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Multiplexing

fdma-tdma-cdma

October 31, 2016
Published By : Pratik Kataria
Categorised in:

Introduction
The multiplexing is used to combined a number of independent signals into a composite signal suitable for transmission over a common channel .
It is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium.
The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire.
A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX or DMX).

Types of Multiplexing

  1. Frequency-Division Multiple Access (FDMA).
  2. Time-Division Multiple Access (TDMA).
  3. Code-division Multiple-Access (CDMA).

fdma-tdma-cdma

 

FDMA

It is a communications technique that divides a communications channel into a number of equally spaced frequency bands .
Common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television.

TDMA

It is a digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel.

CDMA

It is a wireless communications technology that uses the
principle of spread spectrum communication.

There are three ways to spread the bandwidth of the signal:

  • Frequency hopping
  • Time hopping
  • Direct sequence

CDMA is a Direct Sequence Spread Spectrum system

CDMA Features

All users use same frequency and may transmit simultaneously
Narrowband message signal multiplied by wideband spreading signal, or codeword
Each user has its own pseudo-codeword (orthogonal to others).
Receivers detect only the desired codeword. All others appear as noise.
Receivers must know transmitter’s codeword.

cdma

Wavelength Division Multiplexing (WDM)

WDM is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light.

wdm

The term wavelength-division multiplexing is commonly applied to an optical carrier (which is typically described by its wavelength), whereas frequency-division multiplexing typically applies to a radio carrier (which is more often described by frequency).
Wavelength and frequency are tied together through a simple directly inverse relationship, in which the product of frequency and wavelength equals c (the propagation speed of light), the two terms actually describe the same concept.
WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber.

Time Division Synchronous Code Division Multiple Access (TD-SCDMA)

TD-SCDMA is the 3G standard developed for and used by China Mobile on the Chinese mainland. When 3G was just beginning in China, China Mobile decided to develop their own format of 3G so to avoid paying highly expensive patent fees.

Long Term Evolution (LTE)

It is a standard for wireless communication of high- speed data for mobile phones and data terminals (4G LTE).
Peak download rates up to 299.6 Mbit/s and upload
rates up to 75.4 Mbit/s
Low data transfer latencies (5 ms)
LTE has two variants:
–LTE TDD (Time division duplex)
–LTE FDD (Frequency division duplex)

LTE-TDD & LTE-FDD

LTE-FDD uses paired frequencies to upload and
download data.
LTE-TDD uses a single frequency, alternating between uploading and downloading data through time.
The ratio between uploads and downloads on a LTE- TDD network can be changed dynamically, depending on whether more data needs to be sent or received.

TD-LTE and LTE FDD operates on different frequency bands, with LTE-TDD working better at higher frequencies, and LTE-FDD working better at lower frequencies.
Frequencies used for LTE-TDD range from 1850 MHz to 3800 MHz, with several different bands being used.
The LTE-TDD spectrum is generally cheaper to access, and has less traffic.