Efficient utlization of OVSF codes for CDMA networks

Abstract

Wideband code division multiple access (WCDMA) is the main third generation air interface in the world, as per specification created in third generation partnership project (3GPP). Orthogonal Variable Spreading Factor (OVSF) based CDMA networks suffer from limitation of code blocking. The transmission in forward and reverse link is carried out using two codes namely; scrambling and channelization codes. In the forward link, scrambling codes are used for interference mitigation and channelization codes are used for user separation and rate matching. On the other hand, in the reverse link the scrambling codes are used for user identification and interference mitigation while channelization codes are used for rate matching. The channelization codes in WCDMA are OVSF codes. For the forward link, a large number of channels are required corresponding to large mobile stations (MSs). This requires a large number of OVSF codes at each base station (BS). As there are limited number of codes in the code tree, the efficient use of OVSF codes is essential in the forward link. As in the reverse link, the code tree is used by unique user, the number of codes are sufficient. Therefore, the work done in this thesis is concentrated on the forward link optimization. The orthogonal nature of codes in the OVSF code tree requires a careful attention during code assignment. The code assigned is not orthogonal to its parents and children. Hence, a particular assigned code blocks all its parents and children from assignment. This may result in scattering of vacant codes in the OVSF code tree. A situation may be reached where the code tree has enough capacity to handle an incoming call but due to scattered vacant codes (capacity), the call is rejected.