Error vector magnitude

Error vector magnitude (EVM), also known as relative constellation error (RCE), is a metric for assessing the performance of digital radio transmitters and receivers. In an ideal system, all constellation points land exactly at their intended locations, but real-world imperfections—such as carrier leakage, poor image rejection, and phase noise—cause these points to shift. EVM essentially measures how far the actual points stray from those ideal spots.

Definition

Noise, distortion, spurious signals, and phase noise all worsen EVM, making it a broad indicator of signal quality in digital communications. Specialized equipment can measure transmitter EVM by demodulating the signal much like a real radio would. In typical phase-shift keying demodulation, a stream of I-Q points is produced, which serves as a reliable estimate of the ideal transmitted signal for EVM calculation. An error vector is the difference in the I-Q plane between an ideal constellation point and the actual received point.

EVM is the root mean square (RMS) average amplitude of these error vectors, normalized to a reference signal amplitude, and is usually expressed as a percentage (multiplied by 100). The reference amplitude can be either the maximum ideal signal amplitude in the constellation or the RMS average of all possible ideal amplitudes. For common constellations like BPSK, QPSK, and 8PSK, both methods yield the same result. However, for higher-order QAM constellations such as 16QAM, Star 32QAM, 32APSK, and 64QAM, the RMS average and maximum produce different reference values.

EVM is sometimes given in decibels (dB), which relates to the percentage value through a standard formula. The exact definition of EVM depends heavily on the applicable standard—for example, 3GPP LTE documents specify exactly how to measure it. Academics continue to debate certain problems with EVM measurement. Battery life and power consumption are key concerns in system-level RF transmitter design.

The power amplifier (PA) uses a large share of total DC power, so techniques are used to reduce its consumption. Many PAs allow an adjustable DC supply voltage to balance maximum RF output power against DC power use. Most PAs can also be powered down when idle (e.g., during reception or between packets) to save energy.