SPREAD-SPECTRUM COMMUNICATIONS

12.8 SPREAD-SPECTRUM COMMUNICATIONS

Spread-spectrum signals are often used in the transmission of digital data over communication channels that are corrupted by interference due to intentional jamming or from other users of the channel (e.g., cellular tele- phones and other wireless applications). In applications other than com- munications, spread-spectrum signals are used to obtain accurate range (time delay) and range rate (velocity) measurements in radar and navi- gation. For the sake of brevity we shall limit our discussion to the use of spread spectrum for digital communications. Such signals have the char- acteristic that their bandwidth is much greater than the information rate in bits per second.

In combatting intentional interference (jamming), it is important to the communicators that the jammer who is trying to disrupt their com- munication does not have prior knowledge of the signal characteristics. To accomplish this, the transmitter introduces an element of unpredictability or randomness (pseudo-randomness) in each of the possible transmitted signal waveforms, which is known to the intended receiver, but not to the jammer. As a consequence, the jammer must transmit an interfering sig- nal without knowledge of the pseudo-random characteristics of the desired signal.

Interference from other users arises in multiple-access communica- tions systems in which a number of users share a common communications channel. At any given time a subset of these users may transmit informa- tion simultaneously over a common channel to corresponding receivers. The transmitted signals in this common channel may be distinguished from one another by superimposing a different pseudo-random pattern, called a multiple-access code, in each transmitted signal. Thus a particular receiver can recover the transmitted data intended for it by knowing the pseudo-random pattern, that is, the key used by the corresponding trans- mitter. This type of communication technique, which allows multiple users to simultaneously use a common channel for data transmission, is called code division multiple access (CDMA).

The block diagram shown in Figure 12.17 illustrates the basic el- ements of a spread-spectrum digital communications system. It differs

FIGURE 12.17 Basic spread spectrum digital communications system

Spread-Spectrum Communications 633

from a conventional digital communications system by the inclusion of two identical pseudo-random pattern generators, one that interfaces with the modulator at the transmitting end and the second that interfaces with the demodulator at the receiving end. The generators generate a pseudo- random or pseudo-noise (PN) binary-valued sequence (±1’s), which is impressed on the transmitted signal at the modulator and removed from the received signal at the demodulator.

Synchronization of the PN sequence generated at the demodula- tor with the PN sequence contained in the incoming received signal is required in order to demodulate the received signal. Initially, prior to the transmission of data, synchronization is achieved by transmitting a short fixed PN sequence to the receiver for purposes of establishing synchro- nization. After time synchronization of the PN generators is established, the transmission of data commences.

12.8.1 PROJECT 12.8: BINARY SPREAD-SPECTRUM COMMUNICATIONS The objective of this project is to demonstrate the effectiveness of a PN spread-spectrum signal in suppressing sinusoidal interference. Let us con- sider the binary communication system described in Project 12.7, and let us multiply the output of the modulator by a binary (±1) PN sequence. The same binary PN sequence is used to multiply the input to the demod- ulator and thus to remove the effect of the PN sequence in the desired signal. The channel corrupts the transmitted signal by the addition of a

FIGURE 12.18 Block diagram of binary PN spread-spectrum system for simula- tion experiment

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Chapter 12

APPLICATIONS IN COMMUNICATIONS

wideband noise sequence {w(n)} and a sinusoidal interference sequence of the form i(n) = A sin ω 0 n, where 0 < ω 0 < π. We may assume that

A ≥ M , where M is the number of samples per bit from the modula- tor. The basic binary spread spectrum-system is shown in Figure 12.18. As can be observed, this is just the binary digital communication system shown in Figure 12.16, to which we have added the sinusoidal interference and the PN sequence generators. The PN sequence may be generated by using a random-number generator to generate a sequence of equally prob- able ±1’s.

Execute the simulated system with and without the use of the PN sequence, and measure the error rate under the condition that A ≥ M for different values of M, such as M = 50, 100, 500, 1000. Explain the effect of the PN sequence on the sinusoidal interference signal. Thus ex- plain why the PN spread-spectrum system outperforms the conventional binary communication system in the presence of the sinusoidal jamming signal.