Multiple Access Techniques for
Wireless Communication
FDMA
TDMA
CDMA
SDMA
A Presentation by Schäffner Harald
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
reverse channel forward channel
frequency seperation f
Time division duplexing (TDD)
• uses time for forward and reverse link
• multiple users share a single radio channel
• forward time slot
• reverse time slot
• duplexer is required
reverse channel forward channel
t
time seperation
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
• Narrowband FDMA
• Narrowband TDMA
• FDMA/FDD
• FDMA/TDD
• TDMA/FDD
• TDMA/TDD
Logical separation FDMA/FDD
forward channel
user 1
reverse channel
... f
forward channel
user n
reverse channel
t
Logical separation FDMA/TDD
user 1
forward channel reverse channel
... f
user n
forward channel reverse channel
t
Logical separation TDMA/FDD
forward forward
channel channel
user 1 ... user n f
reverse reverse
channel channel
t
Logical separation TDMA/TDD
user 1 user n
forward reverse ... forward reverse f
channel channel channel channel
t
Wideband systems
• CDMA/FDD
• CDMA/TDD
Logical separation CDMA/FDD
user 1
forward channel reverse channel
... code
user n
forward channel reverse channel
f
Logical separation CDMA/TDD
user 1
forward channel reverse channel
... code
user n
forward channel reverse channel
t
Multiple Access Techniques in use
Multiple Access
Cellular System
Technique
Advanced Mobile Phone System (AMPS) FDMA/FDD
Global System for Mobile (GSM) TDMA/FDD
US Digital Cellular (USDC) TDMA/FDD
Digital European Cordless Telephone (DECT) FDMA/TDD
US Narrowband Spread Spectrum (IS-95) CDMA/FDD
Frequency division multiple access FDMA
• one phone circuit per channel
• idle time causes wasting of resources
• simultaneously and continuously
transmitting
Time Division Multiple Access
• time slots
• one user per slot
• buffer and burst method
• Non-continuous transmission
Repeating Frame Structure
One TDMA Frame
Preamble Information Message Trail Bits
Slot 1 Slot 2 Slot 3 … Slot N
Trail Bits Sync. Bits Information Data Guard
Bits
The frame is cyclically repeated over time.
• Preamble 🡪 Address and synchronization
information for base station and subscriber
identification
• Guard times 🡪 Synchronization of
receivers between different slots and
frames
Features of TDMA
• a single carrier frequency for several users
• transmission in bursts
• low battery consumption
• very high transmission rate
• guard slots necessary
Space Division Multiple Access
• Controls radiated energy for each user in space
• using spot beam antennas
• base station tracks user when moving
Space Division Multiple Access
• primitive applications are
“Sectorized antennas”
