Spread Spectrum-Data Communication and Computer Networks-Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Data and Computer

Communications

Spread Spectrum

Spread Spectrum

 important encoding method for wireless communications

 analog & digital data with analog signal

 spreads data over wide bandwidth

 makes jamming and interception harder

 two approaches, both in use:

 Frequency Hopping  Direct Sequence

Spread Spectrum Advantages

 immunity from noise and multipath distortion

 can hide / encrypt signals

 several users can share same higher bandwidth with little interference  CDM/CDMA Mobile telephones

Pseudorandom Numbers

 generated by a deterministic algorithm

 not actually random  but if algorithm good, results pass reasonable tests of randomness

 starting from an initial seed

 need to know algorithm and seed to predict sequence

 hence only receiver can decode signal

Frequency Hopping Example

FHSS (Transmitter)

Slow and Fast FHSS

 commonly use multiple FSK (MFSK)

 have frequency shifted every Tc seconds

 duration of signal element is Ts seconds

 Slow FHSS has Tc  Ts

 Fast FHSS has Tc < Ts

 FHSS quite resistant to noise or jamming

 with fast FHSS giving better performance

Slow MFSK FHSS

Direct Sequence Spread Spectrum (DSSS)

 each bit is represented by multiple bits using a spreading code

 this spreads signal across a wider frequency band

 has performance similar to FHSS

Direct Sequence Spread

Spectrum Example

DSSS Example Using BPSK

Approximate

Spectrum of DSSS Signal

CDMA Example

CDMA Example

(a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal Docsity.com