1. 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
On receiving end, digit sequence is used to demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover data

2. 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
Several users can independently use the same higher bandwidth with very little interference
Several users:---- CDMA

3. Frequency Hoping 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

4. 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

5. Frequency Hoping Spread Spectrum
Pseudo-random number: ,

7. 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 (L = number of bits per signal element)
Tc  Ts - slow-frequency-hop spread spectrum
Tc < Ts - fast-frequency-hop spread spectrum
Multiple Frequency shift keying (MFSK)– more than two frequencies are used. Each signaling element represents more than one bit.

8. MFSK with M=4. An input bit stream is encoded 2 bits at a time, with each of the four possible 2-bit combinations transmitted as a different frequency.

10. 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
Jamming: Deliberate radiation or reradiation of electromagnetic waves so as to impair the usefulness of a specific segment of the radio spectrum that is being used by the enemy for communication or radar.

11. 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 (XOR )

13. Code-Division Multiple Access (CDMA)
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
If k=6 and code is a sequence of ‘1’s and ‘-1’s
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
<d1, d2, d3, d4, d5, d6> = received chip pattern
<c1, c2, c3, c4, c5, c6> = sender’s code

14. Categories of Spreading Sequences
Spreading Sequence Categories
PN sequences
Orthogonal codes
For FHSS systems
PN sequences most common
For DSSS systems not employing CDMA
For DSSS CDMA systems
As was mentioned the spreading sequence, c(t), is a sequence of binary digits shared by transmitter and receiver. Spreading consists of multiplying (XOR) the input data by the spreading sequence, where the bit rate of the spreading sequence is higher than that of the input data. When the signal is received, the spreading is removed by multiplying with the same spreading code, exactly synchronized with the received signal.
The resulting data rate is consequently that of the spreading sequence. This increases the transmitted data rate and therefore increases the required bandwidth. The redundancy of the system is also increased. The spreading codes are chosen so that the resulting signal is noise-like; therefore, there should be an approximately equal number of ones and zeros in the spreading code and few or no repeated patterns. When spreading codes are used in a CDMA application, then there is the further requirement of lack of correlation. When multiple signals are received, each spread with a different spreading code, the receiver should be able to pick out any individual signal using that signal's spreading code. The spread signals should behave as if they were uncorrelated with each other, so that other signals will appear as noise and not interfere with the despreading of a particular signal. Because of the high degree of redundancy provided by the spreading operation, the despreading operation is able to cope with the interference of other signals in the same bandwidth.
Two general categories of spreading sequences have been used: PN sequences and orthogonal codes. PN sequences are the most common ones used in FHSS systems and in DSSS systems not employing CDMA. In DSSS CDMA systems, both PN and orthogonal codes have been used. We examine each of these approaches in turn.

15. 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
Ideal sequence is a random sequence of binary ones and zeros but it is difficult to synchronise the transmitter and receiver. (unpredictable)
Only the receiver that share this information with a transmitter will be able to decode the signal successfully.

16. Important PN Properties
Randomness
Unpredictability

17. Gold Sequences Gold Sequences
Gold sequences constructed by the XOR of two m-sequences with the same clocking
Codes have well-defined cross correlation properties
Only simple circuitry needed to generate large number of unique codes

18. Orthogonal Codes Orthogonal codes Types
All pairwise cross correlations are zero
Fixed- and variable-length codes used in CDMA systems
For CDMA application, each mobile user uses one sequence in the set as a spreading code
Provides zero cross correlation among all users
Types
Welsh codes
Variable-Length Orthogonal codes

19. Walsh Codes
W1 = (0)
Every row is orthogonal to every other row and to the logical not of every other row
Requires tight synchronization
Cross correlation between different shifts of Walsh sequences is not zero
n = dimension of the matrix

21. 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

22. Thus, the unwanted signal (from B) does not show up at all
Thus, the unwanted signal (from B) does not show up at all. You can easily verify that if B had sent a 0 bit, the decoder would produce a value of 0 for SA again. This means that if the decoder is linear and if A and B transmit signals SA and SB, respectively, at the same time, then SA(sA + SB) = SA(SA) + SA(SB) = SA(sA) since the decoder ignores B when it is using A's code. The codes of A and B that have the property that SA (SB) = SB (SA) = 0 are called orthogonal. Such codes are very nice to have but there are not all that many of them. More common is the case when Sx(cy) is small in absolute value when X ≠ Y Then it is easy to distinguish between the two cases when X = Y and when X ≠Y. In our example SA (Sc) = SC (SA) = 0 but SB (Sc) = SC (SB) = 2. In the latter case the C signal would make a small contribution to the decoded signal instead of 0. Using the decoder, Su, the receiver can sort out transmission from u even when there may be other users broadcasting in the same cell.
In practice, the CDMA receiver can filter out the contribution from unwanted users or they appear as low-level noise.