## What Is Modulation?

In wireless communication systems, modulation alters a carrier signal according to
information in a message signal for transmission over a wireless channel. The alteration
depends on the modulation method. To understand the process, consider the general form
of a carrier signal, *s*(*t*), as

*s*(*t*) =
*A*(*t*)cos[2π*f*_{0}*t*+ϕ(*t*)]

In this equation,the information-carrying component in the message signal is amplitude
(*A*), frequency (*f*_{0}),
and phase (ϕ), which can be part of the signal individually or in combination.

Wireless communication systems can use single carrier or multiple carrier modulation schemes to transmit the information contained in message signals.

### Single Carrier Modulation

In single carrier modulation, the process alters the characteristics of one
carrier sine wave by combining it with the message signal. For
*analog* modulation, an analog message signal modulates the
carrier signal. Common analog modulation methods include amplitude modulation (AM)
and frequency modulation (FM). For *digital* modulation, a
digital message signal modulates the carrier signal. Common digital modulation
techniques include quadrature amplitude modulation (QAM) and frequency shift keying
(FSK), which are digital equivalents of AM and FM.

#### View Frequency Modulation

To observe the effect of modulation, combine a carrier sine wave with a message signal. Plot the carrier, message, and frequency-modulated signals.

### Multicarrier Modulation

In multicarrier modulation, the process alters characteristics of a collection of
carrier sine waves by combining each with a different message signal. For 5G and
IEEE^{®}
802.11™ communications systems, the standards specify the use of multicarrier
modulation. Specifically, they use orthogonal frequency division multiplexing
(OFDM).

OFDM modulation divides the information to be transmitted into multiple
bitstreams. For this technique, the process first codes and modulates the bitstreams
into symbols, usually QAM symbols. Then, it loads the symbols into equally spaced
frequency bins and applies an inverse fast Fourier transform (IFFT) to transform the
signal into low symbol rate, orthogonal overlapping sinusoidal subcarriers in the
frequency domain. This equation is a scaled version of the inverse discrete Fourier
transform (IDFT) of the QAM symbol stream, *a*_{m,n}, for the m^{th}
subcarrier in the n^{th} OFDM time symbol.

$$s(k)=\underset{m=0}{\overset{N-1}{{{\displaystyle \sum}}^{\text{}}}}{a}_{m,n}\text{\hspace{0.17em}}{e}^{\text{j2}\pi (\frac{mk}{N})}$$

Here, *s(k)* represents the sinusoidal
subcarriers, with *N* samples at the output of the IFFT making up
one OFDM symbol. This plot shows how OFDM modulation orthogonally spaces the
overlapping subcarriers.