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The aim of project to design and simulate a wireless system which can provide high data rate coverage while maintaining the quality of data and effectively eliminating the interferences by using MC-CDMA relaying. Multi Carrier-Code Division Multiple Access (MC-CDMA) is a multiple access scheme. As Multi Carrier-Code Division Multiple Access (MC-CDMA) is a combination of Orthogonal Frequency Domain Multiplexing (OFDM) signalling and Code Division Multiple Access (CDMA) scheme, this technique can achieve high data rate for multimedia services and it has the capability of handling diverse multimedia traffic. The main reason for its popularity in recent research is its capability to support high data coverage. This scheme is regarded for beyond 4G mobile communication systems because of the efficiency in counteracting frequency selective fading and multi-user interference for high data rate communications. In wireless networks like cellular networks and ad-hoc networks etc, it is very hard to have multiple antennas mounted at the terminals because of the restriction of space and antenna cross-correlation particularly in mobile station. Relaying is a technique in which an electronic device that receives a signal and resends it at a higher level and/or higher power, or onto the other side of a barrier, so that the signal can cover longer distances without performance deterioration. Relay system break the communication system into shorter communication system instead of conventional point-to-point transmission. Multiple carrier Code division multiple access (MC-CDMA) is emerging as one of the most common methods for multi-user access. Combining MC-CDMA with relaying provides a system that supports better performance and cost-effectively.
Declaration of originality
I hereby declare that the research recorded in this thesis and the thesis itself was composed and
originated entirely by myself in the Department of Electronics and Electrical Engineering at
The University of Edinburgh.
In the rapidly increase world of wireless communications in cellular communications, a number of new communication technologies came. Among these multi-carrier communication and its applications are plays an important role present multiple-access communication. The main advantage of multi-carrier technique is, it is robustness to the frequency selective fading. It has synergistic effects when combined with Code Division Multiple Access (CDMA).
Multi-carrier Code Division Multiple Access is a combination of OFDM and CDMA. It has attention in the field of wireless communication due to its high data rate services. These signals are modulated and demodulated by using Fast Fourier Transform (FFT) devices without increasing the receiver's complexity. In order to achieve frequency diversity, the combination of CDMA and OFDM can combat the effects of fading channels by spreading signals over several carriers.
MC-CDMA has been widely used in wireless telecommunication systems to multiplex information signals from various users and transmit the information by using a single carrier. The signals of Multi-carrier CDMA are spreaded in the frequency domain where different chips of the same symbols are transmitted in parallel by using various orthogonal spreading codes. In MC-CDMA systems, Orthogonal Walsh-Hadamard spreading wave form is providing better performance in the data transmission. These systems have an ability to achieve high diversity gains over frequency selective fading channels due to time domain equalization.
In this present document, the author clearly explains the different types of multiple access techniques and different spread spectrum methods.
Aim of the project:
The main aim of my project is to design and simulate a wireless system which can provide high data rate coverage while maintaining the quality of data and effectively combating against the interferences. Multi Carrier-Code Division Multiple Access (MC-CDMA) is a multiple access scheme. If we implement MC-CDMA on Relays it not only maintains the quality of data and it also improves the system performance.
There were various milestones that had to be completed in order to reach the completion of this project. One of them was to have the basic understanding of the application to be built and the features needed to support the same. This has been achieved by a detailed literature review that has been done on various aspects such as the basic functionality of a MC-CDMA and its implementation in wireless networks, knowing the basic concepts of performance characteristic of this system like error probability and capacity.
Identifying the relevant Project deliverables is the most important task for the successful completion of the project. The deliverables for analyzing the performance of MC-CDMA Relaying are
Explanation of different code division multiple access techniques such as Direct Sequence Code Division Multiple Access (DS-CDMA), Wideband Code Division Multiple Access (W-CDMA) and Multi-carrier Code Division Multiple Access (MC-CDMA).
Calculation of error probability before and after relaying, capacity before and after relaying.
Analyzing the theoretical calculation.
The concept of multiple access technique plays very important role in present wireless communication systems. This concept is used to allow more mobile users to share simultaneously in a fixed amount of radio spectrum. The sharing of spectrum is very important to reach the high capacity by simultaneously allocating the existing channels to multiple users. This process will be done without severe degradation in the performance of the system to achieve high quality communication. Cellular systems break up geographic region as cells where a mobile unit in every cell communicates with a control station or base station (BS). The main reason for implementation of cellular systems is capable to maintain as many number of calla as possible (It is represented as capacity in cellular terminology) in a specific bandwidth with some consistency.
Multiple-access has the ability of a large number of base stations simultaneously interconnected their respective data, voice, teletype and TV links through a satellite. Multiple access means a large number of users can share a common bandwidth of radio channels and any user can use access to any channel (each user always not assigned same channel). A channel is a portion of the limited radio resource which is temporarily allocated for a specific purpose each as a phone call. Multiple Access method is a definition of how radio spectrum is separated in to channels and how channels are allocated to many users of the system.
Three basic "dimensions" that can be allocated to provide multiple accesses:
Spatial allocations are largely fixed by significant infrastructure deployment decisions. Time and frequency multiple-access techniques are very important in wireless communication. Time and frequency can be allocated more flexibly. In this documentation author mainly concentrate on time and frequency based multiple-access methods.
By theoretically in ideal environment any multiple Access technique offers the same capacity. But in cellular communication, some multiple-access techniques provide better capacity than the other techniques. The capacity limitation of earlier analog cellular systems employing frequency modulation and digital methods offering more capacity were proposed for overcoming the limitation. Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) were the primary digital transmission methods that were researched and it was found that CDMA systems offer the highest capacity than the other competing digital technologies (like TDMA) and analog technologies.
Frequency and time division multiple access are the two major techniques used to share the available bandwidth in a conventional mobile radio communication system . FDMA was the primary multiple- access techniques for cellular systems. In this technique a mobile user is allocate a pair of frequencies when receiving a call. One frequency is used for downlink and other one is used for uplink. FDMA assign individual channels (frequency bands) to individual users. The frequency bands are assigned to users when they request the service. Second generation cellular systems (IS-54, GSM) utilize time/frequency multiple-access whereby the available spectrum is separated into frequency slots (30 kHz) but then every frequency slot is separated into time slots. Various users can utilize the same frequency in the same cell except that they should transmit at various times. This method is also being utilized in 3rd generation wireless systems.
Code division multiple-access method allows maximum users to concurrently access a provided frequency allocation. At the receiver user division is possible due to every user increases the transform waveform over a wide bandwidth by using distinctive spreading codes. CDMA has two basic types. Direct-sequence CDMA (DS-CDMA) increases the signal by multiplying the data waveform with a user-distinctive high bandwidth pseudo-noise binary sequence.
This multiple access interference can give a considerable problem if the power-level of the preferred signal is drastically lower than the power-level of the interfering user. This is represented as the near-far problem. Over the last 15 years there has been significant theoretical research on solutions to the near-far problem beginning with the origin of the optimal multi user receiver and now with max organizations structural suboptimal reduced complexity multi-user receivers. The process being measured by organizations is also successive interference cancellation or parallel interference postponement. One of the advantages of these methods is that they generally do not needed spreading codes with period equal to the bit duration. One more advantage is that they do not needed significant complexity. These interference cancellation detectors can also easily be enhanced by cascading numerous stages together.
There are various ways to permit access to the channel. These consist:
Time division multiple access: TDMA
Frequency division multiple access: FDMA
Orthogonal Multiple Division Multiplexing: (OFDM)
Code division multiple access: CDMA
direct-sequence (DS) CDMA
frequency-hop (FH) CDMA
Multi-carrier CDMA: FH or DS
Orthogonal Frequency Division Multiplexing (OFDM):
Frequency Division Multiplexing is a technology that can transmit multiple signals at a time over a single transmission path such as cable or wireless system. In this method each signal travels within its own unique frequency range (carrier), which is modulated by the data such as voice, video.
Orthogonal Frequency Division Multiplexing (OFDM) is a spread spectrum technique which distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The main advantages of this technique are it has high spectral efficiency, resilience to RF interference and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario their exit multipath-channels that is the transmitted signal arrives at the receiver using different paths with different lengths. Since multiple versions of the signal interfere with each other (inter symbol interference (ISI) it becomes very hard to extract the original information. OFDM is also called multi-carrier or discrete multi-tone modulation.
Frequency Division Multiple Access (FDMA)
In FDMA, the available radio spectrum is divided into channels of fixed bandwidth. These fixed bandwidths are then assigned to different users. It was utilized as the basis for 1st generation (analog) cellular phone systems. With FDMA, only one subscriber is assigned to a channel at a time. Other conversations can access this channel only after the subscriber's call has terminated or after the original call is handed off to a various channel by the system. Frequency division multiple access (FDMA) is characterized by continuous access to the satellite in a provided frequency band.
C1 C2 C3 C4
C1, C2, C3, C4-Channels
Total Available Bandwidth
The main advantage of FDMA has its simplicity and relies on the utilization of proven equipment.
Lack of flexibility in case of reconfiguration; to accommodate capacity variations it is necessary to change the frequency plan and this implies modification of transmitting frequencies, receiving frequencies and filter Bandwidths of the earth stations.
Loss of capacity when the number of accesses increases due to the generation of intermodulation products and the need to operate at a reduced satellite animating power (back-off).
The need to control the transmitting power of earth stations in such a way that the carrier powers at the satellite input are the same in order to circumvent the capture effect. This control should be performed in real time and should adapt to attenuation caused by rain on the uplinks.
TIME DIVISION MULTIPLE ACCESS
In TDMA, time is separated into intervals of regular length, and then each interval is subdivided into slots. Each user is assigned a slot number, and can transmit over the entire bandwidth during its slot within each interval.
S1 S2 S3 S4 â€¦..
Interval1 Interval2 â€¦.
Time division multiple access (TDMA) is characterized by access to the channel during a time slot.2G Systems like GSM and Digital-AMPS employ TDMA as the multiple access Method.
At each instant the channel amplifies only a single carrier, which occupies all of the channel bandwidth: no intermediation products and the carrier benefits from the saturation power of the channel. However nonlinearly exists and, combined with the effects of filtering on transmission and reception, introduces degradation with respect to the ideal digital transmission.
There is no need to control the transmitting power of the stations.
All stations transmit and receive on the same frequency whatever the Origin or destination of the burst; this simplifies tuning.
The need to dimension the station for transmission at high Throughput.
The need for synchronization
Poor voice quality
CODE DIVISION MULTIPLE ACCESS:
In a code-division multiple-access communication system, a communication channel with a provided band-width is accessed by all the users at the same time. The various mobile users are distinguished at the base station receiver by the distinctive spreading code assigned to the users to modulate the transmitted signals. Hence, the CDMA signal transmitted by any provided user consists of that user's data which modulates the distinctive spreading code assigned to that user, which in turn modulates a carrier utilizing any well-known modulation scheme. The frequency of this carrier is the same for all users. At the receiver division is possible due to each user increases the transformed waveform over a wide bandwidth utilizing distinctive spreading codes.
The set of codes utilized should have the following correlation properties:
Each code should be easily distinguishable from a replica of itself shifted in time.
Each code should be easily distinguishable regardless of other codes utilized on the network.
Code division multiple access is very important and most suitable transmission technology for mobile communications. The reason behind for the success of this technique is the huge increase in capacity covered by CDMA system when compared to other transmission techniques. CDMA is a "spread spectrum" technology, it assign different codes to each communication to separate it from the others in the same spectrum. CDMA shows better capacity for voice and data communications. In CDMA system every user has the same carrier frequency. Every user has its own codeword and this codeword is almost orthogonal to all other code words. The receiver does a time correlation procedure to identify only the specific required codeword. For the detection of the signal, the receiver requires to know the codeword used by the transmitter.
In all types of CDMA spread spectrum method is used to permit receivers to partially distinguish against the unwanted signal. Signals with the specific spreading code and timing are received at the receiver and signals with different spreading codes or the same spreading code but different timing offset appear as noise and this noise reduced by the process gain.
The way it working is like each station is assigned a spreading code or chip sequence. This chip sequence as expressed a sequence of -1 and +1 values. The dot product of each chip sequence with itself is 1and dot product with its complement is -1. And dot product of two various chip sequences is 0.
If C1 = (+1,-1,-1,+1) and C2=(-1,-1,+1,+1)
C1 . C1 = (+1,-1,-1,+1) . (+1,-1,-1,+1) = +1
C1 . -C1 = (+1,-1,-1,+1) . (-1,+1,+1,-1) = -1
C1 . C2 = (+1,-1,-1,+1) . (-1,-1,+1,+1) = 0
C1 . -C2 = (+1,-1,-1,+1) . (-1,-1,+1,+1) = 0
This property is called orthogonality.
These sequences are Walsh codes and these codes are derived from the Hadamard matrix. Hadamard matrix is a square matrix in which each row is orthogonal to all other rows and each column in the matrix is orthogonal to all other columns.
Direct Sequence CDMA:
In direct sequence method, the data signal is multiplied by code sequence and these code sequences are mostly binary sequence. The duration of an element in the code is called the "chip time". The ratio of user symbol time and chip time is called spread factor. In this method signals are spreaded from or to different users with different codes. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data. This method is one of the most used techniques of CDMA.
A DS-CDMA signal is generated by multiplication of a user data signal by a code sequence.
In the receiver side, the received signal is again multiplied by the same (syncronized) code. With this operation the code will remove and receiver can easily recover the transmitted user data.
SPREAD SPECTRUM METHODS:
In Code Division Multiple Access system every user sends the data in the same bandwidth simultaneously. Spread spectrum method has been successfully used by the military from decades recently spread spectrum based coded division multiple access has taken on a significant role in cellular and mobile communications. Spread spectrum method main applications in cellular mobile systems. This method is resistance to signal interference from multi transmission paths and potentially higher bandwidth efficiency in multiple access communication than in other technologies. In this transmission method, the signal in the frequency spectrum is spread by using a code uncorrelated with that signal. This leads to the bandwidth occupancy is much higher than required. The main reason that the receiver has knowledge about the code of the intended transmitter is capable of selecting desired signal is in spreading the signal some codes are used. These codes have low cross-correlation values which are different to every user. This method has more advantages when compared to other methods.
Interference limited operation because in all scenarios the total frequency-spectrum is used.
This method has high squired due to random codes. The codes using in this process are in principle unknown to a hostile user. With this process no one detect the message of other users.
By using this spread spectrum the effect of multi-path reduced.
With this method users can start their transmission at any time due to random access possibilities.
This method is used in military communication for the purpose of anti-jamming.
In the present wireless communications spread spectrum signals are very popular in commercial applications.
The ratio of transmission bandwidth to the information bandwidth is called processing gain. It is also called spreading factor. Processing gain is one of the important parameter in spread spectrum.
Where G p= processing gain
BWt = transmitter bandwidth
And BWi= information bandwidth.
Processing gain determines the number of users that can be allowed in a system, the difficulty to detect a signal and the effect of multi-path effect reduction. It is very advantage for spread spectrum that processing gain as high as possible.
There are mainly exist three different types to spread a signal.
Hybrid systems: DS/(F)FH
It is a very popular spread spectrum method. In Direct Sequence the data signals are multiplied by a code sequence called Pseudo Random Noise Code (PNcode). A PNcode is a sequence of chips indicated by values -1and 1 or 0 and 1. An easy way to create a PNcode is by means of at least one shift register. If the length of a shift-register is n then the length of the code is given by
In direct sequence system the spreading factor and length of the code is equal that is
Fig 1 direct-sequence spreading
The above diagram shows how PNcode is combined with the data signal. The above figure also shows that the length of the code is equal to the spreading factor and in this example. Then the bandwidth of the data signal is now multiplied by a factor.
It is very easy to generate PNcodes; a number of shift-registers are all that is required. With this reason it is easy to introduce a large processing-gain in Direct-Sequence system.
Figure 2: DS-concept, before and after despreading.
In the receiver side, the received signal is again multiplied by the same (synchronized) PNcode. With this operation it is easy to get the original data signals because PNcode existed of +1s and -1s. In direct sequence method the concept of spreading operation is same as dispreading operation. This leads that a possible jamming-signal in the radio channel will be spread before data-detection is performed. The main disadvantage of Direct Sequence method is Near-Far effect which is clearly explained with the figure 3.
It is a method of transmitting radio signals by rapidly switching a carrier frequency among many frequency channels. This method also uses the Pncodes for spreading the data signal known to both transmitter and receiver. The carrier frequency is "hopping" according to the distinctive sequence when applying Frequency Hopping. If the length of the FH sequence is Nfh. The bandwidth is increased by a factor N if the channels are non-overlapping.
The main drawback of Frequency-Hopping method is it is very hard to obtain a high processing gain when compared to Direct-Sequence. For obtaining fast-hopping over the carrier frequencies it is essential to perform frequency-synthesizer. The faster the "hopping-gain" is, the higher the processing gain.
Frequency Hopping is less effected by the Near-Far effect when compared to the Direct-Sequence. FH sequence has only a limited number of "hits" with each other. This means that if a near-interferer is present, only a number of "frequency-hops" will be blocked instead of the whole signal. From the "hops" that are not blocked it should be possible to recover the original data message.
Hybrid System: DS/(F)FH
Hybrid System is a combination of direct-sequence and frequency-hopping. One data bit is separated over frequency-hop channel is that is carrier frequencies. One complete PNcode of length is multiplied with the data in each frequency-hop channel.
As the FH-sequence and the PN-codes are coupled, an address is a combination of an FH-sequence and PN-codes. To bind the hit-chance (the chance that two users share the same frequency channel in the same time) the frequency-hop sequences are chosen in such a way that two transmitters with various FH-sequences share at most two frequencies at the same time (time-shift is random).
Wideband Code Division Multiple Acces (W-CDMA):
W-CDMA is used in 3G cellular systems. It is used a direct-sequence spread spectrum technique of asynchronous code division multiple access to get high speed spectrum and carry more users compared to the performance of Time Division Multiplexing used in 2G GSM networks. It used coherent detection on both the uplink and downlink based on the use of pilot systems and channels. It supports inter-cell asynchronous function.
Objectives of Wide Band CDMA
Full coverage and mobility for 144 kbps, preferably 384 kbps.
Limited coverage and mobility for 2 Mbps.
High spectral efficiency compared with 2G.
High flexibility to introduce new services.
An early goal, now largely abandoned, was a global air-interface standard.
First systems should start emerging (in Japan) next year.
Wideband CDMA Features:
Bandwidth of 5 MHz (or more, up to 20 MHz proposed).
Chip rates from 4 million chips-per-second (up to 15 Mcps proposed).
Support of multiple data rates & packet data.
Coherent uplink (with uplink pilot channel).
Base-station antenna beam forming.
Advanced interfering suppression methods
Multi-carrier CDMA is a multiple Access technique, which permit the system to support multiple users at the same time. It combines multicarrier transmission with the direct sequence spread spectrum.
Importance of MC-CDMA Communication System:
In the present days, there has been a considerable change in the signal processing procedures like interference cancellation methods and antenna structure. These antennas are normally considered as small antennas like adaptive antennas and MIMO antennas. Even though these schemes are vital for future wireless schemes but for practical applications these schemes does not meet the present requirements of allowing fast data coverage in everywhere. For example, it is very hard to install complex antenna system at wireless terminals. In addition, the smart antennas will not able to success in the presence of more shadowing. Based on these reasons, additional major improvements are required, for determined capacity, throughput and coverage requirements for future wireless communication systems.
Some major changes in the wireless systems are needed itself in such a way that it will facilitate useful supply and collection of signals to and from wireless users. The combination of multi-hop capability in conventional wireless communication systems will be one of the successful improvements in architecture up gradation. One scheme which is being extensively studied and getting more attention is Multi-carrier Code Division Multiple Access (MC-CDMA). This is a multiple access scheme. The main reason for its popularity in recent research is its capability to support high data coverage. This scheme is regarded for beyond 4G mobile communication systems due to its efficiency in counterattacking to frequency selective fading and multi-user interference for high data rate communications. As Multi Carrier-Code Division Multiple Access (MC-CDMA) is considered as the combination of Orthogonal Frequency Domain Multiplexing (OFDM) signaling and Code Division Multiple Access (CDMA) scheme, this technique can achieve high data rate for multimedia services and it has the capability of handling diverse multimedia traffic.
The transmitter model of MC-CDMA model used in this work is shown in below fig. In this system model the author specifically addresses the synchronous downlink case and assumes that the transmitter and receiver are synchronised. Walsh-hadamard codes are used for user separation due to its simple encoding process and provide zero cross-correlation.
Relaying is a technique in which an electronic device that receives a signal and resends it at a higher level and/or higher power, or onto the other side of a barrier, so that the signal can cover longer distances without deterioration.
Importance Relaying Networks:
In wireless networks like cellular networks and ad-hoc networks etc, it is very hard to mountain multiple antennas at the terminals because of the restriction of space and cross-correlation particularly in mobile station. Multi-hop networks have many advantages as compared to usual communication systems foe connectivity, deployment capacity to minimize the need for the fixed infrastructure. Corporative communication is one example of multi-hop communication in which receiver terminal combines the received signals from the transmitter as well as the relay terminals. To get advantage from the multiple signal models available the receiver terminal can use a board range of diversity schemes like maximal-ratio, selection diversity or equal gain. There are number of protocols that attain the advantage of user cooperation and relaying is one of them. The receiver terminal can use a broad range of diversity schemes like maximal-ratio, selection diversity or equal-gain to get advantage from the multiple signal models available. There are several protocols that attain the benefit of user cooperation. One of them is relaying.
Recently relaying has emerged as a field of growing field for wireless systems. The use of relays between the source and destination promises improvements in increase connectivity and reduce transmit powers to diversity gains.
Relays are mainly used in the long distance wireless communications. Relays are also used within the cellular systems between the base station and user terminal which increases the transmission range. Wires relays are used between the base stations as a cost effective and alternative to cable links. Relays are very important in wireless ad-hoc networks, which decrease the power consumption of the terminals. This improves the lifetime of the network.
In conventional wireless cellular system all terminals are directly connected to the backbone infrastructure through a single hop. But in the relay network, the use of intermediate nodes to help transmit information from one node to other which makes number of improvements like the connectivity of nodes can be improved, increase network coverage in cellular systems.
The idea of relaying in wireless networks has gain more attracting for theoretical up to now and recently relaying is considered for practical systems. A flexible relay based wireless network is used for to extend the conventional point communication to multipoint communication. In addition to this it also aimed to provide a broadband wireless access for the telecommunication user. The placing of relays is used for different purposes. If relays are placed within the base stations, relays increase the capacity and relays are extend the coverage range when they placed on the cell boundary.
Relay channels will play key role in the future wireless communication systems. Relays increase the dimensionality of a channel at the same time it boost the data rates of a channel under certain circumstances. Relays are not always able to advance the system's performance under traditional SISO (single-input single-output) system configuration. Relay network allows a source communicate with the destination through a number of relay nodes.
Relay network are mainly divided in two types.
In amplify-and-forward relay network, the relays amplify the received signal from the source and forward it to the destination.
In decode-and -forward relay network, the relay decodes the received data and retransmits it to the destination. Relay forward the received data if and only if it decodes the message.
In both Decode-and-forward and Amplify-and-forward modes multiplexing gain is same in simple diversity versus multiplexing analysis but Amplify-and-Forward provide better diversity order. While BER analysis shows that Decode-and-Forward method is found to be better than Amplify-and-Forward method. Amplify-and-Forward and Decode-and-Forward both methods can do better than the other relying on the fundamental condition.
4.2. The Gain of the Relay:
Antenna arrays placed into the corners of an isolated triangle with base angle of 30Â° and top angle 120Â°.
Lower case letters specifies the distance between the corners and capital letters specifies the angles.
In this system model the author assume that the angle between the relay and the receiver is 30Â° and also assume that a single relay is used between the source and destination.
From the triangle abc
c2 = a2 + b2 - 2abcosC
but C = 1200
and cosC = -1/2
c2 = 3a2 => c = âˆš3a2 => c = 1.732a
After doing these geometrical calculations, the SNR gain of the system can be found. The system without the relay has the following SNR.
SNR1 = Pr1 / N
Pr1 = Pt1 K c- Î³ => Pr1 = Pt1K (1.732a) - Î³
Where Pt is the total power transmitted, Pr the total power received and K the total gains and losses of the system. The path loss index Î³ normally varies from 2 to 6 where 2 is the open space path loss index and 6 is the path loss index of urban environments full with scatterers and shadows. For this calculations, path loss index Î³ = 3 was used as it is a median case and shows a sub-urban environment.
The system with the relay has the following SNR and assuming that half of the total power is used to transmit from the transmitter to the relay and the second half is used to transmit from the relay to the receiver.
SNR2 = Pr2 / N
Pr2 = (Pt1 / 2) K a- Î³
By comparing the two SNRs found above the author observes that SNR after relay is 2.59 times the SNR without relay that is SNR2 = 2.59 Ã- SNR1. In practice though it was transmitted in two time slots and so we use double channel resources. Hence the channel capacity will be half of the minimum among the capacities of the two sub-channels. 
Multi-Carrier modulation is very successful in the broadcast applications. This makes the researchers investigate the stability of MC modulation in wireless mobile communications. There are mainly two different concepts were introduce when DS-CDMA combined with MC modulation. One concept is MC-CDMA (frequency domain spreading) is also called combination of CDMA and OFDM. Second concept is referred as MC-DS-CDMA (time domain spreading).
One way of looking at MC-CDMA is as a combination of CDMA and OFDM, resulting in better frequency diversity and higher data rates. In MC-CDMA, every symbol is spread utilizing code chips and transmitted on numerous subcarriers. There is no necessity for the number of carriers to be equal to the code length; thus offering a degree of flexibility in the construction. The basic principle is explained in the case of a single-user scenario where the data is spread utilizing a code of length 4 and number of subcarriers is the same.
In MC-CDMA, the multiple access possible through proper system construction by using orthogonal codes. This makes MC-CDMA has high bandwidth efficiency. MC-CDMA offers greater resilience to error due to its additional flexibility offered by the possibility to using different length codes. In MC-CDMA, it is possible to interleave the data in both frequency and time domain to execute both time and frequency diversity and employ time-frequency spreading. MC-CDMA and OFDM offers high peak to average ratios, the challenges of synchronization in both time and frequency domains and dealing with the carrier frequency offset and multiple access interference (MAI).
In the recent researches the use of MC-CDMA for mobile multi-user communication has gain more attraction. Up to now, major researches are down on MC-CDMA to investigate data detection methods suitable for mobile radio systems in the down link. In this document the author gave an overview of Adaptive modulation technique for a multi-user downlink MC-CDMA system that employs frequency domain spreading.
This modulation is done by assembling the subcarriers into groups, the spreading codes align themselves in a synchronous manner, and thus with the appropriate equalization pair wise orthogonality is preserved and adaptive modulation can be performed. Such a configuration leads to the origin of an analytical expression for the instantaneous SNR of a group of sub-carriers utilized for adaptive modulation and resource allocation purposes.
An effective channel function might be evaluated for every group of sub-carriers for every user operating in that said group, and every group might then be interpreted as an equivalent sub-carrier of a conventional OFDM modem, thereby enabling any existing adaptive schemes originally intended for OFDM to be deployed to MC-CDMA. Based on the equivalent sub-carrier concept, the authors introduce and adaptive modulation scheme for multi-user MC-CDMA.
This scheme employs the target BER process and attempts to let every user to transmit as maximum symbols as his channel allows, it has no rate constraint or matching imposed. In general, every user will have various channel conditions than that of other users operating with a particular control/base station. Thus, the data rates that every of these channels can support will naturally vary from user to user, and this scheme allows any particular user to reserve none, or one or more orthogonal codes in one or more sub-carrier groups.
Let K be the number of modulation schemes (MS) available for utilize between the control/base station and every user (predetermined by construction according to some criteria, e.g. complexity). For every group of sub-carriers, provided the channel state information for every user, the transmitter can compute the effective instantaneous SNR for every spreading code utilized for every user. This allocation process determines how maximum data bits are accommodated in an OFDM symbol for the user. The classification and allocation process is done for every spreading code assigned to every user. The chips generated are synchronously added together in every group across the N subcarriers before the IFFT operation.
The MC-CDMA air interface allows high-capacity networks and robustness in the case of frequency-selective channels, taking benefits from CDMA ability offered by the spread spectrum method, and MC modulation as orthogonal frequency division multiplex (OFDM). A possible generic downlink transmission scheme is depicted in Figure.
Each user data can be concurrently processed at the spreading step before MC modulation. In the following, due to their good properties for the downlink , Walsh-
Hadamard (WH) spreading sequences will be considered. The presented MC-CDMA configuration is based on the transmission of multiple data per MC-CDMA symbol for each user. Data di j (n) denotes the ith, 1 â‰¤ i â‰¤ Nb, data transmitted by user j, 1 â‰¤ j â‰¤ Nu, in the nth MC-CDMA symbol.
The maximum number of available users, which is also equal to the length of the WH spreading sequences, will be denoted Nu. The total number of subcarriers is Nc = Nz + Ncu, where Nz and Ncu are the number of unused and utilized subcarriers, respectively. Therefore, the number of data transmitted by each user in one MC-CDMA symbol is Nb = Ncu/Nu. Frequency interleaving is performed in order to fully exploit the frequency diversity offered by OFDM modulation.
At the receiver part, despreading is done according to the specific user sequence after equalization in the frequency domain. Among various equalisation methods, we especially focus on single-user detection methods. Channel estimation function can efficiently be performed by utilizing pilot subcarriers insertion. The arrangement of these pilots should guarantee an optimum sampling of the channel transfer function in time and in frequency, depending on the bandwidth coherence and on the time coherence of the channel.
Obviously, MC-CDMA system offers high flexibility in resources (spectral efficiency, number of users) allocation which consequently induces large design spaces. As a result, high-level design methods are convenient in order to deal with such complexity and for efficient implementation.
MC-CDMA addresses the issue of how to spread the signal bandwidth without increasing the adverse effect of the delay spread. As a MC-CDMA signal is composed of N narrowband subcarrier signals each of which has a symbol duration much larger than the delay spread, a MC-CDMA signal will not experience an increase in susceptibility to delay increases and ISI as does DS-CDMA. In addition, since the F-parameter can be chosen to determine the spacing between subcarrier frequencies, a smaller spreading factor than one required by DS-CDMA can be utilized to make it unlikely that all of the subcarriers are located in a deep fade in frequency and consequently achieve frequency diversity.
In MC-CDMA, each data symbol is transmitted at N binary Phase Shift-Keying (BPSK) narrowband subcarriers at the same time. Distance between the each symbol is F/Tb Hz where F is an integer. MC-CDMA signals does not effect with the linear distortion due to its special signal structure in fading channel if and only if the symbol duration Tb is very higher than the delay spread Td. From the figure, each of the N subcarriers are modulated or multiplied by a single chip with respect to a spreading code of length N. various users can transmit same set of subcarrier at the same time but with a different spreading code in the frequency domain. In MC-CDMA the transmitted signals are not effected by the delay spreads due to its narrow band composition. Direct-Sequence CDMA and Wideband CDMA techniques are more effect by the delay spreads. The signal structure of MC-CDMA is same as that of the OFDM but the process in which the signals are used is very difficult.
Let the input data am[k] is binary antipodal in which m denotes the mth user and k denotes the kth bit interval. In the analysis, let us assume am[k] takes the values of -1 and +1 with equal probability. The generation of an MC-CDMA signal can be considered as follows. A single data symbol is replicated into N parallel copies. The ith branch (subcarrier) of the parallel stream is multiplied by a chip, cm [i], from a pseudo-random (PN) code or some other orthogonal code of length N and then BPSK transformed to a subcarrier spaced apart from its neighboring subcarriers by F/Tb where F is an integer number. The transmitted signal consists of the sum of the outputs of these branches. This process yields a multicarrier signal with the subcarriers containing the PN-coded data symbol.
Figure: possible implementation of a Multi-Carrier spread-spectrum transmitter.
In the analysis, we will assume a continuous-time receiver model. This model makes the analysis simpler and more instructive.
When M active users, the received signal is
To extract the desired signal's component, the orthogonality of the codes is utilized. For the ith subcarrier of the desired signal, the corresponding chip, c0[i] from the desired user's code is multiplied with it to undo the code. If the signal is undistorted by the channel, the interference terms will cancel out in the decision variable due to the orthogonality of the codes. As the channel will distort the subcarrier components, an equalization gain, d0, i may be included for each of the receiver. Applying the receiver model to the received signal provided in Eq. yields the following decision variable for the kth data symbol assuming the users are synchronized in time.
Implementation of MC-CDMA system:
Below block diagram is a possible implementation of transmitter model of MC-CDMA.
Walsh-Hadamard Code Matrix
Parallel to serial converter
Inverse Fast Fourier Transform (I-FFT)
Figure: FFT implementation of an MC-CDMA base station multiplexer and transmitter.
In the implementation of MC-CDMA transmitter model, user signals have been given to Walsh-hadamard code matrix. In this user signals are converted into orthogonal codes. These orthogonal codes are assigned to Inverse Fourier Transform (I-FFT). By doing this we can get a matrix form. Finally this matrix is converted from parallel to serial converter then signals are sending through the transmitter.
For the possible receiver model of MC-CDMA, the same process can be done in the reverse or opposite order.
EVLUATION AND TESTING
We estimated the upper-bound BER of Approach I upon combining paths in the receiver. The BER of hard-detection based on the approach was also plotted as a benchmarked, assuming that the receiver exploited the explicit knowledge of the DS patterns. The parameters related to the computations were shown in the figures. The results demonstrate that the system provides dramatic BER improvements, when the number of combined diversity paths, increases.
The proposed adaptive schemes may be applied to any multi-user frequency domain spreading downlink MC-CDMA system. The sub-carrier grouping structure and spreading confinement ensure synchronization between various users' that is essential to the recovering and division of user symbols at the receiver. There is no limit imposed on the group size, this permits various spreading code lengths to be utilized for various scenarios or for performance tuning etc. The equivalent sub-carrier concept further allows a group of subcarriers to be replaced by an equivalent sub-carrier of a conventional OFDM modem for the purpose of bit/power loading. This enables various powerful bit-loading schemes, originally developed for OFDM, to be directly deployed to MC-CDMA systems.
5. Chapter 5
5.1. Results and Discussion:
5.1.1. Probability of Error:
22.214.171.124. Probability of Error before Relaying:
126.96.36.199. Probability of Error after relaying:
As it can be seen, these two graphs plotted between Probability of Error and SNR before and after relaying is almost identical, showing that as the SNR increases the probability of error decreases. Decode-and-Forward relaying scheme was used.
188.8.131.52. Capacity before Relaying:
184.108.40.206. Capacity after relaying:
As it can be shown in the graphs plotted between capacity and SNR before and after relaying, the initial capacity was increased after relaying. Therefore, more signals can be transmitted through channel. As the SNR increases, the capacity of channel also increases. Decode-and-Forward relaying technique was used.
6. Chapter 6
As MC-CDMA will be the driving force in 4th Generation in wireless communication systems to provide high data rate. If relaying technique is used with MC-CDMA, it not only maintains the quality of signals/data which is being sent but it also combats efficiently with the interferences (such as noise interference, path loss, shadowing and multipath fading etc.).
6.2. Future Work:
For future enhancements it is suggested that Amplify-and-Forward relaying schemes should be implemented with MC-CDMA. Because in some environment Decode-and-Forward relaying works better and in some scenarios Amplify-and-Forward relaying works better. Multi-user techniques can be implemented to detect multiple users.