Abstract: A transmitter encodes and a receiver decodes a signal using M-ary bi-orthogonal keying. The transmitter receives a stream of data bits and breaks a number of data bits off of the stream to form a bit sequence. The transmitter then chooses a code that corresponds the bit sequence, and transmits the chosen code. The receiver receives the code and correlates it with all possible codes to generate a plurality of correlation values. It then compares these correlation values to determine which code was sent, and therefore what the received bit sequence is. The codes are all mutually orthogonal, and preferable include a plurality of code words and an equal number of code word inverses. The code may be scrambled by multiplying it with a pseudo-random sequence at the transmitter, and descrambled by multiplying it by the same pseudo-random sequence at the receiver.