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Thursday, March 5, 2009

Wireless evolution ...GSM and CDMA

Well, let me take you the tour about wireless technology. Even if you are a non technical person, you can enjoy it.
As of now, we all are use to with cell phone, so, it will not be difficult to understand the
technology behind.
Well, before actually start to go into nore detail with GSM technology, let’s have a look about what and how wireless technology evolved?
Yes, you are right, we can start with a cordless telephone.
It’s a telephone with a wireless handset that communicates via “radio waves” and the base station connected to a fixed telephone line.
The cordless handset is powered by rechargeable battery, which is charged when the handset sits in the cradle.
Same way, wireless services expanded gradually like
Wireless PBX =>will allow you to use wireless handset within that wireless PBX coverage area.
then Paging and Cellular telephones and Mobile satellite service etc.
So, ultimately, what drives to expand wireless services? From one technology to other? From one generation to other?

It’s coverage and mobility. => from cordless to cell phone => what’s the service or attraction?
1) More coverage (from home coverage to worldwide coverage)
2) More mobility (from very low area (walk able) to high speeds even if you are driving a car).

Now let’s talk about GSM...let’s start from it’s background.
In Earlier days large number of analog systems were used. But it was not matching ideal situation from subscriber point of view. Then to accommodate an increasing number of users and to establish compatibility with the evolution of fixed network toward digital, CEPT (Conference Europeenne des Postset Telecommunications) establish a “Group Special Mobile” in 1982. The work of that group became GSM system and now it is a Global System for Mobile communication.

Clear objective of GSM standards are.
1) Common radio spectrum in all countries 2) Integrated European system with international roaming 3) Create large single market 4) Increase available cellular radio capacity 4) Standardization of network components and network interfaces 5) Better security functions 6) Compatibility with evolution of fixed digital networks 7) Accommodate new services.

Now let’s talk about how wireless technology works? How my voice can reach to the destination? How do we use RF spectrum? What is air interface and how do we process it to support multiple users?

Well, when we talk about mobile communication. It is nothing but an air interface.
1) Mobile telephone is nothing more than an FM radio with over 400 pairs of radio channels.
2) These channels are paired . so that one channel is from mobile to base and other is from base to mobile. This makes possible the full duplex communication. In technical term we refereed them as uplink and downlink frequencies.
3) There is set of two way control channels that control voice channel.
4) Air interface need process by which voice channel allocated to multiple users simultaneously.

Early systems now referred as first generation (1G), used analog technology called Frequency division multiplexing (FDMA). Which means each call is carried over a separate frequency channel.
When you dial a number, a channel pair is assigned to a mobile station for duration of one voice call.
You are right, major problem with that was fewer users can support with a single cell. More technically it has poor quality, less coverage as well. So, second generation (2G) was deployed with digital technology. In early 1990, TDMA was used to introduce the GSM into Europe.
In TDMA, each frequency channel is further divided into a set of time slot. Each time slots further carry a set of data. Result, more users can support, better voice quality, more coverage.
To use these time slots, the analog voice had to be converted to digital. A voice coder, known as a vocoder, performs this process. The number of voice channels per radio channel could be increased significantly.
In mid 1990, CDMA became second type of digital 2G system.
In CDMA the approach was different. Instead of time slot, each voice channel has assigned unique code before being added into radio channel. In terms of analogy imagine an international airport. When you enter the air port, there is announcement in different languages.
English men ears will hear English announcement, germen men will hear in German voice and so on through the languages of the world. You can pick the appropriate signals and overall noise signals as well..Compared with TDMA, CDMA offers more capacity at the same or better voice quality level.
But as we move on......and wireless industry evolve,
TDMA, and FDMA will be sent to the dustbin of the history. GSM path ends with the WCDMA (wide band CDMA) where the CDMA ends with the CDMA2000.
.....stay tuned to know more about 3G, 4G. :-))

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