22-08-2012, 02:30 PM
Communication Multiple Access Techniques for Wireless
Communication Multiple Access Techniques for Wireless.ppt (Size: 898.5 KB / Downloads: 48)
Introduction
many users at same time
share a finite amount of radio spectrum
high performance
duplexing generally required
frequency domain
time domain
Frequency division duplexing (FDD)
two bands of frequencies for every user
forward band
reverse band
duplexer needed
frequency seperation between forward band and reverse band is constant
Time division duplexing (TDD)
uses time for forward and reverse link
multiple users share a single radio channel
forward time slot
reverse time slot
no duplexer is required
Multiple Access Techniques
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Space division multiple access (SDMA)
grouped as:
narrowband systems
wideband systems
Narrowband systems
large number of narrowband channels
usually FDD
Narrowband FDMA
Narrowband TDMA
FDMA/FDD
FDMA/TDD
TDMA/FDD
TDMA/TDD
Wideband systems
large number of transmitters on one channel
TDMA techniques
CDMA techniques
FDD or TDD multiplexing techniques
TDMA/FDD
TDMA/TDD
CDMA/FDD
CDMA/TDD
Frequency division multiple access FDMA
one phone circuit per channel
idle time causes wasting of resources
simultaneously and continuously transmitting
usually implemented in narrowband systems
for example: in AMPS is a FDMA bandwidth of 30 kHz implemented
FDMA compared to TDMA
fewer bits for synchronization
fewer bits for framing
higher cell site system costs
higher costs for duplexer used in base station and subscriber units
FDMA requires RF filtering to minimize adjacent channel interference
Nonlinear Effects in FDMA
many channels - same antenna
for maximum power efficiency operate near saturation
near saturation power amplifiers are nonlinear
nonlinearities causes signal spreading
intermodulation frequencies
Nonlinear Effects in FDMA
IM are undesired harmonics
interference with other channels in the FDMA system
decreases user C/I - decreases performance
interference outside the mobile radio band: adjacent-channel interference
RF filters needed - higher costs
Number of channels in a FDMA system
N … number of channels
Bt … total spectrum allocation
Bguard … guard band
Bc … channel bandwidth
Example: Advanced Mobile Phone System
AMPS
FDMA/FDD
analog cellular system
12.5 MHz per simplex band - Bt
Bguard = 10 kHz ; Bc = 30 kHz
Time Division Multiple Access
time slots
one user per slot
buffer and burst method
noncontinuous transmission
digital data
digital modulation