The Use Of Wired Communication System

Published Date: 02 Nov 2017

Introduction

Today the use of wired communication system is fading out and at the same time there is a tremendous increase in the utilization of wireless communication systems and networks. The reasons behind this change are linked with the advantages that wireless communication systems provide. The advantages include increased mobility and also significant cost saving, as there is no need to install cabling and fixed communication infrastructure.

People desires for higher internet speeds, easy access anywhere and at any time to enjoy multimedia on the move are constantly increasing. Thus future wireless communication system will have to support data with different quality of service (QOS) and also mobile Ad-hoc Networks are a good alternative where no infrastructure is required. The quality of the received signals using a wireless ad-hoc network is extremely variable due to the motion of the both the transmitter and receiver, changing path attenuation, transmission deterioration due to induced interference, and limited battery resources. Thus wireless communication is one of the most important technological research areas.

The general communication system proposed by Shannon in his work "A Mathematical Theory of Communication" can be used to understand the wireless communication system [1]. Figure 1 shows the schematic diagram of a general communication system proposed by Shannon.

Figure 1: schematic of a general communication system

He described the information source as one which produces the messages to be transmitted to the receiver. The message may be of various types, but for the case of digital communication system the messages are converted to bits.

A transmitter is the next step which is involved in the communication system; it takes the message and processes it in ways such as to produce a signal proper for transmission over the channel. For example, in a digital communication system ,voice is converted to bits by the use of pulse code modulation (PCM).Other operations in the transmitter involve encoding and modulation, encoding may be further classified as source encoding and channel encoding. Source encoding removes redundant information that may result in an unwanted delay during transmission and hence consume bandwidth.

The channel is simply the medium through which signal is transmitted to receiver and for the case of wireless communication the medium is a band of radio frequencies. Channel coding is a way to reduce the effect of noise on the signal transmitted. Channel coding is the addition of redundant bits which are used at the receiver side to reconstruct the sent message with minimum errors. Furthermore, modulation is another operation carried out at the transmitter which reduces the effect of noise on the transmitted signal. Modulation is the process which enables a message to be physically transmitted over a noisy channel. By using modulation a low frequency audio signal representing the message is conveyed in a high frequency signal by varying one or more of its characteristics

In order to satisfy the growing demand the smallest technological advancement in the field of wireless communication must be explored to construct robust transmission schemes. The factors that can be used to add capacity to wireless communication systems are radio bandwidth and transmitted power. Unfortunately, both bandwidth and power are among the most strictly limited in the operation of modern wireless networks .The allowable transmission bandwidth to be used is provided by the frequency regulator and is limited with regard to useful radio spectrum. Mobiles and other portable services such as laptops require the use of battery for power, only a small power may be supplied with the tiny battery. These two resources are merely not on the rise or improving at rates that can hold predicted demands for wireless capacity. One resource that can be used to increase the capacity of wireless system is the rapidly growing processing power, which may be used to achieve the capacity. [2]

This work focuses on increasing the capacity of the wireless communication system as well as providing unequal error protection to information with different priorities. The Shannon capacity can be approached by adapting the transmit power, data rate, and coding scheme according to the channel fade level. [3] Most applications necessitate that the bit error rate (BER) be above a maximum value. Fixed modulation has the main drawback of varying BER due to the changing signal to noise ratio (SNR) of the channel. Adaptive technique such as adaptive modulation scheme can provide a constant BER although the SNR of the channel varies. Adaptive modulation schemes give a better average spectral efficiency performance compare to fixed modulation. Depending on the channel condition the size of the modulation may be adapted in order to achieve higher throughputs [4].Often some parts of the information to be transmitted are more important, hence unequal error protection is required. For example in wireless networks ,control signals like channel state , power control ,and scheduling information often need more protection than the payload data. UEP will be required for wireless communication system to support a multitude of service[5].Hierarchical, embedded constellation also called multiresolution modulation is a practical way of achieving UEP,which is based on uniform signal constellation[6].

OVERVIEW OF ADAPTIVE MULTIRESOLUTION MODULATION

The idea of transmitting two dissimilar classes of data, a basic message and an additional message using an adaptive non-uniform PSK was proposed in [7]. Both the dimension and shape of the PSK constellations were adjusted according to the CNR estimates. In the proposed scheme it was considered that the non-uniform M-PSK constellations incorporated two parts: coarse and fine. The coarse part was uniform M-PSK of alphabet size N= M/2. The least significant bit (LSB) of the symbol was allocated to the fine part and has a different probability of BER. One parameter, the phase-offset ratio, was proposed to change according to the CNR estimate to increase the throughput for the additional message.

In [8], the authors used the idea of multi-resolution QAM constellations and their BER expressions to propose a new technique to transmit voice and two different types of data at a smaller BER each compare to their respective target BERs. The proposed system adapted not only the alphabet size but also the priority parameters of the multi-resolution signal constellation, which control the relative BERs of dissimilar bit positions. The voice bits are transmitted in the LSB-position and the remaining bit positions within the constellation are used for transmitting data .The bit position to be allocated to different classes of data depends on the selected priority parameters.

A multi-layered video transport scheme for wireless channels capable of adapting to channel conditions in order to maximize end-to-end quality of service (QoS) was proposed in [9]. This scheme employs UEP to transmit the different priority layers, UEP is achieved by using different channel codes collectively with the multiresolution modulation concept. To adapt to channel conditions joint source-channel coding (JSCC) is used ,which tries to jointly optimize the source and channel coding rates along with the modulation parameters(M PSK) to obtain the maximum achievable end-to-end QoS for the prevailing channel conditions.

In [10] signaling methods have been proposed to deliver a basic message at a particular error rate and at the same time deliver another message by exploiting any extra capability that is available. It is shown that by adapting the location of the points in a PSK constellation, the throughput can be increased for the additional message while maintaining an acceptable error rate for the basic message. Remarkable changes in channel quality are taken into consideration by adapting the PSK constellation size, signaling rate, and error-correcting code.

A simple receiver-oriented rate-adaptation approach is proposed in [11] for improving the throughput of mobile ad-hoc networks (MANETs) while transmitting multimedia data. The quality of the randomly varying wireless link is maintained at a desired level by multiplexing and mapping different traffic sources that require unequal error protection into a multiresolution modulation and dropping the less important bits when the channel condition deteriorates. The throughput is augmented by not removing the correctly received bits within the noisy received symbols with a high probability. Only 8 PSK modulation was used for transmitting the multimedia data.

To find the optimal shape of the multicast constellation, exact bit error rate expressions was found in [12] [13] for Hierarchical M-QAM and M-PSK .These expressions are not easily invertible with respect to its symbol CNR . Motivated by this fact, the efficacy of the exponential approximation for the Adaptive Multiresolution Modulation (AMM) scheme in, terms of PER using the asymmetric Phase-Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM) was proposed in [14].later on the effectiveness of the complimentary error function approximation as a superior curve fitting approximation in contrast to exponential approximation was investigated [15].

A receiver-transmitter rate adaptation strategy was proposed in [16], which employs the average packet error rate (PER) probability curve fitting approximation to establish which modulation scheme to apply for a specified PER requirement. The individual [16]PER probability curve fitting approximations are then utilized to determine which bit of the asymmetric PSK or QAM constellation to assign to each distinct queue class so that the prescribed PER requirements are satisfied. Furthermore a simple algorithm was developed for determining the optimum phase offset ratio when more than two UEP classes are considered.

Motivation

Most of the adaptive multiresolution modulation schemes for providing UEP to different classes of data that have been developed till now are based on the M PSK modulation. In the few works making use of QAM constellation the authors has not shown much interest in the channel coding. Amongst the few works that have considered channel coding along with AMM, the authors have not specified if they have taken the channel coding into consideration for choosing the shape and size of the constellation. Additionally previously proposed model have not taken into account the case when the noise power exceeds the transmitted signal power, that is when the SNR (dB) is negative. Moreover LDPC codes have not yet been used as a FEC code in conjunction with AMM.