Transcript Document

Lecture 8: Spread Spectrum
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Principle of spread spectrum
Frequency hopping spread spectrum
Direct sequence spread spectrum
Direct sequence CDMA systems
Ben Slimane
[email protected]
What is Spread Spectrum?
• Spread spectrum techniques are methods by
which electromagnetic energy generated in a
particular bandwidth is deliberately spread in the
frequency domain, resulting in a signal with a
wider bandwidth
• Spread spectrum methods:
– Frequency hopping spread spectrum
– Direct sequence spread spectrum
– Time hopping spread spectrum
Spread Spectrum
• At the transmitter side:
– 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
• At the receiving end:
– digit sequence is used to demodulate the spread
spectrum signal
– Signal is fed into a channel decoder to recover data
Spread Spectrum
Spread Spectrum
• 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
– Multiple access capability
• Several users can independently use the same wider
bandwidth with very little interference
Frequency Hopping Spread
Spectrum (FHSS)
• Signal is broadcast over seemingly random series of
radio frequencies
– A number of channels allocated for the FH signal
– 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, a new carrier frequency is
selected
Frequency Hoping Spread
Spectrum
• The frequency sequence is dictated by the spreading
code
• Receiver should hop synchronously with the
transmitter to be able to recover the 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 Hopping Spread
Spectrum
Frequency Hopping Spread
Spectrum
• Slow-frequency-hop spread spectrum
– The hopping duration is larger or equal to the
symbol duration of the modulated signal
Tc >= Ts
• Fast-frequency-hop spread spectrum
– The hopping duration is smaller than the
symbol duration of the modulated signal
Tc < Ts
Slow Frequency-Hop SS
Fast Frequency-Hop SS
FHSS Performance
Considerations
• Large number of frequencies used
• 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
Direct Sequence Spread
Spectrum (DSSS)
• The modulated signal is spread by a spreading
waveform (spreading code)
• The spreading code spreads the signal over a wider
frequency band
– Spread is in direct proportion to number of bits per symbol
used
• The spreading code is usually periodic with a period
larger or equal to the symbol duration of the
modulated signal
DSSS Using BPSK
Direct Sequence Spread
Spectrum (DSSS)
Spectrum of DS Spread
Spectrum Signal
Code-Division Multiple
Access (CDMA)
• CDMA is multiple access scheme that allows
many users to share the same bandwidth
– 3G (WCDMA), IS-95
• Basic Principles of CDMA
– Each user is assigned a unique spreading code
– The processing gain protects the useful signal and
reduces interference between the different users
PG = (Bandwidth after spreading)/(Bandwidth before spreading)
CDMA for Direct Sequence
Spread Spectrum
CDMA Example
Spreading Sequences
• Spreading sequences are very important in the design
of spread spectrum communication
• Two categories of Spreading Sequences
– PN sequences
– Orthogonal codes
• FHSS systems
– PN sequences most common
• DSSS CDMA systems
– PN sequences
– Orthogonal codes
PN Sequences
• PN sequences are periodic but appear random within
one period
• PN sequences are very easy to generate
– Generated using LFSR
• PN sequences are easy to re-generate and synchronize
at the receiver
• PN sequences have good random properties
• PN sequences converge to a Gaussian process when
the period tends to infinity
Spreading in Cellular CDMA
Systems
• Cellular CDMA systems use two layers of
spreading
• Channelization codes (orthogonal codes)
– Provides orthogonality among users within the
same cell
• Long PN sequences (scrambling code)
– Provides good randomness properties (low cross
correlation)
– Reduces interference from other cells