Spread Spectrum(SS) and multiple access (MA) techniques : FDMA, TDMA, CDMA, SDMA application in wire and wireless communication : Advantages (merits) :

1. EC6651 COMMUNICATION ENGINEERING
UNIT 4
Dr Gnanasekaran Thangavel
Professor and Head
Electronics and Instrumentation Engineering
R M K Engineering College
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2. UNIT IV MULTIPLE ACCESS TECHNIQUES
Spread Spectrum(SS) and multiple access (MA)
techniques : FDMA, TDMA, CDMA, SDMA application in
wire and wireless communication : Advantages (merits) :
2Dr Gnanasekaran Thangavel2/16/2018
YouTube Video Presentation
1. https://www.youtube.com/watch?v=qZAPEoPJn0Y
2. https://www.youtube.com/watch?v=4QUM0Sb0YFA
3. https://www.youtube.com/watch?v=33Cqp6Lduj8

3. MULTIPLE ACCESS
 Since the spectrum is limited, the sharing is necessary to improve the overall
capacity over a geographical area.
 This is carried out by permitting the available bandwidth to be used simultaneously
by different users.
 In computer networks and telecommunications, the multiple access method permits
various terminals to connect to the same multi-point transmission medium to
transmit over it and share its capacity.
 A few examples of shared physical media include bus networks, wireless networks,
star networks, ring networks, half-duplex point-to-point links, etc.
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4. MEDIA ACCESS CONTROL(MAC)
A channel-access scheme is also based on a multiple access
protocol and control mechanism, also known as media access
control (MAC). This protocol deals with issues such as
addressing, assigning multiplex channels to different users, and
avoiding collisions.
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5. 5
Frequency Division Multiple Access
(FDMA)

6. Frequency Division Multiple Access
(FDMA)
 In an FDMA system, each user has its own frequency channel.
This implies that relatively narrow filters are needed in each
receiver and transmitter.
 Most duplex FDMA systems must transmit and receive
simultaneously. (Frequency Division Duplex, FDD)
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7. FDMA
Time
Frequency
Channel
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8. Time Division Multiple Access
(TDMA)
 In TDMA, a set of N users share the same radio channel,
but each user only uses the channel during
predetermined slots.
 A frame consists of N slots, one for each user. Frames
are repeated continuously
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9. 9
Time Division Multiple Access (TDMA)

10. TDMA
Time
Frequency
0 1 2 3 4 5 6 7
Channel
Time Slot
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12. Spread Spectrum Transmission
 In Spread Spectrum communication, the bandwidth occupancy of a
single transmitted signal is much higher than in systems using
conventional modulation methods.
 This band-spreading is achieved by selecting appropriate
transmission waveforms with a wide bandwidth.
 A very popular method is to multiply the user data signal with a fast
code sequence, which mostly is independent of the transmitted data
message.
 In the case that multiple users share the same portion of the radio
spectrum but use different codes to distinguish their transmissions,
we speak of Code Division Multiple Access (CDMA)
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13. CDMA
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14. CDMA
Frequency
Time
Code
Code 1
Code 2
Code 3
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15. Spread Spectrum
 Important encoding method for wireless communications
 analog & digital data with analog signal
 Spreads data over wide bandwidth
 Makes jamming and interception harder
 Two approaches, both in use:
 Frequency Hopping
 Direct Sequence

16. General Model of Spread Spectrum
System

17. Spread Spectrum Advantages
 Immunity from noise and multipath distortion
 Can hide / encrypt signals
 Several users can share same higher bandwidth with little
interference
 CDM/CDMA Mobile telephones

18. 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

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 Tc seconds
 duration of signal element is Ts seconds
 Slow FHSS has Tc  Ts
 Fast FHSS has Tc < Ts
 FHSS quite resistant to noise or jamming
 with fast FHSS giving better performance

24. Slow MFSK FHSS

25. Fast MFSK FHSS

26. Direct Sequence Spread Spectrum
(DSSS)
 each bit is represented by multiple bits using a spreading code
 this spreads signal across a wider frequency band
 has performance similar to FHSS

27. Direct Sequence Spread Spectrum
Example

28. Direct Sequence Spread Spectrum System

29. DSSS Example Using BPSK

30. Approximate
Spectrum of
DSSS Signal

31. 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

32. CDMA Example

33. CDMA for DSSS

34. References
Book:
1. Wireless Communications and Networks by William Stallings .
2. Taub & Schiling “Principles of Communication Systems” Tata McGraw hill 2007.
3. Kennedy and Davis “Electronic Communication Systems” Tata McGraw hill, 4th Edition, 1993.
4. Sklar “Digital Communication Fundamentals and Applications“ Pearson Education, 2001.
5. TG Thomas and S Chandra Sekhar, “Communication Theory” Tata McGraw hill 2006.
Web: http://www.wirelesscommunication.nl/reference/about.htm
PPT:
 https://www.ics.uci.edu/~magda/Courses/netsys270/ch6_2_v1.ppt
 www.cs.utsa.edu/~korkmaz/teaching/cs3413/ppt/09-SpreadSpectrum.ppt
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35. Other presentations
http://www.slideshare.net/drgst/presentations
35Dr Gnanasekaran Thangavel2/16/2018

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Thank You
Questions and Comments?