What does Vector Error (VE) mean and what are the practical implications?

Vector Error (VE) provides a useful indication of the quality of the received signal. VE is the normalised error between the received QAM signal and a set of ideal QAM constellation points. VE has units of dB. A more negative number (e.g. -30 dB) indicates a better signal than a more positive number (e.g. -10 dB). VE is rarely a positive number, which represents a very poor signal.

Sources of VE are thermal noise, distortion, interference, and oscillator phase noise. With a low receive power, VE indicates the approximate signal to noise of the receive signal. At higher receive power, VE can be degraded by overload of the transmitter at the remote end of the link or overload of the receiver at the local end of the link. Oscillator phase noise and the noise floor of the receiver set a lower limit on the VE that can be measured, at about -33 dB.

The System Status page displays the maximum, minimum, mean and latest value for Vector Error. Also, the Diagnostics Plotter has an option to display Vector Error.

In this ODU, the receive power is -53.3 dBm which is a good size signal, but not enough to overload the receiver. The remote transmit power is 7 dBm, which does not introduce significant transmit distortion. The receiver is measuring -30.8 dB VE which is close to the measurement floor, and good enough to allow the unit to operate at the maximum modulation mode.


Here, I’ve reduced the transmit power at the remote unit to -3 dBm (a reduction of 10 dB):


You can see that the receive power has fallen to -62.8 dBm, a fall of about 10 dB as we would expect. VE has increased to -26.5 dB, which is clearly above the measurement floor, and in this case it is related to the signal to noise ratio for the received signal. A VE of -26.5 dB is still enough to keep the receiver in the top modulation mode.

With a further 10 dB reduction in the remote power we have this:


VE is now -18.9 dB, almost 10 dB worse than in the second step. The receiver has adapted to the lower VE by selecting 16QAM0.63 modulation mode.

Mark Thomas

System Engineer