The Cellular Concepts: Multimedia System Design Issues Multimedia Systems 1Telecom Management ---- Engr. Bilal Ahmad

Cellular Systems- Basic Concepts High capacity is achieved by limiting the coverage of each Base Station to a small geographic region called a Cell. Same Frequencies/ Time slots/ Codes are re-used by spatially separated base stations. A switching technique called a Hand off/ Hand over enables a call to proceed uninterrupted when one users move from one cell to another. Resolves problem of limited Radio Spectrum. 2Telecom Management "Engr. Bilal Ahmad"

Basic Concepts of Cellular Systems Neighboring Base Stations are assigned different groups of channels so as to minimize the interference. By systematically spacing base stations and the channel groups may be re-used as many number of times as necessary. As demand increases, the number of Base Stations may be increased. Thereby providing additional capacity. 3Telecom Management "Engr. Bilal Ahmad"

Introduction Antenna height and maximum allowed transmitter power is limited. Coverage can be improved through the use of cellular structure. Frequency re-use within the cellular structure offers a large overall system capacity even with limited bandwidth. Frequency re-use comes, however, at the expense of co-channel interference. 4Telecom Management "Engr. Bilal Ahmad"

Frequency re-use The same frequency is re-used provided a certain minimum radius between identical channels is maintained. 5Telecom Management "Engr. Bilal Ahmad"

Co-channel interference Power from the primary channel and from all the interfering channels. 6Telecom Management "Engr. Bilal Ahmad"

Channel re-use In Urban areas where the number of users is large, it is necessary to reduce the size of the cell so that; I.The frequency channels can be used more often, thus increasing the capacity. II.The number of users per cell is minimized thus improving the call initiation and call handover etc. III.The Transmitter power needed is reduced. The technique is called cell splitting. 7Telecom Management "Engr. Bilal Ahmad"

Cell Geometry 8Telecom Management "Engr. Bilal Ahmad"

Cell Geometry The Problem with circular cells is the creation of uncovered zones. Rectangular and triangular cells have uneven spacing from the cell center to the edges. Hexagonal cells are most widely used (analysis). NB: It is the antenna that provides coverage. 9Telecom Management "Engr. Bilal Ahmad"

Forward and Reverse Channels Forward Voice Channel is used for Voice transmission from BS to MS. Reverse Voice Channel is used for Voice transmission from MS to BS. Forward Control Channel is used for initiating a call from BS to MS. Reverse Control Channel is used for initiating the call from MS to BS. FCC and RCC are also called as setup channels. 10Telecom Management "Engr. Bilal Ahmad"

Anatomy of a Cellular Call A cell phone when turned on scans a group of FCC to determine the one with the strongest signal. It monitors the channel until drops below a useable threshold. It then scans for the next available strongest channel. Control channels are defined and standardized over the entire area of service. Typically 5% of the total number of channels are used by control channels. 11Telecom Management "Engr. Bilal Ahmad"

A Call to a Mobile User The MSC dispatches the request to all the BS’s. The Mobile identification number is broadcast as a paging message over all FCC throughout the service area. The MS receives the paging message from the BS it is monitoring. It responds by identifying itself over the RCC. The BS conveys the handshake to the MSC. The MSC instructs the BS to move to an unused voice channel. 12Telecom Management "Engr. Bilal Ahmad"

A Call to a Mobile User (Continued) The BS signals the MS to change over to an unused FVC and RVC. A Data Message called an Alert is transmitted over FVC to instruct the mobile to ring. All of these sequence of events occurs in just a few seconds, and are not noticeable to you all as a user. While the call is in progress the MSC adjusts the transmitted power in order to maintain the call quality. 13Telecom Management "Engr. Bilal Ahmad"

A Call from a Mobile User A call initiated request is sent to the RCC. Along with this the MS transmits the ESN, Electronic Serial Number and the phone number of the receiving party. The MS also transmits the stations Class Mark SCM which indicated the maximum transmitter power level for the particular user. The BS forwards the data to MSC which validates the data and makes connection to the called party through PSTN. 14Telecom Management "Engr. Bilal Ahmad"

Channel re-use 15Telecom Management "Engr. Bilal Ahmad"

Geometry of Hexagonal cell The x-y axis and u-v axis does not match. There is a pi/6 phase shift from x to u. 16Telecom Management "Engr. Bilal Ahmad"

Geometry of a Hexagonal Cell The Center to Center Distance between two neighboring cells is 2 R Cos (pi/ 6) 17Telecom Management "Engr. Bilal Ahmad"

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Relation between I, j and the Number of Cells in a Cluster 19Telecom Management "Engr. Bilal Ahmad"

Co Channel Interference When Frequencies are re-used, the performance is determined by the signal to (noise + Interference) ratio. 20Telecom Management "Engr. Bilal Ahmad"

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