DS-CDMA overlay for a VHF-AM

communication system

Oskar Drugge

Hampus H�aggstr�om

Tomas J�onsson

Magnus Lindgren

30th May 2000

Abstract

The issue of this project is to investigate the possibility of a modernizationof the aeronautical communication system. Presently there are a lot ofusers and the bandwidth does not seem to be su�cient. New features suchas weather radar information transmitted from the ground could be ofvaluable help for the pilots, although there is little possibility to send suchinformation with the existing communication system (analog AM voice).Since there are a lot of users in the present system it would be desirable touse new communication techniques that can coexist with the old system.Users willing to have the extra information provided by the new technologywould be able to install recievers in their aircraft, users not interested couldcontinue using the old system without interference. A candidate for sucha solution would be spread-spectrum communication which transmits itsinformation at low power over a wide spectrum. Possibly this transmissionwill be functional while not causing too much disturbance to the existingsystem. The task has been to investigate and simulate such a scenario.

This project uses Direct Sequence Code Division Multiple Access (DS-CDMA) as the spread spectrum modulation. To be able to investigatethe e�ect on the existing radio-channels when adding a CDMA-channel,simulations in Signal Processing WorkSystem (SPW) and Matlab havebeen used. Simulations have been performed to quantify how the AM-communication is interfered by the CDMA and vice versa. It has beenshown that under certain circumstances it is possible to overlay existingAM-channels with a coexisting digital CDMA-bitstream.

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Contents

1 Introduction 4

2 Spread spectrum communications 4

2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 DS-CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Spreading code . . . . . . . . . . . . . . . . . . . . . . . . . 62.4 Code synchronization . . . . . . . . . . . . . . . . . . . . . 7

3 Simulation scenario 8

3.1 The VHF band . . . . . . . . . . . . . . . . . . . . . . . . . 83.2 The overlay scenario . . . . . . . . . . . . . . . . . . . . . . 8

4 Design of the testsystem 9

5 Simulations and results 9

5.1 S/I-measurement . . . . . . . . . . . . . . . . . . . . . . . . 105.2 AM-voice simulations . . . . . . . . . . . . . . . . . . . . . 10

5.2.1 Subjective measurements . . . . . . . . . . . . . . . 105.2.2 Quantitative measurements . . . . . . . . . . . . . . 10

5.3 CDMA-simulations . . . . . . . . . . . . . . . . . . . . . . . 115.3.1 E�ects of S/I-ratio and bitrate . . . . . . . . . . . . 115.3.2 E�ects of a varying number of AM-channels . . . . . 11

6 Discussion 12

6.1 The near-far problem . . . . . . . . . . . . . . . . . . . . . . 14

7 Conclusions 15

A SPW simulations 17

A.1 Total system . . . . . . . . . . . . . . . . . . . . . . . . . . 17A.2 AM-transmitter . . . . . . . . . . . . . . . . . . . . . . . . . 17A.3 AM-receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . 17A.4 CDMA-system . . . . . . . . . . . . . . . . . . . . . . . . . 17A.5 CDMA-transmitter . . . . . . . . . . . . . . . . . . . . . . . 18A.6 CDMA-receiver . . . . . . . . . . . . . . . . . . . . . . . . . 18

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1 Introduction

Today's aeronautical communication system uses AM modulated voice andis really quite ine�ective considering the techniques now available. As anexample it would be a good idea to transmit weather radar from ground toair. The whole aeronautical band (108-137MHz) is occupied with 25kHzwide AM voice channels so there is really only two possible ways to getmore information through to the aircraft. The best idea would probablybe to throw the old equipment out the window and design a brand newmodern e�ectice communication system. This idea is not easy to realizethough. Every aircraft in the whole world uses the present system and tochange this system to a new incompatible system would be very di�cultand expensive, if at all viable.

The second idea is to develop a communication system that can coexistwith the old one. Users willing to have the extra information providedby the new system would be able to install additional equipment in theiraircraft, users not interested could continue using the old system withoutinterference.

A candidate for such a solution could be a spread-spectrum systemwhich transmits its information at low power over a very wide bandwidth.This kind of spread spectrum systems are very tolerant to interference andjammers and it is likely that it would be possible for an AM and a spread-spectrum system to coexist on the same bandwidth without disturbing eachother. For example this could mean that the AM system operates as beforeand a digital bitstream can be added and broadcast new information.

There has been some research on applications similar to the one sug-gested above. For example in the articles [4, 5, 6] it is discussed if andhow it is possible to overlay a FM communication system, in this case theeuropean GSM system and the american AMPS system are investigated.These articles state that the overlay approach is a possible solution andsome ideas how to improve the overlay results are presented.

This project investigates the possibility of an overlay solution to thecrowded aeronautical band environment. The simulation software SPW(Signal Processing Worksystem) by Comdisco Systems Inc has been used tosimulate a proposed scenario where a spread-spectrum broadcast channelis added to the aeronautical AM-spectrum.

2 Spread spectrum communications

2.1 History

Spread spectrum techniques was developed during the 40's, by the US mili-tary. Its main advantage at this time was a superior resistance to jamming.There are generally two spread spectrum techniques, frequency hoppingand direct sequence spread spectrum. Frequency hopping was originally

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invented by the female american actor Hedy Lamarr and the male pianoplayer George Antheil, believe it or not. They actually patented the ideain 1942, [3]. This technique has been used for quite some time for examplein military radio and to some extent the European GSM system. Recently,direct-sequence spread spectrum techniques (DS-SS) have shown very use-ful in multiple-access systems (DS-CDMA) providing an e�ective use ofbandwidth, resistance to multipath interference and often high bitrates.A nice example of a modern CDMA system is the third generation 3Gcellular phone systems that are beeing developed.

2.2 DS-CDMA

In this project we focus mainly on direct sequence code division multipleaccess (DS-CDMA). The idea is to multiply a digitally modulated signal(e.g BPSK) with a pseudo-random code-signal of a much higher frequencythan the message. The digitally modulated signal is then multiplied withthe code to spread it over a very wide frequency band, at the same time asthe transmitted power per unit frequency is lowered substantially. Possiblythe transmitted power can be lowered to the same level or lower than thenoise-level of the channel.

The code mentioned above is referred to as the spreading code or chip-sequence. The smallest pulsewidth of the code is referred to as a chip.The spreading code takes on the values � 1 in an order which is onlyknown by the transmitter and the intended reciever. Further the codeis periodic with a su�ciently long period to make it seem random. Thereceiver will despread the signal by multiplying the wideband signal withan exact replica of the spreading code. Since the spreading code consistsof � 1 values, it will produce a constant one if it is multiplied with anexact replica. This will despread the signal, contracting the bandwidthand lifting the power above the noise oor to a detectable level. Whena jamming signal reaches the receiver, it will also be multiplied with thespreading code and therefore spread over a wide bandwidth, pushing thepower below the noise oor. This is why DS-CDMA is resistant to jamming.

Every user in a CDMA-system is provided with a unique code which is\orthogonal" to codes used by other users. This means that the di�erentcodes ideally have no cross correlation, enabling receiver to exclusively de-tect the signal modulated with its own code. Signals modulated with othercodes will not despread in the demodulating process, leaving the powerlevel well below the noise oor. It is also desirable to have code-sequencesthat are uncorrelated with themselves when they are not perfectly linedup (in phase), autocorrelation function as in Figure 1. Autocorrelationproperties are of great importance when choosing code sequences that givethe least probability of false synchronization.

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Figure 1: Autocorrelation function for an M-sequence, chip period Tc.

2.3 Spreading code

The �rst part of the CDMA signal generation is the previously mentionedspreading code. A summary of the properties that is of primary interestis:

� Interference protection. This is a tradeo� between needed bandwidthand achieved processing gain against interfering signals.

� Privacy. If a \secure" code is used, the signals can not be despreadby users not familiar with the code.

� Code families. If there is a set of codes with small crosscorrelation itis possible to have multiple access with code division.

Protection against interference is achieved in the receiver despreading pro-cess. When the receiver despreads the coded signal it also spreads allsignals that are uncoded. The metric processing gain is used to measurethe interference performance. Processing gain is de�ned as

Gp =T

Tc; (1)

where T is the period of the message and Tc is the chip period. In otherwordsGp is a bandwidth ratio, in some references, e.g. [6] the ratio betweenthe spread CDMA bandwith and the bandwidth of the jammer is usedinstead.

There are many types of codes, and implementing a good code is noteasy. Two families of codes are mentioned in most literature, Maximal-length sequences and Gold-codes.

M-sequences are by de�nition the longest codes that can be generatedby a given delay-element, or shift register. The code generator is a setof shift registers and some logic which feeds back shift register states. Ifmultiple access is desired it is possible to generate a long M-sequence andshift it in phase to get multiple codes with little cross correlation. Adrawback of maximal-length codes is that they are not very secure codes.

Gold-codes were invented speci�cally for the multiple-access applica-tions of spread spectrum. As most codes they are generated in the same

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way. Shift registers and feedback. The interesting part is that a typicalGold-coder consists of two M-sequence coders. The outputs of the twoare phase shifted and modulo-2 added. In this way the Gold-coders cangenerate large sets of codes with low cross correlation, just by changingthe phase shifts of the M-sequences.

2.4 Code synchronization

When the CDMA receiver is turned on, it must syncronize with the desiredsignal in order to despread it. This synchronization is performed in twosteps, code aquisition and code tracking.

Aquisition is the process of �nding the initial phase of the receivedsignal, enabling the correlation code in the receiver to be lined up perfectly.A basic way to do this is so called serial search.

The receiver sets an initial phase and tries do despread the message. Ifit succeeds, the despread signal will proceed through a bandpass-�lter thathas a bandwith that suits the unspread signal. After the �lter, the despreadsignal will be sensed by an energy-detector. If the energy detector does notsense the signal, the phase of the correlation code has to be changed bye.g. one chip, and the receiver will try to despread the signal with the newphase. The receiver searches through all possible phase references until themessage despreads.

Code tracking is the process of maintaining the syncronization whichwas achieved in aquisition. Tracking is needed since Doppler-shift will makethe period of the spreading-sequence change with time. The carrier willalso be a�ected by Doppler e�ects which sometimes can make it di�cultto achieve aquisition before the carrier frequency has been \tracked".

Tracking utilizes the properties of the autocorrelation function, Figure1. Mostly, tracking is done by generating three replicas of the code. the �rstone called prompt, which is the best estimate that we have from aquisition.The second is called early , which is prompt shifted half a chip forward intime. The last one is called late, which is prompt shifted half a chip backin time.

By calculating the correlation between these three replicas and the codethat is received, resulting in three correlation values that we can co mparewith the autocorrelation function, Figure 1. The ideal situation is whenprompt and early generate correlation values with the same amplitude.This means that prompt is right in phase with the received signal, (theautocorrelation function is triangular). If for example late generates ahigher correlation value, we know that late is closer to the received message,all three replicas must be phase-shifted back in time so that prompt getsin phase with the received signal. This comparison is illustrated in Figure2.

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Figure 2: Illustration of tracking.

3 Simulation scenario

To be able to investigate the e�ect on the existing radio-channels whenadding a CDMA-channel, simulations in Signal Processing WorkSystem(SPW) have been used. This section will start with a summary of the waythe VHF-band is used for air tra�c control and navigation. Provided thebackground a brief speci�cation of the simulations is introduced.

3.1 The VHF band

Aircraft communication of today uses the bandwidth 108-137 MHz, sit-uated in the portion of the electromagnetic spectrum called VHF (VeryHigh Frequency). The upper part of this bandwidth (118-137 MHz) makesroom for 760 AM-channels each 25KHz wide. The international airspaceis controlled by air tra�c control centers (ATCC's) each controlling onedesignated part of the airspace. Each control center is provided with one ormore speci�c frequencies over which it can communicate with the aircraftpresently within its airspace. As an aircraft ies out of the scope of its cur-rent control center, it changes frequency to be able to communicate withthe control center responsible for the area it enters. In addition to the vo-cal transmissions mentioned above, weather information is provided withcontinous computer-synthesized vocal transmission at certain frequencies.

The lower part of the bandwidth (108-117.950 MHz) is used by a navi-gational system called VOR (Very high frequency Omnidirectional Range).VOR stations placed at known positions help the aircraft not only to keep adesired course, but also to determine its position. The navigational trans-mitters are expected to use their dedicated spectrum continously, eachstation using one speci�c frequency.

3.2 The overlay scenario

The simulation-scenario has been set up as follows: One CDMA groundto air broadcast channel coexisting with a number of AM channels trans-mitting continuously. Starting out simple the digital signal is modulatedusing BPSK. The CDMA-channel is designed to be 20MHz wide in fre-quency and to have an initial bitrate of 100kbit/s. In the simulations the

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e�ect on the AM-communication versus CDMA performance was evalu-ated using both listening tests and by calculating the mean squared error(MSE). A preliminary schematic of this scenario is presented in Figure 3.

Figure 3: Block diagram of the simulation scenario.

As shown in Figure 3, The sum of the CDMA signal and the AM signalswill be demodulated separately in the CDMA and AM receivers. If boththe CDMA and the AM systems can retrieve their information sucessfully,the communication system has been improved.

4 Design of the testsystem

The simulation scenario was tested with a testsystem designed in SPW. It isa simple system only to investigate the possibilities of the overlay method.Optimization of system parts as receivers and transmitters was left for laterinvestigation. Aquisition and tracking was omitted from the system sincethis has been solved. Instead a priori knowledge of the chipping code phasewas used. The focus of the project has been on issues that are not o�ciallysolved. The design and block diagrams of all parts in the testsystem arepresented and explained in the Appendix.

5 Simulations and results

Firstly, simulations were performed to investigate the e�ects when trans-mitting voice over an AM-channel. A short voice-sample was transmittedover the channel while at the same time sending data with the CDMA-system. The issue of these simulations was to determine approximatelyhow much noise that could be added to the signal before the degradationof the speech was too severe. Two types of simulations examining thedegradation subjectively as well as quantitatively were carried out.

Secondly, simulations investigating how the CDMA-signal is a�ectedwere performed. Bit errors were introduced by the interference from theAM-channels. In the simulations three e�ects on the CDMA-performancewere investigated:

� The e�ect of a varying S/I.

� The e�ect of a varying bitrate.

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� The e�ect of a varying number of AM-channels.

5.1 S/I-measurement

In the simulations di�erent relative powers of the CDMA-signal and theAM-signals were investigated. In order to determine the S/I ratio a shortsimulation was done. A FFT of the signal transmitted through the air wascalculated and plotted in dB. The S/I ratio was then calculated by takingthe di�erence between the AM-peaks and the peak of the CDMA-mainlobe. This method was chosen since it was quick, easy and quite intuitive.This method would also make it easier to compare the simulated spectrumto an actual spectrum measured in reality. It is important to note that weused a 4096 FFT since a broadband signal is supressed and a narrowbandsignal is ampli�ed if the size of the FFT increases.

5.2 AM-voice simulations

5.2.1 Subjective measurements

At a 25dB S/I-ratio the noise is audible but not overly annoying. Aninteresting question here is whether this noise would be audible in an aero-plane where the background acoustic noise in the cockpit can be expectedto be higher. For this simulation a four seconds long voice�le was used.This voice�le was distorted in three di�rent ways: with interference fromAWGN, with an interfering CDMA channel and with both AWGN andCDMA[1]. The variance of the AWGN and the CDMA were 0.5 in allsimulations. This might not be fair variance levels compared with a realnoise environment, where the noise probably has a lower variance. Butthese simulations where performed to investigate the relative power ratiosbetween the di�erent signals, so it is not crucial if the power levels arenot completely concordant with reality. Listening tests showed that therewere almost no audible di�erence between the three cases. This is goodsince this indicates that the CDMA-system can overlay the AM-systemwithout interfering too much. It also shows that the noiselevel does notrise mentionable when CDMA and AWGN are added together.

5.2.2 Quantitative measurements

In order to quantify the distortion made to the AM-channels an ordinaryMean Squared Error estimate (MSE) was introduced. This metric is ob-tained by comparing two versions of an AM-signal. The �rst version is a4kHz test signal modulated by the AM-transmitter and demodulated bythe AM-receiver. This signal will be distorted only by imperfections in theAM-equipment and numerical simulation errors. Thus undistorted in theCDMA sense. The second version is the same test tone modulated by the

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AM-transmitter and added together with a CDMA signal and demodu-lated by the AM-receiver. This signal will be distorted by the CDMA andthis is the error that we would like to quantify. The MSE is calculated as

MSE = Ef[S1(t)� S2(t)]2g;

Where S1(t) is the undistorted signal and S2(t) is the distorted one. Sim-ulations investigating if the MSE was somehow correlated to the S/I andthe bitrate respectively were carried out.

From the results of these simulations there were no conclusions to bedrawn. The expected results would have been a decreasing MSE with anincreasing S/I and an increasing MSE with an increasing bitrate. The datagathered did however not really seem to comply with these assumptionsfor reasons mentioned below.

Because of the long simulation time, the length of the test signal waskept rather short making the statistical reliability rather low. Furthermore,more combinations of bitrates and S/I-ratios over a bigger range (moresimulations) would be needed to be able to draw any reliable conclusionsof the simulations.

5.3 CDMA-simulations

In these simulations a 4kHz sinusoidal signal was used as input to the AM-transmitter. The reason for not using an authentic voice-signal was to keepthe simulation times at reasonable levels.

5.3.1 E�ects of S/I-ratio and bitrate

We chose to overlay 18 AM-channels evenly spaced over the CDMA band-width (20MHz) with equal transmitted power. In each simulation 40000bits were sent. The results are shown in Figure 4, the exact values can befound in Table 1. One simulation with 100000 bits, S/I = 22 dB at 100kbit/s was also done, but there were still no errors. Thus, the bit errorrate is at least smaller than 1� 10�5 in this case.

5.3.2 E�ects of a varying number of AM-channels

These simulations were performed in the same way as before but in thiscase the number of AM-channels were varying and the S/I ratio was keptconstant at 28 dB. The results are shown in Figure 5, the exact valuescan be found in Table 2. As expected the bit error rate is a�ected by thenumber of AM-channels.

1N/D: Not Detectable

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Figure 4: CDMA performance vs. S/I ratio and bitrate.

6 Discussion

There are a couple of interesting enhancements that should be investigated.Later simulations have shown that an AM transmitter transmitting voicecreates a lot more distortion than a test tone and we know that the AMtransceiver situated in the same aircraft as the CDMA receiver will be avery strong jammer to it, since they are so close to each other. It is gener-ally very important to consider the problem where the possible interfereris closer to the receiver than the transmitter is. This problem is morethoroughly described in section 6.1.

The problematic situations mentioned above are probably signi�cantlyimproved if some kind of notch method is employed, see [4, 5, 6]. The arti-cle [7] states that \combining spread spectrum techniques with pre�ltering

yields dramatic signal-to-interference immunity". Notch �lters could beused as follows:

� By notching out the CDMA power at the critical AM frequencies theCDMA signal will not interfere with the AM signals.

� By notching out the AM frequencies at the CDMA-receiver the AMsignal will not interfere with the CDMA-receiver.

The notch �ltering in the CDMA-transmitter does not need to be verycomplex since the frequencies of the ight tower are stationary. This could

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Table 1: BER simulations with varying S/I-ratio and varying bitrate.

S/I (dB) Bit error rate100kbit/s 250kbit/s 500kbit/s

22 N/D1 2:50� 10�3 2:47 � 10�2

23 2:25 � 10�4 9:90� 10�3 4:94 � 10�2

24 4:25 � 10�4 1:29� 10�2 5:56 � 10�2

25 3:15 � 10�3 3:07� 10�2 8:42 � 10�2

26 4:53 � 10�3 3:87� 10�2 9:64 � 10�2

27 8:40 � 10�3 5:12� 10�2 1:15 � 10�1

28 1:64 � 10�2 7:19� 10�2 1:45 � 10�1

Table 2: BER simulations with varying number of AM-channels.

# AM BER1 N/D2 2:50 � 10�5

3 2:50 � 10�5

6 4:24 � 10�4

9 2:00 � 10�3

12 5:85 � 10�3

15 1:09 � 10�2

18 1:64 � 10�2

for example be implemented as a �lter bank with �xed AM-channel notch-�lters which are tuned to the local frequency plan. The �ltering in theCDMA-receiver could be harder to implement since aeroplanes change air-ports and thus change frequencies. These �lters may need to be adaptivenotch-�lters that automatically scan the spectrum and place notches atthe needed positions.

Furthermore the bitrates that are presented in this report are all un-coded bitrates. If error correcting codes are applied, the bit error rates willmost certainly be improved dramatically. Information from [8] suggeststhat a communications system with an uncoded BER of about 1 � 10�2

can be improved to around 1 � 10�6 with Reed-Solomon coding. Suchbiterror-rates are quite adequate for data-transfer. Naturally coding re-duces the e�ective bit-rate since not all of the bits can be used for infor-mation. A coding scheme that optimizes the bitrate while maintaining anacceptable BER should be possible to �nd. This issue would certainly beworth investigating when time permits.

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Figure 5: CDMA performance vs. number of AM-channels.

6.1 The near-far problem

Figure 6 shows a situation with one base-station and two aeroplanes A, andB. It is assumed that all AM-transmitters use the same power. We begin byconsidering a simple transmission between the base-station and aeroplaneA. If the system is supposed to work, the relative power-level must beadjusted to a workable level at the base-station transmitter (Around 30dB). Assuming this is done, both signals will be attenuated equally asthey propagate towards the aeroplane, keeping their relative power-levels.This will make the reception of both signals at aeroplane A possible withacceptable interference. The situation described above must however beconsidered as a special case of the real scenario. As soon as transmissions inother directions and from other aeroplanes/stations are taken into accounta number of problems arises:

� Interference at the base-stationAM-transmissions from aeroplanes are attenuated by the distanceand are likely to be interferred by the CDMA-transmitter at thebase-station.

� Interference at the aeroplanesCDMA-transmissions from the base-station are attenuated by thedistance and likely to be jammed by the AM-transmitter in the aero-plane.

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� Interference between aeroplanesCDMA-reception at aeroplane A is interferred by the AM-transmitteron aeroplane B. When aeroplane B enters the area within the circledrawn in the picture (Figure 6), it will be the closest of the two AM-transmitters in the area. The received AM-signal from aeroplane Bwill be stronger in power than the signal received from the base-station.

In all three cases listed above, the relative power-levels that was as-sumed to be adjusted at the base-station are not valid anymore. This willin turn cause the quality of both transmissions to degrade below acceptablelevels.

Figure 6: Schematic view over an airport with two aeroplanes in the area.

7 Conclusions

The simulation results show that both the S/I ratio, the bitrate and thenumber of active AM-channels a�ects the CDMA performance. Consid-ering that fact, the e�ect of individual ranges between the receivers andinterferers must be further investigated. If an e�ective notching techniqueis used though, the ranges will most certainly have a negligible e�ect.

The conclusion so far is that the overlay technique could be a verysuitable solution for the crowded aeronautical band. This modernisationwould provide a potential bit-pipe from ground to air while maintaining

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the AM communication system in its current state. Possibly this couldlead the way for further developments and a smooth transition to a fullymodernized e�ective communications system.

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A SPW simulations

These sections describes the systems and blocks that has been designed forusage in the simulation of the AM-CDMA scenario.

A.1 Total system

In Figure 7 the top view of the SPW-system is shown. The interesting partare the three big blocks at the top, which are arrays of AM-transmitters.Below them are the CDMA system, transmitter and receiver and one AM-receiver where the AM quality is measured. A bigger version of the mainblock diagram is presented in BILAGA.

A.2 AM-transmitter

The interior of the AM-transmitter is shown in Figure 8. This is really theDSB+C modulator from SPW with an on/o� feature added to be able tocontrol the transmissions of the transmitter.

A.3 AM-receiver

The interior of the AM-receiver is shown in Figure 9. The receiver bandpass�lters the incoming signal to cut out noise and interference at frequencieswhich are irrelevant to the speci�c AM-channel beeing demodulated. This�lter is centered at the desired AM-channel center frequency and has a totalbandwidth of 25kHz, assuming 25kHz channel spacing. In the followingblock the signal is multiplied with a cosine at the same frequencey as thedemodulated carrier frecuency. Finally the signal is bandpass �ltered againwith a �lter centered at 6.25kHz with a total bandwith of 12.4kHz. Thus�ltering out one sideband of a 25kHz wide AM-channel omitting 50Hz oneach side to get rid of the DC-component. Finally the received signal isampli�ed by a factor two, since half of the received energy is \lost" inthe baseband conversion. This results in a replica of the received signal.Figure 10 shows the spectrum of the AM signal when modulated withvoice. Figure 11 shows the spectrum of the AM signal when modulatedwith the 4 kHz testtone.

A.4 CDMA-system

The CDMA-system, Figure 12, consists of a random data generator whichgenerates information bits. A CDMA-transmitter that based on the in-formation bits generates a BPSK signal, spreads it using a general PN-sequence and transmits the modulated and spread data across a noiselesschannel. The design and function of the CDMA-blocks is presented in thefollowing subsections.

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A.5 CDMA-transmitter

The transmitter, Figure 14, uses the SPW-block BPSK-mod to generatea BPSK-signal modulated with the random data (information bits). The\complex to real/imag" block is used to convert the complex output formatof the BPSK-modulator to the purely real sinusoid that is the usual out-put from a BPSK-modulator. The BPSK-signal is multiplied with a PN-sequence (chipping code). The block pn seq, shown in Figure 13 consistsof the SPW-block \PN-sequence generator" and a converter from the lev-els [0; 1] to [�1; 1]. The PN-sequence and the BPSK-signal are multiplied,producing the spread BPSK-signal which is the output from the CDMA-transmitter. Figures 15 shows the input data to the CDMA-transmitterand the output data from the CDMA-receiver. These plots show that thesystem works. The output data is a slightly delayed replica of the inputdata. Figure 16 shows the spectrum of the CDMA-signal, the main lobeand the �rst sidelobes.

A.6 CDMA-receiver

The received signal is multiplied (correlated) with the same PN-sequenceas in the transmitter. A priori knowledge of the code phase is used insteadof implementing aquisition and tracking stages. After the correlation stagethe signal is mixed down from the carrier frequency. The resulting signal isthen fed into the BPSK-demodulator. Since the bpsk-demodulator needsa complex input we added a block to transform the received signal fromreal to complex. The block-diagram for the receiver is shown in Figure 17.

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ING

hold

1000

0

Val

ue:

outC

1.0

Lib

rary

/Fil

e:

s_ti

me

hold

s_fr

eq

x

SINK

SIGN

AL

’cdm

a2/t

omas

cdm

a/er

r_ra

te’

hold

>=RE

ALth

res

xz

-1Z h

old

out

in

Val

ue:

outC

1000

0000

0.0

STO

Pin

Lib

rary

/Fil

e:

s_ti

me

hold

s_fr

eq

xSI

NKSI

GNAL

’cdm

a2/to

mas

cdm

a/nu

mbe

r_bi

ts’

hold

in1

in2

out

KValue:

in out

1.0

hold

in1

in2

out

hold

in1

in2

out

BER

del

ay

N

errs

rate

bia

s

ref

in

cdm

a_tx

out

in

cdm

a_rx

out

inx

DATA

RAND

OM

hold

100e

3

10e6

5512

5e3

15e6

Bit

rat

e

CD

MA

chi

ppin

g fr

eque

ncy

Sam

plin

g fr

eque

ncy

CD

MA

car

rier

fre

quen

cy

Figure 7: Complete simulation system.

19

on/off

outin

hold

in1

in2

out2

hold

X

REAL/IMAG

COMPLEX

TOreal

imag

2

hold

outMOD

DSB+Cin

Figure 8: Block-diagram for the AM-transmitter.

out

in

KValue:

in out

2.0

BANDPASSIIR FILTER

BUTTERWORTHfreq

center

2n

bw

2nB

A

y

x

lockhold

s_freqpassband

BANDPASSIIR FILTER

BUTTERWORTHfreq

center

2n

bw

2nB

A

y

x

lockhold

s_freqpassband

2

hold

X

REAL/IMAG

COMPLEX

TOreal

imag

2

TONEhold

X

COMPLEX

hold

in1

in2

out

Figure 9: Block-diagram for the AM-receiver.

20

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

x 104

−250

−200

−150

−100

−50

0

Frequency (Hz)

Pow

er (

dB)

Figure 10: Spectrum for a voice modulated AM-signal at 20kHz carrier frequency.

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

x 104

−300

−250

−200

−150

−100

−50

0

Frequency (Hz)

Pow

er (

dB)

Figure 11: Spectrum for a tone modulated AM-signal at 20kHz carrier frequency.

21

Library/File:

s_timehold

s_freq

x

SINKSIGNAL

’cdma_out’

x

DATA

RANDOM

hold

cdma_rx

outin

cdma_tx

outin

Figure 12: Block-diagram for the CDMA-system.

out

PN_SEQUENCE

GENERATOR

x

hold

NUMERIC

TO

BINARY

hold

x y

Figure 13: Block-diagram for the PN-sequence generator.

out

in

2

hold

X

REAL/IMAG

COMPLEX

TOreal

imag

pn_seq

out

hold

in1

in2

out2

hold

outMOD

BPSKin

Figure 14: Block-diagram for the CDMA-transmitter.

22

0 2 4 6 8 10 12 14 16 18

x 104

0

0.5

1

1.5Received data

Time (s)

0 2 4 6 8 10 12 14 16 18

x 104

0

0.5

1

1.5Sent data

Figure 15: Received and sent CDMA-data.

23

0.5 1 1.5 2 2.5

x 107

−120

−110

−100

−90

−80

−70

−60

−50

−40

Frequency (Hz)

Pow

er (

dB)

Figure 16: Spectrum of the CDMA-signal, fchip = 10MHz.

out

in

timing

ready

sym2

DEMOD

PSK2

hold

num

r

2

2

hold

X

REAL/IMAG

COMPLEX

TOreal

imag

2

TONEhold

X

COMPLEX

hold

in1

in2

out2

hold

Z

COMPLEX

MAKEreal

imaghold

in1

in2

out

pn_seq

out

Figure 17: Block-diagram for the CDMA-reciever.

24

References

[1] Contact magnus.lindgren@sverige.nu

[2] R.L. Peterson, R.E. Ziemer, D.E. Borth, 1995, Introductionto spread spectrum communications Prentice-Hall. ISBN 0-02-431623-7

[3] R.C. Dixon, 1995, Spread spectrum systems with commercial

applications, John Wiley & sons, inc. ISBN 0-471-59342-7

[4] L.B. Milstein, D.L. Schilling \The CDMA Overlay concept"IEEE 0-7803-3567-8/96/ p 476-480, 1996.

[5] D.M. Grieco, D.L. Schilling \The capacity of BroadbandCDMA overlaying a GSM cellular system" IEEE 0-7803-1927-3/94/, p 31-35, 1994.

[6] D.L. Schilling, G.R. Lomp, J. Garodnick 1993, \Broadband-CDMA overlay" IEEE Vehicular Technology Conference p452-455.

[7] Comdisco Systems, INC Application note \Designing a di-rect sequence spread spectrum communications system withSPW" CS-5007 12/90.

[8] Changwon national university (jul-1996). Metadata:SPW Tutorial on Bit Error Rate Estimation URL:http://bjko.changwon.ac.kr/~ovidius/main/sub/jewel/spw/index.html (2000-05-13)

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