Radio frequency (RF) modulation is a technique for passing data between distant places using an electromagnetic wave as a carrier. Cables can also be used, but RF is mostly wireless. Modulation and demodulation are how we imprint and extract data from radio frequency waves.
This seven-part RF Modulation Schemes series will explore several basic modulation schemes, multilevel coding, and combined modulation techniques. Then, we will take a closer look at implementation for analog and digital messages. Later in the series, I’ll introduce I&Q principles and cover some complex modulation techniques using non-conventional carriers.
But first: A refresh on modulation basics. This first module will highlight various RF modulation schemes, which are foundational to understanding the series as a whole.
Figure 1: carrier and information analogy
A carrier is required because information signals have characteristics (such as frequency and power) that are not suited with the transmission medium (such as free space, copper, fiber, etc.). Only electromagnetic waves with a certain frequency can travel long distances through space.
We can use the analogy of a letter. In this case, the information (the words) is imprinted on a carrier (paper) for transmission (delivery) by the postal service. In RF, the carrier is a sine wave that is determined by three parameters: amplitude (A), frequency (f or ω) and phase (ɸ).
Figure 2: Characteristics of a carrier sinewave
In electrical terms, the carrier is a high frequency electromagnetic wave, and the information is a signal that can be in analog or digital form. The selection of the carrier can vary based on requirements such as speed, robustness, and need for multiplexing.
Modulation describes the way a data signal is imprinted on a carrier. The reverse action of extracting information from a modulated signal is called de-modulation. The information can be very simple (such as a single letter) or very complex (such as a video).
Figure 3: Carrier, data, and combined (modulated) signals. Graphic created by אליסף and distributed via Wikimedia Commons under the GNU Free Documentation License. Labels have been translated to English and the aspect ratio has been adjusted for easier viewing.
Remember that the carrier sine wave has three characteristics, amplitude (A), frequency (f or ω), and phase (ɸ). This gives us three modulation options. To imprint the information onto the carrier, we can modify A, F or ɸ. In each case, the carrier signal changes based on the information signal.
Figure 4: ASK, FSK and PSK Modulations. Graphic created by Jamj2000 and distributed via Wikimedia Commons under the Creative Commons Attribution-Share Alike 4.0 International license. Aspect ratio adjusted for easier viewing.
The three basic digital modulation techniques ASK, FSK and PSK may be used to send one bit of data (0 or 1) per unit transmission time. That technique is called binary shift keying (B-ASK, B-FSK or B-PSK). But, since carrier frequency is a limited resource, the technique to modulate several bits in one time slot is often more advantageous.
In this technique, several data bits are grouped together and coded into symbols. Each symbol is assigned to a particular modulation level. We call these multiple level modulation schemes, or M-ary with M values >2. The M-ary technique can be applied to ASK, FSK, or PSK modulation schemes.
Figures 5 and 6 show graphical representations of M-ary amplitude modulation schemes. Each uses four symbols. In 4-ASK, four amplitude levels are used: V0 for symbol 00, V1 for symbol 01, V2 for symbol 10 and V3 for symbol 11 (Figure 5). In 4-FSK, four different carrier frequencies will be used: F0 for symbol 00, F1 for symbol 01, etc.
Figure 5: Example of M-ary amplitude modulation. This is a 4-ASK scheme.
Figure 6: Example of M-ary frequency modulation. This is a 4-FSK scheme.
If we group bits two by two, then each symbol contains 2 bits.
If modulation is made on the symbol, the resulting bit rate is doubled, and we then have four different symbols: 00, 01, 10 and 11.
If we group the bit stream by fours, then we have 16 different symbols: 0000, 0001, 0010… to 1111.
All of this determines the data transmission speed—which is also referred to as bit rate (bits per second or bps), symbol rate (symbol per second or sps), or baud rate. We can attempt to push the bit rate/symbol rate even further, but the more complex the symbols, the more difficult it becomes to differentiate them—especially at the receiving side.
Figure 7: M-ary symbols and data flows
To go even further with the data coding, and thus increase the data flow, it is possible to combine different modulation schemes. For example, coding the different symbols with both phase and amplitude modulations will dramatically increase the data transmission and reception. This is called quadrature and amplitude modulation (QAM).
We’ll dive deeper into QAM and other combined modulation schemes in an upcoming post, so be sure to check back next month to learn more about this technique for improving data transmission speed.
