6.7  Operation modes: RA, MA and PA (FM)

As discussed in Chapter 4, there is always a restriction on the amount of power that can be used for transmission. Because multicarrier modulation allows to assign distinct power values for different tones (subcarriers), there are many power loading algorithms for distributing the total power among the tones, which corresponds to defining a PSD for the transmitted signal. A related problem is bitloading, which assigns the number of bits that each tone will carry.

When choosing a transmit PSD, three parameters are of major interest: rate $R$, total power $P^{\mathrm{\text{tot}}}$ and margin $\gamma$. Given two of them, the third can be determined. Choosing the parameter that will vary (be maximized or minimized) while keeping the other two at chosen values defines three modes of operating a multicarrier transceiver:

• rate-adaptive (RA) mode: having pre-specified (fixed) total power and margin, the transceiver maximizes the bit rate, while maintaining $P^{\mathrm{\text{tot}}}$ and $\gamma$ fixed,
• margin-adaptive (MA) mode: $P^{\mathrm{\text{tot}}}$ and $R$ are given, and the transceiver maximizes the noise margin, while maintaining a fixed bit rate,
• power-adaptive (PA) or fixed-margin (FM) mode: given $R$ and $\gamma$, the transceiver minimizes the power consumption.

The PA mode is useful for minimizing power and favor “green” communications. The MA mode is the natural choice if the system must operate at full power and the user should have a given bit rate. The RA is the one that maximizes bit rate.

When designing a DMT system, after estimating the channel, one can then choose how many bits $b_n$ and energy ${𝜀}_n$ each tone will use. There are basically two optimizations:

• a) Rate-adaptive: the modem is restricted to a maximum transmit power and the optimization is to find the ${𝜀}_n$ that maximizes the rate
• b) Margin-adaptive: the modem should transmit in a given total rate and the optimization is to find the minimum total power that achieves this target rate and also maximizes the margin.