Vacant Code Precedence and Call Blocking Reduction in WCDMA Systems based on OVSF Codes

In this paper a code assignment scheme called Next Code Precedence High (NCPH) is proposed for UTRA-FDD (universal terrestrial radio access frequency division duplex) systems based on OVSF (orthogonal variable spreading factor) channelization codes. Using the proposed scheme, the number of codes searched before finding suitable vacant code is least. Also the external code fragmentation which can lead to code blocking is less because of the compact nature of code assignment scheme. Simulation results are presented to compare the reduction in call blocking probability of the proposed scheme with the existing novel assignment schemes.

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. Fig. 1. 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. TABLE I 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]