We analyze and compare the error performances of two multirate transmission schemes in W-CDMA: multicode (MCD) and variable spreading gain (VSG). First, we show that in an AWGN channel, the VSG data user performs better than the MCD user if the number of in-cell voice users are low and the background noise is weak. However, as the noise power or the number of voice subscribers increases, the performances of the two schemes become similar. We then show that in a multipath fading channel, the two schemes achieve very similar error performances as long as their receivers are able to track all resolvable signal paths. However, when the receiver can only track multipath components within the current symbol interval, the VSG user yields a much worse error probability than the MCD user. This is because the VSG data user has a much shorter symbol interval. Simulation results are illustrated to support the analysis.

This paper presents a new protocol for statistical multiplexing of bursty data traffic in the forward (base-to-mobile) link of a wireless wideband code division multiple access (W-CDMA) system using orthogonal variable spreading factor (OVSF) codes. At the heart of the protocol is an efficient scheduling algorithm that dynamically assigns an OVSF code to a mobile user on a timeslot-by-timeslot basis and allows many users with bursty traffic to share a limited set of OVSF codes. An important feature of our protocol is that it can provide a heterogeneous data rate guarantee to each mobile user and fully utilize the system capacity. Moreover, the unreserved bandwidth of the network can be shared fairly among competing mobile users.

In a single-cell DS-CDMA system, linear MMSE detectors has been shown to significantly outperform conventional detectors in terms of spectral efficiency. Furthermore, simulation shows that the spectral efficiency of asynchronous CDMA systems using MMSE detection and random spreading codes can reach four times that of orthogonal CDMA. In the case of universal frequency reuse, however, this is not necessarily true because of the presence of strong inter-cell interference (ICI). This paper analyzes the reason for the superior performance of MMSE detection in single-cell asynchronous CDMA systems and investigates the potential for performance enhancement using MMSE detection in multi-cell environment with sectorization and a higher frequency reuse factor. Our results indicate that, with sectorization and reduced frequency reuse efficiency, using the MMSE detector can still significantly improve the overall system throughput surpassing that of CDMA systems using conventional detection with universal frequency reuse.

This paper presents an optimal dynamic code assignment (DCA) scheme using orthogonal variable-spreading-factor (OVSF) codes. The objective of dynamic code assignment is to enhance statistical multiplexing and spectral efficiency of W-CDMA systems supporting variable user data rates. Our scheme is optimal in the sense that it minimizes the number of OVSF codes that must be reassigned to support a new call. By admitting calls that would normally be blocked without code reassignments, the spectral efficiency of the system is also maximized. Simulation results are presented to show the performance gain of dynamic code assignment compared to a static assignment scheme in terms of call blocking rate and spectral efficiency. We also discuss various signaling techniques of implementing our proposed DCA scheme in third-generation wideband CDMA systems.