OVSF Code Allocation in the Forward Link of CDMA Wireless Networks

Abstract

Today is an era of wireless communication, which has drawn the attention of people. newlineWireless scenario of today has travelled a long way since its inception and at present there are more than two billion wireless users worldwide. The main aim of every technology introduced in communication is now wider connectivity with as many services on the same set as compared to earlier schemes. After 1G and 2G, the 3GPP proposed CDMA with a wider band, known as WCDMA. WCDMA was chosen as basic radio-access technology for UMTS/IMT-2000 in all newlinemajor areas of the world. Compared to second generation narrowband CDMA, the WCDMA newlineradio interface offers significant improvements, in addition to the support of higher rate services. These included improved coverage and capacity due to a higher bandwidth and coherent uplink newlinedetection. WCDMA technology uses orthogonal concatenated channelization and scrambling newlinecodes for the simultaneous transmission of data to multiple users in the downlink. These codes are known as OVSF codes. The use of OVSF codes provides variable data rates which support applications with different bandwidth requirements. The SF chosen in these codes depends upon the rate of the incoming call, provided that the product of SF and data rate always equals to 3.84 newlineMcps. OVSF codes are represented as a binary code tree in which the spreading factor of the downstream level is a power of two of the current level, while the relationship of code rates for these two levels is reverse. Any two codes are mutually orthogonal if and only if one code is not the ancestor or descendant code of the other. Avoiding simultaneous use of ancestor and newlinedescendant codes, the orthogonality can be maintained. While handling variable rate calls, OVSF codes exhibit a property known as code blocking, due to which more number of calls can not be handled even if the tree has enough capacity to handle the new call. Most work in the literature is confined to the efficient utilization of the code tree while handling maximum number of calls.