PERFORMANCE OF MULTICARRIER DS-CDMA SYSTEMS

A Project Work

ABSTRACT

In “Performance of multi-carrier DS CDMA Systems” we apply a multi-carrier

signaling technique to a direct-sequence CDMA system, where a data sequence

multiplied by a spreading sequence modulates multiple carriers, rather than a single

carrier. The receiver provides a correlator for each carrier, and the outputs of the

correlators are combined with a maximal-ratio combiner. This type of signaling has the

desirable properties of exhibiting a narrowband interference suppression effect, along

with robustness to fading, without requiring the use of either an explicit RAKE structure

or an interference suppression filter.

We use band limited spreading waveforms to prevent self-interference, and we evaluate

system performance over a frequency selective Rayleigh channel in the presence of

partial band interference. There is no interference from the CDMA signals to the existing

microwave systems. Thus, there is no need for either a narrowband suppression filter at

the receiver or a notch filter at the transmitter. Simulation results compare system

performance with that of a single-carrier RAKE system.

ii

CHAPTER 1

1.1 INTRODUCTION:

Direct sequence spread spectrum (DS-SS) techniques to multiple access communications.

This is partly due to its multiple access capability, robustness against fading, and anti-

interference capability.

In direct sequence spread spectrum, the stream of information to be transmitted is divided

into small pieces, each of which is allocated across to a frequency channel across the

spectrum. A data signal at the point of transmission is combined with a higher data-rate

bit sequence (also known as a chipping code) that divides the data according to a

spreading ratio. The redundant chipping code helps the signal resist interference and also

enables the original data to be recovered if data bits are damaged during transmission.

1.2 PROBLEM STATEMENT:

Direct sequence (DS) waveforms have a wide bandwidth, anytime that bandwidth

exceeds the coherence bandwidth of the channel, the fading tends to be frequency

selective, and a RAKE receiver can be used to enhance system performance. Anti-

interference capability in a DS system is achieved by correlating the received signal with

the predetermined spreading sequence, thus allowing the inherent processing gain of the

system to attenuate the interference. Further, an interference suppression filter can be

utilized to reject strong narrowband interference.

We need to design a system with less interference without using any external circuitry.

1.3 OBJECTIVE

• The main objective is to find the probability of Bit Error Rate (BER) for the

Multi-Carrier CDMA.

• Simulation Results shows BER is less for Multi Carrier as compared to single

Carrier CDMA

1.4 EXSISTED METHOD (SC-CDMA):

Single-carrier code division multiple access (SC-CDMA), also named cyclic-prefix

CDMA in the literature, is a promising air interface for the uplink of the 4G cellular

wireless communication systems. It enables the high capacity intrinsically offered by

CDMA by making the equalization of the multipath channels and the mitigation of the

resulting interference possible at a low complexity. SC-CDMA technique has flexibility

for providing variable rate transmissions, yet retaining multiple access capability.

In singlecarier DS SS system a data sequence multiplied by a spreading sequence

modulates a single carrier. The receiver provides a correlator by which original data

sequence can be extracted. This type of system has the following advantages: the fading

tends to be frequency selective, and a RAKE receiver can be used to enhance system

performance. An interference suppression filter can be utilized to reject strong

narrowband interference.

In single carrier DS SS system multipath interference is because the delayed versions of

the transmitted pseudorandom codes will have poor correlation with the original

pseudorandom code, and will thus appear as another user, which is ignored at the

receiver. In other words, as long as the multipath channel induces at least one chip of

delay, the multipath signals will arrive at the receiver such that they are shifted in time by

at least one chip from the intended signal. The correlation properties of the

pseudorandom codes are such that this slight delay causes the multipath to appear

uncorrelated with the intended signal, and it is thus ignored.

Hence this system uses a rake receiver which is a radio receiver designed to counter the

effects of multipath fading. It does this by using several "sub-receivers" called fingers,

that is, several correlators each assigned to a different multipath component. Each finger

independently decodes a single multipath component; at a later stage the contribution of

all fingers are combined in order to make the most use of the different transmission

characteristics of each transmission path i.e., rake receiver combines the information

from several correlators, each one tuned to a different path delay, producing a stronger

version of the signal than a simple receiver with a single correlator tuned to the path

delay of the strongest signal.

The multipath channel through which a radio wave transmits can be viewed as

transmitting the original (line of sight) wave plus a number of multipath components.

Multipath components are delayed copies of the original transmitted wave traveling

through a different echo path, each with a different magnitude and time-of-arrival at the

receiver. Since each component contains the original information, if the magnitude and

time-of-arrival (phase) of each component is computed at the receiver (through a process

called channel estimation), then all the components can be added coherently to improve

the information reliability.

But on the other hand, spreading codes that give perfect isolation from other users at the

output of a correlation receiver are called orthogonal codes, and a number of such code

sets exist. The problem in practice is that orthogonality between codes is impossible to

maintain at the receiver, because of asynchronism and channel delay spread. Both effects

are present on the uplink (or reverse link), while only the latter is seen on the downlink of

a wireless channel. The correlation receiver, which in a multipath channel becomes the

celebrated Rake receiver, cannot then perfectly separate the signals for the multiple users.

The resulting multiaccess interference (MAI) causes a severe degradation in

performance, and may render the system useless for even moderate user loads with equal

power received from each user. The need for MAI suppression is therefore clear. Hence

we go for MC-CDMA.

1.5 PROPOSED METHOD (MC-CDMA):

Multi-Carrier Code Division Multiple Access (MC-CDMA) is a multiple access

scheme used in OFDM-based telecommunication systems, allowing the system to support

multiple users at the same time.

MC-CDMA spreads each user symbol in the frequency domain. That is, each user

symbol is carried over multiple parallel subcarriers, but it is phase shifted (typically 0 or

180 degrees) according to a code value. The code values differ per subcarrier and per

user. The receiver combines all subcarrier signals, by weighing these to compensate

varying signal strengths and undo the code shift. The receiver can separate signals of

different users, because these have different (e.g. orthogonal) code values.

Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in

bit/s), a signal-to-noise-plus-interference ratio (if defined as signal power divided by total

noise plus interference power in the entire transmission band) of less than 0 dB is

feasible.

For the case of MC-DS-CDMA where OFDM is used as the modulation scheme, the data

symbols on the individual subcarriers are spread in time by multiplying the chips on a PN

code by the data symbol on the subcarrier. By using a multicarrier DS system we can

reduce multiaccess interference MAI because here the entire bandwidth of the system is

divided into M (not necessarily contiguous) equi-width frequency bands, and thus each

carrier frequency is modulated by a spreading sequence with a chip duration which is M

times as long as that of a single-carrier system. Hence in this system the codes need not

be orthogonal to each other.

This type of system has the following advantages:

Multi-carrier DS SS system is robust to multipath fading.

Multi-carrier system has a narrowband interference suppression effect and finally,

a lower chip rate is required.

1.6 THESIS ORGANIZATION:

First chapter described the problem statement and how the proposed method

advantageous over the previous methods. Chapter 2 giving background information for

all the keywords which we have described in proposed method. Chapter 3 gives the

detail description for the proposed method. Chapter 4 is about communication tools in

MATLAB which we have used in this project .Chapter 5 showing simulation results and

Chapter 6 giving conclusion and future scope

CHAPTER 2

MULTIPLE ACCESS TECHNIQUES

One of the basic concepts in data communication is the idea of allowing several

transmitters to send information simultaneously over a single communication channel.

This allows several users to share a bandwidth of frequencies. This concept is called

multiplexing. There are three basic multiplexing techniques,

They are:

1. Frequency division multiple access (FDMA).

2. Time division multiple access (TDMA).

3. Code division multiple access (CDMA).

2.1 MULTIPLEXING TECHNIQUES:

2.1.1 FDMA:

Frequency Division Multiple Access or FDMA is a channel access method used in

multiple-access protocols as a channelization protocol. FDMA gives users an individual

allocation of one or several frequency bands, allowing them to utilize the allocated radio

spectrum without interfering with each other. Multiple Access systems coordinate access

between multiple users.

FDMA requires high-performing filters in the radio hardware, in contrast to

TDMA and CDMA.

FDMA is not vulnerable to timing problems as TDMA. Since a predetermined

frequency band is available for the entire period of communication, stream data (a

continuous flow of data that may not be packetized) can easily be used with

FDMA.

Due to the frequency filtering, FDMA is not sensitive to near-far problem which

is pronounced for CDMA.

It is important to distinguish between FDMA and frequency-division duplexing (FDD).

While FDMA allows multiple users simultaneous access a certain system, FDD refers to

how the radio channel is shared between the uplink and downlink (for instance, the traffic

going back and forth between a mobile-phone and a base-station). Furthermore,

frequency-division multiplexing (FDM) should not be confused with FDMA. The former

is a physical layer technique that combines and transmits low-bandwidth channels

through a high-bandwidth channel. FDMA, on the other hand, is an access method in the

data link layer.

FDMA also supports demand assignment in addition to fixed assignment. Demand

assignment allows all users apparently continuous access of the radio spectrum by

assigning carrier frequencies on a temporary basis using a statistical assignment process.

2.1.2 TDMA:

Time division multiple access (TDMA) is a channel access method for shared medium

networks. It allows several users to share the same frequency channel by dividing the

signal into different time slots. The users transmit in rapid succession, one after the other,

each using his own time slot. This allows multiple stations to share the same transmission

medium (e.g. radio frequency channel) while using only a part of its channel capacity.

TDMA is used in the digital 2G cellular systems such as Global System for Mobile

Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the

Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It

is also used extensively in satellite systems, and combat-net radio systems. TDMA frame

structure showing a data stream divided into frames and those frames divided into time

slots.

TDMA is a type of Time-division multiplexing, with the special point that instead of

having one transmitter connected to one receiver, there are multiple transmitters. In the

case of the uplink from a mobile phone to a base station this becomes particularly

difficult because the mobile phone can move around and vary the timing advance

required to make its transmission match the gap in transmission from its peers.

2.1.3 CDMA:

Code division multiple access (CDMA) is a form of multiplexing, which allows

numerous signals to occupy a single transmission channel, optimizing the use of available

bandwidth i.e., in CDMA both data and voice are separated from signals using codes and

then transmitted using a wide frequency range. Because of this, there are more space left

for data transfer and this was one of the major reasons why CDMA is the preferred

technology for the 3G generation, which is broadband access and the use of big

multimedia messages. 14% of the worldwide market goes to CDMA. It is a channel

access method utilized by various radio communication technologies. The technology is

used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-

GHz bands.

Code division multiple access (CDMA) has advantages over traditional multiple access

techniques such as time division multiple access (TDMA), or frequency division multiple

access (FDMA), in that it does not require coordination of the users in time. This has

recently been exposed as an apparent advantage as a technique for use in a cellular

context because it is an efficient technique to achieve spatial re-use.

These multiple access schemes can be shown as follows:

Fig 2.1: Multiple Access Schemes

As shown in fig 2.1, FDMA puts each call on a separate frequency and TDMA assigns

each call a certain portion of time on a designated frequency for the above mentioned

purpose. But CDMA gives a unique code to each call and spreads it over the available

frequencies i.e., CDMA employs spread-spectrum technology and a special coding

scheme (where each transmitter is assigned a code) to allow multiple users to be

multiplexed over the same physical channel. Thus CDMA is a form of "spread-spectrum"

signaling, since the modulated coded signal has a much higher data bandwidth than the

data being communicated. All these are multiple access systems because more than one

user can use the specified CELL (not cell phone!) at a time.

An analogy to the problem of multiple access is a room (channel) in which people wish to

communicate with each other. To avoid confusion, people could take turns speaking

(time division), speak at different pitches (frequency division), or speak in different

languages (code division). CDMA is analogous to the last example where people

speaking the same language can understand each other, but not other people. Similarly, in

radio CDMA, each group of users is given a shared code. Many codes occupy the same

channel, but only users associated with a particular code can understand each other.

Advantages of CDMA:

Higher bandwidth.

Greater level of privacy and security to the communication.

Lesser radiation while using cell phones.

Due to these advantages there is high probability that CDMA technology will dominate

the future of mobile communications. It is expected that in future CDMA will become

superior and all the major cell operators will have a migration to CDMA.

2.2 SPREAD SPECTRUM TECHNIQUES:

Spread spectrum systems are becoming more and more popular, especially as the

available frequency spectrum is becoming more and more crammed. Although these

systems are extremely complex and expensive, they have numerous advantages over

conventional systems. The major advantage of these systems in the signal security that

the systems present. Unlike conventional systems it is extremely difficult to eavesdrop on

a conversation that takes place over a spread spectrum system.

Spread spectrum systems spread the signal over as wide bandwidth as possible. Also they

try to hide the transmitted signal as close to the background noise as possible. This makes

the communication very difficult to find in the frequency spectrum and cannot be easily

tracked and more difficult to jam. There are three different types of spread spectrum

techniques that can be used. These are:

Frequency hopping

Time hopping

Direct sequence

2.2.1 FREQUENCY HOPPING:

In frequency hopping CDMA, the carrier frequency of the modulated information signal

is not constant but changes periodically. During time intervals T the carrier frequency

remains the same, but after each time interval the carrier hops to another (or possibly the

same) frequency. The hopping pattern is decided by the code signal. The set of available

frequencies the carrier can attain is called the hop-set.

The frequency occupation of an FH-SS system differs considerably from a DS-SS

system. A DS system occupies the whole frequency band when it transmits, whereas an

FH system uses only a small part of the bandwidth when it transmits, but the location of

this part differs in time.

Within frequency hopping CDMA a distinction is made that is based on the hopping rate

of the carrier. If the hopping rate is (much) greater than the symbol rate, one speaks of a

fast frequency hopping (F-FH). In this case the carrier frequency changes a number of

times during the transmission of one symbol, so that one bit is transmitted in different

frequencies. If the hopping rate is (much) smaller than the symbol rate, one speaks of

slow frequency hopping (S-FH). In this case multiple symbols are transmitted at the same

frequency.

The occupied bandwidth of the signal on one of the hopping frequencies depends not

only on the bandwidth of the information signal but also on the shape of the hopping

signal and the hopping frequency. If the hopping frequency is much smaller than the

information bandwidth (which is the case in slow frequency hopping), then the

information bandwidth is the main factor that decides the occupied bandwidth. If,

however, the hopping frequency is much greater than the information bandwidth, the

pulse shape of the hopping signal will decide the occupied bandwidth at one hopping

frequency. If this pulse shape is very abrupt (resulting in very abrupt frequency changes),

the frequency band will be very broad, limiting the number of hop frequencies. If we

make sure that the frequency changes are smooth, the frequency band at each hopping

frequency will be about 1/Th times the frequency bandwidth, where Th is equal to the

hopping frequency. We can make the frequency changes smooth by decreasing the

transmitted power before a frequency hop and increasing it again when the hopping

frequency has changed.

2.2.2 TIME HOPPING:

In time hopping CDMA the data signal is transmitted in rapid bursts at time intervals

determined by the code assigned to the user. The time axis is divided into frames, and

each frame is divided into M time slots. During each frame the user will transmit in one

of the M time slots. Which of the M time slots is transmitted depends on the code signal

assigned to the user. Since a user transmits all of its data in one, instead of M time slots,

the frequency it needs for its transmission has increased by a factor M.

The multiple access capability of TH-SS signals is acquired in the same manner as that of

the FH-SS signals; namely, by making the probability of users' transmissions in the same

frequency band at the same time small. In the case of time hopping all transmissions are

in the same frequency band, so the probability of more than one transmission at the same

time must be small. This is again achieved by assigning different codes to different users.

If multiple transmissions do occur, error-correcting codes ensure that the desired signal

can still be recovered. If there is synchronization among the users, and the assigned codes

are such that no more than one user transmits at a particular slot, then the TH-CDMA

reduces to a TDMA scheme where the slot in which a user transmits is not fixed but

changes from frame to frame.

Multipath interference:

In the time hopping CDMA, a signal is transmitted in reduced time. The signaling rate,

therefore, increases and dispersion of the signal will now lead to overlap of adjacent bits.

Therefore, no advantage is to be gained with respect to multipath interference rejection.

Narrowband interference:

A TH-CDMA signal is transmitted in reduced time. This reduction is equal to 1/Gp,

where Gp is the processing gain. At the receiver we will only receive an interfering signal

during the reception of the desired signal. Thus, we only receive the interfering signal

1/Gp percent of the time, reducing the interfering power by a factor Gp.

2.2.3 DIRECT SEQUENCE:Direct sequence spread spectrum, also known as direct sequence code division multiple

access (DS-CDMA), and is one of two approaches to spread spectrum modulation for

digital signal transmission over the airwaves.

Direct sequence contrasts with the other spread spectrum process, known as frequency

hopping spread spectrum, or frequency hopping code division multiple access (FH-

CDMA), in which a broad slice of the bandwidth spectrum is divided into many possible

broadcast frequencies. In general, frequency-hopping devices use less power and are

cheaper, but the performance of DS-CDMA systems is usually better and more reliable.

Spread spectrum first was developed for use by the military because it uses wideband

signals that are difficult to detect and that resist attempts at jamming. In recent years,

researchers have turned their attention to applying spread spectrum processes for

commercial purposes, especially in local area wireless networks.

Signal transmission consists of the following steps:

1. A pseudo-random code is generated, different for each channel and each successive connection.

2. The Information data modulates the pseudo-random code (the Information data is “spread”).

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and broadcast.

Signal reception consists of the following steps:

1. The carrier is received and amplified.

2. The received signal is mixed with a local carrier to recover the spread digital signal.

3. A pseudo-random code is generated, matching the anticipated signal.

4. The receiver acquires the received code and phase locks its own code to it.

5. The received signal is correlated with the generated code, extracting the Information data.

Generating Pseudo-Random Codes:

For each channel the base station generates a unique code that changes for every

connection. The base station adds together all the coded transmissions for every

subscriber. The subscriber unit correctly generates its own matching code and uses it to

extract the appropriate signals. Note that each subscriber uses several independent

channels.

In order for all this to occur, the pseudo-random code must have the following properties:

1. It must be deterministic. The subscriber station must be able to independently generate the code that matches the base station code.

2. It must appear random to a listener without prior knowledge of the code (i.e. it has the statistical properties of sampled white noise).

3. The cross-correlation between any two codes must be small (see below for more information on code correlation).

4. The code must have a long period (i.e. a long time before the code repeats itself).

Fig 2.2(a): Pseudo-noise spreading

Here the duration of an element in the code is called the "chip time". The ratio between

the user symbol time and the chip time is called the spread factor or processing gain. This

is a theoretical system gain that reflects the relative advantage that frequency spreading

provides.

Code Correlation:

In this context, correlation has a specific mathematical meaning. In general the

correlation function has these properties:

It equals 1 if the two codes are identical

It equals 0 of the two codes have nothing in common

Intermediate values indicate how much the codes have in common. The more they have

in common, the harder it is for the receiver to extract the appropriate signal.

There are two correlation functions:

Cross-Correlation: The correlation of two different codes. As we’ve said, this

should be as small as possible.

Auto-Correlation: The correlation of a code with a time-delayed version of itself.

In order to reject multi-path interference, this function should equal 0 for any time

delay other than zero.

The receiver uses cross-correlation to separate the appropriate signal from signals meant

for other receivers, and auto-correlation to reject multi-path interference.

Now, in Direct Sequence spread spectrum transmission, the user data signal is multiplied

by a code sequence. Mostly, binary sequences are used. This noise signal is a

pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the

original signal, thereby spreading the energy of the original signal into a much wider

band.

Fig 2.2(b): Frequency-Spreading

The transmit signal occupies a bandwidth that equals the spread factor times the

bandwidth of the user data.

In the receiver, the received signal is again multiplied by the same (synchronized) code.

This operation removes the code, so we recover the transmitted user data.

Different CDMA users use different codes. In this example the receiver sees the signal

from user 1, while the signal from user 2 is heavily attenuated by the correlator

(multiplier and integrator) in the receiver

CHAPTER 3

LIMITATIONS OF NOISE

3.1 INTERFERENCE IN CDMA:

The colored interference model used here is more suitable than the white noise model

when jamming signals and/or co-channel interference (CCI) are present in the wireless

communication system.

3.1.1 CO-CHANNEL INTERFERENCE (CCI):

In frequency reuse two co-channels use same set of frequencies. In this case a mobile unit

in one cell may receive a weak signal from another co-channel cell. This kind of

interference is called co-channel interference. A minimum amount of distance must be

maintained between co-channels in order to reduce the co-channel interference.

Fig 3.1: Co-channel interference at the mobile unit

We can express the interfering signal transmitted from the ith transmit antenna of the mth

interferer as

Where Pm is the transmit power of the mth interferer, and are data symbols

transmitted from the ith transmit antenna of the mth interferer. The data symbols are

assumed to be i.i.d. binary random variables with zero mean and unit variance. In

addition, is the symbol waveform and T is the symbol duration. It is assumed that

the receiver is synchronized to the desired user but not necessarily to the interfering

signals and is the symbol timing difference between the mth interferer and the

desired user signal. Without loss of generality, we assume 0 < <T. The elements of the

interference symbol matrix Si are samples at the matched filter output at the receiver at

time index jT . The (j,i)th element of Si is

Where

is the autocorrelation of the symbol waveform. For the CCI, the temporal interference

correlation is due to the intersymbol interference in the sampled interfering signals.

Jamming signals:

We assume that there are two jammers, each with three transmit antennas, in the system.

The jamming signals are modeled as first-order AR processes driven by temporally white

Gaussian processes

where represents the jamming signal transmitted from the ith transmit antenna of

the mth jammer at the ith time index, is the temporal correlation coefficient,

and has zero mean with variance , which decides the transmit power of the mth

jammer. The signal-to-interference ratio is set to be 3 dB. The total channel estimation

MSEs for the high spatial correlation channel and low spatial correlation channel,

respectively. For the AR jammers, similar conclusions on the estimation performance

achieved by different training sequences can be made as in the case of CCI.

3.1.2 MULTIPLE ACCESS INTERFERENCE (MAI):

Direct-sequence spread spectrum (DS-SS) will be used in most implementations of the

third generation of cellular mobile communication systems (popularly known as 3G) for

fixed-assignment multiple access. This means that a central controller (or base station) is

responsible for the allocation of channel resources to all users who want to communicate

at the same time within its physical domain.

Although nowadays CDMA is commonly linked with 3G because of the latter's

increasing public visibility, its applications are certainly not limited to this. For instance,

wireless local area networks (WLANs) and wireless local loops (WLLs) can also make

use of this technology and are already starting to do so. This makes an understanding of

the strengths and limitations of CDMA important to the engineer. One of those

limitations is the existence of multi-access interference (MAI).

The multi-user systems have the interesting property that their capacity is typically

limited by multiple-access interference (MAI) or multi-user interference (MUI), rather

than noise. Two quite different, suboptimal approaches for multi-user detection (MUD)

emerged, which had much lower complexity than the optimum multi-user detector:

Interference cancellation (IC).

Adaptive filtering.

3.2 TYPES OF FADING CHANNELS:

Fading is the phenomena of reduction in the signal power means of reflection refraction

and diffraction.

The terms slow and fast fading refer to the rate at which the magnitude and phase change

imposed by the channel on the signal changes. The coherence time is a measure of the

minimum time required for the magnitude change of the channel to become decorrelated

from its previous value.

3.2.1 SLOW FADING:

When the coherence time of the channel is large relative to the delay constraint of the

channel. In this regime, the amplitude and phase change imposed by the channel can be

considered roughly constant over the period of use. Slow fading can be caused by events

such as shadowing, where a large obstruction such as a hill or large building obscures the

main signal path between the transmitter and the receiver. The amplitude change caused

by shadowing is often modeled using a log-normal distribution with a standard deviation

according to the log-distance path loss model.

3.2.2 FAST FADING:

Fast fading occurs when the coherence time of the channel is small relative to the delay

constraint of the channel. In this regime, the amplitude and phase change imposed by the

channel varies considerably over the period of use.

In a fast-fading channel, the transmitter may take advantage of the variations in the

channel conditions using time diversity to help increase robustness of the communication

to a temporary deep fade. Although a deep fade may temporarily erase some of the

information transmitted, use of an error-correcting code coupled with successfully

transmitted bits during other time instances (interleaving) can allow for the erased bits to

be recovered. In a slow-fading channel, it is not possible to use time diversity because the

transmitter sees only a single realization of the channel within its delay constraint. A deep

fade therefore lasts the entire duration of transmission and cannot be mitigated using

coding.

Fading models:

Examples of fading models for the distribution of the attenuation are:

Nakagami fading

Weibull fading

Rayleigh fading

Rician fading

Log-normal shadow fading

3.3 RAKE RECEIVER:

Multipath channel:

Due to reflections from obstacles a radio channel can consist of many copies of originally

transmitted signals having different amplitudes, phases, and delays Multipath can occur

in radio channel in various ways Reflection, diffraction, scattering The RAKE receiver

uses a multipath diversity principle – It rakes the energy from the multipath propagated

signal components

Fig 3.3 (a): Multipath propagation of signal

A rake receiver is a radio receiver designed to counter the effects of multipath fading. It

does this by using several "sub-receivers" each delayed slightly in order to tune in to the

individual multipath components. Each component is decoded independently, but at a

later stage combined in order to make the most use of the different transmission

characteristics of each transmission path. This could very well result in higher SNR (or

Eb/No) in a multipath environment than in a "clean" environment.

RAKE Receiver Requirements:

RAKE receiver has to know

Multipath delays: Time delay synchronization

Phases of the multipath components: carrier phase synchronization

Amplitudes of the multipath components: Amplitude tracking

Number of multi-path components: RAKE allocation Time delay synchronization

is based on correlation measurements

Delay acquisition

Delay tracking by feedback loops (delay-locked loops, DLL)

Due to fading channels conventional phase-locked loop (PLL) cannot be used in carrier

and amplitude tracking Number of available fingers depends on the channel profile and

the chip rate. The main challenges for RAKE receivers operating in fading channels are

in receiver synchronization de called Short Code and then with one called Long code.

Fig 3.3(b): Structure of rake receiver

3.4 PROPERTIES OF SPREADING CODES:

Multiplication with the code sequence which is of a higher bit rate, results in a much

wider spectrum. The ratio of the code rate to the information bit rate is called both the

spreading factor and the processing gain of the CDMA system. In IS-95, the chipping

rate is 1.2288 and the spreading factor is 64. Processing gain is usually given in dBs. To

distinguish the information bit rate from the code rate, we call the code rate, chipping

rate. In effect, we take each data bit and convert it into k chips, which is the code

sequence. We call it the chipping rate because the code sequence applied to each bit is as

you can imagine it chipping the original bit into many smaller bits. For CDMA spreading

code, we need a random sequence that passes certain “quality” criterion for randomness.

These criterions are:

1. The number of runs of 0’s and 1’s is equal. We want equal number of two 0’s and 1’s,

a length of three 0’s and 1’s and four 0’s and 1’s etc. This property gives us a perfectly

random sequence.

2. There are equal number of runs of 0’s and 1’s. This ensures that the sequence is

balanced.

3. The periodic autocorrelation function (ACF) is nearly two valued with peaks at 0 shift

and is zero elsewhere. This allows us to encrypt the signal effectively and using the ACF

peak to demodulate quickly. Binary sequences that can meet these properties are called

optimal binary sequences, or pseudo-random sequences. There are many classes of

sequences that mostly meet these requirements, with m-sequences the only ones that meet

all three requirements strictly.

These sequences can be created using a shift-register with feedback-taps. By using a

single shift-register, maximum length sequences can be created and called often by their

shorter name of m-sequence, where m stands for maximum.

m-sequences and the Linear Feed Shift-Register:

3 stage LFSR generating m-sequence of period 7, using taps 1 and 3.

Another 3 stage LFSR generating m-sequence of period 7, using taps 2 and 3

Fig 3.4(a): The structure of linear feedback registers (LFSR) from which

m-sequences can be created

m–sequences are created using linear feedback registers (LFSR). Figure shows a three

register LFSR with two different tap connection arrangements. The tap connections are

based on primitive polynomials on the order of the number of registers and unless the

polynomial is irreducible, the sequence will not be a m-sequence and will not have the

desired properties.

Each configuration of N registers produces one sequence of length 2N. If taps are

changed, a new sequence is produced of the same length. There are only a limited

number of m-sequences of a particular size.

The cross correlation between an m-sequences and noise is low which is very useful in

filtering out noise at the receiver. The cross correlation between any two different m

sequences is also low and is useful in providing both encryption and spreading. The low

amount of cross-correlation is used by the receiver to discriminate among user signals

generated by different m-sequences. Think of m-sequence as a code applied to each

message. Each letter (bit) of the message is changed by the code sequence. The spreading

quality of the sequence is an added dimensionality and benefit in CDMA systems.

3.4.1 GOLD SEQUENCES:

Combining two m-sequences creates Gold codes. These codes are used in asynchronous

CDMA systems Gold sequences are an important class of sequences that allow

construction of long sequences with three valued Auto Correlation Function ACFs. Gold

sequences are constructed from pairs of preferred m-sequences by modulo-2 addition of

two maximal sequences of the same length. Gold sequences are in useful in non-

orthogonal CDMA. (CDMA 2000 is mostly an orthogonal CDMA system) Gold

sequences have only three cross-correlation peaks, which tend to get less important as the

length of the code increases. They also have a single auto-correlation peak at zero, just

like ordinary PN sequences. The use of Gold sequences permits the transmission to be

asynchronous. The receiver can synchronize using the auto-correlation property of the

Gold sequence.

Fig 3.4(b): Generating Gold codes by combining two preferred pairs of m-sequences

More codes:

IS-95 and IS-2000 use two particular codes that are really m-sequences but have special

names and uses. These are called long codes and short codes.

3.4.2 LONG CODE:

The Long Codes are 242 bits (created from a LFSR of 42 registers) long and run at

1.2288 Mb/s. The time it takes to recycle this length of code at this speed is 41.2 days. It

is used to both spread the signal and to encrypt it. A cyclically shifted version of the long

code is generated by the cell phone during call setup. The shift is called the Long Code

Mask and is unique to each phone call. CDMA networks have a security protocol called

CAVE that requires a 64-bit authentication key, called A-key and the unique ESN

(Electronic Serial Number, assigned to mobile based on the phone number). The network

uses both of these to create a random number that is then used to create a mask for the

long code used to encrypt and spread each phone call. This number, the long code mask

is not fixed but changes each time a connection is created.

There is a Public long code and a Private long code. The Public long code is used by the

mobile to communicate with the base during the call setup phase. The private long code

is one generated for each call then abandoned after the call is completed.

3.4.3 SHORT CODE:

The short code used in CDMA system is based on a m-sequence (created from a LFSR of

15 registers) of length 215 – 1 = 32,767 codes. These codes are used for synchronization

in the forward and reverse links and for cell/base station identification in the forward link

the short code repeats every 26.666 milliseconds. The sequences repeat exactly 75 times

in every 2 seconds. We want this sequence to be fairly short because during call setup,

the mobile is looking for a short code and needs to be able find it fairly quickly. Two

seconds is the maximum time that a mobile will need to find a base station, if one is

present because in 2 seconds the mobile has checked each of the allowed base stations in

its database against the network signal it is receiving. Each base station is assigned one of

these codes. Since short code is only one sequence, how do we assign it to all the stations

we cyclically shift it. Each station gets the same sequence but it is shifted.

From properties of the m-sequences, the shifted version of a m-sequences has a very

small cross correlation and so each shifted code is an independent code. For CDMA this

shift is 512 chips for each adjacent station. Different cells and cell sectors all use the

same short code, but use different phases or shifts, which is how the mobile

differentiates one base station from another. The phase shift is known as the PN Offset.

The moment when the Short code wraps around and begins again is called a PN Roll. If I

call the word “please” a short code, then I can assign, “leasep” to one user, “easepl” to

another and so on. The shift by one letter would be my PN Offset. So if I say your ID is

3, then you would use the code “aseple”. A mobile is assigned a short code PN offset by

the base station to which it is transmitting. The mobile adds the short code at the

specified PN offset to its traffic message, so that the base station in the region knows that

the particular message is meant for it and not to the adjacent base station. This is

essentially the way the primary base station is identified in a phone call. The base station

maintains a list of nearby base stations and during handoff; the mobile is notified of the

change in the short code. There are actually two short codes per base station. One for

each I and Q channels to be used in the quadrature spreading and despreading of CDMA

signals.

CHAPTER 4

PERFORMANCE ANALYSIS

In this project, we propose a multicarrier DS SS system in which a data sequence

multiplied by a spreading sequence modulates M carriers, rather than a single carrier. The

receiver provides a correlator for each carrier, and the outputs of the correlators are

combined with a maximal-ratio combiner. This type of system has the following

advantages: First, a multicarrier DS SS system is robust to multipath fading; second, a

multicarrier system has a narrowband interference suppression effect; and finally, a lower

chip rate is required, since, in a multicarrier DS system with M carriers, the entire

bandwidth of the system is divided into M (not necessarily contiguous) equi-width

frequency bands, and thus each carrier frequency is modulated by a spreading sequence

with a chip duration which is M times as long as that of a single-carrier system. In other

words, a multicarrier system requires a lower speed, parallel-type of signal processing, in

contrast to a fast, serial-type of signal processing in a singlecarrier RAKE receiver. This,

in turn, might be helpful for use with a low power consumption device.

In fact, multicarrier DS systems have already been proposed, and these proposed

techniques can be categorized into two types, a combination of orthogonal frequency

division multiplexing (OFDM) and CDMA or a parallel transmission scheme of

narrowband DS waveforms in the frequency domain. In the former system, a spreading

sequence is serial-to-parallel converted, and each chip modulates a different carrier

frequency. This implies that the number of carriers should be equal to the processing

gain, and each carrier conveys a narrowband waveform, rather than a DS waveform. In

other words, the resulting signal has a PN coded structure in the frequency domain. In the

latter system, the available frequency spectrum is divided into M equi-width frequency

bands, where M is the number of carriers, typically much less than the processing gain,

and each frequency band is used to transmit a narrowband DS waveform. In fact, both

systems show a similar fading mitigation effect over a frequency selective channel.

However, the latter system requires only M adaptive gain amplifiers in the maximal ratio

combiner, which may simplify the receiver. The system described in this project belongs

to the second group.

In a multicarrier system, carrier frequencies are usually chosen to be orthogonal to each

other, i.e., carrier frequencies satisfy the following condition:

Where is the chip duration, and are, respectively, the ith and jth carrier

frequencies, and and are arbitrary carrier phases, respectively. This is done so that a

signal in the jth frequency band does not cause interference in the correlation receiver for

the ith frequency band. However, in an asynchronous CDMA system, signals from other

users are no longer orthogonal to the desired signal, even if (1) is satisfied. In addition,

orthogonality might be lost because of multipath propagation or Doppler frequency shift

even for a synchronous system. This implies that co-channel interference in one

frequency band causes interference not only at the output of the correlator for that

frequency band, but also in the signals out of all the other correlators. In this paper, we

use bandlimited multicarrier DS waveforms to minimize such unnecessary self-

interference, and so orthogonality among carriers is not required. Also, this signaling

scheme prevents narrowband waveforms from causing interference to all frequency

bands.

4.1 SYSTEM OVERVIEW:

In recent years, several wideband CDMA systems have been proposed either to realize an

overlay system, where DS CDMA waveforms are overlaid onto existing narrowband

signals to enhance the overall capacity, or to combat multipath. A multicarrier system can

be considered as one realization of such a wideband DS system.

PERFORMANCE OF MULTICARRIER DS CDMA SYSTEMS:

Fig 4.1(a): psd of a single-carrier

Fig 4.1(b): psd a multicarrier DS waveform

Fig 3.l(a) shows a bandlimited single-carrier wideband DS waveform in the frequency

domain, where the bandwidth, BW1, is given by

In (2), and T, is the chip duration of the singlecarrier system. In a multicarrier

system, we divide BW1 into M equi-width frequency bands as shown in Fig. l(b), where

all bands are disjoint. Then the bandwidth of each frequency band, BWM, is given by

4.1.1 TRANSMITTER:

The transmitter has input a random binary sequence representing data, and pseudo-

random spreading signature sequences are given to multiplier. We assume that there are

N chips per symbol, and that each user has a different signature sequence. The sequence

modulates an impulse train, where the energy per chip is E,. After passing through a chip

wave-shaping filter, the signal out of the filter modulates the multiple carrier signals and

is transmitted. Note that the transmitter and receiver block diagrams for the proposed

multicarrier system can be effectively implemented by using a DFT technique.

Fig 4.1(c): Block diagram of transmitter

Pseudo-Noise Sequences:

PN sequences are periodic sequences that have a noise like behavior. They are generated

using shift registers, modulo-2 adders (XOR gates) and feedback loops.

So far we haven't discussed what properties we would want the spreading signal to have.

This depends on the type of system we want to implement. Let's first consider a system

where we want to use spread spectrum to avoid jamming or narrow band interference.

If we want the signal to overcome narrow band interference, the spreading function needs

to behave like noise. Random binary sequences are such functions. They have the

following important properties:

Balanced: they have an equal number of 1's and 0's

Single Peak auto-correlation function

Carrier 1

Carrier n

Impulse modulatori/p Binary

Sequence

P.N sequence

Wave shaping

filter

In fact, the auto-correlation function of a random binary sequence is a triangular

waveform as in the following figure, where TC is the period of one chip:

Fig 4.1(d): Auto-correlation function

Hence the spectral density of such a waveform is a sinc function squared, with first zeros

at ± 1/TC

PN sequences are periodic sequences that have a noise like behavior. They are generated

using shift registers, modulo-2 adders (XOR gates) and feedback loops. The following

diagram illustrates this:

Fig 4.1(e): Generation of PN sequences

The maximum length of a PN sequence is determined by the length of the register and the

configuration of the feedback network. An N bits register can take up to 2N different

combinations of zeros and ones. Since the feedback network performs linear operations,

if all the inputs (i.e. the content of the flip-flops) are zero, the output of the feedback

network will also be zero. Therefore, the all zero combination will always give zero

output for all subsequent clock cycles, so we do not include it in the sequence. Thus, the

maximum length of any PN sequence is 2N-1 and sequences of that length are called

Maximum-Length Sequences or m-sequences.

Impulse Modulator: It is used to modulate the impulse train coming from the multiplier,

where the energy per chip is E.

Wave shaping filter: This filter is used to modify the shape of the waveform i.e., it is used

to adjust the energy levels of the waveform before the sequence modulates the multiple

carriers.

4.1.2 CHANNEL:

The channel is assumed to be a slowly varying frequency selective Rayleigh channel with

delay spread of Tm In order to facilitate a performance comparison between a single-

carrier RAKE system and the proposed multicarrier system, we define two channel

models; one is a time domain model for a RAKE system, and the other is a frequency

domain model for a multicarrier system. For a single-carrier system, the key parameter is

the number of resolvable paths of the channel, L1, which is given by

………… (4)

Then the complex lowpass equivalent impulse response of the channel can be modeled as

……….. (5)

where is a zero-mean, complex Gaussian random variable. On the other hand, a

frequency domain channel model can be characterized by the coherence bandwidth,

which is given below

……….. (6)

We are going to choose M so as to meet the following Conditions

……….. (7)

Each sub-band of a multicarrier system has no selectivity; all sub-bands are subject to

independent fading

………… (8)

From (3) and (6), these two conditions are satisfied if

……… (9)

The received signal is then given by

………….. (10)

The receiver of the first user (k = 1) is shown in Fig. 2(b), n’=O where we assume the

following characteristics for the chip wave-shaping filter:

………. (11)

Satisfies the Nyquist criterion

The CDMA standard describes the processing performed in the terminal receiver as being

complementary to those of the base station modulation processes on the Forward CDMA

Channel". The input signal of the receiver is given to serial to parallel converter. The

signal out of the converter demodulates the multiple carrier signals so that each sequence

modulates an impulse train and pseudo-random spreading signature sequence are given

to multiplier. Now the outputs received are combined by using maximal ratio combiner.

4.1.3 RECEIVER:

Fig 4.1(f): Block diagram of receiver

Serial to parallel converter: At the transmitter end N bits are sent simultaneously over

N sub-carriers. Each sub-carrier transmit different symbol with spreading code in time-

domain. In order to separate different symbol of each subscriber we use this converter.

Maximal ratio combiner: Maximal Ratio Combining is defined as all paths co-phased

and summed with optimal weighting to maximize combiner output SNR MRC is the

optimum linear combining technique for coherent reception with independent fading. Its

main advantage is the reduction of the probability of deep fades

Carrier 1

Carrier n

Low pass Impulse

modulator

Low pass Impulse

modulator

P.N sequence

P.N sequence

MaximalRatioCombiner

Received signal

Serial to parallel converter

.

Fig 4.1(g): Maximal ratio combiner

………… (12)

Also, we assume that is bandlimited to W’, where

W’ is

…….. (13)

and fi is the ith carrier frequency. This implies that the DS waveforms do not overlap.

CHAPTER 5

SOURCE CODE

function varargout = DSCDMA(varargin)

% DSCDMA M-file for DSCDMA.fig

% DSCDMA, by itself, creates a new DSCDMA or raises the existing

% singleton*.

%

% H = DSCDMA returns the handle to a new DSCDMA or the handle to

% the existing singleton*.

%

% DSCDMA('CALLBACK',hObject,eventData,handles,...) calls the local

% function named CALLBACK in DSCDMA.M with the given input arguments.

%

% DSCDMA('Property','Value',...) creates a new DSCDMA or raises the

% existing singleton*. Starting from the left, property value pairs are

% applied to the GUI before DSCDMA_OpeningFunction gets called. An

% unrecognized property name or invalid value makes property application

% stop. All inputs are passed to DSCDMA_OpeningFcn via varargin.

%

% *See GUI Options on GUIDE's Tools menu. Choose "GUI allows only one

% instance to run (singleton)".

%

% See also: GUIDE, GUIDATA, GUIHANDLES

% Copyright 2002-2003 The MathWorks, Inc.

% Edit the above text to modify the response to help DSCDMA

% Last Modified by GUIDE v2.5 22-Mar-2009 13:32:56

% Begin initialization code - DO NOT EDIT

gui_Singleton = 1;

gui_State = struct('gui_Name', mfilename, ...

'gui_Singleton', gui_Singleton, ...

'gui_OpeningFcn', @DSCDMA_OpeningFcn, ...

'gui_OutputFcn', @DSCDMA_OutputFcn, ...

'gui_LayoutFcn', [] , ...

'gui_Callback', []);

if nargin && ischar(varargin{1})

gui_State.gui_Callback = str2func(varargin{1});

end

if nargout

[varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});

else

gui_mainfcn(gui_State, varargin{:});

end

% End initialization code - DO NOT EDIT

% --- Executes just before DSCDMA is made visible.

function DSCDMA_OpeningFcn(hObject, eventdata, handles, varargin)

% This function has no output args, see OutputFcn.

% hObject handle to figure

% eventdata reserved - to be defined in a future version of MATLAB

% handles structure with handles and user data (see GUIDATA)

% varargin command line arguments to DSCDMA (see VARARGIN)

% Choose default command line output for DSCDMA

handles.output = hObject;

global err

load err

% Update handles structure

guidata(hObject, handles);

% UIWAIT makes DSCDMA wait for user response (see UIRESUME)

% uiwait(handles.figure1);

% --- Outputs from this function are returned to the command line.

function varargout = DSCDMA_OutputFcn(hObject, eventdata, handles)

% varargout cell array for returning output args (see VARARGOUT);

% hObject handle to figure

% eventdata reserved - to be defined in a future version of MATLAB

% handles structure with handles and user data (see GUIDATA)

% Get default command line output from handles structure

varargout{1} = handles.output;

% --- Executes on button press in pushbutton1.

function pushbutton1_Callback(hObject, eventdata, handles)

% hObject handle to pushbutton1 (see GCBO)

% eventdata reserved - to be defined in a future version of MATLAB

% handles structure with handles and user data (see GUIDATA)

% ----------------------------------------------------------------------- %

snr_in_dB = 0:5:30;

u = max_length([1 0 0 1 0 1]);

v = max_length([1 1 0 1 1 1]);

gold = mod(u+v,2);

sequence = gold;

M = 4; Nusers = 3;

for no = 1:Nusers

pn(no,:) = (-1).^sequence(no,:);

end;

h = zeros(1,M);

for path = 1:M

h(path) = randn+j*randn;

end;

err = [];

N = 100;

data = round(rand(N,Nusers));

err = cell(1);

nn = 1;

for i = 1:length(snr_in_dB)

[err_prob1 err_prob2] = errprob(data,snr_in_dB(i),M,pn,h);

end

% ----------------------------------------------------------------------- %

% Plot

err = err_prob1;

axes(handles.axes1)

err1 = err{1};

semilogy(snr_in_dB,err1,'r-+')

hold on

err2 = err{2};

semilogy(snr_in_dB,err2,'k-*')

hold on

err3 = err{3};

semilogy(snr_in_dB,err3,'g-+')

hold on

err4 = err{4};

semilogy(snr_in_dB,err4,'c-*')

hold off

axis([0 30 0.000001 1])

grid on

legend('M = 1 N = 512','M = 2 N = 256','M = 3 N = 128','M = 4 N = 64')

xlabel('Eb/No(dB)')

ylabel('Probability of Bit Error')

%

***********************************************************************

%

PROBABILITY ERROR:

function [err avp n1] = errprob(data,snr_in_dB,M,pn,h)

% ----------------------------------------------------------------------- %

global err

snr = 10^(snr_in_dB/10);;

Nusers = 3;

N = 100;

Nc = 31;

T = 1;

Tc = (T/Nc);

%chip duration of MC-DS-CDMA

Nsample = 1; ts = Tc/Nsample;

fs = 1/ts;

%To save run time, we can select fc to be [1:M]*10000;

%To ensure that bandwidth of subcarrier doesn't overlap each other.

for i = 1:M

fc(i) = 10000+(1/(2*Tc));

end;

error = 0;

iteration = 2;

for n = 1:iteration

for i = 1:N,

%Sampling time

t = [(i-1)*T:ts:i*T-ts];

datat = data(i,:);

%Transmitter

trans = transmitter(datat,t,Nsample,Nc,pn,M,fc,Nusers);

E = sum(trans.^2);

simga = sqrt(E/snr);

%For multicarrier system

hno=(randn(1,Nc)+j*randn(1,Nc));

for path = 1:M

noise = simga*hno;

rec(path,:) = conv(h(path),trans)+noise;

end;

%Receiver

z = multirece(rec,h,t,Nsample,Nc,pn,M,fc,Nusers);

%decision

if z(1)<1

decision = 0;

else

decision = 1;

end;

if decision ~= datat(1)

error = error+1;

end;

end;

p(n) = error/(Nusers*N);

end;

avp = sum(p(n))/iteration;

% ----------------------------------------------------------------------- %

FILTER:

function [g_T] = Fil_ter(N,T,alpha,Fs,fc)

% ----------------------------------------------------------------------- %

n = -(N-1)/2:1:(N-1)/2;

for k = 1:1:length(n)

g_T(k) = 0;

for m = -(N-1)/2:1:(N-1)/2,

g_T(k) = g_T(k)+sqrt(RCF(Fs*m/(N),alpha,T,fc))*exp((j*2*pi*m*n(k)/N));

end

end

g_T = real(g_T);

g_T = g_T/max(abs(g_T));

% ----------------------------------------------------------------------- %

MAXIMUM LENGTH:

function [seq] = max_length(connection);

% ----------------------------------------------------------------------- %

connection = fliplr(connection);

% number of registers

m = length(connection)-1;

% length of sequences

L = 2^m-1;

%initial condition

register = [zeros(1,m-1),1];

initial = register;

seq(1) = register(1);

for i=2:L

for j=1:m-1,

new_reg_cont(j) = register(j+1);

end;

new_reg_cont(m) = register(1);

for k=2:m

new_reg_cont(m) = new_reg_cont(m)+register(k)*connection(k);

new_reg_cont(m) = mod(new_reg_cont(m),2);

end;

register = new_reg_cont;

seq(i) = register(1);

if (initial==register)

disp('the connection is not valid to generate maximum length sequences');

break

end

end

%find other all sequences

for a = 2:L

seq(a,:) = wshift('1D',seq(a-1,:),1);

end

% ----------------------------------------------------------------------- %

MULTIPLE RECEIVERS:

function [z]= multirece(rec,h,t,Nsample,Nc,pn,M,fc,Nusers)

% ----------------------------------------------------------------------- %

Nf = 31;

alpha = 0.5;

T = 1;

Tc = (T/Nc);

Fs = Nsample/Tc;

%Multiplying multicarrier M

for a = 1:Nusers

for b = 1:M,

demod(b,:) = sqrt(2)*rec(a,:).*cos(2*pi*fc(b)*t);

[ht] = Fil_ter(Nf,Tc,alpha,Fs,0);

aa = conv(demod(b,:),ht);

filter_output(b,:) = aa(1+(Nf-1)/2:length(aa)-(Nf-1)/2);

%sampling

for k = 1:Nc,

rec1(b,k) = filter_output(b,((k-1)*Nsample+1));

end;

%despread

temp(b) = conj(h(b))*sum(rec1(b,:).*pn(1,:));

end;

y(a) = sum(real(temp));

end

z = y; %Decision

% ----------------------------------------------------------------------- %

RCF:

function [y] = RCF(f,alpha,T,fc)

% ----------------------------------------------------------------------- %

if (abs(f-fc) > ((1+alpha)/(2*T))),

y = 0;

elseif (abs(f-fc) > ((1-alpha)/(2*T))),

y = (T/2)*(1+cos((pi*T/alpha)*(abs(f-fc)-(1-alpha)/(2*T))));

else

y = T;

end

% ----------------------------------------------------------------------- %

TRANSMITTER:

function [mc_cdma] = transmitter(data,t,Nsample,Nc,pn,M,fc,Nusers)

% ----------------------------------------------------------------------- %

Nf = 31;

alpha = 0.5;

T = 1;

Tc = (T/Nc);

Fs = Nsample/Tc;

for no=1:Nusers

repeated_data(no,:) = kron(data(no),ones(1,Nc));

dsss(no,:) = repeated_data(no,:).*pn(no,:);

[ht] = Fil_ter(Nf,Tc,alpha,Fs,0);

sampled_dsss(no,:) = kron(dsss(no,:),ones(1,Nsample));

a(no,:) = conv(sampled_dsss(no,:),ht);

filter_output(no,:) = a(1+(Nf-1)/2:length(a)-(Nf-1)/2);

for d = 1:M,

mod(d,:) = sqrt(2)*filter_output(no,:).*cos(2*pi*fc(d)*t);

end;

%transmitted_signal

trans(no,:) = sum(mod,1);

end;

mc_cdma = sum(trans,1);

% ----------------------------------------------------------------------- %

CHAPTER 6

SIMULATION RESULT

Fig 6(a): simulation result

The above figure shows the BER versus Eb/No for various values of M and N, where N

is the processing gain and M is the number of carriers. Note that M=1 is nothing but

Single carrier CDMA system. Here the performance of a multicarrier system will be the

same as that of a single-carrier system if M=1 i.e., the no of carriers is equal to one.

Simulation results show that single carrier system having high probability of BER as

compared to the multi-carrier DS-CDMA

CHAPTER 7

CONCLUSION AND FUTURE SCOPE

7.1 CONCLUSION:

In Multi Carrier DS CDMA we discussed a new way of realizing a wideband CDMA

system. As shown above, the system has a narrowband interference suppression effect,

along with a robust-ness to multipath fading. As a consequence, this system might be

suitable for a CDMA overlay scheme as described. In a CDMA overlay system, DS SS

signals are overlaid onto existing narrowband microwave signals-as a consequence, to

obtain the best performance, it is necessary to eliminate interference from narrowband

users to CDMA users and vice versa. To reject the interference from a narrowband signal

to a CDMA signal, the narrowband interference suppression capability of this multi-

carrier system is useful. On the other hand, to eliminate the interference from the DS SS

signals to a narrowband user, one can just stop transmitting at those frequencies which

occupy the same spectrum as the narrowband user.

Simulation Results shows that MC-CDMA advantageous in terms of BER as compared to

the SC-CDMA.

7.2 FUTURE SCOPE:

There is no interference from the CDMA signals to the existing microwave systems.

Thus, there is no need for either a narrowband suppression filter at the receiver or a notch

filter at the transmitter. We need to design a system to full fill the above requirement.

BIBLIOGRAPHY

BOOKS:

G. L. Turin, “Introduction to spread-spectrum antkultipath techniques and their

application to urban digital radio,” Proc. IEEE, vol. 68, Mar. 1980.

R. E. Ziemer and R. L. Peterson, Digital Communications and Spread Spectrum

Systems. New York Macmillan, 1985.

R. L. Pickholtz, D. L. Schilling, and L. B. Milstein, ‘Theory of spreadspectrum

communications-A tutorial,” IEEE Trans. Commun., vol. COM-30, no. 5, May

1982.

S. Kondo and L. B. Milstein, “Multicarrier CDMA system with cochanne1

interference cancellation,” in Proc. VTC ’94, Stockholm, Sweden, June 1994, pp.

164C-1644.

“Multicarrier DS CDMA systems in the presence of partial band interference,” in

Proc. MILCOM ’94, Fort Monmouth, NJ, Oct. 1994.

WEB SITES:

www.wikepedia.com www.atmel.com www.circuitcellar.com

.APPENDIX A

INTRODUCTION TO MATLAB

MATLAB is a high-level language and interactive environment that enables to perform

computationally intensive tasks faster than with traditional programming languages such

as C, C++, and FORTRAN.

We can use MATLAB in a wide range of applications, including signal and image

processing, communications, control design, test and measurement, financial modeling

and analysis, and computational biology. Add-on toolboxes (collections of special-

purpose MATLAB functions, available separately) extend the MATLAB environment to

solve particular classes of problems in these application areas.

MATLAB provides a number of features for documenting and sharing the work. We can

integrate the MATLAB code with other languages and applications, and distribute the

MATLAB algorithms and applications.

WHAT IS MATLAB?

MATLAB is a high-performance language for technical computing. It integrates

computation, visualization, and programming in an easy-to-use environment where

problems and solutions are expressed in familiar mathematical notation. Typical uses

include

1. Math and computation

2. Algorithm development

3. Data acquisition

4. Modeling, simulation, and prototyping

5. Data analysis, exploration, and visualization

6. Scientific and engineering graphics

7. Application development, including graphical user interface building.

MATLAB is an interactive system whose basic data element is an array that does not

require dimensioning. This allows you to solve many technical computing problems,

especially those with matrix and vector formulations, in a fraction of the time it would

take to write a program in a scalar non interactive language such as C or FORTRAN.

The name MATLAB stands for matrix laboratory. MATLAB was originally written to

provide easy access to matrix software developed by the LINPACK and EISPACK

projects. Today, MATLAB engines incorporate the LAPACK and BLAS libraries,

embedding the state of the art in software for matrix computation.

MATLAB has evolved over a period of years with input from many users. In university

environments, it is the standard instructional tool for introductory and advanced courses

in mathematics, engineering, and science. In industry, MATLAB is the tool of choice for

high-productivity research, development, and analysis.

MATLAB features a family of add-on application-specific solutions called toolboxes.

Very important to most users of MATLAB, toolboxes allow you to learn and apply

specialized technology. Toolboxes are comprehensive collections of MATLAB functions

(M-files) that extend the MATLAB environment to solve particular classes of problems.

Areas in which toolboxes are available include signal processing, control systems, neural

networks, fuzzy logic, wavelets, simulation, and many others.

THE MATLAB SYSTEM:

The MATLAB system consists of five main parts:

DEVELOPMENT ENVIRONMENT:

This is the set of tools and facilities that help you use MATLAB functions and files.

Many of these tools are graphical user interfaces. It includes the MATLAB desktop and

Command Window, a command history, an editor and debugger, and browsers for

viewing help, the workspace, files, and the search path.

THE MATLAB MATHEMATICAL FUNCTION:

This is a vast collection of computational algorithms ranging from elementary functions

like sum, sine, cosine, and complex arithmetic, to more sophisticated functions like

matrix inverse, matrix eigen values, Bessel functions, and fast Fourier transforms.

THE MATLAB LANGUAGE:

This is a high-level matrix/array language with control flow statements, functions, data

structures, input/output, and object-oriented programming features. It allows both

"programming in the small" to rapidly create quick and dirty throw-away programs, and

"programming in the large" to create complete large and complex application programs.

GRAPHICS:

MATLAB has extensive facilities for displaying vectors and matrices as graphs, as well

as annotating and printing these graphs. It includes high-level functions for two-

dimensional and three-dimensional data visualization, image processing, animation, and

presentation graphics. It also includes low-level functions that allow you to fully

customize the appearance of graphics as well as to build complete graphical user

interfaces on your MATLAB applications.

THE MATLAB APPLICATION PROGRAM INTERFACE (API):

This is a library that allows you to write C and Fortran programs that interact with

MATLAB. It includes facilities for calling routines from MATLAB (dynamic linking),

calling MATLAB as a computational engine, and for reading and writing MAT-files.

MATLAB WORKING ENVIRONMENT:

MATLAB DESKTOP:

Matlab Desktop is the main Matlab application window. The desktop contains five sub

windows, the command window, the workspace browser, the current directory window,

the command history window, and one or more figure windows, which are shown only

when the user displays a graphic.

The command window is where the user types MATLAB commands and expressions at

the prompt (>>) and where the output of those commands is displayed. MATLAB defines

the workspace as the set of variables that the user creates in a work session. The

workspace browser shows these variables and some information about them. Double

clicking on a variable in the workspace browser launches the Array Editor, which can be

used to obtain information and income instances edit certain properties of the variable.

The current Directory tab above the workspace tab shows the contents of the current

directory, whose path is shown in the current directory window. For example, in the

windows operating system the path might be as follows: C:\MATLAB\Work, indicating

that directory “work” is a subdirectory of the main directory “MATLAB”; WHICH IS

INSTALLED IN DRIVE C. clicking on the arrow in the current directory window shows

a list of recently used paths. Clicking on the button to the right of the window allows the

user to change the current directory.

MATLAB uses a search path to find M-files and other MATLAB related files, which are

organize in directories in the computer file system. Any file run in MATLAB must reside

in the current directory or in a directory that is on search path. By default, the files

supplied with MATLAB and math works toolboxes are included in the search path. The

easiest way to see which directories are on the search path. The easiest way to see which

directories are soon the search paths, or to add or modify a search path, is to select set

path from the File menu the desktop, and then use the set path dialog box. It is good

practice to add any commonly used directories to the search path to avoid repeatedly

having the change the current directory.

The Command History Window contains a record of the commands a user has entered in

the command window, including both current and previous MATLAB sessions.

Previously entered MATLAB commands can be selected and re-executed from the

command history window by right clicking on a command or sequence of commands.

This action launches a menu from which to select various options in addition to executing

the commands. This is useful to select various options in addition to executing the

commands. This is a useful feature when experimenting with various commands in a

work session.

USING THE MATLAB EDITOR TO CREATE M-FILES:

The MATLAB editor is both a text editor specialized for creating M-files and a graphical

MATLAB debugger. The editor can appear in a window by itself, or it can be a sub

window in the desktop. M-files are denoted by the extension .m, as in pixelup.m. The

MATLAB editor window has numerous pull-down menus for tasks such as saving,

viewing, and debugging files. Because it performs some simple checks and also uses

color to differentiate between various elements of code, this text editor is recommended

as the tool of choice for writing and editing M-functions. To open the editor, type edit at

the prompt opens the M-file filename.m in an editor window, ready for editing. As noted

earlier, the file must be in the current directory, or in a directory in the search path.

GETTING HELP:

The principal way to get help online is to use the MATLAB help browser, opened as a

separate window either by clicking on the question mark symbol (?) on the desktop

toolbar, or by typing help browser at the prompt in the command window. The help

Browser is a web browser integrated into the MATLAB desktop that displays a Hypertext

Markup Language (HTML) documents. The Help Browser consists of two panes, the

help navigator pane, used to find information, and the display pane, used to view the

information. Self-explanatory tabs other than navigator pane are used to perform a search.