Peak Power Reduction of OFDM Signals with Sign Adjustment

It has recently been shown that significant reduction in the peak to mean envelope power (PMEPR) can be obtained by altering the sign of each subcarrier in a multicarrier system with n subcarriers. However, finding the best sign not only requires a search over 2n possible signs but also may lead to a substantial rate loss for small size constellations. In this paper, we first propose a greedy algorithm to choose the signs based on p-norm minimization and prove that the resulting PMEPR is guaranteed to be less than c log n where c is a constant independent of n for any n. This approach has lower complexity in each iteration compared to the derandomization approach of while achieving similar PMEPR reduction. We further improve the performance of the proposed algorithm by enlarging the search space using pruning. Simulation results show that PMEPR of a multicarrier signal with 128 subcarriers can be reduced to within 1.6 dB of the PMEPR of a single carrier system. In the second part of the paper, we address the rate loss by proposing a block coding scheme in which only one sign vector is chosen for K different modulating vectors. The sign vector can be computed using the greedy algorithm in n iterations. We show that the multi-symbol encoding approach can reduce the rate loss by a factor of K while achieving the PMEPR of c logKn, i.e., only logarithmic growth in K. Simulation results show that the rate loss can be made smaller than %10 at the cost of only 1 db increase in the resulting PMEPR for a system with 128 subcarriers

Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Comparison of Channel State Information Estimation Using SLM and Clipping-based PAPR Reduction Methods

AbstractChannel estimation is a crucial issue in orthogonal frequency-division multiplexing (OFDM) as well as in all multicarrier systems. However, OFDM suffers from a major setback, the peak-to-average power ratio (PAPR). PAPR can be solved using a number of available techniques in literature, such as coding, active constellation extension, amplitude clipping, and selected mapping. The coding approach presents a disadvantage, represented by redundant data that significantly reduce the bit rate. The active constellation extension is an effective method; however, it requires higher transmission power. The clipping method is the simplest, but it produces high bit error rate (BER) degradation. Selected mapping (SLM) is the best among the available methods; however, it sends several bits as side information. In this study, we compare the clipping and SLM methods and show how the channel state information (CSI) estimation is affected in both techniques. Simulation results show that the SLM method is more effective than the clipping technique. The BER significantly increases when the clipping method is used because of the inaccurate estimation of CSI when the high peaks are clipped, such as in the case of the inserted pilots

Comparison of Channel State Information Estimation Using SLM and Clipping-based PAPR Reduction Methods

AbstractChannel estimation is a crucial issue in orthogonal frequency-division multiplexing (OFDM) as well as in all multicarrier systems. However, OFDM suffers from a major setback, the peak-to-average power ratio (PAPR). PAPR can be solved using a number of available techniques in literature, such as coding, active constellation extension, amplitude clipping, and selected mapping. The coding approach presents a disadvantage, represented by redundant data that significantly reduce the bit rate. The active constellation extension is an effective method; however, it requires higher transmission power. The clipping method is the simplest, but it produces high bit error rate (BER) degradation. Selected mapping (SLM) is the best among the available methods; however, it sends several bits as side information. In this study, we compare the clipping and SLM methods and show how the channel state information (CSI) estimation is affected in both techniques. Simulation results show that the SLM method is more effective than the clipping technique. The BER significantly increases when the clipping method is used because of the inaccurate estimation of CSI when the high peaks are clipped, such as in the case of the inserted pilots

A novel PAPR reduction scheme based on selective mapping and a random-like coding with no explicit side information in OFDM

Orthogonal Frequency Division Multiplexing (OFDM) is a promising technique for high data rate and reliable communication over fading channels. The main implementation drawback of this system is the possibility of high Peak to Average Power Ratio (PAPR). In this paper, we develop a novel Selective Mapping (SLM) PAPR reduction technique. In the novel proposed scheme, the alternative symbol sequences are generated by module 2 additions of data with the rows of cyclic Hadamard matrix with the same size, inserting the selected row's number to avoid transmitting any side information and specially using a random-like Irregular Repeat Accumulate (IRA) encoder for both PAPR and Bit Error Rate (BER) better performance. Keywords: IRA Codes, OFDM, PAPR, SLM method