The major issue in wireless communication system is how to accommodate many users within a fixed bandwidth. This of course is accomplished in the Code Division Multiple Access (CDMA) system by using certain coding properties, modulation techniques and optimization algorithms. The authors, Kishor G Maradia, S M Joshi and Jayesh S Patel, in the first paper, “Genetic Algorithm for CDMA-Based MUD Techniques Under Rayleigh Fading Environment”, explore usage of the concept of genetic algorithm for the Multi-User Detection (MUD) model—intelligent estimation/demodulation of transmitted bits in the presence of multiple access interference—in multipath Rayleigh fading environment. However, in order to reduce interference from other users sharing the same frequency band, the authors exploit genetic algorithms by analyzing and comparing simulated results for different number of users, modulation techniques and spreading codes, using Bit Error Rate (BER) and time profile as the measures of performance. Based on the results, they conclude that genetic algorithm is the best algorithm for MUD in CDMA-based wireless communications.

This is followed by the paper, “Speech Enhancement in Non-Stationary Noise Environments: An Efficient Approach”, by Ch V Rama Rao, M B Rama Murthy and K Srinivasa Rao. Here, the authors attempt to find a way to reduce noise in mobile communications for improving the quality of speech communication using a two-stage noise reduction algorithm that suits well for both quasi-stationary and non-stationary noise environments. They use the algorithms to filter noise using spectral gain and perceptually motivated weighting techniques. Comparing the results so obtained, the authors conclude that the algorithms devised by them are suitable for speech enhancement even in non-stationary noise environments.

In the next paper, “Performance Comparison of Adaptive Cross-Layer Schemes in Wireless Ad Hoc Networks”, the authors, T Aruna and M Suganthi, investigate the performance of the cross-layer design combining AMC-TARQ and AMC-HARQ under fading channels, so as to enhance spectrum efficiency and channel utilization under high-speed wireless multimedia networks. They choose the physical layer modes based on the SNR thresholds. Their study reveals that the AMC-TARQ system would provide better spectral efficiency than both the AMC and AMC-HARQ systems.

In the next paper, “Impact of SRS Nonlinear Effect of Optical Fiber in the Presence of Ultra Wide Band EDFAs in an OFDM Communication System”, the author, Neelam Srivastava, explores the scope for practical and cost-effective approaches that can be deployed rapidly and easily to create long, repeaterless, point-to-point links, taking into consideration the Optical Frequency Division Multiplexing (OFDM) system, where ultra wide band Erbium Doped Fiber Amplifier (EDFA) is used as an in-line amplifier. The study takes into consideration only the Stimulated Raman Scattering (SRS) nonlinear effect. The author concludes that the total number of channels that can simultaneously be transmitted through EDFAs will be 700 in the Wavelength Division Multiplexing (WDM) system. The maximum repeaterless distance is governed by SRS and the noise generated in EDFA itself. It is, however, concluded that increasing the optical amplifier initially increases the span length, but due to accumulation of amplifier noise in cascaded states, the distance between two amplifiers decreases as the over loss factor increases. It is also said that after a particular distance, an increasing number of optical amplifiers does not increase the overall repeaterless distance.

Commercial cellular networks such as Direct Sequence-Code Division Multiple Access (DS-CDMA) suffer from many types of interferences impacting the efficiency of communication. To improve the efficiency of communication systems, the authors, S D Parmar and Bhuvan Unhelkar, in the next paper, “An Application of PearsonICA to DS-CDMA Detection”, evaluate the performance of independent component analysis algorithm to solve the symbol estimation problem of the multiusers in a DS-CDMA communication system. The focus of the paper has been blind separation of convolved CDMA mixture and improvement in downlink symbol estimation. The results of the numerical experiment are then compared with the results obtained using the SUD receiver, the ICA detector and the combined SUD-ICA detector. The authors conclude that the ICA detector could solve the symbol estimation problem with no spreading code required.

In the next paper, “A VHDL Implementation of Orthogonal Interleavers for the IDMA Scheme”, the authors, M Shukla, V K Srivastava and S Tiwari, discuss the role of interleavers in CDMA for coding gain as the means of user separation in IDMA and as the periodic optical filters that combine or separate a comb of dense wavelength division multiplexed signals in optical fiber communication. The authors compare the performance of RI, MRI and TBI using VHDL on FPGA, based on their hardware requirement and time constraints, and conclude that the tree-based interleaver has the best performance.

The authors, S Swapna Rani, Rajkumar Goswami and G Sasi Bhushana Rao, of the last paper, “Tracking of Objects in the Images Received from Imaging Sonar of Autonomous Underwater Vehicles (AUVs)”, present a simple method of tracking objects based on their centroids by undertaking simulations in the MATLAB environment. Based on the results, the authors conclude that the smoothed estimate using the Kalman filter is better with smaller uncertainties and work faster when the vector state is small. The authors also comment that there is a need to include acceleration too as a part of the state, for having the best estimation of trajectory.

-- GRK Murty
Consulting Editor