Tree-structured generation of orthogonal spreading codes with different lengths for forward link of - Electronics Letters

Introduction: Improving the capabilities of multimedia communications is the target for third generation mobile communications systems. A variety of data services from low to very high bit rates is required; most services offered by current mobile radio systems are limited to voice and voice-band data services. Recently, direct sequence code division multiple access @S-CDMA) [l] has been drawing much attention for third generation systems [2, 31. The forward radio link (cell site-to-mobile) can use orthogonal spreading because all code-channels are synchronous, where the codechannels are distinguished by the combination of different short spreading codes (with the same code length) and a single long spreading code [l, 31. Conversely, for the reverse radio link (mobile-to-cell site), all users are asynchronous and therefore, random long spreading codes unique to each user are used. One way to flexibly provide data services of different rates is to assign the multiple code-channels in the forward link to each user according to the data rate requested [3]. This orthogonal multicode assingment requires multiple RAKE combiners at the mobile receivers, each combiner belonging to a different code channel. This increases the complexity of the mobile receivers. Of course, for the reverse link, either orthogonal multicode transmission [3] or variable rate transmission using a single long spreading code [2] can be applied. In this Letter, we propose a tree-structured generation method of orthogonal spreading codes with different code lengths for forward link applications and we evaluate its bit error rate (BER) performance by computer simulation under a multi-user environment wherein multiple users requesting different data rates are active under multipath Rayleigh fading.