1. 1 CMPT 371 Data Communications and Networking Spread Spectrum

2. Janice Regan © 2005 2 Spread Spectrum  Analog signal based on analog or digital data  Begin with data encoded in a narrow band signal.  Spread data over wider bandwidth  Can move from one narrow band to another  Can share multiple narrow bands with other signals

3. Janice Regan © 2005 3 Spread Spectrum Concept  Input fed into channel encoder  Narrow bandwidth analog signal around central frequency  Signal modulated using a spreading sequence/code  Sequence often generated by pseudorandom number generator  Increases bandwidth significantly  Spreads each signal throughout the spectrum  Receiver uses same sequence/code to demodulate signal  Demodulated signal fed into channel decoder

4. Janice Regan © 2005 4 Model Spread Spectrum System Stallings 2003: Figure 9.1

5. Janice Regan © 2005 5 Gains  Many users can share same higher bandwidth with little interference  Can hide/encrypt signals  Can reduce each signal’s susceptibility to jamming, noise and interference  Only receiver who knows spreading code can retrieve signal  Need to jam a wide bandwidth to guarantee that a particular narrow band signal is jammed  Noise or interference at a particular frequency does less damage to the signal

6. Janice Regan © 2005 6 Spread Spectrum  Frequency hopping  Broadcast in a narrow frequency band whose central frequency moves from one narrow frequency band to another on a regular basis  Order of switching between bands is based on a pseudorandom sequence  Direct Sequence  Signal is multiplied by a chipping code (multiple chips per input signal bit)  Code Division Multiple Access (CDMA)  Signal is spread over a wide band shared with many other similar signals

7. Janice Regan © 2005 7 Frequency Hopping Spread Spectrum (FHSS)  Signal broadcast over pseudorandom series of frequencies  Receiver uses same pseudorandom series to know which frequencies to listen for  Receivers often use autocorrelation to synchronize pseudorandom sequence with transmitter.  Noise or jamming on one frequency affects only a few bits being transmitted at that frequency (usually these bits can be recovered using error correction)

8. Janice Regan © 2005 8 Frequency Hopping Stallings 2003: Figure 9.2

9. Janice Regan © 2005 9 Pseudorandom Sequences  Generated by algorithm which repeatedly produces a particular known series of numbers of length n that appear to have random properties  An initial seed is used to choose where in the known sequence the particular random sequence will begin.  Algorithm is deterministic but resulting pseudorandom sequence will pass reasonable tests of randomness  Need to know algorithm and seed to predict sequence  Algorithm will be part of protocol definition  Seed will be determined based on assigned channel, serial number or other basis

10. Janice Regan © 2005 10 FHSS Operation  Typically 2 k carrier frequencies/channels  Channel spacing corresponds with bandwidth of input  The channel to use in a particular time interval, T c, is determined by the next k digits in the pseudorandom spreading sequence  Each channel used for fixed time interval (300 ms in IEEE 802.11)  During each time interval L (L may be fractional) bits are transmitted using some M=2 L level encoding scheme on one of the 2 k carrier frequencies

11. Janice Regan © 2005 11 FHSS System (Transmitter) Stallings 2003: Figure 9.3

12. Janice Regan © 2005 12 FHSS using BFSK   Filtering for a single sideband

13. Janice Regan © 2005 13 FHSS System (Receiver) Stallings 2003: Figure 9.3

14. Janice Regan © 2005 14 FHSS using BFSK   Filtering for a single sideband

15. Janice Regan © 2005 15 Slow and Fast FHSS  T c is the chipping duration or the length of time each channel is used for a single series of k bits in the spreading or chipping sequence. Frequency is shifted every T c seconds  Duration of signal element is T s seconds  If T c  T s transmission by slow FHSS  If T c < T s transmission by fast FHSS  Generally fast FHSS gives improved performance in noise (or jamming)

16. Janice Regan © 2005 16 Slow FHSS: MFSK (M=4, k=2) Stallings 2003: Figure 9.4

17. Janice Regan © 2005 17 Fast FHSS MFSK (M=4, k=2) Stallings 2003: Figure 9.5

18. Janice Regan © 2005 18 FHSS Performance Considerations  Typically large number of frequencies used so there are many more channels than levels  Provides excellent protection against jamming, noise and interference  To jam a single channel need to broadcast a signal with the bandwidth of the channel and some power level P  To jam FHSS must jam all channels simultaneously, broadcasting a power equal to the number of channels * P (expensive and difficult)

19. Janice Regan © 2005 19 Direct Sequence Spread Spectrum (DSSS)  Each bit multiplied by multiple bits of a spreading sequence  Signal is spread across a frequency band wider than that of the original signal. If each data bit is multiplied by n bits of the spreading sequence the bandwidth of the spread signal is n times the bandwidth of the original signal  One method:  Combine input with spreading code using XOR  Data rate equal to data rate of the original spreading code  Performance similar to FHSS

20. Janice Regan © 2005 20 DSSS Transmitter Stallings 2003: Figure 9.7

21. Janice Regan © 2005 21 Direct Sequence Spread Spectrum Stallings 2003: Figure 9.6

22. Janice Regan © 2005 22 DSSS Receiver

23. Janice Regan © 2005 23 Direct Sequence Spread Spectrum Stallings 2003: Figure 9.6

24. Janice Regan © 2005 24 DSSS using BPSK   Multiply data by chipping sequence to get transmitted data  Multiply received data by chipping sequence to recover initial data

25. Janice Regan © 2005 25 DSSS Using BPSK Stallings 2003: Figure 9.8

26. Janice Regan © 2005 26 Approximate Spectrum: DSSS Signal Stallings 2003: Figure 9.9

27. Janice Regan © 2005 27 CDMA  Code Division Multiple Access  Start with data signal rate D (Called bit data rate)  Break each bit into k chips by multiplying by a k bit user code (known as a Walsh code)  Channel has chip data rate kD chips per second  User code (Walsh code) is orthogonal to all other possible user codes  User code 1 * User code 2 = 0  User code 1 * User code 1 = signal for user 1  Signals for several users can be added and sent as a single signal within the same band (multiplexed)

28. Janice Regan © 2005 28 CDMA user code and data Stallings 2003: Figure 9.10

29. Janice Regan © 2005 29 CDMA Explanation  Consider a user communicating with a base station  Base station knows user A’s code  Assume communication already synchronized  Base station receives a message from A and wants to decode it. To extract the signal from A the basestation multiplies the signal by A’s code  Decoder ignores other sources by using A’s code to decode  For all other stations code station I * code station A = 0 so only the signal for station A remains

30. Janice Regan © 2005 30 CDMA for DSSS  When the basestation sends messages to n users each message multiplied by a different orthogonal Walsh code sequence, those signals are added before transmission.  At each receiving station, the signal for that station is extracted by multiplying by that stations Walsh code.

31. Janice Regan © 2005 31 CDMA: two-senders, eight bit Walsh codes Walsh Code 2 Walsh Code 1 Data Station 1 Data Station 2 Data multiplied by Walsh Code (Sum of all stations) Transmitted data

32. Janice Regan © 2005 32 CDMA: eight bit Walsh codes Walsh Code 1 Received Data multiplied by Walsh Code Decoded Received Data Station 1 -2 2 222 Receive Data (Sum)

33. Janice Regan © 2005 33 Summary of Channel Partitioning CDMA (Code Division Multiple Access)  Used mostly in wireless broadcast channels such as cellular phones  All users share same frequency band. Information from each user is spread throughout that frequency band  Each user has their own orthogonal Walsh code ‘chipping’ sequence to encode data.  encoded signal = (original data) X (Walsh code)  Encoded signals from each channel are added, the summed signal is transmitted  The orthogonal property of Walsh codes guarantees that (ignoring transmission errors) multiplying the received signal by a Walsh code will extract the data for the channel encoded using that Walsh code from the received (summed) signal.  Decoded signal = (received summed signal X Walsh code)

34. Janice Regan © 2005 34 CDMA in a DSSS Environment Stallings 2003: Figure 9.11