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Note for Cellular Mobile Communication - CMC By Kumar Siva

  • Cellular Mobile Communication - CMC
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(Refer Slide Time: 00:04:55 min) First we start with a brief introduction. The mobile radio channel places fundamental limitations on the performance of wireless communication system. This is the very important point. Wireless by definition, is a very hostile environment. We do not have the luxury of a fixed line copper or the luxury of a large bandwidth like fiber. We have to deal with the uncertainties of the channel. Coupled with that is multipath propagation, attenuation, scattering and host of other problems. Now the widest transmission path may either be line of sight, if I am lucky. That is, I have a direct line of sight from the transmitted to the receiver or I can have a non-line of sight in which case my signal is actually obstructed either by building or foliage or hills or even cars on the streets. In general, we deal with the non-line of sight situations in our cellular mobile systems. We do not have the luxury to be in direct line of sight with the base station. Radio channels are random and often time varying. This is important. Not only is it random, that is, you can take enough measurements; come up with a statistics to model it but with time, the statistics might change. This is a fundamental issue. How to model a radio channel effectively and remember radio channels behave differently in different frequencies. So for example, the model that I come up with for 900 MHz frequency band may not be entirely applicable for 2.4GHz band or for the LMDS28GHz band. We will realize why it is different the lambda, the wave length of transmission is going to be effected and how it gets reflected, obstructed, absorbed or diffracted depends on the size of the wavelength. Modeling radio channels have been one of the most difficult parts of the mobile radio designs. This is because when we have to come up with a whole system model, I must plug in the channel characteristics. I should have a reliable channel model so that I can simulate my system before actually implementing it. Remember mobile systems are expensive systems. I randomly cannot put a base station, measure the power and then decide, “Look, I didn‟t do a good job. Let me shift the base station because I didn‟t get enough power”. So measurements are carried based on these fundamental measurements. You try to come up with a realistic channel model. Most of the channel models are random but as we will see, as you move to higher and higher frequencies that 3

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is, lower band wavelengths, you will have to go to deterministic channel modeling. so random channel models are good for lower frequency bands. By lower I mean 900MHz, 1000MHz and 2.4GHz and by higher, I mean above 10GHz.let us now look at some propagation basics. (Refer Slide Time: 00:08:47 min) We start right from this start. That is, when electrons move, they create electromagnetic waves that can propagate through space. So we start with a circuit where we make the electron move in a certain fashion so as to generate a certain kind of hopefully a sinusoidal wave form. By attaching an antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and on the other hand, received by a receiver at a distance away. So antenna is an interface between the circuit and the wireless channel. Please remember the size of the antenna has a lot to do with the wavelength. In fact many times we measure the size of the antenna in terms of wavelengths. If I deal with a lower frequency band that is, a larger wavelength, then my antenna size will be larger and wise verse. So as we are translating to higher and higher frequencies, our antenna size is diminishing. This is good for us. As we move to higher spectral band, my mobile handset will become smaller and so will the antenna inside it. The radio, microwave, infrared and visible light portions of the electromagnetic spectrum can all be used to transmit information. Remember all of this is wireless. Wireless radio, point to point microwave link, infrared links right inter-building intra-building and visible light not guided but unguided. May be across buildings I can have links which are visible light links. That‟s also wireless channel but clearly visible light has to have a line of sight. I cannot have any obstructions. Infrared too must have a line of sight. Microwave gets very highly diffracted if you have scatters around. So it‟s preferred to have a microwave line of sight link. However for radio which is at a much lower frequency than the above 3 can propagate without having a clear line of sight. Information can be sent by modulating one of the properties of the waveform. It could be the amplitude, frequency, phase and these are not the only possibilities. 4

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for example in ultra wideband, we use pulse position modulation because we have luxury to send very narrow pulses and where the pulses are placed with respect to a reference can tell me what kind of bit pattern I am trying to convey. (Refer Slide Time: 00:11:55 min) Properties of radio waves here are some of the reasons why we would like to use radio waves. They are easy to generate. The technology is 50 years old. We have the technology. They can travel long distances usually without line of site because the moment we talk about long distances, we should start thinking about no- line of sight they can penetrate buildings. So I can sit in the basement and still talk on my mobile phone. May be used both for indoor and outdoor communications. They are omnidirectional and we can narrowly focus them at higher frequencies. So we can actually communicate from one satellite to another satellite or from satellite to a base station on earth because they can be focused at a small point. Let us look at some other properties. The first and the foremost is frequency dependence. The radio waves at higher frequencies tend to behave more like light. That is, they have difficulty penetrating obstacles. They rather move in direct path that is, rectilinear propagation and they can get absorbed by rain fog particles, dust particles, etc. For example, if I am working for IEEE802.16 metropolitan area network frequencies or frequencies above 10GHz, my propagation will be affected by rain. If suddenly during the transmission there is snow fall or rainfall, then my signal power at the receiving end will go down significantly. This is not the case when I am using my mobile phone. I will not perceive any perceptible drop in the quality of the voice received on my mobile phone whether it is raining or not. But at the lower frequencies, they can pass obstacles and the power falls sharply as we move away from the source. How the power falls off has already been studied. It depends simply not on the inverse square law formula. It also depends on the path loss exponent which is related to your density of buildings whether it‟s a concrete jungle whether you 5

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are foliage or any other parameter subject to interference from other radio wave sources. So radio waves we have a very high probability that some other sources radiating in a closer band and I have the chances of emissions or even out-of- band emissions. (Refer Slide Time: 00:14:59 min) Let‟s look at some basics of the mobile radio propagation. So here I am talking about VLF, LF and MF bands. These are the various frequency bands depending upon what is the frequency you choose. Radio waves follow the ground. So here let‟s look at an example. This is a contour of the earth and I have put two towers. Let‟s not call them base stations. Right now they are just trying to communicate with each other. What they are trying to do is send some signal and take it back. Usually the surface of the earth is working as a guide. It directs and helps it to propagate. So please note that the curvature is taken into consideration and still I am able to communicate. On the other hand at HF, high frequency bands the grounds waves tend to be absorbed by the earth. I did not have this luxury that we saw earlier. So we use ionospheric reflections. Let‟s look at the contour of the earth. Here is an ionosphere. It has lot of charged particles. Let us again put two towers, not necessarily base stations. If they want to communicate, they use this as a reflection. Please note that the ionosphere shown here is not really a continuous band. It has many bands from which the waves can be reflected. So clearly this path is a multipath propagation. The reflection here is not through a single reflecting surface. There is a continuum of reflecting surfaces here. So what you receive here is actually a faded signal we will see later on that fading result basically from multipath. Your signals can reach the receiver not only from a direct path but by reflections. All these signals add up either constructively or destructively and generate something which is entirely different from what you send. Therefore you tend to get something called as a faded signal. We will talk about fading and types of fading in later lectures. The problem with HF band is when you transmit it and it hits the earth, it gets absorbed. So I have no choice but to direct to my antenna upwards for reflection and comes back. 6

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