AN INTRODUCTION OF ANALOG AND DIGITAL MODULATION TECHNIQUES IN COMMUNICATION SYSTEM Rashmi Pandey Vedica Institute of Technology, Bhopal Department of Electronics & Communication rashmipandey07@rediffmail.com Kuldeep Pandey Bhabha College of Engineering, Bhopal Department of Electronics & Communication kuldeeppandey23@gmail.com ABSTRACT Wireless communications has become one of the fastest growing areas in our modern life and creates enormous impact on nearly every feature of our daily life. A tremendous technological transformation during the last two decades has provided a potential growth in the area of digital communication and lot of newer applications and technologies are coming up everyday due to these reasons. Restricting over self to the domain of modulation techniques a brief overview over different analog and digital modulation techniques has been provided in this article through extensive literature survey in a tabular manner enabling to analyze and establish the superiority at a glance of a specific modulation technique for a particular application. Keywords: Analog Signal, Digital Signal, Modulation, Modulation Techniques INTRODUCTION: Living in the era of communication everything may be video, audio or any information in the form of electrical signal is termed as data and there is an enormous requirement of data transfer between two or more point through the world wide web, every moment of the clock, which is a big threaten to the existing communication systems because of the problems like spectral congestion, severe adjacent & co-channel interference problems and noise corrupted data reception etc. This has resulted in serious need for the research work all around the world for the development of the communication systems which can handle the above said problems, where each aspect of the communication systems is dealt with the development of new encoding techniques, modulation techniques, possibilities for newer transmission channels and off course the demodulation and decoding techniques [1, 2]. The design of a communication system is application oriented and is dependent on the type of the signal. The choice of digital communication technique over its analog counterpart becomes more evident of the fact that it provide larger immunity to noise for even at the price of large bandwidth requirements, where as the requirement of video, 80
Audio and data over the computer network or the mobile telephony network termed as the third generation (3G) mobile communication poses a serious problem for the bandwidth so The existing modulation techniques need to be modified for the purpose where it can handle both the situations of noise and bandwidth efficiency [3, 4]. The major advantage of using digital modulation technique is that the use of digital signals reduces hardware, noise and interference problems as compared to the analogue signal where large number of waveforms will be required resulting in a larger bandwidth for the symbol to be transmitted [5]. Over the past years various modulation techniques have been designed and extensively used for various applications but the modern communication system requires data transmission at a higher rate, larger bandwidth n order to have multimedia transmission, hence the existing modulation techniques are not able to provide a complete solution keeping this in the view the authors of this article have tried to draw a sketch within the existing modulation techniques to derive out exactly what modifications or the alterations in the present techniques may sort out the problem or there is still a need for designing a new modulation technique for the purpose of the present communication system requirements. CLASSIFICATION OF MODULATION TECHNIQUES: Modulation is the process of varying some parameter of a periodic waveform in order to use that signal to convey a message. Normally a highfrequency sinusoidal waveform is used as carrier signal. For this purpose,if the variation in the parameter of the carrier is continuous in accordance to the input analog signal the modulation technique is termed as analog modulation scheme if the variation is discrete then it is termed as Digital Modulation Technique. Figure 1: Types of Modulation Techniques Analog Modulation Techniques:- There are basically three type of analog modulation schemes the amplitude modulation, the Frequency modulation and the phase modulation schemes which have in turn lot of class, subclass or derivatives as listed in Table-2. In case of the Amplitude Modulation there are several derivatives and it is evident from the comparative table -3 that the Single Side Band Suppressed Carrier (SSS-SC) has smaller bandwidth and power requirements in contrast with Double Side Band Suppressed Carrier (DSB SC) and Double Side Band Full Carrier (DSB FC) and Single Side Band Full Carrier (SSB FC) but for detection of this signal, we require sharp cutoff Low Pass Filter (LPF) which is not practically viable. Using the Vestigial Side Band (VSB) technique in place of (SSB SC), we can achieve a low pass filter with a gradual cut off but it requires more BW and power than SSB-SC and less then the DSBSC and DSB-FC and hence ideally SSB- SC is proves to be better than other AM schemes but practically, VSB proves to be a much better candidate then the other amplitude modulation techniques. 81
The Amplitude modulated signals require nonlinear amplifiers which generate spurious outof-band spectral components which are filtered out with a great difficulty. Frequency Modulation proves to be better in comparison to amplitude modulation and phase modulation, and the derivative of frequency modulation, narrow band FM (NBFM) is usually employed to overcome above mentioned problems in the communication system. Digital Modulation Techniques:- After the conversion of an Analog signal to digital by sampling different type of digital modulation schemes can be achieved by the variation of different parameter of the carrier signal for example the Amplitude variation gives BASK, Frequency variation gives BFSK and the phase variation gives BPSK. Also sometimes a combinational variation of this parameter is done to generate the hybrid modulation technique viz. a combinational variation of Amplitude and Phase Shift Keying (APSK). Many more digital modulation techniques are available and can also be designed depending upon the type of signal and the application. Thus a better digital modulation technique is to be thought over by the designer which has an ability of exploiting the available transmitted power and the bandwidth to its full extent. In order to achieve a discrete signal it is essential to have the modulating signal of the form of a NRZ rectangular pulse thus yielding the modulated parameter as a discrete signal switching or keying between two discrete values. However, ASK, FSK, and PSK with Nyquist pulse shaping at the base band form the basic technique of digital modulation, but other methods are also possible with hybridization of two or more basic digital modulation schemes with or without pulse shaping. CLASSIFICATION OF DIGITAL MODULATION: These digital modulation techniques can be classified basically either on the basis of their detection characteristics or in terms of their bandwidth compaction characteristics. Various types of digital modulation techniques are listed in Table-4 and few of them have been comprehensively emphasized here in details providing a comparative analysis. Binary Amplitude Shift Keying [BASK] The BASK is obtained by the alteration of the amplitude of the carrier wave [1]. It is a coherent modulation technique hence the concept of the corelation between the signal, number of basic functions, the I and Q components and the symbol shaping are not applicable here. It has very poor Bandwidth efficiency. The basic merit of this technique is its simple implementations but is highly prone to noise and the performance is well established only in the linear region which does not make it a viable digital modulation technique for wireless or mobile application in the present scenario. The combination with PSK yields derivatives like QAM and Mary ASK, which have much important application with improved parameters. Binary Frequency Shift Keying [BFSK] When two different frequencies are used to represent two different symbols, then the modulation technique is termed as BFSK.BFSK can be a wideband or a narrow band digital modulation technique depending upon the separation between the two carrier frequencies, though cost effective and provides simple implementations but is not a bandwidth efficient technique and is normally ruled out because of the receiver design complexities. 82 Binary Phase Shift Keying [BPSK]
When the phase of the carrier wave is altered with reference of the modulating signal then the resultant modulation scheme is termed as Phase Shift Keying. The digital modulation technique can be said to be the simplest form of phase modulation and is known as binary because the carrier phase represents only two phase states. It is normally used for high speed data transfer application, provides a 3dB power advantage over the BASK modulation technique and is robust and simple in implementation but proves to be an inefficient user of the provided bandwidth and is normally termed as a non-linear modulation scheme. It provides small error rates than any other systems. The modulation techniques provide a number of derivatives. Differential Phase Shift Keying [DPSK] For the perfect detection of a phase modulated signal, it become evident that the receiver needs a coherent reference signal but if differential encoding and phase shift keying are incorporated together at the transmitter then the digital modulation technique evolved is termed as Differential Phase Shift Keying [1, 14]. For the transmission of a symbol 1, the phase is unchanged whereas for transmission of symbol 0, the phase of the signal is advanced by. The track of the phase change information which becomes essential in determining the relative phase change between the symbols transmitted. The whole process is based on the assumption that the change of phase is very slow to an extent that it can be considered to be almost constant over two bit intervals (7). Quadrature Phase Shift Keying [QPSK] Another extension of the PSK digital modulation technique is the division of the phase of the carrier signal designed by allotting four equally spaced values for the phase angle. QPSK has four message points in the constellation diagram and so it becomes a highly bandwidth efficient digital modulation technique. But the exact phase retrieval becomes a very important factor for the receiver design considerations, failing which can give rise to erroneous detection of the signal. This factor increases the receiver design complexities. To compensate for these problems, normally the idea of pulse shaping the carrier modulated signal is employed with the Root Raised Cosine Pulse shaping for achieving better performances which in turn provides a demerits that the constant envelope property of the signal is lost but then there is a lost but there is a remarkable improvement in the ISI performance of this digital modulation technique. Minimum Shift Keying [MSK] Minimum Shift Keying (MSK) is a modified form of continuous phase FSK. Here, in this case, the spacing between the two carrier frequencies is equal to half of the bit rate which is the minimum spacing that allows the two frequencies states to be orthogonal [1-3]. An MSK signal can e said to be derived from either an Offset Quadrature Phase Shift Keying (OQPSK) signal by replacing a square pulse by ½ co sinusoidal pulse or alternatively from an FSK signal. The information capacity of an MSK signal is equal to that of QPSK signal but due to the ½ cosine pulse shaping the bandwidth requirement is lesser than that required by QPSK. It achieved smooth phase transitions thus providing a constant envelope. It has lower out of band power and can be said to be more spectrally efficient than the QPSK modulation technique. The major demerits which this digital modulation scheme suffer s is that it is in the class of linear modulation. The spectrum is not enough compact to realize data rate approximating RF channel bandwidth. Table-2 summarizes representation and different properties of this technique. 83
Orthogonal Frequency Division Multiplexing (OFDM):- The OFDM is a modulation scheme having multicarrier transmission techniques here the available spectrum is divided into many carriers each one being modulated at a low rate data stream. The spacing between the carriers is closer and the carriers are orthogonal to one another preventing interferences between the closely spaced carriers hence OFDM can be thought of as a combination of modulation and multiplexing techniques, each carrier in a OFDM signal has very narrow bandwidth so the resulting symbol rate is low which means that the signal has high tolerance to multipath delay spread reducing the possibility of inter symbol interferences (ISI) which is the requirement for The higher is the transmission rate, the large will be the bandwidth of the signal as compared with the coherence bandwidth of the propagation channel, at this stage the different spectral components present in the signal will experience different fading characteristics, this frequency selective fading has to be characterized using appropriate techniques in order to achieve acceptable error rate at the detection or output in order to achieve characterization in frequency selective fading the 84 basic approach is to partition the signal into frequency bands, each one of which is narrow as compared to the coherence bandwidth of the channel and subsequently each of this signal component is then modulated onto a different sub carrier and the signal components are sent parallel over the channel. Hence, each signal component will now experience non- frequency- selective fading because now the high rate serial data sequence is converted into a number of lower rate parallel sequences and then each of them is modulated onto a sub carrier, the effective method to achieve this is orthogonal frequency division multiplexing (OFDM). Conclusions: An analysis of the digital modulation technique carried out in this article reveals that the selection of a digital modulation technique is solely dependent on the type of application. This is because of the fact that some of the technique provide lesser complexities in the design of the modulation and demodulation system and prove economic like the BASK, BFSK, BPSK and DPSK techniques and can be visualized for the systems which really does not require high amount of precisions or when economy is the major aspect and the BER performances can be tolerated. Criterion for higher data rate communication is taking the lead in almost every area of communication and thus the ISI and BER realization become very important and crucial aspect for any future digital modulation technique. Taking the above facts into consideration, the design of a digital communication system is very trivial and is very much applications oriented, as one application may require higher precision in data reception where as the other may compromise on this aspect but may be rigid on the aspect of the available bandwidth or power, thus the parameters like the modulation bandwidth, power, channel noise and the bit error rate become very important parameters in the designing of digital/wireless communication system.
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