Adjacent-channel interference
A second impairment common in a wireless communications channel is interference from adjacent carriers. While government regulations and standards bodies regulate emissions into adjacent bands to limit interference, regulations do not entirely eliminate it. As a result, modem receivers apply filtering to account for adjacent-channel interference. In Figure 3, a single tone adjacent to a WCDMA carrier is illustrated.
The impact of interference on the modulated signal depends upon both the interference power and its frequency offset. In general, interference is more problematic when it is higher in power and closer to the band of interest. To illustrate this, we compare constellation plots of a 16-QAM signal in the presence of a single-tone carrier that is 4 MHz from the center frequency.
As Figure 4 illustrates, EVM increases with the power of the adjacent-channel interference. In the rightmost graph, we see that as EVM increases dramatically the receiver loses carrier lock.