WIRELESS NETWORKS AND MOBILE COMMUNICATIONS

ANALOG DIGITAL MODULATION

Modulation is the process of embedding information onto a high-frequency carrier wave for transmission over long distances. It involves changing a characteristic of the carrier wave—its amplitude, frequency, or phase—in a way that corresponds to the information signal. 

Modulation can be either analog or digital as illustrated on Figure 1 below

Figure 1: Categorization of Modulation techniques

ANALOG MODULATION

Analog modulation is a technique used to encode information from a low-frequency analog signal onto a high-frequency carrier wave. The goal of modulation is to modify a specific characteristic of the carrier wave (frequency, amplitude, phase) in proportion to changes in the modulating signal. Modulation is essential for efficient transmission over long distances. It also allows multiple signals to share the same transmission medium without interfering with each other (Compare with notes on Multi-Access). There are three major categories of analog modulation techniques: Amplitude, frequency, and phase modulation techniques

1.Amplitude Modulation (AM)

In Amplitude Modulation (AM) is the varying of carrier wave in accordance with the changes in amplitude of the modulating signal. The frequency and phase of the carrier wave remain constant. AM is one of the oldest and simplest forms of analog modulation and was widely used for radio broadcasting.

When one speaks into a microphone, the voice signal/modulating signal causes peaks and troughs of the carrier wave to change.

Figure 2: Amplitude modulation (AM)

The mathematical representation of an AM signal is given by:

Modulation Index is the ratio of the modulating signal amplitude to the carrier amplitude. It indicates the depth of modulation. If it is greater than 1, it results in over-modulation, which causes distortion.

The main drawbacks of AM are that it is a highly susceptibly to noise and interference because most noise sources (e.g., lightning and electrical sparks) are amplitude-based and can easily to mistaken for part of the signal.

2. Frequency Modulation (FM)

Frequency Modulation (FM) varies the instantaneous frequency of the carrier wave in proportion to the change in amplitude of the modulating signal. The amplitude of the carrier remains constant. FM is known for its high fidelity and resistance to noise.

Figure 3: Frequency Modulation

FM has the advantage of being much more immune to noise than AM. Since most noise affects the amplitude of a signal, this is the reason why FM radio stations are preferred for broadcasting music.

FM signal also has a wider bandwidth than an AM signal. The required bandwidth is approximated by Carson's Rule: