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Spread Spectrum Input is fed into a channel encoder

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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.

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