Abstract
We consider a two-way half-duplex decode-and-forward (DF) relaying system with multiple pairs of single-antenna users assisted by a cell-free (CF) massive multiple-input multiple-output (mMIMO) architecture with multiple-antenna access points (APs). Under the practical constraint of imperfect channel state information (CSI), we derive the achievable sum spectral efficiency (SE) for a finite number of APs with maximum ratio (MR) linear processing for both reception and transmission in closed-form. Notably, the proposed CF mMIMO relaying architecture, exploiting the spatial diversity, and providing better coverage, outperforms the conventional collocated mMIMO deployment. Moreover, we shed light on the power-scaling laws maintaining a specific SE as the number of APs grows. A thorough examination of the interplay between the transmit powers per pilot symbol and user/APs takes place, and useful conclusions are extracted. Finally, differently to the common approach for power control in CF mMIMO systems, we design a power allocation scheme maximizing the sum SE.
Highlights
M ASSIVE multiple-input multiple-output systems, where a large number of antennas in both collocated and distributed setups serves simultaneously a lower number of users, has become one of the key fifthgeneration (5G) physical-layer technologies towards higher throughput and energy efficiency by means of simple linear signal processing [1], [2]
A CF M ASSIVE multiple-input multiple-output (mMIMO) system includes a large number of single-antenna access points (APs) that is connected to a central processing unit (CPU) and serves jointly all users by means of coherent joint transmission/reception
Authors in [5] achieved better data rates by means of a user-centric approach, where the APs serve a group of users instead of all of them, while in [9], the realistic spatial randomness of the APs was taken into account by means of a Poisson point process (PPP) to obtain the coverage probability in CF mMIMO systems
Summary
M ASSIVE multiple-input multiple-output (mMIMO) systems, where a large number of antennas in both collocated and distributed setups serves simultaneously a lower number of users, has become one of the key fifthgeneration (5G) physical-layer technologies towards higher throughput and energy efficiency by means of simple linear signal processing [1], [2]. We derive the achievable sum SE of a two-way CF mMIMO relaying network employing the DF protocol with imperfect CSI and linear processing by means. We carry out an asymptotic analysis for this architecture to investigate the power-scaling laws maintaining a specific SE as the number of APs increases This is the unique work on CF mMIMO relaying that obtains power scaling laws, which are different compared to the scaling laws in [21] since they include summations with respect to the number of APs and the relevant variables correspond to different APs instead of a relay with collocated antennas. The application of other linear techniques such as zero-forcing presents certain trade-offs between complexity and performance They demand more backhaul, which might be prohibitive in the case of large distributed networks (CF mMIMO systems). B ∼ CN (0, ) represents a circularly symmetric complex Gaussian vector with zero mean and covariance matrix
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