Adaptive rate QS-CDMA UWB systems using ternary OVSF codes with a zero-correlation zone

For a quasi-synchronous code-division multiple access (QS-CDMA) system, the relative time delays between the signals of different users can be maintained with a given time certainty. In this paper, the time delay is modelled as a uniformly distributed random variable on (-tau, tau). It is known that the multiple access interference (MAI) of the QS-CDMA system can be significantly reduced by employing spreading sequences with a zero-correlation zone (ZCZ). Hence, we propose a construction method for the design of ternary orthogonal variable-spreading-factor (OVSF) codes with a ZCZ of arbitrary lengths. Based on the knowledge of tau, we demonstrate how to select the OVSF code tree with different ZCZ to increase the aggregate throughput of DS-UWB systems with a loose synchronization. Once the OVSF code tree is selected, an adaptive date rate transmission scheme is proposed which can further increase the aggregate throughput by allocating OVSF codes to the active users