Cell (cell-id), BTS and BSS in GSM network

Question

1. what is the relation between BTS and cell? I think one BTS hardware can cover few cells and also some cells could be covered by more than one BTS isn't it?

2. Is part of information, that mobile receives from GSM network identification of concrete BTS or mobile phone knows only cell-id?

3. Is part of information, that mobile receives from GSM network identification of BSC?

Answer 1

Ad 1: Typically one BTS can handle several cells. Common patterns are a one BTS covering a circular area with one round-radiating antenna or a three-sector BTS which covers three cells with sector-radiating antennas. One cell can only be handed by one BTS at a time. Two or more BTSes are not possible since the radio communication would interfere with each other. Note that this is completely different in WCDMA/UMTS since there is no concept of cells.

Ad 2: Since one cell is covered by exactly one BTS, the cell id uniquely identified the concrete BTS.

Ad 3: Since the BTS does not contain any control logic, the mobile communicates directly with the BSC, e.g. about radio resources.

Edit after comment:

1/ The BTS is "dumb" to say it simply. It does only what the BSC instructs it to do. E.g. The BSC tells the BTS as well as the mobile which frequencies to use for the radio communication. A BTS does not route traffic as it is hooked to exactly one BSC. It even does not route traffic to one of several mobiles attached to the BTS as this is done by the BSC. Think of the BTS as a Um-to-Abis physical layer and protocol transcoder.

2/ Actually my earlier statement that UMTS has no cell concept is not exactly true, it's just different.

GSM is FTDMA (frequency and time division multiple access). The radio channel is shared by using different frequecies (per cell) and timeslots (per mobile). Since radio frequency is used to distinguish participants, great care must be taken that not two GSM participants use the same frequency at the same time at the same location. The solution to this is cells, where geographic areas have different frequencies assigned. Network planning must ensure that no two neighbouring cells use the same frequencies as this may lead to interference since you cannot control exactly the size of a cell (e.g. due to absorption and reflection). In GSM, a BTS has a fixed number of radio transmission channels, the number depends on the BTS hardware configuration. If all channels are in use, the cell is full, this is indpendent of the location of a mobile in the cell.

UMTS is CDMA (code division multiple access). The radio channel is shared by encoding the payload in a way that allows to decode it later even if several senders use the same frequency range. That requires coding schemes which are collision free (all codes are different from each other to avoid senders using too similar codes) and a great deal of signal processing. As an analogy: on a party you can understand someone accross the room, even if ten people are talking. The more senders communicate within the cell, the smaller the cell gets in order to allow the BTS/Node-B distinguishing between senders. Therefore, in UMTS a cell size is not geographically fixed. The cell "breathes" depending on its load.

Answer 2

OK, this thread is quite old, but requires some further clarifications for next generations.

When talking about GSM physical network architecture, the term BTS (Base Transceiver System) refers to the physical site itself - the 'small house with the tower' (although modern small BTSs are just boxes hanged on walls or placed on roof tops). Each such physical site can host one omni-directional cell, or several sector cells.

In GSM logical network architecture, there is some confusion. The terms 'Cell' and 'Base Station' actually refer to the same physical entity (a set of transceiver units, each used to receive and transmit one of the paired UL/DL carrier frequencies allocated in the BA frequency set). Let's call this entity 'physical cell' just for clarification.

The term Base Station is used for radio resource management. A BSIC (BS Id Code, or BTS Id Code) is allocated for the 'physical cell' and is used in the radio-related conversations between the MS (Mobile Station) and the BSS (BTS and BSC), e.g. for measurement reports.

The BSIC is composed of 'local' parameters - Network Color Code (NCC) and BS Color Code (BCC), and is therefore unknown outside the network. This is where the term Cell comes in:

The term Cell is used for Mobility Management. A Cell Identity (CI) is defined as a refinement of the Routing Area - one RA will include several cells in it. The Global Cell Identifier (GCI) is composed of network, RA and CI, and is used for handovers inside and outside the network.

It is up to the BSC to convert the BSIC to the Cell Identity (the BSC may convert the BSIC directly to GCI, or the BSC converts to CI, and the MSC will convert it to GCI).

Hope that helps a bit.