Overview
Today we discuss modulation. Common terms associated with modulation include BPSK, QPSK, AM, PM, QAM, and constellation diagrams.
Why Modulate?
Modulation and demodulation are fundamental processes in communication. Without them, wireless and mobile communications would not function. What is modulation and why is it needed? How does 5G perform modulation?
Analogy: Transportation and Communication
Consider a daily travel example: when we travel we choose a transport mode based on the trip. Different vehicles have different speeds. This forms a simple model of moving a person from origin to destination.
Communication systems are similar: they move data signals from a transmitter to a receiver. In this analogy, modulation is like choosing a transport mode for the signal so it can carry information through the channel to its destination.
How Does a Carrier Carry Information?
Wireless signals propagate as electromagnetic waves. How can a wave carry information? A simple analogy uses fruit to represent bits. For example, apple could stand for 0 and banana for 1; the receiver identifies the fruit and recovers the bit.
If only apples are available, different attributes can be used: red apple for 0 and green apple for 1, or large apple for 0 and small apple for 1. These examples use type, color, and size to encode information.
Similarly, a sine wave has three main attributes: amplitude, phase, and frequency. We can use these attributes of the carrier to transmit information.
Amplitude, Frequency, and Phase Modulation
Amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM) modify those carrier attributes to represent data.
5G Supported Modulation Methods
According to 3GPP (TS 38.201), 5G supports several modulation schemes. These can be grouped by which carrier attributes change:
- Phase changes while amplitude remains constant: pi/2-BPSK, QPSK. This class is phase-shift keying (PSK).
- Both amplitude and phase change: 16QAM, 64QAM, 256QAM. This class is quadrature amplitude modulation (QAM).
Constellation Diagrams
To visualize modulation states, constellation diagrams plot symbol states indicating amplitude and phase. Constellation points represent possible modulation states.
BPSK defines two phases to represent 0 and 1, so it carries 1 bit per carrier.
Pi/2-BPSK alternates a phase offset of pi/2 on odd symbol positions and uses standard BPSK phase on even positions, effectively defining four phase states to represent bits.
QPSK defines four phase states corresponding to bit pairs 00, 01, 10, 11, so it carries 2 bits per carrier.
16QAM maps one symbol to 4 bits, 64QAM maps one symbol to 6 bits, and 256QAM maps one symbol to 8 bits. As the bits-per-symbol increases, the data capacity increases, enabling higher throughput.
Modulation and Demodulation Principles
All 5G modulation schemes can be implemented using IQ modulation and demodulation. Starting from a sinusoidal carrier, mathematical manipulations produce the IQ representation used in practical systems.
Demodulation extracts the transmitted symbol values from the received modulated signal. Multiplying the received signal by carriers with the appropriate phase and integrating recovers the original baseband components. This is the IQ demodulation process.
Combining the modulation and demodulation diagrams yields a complete IQ mod/demod view used in 5G implementations.
The IQ representation can be expressed with complex numbers. The modulation formulas and the corresponding demodulation formulas are commonly represented in complex form, which leads to the complex IQ block diagrams used in practical transceivers.
Summary
Modulation maps digital bits to changes in carrier amplitude, phase, or frequency. Constellation diagrams visualize these states. In 5G, PSK and QAM variants are used to trade off robustness and spectral efficiency. IQ modulation and demodulation provide the practical framework for implementing these schemes in modern transceivers.
