Anastasios K. Papazafeiropoulos

Sum-Rate and Power Scaling of Massive MIMO Systems With Channel Aging

This paper investigates the achievable sum-rate of massive multiple-input multiple-output (MIMO) systems in the presence of channel aging. For the uplink, by assuming that the base station (BS) deploys maximum ratio combining (MRC) or zero-forcing (ZF) receivers, we present tight closed-form lower bounds on the achievable sum-rate for both receivers with aged channel state information (CSI). In addition, the benefit of implementing channel prediction methods on the sum-rate is examined, and closed-form sum-rate lower bounds are derived. Moreover, the impact of channel aging and channel prediction on the power scaling law is characterized. Extension to the downlink scenario and multicell scenario is also considered. It is found that, for a system with/without channel prediction, the transmit power of each user can be scaled down at most by 1/√M (where M is the number of BS antennas), which indicates that aged CSI does not degrade the power scaling law, and channel prediction does not enhance the power scaling law; instead, these phenomena affect the achievable sum-rate by degrading or enhancing the effective signal to interference and noise ratio, respectively.

The α - λ - μ and α - η - μ Small-Scale General Fading Distributions: A Unified Approach

In this paper, a general small-scale fading model for wireless communications, that explores the nonlinearity and at the same time the inhomogeneous nature of the propagation medium, is presented, studied in terms of its first-order statistics of the envelope, and validated by means of field measurements and the Monte Carlo simulation. It is indeed a novel distribution with many advantages such as its generality, its physical interpretation that is directly associated with the propagation channel, and its mathematical tractability due to its simple and closed-form expression. By fitting to measurement data, it has been shown that the proposed distribution outperforms the widely known fading distributions. Namely, the α − λ − μ model, which can be in fact called α − η − μ format 2 model, can also be obtained from the α − η − μ format 1 model by a rotation of the axes. Both formats are combined, in order to result to a unified model in a closed form that may describe the propagation environment in a variety of different fading conditions. Its physical background is hidden behind the names of its parameters. The unified model includes the already known general distributions α − μ′, η − μ, λ − μ (η − μ format 2), and their inclusive ones as special cases.

Uplink performance of massive MIMO subject to delayed CSIT and anticipated channel prediction

We consider a realistic cellular multi-user uplink channel with an excess of base station (BS) antennas, where the number of BS antennas and the number of users per cell increase at the same rate. Specifically, we investigate the negative impact of delayed channel state information at the transmitter (CSIT), when a minimum-mean-square-error (MMSE) detector is applied. Nevertheless, channel prediction is used for overcoming delayed CSIT degradation. We provide the asymptotic signal-to-interference-plus-noise ratios (SINRs) that demonstrate not only their dependence on delayed and predicted CSIT, but also the outperformance of MMSE against maximal ratio combiner (MRC). The deterministic nature of the results renders them easily computed, while simulations show their accuracy even for practical system dimensions.

Effect of channel aging on the sum rate of uplink massive MIMO systems

This paper investigates the achievable sum-rate of uplink massive multiple-input multiple-output (MIMO) systems considering a practical channel impairment, namely, aged channel state information (CSI). Taking into account both maximum ratio combining (MRC) and zero-forcing (ZF) receivers at the base station, we present tight closed-form lower bounds on the sum-rate for both receivers, which provide efficient means to evaluate the sum-rate of the system. More importantly, we characterize the impact of channel aging on the power scaling law. Specifically, we show that the transmit power of each user can be scaled down by 1/√(M), which indicates that aged CSI does not affect the power scaling law; instead, it causes only a reduction on the sum rate by reducing the effective signal-to-interference-and-noise ratio (SINR).

Linear precoding for downlink massive MIMO with delayed CSIT and channel prediction

We consider a multi-cell multi-user downlink channel of a time-division duplex (TDD) MIMO system, where the base stations (BSs) employ the concept of massive MIMO, i.e., they are equipped with a large number of antennas. In addition, the number of users increases with the same speed. Focusing on the practical impairments of the channel such as pilot contamination and, in particular, delayed channel state information at the transmitter (CSIT), we derive an approximation of the sum rate with regularized zero-forcing (RZF) precoding, which provides a quantification of the capacity loss. As a result, it is deemed necessary to obtain the deterministic equivalent sum rate by incorporating in our analysis channel prediction circumventing the degradation due to delayed CSIT. The proposed results are accurate for realistic system dimensions, as simulations testify. Finally, we show the benefits of applying RZF in the sum rate against using eigenbeamforming (BF) for the same Doppler shift with no extra computational complexity.

Impact of Transceiver Impairments on the Capacity of Dual-Hop Relay Massive MIMO Systems

Despite the deleterious effect of hardware impairments on communication systems, most prior works have not investigated their impact on widely used relay systems. Most importantly, the application of inexpensive transceivers, being prone to hardware impairments, is the most cost- efficient way for the implementation of massive multiple-input multiple-output (MIMO) systems. Consequently, the direction of this paper is towards the investigation of the impact of hardware impairments on MIMO relay networks with large number of antennas. Specifically, we obtain the general expression for the ergodic capacity of dual-hop (DH) amplify-and-forward (AF) relay systems. Next, given the advantages of the free probability (FP) theory with comparison to other known techniques in the area of large random matrix theory, we pursue a large limit analysis in terms of number of antennas and users by shedding light to the behavior of relay systems inflicted by hardware impairments.

Impact of user mobility on optimal linear receivers in cellular networks

We consider the uplink of non-cooperative multicellular systems deploying multiple antenna elements at the base stations (BS), covering both the cases of conventional and very large number of antennas. Given the inevitable pilot contamination and an arbitrary path-loss for each link, we address the impact of time variation of the channel due to the relative movement between users and BS antennas, which limits systems performance even if the number antennas is increased, as shown. In particular, we propose an optimal linear receiver (OLR) maximizing the received signal-to-interference-plus-noise (SINR). Closed-form lower and upper bounds are derived as well as the deterministic equivalent of the OLR is obtained. Numerical results reveal the outperformance of the proposed OLR against known linear receivers, mostly in environments with high interference and certain user mobility, as well as that massive MIMO is preferable even in time-varying channel conditions.

MMSE filtering performance of DH-AF massive MIMO relay systems with residual transceiver impairments

The emerging requirements of the fifth generation (5G) wireless communications are high spectral efficiency, low latency, and ubiquitous coverage. In this direction, Dual-Hop (DH) Amplify-and-Forward (AF) relaying has been widely investigated due to its simplicity, low implementation complexity and low transmission delay. However, most existing works assume ideal transceiver hardware which is impractical. In practice, a cost-efficient wireless transceiver has to combat the effects of several inevitable impairments such as high power amplifier nonlinearities, In-phase/Quadrature-phase (I/Q)-imbalance, and oscillator phase noise, which can be only partially compensated using calibration algorithms. In this direction, this paper analyzes the Minimum Mean Square Error (MMSE) filtering performance of a DH-AF Multiple-Input-Multiple-Output (MIMO) wireless system considering the effects of the residual additive impairments at the transmitter and receiver of both hops. Using free probability principles, the MMSE filtering performance of the considered system is studied and a tight lower bound is proposed by taking the effects of residual additive transceiver impairments into account. Our numerical results show that the MMSE filtering performance of the DH-AF massive MIMO relay system significantly degrades and results to saturation in the presence of residual additive transceiver impairments.

An Extended Generalized Rice Model for Wireless Communications

The multipath component arriving at the mobile receiver cannot explicitly be of homogeneous nature due to nonuniform scattering caused by objects with irregular surfaces having various scattering properties. Another reason, which may contribute to this inhomogeneity, is the nonlinearity of the propagation medium. In this paper, a new general statistical small-scale model for wireless channels, which is affected by nonfrequency-selective fading, is proposed to describe the received signal level by taking into account the aforementioned effects. The advantageous physical interpretation of the model is introduced, and solutions for the probability density functions (PDFs) of the envelope and the phase as well as the cumulative distribution function (CDF) of the envelope are derived. In addition, the second-order statistics have been studied, and solutions for the level-crossing rate (LCR) and the average duration of fades (ADF) are obtained. The usefulness of the model is demonstrated by providing the best fit, compared with widely known models, to certain measurement data corresponding to a variety of real-world channel conditions. Furthermore, its validation is confirmed by simulation. Consequently, the extended generalized Rice model is applicable in wireless channels subjected to various conditions, and it can be preferable because of its outperformance.

Second-Order Statistics for the Envelope of α - κ - μ Fading Channels

Exact closed-form expressions for the level crossing rate (LCR) and the average duration of fades (ADF) of α - κ - μ fading channels are presented. Confirmation of the derived analytical formulations is attained by reducing the general results to some widely known cases (Nakagami-m, Rice, and Weibull). Extra validation is performed by comparison with measurement data and by means of a sample of numerical results obtained by the Monte Carlo simulation.