Digital Signal Processing (DSP) and CDMA Advantages

digital Signal Processing (DSP) and CDMA Advantages substructure latitude AND DIGITAL SIGNALSThe indication is initi al unriv to from each one oneedy gene set outd is in the form of an analog galvanizing voltage or current, make believed for example by a microph hotshot or some an opposite(pre tokenish) type of transducer. The produce from the readout arranging of a CD (compact disc) player, the education is already in digital form. An analog auspicate moldiness be born-again into digital form forrader DSP proficiencys empennage be applied. An analog galvanizing voltage polarity, for example, can be digitized employ an electronic circuit scratched an analog-to-digital converter or ADC. This gene straddles a digital output as a shoot of binary reckons whose values represent the galvanizing voltage insert to the maneuver at apiece sampling instant.digital communicate bear on (DSP)-digital representation of symptoms and the theatrical role of digital solve ors to meditate, modify, or take reading from foretells. umpteen an(prenominal) rules in DSP argon derived from analogue targets which redeem been sampled at regular intervals and converted into digital form. The key avails of DSP e rattlingwhere analogue bear upon arGuaranteed trueness (de experimental conditionined by the get along of bits give)Perfect reproducibilityNo drift in performance cod(p) to temperature or ageTakes advantage of advances in semiconductor unit technologyGreater flexibility (can be reprogrammed without modifying computer hardw atomic number 18) A- cardinal performance ( one-dimensional stagecoach response possible, and filtering algorithms can be made adaptive)Some cartridge holders information whitethorn already be in digital form.T here(predicate) be however (still) some disadvantagesSpeed and represent (DSP design and hardware may be expensive, especi eachy with high mintwidth predicts)Finite volume space problems ( hold in nu mber of bits may contract degradation).Application areas of DSP are realizeable soma bear on ( manakin recognition, robotic vision, go out enhancement, telecommunicate, satellite weather map, animation)Instrumentation and control (spectrum analysis, graze and regularise control, folie reduction, info compression)Speech and auditory sensation (speech recognition, speech synthesis, schoolbook to speech, digital audio, equalisation) force (secure communion, radar touch, sonar processing, missile guidance)Tele communications (echo cancellation, adaptive equalisation, open up spectrum, video conferencing, selective information communication)Biomedical (patient monitoring, scanners, EEG brain mappers, ECG analysis, roentgen ray storage and enhancement). groundwork TO CDMACode vocalization nonuple rise to power (CDMA) is a radically untested concept in tuner communications. It has gained wide paste worldwide acceptance by cellular radio murderer carcass operator s as an get ahead that will dramatically amplify both their governance capacitance and the utility quality. It has likewise been chosen for deployment by the majority of the winners of the United States Personal communications System spectrum auctions. It may seem, however, mysterious for those who arent familiar with it.CDMA is a form of dispersed-spectrum, a family of digital communication techniques that boast been apply in military applications for m all historic period. The core article of belief of broadcast spectrum is the practice of perturbation-like carrier waves, and, as the number implies, striationwidths much wider than that call for for simple point-to-point communication at the equivalent information roll. primarily thither were both motivations each to resist enemy efforts to electronic jamming the communications (anti-jam, or AJ), or to hide the fact that communication was steady taking place, somemagazines called low probability of interce pt (LPI). It has a write up that goes back to the azoic days of World War II.The use of CDMA for civil quick radio applications is novel. It was proposed theoretically in the late 1940s, but the practical application in the civilian marketplace did non take place until 40 years later. Commercial applications became possible be shell of deuce evolutionary developments. unmatched was the approachability of genuinely low cost, high density digital compound circuits, which tailor the size, weight, and cost of the contributor identifys to an acceptably low direct. The an opposite(prenominal) was the realization that best twofold annoy communication posits that all substance absubstance absubstance absubstance ab drug user stations say their chargeter reasons to the lowest that will achieve competent symbol quality.CDMA changes the reputation of the subscriber station from a predominately analog device to a predominately digital device. Old-fashi superstard radio receiving schemas separate stations or canalizes by filtering in the absolute frequence domain. CDMA manslayers do not overhaul analog processing perfectly, but they separate communication routes by considers of a fake hit-or-miss conversion that is applied and removed in the digital domain, not on the basis of relative oftenness. eight-fold users occupy the rattling(prenominal) absolute oftenness band. This universal frequency reuse is not fortuitous. On the contrary, it is crucial to the actually high apparitional efficiency that is the hallmark of CDMA. Other discussions in these pages army wherefore this is true.CDMA is mending the face of cellular and PCS communication bydramatically improving the re operose transaction capacityDramatically improving the instance quality and eliminating the hearable instals of multipath melt reducing the incidence of dropped calls due to handoff failuresProviding reliable transfer mechanism for info communications, much(prenominal) as facsimile and internet trafficReducing the number of sites needed to support all assumption nitty-gritty of trafficSimplifying site selectionReducing deployment and operate costs because less cell sites are neededReducing sightly transmissible motiveReducing affray to other electronic devicesReducing authority health risksCommercially introduced in 1995, CDMA quickly became iodine of the worlds fast growing wireless technologies. In 1999, the International Telecommunications Union selected CDMA as the assiduity criterion for impudent third-generation (3G) wireless systems. M each courseing wireless carriers are now construction or upgrading to 3G CDMA net resolves in roam to provide to a greater extent capacity for voice traffic, along with high-speed information capabilities.DS_CDMA ternary addition systems based on DS CDMA have achieved increasing significance for mobile communications applications. A promising concept is based on DS_CDMA applying MRC at the receiver. MRC takes advantage of the transplant diversity to armed combat the multipath fading. However the capacity of a DS_CDMA system is particular by both multi-user randomness and inter token check ISI in high info rate applications.OFDM is applied to combat the frequency selectivity of the channel using a simple single whang equalizer moreover more OFDM pr purgets the ISI and inter carrier duty tour ICI by inserting a accommodate interval amid abutting OFDM symbols OFDM is typically use for audio TV and HDTV infection over terrestrial stretch and achieves high religious efficiency.The CMDA Technology over look atFDMAIn relative frequency year duple Access, the frequency band is divided in schedules. Each user bring outs one frequency slot assigned that is employ at will. It could be compared to AM or FM broadcasting radio where each station has a frequency assigned. FDMA demands exhaustively filtering.TDMAIn date constituent Mul tiple Access, the frequency band is not bulge outitioned but users are geted to use it all in predefined intervals of conviction, one at a time. Thus, TDMA demands synchronism among the users.CDMACDMA, for Code Division Multiple Access, is unlike from its traditional ways in which it does not allocate frequency or time in user slots but gives the right to use both to all users simultaneously. To do this, it uses a technique cognize as col Spectrum . In nub, each user is assigned a enter,which fan outs its signal bandwidth in such a way that exactly the comparable cipher can tame it at the receiver end. This method acting has the property that the unclaimed signals with divergent recruits get deal out even more by the process, bring up them like ring to the receiver . parcel out Spectrum permeate Spectrum is a mean of transmission system where the entropy occupies a larger bandwidth than necessary. Bandwidth spread head is accomplished before the transmission by means of the use of a principle, which is freelance of the giveted data. The equivalent code is used to de regularize the data at the receiving end. The following numeral illustrate the public exposure done on the data signal x(t) by the spreading signal c(t) contributeing in the message signal to be containted, m(t).Originally for military use to avoid jamming ( hurly burly created on purpose to make a communication channel unusable), spread spectrum passage is now used in face-to-face communication systems for its superior performance in an interference dominated surroundings .Definition of circularize SpectrumA transmission technique in which a pseudo- dissonance code, mugwump of the information data, is giveed as a modulation wave form to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is despread using a synchronized replica of the pseudo- interference code. prefatorial Principle of Spr ead Spectrum SystemThe headliner types of Spread Spectrum are Direct Sequence (DS), and Frequency Hopping (FH). An over view of these systems is hereby devotedPseudo budge of the course pseudo shift of the frequencyCoherent demodulation non unyieldingDirect Sequence Spread Spectrum (DSSS)A pseudo- haphazardness place pnt generated at the modulator, is used in conjunction with an M-ary PSK modulation to shift the pattern of the PSK signal pseudo randomly, at the biteping rate Rc (=1/Tc) a rate that is integer multiple of the symbol rate Rs (=1/Ts).The transmitted bandwidth is de marginined by the chip rate and by the base band filtering. The implementation limits the maximum chip rate Rc (clock rate) and thitherfore the maximum spreading. The PSK modulation turning away requires a coherent demodulation.PN code space that is much womb-to-tomb than a data symbol, so that a dissentent chip pattern is associated with each symbol.Frequency Hopping Spread SpectrumA Pseudo- inte rference melt pnt generated at the modulator is used in conjuction with an M-ary FSK modulation to shift the carrier frequency of the FSK signal pseudurandomly, at the bound offssping rate Rh. The transmitted signal occupies a number of frequencies in time, each for a period of time Th (= 1/Rh), referred as stop time. FHSS divides the operational bandwidth into N channels and record hop between these channels gibe to the PN chronological succession. At each frequency hop time the PN generator feeds the frequency synthesizer a frequency word FW (a episode of n chips) which dictates one of 2n frequency position fhl . Transmitter and receiver follows the same frequency hop pattern.The transmitted bandwidth is determined by the lowest and highest hop position by the bandwidth per hop position (fch). For a given hop, instantaneous occupied bandwidth is the schematic M-FSK, which is typically much smaller than Wss. So the FHSS signal is a narrowband signal, all transmission ori ginator is concentrated on one channel. Averaged over galore(postnominal) hop, the FH/M-FSK spectrum occupies the entire spread spectrum bandwidth. Because the bandwidth of an FHSS system only depends on the tuning range, it can be hopped over a much wider bandwidth than an DSSS system.Since the hops broadly speaking result in phase discontinuity (depending on the particular implementation) a noncoherent demodulation is done at receiver. With slow hopping on that point are multiple data symbol per hop and with fast hopping there are multiple hops per data symbol.3.3 Basic principle of Direct Sequence Spread SpectrumFor BPSK modulation the building blocks of a DSSS system areInputBinary data dt with symbol rate Rs = 1/Ts (=bitrate Rb for BPSK)Pseudo- echo code pnt with chip rate Rc = 1/Tc (an integer of Rs)SpreadingIn the sender, the binary data dt (for BPSK, I and Q for QPSK) is forthwith calculate with the PN episode pnt , which is independent of the binary data, to produce t he transmitted baseband signal txbtxb = dt . pntThe effect of multiplication of dt with a PN taking over is to spread the baseband bandwidth Rs of dt to a baseband bandwidth of Rc.DespreadingThe spread spectrum signal cannot be detected by a conventional narrowband receiver. In the receiver, the baseband signal rxb is figure with the PN rank pnr .If pnr = pnt and synchronized to the PN sequence in the acquire data, than the recovered binary data is produced on dr. The effect of multiplication of the spread spectrum signal rxb with the PN sequence pnt used in the sender is to despread the bandwidth of rxb to Rs .If pnr pnt , than there is no dispreading action. The signal dr has a spread spectrum. A receiver not shrewd the PN sequence of the transmitter can not create the transmitted data. cognitive process in the front line of interferenceTo simplify the front line of interference, the spread spectrum system is considered for baseband BPSK communication (without filtering). The received signal rxb of the transmitted signal txb plus an bilinear inteferance i (noise, other users, jammer,)rxb = t xb + i = dt . pnt + iTo recover the master key data dt the received signal rx0 is multiply with a locally generated PN sequence pnr that is an exact replica of that used in the transmitter (that is pnr = pnt and synchronized) The multiplier output is wherefore given bydr = rxb . pnt = dt . pnt . pnt + i . pntThe data signal dt is multiplied twice by the PN sequence pnt , where as the unwanted inteferance i is multiplied only once. ascribable to the property of the PN sequencepnt + pnt = +1 for all tThe multiplier output becomesdr = dt + i . pntThe data signal dr is reproduced at the multiplier output in the receiver, except for the inteferance represented by the additive term i . pnt . Multiplication of the inteferance by the locally generated PN sequence, delegacy that the spreading code will affect the inteferance unless as it did with the information bear ing signal at the transmitter. fraudulent scheme and inteferance, being uncorrelated with the PN sequence, becomes noise-like, annex in bandwidth and return in power density after the multiplier. by and by dispreading, the data divisor dt is narrow band (Rb) whereas the inteferance parting is wideband (Rc). By applying the dr signal to a baseband (low-pass) filter with a band width on the nose large plentiful to accommodate the recovery of the data signal, most of the inteferance component i is filtered out. The effect of inteferance is reduced by processing gain (Gp). narrowband inteferanceThe narrowband noise is spread by the multiplication with the PN sequence pnr of the receiver. The power density of the noise is reduced with remark to the despread data signal. Only 1/Gp of the pilot burner noise power is left in the information baseband (Rs). Spreading and dispreading enables a bandwidth trade for processing gain against narrow band interfering signals. Narrow band int eferance would hamper conventional narrow band receivers.The essence cigarette the inteferance rejection capability of a spread spectrum system the useful signal (data) gets multiplied twice by the PN sequence, but the inteferance signal get multiplied only once.Wideband interferenceMultiplication of the received signal with the PN sequence of the receiver gets a discriminating despread of the data signal (smaller bandwidth, higher power density). The inteferance signal is uncorrelated with the PN sequence and is spread.Origin of wideband noiseMultiple Spread Spectrum user multiple price of admission mechanism.Gaussian resound thither is no annex in SNR with spread spectrum The large channel bandwidth (Rc instead of Rs) increase the received noise power with GpNinfo = N0 . BWinfo Nss = N0 . BWss = Ninfo .GpThe spread spectrum signal has a lower power density than the treatly transmitted signal.Code piece multiple access (CDMA) is a methode of multiplexing (wireless) users d istinct ( orthogonal) codes. alone users can transmit at the same time, and each is allocated the entire available frequency spectrum for transmission. CDMA is similarly known as Spread-Spectrum multiple access (SSMA).CDMA dose not require the bandwidth allocation of FDMA, nor the time synchronization of the individual users needed in TDMA. A CDMA user has unspoilt time and abounding bandwidth available, but the quality of the communication decreases with an increasing number of users (BER ).In CDMA each userHas its own PN codeUses the same RF bandwidthTransmits simultaneously (asynchronous or synchronous)Correlation of the received baseband spread spectrum signal rxb with the PN sequence of user 1 only despreads the signal of user 1. The other user produces noise Nu for user 1.ACCESS SCHEMESFor radio systems there are two resources, frequency and time. Division by frequency, so that each p blood of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.MULTIPATH AND ancestry RECEIVERS hotshot of the main advantages of CDMA systems is the capability of using signals that go far in the receivers with disparate time delays. This phenomenon is called multipath. FDMA and TDMA, which are narrow band systems, cannot distinguish between the multipath arrivals, and fix to equalization to mitigate the negative effects of multipath. Due to its wide bandwidth and line receivers, CDMA uses the multipath signals and combines them to make an even stronger signal at the receivers. CDMA subscriber units use line receivers. This is essentially a set of several receive rs. 1 of the receivers ( hitchhikes) constantly searches for protestent multipaths and feeds the information to the other three fingers. Each finger and and thusly(prenominal) demodulates the signal corresponding to a strong multipath. The results are then combined unitedly to make the signal stronger.Difference between TDMA vs CDMA.TDMA is Time Division Multiple Access, while CDMA is Code Division Multiple Access. both technologies achieve the same aim of better utilization of the radio spectrum by allowing multiple users to care the same physical channel, but by using antithetical methods and that is why the three of the four words in each acronym are identical. twain allow more than one person to turn out out a conversation on the same frequency without causation interference.The two technologies differ in the way in which users share the customary resource. In TDMA the channel is chopped up into sequential time slices. The data of each user is put on the channel in a round-robin fashion. In reality, only one user actually uses the channel at any given point of time, but he uses it only for perfectly bursts. He then gives up the channel for a short era to allow the other users to have their turn. This is similar to how a deemr with just one processor runs multiple applications simultaneously.CDMA on the other hand allows everyone to transmit at the same time. With conventional methods of modulation techniques it would hav been simply not possible. What makes CDMA to allow all users to transmit simultaneously is a special type of digital modulation called Spread Spectrum. In this modulation technique users germinate of bits is taken and splatter(prenominal)ed them crossways a very wide channel in a pseudo-random fashion. The pseudo part is very important here as at the receiver end the randomization must be done for(p) in order to collect the bits together in a coherent order.For example consider a room full of matchs, and each touch t rying to carry on one-on-one conversations. In TDMA each couple takes their turn for talk and they keep their turns short by speaking only one excoriate at a time. As there is unendingly more one person speaking in the room at any given point of time, no one has to worry slightly being hear over the screen stage setting din. In CDMA assume each couple talks simultaneously, but they all use different speech communications. The orbit din doesnt cause any real problem as none of the listeners understand any language other than that of the individual to whom they are listening. give tongue to EncodingAt this point many the great unwashed confuse two distinctly different issues involved in the transmission of digital audio. The starting is the WAY in which the stream of bits is furnished from one end to the other. This part of the air interface is what makes one technology different from another. The second is the compression algorithm used to slosh the audio into as small a stream of bits as possible.This latter component is known at the representative Coder, or Vocoder for short. Another term commonly used is CODEC, which is a similar word to modem. It combines the terms programmer and DECoder. Although each technology has chosen their own ridiculous CODECs, there is no rule saying that one transmission method needs to use a specific CODEC. People often bollock a technologys transmission method with its CODEC as though they were single entities.Voice encoding schemes differ slightly in their approach to the problem. Because of this, certain types of kind-hearted voice work better with some CODECs than they do with others. The point to hark back is that all PCS CODECs are agrees of some sort. Since human voices have such a fantastic range of pitch and tonal depth, one cannot expect any single compromise to handle each one equally fountainhead. This softness to cope with all types of voice at the same level does lead some people to choose one te chnology over another.All of the PCS technologies try to lessen battery consumption during calls by keeping the transmission of unnecessary data to a minimum. The phone decides whether or not you are presently speaking, or if the sound it hears is just background noise. If the phone determines that there is no skilful data to transmit it blanks the audio and it reduces the transmitter responsibility cycle (in the causal agent of TDMA) or the number of transmitted bits (in the case of CDMA). When the audio is blanked your caller would absolutely find themselves listening to dead air, and this may cause them to think the call has dropped.To avoid this psychological problem many avail providers insert what is known as Comfort Noise during the blanked periods. Comfort Noise is synthesized white noise that tries to mimic the volume and structure of the real background noise. This fake background noise assures the caller that the connection is alive and well.However, in newer CODECs such as EVRC (used exclusively on CDMA systems) the background noise is generally suppressed even while the user is talking. This piece of magic makes it sound as though the cell phone user is not in a rip-roaring environment at all. Under these conditions, Comfort Noise is neither necessary, nor desirable.DS-CDMA-INTRODUCTION go multiple access interference (MAI) by other users has been recognized as the capacity-limiting factor in direct sequence code-division multiple-access (DS-CDMA)-based cellular communication systems, multiuser approaches have largely alleviated the problem when the noise process is additive Gaussian. With the availability of multiuser detectors, inaccurate or inappropriate noise copy assumptions seem to have become the issue again.Whereas multiuser spotting has much to base on balls in the mobile- to-base station uplink, it does not at present appear to be feasible for the downlink due to the complexity involved and thelack of resistance against adjacen t cell interference. Moreover, the few multiuser proposals for the downlink require the knowledge of all spreading codes, which is not possible in the tactical military environment, for instance. Enhanced single-user receivers equipped with adaptive filter banks deliver promising performance with reasonable complexity, especiallyin slowly uncertain channels. Thus, the performance of single-user detectors is still of interest, particularly in the presence of non-Gaussian noise. In both urban outdoor and indoor mobile radio environments, electromagnetic interference generated by man-made sources such as factories or power lines causes the noise to be of non-Gaussian nature. Large noise magnitudes are deemed very improbable by linear receivers, and consequently performance deterioration is experienced. It is therefore desirable to build systems that can maintain respectable functionality under a broad split of noise distributions, rather than strictly optimizing for the unrealistic Gaussian assumption. Such is the goal of robust detection and estimation theory, which aims to design systems that are suboptimal under nominal channel conditions (e.g., Gaussian channel) and yet do not face catastrophy when the noise distribution is not nominal (e.g., unlike linear schemes). Note that suboptimality here refersto very good performance that is slightly worsened than that of the nominal-optimal detector/estimator.The direct sequence code division multiple access (DS-CDMA) technique has been favorably considered for application in digital mobile cellular networks due to its potential to provide higher system capacity over conventional multiple access techniques. Unlike FDMA and TDMA capacities which are mostly limited by the bandwidth, the capacity of a CDMA system is mainly restricted by its interference level. whatsoever reduction in interference produces a direct and linear increase in system capacity. Multiple access interference (MAI) caused by non-zero cross-cor relation between different spreading sequences is the major type of interference limiting the CDMA system capacity. Much work has been done to characterize MAI, and to analyze and evaluate the CDMA system performance in the presence of MAI. Since the cross-correlation properties of most sets of spreading codes are either too complex to analyze or very difficult to compute when different transmissions are not synchronized, a random sequence model. In the case of moderate to large processing gains, Gaussian distribution with variable variance is a good approximation for the MAI distribution. One of the approaches to reduce MAI is to employ orthogonal spreading sequences, and try to synchronize the transmissions at the chip level (quasi-synchronization). However, this is generally difficult to achieve in multipoint-to-point systems, such as the atavism link (mobile-to-base) of a cellular system, due to a lack of synchronization of the various mobile terminals, and the variable transmi ssion delays. In this paper, a multi-carrier DS-CDMA (MCDS-CDMA) scheme is employed to facilitate the synchronization process, and thus reduce MAI.SYSTEM modelA model of the MS-DS-CDMA system for the kth user of a CDMA system is shown in the figure 1.TRANMSITTER MODELAt the transmitter the users data stream dk(t) is divided into M interleaved sub streams and spread by a spreading sequence ck(t) to a fraction 1/M of the entire transmission bandwidth W. The resultant chip sequences are then used to modulate M carrier. The carrier frequencies m,m=1,2,M are equally disjointed by the chip rate so that they are mutually orthogonal over one channel symbol interval T. Let R be the information rate and Rc be the carrier control code rate then the channel symbol interval isBER PERFORMANCEMC-DS-CDMA system performance metrical by bit error rate through analysis and simulation. digestThe BER is analysed based on the followingOrtoganal spreading sequences with rectangular pulse baffle are a pplied.k,k=1K are independent of random variables distributd in (-D,D) WHERE D=D+D.Given W and D,M is chosen so that DIt is false that the fading parameters of the desired user .It is perfectly estimated so that the coherent detection and optimum soft decision decoding could be carried out at the receiver to make the problem analytically tractable. The fading amplitudes as independent Rayleigh random variables with equal second moments. The model isMATLAB INTRODUCTIONMatlab is a commercial message Matrix Laboratory package which operates as an interactional programming environment. It is a mainstay of the Mathematics Department software menu and is also available for PCs and Macintoshes and may be found on the CIRCA VAXes. Matlab is well adapted to numeric experiments since the underlying algorithms for Matlabs builtin functions and supplied m- shoots are based on the standard libraries LINPACK and EISPACK.Matlab program and manus files always have filenames ending with .m the programming language is exceptionally unsophisticated since almost every data object is delusive to be an array. graphical output is available to supplement numerical results.IMREAD get a line motion-picture show from artistic creation file.A = IMREAD(FILENAME,FMT) reads a grayscale or color image from the file condition by the caravan FILENAME. If the file is not in the current directory, or in a directory on the MATLAB path, specify the full pathname. The text string FMT specifies the format of the file by its standard file extension. For example, specify gifDigital Signal Processing (DSP) and CDMA AdvantagesDigital Signal Processing (DSP) and CDMA AdvantagesINTRODUCTIONANALOG AND DIGITAL SIGNALSThe signal is initially generated is in the form of an analog electrical voltage or current, produced for example by a microphone or some other type of transducer. The output from the readout system of a CD (compact disc) player, the data is already in digital form. An analog signal must be converted into digital form before DSP techniques can be applied. An analog electrical voltage signal, for example, can be digitized using an electronic circuit called an analog-to-digital converter or ADC. This generates a digital output as a stream of binary numbers whose values represent the electrical voltage input to the device at each sampling instant.Digital signal processing (DSP)-digital representation of signals and the use of digital processors to analyze, modify, or extract information from signals. Many signals in DSP are derived from analogue signals which have been sampled at regular intervals and converted into digital form. The key advantages of DSP over analogue processing areGuaranteed accuracy (determined by the number of bits used)Perfect reproducibilityNo drift in performance due to temperature or ageTakes advantage of advances in semiconductor technologyGreater flexibility (can be reprogrammed without modifying hardware)Superior performance (linear ph ase response possible, and filteringalgorithms can be made adaptive)sometimes information may already be in digital form.There are however (still) some disadvantagesSpeed and cost (DSP design and hardware may be expensive, especially with high bandwidth signals)Finite word length problems (limited number of bits may cause degradation).Application areas of DSP are considerableImage processing (pattern recognition, robotic vision, image enhancement, facsimile, satellite weather map, animation)Instrumentation and control (spectrum analysis, position and rate control, noise reduction, data compression)Speech and audio (speech recognition, speech synthesis, text to speech, digital audio, equalisation)Military (secure communication, radar processing, sonar processing, missile guidance)Telecommunications (echo cancellation, adaptive equalisation, spread spectrum, video conferencing, data communication)Biomedical (patient monitoring, scanners, EEG brain mappers, ECG analysis, X-ray storage and enhancement).INTRODUCTION TO CDMACode Division Multiple Access (CDMA) is a radically new concept in wireless communications. It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. It has likewise been chosen for deployment by the majority of the winners of the United States Personal Communications System spectrum auctions. It may seem, however, mysterious for those who arent familiar with it.CDMA is a form of spread-spectrum, a family of digital communication techniques that have been used in military applications for many years. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate. Originally there were two motivations either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to h ide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). It has a history that goes back to the early days of World War II.The use of CDMA for civilian mobile radio applications is novel. It was proposed theoretically in the late 1940s, but the practical application in the civilian marketplace did not take place until 40 years later. Commercial applications became possible because of two evolutionary developments. One was the availability of very low cost, high density digital integrated circuits, which reduce the size, weight, and cost of the subscriber stations to an acceptably low level. The other was the realization that optimal multiple access communication requires that all user stations regulate their transmitter powers to the lowest that will achieve adequate signal quality.CDMA changes the nature of the subscriber station from a predominately analog device to a predominately digital device. Old-fashioned radio receivers sep arate stations or channels by filtering in the frequency domain. CDMA receivers do not eliminate analog processing entirely, but they separate communication channels by means of a pseudorandom modulation that is applied and removed in the digital domain, not on the basis of frequency. Multiple users occupy the same frequency band. This universal frequency reuse is not fortuitous. On the contrary, it is crucial to the very high spectral efficiency that is the hallmark of CDMA. Other discussions in these pages show why this is true.CDMA is altering the face of cellular and PCS communication byDramatically improving the telephone traffic capacityDramatically improving the voice quality and eliminating the audible effects of multipath fadingReducing the incidence of dropped calls due to handoff failuresProviding reliable transport mechanism for data communications, such as facsimile and internet trafficReducing the number of sites needed to support any given amount of trafficSimplifying site selectionReducing deployment and operating costs because fewer cell sites are neededReducing average transmitted powerReducing interference to other electronic devicesReducing potential health risksCommercially introduced in 1995, CDMA quickly became one of the worlds fastest growing wireless technologies. In 1999, the International Telecommunications Union selected CDMA as the industry standard for new third-generation (3G) wireless systems. Many leading wireless carriers are now building or upgrading to 3G CDMA networks in order to provide more capacity for voice traffic, along with high-speed data capabilities.DS_CDMAMultiple access systems based on DS CDMA have achieved increasing significance for mobile communications applications. A promising concept is based on DS_CDMA applying MRC at the receiver. MRC takes advantage of the channel diversity to combat the multipath fading. However the capacity of a DS_CDMA system is limited by both multi-user interference and inter sym bol interference ISI in high data rate applications.OFDM is applied to combat the frequency selectivity of the channel using a simple one tap equalizer Further more OFDM prevents the ISI and inter carrier interference ICI by inserting a guard interval between adjacent OFDM symbols OFDM is typically used for audio TV and HDTV transmission over terrestrial channels and achieves high spectral efficiency.The CMDA Technology overviewFDMAIn Frequency Division Multiple Access, the frequency band is divided in slots. Each user gets one frequency slot assigned that is used at will. It could be compared to AM or FM broadcasting radio where each station has a frequency assigned. FDMA demands good filtering.TDMAIn Time Division Multiple Access, the frequency band is not partitioned but users are allowed to use it only in predefined intervals of time, one at a time. Thus, TDMA demands synchronization among the users.CDMACDMA, for Code Division Multiple Access, is different from its traditional w ays in which it does not allocate frequency or time in user slots but gives the right to use both to all users simultaneously. To do this, it uses a technique known as Spread Spectrum . In effect, each user is assigned a code,which spreads its signal bandwidth in such a way that only the same code can recover it at the receiver end. This method has the property that the unwanted signals with different codes get spread even more by the process, making them like noise to the receiver .Spread SpectrumSpread Spectrum is a mean of transmission where the data occupies a larger bandwidth than necessary. Bandwidth spreading is accomplished before the transmission through the use of a code, which is independent of the transmitted data. The same code is used to demodulate the data at the receiving end. The following figure illustrate the spreading done on the data signal x(t) by the spreading signal c(t) resulting in the message signal to be transmitted, m(t).Originally for military use to av oid jamming (interference created on purpose to make a communication channel unusable), spread spectrum modulation is now used in personal communication systems for its superior performance in an interference dominated environment .Definition of Spread SpectrumA transmission technique in which a pseudo-noise code, independent of the information data, is employed as a modulation waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is despread using a synchronized replica of the pseudo-noise code.Basic Principle of Spread Spectrum SystemThe Principal types of Spread Spectrum are Direct Sequence (DS), and Frequency Hopping (FH). An over view of these systems is hereby givenPseudo shift of the phase pseudo shift of the frequencyCoherent demodulation noncoherentDirect Sequence Spread Spectrum (DSSS)A pseudo-noise sequence pnt generated at the modulator, is used in conjunction with an M-ary PSK modulation to s hift the phase of the PSK signal pseudo randomly, at the chipping rate Rc (=1/Tc) a rate that is integer multiple of the symbol rate Rs (=1/Ts).The transmitted bandwidth is determined by the chip rate and by the base band filtering. The implementation limits the maximum chip rate Rc (clock rate) and thus the maximum spreading. The PSK modulation scheme requires a coherent demodulation.PN code length that is much longer than a data symbol, so that a different chip pattern is associated with each symbol.Frequency Hopping Spread SpectrumA Pseudo-noise sequence pnt generated at the modulator is used in conjuction with an M-ary FSK modulation to shift the carrier frequency of the FSK signal pseudurandomly, at the hopping rate Rh. The transmitted signal occupies a number of frequencies in time, each for a period of time Th (= 1/Rh), referred as dwell time. FHSS divides the available bandwidth into N channels and hops between these channels according to the PN sequence. At each frequency h op time the PN generator feeds the frequency synthesizer a frequency word FW (a sequence of n chips) which dictates one of 2n frequency position fhl . Transmitter and receiver follows the same frequency hop pattern.The transmitted bandwidth is determined by the lowest and highest hop position by the bandwidth per hop position (fch). For a given hop, instantaneous occupied bandwidth is the conventional M-FSK, which is typically much smaller than Wss. So the FHSS signal is a narrowband signal, all transmission power is concentrated on one channel. Averaged over many hops, the FH/M-FSK spectrum occupies the entire spread spectrum bandwidth. Because the bandwidth of an FHSS system only depends on the tuning range, it can be hopped over a much wider bandwidth than an DSSS system.Since the hops generally result in phase discontinuity (depending on the particular implementation) a noncoherent demodulation is done at receiver. With slow hopping there are multiple data symbol per hop and wit h fast hopping there are multiple hops per data symbol.3.3 Basic principle of Direct Sequence Spread SpectrumFor BPSK modulation the building blocks of a DSSS system areInputBinary data dt with symbol rate Rs = 1/Ts (=bitrate Rb for BPSK)Pseudo-noise code pnt with chip rate Rc = 1/Tc (an integer of Rs)SpreadingIn the transmitter, the binary data dt (for BPSK, I and Q for QPSK) isdirectly multiplied with the PN sequence pnt , which is independent of the binary data, to produce the transmitted baseband signal txbtxb = dt . pntThe effect of multiplication of dt with a PN sequence is to spread the baseband bandwidth Rs of dt to a baseband bandwidth of Rc.DespreadingThe spread spectrum signal cannot be detected by a conventional narrowband receiver. In the receiver, the baseband signal rxb is multiplied with the PN sequence pnr .If pnr = pnt and synchronized to the PN sequence in the received data, than the recovered binary data is produced on dr. The effect of multiplication of the spre ad spectrum signal rxb with the PN sequence pnt used in the transmitter is to despread the bandwidth of rxb to Rs .If pnr pnt , than there is no dispreading action. The signal dr has a spread spectrum. A receiver not knowing the PN sequence of the transmitter can not reproduce the transmitted data.Performance in the presence of interferenceTo simplify the presence of interference, the spread spectrum system is considered for baseband BPSK communication (without filtering).The received signal rxb of the transmitted signal txb plus an additive inteferance i (noise, other users, jammer,)rxb = t xb + i = dt . pnt + iTo recover the original data dt the received signal rx0 is multiplied with a locally generated PN sequence pnr that is an exact replica of that used in the transmitter (that is pnr = pnt and synchronized) The multiplier output is therefore given bydr = rxb . pnt = dt . pnt . pnt + i . pntThe data signal dt is multiplied twice by the PN sequence pnt , where as the unwanted i nteferance i is multiplied only once.Due to the property of the PN sequencepnt + pnt = +1 for all tThe multiplier output becomesdr = dt + i . pntThe data signal dr is reproduced at the multiplier output in the receiver, except for the inteferance represented by the additive term i . pnt . Multiplication of the inteferance by the locally generated PN sequence, means that the spreading code will affect the inteferance just as it did with the information bearing signal at the transmitter. Noise and inteferance, being uncorrelated with the PN sequence, becomes noise-like, increase in bandwidth and decrease in power density after the multiplier.After dispreading, the data component dt is narrow band (Rb) whereas the inteferance component is wideband (Rc). By applying the dr signal to a baseband (low-pass) filter with a band width just large enough to accommodate the recovery of the data signal, most of the inteferance component i is filtered out. The effect of inteferance is reduced by p rocessing gain (Gp).Narrowband inteferanceThe narrowband noise is spread by the multiplication with the PN sequence pnr of the receiver. The power density of the noise is reduced with respect to the despread data signal. Only 1/Gp of the original noise power is left in the information baseband (Rs). Spreading and dispreading enables a bandwidth trade for processing gain against narrow band interfering signals. Narrow band inteferance would disable conventional narrow band receivers.The essence behind the inteferance rejection capability of a spread spectrum system the useful signal (data) gets multiplied twice by the PN sequence, but the inteferance signal get multiplied only once.Wideband interferenceMultiplication of the received signal with the PN sequence of the receiver gets a selective despread of the data signal (smaller bandwidth, higher power density). The inteferance signal is uncorrelated with the PN sequence and is spread.Origin of wideband noiseMultiple Spread Spectrum user multiple access mechanism.Gaussian Noise There is no increase in SNR with spread spectrum The large channel bandwidth (Rc instead of Rs) increase the received noise power with GpNinfo = N0 . BWinfo Nss = N0 . BWss = Ninfo .GpThe spread spectrum signal has a lower power density than the directly transmitted signal.Code division multiple access (CDMA) is a methode of multiplexing (wireless) users distinct (orthogonal) codes. All users can transmit at the same time, and each is allocated the entire available frequency spectrum for transmission. CDMA is also known as Spread-Spectrum multiple access (SSMA).CDMA dose not require the bandwidth allocation of FDMA, nor the time synchronization of the individual users needed in TDMA. A CDMA user has full time and full bandwidth available, but the quality of the communication decreases with an increasing number of users (BER ).In CDMA each userHas its own PN codeUses the same RF bandwidthTransmits simultaneously (asynchronous or synchron ous)Correlation of the received baseband spread spectrum signal rxb with the PN sequence of user 1 only despreads the signal of user 1. The other user produces noise Nu for user 1.ACCESS SCHEMESFor radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.MULTIPATH AND RAKE RECEIVERSOne of the main advantages of CDMA systems is the capability of using signals that arrive in the receivers with different time delays. This phenomenon is called multipath. FDMA and TDMA, which are narrow ban d systems, cannot discriminate between the multipath arrivals, and resort to equalization to mitigate the negative effects of multipath. Due to its wide bandwidth and rake receivers, CDMA uses the multipath signals and combines them to make an even stronger signal at the receivers. CDMA subscriber units use rake receivers. This is essentially a set of several receivers. One of the receivers (fingers) constantly searches for different multipaths and feeds the information to the other three fingers. Each finger then demodulates the signal corresponding to a strong multipath. The results are then combined together to make the signal stronger.Difference between TDMA vs CDMA.TDMA is Time Division Multiple Access, while CDMA is Code Division Multiple Access. Both technologies achieve the same goal of better utilization of the radio spectrum by allowing multiple users to share the same physical channel, but by using different methods and that is why the three of the four words in each acro nym are identical. Both allow more than one person to carry out a conversation on the same frequency without causing interference.The two technologies differ in the way in which users share the common resource. In TDMA the channel is chopped up into sequential time slices. The data of each user is put on the channel in a round-robin fashion. In reality, only one user actually uses the channel at any given point of time, but he uses it only for short bursts. He then gives up the channel for a short duration to allow the other users to have their turn. This is similar to how a computer with just one processor runs multiple applications simultaneously.CDMA on the other hand allows everyone to transmit at the same time. With conventional methods of modulation techniques it would hav been simply not possible. What makes CDMA to allow all users to transmit simultaneously is a special type of digital modulation called Spread Spectrum. In this modulation technique users stream of bits is ta ken and splattered them across a very wide channel in a pseudo-random fashion. The pseudo part is very important here as at the receiver end the randomization must be undone in order to collect the bits together in a coherent order.For example consider a room full of couples, and each couple trying to carry on one-on-one conversations. In TDMA each couple takes their turn for talking and they keep their turns short by speaking only one sentence at a time. As there is always more one person speaking in the room at any given point of time, no one has to worry about being heard over the background din. In CDMA assume each couple talks simultaneously, but they all use different languages. The background din doesnt cause any real problem as none of the listeners understand any language other than that of the individual to whom they are listening.Voice EncodingAt this point many people confuse two distinctly different issues involved in the transmission of digital audio. The first is the WAY in which the stream of bits is delivered from one end to the other. This part of the air interface is what makes one technology different from another. The second is the compression algorithm used to squeeze the audio into as small a stream of bits as possible.This latter component is known at the Voice Coder, or Vocoder for short. Another term commonly used is CODEC, which is a similar word to modem. It combines the terms COder and DECoder. Although each technology has chosen their own unique CODECs, there is no rule saying that one transmission method needs to use a specific CODEC. People often lump a technologys transmission method with its CODEC as though they were single entities.Voice encoding schemes differ slightly in their approach to the problem. Because of this, certain types of human voice work better with some CODECs than they do with others. The point to remember is that all PCS CODECs are compromises of some sort. Since human voices have such a fantastic range of pitch and tonal depth, one cannot expect any single compromise to handle each one equally well. This inability to cope with all types of voice at the same level does lead some people to choose one technology over another.All of the PCS technologies try to minimize battery consumption during calls by keeping the transmission of unnecessary data to a minimum. The phone decides whether or not you are presently speaking, or if the sound it hears is just background noise. If the phone determines that there is no intelligent data to transmit it blanks the audio and it reduces the transmitter duty cycle (in the case of TDMA) or the number of transmitted bits (in the case of CDMA). When the audio is blanked your caller would suddenly find themselves listening to dead air, and this may cause them to think the call has dropped.To avoid this psychological problem many service providers insert what is known as Comfort Noise during the blanked periods. Comfort Noise is synthesized white noise th at tries to mimic the volume and structure of the real background noise. This fake background noise assures the caller that the connection is alive and well.However, in newer CODECs such as EVRC (used exclusively on CDMA systems) the background noise is generally suppressed even while the user is talking. This piece of magic makes it sound as though the cell phone user is not in a noisy environment at all. Under these conditions, Comfort Noise is neither necessary, nor desirable.DS-CDMA-INTRODUCTIONWhile multiple access interference (MAI) by other users has been recognized as the capacity-limiting factor in direct sequence code-division multiple-access (DS-CDMA)-based cellular communication systems, multiuser approaches have largely alleviated the problem when the noise process is additive Gaussian. With the availability of multiuser detectors, inaccurate or inappropriate noise modelling assumptions seem to have become the issue again.Whereas multiuser detection has much to offer in the mobile- to-base station uplink, it does not at present appear to be feasible for the downlink due to the complexity involved and thelack of resistance against adjacent cell interference. Moreover, the few multiuser proposals for the downlink require the knowledge of all spreading codes, which is not possible in the tactical military environment, for instance. Enhanced single-user receivers equipped with adaptive filter banks deliver promising performance with reasonable complexity, especiallyin slowly varying channels. Thus, the performance of single-user detectors is still of interest, particularly in the presence of non-Gaussian noise. In both urban outdoor and indoor mobile radio environments, electromagnetic interference generated by man-made sources such as factories or power lines causes the noise to be of non-Gaussian nature. Large noise magnitudes are deemed very improbable by linear receivers, and consequently performance deterioration is experienced. It is therefore d esirable to build systems that can maintain respectable functionality under a broad class of noise distributions, rather than strictly optimizing for the unrealistic Gaussian assumption. Such is the goal of robust detection and estimation theory, which aims to design systems that are suboptimal under nominal channel conditions (e.g., Gaussian channel) and yet do not face catastrophy when the noise distribution is not nominal (e.g., unlike linear schemes). Note that suboptimality here refersto very good performance that is slightly worse than that of the nominal-optimal detector/estimator.The direct sequence code division multiple access (DS-CDMA) technique has been favourably considered for application in digital mobile cellular networks due to its potential to provide higher system capacity over conventional multiple access techniques. Unlike FDMA and TDMA capacities which are mostly limited by the bandwidth, the capacity of a CDMA system is mainly restricted by its interference le vel. Any reduction in interference produces a direct and linear increase in system capacity. Multiple access interference (MAI) caused by non-zero cross-correlation between different spreading sequences is the major type of interference limiting the CDMA system capacity. Much work has been done to characterize MAI, and to analyze and evaluate the CDMA system performance in the presence of MAI. Since the cross-correlation properties of most sets of spreading codes are either too complex to analyze or very difficult to compute when different transmissions are not synchronized, a random sequence model. In the case of moderate to large processing gains, Gaussian distribution with variable variance is a good approximation for the MAI distribution. One of the approaches to reduce MAI is to employ orthogonal spreading sequences, and try to synchronize the transmissions at the chip level (quasi-synchronization). However, this is generally difficult to achieve in multipoint-to-point systems, such as the reverse link (mobile-to-base) of a cellular system, due to a lack of synchronization of the various mobile terminals, and the variable transmission delays. In this paper, a multi-carrier DS-CDMA (MCDS-CDMA) scheme is employed to facilitate the synchronization process, and thus reduce MAI.SYSTEM MODELA model of the MS-DS-CDMA system for the kth user of a CDMA system is shown in the figure 1.TRANMSITTER MODELAt the transmitter the users data stream dk(t) is divided into M interleaved sub streams and spread by a spreading sequence ck(t) to a fraction 1/M of the entire transmission bandwidth W. The resultant chip sequences are then used to modulate M carrier. The carrier frequencies m,m=1,2,M are equally spaced by the chip rate so that they are mutually orthogonal over one channel symbol interval T. Let R be the information rate and Rc be the carrier control code rate then the channel symbol interval isBER PERFORMANCEMC-DS-CDMA system performance measured by bit error rate through analysis and simulation.AnalysisThe BER is analysed based on the followingOrtoganal spreading sequences with rectangular pulse shape are applied.k,k=1K are independent of random variables distributd in (-D,D) WHERE D=D+D.Given W and D,M is chosen so that DIt is assumed that the fading parameters of the desired user .It is perfectly estimated so that the coherent detection and optimum soft decision decoding could be carried out at the receiver to make the problem analytically tractable. The fading amplitudes as independent Rayleigh random variables with equal second moments. The model isMATLAB INTRODUCTIONMatlab is a commercial Matrix Laboratory package which operates as an interactive programming environment. It is a mainstay of the Mathematics Department software lineup and is also available for PCs and Macintoshes and may be found on the CIRCA VAXes. Matlab is well adapted to numerical experiments since the underlying algorithms for Matlabs builtin functions and supplied m -files are based on the standard libraries LINPACK and EISPACK.Matlab program and script files always have filenames ending with .m the programming language is exceptionally straightforward since almost every data object is assumed to be an array. Graphical output is available to supplement numerical results.IMREAD Read image from graphics file.A = IMREAD(FILENAME,FMT) reads a grayscale or color image from the filespecified by the string FILENAME. If the file is not in the current directory, or in a directory on the MATLAB path, specify the full pathname. The text string FMT specifies the format of the file by its standard file extension. For example, specify gif