1. Introduction
UTRA-FDD/WCDMA [1], [2] system differs from existing and systems in the use of direct sequence spread spectrum multiple access method. In WCDMA, the channels in the forward link and reverse link use two set of codes known as scrambling codes and channelization codes. In forward link, scrambling codes are used for interference mitigation while the channelization codes are used for rate matching and user separation. In the reverse link, scrambling codes are used for user identification and interference mitigation while channelization codes are used for rate matching. The use of these codes is illustrated in Fig. 1. The channelization codes used in WCDMA are OVSF codes. The code generation in OVSF code tree is given in [3]. The spreading factor of OVSF codes varies as 4,8.,512 in the forward link and 4,8….,256 in the reverse link. The possible data rates in the forward link are is 7.5kbps). The total chip rate transmitted through channel is 3.84Mcps. Due to the orthogonality condition in OVSF code tree, only one code can be used from root to leaf. This gives code blocking in OVSF codes resulting in new call blocking. Therefore the spectral efficiency and throughput decreases if code blocking problem is kept unsolved. Scrambling and Channelization code usage in WCDMA. Figure l(a) shows the forward link transmission from node (Node ) to three UEs. All the channels use same scrambling code and different channelization code. Figure l(b) shows reverse link link transmission from three UEs to Node . All channels/users uses different scrambling codes. Channelization codes need not to be different. Code Assignment Schemes
S. No. | Type | References |
---|---|---|
1. | Single Code Assignment without time multiplexing and without reassignments | Random Assignment [4], Leftmost Code Assignment [4], Fixed Set Partitioning [5], Adaptive Assignment [7], Ours |
2 | Single Code Assignment without time multiplexing and with reassignments | Dynamic Code Assignment [6] |
3. | Single Code Assignment with time multiplexing | NoN-Blocking OVSF codes [8] |
4 | Multiple Code assignment scheme for quantized rates | Multiple rake Combiners [9] |