Making the Foundation

Welcome.

Let’s make a good and solid foundation for understanding the cellular communication. We believe this is very much needed before we dwell into any of the technology.

Billions of humans (I didn’t see any animal using it) use mobile phone today. But very few know what is inside, how it works, how the hell my voice is reaching the other party, how I can connect to inter’net’ without any physical connection. For many, this may not be of interest. They are much interested in cool mobile designs, typing and forwarding the funny smses etc.. But we, as technical people and being in this wireless (I never said worth less) industry should know how the system works? What are the building blocks? So let’s start..

Telecommunication by definition is a system for transmitting the information over a distance electronically. In case of wire line communications, this is done with the help of electric signals. In case of wireless communication, this is achieved by Electro Magnetic Waves.

The basic elements of a telecommunication system are
- A transmitter (Tx) – that takes information and converts it to signal
- A transmission medium – that carries the signal
- A receiver – that receives the signal and converts it back to the usable information.

When you make a call using your mobile phone, your phone is the transmitter and the big towers (which we can see on top of some buildings) act as a receiver. When you receive a call, these towers act as transmitter and your phone is now a receiver. In technical terms this tower is called as BTS (Base Transceiver System). The word transceiver is because it can work as both transmitter and receiver. These BTSs belong to some operator like Airtel, Reliance, BSNL etc.,

Having said about the transmitter and receiver, let’s look at the third and most important element of the communication system – the transmission medium. In wireless communication RF signals are used to transmit the information over the air. Let’s see it in the electromagnetic spectrum

The RF spectrum lies in the frequency range of 3Hz – 300GHz. This is divided into many sub bands like low frequency, medium frequency, high frequency, very high frequency, super high frequency…… huh…etc.,

The frequency band used for mobile phones is the UHF band which ranges from 300-3000 MHz. Some wireless technologies like wireless LAN uses the frequency in the range of 5GHz.

People cannot use this frequency for their own use freely. Government controls and sells the license to use these frequencies. Operators like Airtel, reliance buy the license from government by paying lot of money. As this is of very high cost, the operator want to use this frequency very effectively, efficiently. They try to accommodate more and more users in the frequency band they bought. Also the frequency band is limited. So this is a scarce and very costly resource in cellular communication. All the technologies that we are going to talk later came up to do this job more and more efficiently.

Now let’s look at in what manner the BTSs are placed over a region. It is as shown in the below figure.

The basic geographical unit served by one BTS is called cell. (That’s why it is called cellular communication system). The total geographical area is divided into number of cells. A group of cells is called a cluster. The shape of coverage area of one BTS is shown as a hexagon shape but in reality due to the constraints of the geographic terrain, the actual shape is not hexagon.

Now, one can ask a question why hexagon shape is selected to represent a cell?
The answer is, when we arrange the cells uniformly over the region, it should cover whole of the geographic area, and at the same time, we should minimize the overlapping area between the cells. If we have the large overlapping area, then we need more base stations to cover the total area. In general when we use the Omni directional antenna, the BTS coverage region is circular in shape. So there are three possible shapes to represent this. See the below figure.

In the overlap region the mobile phone switches from one BTS to other BTS. This is called hand over in technical terms. This is a complex process and should be minimized in the cellular system. So to minimize the overlap region and to cover the area with minimum BTSs the Hexagon shape is the only possible shape to represent the cell.

Now, let’s go back and discuss a bit on the cluster. Why this is important to know?
In FDMA, and TDMA we differentiate users based on frequency and time slots respectively. So in a cluster, if we use the same frequency in the neighbor cells, then there is a interference between the users in the two cells because both are using same frequency. (Here we are not mentioning about time slots because, when we use different frequency, obviously we can differentiate the time slots on two different frequencies).

To put in simple terms, let’s assume that two group of students are sitting side by side and one teacher for each group in the middle, delivering some science lesson. When both teachers start the lesson in the same language, same pitch etc., the students cannot follow the teacher because of interference from the other teacher’s lesson.
Now let’s say if in the two groups, one group is normal students and a teacher, and the other group consists of deaf and dump students and teacher. Now when the teachers start the lessons the students can follow easily in their own way.

Similarly in cellular systems, to avoid this interference problem we cannot use the same frequency in the neighbor cell. Then? How many different frequencies we should buy? No problem, because the frequencies can be reused in other clusters. So to summarize in FDMA and TDMA we cannot use the same frequency for different cells in a cluster, but we can re use the frequency in different clusters. This is called frequency reuse and it is denoted by frequency reuse factor. In the GSM this factor is 1/7 (7 is because the cluster consists of 7 cells).

Now, let’s discuss about how it is decided that which frequencies to use in cellular communication and other services, who decides it and how it is decided.

As we already mentioned frequency is a finite resource and can accommodate a limited number of simultaneous users at one time. Due to this reason, the governments divide the radio spectrum into non-overlapping blocks. And then different blocks are allocated to different class of services for example broadcast services (TV,radio etc), military services (Defense related services), mobile services (for the commercial mobile communication purpose) etc., I have put some important services and their frequencies in the below table.

Note: As you can see from the above table, the frequency bands used for cellular communication are 824-849 & 869 – 894MHz which is well known as 800 MHz band and 1850-1910 and 1930-1990 MHz which is well known as 1900 MHz band in cellular communication.


These allocations can be specific for each country. But due to the fact that radio signal propagation do not stop at national boundaries, if there is no coordination between countries it may lead to dangerous situations. Want to know some examples?

Assume that some frequency xHz is allocated for some defense communication by Indian government. And our neighbor country doesn’t know about it or they know but they don’t care and they allocated the same xHz frequency for some mobile operator. Now with this, that mobile operator can easily decode Indian defense communication signals legally in his country. Don’t you think it is a danger?

To avoid this some standard bodies like ITU (International Telecommunication Union) are formed which regulates the frequency allocations for different class of services in different regions. All countries governments should agree and follow the regulations made by the ITU for the use of frequency for different services. ITU regulatory body meets once in every two years to review the regulations, add /modify any class of services etc., and to take necessary actions.

Apart from ITU which governs this process at international level, there is a regulatory body in every country to do frequency allocation for different services, for different operators etc.,
In India we have TRAI (Telecom Regulatory Authority of India) for this purpose. TRAI allocates frequencies and they sell the licenses to use the frequency bands for different operators like Airtel, Reliance etc., Again because there are few frequencies and many operators, this selling is done by auction.

Now you might be wondering why AM radio / TV takes those initial range of frequencies? why not other frequencies? Any specific technical reason? The answer is No. It is just evolution, whenever some new class of service comes people allocate the next available frequency band. AM Radio came first.. so it took the initial range of frequencies, then TV and then Cell Phone etc..


This is pretty much about the frequency allocation, frequency reuse etc., that I wanted to share with you. In our next article.. we will discuss about the evolution of different RF technologies like GSM, CDMA etc., and their technology lines, what frequencies they use and some basic concepts of modulation techniques..

CDMA - Prologue

This is a prologue to a series of articles we are planning to write here on CDMA. The articles basically take one from the basics of CDMA to advanced stages of the technology like Ev-Do etc. We are couple of people writing the articles here, and you may find a bit of difference in the style of writing in these articles. Let's start the journey.

I remember one of my friend who is really good in java and related technologies once asking me "what is this CDMA phone and GSM phone and how different they are. As both phones are transmitting through air gets sound to my ear :), would there be lot different the way they work?". Sounded to me quite right :). But the point here is, if a techie geek like him doesn't know much about CDMA, I felt lot many people also who are new to the IT and telecom industry may not know much about it. Hence, thought of starting this category in this blog. In this blog, we are planning to write a series of articles which would serve as a tutorial for CDMA that may be helpful for people who are newcomers to this field.

CDMA - Code Division Multiple Access

CDMA is a technology that gets used in one type of Cellular system for communication over the air. This is one of the technologies that gets used in your cell phones for the communication. Let's look at what exactly the name CDMA relates to.

The full form of CDMA - Code Division Multiple Access. As the name contains "Multiple Access", it gives a clue that this is something to do with accessing something by a multiple things. Let's see what these “things” are. These things are basically, the air interface communication channel and multiple users using it for communication. In a nutshell, the communication channel is shared by multiple users, by using some Codes that will divide this channel into multiple unique channels that are relevant only to that particular user.

There are some more multiple access techniques which are also widely used. They are TDMA (Time division Multiple Access) and FDMA (Frequency division Multiple Access).

Following is a crude analogy to understanding all these three techniques.

Let’s go back to our childhood, where we used to take some plastic PVC pipes to talk to a friend on the other side.

Similarly, let’s say we have a big plastic pipe through which people on the both ends talk to each other to communicate.

TDMA : Time Division Multiple Access

This is the technique, where each person would be allocated some time slot to talk through the pipe. Once, his timeslot is over, the chance will go to the next user. The first user needs to wait for his next turn. Here, each user is limited by the amount of time he has been allotted. Total number of users that can use this pipe is limited by the number of time slots we have. In actual TDMA also, each user is allocated a predefined time slot to use the air interface.

FDMA: Frequency Division Multiple Access

This is the technique, where, there are some small pipes that are inserted into the big pipe. Each small pipe can be used by each user without any limit on the time. But each user is limited by the lesser size of the pipe. Total number of users that can use this pipe is limited by the number of smaller pipes that can fit in. In actual FDMA, each user is allocated a frequency out of a frequency band, which is like the smaller pipe that we talked about.

CDMA: Code Division Multiple Access

Here, each user talks in different languages in the same pipe at the same time. Each person uses a unique language that can be understood by only by his pair on the other end. Hence, there wouldn’t be any problem to the other pair of people. Here, the users can use the whole pipe without any limit due to the time slot etc. But, here the users are limited by the noise created. If there are three pairs talking to each other at the same time. The noise levels might not be high and each person can listen to their counterpart properly. If number of users increase, then the noise level can go high and can create lot of interference. Hence, in CDMA, the users are limited by the noise levels. In actual CDMA, each user is assigned a unique secret code, using which it communicates to the other side.

FDMA and TDMA are used together used in GSM cellular systems. CDMA is used in CDMA cellular systems. So, if you have a GSM phone/service, that will be using FDMA and TDMA techniques to access the air interface, whereas if it’s a CDMA phone, it uses CDMA technique to access the air interface.

The next article talks about why someone invented CDMA when TDMA and FDMA were already ruling the space. Any big advantages of CDMA over the other technologies? The next article also gives a taste of what kinds of codes we use in CDMA etc. So ..stay glued.