Basics of FDM and OFDM

 Note : All figures/images taken from internet 

Multiplexing

Multiplexing is a technique used to share a Medium ( Cable, Wire, optical fiber, air). This allows simultaneous transfer of multiple signals in a single data link. The most simplest method of using the medium is to transmit one signal at a time. Multiplexing is a technique used to combine signals or information from different users/devices and transmit at the same time.

Time Division multiplexing:

Transmitting different users' digital signal over a single link by dividing the time into intervals called slots and allocating equal time slots to each user.

  

 

Statistical Time Division Multiplexing

In this type of multiplexing, some users may have high priority or may have more data to transmit. Instead of allocating equal time slots to all users, depending on the priority, more time slots may be allotted to a particular use.

Baseband systems can tranmsit only only signal at a time ( from one user) . Base band systems require the exclusive use of channel capacity. In this, when a system uses the wire, no one else can use the wire. Ethernet is an example of baseband system. TDM and STDM are used in Baseband system

Broadband deals with analog signals which are in the form of RF or optical waves. They can share a chanel by using different frequency range. FDM is used in broadband systems. 

 Frequency Division Multiplexing

In FDM , more than one signal is sent over the shared medium. This is an analog multiplexing technique that divides the available bandwidth ( or a frequency band) into number of non overlapping sub channels ( set of frequency bands) . FDM allows multiple users to share a single link where each user is assigned a separate subchannel.. These subchannels are separated by unused frequecies called guard band so that the signals do not overlap. 

 

    

 

 

In the above each device is allocated a frequency band. Signal from the device are sent in the allocated frequency band. At the sending end, the 4 frequency bands are multiplexed and transitted via the communication channel

Example of FDM

1. It is used in radio station broadcasting where multiple radio signals from different radio stations at different frequencies pass through the air at the same time.

2. It is also used in cable modems to broadcast differnet TV channels at different frequencies.

 

FDM Process

 

 

In the above each device generates a signal of a particular frequency. Each signal modulates a different carrier frequency ( f1,f2 and f3).

Carrier frequncies are separated by sufficient bandwidth so that the modulated signals do not overlap

The resulting modulated signals are combined into a single composite signal using a multiplier and sent over the media.

The bandwidth ranges are the channels through which the signals travel.

These channels are separated by unused frequecny band called guard bands to prevent signald from overlapping.

OFDM

In FDM the sub channels are far apart and have guard bands between them. But in OFDM , the sub channels are closely spaced and overlap each other. OFDM prevenst interference by a unique method of combining the signals. It combines the signals such that each signal/carrier is orthogonal to every other signal. Each signal or carrier operates without interfering with the neighbour signal.

How does OFDM prevent interference ?

The carriers that are multiplexed are orthogonal to each other. This means that the peak of one signal occurs at the zero or null point of neighbouring signals. At the receiving end, the demultiplexer uses this orthogonal feature to separate the signals.Ofdm uses the available bandwidth more efficiently than FDM .

 

In the above figure when the  peak of sub-carrier 1 occurs , all other sub carriers are at 0 or null point.

OFDM data transission in 2.4 GHz

The 2.4GHz channel has a 20 Mhz bandwidth. This uses a single carrier at Center Frequency to modulate and send signals. 

 

In 802.11a , the  spectrum is divided into operating channels each of 20MHz,  Each 20MHz channel is divided in N (52) orthogonal subcarriers. 4 subcarriers are used as pilot carriers while the other 48 subcarriers are used to transmit data. Sub carriers are spaced at .3125 MHz apart. Data is modulated using one of the Digital Modulation techniques and sent on each of the subcarrier. All modulated carrier waves are multiplexed and transmitted

802.11a uses QAM modulation on each of the subcarrier to transmit data.