Numerous technologies were evolved extensively

Numerous technologies were evolved extensively

CHAPTER 1: INTRODUCTION 1.1 OVERVIEW

In the earliest 20th century, numerous technologies were evolved extensively to provide novel methods and new products for the wireless communications, more significantly in the last one or two decades, wireless communication services had been penetrating in our society with tremendous growth rate. In the thesis the main concept of spread spectrum communication, particularly the cellular communication systems based on it is discussed, in order to understand the concepts of multiple access technologies the below section discuss the generation of the Wireless Communication Systems in detail.

1.2Generations of Wireless Communication Systems

Cellular generations are basically classifies on the accessing technologies they use the detailed description of the generation of wireless communication systems is provided below:

1.2.1First Generation (1G) Wireless Communication Systems:

The very first cellular systems that were launched in the world were AMPS (i.e. Advanced Mobile Phone System), NMT (i.e. Nordic Mobile Telephone), and the TACS (i.e. Total Access Communication) [1]. These systems were designed such that these could carry voice transmission only. And these systems were based on assigning a unique frequency band to individual user. The way users could access the system was based on the principle of separating the signals of multiple users into the frequency domain known as Frequency Division Multiple Access (FDMA). In those days the market experienced tremendous growth rates which were from 30% to 50% and the user density was about 20 million by the end of 1990[2]. Second Generation (2G) Wireless Communication Systems:

Digital technologies shifted the 1st generation technologies to the 2nd generation technologies, 2G system were based on the multiple access techniques known as Time Division Multiple Access (TDMA) along with Code Division Multiple Access (CMDA). The first commercial system based on TDMA was known as Global System for Mobile Communication (GSM) in the year 1991 in Europe, other 2nd generation systems are IS-54 (i.e. Interim Standard 54) in North America and the PDC (i.e. Pacific Digital Cellular) in Japan, and the 2nd generation system based on CDMA was known as IS-95 (i.e. Interim Standard 95). In the mid 1990s cellular communication industry experienced extensive growth, there were about 600 million cellular subscribers worldwide at the end of 2001 [3]. Third Generation (3G) Wireless Communication Systems:

3G wireless communication systems were actually developed to support high rate data services for example high speed internet access, videoconferencing, and the high quality image transmission). 3G wireless communication systems were initiated by the IMT-2000 (i.e. International Mobile Telecommunication 2000), which is defined by a set of independent ITU (i.e. International Telecommunication Union) Recommendations. In the meeting of the IMT-2000 a framework or particular rules for worldwide wireless access were devised by linking diversified systems based networks [4]. The important systems that were recognized were the European and Japanese Wideband-CDMA (i.e. WCDMA), besides these systems American CDMA2000 was also recognized, and also the Chinese Time Division Synchronous CDMA (i.e. TD-SCDMA) was also recognized as the 3G system. The above mentioned standards are basically based on CDMA and the operating range is about 2GHz. If we look at all over the world there are mainly four companies which have planned to launch 3G systems, which are Hutchison 3G Limited, Sunday 3G(Hong Kong), SmarTone 3G Limited and CSL Limited, these are the companies who have obtained the license from telecommunications authority, Hong Kong in the end of 2001 [5]. And CSL Limited was the first operator to launch 3G services in Hong Kong in the end of 2004Fourth Generation (4G) Wireless Communication Systems:

The technologies never stop from evolving so the 4G wireless communication Systems are being researched, before the 3G Systems were deployed, there was a debate within the researchers about the systems beyond 3G, so the researchers started investigating about the 4G systems. The standardization for the 4G systems is still on the way. In June, 2003 a recommendation was approved by ITU (i.e. ITU-R M.1645, “Frameworks and overall objectives of the future development of IMT-2000 and systems beyond IMT-2000”). This document details the expected capabilities along with the possible solutions for the 4G wireless communications systems.

Features of 4G Wireless Communication Systems:

4G systems are expected to provide high data rates up to 100 Mbps for the mobility purpose, as an example for the mobile access, for the local wireless access with low mobility it is expected to provide a data rate up to 1Gbps

4G systems are expected to use a packet based architecture which will be offering an increase in the system security along with the reliability, also the intersystem mobility and finally the interoperability capabilities.

4G systems are expected to satisfy the future requirements for the wireless network universally so that it will provide a high data rates and seamless interface with the wired backbone network

1.3MOTIVATION

The Multiple Access (MA) schemes are normally differentiated on the principle that how user would be accessing the system resources, and whether that access is orthogonal or not. There are two broad categories of MA, Orthogonal MA schemes and Non-orthogonal MA schemes, the examples of orthogonal MA schemes are TDMA and FDMA whereas the examples of non-orthogonal MA schemes are CDMA and recently developed scheme known as IDMA (Interleave Division Multiple Access).

IDMA scheme has recently been developed as the alternate solution to the CDMA scheme. The concept of IDMA was actually developed with the combination of the code spread CDMA [6], and also with the concept of and it potentially inherits many attractive properties of CDMA. The IDMA philosophy was then further developed by Ping and his team.

1.4.AIMS & OBJECTIVES

The aims and objectives of thesis are divided as follow:

The first objective of the thesis is to investigate the performance of the MC-IDMA system into fading channel using BPSK and QPSK modulation method and also to compare the performance.

Secondly the objective of this work is to investigate the performance of the MC-CDMA systems and compare the performance results to the MC-IDMA based system.

1.5 ORGANIZATION OF REPORT

The further chapters in this thesis work are organised as follow:

The second chapter discuss the fundamental concepts of MC-CDMA systems, furthermore the transmitter and receiver structure along with the detailed explanation. Also the key concepts of IDMA system in the multipath fading channels are discussed in order to provide the base line for the next chapter.

In the third chapter, the proposed system model is discussed along with detail discussion of the transceiver structure also the derivation is discussed in order to carry the detection analysis, finally the methodology is discussed in order to provide the necessary concepts for the simulation.

In this thesis the fourth chapter discuss the simulation results based on the proposed system model, this chapter is divided into the three scenarios, the first scenario analyze the performance of the MC-IDMA system using the BPSK modulation technique using the real channel parameters, second scenario provide the MC-IDMA system analysis using the BPSK and QPSK modulation technique and also the comparison of the first and second scenario is provided. The final scenario compares the performance of the MC-CDMA system with the MC-IDMA system and the results are discussed.

The final chapter of this thesis work discuss the overall conclusion of the simulation results along with the future recommendation.

CHAPTER 2: BACKGROUND & LITERATURE REVIEW 1.5 BACKGROUND

One of the promising candidate for implementing the physical layer for the 4G wireless communication system is known as MC-CDMA or OFDM-CDMA. Allot of research has already been carried on OFDM-CDMA. But the problem is there are few disadvantages and also difficulties in the systems based on OFDM-CDMA. This scheme OFDM-CDMA employs the concept of mutual orthogonal codes to identify multiple users, the problem with this scheme is frequency selective channels can compromise the orthogonality among multiple users, which eventually results in the problem known as MAI.as far as the practical applications are concerned

It is considered in , that the MAP (i.e. maximum a posteriori) has exponential complexity level with respect to the number of users (i.e. K). besides this, linear multiuser detectors of the conventional OFDM-CDMA system for examples, the decorrelator and the linear MMSE (i.e. minimum mean square error) have complexity level quadratic in nature with respect to the number of users (i.e. K), this complexity is due to the number of operations presented in resolving the phenomenon of correlation between the spreading sequences. If the K is large enough, it can be computationally prohibited for implementing practically.

Systems based on MC-CDMA normally works on the concept related to the spreading sequence, which is utilized to identify different users. In another way if we discuss it in the coding theory point of view, it is not a considerable option to utilize spreading sequence concept for separating the users, because spreading will result in the bandwidth expansion without having suitable coding gain. In [18] and some theoretical analysis are carried to describe that the capacity of multiple access channel can be approached only if the bandwidth expansion is entirely devoted to the Forward Error Correction coding.

A new concept of developing a hybrid communication scheme is recently proposed in which OFDM is combined with the IDMA known as OFDM-IDMA in [20] and [21]. In this scheme IDMA is employed instead of CDMA as in OFDM-CDMA, so this scheme potentially inherits many features of the OFDM-CDMA based systems, for example this scheme has simple treatment for ISI and has effective mitigation against the cross cell interference as well. And also some additional advantages are also incorporated because of IDMA, particularly IDMA has a very simple and effective turbo type iterative Multi User Detection algorithm which is applicable to systems having large number of users, which in face is crucial for systems in order to achieve high throughput. The algorithm is known as Chip by Chip detection (i.e. CBC), this algorithm has a complexity level linear in nature with respect to the number of user (i.e. K), because there are random interleavers to distinguish multiple signals from the multiple users due to this the spreading operation can simply be avoided in this scheme.

In this situation, the OFDM-IDMA scheme devotes entire bandwidth expansion to FEC coding and obtains additional coding gain. It is resolve this problem, it is required is always dependent on the options of signature sequences

As it is a fact that the performance is not actually sensitive to the available options of spreading sequence when we have long random spreading codes, this phenomenon makes it mathematically demanding to analyze the performance using large random matrix theory as described is a system which does not involve signature sequences, and because of because of this MC-IDMA system problems.

OFDM based system can be analyzed simply by a method known as SNR evolution, this method can be used find the optimization of the system performance can be further optimized by the power allocation.

2.2 Multicarrier CDMA (MC-CDMA) MC-CDMA is basically a technology which is based upon the concept spread spectrum principle along with the concept of combining multicarrier modulation and CDMA.

By the beginning of 1993, various multicarrier CDMA based schemes were proposed; those schemes can be differentiated on the basis of spreading and modulation type, so there are three categories: MC-CDMA, MC-DS-CDMA (i.e. Multicarrier Direct Sequence CDMA) and MT-CDMA (i.e. Multitone CDMA as in [23]. In this work the focus is only on the MC-CDMA which is also known as OFDM-CDMA. This scheme combines the benefits of OFDM and CDMA and by this it provides very effective solution for multiuser communication systems especially over the multipath channels in any cellular environment.

OFDM with sufficient Guard Interval can completely remove Intersymbol Interference. CDMA scheme can provide effective solution to mitigate the cross cell MAI problem if we compare it with other orthogonal multiple access schemes, and due to its spread spectrum nature CDMA is more robust against fading channels, so it is considered as a promising candidate for the implementation of physical layer of the future wireless systems. Allot of work is carried on this concept in.

The transmitter block diagram of the OFDM-CDMA considering the uplink scenario with the simultaneous numbers of active users (i.e. K) is illustrated in the

Transmitter block diagram of OFDM-CDMA considering the uplink scenario with the simultaneous numbers of active users (i.e. K)

Consider the data stream of K users. Following the channel coding as well as the symbol mapping, one can obtain the complex valued symbols sequences , where the symbol J is representing the frame length. is spread by a spreading sequence. The resulting effect of the spreading process provides sequence with elements given as

denoted the spreading sequence for K users having length represented by S.

the developed sequence is actually OFDM modulated onto individual subcarriers. This spreading operation is carried out in frequency domain for OFDM-CDMA. This must be modulated to more than single OFDM symbol. If several elements of can be modulated in parallel. By specifying S as the factor of ), the complexity of transmission can be reduced. To make the derivation simple, we assume .

This is only considered to make a simpler notation.

2.4Receiver block diagram of OFDM-CDMA system

The receiver block diagram of OFDM-CDMA based on soft PIC (i.e. Parallel Interference Cancellation) is depicted in the figure 2.2. the detector in Figure 2.2 is comprised of a detector known as MMSE (i.e. Minimum Mean Square Error) detector block, K, MRC (i.e. Maximum Ratio Combining) single user detector blocks and the interference cancellation process .………………………………………………..Equation 2.3

If we consider the transmission into the frequency selective channels, there is a chance that the orthogaonality among various users can be destroyed and MAI can be occurred. Various data detection techniques have been introduced to suppress the effects of MAI, in general these data detection techniques are classified into two categories: SUD (i.e. Single User Detection) and MUD (i.e. Multi User Detection) techniques.

Single User Detection is designed by an adaptive single tap equalizer which compensates the amplitude and the phase distortions, which are imposed on the received signal during the transmission. SUD does not need to know about the information of Multi Access Interference. The block of the SUD in the Figure 2.2 utilizes the Maximum Ratio Combining equalization

Multi User Detection technique has the capability to improve the uplink OFDM-CDMA performance effectively by exploiting the MAI information. There are two MUD techniques depicted in Figure 2.2, PIC and MMSE [28]. The process of soft PIC of OFDM-CDMA system depicted in Figure 2.2 is summarized as follow:

  1. At the first, MMSE MUD is implemented to alleviate MAI and the desired signal is detected. It should be noted that MMSE MUD can only work during th entire detection process.
  2. The resulting interference is reconstructed and subtracted from original signal for each individual user
  3. MRC SUDs having low complexities are applied to detect the deseired signal

And finally the iterations are repeated out between second and third steps. Now the details of individual data detection are described as follow:

Maximum Ration Combining:

in the MRC SUD equalization is used to maximize the SNR (i.e. Signal to Noise Ratio) for the case of single user. But when the system has various users simultaneously activated, the MAI can not be suppressed so eventually MRC performance is poor, but due to iterative process, the received signal is MAI free, MRC can become more efficient with low cost in that particular case. The resulting equalization coefficients can be represented as:

2.6Conclusions

if we try to summarize all the points in this chapter, then initially some preliminaries were discussed for this thesis, we started with the fundamentals of OFDM modulation, then a review of fundamentals of OFDM-CDMA systems was presented along with the detection algorithms based on PIC was also discussed. Then we presented the basic architecture of IDMA along with its detection algorithm particularly in fading channels. By understanding these techniques it is necessary to investigate the spread spectrum multiple access communication system based on the combination of OFDM and IDMA.

CHAPTER - 03: Proposed System Model for MC-IDMA based System

In this chapter the uplink system model for the MC-IDMA (Multi-Carrier Interleave Division Multiple Access) is proposed along with the transmitter and receiver structure. The chapter also discusses the iterative detection process used by this system known as Chip-by-Chip (CBC) detection algorithm.

3.1 Transceiver structure of MC-IDMA based system

where we have assumed that the extrinsic LLRs for the complex parts i-e, real and imaginary parts of are uncorrelated, and thus the off-diagonal entries of are considered zeros.

In the iterative process, ESE and DEC-k exchange the extrinsic information about The CBC detection for OFDM-IDMA can be concluded as follows

ESE generates by equation 3.11 for DEC-k.

DEC-k generates , which are used to update mean and variance of

CHAPTER - 04: Simulation Results & Analysis

This chapter presents the simulation results based upon the analytical model for the MC-IDMA based system presented in the chapter-03, Also the detailed description about the multiple scenarios is also discussed keeping in the mind that the scenario are only subjected to the uplink case of a mobile network. The brief description of the simulation scenarios is given below:

In the first scenario the initial parameters for the IDMA and OFDM are set then the calculation of the performance of proposed MC-IDMA model using the BPSK modulation with real channel parameters is carried out.

In the second scenario the initial parameters for the IDMA and OFDM remain same but the analysis is extended to the QPSK modulation.

In the third scenario the performance of MC-CDMA based system is compared with the performance of the MC-IDMA based system.

The detailed description about the scenarios and the simulation parameters is discussed below:

4.1 First Scenario

In the first scenario of the simulation, the initial parameters for the IDMA and OFDM are provided to calculate the analytical performance of the MC-IDMA based system in the multipath fading channel, the modulation technique considered for this scenario is Binary Phase shift keying (BPSK) ,the detailed parameters for the simulation are provide in the Table-4.1.

Fig-4.1 is the Bit Error Rate (BER) performance of the MC-IDMA based system using BPSK modulation and real channel parameters (i-e., fading coefficients), these parameters are calculated by special function known as the damping method. So if we analyze the response of the Fig-4.1 one can clearly observe that the Bit Error Rate (BER) performance of the MC-IDMA based system when Data=128 and SL=16 is much better as compared to the performance at Data=256 and SL=32, and furthermore the performance of MC-IDMA based system at Data=512 and SL=32 is even worse. So one can conclude that the MC-IDMA based system work efficiently at the values Data=128 and SL=16 if you increase these values the performance degrades.

IDMA Parameters

OFDM Parameters

NUser

1

Number of parallel channel

128

DataLen

128

256

512

FFT length

128

SpreadLen

16

32

64

Number of carrier

128

ItNum

10

Modulation

BPSK

Block

200

Bit rate per carrier

256000

EbNoNum

10

Guard interval

32

EbNoStart

0

EbNoStep

2

Table-4.1: Simulation Parameters for the First Scenario.

4.2 Second Scenario

In the second scenario of the simulation, the initial parameters for the IDMA and OFDM are provided to calculate the performance of the MC-IDMA based system in the multipath fading channel, the modulation technique considered for this scenario is Binary Phase shift keying (BPSK) as well as the Quadrature Phase shift keying (QPSK), the detailed simulation parameters are provided in the Table-4.2. Furthermore the performance of the MC-IDMA using BPSK is compared with MC-IDMA using QPSK.

IDMA Parameters

OFDM Parameters

NUser

1

Number of parallel channel

128

DataLen

128

256

512

FFT length

128

SpreadLen

16

32

64

Number of carrier

128

ItNum

10

Modulation

BPSK & QPSK

Block

200

Bit rate per carrier

256000

EbNoNum

10

Guard interval

32

EbNoStart

0

EbNoStep

2

Table-4.2: Simulation Parameters for the Second Scenario. 4.3 Final Scenario

In the final scenario of the simulation, initially the parameters for the CDMA and OFDM are set to simulate the performance of the MC-CDMA system using the BPSK modulation, the result of the MC-IDMA are considered same as the 1st scenario. The initial parameters are provide in the Table-4.3, if we carefully observe the Fig-4.3 one can observe that the MC-IDMA system outperforms the MC-CDMA system in terms of the BER which is a very useful result in order to consider a more potential candidate for the 4th generation mobile networks.

CDMA Parameters

OFDM Parameters

NUser

1

Number of parallel channel

128

DataLen

128

FFT length

128

SpreadLen

16

Number of carrier

128

ItNum

10

Modulation

BPSK

Block

200

Bit rate per carrier

256000

EbNoNum

10

EbNoStart

0

EbNoStep

2

Table-4.3: Simulation Parameters for the Final Scenario.

CHAPTER - 05: CONCLUSION AND FUTURE RECOMMENDATION 5.1 CONCLUSION OF WORK

In this thesis a hybrid multiple access schemes is proposed which is known as MC-IDMA, this schemes has potentially the advantage of OFDM and IDMA for example it can produce an effective way to mitigate the problems known as ISI and MAI. If we place sufficient guard intervals with the help of OFDM ISI can be completely removed. And with the help of IDMA with its special iterative detection process the intercell and intracell MAI can easily be compensated. In this thesis an effort of comparison between the MC-CDMA and MC-IDMA is performed, it is observed that in term of the Bit Error Rate analysis MC-IDMA can outperform the performance of MC-CDMA, because MC-IDMA has very effective iterative detection process known as CBC detection algorithm. Another advantage of the proposed scheme is that it can achieve additional coding gain as compared to MC-CDMA, which is achieved by allocation bandwidth expansion to FEC coding. It can be concluded here that MC-IDMA can provide an effective solution to the needs of high rate multiuser communication systems in the channels having multipath fading.

5.2 FUTURE RECOMMENDATION

when we consider OFDM, every single user utilize all subcarriers equally by doing this the characteristics of frequency selective channels a re transforms into the frequency flat channels, which can be very efficient to analyze the channel frequency selective nature, so the performance can be improved by careful subcarrier and rate allocation. No system is perfect so the proposed system also has problems, major problem is utilizing the OFDM which results in the phenomenon known as PAPR (i.e. Peak to Average Power Ratio, in this phenomena there are large peaks in the transmitted signal, when we have signal from number of subcarriers adding constructively in phase there will be large signal peaks which will result in the saturation at the power amplifiers at the transmitter side, and there will be distortions in the transmitted signal. To alleviate this problem it is desirable that the power amplifiers operate in the linear range, also it is desirable to alleviate the power of transmitted signal. Also it is desirable to design clipping-compensation algorithm to compensate the PAPR problem.

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