A Stochastic Conjugate Gradient Method for Approximation of Functions

A stochastic conjugate gradient method for approximation of a function is proposed. The proposed method avoids computing and storing the covariance matrix in the normal equations for the least squares solution. In addition, the method performs the conjugate gradient steps by using an inner product that is based stochastic sampling. Theoretical analysis shows that the method is convergent in probability. The method has applications in such fields as predistortion for the linearization of power amplifiers.Comment: 21 pages, 5 figure

Iterative pre-distortion of the non-linear satellite channel

Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) is the current European standard for satellite broadcast and broadband communications. It relies on high order modulations up to 32-amplitude/phase-shift-keying (APSK) in order to increase the system spectral efficiency. Unfortunately, as the modulation order increases, the receiver becomes more sensitive to physical layer impairments, and notably to the distortions induced by the power amplifier and the channelizing filters aboard the satellite. Pre-distortion of the non-linear satellite channel has been studied for many years. However, the performance of existing pre-distortion algorithms generally becomes poor when high-order modulations are used on a non-linear channel with a long memory. In this paper, we investigate a new iterative method that pre-distorts blocks of transmitted symbols so as to minimize the Euclidian distance between the transmitted and received symbols. We also propose approximations to relax the pre-distorter complexity while keeping its performance acceptable

High power amplifier pre-distorter based on neural-fuzzy systems for OFDM signals

In this paper, a novel High Power Amplifier (HPA) pre-distorter based on Adaptive Networks - Fuzzy Inference Systems (ANFIS) for Orthogonal Frequency Division Multiplexing (OFDM) signals is proposed and analyzed. Models of Traveling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA), both memoryless and with memory, have been used for evaluation of the proposed technique. After training, the ANFIS linearizes the HPA response and thus, the obtained signal is extremely similar to the original. An average Error Vector Magnitude (EVM) of 10-6 can be easily obtained with our proposal. As a consequence, the Bit Error Rate (BER) degradation is negligible showing a better performance than what can be achieved with other methods available in the literature. Moreover, the complexity of the proposed scheme is reducedThis work was supported in part by projectsMULTIADAPTIVE (TEC2008-06327-C03-02) and AECI Program of Research Cooperation with MoroccoPublicad

Improving the Spectral Efficiency of Nonlinear Satellite Systems through Time-Frequency Packing and Advanced Processing

We consider realistic satellite communications systems for broadband and broadcasting applications, based on frequency-division-multiplexed linear modulations, where spectral efficiency is one of the main figures of merit. For these systems, we investigate their ultimate performance limits by using a framework to compute the spectral efficiency when suboptimal receivers are adopted and evaluating the performance improvements that can be obtained through the adoption of the time-frequency packing technique. Our analysis reveals that introducing controlled interference can significantly increase the efficiency of these systems. Moreover, if a receiver which is able to account for the interference and the nonlinear impairments is adopted, rather than a classical predistorter at the transmitter coupled with a simpler receiver, the benefits in terms of spectral efficiency can be even larger. Finally, we consider practical coded schemes and show the potential advantages of the optimized signaling formats when combined with iterative detection/decoding.Comment: 8 pages, 8 figure

A Digital Predistortion Scheme Exploiting Degrees-of-Freedom for Massive MIMO Systems

The primary source of nonlinear distortion in wireless transmitters is the power amplifier (PA). Conventional digital predistortion (DPD) schemes use high-order polynomials to accurately approximate and compensate for the nonlinearity of the PA. This is not practical for scaling to tens or hundreds of PAs in massive multiple-input multiple-output (MIMO) systems. There is more than one candidate precoding matrix in a massive MIMO system because of the excess degrees-of-freedom (DoFs), and each precoding matrix requires a different DPD polynomial order to compensate for the PA nonlinearity. This paper proposes a low-order DPD method achieved by exploiting massive DoFs of next-generation front ends. We propose a novel indirect learning structure which adapts the channel and PA distortion iteratively by cascading adaptive zero forcing precoding and DPD. Our solution uses a 3rd order polynomial to achieve the same performance as the conventional DPD using an 11th order polynomial for a 100x10 massive MIMO configuration. Experimental results show a 70% reduction in computational complexity, enabling ultra-low latency communications.Comment: IEEE International Conference on Communications 201

Enhanced Multicarrier Techniques for Professional Ad-Hoc and Cell-Based Communications (EMPhAtiC) Document Number D3.3 Reduction of PAPR and non linearities effects

Livrable d'un projet Européen EMPHATICLike other multicarrier modulation techniques, FBMC suffers from high peak-to-average power ratio (PAPR), impacting its performance in the presence of a nonlinear high power amplifier (HPA) in two ways. The first impact is an in-band distortion affecting the error rate performance of the link. The second impact is an out-of-band effect appearing as power spectral density (PSD) regrowth, making the coexistence between FBMC based broad-band Professional Mobile Radio (PMR) systems with existing narrowband systems difficult to achieve. This report addresses first the theoretical analysis of in-band HPA distortions in terms of Bit Error Rate. Also, the out-of band impact of HPA nonlinearities is studied in terms of PSD regrowth prediction. Furthermore, the problem of PAPR reduction is addressed along with some HPA linearization techniques and nonlinearity compensation approaches

Efficient digital predistortion techniques for power amplifier linearization

The importance of spectral efficiency in mobile communications often requires the use of non-constant-envelop linear digital modulation schemes. These modulation techniques carry signal information in both magnitude and phase, thus they must be linearly amplified to avoid nonlinear signal distortion which is not correctable in a typical receiver. A second difficulty in utilizing these modulation formats is that nonlinear amplification generates out-of-band power (spectral regrowth). Therefore, to achieve both high energy efficiency and spectral efficiency, some forms of linearization must be used to compensate for the nonlinearity of power amplifiers. One powerful technique that is amenable to monolithic integration is digital signal predistortion. Most predistorters try to achieve the inverse nonlinear characteristic of High Power Amplifier(HPA). In this thesis a new multi-stage digital adaptive signal predistorter is presented. The scheme is developed from the direct iterative method with low memory requirement proposed by Cavers [1] in combination with the multi-stage predistortion proposed by Stonick [2]. To make the predistorter more compact a very simple and fast method called the complementary method is proposed. The complementary method has prominent advantages over other digital predistorters in terms of stability of the algorithm, complexity of the algorithm and computational load

The Research of Predistortion in DTMB Digital TV Transmitter

在数字电视发射系统中，发射机末级功率放大器对系统的性能和设备的可靠性有着重要的影响。高功率放大器因其具有非线性特性，这种非线性将会造成信号的畸变，使信号的输出频谱发生变化，产生带内失真和带外干扰。因此，在高质量的发射系统中一般采取线性化技术补偿功放的非线性对数字电视信号的影响。另外由于功放具有记忆效应，即功放的输出不仅依赖于当前的输入信号也与过去的输入信号有关，为了实现更好的校正功能，还必须研究带有记忆功能的数字基带预失真技术。 本文以中国数字电视地面广播传输标准的TDS-OFDM系统为仿真平台，采用数字预失真技术对数字电视发射机的功率放大器进行线性化。目前针对数字电视激励器所用的预失真方法...In the digital television transmission system, the power amplifier has a significant effect to the system reliability and performance. It is well known that HPAs(High Power Amplifiers)are inherently nonlinear devices which affect the communication systems directly, whose nonlinear distortions result in spectrum expansion and adjacent channel interference, then worsen the bit error rate. Therefore,...学位：工学硕士院系专业：信息科学与技术学院通信工程系_通信与信息系统学号：2332007115218