There are two popular mechanisms which are used to transmit data/voice through available frequency spectrum. They are CDMA and GSM. I will try to explain these technologies using analogies. Initially we had something called FDMA (FDMA - I think expands to Frequency Division Multiple Access). We need to start with that to explain the subsequent successors.
The concept is pretty straight forward. On a Friday night say you decide to go bowling along with some of your friend. You reach a bowling place and you see it is pretty crowded, but then you notice some alleys are free. You go to the person in charge and he locates a free alley and allots it to you and then you start bowling in that alley. There are around 20 bowling alleys. Each alley is clearly separated from the adjacent bowling alley so that your bowling ball doesn't interfere with the balls in the other bowling alley.
Similar to that, in FDMA there is an allocated frequency spectrum which can be used for mobile communications. This frequency spectrum is divided into smaller frequency bands (subdivisions), each band/channel separated from each other to avoid interference. Each frequency band is allotted to a conversation/ call.
This is similar to how your radio channels share bandwidth, but then radio channels are different in the sense, they occupy the channel for a greater area depending on the power of the transmitter. In the case of cellular communications, the frequency sharing is limited to a defined area called cell and in a different cell the same frequency band is used by a different call.
We will get to these details later. But clearly we don't have infinite frequency spectrum width and definite allotted width. So as the allotted spectrum is limited, when demand (no of calls in a cell region) increase, the channels get used up in that region and there is no room left for more conversations, similar to the days when you went late and didn't get a free bowling alley. The number of simultaneous conversations with in a cell is limited to the number of frequency bands.
The FCC licenses these frequency bands to specific companies who bid for them. These licenses are issued territory wise for these frequency bands. So frequency band in one state might be owned by one cellular company while in another region by another cellular company.
So the obvious question is why don't we make the frequency bands smaller and accommodate all people or in other words why don't we have smaller bowling alleys??!!
We will answer the question in detail in the next blog ....
