SPREAD SPECTRUM In spread spectrum SS we combine
SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. Topics discussed in this section: Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum Synchronous (DSSS)
Spread Spectrum Ø important encoding method for wireless communications Ø analog & digital data with analog signal Ø spreads data over wide bandwidth Ø makes jamming and interception harder Ø two approaches, both in use: l l Frequency Hopping Direct Sequence
Figure 6. 27 Spread spectrum
General Model of Spread Spectrum System
Spread Spectrum Advantages Ø immunity from noise and multipath distortion Ø can hide / encrypt signals Ø several users can share same higher bandwidth with little interference l CDM/CDMA Mobile telephones
Pseudorandom Numbers Ø generated by a deterministic algorithm l l not actually random but if algorithm good, results pass reasonable tests of randomness Ø starting from an initial seed Ø need to know algorithm and seed to predict sequence Ø hence only receiver can decode signal
Frequency Hopping Spread Spectrum (FHSS) Ø signal is broadcast over seemingly random series of frequencies Ø receiver hops between frequencies in sync with transmitter Ø eavesdroppers hear unintelligible blips Ø jamming on one frequency affects only a few bits
Figure 6. 28 Frequency hopping spread spectrum (FHSS)
Figure 6. 29 Frequency selection in FHSS
Figure 6. 30 FHSS cycles
Figure 6. 31 Bandwidth sharing
Frequency Hopping Example
FHSS (Transmitter)
Frequency Hopping Spread Spectrum System (Receiver)
Slow and Fast FHSS Ø commonly use multiple FSK (MFSK) Ø have frequency shifted every Tc seconds Ø duration of signal element is Ts seconds Ø Slow FHSS has Tc Ts Ø Fast FHSS has Tc < Ts Ø FHSS quite resistant to noise or jamming l with fast FHSS giving better performance
Slow MFSK FHSS
Fast MFSK FHSS
Direct Sequence Spread Spectrum (DSSS) Ø each bit is represented by multiple bits using a spreading code Ø this spreads signal across a wider frequency band Ø has performance similar to FHSS
Figure 6. 32 DSSS
Figure 6. 33 DSSS example
Direct Sequence Spread Spectrum Example
Direct Sequence Spread Spectrum System
DSSS Example Using BPSK
Approximate Spectrum of DSSS Signal
Code Division Multiple Access (CDMA) Ø a multiplexing technique used with spread spectrum Ø given a data signal rate D Ø break each bit into k chips according to a fixed chipping code specific to each user Ø resulting new channel has chip data rate k. D chips per second Ø can have multiple channels superimposed
CDMA Example
CDMA for DSSS
Summary Ø looked at use of spread spectrum techniques: Ø FHSS Ø DSSS Ø CDMA
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