Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

8.5 CODE DIVISION MULTIPLE ACCESS

Code division multiple access (CDMA) technology has been developed for defense applications where secure communication is very important. In CDMA systems, a very large bandwidth channel is required, many times more than the bandwidth occupied by the information to be transmitted. For instance, if the actual bandwidth required is 1MHz, in CDMA systems, perhaps 80MHz is allocated. Such large bandwidths were available only with defense organizations, and hence CDMA was used initially only for defense applications. Because the spectrum is spread, these systems are also known as spread spectrum multiple access (SSMA) systems. In this category, there are two types of techniques: frequency hopping and direct sequence.

In spread spectrum multiple access, a wide bandwidth channel is used. Frequency hopping and direct sequence CDMA are the two types of SSMA techniques.

8.5.1 Frequency Hopping (FH)

Consider a system in which 1MHz bandwidth is required to transmit the data. Instead of allocating a radio channel of 1MHz only, a number of radio channels (say 79) will be allocated, each channel with 1MHz bandwidth. We need a very large spectrum, 79 times that of the actual requirement. When a station has to transmit its data, it will send the data in one channel for some time, switch over to another channel and transmit some more data, and again switch over to another channel and so on. This is known as frequency hopping (FH). When the transmitting station hops its frequency of transmission, only those stations that know the hopping sequence can receive the data. This will be a secure communication system if the hopping sequence is kept a secret between the transmitting and the receiving stations.

Frequency hopping, as used in Bluetooth radio system, is illustrated in Figure 8.8. Here the frequency hopping is done at the rate of 1600 hops per second. Every 0.625 milliseconds, the frequency of operation will change. The terminal will receive the data for 0.625 msec in frequency f1, for 0.625 msec in f20, for 0.625 msec in f32, and so on. The hopping sequence (f1, f20, f32, f41…) is decided between the transmitting and receiving stations and is kept secret.

Figure 8.8: Frequency hopping.

In frequency hopping (FH) systems, each packet of data is transmitted using a different frequency. A pseudo-random sequence generation algorithm decides the sequence of hopping.

Frequency hopping is used in Global System for Mobile Communications (GSM) and Bluetooth radio systems.

8.5.2 Direct Sequence CDMA

In direct sequence CDMA (DS-CDMA), each bit to be transmitted is represented by multiple bits. For instance, instead of transmitting a 1, a pattern of say 16 ones and zeros is transmitted, and instead of transmitting a 0, another pattern of 16 ones and zeros is transmitted. Effectively, we are increasing the data rate and hence the bandwidth requirement by 16 times. The number of bits to be transmitted in place of 1 or 0 is known as chipping rate. If the chipping code is kept a secret, only those stations that have the chipping code can decode the information. When multiple stations have to transmit, the chipping codes will be different for each station. If they are chosen in such a way that they are orthogonal to each other, then the data from different stations can be pushed on to the channel simultaneously without interference.

As shown in Figure 8.9, in DS-CDMA, multiple terminals transmit on to the channel simultaneously. Because these terminals will have different chipping codes, there will be no interference.

Figure 8.9: DS-CDMA.

CDMA systems are now being widely deployed for cellular communications as well as 3G systems for accessing the Internet through wireless networks. CDMA systems are used in wireless local loops.

In DS-CDMA, multiple terminals transmit on the same channel simultaneously, with different chipping codes. If the chipping code length is say 11 bits, both 1 and 0 are replaced by the 11-bit sequence of ones and zeros. This sequence is unique for each terminal.