AProject report

on

“CDMA IMPLEMENTATION USING QPSK MODULATION & DEMODULATION”

[Submission in partial fullfillment of Bachelor of engineering degree of

the Rajasthan Technical University]

SUBMITTED By:

GUIDED By:Isha Bhatnagar Dr. Neelam Sharma Neha Soni Prof. & Vice PrincipalNikita KhandelwalSugandha Saxena

Department of Electronics & Comm. Engineering.Institute Of Engineering &Technology

Alwar(Rajasthan)-301030Session 2009-2010

CONTENTSChapter1……………………………………………

Introduction Technologies of 2G Modulation

a.PSKb.BPSK

Modulation &Demodulation by QPSK

Chapter2…………………………………………..

CDMA Design Synthesis Significance

Chapter3…………………………………………..

Software to be used Simulation on Matlab Simulink Objective & Scope

a.Modulationb.SDRc.Benefits of SDR

Chapter4…………………………………………..

Features of CDMA Advantages & Disadvantages CDMA Vs. GSM

ACKNOWLEDGEMENT

We proudly express our respectful thanks to our esteemed educational institution I.E.T which has blessed us to continue our technical education and has provided us a bright future.We respectfully express our earnest thanks to Prof. Dr. Neelam Sharma Madam, Electronics Instrumentation &Control Department, who has taken care of all the students in the aspects and has guided us professionally and technically helped in completing my training.Finally last but not the least all the mental support of the respected lecturers as well as the friends who gave us the all kind of support available.

STUDENTS

Isha Bhatnagar

Neha Soni

Nikita Khandelwal

CERTIFICATE

This is to certify that, work project report entitled ”Environmental Explorer” Submitted by Isha Bhatnagar, Neha Soni, Nikita Khandelwal & Sugandha Saxena under my supervision is student own work & has not been submitted elsewhere for the award of any degree to the best of my knowledge and belief.

Signature

Dr. Neelam Sharma

Professor & Vice Principal

Abstract

METHODOGY:

The QPSK modulator and demodulator will be first simulated on MATLAB simulink. After it successful simulation on simulink system generator on Xilinx will

be use to simulate and generate the bit stream. Walsh code odd and even bits (of double the bit period of original data) will be multiply the orthogonal carrier waves (generated by sine block of simulink). Both will then the added to give the modulated wave .At the receiving end, the modulated wave will be multiply by two orthogonal wave, the output will be passed through low pass filter to remove the twice the carrier frequency present on it. Finally the original bit stream will be discovered.

Objective

MODULATION:

It is a process of varying a periodic waveform. In order to use that signal to convey a message. The periodic waveform is known as the carrier wave and it is modulated according to the message waveform which ensures minimum attenuation while transmission. If two or more bits are combined in some symbols then the signalling rate will be reduced. Thus, the frequency of the carrier needed is also reduced. Therefore, so is with the channel transmission bandwidth. Thus, we are using QPSK modulation and demodulation technique for the implementation of the CDMA in our project.

Introduction

This project is aimed at implementing QPSK modulator and

demodulator of CDMA technology of wireless communications on

Xilinx FPGA (field programmable gate arrays) through system

generator which would serve as a module for software defined

radio. A Software Defined Radio (SDR) is a radio communication

system which can potentially tune to any frequency band and

receive any modulation across a large frequency spectrum by

means of as little hardware as possible and processing the signals

through software.

In cellular service there are two main competing network

technologies: Global System for Mobile Communications (GSM)

and Code Division Multiple Access (CDMA). CDMA operates in

800 MHz and 1900 MHz band with a channel separation i.e.

carrier bandwidth of 1.25MHz. It uses FDD (frequency division

duplexing) and QPSK with quadrature spreading to modulate the

signals. In QPSK, information in conveyed through phase

variations. In each time period, the phase can change once. Since

there are four possible phases, there are 2 bits of information

conveyed within each time slot. The rate of change (baud) in this

signal determines the signal bandwidth, but the throughput or bit

rate for QPSK is twice the baud rate.

This project will implement QPSK modulator and demodulator on

FPGA. The reconfigurablilty of FPGA allows to use it for all

standards existing simply by updation of software.

1.2. Technologies of 2G

1.2.1. IS - 95 (CDMA-ONE)

Interim Standard 95 (IS-95), is the first CDMA-based (code

division multiple access) digital cellular standard pioneered by

Qualcomm. The brand name for IS-95 is CDMAOne. It is a 2G

mobile telecommunications standard that uses CDMA, a multiple

access scheme for digital radio, to send voice, data and signalling

data (such as a dialled telephone number) between mobile

telephones and cell sites.

IS - 95 operates in 800 MHz and 1900 MHz band with a channel

separation i.e. carrier bandwidth of 1.25MHz. It uses FDD

(frequency division duplexing) and QPSK with quadrature

spreading to modulate the signals. It has 64 voice channels per

carrier. While the AMPS systems requires that the signal be a at

least be 18dB above the co-channel interference to provide

acceptable sound quality, CDMA systems can operate at much

larger interfere levels because of their inherent interference

resistance properties. The ability of CDMA to operate with a much

smaller signal to noise ratio (SNR) than conventional narrowband

FM techniques allows CDMA to use the same set of frequencies in

every cell, which provides a large improvement in capacity.

1.2.2. GSM

The Global System for Mobile Communications, GSM, is the most

popular standard for mobile phones in the world. Around 78% of

the total market share is occupied by GSM and its derivatives as of

today. The key advantage of GSM systems has been higher digital

voice quality and low cost alternatives to making calls, such as text

messaging. GSM uses TDMA, i.e. time division multiple access.

Most GSM networks operate in the 900 MHz or 1800 MHz bands

with carrier bandwidth of 200 KHz . The channel data rate is

270.833 kbit/s, and the frame duration is 4.615 ms, supporting 8

voice chan

nels per carrier. The modulation used in GSM is 0.3 Gaussian

minimum shift keying (GMSK). One of the key features of GSM is

the Subscriber Identity Module (SIM), commonly known as a SIM

card. The SIM is a detachable smart card containing the user's

subscription information and phonebook.

Modulation

It is the process of varying a periodic waveform in order to use that

signal to convey a message. The periodic waveform is known as

carrier wave, and it is modulated according to the message signal.

The main purpose of modulation is to send the message to longer

distance, with minimum attenuation. The carrier wave is a high

energy (high frequency) wave. Higher the energy, more the

distance the wave can travel.

Demodulation – As the name implies, it is the process to get back

the original message signal from the modulated signal which is

received at the receiving end.

2.1. Phase shift keying (PSK):

PSK is a digital modulation scheme that conveys data by changing,

or modulating, the phase of a reference signal (the carrier wave). If

it uses two phases, it is called Binary phase shift keying. For four

phases, it is called quadrature phase shift keying. It can be of

higher order also.

Fig 1.1 : Modulation Techniques

For binary PSK (BPSK)

S0(t) = A cos (wt) represents binary “0”

S1 (t) = A cos (wt + π) represents binary “1”

For M-ary PSK, M different phases are required, and every n

(where M=2^n) bits of the

binary bit stream are coded as one signal that is transmitted as

A sin (wt + θj) j=1, 2……M

QPSK signal is an extension of BPSK signal. Both of these are a

type of M-ary signals.

The process that describes the modulated signal in a polar form is

Si (t) = AcPs(t)cos(2 π fct+2 π i/M)

Where ps(t) is the pulse shaping function. In digital phase

modulation, the phase of the sinusoid is modified in response to a

received bit. The allowed phases are given by

Modulation Angles θi = 2πi/M.

M stands for the order of the modulation. M=2, makes this a

BPSK, M=4 is QPSK, M=8, 8PSK and so on. Following diagram

shows three of these modulations and their “constellations”. A

rotation of the second resulting in the third figure does not change

the modulation, its power or performance. These modulations are

called rotationally invariant.

As QPSK has 4 possible states, QPSK is able to encode two

bits per symbol.

Phase Data

45

degrees Binary 00

135

degrees Binary 01

225

degrees Binary 11

315

degrees Binary 10

In QPSK, information in conveyed through phase variations. In

each time period, the phase can change once. Since there are four

possible phases, there are 2 bits of information conveyed within

each time slot. The rate of change (baud) in this signal determines

the signal bandwidth, but the throughput or bit rate for QPSK is

twice the baud rate.

2.2. Modulation and Demodulation by QPSK:

In communication system we have two main resources; these are

the transmission power and the channel bandwidth. The channel

bandwidth depends upon the bit rate or signalling rate (fb). In

digital bandpass transmission we use the carrier for transmission.

This carrier is transmitted over a channel. If two or more bits are

combined in some symbols, then the signalling rate will be

reduced. Thus the frequency of the carrier needed is also reduced.

This reduces the transmission channel bandwidth. Hence because

of grouping of bits in symbols the transmission bandwidth, the

transmission channel bandwidth can be reduced. In quadrature

phase shift keying, two successive bits in data sequence are

grouped together. This reduces the bit rate or the signalling rate

and thus the bandwidth of the channel.

If we define four signals, each with a phase shift differing by 90 º,

then we have Quadrature phase shift keying (QPSK)

2.2.1. Modulation

The input binary bit stream {d }, dk = 0, 1,2,..... Arrives at

the modulator input at a rate 1/T bits/sec sec and is separated into

two data streams dI(t) and dQ(t) containing odd and even bits

respectively

dI(t) = d0, d2 , d4 ,... dQ(t) = d1, d3 , d5 , ...

Fig 2.1: Input bit sequence & Even ,Odd bit

sequnces

A convenient orthogonal realization of a QPSK waveform , s(t) is

achieved by amplitude modulating the in-phase and quadrature

data streams onto the cosine and sine functions of a carrier wave as

follows:

S(t)=1/ 2 d I (t) cos (2πft + π/4) + 1/ 2dQ(t) sin (2πft + π/4)

Using trigonometric identities this can also be written as

s(t)=A cos [2πft +π/4 + q(t)].

The pulse stream dI(t) modulates the cosine function with an

amplitude of ± 1. This is equivalent to shifting the phase of the

cosine function by 0 or π; consequently this produces a BPSK

waveform. Similarly the pulse stream dQ (t) modulates the sine

Function, yielding a BPSK waveform orthogonal to the cosine

function. The summation of these two orthogonal waveforms is

the QPSK waveform.

The values of q(t) = 0, -( π /2), π /2,

here π represent the four possible combinations of a I (t) and

aQ(t)

Each of the four possible phases of carriers represents two bits of

data. Thus there are two bits per symbol. Since the symbol rate

for QPSK is half the bit rate, twice as much data can be carried in

the same amount of channel bandwidth as compared to BPSK.

This is possible because the two signals I and Q are orthogonal to

each other and can be transmitted without interfering with each

other.

In QPSK the carrier phase can change only once every 2T

secs. If from one T interval to the next one, neither bit stream

changes sign, the carrier phase remains unchanged. If one

component aI(t) or ad(t) changes sign, a phase change of π /2

occurs. π occurs However if both components change sign then a

phase shift of π occurs.

Fig.2.2 : QPSK Modulator and it’s Constellation

2.2.2 Demodulation:

The QPSK Demodulator demodulates a quaternary phase shift

keyed (QPSK) signal.  The input is a baseband representation of

the modulated signal.

Decomposes the modulated input signal by using two fixed-

frequency driven multipliers with outputs are 90° apart, producing

values twice as fast a oscillator.  Two low-pass filters are also

utilized, along with a parallel-to-serial converter which translates

the dibits to bits, which are the PCM signal.

Alternative QPSK demodulator

Four modified carrier phases of 0°, 90°, 180°, and 270° (obtained

in the modulator by rotating the angles of the QPSK constellation)

are generated.  An XNOR operation is executed between each

phase and if both coincide, the modulated signal produces logic

"1".  If the received phase is 0° (related to dibit 00), all the XNORs

are at logic "0." If the phase is 90° (dibit 10), the upper branch is a

"1".  For 270° (dibit 01) an inversion is observed as logic "1" is

produced at the lower branch. In the case of the phase being 180°

(dibit 11), a logic "1" is observed on both

branches, resulting in a demodulated PCM signal.

A phase-references generator can produce inputs to the XNOR

gates.

This report talks about first method to demodulate the received

QPSK modulated signal. The modulated signal is received at

receiving end and then multiplied by two orthogonal carrier waves

of the frequency same as used in transmission end. The output of

the multiplier consists of frequencies twice as fast as carrier waves

having envelope of the original data bits sent. Thus the inside high

frequency is removed by using a low pass filter of desired cutoff

frequency, equal to maximum frequency at which data can come.

The two streams of data are then combined using a multiplexer.

CDMA:

Basically code division multiple access system are an extension of

direct sequence spread spectrum and frequency hopping spread

spectrum technique. In CDMA each user is assigned a unique code

sequence (spreading code). It is used to encode its information

bearing signal. The receiver, knowing the code sequence of the

user, decode e receive signal after reception and recovers the

original data. This possible since the cross correlation between the

code of the desire user and the code of the other users are small.

Since the bandwidth of the code signal is chosen to be much larger

than the bandwidth of the information bearing signal, the encoding

process spreads the spectrum of the signal and is therefore also

known as spread spectrum modulation. The resulting signal is also

called a spread spectrum signal and CDMA is often denoted as

spread spectrum multiple access.

A spread spectrum modulation technique must fulfill two criteria:

1. The transmission bandwidth must be larger than the

information bandwidth.

2. The resulting radio frequency bandwidth is determined by a

function other than the information being sent, thus this

excludes modulation techniques like FM and PM.

DESIGN SYNTHSIS:

The QPSK modulator and demodulator were first simulated on

matlab simulink. At the transmitter end there are two walsh code

generator producing the walsh codes from orthogonal set of codes

which are converted to bipolar NRZ type signal. The wash codes

are used in CDMA communication. The length of a walsh code is

divided into two codes of length n/2. All n/2 length codes,

generated from length n codes, are orthogonal to each other. Walsh

codes are the examples of orthogonal codes. Orthogonal codes are

the sets of sequences in which all pair-wise cross correlations are

zero. They are completely orthogonal for zero delay, and for others

they have very bad cross correlation properties. They are divided

into two types, first fixed length codes and second variable length

codes. The two walsh code generator divides the signal into two

separate bit streams of the odd numbered and the even numbered

bits. The symbol duration of both of these odd and even number

sequences is 2T. Each symbol duration consists of two bits. The

even bit stream modulates carrier cos(2πfct) and odd bit stream

modulates sin(2π f ct). These modulators are balanced modulator.

The two carriers are called quadrature carriers. If there is any phase

change, it occurs at minimum duration of T. thus, the phase shift in

QPSK is π /2. The two bit streams are then added in the adder and

then sent to gain for its amplification that ensures long distance

transmission. On the other

end is the receiver that carries out synchronous reception.

The two coherent quadrature carriers i.e. cos(2πfct) and sin(2π f ct)

along received signal are applied to both the multipliers. It is then

sent to analog filter for obtaining pass band signal and then fed into

the sign block for removing zero crossing detection and then

multiplexed using a switch. After that the reqired waveform can be

seen in the scope2.

After its successful simulation on simulink, system generator on

Xilinx ISE 10.01 was used to simulate and generate bit stream.

Walsh code odd an even bits (of double the bit of original data) are

multiplied by orthogonal carrier waves (generated by sine block of

simulink). Both are then added to give modulated wave. At the

receiving end the modulated wave is multiplied by two orthogonal

waves. The output is passed through low pass filter to remove the

twice the carrier frequency present in it. Finally the original bits

stream is recovered.

SIGNIFICANCE:

CDMA is far more convenient a communication system as compared to the

eminent GSM technology. In CDMA, owing to the use of orthogonal codes,

the vector summation of the entire set of the message signal to be

transmitted to the respective destination is sent together saving time by

many folds. CDMA has the advantage of universal frequency reuse.

Thereby, the same frequency can be used by all the user of the network and

like the GSM technology in which each user is allotted a distinct frequency.

Another advantage is soft hand off i.e. owing to the use of same frequency

by all user there is no change of hardware. Contrarily, GSM suffers with a

hard hand off that is there is a change of frequency as we move from one

cell to another making the change of hardware mandatory. CDMA has a

soft capacity, as the traffic increases the quality of signal can be

compromised with to accommodate the great number of users.

Features of CDMA

1.Universal frequency reuse

2.Fast and accurate power control

3.Diffrent type of handoff

Advantages of CDMA

1.It’s highly secure.

2.Call processing speed is hard.

3.CDMA can have 4 times the TDMA capacity and 20 times the

FDMA capacity per channel.

5.When the transmission rate is much higher than 10kb/s in both

FDMA and TDMA, an equalizer is required. CDMA only needs

correlator ,which is cheaper than equalizer.

6.Low power requirement.

7.Less fading is observed in the CDMA mobile technology.

8.Smaller phones.

9.Increased cellular communication security.

10.Simultaneous conversation.

11.Extended reach-beneficial to rural usre far from cells.

Disadvantages of CDMA

1.Due to its proprietary nature, all of CDMA's flaws are not known to the engineering community.

2.CDMA is relatively new, and the network is not as mature as GSM.

3.CDMA cannot offer international roaming, a large GSM advantage.

Simulation on MATLAB Simulink

The QPSK modulator and demodulator were first simulated on

MATLAB simulink. After its successful simulation on simulink,

SYSTEM GENERATOR of XILINX ISE 10.1 was used to

simulate and generate bit stream.

Walsh code odd and even bits (of double the bit period of original

data) are multiplied with orthogonal carrier waves (generated by

sine block of simulink) .Both are then added to give modulated

wave. At the receiving end, the modulated wave is multiplied by

two orthogonal waves, the output is passed through low pass filter

to remove the twice the carrier frequency present in it. Finally the

original bit stream is recorded.

OBJECTIVE & SCOPE OF PROJECT:

1. MODULATION:

It is a process of varying a periodic waveform. In order to use that signal to

convey a message. The periodic waveform is known as the carrier wave and

it is modulated according to the message waveform which ensures

minimum attenuation while transmission. If two or more bits are combined

in some symbols then the signalling rate will be reduced. Thus, the

frequency of the carrier needed is also reduced. Therefore, so is with the

channel transmission bandwidth. Thus, we are using QPSK modulation and

demodulation technique for the implementation of the CDMA in our

project.

We’ll be using software defined radio as our radio communication system

which can potentially tuned to any frequency band and receive any

modulation across a large frequency spectrum by means of as little

hardware as possible in processing signals through software.

2.Software Defined Radio

Today’s rapid technological advances make communication

devices become

obsolete shortly after they are produced. Hence, there is a need to

design a

communication system which is transparent to the technological

changes t

o be made for improved performance. Emphasis should be on

developing

systems wherein upgraded devices should still be able to

communicate with

each other as well as traditional systems. In the traditional systems,

tuning

was facilitated by hardware and hence used a limited number of

waveforms

because of which

were not able to communicate since they were incompatible with

each

other. Thus equipment designed to work according to one

particular

protocol will not work with another standard. Making them

compatible

requires making changes in hardware which is not cost effective.

The need

to communicate with people using different types of equipment

can only be

solved using software programmable radios because of its flexible

architecture.

The term software defined radio refers to reconfigurable or

reprogrammable radios that can show different functionality with

the same

hardware. Because the functionality is defined in software, a new

technology

can easily be implemented in a software radio with a software

upgrade.

A Software Defined Radio (SDR) is a radio communication system

which

can potentially tune to any frequency band and receive any

modulation

across a large frequency spectrum by means of as little hardware as

possible and processing the signals through software. In an SDR,

the

transmitter modulation is generated or defined by a computer. The

receiver

then also uses a computer to recover the signal intelligence. The

primary

interface, modulation and coding schemes, analog-to-digital

converter

(ADC), and digital-to-analog converter (DAC).goal of SDR is to

replace as

many analog components and hardwired digital VLSI devices of

the

transceiver as possible with programmable devices. These include

the air

Software radios have significant utility for the military and cell

phone

services, both of which must serve a wide variety of changing

radio

protocols in real time. On a larger scale, SDR is an enabling

technology

that is useful in a wide range of areas within wireless systems.

SDR is thus,

an emerging technology that spans all radio network topologies in

the

commercial, military and civil government sectors, and enables

highly

flexible solutions with benefits to operators, manufacturers and

consumers.

3. Benefits of SDR

SDR technologies provide software control of a variety of

modulation,

interference management and capacity enhancement techniques

over a

broad frequency spectrum (wide and narrow band), while ensuring

secure

communications management. Radios built using SDR concepts

offer:

-Standard architecture for a wide range of communications

products.

-Non-restrictive wireless roaming for consumers by extending the

capabilities of current and emerging commercial air-interface

standards.

-Uniform communication across commercial, civil, federal and

military organizations

-Flexibility and adaptability

-Potential for significant life-cycle cost reductions

-Over the air downloads of new features and services as well as

software patches

-Advanced networking capabilities to allow truly "portable"

networks

Worldwide interest and investment in the SDR technologies is

growing

significantly, with key standardization and development efforts

now taking

place throughout Europe, North America, Japan, Korea, and China.

Conclusion

Hence, the QPSK modulator and demodulator of CDMA

technology has been designed and implemented. Later, QPSK

modulator and demodulator form a major module of SDR and

XILINX system generator will be used to do the simulation and

generate the bit stream to be transferred to FPGA kit.

References

1.[Http://www.mathworks.com]

[Http://www.adapti.com/QPSK Demodulator.html]

3.[”Wireless Communication’’-RP YADAV, 2008, 3rd

edition, Ashirwaad Publications]

4.[’’Communication System (Analog and Digital)’’

SANJAY SHARMA, 2008, 5th edition, S.K.Kataria &

Sons]

5.[’’Wireless Networked Communications’’-BATES.R.J,

2009,Tata McGrawhill Publication]

Appendix

Software used for simulation of circuit:

1. Matlab Simulink