SPREAD SPECTRUM

SPREAD SPECTRUM

An increasingly popular form of communications is known as spread spectrum. This

technique does not fit neatly into the categories defined in this lesson, as it can be

used to transmit either analog or digital data, using an analog signal.

The spread spectrum technique was developed initially for military and intelligence

requirements. The essential idea is to spread the information signal over a

wider bandwidth in order to make jamming and interception more difficult. The

first type of spread spectrum developed became known as frequency-hopping.4 A

more recent version is direct-sequence spread spectrum. Both of these techniques

are used in various wireless data-network products. They also find use in other communications

applications, such as cordless telephones.

Figure 4.21 highlights the key characteristics of any spread spectrum system.

Input is fed into a channel encoder that produces an analog signal with a relatively

narrow bandwidth around some center frequency. This signal is further modulated

using a sequence of seemingly random digits known as a pseudorandom sequence.

The effect of this modulation is to significantly increase the bandwidth (spread the

spectrum) of the signal to be transmitted. On the receiving end, the same digit

sequence is used to demodulate the spread spectrum signal. Finally, the signal is fed

into a channel decoder to recover the data.

A comment about pseudorandom numbers is in order. These numbers are

generated by an algorithm using some initial value called the seed. The algorithm is

deterministic and therefore produces sequences of numbers that are not statistically

random. However, if the algorithm is good, the resulting sequences will pass many

reasonable tests of randomness. Such numbers are often referred to as pseudorandom

number. The important point is that unless you know the algorithm and

the seed, it is impractical to predict the sequence. Hence, only a receiver that shares

this information with a transmitter will be able to successfully decode the signal.

Frequency-Hopping

Under this scheme, the signal is broadcast over a seemingly random series of radio

frequencies, hopping from frequency to frequency at split-second intervals. A

receiver, hopping between frequencies in synchronization with the transmitter,

picks up the message. Would-be eavesdroppers hear only unintelligible blips.

Attempts to jam the signal succeed only at knocking out a few bits of it.

A typical block diagram for a frequency-hopping system is shown in Figure

4.22. For transmission, binary data is fed into a modulator using some digital-to-

analog encoding scheme, such as frequency-shift keying (FSK) or binary-phase shift

keying (BPSK). The resulting signal is centered around some base frequency. A

pseudorandom number source serves as an index into a table of frequencies. At

each successive interval, a new frequency is selected from the table. This frequency

is then modulated by the signal produced from the initial modulator to produce a

new signal with the same shape but now centered on the frequency chosen from the

table.

On reception, the spread-spectrum signal is demodulated using the same

sequence of table-derived frequencies and then demodulated to produce the output

data.

 

Direct Sequence

Under this scheme, each bit in the original signal is represented by multiple bits in

the transmitted signal, known as a chipping code. The chipping code spreads the signal

across a wider frequency band in direct proportion to the number of bits used.

Therefore, a 10-bit chipping code spreads the signal across a frequency band that is

10 times greater than a 1-bit chipping code.

One technique with direct-sequence spread spectrum is to combine the digital

information stream with the pseudorandom bit stream using an exclusive-or. Figure

4.23 shows an example. Note that an information bit of 1 inverts the pseudorandom

bits in the combination, while an information bit of 0 causes the pseudorandom bits

to be transmitted without inversion. The combination bit stream has the data rate

of the original pseudorandom sequence, so it has a wider bandwidth than the information

stream. In this example, the pseudorandom bit stream is clocked at four

times the information rate.

Figure 4.24 shows a typical direct sequence implementation. In this case, the

information stream and the pseudorandom stream are both converted to analog signals

and then combined, rather than performing the exclusive-or of the two streams

and then modulating.

The spectrum spreading achieved by the direct sequence technique is easily

determined. For example, suppose the information signal has a bit width of Tb

which is equivalent to a data rate of l/Tb. In that case, the bandwidth of the signal,

depending on encoding technique, is roughly 2/Tb. Similarly, the bandwidth of the

pseudorandom signal is 2/Tc,, where Tc is the bit width of the pseudorandom input.

The bandwidth of the combined signal is approximately the sum of the two bandwidths,

or 2/(Tb + T,). The amount of spreading that is achieved is a direct result of

the data rate of the pseudorandom stream; the greater the data rate of the pseudorandom

input, the greater the amount of spreading.