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Draw the block diagram of FHSS transmitter and receiver. Differentiate between slow hopping and fast hopping.
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Frequency-Hopping Spread Spectrum (FHSS)

  • FHSS is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom or pseudonoise sequence known to both transmitter and receiver.
  • The advantage of this system is that the signal sees a different channel and a different set of interfering signals during each hop.
  • This system helps to avoid the problem of failing communication at a particular frequency, because of a fade or a particular or unintentional interference.
  • It is utilized as a multiple access method in the frequency-hopping Code Division Multiple Access (FH-CDMA) schemes.
  • FHSS is also useful to counter eavesdropping, as well as to obstruct the frequency jamming of telecommunications

Block Diagram for FHSS Transmitter:

FHSS T

Block Diagram for FHSS Receiver:

FHSS R

  • The above figure shows the Block diagram of the FHSS system for both transmitter and receiver.
  • For transmission, binary data are fed into a modulator using some digital-to-analog encoding scheme, such as Frequency Shift Keying (FSK) or Binary Phase Shift Keying (BPSK).
  • A PN source serves as an index into a table of frequencies each K bit on the PN source specifies one of the 2k carrier frequencies.
  • At each successive interval, a new carrier frequency is selected.
  • This frequency is then modulated by the signal produced from the initial modulator to produce a new signal with the same shape.
  • On reception, the spread spectrum signal is demodulated using the same sequence of PN-derived frequencies and then demodulated to produce the output data.

Difference between Fast Hopping and Slow Hopping

Sr. No. Slow Frequency Hopping (SFH) Fast Frequency Hopping (FFH)
1 As the name suggests frequency hopping takes place slowly. As the name suggests frequency hopping takes place at a fast rate.
2 In this case, one or more data bits are transmitted within one frequency hop. In this case one data bit is divided over multiple frequency hops.
3 One or more data bits are transmitted over the same carrier frequency. One data bit is transmitted over multiple carriers in different frequency hops.
4 In this symbol rate of the MFSK signal is an integer multiple of the hop rate. In this hop rate is an integer multiple of the MFSK symbol rate.
5 Symbol rate is equal to chip rate. Hop rate is higher than the symbol rate.
6 Hop rate is lower than the symbol rate. Hop rate is higher than the symbol rate.
7 A jammer can detect this signal if carrier frequency in one hop is known. A jammer can’t detect this signal because one symbol is transmitted using more than one carrier frequency.
8 It supports coherent data detection. It does not support coherent signal detection. It is very difficult in FFH.
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