Cellular Systems

Now that we will be talking about 2G systems next month,and move to 3G soon, it is important to understand the cellular concept in order to understand how is the capacity of systems is improved. The problem of limited spectrum Vs higher capacity was solved using this concept.
In a very simple language, one single high power transmitter was replaced by multiple low power transmitters!! To answer the question how exactly the system capacity is increased by increasing the number of transmitters when the allocated spectrum still remains the same?
The answer is frequency reuse!! Every base station is allocated certain number of radio channels, and nearby base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations. Neighboring base stations are assigned different groups of channels so that the interference between base stations (and the mobile users under their control) is minimized.

This channel assignment can be fixed or dynamic. Another concern in cellular systems (now we know why these are called "cellular systems!!")  is handoffs. When a user moves from one cell to another, the call should be transferred to another base station this is called handoff..


People have come up with creating different sized cells in order to improve the capacity further. The picture shows an example of the same.

The factor that limits cell sizes and hence capacity is "Interference". Sources of interference are other calls in same or different cells, other base stations and systems. It is necessary to place co-channel cells(cells using same set of frequencies) far apart so as to minimize the interference caused by them. This distance (lets say D) can not be decreased beyond certain limit and hence the capacity can not be increased.

If the interference is caused due to adjacent channels (mostly due to imperfect RF filters) following steps are taken to minimize it.
1. Use of better filters
2. careful channel assignment
3. More frequency separation between channels

Nest month we will start with 3G systems and some concepts related to Modulation techniques.

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Wireless Propagation Basics

We have already looked at the First generation of telecommunication system. I think it is necessary to understand the basic principles of Mobile radio Propagation. Without the basic understanding of the principles of mobile radio propagation it is impossible to understand the practical problems related any communication system. Trust me, even with complete understanding of these principles solving the problems in network is difficult!! Lets say for safer side that with understanding of these principles, life will be easier!!
Wireless propagation can be split into three parts

1. Mechanisms of Propagation
2. Large scale Path Loss
3. Small scale Fading

There are numerous great books describing these phenomena. I am just going to present a brief idea so that one is not overwhelmed with the amount of information presented by enormous resources.

Reflection : When the electromagnetic wave impinges upon an object with very large dimensions as compared to its wavelength.(like buildings, walls)

Diffraction : When  the path between Transmitter and Receiver is obstructed by a surface with sharp irregularities.

Scattering : When the medium has objects smaller compared to the wavelength.


Now lets get to Propagation.If you go the wiki page of Radio Propagation, it says, " Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere."
Large scale propagation models deal with predicting mean signal strength for an arbitrary Transmitter-Receiver separation distance, they are called large scale as they characterize signal strength over a large distances. One most important formula one needs to know is the Friis equation. This describes the signal strength over the distance. 

 Small scale propagation models characterize the rapid fluctuations of received signal strength over very short distances or short duration of time.
Small scale fading can be further classified into Flat and frequency selective fading. Another way pf classification is fast and slow fading.

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