Transcript No Slide Title

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
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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.
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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
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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
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
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
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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;
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




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
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Mobile Radio Terminology
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
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
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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
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 No information as to user location!
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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!
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Paging Systems

Why does low data rate yield good S/N?
 Data rate Rd  signal BW
» PSD of rectangular pulse train is (sin x / x)2
0
1
0
0
1
0
1
0
Symbol Period = Ts = Tb
= Bit Period
Signal BW  1 / Tb  Rd
PSD
f
Rd = 1 / Ts = FNBW
 Noise in paging Rx is due to thermal noise, NT ,
where NT = k T BW
 Small data rate  small BW  low NT  good S/N
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Paging Systems
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Cordless Telephones
Primarily in-home use
 Low power, limited range (~ 100 m) and coverage,
and limited mobility

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Cellular Systems
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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
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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)
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Cellular Systems
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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
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Cellular Systems
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Regular Handoff
 1G Analog AMPS systems
 Base stations (current server + adjacent stations) monitor
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
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
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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
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Cellular Call Timing
Initiated
by
Landline
(PSTN)
User
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Cellular Call Timing
Initiated
by
Mobile
User
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Mobile System Comparisons
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Mobile System Comparisons
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