Method and apparatus for despreading spread spectrum signals

Abstract

A system for despreading and/or demodulating spread spectrum codes is provided. A data signal is encoded using an M-ary transmission technique, wherein predefined symbol codes are associated with a particular series of data bits. A receiver comprises a plurality of delay elements or a shift register with a plurality of taps configured to recognize one or more the predetermined symbol codes. Delayed representations of the received signal are generated and combined to produce a known output sequence, such as a string of consecutive one or consecutive zeroes, when a recognizable spread spectrum code is received. The receiver shift register may be preloaded such that recognition may occur starting with the first received chip. More than one set of taps may be coupled to the same shift register, so that more than one of the M spreading codes may be recognized by the same shift register. A received set of chips may be identified with a code sequence that has a minimal cross-correlation distance or Hamming distance, whereby a received code sequence with imperfect correlation may be recognized as corresponding to a defined symbol. More than one symbol may be simultaneously transmitted using quadrature phase modulation or like techniques, and recognized by the receiver.