April 2012
Beginner
232 pages
5h 51m
English
Appendix A
Figure A.1 shows how a spreading code of order 4 can be produced for a pair of messages to be sent via a CDMA link. This example is for illustration only, as 4 bits is not a practical length of code. Figure A.2 shows how the received signal (effectively the sum of the two signals from Figure A.1) can be decoded to reveal the two messages. The ‘correlation’ process is to take the spreading code used to encode the message to be decoded (in the case of GPS it is the spreading code for a particular satellite) and multiply it by the received signal. Over the length of the spreading code the result is then integrated and divided by the length of the spreading code and if a message exists a non-zero result is achieved. In the example in the second row of Figure A.2 the message ( + 1, − 1, + 1) is successfully decoded. In a similar way the message sent via the second spreading code is decoded correctly in the third row of Figure A.2. In the fourth row of Figure A.2 another spreading code (which was not used to encode anything) is used and correctly shows a null message.
Figure A.1 Forming a simple CDMA signal with a spreading code of order 4.
Figure A.2 Receiving the CDMA signal.
This is a very simple illustration and assumes that the spreading code and the received signal ...
