Heng Siong Lim

Multiuser detection for DS-CDMA systems using evolutionary programming

This article introduces a new multiuser detection scheme which uses evolutionary programming (EP) to detect the user bits based on the maximum-likelihood decision rule. The major advantage of the proposed detector is that it has a lower computational complexity compared to other popular evolutionary-algorithm-based detectors. The simulation results show that the EP has always converged to the optimum solution with a small number of generations. The simulated average computational time performance demonstrates that this approach achieves practical ML performance with polynomial complexity in the number of users.

An efficient local search heuristics for asynchronous multiuser detection

This letter considers the application of k-opt local search to the detection of data transmitted through the additive white Gaussian noise channel by K asynchronous users using direct-sequence code-division multiple access. The algorithm is based on a general approach to heuristics that is highly efficient in solving large combinatorial optimization problems. It is shown that the algorithm manages to produce optimum solutions with high frequency, in running time that grows about O(n/sup 2/).

On the Optimal Alpha-

The alpha-k (alpha - k) curve is an important tool for real-time myriad filtering problems in channels with time-varying impulsiveness. In this letter, we use the asymptotic variance as the criterion to derive the optimal alpha - k curve. We then suggest a new expression for the optimal curve and compare it to the conjectured expression in earlier works.

k

Evolutionary computation was used to train neural networks to learn the play the game of Othello. Each neural network represents a strategy based on board evaluations of the game tree generated by a minimax search algorithm. Networks competed against each other in tournament play and selection used to eliminate those that performed poorly relative to other networks. Self-adaptation was used to mutate the weights and biases of surviving neural networks to generate offspring. By monitoring the evolutionary behavior over 1000 generations through game competitions with computer players playing at higher ply-depths using deterministic evaluations, the networks are shown to coevolve with the style of game play progressing from random to positional and finally to mobility strategy.

Curve of the Sample Myriad

This paper investigates the decoding of turbo codes in impulsive symmetric α-stable (SαS) noise. Due to the nonexistence of a closed-form expression for the probability density function (pdf) of α-stable processes, numerical-based SαS pdf is used to derive branch transition probability (btp) for the maximum a posteriori turbo decoder. Results show that in Gaussian noise, the turbo decoder achieves similar performance using both the conventional and the proposed btps, but in impulsive channels, the turbo decoder with the proposed btp substantially outperforms the turbo decoder utilizing the conventional btp. Results also confirm that the turbo decoder incorporating the proposed btp outperforms the existing Cauchy-based turbo decoder in non-Cauchy impulsive noise, while the two decoders accomplish similar performance in Cauchy noise.

Evolved neural networks learning Othello strategies

Compressed image will always reduce the image fidelity. This is especially so if the image is compressed at lower bit rates, which most of the time cannot be tolerated. Compression is necessary in medical field due to the limited transmission bandwidth and storage capacity. In this paper, performance of the controversial lossy compression on medical modalities using the new JPEG2000 and the more conventional JPEG is being studied. The common objective measure, peak signal-to-noise ratio (PSNR) is definitely not adequate as a perceptual quality measurement. Therefore, subjective measures such as picture quality scale (PQS) and mean opinion score (MOS) are used as the benchmark instead, along with the statistical and frequency measures for the image characteristics evaluation. Three types of medical modalities are used - X-ray, ultrasound and computed tomography (CT). From these various measurements, the general acceptable compression bit rate for each of the medical modality is determined.

Decoding of Turbo Codes in Symmetric Alpha-Stable Noise

The sample myriad and the weighted myriad filter are normally computed using the batch processing fixed-point algorithm. Since a block of input samples has to be gathered first before the algorithm can perform estimation, significant delay may arise if the block size is large. In this correspondence, we derive the sequential sample myriad and sequential weighted myriad that compute the estimate in real-time by updating the current estimate whenever a new input sample becomes available. Simulation results show that the proposed sequential techniques which have a lower computational complexity, achieve almost the same convergence speed and accuracy as the fixed-point algorithm.

Medical Image Processing: From Lossless to Lossy Compression

The problem of impulsive noise suppression for direct-sequence code-division multiple-access (DS-CDMA) systems employing multiuser detection in impulsive channels is treated. Prior to the detector, linear chip-matched filter is commonly used for continuous-to-discrete-time conversion. The effects of the non-Gaussian impulsive channel in an asynchronous system are studied by modeling the samples of noise before front-end filtering as a symmetric @a-stable process. The error probability performance of the linear correlation receiver under these conditions is compared to that for additive white Gaussian noise (AWGN) channels. It is found that the approach performs poorly in the presence of impulsive noise. To overcome this problem, an impulse mitigation strategy derived from the myriad filtering framework is employed to produce reliable soft channel outputs over a wide range of noise characteristics. The asymptotic error probability performance of the chip-matched myriad filter (CMMyF)-based receivers is then computed and compared to the computer simulated expression. The results show that, in contrast to the linear filter, the CMMyF is effective in removing the impulsive noise and leads to a reliable recovery of the desired chips with low estimation error. The proposed method proves to be fruitful in that the subsequent k-opt multiuser detector is safe to process the chip samples obtained hereafter, thereby retaining all its optimality and advantages.

Sequential Algorithms for Sample Myriad and Weighted Myriad Filter

Fuzzy logic is the principles of imprecise knowledge. Fuzzy adaptive equalizers are adaptive equalizers that apply the concepts of fuzzy logic. The main merit of applying fuzzy adaptive equalizers in powerline channel equalization is that linguistic information (fuzzy IF-THEN rules) and numerical information (input-output pairs) can be combined into the equalizers. The adaptive algorithms adjust the parameters of the membership functions, which characterize the fuzzy concepts in the IF-THEN rules, by minimizing some criterion function. In this paper, we introduce a new type of fuzzy adaptive equalizer using extended Kalman filter (EKF) algorithm for powerline channel equalization. The performance for this type of fuzzy adaptive equalizer is compared with two other types of fuzzy adaptive equalizers using recursive least squares (RLS) and least mean squares (LMS) adaption algorithm. The simulation results show that extended Kalman filter based fuzzy adaptive equalizer has faster convergent speed compared to the other two fuzzy adaptive equalizers. The bit error rate of extended Kalman filter based fuzzy adaptive equalizer is close to that of the optimal equalizer.

Robust chip-matched myriad filter-based multiuser receiver for impulsive channels

This work presents a new two-phase, memetic algorithm to solve the maximum-likelihood multiuser detection problem. The algorithm uses a standard evolutionary programming (EP) for the base level search, which makes a quick decision to direct the search towards the optimal region (phase-1). The local optimization by k-opt heuristic local search is then employed to do fine tuning (phase-2). In order to validate the effectiveness of the proposed method, the algorithm is applied to 10- and 20-user synchronous direct-sequence code-division multiple-access (DS-CDMA) systems. The results clearly show that the algorithm not only improves the solution quality but also makes the EP more efficient in terms of number of iterations to reach the global optimum.