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7 Mobile Communication
Business Research Methods (DMS 502)
University of Nairobi
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Mobile Communication Since the time of wireless telegraphy, radio communication has been used extensively. Initially the mobile communication was limited between one pair of users on single channel pair. The range of mobility was defined by the transmitter power, type of antenna used and the frequency of operation. With the increase in the number of users, accommodating them within the limited available frequency spectrum became a major problem. To resolve this problem, the concept of cellular communication was evolved. The present day cellular communication uses a basic unit called cell. Areas are divided into cells and each is served by its own antenna. It is also served by base station consisting of transmitter, receiver, and control unit. Cells are allocated bands of frequencies. The cells are set up such that antennas of all neighbors are equidistant (hexagonal pattern). Each cell consists of small hexagonal area with a base station located at the center of the cell which communicates with the user.
Wireless Communication System Definitions: - To understand the subject “Wireless Communication”, it is better to go through some important definitions, which we will come across frequently.
Base Station: It is a fixed mobile radio system and it is used for radio Communication with mobile stations. Base stations are situated at the center on the edge of coverage area.
Transceiver: It is a device which is capable to transmit and receive the signal at the same time.
Mobile Station: It is a station in the cellular radio service and it is used in motion at unspecified locations.
Control Channel: This Channel is used for the control of call set up, call request, call initiate and other signal for control purposes.
Forward Channel: This channel is used for the transmission of signal from the base station to mobile.
Reverse Channel: Function of the reverse Channel is opposite of the forward channel i. its function is to transmit signal from mobile to base station.
Half Duplex Systems: These systems allow one way communication i. user can only either transmit or receive information.
Full Duplex Systems: - Communication system which allows two way communication i. user can transmit and receive the information at the same time, using this system.
Hand Off: It is the process of transferring the call of a mobile station from one channel or base station to another.
Mobile Switching Center (MSC) :- Its function is to provide route to the calls in large service area.
Subscriber: - It is the name given to the user who pays subscription charges for using mobile communication system.
Page: - It is the brief message which is sent over the entire service area simultaneously by many base stations.
Simplex Systems: - These systems provide one way communication.
Roamer: - It is basically a mobile station and it operates in a service area other than that for which services have been prescribed.
Mobile: - It is the term used to classify any radio channel that can be moved during operation
Mobile or users: - Collective group of users in a wireless system are called mobiles or users.
How to cope with increasing capacity of cells Cell splitting: Cell Splitting is based on the cell radius reduction and minimizes the need to modify the existing cell parameters. Cell splitting involves the process of sub-dividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna size and transmitting power. This increases the capacity of a cellular system since it increases the number of times that channels are reused. Cell sectoring: Sectoring is basically a technique which can increase the SIR without necessitating an increase in the cluster size. By adding new channels Frequency borrowing from adjacent cells Microcells: Antennas can be moved to buildings, hills, and lamp posts
Each cell has a low power transmitter with a coverage area equal to the area of the cell. This technique of substituting a single high powered transmitter by several low powered transmitters to support many users is the backbone of the cellular concept.
Operational Channels In each cell, there are four types of channels that take active part during a mobile call. These are: Forward Voice Channel (FVC): This channel is used for the voice transmission from the BS to the MS. Reverse Voice Channel (RVC): This is used for the voice transmission from the mobile station (MS) to the BS. Forward Control Channel (FCC): Control channels are generally used for controlling the activity of the call, i., they are used for setting up calls and to divert the call to unused voice channels. Hence these are also called setup channels. These channels transmit and receive call initiation and service request messages. The FCC is used for control signaling purpose from the BS to MS. Reverse Control Channel (RCC): This is used for the call control purpose from the MS to the BS. Control channels are usually monitored by mobiles.
Making a Call When a mobile is idle, i., it is not experiencing the process of a call, then it searches all the FCCs to determine the one with the highest signal strength. The mobile then monitors this particular FCC. However, when the signal strength falls below a particular threshold that is insufficient for a call to take place, the mobile again searches all the FCCs for the one with the highest signal strength. For a particular country or continent, the control channels will be the same. So all mobiles in that country or continent will search among the same set of control channels. However, when mobile moves to a different country or continent, then the control channels for that particular location will be different and hence the mobile will not work. Each mobile has a mobile identification number (MIN). When a user wants to make a call, he
sends a call request to the Mobile Switching Center (MSC) on the reverse control channel. He
also sends the MIN of the person to whom the call has to be made. The MSC then sends this
MIN to all the base stations. The base station transmits this MIN and all the mobiles within the coverage area of that base station receive the MIN and match it with their own. If the MIN matches with a particular MS, that mobile sends an acknowledgment to the BS. The BS then informs the MSC that the mobile is within its coverage area. The MSC then instructs the base station to access specific unused voice channel pair. The base station then sends a message to the mobile to move to the particular channels and it also sends a signal to the mobile for ringing. In order to maintain the quality of the call, the MSC adjusts the transmitted power of the mobile which is usually expressed in dB or dBm. When a mobile moves from the coverage area of one base station to the coverage area of another base station i., from one cell to another cell, then the signal strength of the initial base station may not be sufficient to continue the call in progress. So the call has to be transferred to the other base station. This is called handoff. In such cases, in order to maintain the call, the Mobile Switching Center (MSC also called Mobile Telephone Switching Office (MTSO)) transfers the call to one of the unused voice channels of the new base station or it transfers the control of the current voice channels to the new base station.
Mobile propagation effects Signal strength: The mobile signal must be strong enough between base station and mobile unit to maintain signal quality at the receiver. The strength must be so strong so as to create co-channel interference with channels in another cell using the same frequency band. Fading: This is where the signal propagation effects may disrupt the signal and cause errors. Handoff: A handoff is the term used to describe migration from one BS to another in a mobile network. Soft Handoff: commences communication with a new BS without interrupting communication with old BS. It uses the same frequency assignment between old and new BS provides different site selection diversity. Hard handoff: this is the transmission between two base stations with different frequency assignment.
Mobile communication techniques By definition, mobile radio terminal means any radio terminal that could be moved during its operation. Depending on the radio channel, there can be three different types of mobile communication. In general, however, a Mobile Station (MS) or subscriber unit communicates to a fixed Base Station (BS) which in turn communicates to the desired user at the other end. The MS consists of transceiver, control circuitry, duplexer and an antenna while the BS consists of transceiver and channel multiplexer along with antennas mounted on the tower. The BS are also linked to a power source for the transmission of the radio signals for communication and are connected to a fixed backbone network.
The figure above shows a basic mobile communication with low power transmitters/receivers at the BS, the MS and also the Mobile Switching Center (MSC). The MSC is sometimes also called Mobile Telephone Switching Office (MTSO). The radio signals emitted by the BS decay as the signals travel away from it. A minimum amount of signal strength is needed in order to be detected by the mobile stations or mobile sets which are the hand-held personal units (portables) or those installed in the vehicles (mobiles). The region over which the signal strength lies above such a threshold value is known as the coverage area of a BS. The fixed backbone network is a wired network that links all the base stations and also the landline and other telephone networks through wires.
Existing Mobile Communication Technologies The cellular radio network system facilitates mobility in communication. Systems achieve mobility by transmitting data via radio waves. The following are some examples of mobile communication systems currently in use. Paging (SMS) A simple and inexpensive form of mobile communication. An antenna or satellite broadcasts short messages to subscribers. Receivers are usually devices such as beepers, which display messages on a small screen. Transmission of data is one way. Paging systems are designed to provide reliable communication to subscribers wherever they are. This necessitates high-powered transmitters and low data rates for maximum coverage of each transmitter’s designated area.
Communication Satellites Satellites consist of large transponders that listen to a particular radio frequency, amplify the signal, and then rebroadcast it at another frequency. They are inherently broadcast devices. A drawback of satellites is that they have quite a large propagation delay due to the distances traveled by radio waves.
Cellular Radio Networks Cellular networks are divided up into cells, each cell being serviced by one or more radio transceivers (transmitter/receiver). Communication in a cellular network is full duplex, where communication is attained by sending and receiving messages on two different frequencies. The reason for the cellular topology of the network is to enable frequency reuse. Cells, a certain distance apart, can reuse the same frequencies, which ensure the efficient usage of limited radio resources.
Personal Handyphone The Personal Handyphone System (PHS) is used in Japan. It is similar to cellular networks; however phones can also communicate directly with one another when in range. This is an advantage over cellular phones, which can only communicate with one another via base station transceivers. This system is very popular within heavily populated metropolitan areas.
7 Mobile Communication
Course: Business Research Methods (DMS 502)
University: University of Nairobi
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