2. Spread Spectrum
• Data is spread data over wide bandwidth
• Makes jamming and interception harder.
— It is easier to jam a narrowband signal than wideband signal.
— Difficult to intercept a spread spectrum signal because energy is spread
over larger bandwidth.
• Two types: Frequency hopping and direct sequence
• Frequency hoping
— Signal broadcast over seemingly random series of frequencies
— Transmission hops from one frequency to another
• Direct Sequence
— Each bit is represented by multiple bits in transmitted signal
— Chipping code:
• Mixed with signal to spread signal over wider bandwidth.
• Chipping code is also known as spreading signal.
6. Spread Spectrum Concept
• Input fed into channel encoder
— Add error detection and correction bits.
• Signal mixed sequence of digits
— Spreading code/sequence
— Typically generated by pseudonoise/pseudorandom number
generator
• Increases bandwidth significantly
— Spreads spectrum
• Receiver uses same sequence to demodulate signal
• Demodulated signal fed into channel decoder
7. Advantages of Spread
Spectrum
• Immunity from various noise and multipath
distortion
—Including jamming
• Can hide/encrypt signals
—Only receiver who knows spreading code can retrieve
signal
• Multiple access: Several users can share same
higher bandwidth with little interference
—Cellular telephones
—Code division multiple access (CDMA)
—Results in higher data rates
8. Frequency Hopping Spread
Spectrum (FHSS)
• Signal broadcast over seemingly random series
of frequencies
• Receiver hops between frequencies in sync with
transmitter
• FHSS not severely affected by jamming:
—Jamming on one frequency affects only a few bits
10. Basic Operation
• A pseudonoise (PN), or pseudorandom number,
source serves as an index into a table
of frequencies;
—This is the spreading code referred to previously.
• PN sequence is of length k bits
• Typically 2k carriers frequencies forming 2k
channels
• Each k bits of the PN source specifies one of the
carrier frequencies.
12. Basic Operation
• At each successive interval (each k PN bits), a
new carrier frequency is selected.
• This frequency is then modulated by the signal
produced from the initial modulator to produce
a new signal with the same shape but now
centered on the selected carrier frequency.
• On reception the spread spectrum signal is
demodulated using the same sequence of PN-
derived frequencies and then demodulated to
produce the output data.
15. Direct Sequence Spread
Spectrum (DSSS)
• Each bit represented by multiple bits using spreading
code
• Spreading code spreads signal across wider frequency
band
— Spreading is proportional to number of bits used
— E.g 10 bit spreading code spreads signal across 10 times
bandwidth of 1 bit code
• Example:
— Combine input with spreading code using XOR
• The combination bit stream has
the data rate of the original spreading code
sequence, so it has a wider bandwidth
than the information stream.
20. Code Division Multiple Access
(CDMA)
• Multiplexing Technique used with spread spectrum
• Start with data signal rate D
— Called bit data rate
• Break each bit into k chips according to fixed pattern
specific to each user
— User’s code
— Also known as channelization code.
• New channel has chip data rate kD chips per second
• E.g. k=6, three users (A,B,C) communicating with base
receiver R
• Code for A = <1,-1,-1,1,-1,1>
• Code for B = <1,1,-1,-1,1,1>
• Code for C = <1,1,-1,1,1,-1>
22. CDMA Explanation
• Consider A communicating with base
• Base station knows A’s code
• Assume communication already synchronized
• A wants to send a 1
— Send chip pattern <1,-1,-1,1,-1,1>
• A’s code
• A wants to send 0
— Send chip[ pattern <-1,1,1,-1,1,-1>
• Complement of A’s code
23. CDMA Explanation
• Decoder ignores other sources when using A’s
code to decode because the chipping codes are
orthogonal
• The codes of A and B that have the property
that are called orthogonal if, for example:
24. CDMA for DSSS
• n users each using different orthogonal PN
sequence
• Modulate each users data stream
—Using BPSK
• Multiply by spreading code of user
