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 bandwidthSignal is further modulated using sequence of digitsSpreading code or spreading sequenceGenerated by pseudonoise, or pseudo-random number generatorEffect of modulation is to increase bandwidth of signal to be transmittedOn receiving end, digit sequence is used to demodulate the spread spectrum signalSignal 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 distortionCan be used for hiding and encrypting signalsSeveral users can independently use the same higher bandwidth with very little interferenceSeveral users:---- CDMA
3 Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of radio frequenciesA number of channels allocated for the FH signalWidth of each channel corresponds to bandwidth of input signalSignal hops from frequency to frequency at fixed intervalsTransmitter operates in one channel at a timeBits are transmitted using some encoding schemeAt each successive interval, a new carrier frequency is selected
4 Frequency Hoping Spread Spectrum Channel sequence dictated by spreading codeReceiver, hopping between frequencies in synchronization with transmitter, picks up messageAdvantagesEavesdroppers hear only unintelligible blipsAttempts 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 MFSKMFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signalFor data rate of R:duration of a bit: T = 1/R secondsduration of signal element: Ts = LT seconds (L = number of bits per signal element)Tc Ts - slow-frequency-hop spread spectrumTc < Ts - fast-frequency-hop spread spectrumMultiple 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 usedResults in a system that is quite resistant to jammingJammer must jam all frequenciesWith fixed power, this reduces the jamming power in any one frequency bandJamming: 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 signalSpreading code spreads signal across a wider frequency bandSpread is in direct proportion to number of bits usedOne technique combines digital information stream with the spreading code bit stream using exclusive-OR (XOR )
13 Code-Division Multiple Access (CDMA) Basic Principles of CDMAD = rate of data signalBreak each bit into k chipsChips are a user-specific fixed patternChip data rate of new channel = kDIf k=6 and code is a sequence of ‘1’s and ‘-1’sFor 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 CategoriesPN sequencesOrthogonal codesFor FHSS systemsPN sequences most commonFor DSSS systems not employing CDMAFor DSSS CDMA systemsAs 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 SequencesPN generator produces periodic sequence that appears to be randomPN SequencesGenerated by an algorithm using initial seedSequence isn’t statistically random but will pass many test of randomnessSequences referred to as pseudorandom numbers or pseudonoise sequencesUnless algorithm and seed are known, the sequence is impractical to predictIdeal 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 RandomnessUnpredictability
17 Gold Sequences Gold Sequences Gold sequences constructed by the XOR of two m-sequences with the same clockingCodes have well-defined cross correlation propertiesOnly simple circuitry needed to generate large number of unique codes
18 Orthogonal Codes Orthogonal codes Types All pairwise cross correlations are zeroFixed- and variable-length codes used in CDMA systemsFor CDMA application, each mobile user uses one sequence in the set as a spreading codeProvides zero cross correlation among all usersTypesWelsh codesVariable-Length Orthogonal codes
19 Walsh CodesW1 = (0)Every row is orthogonal to every other row and to the logical not of every other rowRequires tight synchronizationCross correlation between different shifts of Walsh sequences is not zeron = dimension of the matrix
21 CDMA ExampleUser 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 -> 1User A ‘0’ bit: -6 -> 0User 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.