From: Application of cell-free massive MIMO in 5G and beyond 5G wireless networks: a survey
Ref. | Focus and coverage | Key findings | Limitations | Year |
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[62] | This survey dwells on incorporating channel hardening in CF-MIMO using stochastic geometry while evaluating potential constraints to its practical implementation. A homogenous PPP is explored to model the AP distribution. In addition, the effect of channel hardening on AP density is evaluated. | â–ª The study reveals that channel hardening in CF-mMIMO mainly depends on the number of antennas per AP and the pathloss exponent of the propagation environment. â–ª A synergy between smaller pathloss exponent and multiple antennas per AP provides a modest level of channel hardening. | â–ª The capacity bounds obtained from cellular massive MIMO are not so tight in CF-mMIMO. â–ª Channel hardening comes at the cost of reduced macro-diversity, which is undesirable in practice. | 2017 |
[64] | The authors considered the channel hardening effect extensively in CF massive MIMO under practical operational conditions. Closed-form expressions are presented for the hardening coefficients, taking cognizance of pilot contamination, power loading, and MMSE channel estimation. | â–ª Channel hardening is substantially improved with the NCB precoder compared to the CB scheme. | â–ª The effects of spatial correlation at the APs are not considered. â–ª The system is largely affected by pilot contamination, given a small coverage area. | 2020 |
[133] | An enhanced normalized CB (ECB) precoding is proposed to boost the channel hardening effect in CF massive MIMO. An exact closed-form expression for the achievable DL SE concerning the pilot contamination, channel estimation errors, lack of CSI at the user side, along with an optimal max-min fairness power allocation scheme, is presented. | â–ª The effective channel is nearly deterministic, thus, validating the effectiveness of the proposed ECB. â–ª The DL SE is significantly improved compared to the case with CB. | â–ª The proposed scheme does not support interference suppression. â–ª Acquiring CSI for the users is quite challenging. | 2021 |