Basic Concept of FDM and TDM

Why do we need Multiplexing technique?

In communication, under the simplest conditions, a medium can carry only one signal at any moment in time.For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth. This is where the concept of Multiplexing comes!

Multiplexing means “sharing a medium”. It is a form of data transmission in which one communication channel carries several transmissions at the same time. In simple words, the method of dividing a single channel into many channels so that a number of independent signals may be transmitted on it is known as Multiplexing.

Multiplexing techniques can be divided into two basic categories:

Frequency Division Multiplexing-FDM

Time Division Multiplexing-TDM

Frequency Division Multiplexing-FDM

In FDM the available bandwidth is divided into a number of smaller independent logical channels with each channel having a small bandwidth. It assigns “frequency ranges” to each “user” or “signal” on a medium. Thus, all signals are transmitted at the same time, each using different frequencies.The method of using a number of carrier frequencies, each of which is modulated by an independent signal is in fact frequency division multiplexing.

Time Division Multiplexing-TDM

In TDM, sharing is accomplished by dividing available “transmission time” on a medium/channel among users.

Each user of the channel is allotted a small time interval during which he transmits a message. Total time available in the channel is divided, and each user is allocated a time slice. In TDM, users send message sequentially one after another. Each user can use the full channel bandwidth during the period he has control over the channel.

20:13 | Posted in Communication Basics, Digital Communication System, FDM, TDM | Read More »

Introduction to FDM, OFDM, OFDMA

Frequency Division Multiplexing (FDM)

In FDM system, signals from multiple users/transmitters are transmitted simultaneously (at the same time slot) over multiple frequencies. That is each user is assigned a different frequency for communication. Each frequency range is called as sub-carrier, n it is modulated separately by different data stream, that is there are different sources of data (users/transmitters) for each subcarrier frequency. A spacing (guard band) is placed between sub-carriers to avoid signal overlap.

Orthogonal Frequency Division Multiplexing (OFDM)

Like FDM, OFDM also uses multiple sub-carriers but the sub-carriers are closely spaced to each other without causing interference, removing guard bands between adjacent sub-carriers. This is possible because the frequencies (sub-carriers) are orthogonal. In OFDM multiple subcarriers are modulated by single source of data. If I am a transmitter, n using OFDM technique, my data will be modulated by multiple subcarrier frequencies , instead of single carrier frequency ,as it happens in normal modulation .if I assume that 3 subcarriers are being used for modulation so, they will modulate the data of just one Transmitter ,unlike FDM where 3 frequencies will b used to modulate three different sources of data (three transmitters).

In an OFDM system, a very high rate data stream is divided into multiple parallel low rate data streams. Each smaller data stream is then mapped to individual data sub-carrier and modulated using some sorts of PSK or QAM modulation . let’s assume that I have a high rate data stream, X=[abcdefghi] ,

I divide it into three low rate data streams [p=abc, q=def, r=ghi ].Assume that there are three sub-carrier frequencies: f1,f2 n f3

Each low data rate stream will be modulated individually by each sub-carrier,that is data stream ‘p’ will be modulated by carrier f1, data stream ‘q’ by f2 n similarly data stream ‘r’ by f3 and finally all are combined.If it were FDM technique ,the data stream X would have been modulated by a single carrier frequency instead of multiple carriers.Therefor OFDM is called as multi-carrier modulation

Orthogonal Frequency Division Multiple Access (OFDMA)

Like OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub-carriers are further divided into groups of sub-carriers. Each group is named a sub-channel, different colors in figure show a different sub channel. The sub-carriers that form a sub-channel need not be adjacent. Sub-channels actually used for multiple access (multi users communication). Each sub-channel consists of multiple carrier frequencies. For instance I have two sub-channels, S1 n S2. S1 has carrier frequencies f1 ,f2,f3, and S2 has carrier frequencies, f4,f5, n f6 . sub-channel 1 will b used to entertain one user/transmitter and sub-channel 2 will b used by another user/transmitter.

05:48 | Posted in Communication Basics, Digital Communication System, FDM, OFDM, OFDMA | Read More »