MULTIPLEXINGFDMATDMA

WDMACDMA

By: Shadab SiddiquiM.Tech LEOCDIAT, Pune

MULTIPEXING:• One path many user.• The process of combining multiple signal onto

a single transmission link.• It is used for maximum utilization of channel

capacity.• To accommodate more data channel user.• It is used to increase the bandwidth to exploit

the capacity of an optical fiber.

Types of multiplexing:

• FDM: Frequency division multiplexing (oldest multiplexing technique)

• WDM: Wavelength division multiplexing• TDM: Time division multiplexing• CDM: Code division multiplexing

Time Division Multiple Access (TDMA)

• Each user is allowed to transmit only within specified time intervals (Time Slots). Different users transmit in different Time Slots.

• When users transmit, they occupy the whole frequency bandwidth(separation among users is performed in the time domain).

TDM is used to multiplex digital signals.

LPF is used to band limit the given message signals.

Commutator is a rotating switch which rotates in anti-clockwise

direction with uniform speed.

Commutator is used to sample multiple number of continuous

signal simultaneously

Time taken by commutator to make one complete rotation is

known as frame time Ts

TDMA requires a centralized control node i.e Time frame is

required to synchronization of all the users.

TDMA:• Speed synchronization has to be maintained

between comm. and decomm.• The frame so-defined is divided into time

slots, and each user is assigned a Time Slot in which to transmit its information.

Frequency Division Multiple Access (FDMA)• FDM is used to multiplex

continuous signal.• Each user transmits with no

limitations in time, but using only a portion of the whole available frequency bandwidth.

• Different users are separated in the frequency domain.

The major disadvantage of FDMA is the relatively expensive and complicated bandpass filters

required.

FDMA:

WDM(Wavelength Division Multiplexing)

Basic Difference:• WDM and FDM are interrelated.• WDM is commonly applied to an optical carrier

while FDM typically applies to a radio carrier.• In WDM all the signals will arrive at the same time

while in TDM they will arrive serially ie one after other.

• In WDM single fiber multiple wavelength but in TDM single fiber one wavelength(uses single RF carrier).

• The problem with FDM is that it can not utilize the full capacity of the system because we need to ensure the adjacent band do not overlap each other (wastage with the guard band).

• Although system has the capacity then also in some cases the channel can not pass the actual signal.

• In TDM the time is divided into multiple smaller units called slots and each user is given a slots to transmit its signal. Each user have the entire bandwidth of the channel for the short duration of time.

Fig: WDM system

WDM:• The concept was firs published in 1978 and by

1980 WDM system were being realized in the laboratory.

• The first WDM system combined only two signals.• Modern system can hand-up upto 160 signals.• More popular in telecom because they allow to

expand the capacity of the network without laying more fiber.

In optical fiber:• As data transmission gets more and more heavy,

the need of high rate transmission technology is increasing, the target is high speed high capacity transmission technology.

• These requirements are achieved using multiplexing in optical fiber, where high speed and great capacity demands are met.

• Different signal with specific wavelength are multiplexed into a fiber for transmission.

WDM SYSTEM:

• Low loss BW of fiber 1200 to 1600 nm.30 THz Bandwidth

• Capable of carrying 300,000 channel of 10Mbps.(standard BW of video signal)

• Pulse width required few tens of femto second in TDM which is impossible to achieve.

• Using high quality LASER light of different wavelength it is possible to achieve the BW in WDM.

• As the time progress the LASER technology is far more matured and spectral width is significantly reduced one can essentially put multiple channel even within a window of optical fiber.

• Now the channels are closely packed this system are now called dense WDM (DWDM) means within a band/window we have multiple carrier transmission.

WDM Standard:• ITU specified G.692 standard for WDM the

reference frequency is taken as 193.10 THz. This corresponds to 1552.524 nm.

• ITU has specified that every 100GHz we can put another channel so spacing specified for DWM channel is 100Ghz(=0.8 nm at 1552nm).

WDM systems are divided into two different wavelength pattern coarse and dense wdm.

CWDM• Short-range communications• Wide range frequencies• Wavelength spread far apart 20nm• Breaks the spectrum into big chunks• Light signal is not amplified• Low cost compared to DWDM• Simple implementation

DWDM• Long-haul transmission• Narrow frequencies• Tightly packed wavelengths 0.8nm• Breaks the spectrum into small pieces• Signal amplification is used• High cost• Complex Implementation

Code Division Multiple Access (CDMA)

• In CDMA each user is assigned a unique code sequence (spreading code), which it uses to encode its data signal.

• The receiver, knowing the code sequence of the user, decodes the received signal and recovers the original data.

CDMA• Different user use same frequency at the same time , but with

different spreading code.• Generate a pseudo random code with a higher rate than the data

to be transmitted.• Each user is assigned a unique PR code.• Each user transmits it’s information by spreading with unique

code. Direct sequence spread spectrum is used.• Users are separated by code not by time slots and frequency

slots.• CDMA is power limited system.• The bandwidth of the coded data signal is chosen to be much

larger than the bandwidth of the original data signal, that is, the encoding process enlarges (spreads) the spectrum of the data signal.

Thank You!