In the direct sequence spread spectrum modulation, the pseudo-noise sequence of large bandwidth is multiplied with the narrowband data signal. Therefore the output signal is spreaded over the complete output bandwidth at every instant. The ability to overcome jamming is determined by the processing gain of system. We know that processing Gain = N; length of pseudo-noise sequence. As the number of bits ‘N’ of pseudo-noise sequence (chips) are increased per data bits, the bandwidth of output signal becomes more. Hence processing gain is also more. But there is limitation of the physical devices which generates pseudo-noise sequence. Hence very large bandwidth are not possible with direct sequence modulation. To overcome this problem, frequency hop spread spectrum is used.
Principle of Frequency Hop Spread Spectrum
Frequency hopping means to transmit the data bits in different frequency slots. The total bandwidth of the output signal is equal to sum of all these frequency slots or ‘hops’. Since the frequency hopping is random (which is known only to the transmitter and recognized receiver) unwanted receivers has to cover the complete output bandwidth. This bandwidth is in GHz and hence unwanted receivers find it difficult to receive the frequency hop signal.
Thus in the frequency hop spread spectrum the carrier frequency hops randomly from one frequency to another. Normally M-ary FSK is used along with frequency hop spread spectrum. There are to basic types of frequency hop spread spectrum.
- Slow frequency hopping and
- Fast frequency hopping.
Slow Frequency Hopping
Normally ‘k’ successive bits of input data sequence represent 2k = M symbols. Those distinct M symbols are transmitted with the help of M-ary FSK modulation system. When spread spectrum modulation is to be used, then the M-ary FSK signal is further modulated to generate wideband signal. The spread spectrum carrier frequency ‘hops’ randomly from one frequency to another.
Hop rate (Rh) : The rate of change of frequency ‘hops’ is called hop rate.
Symbol rate (Rs) : The rate at which K-bit symbols of data input sequence are generated is called symbol rate.
Definition of slow frequency hopping :
When several symbols of data are transmitted in one frequency hop (slot), then it is called slow frequency hopping. This means symbol rate is higher than hop rate. Here hop rate is slower (than Rs ), hence it is called slow frequency hopping.
Transmitter of FH / MFSK
Fig shows the block diagram of frequency hopping M-ary FSK transmitter.
The input binary data sequence is applied to the M-ary FSK modulator. This modulator output is the particular frequency (out of ‘M’ frequencies) depending upon the input symbol. The output of FSK modulator is then applied to a mixer. The other input to the mixer is particular frequency from frequency synthesizer. The output of frequency synthesizer at particular instant is the frequency slot or ‘hop’. This hop is mixed with the FSK signal. The output of the mixer is the ‘sum’ frequency component of FSK signal and frequency hop from synthesizer. This signal is FH/MFSK signal and is transmitted over the wideband channel.
The frequency hops (or slots) given to the mixer are generated by the frequency synthesizer. The inputs of the frequency synthesizer are controlled by pseudo-noise (PN) sequence generator. The ‘t’ successive bits of PN sequence generator control the frequency hops generated by synthesizer. Since the bits of PN sequence generator change randomly, the frequency hops generated also change randomly. Since ‘t’ bits of PN sequence generator control the frequency hops, there will be distinct ‘2t‘ frequency hops generated. Therefore the total bandwidth of the output signal will be equal to the sum of individual frequency hops. This bandwidth goes in the range of GHz. Therefore the bandwidth of frequency hops signals are very large.
Receiver of FH/MFSK
The received FH/MFSK signal is applied to the mixer. The output of frequency synthesizer is also given to the mixer. The sum and difference frequencies are generated by the mixer. Only difference frequencies are allowed to pass out of the mixer. Those difference frequencies are exactly the M-ary FSK signals. These signals are given to the non-coherent M-ary FSK detector. The detector detects particular symbol transmitted.
Reason for noncoherent detection : The local PN sequence generator exactly generates the pseudo-noise sequence which was generated in the receiver at particular instant of time. That is the local PN sequence generator operates synchronously with the transmitter. But the frequency synthesizer doesn’t generate the ‘inphase’ (or phase locked) frequencies with that of synthesizer at transmitter. That is synthesizer frequencies in transmitter and receiver are exactly same but they are not in phase. Hence coherent detection is not possible in the receiver. Therefore the receiver uses non-coherent detection of FSK signals.