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This document shows the study on amplitude modulation. Way of modeling an amplitude modulation signal will be shown. At the same time, the simulation, characterization and analysis of an amplitude modulation signal will be done based on the amplitude modulation.
Modulation- A process where one signal is combined with another signal to form a new signal.
Amplitude modulation is a simplest way to transmit an information signal from one place to another place of long distances. It is a process of transforming the information signal from low frequency band to high frequency band by mixing the information signal with high frequency carrier signal. Hence, a low frequency signal which cannot travel far can be transmitted to a far distances place after transformed to a higher frequency signal.
There are different types of amplitude modulation such as Double sideband suppressed carrier (DSB-SC) amplitude modulation, Single sideband (SSB) amplitude modulation. In this content, we will be more concern on the double sideband suppressed carrier (BSB-SC) AM.
Amplitude Modulated signal
Figure 1: A Simple Block Diagram of a Modulator
Information signal: Vm(t) = Em cos wmt
Carrier signal: Vc (t) = Ec cos wct; fc >>fm
In the modulator, the information and the carrier signal is combined by a multiplier to form a modulated amplitude signal which can be shown through the following equation.
VAM(t) = Vm(t) X Vc(t)
= (Ec + Vm(t))cos wct
The sample of amplitude modulation in form of diagram can be shown as below which can be in term of time domain and frequency domain.
In Time Domain
Figure 2.1: Information signal
Figure 2.2: Carrier signal
Figure 2.3: Amplitude Modulated Signal
In Frequency Domain
Figure 3.1: Information signal
Figure 3.2: Carrier signal
Figure 3.3: Amplitude Modulated Signal
Envelope= Ec + Vm(t)
Figure 4.1: Amplitude Modulated Signal in Time Domain
Figure 3.1 shows the output waveform of amplitude modulation signal which contains the frequencies that produce the AM signal which will be transmitted through the transmitter. The shape of modulated wave observed from figure 5 is called the AM envelope which can be represent with the equation,
Envelope = Ec + Vm(t)
The amplitude of output wave is varies in accordance with the modulating (information) signal.
Figure 4.2: Amplitude Modulated Signal in Frequency Domain
Modulation index also known as depth of modulation. It represents the amount of amplitude change in the output amplitude modulation waveform. Mathematically, the modulation index is,
m = (Emax + Emin) / (Emax + Emin)
= ( Em / Ec )
MODELLING AND SIMULATION
Figure 5.0: Modelling Diagram of Amplitude Modulation Using Simulink
Figure 5.0 shows the basic modelling diagram of amplitude modulation using Simulink. First of all, the information signal and carrier signal are generated using the signal generator. To perform AM modulation, the information and carrier signal have to be multiplied together as equation shown below.
VAM(t) = Vm(t) X Vc(t)
= (Ec + Vm(t))cos wct
Oscilloscopes are used to monitor the waveforms of information, carrier and output signal.
Simulation by Varying the Carrier Frequency
Simulation of the amplitude modulation is done by using three different frequency of carrier signal and they are 10 Hz, 100Hz with amplitude equals to 1V. The information signal is being fixed to sine wave signal with amplitude 1V and 1Hz. The diagrams below show the different results of AM by using two different frequency carrier signals.
Figure 5.1: AM with 10Hz carrier frequency
Figure 5.2: AM with 100Hz carrier frequency
Simulation by Using Different Information Signal
The following is the output waveform by using different types of information signal. Three types of waveform that is sine wave, square wave and saw wave had been used for the simulation and all with 1V peak voltage.
Figure 6.1: AM by Using Saw Wave as Information Signal
Figure 6.2: AM by Using Square Wave as Information Signal
Simulation by Varying the Modulation Index.
Simulation is then carrying out by varying the modulation index of AM. Three types of is chosen for the simulation that is m<0, m=0 and m>0. The output waveform is shown below.
Figure 7.1: AM with modulation index, m<100%
Figure 7.2: AM with modulation index, m=100%
Figure 7.3: AM with modulation index, m>100%
Two methods have been carryout in order to simulate the correct modulation of AM. From the results obtained using both method of simulation that is by using Matlab Simulink and Matlab Coding, it shows both of the result are the same as the theoretical result of amplitude modulation. This shows that the modeling and simulation technique used is correct. During the simulation, the amplitude of information and carrier signal is set to 1V and both frequencies are set to 1Hz in order to simply the simulation and make the understanding easier. Then, the information signal that multiplies with the carrier signal must always be positive or negative but not intermediately (positive or negative sometime) in order to avoid the touching or crossing of the upper and lower side of envelopes of the modulated signal. This can be done or achieved by simply adding a constant as shown in Figure 5.0 to the original information signal to shift or lift it up the information signal so that the sum of the constant and the information signal is always positive or always negative.
Meanwhile, the carrier signals with different frequencies will produce modulated signals with different frequencies as shown in the Figure 5.1 and Figure 5.2. From the results, the higher the frequency of the carrier signal, the higher the frequency of the modulated signal. On the other hand, different information signal also being simulated using the simulation model. By using different types of information signals such as sinusoidal wave, saw wave and square wave, modulated signal with different envelopes will be produced. From the results shown in Figure 6.1 and Figure 6.2, it shows that the envelope will depend on the type of the information signals used. Besides, different modulated signal will be produced by varying the modulation index, m as shown in Figure 7.1 Figure 7.2 and Figure 7.3. Overmodulation that is modulation index greater than 100% should be avoided as it will cause distortion of the modulation envelope.
In conclusion, amplitude modulation (AM) is the process of changing the amplitude of carrier signal with high band frequency in proportion with the instantaneous value of information signal. During the process, the information signal is translated from low band frequency to high band frequency. The amplitude modulation plays a very important role in the transmission process of signal because normally the information signal with low band frequency cannot travel for a long distance. With the technique of amplitude modulation (AM), the information signal can be translated to higher frequency signal and make the signal able to travel far. If compared to other types of modulation, amplitude modulation is really a relatively low cost modulation which can be used for broadcasting of audio and video signals in the commercial field.p>
ABTRACT- In this paper we review about indoor wireless optical communication in Malaysia. Type of indoor wireless and application also been reviewed in this article. We also discuss the medium characteristic of indoor wireless application optical. Further research or ongoing research has been included in this paper.
Usually, one may see that computer terminals are clustered within office environment, labs, conference room, educations institutions, libraries, hospitals or production floor. In all these environment, the inconvenience and high cost of maintaining and reconfiguring wired systems has lead to alternative used of wireless communication. Wireless offer flexibility in the placement of terminals in work environment and avoids the waste of time and cost that reconfiguring a wired system imply.
Figure 1: An Indoor Wireless Optical Communication System
Figure 1 shows an example of a simple indoor wireless optical communication. The terminals are allowed to roam inside the room to establish a links with a ceiling as a basestation as well as with other terminal.
A way to achieved high speed indoor wireless communication is by using infrared radiation. The idea of using infrared as a medium to communicate in house environment was proposed about two decades ago, but it has been in the last few years that the interest in optical wireless communication has growth. As optical system operates in the near-infrared part of the spectrum, they make use of very low cost optoelectronic component available today. These components are generally small and consume little power, which is very important when manufacturing mobile terminal for telecommunication in large quantities.
Type of indoor wireless
Now days, wireless optical communication has been widely use in Malaysia. Gigantic Communication company such as Celcom., Maxis, Digi and Telekom Malaysia implement wireless optical communication iin their product. Broadband is the best examples that use indoor wireless optical communication. Wireless is a term used to describe telecommunication where electromagnetic waves carry the signal over all of the communication path. Optical communication is defined as the transmission or the reception information using optic signal. Optical communication may use guides of free space transmission to transfer optic signal. In Malaysia, we have used four types of indoor wireless which are fixed wireless, mobile wireless, portable wireless and IR wireless. Fixed wireless is the operation of wireless devices or systems in home and in particular equipment connected to the internet via specialized modem. Mobile wireless is the use of indoor wireless devices or system in moving vehicle, example include the automotive cell phone and PCS (personal communication services). Portable wireless is the operation of battery powered wireless devices or system outside the office, home and vehicle. The example of portable wireless includes handheld, cell phone and PCS units. The fourth type of indoor wireless is IR(Infrared Radiation) wireless that use of device that convey data via IR, employed in certain limited range communication and control system.
Advantage of indoor wireless optical communication
Optical wirelesses have the ability to provide wider bandwidth in excess of those available with current or planned Radio Frequency network. There are few approaches or methods to implement a optical wireless system, however these usually involve the integration of optical wireless system, optoelectronic and electronic component to create a transceiver such systems which normally are very complex and the wide use of optical wireless is very dependent on the ability to fabricate they require transceiver component of low cost.
The main reasons why indoor wireless communication has be widely used in Malaysia is due to the benefits of optical communication are threefold. Firstly, the high frequency of the carrier of optical which is typically of the order 300,000GHz that allows much more information to be transmitted over a single channel that is impossible with a conventional radio or microwave system. Secondly, the optical carrier has shorter wavelength which typically of the order in micrometer allow the realization of very small compact component. Third, the highest transparency for electromagnetic radiation that is achieved in any solid materials such as silica is in the wavelength region of 1- 1.5Î¼m.
II. Indoor wireless optical propagation.
In indoor wireless communication environment, reflection from the floor, wall, the ceiling or any surface will cause many delays and signal propagation paths, which consequently decrease the performance of the received signal and its quality. One of the solution is that a beam forming technique to direct antenna main beam towards a transmitter which is to suppressed the reflected incoming signal by direct null towards interference or multipath signal direction while increasing the gain of antenna for a desired signal direction.
The vehicular cellular phone system is a factor which initiated a rapid development of wireless communication. However with the development of these system cell sizes which are made smaller in order to increase the user capacity. In the same time, the interest towards indoor systems of telephony (cordless phone) and data services such as wireless LAN'S also started. Currently, there are more and more research is being conducted based on the indoor and outdoor propagation.
2.1 Comparison of infrared with radio system
Infrared radiation appears to be viable alternative to radio for indoor wireless optical communication in Malaysia. This is because for indoor short range communication applications, infrared present certain advantage when compared with radio frequency. The infrared region of the spectrum on the other hand offers the large bandwidth potential that is unregulated all over the world.
Table 1: Comparison Of The Infrared Red Radio Medium Characteristic For Indoor Application.
2.2 Advantage of infrared radiation
Infrared radiation, just like visible light is considered to the room in which it is generated. Hence it cannot be detected outside, securing transmission against eavesdropping. Besides that, infrared radiation does not interference with system of the same nature operating in neighbouring room and does not interference with the radio frequency either. Another advantage is that the infrared components is inexpensive small and consume little power and it very important for mobile terminal system. In spite of advantage presented by infrared over medium for different application, it has some draws back as well. Infrared may suffer from blocking a person and objects, resulting in problems on the communication link. In infrared system the transmitted power level is limited due to eye safety considerations and this is implies that the range of the system is restricted as well. It is possible to conclude that radio and infrared technologies can operate in complementary way, but one may be preferred over the other depending on the application.
The indoor channels are not easily to be captured in rough path loss exponent. While delay spread are often much smaller than outdoor, the indoor system often need to transfer very big data rate to support wireless multimedia computing. These are several cause of signal corruption in wireless communication channel. The main cause of attenuation are distance, penetrate loss through floor, wall and multipath propagation. Outdoor macro cellular network propagation is fairly predictable is the main difference between an indoor and outdoor propagation. A topographical database can be used to determine what will be shape of a cell if we put a base station somewhere. When the mean is attenuated to be function of the distance, signal attenuation over distance is observed. The signal will facing decay problem as a result to ground wave loss although it only comes into play for very large distance. For indoor propagation this mechanism is relevant, but effect on wave guidance through outdoor can occurs.
III. Configurations of infrared link
The different kind of links for indoor optical wireless communication has been classified depends on the existence of a line of sight (LOS) pat between the transmitter and receiver. It also depend on the degree of directionality (directed, non directed or hybrid).
Figure 2: Configuration of infrared links
Figure 2 show the example and the differences between each of 6 basic configurations of infrared links. In line-of-sight (LOS) topology, there is no obstacle in between the transmitter and the receiver. However, for the non-LOS, the obstacle exists between the transmitter and receiver, hence it needs to use reflective surface in order to transmit the signal.
Line-of-sight (LOS) improves the power efficiency of the system and reduces multipath distortion in the system. However, due to fact that alignment of the transmitter and the receiver, the flexibility of some LOS-based systems will be forbidden. For non-line-of-sight (non-LOS), it increases link robustness so that the system will function even if the obstacle exists between the transmitter and the receiver. It used widely the reflection from the surface to find an alternative path between the transmitter and receiver. Non-LOS configurations also facing multipath distortion since it widely use the reflection of the surface to transmit data.
Directed-LOS links improves power efficiency of the system where the path loss is reduced because the power transmitted is focus into narrow optical beam. Directed-LOS topology also known as "directed bean infrared" (DBIR). This system does not suffer from multipath distortion due to fact that it allow the use of narrower field-of-view (FOV) receiver which also minimize the unwanted energy unlike the hybrid-non-Los which suffer the multipath distortion which proportional to the area. Besides that, it only needs lower power to transmit data since the emitted beam's divergence is small. However, it faced the blocking problem, limited mobility and it requires aiming to the transmitter and receiver.
For hybrid-LOS, it has wider field-of-view for both transmitter and receiver. It able to optimizes the power budget, the distances of the link, and allow the reception of signal for another transmitters as long as it is in the range of field-of-view since it has a very directive transmitter and a wide FOV receiver. Hybrid-non-LOS has a very directive property either in transmitter or receiver. As a result, it will not have the problem of alignment but in the mean time suffer the multipath distortion problem which increases with the area of the room.
Directed-non LOS has a very narrow emission angle and FOV transmitter and receiver. It is using the reflective surface to transfer data which enable the link to solve the existence of obstacles or barrier. It does not facing the big problem of multipath dispersion since the signal is transmitted in a single reflection. However, the problem of alignment is exists due to a very directive characteristic of the transmitter and receiver.
For the non-directed and non-LOS which also known as "diffuse infrared" (DFIR), it is an attractive configuration among the other. The system does not need a direct line or sight or even alignment due to the optical waves that spread uniformly since it uses a wide emission beam transmitter and larger range of FOV receiver. This kind of links have the most advantages that it can function even there is an obstacles between transmitter and receiver. However, it facing a multipath dispersion and high optic losses compared to LOS and hybrid-LOS.
IV. Latest Infrared Communication System
Optical wireless communication systems had undergone a huge evolution since the last few decades. Since the use of IR was proposed as an alternative way despite of radio frequency, many manufacturers have developed varies systems to communicate either indoor or outdoor environment. Nowadays, most of the designs were based on the directed-LOS and hybrid-LOS configurations since these topologies only need a very low cost due to its simple structure of receiver and less power consumption.
The latest technology of indoor wireless optical communication is WI-FI at the speed f light. From the research, it shows that wireless optical network could provide gigabit per second data rate transfer. A wireless network that uses the reflected infrared instead of radio waves has transmitted data through the air at the speed one gigabit per second and six to 14 times faster that the WI-FI network . This optical network could provide faster and more secure communication would be especially suitable for use in hospital, aircraft and factories when radio frequency transmission can interference with navigation equipment, medical devices and control system. Besides that, another possible application is wireless networking for home theatre, a system that transmit data at the 1.6 gigabit per second could broadcast two separate high destination TV channel across rooms, a capacity that exceed the band width or any existing radio frequency.
Figure 2 shows the model of Bright Light. 
This experimental system is capable to transfer data at one gigabit per second. An infrared laser of the system (the black devices on the right) is used to transmit the data. Setup of the model sent a data across a room by modulating abeam of infrared light that was focused on the ceiling and picking up the reflection using a special modified photo detector. The measurement show the system could support data rates well beyond the one gigabit per second.
As a conclusion, in spite of the advances achieved so far there is still a lot of work to be done to exploit completely the advantages and the potential offered by the optical medium. For indoor wireless system application, the use of optical communication offers an important alternative for the growing area of mobile computers and communications. Optical wireless network also could offer less interference and greater security than radio frequency network. While radio signal passes through wall and door, light does not passes and making it easier to reuse frequency and more difficult to intercept transmission. Unlike radio frequency spectral region for all light infrared, visible, and ultraviolet is unregulated worldwide. This could make easier to commercialize optical wireless network.