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Data and Computer Communications. Chapter 9 – Spread Spectrum. Eighth Edition by William Stallings Lecture slides by Lawrie Brown. Spread Spectrum. important encoding method for wireless communications analog & digital data with analog signal spreads data over wide bandwidth - PowerPoint PPT Presentation
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Data and Computer Communications
Eighth Editionby William Stallings
Lecture slides by Lawrie Brown
Chapter 9 – Spread SpectrumChapter 9 – Spread Spectrum
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
Spread Spectrum Input is fed into a channel encoder
Produces analog signal with narrow bandwidth Signal is further modulated using sequence of
digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number
generator Effect of modulation is to increase bandwidth of
signal to be transmitted
Spread Spectrum On receiving end, digit sequence is used to
demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data
Spread Spectrum Advantages
What can be gained from apparent waste of spectrum? Immunity from various kinds of noise and multipath
distortion Can be used for hiding and encrypting signals Several users can independently use the same higher
bandwidth with very little interference CDM/CDMA Mobile telephones
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
Frequency Hoping Spread Spectrum (FHSS)
Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal (2k) Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval (300 ms for LAN 802.11), a new
carrier frequency is selected
Frequency Hoping Spread Spectrum
Channel sequence dictated by spreading code Receiver, hopping between frequencies in
synchronization with transmitter, picks up message
Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only
at knocking out a few bits
Frequency Hoping Spread Spectrum
Frequency Hoping Spread Spectrum
Frequency Hoping Spread Spectrum
Frequency Hoping Spread Spectrum
Define the FSK input to the FHSS system as
Frequency Hoping Spread Spectrum Assume the duration of one hop is the same as the
duration of one bit ignore phase differences between the data signal and
the spreading signal, also called a chipping signal, c(t) .
The product signal during the ith hop
Using the trigonometric identity
A bandpass filter is used to block the difference frequency and pass the sum frequency, yielding an FHSS signal of
Frequency Hoping Spread Spectrum At a receiver
A bandpass filter is used to block the sum frequency and pass the difference frequency
The same form as
Multiple Frequency-Shift Keying (MFSK)
More than two frequencies are used More bandwidth efficient but more susceptible to error
f i = f c + (2i – 1 – M)f d
f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L
L = number of bits per signal element
tfAts ii 2cos Mi 1
FHSS Using MFSK MFSK signal is translated to a new frequency
every Tc seconds by modulating the MFSK signal with the FHSS carrier signal
For data rate of R: duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds
Tc Ts - slow-frequency-hop spread spectrum multiple data bits per frequency hop
Tc < Ts - fast-frequency-hop spread spectrum multiple frequency hops per data bit
Slow Frequency Hop Spread Spectrum Using MFSK (M = 4, k = 2)
Multiple Frequency-Shift Keying (MFSK)
total MFSK bandwidth Wd = Mfd
Total FHSS bandwidth Ws = 2k Wd
Fast Frequency Hop Spread Spectrum Using MFSK (M = 4, k = 2)
FHSS Performance Considerations Large number of frequencies used Large value of k results in a system that is quite resistant to
jamming Jammer must jam all frequencies With fixed power, this reduces the jamming power in any one frequency
band Ex. MFSK transmitter with BW Wd and noise jammer of the same
BW then Eb/Nj = EbWd/Sj
If frequency hopping is used, jammer must jam 2k frequencies. With fixed power, jammed frequency will be reduced Sj/2k.
The gain in SNR or processing gain is Gp = 2k = Ws/Wd
Direct Sequence Spread Spectrum (DSSS)
Each bit in original signal is represented by multiple bits in the transmitted signal
Spreading code spreads signal across a wider frequency band Spread is in direct proportion to number of bits used
One technique combines digital information stream with the spreading code bit stream using exclusive-OR (Figure 7.6)
Direct Sequence Spread Spectrum (DSSS)
Phase-Shift Keying (PSK) Two-level PSK (BPSK)
Uses two phases to represent binary digits
ts tfA c2cos tfA c2cos
1binary 0binary
tfA c2cos
tfA c2cos1binary 0binary
Phase-Shift Keying (PSK) BPSK signal can be represented
DSSS Using BPSK Multiply BPSK signal,
sd(t) = A d(t) cos(2 fct)
by c(t) [takes values +1, -1] to gets(t) = A d(t)c(t) cos(2 fct)
At receiver, incoming signal multiplied by c(t) Since, c(t) x c(t) = 1, incoming signal is recovered
DSSS Using BPSK
DSSS Using BPSK
DSSS Performance Consideration
DSSS Performance Consideration
assume a simple jamming signal at the center frequency of the DSSS system with a form
the received signal is
where
DSSS Performance Consideration
The despreader at the receiver multiplies sr(t) by c(t), for jamming
Which is simply a BPSK modulation of the carrier tone
the carrier power Sj is spread over a bandwidth of approximately 2/Tc
Using Bandpass filter with BW 2/T, most of the jamming power is filtered
DSSS Performance Consideration
as an approximation, we can say that the jamming power passed by the filter is
The jamming is reduced by (Tc/T) The inverse of this factor is the gain in SNR
Code-Division Multiple Access (CDMA)
CDMA is a multiplexing technique used with spread spectrum
Basic Principles of CDMA D = rate of data signal Break each bit into k chips
Chips are a user-specific fixed pattern Chip data rate of new channel = kD
CDMA Example
CDMA Example If k=6 and code is a sequence of 1s and -1s
For a ‘1’ bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6>
For a ‘0’ bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6>
Receiver knows sender’s code and performs electronic decode function
u is the user that we are interested in <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code
665544332211 cdcdcdcdcdcddSu
CDMA Example User A code = <1, –1, –1, 1, –1, 1>
To send a 1 bit = <1, –1, –1, 1, –1, 1> To send a 0 bit = <–1, 1, 1, –1, 1, –1>
User B code = <1, 1, –1, – 1, 1, 1> To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code (A’s code) x (received chip pattern)
User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 -> unwanted signal ignored
CDMA Example the preceding computation using SA
becomes
If A sends a 0 bit that corresponds to
no matter what sequence of 1s and -1s, it is always
CDMA Example If B sends a 1 bit, then
Thus, the unwanted signal (from B) does not show up at all
if the decoder is linear and if A and B transmit signals SA and SB , respectively, at the same time, then
The codes of A and B that have the property that
are called orthogonal
In practice, the CDMA receiver can filter out the contribution from unwanted users or they appear as low-level noise
CDMA Example User’s codes
Transmission from A
CDMA Example Transmission from B, receiver attempts to recover
A’s transmission
Transmission from C, receiver attempts to recover B’s transmission
CDMA Example Transmission from B and C, receiver
attempts to recover B’s transmission
CDMA for Direct Sequence Spread Spectrum
Categories of Spreading Sequences
Spreading Sequence Categories PN sequences Orthogonal codes
For FHSS systems PN sequences most common
For DSSS systems not employing CDMA PN sequences most common
For DSSS CDMA systems PN sequences Orthogonal codes
PN Sequences PN generator produces periodic sequence that
appears to be random PN Sequences
Generated by an algorithm using initial seed Sequence isn’t statistically random but will pass
many test of randomness Sequences referred to as pseudorandom numbers
or pseudonoise sequences Unless algorithm and seed are known, the
sequence is impractical to predict
Important PN Properties Randomness
Uniform distribution Balance property :
in long sequence the fraction of binary ones should approach 1/2 Run property: run is defined as a sequence of all 1-s or a sequence
of all 0-s about 1/2 of runs of each type should be of length 1,1/4 of length 2,
1/8 of length 3, etc. Independence
no one value in sequence can be inferredfrom the others Correlation property
good auto-and cross-correlation properties
Unpredictability
Implementation of PN sequences
Linear Feedback Shift Register Implementation
Problem Assignments
Solve all the review questions Try the following problems: 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7