1. What is Multiplexing?
Multiplexing is a technique in which multiple signals are combined and transmitted simultaneously over a single communication channel.
The device that combines the signals is called a Multiplexer (MUX), and the device that separates them at the receiving end is called a Demultiplexer (DEMUX).
Basic idea
Signal 1 ──┐
Signal 2 ──┤
Signal 3 ──┤──→ MUX ──→ Single Communication Channel ──→ DEMUX ──┬→ Signal 1
Signal 4 ──┘ ├→ Signal 2
├→ Signal 3
└→ Signal 4Main purpose: To use the available communication medium efficiently and allow multiple users/signals to share the same medium.
2. Frequency Division Multiplexing (FDM)
Definition
Frequency Division Multiplexing (FDM) is a multiplexing technique in which the total bandwidth of a communication channel is divided into several non-overlapping frequency bands, and each signal is transmitted using a different frequency band.
In simple words:
FDM divides the available frequency range into separate frequency channels.
Each signal gets its own frequency band.
3. Working of FDM
Suppose four signals need to be transmitted.
Each signal is assigned a different frequency range:
Frequency
↑
|
| Signal 4
| ┌─────────────┐
| │ │
| └─────────────┘
|
| Signal 3
| ┌─────────────┐
| │ │
| └─────────────┘
|
| Signal 2
| ┌─────────────┐
| │ │
| └─────────────┘
|
| Signal 1
| ┌─────────────┐
| │ │
| └─────────────┘
+--------------------------------→ FrequencyThe frequency bands are separated so that signals do not interfere with each other.
4. Guard Bands
A small unused frequency range called a guard band is placed between adjacent channels.
Frequency →
┌────────┐ ┌────────┐ ┌────────┐
│Signal 1│GB│Signal 2│GB│Signal 3│
└────────┘ └────────┘ └────────┘GB = Guard Band
Why are guard bands needed?
They prevent the frequency bands from overlapping and reduce interference between adjacent channels.
5. FDM Block Diagram
Input Signals
Signal 1 ──────┐
│
Signal 2 ──────┤
│
Signal 3 ──────┤──→ MULTIPLEXER ──→ Shared Channel ──→ DEMULTIPLEXER
│ │
Signal 4 ──────┘ │
↓
┌───────────────┼──────────────┐
↓ ↓ ↓
Signal 1 Signal 2 Signal 3
↓
Signal 4At the transmitter, the multiplexer combines the signals.
At the receiver, the demultiplexer separates them using their assigned frequency bands.
6. Example of FDM
Consider a communication channel having a bandwidth of 100 kHz.
Suppose the channel is divided into four frequency bands:
| Signal | Frequency Range |
|---|---|
| Signal 1 | 0–20 kHz |
| Signal 2 | 25–45 kHz |
| Signal 3 | 50–70 kHz |
| Signal 4 | 75–95 kHz |
The gaps between the channels act as guard bands.
Thus, multiple signals can travel simultaneously through the same physical medium.
7. Real-Life Applications of FDM
1. Radio Broadcasting
Different radio stations use different frequency bands.
For example:
Station A → 90 MHz
Station B → 95 MHz
Station C → 100 MHzAll stations can broadcast simultaneously without using separate physical cables.
2. Television Broadcasting
Different TV channels are assigned different frequency bands.
Channel 1 → Frequency Band 1
Channel 2 → Frequency Band 2
Channel 3 → Frequency Band 3The receiver selects the required frequency.
3. Telephone Systems
Traditional telephone systems used FDM to allow multiple voice channels to share the same transmission medium.
4. Cable Television
Cable TV systems can transmit many channels simultaneously by assigning different frequency bands to different channels.
8. Advantages of FDM
1. Simultaneous Transmission
Multiple signals can be transmitted at the same time.
2. Efficient Use of Communication Medium
Several users can share one physical communication channel.
3. No Strict Synchronization Required
Unlike TDM, FDM does not require precise time synchronization between transmitters.
4. Suitable for Continuous Signals
FDM is particularly useful for analog signals and continuous data streams.
5. Established Technology
FDM has been widely used in radio, television and traditional telephone systems.
9. Disadvantages of FDM
1. Requires Large Bandwidth
The total bandwidth must accommodate all frequency channels and guard bands.
2. Guard Bands Waste Bandwidth
Some frequency space is deliberately left unused to prevent interference.
3. Interference
If frequency bands are not properly separated, cross-talk/interference can occur.
4. Hardware Complexity
Filters and other frequency-selective circuits are required.
5. Not Ideal for Bursty Digital Data
For data that is transmitted only occasionally, permanently assigning a frequency band can waste capacity.
10. FDM vs TDM
| Feature | FDM | TDM |
|---|---|---|
| Full form | Frequency Division Multiplexing | Time Division Multiplexing |
| Resource divided by | Frequency | Time |
| Users transmit | Simultaneously | In different time slots |
| Mainly suitable for | Continuous/analog signals | Digital signals |
| Guard bands | Generally required | Not normally required |
| Synchronization | Less critical | Important |
| Example | Radio/TV broadcasting | Digital telephone/data systems |
Easy way to remember
FDM → Frequency is divided
TDM → Time is divided
11. FDM in One Diagram
FDM
Signal 1 ──┐
Signal 2 ──┤
Signal 3 ──┼──→ [ MUX ] ──→
Signal 4 ──┘ │
│
Different Frequency Bands
│
↓
[ DEMUX ]
│
┌─────────────┼─────────────┐
↓ ↓ ↓
Signal 1 Signal 2 Signal 3
↓
Signal 4
Conclusion
Frequency Division Multiplexing (FDM) divides the available bandwidth of a communication channel into multiple frequency bands. Each signal is assigned a separate frequency band, and guard bands are used to prevent interference. FDM allows multiple signals to be transmitted simultaneously over the same communication medium.
⭐ MCA 10-Mark Answer Structure
For an exam, write:
- Definition of multiplexing
- Definition of FDM
- FDM working
- FDM diagram
- Frequency bands and guard bands
- Examples/applications
- Advantages
- Disadvantages
- FDM vs TDM
- Conclusion