ICT Projects: INTRODUCTION of WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS-INTRODUCTION

Saturday, April 28, 2012

INTRODUCTION of WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS-INTRODUCTION

PERFORMANCE ANALYSIS ON MODULATION TECHNIQUES IN

WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS

INTRODUCTION

1.1     Introduction

The growing demands for voice and multimedia services on mobile wireless communications spur the advancement of the wireless communication field in the recent decade. The evolving technologies enable this to happen. One of the major underlying technologies is the digital modulation technique which allows digitized data to be carried or transmitted via the analog radio frequency (RF) channels.

Digital modulation techniques contribute to the evolution of our mobile wireless communications by increasing the capacity, data rate as well as the quality of the wireless network.

One main issue of these high data rate networks is the WCDMA system that uses modulation technique and error correction mechanism for delivery high data rate in multipath fading environment.

1.2     Background of WCDMA

WCDMA is based on CDMA technology, where all users have access to the entire bandwidth. This is accomplished by giving each user their own unique code. Two other multiple access schemes that have been used in first and second generation cellular mobile systems for sharing the spectrum are Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). In TDMA the users have been assigned time slots and in FDMA they have been given their own frequency band to send the information in.

The user specific code in a CDMA system can be used in two different ways, either the user's code is applied directly on the signal, direct sequence, or it is used to determine a frequency utilization pattern, frequency hopping. The disadvantage with frequency hopping schemes is that they require advanced modulators [1]. Similar to the American standard IS-95 for second generation of mobile systems, WCDMA utilizes direct sequence CDMA.

WCDMA has a bandwidth of 5 MHz, while IS-95 with a bandwidth of 1.25 MHz is referred to a narrow-band CDMA system. WCDMA uses frequency division duplex (FDD) and has a flexible carrier spacing of 4.4-5.0 MHz with a carrier raster of 200 kHz. The carrier raster has been chosen to provide coexistence and inter-operability with GSM.

The key features of WCDMA include full support of the third generation service requirements including user rates up to 2 Mbps and efficient packet transport; improved capacity and coverage compared to second generation systems due to a higher bandwidth and coherent uplink detection; support for technologies such as adaptive antennas and multiuser detection; support for hierarchical cell structures (HCS), and possibility for asynchronous base station operation [2].

To combat the problem of fading dips, WCDMA uses fast power control and the inherent diversity reception of the Rake receiver to mitigate the problem of fading signal power. Strong coding and modulation techniques are used to enhance performance of WCDMA system.

1.3     Modulation Technique

There are several reasons for using a modulator when transmitting data. One of them is that the system can easily convert binary data into other data types such as complex numbers. This is a more appropriate data type to send to a transmitter because a complex number can both state the signals amplitude and phase. Another reason for using a modulator is that it can reduce the sample rate, depending on what kind of modulation is used [3].

In this thesis we use, the QPSK and 16-QAM modulation. Basically both schemes work in the same way, the modulator maps binary data bits to complex numbers. In QPSK the modulator maps two binary data bits into one predefined complex number resulting in four specified constellation points. In 16-QAM four binary data bits are mapped to one complex number resulting in 16 specified constellation points.

1.4     Problem Background

The revolution of wireless networking continues to unfold with the gradual emergence of WCDMA-based third generation (3G) cellular technologies, such as Universal Mobile Telecommunications System (UMTS). W-CDMA uses noise-like broadband frequency spectrum where it has high resistance to multipath fading where as this was not present in conventional narrowband signal of 2nd generation (2G) communication system. High data rate signal transmission can be transmitted over the air by using W-CDMA System. So we need suitable modulation technique and error correction mechanism to be used in W-CDMA system for transmitting   multimedia rich applications such as video streams and high 1resolution pictures to end users.

In 2G networks, GMSK modulation scheme is widely used in GSM (Global System for Mobile Communication). This modulation can only transmit data rate of 1 bit per symbol. So it is quite sure that this kind of modulation scheme is not suitable for the next generation communication system. So, there is a need to study the performance of new modulation technique that could deliver higher data rate effectively in a multipath fading channel.

1.5     Problem Definition

High data rate modulation scheme is one of the important criteria besides good error coding, to deliver multimedia content application over the cellular networks. However, the implementation of high data rate modulation techniques that have good bandwidth efficiency in W-CDMA cellular communication requires perfect modulators, demodulators, filter and transmission path that are difficult to achieve in practical radio environment. Modulation schemes which are capable of delivering more bits per symbol are more immune to error s caused by noise and interference in the channel. Moreover, errors can be easily produced as the number of users is increased and the mobile terminal is subjected to mobility.

1.6     Previous Work

The current trend to achieve high data rate cellular communication drives the interest of this research. There are many ways one can improve data rate in a W-CDMA system. However, two significant areas that could give significant boost to the improvement of W-CDMA system is modulation scheme and error correction or channel coding.

There are many modulation schemes that have the potential to deliver higher data rate but there is a trade off between data rate and multipath environment. Modulation techniques that can deliver more bits per symbol normally generate lots of error when they are subjected to multipath channels. Recently, there is intensifying research about Adaptive Modulation and Coding (AMC) [4]-[6]. The principle of AMC is to change the modulation and coding format (transport format) in accordance with instantaneous variations in the channel conditions, subject to system restrictions. AMC extends the systems ability to adapt to good channel conditions. Channel conditions should be estimated based on feedback from the receiver. For a system with AMC, users closed to the cell site are typically assigned higher order modulation with higher code rates (e.g. 64 QAM with R=3/4 Turbo Codes). On the other hand, users closed to the cell boundary, are assigned lower order modulation with lower code rates (e.g. QPSK with R=1/2 Turbo Codes). AMC allows different data rates to be assigned to different users depending on their channel conditions. Since the channel conditions vary over time, the receiver collects a set of channel statistics which are used both by the transmitter and receiver to optimize system parameters such as modulation and coding, signal bandwidth, signal power, training period, channel estimation filters, automatic gain control, etc [7].

Thus, this thesis will scrutinize suitable modulation techniques that are capable to deliver highest data rate without compromising errors in multipath fading environment. The performance of these modulation techniques will be simulated by using MATLAB.

1.7     Objectives

The objective of this research is focus on the study and the performance measurement of high data rate modulation schemes at those channels which are subjected to Multipath Rayleigh Fading and Additive White Gaussian Noise (AWGN). Modulation Schemes that will be studied are 16-ary QAM (Quadrature Amplitude Modulation) and QPSK (Quadrature Phase Shift Keying).The performance study will be carried out by varying the chip rate of pseudo noise generator. W-CDMA (Wideband Code Division Multiple Access) scheme will also be studied by comparing some certain number of users under static and dynamic environment that are subjected to AWGN and multipath Rayleigh fading. The performance of fading channels in W-CDMA system are based on Bit Error Rate (BER) W-CDMA system at downlink transmission and Signal-to-Noise ratio (SNR).There will be three W-CDMA wireless cellular system models that will used in this research. The models are

1. W-CDMA system in AWGN channel.

2. W-CDMA system in AWGN and Multipath Rayleigh Fading.

3. Multi-user W-CDMA system in AWGN and Multipath Rayleigh Fading.

There are some parameters for multiple rays using QPSK and QAM in W-CDMA system

models that will be obtained using MatLab. They are

1. Bit Error Rate (BER) versus Signal-to-Noise ratio (SNR) in AWGN channel for QPSK modulation technique.

2. BER versus SNR in AGWN channel for 16-QAM modulation scheme.

3. BER versus SNR in AWGN and multipath Rayleigh fading channel with Doppler shift (60kmph, 90 kmph,120kmph) for QPSK modulation scheme.

4. BER versus SNR to compare between AWGN channel and Rayleigh fading channel for different number of user for 16-ary QAM modulation technique.

5. BER versus SNR to compare between AWGN channel and Rayleigh fading channel for different number of user for QPSK modulation technique.

Once data for BER and SNR under various parameters are obtained, the data are tabulated. Graphs of BER as a function of SNR under different modulation techniques as well as different velocities of mobile terminal subjected to noise and interference channel will be plotted. These graphs will be studied and compared.

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Saturday, April 28, 2012

INTRODUCTION of WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS-INTRODUCTION

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