We have described the three fundamental modulation schemes used for digital message communication, which are ASK, FSK and PSK. We learned how these reflect their actions on the carrier amplitude, frequency, and phase, respectively, with their pros, cons, and suited applications in the previous posts. These three parameters fully determine the behavior of a sinewave signal:
We have seen the way to increase the data flow is to adopt multiple possible values for the amplitudes (M_ary ASK), frequencies (M_ary FSK), or phases (M_ary PSK). Now, there is no reason to not combine amplitudes, frequencies and phases at the same time on the carrier.
A symbol can then carry much more information. The price to pay is a much more complex circuitry to modulate and demodulate signals, with a higher risk for symbol overlaps—increasing Bit Error Rate (BER).
When we combine amplitude and phase modulation, it is called Quadrature and Amplitude Modulation or QAM. The origin of the word “quadratic” will be better clarified in the next episode when we explain the I&Q for “in-phase and quadrature” concept, which is particularly associated with QAM in nearly all the existing literature. At this stage, “quadrature” is just related to angle or phase.
In QAM signal, the carrier can have a number #A of amplitudes and a number #P of phases. The number of total possible values for the symbol is (#A) * (#P). For example, with two different amplitudes and four different angles, one obtains 2*4 = 8 different symbols, which allows the coding of 3 bits (8 = 23).
In this example, it would be more precise to say we have an 8-QAM because it provides 8 symbols:
Figure 1 – Example of an 8-QAM with symbols
There’s another way to define an 8-QAM: Instead of only two different amplitudes, one can adopt four different amplitudes and only two different phases. The choice of QAM coding structures influences the quality of modulated signals during the modulation and demodulation processes—hence influencing the quality of the communications (i.e. BER, circuit complexity, distance, power).
Figure 2 – Typical 8-QAM signal (only 6 symbols out of 8 appear)
QAM is, de facto, a M_ary scheme since its symbols carries several bits. As a direct consequence, the minimum QAM coding is a 4-QAM (minimum of two amplitudes and two angles). Modern digital communications—digital TV broadcasting, high data rate WiFi such as the 802.11 standards—are continuously pushing for more complex symbols. It is becoming common to see 256-QAM, 512-QAM and even 4096-QAM.
As QAM modulates amplitude and phase, a carrier can also be modulated by amplitude and frequency. In such cases, we have the ASK-FSK combo. This scheme is rare in RF communication, but it can be found in optical links (i.e. fiber) where the carrier frequency is in the visible light domain.
The main reason this combination is not popular is the demodulator circuit. An FSK modulated signal has a certain impact on the carrier amplitude as well (and vice versa). Therefore, the ASK/FSK demodulator will have a hard time distinguishing mutual modulation effects. This technique will not be analyzed further here.
This combination, though theoretically possible, is extremely difficult to implement since frequency and phase are strongly linked: frequency is the derivative of phase: ω = dɸ/dt. This means changing the carrier frequency will also modify its phase and vice versa.
It would be quasi-impossible to construct a modulator or demodulator that could handle both FSK and PSK with today’s technologies. This is why one will not find such modulation schemes in practice.
Knowing the FSK/PSK combination is quasi-impossible to realize, a scheme combining all three basic schemes ASK, FSK and PSK will be even more problematic. If one really needs to increase data flow speed with good frequency efficiency and link reliability, it is better to adopt a M_ary QAM such as 4096-QAM.
With what has been described earlier, it appears that QAM schemes are the only mixed modulation option. A possible theoretical implementation could be:
Figure 3 – Block diagram showing how an 8-QAM modulator can be implemented
You may observe the extreme difficulty of having a system able to imprint so many parameters onto the carrier at once. And here, we are only considering a quite simple 8-QAM. What if you had to build a 4096-QAM?
This is why another way to handle a signal must be considered: The I&Q approach. Many associate I&Q only with QAM, but in the next episode we will see that the I&Q approach is perfectly suited for all the modulation schemes: ASK, FSK, PSK, and all their derivatives. Check back soon to read about it!
Figure 4 – Animation of a phase shifter using IQ modulator. In the next episode, we'll explore how changing the amplitude of I and Q waves can change the phase of the I+Q wave. CC via Wikimedia Commons
Next post—coming Dec. 10!
