributed, wherein antennas are spread over a large area [2, and the references therein], [3]– [5]
We model the uplink downlink interference (UDI) on the downlink and the residual intra-/inter-AP interference (RI) on the uplink, but unlike existing FD CF mMIMO literature [12]–[14], [21], we consider the quantization distortion due to limited-capacity fronthaul links, as modelled in the total quantization distortion (TQD) terms
Summary
Massive multiple-input-multiple-output (mMIMO) wireless systems employ a large number of antennas at the base stations (BSs), and achieve higher spectral efficiency (SE) and energy efficiency (EE) with relatively simple signal processing [1], [2]. Unlike existing FD CF mMIMO literature [12]–[14], [21], which consider perfect high-capacity fronthaul links, it is critical to model and analyze the UDI and inter-/intra-AP interferences and limited-capacity impairments while deriving lower bounds for both uplink and downlink UEs SE, which are valid for arbitrary number of antennas at each AP. Reference [21] is the only work so far which optimized the EE of FD CF mMIMO It considered a novel SE-GEE objective, which reduces to a PC function and is maximized using a Dinkelbach-like algorithm. We introduce separate Lagrangian parameters for the downlink and uplink UEs, and separate penalty parameters for the downlink and uplink power control variables This enables us to properly define the augmented Lagrangian and decouple the respective sub-problems at the D-servers which calculate the local solutions, and eventually coordinate them into the globally optimal solution at the C-server.
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