Modulation

[2] For low frequency radio waves, wavelength is on the scale of kilometers and building such a large antenna is not practical.

Another purpose of modulation is to transmit multiple channels of information through a single communication medium, using frequency-division multiplexing (FDM).

These were the earliest types of modulation[citation needed], and are used to transmit an audio signal representing sound in AM and FM radio broadcasting.

This alphabet can consist of a set of real or complex numbers, or sequences, like oscillations of different frequencies, so-called frequency-shift keying (FSK) modulation.

The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).

Computers may, however, communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols.

[citation needed] The most fundamental digital modulation techniques are based on keying: In QAM, an in-phase signal (or I, with one example being a cosine waveform) and a quadrature phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite number of amplitudes and then summed.

In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits.

Digital modulation schemes are possible because the transmitter-receiver pair has prior knowledge of how data is encoded and represented in the communications system.

In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.

In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred.

Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one-symbol-time duration (total response signaling).

However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA, but not with QAM and OFDM.

According to incremental expanse of intelligent receivers, automatic modulation recognition becomes a challenging topic in telecommunication systems and computer engineering.

Moreover, blind recognition of modulation type is an important problem in commercial systems, especially in software-defined radio.

Obviously, with no knowledge of the transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information, etc., blind identification of the modulation is made fairly difficult.

The latter methods both involve relatively simple line codes, as often used in local buses, and complicated baseband signalling schemes such as used in DSL.