Why communication systems use a carrier
An information source such as speech, video, or computer data produces a baseband signal. The transmitter prepares this signal for a channel using a higher-frequency sinusoid as the carrier wave. The operation that changes a carrier parameter according to the information is modulation.
Baseband content may not fit the allocated radio channel, and direct radiation at that frequency could require an impractically large antenna. Modulation with a carrier translates the information spectrum to a selected passband, where filters and antennas can be designed for the intended service. The receiver performs demodulation to recover the information signal from the received waveform.
Key terms in modulation
- Information signal (or modulating signal): The signal that contains the data to transmit, for example voice in a phone call. Its spectrum before modulation is commonly called the baseband.
- Carrier wave: A high-frequency, usually sinusoidal, signal used as a reference for transmission. Before modulation, the unmodulated carrier contains no user data.
- Modulated signal: The waveform produced when information changes a carrier parameter such as amplitude, frequency, or phase. This waveform is transmitted through the channel.
- Demodulation: A receiver operation that estimates the information signal or transmitted symbols from the received modulated waveform.
Why communication systems use modulation
Modulation translates a baseband signal to a passband suitable for the channel. It also supports filtering, antenna design, and simultaneous use of one medium by multiple signals. The main reasons are:
- Efficient long-distance transmission and antenna size:
An antenna interacts with a signal according to its wavelength, λ = c/f. For a 15 kHz signal, the wavelength in free space is about 20 kilometers, so a quarter-wave or half-wave reference antenna would be long. A 100 MHz carrier has a wavelength of about 3 meters, which permits antennas with practical dimensions. Actual antenna size also depends on geometry, matching, bandwidth, and the environment.
- Managing interference:
Natural and man-made noise is distributed unevenly across the spectrum. Moving a signal to an allocated band can place it where channel filters reject more unwanted energy. This does not remove interference by itself: the selected band, power, bandwidth, and filtering determine the result.
- Enabling :
Modulation can place different information streams on separated carrier frequencies so they share a medium, such as air or a cable, at the same time. In broadcast radio, each station is assigned a carrier frequency such as 94.5 MHz or 101.1 MHz. A receiver selects the wanted channel with a band-pass filter and local oscillator while attenuating other channels.
Modulation categories
A modulation class is determined by the carrier parameter that changes, such as amplitude, frequency, or phase, and by whether the information signal is analog or digital. The categories below describe common groupings:
- :Used when the information signal is analog and continuous. This includes:
- Amplitude Modulation (AM): The amplitude of the carrier is varied.
- Frequency Modulation (FM): The frequency of the carrier is varied.
- Phase Modulation (PM): The phase of the carrier is varied.
- :Used when the information signal is digital, such as a stream of bits. This includes:
- Amplitude-Shift Keying (ASK): Discrete amplitude levels represent the data.
- Frequency-Shift Keying (FSK): Discrete frequency levels represent the data.
- Phase-Shift Keying (PSK): Discrete phase shifts represent the data.
- Quadrature Amplitude Modulation (QAM): A method that varies both amplitude and phase to encode data.
- :The information is encoded into a train of pulses. This includes:
- Pulse Amplitude Modulation (PAM): The amplitude of each pulse is varied.
- Pulse Width Modulation (PWM): The duration (width) of each pulse is varied.
- Pulse Position Modulation (PPM): The time position of each pulse is varied.
- Pulse Code Modulation (PCM): An extension of PAM in which sampled amplitudes are quantized and converted into binary code. It is used as a basis for digital audio.