The Coexistence Structure of the OFDMA and the WCDMA for the Delay Sensitive Services in Future Moblie Multimeida Systems

Abstract

With the explosive demands on wireless communications and with the emergence of bandwidth intensive multimedia applications, satisfying the quality of service (QoS) in wireless multimedia networks is becoming more and more important. Providing the heterogeneous service classes with different QoS requirements such as the maximum allowable delay and the average target data rate is needed in the future wireless communication systems. The orthogonal frequency division multiple access (OFDMA) is one of the candidate systems for the next generation communication systems due to its high data rate capability even in the rough wireless communication environment. In the OFDMA systems, multiple users simultaneously transmit data on the different subcarriers per symbol. Since the probability that all users experience a deep fade in a particular subcarrier tend to be low, the performance gain can be achieved by assigning subcarriers to the users who have good channel gains on those subcarrier. However, the subcarrier assignment is not efficient enough to the bursty delay sensitive services due to its long processing time and its complexity. Furthermore, since the OFDMA systems are sensitive to the misalignment of timing and the frequency offset, the synchronization is required between the transmitter and the receiver. All the processes such as the subcarrier assignment and the synchronization cause a significant time delay in the OFDMA system. Due to this reason, random access schemes which do not need the long processing delay are considered as the solution for the bursty delay sensitive services. In this thesis, we propose the coexistence system structure of the OFDMA and the WCDMA for providing the QoS of delay sensitive services. By utilizing the spread spectrum technique of the WCDMA which is less sensitive to the offset problem, the delay sensitive signal is spread into a part of the OFDMA frequency band, and then transmitted in the random access fashion. Because of the interference between the spreading signal in the WCDMA and the OFDMA subcarriers within the same frequency band, the modulation mode of the OFDMA should be controlled adaptively, and the acceptable number of users for the WCDMA is limited. The simulation results show that the delay performance of the proposed system is better than that of the conventional scheme while maintaining the comparable throughput performance.