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Cellular & Satellite Network Dr. Nguyen Tuan Nam History of Cellular Network •  First telephone system: –  A single radio transmitter –  A single channel was used –  Half-duplex: press button to talk, and release it to listen •  Cellular radio system –  Analogue –  Digital Nguyen Tuan Nam/WNC 2011 2 The First-generation System •  Analogue •  Started in the 1980s •  No worldwide coordination for the development of technical standards for the system •  Roaming was not possible •  Efficient use of the frequency spectrum was not there Nguyen Tuan Nam/WNC 2011 3 The Second-generation System •  Mid-1980s, the European commission started a series of activities to liberalize the communication sector, including mobile communication •  Global System for Mobile Communication (GSM) –  –  –  –  First specifications The network is based on digital technology 1990s Capable of providing all the basic services such as speech and data services •  GSM and VAS (Value Added Service) –  Voice Mail System (VMS) –  Short Message Service Center (SMSC): proved to be incredibly commercially successful •  GSM and GPRS (General Packet Radio Services) –  Possible to send packet data on the air-interface •  GSM and EDGE (Enhancement Data rates in GSM Environment) –  More sophisticated coding methods over the Internet to increase the data rate •  2G phone systems were characterized by –  Digital circuit switched transmission –  Introduction of advanced and fast phone-to-network signaling •  Smaller phone. Why? Nguyen Tuan Nam/WNC 2011 4 The Third-generation Networks •  In EDGE –  Packet transfer on air-interface behaves like a circuit switch call à low efficiency –  Standards for developing the networks were different for different parts of the world •  Standardized in the IMT-2000 standardization processing –  Not standardize on a technology –  Instead, standardize in a set of requirements (data rate) •  WCDMA for UMTS Nguyen Tuan Nam/WNC 2011 5 The Fourth-generation Networks •  Not discussed here Nguyen Tuan Nam/WNC 2011 6 Cellular Network •  Radio network made up of a number of radio cells (cells) •  Each cell served by at least one fixed-location transceiver (known as cell site or base station) •  These cells cover different land areas à provide radio coverage over a wider area than a cell •  A variable number of portable transceivers can be used in any one cell and moved through more than one during transmission Nguyen Tuan Nam/WNC 2011 7 Cellular Network Architecture •  Mobile station (MS): A device used to communicate over the cellular •  network. Base station transceiver (BST): –  A transmitter/receiver used to transmit/receive signals over the radio interface section of the network. –  Fair amount of computing power, correcting errors in the received signal and encrypting the conversation •  Mobile switching center (MSC): The heart of the network which sets •  •  up and maintains calls made over the network. Base station controller (BSC): Controls communication between a group of BSTs and a single MSC. Public switched telephone network (PSTN): The land based section of the network. Nguyen Tuan Nam/WNC 2011 8 Cellular Network Architecture Hierarchical fashion. Why? Nguyen Tuan Nam/WNC 2011 9 Switches •  The very first switches –  Human manning a switchboard –  You phoned the switchboard –  Told them which line you wanted to be connected –  They plugged your phone line into the appropriate socket •  Automatic switch –  Invented by Strowger –  Relay Nguyen Tuan Nam/WNC 2011 10 The Structure of a Simple Switch Nguyen Tuan Nam/WNC 2011 11 Frequency reuse (FDMA) •  Problem: –  Only one transmission can be used on any given frequency –  Number of available frequencies is very limited •  “Spectrum is like real estate – they just don’t make it anymore” –  A cellular operator has typically been given •  25MHz of radio spectrum •  Each individual requires 25KHz in order to make a call à Maximum 1,000 subscribers •  Solution: –  Same frequency can be reused in a different area for a completely different transmission –  But some level of interference from cells sharing the same frequencies à There must be at least a one cell gap between cells which reuse the same frequency Nguyen Tuan Nam/WNC 2011 12 Frequency reuse •  The frequency reuse factor: –  The rate at which the same frequency can be used in the network. –  It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. –  Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation). •  N sector antennas –  One base station can have N sector antennas, each with different direction Nguyen Tuan Nam/WNC 2011 13 Examples F1 F1 F1 F3 F2 F2 F3 F4 F1 F2 F1 F3 F2 F2 F4 Nguyen Tuan Nam/WNC 2011 14 CDMA •  Adjacent base station sites use the same frequencies •  The different base stations and users are separated by codes rather than frequencies •  Requires certain signal-to-noise ratio to operate •  Near-far problem: –  As receiver moves away from transmitter à the power transmitted is reduced à signal becomes corrupted and unusable Nguyen Tuan Nam/WNC 2011 15 Advantages of Cellular Network •  Reduce power usage •  Larger coverage area •  Increase capacity •  Reduce interference from other signals Since most mobile phones use cellular network à cell phones In cities, cell site range is shorter compared to rural areas Nguyen Tuan Nam/WNC 2011 16 Setting up a Call •  Polling scheme –  A control channel exists permanently between mobile stations and base station –  Base station send a request for a call setup to mobile station using the control channel –  Mobile station continually poll channel for connection request –  If mobile station and radio resources are available àa dedicated voice circuit is setup à voice circuit only exist when necessary, and then get destroyed •  Original analogue representation of speech is encoded into digital representation using a speech coder: –  Well known pulse code modulation (PCM) encoding scheme used in most public switched telephone networks (PSTN) output data at a rate of 64 kbps –  GSM only 34kbps àRPE-LC encoding scheme at 25kbps (added overhead) •  GSM systems operate in a discontinuous transmission mode: –  No data is transmitted during a user's silent period –  Each person on average speaks no more than 40% of the time à effective Nguyen Tuan Nam/WNC 2011 17 Handover/Handoff •  Mobile transceiver moves from one cell to another during ongoing continuous communication à switch from one cell frequency to a different cell frequency •  Intra-cell handoff vs. inter-cell handoff •  Soft-handoff vs. Hard handoff Nguyen Tuan Nam/WNC 2011 18 Registering Per-user location caching Pointer forwarding Local anchoring Nguyen Tuan Nam/WNC 2011 19 Roaming •  A service that enables customers of a particular network operator to make calls from areas not served by that network operator •  Network operator within the area of the call initiation contacts a Gateway MSC (GMSC), which links the user to its own network operator •  Using mobile station ISDN number (MSISDN), which uniquely identifies a mobile station •  MSISDN consists of a country code (CC), national destination code (NDC) and a subscriber number (SN). Nguyen Tuan Nam/WNC 2011 20
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