Why do we ever use low-order modulation schemes?

Question

Many communication standards implement adaptive modulation and coding with modulation and coding scheme (MCS) tables. In these tables, we typically use low order modulation for low spectral efficiency. For example, in the "MCS index table 1 for PDSCH" of 5G NR, QPSK is used to deliver spectral efficiency from 0.2344 to 1.3262 bits; beyond these, we use 16-QAM for spectral efficiency from 1.3281 to 2.5703 bits; and so on, c.f. 3GPP TS 38.214 Table 5.1.3.1-1.

It is, however, well known that to achieve the same data rate, higher-order QAM is always more energy efficient than lower-order QAM, c.f. Figure 2 of Ungerboeck's celebrated paper on TCM. For example, to transmit 1.5 bits with 16-QAM requires ~0.6dB less (SNR) than with 4-QAM (QPSK).

Why do we ever want to use QPSK or BPSK? Why don't we just use 64-QAM (or 256-QAM) together with low rate channel coding? Is it because channel coding/decoding complexity is higher when we use only high-order modulation?

Answer 1

As we go for higher modulation order, detection complexity increases by exponentially , Hypothesis grows and detection becomes computationally intensive task.

Also in use cases where channel condition is poor, bandwidth efficiency has to sacrificed switching to lower modulation schemes as channel becomes susceptible to noise and interference.

One of the Key factor is power consumption too, transmission by lower modulation order has less power consumption. In short, there is a tradeoff always depending upon the requirements and each scheme comes with its pros and cons

Answer 2

As indicated by the figure quoted by the OP, indeed, Ungerboeck discovered that in AWGN channel, with high-order modulation scheme to transmit lower number of bits per symbol is more power efficient than using the lower order modulation. For instance, while usually we use QPSK modulation for the purpose of transmitting 2 bits per symbol interval, Ungerbock pointed out that for the same purpose, if we use 8PSK (or 16QAM) constellation for modulation, we can get some power gain, that is, we can achieve the same BER under a lower SNR without sacrifice of bandwidth efficiency. From there, in 1980's, Ungerbock officially published his famous paper to propose the new coding technique, so-called "Trellis coding modulation (TCM)". Following his paper, TCM became a very hot research topic in the whole world and soon got many applications, such as in satellite communications, modem designs, wireline data transmission systems, and soon.

It should be noted that the gain indicated by the above figure is a theoretical result. It is not available automatically without extra cost. Instead, in reality it needs some sort of coding techniques with a certain complexity.

Later after turbo coding (TC) (as well LDPC) appeared and getting hot, TCM is eventually cooled down, since the former provides more coding gain with lower complexity.

Answer 3

One major motivator for choosing a low-order modulation scheme such as BPSK or QPSK over 16-QAM or 256-QAM is improved energy efficiency. Different modulation schemes require different Eb/N0 for a given BER. Eb is the RF signal energy per bit, and N0 is the noise power spectral density. Assuming that N0 does not depend on the modulation scheme, this simply means that the schemes require different amounts of received signal energy per bit to achieve a given BER. A lower Eb/N0 requirement thus implies that less signal energy needs to be transmitted per bit.

For better TX energy efficiency, it is thus good to use a modulation scheme that requires as low Eb/N0 as possible for the targeted error probability. From this perspective, it is good to choose BPSK, QPSK or, even better, an orthogonal modulation scheme such as M-PPM or M-FSK with a very high M. The improved energy efficiency, however, often comes at the cost of spectral efficiency.

For a battery-powered wireless device, choosing BPSK over 256-QAM would generally improve the device lifetime. On the contrary, in an application where power and energy consumption are no concern at all, choosing 256-QAM or even 1024-QAM would of course make more sense because of the higher data rate.

In contrast to what the OP says, use of higher-order QAM is generally not more energy-efficient than use of lower-order QAM -- the higher the order, the greater Eb/N0 is required.

One can reach these conclusions by reading communications literature and by performing waveform-level RF signal reception simulations using Matlab or Octave. Particularly performing the simulations is recommended for understanding the differences between modulation schemes.

Additionally note that, compared to the simpler modulation schemes, the generation of QAM signals may require more complex electronic circuitry (i.e. a greater amount of high-power-consuming circuit blocks). It is easier to achieve high TX/RX power-efficiency using OOK/BPSK/BFSK compared to 1024-QAM.