Presentation on theme: "ECE 4730: Lecture #2 1 Cellular Concept Early mobile phone systems used a single high power Tx to cover a large spatial area ( R = 50 km) Half-duplex."— Presentation transcript:
ECE 4730: Lecture #2 1 Cellular Concept Early mobile phone systems used a single high power Tx to cover a large spatial area ( R = 50 km) Half-duplex (HDX) operation »Two-way communication using same radio channel »Transmit or receive only at a given time (HDX) »“Push-to-talk” system CB or ham radio Allocated spectrum determines maximum # of simultaneous users »Example: 10 MHz allocated BW with 100 kHz channel BW/user = 100 simultaneous users/market Demand was great in large cities and this led to poor service (many blocked calls) Spectrally inefficient system »Allocated spectrum supports small # of users
ECE 4730: Lecture #2 2 Cellular Concept Cellular Concept break coverage area (market) into many small cells (many Tx’s) where each cell will reuse a portion (not entire) of allocated spectrum Increase spectrum efficiency »Many users share same channels! Increase in required system infrastructure (base stations) »More capital costs to provide adequate coverage AMPS Advanced Mobile Phone Service »First cellular standard in U.S.
ECE 4730: Lecture #2 3 Cellular Concept AMPS Advanced Mobile Phone Service AMPS spectrum allocated by FCC in 1983 Full Duplex (FDX) operation : simultaneous two-way communication »Two 30 kHz channels (forward & reverse) Two providers for each market duopoly »Limited competition Analog frequency modulation (FM) used exclusively Frequency Division Multiple Access (FDMA) »One channel/user
ECE 4730: Lecture #2 4 Cellular Concept USDC US Digital Cellular proposed in 1991 Also called D AMPS or IS 54 Replace single user analog channel with digital channels that support 3 users/30 kHz channel BW! User capacity is 3 greater than AMPS more provider revenue ($$) Digital modulation & speech coding allow Time Division Multiple Access (TDMA) »3 users share one FDX frequency channel by using different time slots First digital 2G cellular standard in U.S. »Not widely adopted until late 1990’s »No competition in FCC MTA duopoly!! »Auction of PCS licenses in 1998 by FCC forced duopoly providers to upgrade to digital services
ECE 4730: Lecture #2 5 Mobile Radio Terminology Table 1.4, pg. 10 everyone responsible for these definitions Mobile: high speed motion (e.g. cell phone in car) Portable: low speed motion (cordless phone in home; laptop on wifi, etc.) Mobile unit = subscriber unit = user communication device Transmitter (Tx) and Receiver (Rx) Base Station (BS): Tx/Rx on tower at center of cell that provides service to group of mobile users Simplex (SX), Half Duplex (HDX), & Full Duplex (FDX) Forward/Reverse Channels (a.k.a. downlink/uplink) PSTN: Public Switched Telephone Network
ECE 4730: Lecture #2 6 Mobile Radio Terminology MSC: Mobile Switching Center Brains of cellular network Controls all base stations, call initiation & routing, handoffs, etc. Connects cellular network to PSTN FDX systems Frequency Division Duplexing (FDD) »Two simultaneous but separate frequency channels »Base station: separate Tx & Rx antennas »Mobile unit: single antenna + duplexer (Tx/Rx) Time Division Duplexing (TDD) »Share single radio channel in time »One time slot base to mobile »Next time slot mobile to base »Must use digital modulation
ECE 4730: Lecture #2 7 Paging Systems One-way communication (SX) Send short message to mobile unit (pager) Wide area coverage Page broadcast from many base stations simultaneously to remote units No information as to user location! Reliable communication everywhere (good S / N ) Requires large Tx power and low data rate (~ 2 8 kbps) Coverage even inside buildings w/ 20 30 dB signal attenuation!
ECE 4730: Lecture #2 8 Paging Systems Why does low data rate yield good S / N ? Data rate R d signal BW »PSD of rectangular pulse train is (sin x / x ) 2 Noise in paging Rx is due to thermal noise, N T, where N T = k T BW Small data rate small BW low N T good S / N f PSD R d = 1 / T s = FNBW 0 00 0 0 1 1 1 Symbol Period = T s = T b = Bit Period Signal BW 1 / T b R d
ECE 4730: Lecture #2 10 Cordless Telephones Primarily in-home use Low power, limited range (~ 100 m) and coverage, and limited mobility
ECE 4730: Lecture #2 11 Cellular Systems Large geographic coverage Limited frequency spectrum High user mobility High system capacity Large # of simultaneous users Obtained by limiting coverage of each base station to small area (cell) Limited frequency spectrum can be reused by other cells in network Base station Serves mobile users in each cell Bridge between mobile unit and MSC Connected to MSC via phone line (T1/T3) or LOS microwave link
ECE 4730: Lecture #2 12 Cellular Systems MSC: Mobile Switching Center Coordinates activities of all base stations Connects cellular system to PSTN Cellular network brains »Call initiation/setup »Base station handoffs »Mobile unit power control »Billing information »Roaming user ID and verification Typically handles 5000 simultaneous calls supporting 100,000 cellular users (for reasonable GOS)
ECE 4730: Lecture #2 14 Cellular Systems Common Air Interface (CAI) Physical Layer Forward/Reverse Voice Channels FVC/RVC »FDX communication Forward/Reverse Control Channels FCC/RCC »Call initiation & setup »Makes up ~5% of total # of available channels One AMPS cell contains 10 60 voice channels and only 1 3 control channel pairs (F+R) MSC broadcasts call request from PSTN over FCC of all base stations Mobile unit monitors FCC looking for strongest base station (closest) and incoming call
ECE 4730: Lecture #2 15 Cellular Systems Regular Handoff 1G Analog AMPS systems Base stations (current server + adjacent stations) monitor RVC and report mobile unit signal strength to MSC If RVC signal < acceptable level MSC initiates handoff to one of neighboring base stations Neighboring base stations must use different frequencies for FVC/RVC
ECE 4730: Lecture #2 16 Cellular Systems Mobile Assisted HandOff (MAHO) Digital / PCS systems only ! Mobile monitors FCC of serving BS and neigboring BSs If FCC signal < acceptable level mobile looks for another base station Neighboring base stations must use different frequencies for FCC/RCC Handoff from one base station to another occurs when FCC signal < acceptable level
ECE 4730: Lecture #2 17 Cellular Call Timing Initiated by Landline (PSTN) User
ECE 4730: Lecture #2 18 Cellular Call Timing Initiated by Mobile User
ECE 4730: Lecture #2 19 Mobile System Comparisons
ECE 4730: Lecture #2 20 Mobile System Comparisons
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