Lecture 12 FHSS CDMA 26th lecture January.pptx

Advance communication systems

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2. Grading Policy Mid Term Exam weightage 25 % Quizzes 15% Assignments 10 % Terminal Exam 50 % Total 100 2

3. Instructor: Dr. Mustafa Shakir Advanced Digital Communication Systems Code: TCW705310-F24-PB-GCL-MSEEW 3

4. Course Material Access All the lecture slides and helping material, and course information would be available in following yahoo or gmail group. (email your name and class section to [email protected]) Group home page: https://groups.google.com/forum/#!forum /fa24-advanced-digital-communication-systems-superior.engineering 4

5. Course Catalog Description Power Spectral Density, Formatting , Modulation and Demodulation /detection techniques, link, channel coding , probability of bit error performance, bandwidth efficiency, synchronization, phase locked loop implementation, multiplexing and multiple access, encryption/decryption, fading channels OTHER TOPICS ACCORDING TO CLOs of the course 5

6. 6 Advanced Digital Communication Systems

7. Text: Digital Communications: Fundamentals and Applications, By “ Bernard Sklar ” , Prentice Hall,   Probability and Random Signals for Electrical Engineers, Neon Garcia References: Digital Communications, Fourth Edition, J.G. Proakis , McGraw Hill, 2000.  Relevant Course Books 7

8. Spread Spectrum 8

9. 9 Direct Sequence User data stream is multiplied by a high rate (fast) code sequence Example: User bits 101 (+ - +) Code 1110100 (+ + + - + - -); spread factor = 7 EXOR User Bits Code Sequence 1 -1 -1 -1 1 1 1 -1 1 1 1 -1 -1 -1 1 -1 -1 -1 1 1 1 User bit -1 = 1 User bit = -1 User bit +1 = 1

10. Direct Sequence Spread Spectrum Example 10

11. Direct Sequence Spread Spectrum System

12. DSSS Example Using BPSK

13. Approximate Spectrum of DSSS Signal

14. Frequency Hopping Spread Spectrum (FHSS) Frequency-hopping spread spectrum ( FHSS ) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. A pseudo-noise sequence generated at the modulator is used in conjunction with an M- ary FSK modulation to shift the carrier frequency of the FSK signal pseudo-randomly, at the hopping rate R h . Signal broadcast over seemingly random series of frequencies. The transmitted signal occupies a number of frequencies in time, each for a period of time T h = 1/ R h , referred to as Dwell Time . FHSS divides the available bandwidth into N channels and hops between these channels according to the PN sequence. Receiver hops between frequencies in sync with transmitter. Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits 14

15. Frequency Hopping Example 15 Receiver will be hopping between frequencies in sync with Tx Hopping pattern dictated by Spreading Code Width of each channel corresponds to BW of the input signal

16. Terminology of FHSS Hop Set This is the number of channels that are used by the system (i.e. the number of different frequencies utilized). Dwell time This is the length of time that the system transmits on an individual channel (i.e., the length of time spent on one frequency). Hop rate This is the rate at which the hopping takes place (i.e. how fast the system changes from one channel to another or from one frequency to another). 16

17. Frequency Hopping Spread Spectrum (FHSS) The transmitted bandwidth is determined by the lowest and highest hop positions and by the bandwidth per hop position ( Δ f ch ). FHSS signal is a narrowband signal because for a given hop, the instantaneous occupied bandwidth is identical to bandwidth of the conventional M-FSK, which is typically much smaller than W ss . Averaged over many hops, the FH/M-FSK spectrum occupies the entire spread spectrum bandwidth W ss . Because FHSS bandwidth only depends on tuning range, it can be hopped over much wider bandwidth than an DSSS system. Since, the hops generally results in phase discontinuities a non-coherent demodulation is done at the receiver. With slow hopping there are multiple data symbols per hop and with fast hopping there are multiple hops per data symbol. 17

18. Bluetooth uses FH Bluetooth is a FH-SS system, which achieves a (coded) bit rate of 1 Mbps (potentially up to 3 Mbps), but uses 80 MHz of spectrum, in 79 different center frequencies, with a hopping period T h = 1/1600 s/hop. 18

19. Frequency Hopping Spread Spectrum (FHSS) signal is broadcast over seemingly random series of frequencies receiver hops between frequencies in sync with transmitter eavesdroppers hear unintelligible blips jamming on one frequency affects only a few bits

20. Frequency Hopping Example

21. FHSS (Transmitter)

22. Frequency Hopping Spread Spectrum System (Receiver)

23. Slow and Fast FHSS commonly use multiple FSK (MFSK) have frequency shifted every T c seconds duration of signal element is T s seconds Slow FHSS has T c  T s Fast FHSS has T c < T s FHSS quite resistant to noise or jamming with fast FHSS giving better performance

24. Slow MFSK FHSS

25. Fast MFSK FHSS

26. Benefits of FHSS Three benefits of FH-SS are: 1. Interference avoidance : There may be significant interference at a few of the center frequencies. But even if we totally lose all bits during those hops, we will be able to recover using the bits received during successful (non-interfered) hops. We also avoid being an interferer to someone else’s signal for too long. 2. Multiple Access : Two devices can occupy the same spectrum and operate without coordinating medium access at all. Their transmissions will “collide” some small fraction of the time, but not often enough to cause failure. 3. Stealth : There is an advantage to switching randomly among frequencies when an eavesdropper doesn’t know your hopping pattern – they will not be able to easily follow your signal. This was the original reason for the discovery and use of FHSS (by actor and inventor Hedy Lamarr , in 1940). 26

27. Bandwidth Sharing 27

28. Code Division Multiple Access (CDMA) a multiplexing technique used with spread spectrum given a data signal rate D break each bit into k chips according to a fixed chipping code specific to each user resulting new channel has chip data rate kD chips per second can have multiple channels superimposed

29. CDMA Example

30. CDMA for DSSS

31. 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 Electrical Engineering Department 31

32. CDMA for DSSS n users each using different orthogonal PN sequence At transmitter, each users will transmit modulate data stream Using BPSK to produce a signal with a bandwidth of W s (FHSS bandwidth) then multiply by spreading code of user Signal plus noise reach at receiver’s antenna At receiver, suppose attempt to recover data of user 1 Incoming signal multiplied by the spreading code then demodulated Electrical Engineering Department 32

33. CDMA in a DSSS Environment Electrical Engineering Department 33

34. Seven Channel CDMA Encoding and Decoding Electrical Engineering Department 34

35. Introduction to PN Sequences 35

36. Pseudorandom Numbers generated by a deterministic algorithm not actually random but if algorithm good, results pass reasonable tests of randomness starting from an initial seed need to know algorithm and seed to predict sequence hence only receiver can decode signal

37. Pseudo-noise (PN) Sequences In order to de-spread the signal, the receiver needs a replica of the transmitted sequence (in time synchronism). If a truly random sequence is employed at the transmitter, we have to use a separate channel to transmit the spreading sequence. Obviously, it is not convenient. In practice, we use PN sequences so that the same sequences can be easily generated at the transmitter and receiver independently. 37

38. PN Sequences and Truly Random Sequences How does a PN Sequence differ from a truly random sequence? A truly random sequence cannot be predicted. However, its future variations can be described in a statistical manner. A PN sequence is deterministic and not random at all. However, even though it is not random, it has certain statistical properties similar to those of a truly random sequence. So, what are those properties? 38

39. Pseudo-Random Properties Balance Property Over the sequence period, the number of 1’s and 0’s differs by at most 1 . For eg ., a 15 chip PN sequence 111100010011010 Number of 1’s = 8 Number of 0’s = 7 Note : A sequence is balanced if and only if there are exactly equal number of 1’s and 0’s. Otherwise, it is only near-balanced. 39

40. Pseudo-Random Properties Correlation Properties If a shift version of itself is compared term by term with the original sequence, the number of agreements differs from the number of disagreements by not more than one count. … 1111 00010011010 … … 00010011010 1111 … ____________________ … dddsssdssddsdsd … Number of disagreements (ds) = 8 Number of Similarities ( Ss ) = 7 40

41. Text: Digital Communications: Fundamentals and Applications, By “ Bernard Sklar ” , Prentice Hall,   Probability and Random Signals for Electrical Engineers, Neon Garcia References: Digital Communications, Fourth Edition, J.G. Proakis , McGraw Hill, 2000.  Relevant Course Books 41

42. Communication Systems Spread Spectrum 42

43. What are PN Sequences A simple definition of PN Sequence Pseudo-random noise sequences or PN sequences are known sequences which exhibit the properties or characteristics of random sequences. PN Sequences can be used to logically isolate users on the same physical (frequency) channel. They can also be used to perform scrambling as well as spreading and de-spreading functions. If the Code sequence were deterministic, everybody could access the channel; If the Code sequence were truly random, then nobody, including the intended receiver, could access the channel; So, PN sequences appear as random noise to everybody else, except to the transmitter and the intended receiver. 43

44. Processing Gain (PG) It is the number of chips per bit . Where R c = 1/ T c is the chip rate . For example, 802.11b has a chip rate of 11 M-cps (chips per second) and a symbol rate of 1 M- sbs (symbols per second). PG is the ratio by which unwanted signals or interference can be suppressed relative to the desired signal when both share the same frequency channel. PG has no effect on wideband thermal noise. 44

45. Spreading Factor or Processing Gain Bandwidth of the DS-SS signal is proportional to N (for a fixed duration of the signature waveform) For a given signal-to-noise ratio , the single-user bit-error-rate in a white Gaussian noise is independent of N Bandwidth ratio G = W/B is called a spreading factor Military applications, G ≈ 100 : : : 1000 WCDMA, G = 4 – 256 45 The number of chips N per bit is also called spreading factor or a processing gain, and it has the following properties:

46. Generation of PN Sequences 46

47. Modulo-2 Addition The modulo-2 sum of two 1-bit binary numbers yields 0 if the two numbers are identical, and 1 if they differ: 0+0=0, 0+1=1, 1+0=1, 1+1=0. 47