Transcript ECE4730_L22
Multiple Access Methods
Primary Goal : maximize the # users sharing limited
frequency spectrum while at the same time providing
quality communications with reasonable system costs.
For any type of cellular system the service quality
as the # users
Same
system interference (ACI, CCI, etc.) limit cellular
capacity
Cellular capacity is related to QoS and revenue $$ for
service provider
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Multiple Access Methods
Duplexing
“Simultaneous” 2-way communication
FDD : Frequency Division Duplexing
Two separate SX frequency channels for each user
Frequency
Separation
FVC
RVC
f
Duplexer : device that allows Tx & Rx to operate
simultaneously using same antenna
Desire large frequency separation for good Tx/Rx isolation
Frequency separation is a fixed or constant value
» Does NOT depend on channel #
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Multiple Access Methods
TDD : Time Division Duplexing
Two separate time slots (forward & reverse for each user on same
frequency channel
FVC
Time
Separation
RVC
t
Time separation must be small (~ 10 msec) for real-time services (e.g.
voice) so that service appears to be continuous
Simpler mobile unit Tx/Rx since duplexer not needed
» Antenna is shared in time by Tx/Rx
» Duplexer is complicated & expensive component
TDD/FDD hybrid many users share FVC frequency using TDD and
different RVC frequency also using TDD GSM
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Multiple Access Methods
Multiple Access (MA) Methods
FDMA, TDMA, & CDMA 3 major types
» Many MA schemes are hybrids of these basic types
Narrowband vs. Wideband
» Signal BW (Bs) < channel BW (Bc) narrowband
Large # of narrow frequency channels FDMA (e.g. AMPS)
» Bs >> Bc wideband
Multipath fading only affects small % of signal frequency content
no equalization needed
Large # users share same channel CDMA (Sprint, Verizon)
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Multiple Access Methods
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Multiple Access Methods
FDMA : Frequency Division Multiple Access
1 user/channel with FDD channel pair
FDMA Features :
1) FDMA channel
Carries only one user at a time
Relatively narrow ~ 30 kHz for AMPS
Idle when not in use wasted resource
2) Base station and mobile transmit simultaneously and continuously
3) Narrowband system usually
Only way to support large # of users in limited spectrum
4) Normally use Analog FM for mobile communications
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Multiple Access Methods
FDMA : Frequency Division Multiple Access
FDMA Features (continued) :
5) FDMA Mobile Systems
Low complexity compared to TDMA or CDMA
Fewer overhead bits in digital systems
Less timing and synchronization
6) Higher Cell Site Costs (relative to TDMA)
Lower channel utilization (1 channel/user)
Rejection of ACI requires expensive BPF in base station
Duplexers needed so Tx and Rx can use antenna simultaneously
7) Adjacent Channel Interference
Significant interference source need good BPF @ base station
Power amplifiers are non-linear and cause :
spectral regeneration of modulation sidebands
increased ACI
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Multiple Access Methods
FDMA : Frequency Division Multiple Access
Non-linear power amplifiers
» Spectral broadening increased ACI
» Non-linear Tx emission byproducts intermodulation (IM)
Same-system interference (ACI)
Out of band interference system -to-system
U.S. AMPS
1G System
» Narrowband analog FM
» FDMA/FDD
» 45 MHz separation between FVC & RVC
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Multiple Access Methods
FDMA : Frequency Division Multiple Access
U.S. AMPS
1G System
» # FDMA/FDD channels =
Bt 2Bg
Bs
Nc
Bt : total spectrum allocation
Bs : signal BW (specification)
Bg : guard band which provides separation to minimize ACI to
other provider
» For AMPS Bt = 12.5 MHz Bs = 30 kHz Bg = 10 kHz & Nc =
416 (21 control and 395 voice channels)
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Multiple Access Methods
TDMA : Time Division Multiple Access
Divide
radio channel into N time slots for one “frame” of
data to support multiple users/channel
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Multiple Access Methods
TDMA : Time Division Multiple Access
TDD mobile & base share (50/50) same channel
» Not widely used
» Used in some digital cordless phone (Table 9.1)
FDD
different channels with multiple users for forward
& reverse links
» Most widely used
» Several time slot delay between forward & reverse link!
Mobile unit not simultaneously using Tx/Rx
no duplexer required
Simple Tx/Rx design
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Multiple Access Methods
TDMA : Time Division Multiple Access
Frame structure
» Depends on wireless standard (GSM, USDC, etc.)
» Preamble
Control, setup, MIN, & synchronization (timing)
Not all standards have this! (e.g. GSM)
Guard bits needed for proper timing
Efficiency % of information data bits
overhead, control, & timing bits hf
as opposed to
» Doesn’t include coding actual efficiency (info. data) < hf
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Multiple Access Methods
TDMA : Time Division Multiple Access
Frame Efficiency
boh
100%
h f 1
bT
where
boh = # overhead bits/frame
bT = # total bits/frame
# TDMA channels =
m Bt 2 Bg
Bs
where m = max. # users/channel
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Multiple Access Methods
TDMA : Time Division Multiple Access
TDMA Features:
1) TDMA channel
Multiple users share same channel
Non-overlapping time slots in frame structure
User data communication done in periodic bursts
must use digital modulation
2) Lower battery consumption than FDMA
Mobile Tx is off most of the time (e.g. ~ 7/8 for GSM)
3) Mobile Assisted Handoffs
Mobile Rx monitors multiple base stations during idle time slots
Relay information back to MSC for handoff execution
4) Tx and Rx of mobile unit never on at same time
No duplexer required
Simple Tx/Rx switch less costly mobile architecture
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Multiple Access Methods
TDMA : Time Division Multiple Access
TDMA Features:
5) Higher data rates relative to FDMA
Bs is sometimes > Bc
Equalization often required
Training bit overhead
6) Timing is critical
Synchronization between base and mobile and between Tx
and Rx
Timing data bits required in each slot (many slots/frame)
Guard bits to separate users (limit multipath impact)
7) Bandwidth on demand:
Allocate multiple time slots per frame for a higher rate user
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