Spread Spectrum Communication Systems

1
Spread Spectrum Communication Systems
2
Overview of Spread Spectrum Communications

• Spread Spectrum is a means of transmission in
  which the data sequence occupies a bandwidth in
  excess of the minimum bandwidth necessary to send
  it.
• Effectively the signal is mapped to a higher
  dimension signal space
• The signal component in each direction is
  sqrt(E/N)
• Signal spreading is done before transmission by
  using a spreading sequence. The same sequence is
  used at the receiver to retrieve the signal
• Spread Spectrum is most effective against
  interference (intentional or non-intentional)
  with fixed energy.
• Main commercial applications in wireless and GPS.

3
Spread Spectrum- Main Operation
b(t)
m(t)
x
c(t)
B(f)
M(f)
4
Spread Spectrum Main Operation
m(t)
r(t)
u(t)

x
c(t)
j(t)
5
Generating the Spreading (Pseudo-Noise) Sequence

• The pseudo-noise (PN) sequence is a periodic
  binary sequence with a noise like waveform that
  is generated by means of a feedback shift
  register.
• The feedback shift register consists of m-stage
  shift registers and a logic circuit that perform
  modulo-2 (X-OR) arithmetic.
• A sequence with period 2m-1 is called
  Maximal-Length sequence

6
PN Sequence Example
s1 s2 s3 1 0 0 1 1 0 1 1 1 0 1 1 1 0 1 0 1 0 0 0 1
1 0 0
Spreading code ? 0 0 1 1 1 0 1 0 . . .
7
Properties of Maximal-Length Sequences

• In each period of a maximal-length sequence, the
  number of 1s and the number of 0s in the
  sequence always differ by 1.
• The autocorrelation function of a maximal-length
  sequence is periodic and binary valued.

8
(No Transcript)
9
Direct Sequence Spread Spectrum with Coherent
BPSK (DS/BPSK)
10
(No Transcript)
11
Analysis

• The objective is to find the ratio of the SNR
  (SNRI) at the input of the receiver to the SNR
  (SNRO) at the output of the receiver

Jamming Component
Signal Component
12
Analysis
Define the set of orthonormal basis
Tc is the chip duration and N is the number of
chips per bit (Tb).
Signal Component
Jamming Component
13
Tb/Tc is called the processing gain (PG)
14
Probability of Error

• vcj is a random variable with zero mean and
  variance JTc/2
• vcj can be viewed as a white noise with zero mean
  and variance N0/2 JTc/2
• Assuming Gaussian distribution for the jamming
  component, the probability of error due to
  jamming
• When noise and jamming exist the probability of
  error is

15
Jamming Margin
Jamming margin is the relative power advantage
that an interference may have without disrupting
the communication system.
16
Frequency-Hopped (FH) Spread Spectrum
Information Sequence
Mixer
Encoder
FSK Modulator
Mixer
Channel
FSK Modulator
Decoder
Frequency Synthesizer
Time Sync
PN Sequence Generator
Frequency Synthesizer
PN Sequence Generator

• FH/SS is usually used with Binary FSK or M-ary
  FSK
• The carrier frequency is determined by the output
  sequence from a PN generator
• Slow hopping system has a hopping rate that is
  lower than the information rate (symbol rate)
• Several information symbols are transmitted by
  the same carrier frequency
• Fast hopping system has a hopping rate that is
  higher than the information rate
• One information symbol is transmitted by
  different carrier frequencies.

17
Slow Frequency Hopping Example
Number of bits per MFSK symbol 2 ? M 4 Rs
Rb/2 Rc max(Rh, Rs) Rs Length of PN segment
per hop 3 Total number of frequency hops 23
8
18
Fast Frequency Hopping Example
Number of bits per MFSK symbol 2 ? M 4 Rs
Rb/2 Rc max(Rh, Rs) Rh Length of PN segment
per hop 3 Total number of frequency hops 23
8
19
Code Division Multiple Access (CDMA)

• Multiple users can use the same channel as long
  as different users are assigned different PN
  sequence (code)
• Several users can transmit simultaneously on the
  same channel
• The transmissions from other users will look like
  interference.
• CDMA main application is wireless communication.
• CDMA is the wireless standard for North America
• Assuming all users have the same power, Ps, the
  number of user Nu that can be supported is given
  by