DMT and OFDM Multicarrier Modulations

6.2 DMT and OFDM Multicarrier Modulations

There are several multicarrier systems and two important examples are DMT and OFDM. Both use IFFT and FFT to perform modulation and demodulation, respectively. Some texts consider DMT as a special case of OFDM while others make the distinction. Basically, OFDM can be seen as a DMT modulated by a carrier (sinusoidal signal). In this sense, an analogy can be drawn between the pair PAM / QAM and the pair DMT / OFDM. In time-domain, PAM and DMT are real-valued signals. Similarly, QAM and OFDM are complex-values signals. This text assumes the following convention: the DMT modulator generates a real-valued baseband signal while the OFDM modulator generates a complex-valued envelop (that does not need to have a spectrum with Hermitian symmetry) and uses a carrier frequency to perform frequency upconversion for all “OFDM subcarriers” and generate a real-valued passband signal.

In OFDM, the signal after the IFFT corresponds to a baseband complex envelope $x_{ce} (t)$ that is then upconverted via a carrier of frequency $f_{c}$ to create a real-valued passband signal

x_{OFDM} (t) = R eal{x_{}

ce(t) e^j 2

π f_{c} t}. (6.1)

This scheme is similar to the one used in Eq. (3.4) for QAM.

In OFDM, all values in the IFFT modulator’s input (interpreted as QAM symbols) are independent and can be chosen to carry information because the IFFT output can be a complex-valued signal. In contrast, in DMT there is no upconversion by $f_{c}$ (i. e., $f_{c} = 0$ ) and the QAM symbols corresponding to “negative frequencies” need to be the complex conjugate of their corresponding symbols (in “positive frequencies”) such that the IFFT output is a real-valued signal. Another restriction in DMT systems is that the symbols at subcarriers DC and Nyquist frequency must be real (i. e. PAM, not QAM symbols).

Another distinction that has been adopted in the past is that OFDM used the same number of bits per tone while DMT uses a bit loading procedure to assign bits per tone according to their SNRs. However, modern OFDM systems may not use a flat bit allocation.

It should be noted that, while DMT has been widely used to describe some DSL technologies (ADSL, VDSL, etc.), in more recent standards such as G.fast, the term OFDM has been adopted instead of DMT.

The next section discusses DMT, but most of the concepts are valid for OFDM systems too.