Kwok-Kai Soo

PAPR reduction of an OFDM signal by use of PTS with low computational complexity

One of the major drawbacks of orthogonal frequency division multiplexing (OFDM) is the high peak-to-average power ratio (PAPR) of the transmitted OFDM signal. Partial transmit sequence (PTS) technique can improve the PAPR statistics of an OFDM signal. However optimum PTS (OPTS) technique requires an exhaustive search over all combinations of allowed phase factors, the search complexity increases exponentially with the number of sub-blocks. By combining sub-optimal PTS with a preset threshold, a novel reduced complexity PTS (RC-PTS) technique is presented to decrease the computational complexity. Numerical results show that the proposed approach can achieve better performance with lower computational complexity when compared to that of other PTS approaches.

Particle-Swarm-Optimization-Based Multiuser Detector for CDMA Communications

In a code-division multiple-access system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase system capacity. However, the optimum MUD can be characterized as an NP-hard combinatorial optimization problem such that the computational complexity increases exponentially with the number of users. In this paper, we apply a new evolutionary algorithm, called particle swarm optimization (PSO), to develop a suboptimal MUD strategy. The decorrelating detector (DD) or linear minimum mean square error (LMMSE) detector is used as the first stage to initialize the PSO-based MUD. Then, the PSO algorithm is applied to detect the received data bit by optimizing an objective function incorporating the linear system of the DD or LMMSE detector. Simulation results show that the performance of our proposed decorrelating PSO and LMMSE-PSO MUD are promising and outperform the decorrelating and LMMSE MUD with a slight increase in computation.

A Low Complexity Selected Mapping Scheme by Use of Time Domain Sequence Superposition Technique for PAPR Reduction in OFDM System

By using the time domain sequence superposition (TDSS) technique, in this paper, we propose a low complexity selected mapping (LC-SLM) scheme for the peak-to-average power ratio (PAPR) reduction of the orthogonal frequency division multiplexing (OFDM) system. Unlike the conventional selected mapping (SLM) scheme which needs several inverse fast Fourier transform (IFFT) operations for an OFDM signal, the proposed scheme requires to implement two IFFT operations only. Thus, it can remarkably reduce the computational complexity. Simulation results show that the proposed scheme can achieve similar PAPR performance as the conventional SLM scheme.

PAPR Reduction Using Low Complexity PTS to Construct of OFDM Signals Without Side Information

Partial transmit sequence (PTS) is one of the most well-known peak-to-average power ratio (PAPR) reduction techniques proposed for orthogonal frequency-division multiplexing (OFDM) systems. The main drawbacks of the conventional PTS (C-PTS) are high computational complexity and transmission of several side information bits. A new PTS with simple detector is proposed in this paper to deal with these drawbacks of C-PTS. The candidates can be generated through cyclically shifting each sub-block sequence in time domain and combining them in a recursive manner. At the receiver, by utilizing the natural diversity of phase constellation for different candidates, the detector can successfully recover the original signal without side information. Numerical simulation shows that the proposed scheme performs very well in terms of PAPR. The probability of detection failure of the side information demonstrates that the detector could work without any side information with high reliability. The proposed scheme achieves almost the same bit error rate (BER) performance as the C-PTS with perfect side information, under both additive white Gaussian noise (AWGN) channel and Rayleigh fading channel.

SSOR preconditioned CG method for the linear interference cancellation of asynchronous CDMA systems

An ideal computation of the decorrelating or the linear minimum mean-squared-error (LMMSE) detector requires computational complexity of order K 3 when there K is the number of users. To alleviate the computational complexity, iterative decorrelating and LMMSE detectors are proposed for solving a set of linear equations corresponding to linear interference cancellation structures. Iterative conjugate gradient (CG) method has been used for the linear interference cancellation detectors. Its main advantages are to reduce the order of computation complexity and their suitability to highly parallel implementations. In this paper, the symmetric successive overrelaxation (SSOR) preconditioning scheme is applied to the CG method. The performance of the detectors is investigated and it is found that the SSOR preconditioned CG method can provide significantly faster convergence than CG method.

SLM with non-unit magnitude phase factors for PAPR reduction in OFDM

Selected mapping (SLM) scheme with non-unit magnitude phase factor is developed for the peak-to-average power ratio (PAPR) reduction of the orthogonal frequency division multiplexing (OFDM) system. A new low-complexity version of SLM is introduced, which employs the shifting time-domain signal combination technique to generate new candidates. Unlike the conventional SLM scheme, which needs several inverse fast Fourier transform (IFFT) operations for an OFDM signal, the proposed scheme requires to implement one IFFT operation only. Thus, it can significantly decrease the computational complexity. The simulation results show that the proposed scheme can obtain slightly poorer PAPR reduction performance than the conventional SLM scheme with the same number of candidates.

System Performance Enhancement in CDMA Mobile System with Uplink Adaptive Power Control Based on Fuzzy Logic

In this paper, an uplink adaptive SIR (signal-to-interference)-based powercontrol mechanism fora CDMA mobile system using fuzzy logic techniques is developed and analyzed.The proposed algorithm providesbetter system performance in terms of outage probability, response time, andstability than current power controlalgorithms and has the advantage of easy implementation.

Linear Interference Cancellation of Asynchronous CDMA Systems by Preconditioned CG Method

It is well known that the iterative method is used for solving a set of linear equations corresponding to linear interference cancellation structures. An ideal computation of the decorrelating or the linear minimum mean-squared-error (LMMSE) detector requires order K3 flops, where K is the number of users. To alleviate the computational complexity, iterative decorrelating and the LMMSE detectors are proposed. In this paper, the symmetric successive over relaxation (SSOR) preconditioning scheme is applied to the conjugate gradient (CG) method. The performance of the detectors is investigated and it is found that the SSOR preconditioned CG (PCG) method can provide significantly faster convergence than CG method.

New Construction Scheme to Reduce the PAPR of M-QAM OFDM Signal

A novel time domain combination scheme is proposed to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) systems. in this paper, a high-order quadrature amplitude modulation (QAM) OFDM signal sequence is constructed with several Quaternary/Binary phase shift Keying (QPSK/BPSK) signal sequences. As this operation can be implemented after the inverse fast fourier transform (IFFT), more candidates can be accommodated by the proposed scheme with fewer numbers of IFFT operations. Although for the same number of candidates, the PAPR performance of our proposed scheme is slightly lower than the conventional selected mapping (C-SLM), our scheme significantly decreases the computational complexity. Simulation results also show that the proposed scheme almost achieves the same bit error rate of the C-SLM when they pass through the nonlinear high power amplifier.

Hybrid Reduced-Complexity Multiuser Detector for CDMA Communication Systems

In a code-division multiple-access (CDMA) system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase system capacity. However, the optimum MUD for CDMA systems requires the solution of an NP-hard combinatorial optimization problem. It is well known that the computational complexity of the optimum MUD is exponential with the number of active users in the system. In this paper, we apply a hybrid algorithm to develop a suboptimal MUD strategy. The result of the symmetric-successive-overrelaxation-preconditioned conjugate-gradient (SSOR-CG) method is first used to initialize the reduced-complexity recursive (RCR) MUD. Then, the RCR algorithm is applied to detect the received data bit by optimizing the likelihood function. Simulation results for the synchronous and asynchronous case show that the performance of our proposed SSOR-RCR MUD is promising and outperforms the decorrelator, linear minimum mean-squared error, and original RCR MUD.