e selection occurs. For signals such as AM and FM, different frequencies are used, and the receiver can be set to a given frequency to select the required signal. Other systems use differences in time. For example, using pulse code modulation, pulses from different signals are interleaved in time, and by synchronizing the receiver and transmitter to look at the overall signal at a given time, the required signal can be selected. CDMA uses different codes to distinguish between one signal and another. To illustrate this, take the analogy of a room full of people speaking different languages. Although there is a large level of noise, it is possible to pick out the person speaking English, even when there may be people who are just as loud, or may be even louder speaking a different language you may not be able to understand. The system enables several sets of data to be placed onto a carrier and transmitted from one base station, as in the case of a cellular tele- communications base station. It also allows for individual units to send data to a receiver that can receive one of more of the signals in the presence of a large number of others. To achieve this the signal is spread over a given bandwidth. This is achieved by using a spreading code. This operates at a higher rate than the data. The codes for this can either be random, of they can be what is known as orthogonal. Orthogonal codes are ones which when multiplied together and then added up over a period of time have a sum of zero. To illustrate this, take the example of two codes: Modulation 73 Code A 1 -1 Code B Product 1 1 -1 -1 summed over a period of time = 0, i.e. 1 + 1 - 1 1 = 0 Using orthogonal codes it is possible to transmit a large number of data channels on the same signal. To achieve this the data is multiplied with the chip stream. This chip stream consists of the codes being sent repeatedly SO that each data bit is multiplied with the complete code in the chip stream. In other words if the chip stream code consists of four bits then each data b