Igor Gutman

Iterative Decoding of Iterative Clipped and Filtered OFDM Signal

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.

Performance Analysis of Communication System with Nonlinear Power Amplifier

A communication system with nonlinear Power Amplifier (PA) is considered. The PA nonlinearity forces the designer of communication system to choose proper Back-Off, which influences three major parameters of the communication system: (i) power efficiency, (ii) Adjacent Channel Power Ratio (ACPR), and (iii) bit rate expressed by Mutual Information (MI). Having these three parameters, we propose to evaluate the performance of a communication system with nonlinear PA using the fact that ACPR is dictated by the regulator/communication standard. In what follows we propose to find power efficiency and MI as a function of ACPR. To illustrate the usefulness of the proposed method, the Solid State Power Amplifier (SSPA), and two linearized versions of SSPA are considered. The first version is a perfectly linearized PA using pre-distortion resulting with Soft Envelope Limiter (SEL), and the second is the Feed-Forward (FF) architecture based on SSPA. For each of the PAs ACPR, power efficiency and MI are found. While the analysis of SSPA and SEL is mainly based on the existing literature, the analysis of FF may be considered as a new contribution. From the results of the analysis it is concluded that linearization improves the overall performance of communication system. It is also shown that the pre-distortion method (SEL) shows better performance than FF.

Distribution of Instantaneous Power in Single Carrier Signal with Independent I/Q Components

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.

Rate increase in digital communications by using master-slave power amplifiers

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.

Impact of Linear Power Amplifier on power loading in OFDM. Part I: Principles

It is proposed to modify any existing power loading algorithms by linear scaling of the OFDM signal according to the maximal PAPR observed during the quasi-static period of the channel. Such scaled signal is then fed to the Linear Power Amplifier in order to use its whole linear dynamic range. It is proved that the average power to be allocated among subcarriers of the modified power loading algorithm is larger than in the standard algorithm. It is therefore expected that the modified algorithm will perform better than the standard one.

On the distribution of instantaneous power in single carrier: Effective numerical approach

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.

Efficient Power Amplification for Wireless Communication Based on Single-Carrier

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 amplifiers 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.

Distribution of PAPR in low order OFDM with non equal amplitudes

It has been widely known that one of the key design parameters in orthogonal frequency division multiplexing (OFDM) systems is the distribution of peak-to-average power ratio (PAPR). Recently some theoretical approaches to determine the PAPR distribution have been proposed based on the assumption that all subcarriers are allocated with equal power, and that number of subcarriers is large. As a result Gaussian approximations may be used. However, these assumptions may not be valid due to the following facts: (i) in all realistic OFDM systems, usually only a subset of subcarriers is used to carry information and the rest are set to zero, (ii) power allocation may not be uniform and may depend on bit and power loading. As a result the Gaussian approximation is not longer valid to compute the PAPR distribution. In this paper we derive an analytical expression of the PAPR distribution of low order OFDM signal having non-equal amplitudes. To validate the analytical results, extensive numerical analysis have been conducted, showing a very good match between the analytically results and that of real OFDM systems.

On distortion-yes PAPR reduction methods

Three versions of Clipping & Filtering (CF) and just controlled clipping without filtering are compared using extensive simulations. The comparison is performed according to minimum BER criteria while taking into account the spectral mask constraint. The clipping, no matter how implemented, introduces in-band distortions; therefore we will name it “distortion-yes” technique to reduce PAPR. It is obtained that CF gives non-significant better performance than controlled clipping for spectral masks requiring very high spectral purity. However, for spectral masks of practical systems, such as WiMAX, the performance of CF (or its iterative version) and just clipping are exactly the same. It may be concluded that for such cases there is no justification for the use of CF or Iterative CF.