Adjacent Channel Power Ratio (ACPR) is ratio between the total power of adjacent channel (intermodulation signal) to the main channel's power (useful signal).
[1] The ratio between the total power adjacent channel (intermodulation signal) to the main channel's power (useful signal).
The first way is by finding 10*log of the ratio of the total output power to the power in adjacent channel.
[2] The second (and much more popular method) is to find the ratio of the output power in a smaller bandwidth around the center of carrier to the power in the adjacent channel.
A high ACPR indicates that significant spectral spreading has occurred.