The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum. Each cellular base station is allotted a group of radio channels to be used within a small geographic area called cells.
Contd. Use multiple low-power transmitters. Areas divided into cells. Each served by its own antenna. Served by base station consisting of transmitter, receiver, and control unit.
Determined by engineers experienced in cellular system architecture. Provisioning for each region is planned according to an engineering plan that includes cells, clusters, frequency reuse, and handovers. Cells A cell is the basic geographic unit of a cellular system.
Band of frequencies allocated. Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern). CELLULAR NETWORK ARCHITECTURE. CELLS STRUCTURE. Determined by the desired received signal level by the mobile subscriber from its base station transmitter in its operating area ideal, actual and fictitious cell models.
Contd. A CELL WITH A CS AND MOBILE. HEXAGONAL CELLULAR GEOMETRY. Offers best possible non-overlapped cell radio coverage. Multiple hexagons can be arranged next to each other.
The hexagons covers the large area. Simplifies the planning and design of the cellular system. HEXAGONAL CELLULAR GEOMETRY. CELLULAR CLUSTER. A group of cells that use a different set of frequencies in each cell.
Only selected number of cells can form a cluster. Can be repeated any number of times in a systematic manner.
The cluster size is the number of cells with in it and designated by K. HEXAGONALS CLUSTER PATTERNS. CONVENTIONAL MOBILE COMMUNICATION. One very powerful transmitter located at the highest spot in an area would broadcast in a radius of upto 50 kilometers.
LIMITATIONS:. High power consumption.Low capacity.Large size of the mobile. SYSTEM DESIGN PROBLEM Main limitations of a conventional mobile wireless communication system is:. Minimum availability of FREQUENCY SPECTRUM!!! So the big challenge is to serve large no. Of mobile users.
within limited allocated frequency spectrum. with a specified system quality. SYSTEM CAPACITY AND & SPECTRUM UTILIZATION How to increase the capacity and Spectrum utilization?
The Need:. optimum spectrum usage. More capacity. High quality of service. Low cost. POSSIBLE SOLUTION- FREQUENCY REUSE.
Reuse allocated RF spectrum or a given set of frequencies in a given large geographical service area without increasing the interference. Divide the service area into a number of small areas called cells. Allocate a subset of frequencies to each cell.
Use low-power transmitters with lower height antennas at the base stations. FREQUENCY REUSE CONCEPT. Large coverage area, efficient spectrum utilization and enhanced system capacity are the major attributes of cellular communications. Frequency reuse is the core concept of cellular communications. The design process of selecting and allocating channel groups for all the cellular base stations within a system is called frequency reuse. IILUSTRATION OF FREQUENCY REUSE.
CO-CHANNEL & ADJACENT CHANNEL CELLS. Cells, which use the same set of frequencies are referred to as co-channel cells. Co-channel cells are located sufficiently physically apart so as not to cause co-channel interference. The space between adjacent co-channel cells filled with other cells that use different frequencies to provide frequency isolation. TECHNICAL ISSUES Technical issues for proper design & planning of a cellular network:. Selection of a suitable frequency reuse pattern. Analysis of the relationship between the capacity, cell size & the cost of the infra structure.
CLUSTER SIZE AND CELL CAPACITY. In a cellular system, the whole geographical service area is divided into a number of clusters having finite number of cells. The K number of cells in a cluster use the complete set of available frequency channels, N. Each cell in the cluster contain J = (N/K) number of channels only. Therefore, N = J. K; where J ≤ N.
CLUSTER SIZE & SYSTEM CAPACITY. The cluster can be replicated many times to cover the desired geographical area by a cellular communication system.Let M be a number of clusters in the system, then overall system capacity, C is given as: C = M. N C = M.
J. K (N = J. K). When K is reduced, J is proportionally increased since N = J. K is constant. To increase co-channel interference!.
CO-CHANNEL CELLS. Cells which use the same set of frequencies are referred to as co-channel cells. The interference between co-channel cells is referred to as co-channel interference. The space between adjacent co-channel cells are filled with cells using different frequencies. Contd. Shift Parameters I and j in a Hexagonal Geometry.
The Shift parameters i and j are separated by 60 degrees in a hexagonal geometry. It can have any integer value 0,1,2.
These can be used to determine the location of co- channel cells. Contd. METHOD TO LOCATE THE CO-CHANNEL CELLS Rules for determining the nearest co-channel cell using “Shift parameters” (i, j) to lay out a cellular system is:.
Step 1: Move I cells along any side of a hexagon. Step 2: Turn 60 degrees anticlockwise. Step 3: Move j cells. Where I and j are shift parameters and can have integer value 0,1,2,3 & so on.
Co-Channel cells for i=3, j=2. FREQUENCY REUSE DISTANCE, D. Reusing an identical frequency channel in different cells is limited by co-channel interference between cells.The co-channel interference can become a major problem in cellular communication.It is desirable to find the minimum frequency reuse distance D in order to reduce this co-channel interference. Factors Which Influence ‘D’. The number of co-channels in the vicinity of the center cell.The antenna height. The transmitted power at each cell site NOTE: As long as the cell size is fixed, co-channel interference is independent of transmitter power of each cell. Frequency Reuse Ratio, q q = D/R The frequency reuse ratio, q is also referred to as:.
The co-channel reuse ratio. The co-channel reuse factor. Co-channel interference reduction factor. q = D/R Ratio The real power of the cellular concept is that interference is not related to the absolute distance between cells but related to ratio of the distance between co-channel (same frequency) cells to the cell radius. FREQUENCY REUSE ADVANTAGES DISADVANTAGES The frequency reuse system can be increase the spectrum efficiency, thereby increasing the system capacity.
If the system is not properly designed, co- channel interference may occur due to the simultaneous use of the same channel. CO-CHANNEL INTERFERENCE The interference between signals from co-channel cells channel cells. Unlike thermal noise which can be overcome by increasing the signal-to- noise ration (SNR), co-channel interference cannot be combated by simply increasing the carrier power of a transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells. How it can be reduced??.
Increasing the separation between two co-channel cells. Using directional antennas at the cell site.
Lowering the antenna heights at the cell site. Use of diversity scheme at receiver.
ADJACENT CHANNEL INTERFERENCE 1. What is adjacent channel interference?
Interference resulting from signals which are adjacent in frequency to desired signal. Why does it occur?
This results from imperfect receiver filters which allow nearby frequencies to leak into the pass band. How it can be reduced?? Careful filtering and channel assignment By keeping the frequency separation between each channel in a given cell as large as possible By sequentially assigning successive channels in the frequency band to different cells. Co-channel cells Adjacent- channel cells Co-channel interference Adjacent- channel interference INTERFERENCE. HANDOFF. When a mobile user travels from one cell to another cell within a call’s duration the call should be transferred to the new cell’s base station.
Otherwise, the call will be dropped because the link with the current base station becomes too weak as the mobile recedes. This ability for transference is a design matter in mobile cellular system design and is called handoff. Contd.
HANDOFF STRATIGIES. When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. Handoff operation identifying a new base station re-allocating the voice and control channels with the new base station. Contd. Handoff Threshold Minimum usable signal for acceptable voice quality (- 90dBm to -100dBm) Handoff margin ( = Pr Handoff – Pr minimum useable) cannot be too large or too small.
If is too large, unnecessary handoffs burden the MSC If is too small, there may be insufficient time to complete handoff before a call is lost. Contd. Contd.
Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station. Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided. Depends on the speed at which the vehicle is moving. Steep short term average - the hand off should be made quickly The speed can be estimated from the statistics of the received short-term fading signal at the base station. Contd.
Dwell time The time over which a call may be maintained within a cell without handoff. Dwell time depends on propagation interference distance speed. PRACTICAL HANDOFF CONSIDERATIONS.
Different type of users High speed users need frequent handoff during a call. Low speed users may never need a handoff during a call.The MSC becomes burdened if high speed users are constantly being passed between very small cells.To Minimize handoff intervention handle the simultaneous traffic of high speed and low speed users.
CELLULAR COMMUNICATION SYSTEM. 1. CHAPTER 4 CELLULAR COMMUNICATION SYSTEM. CHAPTER OUTLINE CELLULAR COMMUNICATION COMPONENT ANTENNA IN CELLULAR COMMUNICATION CELLULAR COMMUNICATION CONCEPT HAND OVER PROCESS IN CELLULAR COMMUNICATION SYSTEM RADIO CHANNEL & MODULATION TECHNIQUE IN CELLULAR COMMUNICATION. PART 1 MAIN COMPONENT IN CELLULAR COMMUNICATION SYSTEM.
AT THE END OF THIS TOPIC STUDENT SHOULD BE ABLE TO: 4.1 Understand the main component in cellular communication system. 4.1.1 Describe the Mobile Switching Centre (MSC). 4.1.2 Describe the elements connected to the MSC. Home Location Register. Visitor Location Register. Equipment Identity Register (EIR). Authentication Centre (AuC) e.
Gateway Mobile Switching Centre (GMSC). SMS Gateway (SMS-G). AT THE END OF THIS TOPIC STUDENT SHOULD BE ABLE TO: 4.1.3 Describe the Base Station Subsystem (BSS). 4.1.4 Describe the types of Base Transceiver. Base Transceiver Station (BTS). Base Station Controller (BSC). 4.1.5 Describe the Mobile Unit that functions as a transceiver.
4.1.6 Explain the function of sim card used in mobile units. 4.1.7 Relate between MSC, BSS, BTS, BSC and mobile unit in the form of diagrams. Wireless Communications. Multimedia wireless Communications at any Time and Anywhere. Brief history. Ancient Systems: Smoke Signals, Carrier Pigeons.
Cellular has enjoyed exponential growth since 1988, with more than 2 billion users worldwide today. Ignited the recent wireless revolution, 1980-2003 Radio invented in the 1880s by Marconi Many sophisticated military radio systems were developed during and after WW2 Growth rate tapering off Is there a future for wireless?. Current Wireless Systems.
Cellular systems Wireless LANs Satellite Systems Paging Systems Bluetooth Ultrawideband Radios Zigbee Radios. What is Cellular Communication System.
Cellular communication is designed to provide communications between two moving units, or between one mobile unit and one stationary phone or land unit (PSTN). A service provider must be able to locate and track a caller, assign a channel to the call, and transfer the channel from base station to base station as the caller moves out of range (handover/handoff). To make this tracking possible. Each cellular service area is divided into small regions called cells. Each cell contains an antenna and is controlled by powered network station, called the base station (BS). Each base station is controlled by a switching office, called a mobile switching center (MSC).
The MSC coordinates communication between all the base stations and the telephone central office (exchange). It is a computerized center that is responsible for connecting calls, recording call information, and billing. COMPONENTS IN CELLULAR COMMUNICATION SYSTEM 3 main components:. Mobile Station (MS) – UE, SIM. Base Station Subsystem (BSS) – BTS, RBS, BSC.
Network and Switching Subsystem (NSS) – MSC, VLR, HLR,. Network & Switching Subsystem (NSS). Mobile Switching Center (MSC).
Authentication Centre (AuC). Gateway Mobile Switching Center (GMSC). SMS Gateway (SMS-G) Home Location Register (HLR) Visitor Location Register (VLR) Equipment Identify Register (EIR). Mobile Switching Center (MSC).
The MSC is the heart of the GSM network. From technical perspective MSC is just an ordinary Integrated Services Digital Network (ISDN) exchange. One MSC can handles multiple BSCs and also interfaces with other MSC's (Using E-Interface).
It also handles inter-BSC handoffs as well as coordinates handoffs. With other MSC's for inter-MSC. Mobile Switching Center (MSC). Mobile Switching Center (MSC). Mobile Switching Center (MSC). MSC performs the telephony switching functions of the system.
Controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN) and Public Land Mobile Network (PLMN). Mobile Switching Center (MSC). Difference between a MSC and an exchange in a fixed network, MSC has to take into account the impact of the allocation of radio resources and the mobile nature of the subscribers and has to perform in addition, at least the following procedures:. required for location registration.
procedures required for handover. Mobile Switching Center (MSC). MSC can be connected to only one VLR or more VLR. Therefore, all mobile stations that move around under base stations connected to the MSC are always managed by the same VLR. MSC would communicate typically with one EIR.
While it is possible for an MSC to communicate to multiple EIRs, this is highly unlikely since the EIR provides a centralized and geographic independent function. Ericsson Mobile Switching Center Server (MSC-S) The Mobile Switching Center Server (MSC-S) provides control of high-capacity switching in mobile circuit core networks. The Elements Connected to The MSC The MSC connects to the following elements: a. The Home Location Register (HLR) b.
The Visitor Location Register (VLR) c. Equipment Identify Register (EIR) d. Authentication Centre (AuC) e. Gateway Mobile Switching Center (GMSC) f. SMS Gateway (SMS-G). Home location register (HLR).
HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. The HLRs store details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record. Visitor Location Register (VLR).
When the mobile user visits a PCS network other than the home system, a temporary record for the mobile user is created in the visitor location register (VLR) of the visited system. The VLR temporarily stores subscription information for the visiting subscribers so that the corresponding MSC can provide service. In other words, the VLR is the 'other' location register used to retrieve information for handling calls to or from a visiting mobile user.
Home Location Register (HLR). Examples of other data stored in the:. GSM services that the subscriber has requested or been given. GPRS settings to allow the subscriber to access packet services. Current location of subscriber (VLR and serving GPRS support node/SGSN).
Call divert settings applicable for each associated MSISDN. Responsibilities of the HLR include:. management of service profiles. mapping of subscriber identities (MISDN, IMSI). supplementary service control and profile updates.
execution of supplementary service logic e.g. Incoming calls barred. passing subscription records to VLR. directly receives and processes MAP transactions and messages from elements in the GSM network, for example, the location update messages received as mobile phones roam around. Visitor Location Register (VLR).
VLR is a database as same as HLR that contains all subscriber information data for call handling and mobility management. VLR provide dynamic data management (HLR static data management). The VLR keeps track of all subscribers roaming in the VLR service area. In GSM system the VLR is integrated with the MSC. Visitor Location Register (VLR). VLR contains:. Selective information function from the HLR.
IMSI (the subscriber's identity number). Authentication data. MSISDN (the subscriber's phone number). GSM services that the subscriber is allowed to access. access point (GPRS) subscribed.
The HLR address of the subscriber. Function of the VLR include:. Executing supplementary service programs (outgoing calls barred). Initiating authentication and ciphering. Initiating paging.
Mapping of various identities (MSISDN, IMSI, TMSI, MSRN). Passing location information to HLR. Function of the VLR include:. To inform the HLR when subscriber has arrived in the area covered by the VLR. To track where the subscriber when idle mode.
To allow or disallow which services the subscriber may use. To allocate roaming numbers during the processing of incoming calls.
To purge the subscriber record if becomes inactive whilst in the area and deletes the subscriber's data after some period and informs the HLR. To delete the subscriber record when a subscriber explicitly moves to another, as instructed by the HLR. Visitor Location Register (VLR). Equipment Identity Register (EIR). The EIR is a database that keeps tracks of handsets on the network using the IMEI.
The EIR was introduced to identify, track and bar such equipment from being used in the network. There is only one EIR per network. Composed of three lists. The White List. The Gray List.
The Black List. Equipment Identity Register (EIR). Authentication Centre (AuC). AUC is always integrated with HLR for the purpose of the authentication. The Subscriber Authentication Key (Ki) is allocated to the subscriber, together with the IMSI. The Ki is stored in the AUC and used to provide the triplets, same Ki is also stored in the SIM.
AUC stores the following information for each subscriber. The IMSI number,. The individual authentication key Ki. A version of A3 and A8 algorithm. Authentication Centre (AuC) In AUC following steps are used to produce one triplet: 1.
A non- predictable random number, RAND, is produced RAND & Ki are used to calculate the Signed Response (SRES) and the Ciphering Key (Kc) RAND, SRES and Kc are delivered together to HLR as one triplet. HLR delivers these triplets to MSC/VLR on request in such a way that VLR always has at least one triplet. Gateway Mobile Switching Center (GMSC). There is another important type of MSC, called a Gateway Mobile Switching Center (GMSC). The GMSC functions as a gateway between two networks.
If a mobile subscriber wants to place a call to a regular land line, then the call would have to go through a GMSC in order to switch to the Public Switched Telephone Network (PSTN). Gateway Mobile Switching Center (GMSC). SMS Gateway (SMS-G). The SMS GMSC (SMS gateway MSC) is a gateway MSC that can also receive short messages.
The gateway MSC is a mobile network‟s point of contact with other networks. Base Station Subsystem (BSS). BSS is the section of a traditional cellular telephone network which is responsible for handling traffic and signaling between a mobile phone and the NSS.
The BSS performs all the radio-related functions. The BSS is comprised of the following functional units:. Base Station Controller (BSC). Base Transceiver Station (BTS). BSS communicate to Mobile Station (MS) using Air Interface.
Base Station Subsystem (BSS). Function of BSS. transcoding of speech channels,. allocation of radio channels to mobile phones,. paging,. transmission and reception over the air interface.
and many other tasks related to the radio network. BTS + BSC = BSS.