RECEIVER STRUCTURE IN A DIGITAL SPREAD SPECTRUM COMMUNICATION SYSTEM

摘要

A receiver structure (1) for a DS/CDMA (direct sequence/code division multiple access) digital communication system comprises a correlator (3) with a plurality of correlator-units (4.1 to 4.3) for code acquisition as the first step of code synchronisation of an incoming signal which is spread on the sender side with a spreading code. Since the communication system channel shows large frequency offsets, each correlator unit is made for searching a specific time- and frequency-region by using a separate despreading code for correlation against the incoming signal (2). The first correlator-unit (4.1) uses a replica (5) of the original spreading code which means no frequency offset. The second correlator-unit (4.2) uses a despreading code which is a swivelled version of the replica where a swivelling operator (7) rotates each segment of the replica by +/- pi /4 or a multiple thereof. In the chosen example, a frequency offset of +/-7 kHz of the incoming signal corresponds to a rotation of the n-th segment by an angle of +/-((n-1) * pi /4). The third correlator-unit (4.3) also uses a swivelled code. The swivelling operator (6) rotates the n-th segment by an angle of +/-((n-1) * pi /4), corresponding in the example to a frequency offset of +/-12 k Hz. For easier implementation of the correlator the symbol values of the symbols at phase offsets of +/- pi /4, +/-3* pi /4, +/-5* pi /4 and +/-7* pi /4 are taken as (+/-1 +/-j) instead of 0.5* 2ROOT 2* (+/-1 +/-j).