Wireless Communication: the Modulation Technique - Book Report/Review Example

This report is an in depth study of modulation which is a technique used in wireless communication. First an introduction to wireless communication is given. In the next section we define the term Modulation and give its types. Then in the subsequent sections a comprehensive study about modulation and its types is presented. We begin with the Amplitude Modulation and study its types i.e. Double side band with suppressed carrier (DSB-SC) Amplitude Modulation, Quadrature Amplitude Modulation (QAM), Single Side Band (SSB) Amplitude Modulation and Vestigial Side Band (VSB) Amplitude Modulation. Then Angle modulation is discussed with its two types i.e. frequency modulation and phase Modulation. Advantages and disadvantages of each technique are discussed. Generation of modulated signals is also discussed briefly along with useful equations and figures. In the end an appendix is presented in which MATLAB program is presented for Amplitude Modulation with the snapshot of the resulting graphs of signals. A similar program for Frequency Modulation is presented with resulting graphs of signals. Table of contents: - S. NO Title Page NO 1- Introduction to wireless communication. 1 1.1- History of wireless communication 1.2- Techniques used for wireless communication 2- Modulation 2.1- Amplitude Modulation (AM) 2.1.1- DSB-SC Amplitude Modulation 2.1.2- Quadrature Amplitude Modulation (QAM) 2.1.3- Single Side Band Amplitude (SSB) Modulation 2.1.4- Vestigial Side Band (VSB) Amplitude Modulation 2.2- Angle Modulation 2.2.1- Frequency Modulation (FM) 2.2.2- Generation of FM signals. 2.2.3- Phase Modulation (PM) 2.3- Demodulation Techniques for AM, FM and PM 3- Appendix 3.1- MATLAB program for AM 3.2- MATLAB program for FM 1- Introduction to Wireless Communication: - In today’s communication, wireless communication has become the main area for researchers and has become an industry and also the governments of different countries are interested in it as wireless sensors have an enormous range of both commercial and military applications. Commercial applications include monitoring of fire hazards, stress and strain in building, carbon dioxide movement and gases at a disaster side. Military application includes identification and tracking of enemies target, direction of chemical and biological attacks, support of unmanned robotic vehicles, and counter-terrorism. There are many factors of growing of this field. One of which is the tremendous increase in demand for tether less connectivity, driven so far mainly by cellular telephony but is expected to be soon eclipsed by wireless data applications. Wireless communication has also captured the attention of media and imagination of the public. Not only this, the Cellular systems have experienced exponential growth over last decade and there are currently about two billion subscribers around the world and cellular phones have become critical business tool and part of everyday life and are rapidly replacing the wired systems in many developing countries. In addition, wireless local area networks are currently replacing the wired networks in many homes, businesses and campuses. This is because by using wireless things it is easy to move and to get rid from junk of wires. The explosive growth of wireless systems in laptop and palmtop computers indicates a bright future of wireless networks. 1.1- History of wireless communication When we talk about the history of wireless communications then one must know it has history of hundreds of years. In the Pre-industrial age the people were used to transmit information over line-of-sight distances using torch signaling, flashing mirrors, flags of different colors and etc. To convey complex message, they developed sets of different signals. The observation stations were built on tops of the hills to relay these messages over large distances. These early communication networks were replaced first by the telegraph network (invented by Samuel Morse in 1838) and later by the telephone. In 1895, a few decades after the telephone was invented, Marconi demonstrated the first radio transmission from the Isle of Wight to a tugboat 18 miles away, and radio communications was born. Radio technology advanced rapidly to enable transmissions over larger distances with better quality and less power with smaller and cheaper devices, thereby enabling public and private radio communications, television, and wireless networking. In early radio systems, analog signals were used for transmission. However, in today’s radio systems, most of them transmit digital signals (due its many advantages) composed of binary bits, where the bits are obtained directly from a data signal or by converting an analog signal to digital with the help of analog to digital converters. The introduction of wired Ethernet technology in the 1970’s steered many commercial companies away from radio-based networking. Ethernet’s 10 Mbps data rate was tremendous and far exceeded anything available using radio, and companies did not mind running cables within and between their facilities to take advantage of this high rate. 1.2- Techniques Involved in Wireless Communication: - To accomplish wireless communication following are the techniques used Modulation Coding Transmission Path loss calculation Amplification (for analogue data) Regenerative repeating (for digital data) Reception Decoding Demodulation In this report only the Modulation technique with its details and types is discussed. 2- Modulation: - It can be defined as “The process of shifting the range of frequencies in a message signal”. There are several reasons for performing modulation: - Baseband signals are not suitable for transmission over long distances. Their frequency is very low which means that they posses not enough energy. The wavelength of base band signals is very large as their frequency is low. In order to transmit those signals the antenna size should at least be one-quarter of the wavelength. Such a large antenna size is practically not possible. As message signals from different sources are almost of same frequency several problems occur i.e. the frequency bands of the signals overlap and crosstalk is produced. Only one source can transmit its signal at a time due to crosstalk. So a large bandwidth of the channel is lost. Bandwidth of the channel is not utilized efficiently i.e. FDM (Frequency division Multiplexing) is not possible. For Modulation we modify the parameters of a high frequency carrier. The parameters are amplitude, frequency or phase. If amplitude of the carrier is varied in proportion to the baseband signal then modulation is called Amplitude Modulation. Similarly if Frequency of the carrier is varied in proportion to the baseband signal then the modulation is called Frequency Modulation. Similarly phase modulation is one in which phase of the carrier is modified according to the baseband signal. Frequency modulation and phase modulation are together known as Angle Modulation. Modulation gives the following advantages: - High frequency carrier has high energy so it can travel longer distances. After modulation only high frequency signal is to be transmitted. This allows the use of smaller antenna sizes. As for different sources different carrier signals can be modulated each with different frequency, so signals from many sources can be transmitted over the same channel at the same time. NO crosstalk will occur as frequency band captured by each signal is different. FDM frequency division multiplexing of several sources is possible via modulation. There are other advantages as well but the above mentioned points were worth noting here. In the subsequent sections of this report we will study different modulation techniques in depth. We will analyze each technique with its merits and demerits. Demodulation of modulated signals is also presented. In the end MATLAB coding for Amplitude Modulation and Frequency Modulation are shown with results. 2.1- Amplitude Modulation (AM): - In this type of modulation amplitude of the carrier signal is varied in proportion to the baseband signal whereas frequency and phase are kept constant. 2.1.1. Double Side Band with Suppressed carrier (DSB-SC) AM: - Amplitude modulation is characterized by the fact that amplitude of the carrier Acos (ωct + θc) is varied in proportion to the baseband signal or message signal or modulating signal. Carrier amplitude is A and is made directly proportional to modulating signal. The result is a modulated signal i.e. m(t)×cos (ωct). let us consider modulating signal as m(t) and its frequency spectrum as M(ω) then m(t)↔M(ω) and m(t)×cos (ωct)↔ ½[M(ω+ωc) + M(ω-ωc)] From above relations we see that spectrum of the modulated signal is the same as message signal but is shifted to ω+ωc and ω-ωc. which means that process of modulation shifts the spectrum of m(t) to left and right by ωc. Following are the characteristics of DSB-SC: - Spectrum of modulated signal is same as that of m(t) shifted to left and right by ωc. If bandwidth of m(t) is B Hz then bandwidth of modulated signal is 2B Hz. There are two sidebands in modulated signal. One centered at ωc is called upper side band (USB) and the one at - ωc is called lower side band (LSB). For this reason it is called DSB AM. Carrier frequency is not present in the modulated signal i.e. it is suppressed so this modulation technique is also called double sideband with suppressed carrier (DSB-SC) Amplitude Modulation. 2.1.2. Quadrature Amplitude Modulation (QAM): - The DSB signals occupy twice the bandwidth required by baseband signals. This is a major disadvantage which can be overcome by transmitting two DSB signals using carriers of the same frequency but in phase quadrature as shown in the figure below, Box labeled –π/2 is phase shifter. If the two signals to be transmitted are a(t) and b(t) than mQAM(t) is the sum of both signals. The advantage we find in QAM is that by this technique two signals which occupy the same band are transmitted simultaneously. Both can also be separated at the receiver. 2.1.3. Single Side Band (SSB) Amplitude Modulation: - There are two sidebands in modulated signal for DSB signals. One centered at ωc is called upper side band (USB) and the one at - ωc is called lower side band (LSB). A scheme in which only one sideband is transmitted is called Single Side Band (SSB) Amplitude Modulation. SSB requires only one sideband and thus only one-half of the bandwidth of the DSB signal. Two methods are generally used for the generation of SSB signals. i- Selective filtering method: - In this method a DSB-SC signal is passed through a sharp cut-off filter to eliminate the undesired sideband. To obtain USB the filter should pass all frequency components above ωc un-attenuated and completely suppress all components below ωc. This operation requires an ideal filter which is unrealizable. It can be realized closely if there is some separation between pass band and stop band. The voice signal provides this condition but its spectrum allows little power content at the origin, as frequencies below 300Hz are not significant. In other words we may suppress al speech components below 300Hz without affecting the intelligibility. Thus filtering the unwanted band becomes relatively easy. ii- Phase shift method: - Following equation is the basis for this method. musb(t) = m(t)×cos (ωct) – mh(t)×sin (ωct) This equation is for upper side band USB of the DSB signal. Where mh(t) is the Hilbert transform of m(t). Hilbert transform every spectral component is delayed by π/2 which is the requirement in our case. An ideal phase shifter is un realizable. We canm at most approximate it over a finite band. However it is possible to realiz a filter with two outputs such that both outputs have the same amplitude spectrum, but their phase spectra differ by π/2 rad over a band of frequencies. In terms of bandwidth requirements SSB is similar to QAM but less exacting in terms of carrier frequency and phase. It is also less exacting in terms that it requires a distortion less transmission medium. However SSB is difficult to generate if the baseband signal has no DC null in its spectrum. It is easy to build a circuit to shift the phase of a single frequency component by π/2 rad. But a device to achieve a π/2 shift of all the spectral components over a band of frequencies is unrealizable. We can at least approximate it over a finite band. 2.1.4. Vestigial Side Band (VSB) Amplitude Modulation: - As seen earlier the generation of SSB signals is rather difficult. The selective filtering method demands DC null in the modulating signal spectrum. A phase shifter required in the phase shifter method is unrealizable or realizable only approximately. Similarly the generation of DSB signals is simpler but requires twice the signal bandwidth. A vestigial sideband (VSB) is a compromise between DSB and SSB. It inherits the advantages of both but avoids their disadvantages at a small cost. VSB signals are relatively easy to generate, and, at the same time, their bandwidth is only 25% greater than the SSB signals. In VSB, instead of rejecting one sideband completely (as in SSB), a gradual cut off of one sideband is accepted. The baseband signal can be recovered exactly by a synchronous detector. An equalizer filter is used in conjunction at the receiver output. If the vestigial shaping filter that produces VSB from DSB is Hi(ω) than the resulting VSB signal spectrum is [M (ω+ωc) + M (ω-ωc)] Hi(ω) This VSB shaping filter allows the transmission of only one sideband, but suppresses the other sideband, not completely but gradually. This makes it easy to realize such a filter. The transmission bandwidth is now somewhat higher than the SSB signal. The bandwidth of the VSB signal is typically about 25-33% of SSB signals. 2.2- Angle Modulation: - A sinusoidal signal is described by amplitude and angle. The angle includes frequency and phase. There exists a possibility of carrying the information in the baseband signal by varying the angle (frequency or phase) of the carrier. We will see later that PM and FM are not only very similar but are inseparable i.e. if we have a PM wave and we replace m(t) by ∫m(t), it will be changed into FM. So it is concluded that a signal that is FM wave corresponding to m(t) is also a PM wave corresponding to ∫m(t). Therefore by looking at an angle modulated carrier, there is no way of telling whether the signal is FM or PM. In fact it is meaningless to ask an angle modulated wave that whether it is FM or PM. An analogous situation will be to ask a person (who has children) whether he is father or a son. The person will be puzzled because he is both, a father of his child and a son of his father. 2.2.1. Phase Modulation: - In PM the angle ‘θ’ is varied linearly with m(t)/modulating signal i.e. θ(t) = ωct + θo + kp m(t) Where kp is a constant and ωc is carrier frequency. Assuming θo = 0 with out loss of generality θ(t) = ωct + kp m(t) The resulting PM wave is Spm(t) = Acos (ωct + kp m(t)) The instantaneous frequency in this case is given by, ωi(t) = dθ/dt = ωc + kp d(m(t))/dt Hence in PM the instantaneous frequency varies linearly with the derivative of the modulating signal. FM instantaneous frequency varied linearly with the modulating signal NOT with its derivative. Following figure tells the process of PM, The below figure is a phase modulated wave and modulating signal. 2.2-2. Frequency Modulation: - In FM instantaneous frequency is varied linearly with the modulating signal NOT with its derivative as was the case with PM. Thus in FM the instantaneous frequency is given by, ωi(t) = ωc + kf m(t) Where ‘kf’ is a constant called modulation sensitivity. The angle ‘θ(t)’ is now θ(t) = ∫ [ωc + kf m(ά)] dά Integrated over the interval, -∞ to t. θ(t) = ωc t + kf ∫ m(ά) m(ά)] dά integral is over the interval -∞ to t. Hence the generated FM wave is Sfm(t) = A cos [ωc t + kf ∫ m(ά)] Following figure shows the process of FM Graphically, An important concept in the understanding of FM is that of frequency deviation. The amount of frequency deviation a signal experiences is a measure of the change in transmitter output frequency from the rest frequency of the transmitter. The rest frequency of a transmitter is defined as the output frequency with no modulating signal applied. For a transmitter with linear modulation characteristics, the frequency deviation of the carrier is directly proportional to the amplitude of the applied modulating signal. Thus an FM transmitter is said to have a modulation sensitivity, represented by a constant, kf, of so many kHz/V, kf = frequency deviation/V = kf kHz/V typical FM signal shown with the modulating signal For a single modulating tone of eM(t) = eM sin(ωMt), the amount of frequency deviation is given by δ(t) = kf × eM(t) where δ(t) is the instantaneous frequency deviation and eM(t) represents the modulating signal. The peak deviation is given by δ = kf × EM 4.2 where both δ and EM are peak values. References [1]- B.P Lathi, "Modern Digital and Analog Communication Systems", 1988, Ch.4-5 Appendix MATLAB program to show Amplitude Modulation: - clear all; f = 5; ff = 2/f; w = 2*pi*f; t = 0:.001:ff ym = 10*sin(w*t); subplot(3,1,1) plot(t,ym,r) title(Modulating or Baseband signal); xlabel(Time) ylabel(amplitude) yc = 20*sin(20*w*t); subplot(3,1,2) plot(t,yc,g) title(carrier signal); xlabel(time) ylabel(amplitude) y=ym.*yc; subplot(3,1,3) plot(t,y,b) title(Modulated signal); xlabel(Time) ylabel(Amplitude) Snapshot of the results is as under: - MATLAB program to show Frequency Modulation: - f=5; ff=2/f; w=2*pi*f; t=0:0.001:ff; ym=10*sin(w*t); subplot(3,1,1); plot(t,ym,r); title(Modulating or Baseband signal); xlabel(Time) ylabel(amplitude) yc = 20*cos(20*w*t); subplot(3,1,2) plot(t,yc,g) title(carrier signal); xlabel(time) ylabel(amplitude) y=20*cos(20*w*t+10*sin(w*t)); subplot(3,1,3) plot(t,y,b) title(Modulated signal); xlabel(Time) ylabel(Amplitude) Snapshot of the results is as under: - At modulation index ß=10 At modulation index ß=20 Read More