ECE 4331, Fall, 2009spread-spectrum transmissionPN Sequence GeneratorDirect Sequence (DS)-CDMACode Division Multiple Access (CDMA)CDMA AdvantagesDirect Sequence Spread SpectrumSpreading & DespreadingDirect Sequence SpreadingSlide 10CDMA – Multiple UsersCDMA PrincipleProcessing GainNear Far Problem and Power ControlDSSS TransmitterDSSS ReceiverSpectra of Received SignalCDMA ExampleCDMA Example – transmission from two sourcesCDMA Example – recovering signal A at the receiverCDMA Example – recovering signal B at the receiverCDMA Example – using wrong codeword at the receiverFrequency Hopping Spread SpectrumHybrid Spread Spectrum TechniquesSlide 25Road Map2G: IS-95A (1995)2.5G: IS-95B (1998)3G Technology3G: CDMA2000 (2000)3G: CDMA2000 Spreading Codes3G: W-CDMA (2000)Orthogonal frequency-division multiplexingOFDM bit loadingOFDM Time and Frequency GridGuard Time and Cyclic Extension...OFDM Transmitter and ReceiverPro and ConOFDM ApplicationsThe IEEE 802.11a/g StandardECE 4331, Fall, 2009Zhu HanDepartment of Electrical and Computer EngineeringClass 25Nov. 17th, 2009spread-spectrum transmission spread-spectrum transmission Three advantages over fixed spectrum –Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background. –Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter. –Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.PN Sequence GeneratorPN Sequence GeneratorPseudorandom sequence–Randomness and noise properties–Walsh, M-sequence, Gold, Kasami, Z4–Provide signal privacyDirect Sequence (DS)-CDMADirect Sequence (DS)-CDMAIt phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.Code Division Multiple AccessCode Division Multiple Access (CDMA) (CDMA)In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term. Message consists of symbols –Has symbol period and hence, symbol rateSpreading signal (code) consists of chips–Has Chip period and and hence, chip rate–Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence)–PN sequence is called a codeword–Each user has its own cordword–Codewords are orthogonal. (low autocorrelation)–Chip rate is oder of magnitude larger than the symbol rate. The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code The sent signal is recovered by despreading process at the receiver.CDMA AdvantagesCDMA AdvantagesLow power spectral density. –Signal is spread over a larger frequency band–Other systems suffer less from the transmitterInterference limited operation–All frequency spectrum is usedPrivacy–The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transitReduction of multipath affects by using a larger spectrumRandom access possible–Users can start their transmission at any timeCell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacityHigher capacity than TDMA and FDMA No frequency managementNo equalizers neededNo guard time neededEnables soft handoffDirect Sequence Spread SpectrumDirect Sequence Spread SpectrumUnique code to differentiate all usersSequence used for spreading have low cross-correlations Allow many users to occupy all the frequency/bandwidth allocations at that same timeProcessing gain is the system capacity–How many users the system can supportSpreading & DespreadingSpreading & DespreadingSpreading–Source signal is multiplied by a PN signal: 6.134, 6.135Processing Gain:Despreading–Spread signal is multiplied by the spreading codePolar {±1} signal representationDataRateChipRateTTGcspDirect Sequence SpreadingDirect Sequence SpreadingSpreading & DespreadingSpreading & DespreadingCDMA – Multiple UsersCDMA – Multiple UsersOne user’s information is the other’s interferencesIf the interference structure can be explored, multiuser detection–Match filter–Decorrelator–MMSE decodor–Successive cancellation–Decision feedbackCDMA PrincipleCDMA Principle1011 1 1 0 1 0 1 11 1 1 0 1 0 1 1Chip periodOne bit period (symbol period)Data Coded Signal Input to the modulator (phase modulation)Represent bit 1 with +1Represent bit 0 with -1Processing GainProcessing GainMain parameter of CDMA is the processing gain that is defined as: RBRBGchipspreadpGp: processing gainBspread: PN code rate Bchip: Chip rateR: Data rateIS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. 1.228 Mhz chipping rate1.25 MHz spread bandwidthNear Far Problem and Power ControlNear Far Problem and Power ControlAt a receiver, the signals may come from various (multiple sources. –The strongest signal usually captures the modulator. The other signals are considered as noise–Each source may have different distances to the base stationIn CDMA, we want a base station to receive CDMA coded signals from various