Today’s Cellular Telephone Systems

Presenting by…SHANTANU KRISHNAREG. No. :1791210007Branch : TCN

Today’s Cellular Telephone Systems

Contents• History• Cell concept• Frequency Reuse• Handoff• Operation

History

• In 1895, the telephone was invented by MARCONI demonstrated the first radio transmission from the Isle of Wight to a tugboat 18 miles away, and radio communications was born.

• Today most radio systems transmit digital signals composed of binary bits, where the bits are obtained directly from a data signal or by digitizing an analog voice or music signal.

Cellular Telephone Systems

• Cellular telephone systems are designed to provide two-way voice.

• Cellular systems were initially designed for mobile terminals inside vehicles with antennas mounted on the vehicle roof.

• The basic feature of the cellular system is frequency reuse.

• In a cellular system, the signal from a mobile unit (cell phone) to a base station is transmitted by radio waves through the air, instead of through metallic wires, However, the signal from the base station is sent to a mobile switching center and possibly to a telephone central office through electrical wires.

• The antenna at the base station converts the radio waves to electrical signals and circuits in the base station send the signal to the appropriate mobile switching center.

Cell concept

• Initial cellular system designs were mainly driven by the high cost of base stations, about one million dollars each. For this reason early cellular systems used a relatively small number of cells to cover an entire city or region. The cell base stations were placed on tall buildings or mountains and transmitted at very high power with cell coverage areas of several square miles.

• These large cells are called macrocells.

• Signals propagated out from base stations uniformly in all directions, so a mobile moving in a circle around the base station would have approximately constant received power.

• Cellular telephone systems are now evolving to smaller cells with base stations close to street level or inside buildings transmitting at much lower power. These smaller cells are called microcells or picocells, depending on their size.

• In a cellular radio system, a land area to be supplied with radio service is divided into regular shaped cells, which can be hexagonal, square, circular or some other regular shapes. Each of these cells is assigned multiple frequencies (f1 - f6) which have corresponding radio base stations. The group of frequencies can be reused in other cells Cellular Concept to increase both coverage and capacity.

Frequencies reused

Handoff

• The term handover or handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another.

• When the phone is moving away from the area covered by one cell and entering the area covered by another cell the call is transferred to the second cell in order to avoid call termination when the phone gets outside the range of the first cell.

Types of handoff

• Hard handoff is one in which the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to the source is broken before the connection to the target is made.

• Soft handoff is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken.

Handoff Process

Operation

• The area (a city, or a part of town) is divided into a number of cells and a base station within each cell.

• If a user (mobile phone) is within a particular cell, the call is handled by the corresponding base station within that cell.

• The base station transmits the signal to the mobile switching center (MTSO) which switches the signal to another base station, or to a Public Switched Telephone Network (PSTN) depending on the destination of the call.

• As a user moves from one cell to another, the call is “handed over” to the base station of the other cell.

• All base stations in a city are connected via a high-speed link to a mobile telephone switching office (MTSO).

• The MTSO acts as a central controller for the network, allocating channels within each cell, coordinating handoffs between cells when a mobile traverses a cell boundary, and routing calls to and from mobile users in conjunction with the public switched telephone network (PSTN).

• A new user located in a given cell requests a channel by sending a call request to the cell’s base station over a separate control channel. The request is relayed to the MTSO, which accepts the call request if a channel is available in that cell. If no channel is available, the call request is rejected.

Operation

• All cellular systems being deployed today are digital, and these systems provide voice mail, paging, and email services in addition to voice. Due to their lower cost and higher efficiency.

• Digital cellular systems can use any of the multiple access techniques TDMA ,FDMA or CDMA.

• There are two standards in the 900 MHz (cellular) frequency band: IS-54 (Interim Standard), which uses a combination of TDMA and FDMA, and IS-95, which uses semi-orthogonal CDMA.

• IS-95 is a digital cellular phone system using CDMA and FDMA.

• GSM is a digital cellular phone system using TDMA and FDMA.

Thank you…