Spread Spectrum - Lecture 21

Spread SpectrumLecture 21OverviewSpread Spectrum IntroSpread Spectrum ModelPseudorandom SequencesPN Sequence GeneratorFrequency Hopping Spread SpectrumDirect Sequence Spread SpectrumProcessing GainDSSS Using BPSK2Spread SpectrumSpread spectrum technology has blossomed from a military technology into one of the fundamental building blocks in current and next-generation wireless systems. From cellular to cordless to wireless LAN (WLAN) systems, spectrum is a vital component in the system design process.3Spread Spectrum Wideband ModulationBenefitsInformation SecurityInterference ResistanceBand SharingFrequency Hopping Spread Spectrum (FHSS)Data are constantFrequencies are randomizedDirect Sequence Spread Spectrum (DSSS)Frequency is constantData are randomizedimmunity from noise and multipath distortioncan hide / encrypt signalsseveral users can share same higher bandwidth with little interferenceCDM/CDMA Mobile telephones4Spread SpectrumImportant encoding method for wireless communicationsAnalog & digital data with analog signalSpreads data over wide bandwidthMakes jamming and interception harderTwo approaches, both in use:Frequency HoppingDirect Sequence5Communication Systems6General Model of Spread Spectrum System7Pseudorandom SequencesThe spread of the “random” sequence of frequencies is determined by a pseudonoise (PN) sequence generator.A PN generator outputs a stream of bits (1s and 0s) that appears random (has no apparent pattern).PN sequence generators are easy to construct using simple logic components: XOR gates, and a shift register, made up of flip flopsA PN sequence is not truly random (hence, “pseudo”), but is periodic and repeats at a fixed interval.8Pseudorandom NumbersGenerated by a deterministic algorithmnot actually randombut if algorithm good, results pass reasonable tests of randomnessStarting from an initial seedNeed to know algorithm and seed to predict sequenceHence only receiver can decode signal9PN Sequence GeneratorThe PN sequence length is the number of iterations (the number of 1s and 0s) before the sequence repeats. The sequence length is determined by the: number of flip flops, n, in the shift registerselection of feedback taps that are applied to one or more XOR gates.The sequence length can have a maximum value of:A PN sequence that has this length is said to be a maximal length sequence.number of flip flops10PN Sequence GeneratorFor the PN sequence generator below, the number of flip flops, n, is 3.This generator will generate a maximal length sequence, and the length is determined by: maximum length = 2n – 1 = 23 –1 = 711Example Problem 1Assuming the circuit below is a maximal length PN sequence generator, how many outputs (1s and 0s) would it produce before the sequence repeats?Example Problem 1Assuming the circuit below is a maximal length PN sequence generator, how many outputs (1s and 0s) would it produce before the sequence repeats?maximum length = 2n – 1 = 25 –1 = 3114Frequency Hopping Spread Spectrum (FHSS)Signal is broadcast over seemingly random series of frequenciesReceiver hops between frequencies in sync with transmitterEavesdroppers hear unintelligible blipsJamming on one frequency affects only a few bits15Frequency Hoping Spread SpectrumChannel 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 bits16Frequency Hopping Example17FHSS (Transmitter)18Frequency Hopping Spread Spectrum System (Receiver)19Slow and Fast FHSScommonly use multiple FSK (MFSK)have frequency shifted every Tc secondsduration of signal element is Ts secondsSlow FHSS has Tc  TsFast FHSS has Tc < TsFHSS quite resistant to noise or jammingwith fast FHSS giving better performance20FHSS 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 secondsTc  Ts - slow-frequency-hop spread spectrumTc < Ts - fast-frequency-hop spread spectrum21Slow MFSK FHSS22Fast MFSK FHSS23FHSS Performance ConsiderationsLarge 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 band2425Direct Sequence Spread Spectrum (DSSS)Each bit is represented by multiple bits using a spreading codeThis spreads signal across a wider frequency bandHas performance similar to FHSS26In a DSSS system the message bit stream is modified by a higher rate pseudonoise (PN) sequence (called a chip sequence).Direct Sequence Spread Spectrum27Direct Sequence Spread Spectrum –DSSSIn direct-sequence spread spectrum (DSSS), the serial binary data is XORed with a pseudo-random binary code which has a bit rate faster than the binary data rate, and the result is used to phase-modulate a carrier.chipping rate – bit rate of the pseudorandom codethe faster you change the phase of a carrier, the more BW the signal takes up – looks like noiseUNMODULATED CARRIERSLOW SPEED PSKHIGH SPEED PSKmany clock (chipping rate) pulses in one data bit time28DSSSdataPseudo RandomSequencedata  PRS1101UNMODULATED CARRIERSLOW SPEED PSKHIGH SPEED PSK“chip”time of one data bitfrequencypowercarrier modulated by the datacarrier modulated by the data  PRSXOR 29Direct Sequence Spread Spectrum (cont’d)ObservationsA signal that would normally occupy a few kHz BW is spread out 10 to 10,000 times its BW.The fast phase modulation spreads the energy of the signal over a wide BW – appears as noise in a conventional receiver.Also called CDMA – Code Division Multiple Accessused in satellites – many signals can use the same transponderused in cell phones – many users in same BW30Direct Sequence Spread SpectrumReceiverReceiver must know the pseudorandom sequence of the transmitter and have a synchronizing circuit to get in step with this pseudorandom digital signal.The receiver using an identically programmed PN sequence compares incoming signals and picks out the one with the highest correlation.Other signals using different PN sequences appear as noise to the receiver and it doesn’t recognize them. 31The measure of the spreading is called the processing gain, G, which is the ratio of the DSSS bandwidth, BW, divided by the data rate, fb .The higher the processing gain, the greater the DSSS signal’s ability to fight interference.Processing gain32ExampleInformation signal is 13 kbps, spread over 1.25 MHz of bandwidth (BPSK)GdB=10Log(96.15)=19.83 dBThe higher the gain, the greater the system’s ability to fight interference3334Direct Sequence Spread Spectrum Example35Direct Sequence Spread Spectrum System36DSSS Example Using BPSK37ApproximateSpectrum of DSSS Signal38Code Division Multiple Access (CDMA)A multiplexing technique used with spread spectrumGiven a data signal rate DBreak each bit into k chips according to a fixed chipping code specific to each userResulting new channel has chip data rate kD chips per secondCan have multiple channels superimposed39CDMA Example40CDMA for DSSS4142Secure CommunicationSpread spectrum uses wideband, noise-like signals that are hard to detect, intercept, or demodulate. Additionally, spread-spectrum signals are harder to jam (interfere with) than narrow band signals.These low probability of intercept (LPI) and anti-jam (AJ) features are why the military has used spread spectrum for so many years. Spread-spectrum signals are intentionally made to be a much wider band than the information they are carrying to make them more noise-like. 43Spectral DensitySpread-spectrum transmitters use similar transmit power levels to narrowband transmitters. Because spread-spectrum signals are so wide, they transmit at a much lower spectral power density, measured in watts per hertz, than narrow band transmitters. This lower transmitted power density characteristic gives spread-spectrum signals a big plus. Spread-spectrum and narrowband signals can occupy the same band, with little or no interference. This capability is the main reason for all the interest in spread spectrum today.44Noise Like AppearanceThe use of special pseudo noise (PN) codes in spread-spectrum communications makes signals appear wide band and noise-like. It is this very characteristic that makes spread-spectrum signals possess a low LPI. Spread-spectrum signals are hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times the information bandwidth45WidebandIn a spread-spectrum system, signals are spread across a wide bandwidth, making them difficult to intercept, demodulate, and intercept.46Less Interference with NarrowbandThe spread of energy over a wide band, or lower spectral power density, also makes spread-spectrum signals less likely to interfere with narrowband communications. Narrowband communications, conversely, cause little to no interference to spread spectrum systems because the correlation receiver effectively integrates over a very wide bandwidth to recover a spread spectrum signal. The correlator then "spreads" out a narrowband interferer over the receiver's total detection bandwidth. 47Tolerance Level (Threshold)Since the total integrated signal density or signal-to-noise ratio (SNR) at the correlator's input determines whether there will be interference or not. All spread spectrum systems have a threshold or tolerance level of interference beyond which useful communication ceases. This tolerance or threshold is related to the spread-spectrum processing gain, which is essentially the ratio of the RF bandwidth to the information bandwidth. 48Direct Sequence and Frequency HoppingDirect sequence and frequency hopping are the most commonly used methods for the spread spectrum technology. Although the basic idea is the same, these two methods have many distinctive characteristics that result in complete different radio performances. The carrier of the direct-sequence radio stays at a fixed frequency. Narrowband information is spread out into a much larger (at least 10 times) bandwidth by using a pseudo-random chip sequence. The generation of the direct sequence spread spectrum signal (spreading) is shown in Figure49DSSS or DS-CDMADirect sequence spread spectrum, also known as direct sequence code division multiple access (DS-CDMA), is one of two approaches to spread spectrum modulation for digital signal transmission over the airwavesIn direct sequence spread spectrum, the stream of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum. A data signal at the point of transmission is combined with a higher data-rate bit sequence (also known as a chipping code) that divides the data according to a spreading ratio. The redundant chipping code helps the signal resist interference and also enables the original data to be recovered if data bits are damaged during transmission.50Power DensityIn Figure , the narrowband signal and the spread-spectrum signal both use the same amount of transmit power and carry the same information. However, the power density of the spread-spectrum signal is much lower than the narrowband signal. As a result, it is more difficult to detect the presence of the spread spectrum signal. The power density is the amount of power over a certain frequency. In the case of Figure 2, the narrowband signal's power density is 10 times higher than the spread spectrum signal, assuming the spread ratio is 10.51DSSS (Receiver End)At the receiving end of a direct-sequence system, the spread spectrum signal is de-spread to generate the original narrowband signal.52InterferenceIf there is an interference jammer in the same band, it will be spread out during the de-spreading. As a result, the jammer's impact is greatly reduced. This is the way that the direct-sequence spread-spectrum (DSSS) radio fights the interference. It spreads out the offending jammer by the spreading factor (Figure ). Since the spreading factor is at least a factor of 10, the offending jammer's amplitude is greatly reduced by at least 90%.Direct-sequence systems combat noise problems by spreading jammers across a wideband as shown in the figure above.53DSSS for Navigation PurposesFor de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process.However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver's position if the transmitters' positions are known. This is the basis for many satellite navigation systems.54Process GainThe resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.55De-Spreading ProcessAt the receiving end of a direct-sequence system, the spread spectrum signal is de-spread to generate the original narrowband signal. Figure shows the de-spreading process. 56Offending Jammer SpreadingIf there is an interference jammer in the same band, it will be spread out during the de-spreading. As a result, the jammer's impact is greatly reduced. This is the way that the direct-sequence spread-spectrum (DSSS) radio fights the interference. It spreads out the offending jammer by the spreading factor (Figure). Since the spreading factor is at least a factor of 10, the offending jammer's amplitude is greatly reduced by at least 90%. 57The Hopping ApproachFrequency-hopping systems achieve the same results provided by direct-sequence systems by using different carrier frequency at different time. The frequency-hop system's carrier will hop around within the band so that hopefully it will avoid the jammer at some frequencies. 58Processing Gain (no)The frequency-hopping technique does not spread the signal, as a result, there is no processing gain. The processing gain is the increase in power density when the signal is de-spread and it will improve the received signal's Signal-to-noise ratio (SNR). In other words, the frequency hopper needs to put out more power in order to have the same SNR as a direct-sequence radio. 59Time and Frequency SynchronizationThe frequency hopper, however, is more difficult to synchronize. In these architectures, the receiver and the transmitter must be synchronized in time and frequency in order to ensure proper transmission and reception of signals. In a direct-sequence radio, on the other hand, only the timing of the chips needs to be synchronized.The frequency hopper also needs more time to search the signal and lock to it. As a result, the latency time is usually longer. While a direct-sequence radio can lock in the chip sequence in just a few bits. 60Fixed Frequency ParkingTo make the initial synchronization possible, the frequency hopper will typically park at a fixed frequency before hopping or communication begin. If the jammer happens to locate at the same frequency as the parking frequency, the hopper will not be able to hop at all. And once it hops, it will be very difficult, if not impossible to re-synchronize if the receiver ever lost the sync. 61Multipath FadingThe frequency hopper, however, is better than the direct-sequence radio when dealing with multipath. Since the hopper does not stay at the same frequency and a null at one frequency is usually not a null at another frequency if it is not too close to the original frequency. So a hopper can usually deal with multipath fading issues better than direct-sequence radio. 62Survival of the FittestThe hopper itself, however, could suffer performance problems if it interferes with another radio. In these scenarios, the system that survives depends upon which can suffer more data loss.In general, a voice system can survive an error rate as high as 10-2 while a data system must have an error rate better than 10-4. Voice system can tolerate more data loss because human brain can "guess" between the words while a dumb microprocessor can't. 63Modulation and DemodulationFor direct-sequence systems the encoding signal is used to modulate a carrier, usually by phase-shift keying (PSK; for example, bi-phase or quad-phase) at the code rate. Frequency-hopping systems generate their wide band by transmitting at different frequencies, hopping from one frequency to another according to the code sequence.Typically such a system may have a few thousand frequencies to choose from, and unlike direct sequence signal, it has only one output rather than symmetrically distributed outputs. 64Code Sequence Generator for DSSS and FHIt's important to note that for both direct-sequencing and frequency-hopping, systems generate wideband signals controlled by the code sequence generator. For one the code is the direct carrier modulation (direct sequence) and the other commands the carrier frequency (frequency hopping). 65Clock ModulationClock modulation, which is actually frequency modulation of the code clock, is another option in spread-spectrum designs. In most cases (including frequency hopping), clock modulation is not used because of the loss in correlation due to phase slippage between received and local clocks, could cause degraded performance.Code modification is another modulation technique that designers can use when building a spread-spectrum system. Under this approach, the code is changed in such a way that the information is embedded in it, then modulated by phase transitions on a RF carrier. 66Balanced ModulationIn direct-sequence designs, balance modulation can be used in any suppressed carrier system used to generate the transmitted signal. Balanced modulation helps to hide the signal, as well as there are no power wasted in transmitting a carrier that would contribute to interference rejection or information transfer. When a signal has poor balance in either code or carrier, spikes are seen in its spectrum. With these spikes, or spurs, the signal is easily detectable, since these spikes are noticed above the noise and thus provide a path for detecting the hidden signal. 67Demodulation StepsOnce the signal is coded, modulated and then sent, the receiver must demodulate the signal. This is usually done in two steps. The first step entails removing the spectrum-spreading modulation. Then, the remaining information-bearing signal is demodulated by multiplying with a local reference identical in structure and synchronized with the received signal. 68Coding TechniquesIn order to transmit anything, codes used for data transmission have to be considered. However, this section will not discuss the coding of information (like error correction coding) but those that act as noise-like carriers for the information being transferred. These codes are of much greater length than those for the usual areas of data transfer, since it is intended for bandwidth spreading. 69Spread Spectrum CodesCodes in a spread-spectrum system are used for: Protection against interference: Coding enables a bandwidth trade for processing gain against interfering signals. Provision for privacy: Coding enables protection of signals from eaves dropping, so that even the code is secure. Noise-effect reduction: error-detection and correction codes can reduce the effects of noise and interference.70Error Detection and CorrectionError detection and correction codes (EDAC) must be used in frequency-hopping systems in order to overcome the high rates of error induced by partial band jamming. These codes usefulness has a threshold that must be exceeded before satisfactory performance is achieved.In direct-sequence systems, EDACs may not be advisable because of the effect it has on the code, increasing the apparent data transmission rate, and may increase jamming threshold. Some demodulators can operate detecting errors at the approximately the same accuracy as an EDAC, so it may not be worthwhile to include a complex coding/decoding scheme in the system. 71Advantages of Spread Spectrum Reduced crosstalk interferenceBetter voice quality/data integrity and less static noiseLowered susceptibility to multipath fadingInherent security:Co-existenceLonger operating distancesHard to detectHard to intercept or demodulateHarder to jam7273SummarySpread Spectrum (SS)SS Techniques:FHSSDSSSCDMABenefits of SS74