Question

Report about Frequency hopping in wireless communication systems

With refrences: citation

Answer 1

Frequency hopping is type of spread spectrum encoding method which has a advantages of spreading data over wide bandwidth, makin jamming and interception harder and immunity from various noises and distortions.

Under Spread spectrum concept, input is provided to channel encoder which produces narrow bandwidth analog signal around central frequency. Signal is modulated using sequence of digits by typically spreading code and it is generated by pseudonoise or pseudorandom nuber generator. Receiver uses same sequence to demodulate signal. A typical block diagram is shown below

nput data Channel De- Channel Output data Modulator Channel Spreading code Spreading code Pseudonoise generator Pseudonoise generator

Frequency hopping is sigal broadcast over seemingly random series of frequencies. Receiver as well hops between frequencies to be in synchronization with transmitter. Advantages are eavesdropping becomes very difficult and jamming one frequency will make very less impact.

Basic operation is typically 2k carriers frequencies forming 2k channels, in which channel crossing corresponds with bandwidth of input, Each channel is used for fixed interval i.e. 300 ms in IEEE 802.11, some number of bits transmitted using some encoding scheme (May be fractions of bit). Sequence is dictated by spreading code.

Below are the pictures of Frequency hopping example, transmitter and receiver.

Frequency Energy Time 1112 3 4 s Frequency (b) Channel use (a) Channel assignment

Transmitter:

FH Spreader binary data Modulatord(t) I Bandpass Spread spectrum signal (FSK or filter (about sum frequency) s c(t) Frequency TPseudonoise- Table bit sourceChannel

Receiver:

FH Despreader Spread spectrum( Bandpass Demodulato binary data s(t) BPSK) frequency) ctt) I I Frequency Synthesizer I PseudonoiseTable bit source ChannelI

The nature of FHSS depends on Tc (Frequency shifter every Tc seconds) and Ts (Duration of signal element). slow FHSS has Tc>Ts and vice versa.