8 spread spectrum
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The document discusses different types of spread spectrum techniques used in wireless communications including frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and code division multiple access (CDMA). It explains that spread spectrum works by spreading data over a wider bandwidth through techniques like frequency hopping or spreading each bit into multiple bits, making the signal harder to intercept or jam. The receiver is able to recover the original data by using the same spreading code or sequence as the transmitter.
8 spread spectrum
- 1. Data Communications - Spread Spectrum
- 2. Introduction - Spread Spectrum Important form of encoding for wireless comm. Transmit either analog or digital data Analog signal (transmission) Developed initially for military and intelligence requirements Spread data over wider bandwidth Makes jamming and interception harder Types of spread spectrum Frequency hoping Signal broadcast over seemingly random series of frequencies Direct Sequence Each bit is represented by multiple bits in transmitted signal Chipping code
- 3. Spread Spectrum Concept Input fed into channel encoder Produces narrow bandwidth analog signal around central frequency Signal modulated using sequence of digits Spreading code/sequence Typically generated by pseudonoise/pseudorandom number generator Increases bandwidth significantly Spreads the spectrum Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder
- 4. General Model of Spread Spectrum System
- 5. Gains Immunity from various noise and multipath distortion Including jamming Can hide/encrypt signals Only receiver who knows spreading code can retrieve signal Several users can share same higher bandwidth with little interference Cellular telephones Code division multiplexing (CDM) or Code division multiple access (CDMA)
- 6. Pseudorandom Numbers Generated by algorithm using initial seed (initial value) Deterministic algorithm Not actually random If algorithm good, results pass reasonable tests of randomness Pseudorandom numbers Need to know algorithm and seed to predict sequence Only receiver sharing info with transmitter able to decode the signal successfully.
- 7. Frequency Hopping Spread Spectrum (FHSS) Signal broadcast over seemingly random series of frequencies at fixed interval Receiver hops between frequencies in sync with transmitter to pick msg Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits
- 8. Basic Operation Typically 2 k carriers frequencies forming 2 k channels Channel frequency spacing corresponds with bandwidth of input signal Each channel used for fixed interval Eg:300 ms in IEEE 802.11 Some number of bits transmitted using some encoding scheme May be fractions of bit (see later) Sequence dictated by spreading code Both transmitter & receiver use same code
- 10. FHSS Performance Considerations Typically large number of frequencies used Improved resistance to jamming
- 11. Direct Sequence Spread Spectrum (DSSS) Each bit represented by multiple bits using spreading code Spreading code spreads signal across wider frequency band In proportion to number of bits used 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code One method: Combine input with spreading code using XOR (see figure) Performance similar to FHSS
- 12. Direct Sequence Spread Spectrum Example
- 13. Direct Sequence Spread Spectrum Using BPSK Example
- 14. Code Division Multiple Access (CDMA) Multiplexing technique used with spread spectrum Start with data signal rate D Called bit data rate Break each bit into k chips according to fixed pattern specific to each user User’s code New channel has chip data rate kD chips per second E.g. k =6, three users (A,B,C) communicating with base receiver R Code for A = <1,-1,-1,1,-1,1> Code for B = <1,1,-1,-1,1,1> Code for C = <1,1,-1,1,1,-1>
- 15. CDMA Example
- 16. CDMA Explanation Consider A communicating with base Base knows A’s code Assume communication already synchronized A wants to send a 1 Send chip pattern <1,-1,-1,1,-1,1> A’s code A wants to send 0 Send chip pattern <-1,1,1,-1,1,-1> Complement of A’s code Decoder ignores other sources when using A’s code to decode Orthogonal codes