The high capacity demands of 5G communications require multiple input multiple output (MIMO) technologies and millimeter wave communications to enables the required multi-Gbps. The small wavelengths of millimeter wave communications allow packing a huge number of antennas in a small space, enabling massive MIMO schemes, with the associated high capacity gains. The hardware impairments inherent in low complexity hardware implementations are characteristic of these systems. This means strong nonlinear effects throughout the transmission chain that will have negative impact on the system's performance. In this paper we study the impacts of these nonlinear effects on the overall performance of an OFDM massive MIMO communication employing a hybrid analog/digital architecture. For this purpose, we develop analytical and semi-analytical tools for performance evaluation of nonlinear effects in those systems. These tools are shown to be very accurate. They make it possible to predict the nonlinearities' penalties in hybrid systems and/or to assess the optimal operation point for a specific nonlinear amplifier.

This paper considers a hybrid two-step space-frequency receiver structure for multiuser uplink millimeter-wave (mmWave) massive MIMO based systems. We adopt constant envelope OFDM (CE-OFDM) since it is a promising modulation technique for future systems, which allows very efficient power amplification. We consider low-complexity user terminals employing a single RF chain and using an analog precoding based only on the average angles of departure of the channel. At the base station a non-linear multi-user equalizer, based on the iterative block decision feedback equalization (IB-DFE) principle, and using a hybrid analog-digital structure, is designed to efficiently remove the inter-user and inter-carrier interferences. This equalizer is designed to minimize the sum of the mean square errors of all subcarriers. Numerical results have shown that the proposed scheme outperforms the linear equalizers. Moreover, performance is already close to the full digital counterpart after only a few iterations.

In order to fulfil the high capacity demands of 5G communications, massive MIMO and millimetre wave are two enabling key technologies considered for future wireless communication. Although these technologies allow huge capacity gains, the radio frequency transceivers employed in this systems induce several hardware impairments as noise quantization (due to resolution quantizers), nonlinear power amplifiers, frequency and phase offsets and in-phase and quadrature imbalance, which lead to a strong nonlinear distortion in the transmitted signal. The impact of these nonlinear effects on the overall performance of hybrid analog-digital massive MIMO millimetre wave OFDM systems is studied in this work, highlighting the importance of its knowledge to improve communication quality. Theoretical analysis of the distortion spectral signal in transmission is assessed and validated for these communication systems, using the quasi-Gaussian nature of the input nonlinear signals. The nonlinearity impact in these systems was observed for different scenarios, where the value of saturation level shows to have a strong influence on the overall system performance.

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative frequency-domain detection receivers based on the IB-DFE concept (Iterative Block Decision Feedback Equalization) can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In this paper we combine iterative IA precoding at the transmitter with IB-DFE based processing at the receiver for MC-CDMA systems. The receiver is designed in two steps: first a linear filter is used to mitigate the inter-user aligned interference, and then an iterative frequency-domain receiver is designed to efficiently separate the spatial streams in the presence of residual inter-user aligned interference at the output of the filter. The matrices for this non-linear space-frequency equalizer are obtained by minimizing the overall mean square error (MSE) of all data streams at each subcarrier. Our receiver structure is explicitly designed taking into account the residual inter-user interference, allowing both an efficient separation of the spatial streams and a reduction in the number of iterations of the IA procedure. We also propose a simple, yet accurate analytical approach for obtaining the performance of the proposed receiver structure. Our scheme achieves the maximum degrees of freedom provided by the IA precoding, while allowing an almost optimum space-diversity gain, with performance close to the matched filter bound (MFB).

We propose a data precoded relay-assisted scheme, for a system cooperating with L relays, each one equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where either the direct path is not available or has poor quality. However because of the half duplex constraint at the relays, transmission of a data rate that would be possible using a modulation technique with m bits per symbol in the case a continuous link was available from the BS to the UT, requires the use of a constellation with 2m bits per symbol. This implies a penalty in the power efficiency. In this communication we propose a simple precoding scheme that exploits the relation between QPSK and 4L-QAM, by alternately transmitting through the L relays, keeping the diversity of order L. Numerical results are presented for the case where 2 relay nodes are used, for realistic scenarios with LTE parameters and using turbo coding, showing significant performance improvements relatively to the case of a relay based distributed Alamouti employing 16-QAM.

We propose a data precoded relay-assisted scheme, for a system cooperating with 2 relays. We assume that all the terminals are equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where a direct path is not available. However because of the half duplex constraint at the relays, transmission of a data rate equivalent to that of a modulation technique with m bits per symbol in the case a continuous link would be available from the BS to the UT requires the use of a constellation with 2m bits per symbol. This implies a penalty in power efficiency. The proposed simple precoding scheme exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, keeping the diversity of order 2. Numerical results show significant performance improvements relatively to the case of a relay-based distributed Alamouti employing 16-QAM, and a performance very close to the non-cooperative system employing 2x1 QPSK Alamouti coding with a continuous link available, when all the links have the same SNR.

In this paper we investigate the performance of single-relay cooperative scheme, where the source is equipped with a 2-antenna array and both the relay and destination with a single antenna. We consider Amplify-and-Forward, Decode-and-Forward and Selective Decode-and-Forward relaying protocols and a space-frequency block coding designed for DL OFDM based systems is employed. Signals expressions are derived for each relay protocol under various scenarios, considering power constraint imposed on cooperating nodes. The aim of this work is to evaluate this Virtual MIMO scheme in realistic scenarios (e.g. correlated antennas, precise channel models, Doppler effects) for implementation purposes in a system with LTE based parameters and considering channel turbo coding. The coded performance of the proposed Virtual MIMO scheme is evaluated, and compared against non-cooperative OFDM based systems, for typical pedestrian scenarios based on LTE specifications. The proposed cooperative scheme outperforms the co-located MISO and MIMO for some scenarios.