1. Chapter 6: Spread-Spectrum Communications

2. Learning Objectives
LO Understand the principle of spread-spectrum modulation including pseudo-noise sequence.
LO 6.2 – Illustrate the concept of frequency-hopping spread-spectrum (FHSS) and direct-sequence spread-spectrum (DSSS) with coherent BPSK.
LO 6.3 – Implement DSSS in code division multiple access (CDMA) multiuser communication systems.
LO 6.4 – Outline multiples access techniques including FDMA, TDMA, CDMA, and Multi-Carrier FDMA.

3. 6.1 Principles of Spread-Spectrum Modulation

5. Processing Gain
Processing gain is the ratio of the spread RF bandwidth to the original information bandwidth.
The effect of multipath fading as well as interference can be reduced by a factor equivalent to the processing gain.
In fact, it quantifies the degree of interference rejection.

8. Pseudo-Noise (PN) Sequence
Spreading of narrow bandwidth of baseband signal is accomplished through the use of a spreading code, called pseudo-noise (PN) code.
A PN code is independent of the baseband signal. The same code is used to demodulate the received data at the receiving end.
Pseudo-noise sequence or pseudo-random numbers is a random periodic sequence of binary ones and zeros.

11. 6.2 Spread-Spectrum Techniques
Frequency Hopping Spread-Spectrum (FHSS)
FHSS with BFSK or M-ary FSK
Performance of FHSS System
Direct Sequence Spread-Spectrum (DSSS)
6.2.5 Comparison of FHSS and DSSS
6.2.6 Salient Features of Spread-Spectrum Systems

12. 6.2.1 Frequency Hopping Spread-Spectrum (FHSS)
Frequency hopping involves a periodic change of transmission frequency over a wide band.
The set of possible time-varying, pseudorandom carrier frequencies is called the hopset.
In FHSS technique, the spectrum of the transmitted signal is spread sequentially (pseudo-random-ordered sequence of the frequency hops) rather than instantaneously.

16. 6.2.2 FHSS with BFSK or M-ary FSK

18. 6.2.3 Performance of FHSS System
The signal-to-interference ratio (SIR) per user in FHSS system is given by
Where B is the system bandwidth (Hz)
M is the number of users in FHSS system
L is the number of orthogonal codes used
Rb is the signaling rate (bps)

19. FHSS System - Advantages and Disadvantages
High tolerance of narrowband interference
Relative interference avoidance
Large frequency-hopped bandwidths
More interference tolerance
Non-coherent detection
Probability of detection

20. 6.2.4 Direct Sequence Spread-Spectrum (DSSS)

22. 6.2.5 Comparison of FHSS and DSSS

23. 6.2.6 Salient Features of Spread-Spectrum Systems
- High Processing Gain
- Immunity to Jamming
- Greater Security
- Low Probability of Interception
- Increased Tolerance to Multipath
- Low Interference
- Easy Encryption
- Multiple Access
- Increased Tolerance to Interference
- Increased Ranging Capability

24. 6.3 Application of DSSS in a Multiuser System
Multiuser DS-CDMA System
Code Acquisition and Tracking
Multiple-Access and Near-far Interference
Performance of DS-CDMA Multiuser System

25. 6.3.1 Multiuser DS-CDMA System
Spread spectrum is a modulation technique that forms the basis for spread spectrum multiple access, known as code-division multiple access (CDMA).
The input and output signal-to-noise power ratios are related as

28. 6.3.2 Code Acquisition and Tracking

30. 6.3.3 Multiple-Access and Near-far Interference
where M is the number of simultaneous users, Q is number of chips per time period, and Eb is the common received power level.

31. Intercell Interference Factor and Multiple-Access Interference (MAI)
MAI is directly proportional to the channel loading or capacity, M/Q

32. Signal-to-Interference-plus-Noise ratio (SINR)

36. RF Power Control Mechanisms
Open-loop RF power control refers to the procedure whereby the mobile user measures its received signal level and adjusts its transmit power accordingly.
An adaptive open-loop RF power control is based on the measured signal strength of the received pilot signal by the mobile user.
Closed-loop power control refers to the procedure whereby the cell site measures its received signal level and then sends corresponding message to the mobile user to adjust its transmit power to the desired level.

37. 6.3.4 Performance of DS-CDMA Multiuser System

38. Jamming Margin
It is the ratio of average interference power and average signal power.

39. Capacity of a Single-Cell CDMA System
The received signal-to-interference ratio, Sr for the target mobile receiver will be

40. Performance Improvement Factor (Pf)

41. 6.4 Multiple Access Techniques
Frequency Division Multiples Access (FDMA)
Time Division Multiples Access (TDMA)
Code Division Multiples Access (CDMA)
Multi-Carrier Multiples Access

42. 6.4.1 Frequency Division Multiple Access (FDMA)
Frequency division multiple access (FDMA) technique refers to sharing the available radio spectrum by assigning specific frequency channels to users on permanent or temporary basis.

43. Frequency Division Duplexing (FDD)
A duplex spacing is used between the forward channels (i.e., downlink – base station to user transmission) and reverse channels (i.e., uplink – user to base station transmission).

44. FDMA Channels
Where Bg is the guard band allocated at the edge of the allocated spectrum band

45. 6.4.2 Time Division Multiple Access (TDMA)
Time division multiple access (TDMA) technique refers to allowing a number of users to access a specified channel bandwidth on a time-shared basis.

48. The most critical feature of TDMA operation is time synchronization.
TDMA Channels
Where m is the number of time slots per carrier channel
The frame efficiency of a TDMA system is defined as the number of bits representing digitized speech, expressed as a percentage of the total number of bits including the control overhead bits that are transmitted in a frame.
The most critical feature of TDMA operation is time synchronization.

49. 6.4.3 Code Division Multiple Access (CDMA)
CDMA refers to a multiple-access technique in which the individual users occupy the complete frequency spectrum whenever they transmit and different spread-spectrum codes are generated by PN code generator and assigned to each user.

53. 6.4.4 Multi Carrier Multiple Access
Orthogonal frequency division multiple access (OFDMA) is one of multicarrier multiple access schemes which use multiple carrier signals at different frequencies, sending some of the bits on each channel.
In OFDM technique, data is distributed over multiple carriers at precise frequencies. The precise relationship among the subcarriers is referred to as orthogonality.

56. Benefits of Spread Spectrum Communications

57. About the Author
T. L. Singal graduated from National Institute of Technology, Kurukshetra and post-graduated from Punjab Technical university in Electronics & Communication Engineering. He began his career with Avionics Design Bureau, HAL, Hyderabad in 1981 and worked on Radar Communication Systems. Then he led R&D group in a Telecom company and successfully developed Multi- Access VHF Wireless Communication Systems. He visited Germany during
He executed international assignment as Senior Network Consultant with Flextronics Network Services, Texas, USA during He was associated with Nokia, AT&T, Cingular Wireless and Nortel Networks, for optimization of 2G/3G Cellular Networks in USA. Since 2003, he is in teaching profession in reputed engineering colleges in India. He has number of technical research papers published in the IEEE Proceedings, Journals, and International/National Conferences. He has authored three text-books `Wireless Communications (2010)’, `Analog & Digital Communications (2012)’, and `Digital Communication (2015) with internationally renowned publisher McGraw-Hill Education.

58. THANKS!