Walsh-like Nonlinear Phase Orthogonal Transforms for CDMA Communications

Historically Walsh codes and Gold codes have been used as spread spectrum codes in CDMA communications. Popularity of these codes is more from the ease of generation from standard kernels rather than the efficiency of the codes. Availability of these codes is limited in number and in their lengths. By relaxing the restrictions like zero crossings, linear phase etc on the nature of codes, number of new sets of codes can be generated. In this paper, methodology has been proposed for generating new Walsh-like family codes that can be used for both synchronous and asynchronous communications. Performance characteristics of these codes have been studied in AWGN and in Rayleigh fading channels. These codes outperform Walsh codes in their asynchronous channel behavior and closely match with Gold codes performance. In addition, we present in this paper that there are a good number of such desirable code sets available in the binary sample space with different transform sizes. These new binary sets with good performance and flexible code lengths might help us to improve the spread spectrum capabilities of future CDMA communications systems.