Performance Analysis of Cell-free and User-Centric MIMO Networks with Optical Fronthaul and Backhaul Links
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Cell-free massive MIMO (CF-mMIMO) network and its variations such the user-centric (UC) one are considered as promising techniques for the forthcoming wireless networks. Also, investigating effects of fronthaul and backhaul links connecting access points (APs) to central processing unit (CPU) is of interest to make it more practical. Thus, we study uplink of CF- and UC-mMIMO networks wherein free space optical fronthauls connect APs to serving aggregation nodes (ANs), and the ANs are connected to the CPU by fiber optic backhauls. To derive the achievable rates, we firstly obtain channel state information by use of uplink pilot transmissions, and then data recovery is performed at the CPU. Moreover, it is assumed that devices at APs and ANs are not ideal and distort the received information/pilot signals. For the CF- and UC-mMIMO setups, optimal transmission power allocations at the users, and optical gain coefficients at the APs and ANs are derived to maximize the achievable sum rates in presence of non-ideal hardware devices by investigating two hardware impairment models. Through numerical results, spectral- and energy-efficiency of the proposed networks are analysed and compared in various circumstances to highlight the effects of resource allocations, hardware impairments, size of the network and weather conditions. It is shown that the performance of the UC-mMIMO network outperforms that of the CF-mMIMO one thanks to the less effective hardware impairments.
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