This paper deals with a Maximum Likelihood (ML) receiver for a Single Carrier (SC) signal distorted by a nonlinear Power Amplifier (PA). The well known optimal Matched Filter (MF) loses its optimality when a nonlinear PA is considered. We first propose an optimal receiver based on the ML criterion taking into account the PA nonlinearity. In case of a Additive White Gaussian Noise (AWGN), the ML receiver has to find the minimum Euclidean distance between the received vector and all nonlinearly distorted vectors that the transmitter may send. The exhaustive search solution of this problem has exponential complexity. To reduce the complexity, we propose a sub-optimal receiver that minimizes the Euclidean distance, seen as a cost function, using the Gradient Descent (GD) algorithm. We also present a low complexity version of the GD based receiver which is suitable for a parallel processing implementation. Comparative study of Symbol Error Rate (SER) performance of the proposed receiver against the optimal SC MF based receiver is given.

A multiple input multiple output (MIMO) system with nonlinear power amplifiers (PAs) is considered. The nonlinear PAs distort the transmitted signal causing performance degradation. The common approach for the mitigation of nonlinear distortions is the linearization of each one of the PAs in the MIMO system separately. We propose to apply a more generic approach in the form of a global linearization of the entirenonlinear MIMO channel. The aim of the global linearization is to reduce the nonlinear distortions seen at the receiver. An iterative technique is proposed for the implementation of such a linearization. The proposed technique improves the system performance, including: 1) power efficiency of the PAs, 2) quality of the received signal, measured in terms of symbol error rate, and 3) spectral purity of the transmitted signals.

A Multiple Input Single Output (MISO) system based on Maximum Ratio Transmission (MRT) precoding with nonlinear Power Amplifiers (PAs) is considered. The nonlinear PAs distort the transmitted signal causing performance degradation. The common approach to mitigating nonlinear distortions is linearization of the PAs. For the MISO system we propose to apply a more generic approach in the form of an entire nonlinear MISO channel linearization, applied jointly for all PAs. It is shown that the results of this approach outperform those of the approach based on local linearization of each PA separately. An iterative technique is proposed to implement the entire MISO channel linearization. This technique requires knowledge of the channel state information and a model of the PAs at the transmitter. The performance of separate local and collective joint linearization is evaluated and compared in terms of the Symbol Error Rate (SER).

This paper deals with a Maximum Likelihood receiver for a nonlinearly distorted OFDM signal over a flat channel with AWGN. The nonlinearity destroys the orthogonality between subcarriers, consequently, a per subcarrier decision, used when the linear PA is considered, is no longer optimal. We propose a sub-optimal receiver based on the Maximum Likelihood (ML) criterion. The ML receiver has to find the minimum Euclidean distance between the received vector and a set of all possible OFDM symbols passed through the same nonlinearity. This approach has exponential complexity. To reduce the complexity, we propose a sub-optimal receiver that minimizes the Euclidean distance, seen as a cost function, by the gradient descent algorithm. Unfortunately, due to the nonlinearity, the cost function is non-convex. In order to overcome this obstacle, we propose a method to classify the solution, i.e., to decide if the achieved minimum is local or global. We modify the gradient descent algorithm to avoid convergence to a local minimum. It is shown that the proposed receiver outperforms the simple OFDM and iterative receivers in terms of symbol error rate (SER) performance.

A novel iterative algorithm for transmitter (TX) precoding for a Orthogonal Frequency Division Multiplexing - (OFDM-) Multiple Input Single Output (MISO) system with nonlinear Power Amplifiers (PAs) is proposed. Having a plurality of PAs at the TX, it is possible to synthesize peak power limited signals, transmitted at each TX branch, such that the total distortions seen at the receiver (RX) will be minimal. Two versions of the algorithm are proposed. The first one reduces nonlinear distortions seen at the RX antenna without considering the spectral purity requirements. The second one reduces nonlinear distortions at the RX and also keeps the out-of-band radiation of the TX signals at a low level in order to satisfy spectral purity requirements. For a low Back-Off (BO) scenario, where the nonlinear distortions are strong, the proposed algorithms show a much lower Symbol Error Rate (SER) and much higher power efficiency than the well known Maximum Ratio Transmission approach.

In this work we consider an OFDM-SDMA downlink with a real life, nonlinear Power Amplifier (PA). It is well known that a nonlinear PA produces nonlinear distortions. We propose a novel iterative algorithm based on Zero Forcing (ZF) precoding to cancel the nonlinear distortions seen at the mobile devices while preserving the interference-free reception due to ZF. The proposed algorithm also keeps the out-of-band radiation of the transmitted signals low, in order to satisfy spectral purity requirements. For a low Back-Off (BO) scenario, where the nonlinear distortions are strong, the proposed algorithm shows a much lower Symbol Error Rate (SER) and much higher power efficiency than the well known ZF-SDMA with a real life PA.

The influence of nonlinear power amplifiers (PAs) on the performance of multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems is investigated. A “full” MIMO scheme with transmitters (TX) precoding and receivers (RX) decoding is considered. It is shown, under assumption of high order OFDM and frequency selective channel, that the ratio between the useful signal and the nonlinear distortions is proportional to the number of TX antennas, i.e., the influence of the nonlinear distortions decreases as the number of TX antennas increases. The distortion reduction factor (DRF) is proposed as a performance metric for MIMO-OFDM systems with nonlinear PAs. A general formula for signal to distortion and noise ratio is obtained for the nonlinear PA with memory, described by the Wiener-Hammerstein (WH) model. Special cases of MIMO, such as maximum ratio transmission (MRT) and maximum ratio combining (MRC), are considered. A full agreement between the theoretical and simulation results is obtained. The main contribution of this paper is in a statement that TX processing reduces the effect of nonlinear distortions. This is obtained by novel approach for analysis of the MIMO-OFDM systems with nonlinear PAs with memory.

The influence of a nonlinear power amplifier on the performance of OFDM-MRT is investigated. It is shown that the ratio between the useful signal and the nonlinear distortions is proportional to the number of TX antennas, i.e., the influence of the nonlinear distortions decreases as the number of TX antennas increases. The Distortion Reduction Factor (DRF) is proposed as a performance metric for multi-antenna systems with nonlinear power amplifier. Considering the nonlinear power amplifier in each transmitting branch, a general formula for signal to distortions and noise ratio in the receiver is obtained and symbol error rate is computed. The MRT is compared to MRC and SISO. It is shown that for a low noise scenario the performance of MRT is much better than that of MRC. Full agreement of the theoretical and simulation results is obtained.

A communication system with non-linear power amplifier (PA) is considered. We introduce three major performance criteria of the system: (i) power efficiency of the PA, (ii) spectral purity of the transmitted signal expressed by adjacent channel interference, and (iii) transmission performance expressed by mutual information or symbol error rate. In order to fulfill the first two criteria it is proposed to use iterative clipping and filtering (ICF) at the transmitter together with low input back-off, while the third criterion is obtained by modified iterative receiver (MIR). The proposed MIR is based on the well-known iterative receiver, but with ICF in the feedback path. The obtained results show that the proposed system is better than its competitors when considering the three performance criteria together. The analysis and results are focused on OFDM; however, the proposed approach may be applied to single carrier.

In this paper we propose to use a so-called “Slave” power amplifier (PA) to remove the distortions caused by the non-linear “Master” high power amplifier (HPA) in multicarrier systems. We derive the mutual information (MI) expressions for the proposed scheme and perform simulations to obtain the spectral mask. It is shown that by adding an additional “Slave” PA we increase the MI and improve the spectral mask, while maintaining the same supplied power as in regular multicarrier system with a single “Master” HPA.