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

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

2 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 MHz GHz GHz

3 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 4 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 Digital Signal Analog Signal Digital Signal Analog Signal

5 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 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 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 8 Types of Spread Spectrum Frequency Hopping Spread Spectrum (FHSS) First type developed Direct Sequence Spread Spectrum (DSSS) More recent technology

9 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 10 Frequency Hopping SS Source:

11 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 standard  300 ms Chipping rate Hopping rate

12 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 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 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 15 Direct Sequence SS Source: Source:

16 16 Direct Sequence SS

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

18 18 Processing Gain in FHSS

19 19 Processing Gain in DHSS In a DS system Random binary data has a bit rate of R b The pseudorandom binary waveform has a rate of R c

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

21 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 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 For a ‘0’ bit, A sends complement of code Receiver knows sender’s code and performs electronic decode function = received chip pattern = sender’s code

23 23 CDMA Example User A code = To send a 1 bit = To send a 0 bit = User B code = To send a 1 bit = 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 24 CDMA for DSSS

25 25 Spread Spectrum


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