Performance analysis and optimization of DCT-based multicarrier system on frequency-selective fading channels

Regarded as one of the most promising transmission techniques for future wireless communications, the discrete cosine transform (DCT) based multicarrier modulation (MCM) system employs cosine basis as orthogonal functions for real-modulated symbols multiplexing, by which the minimum orthogonal frequency spacing can be reduced by half compared to discrete Fourier transform (DFT) based one. With a time-reversed pre-filter employed at the front of the receiver, interference-free one-tap equalization is achievable for the DCT-based systems. However, due to the correlated pre-filtering operation in time domain, the signal-to-noise ratio (SNR) is enhanced as a result at the output. This leads to reformulated detection criterion to compensate for such filtering effect, rendering minimum-mean-square-error (MMSE) and maximum likelihood (ML) detections applicable to the DCT-based multicarrier system. In this paper, following on the pre-filtering based DCT-MCM model that build in the literature work, we extend the overall system by considering both transceiver perfections and imperfections, where frequency offset, time offset and insufficient guard sequence are included. In the presence of those imperfection errors, the DCT-MCM systems are analysed in terms of desired signal power, inter-carrier interference (ICI) and inter-symbol interference (ISI). Thereafter, new detection algorithms based on zero forcing (ZF) iterative results are proposed to mitigate the imperfection effect. Numerical results show that the theoretical analysis match the simulation results, and the proposed iterative detection algorithms are able to improve the overall system performance significantly

Reliability of Trigonometric Transform-based Multi-Carrier Scheme

This work is looking for a new physical layer of a multi-carrier wireless communication system to be implemented in low complexity way, resorting to suitable fast transform. The work presents and assesses a scheme based on Discrete Trigonometric Transform with appending symmetric redundancy either in each or multiple consecutive transformed blocks. A receiver front-end filter is proposed to enforce whole symmetry in the channel impulse response, and bank of one tap filter per sub-carrier is applied as an equalizer in the transform domain. The behaviour of the transceiver is studied in the context of practical impairments like fading channel, carrier frequency offset and narrow band interference. Moreover, the performance is evaluated in contrast with the state of art methods by means of computer simulations, and it has been found that the new scheme improves robustness and reliability of communication signal, and record lower peak to average power ratio. The study demonstrates that front-end matched filter effectively performs frequency synchronization to compensate the carrier frequency offset in the received signal

Enhanced DCT-OFDM system with index modulation

Discrete cosine transform (DCT) based orthogonal frequency division multiplexing (OFDM), which has double number of subcarrier compared to the classic discrete fourier transform (DFT) based OFDM (DFT-OFDM) at the same bandwidth, is a promising high spectral efficiency multicarrier techniques for future wireless communication. In this paper, an enhanced DCT-OFDM with index modulation (IM) (EDCT-OFDM-IM) is proposed to further exploit the benefits of the DCT-OFDM and IM techniques. To be more specific, a pre-filtering method based DCT-OFDM-IM transmitter is first designed and the non-linear maximum likelihood (ML) is developed for our EDCT-OFDM-IM system. Moreover, the average bit error probability (ABEP) of the proposed EDCT-OFDM-IM system is derived, which is confirmed by our simulation results. Both simulation and theoretical results are shown that the proposed EDCT-OFDM-IM system exhibits better bit error rate (BER) performance over the conventional DFT-OFDM-IM and DCT-OFDM-IM counterparts

A Review on PAPR Reduction in Perspective of BER Performance in MIMO-OFDM Based Next Generation Wireless Systems.

Today, high speed and trustworthy wireless communication over mobile is the requirement of society. As the mobile applications and the users are rapidly increasing, it is obligatory to have more reliable, high speed wireless network with high throughput, which will combat the disadvantages in existing system in this multiuser environment. In wireless system the received signal may be corrupted due to noise and interferences such as ‘inter symbol interference’ and ‘inter carrier interference’ when subjected to multi-path fading. Also the performance the system may be affected due to poor ‘bit error rate’ and high ‘peak to average power ratio’ value, which further affect the signal power and spectral efficiency of transmitted signal. The blend of ‘orthogonal frequency division multiplexing’ and ‘multi input multi output’ antenna system referred as MIMO-OFDM system, which offers the improvement in quality of service and higher throughput to satisfy the tomorrow’s need. This review article mainly focuses on various technologies adopted by different researchers for enhancing the ‘bit error rates’, ‘peak to average power ratio’, ‘signal to noise ratio’ and ‘spectral efficiency’ performances in wireless systems. We continue by highlighting the limitations and comparing results of conventional methods, schemes and algorithms proposed by different researchers.  We also focus on the multiple antenna system (MIMO), which is designed for future multiuser environment to enhance the capacity or to have high throughput along with good quality services

Performance evaluation of T-transform based OFDM in underwater acoustic channels

PhD ThesisRecently there has been an increasing trend towards the implementation of orthogonal frequency division multiplexing (OFDM) based multicarrier communication systems in underwater acoustic communications. By dividing the available bandwidth into multiple sub-bands, OFDM systems enable reliable transmission over long range dispersive channels. However OFDM is prone to impairments such as severe frequency selective fading channels, motioned induced Doppler shift and high peak-to-average-power ratio (PAPR). In order to fully exploit the potential of OFDM in UWA channels, those issues have received a great deal of attention in recent research. With the aim of improving OFDM's performance in UWA channels, a T-transformed based OFDM system is introduced using a low computational complexity T-transform that combines the Walsh-Hadamard transform (WHT) and the discrete Fourier transform (DFT) into a single fast orthonormal unitary transform. Through real-world experiment, performance comparison between the proposed T-OFDM system and conventional OFDM system revealed that T-OFDM performs better than OFDM with high code rate in frequency selective fading channels. Furthermore, investigation of different equalizer techniques have shown that the limitation of ZF equalizers affect the T-OFDM more (one bad equalizer coefficient affects all symbols) and so developed a modified ZF equalizer with outlier detection which provides major performance gain without excessive computation load. Lastly, investigation of PAPR reduction methods delineated that T-OFDM has inherently lower PAPR and it is also far more tolerant of distortions introduced by the simple clipping method. As a result, lower PAPR can be achieved with minimal overhead and so outperforming OFDM for a given power limit at the transmitter

Power Allocation for Adaptive OFDM Index Modulation in Cooperative Networks

In this paper, we propose a power allocation strategy for the adaptive orthogonal frequency-division multiplexing (OFDM) index modulation (IM) in cooperative networks. The allocation strategy is based on the Karush-Kuhn-Tucker (KKT) conditions, and aims at maximizing the average network capacity according to the instantaneous channel state information (CSI). As the transmit power at source and relay is constrained separately, we can thus formulate an optimization problem by allocating power to active subcarriers. Compared to the conventional uniform power allocation strategy, the proposed dynamic strategy can lead to a higher average network capacity, especially in the low signal-to-noise ratio (SNR) region. The analysis is also verified by numerical results produced by Monte Carlo simulations. By applying the proposed power allocation strategy, the efficiency of adaptive OFDM IM can be enhanced in practice, which paves the way for its implementation in the future, especially for cell-edge communications

Generalized discrete Fourier transform with non-linear phase : theory and design

Constant modulus transforms like discrete Fourier transform (DFT), Walsh transform, and Gold codes have been successfully used over several decades in various engineering applications, including discrete multi-tone (DMT), orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) communications systems. Among these popular transforms, DFT is a linear phase transform and widely used in multicarrier communications due to its performance and fast algorithms. In this thesis, a theoretical framework for Generalized DFT (GDFT) with nonlinear phase exploiting the phase space is developed. It is shown that GDFT offers sizable correlation improvements over DFT, Walsh, and Gold codes. Brute force search algorithm is employed to obtain orthogonal GDFT code sets with improved correlations. Design examples and simulation results on several channel types presented in the thesis show that the proposed GDFT codes, with better auto and cross-correlation properties than DFT, lead to better bit-error-rate performance in all multi-carrier and multi-user communications scenarios investigated. It is also highlighted how known constant modulus code families such as Walsh, Walsh-like and other codes are special solutions of the GDFT framework. In addition to theoretical framework, practical design methods with computationally efficient implementations of GDFT as enhancements to DFT are presented in the thesis. The main advantage of the proposed method is its ability to design a wide selection of constant modulus orthogonal code sets based on the desired performance metrics mimicking the engineering .specs of interest. Orthogonal Frequency Division Multiplexing (OFDM) is a leading candidate to be adopted for high speed 4G wireless communications standards due to its high spectral efficiency, strong resistance to multipath fading and ease of implementation with Fast Fourier Transform (FFT) algorithms. However, the main disadvantage of an OFDM based communications technique is of its high PAPR at the RF stage of a transmitter. PAPR dominates the power (battery) efficiency of the radio transceiver. Among the PAPR reduction methods proposed in the literature, Selected Mapping (SLM) method has been successfully used in OFDM communications. In this thesis, an SLM method employing GDFT with closed form phase functions rather than fixed DFT for PAPR reduction is introduced. The performance improvements of GDFT based SLM PAPR reduction for various OFDM communications scenarios including the WiMAX standard based system are evaluated by simulations. Moreover, an efficient implementation of GDFT based SLM method reducing computational cost of multiple transform operations is forwarded. Performance simulation results show that power efficiency of non-linear RF amplifier in an OFDM system employing proposed method significantly improved

A novel scheme of multicarrier modulation with the discrete cosine transform

In this work, we derive a novel multicarrier modulation based on the Type-I even discrete cosine transform (DCT1e), which includes new procedures to carry out both the channel estimation and the signal reconstruction. By using a small number of training symbols, we achieve an accurate estimation of the channel's impulse response (CIR) using a novel mirror, replicate and add (MIRA) procedure. The proposed scheme does not require knowing the length of the CIR and is valid even in the presence of spectral nones. We provide the theoretical results that guarantee the validity of the developed technique. After the estimation process, the transmitted symbols are also reconstructed by means of the DCT1e using the same novel MIRA scheme. The conditions that ensure a perfect reconstruction in the absence of noise are also provided in this case. Numerical simulations illustrate the excellent behaviour of the proposed approach, both in terms of channel estimation and recovery of the transmitted information.Ministerio de Economía y Competitivida