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.