Modulation : Why do we require modulation in wireless communication?
Wireless communication relies heavily on modulation techniques. Coaxial cable, twisted pair, and other types of wired communication are commonly used. Wired communication, on the other hand, is best for short-distance communication. An antenna is required for wireless communication to transfer signals. Now, if we want to send a baseband signal, we'll need a very large antenna with a range of many kilometers. Baseband signals are ones that typically contain a low or medium frequency message signal. It's also known as a message signal without modulation. Modulation is a technique for increasing the frequency of a message signal by the carrier frequency to a significantly higher frequency.
So, now I'll explain why modulation is necessary. The main two goals of modulation techniques are to reduce antenna height and to multiplex data (Multiplexing).
Long-Distance Communication
Low-frequency base-band signals are not suitable for long-distance communication because they cannot generate a strong far field. The far field is the region where electromagnetic waves become plane waves and true wireless transmission occurs.
At low frequencies, the signal primarily creates a strong near field, which stores energy close to the antenna instead of radiating it efficiently into space. As a result, most of the energy remains near the source and does not propagate over long distances.
Reducing the height of antenna:
For short-range baseband communication, wired communication is sufficient. However, for long-distance communication, wireless is the best option. We know that the transmitter antenna emits a specific radiation pattern. The signal then travels through the earth's atmosphere until it reaches the receiver. We should be aware that antenna height is important for reliable transmission. The wavelength of the working frequency determines the antenna height. The antenna height should be
Ht = λ/4 and λ = c/f
Where, Ht = height of antenna
λ = wavelength of operating frequency
c = speed of light
f = operating frequency
We can also derive the following equations from the above equation:
As Ht = λ/4,
Therefore, Ht =c/4f; or Ht α 1/f
So, we can say that antenna’s height is inversely proportional to the operating frequency. If frequency is more, height of antenna should be smaller and vice-versa for reliable communication.
Example:
Let assume, operating frequency of a communication band is 20 KHz. Then the height of antenna, Ht, should be
Ht = c/4f = (3*10^8) / (4*20*10^3) = 3.75 kilometers
We can observe that the antenna height for reliable 20 KHz band communication should be 3.75 kilometers. As a result, we choose modulation, in which the lower frequency is shifted to the higher frequency. It is a method of enhancing transmission frequency while lowering antenna height.
On the other hand, if you use 100 MHz carrier signal then you only need a 0.75 meter antenna, which is perfectly practical for wireless communication (like FM radios, smartphones, etc.).
Multiplexing:
Multiplexing is another significant advantage of modulation techniques. The parallel data streams are multiplexed into a serial data stream. Simply put, modulation techniques allow us to deliver many simultaneous data streams from the transmitter to the receiver across a single channel. To your knowledge, we use the modulation technology to enable multiplexing in wired connection as well.
Noise Immunity and Signal Strength
Modulation schemes can be designed to make signals more resistant to noise, distortion, and interference.
For example, digital modulation techniques like QAM, PSK, or FSK improve bit error rate performance under noisy conditions.
Bandwidth Utilization
Modulation allows better utilization of available bandwidth, especially with advanced techniques like OFDM. It enables the transmission of high data rates in limited spectral resources.
Adaptability
Adaptive modulation helps wireless systems dynamically change their modulation scheme depending on channel conditions (e.g., switching from QPSK to 64-QAM in 4G/5G).
Further Reading
