it will be successively multiplied by four chip bits. It is also worth noting that the spread rate is the number of data bits in the chip code, i.e. the number of bits that each data bit is multiplied by. In this example the spread rate is four because there are four bits in the chip code. Spread Spreading or chip code Figure 3.19 Multiplying the data stream with the chip stream To produce the final signal that carries several data streams the outputs from the individual multiplication processes are summed. This signal is then converted up to the transmission frequency and transmitted. Spread Output signal for modulation signals onto a carrier Summing circuit Figure 3.20 Generating a signal that carries several sets of data 74 Newnes Guide to Radio and Communications Technology At the receiver the reverse process is adopted. The signal is converted down to the base-band frequency. Here the signal is multiplied by the relevant chip code to extract the relevant data in a process known as correlation. By multiplying by a different chip code a different set of data will be extracted. To illustrate how this operates it is best to provide an example showing how the signal is generated, and then the original data extracted from the received signal. Chip code A 1 -1 -1 1 Chip code B -1 1 -1 Data stream 1 This makes chip stream: -111-1 1-1 -1 Data stream 2 This makes the chip stream: -1 1-11 1 -1 1 -1 1-1 1-1 1 -1 1 -1 Adding the two chip streams the sum that is transmitted becomes: 0 0 -2 2 0 0 2 -2 2 -200 -2 0 0 In the receiver this needs to be multiplied by the relevant code. For the example take code A: 0 0 -2 0 0 2 -2 2-2 200 2 -2 0 0 multiply by 1-1-1 1 1 -1-11 1-1-1 1 1-1-1 1 and this becomes 0 0 -2 -2 add the sum of each group This equates to the original signal of When random or more correctly a pseudo-random spreading code is used a similar process is followed. Instead of using the orthogonal codes, a pseudo-random spreading sequence is used. Both the transmitter and receiver will need to be able to generate the