The Orthogonal Frequency Division Multiple Access (OFDMA) is fast gaining importance as a new multiple access technology to meet the future broadband wireless access systems’ demands for support of high data rates, operation in a hostile multipath radio environment, provision of various QoS profiles, and minimal consumption of RF bandwidth and transmit power. Though Multicarrier communications increase the efficiency and capacity of wireless networks, they are very sensitive to carrier frequency synchronization errors. But owing to the nature of multiple-parameter estimation, it is pretty challenging to estimate the carrier frequency offset in the uplink of OFDMA system—particularly in an interleaved carrier assignment scheme.

The authors Miriam Bank, Michael Bank, Boris Hill and Uri Mahlab of the first article “OFDMA Systems, Pilot Signals and Doppler Effect”, make an attempt to trace the enormous difficulties that the cellular systems implementing OFDMA are facing.

The high level of reflected signals found in big cities are making cellular systems more sensitive to Doppler Effect (DE). The authors study the impact of DE on orthogonality disturbance in critical but real situations. The simulation results reveal that due to orthogonality disturbance owing to DE influence, it is almost impossible to develop an OFDMA cellular system based on the principle of pilot signals. They are of the opinion that pilot signals do not improve the decoding process and on the other hand may even lead to deterioration in the decoding process. It may thus become one of the underlying causes for mobile OFDMA system failure.

The second article, “Optimization of Soft Handover Margin in WCDMA Cellular System”, by S Tamilselvan and K Manivannan, analyzes the downlink performance of a Wideband Code Division Multiple Access (WCDMA) system with Site Selection Diversity Transmission power control during soft handover mode so as to improve the system capacity.

The authors also attempt to find the optimum SHO margin in terms of maximum system capacity. Analyzing the system capacity dependency on MSH by considering uniform user distribution and a concentration of all the users in the location that requires maximum transmitter power, they found an increase in the user capacity by about 16-20% for the Optimum MSH of 5-5.5 dB. In the third article, “Implementation and Analysis of OFDM and CDMA-Based MIMO V-BLAST for MUD Using ZF and MMSE”, the authors, Kishor G Maradia, Patil Ajay Kumar V and S M Joshi, compare Orthogonal Frequency Division Multiplexing (OFDM) with Code Division Multiple Access (CDMA) in respect of their error performance in multiuser environment. Both the transmitter techniques are performed for MIMO architecture using two algorithms, ZF and MMSE in V-Blast detection architecture. From the results, the authors conclude that Walsh code result is better in system using CDMA-based MIMO V-BLAST system. They also infer that BER increases as the number of users in system increases in both techniques—OFDM and CDMA-based MIMO V-BLAST system. They also observe that the performance of 16-QAM is comparatively better than that of QPSK, 64-QAM in system using CDMA and OFDM-based MIMO V-BLAST system.

The last article of the issue, “Banyan-Based Switching Fabrics for Next Generation Networks”, by V S Tripathi and S Tiwari, reviews the Banyan architecture- based switching fabrics suitable for Next Generation Networks. The authors introduce the Next Generation Networks, briefly tracing the role of switching fabric therein. Then they present an overview of the existing switching fabrics along with a discussion on the merits and demerits of different kinds of fabrics. They also discuss the various traffic models and performance evaluation techniques as are present in the current literature and offer mathematical analysis of some Banyan-based switching fabrics.

-- GRK Murty
Consulting Editor