Mini Project Report_Rohit Arora.pdf

VOICE LINK OVER SPREAD SPECTRUM USING DSSS TECHNOLOGY

[Spread Spectrum Radio]

Mini Project Report

Submitted in partial fulfillment of the requirements for the award of the degree of

Bachelor of Technology in

Electronics and Communication Engineering by

ROHIT ARORA (Roll No.: B070367EC ) SHYAM ASHISH (Roll No.: B070287EC) AJITH K.G. (Roll No.: B060079EC) PREM NORBU KHRIMEY (Roll No.: B070244EC) TANAY CHAUHAN (Roll No.: B070281EC)

Under the guidance of

Dr. Deepthi P.P.

Assistant Professor

Electronics and Communication Engineering Department

NIT Calicut

Department of Electronics & Communication Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

April 2010

CERTIFICATE

This is to certify that the report entitled “VOICE LINK OVER SPREAD SPECTRUM USING DSSS

TECHNOLOGY” is a bonafide record of the Mini Project done by ROHIT ARORA (Roll No.:

B070367EC), SHYAM ASHISH (Roll No.: B070287EC), AJITH K.G. (Roll No.: B060079EC),

PREM NORBU KHRIMEY (Roll No.: B070244EC), and TANAY CHAUHAN (Roll No.:

B070281EC) under my supervision, in partial fulfilment of the requirements for the award of

the degree of Bachelor of Technology in Electronics & Communication Engineering from

National Institute of Technology, Calicut, and this work has not been submitted elsewhere

for the award of a degree.

Dr. DEEPTHI P.P. Dr. P.C. Subramaniam

(Guide)

Assistant Professor Professor & Head

Dept. of Electronics & Dept. of Electronics And Communication Engineering Communication Engineering

Place : NIT Calicut

Date : 29th April 2010

ACKNOWLEDGEMENT

At this moment of accomplishment, we are presenting our work with great pride and

pleasure. We would like to express our sincere gratitude to all those who helped us in the

successful completion of our venture. First of all we would like to thank our HOD, Dr. PC.

Subramaniam, who provided us with all facilities for the development and accomplishment.

We are also grateful to our project guide Dr. Deepthi P.P. for her timely and valuable

suggestions, which led us to test our models so efficiently. The data recorded from the tests

was so very helpful for us to judge our system. She gave us constant guidance and support

throughout this journey.

Also Anandan Sir, the Lab Assistant in the Embedded Systems Lab, has stood by us, all along,

to help us reach the zenith we have achieved today.

ROHIT ARORA SHYAM ASHISH PREM NORBU KHRIMEY AJITH K.G. TANAY CHAUHAN

CONTENTS

List of Figures

List of Tables

Abstract

1 Introduction

2 How it Works

2.1 Theory

2.2 Modulation & Demodulation

2.3 Coding Techniques

2.4 Advantages of Spread Spectrum

3 Functional Description

3.1 Microphone

3.2 Delta Modulator

3.3 Di‐Phase Encoder

3.4 PN‐Sequence Generator

3.5 Ex‐OR Gates

4 Detailed Circuitry

4.1 PN Sequence Generator

4.2 Di‐Phase Encoder

4.3 Reset Circuitry

4.4 Clock Generation

5 Component Description

5.1 7414

5.2 7404

5.3 7486

5.4 744040

5.5 74164

5.6 7474

5.7 7805

5.8 TLV320AIC1107

6. Results

6.1 Simulation

6.2 Observation

7. Conclusions

8. Scope for Future Work

References

1. ISSCE—Circuits Systems. "Principles of Spread Spectrum Communication" 2. Dixon, Robert C. "Spread Spectrum Techniques". John Wiley & Sons. 3. Internet Magazine. "Spread Spectrum Scene: ABC's of Spread Spectrum",

http://sss-mag.com/ss.html

LIST OF ABBREVIATIONS

DSSS Direct Sequence Spread Spectrum

FHSS Frequency Hopping Spread Spectrum

SMD Surface Mount Devices

PN Pseudo Noise

PCM Pulse Code Modulation

SIPO Serial-In-Parallel-Out

FF Flip-Flop

MIC Microphone

GND Ground

XTAL Crystal

NRZ Non Return to Zero

ISM Industrial Scientific Medical

EDAC Error Detection And Correction

AM Amplitude Modulation

FM Frequency Modulation

SNR Signal to Noise Ratio

LPI Low Probability of Intercept

AJ Anti Jamming

LAN Local Area Networking

WLAN Wireless Local Area Networking

LIST OF FIGURES

FIG NO. TITLE

2.1 Comparison of DS‐Spread Signal and Narrowband Signal_1

2.2 Comparison of DS‐Spread Signal and Narrowband Signal_2

2.3 Despreading Process

3.1 Transmitter Block Diagram

4.1.a Clock Generator and Compander

4.1.b. PN generator and Diphase Encoder

4.1.1 255‐Chip PN Sequence Generator

4.2.1 Di‐Phase Encoder

4.3.1 Reset Circuitry

4.4.1 Clock Generation Circuitry

4.5. PCB Layout

4.5.1. Component Side

4.5.2. Solder Side

5.1. 7404 (NOT Gate)

5.1.1. Functional Diagram

5.1.2. Pin Diagram

5.1.3. Pinning Information

5.2. 7414 (Schmidt triggered NOT Gate)


5.2.2. Pin Diagram


5.3. 7486 (XOR Gate)


5.3.2. Pin Diagram


5.4. CD4040BC (12 stage binary ripple counter)


5.4.2. Pin Diagram


5.5. 74164 (8‐bit Serial‐In‐Parallel‐Out [SIPO] shift register)


5.5.2. Pin Diagram


5.6. 7474 (2 D Flip Flop Set)


5.6.2. Pin Diagram


5.7. 7805 (12 Volt to 5 Volt Voltage regulator)


5.8. TLV320AIC1106 (PCM Compander IC)


5.8.2. Pin Diagram


Voice Link over Spread Spectrum using DSSS April 29, 2010

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Necessity is the mother of invention, and Curiosity is the mother of discovery. During past decades, Humans have faced many events which were triggered either by curiosity or by necessity, which have led to such developments in Technology as we see today. Necessity to simplify things form humans led to the invention of Abacus, the first computing machine. And the curiosity to improve this machine led us to the discovery of better technology to build a computer, which was further triggered by the breakthrough in the transistor technology. Similarly Curiosity has also led to world wars, and the necessity to survive led the enemies to develop better and still better weapons than each other and unintentionally this led to a lot of technology evolution and now we face a threat of a nuclear war again, but it is the nature of Technology to evolve, so whatever be the scenario, world has always benefited from the events occurring in history, be it good or bad.

ABSTRACT

As engineering students, our endeavor is to do our bit to help make world a better place, by the things we have so far learnt. The problem with conventional security systems today is that they are mostly wired, meaning the security station must be placed within a fixed distance from the information capture device. This usually means that the device must be placed in a fixed location because it is difficult to move a wire embedded in the wall or ceiling of a building.

We plan to implement data security in wireless domain by using a security technique, which was used by the military some years back before the oncoming of better cryptographic techniques. This led to transfer of this technology to civilian domain, where it has been used to evolve Communication (i.e. CDMA). We plan to implement the same technique to build a wireless security system where voice data is sent wirelessly to a receiver station and heard manually or recorded. Though, the device which we develop here is not intended to be used at a military level it can be surely used as a civilian application. In a wireless system, where the transfer rate is more restrictive than a wired system, the information must be condensed in some form. So we use some kind of Source-Coding Technique to code the voice data. However, the voice input can be interchanged with any other type of input, with a few modifications involving baseband and passband encoding of the data to be transmitted.

Also in security applications, communication without security is worthless. So we plan to use Direct Sequence Spread Spectrum Technique (DSSS), to increase the bandwidth of the data to be transmitted using a large sequence of bits (PN Sequence) and thus reduce signal power density and yet retain the data intact. Only the receiver having the correct PN sequence will be able to decode the signal. Moreover the de-spreading of data at the reception point will reject any interference that the signal might have faced during transmission and reception. The advantage would be Low Probability of Exploitation (LPE) and Anti Jam (AJ) feature of the transmission leaving no one other than the intended receiver to decode the data and thus get the information.

The transmitter circuit may be placed anywhere to receive an audio account of the secret activity going on anywhere (of course, in range). This project might find a very good application in a secret communication e.g. If confidential information has to be transmitted across a distance about enemy activity in a battlefield, this idea might be very useful, otherwise even in civilian case, if some attacker tries to jam the voice signals, the LPE and AJ characteristics of the signal and the receiver will help combat the attacker as he would have to first cover the particular bandwidth at which the signal is transmitted and then detect the signal out of noise + interference in the channel, and then intercept it, then decode it, and then only he would be able to exploit the voice data. Moreover a jammer will have to first know basic characteristics of the signal to jam it i.e. he will have to cover and detect the signal before he tries to jam it. So our receiver is designed in such a way that it


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combats jamming very effectively, as when the signal is received, as it first de-spreads the received signal. In the process, the jammed signal contracts too and thus very-very minute information might be lost, which would not be significant.

Thus at first our signal is difficult to detect, then its encoded again, moreover it is spread over a large spectrum, so its voltage threshold is still lower that noise threshold. If someone comes to know about the signal frequency range by some means, the voltage level would be too low in the narrow band (interceptor does not know it’s a spread spectrum transmission) that the interceptor will take it to be noise and leave it. Even if someone detects the signal by chance, if would not be easy to recover the data due the LPE attributes we add in the signal using PN Sequence.

We plan to first baseband modulate the microphone input, then encode it using some technique of Source Coding, then XOR it with a PN sequence coming from another source. This PN sequence will be a very huge sequence of bits as compared to the actual no. of bits of data coming from the source encoder. Then we would go on to mix this XOR-red data with a high frequency local oscillator and pass this through a Band pass filter to limit the data bandwidth to a particular range before transmission. Finally we transmit the signal over the channel through an antenna.

At the Receiver, we tend to down-convert the received signal to a lower bandwidth and then pass it through a Band-pass filter again to restrict the signal to a lower bandwidth. After this we plan to divide this signal into three out of which two will be used to synchronize the transmitter and the receiver and one will be used to actually decode and demodulate and then receive the voice data over a speaker.

Synchronization circuit involves a difference amplifier which outputs some voltage corresponding to the difference in the signal strength, which is obtained after correlating the received data with the PN sequence, down-converting to a lower bandwidth, frequency selective amplification and then passing it through a Signal Strength indicator. This voltage undergoes some signal processing after which a Voltage Controlled Oscillator regenerates clock, using which we generate synchronous PN sequence as was generated during transmission, and thus fed to the correlator inputs before passing it on to the Local Oscillator. We expect to get a proper voice link over a wireless channel which is intimately secret in nature and has LPE and AJ feature as described earlier.

As a Mini Project our aim was to develop a transmitter module for the device, which we have successfully implemented.


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Chapter 1 1. Introduction

Spread spectrum technology has blossomed from a military technology into one of the fundamental building blocks in current and next-generation wireless systems. From cellular to cordless to wireless LAN (WLAN) systems, spectrum is a vital component in the system design process.

Since spread-spectrum is such an integral ingredient, it's vital for designers to have an understanding of how this technology. Here, we address the basic operating characteristics of a spread-spectrum (DSSS) system.

Spread spectrum uses wideband, noise-like signals that are hard to detect, intercept, or demodulate. Additionally, spread-spectrum signals are harder to jam (interfere with) than narrow band signals. These low probability of intercept (LPI) and anti-jam (AJ) features are why the military has used spread spectrum for so many years. Spread-spectrum signals are intentionally made to be a much wider band than the information they are carrying to make them more noise-like.

Spread-spectrum transmitters use similar transmit power levels as narrowband transmitters. Because spread-spectrum signals are so wide, they transmit at a much lower spectral power density, measured in watts per hertz, than narrow band transmitters. This lower transmitted power density characteristic gives spread-spectrum signals a big plus. Spread-spectrum and narrowband signals can occupy the same band, with little or no interference. This capability is the main reason for all the interest in spread spectrum today.

The use of special pseudo noise (PN) codes in spread-spectrum communications makes signals appear wide band and noise-like. It is this very characteristic that makes spread-spectrum signals possess a low LPI. Spread-spectrum signals are hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times the information bandwidth.

The spread of energy over a wide band, or lower spectral power density, also makes spread-spectrum signals less likely to interfere with narrowband communications. Narrowband communications, conversely, cause little to no interference to spread spectrum systems because the correlation receiver effectively integrates over a very wide bandwidth to recover a spread spectrum signal. The correlator then "spreads" out a narrowband interferer over the receiver's total detection bandwidth.

Since the total integrated signal density or signal-to-noise ratio (SNR) at the correlator's input determines whether there will be interference or not. All spread spectrum systems have a threshold or tolerance level of interference beyond which useful communication ceases. This tolerance or threshold is related to the spread-spectrum processing gain, which is essentially the ratio of the RF bandwidth to the information bandwidth.


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Chapter 2 2. How it works

2.1. Theory

Direct Sequence (DSSS) and Frequency Hopping (FHSS) are the most commonly used methods for the Spread Spectrum technology. Although the basic idea is the same, these two methods have many distinctive characteristics that result in complete different radio performances.

Narrow Band Information Signal

Power [Before Spreading]

Spread Spectrum Signal [After Spreading]

Frequency

Figure 2.1: In a spread-spectrum system, signals are spread across a wide bandwidth, making them difficult to intercept, demodulate, and intercept.

The carrier of the direct-sequence radio, which we implement stays at a fixed frequency. Narrowband information is spread out into a much larger (at least 10 times) bandwidth by using a pseudo-random chip sequence. The generation of the direct sequence spread spectrum signal (spreading) is shown in Figure 2.1.

Narrow Band Direct Sequence Amplitude Spread Spectrum

Frequency Figure 2.2: Comparison of the generation of a narrowband and direct-sequence spread spectrum signals.

In Figure 2.2, the narrowband signal and the spread-spectrum signal both use the same amount of transmit power and carry the same information. However, the power density of the spread-spectrum signal is much lower than the narrowband signal. As a result, it is more difficult to detect the presence of the spread spectrum signal. The power density is the amount of power over a certain frequency. In the case of Figure 2.2, the narrowband signal's power density is 10 times higher than the spread spectrum signal, assuming the spread ratio is 10.

At the receiving end of a direct-sequence system, the spread spectrum signal is de-spread to generate the original narrowband signal. Figure 2.3 shows the de-spreading process.


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De-spreading

Amplitude Spread Signal Amplitude Despread Signal Interfering Jammer Despread Jammer

Frequency Frequency

Figure 2.3: Diagram illustrating the despreading process in a direct-sequence system.

If there is an interference jammer in the same band, it will be spread out during the de-spreading. As a result, the jammer's impact is greatly reduced. This is the way that the direct-sequence spread-spectrum (DSSS) radio fights the interference. It spreads out the offending jammer by the spreading factor (Figure 2.4). Since the spreading factor is at least a factor of 10, the offending jammer's amplitude is greatly reduced by at least 90%.

Noise Level

Power

Transmitted DSSS Signal

Frequency Figure 2.4: Direct-sequence systems combat noise problems by spreading jammers across a wideband as

shown in this figure.

In case of interference, data loss may occur. In general, a voice system can survive an error rate as high as 10-2 while a data system must have an error rate better than 10-4. Voice system can tolerate more data loss because human brain can "guess" between the words while a dumb microprocessor can't.

2.2. Modulation and Demodulation

For direct-sequence systems the encoding signal is used to modulate a carrier, usually by phase-shift keying (PSK; for example, bi-phase or quad-phase) at the code rate. Direct sequence signals have only one output rather than symmetrically distributed outputs.

It's important to note that for direct-sequencing we generate wideband signals controlled by the code sequence generator. The code is the direct carrier modulation (Direct Sequence).

Spread spectrum signals (whether direct sequence, frequency hopping or their hybrids) can support any conventional analogue or digital modulation scheme to impress data onto the spread spectrum carrier.

There are several different modulation techniques that designers can employ when developing direct-sequence systems. Information modulation can be accomplished using amplitude (AM) of frequency modulation (FM) techniques. AM is normally used because it tends to be detectable when examining the spectrum. FM is more useful because it is a constant-envelope signal, but information is still readily observed. In both AM and FM, no knowledge of the code is needed to receive the transmitted information.


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Clock modulation, which is actually frequency modulation of the code clock, is another option in spread-spectrum designs. In most cases, clock modulation is not used because of the loss in correlation due to phase slippage between received and local clocks, could cause degraded performance.

Amplitude Modulation and its derivatives are the least desirable as their use will destroy the signal's uniform power spectral density. This constant carrier envelope is very desirable for spread spectrum systems designed for covert usage.

Frequency modulation (frequency shift keying for data) is seldom used in direct sequence systems. This is because when a direct sequence signal passes through a squaring or frequency doubling circuit, a carrier at twice the signal's centre frequency is produced. This twice frequency narrowband carrier will contain any modulation impressed on the direct sequence signal. Thus with analog modulation it is possible for the signal to be demodulated without any prior knowledge of the pseudo-random spreading code.

One option is code inversion or modification. The digitised voice or digital data is exclusive OR-ed with the PN spreading code. This will invert the PN code sequence if the data is a "1" or pass the PN code unmodified if it is a "0".

Assuming synchronisation at the receiver, the unmodified code de-spreads the direct sequence signal. This produces a narrowband signal which is still bi-phase shift modulated, but this time with the data or digitised speech. This signal can then be demodulated by conventional demodulators.

Code modification is another modulation technique that designers can use when building a spread-spectrum system. Under this approach, the code is changed in such a way that the information is embedded in it and then modulated by phase transitions on a RF carrier.

One disadvantage of code modification is that voice or other analogue signals require digitisation. As in any system design, the selection of the digitisation technique is very important. The technique selected must use the lowest possible data rate as data rate is inversely proportional to the process gain of the system. The technique selected for the system described uses an enhanced form of delta modulation to digitally encode the voice into a serial data stream.

This is the technique we have used.

In direct-sequence designs, balance modulation can be used in any suppressed carrier system used to generate the transmitted signal. Balanced modulation helps to hide the signal, as well as there are no power wasted in transmitting a carrier that would contribute to interference rejection or information transfer. When a signal has poor balance in either code or carrier, spikes are seen in its spectrum. With these spikes, or spurs, the signal is easily detectable, since these spikes are noticed above the noise and thus provide a path for detecting the hidden signal.

Once the signal is coded, modulated and then sent, the receiver must demodulate the signal. This is usually done in two steps. The first step entails removing the spectrum-spreading modulation. Then, the remaining information-bearing signal is demodulated by multiplying with a local reference identical in structure and synchronized with the received signal.

The receiver de-spreads this wideband signal by using an identical synchronised pseudo-random code to that in the transmitter. The receiver must use a circuit to adjust its clock rate so that the


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receiver's pseudo-random code is at the same point in the code as the transmitter. A tracking circuit is necessary to maintain synchronism once it has been attained.

For digitisation, a PCM Codec (Delta Modulation) is used which uses u-law companding to give a very good quality voice signals in digital form. These signals are then XOR-ed with PCM data (after bi-phase modulation) coming from the PN generator.

2.3. Coding Techniques

In order to transmit anything, codes used for data transmission have to be considered. However, this section will not discuss the coding of information (like error correction coding) but those that act as noise-like carriers for the information being transferred. These codes are of much greater length than those for the usual areas of data transfer, since it is intended for bandwidth spreading.

Codes in a spread-spectrum system are used for:

1. Protection against interference: Coding enables a bandwidth trade for processing gain against interfering signals.

2. Provision for privacy: Coding enables protection of signals from eaves dropping, so that even the code is secure.

3. Noise-effect reduction: error-detection and correction codes can reduce the effects of noise and interference.

Maximal sequencing is one of the more popular coding methods in a spread-spectrum system. Maximal codes can be generated by a given shift register or a delay element of given length. In binary shift register sequence generators, the maximum length sequence is (2𝑛𝑛 − 1) chips, where n is the number of stages in the shift register.

A shift register generator consists of a shift register in conjunction with the appropriate logic, which feeds back a logical combination of the state of two or more of its stages to its input. The output, and its contents of its n stages at any clock time, is its function of the outputs of the stages fed back at the proceeding sample time. Some maximal codes can be of length 7 to (236 − 1) chips.

In Direct-Sequence systems, Error Detection And Correction codes (EDACs) are not advisable because of the effect it has on the code, increasing the apparent data transmission rate, and may increase jamming threshold. Some demodulators can operate detecting errors at the approximately the same accuracy as an EDAC, so it may not be worthwhile to include a complex coding/decoding scheme in the system.

2.4. Advantages of Spread Spectrum

Spread-spectrum systems provide some clear advantages to designers. As a recap, here are nine benefits that designers can expect when using a spread-spectrum-based wireless system.

2.4.1 Reduced crosstalk interference:

In spread-spectrum systems, crosstalk interference is greatly attenuated due to the processing gain of the spread spectrum system as described earlier. The remaining effect of the suppressed crosstalk interference can be essentially removed with digital processing where noise below certain threshold results in negligible bit errors. These negligible bit errors will have little effect on voice transmissions.


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2.4.2. Better voice quality/data integrity and less static noise:

Due to the processing gain and digital processing nature of spread spectrum technology, a spread-spectrum-based system is more immune to interference and noise. This greatly reduces consumer electronic device-induced static noise that is commonly experienced by conventional analog wireless system users.

2.4.3. Lowered susceptibility to multipath fading:

Because of its inherent frequency diversity properties (thanks to wide spectrum spread), a spread spectrum system is much less susceptible to multipath fading.

2.4.4. Inherent security:

In a spread spectrum system, a PN sequence is used to either modulate the signal in the time domain (direct sequence systems) or select the carrier frequency (frequency hopping systems). Due to the pseudo-random nature of the PN sequence, the signal in the air has been "randomized". Only the receiver having the exact same pseudo-random sequence and synchronous timing can de-spread and retrieve the original signal. Consequently, a spread spectrum system provides signal security that is not available to conventional analog wireless systems.

2.4.5. Co-existence:

A spread spectrum system is less susceptible to interference than other non-spread spectrum systems. In addition, with the proper designing of pseudo-random sequences, multiple spread spectrum systems can co-exist without creating severe interference to other systems. This further increases the system capacity for spread spectrum systems or devices.

2.4.6. Longer operating distances:

A spread spectrum device operated in the ISM band is allowed to have higher transmit power due to its non-interfering nature. Because of the higher transmit power, the operating distance of such a device can be significantly longer than that of a traditional analog wireless communication device.

2.4.7. Hard to detect:

Spread-spectrum signals are much wider than conventional narrowband transmission (of the order of 20 to 254 times the bandwidth of narrowband transmissions). Since the communication band is spread, it can be transmitted at a low power without being detrimentally by background noise. This is because when de-spreading takes place, the noise at one frequency is rejected, leaving the desired signal.

2.4.8. Hard to intercept or demodulate:

The very foundation of the spreading technique is the code use to spread the signal. Without knowing the code it is impossible to decipher the transmission. Also, because the codes are so long (and quick) simply viewing the code would still be next to impossible to solve the code, hence interception is very hard.


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2.4.9. Harder to jam:

The most important feature of spread spectrum is its ability to reject interference. At first glance, it may be considered that spread spectrum would be most affected by interference. However, any signal is spread in the bandwidth, and after it passes through the correlator, the bandwidth signal is equal to its original bandwidth, plus the bandwidth of the local interference. An interference signal with 2 MHz bandwidth being input into a direct-sequence receiver whose signal is 10 MHz wide gives an output from the correlator of 12 MHz. The wider the interference bandwidth, the wider the output signal. Thus the wider the input signal, the less its effect on the system because the power density of the signal after processing is lower, and less power falls in the band pass filter.


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CHAPTER 3 3. Functional Description

Figure 3.1: Block Diagram of the digital voice direct sequence transmitter. Detail C ircuitry given below.

The transmitter system is described in functional blocks.

3.1. Microphone:

The analog voice data is input into the system using an electret microphone as shown in the diagram.

3.2. Delta Modulator:

Microphone audio is digitised to NRZ format through a PCM codec IC which converts the audio into serial data stream. This serial binary data stream must be coded into a format which is polarity insensitive because the receiver demodulator cannot recover the de-spread data's absolute phase. Only data transitions are recovered at the receiver, hence there is no way of determining whether the output data stream is inverted or not.

U1A

Delta Modulator

Microphone

PN Code Generator

Diphase Encoder

Transmission Module

4 MHz CLK

High Freq. Carrier


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3.3. Di-Phase Encoder:

The digitised audio is converted from NRZ format into a polarity insensitive diphase (biphase-mark) data stream. This sub-circuit produces a diphase signal, where a logic 1 has start, mid-bit and end transitions and a logic 0 has only start and finish transitions.

In addition to providing phase insensitive data transmission, the format also makes clock recovery at the receiver relatively easy, as unlike NRZ even a continuous stream of diphase encoded 0's results in many start and finish data cell transitions.

3.4. PN Sequence Generator:

The exciter's clock frequencies are provided by a master 4 MHz crystal oscillator. Power-up reset (with manual override) is configured around a Schmidt-trigger. A shift register and exclusive OR gates are configured as a 4 MHz 255 chip (code bit) long maximal pseudo-random code generator.

3.5. Ex-OR Gate:

The diphase encoded delta modulated digital voice signal is ex-ORed with the pseudo-random code producing a code modified PN spreading code.

The output spectrum consists of a series of symmetrical sidebands which have a Sinc2x distribution due to the many frequency components of the pseudo-random code.

As the spreading code has a pseudo-random character, the occurrence of a particular frequency is pseudo-random in time and the direct sequence output appears as noise on a spectrum analyser. This modified data from the ex-OR gate is ready to be transmitted over free space through a Transmitter module.


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CHAPTER 4 4. Detailed Circuitry

4.1.a. Clock Generator and Compander


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4.1.b. PN generator and Diphase Encoder

4.1. PN Sequence Generator We have used a maximal shift register pseudo-random generator which consists of a shift register with selected outputs being exclusive-ORed and fed back into the shift register input. The circuit goes through a number of states (determined by the bits in the shift register at each clock pulse) before it repeats itself after a set number of clock pulses. The maximum number of states for a shift register of length m is 2m, ie for a 8-stage shift register 28 = 256 states. However the all-zero state is not allowable and we get a pseudo-random


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sequence 2m-1 bit long before it repeats itself. To obtain a maximal sequence, the correct shift register outputs (tap points) must be found. The feedback tap points may be taken from the following stages: [7,1] [7,3] [7,3,2,1] [7,4,3,2] [7,6,4,2] [7,6,3,1] [7,6,5,2] [7,6,5,4,2,1] and [7,5,4,3,2,1]

1 2 3 4 5 6 7 8

Figure4.1.1 . 255 chip maximal length pseudorandom code generator

Inverting stages are inserted to avoid all-zero-lockup problem. When the shift register is switched on, a reset pulse is generated which initiates all shift register outputs to logic 0. This would normally lock up the pseudo-random sequence generator. However the input inverter injects logic 1 so that the maximal sequence can commence. The output may be taken as shown. 74HC164 is an 8 bit Serial-In-Parallel-Out (SIPO) register which we have used in our implementation to generate the PN Sequence. 4.2. Di Phase Encoder

Di Phase encoder is a module which converts the data from NRZ format to phase independent Di-phase format, so that it is possible to decode the voice data as well as sample clock while receiving, assuming initial synchronisation.

Figure4.2.1 . Di-phase Encoder taking voice data and sample clock as input and giving the diphase

encoded signal out.

We have used a 2 D flip-flop IC CD74HC74 to give the sample clock and the digital voice data as input to both the flip-flops and taking the outputs and ex-OR-ring them together to get a single diphase encoded signal.

U1A

PN Sequence out

CLR CLK

Shift Register

Vdd

Di Phase Encoder

Data

Sample clock

Out_1

Out_2

Di Phase signal


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4.3. Reset Circuitry

The RESET circuit involves Schmidt triggered NOT gates, so that whenever a pulse signal is applied, depending on the RST pin configuration of the various ICs (i.e. ACTIVE HIGH or ACTIVE LOW), a corresponding RESET signal is generated for each of the ICs (i.e. PCM Codec, SIPO register, Di-Phase Encoder).

Figure 4.3.1 . Reset Circuitry

We use HD74LS14, as the Schmidt Triggered NOT gate IC for the purpose of generating RESET pulses.

4.4. Clock Generation

We use a single 4 Mhz crystal to generate various clocks required in the transmitter. We need a 4 MHz clock for generating the PN sequence, a 2 MHz clock for the PCM Encoder, and a 8 KHz clock for the sampling clock.

Figure 4.4.1 . Clock Generation Circuitry

For the purpose we use a 12 stage binary ripple counter (CD4040BC) which divides the input clock by powers of two, and thus we get required outputs at the various outputs of the ripple counter.

Vcc Schmidt triggered NOT gates

gnd

RST to SIPO register

RST to PCM Codec

RST to Di-phase Encoder

NOT Gates

12 stage binary ripple counter

XTAL

CLK for

PCM Encoder

Sample CLK

PN Generator


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4.5. PCB Layout 4.5.1. Component Side

4.5.2. Solder Side


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Chapter 5 5. Component Description

5.1. 7404 (NOT Gate) 5.1.1. Functional Diagram

5.1.2. Pin Diagram

5.1.3. Pin Information


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5.2. 7414 (Schmidt triggered NOT Gate) 5.2.1. Functional Diagram

5.2.2. Pin Diagram



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5.3. 7486 (XOR Gate) 5.3.1. Functional Diagram

5.3.2. Pin Diagram



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5.4. CD4040BC (12 stage binary ripple counter)


5.4.2. Pin Diagram


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5.5. 74164 (8-bit Serial-In-Parallel-Out [SIPO] shift register) 5.5.1. Functional Diagram

5.5.2. Pin Diagram



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5.6. 7474 (2 D Flip Flop Set) 5.6.1. Functional Diagram

5.6.2. Pin Diagram


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5.7. 7805 (12 Volt to 5 Volt Voltage regulator) 5.7.1. Functional Diagram

5.8. TLV320AIC1106 (PCM Compander IC)



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5.8.2. Pin Diagram



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CHAPTER 6 6. Results

6.1. Simulation 6.1.1. Circuit Diagram

The above given circuit was tested on Simulation first using Circuit Maker 2000. This is a prerequisite for the transmitter circuit. Only transmitter module was tested. The receiver module for this circuit will be implemented as a Major Project and hence the simulation for the receiver circuit has not been prepared as suggested by the guide.

The following waveforms were obtained using Simulation for the rising edge and the falling edge of the PCM input assumed to be at 3.3 KHz. The Clock for Sampling and generation of PN Sequence was obtained directly using signal generators.


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6.1.2. Waveforms [Rising Edge]

[Falling Edge]

As we can see, when the PCM Data is logic 1, we see the PN sequence is transmitted as it is, but when the PCM Data is logic 0, the PN Sequence gets inverted.

This is the basic fundamental of secrecy using DSSS technology, that we do not send the data directly but we send different codes to represent different data symbols.

6.2. Observation

While doing it in the Lab practically, lots of testing was done, first on the breadboard, then on general purpose PCB, and the final product was a custom designed PCB, whose design is shown above in Chapter 5.

Finally we were able to demonstrate our design of spreading the digital data over a large bandwidth, and thus making it ready for transmission over Wireless Channel.


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We have not attempted to make the transmission wireless yet, and also the voice input was not taken through the PCM Compander IC, because of soldering difficulties we faced.

We will take up this as a Major project and we hope to overcome such difficulties as we proceed further.


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CHAPTER 7 7. Conclusions

We were able to generate PN Sequence and use it to spread the digital input, thus making it ready for transmission over Wireless Channel.

We did not take actual voice input yet into the Compander IC, because of design constraints.

Hence we conclude the design of a spreading circuit, which is an essential component for implementing data secrecy.


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CHAPTER 8

8. Scope for Future Work The spreader circuit may be used for securitizing any kind of data in digital form. Be it the DCT coefficients for video data, or Temperature data, or simply voice data. So Future for this technology is not so bright in military domain due to encryption techniques used nowadays, but it can surely be implemented for safeguarding the interests of the civilians, by the law-keepers to ensure error free impart of justice with quality (e.g. in Traffic Management systems).

Mini Project Report_Rohit Arora.pdf

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