Multiple access techniques

MULTIPLE ACCESS TECHNIQUES
BY
G.GNANA PRIYA
AP/ECE
RAMCO INSTITUTE OF
TECHNOLOGY

MULTIPLE ACCESS TECHNIQUES FOR
WIRELESS COMMUNICATION
FDMA
TDMA
CDMA

INTRODUCTION
 Many users at same time
 Share a finite amount of radio spectrum
 High performance
 Duplexing generally required
 Frequency domain
 Time domain
3

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
Frequency seperation
Reverse channel Forward channel
f
4

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
Time seperation
t
Forward channelReverse channel
5

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
6

NARROWBAND SYSTEMS
 Large number of narrowband channels
 Usually FDD
 Frequency seperation - Large
 Narrowband FDMA/FDD
 Narrowband FDMA/TDD
 TDMA/FDD
 TDMA/TDD
7

LOGICAL SEPARATION FDMA/FDD
f
t
user 1
user n
forward channel
reverse channel
forward channel
reverse channel
...
8

LOGICAL SEPARATION FDMA/TDD
f
t
user 1
user n
forward channel reverse channel
...
9

LOGICAL SEPARATION TDMA/FDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
10

LOGICAL SEPARATION TDMA/TDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
11

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
12

LOGICAL SEPARATION CDMA/FDD
code
f
user 1
user n
...
13

LOGICAL SEPARATION CDMA/TDD
code
t
user 1
user n
...
14

MULTIPLE ACCESS TECHNIQUES IN USE
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
Cellular System Multiple Access Technique
15

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 , FDMA bandwidth of 30 kHz
implemented
16

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
17

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 are undesired
harmonics
 Interference with other channels in the FDMA
system
 Interference outside the mobile radio band:
adjacent-channel interference
 RF filters needed - higher costs
18

NUMBER OF CHANNELS IN A FDMA
SYSTEM
 N … Number of channels
 Bt … Total spectrum allocation
 Bguard … Guard band
 Bc … Channel bandwidth
N=
Bt - Bguard
Bc
19

EXAMPLE: ADVANCED MOBILE PHONE
SYSTEM(AMPS)
 AMPS
 FDMA/FDD
 Analog cellular system
 12.5 MHz per simplex band - Bt
 Bguard = 10 kHz ; Bc = 30 kHz
N=
12.5E6 - 2*(10E3)
30E3
= 416 channels
20

TIME DIVISION MULTIPLE ACCESS
 Time slots
 One user per slot
 Buffer and burst method
 Noncontinuous transmission
 Digital data
 Digital modulation
21

Slot 1 Slot 2 Slot 3 … Slot N
REPEATING FRAME STRUCTURE
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time. 22

FEATURES OF TDMA
 A single carrier frequency for several users
 Transmission in bursts
 Low battery consumption
 Handoff process much simpler
 FDD : switch instead of duplexer
 Very high transmission rate
 High synchronization overhead
 Guard slots necessary
23

NUMBER OF CHANNELS IN A TDMA
SYSTEM
 N … Number of channels
 m … Number of TDMA users per radio channel
 Btot … Total spectrum allocation
 Bguard … Guard Band
 Bc … Channel bandwidth
N=
m*(Btot - 2*Bguard)
Bc
24

EXAMPLE: GLOBAL SYSTEM FOR MOBILE
(GSM)
 TDMA/FDD
 Forward link at Btot = 25 MHz
 Radio channels of Bc = 200 kHz
 If m = 8 speech channels supported, and
 If no guard band is assumed :
N= 8*25E6
200E3
= 1000 simultaneous users
25

EFFICIENCY OF TDMA
 Percentage of transmitted data that contain
information
 Frame efficiency f
 Usually end user efficiency < f
 Because of source and channel coding
26

Slot 1 Slot 2 Slot 3 … Slot N
REPEATING FRAME STRUCTURE
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time. 27

EFFICIENCY OF TDMA
 bOH … number of overhead bits
 Nr … number of reference bursts per frame
 br … reference bits per reference burst
 Nt … number of traffic bursts per frame
 bp … overhead bits per preamble in each slot
 bg … equivalent bits in each guard time
intervall
bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg
28

EFFICIENCY OF TDMA
bT = Tf * R
 bT … total number of bits per frame
 Tf … frame duration
 R … channel bit rate
29

EFFICIENCY OF TDMA
 bOH … number of overhead bits per frame
 f … frame efficiency
 bT … total number of bits per frame
f = (1-bOH/bT)*100%
30

Multiple access techniques

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