Peak to average power ratio reduction in NC–OFDM systems

Non contiguous orthogonal frequency division multiplexing (NC-OFDM) is an efficient and adaptable multicarrier modulation scheme to be used in cognitive radio communications. However like OFDM, NC-OFDM also suffers from the main drawback of high peak to average power ratio (PAPR). In this paper PAPR has been reduced by employing three different trigonometric transforms. Discrete cosine transform (DCT), discrete sine transform (DST) and fractional fourier transform (FRFT) has been combined with conventional selected level mapping (SLM) technique to reduce the PAPR of both OFDM and NC-OFDM based systems. The method combines all the transforms with SLM in different ways. Transforms DCT, DST and FRFT have been applied before the SLM block or inside the SLM block before IFFT. Simulation results show the comparative analysis of all the transforms using SLM in case of both OFDM and NC-OFDM based systems

PAPR Reduction and Sidelobe Suppression in Cognitive OFDM - A Survey

Cognitive radio (CR) is one of the key technology providing a new way to enhance the utilization of available spectrum effectively. The multicarrier modulation (MCM) technique which is widely used is Orthogonal Frequency Division Multiplexing (OFDM) system, is an excellent choice for high data rate application. The main two limitations of this technology is the high peak-to-average power ratio (PAPR) of transmission signal and large spectrum sidelobe. This article describes some of the important PAPR reduction techniques and sidelobe suppression techniques

PAPR Reduction and Sidelobe Suppression in Cognitive OFDM - A Survey

Cognitive radio (CR) is one of the key technology providing a new way to enhance the utilization of available spectrum effectively. The multicarrier modulation (MCM) technique which is widely used is Orthogonal Frequency Division Multiplexing (OFDM) system, is an excellent choice for high data rate application. The main two limitations of this technology is the high peak-to-average power ratio (PAPR) of transmission signal and large spectrum sidelobe. This article describes some of the important PAPR reduction techniques and sidelobe suppression techniques

Generalized DFT: extensions in communications

Discrete Fourier Transform (DFT) is a restricted version of Generalized DFT (GDFT) which offers a very limited number of sets to be used in a multicarrier communication system. In contrast, as an extension on Discrete Fourier Transform (DFT) from the linear phase to non-linear phase, the proposed GDFT provides many possible carrier sets of various lengths with comparable or better performance than DFT. The availability of the rich library of orthogonal constant amplitude transforms with good performance allows people to design adaptive systems where user code allocations are made dynamically to exploit the current channel conditions in order to deliver better performance. For MIMO Radar systems, the ideal case to detect a moving target is when all waveforms are orthogonal, which can provide an accurate estimation. But this is not practical in distributed MIMO radars, where sensors are at varying distances from a target. Orthogonal waveforms with low auto- and cross-correlations are of great interest for MIMO radar applications with distributed antennas. Finite length orthogonal codes are required in real-world applications where frequency selectivity and signal correlation features of the optimal subspace are compromised. In the first part of the dissertation, a method is addressed to design optimal waveforms which meets above requirements for various radar systems by designing the phase shaping function (PSF) of GDFT framework with non-linear phase. Multicarrier transmission such as orthogonal frequency-division multiplexing (OFDM) has seen a rise in popularity in wireless communication, as it offers a promising choice for high speed data rate transmission. Meanwhile, high peak-to-average power ratio (PAPR) is one of the well-known drawbacks of the OFDM system due to reduced power efficiency in non-linear modules. Such a situation leads to inefficient amplification and increases the cost of the system, or increases in interference and signal distortion. Therefore, PAPR reduction techniques play an essential role to improve power efficiency in the OFDM systems. There has been a variety of PAPR reduction methods emphasizing different aspects proposed in the literature. The trade-off for PAPR reduction in the existing methods is either increased average power and/or added computational complexity. A new PAPR reduction scheme is proposed that implements a pre-designed symbol alphabet modifier matrix (SAM) to jointly modify the amplitude and phase values of the original data symbol alphabets prior to the IFFT operation of an OFDM system at the transmitter. The method formulated with the GDFT offers a low-complexity framework in four proposed cases devised to be independent of original data symbols. Without degrading the bit error rate (BER) performance, it formulates PAPR reduction problem elegantly and outperforms partial transmit sequences (PTS), selected mapping technique (SLM) and Walsh Hadamard transform (WHT-OFDM) significantly for the communication scenarios considered in the dissertation

Joint design of PAPR, PICR and OBP in OFDM systems

Orthogonal frequency division multiplexing (OFDM) technique has been adopted by many existing and future wireless communication systems for high-speed data transmission. However, a major problem of OFDM systems is the high peak-to-average power ratio (PAPR) of OFDM signals, which results in inefficient operations of nonlinear devices in the system such as power amplifiers (PAs). On the other hand, at the receiver end, frequency offset, caused by Doppler frequency shifts, mismatched oscillators, or a fast fading channel, destroys the orthogonality among subcarriers and results in inter-carrier interference (ICI), thus degrades the detection performance of OFDM systems. Analogous to the definition of PAPR, the peak interference-to-carrier ratio (PICR) is defined to represent the effect of ICI. In addition, due to the sidelobes of modulated subcarriers, OFDM systems also suffer from high out-of-band power (OBP) radiations. High OBP results in the need for wide guard band and thus inefficient usage of frequency band

Intelligent Processing in Wireless Communications Using Particle Swarm Based Methods

There are a lot of optimization needs in the research and design of wireless communica- tion systems. Many of these optimization problems are Nondeterministic Polynomial (NP) hard problems and could not be solved well. Many of other non-NP-hard optimization problems are combinatorial and do not have satisfying solutions either. This dissertation presents a series of Particle Swarm Optimization (PSO) based search and optimization algorithms that solve open research and design problems in wireless communications. These problems are either avoided or solved approximately before. PSO is a bottom-up approach for optimization problems. It imposes no conditions on the underlying problem. Its simple formulation makes it easy to implement, apply, extend and hybridize. The algorithm uses simple operators like adders, and multipliers to travel through the search space and the process requires just five simple steps. PSO is also easy to control because it has limited number of parameters and is less sensitive to parameters than other swarm intelligence algorithms. It is not dependent on initial points and converges very fast. Four types of PSO based approaches are proposed targeting four different kinds of problems in wireless communications. First, we use binary PSO and continuous PSO together to find optimal compositions of Gaussian derivative pulses to form several UWB pulses that not only comply with the FCC spectrum mask, but also best exploit the avail- able spectrum and power. Second, three different PSO based algorithms are developed to solve the NLOS/LOS channel differentiation, NLOS range error mitigation and multilateration problems respectively. Third, a PSO based search method is proposed to find optimal orthogonal code sets to reduce the inter carrier interference effects in an frequency redundant OFDM system. Fourth, a PSO based phase optimization technique is proposed in reducing the PAPR of an frequency redundant OFDM system. The PSO based approaches are compared with other canonical solutions for these communication problems and showed superior performance in many aspects. which are confirmed by analysis and simulation results provided respectively. Open questions and future Open questions and future works for the dissertation are proposed to serve as a guide for the future research efforts

Massive MIMO transmission techniques

Next generation of mobile communication systems must support astounding data traffic increases, higher data rates and lower latency, among other requirements. These requirements should be met while assuring energy efficiency for mobile devices and base stations. Several technologies are being proposed for 5G, but a consensus begins to emerge. Most likely, the future core 5G technologies will include massive MIMO (Multiple Input Multiple Output) and beamforming schemes operating in the millimeter wave spectrum. As soon as the millimeter wave propagation difficulties are overcome, the full potential of massive MIMO structures can be tapped. The present work proposes a new transmission system with bi-dimensional antenna arrays working at millimeter wave frequencies, where the multiple antenna configurations can be used to obtain very high gain and directive transmission in point to point communications. A combination of beamforming with a constellation shaping scheme is proposed, that enables good user isolation and protection against eavesdropping, while simultaneously assuring power efficient amplification of multi-level constellations

OFDM 시스템을 위한 새로운 저 복잡도 SLM 방식 및 클리핑 잡음 제거 기법 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 2. 노종선.In this dissertation, several research results for the peak-to-average power ratio (PAPR) reduction schemes for orthogonal frequency division multiplexing (OFDM) systems are discussed. First, the basic principle and implementation of the OFDM systems are introduced, where high PAPR of OFDM signal is one of main drawbacks of OFDM systems. Thus, many PAPR reduction schemes to solve this problem have been studied such as clipping, selected mapping (SLM), partial transmit sequence (PTS), and tone reservation. In the first part of this dissertation, a low-complexity SLM scheme is proposed, where the proposed SLM scheme generates alternative OFDM signal sequences by cyclically shifting the connections in each subblock at an intermediate stage of inverse fast Fourier transform (IFFT). Compared with the conventional SLM scheme, the proposed SLM scheme achieves similar PAPR reduction performance with much lower computational complexity and no bit error rate (BER) degradation. The performance of the proposed SLM scheme is analyzed mathematically and verified through numerical analysis. Also, it is shown that the proposed SLM scheme has the lowest computational complexity among the existing low-complexity SLM schemes exploiting the signals at an intermediate stage of IFFT. In the second part of this dissertation, an efficient selection (ES) method of the OFDM signal sequence with the minimum PAPR among many alternative OFDM signal sequences is proposed, which can be used for various SLM schemes. The proposed ES method efficiently generates each component of alternative OFDM signal by utilizing the structure of IFFT and calculates its power, and such generation procedure is interrupted if the calculated power is larger than the given threshold. By using the proposed ES method, the average computational complexity of considered SLM schemes is substantially reduced without degradation of PAPR reduction performance, which is confirmed by analytical and numerical results. In the third part of this dissertation, a clipping noise cancellation scheme using compressed sensing (CS) technique is proposed for OFDM systems. The proposed scheme does not need reserved tones or pilot tones, which is different from the previous works using CS technique. Instead, observations of the clipping noise in data tones are exploited, which leads to no loss of data rate. Also, in contrast with the previous works, the proposed scheme selectively exploits the reliable observations of the clipping noise instead of using whole observations, which results in minimizing the bad influence of channel noise. From the selected reliable observations, the clipping noise in time domain is reconstructed and cancelled by using CS technique. Simulation results show that the proposed scheme performs well compared to other conventional clipping noise cancellation schemes and shows the best performance in the severely clipped cases.1. Introduction 1 1.1. Background 1 1.2. Overview of Dissertation 4 2. OFDM Systems 6 2.1. OFDM System Model 7 2.2. Peak-to-Average Power Ratio 8 2.2.1. Definition of PAPR 9 2.2.2. Distribution of PAPR 9 3. PAPR Reduction Schemes 11 3.1. Clipping 11 3.1.1. Clipping at Transmitter 11 3.1.2. A Statistical Model of Clipped Signals 13 3.1.3. Conventional Receiver without Clipping Noise Cancellation Scheme 15 3.2. Selected Mapping 16 3.3. Low-Complexity SLM Schemes 18 3.3.1. Lims SLM Scheme [25] 18 3.3.2. Wangs SLM Scheme [22] 19 3.3.3. Baxleys SLM Scheme [27] 19 3.4. Tone Reservation 20 4. A New Low-Complexity SLM Scheme for OFDM Systems 22 4.1. A New SLM Scheme with Low-Complexity 23 4.1.1. A New SLM Scheme 23 4.1.2. Relation Between the Proposed SLM Scheme and the Conventional SLM Scheme 26 4.1.3. Good Shift Values for the Proposed SLM Scheme 28 4.1.4. Methods to Generate Good Shift Values 31 4.1.5. Computational Complexity 33 4.2. Simulation Results 36 4.3. Conclusions 37 5. An Efficient Selection Method of a Transmitted OFDM Signal Sequence for Various SLM Schemes 42 5.1. ES Method and Its Application to the Conventional SLM Scheme 43 5.1.1. Sequential Generation of OFDM Signal Components in the Conventional SLM Scheme 43 5.1.2. Application of the ES Method to the Conventional SLM Scheme 45 5.1.3. Complexity Analysis for Nyquist Sampling Case 47 5.1.3.1. Characteristics of a Nyquist-Sampled OFDM Signal Sequence 48 5.1.3.2. Derivation of KN(b) 49 5.1.3.3. Distribution of pBu(bu) 51 5.1.4. Complexity Analysis for Oversampling Case 52 5.1.4.1. Characteristics of a Four-Times Oversampled OFDM Signal Sequence 52 5.1.4.2. Derivation of K4N(b) 53 5.1.4.3. Distribution of pBu(bu) 54 5.1.5. Comparison between Analytical and Simulation Results 55 5.2. Application of the ES Method to Various Low-Complexity SLM Schemes 57 5.2.1. Lims SLM Scheme Aided by the ES Method 57 5.2.2. Wangs SLM Scheme Aided by the ES Method 58 5.2.3. Baxelys SLM Scheme Aided by the ES Method 58 5.3. Simulation Results 59 5.3.1. Simulation Results for the Conventional SLM Scheme Aided by the ES Method 59 5.3.2. Simulation Results for Low-Complexity SLM Schemes Aided by the ES Method 60 5.4. Conclusions 62 6. Clipping Noise Cancellation for OFDM Systems Using Reliable Observations Based on Compressed Sensing 68 6.1. Preliminaries 71 6.1.1. Notation 71 6.1.2. Compressed Sensing 71 6.2. Clipping Noise Cancellation for OFDM Systems Based on CS 73 6.2.1. Sparsity of c 73 6.2.1.1. Sparsity of c for Clipping at the Nyquist Sampling Rate 73 6.2.1.2. Sparsity of c for Clipping and Filtering at an Oversampling Rate 74 6.2.2. Reconstruction of the Clipping Noise c by CS 75 6.2.3. Construction of the Compressed Observation Vector Y 77 6.2.3.1. Which Observations Should Be Selected 78 6.2.3.2. Estimation of θ(k) Based on H1(k)Y (k) 78 6.2.3.3. Selection Criterion of Observations 81 6.2.4. Computational Complexity 81 6.3. Simulation Results 82 6.3.1. AWGN Channel 82 6.3.2. Rayleigh Fading Channel 83 6.4. Conclusion 86 7. Conclusions 93 Bibliography 96 초록 104Docto