Wireless Communication Engineering Spread Spectrum Communication Systems

1
Wireless Communication Engineering - Spread
Spectrum Communication Systems

• Li-Der Jeng
• Department of Electronic Engineering
• Chung-Yuan Christian University
• Chung-Li, Taiwan, ROC
• TEL [phone]
• E-mail lider_at_cycu.edu.tw

2
Contents

• Introduction to Spread-Spectrum Systems
• Binary Shift-Register Sequences for
  Spread-Spectrum Systems
• Code-Division Multiple-Access Digital Cellular
  Systems

3
Selected References (I)

• A. J. Viterbi, Principles of spread spectrum
  communication, Reading, Mass., Addison-Wesley
  1995.  
• J. K. Holmes, Coherent spread spectrum systems,
  1982. 
• R. L. Peterson, R. E. Ziemer and D. E. Borth,
  Introduction to spread-spectrum communications,
  Englewood Cliffs, NJ, Prentice Hall, 1995. 
• K. Fazel, G. P. Fettweis, Multi-carrier
  spread-spectrum, Boston, Mass., Kluwer Academic
  Publishers, 1997.
• K. Fazel, S. Kaiser, Multi-carrier
  spread-spectrum related topics, Boston, Kluwer
  Academic Publishers, 2000.  
• A. A. Hassan, J. E. Hershey and G. J. Saulnier,
  Perspectives in spread spectrum, Boston, Kluwer
  Academic Publishers, 1998.  
• A. N. Manikas, Principles of spread spectrum
  theory and systems, Imperial College Press, 2000.
   
• S. Glisic, B. Vucetic, Spread spectrum CDMA
  systems for wireless communications, Boston,
  Artech House, 1997.  
• M. K. Simon and J. K. Omura, R. A. Scholtz and B.
  K. Levitt, Spread spectrum communications,
  Maryland, Computer Science Press, 1985.  
• R. Skaug, J. F. Hjelmstad, Spread spectrum in
  communication, London, Peter Peregrinus, 1985  

4
Selected References (II)

• S. G. Glisic, P A. Leppanen, Wireless
  communications DMA versus CDMA, Boston, Kluwer
  Academic Publishers, 1997.
• O. Berg, Spread spectrum in mobile communication,
  London, UK, Institution of Electrical Engineers,
  1998.
• P. van Rooyen, M. Lotter and D. van Wyk,
  Space-time processing for CDMA mobile
  communications, Boston, Kluwer Academic
  Publishers, 2000.  
• T. Ojanpera, R. Prasad, Wideband CDMA for third
  generation mobile communications, Boston, Artech
  House, 1998.  
• J. J. Caffery, Wireless location in CDMA cellular
  radio systems, Boston, Kluwer Academic
  Publishers, 2000.
• R. C. Dixon, Spread spectrum systems with
  commercial applications, 3rd ed., New York, John
  Wiley and Sons, 1994 . 
• C. E. Cook, Spread-spectrum communications, New
  york, IEEE, 1983. 
• K. Feher, Wireless digital communications
  modulation and spread spectrum applications,
  Upper Saddle River, N.J. Prentice-Hall, 1995.
• S-L. Low and R. Schneider, CDMA internetworking
  deploying the Open A-Interface, Upper Saddle
  River, NJ, Prentice Hall, 2000.
• J. B. Groe, L. E. Larson, CDMA mobile radio
  design, Boston, Artech House, 2000.    

5
Selected References (III)

•  
• J. S. Lee, L. E. Miller, CDMA systems engineering
  handbook, Boston, Mass. Artech House, 1998.
• S. Tantaratana, K. M. Ahmed, Wireless
  applications of spread spectrum systems,
  Piscataway, NJ, IEEE 1998.  
• S. S. H. Wijayasuriya, Alleviation of near-far
  effects in DS-CDMA mobile radio, UK University of
  Bristol, 1994.  
• V. K. Garg, K. F. Smolik and J. E. Wilkes,
  Applications of CDMA in wireless personal
  communications, Upper Saddle River, NJ Prentice
  Hall, 1997.  
• S. C. Yang, CDMA RF system engineering, Boston,
  Artech House, 1998.  
• M. Y. Rhee, CDMA cellular mobile communications
  network security, Upper Saddle River, NJ,
  Prentice Hall, 1998.   
• F. Swarts, CDMA techniques for third generation
  mobile systems, Boston, Kluwer Academic
  Publishers, 1999.
• P. van Rooyen, M. Lotter and D. van Wyk,
  Space-time processing for CDMA mobile
  communications, Boston, Kluwer Academic
  Publishers, 2000.
• K. Kim, Handbook of CDMA system design,
  engineering, and optimization, Upper Saddle
  River, NJ, Prentice Hall, 2000.  
• V. K. Garg, IS-95 CDMA and cdma2000 cellular/PCS
  systems implementation, Upper Saddle River, NJ,
  Prentice Hall, 2000.   

6
Selected References (IV)

• S. Sheng R. Brodersen, Low-power cmos wireless
  communications a wideband CDMA system design,
  Boston, Kluwer Academic Publishers, 1998.  
• H. Liu, Signal processing applications in CDMA
  communications, Boston, Artech House, 2000.  
• J. C. Liberti, T. S. Rappaport, Smart antennas
  for wireless communications IS-95 and third
  generation CDMA applications, Upper Saddle River,
  NJ, Prentice Hall, 1999.  
• T. Ojanpera, R. Prasad, Wideband CDMA for third
  generation mobile communications, Boston, Artech
  House, 1998.  
• S. G. Glisic, P A. Leppanen, Wireless
  communications DMA versus CDMA, Boston, Kluwer
  Academic Publishers, 1997.  
• J. J. Caffery, Wireless location in CDMA cellular
  radio systems, Boston, Kluwer Academic
  Publishers, 2000.

7
Introduction to Spread-Spectrum Systems
8
Introduction

• Channels
• AWGN, fading
• Jamming (military)
• Continuous wave (CW) tone
• Pulsed AWGN
• Spread spectrum system (modem)
• Bandwidth gt required bandwidth (usually much
  larger)
• Demodulation correlation (in part)
• Processing gain

9
Pulse-noise jamming jammer

• Pulse duty factor r
• Average transmitted power J NJJ/W, W bandwidth
• RSNI, IN Gaussian
• Bit error probability for coherent BPSK in AWGN
• Noise power spectral density

10
Pulse-noise jamming jammer

• The average bit error probability
• Operate in a jamming environment
• Eb/NJ must be known ?worst case

11
Low Probability of Detection (LPD)

12
Spread Spectrum and its Applications (1)

• Spread spectrum (SS) modulation maps a lower
  dimension (smaller bandwidth, Rb) signal SL(t) to
  a much higher dimension signal SH (with much
  larger bandwidth, Rc)
• The ratio Rc/RbG is often referred to as the
  system processing gain
• The mapping SL SH is called (spectrum or
  time) spreading while the inverse mapping SH
  SL is called (spectrum or time) despreading

13
Spread Spectrum and its Applications (2)

• Spectrum spreading can be accomplished via
• direct sequence
• frequency-hopping
• time-hopping
• hybrid methods
• Reverse operation is needed in the receiving end
• mapping should be deterministic and yet easy to
  implement
• additional synchronization mechanism is needed

14
Spread Spectrum and its Applications (3)

• High tolerance to intentional or unintentional
  interference
• jamming, RFI, multipath fading
• Position determination and velocity estimation
• GPS, TDRSS
• Low probability of interception and detection
• random or pseudorandom spreading sequence
• Multiple access communication
• orthogonal addresses

15
Types of techniques used for spread spectrum (I)

• Direct Sequence Spread Spectrum (DSSS)

16
Types of techniques used for spread spectrum (II)

• Frequency Hopping Spread Spectrum (FHSS)

17
Types of techniques used for spread spectrum (III)

• Time-Hopped (TH) system

18
Model of spread spectrum digital communication
system
19
Pseudonoise (PN) Sequences (1)

• For a pure random (noise) binary (1,-1) sequence
  ck of period N the periodic autocorrelation
  function defined by
• should have the property

20
Pseudonoise (PN) Sequences (2)

• A coherent receiver would perform
  matched-filtering by correlating the received
  noise-corrupted waveform (samples), r(t),
• with a local replica s(t)
• where we have assumed that

21
Pseudonoise (PN) Sequences (3)

• If the channel exhibits multipath effect and J
  users are present then the received waveform can
  be expressed as
• where L and Lj represent the number of paths
  associated with the desired user and the jth
  interferer while aj, ?i and ?jk, ?jk are the
  corresponding gains (attenuations) and delays.

22
Pseudonoise (PN) Sequences (4)

• The matched filter output becomes
• where dk and dik represent the kth data bit
  of the desired
• user and the jth interferer, respectively.
• Rc(m) is the autocorrelation function defined
  before while
• Rj(m) is the cross-correlation between the
  desired user
• and the jth interferer.

23
Pseudonoise (PN) Sequences (5)

• Matched-filter receiver demands cross-correlation
  between distinct sequences be as small as
  possible, i.e.,
• Cross-correlation leads to multiple access
  (co-channel) interference (MAI) while
  autocorrelation results in self-noise or
  intersymbol interference (ISI)

24
Pseudonoise (PN) Sequences (6)

• Linear sequences
• maximum length sequence - m sequence
• Gold codes
• Barker codes, Generalized Barker sequences
• Walsh-Hadamard code
• Kasami sequences
• Nonlinear sequences
• bent sequences
• chaotic sequences
• nonlinear sequences based on Kerdock code, linear
  trace code or Delsarte-Goethals code
• other nonlinear sequences

25
Pseudonoise (PN) Sequences (7)

• Frequency hopping patterns can be considered as
  non-binary PN sequences
• Sequences based on shortened RS codes
• Sequences based on congruence theory
• Sequences based on adaptive fuzzy rules
• Sequences based on chaotic maps
• Others

26
Frequency-Hopping Sequences

• Want to have a large family of FH sequences
  (codes) with low cross-correlation values
• These sequences (codes) must be easy to generate
• These sequences (codes) must be difficult to
  estimate (break)

27
Implications and Applications of the Correlation
Properties of SS signals

• Low cross-correlation and out-of-phase
  auto-correlation values
• Privacy and security
• Rejection of undesired signals
• Separation and combination of multipath signal
  components
• Ranging and position locating
• Channel identification - measurement and learning

28
Basic Spread Spectrum Technique
29
Direct Sequence Transmitter/Receiver
30
Basic DS/SS System - Spreading
31
Basic DS/SS System - Despreading
32
Direct-Sequence Spread Spectrum (DS-SS)

• BPSK DS-SS transmitter

(spreading/second modulation)
c(t) spreading waveform (code)
33
Direct-Sequence Spread Spectrum (DS-SS)

• BPSK DS-SS receiver

recall that
(despreading)
34
BPSK direct-sequence spreading and despreading
35
Power spectral density of data-modulated carrier

• PSD for BPSK

36
Power spectral density of data- and spreading
code-modulated carrier

37
Power spectral density

• If not BPSK, from chapter 1
• R(t) G(f) autocorrelation function and
  power spectral density form a Fourier transform
  pair
• Ex 2.1 BPSK power spectrum, T100Tc, period
  infinite
• Sol

38
Power spectral density
39
Power spectral density

40
Power spectral density

• Based on this approximation, we investigate the
  advantage of using spread spectrum interference
  suppression

41
The advantages of using spread spectrum (I)

• The advantages of using spread spectrum
• Narrowband interference suppression
• Received signal
• Power spectrum

42
The advantages of using spread spectrum (II)

• After spreading
• After filtering

43
Reveiver power spetral densities with tone
jamming (I)
44
Reveiver power spetral densities with tone
jamming (II)
45
Frequency-Hopping Transmitter/Receiver
46
A Frequency-Hopped (FH) Pattern
47
Block Diagram of an FH Spread Spectrum System
48
Block Diagram of an Independent Tone FH/SS System
49
Frequency-Hop Spread Spectrum (FH-SS)

• Noncoherent Slow-Frequency-Hop Spread Spectrum
• A common data modulation for FH system is M-ary
  frequency shift keying (MFSK)
• Parameters
• T duration of one information bit
• Data modulator each LT (Ts) seconds output a
  tone (from 2L tones)
• Hopping rate each Tc seconds transfer to a new
  frequency (No. of freqencey-hop 2k)
• When slow-frequency-hop
  system
• When fast-frequency-hop
  system

50
Pictorial representation of transmitted/received
signal for an M-ary FSK/SFH system
51
Pictorial representation of transmitted/received
signal for an M-ary FSK/FFH system
52
Hybrid Direct-Sequence/Frequency-Hop Spread
Spectrum-Transmitter
53
Hybrid Direct-Sequence/Frequency-Hop Spread
Spectrum-Receiver
54
Block Diagram of Time-Hopping (TH) SS System
55
Binary Shift-Register Sequences for
Spread-Spectrum Systems
56
PN Sequences

• Pseudo-noise (PN) sequences are used
• To produce (spread) DS/SS time waveform
• For controlling the carrier frequency of an FH/SS
  waveform
• The PN sequence is so called pseudo random
  because its noise-like properties.

57
Properties of PN sequences

• Balanced property
• Relative frequency of 1s and 0s are almost
  equal.
• Run-length property
• Run-length is the number of successive 1s or
  0s.
• Run-lengths of a PN sequence is exponentially
  distributed.
• Delay and Add property
• Equal numbers of agreements and disagreements
  between the origin and shift version a PN code.

58
Categorization of PN Codes

• M-sequences
• Gold sequences
• Gold sequences are popular multiple access
  sequences
• Other PN sequences
• A PN code must satisfy the three random-like
  properties

59
m-sequence (I)

• Maximum-Length Shift Register sequence
• M-sequence is generated by using linear feedback
  shift-register circuits and the generator
  polynomial is g(x).
• g(x) is a primitive polynomial.
• A polynomial is primitive polynomial of order m
  if the smallest integer n for which the
  polynomial divides is .

60
m-sequence (II)

• For a fixed state of shift registers, the longest
  period of the PN codes is n2m-1, where n is the
  length of the sequences.
• The longest sequence is called Maximum-Length
  Sequence, or m-sequence.

61
Maximum-Length Shift Register

• Example of generator polynomial

1
2
3
4
5
62
General m-stage shift register with linear
feedback
63
Example of m-Sequence
The period of the m-sequence is 31 (m5).
64
Binary Primitive Polynomial
The right-most bit represents the coefficient of
the highest degree term, e. g., 1011011 ?
1x2x3x5x6
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Properties of m-sequence (I)

• A cyclic shift of an m-sequence is also a
  m-sequence.
• Shift-and-add Property
• The modulo-2 sum of an m-sequence and any shift
  of the same m-sequence is another shift of the
  same m-sequence.
• Assume b(d)a(D)/g(D) and b(D)a(D)/g(D), where
  b(D) and b(D) is the m-sequence with different
  initial state a(D) and a(D).
• Because the modulo-2 of two different initial
  state is another initial state,
  b(D)b(D)a(D)a(D)/g(D)a(D)/g(D)b(D)
  where b(D) is another shift version of b(D).

66
Properties of m-sequence (II)

• One more 1 than 0s
• Because (0,0,,0) is not a state of the shift
  register.
• It satisfies the balanced property.
• For any m-sequence, 1/2k of the runs have length
  k.

67
Correlation of m-Sequences

• Autocorrelation
• where i is the code delay between two
  m-sequences.
• Cross-correlation
• The cross-correlation function of two m-sequences
  usually has large peaks.

68
Autocorrelation function for a m-sequence

• Spreading waveform c(t)

69
Gold sequences

• Preferred sequences
• The cross-correlation of two sequences of length
  n takes only three values -1,-t(m), and t(m)-2
  periodically and we call this kind of sequences
  preferred sequences, where

70
Gold Sequences (II)

• Take modulo-2 addition between one preferred
  sequence and the shift version of the other
  sequence, then we will get a set of Gold
  sequences.

71
Example of Gold Sequence
Gold sequence
72
Generation of Gold Sequences of Length 31
73
Correlation of Gold Sequences

• The off-peak autocorrelation and
  cross-correlation function are three-valued in
  the set -1,-t(m), and t(m)-2
• The in-phase autocorrelation function is equal to
  n.
• The in-phase autocorrelation is the
  autocorrelation function between the same
  sequence with zero offset.

74
m-Sequences and Gold Codes (I)
75
m-Sequences and Gold Codes (II)

• m is the stage number of the shift register.
• n is the code period/length.
• is the maximum correlation.
• is the in-phase autocorrelation.
• Gold sequence has smaller peak cross-correlation
  ratio than m-sequence.

76
Code-Division Multiple-Access Digital Cellular
Systems
77
Fundamentals of Cellular Radio system (I)

• Cell Base stations are located either in the
  middle of the cell or at the corner of the cell
• Communication
• Mobile base
• Base mobile
• Mobile mobile
• Frequency-resue
• Hand-off (handover) soft or hard

78
Fundamentals of Cellular Radio system (II)

• Overloaded cell is split into small cells
• For cellular system, the base-to-mobile and
  mobile-to-base communication paths are allocated
  in pairs of frequency separated by a fixed
  bandwidth. Such an approach is known as
  frequency-division duplex (FDD)

79
Hexagonal cell pattern for cellular radio system
80
Seven-cell reuse pattern
81
Cell splitting in seven-cell pattern
82
Comparison of First- and Second-Generation
Cellular Radio Systems Analog
83
Comparison of First- and Second-Generation
Cellular Radio Systems Digital
84
Multiple Access Techniques

• Time division multiple access (TDMA)
• Frequency division multiple access (FDMA)
• Code division multiple access (CDMA)

85
Spread Spectrum and Multiple Access (I)

• Shannons Lesson for Multiple Access
• Make each users signal appear as white noise to
  every other user
• Can achieve Shannon channel capacity through
  appropriate channel coding and interference
  cancellation
• Closest Implementation CDMA

86
Spread Spectrum and Multiple Access (II)

• Implementation Consideration
• Everyone uses the whole expanded space
• Each user is assigned an unique address (mapping)
• The addresses must be orthogonal or at least
  quasi-orthogonal to reduce multiple access
  interference (MAI)
• Linear and nonlinear sequences (addresses) have
  been proposed (m sequences, Gold code, Kasami
  code, bent sequences, chaotic sequence, ... etc.)

87
An FDMA System
88
A TDMA System
89
A Typical TDMA Frame Structure
90
What is CDMA ?

• A multiple access technique using pseudo-noise
  (PN) or other MA codes to spread the spectrum of
  each user signal
• Signals share a common wide-band channel at the
  same time
• Signals are distinguished from each other by
  using different PN codes
• Receiver processes only the addressed signal. All
  other user signals appear as noise to the
  addressed signal
• Capacity determined mostly by G and SNRreq

91
Major Attributes of CDMA

• Universal frequency reuse
• Constructive combining of multipath by RAKE
  receiver
• Seamless soft-handover
• Natural exploitation of voice activity and
  sectored and adaptive beam forming antennas
• Fast and accurate C/I measurement and power
  control

92
Block Diagram of IS-95 Forward Link
93
Block Diagram of IS-95 Reverse Link
94
IS-95 DS-CDMA Digital Cellular System Parameters
95
IS-2000 Forward Link
96
An MFSK/FH Multiple Access System
97
An MFSK/FHMA System Time-Frequency Analysis
98
A Comparison of Three MA Schemes
99
Design Issues (1)

• Address design and assignment
• requires a large family of addresses (sequences)
  with desired cross-correlation and
  autocorrelation properties
• autocorrelation-ISI, self-interference
• cross-correlation-MAI
• sequences in the same family need not be of the
  same length
• Power control
• to reduce MAI and enhance system capacity
• Address (code) acquisition and tracking
• initial and essential synchronization
• needed in RAKE-like operations

100
Design Issues (2)

• RAKE receiver design
• diversity (finger) selection and combining
• joint channel estimation, synchronization, and
  signal detection/decoding may be considered
• Soft handover and pilot control
• avoid too many pilots and too frequent handovers
• Nonlinear distortions
• lessen intermodulation, AM/AM, AM/PM and other
  nonlinear effects for wideband signals
• suitable pre- and post-filtering are recommended
• detector design must take them into account

101
Design Issues (3)

• Modulation and coding
• reduce nonlinear effects while achieving high
  spectral and power efficiency
• soft and iterated detection/decoding
• Smart signal processing
• joint estimation/detection
• spatial/temporal (2D) signal processing and
  filtering
• Multiuser detection
• Narrowband and wideband co-channel and adjacent
  channel interference suppression