Understanding Frequency Hopping Spread Spectrum Systems
Frequency Hopping Spread Spectrum (FH-SSS) is a technique where the carrier frequency changes randomly using a Pseudo-Noise (PN) code. It is widely used in mobile civilian and military communications for secure and efficient data transmission. FH-SSS systems are classified as Fast or Slow based on the hop rate in relation to the data rate, with Fast FH-SSS using multiple carrier frequencies per data bit transmission. The modulation of carriers by data and address codes plays a crucial role in FH systems. Synchronization between the transmitter and receiver PN generators is essential for successful operation. Explore the intricacies and applications of FH-SSS for robust communication.
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FREQUENCY HOPPING FH-SSS: In FH-SSS, the carrier frequency is changed randomly according to a PN code. FH-SSS is used for mobile civilian and military tactical communications. According to the hop rate ( hop rate is the speed of hopping, which is the number of times the carrier frequency is changed per second), and its relation to the data rate, FH-SSS systems are classified into: -Fast FH-SSS: when each data bit is transmitted by one or more carrier frequencies, i.e when the hop rate is greater than the data rate. -Slow FH-SSS: : when more than one data bits are transmitted by one carrier frequency. i.e when the hop rate is less than the data rate. Channel spacing f Note that, spreading of the bandwidth is accomplished by randomly hopping the carrier using N hopping frequencies. The spreading bandwidth is BrfSSS= N. f . The channel spacing f is determined by the bandwidth of the information. f BrfSSS
According to the modulation of carrier by data and address code, the following FH systems are given: 1-FH-SSS with conventional modulation: Here, the frequency hopper is controlled by a PN code, while the data signal (may be analog or digital) modulates the hopping carrier. fo= m . fr, where m is the random address set by the k-bit PN code generator 2k> m > 0 fr data K bit fo Analog information At the Tx side, the PN code generator is driven by a clock that determines the hop rate. The k-bit parallel outputs of the PN generator are used to control the instantaneous carrier frequency fo.
The reference frequency fris obtained from a high stable crystal oscillator. This fois then used in any conventional modulator (may be analog or digital) to produce the FH signal. information flocal K bit At the Rx side, there is an identical PN generator controlling the frequency synthesizer. The local frequency flocalhops synchronously with that at the Tx with a constant IF frequency between them, i.e:
fIF= fo- flocal(note: both foand flocalhops randomly but the difference between them is kept constant as the IF frequency) . Note: Suggested Seminar (report)on Frequency Synthesizers(types, operations,..etc): 2-FH system with single channel data modulation: First, the carrier is modulated by a digital information signal and then the modulated carrier hops according to the PN code by a mixer & frequency synthesizer ( up conversion to fo), i.e, the data is first modulated then hopped. mixer fo data fc
Hopping pattern hopping clock 1 2 3 4 5 6 f2 f4 f3 f6 f1 f5 f f1 f2 f3 f4 f5 f6 Hopping pattern f2 f4 f3 f6 f1 f5 Th 2Th 3Th 4Th 5Th 6Th f
If synchronization is achieved between the Tx and Rx PN generators, then the mixer output set at a certain fIF= fo-flocalis applied to the IF amplifier with a bandwidth equals the bandwidth of the PSK signal. data mixer
