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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
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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 DAMPS or IS54
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
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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;
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
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
No information as to user location!
Reliable communication everywhere (good S/N)
Requires large Tx power and low data rate (~ 28 kbps)
Coverage even inside buildings w/ 2030 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
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
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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 1060 voice channels and only
13 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
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
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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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