GSM Core Network: Base Station Subsystem: Frequencies (2)

Ok, so now when we are done with the entities of a Base Station Subsystem and are on verge of getting into details of Network Switching Subsystem, I thought why not to finish a very important part of the network here itself and thats of frequencies which are assigned to each of the cells. I have to be really very careful here because it’s a technical topic and I need to explain the same in simple and kind of non technical wayJ I will try.
Definition (we all have read it in physics):

Frequency is defined as a number of cycles per unit time i.e. it is the number of occurrences of a repeating event per unit time. It is often denoted by f and the unit of frequency is the hertz (Hz) and 1 Hz means that an event repeats once per second.
In layman’s term, frequency is the reciprocal of period
So, lets come back to the telecom gyan.

How the frequencies work in cell sites:

As I mentioned above, each of the cells is assigned with multiple frequencies which are carefully chosen to reduce interference with neighboring cells. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference. So, lets see how it happens with the help of below diagram:

f1: can be used for Cell1, Cell 3, Cell 8, Cell 10
f2: can be used for Cell2, Cell 4, Cell 9, Cell 11
f3: can be used for Cell5, Cell 7
f4: can be used for Cell6
Similar frequency cells are marked with same color. If you observe, you will see that there is no clash between the similar frequencies. The cells using the same frequencies are not sharing the borders

Allocated frequencies:

Below are the frequencies allocated for Telecom as per India’s National Frequency Allocation plan:
-       806-960 Used by GSM and CDMA mobile services
-       1710- 1930 Used for GSM mobile services
In India GSM technology works in the frequency bands of 900 and 1800 MHz and CDMA technology works in the 800 MHz band.
Point to be noted: Also lower frequencies cover large areas and higher frequencies cover small areas.

Multi band and Multi mode phones:
Before, mobiles could not tune themselves automatically to the frequencies they find, they needed the right hardware, not just software, to use different frequencies.
But today, most telephones support multiple bands as used in different countries to facilitate roaming. These are typically referred to as multi-band phones. Do not get confused with multi-modes phones as they are the ones which can operate on GSM as well as on other mobile phone systems using other technical standards or proprietary technologies for eg a phone that can support CDMA as well as GSM technology.
Multi band phones are the phones that can tune themselves to different frequencies. They can Dual-band phones , tri-band phones and quad-band phones
-       Dual band: that covers GSM networks in pairs such as 900 and 1800 MHz frequencies or 850 and 1900
-       Tri-band phones: that covers the 900, 1800 and 1900 or 850, 1800 and 1900
-       Quad-band phones:  that supports all four major GSM bands, allowing for global use.

Handoff or Handover:
As the phone user moves from one cell area to another cell during a call is in progress (imagine if the
user is travelling in a car), the mobile station will search for a new channel to attach to in order not to drop the call. Once a new channel is found, the network will command the mobile unit to switch to the new channel and at the same time switch the call onto the new channel. There is NO indication to the user exactly which frequency you are using. The cell sites use sophisticated switching equipment to transfer your phone call from one cell site to the next. This is called a handoff . As you get too far from one cell site and the call quality becomes unacceptable, the cellular system looks for the next cell site and frequency to hand your call off to. All of this happens in split seconds and is not obvious to the user. So, next time your call drops while on phone on move, understand that the handover between the sites is not effective in that area.

Transmit and Receive Frequencies:

Transmit and receive frequency are often called paired frequencies. That seems logical enough since it takes two frequencies to pass information. Unfortunately, the forward and reverse channels refer to just a single frequency, making a channel definition little confusing
Transmit or Reverse frequency is the one which is used by the mobile to interact with the BSC and Receive or forward frequency is the one used by base station to interact with the mobile. They're what make talking at the same time possible.

Cell Signal Encoding:
To distinguish signals from several different transmitters, frequency division multiple access (FDMA) and code division multiple access (CDMA) were developed.
With FDMA and CDMA, the transmitting and receiving frequencies used in each cell are different from the frequencies used in each neighboring cell as shown above.
But polarization division multiple access (PDMA) and time division multiple access (TDMA) cannot be used to separate signals from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible. Time division multiple access, however, is used in combination with either FDMA or CDMA in a number of systems to give multiple channels within the coverage area of a single cell.
We would be covering CDMA, TDMA, PDMA and FDMA in coming chapters and then, the above will be quite clearJ I promiseJ
Plz let me know your thoughts on this article. Is there anything else that can be added? Was it useful to you?


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