The basic technology for mobile phones and personal communication systems is a cellular network. A service area is divided into cells. A base station is located in the middle of each cell.
Each cell is allocated a frequency and adjacent cells are allocated a different frequency to avoid interference. The transmitting power is controlled to avoid interference with neighbouring cells. The same frequency is used in non-adjacent cells – this is known as frequency reuse.
When the number of users in a cell increases, the frequency allocated to the cell is no longer enough. There are a number of approaches to overcome this problem.
- Add new channels: Usually, when a system is set up, not all channels are utilised. Growth and expansion is systematically managed by adding new channels when the number of users increases.
- Frequency borrowing: A congested cell borrows frequencies from adjacent cells.
- Cell splitting: Cells in areas with high density traffic are divided into smaller cells. For this purpose, the power level must be reduced to limit signal transmission to within one cell.
- Cell sectoring: A cell is divided into 3 or 6 sectors and each sector is allocated a subset of channels for that cell. Involves replacing a single omni-directional antenna with 3 directional antennas (120o sectoring) or 6 directional antennas (60o sectoring). Each sector uses a directional antenna at the BS and is assigned a set of channels. The number of channels in a sector is the number of channels in a cell divided by the number of sectors. Reduces transmission power, longer battery lifetime.
Cellular Network Entities
A mobile station (MS) is a a device used to communicate in the cellular network, e.g. your mobile phone.A base transceiver station (BTS) is a base station consisting of a transceiver that receives/transmits signals over the radio interface. It serves one cell only.
A base station controller (BSC) controls one or more BTSs and is under one mobile switching centre.
A mobile switching centre (MSC) sets up and maintains calls. It connects the cellular network to the PSTN and performs all switching and signalling functions for MSs located in its area.
A base station subsystem (BSS) consists of one BSC and one or more BTSs. The radio equipment of a BSS may support one or more cells.
Calling a Fixed Number
When you press on the call button, your mobile phone sends a call request to your BTS using a special channel, i.e. reverse control channel (RCC). The BTS forwards the request to the MSC which validates it to make sure that you’re authorized to use the service requested. Then it uses your friend’s number to make a connection via the PSTN. A switch in the PSTN sets up a connection between your MSC and your friend.Calling a Mobile Number
A telephone number on a PSTN contains location information that is used by a switch to establish a connection between two subscribers. The mobile phone number cannot be used to determine a mobile user’s current location because a mobile user does not stay at a fixed location. In order to deliver a call to a mobile user, the cellular network has to determine the user’s location using a process termed paging.Let’s say your friend dials your mobile phone number. When the request for connection establishment arrives at the MSC, it sends a broadcast message to all BSs under its control. The BSs then broadcast a paging message, containing your mobile phone number, on all forward control channels (FCCs). Your mobile phone scans the FCC periodically to check if there is a paging message for you. When it detects the paging message, it acknowledges its presence in the cell by sending a message on RCC.
When the MSC receives the acknowledgement via the BS, it instructs the BS to allocate an unused voice channel for you. A data message is sent on FCC to your phone to instruct it to ring. When you accept the call, a connection is established between you and your friend. When you finish the conversation and hang up, the channel allocated to you is freed and can now be allocated to another user.
Location Registration and Update
In order to deliver a call to a mobile service subscriber, the network operator needs to keep track of the location of all subscribers. Location information is stored in a location register. There are 2 types of location register: home location register (HLR) and visitor location register (VLR).HLR is a database that stores information for the management of mobile subscribers. A Public Land Mobile Network (PLMN) may consist of one or more HLRs, depending on the number of subscribers, the capacity of the equipment and the organization of the network. The information stored in a HLR are subscription information and location information to enable the charging and routing of call to the MSC where you are located. Each subscription is associated with an IMSI and one or more mobile station international ISDN numbers (MISDN). It may also store other information such as service restriction and supplementary services.
VLR is a database that stores information required to handle call requests/deliveries made/received by subscribers roaming in its area. A VLR area is the part of the network controlled by a VLR and may consist of one or more MSC areas. An MSC area consists of all base stations under the control of the MSC and may consist of one or more location areas.
The information stored in VLR are IMSI, MSISDN, mobile station roaming number (MSRN), temporary mobile station identity (TMSI), local mobile station identity (LMSI) and the location area where you are registered.
When is a Location Update Triggered? It is triggered only when you move out of a location area (LA) and into another location area. A LA consists of one or more cells, and is associated with a location area identification (LAI). When you roam into a new location area, your MS initiates a registration procedure. When the MSC in charge of the area notices the registration, it transfers the LAI of your location to the VLR.
Handoff / Handover
This is a procedure to allocate a channel to a mobile device when it enters another cell. The decision to execute a handoff is based on the measure of signal strength received by the mobile device. The mobile device provides feedbacks to the network about the received signal strength.
| eNB | base station |
| DeNB | donor eNB |
| HeNB | home eNB |
| MME | mobile management entity |
| RN | relay node |
| SGW | service gateway |
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