What is Cellular telephone system ?

Cellular telephone system

A mobile phone is a portable telephone which receives or makes calls through a cell site (base station), or transmitting tower. Radio waves are used to transfer signals to and from the cell phone. Modern mobile phone networks use cells because radio frequencies are a limited, shared resource.

Cells in a Network

What’s interesting about mobile phone networks is their cellular design. (Hence the terms “cellular network” and “cellular phone.”) By that, I mean that a mobile phone network is divided into thousands of overlapping geographic areas, or cells. A typical cellular network can be envisioned as a mesh of hexagonal cells, as shown in Figure , each with its own base station at the center. The cells slightly overlap at the edges to ensure that users always remain within range of a base station. (You don’t want a dropped call when you’re driving between base stations.)

Definition - What does Handoff mean?

A handoff refers to the process of transferring an active call or data session from one cell in a cellular network to another or from one channel in a cell to another. A well-implemented handoff is important for delivering uninterrupted service to a caller or data session user.

-Handoffs may be classified into two types: 

• Hard Handoff: Characterized by an actual break in the connection while switching from one cell or base station to another. The switch takes place so quickly that it can hardly be noticed by the user. Because only one channel is needed to serve a system designed for hard handoffs, it is the more affordable option. It is also sufficient for services that can allow slight delays, such as mobile broadband Internet.
• Soft Handoff: Entails two connections to the cell phone from two different base stations. This ensures that no break ensues during the handoff. Naturally, it is more costly than a hard handoff.

Frequency Reuse

Because only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels to carry more than one conversation at a time. The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was implemented by restructuring the mobile telephone system architecture into the cellular concept.

The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere.

 Cellular System Components

The cellular system offers mobile and portable telephone stations the same service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands of subscribers in a major metropolitan area. The cellular communications system consists of the following four major components that work together to provide mobile service to subscribers.

·         public switched telephone network (PSTN)

·         mobile telephone switching office (MTSO)

·         cell site with antenna system

·         mobile subscriber unit (MSU)

PSTN

The PSTN is made up of local networks, the exchange area networks, and the long-haul network that interconnect telephones and other communication devices on a worldwide basis.

Mobile Telephone Switching Office (MTSO)

The MTSO is the central office for mobile switching. It houses the mobile switching center (MSC), field monitoring, and relay stations for switching calls from cell sites to wireline central offices (PSTN). In analog cellular networks, the MSC controls the system operation. The MSC controls calls, tracks billing information, and locates cellular subscribers.

The Cell Site

The term cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. A list of hardware located at a cell site includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems.

Mobile Subscriber Units (MSUs)

The mobile subscriber unit consists of a control unit and a transceiver that transmits and receives radio transmissions to and from a cell site. The following three types of MSUs are available:

·         the mobile telephone (typical transmit power is 4.0 watts)

·         the portable (typical transmit power is 0.6 watts)

·         the transportable (typical transmit power is 1.6 watts)

·         The mobile telephone is installed in the trunk of a car, and the handset is installed in a convenient location to the driver. Portable and transportable telephones are hand-held and can be used anywhere.

Disadvantages of Cellular Systems

Some of the disadvantages of cellular systems are:

1.   Infrastructure needed
Small cells require a complex infrastructure to connect all base station. The infrastructure required includes switches for call forwarding, location registers etc.

2.   Handover needed
The mobile station has to perform a handover when changing from one cell to another very frequently.

3.   Frequency planning
To avoid interference, frequency spectrum should be distributed properly with a very less range of frequency spectrum.

Advantages of Cellular Systems:

1.   Higher capacity
Smaller the size of the cell more the number of concurrent users i.e. huge cells do not allow for more concurrent users.

2.   Less transmission power
Huge cells require a greater transmission power than small cells.

3.   Local interference only
For huge cells there are a number of interfering signals, while for small cells there is limited interference only.

4.   Robustness

As cellular systems are decentralized, they are more robust against the failure of single components.