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What Is Phase?

Preview: Learn more about phase and its importance in communications.

Phase describes the position of a repeating waveform within its cycle relative to a chosen reference point. Together with amplitude and frequency, phase is one of the three fundamental properties that completely define a sinusoidal signal. Although it is often less familiar than amplitude or frequency, phase plays a vital role throughout communications engineering, influencing everything from radio transmission and digital modulation to satellite navigation and wireless networking.

A sinusoidal waveform repeats continuously over time. If two sine waves have the same amplitude and frequency, they may nevertheless reach their maximum and minimum values at different times. This difference in timing is described by their phase. Two identical waveforms that rise and fall together are said to be in phase, while waveforms whose peaks and troughs occur at different times are said to be out of phase.

Phase is usually measured as an angle, expressed in degrees or radians. One complete cycle of a sine wave corresponds to 360 degrees, or (2\pi) radians. A phase difference of 180 degrees means that one waveform is exactly opposite the other: whenever one reaches a positive peak, the other reaches a negative peak. A difference of 90 degrees corresponds to one-quarter of a cycle and is commonly encountered in many communication and signal-processing systems.

One useful way of visualizing phase is to imagine two runners moving around the same circular track at exactly the same speed. If they start from different positions, they maintain the same separation even though they complete each lap together. The distance between them around the track is analogous to their phase difference. Similarly, two sine waves may have identical frequencies while remaining permanently offset from one another in phase.

Phase is particularly important because waves combine according to both their amplitude and their phase. When two waves arrive in phase, their amplitudes reinforce one another, producing a stronger signal. When they arrive out of phase, they partially or completely cancel each other. This phenomenon, known as constructive and destructive interference, explains many effects observed in acoustics, optics, antenna systems, and radio propagation.

In communications engineering, phase carries information in several different ways. In Phase Modulation (PM) and Phase Shift Keying (PSK), information is conveyed by deliberately changing the phase of a carrier signal. More advanced techniques such as Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) combine changes in both phase and amplitude to transmit multiple bits of information during every transmitted symbol. These modulation techniques are widely used in satellite communications, Wi-Fi, mobile telephone networks, digital television, and broadband Internet systems.

Phase also plays an important role in antenna systems. Signals arriving at different antennas may experience slightly different phases because they travel different distances. By combining these signals appropriately, engineers can electronically steer antenna beams without physically moving the antenna itself. This principle underpins modern phased-array radar systems, electronically steerable satellite antennas, and many fifth-generation (5G) mobile communication systems.

Satellite navigation systems such as GPS also rely heavily on phase measurements. By comparing the phase of precisely timed signals transmitted by multiple satellites, receivers can determine their position with extraordinary accuracy. High-precision surveying equipment is capable of measuring carrier phase so accurately that positions can be determined to within a few centimetres under favourable conditions.

It is important to distinguish phase from time delay, although the two are closely related. A fixed time delay produces a phase shift that depends upon the signal frequency. At low frequencies, a particular delay corresponds to only a small phase change, while at higher frequencies the same delay may represent many complete cycles. For this reason, engineers often analyse communication systems using both time and phase, depending on the particular application.

Today, phase is a fundamental concept throughout communications engineering. It influences modulation, synchronization, antennas, radar, satellite navigation, signal processing, optical communications, and wireless networking. Although it is invisible to the user, careful control and measurement of phase are essential for achieving the speed, accuracy, and reliability expected of modern communication systems.

Phase therefore represents far more than the position of a waveform within its cycle. It is one of the fundamental properties of every periodic signal and one of the most powerful tools available to communications engineers for transmitting, combining, and recovering information. Understanding phase provides an essential foundation for many of the technologies that make modern communications possible.