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Performance of Cell-Free Massive MIMO with Rician Fading and Phase Shifts

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Performance of Cell-Free Massive MIMO with Rician Fading and Phase Shifts 1 Performance of Cell-Free Massive MIMO with Rician Fading and Phase Shifts Özgecan Özdogan, Student Member, IEEE, Emil Björnson, Senior Member, IEEE, and Jiayi Zhang, Member, IEEE Abstract—In this paper, we study the uplink (UL) and down- In its canonical form, cell-free massive MIMO uses MR link (DL) spectral efficiency (SE) of a cell-free massive multiple- combining because of its low complexity. Due to the fact that input-multiple-output (MIMO) system over Rician fading chan- MR combining cannot suppress the interference well, some nels. The phase of the line-of-sight (LoS) path is modeled as a uniformly distributed random variable to take the phase-shifts APs receive more interference from other UEs than signal due to mobility and phase noise into account. Considering the power from the desired UE. The LSFD method is proposed in availability of prior information at the access points (APs), the [7] and [8] to mitigate the interference for co-located massive phase-aware minimum mean square error (MMSE), non-aware MIMO systems. This method is generalized for more realistic linear MMSE (LMMSE), and least-square (LS) estimators are spatially correlated Rayleigh fading channels with arbitrary derived. The MMSE estimator requires perfectly estimated phase knowledge whereas the LMMSE and LS are derived without it. first-layer decoders in [9], [10]. The two-layer decoding tech- In the UL, a two-layer decoding method is investigated in order nique is first adapted to cell-free massive MIMO networks in to mitigate both coherent and non-coherent interference. Closed- [11] for a Rayleigh fading scenario. In the concurrent paper form UL SE expressions with phase-aware MMSE, LMMSE, and [12], the LSFD method is studied in a setup with Rician fading LS estimators are derived for maximum-ratio (MR) combining channels where the LoS phase is static. in the first layer and optimal large-scale fading decoding (LSFD) in the second layer. In the DL, two different transmission modes Joint transmission from multiple APs can be either coherent are studied: coherent and non-coherent. Closed-form DL SE (same data from all APs) or non-coherent (different data). expressions for both transmission modes with MR precoding Only the former has been considered in cell-free massive are derived for the three estimators. Numerical results show MIMO, but it requires that the APs are phase-synchronized. A that the LSFD improves the UL SE performance and coherent synchronization method is outlined in [5], [13], [14] without transmission mode performs much better than non-coherent transmission in the DL. Besides, the performance loss due to validation. the lack of phase information depends on the pilot length and it In densely deployed systems, like cell-free massive MIMO, is small when the pilot contamination is low. the channels typically consist of a combination of a semi- Index Terms—Cell-free massive MIMO, Rician fading, phase deterministic LoS path and small-scale fading caused by shift, performance analysis. multipath propagation, which can be modeled as Rician fading [12], [15]. A small change in the UE location may result in a significant phase-shift of the LoS component, but no I. INTRODUCTION change in amplitude. For instance, if the UE moves half Cell-free massive MIMO refers to a distributed MIMO a wavelength away from the AP, the phase of the channel system with a large number of APs that jointly serve a response changes by ±π. Similarly, hardware effects such as smaller number of user equipments (UEs) [2]–[5]. The APs phase noise may create severe shift in the phase. These effects cooperate via a fronthaul network [5] to spatially multiplex are usually neglected in the analysis of Rician fading channels arXiv:1903.07335v2 [cs.IT] 1 Nov 2019 the UEs on the same time-frequency resource, using network by assuming a LoS path with static phase. Especially in high MIMO methods that only require locally obtained channel mobility scenarios, the phase shift in LoS path may have a state information (CSI) [6]. large impact on system performance. Recently, [16] studied a cell-free network that supports both unmanned aerial vehicles Manuscript received March 13, 2019; revised June 13, 2019; accepted (UAVs) and ground UEs where the channels between AP-UAV August 3, 2019. This work was supported in part by ELLIIT, in part by pairs have Rician distribution with uniformly distributed phase the Swedish Research Council, in part by the National Natural Science Foundation of China (Grant Nos. 61601020 and U1834210), and in part by on the LoS paths. the Beijing Natural Science Foundation (Grant Nos. 4182049 and L171005). Each AP needs to learn the channel statistics of each A preliminary version of this manuscript was presented at IEEE SPAWC 2019 UE that it serves, as well as the statistics of the combined [1]. The associate editor coordinating the review of this article and approving it for publication was R. Brown. (Corresponding author: Özgecan Özdogan.) interfering signals, if Bayesian channel estimators are to be Ö. Özdogan and E. Björnson are with the Department of Electrical used. The large-scale fading coefficients can be estimated with Engineering (ISY), Linköping University, Linköping SE-581 83, Sweden (e- a negligible overhead since the coefficients are deterministic mail: [email protected]; [email protected]). J. Zhang is with the School of Electronic and Information Engineer- [17]; several practical methods to estimate these coefficients ing, Beijing Jiaotong University, Beijing 100044, China (e-mail: jiay- using uplink pilots are presented in [18, Section IV]. However, [email protected]) the phase-shifts are harder to estimate since they change as Color versions of one or more of the figures in this article are available online at http://ieeexplore.ieee.org. frequently as the small-scale fading. Depending on the avail- Digital Object Identifier 10.1109/TWC.2019.2935434 ability of channel statistics, the phase-aware MMSE estimator 2 which requires all prior information, LMMSE estimator with only the large-scale fading parameters, or LS estimator with no prior information can be utilized. In this paper, the specific technical contributions are as follows: • We consider Rician fading channels between the APs and UEs, where the mean and variance are different for every AP-UE pair. Additionally, the phases of the LoS paths are modeled as independent and identically distributed (i.i.d.) random variables in each coherence block. • We derive the phase-aware MMSE, LMMSE, and LS channel estimators and obtain their statistics. In the UL, using the estimates for MR combining in the first-layer, we compute closed-form UL achievable SE expressions for two-layer decoding scheme. In the DL, we obtain : AP, : UE, : CPU, : Fronthaul closed-form DL achievable SE expressions for both co- connection herent and non-coherent transmission. Fig. 1: Illustration of a cell-free massive MIMO network. The conference version of this paper [1] only considered fading model since jhm;k j is Rice distributed, but hm;k is not the DL transmission and used the phase-aware MMSE and Gaussian distributed as in many prior works that neglected the LMMSE estimators. phase shift. We assume that hm;k is an independent random Reproducible Research: All the simulation results can be variable for every m = 1;:::; M, k = 1;:::; K and the channel reproduced using the Matlab code and data files available at: realization hm;k in different coherence blocks are i.i.d. https://github.com/emilbjornson/rician-cell-free All APs are connected to a central processing unit (CPU) Notation: Lower and upper case bold letters are used for via a fronthaul network that is error free. The system operates vectors and matrices. The transpose and Hermitian transpose T H in time division duplex (TDD) mode and the uplink (UL) of a matrix A are written as A and A . The superscript and DL channels are estimated by exploiting only UL pilot ¹ º∗ × : denotes the complex conjugate operation. The M M- transmission and channel reciprocity. dimensional matrix with the diagonal elements d1; d2;:::; dM is denoted as diag ¹d1; d2;:::; dM º. The diagonal elements of III. UPLINK CHANNEL ESTIMATION a matrix D are extracted to a M × 1 vector as diag¹Dº = T τ »d1; d2;:::; dM ¼ . The notation X = »xi; j : i = 1;:::; M; j = In each coherence block, p samples are reserved for UL 1;:::; N¼ denotes the M × N matrix. The expectation of a pilot-based channel estimation, using a set of τp mutually random variable X is denoted by E fXg. The expectations are orthogonal pilot sequences. The pilot sequence of UE k is τp ×1 2 taken with respect to all sources of randomness. denotedp by φk 2 C and satisfies kφk k = τp. It is scaled by pˆk , with pˆk being the pilot power, and sent to the APs. p;ap The received signal y 2 Cτp ×1 at AP m is II. SYSTEM MODEL m K We consider a cell-free Massive MIMO system with M APs Õ yp;ap = ppˆ h φ + np ; (2) and K UEs. All APs and UEs are equipped with a single m k m;k k m antenna. The multi-antenna AP case can be straightforwardly k=1 covered by treating each antenna as a separate AP, if it is p ∼ N 2 where nm C 0τp ; σulIτp is additive noise. AP m com- assumed that there is no correlation between the small-scale p;ap putes an inner product between ym and φk to get sufficient fading coefficients (or phase-shifts) [19]. However, for a more statistics for estimation of h . This results in realistic analysis, single-antenna results can be generalized m;k K to multiple antenna case by taking the spatial correlations p H p;ap Õ p H H p between antennas into account.
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