  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.
