Thursday, June 29, 2017

Multiple Access Techniques in communication: FDMA, TDMA, CDMA

Multiple access techniques:

In a communication system, it is unrealistic and impossible to provide a single line to an every separate user on the network. So what do we need to do? The answer is multiple access techniques. To overcome these problems different types of methods implemented to share a single limited band or channel into multiple users.

In communication, Multiplexing Techniques used to allow many users to share together a finite amount of radio spectrum. Multiple access techniques are based on multiplexing techniques. It will also reduce the cost and complexity of the network. E.g., Old telephone system uses FDMA to allow multiple calls on the single line.

Communication channels it can be a wireless spectrum or cable connections are of higher price. Different varieties of channels adapted to the mobile communication system, e.g., like time slots into frequency bands, frequency channels, and clear codes.
Basic Communication access techniques are
  1. FDMA
  2. WDMA
  3. OFDMA
  4. TDMA 
  5. CDMA
  6. SSMA
  7. SDMA
  8. PDMA

1) FDMA (Frequency Division Multiple Access)

is based on Frequency Division Multiplexing (FDM). It is the operation of splitting single channel or bandwidth into several bands of frequency. In this channel access scheme, every user gets a separate sub-band of frequency like in FM radio. In this system, the user transmits data onto different frequency at the same time.
It enables many users to transmit at the same time but using different frequency channels. It is necessary to keep the channel wider, to accept the signal spectra of the conveyance to propagate. FDMA is less complicated than TDMA, and it implemented in the narrow band system.

The widest cable system that is approximately placed in every home, hotel, and restaurants which provide a hundred of live TV channels through only one coaxial cable( 4 MHz - 1 GHz). This bandwidth divides into many small wide bands to carry different channels. E.g., At the first one TV station requires a single 6-MHz band.

2) WDMA (Wavelength Division Multiple access )

based on wavelength division multiplexing (WDM) uses in fiber optic communication. At multiplexer side, multiple lights combine into a single light source and at the de-multiplexer, the light source converted again into various sources.
Using FDMA, the bandwidth of an Optical fiber can be subdivided. Different data source gets a different light frequency(referred to Wavelength, λ) to transmit. The FDMA technique in Optical Fiber is known as WDMA.

4) OFDMA (orthogonal frequency division multiplexing)

it is based on orthogonal frequency division multiplexing (OFDM) uses in the 4G mobile communication system. It is the process of accommodating many users in an assigned bandwidth using OFDM modulation technique. OFDM Split given channel into multiple separate narrow-band channels at different bands to neglect interference and cross-talk occurrence.

5) TDMA (Time Division Multiple Access)

in this channel access scheme, every user gets a separate time slot to transmit data onto a standard frequency band. It allows users to send information or data using the same frequency band but different times slot.
In digital systems, we have no place for continuous transmission because the users never use the assigned bandwidth all the time. TDMA is based on Time Division Multiplexing and uses in Global System for Mobile Communication (GSM).
The primary purpose of introducing TDMA technology is to gain capacity over FDMA system by providing one band or frequency channel to multiple users. In TDMA we add time slots in frequency band/channel so that many users can use single band/frequency.
Digital Bits transmitted in a specified time slot on a given frequency channel. Every user gets one or more than one-time slot per frame, and the entire bandwidth is available to use for a finite period as long as needed. For both transmission and reception TDMA uses different time slot.

TDMA slot must be synchronized. It uses the lower number of channel, that why interference can neglect.

6) Space-division multiple access (SDMA)

Space-division multiple access (SDMA) transmits different information in different physical areas. Examples include simple cellular radio systems and more advanced cellular systems which use directional antennas and power modulation to refine spatial transmission patterns.

7) Power-division multiple access (PDMA)

Power-division multiple access (PDMA) scheme is used to provide power to the users according to their needs. PDMA sets variable power transmission between the user in order to share the available power on the channel. For example, SCPC modems on a satellite transponder, where users get on-demand power.

References Book:
1. Frenzel, Louis E., Principles of Electronic Communication Systems, 3rd Edition, McGraw Hill, 2008.
2. Skylar, Bernard, Digital Communications, 2nd Edition, Prentice Hall, 2001.
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