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FH - CDMA

P. G. Department of Electronics & IT 28

9.0 Introduction

Spread Spectrum is a type of modulation that spreads the modulated signal

across available frequency band, in excess of minimum bandwidth required

to transmit the modulating signal [71]. Spreading makes signal resistant to

noise, interference and eavesdropping. Spread Spectrum is commonly used

in personal communication systems including mobile radio communication

and data transmission over LANs. Spread Spectrum has many unique

properties that cannot be found in other techniques of modulation. These

include the ability to eliminate multi-path interference, privacy of message

security, multi-user handling capacity and low power spectral density since

signal is spread over a large frequency band [53]. There are two commonly

used techniques to achieve spread spectrum. Viz, Direct Sequence Spread

Spectrum (DS-SS) and Frequency Hopping Spread Spectrum (FH-SS). A

DS-SS transmitter converts an incoming data (bit) stream into a symbol

stream. Using a digital modulation technique like Binary Phase Shift Keying

(BPSK) or Quadrature Phase Shift Keying (QPSK), a transmitter multiplies

the message symbols with a pseudo random (PN) code. This multiplication

operation increases the modulated signal bandwidth based on length of chip

sequence. A Code Division Multiple Access (CDMA) system is

implemented via these coding. Each user over a CDMA system is assigned

a unique PN code sequence. Hence, more than one signal can be transmitted

at the same time on same frequency.

In this chapter Frequency Hopping (FH-SS) Spread Spectrum modulation

technique has been used with a new spreading code in which conventional

PN code controls a typical chaos oscillator. The resultant chaotic signal has

a wide frequency range from few Khz to Mhz (12.34 kHz to 9.313 Mhz).

The motivation to use chaotic signal in place of conventional PN code has

been because, chaotic systems are nonlinear dynamical systems with certain

distinct characteristics. These systems can generate highly complex

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P. G. Department of Electronics & IT Page 129

waveforms even though the number of interacting variables is minimal [58].

For an iterated map a dynamical system with single variable can result in

chaotic behavior while for a continuous system, three coupled differential

equations can result in a complicated dynamics. Time series generated from

chaotic dynamics have the following three interesting properties: (i)

wide−band spectrum, (ii) noise−like appearance, and (iii) high complexity.

In a chaotic system, trajectories starting from slightly different initial

conditions diverge exponentially in time which is known as sensitive

dependence on the initial conditions. Because of these distinctive properties,

chaotic systems are widely being studied for secure communication and

multiple user communication applications [70].

Basically there are two ways by which chaos can be used in communication

system

(I) To use chaotic time series as wide-band carrier so that coding and

modulation can be accomplished together, these include Chaotic

Shift Keying (CSK) and Frequency Modulated Differential Chaotic

Shift Keying (FM−DCSK).

(II) To use chaotic sequences as an alternative source for spreading

sequences in Direct Sequence Spread Spectrum (DS/SS) and

Frequency Hopping Spread Spectrum (FH/SS) communication

systems.

9.1 Frequency Hopping Spread Spectrum (FH-SS)

Frequency hopping is a radio transmission technique where the signal is

divided into multiple parts and then sent across the air in random pattern of

jumping or hopping frequencies. When transmitting data, these multiple

parts are data packets. The hopping pattern can be from several times per

second to several thousand times per second. Frequency hopping is the

easiest spread spectrum modulation to use. Any radio with a digitally

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controlled frequency synthesizer can, theoretically, be converted to a

frequency hopping radio. This conversion requires the Addition of a pseudo

noise (PN) code generator to select the frequencies for transmission or

reception. Most hopping systems use uniform frequency hopping over a

band of frequencies. This is not absolutely necessary, if both the transmitter

and receiver of the system know in advance what frequencies are to be

skipped. Thus a frequency hopper in two meters could be made that skipped

over commonly used repeater frequency pairs. A frequency hopped system

can use analog or digital carrier modulation and can be designed using

conventional narrow band radio techniques. De-hopping in the receiver is

done by a synchronized pseudo noise code generator that drives the

receiver's local oscillator frequency synthesizer. FH-SS splits the available

frequency band into a series of small sub channels. A transmitter hops from

sub channel to sub channel, transmitting short bursts of data on each channel

for predefined period, referred to as dwell time (the amount of time spent on

each hop). The hopping sequence is obviously synchronized between

transmitter and receiver to enable communications to occur. FCC

regulations define the size of the frequency band, the number of channels

that can be used, and the dwell time and power level of the transmitter. In

the frequency hopping spread spectrum a narrowband signal mover hops

from one frequency to another using a pseudorandom sequence to control

hopping. This result in a signal’s lingering at a predefined frequency for a

short period of time, which limits the possibility of interference from another

signal source generating radiated power at a specific hop frequency.

Types of Frequency Hopping Spread Spectrum

Frequency Hopping Spread Spectrum Systems are categorized into

(a) Slow Frequency Hopping (SFH): In an SFH spread system the hop rate

(fh chip rate) is less than the base band message bit rate fb. Thus two or

more (in several implementations, more than 1000) base band bits are

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transmitted at th same frequency before hopping to the next RF frequency.

The hop duration, TH is related to the bit duration Tb by

TH = KTb for K= 1,2,3.............

And fc = fH = 1/ TC.

(b) Fast Frequency Hopping Spread Spectrum (FFH): In an FFH spread

spectrum the chipping rate, fc, (chipping rate is same as hopping rate) is

greater than the base band data rate fb. In this case one message bit Tb is

transmitted by two or more frequency hopped RF signals. The hop duration

or chip duration (TH = TC), is defined by.

TC = TH = 1/K Tb for k= 1,2,3..........................

And fc = fh = 1/TC

Advantages of Frequency Hopping Spread Spectrum (FH-SS)

Some of the advantages of Frequency Hopping Spread Spectrum

Modulation is as under

a) Frequency Hopping Spread Spectrum has a ability to provide

diversity in fixed wireless access applications or slowly moving

systems.

b) Frequency hopping system has a relatively short acquisition time than

that of Direct Sequence system.

c) It can achieve greatest amount of spreading.

d) It can be programmed to avoid unwanted portions of spectrum.

9.2 Proposed Frequency Hopping Code Division Multiple Access

Systems

The bock diagram of the proposed FH-CDMA system is outlined in Figures

9.1 and 9.2 respectively. The proposed scheme has been implemented using

linear and non–linear ICs. In this scheme a real time voice signal has been

taken as the modulating signal which has been converted into electrical

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signal by means of a requisite transducer (Sound to Voltage converter)

shown in Figure 9 .3. The electrical signal, which represents the message

signal, to be transmitted in the secure mode of communication is converted

into its digital equivalent. The electrical form of voice signal has been

converted into digital form using an analog to digital converter (ADC) with

a specified clock, which gives the eight bit output. The parallel data

available at ADC output is converted into serial form by means of a

multiplexer. The three select lines of the parallel to serial converter

sequentially selects one output at a time out of the eight. A 3-bit counter is

used for the generation of the select lines required by the parallel-to-serial

converter. The clock of the counter should be eight times greater than that of

the analog to digital converter such that eight bit information will be

transmitted sequentially during one complete cycle. This output of the

parallel to serial converter is applied to the Modulo-2 adder.

The second input to Modulo-Two adder has been generated by the frequency

synthesizer driven by chaos oscillator. As such, this signal changes in a

pseudo-random manner. The Chaotic signal generator is the heart of the

proposed FH- CDMA system. The chaotic signal generator is designed

around a Wein bridge oscillator in which digitally controlled variable

resistance technique using Linear Feedback Shift Register (LFSR), decoders

and array of transistors have been use to select randomly the resistor values

and is given in Figure 9.4. The output of the oscillator thus produces the

sustained analog signal with varying frequency and amplitude. This analog

signal is further applied to the analog to digital converter which produces an

8- bit output.

At the receiver, an inverse system has been designed to generate the FH-SS

signal (as that used in the transmitter). This FH-SS signal is thus

regenerated at the receiver by means of the chaos oscillator in association

with a locally generated PN sequence in a similar fashion as that used at the

transmitter for Proper synchronization between transmitter and receiver.

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The FH-SS generated at the receiver is modulo-2-added with the received

modulated signal. The output of the Modulo-2-Adder is converted into

Parallel form by using serial-to-parallel converter (Demultiplexer). The

Demultiplexer output drives a digital to analog converter. The resultant

analog signal is amplified and after low passes filtering to deliver the

transmitted information signal at the receiver. The circuit diagram

representation of the proposed scheme is outlined in Figures 9.4 and 9. 5

respectively.

The waveforms obtained at various places are as shown in Figures 9. 6-9.12.

Figure 9. 6 shows the waveform of the voice signal transmitted over the

proposed FH-CDMA transmitter. The digital form of the voice signal at the

output of ADC of the transmitter is shown in Figure 9.7. The output of the

chaos oscillator at the transmitter is outlined in Figure 9.8. Figure 9.9 shows

the output of frequency synthesizer at the transmitter. This signal hops

whose output hops from one frequency to another according to output of

chaos oscillator. The FH-SS signal after spectrum spread spreaded with

chaotic spreading signal is shown in Figure 9.10.

The resultant FH-SS signal at the receiving end is shown in Figure 9.11.

The output of the receiver after de-spreading with the locally generated

spreading code has been shown in Figure 9.12. It can be seen that the

received signal shown in Figure 9.12 is similar to that of the transmitted

signal shown in Figure 9.6.

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Figure 9.1. Block Diagram of the Proposed Transmitter

Figure.9. 2. Block diagram of the Proposed Receiver

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Figure 9.3. Circuit Description of Voice to voltage Converter

Figure 9.4. Circuit Description of Transmitter of Proposed Scheme.

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Fig 9.5. Circuit Description of Receiver of Proposed Scheme

9.3 Experimental Verification and Results:

The proposed Frequency Hopping Code Division Multiple Access (FH-

CDMA) has been experimentally tested for its performance by transmitting

and receiving a speech signal over the FH- CDMA system. Various

waveforms obtained while transmitting and receiving the speech signal have

been recorded for technical observation by using readily available non-linear

and linear IC’s. The waveforms obtained at various check points have been

found satisfactory and are in conformity with the theoretical observations.

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The waveforms obtained at various check points are shown in Figures 9. 6 to

9.12 as under.

Figure 9.6. Waveform of voice to voltage converter Figure 9.7. Waveform of Analog to Digital

Converter

Figure 9.8. Waveform of Chaos generator at

Transmitting end

Figure 9.9. Waveform of Digital Frequency

Synthesizer at Transmitting end

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Figure 9.12. Waveform Received Voice Signal

Figure 9.10. Waveform of FHSS-Signal

at Transmitting end

Figure 9.11. Waveform of FHSS-Signal at

Receiving end

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9.4 Conclusion

Spread Spectrum based Code Division Multiple Access (CDMA) are

increasingly becoming more popular for multi-user communication systems.

In most of such multi-user systems, the given bandwidth (in a given area) is

to be divided and allocated to various communication channels. However,

in order to share the same bandwidth by many users in a given service area,

an equal number of unique pseudo-random codes with good correlation and

statistical properties are required. Chaotic signal generators are generally

used for generating pseudo-random codes with good correlation properties.

In this chapter a hardware based chaotic signal generator has been proposed

which can be easily programmed to generate a large number of unique

random codes best suited for a multi-user CDMA system. The chaotic

signal generator is typically controlled by a conventional PN code generator

to enable multicode generation. Different random codes can be generated by

simply changing the PN code. The resultant codes generated by the

programmable chaotic signal generators have been found to exhibit excellent

correlation and other statistical properties compared to those of the

conventional PN codes.

The proposed programmable chaotic signal generator has been used to

implement an FH-CDMA communication system and subsequently tested

for the transmission and reception of a voice signal. The results of

experimental verifications have been presented in the chapter and are in

conformity with theoretical observation. The proposed scheme will find a

range of applications in Spread Spectrum modulation, CDMA, Global

Positioning Systems (GPS) etc. Further, the proposed scheme guarantees

adequate security with low system complexity.

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