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.

This paper investigates the statistical distribution of instantaneous power (IP) in pulse-shaped single-carrier (SC) modulation with independent I/Q components. Such knowledge is required due to the nonlinearities that may appear in the whole communication track, especially that of the power amplifier (PA). Exact analytical derivation of the distribution is an open problem. The existing works rely on Monte-Carlo simulations or on complex numerical analysis. In this paper we propose a straightforward method, based on numerical integration, to evaluate the complementary cumulative distribution function (CCDF) of IP by using characteristic function approach. Error analysis of the proposed numerical approach is given. It is shown that by investing relatively small computation power in the proposed numerical solution we can derive the CCDF of IP with high and controlled accuracy.

We propose and analyze two strategies for efficient use of linear power amplifiers in single carrier systems. The first strategy maximizes the mutual information for a given limited power supply by applying variable gain pre- amplification to the transmitted signal before it is fed to the power amplifier. The second strategy minimizes the required power supply while maintaining a fixed power output by adjusting the power amplifier's linear dynamic range using adaptive-biasing. Pre- amplification results in an increase of up to 1.4 times in average mutual information regardless of the power amplifier class while the adaptive- biasing results in a decrease of up to 7 dB in the required power supply for class A and up to 3 dB for class B power amplifiers.

This paper investigates the statistical distribution of instantaneous power (IP) in pulse-shaped single-carrier (SC) modulation. Such knowledge is required due to the nonlinearity of the power amplifier (PA). Exact analytical derivation of the distribution is an open problem. The existing works rely on Monte-Carlo simulations or on an approximated numerical analysis. In this paper we propose a straightforward method, based on numerical integration, to evaluate the complementary cumulative distribution function (CCDF) of IP by using characteristic function approach. Error analysis of the proposed numerical approach is given. It is shown that by investing relatively small computation power in the proposed numerical solution we can derive the CCDF of IP with high and controlled accuracy.

A novel multiple access method for wireless communication is proposed. The method is based on the channel superposition of multiple coherently transmitted signals. The obtained signal constellation at the receiver represents information bits of all transmitters at once. In contrary to other multiple access methods the quality of reception can increase with the number of transmitters. The method may be used on top of other multiple access methods to achieve higher spectral efficiency

A novel approach to power loading for orthogonal frequency-division multiplexing (OFDM) systems is proposed. The corresponding algorithm minimizes the aggregate bit-error rate, but appears to be rather complicated. Consequently, a much simpler, quasi-optimal loading algorithm is developed. The performance of various power loading strategies known in the literature are compared with the proposed quasi-optimal algorithm in the Rayleigh frequency-selective fading channel.

The problem of peak-to-average power ratio (PAPR) of high-order orthogonal frequency-division modulation (OFDM) is considered. Using results on level crossing of random processes, an upper bound on the probability that the PAPR of an OFDM signal will exceed a given value is derived. Numerical computations are used to show that this bound is tight for low-pass OFDM systems. The central limit theorem is used to find an asymptotic expression for the bound when the number of carriers N grows to infinity. The central limit theorem is also used to find an asymptotic expression for another bound that is based on the envelope of the OFDM signal, and is tighter for bandpass systems. It is shown that, effectively, the PAPR grows as 2lnN and not linearly with N, and by developing a lower bound on the probability that the PAPR of an OFDM signal will exceed a given value, it is shown that asymptotically most OFDM symbols have a PAPR close to 2lnN. Some approaches to coping with the PAPR problem are discussed in light of the obtained results.

A multichannel (MC) system with optimal diversity reception and erasures correcting decoding of a block code applied to a Rayleigh fading channel is considered. The bit-error rate (BER) as a function of the signal-to-noise ratio has been found. It is shown that the proposed system has a lower BER than both the MC with forward-error correction and the MC with optimal uncoded diversity reception for the same redundancy.

The asymptotic behavior of level clipped orthogonal frequency-division multiplexing when its order N approaches infinity is analyzed. It is shown that if the clipping level approaches infinity faster than /spl radic/lnN, then a zero bit-error rate penalty and arbitrarily large peak to-average power ratio (PAPR) gain are asymptotically obtained. Moreover, the expression for the probability of clipping is found, and using it, the upper bound of the probability of error is derived. This upper bound can be used to analyze the tradeoff between PAPR and probability of error.

A main disadvantage of orthogonal multicarrier modulation is that it exhibits a high peak-to-average-power ratio (PAPR). Because of this, all circuits between the output of the modulator and the input of the demodulator must be linear within a large dynamic range to avoid distortions and spectral spreading. It is therefore desirable to limit the PAPR of the multicarrier signal. A method based on amplitude limiting and forward error correcting (AL-FEC) coding is proposed for reduction of the PAPR. The performance of the AL-FEC is compared to two methods, based on block coding, specially constructed to reduce the PAPR. The AL-FEC is superior as compared to the existing methods. Moreover, its practical realization is very simple for any number of carriers.

A periodic transmission of a short resetting sequence used to "fill up" the decision-feedback equalizer feedback filter memory with correct symbols is proposed to reduce the propagation error. Two strategies are considered: full and partial reset, where the entire memory and some part of it is reset, respectively. For any given steady-state bit-error rate, the period of the resetting sequence is found by use of the Markov chain limit theorem. It is shown, via an illustrative example, that for low signal-to-noise ratio (SNR), the resetting strategy outperforms Bose-Chaudhuri-Hocquenghen (BCH) code with the same code rate. Moreover, a joint resetting and error correction coding strategy is considered, which is better than BCH code for low and medium SNR.

A novel signal controlled constrained notch filter (SC-CNF) structure with global feedback is presented. The SC-CNF is combined with an adaptive linear enhancer to retrieve a multitone signal corrupted by a strong nonstationary FM interference. Using a time-warping technique, the SC-CNF, by its nature time-varying, is transformed into a CNF which is time variant. Such transformation is implemented by nonequally spaced sampling (NESS) of the input mixture. A closed-loop system is proposed to improve the strategy of NESS. By exploiting the self-synchronization process, nearly complete rejection of FM interference is achieved. The condition for the occurrence of self-synchronization is obtained. The self-synchronized SC-CNF provides significant improvement (up to 30 dB) of the signal-to-interference ratio of the entire system output.>

A study is made of a cross-coupled phase-locked loop (CCPLL) used for separation of close sinusoids with slowly varying frequencies. It is assumed that the sinusoids have equal amplitudes, and it is shown that the CCPLL system can effectively separate two sine signals with an observation interval Tor=15 dB. The result has application in fields such as ranging, symbol synchronization, and carrier recovery for coherent demodulation.>

A method of constructing a frequency synthesizer with a short settling time is suggested. The frequency synthesizer is based on a double-loop phase-locked loop which has a short convergence time and on a continuous-phase divider that matches the VCO (voltage-controlled oscillator) frequency to the reference frequency. >

A new method of constructing phase-lock loop systems with wide capture range and fast response is suggested. The suggested systems employ the relatively well-known double-loop configuration. They are unusual in the sense that the two multipliers (phase detectors) are replaced by one divider.