14 research outputs found
Downlink Training in Cell-Free Massive MIMO: A Blessing in Disguise
Cell-free Massive MIMO (multiple-input multiple-output) refers to a
distributed Massive MIMO system where all the access points (APs) cooperate to
coherently serve all the user equipments (UEs), suppress inter-cell
interference and mitigate the multiuser interference. Recent works demonstrated
that, unlike co-located Massive MIMO, the \textit{channel hardening} is, in
general, less pronounced in cell-free Massive MIMO, thus there is much to
benefit from estimating the downlink channel. In this study, we investigate the
gain introduced by the downlink beamforming training, extending the previously
proposed analysis to non-orthogonal uplink and downlink pilots. Assuming
single-antenna APs, conjugate beamforming and independent Rayleigh fading
channel, we derive a closed-form expression for the per-user achievable
downlink rate that addresses channel estimation errors and pilot contamination
both at the AP and UE side. The performance evaluation includes max-min
fairness power control, greedy pilot assignment methods, and a comparison
between achievable rates obtained from different capacity-bounding techniques.
Numerical results show that downlink beamforming training, although increases
pilot overhead and introduces additional pilot contamination, improves
significantly the achievable downlink rate. Even for large number of APs, it is
not fully efficient for the UE relying on the statistical channel state
information for data decoding.Comment: Published in IEEE Transactions on Wireless Communications on August
14, 2019. {\copyright} 2019 IEEE. Personal use of this material is permitted.
Permission from IEEE must be obtained for all other use
Joint Design of Power Control and Access Point Scheduling for Uplink Cell-Free Massive MIMO Networks
This work proposes a joint power control and access points (APs) scheduling
algorithm for uplink cell-free massive multiple-input multiple-output
(CF-mMIMO) networks without channel hardening assumption. Extensive studies
have done on the joint optimization problem assuming the channel hardening.
However, it has been reported that the channel hardening may not be validated
in some CF-mMIMO environments. In particular, the existing Use-and-then-Forget
(UatF) bound based on the channel hardening often seriously underestimates user
rates in CF-mMIMO. Therefore, a new performance evaluation technique without
resorting to the channel hardening is indispensable for accurate performance
estimations. Motivated by this, we propose a new bound on the achievable rate
of uplink CF-mMIMO. It is demonstrated that the proposed bound provides a more
accurate performance estimate of CF-mMIMO than that of the existing UatF bound.
The proposed bound also enables us to develop a joint power control and APs
scheduling algorithm targeting at both improving fairness and reducing the
resource between APs and a central processing unit (CPU). We conduct extensive
performance evaluations and comparisons for systems designed with the proposed
and existing algorithms. The comparisons show that a considerable performance
improvement is achievable with the proposed algorithm even at reduced resource
between APs and CPU.Comment: 30 pages, 7 Figures. This work has been submitted to the IEEE for
possible publication. Copyright may be transferred without notice, after
which this version may no longer be accessibl