Cellular Access Technology

By: Ian Ball

Multiplexing

Let us consider radio station in a locality A. Let's say its popular, and therefore has loyal customers in localities B and C; and not just because of good music.

Customers in locality B can tune into to get the News and Information that is relevant only to them; ditto for locality C. To achieve this, our radio station sends one signal to users in B and an entirely different signal to the users in C, at the same time. This is done by a neat concept called MULTIPLEXING. It sends the signal for B on one frequency and C on another frequency. This is called Frequency Division Multiplexing (FDM). FDM is however not the only way to multiplex. Another way for multiplexing is time division multiplexing (TDM) - in a span of one second (called the 'Time frame'), the signal for B can be sent for first half second, and that for C can be sent for second half second. This repeats every second. TDM is very popular as you don't need to use different frequencies for different signals.

Multiple Access

Armed with this knowledge, we can now apply this to a mobile telephone network. A good mobile phone network allows many callers at the same time. Because callers access the network when needed rather than be connected to it all the time, the term 'Multiplexing' is now replaced by 'Multiple Access'. A network that separates callers by different frequencies uses Frequency Division Multiple Access (FDMA), one that separates callers by assigning them different time slots uses Time Division Multiple Access (TDMA); and one that separates the callers by assigning them unique random codes is called Code Division Multiple Access (CDMA).

Frequency Division Multiple Access

FDMA is the oldest and the most basic method for multiple accesses on a cellular network. It was used to analogue transmission on the first generation of cellular networks. Though it was used for the analogue, it was perfectly capable of digital transmission as well; it just wasn't considered to be the most efficient way to do so. Each cellular operator would be authorized to use a range of frequencies. The operator would then divide this range into equalized frequency bands, taking into considerations two things: first, each frequency band had to be large enough to accommodate the frequencies of human voice. Keeping the band 30kHz wide seemed to be sufficient. Second, they needed to ensure that two calls on the network did not interfere with each other. To do this there would be a 'guard band' between the frequencies that would actually be used for calls. Each call is assigned a pair of frequencies - one for talking and one for listening. Once the call is done this pair is released and available to be re-used for another call. The most successful networks to employ FDMA were the Advanced Mobile Phone Systems (AMPS) in the USA and the Nordic Mobile Telephone (NMT) in the Scandinavian countries. The NMT system was set up in 1981. It was called NMT 450, because it operated in the 450 MHz range. In 1986, the NMT 900 system was operational. It had more channels and hence more capacity than the existing NMT 450 system. The size of a cell in the NMT network was between 2 km and 30 km. an impressive aspect of the network is how well it covered the unique terrain of these countries.

Disadvantages of FDMA

The biggest disadvantage was the low call capacity. 395 voice channels on the AMPS network meant that if you were to be the 369th caller in a particular cell, you wouldn't be out of luck!
Another important problem with this is the lack of security- calls could be overhead with considerable ease, and one could steal a phone's serial code and make illegal calls on the systems.

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