Providing Higher Throughput for a Single User with M-ary Orthogonal Walsh Codes

Reliable communication is a challenge in a very noisy RF channel further corrupted by severe, multiple narrowband interference. Code Division Multiple Access (CDMA) is a widely used method to both mitigate such interference and support multiple in-band users. The ability to time-control multiple receptions at a receiver permits use of deterministically orthogonal codes. Achieving full capacity of the available frequency band depends, in part, on ensuring that all signals in the band are from the agreed set of orthogonal signals. Each unwanted signal consumes a fraction of the available capacity. In the extreme, as more interferers are added the available bandwidth can support only one user. We adapt CDMA techniques to a single user by assigning a constellation of M symbols built around Walsh codes. This M-ary constellation encodes K bits of data into one of the length N codewords, and gives preference to a non-power-of-two Hadamard length, e.g., 12, 20, and higher multiples. The tradeoff between the modified and conventional Walsh code designs is that one user sends K bits per Walsh symbol interval as opposed to many users sending one bit each per symbol. The throughput sacrifice is a consequence of the severe external interference environment. Keywords—sonobuoy; Walsh codes; Code Division Multiple Access