1 of 23

����PRESENTATION ON �MULTIPLE ACCESS TECHNIQUE

  • BRANCH-E & TC ENGG
  • SUBJECT- ADVANCE COMMUNICATION ENGINEERING
  • CHAPTER-2- SATELLITE COMMUNICATION
  • TOPIC- MULTIPLE ACCESSING
  • SEM-6TH
  • FACULTY – Er. UMASHANKAR PANDA(Sr. LECT. E & TC ENGG DEPARTMENT)
  • AY-2021-2022, SUMMER-2022

​

2 of 23

Multiple Access Techniques for Wireless Communication

FDMA

TDMA

SDMA

PDMA

3 of 23

Introduction

  • many users at same time
  • share a finite amount of radio spectrum
  • high performance
  • duplexing generally required
  • frequency domain
  • time domain

4 of 23

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

5 of 23

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 channel

reverse channel

6 of 23

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

7 of 23

Narrowband systems

  • large number of narrowband channels
  • usually FDD
  • Narrowband FDMA
  • Narrowband TDMA
  • FDMA/FDD
  • FDMA/TDD
  • TDMA/FDD
  • TDMA/TDD

8 of 23

Logical separation FDMA/FDD

f

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...

9 of 23

Logical separation FDMA/TDD

f

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...

10 of 23

Logical separation TDMA/FDD

f

t

user 1

user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...

11 of 23

Logical separation TDMA/TDD

f

t

user 1

user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...

12 of 23

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

13 of 23

Logical separation CDMA/FDD

code

f

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...

14 of 23

Logical separation CDMA/TDD

code

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...

15 of 23

Multiple Access Techniques in use

Multiple Access

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

Cellular System

16 of 23

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

17 of 23

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

18 of 23

Time Division Multiple Access

  • time slots
  • one user per slot
  • buffer and burst method
  • noncontinuous transmission
  • digital data
  • digital modulation

19 of 23

Repeating Frame Structure

Slot 1 Slot 2 Slot 3 … Slot N

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

20 of 23

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

21 of 23

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

22 of 23

Repeating Frame Structure

Slot 1 Slot 2 Slot 3 … Slot N

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

23 of 23

THANK YOU