RF Modulation Techniques - Tradeoffs between different types of modulation

by Sanjay Mishra

In this article we explore the factors to consider when considering different RF Modulation Techniques. Very often the literature on the subject is dense with equations and formulas and the big picture gets lost. Here we try and consider the trade offs between different types of modulation in plain English.

We also investigate Adaptive Modulation Techniques

Constant Power Envelope / Power Efficiency

Some modulation schemes like Amplitude Shift Keying and Quadrature Amplitude Modulation are based on varying the amplitude of the signal. These requirea linear power amplifier in the output stage of the transmitter to avoid frequency splatter. Constant envelope modulation schemes like Gaussian Minimum Shift Keying (GMSK) do not require a linear power amplifer Linear power amplifiers are less power efficient than non linear amplifiers. This is an important consideration for battery powered radios.

Coherent versus Non Coherent Detection

For coherent detection the receiver has to gather information about both the phase and the amplitude of the received signal. Extracting this information takes a bit longer than setting up non coherent detection. Demodulation schemes based on coherent detection give better performance at the cost of longer setup time and more complex electronics. (Since all the electronics is now available in a chip form with little difference in price this is not a big deal any longer.)

For systems which send short bursts of data, the longer set up time may actually lead to reduced throughput even though the data rate once the signal has locked on is more. For longer data packets, coherent detection is well worth it.

Noise Immunity

Different modulation schemes offer differing levels of noise immunity for the same data rate and the same occupied bandwidth.

Data Rate

Most radio communications are bandwidth limited since we only have the use of a narrow spectrum. In these conditions it is advantageous to have a modulation scheme that transfers the maximum data in our allotted bandwidth.

Adaptive Modulation Techniques

Of course it is best if the radio can change its modulation scheme based on the RF environment.

If your radio can change its modulation scheme based on the environmental RF conditions it can give the optimum performance.

The same principle applies to all modems actually. If you remember the days of analog dial up phone line modems, they would try and connect at the highest possible data rate. If a connection at that data rate was not possible it would go to the next lower data rate, and then to the next lower data rate till a connection was finally established. So if you were far off from the phone exchange, or if you had a noisy phone line, your signal to noise ratio on the phone line was low and you got only lower data rates. If you had a noise free phone line you got higher data rates.

This modulation negotiation is only worth it if you are going to stay connected for a long time. If you only have a short burst of data to transfer, it is faster to connect at a low data rate for which the connection is guaranteed, do the transfer and then break off the connection. This is the approach taken by many transaction processing terminals.

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