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MIT 15 020 - Competition in Telecoms Recitation #4

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15.020 Competition in Telecoms Recitation #4 Othman Laraki Credits to Simon HughesWireless Concepts •Cell site: transmitter, receiver, base station•Mobile Switching Center•Frequency reuse through separated cell sites•Roaming & Location ManagementPSTN Mobile Switching Center Cellular Tower Cell Phone User #1 Cell Phone User #2 Cell Phone User #3 etc… Based on: Dell White Paper: Wireless TechnologiesA Bit of History • Cellular Technologies – 1985: Analog (1st Generation, voice): 800 Mhz band – 1991: Digital (2nd Generation, limited data transfer) • 800 and 900 MHz frequency bands • PCS in 1800 and 1900 MHz frequency bands – 2000: 3rd Generation (UMTS) – Access Methods • Multiplex several calls onto single cellular channel – FDMA: Frequency Division Multiple Access (analog) – CDMA: Code Division Multiple Access (codes) – TDMA: Time Division Multiple Access (timeslots)Multiplexing Technologies Allow multiple calls to share a single connection • TDM Time Division Multiplexing (digital) – Each path shared by several channels which have timeslots • SDM Space Division Multiplexing (analog or digital) – Each channel allocated an exclusive physical path • FDM Frequency Division Multiplexing (analog) – Each channel allocated an individual frequency • PDM Packet Division Multiplexing (digital) • CDM Code Division Multiplexing (digital) – Each channel allocated an individual code within same path • WDM: Wave Division Multiplexing (digital & analog!) – Each channel allocated an individual wavelength on a fiber1st & 2nd Generation Variants • Analog (1st Generation – 1G) – AMPS used mainly in the US - also in Latin America, Australia, New Zealand, parts of Russia and Asia-Pacific. – ETACS used in Europe and Asia-Pacific. – NMT used in Scandinavia and some European countries, as well as parts of Russia, the Middle East and Asia. • Digital (2nd Generation – 2G) – GSM 900 (900 MHz) - Europe; also used in Asia-Pacific. – GSM 1800 (1800 MHz) also used in Europe and Asia, not as widely adopted as GSM 900. – GSM 1900 (1900 MHz) GSM system used in Americas and Canada. – TDMA & CDMA digital standards used in US, also Latin America, New Zealand, parts of Russia and Asia-Pacific. – PDC digital standard used in JapanCDMA vs. TDMA CDMA (Carrier Division Multiple Access) • Segment a single (1.25MHz) channel into (64) multiple channels using a code to identify users • Spreads call over entire spectrum: more immune to interference than TDMA; potentially supports more users • Capacity not fixed, depends on coverage, total voice bandwidth • During peak periods, cell size reduces due to spectrum use TDMA (Time Division Multiple Access) • Segment a single (30KHz) channel into (3 x 8kbit/s) timeslots, carrying specific user information • Cell phones for TDMA networks will not work with CDMA networks and vice versa • Capacity dependent on number of available timeslotsGSM • GSM: Groupe Speciale Mobile • GSM: Global System for Mobile Communications • TDMA-based technology developed in late 1980s • Worldwide (outside N America) de-facto digital standard: 200m GSM users; 370+ operators • Two variants: – GSM: Transmit 890-915MHz, Receive: 935-960MHZ – PCN: Transmit 1850-1910MHz, Receive 1930-1990MHzCDMA Technology For this diagram, see Masson, Tim. Wireless Technology Seminar: Third Generation (3G) Basics. Agilent Technologies, 2000, page 18. Available at http://www.agilent.com.Cell Sites For this diagram, see Farley, Tom, with Mark van der Hoek. Cellular Telephone Basics. Privateline.com, page 2. Available at http://www.privateline.com. For this diagram, see Masson, Tim. Wireless Technology Seminar: Third Generation (3G) Basics. Agilent Technologies, 2000, page 22. Available at


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