Necessity of Secure Communication
There are some applications where it is necessary for the system to resist external interference and to make it difficult for unauthorized receivers to receive the message being transmitted. This type of communication is called secure communication such that noise interference and unwanted receivers should not detect the message. Such communication is very very important in military applications where techniques called spread spectrum modulation is used. Even the spread spectrum modulation is used for non-military applications also. The interference in the transmission channel may be unintentional interference caused because the other user may be transmitting through that same channel. Sometimes the interference is created intentionally by a hostile transmitter to ‘jam’ the transmission. Those problems can be solved using spread spectrum modulation.
Definition of Spread Spectrum Modulation
The spread spectrum modulation can be defined in two parts as follows.
- The transmitted data sequence occupies a much more bandwidth than the minimum required bandwidth and,
- The spectrum spreading (i.e. increase of signal bandwidth) at the transmitter and despreading at the receiver is obtained by ‘special code’ which is independent of the data sequence (message signal).
The first part of definition given above is satisfied by other modulation techniques like frequency modulation, PCM etc. But they do not satisfy second part. That is they donot use frequency spectrum spreading and despreading. The bandwidth requirement of spread spectrum modulation techniques is thus very high. The spread spectrum modulation is used in some applications which involve –
- Antijam capability – Particularly for narrow band jamming.
- Interference rejection.
- Multiple access capability.
- Multipath protection.
- Low Probability of Intercept (LPI).
- Secure communications.
- Improved spectral efficiency – In special circumstances.
These applications are discussed in detail at the end of this chapter.
Classification of Spread Spectrum Systems
Spread spectrum modulation techniques are classified depending upon their operating concept. The spread spectrum systems may be averaging type or avoidance type.
Averaging systems : In this systems the interference is reduced by averaging it over the long period.
Avoidance systems : In those systems the interference is reduced by making the signal to avoid the interference a large fraction of time. The spread spectrum techniques are also classified based on the modulation techniques employed.
They are classified as follows
- Direct sequence
- Frequency hopping
- Time hopping
- Hybrid methods
In the above modulation techniques, Direct sequence is an overaging system and remaining techniques are avoidance type. In this chapter we will study the direct sequence and frequency hopping techniques in details.
In direct sequence modulation, two stages of modulation are used. In the first stage, the incoming data sequence modulates the wideband code. This transforms the narrow band incoming data sequence into wide band signal. That is the spectrum of the signal is spreaded. This wideband signal then undergoes the second modulation using PSK.
In the frequency hop spread spectrum technique the spectrum of data modulated carrier is widened by changing the carrier frequency in a pseudo-random manner. For both direct sequence and frequency hop spread spectrum modulation methods we use a noise like spreading code called a pseudo-random or pseudo-noise sequence. Thus Pseudo-Noise (PN) sequence is essential to the operation of spread spectrum modulation. In this chapter we will discuss briefly about pseudo-random (or pseudo-noise) sequences also.
Model of Spread Spectrum Digital Communication System
Fig shows the model of spread spectrum digital communication system. The binary information sequence is input to the channel encoder on transmitter side. The channel encoder encodes this input sequence according to some error control coding technique. The coded sequence is then given to the modulator. The modulator gets pseudo-random or Pseudo-Noise (PN) sequence from the pseudo-random pattern generator. This pseudo-noise sequence spreads the signals randomly over the wide frequency band.
The signal at the output of modulator is the SS modulated signal. This signal is then transmitted over some channel. At the receiver the demodulator gets coded signal back from the SS signal. For this purpose the demodulator requires the same pseudo-noise sequence which was used at the transmitting end. Hence the pseudo-random pattern generators at the transmitter and receiver side operate in synchronization with each other. The channel decoder at the receiver then gets the binary information sequence back.
Thus the receiver can detect the transmitted SS signals only if it knows the pseudo-noise sequence. For any arbitrary receiver it is difficult to know the pseudo-noise sequence since it appears like noise. The pseudo-noise sequence at the modulator is used with the PSK modulation to shift the phase of the PSK signal pseudo-randomly. Such technique is called Direct Sequence (DS) spread spectrum modulation. It is also called Pseudo-Noise (PN) spread spectrum modulation. When the pseudo-noise sequence in the modulator is used in conjunction with M-ary FSK to shift the frequency of FSK signal pseudo randomly, the technique is called Frequency Hopped (FH) spread spectrum method.