IE 419/519 Wireless Networks

1. IE 419/519Wireless Networks Lecture Notes #6 Spread Spectrum

2. Introduction • In 1985, the FCC modified Part 15 of the radio spectrum regulation • Governs unlicensed devices • Attempt to stimulate the production and use of wireless network products • The modification authorized wireless network products to operate in the Industrial, Scientific, and Medical (ISM) bands using spread spectrum modulation • 902 - 928 MHz • 2.4 - 2.4835 GHz • 5.725 - 5.850 GHz

3. Introduction • FCC allows users to operate wireless products without obtaining licenses if the products meet certain requirements • e.g., Operation under 1 watt transmitter output power • This deregulation of the frequency spectrum eliminates • Need to perform costly and time-consuming frequency planning to avoid interference with existing radio systems • Need to license product again at a new location (if equipment is moved)

4. Digital Signal • Analog Signal • Digital Signal • Analog Signal Spread Spectrum Encoding • Digital data • Analog data • Which option to choose? • Requirements to meet • Media & communications facilities • Spread Spectrum • Can be used to transmit either analog or digital data, using an analog signal

5. Spread Spectrum • Input is fed into a channel encoder • Produces analog signal with narrow bandwidth • Signal is further modulated using sequence of digits • Spreading code or spreading sequence • Generated by pseudonoise, or pseudo-random number generator • Effect of modulation is to increase bandwidth of signal to be transmitted

6. Spread Spectrum • On receiving end, digit sequence is used to demodulate the spread spectrum signal • Signal is fed into a channel decoder to recover data

7. Spread Spectrum • What can be gained from apparent waste of spectrum? • Immunity from various kinds of noise and multipath distortion • Anti-jamming performance • Interference immunity • Can be used for hiding and encrypting signals • Low probability of intercept • Low transmit power density • Several users can independently use the same higher bandwidth with very little interference • Multiple access communications • Multiple simultaneous transmissions

8. Types of Spread Spectrum • Frequency Hopping Spread Spectrum (FHSS) • First type developed • Direct Sequence Spread Spectrum (DSSS) • More recent technology

9. Frequency Hopping SS • Signal is broadcast over seemingly random series of radio frequencies • A number of channels allocated for the FH signal • Width of each channel corresponds to bandwidth of input signal • Signal hops from frequency to frequency at fixed intervals • Transmitter operates in one channel at a time • Bits are transmitted using some encoding scheme • At each successive interval, a new carrier frequency is selected

10. Frequency Hopping SS Source: http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Morris/types.htm

11. Frequency Hopping SS • Hopping Sequence • Channel sequence dictated by spreading code • Pseudorandom number serves as an index into a table of frequencies • Chip Period • Time spent on each channel • FCC regulation  maximum dwell time of 400 ms • IEEE 802.11 standard  300 ms • Chipping rate • Hopping rate

12. Frequency Hopping SS • Receiver, hopping between frequencies in synchronization with transmitter, picks up message • Advantages • Eavesdroppers hear only unintelligible blips • Attempts to jam signal on one frequency succeed only at knocking out a few bits

13. FHSS Performance Considerations • Large number of frequencies used • Results in a system that is quite resistant to jamming • Jamming signal must jam all frequencies • With fixed power, this reduces the jamming power in any one frequency band

14. Direct Sequence SS • Each bit in original signal is represented by multiple bits in the transmitted signal • Spreading code spreads signal across a wider frequency band • Spread is in direct proportion to the number of bits used • One technique combines digital information stream with the spreading code bit stream using exclusive-OR

15. Direct Sequence SS Source: http://www.sss-mag.com/primer.html Source: http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Morris/types.htm

16. Direct Sequence SS

17. Processing Gain • Unique property of spread specturm waveforms • Used to measure the performance advantage of spread spectrum against narrowband forms

18. Processing Gain in FHSS

19. Processing Gain in DHSS • In a DS system • Random binary data has a bit rate of Rb • The pseudorandom binary waveform has a rate of Rc

20. Code-Division Multiple Access • Basic Principles of CDMA • Start with a data signal with rate D • Break each bit into kchips • Chips are a user-specific fixed pattern • Chip data rate of new channel = kD

21. Code-Division Multiple Access • Advantage • Good protection against interference and tapping • Disadvantages • Receiver must be precisely synchronized with the transmitter to apply the decoding correctly • Receiver must know the code and must separate the channel with user data from the background noise composed of other signals and environmental noise

22. CDMA Example • If k=6 and code is a sequence of 1s and -1s • For a ‘1’ bit, A sends code as chip pattern • <c1, c2, c3, c4, c5, c6> • For a ‘0’ bit, A sends complement of code • <-c1, -c2, -c3, -c4, -c5, -c6> • Receiver knows sender’s code and performs electronic decode function • <d1, d2, d3, d4, d5, d6> = received chip pattern • <c1, c2, c3, c4, c5, c6> = sender’s code

23. CDMA Example • User A code = <1, –1, –1, 1, –1, 1> • To send a 1 bit = <1, –1, –1, 1, –1, 1> • To send a 0 bit = <–1, 1, 1, –1, 1, –1> • User B code = <1, 1, –1, – 1, 1, 1> • To send a 1 bit = <1, 1, –1, –1, 1, 1> • Receiver receiving with A’s code • (A’s code) x (received chip pattern) • User A ‘1’ bit: 6 -> 1 • User A ‘0’ bit: -6 -> 0 • User B ‘1’ bit: 0 -> unwanted signal ignored

24. CDMA for DSSS

25. Spread Spectrum