A method includes transmitting, from a radio device of a secondary communication system, a signal into a channel of a primary communication system; monitoring channel signal quality-related feedback of the primary system; determining, based on the monitored channel signal quality-related feedback, an amount of mutual coupling between the transmitted signal and the channel, if any, caused by the transmitted signal; and if the amount of determined mutual coupling is less than a threshold amount, using the channel for communication with another radio device of the secondary communication system. The signal that is transmitted may be a probing signal having at least a predetermined temporally varying transmission pattern, and where determining includes correlating at least the predetermined temporally varying transmission pattern with changes, if any, in the monitored channel signal quality-related feedback. The channel signal quality-related feedback that is monitored may include at least one of ACK/NACK feedback, power control feedback, rate control feedback and channel quality indicator feedback.