Certain aspects and features are directed to determining characteristics such as phase margin and gain margin of signals such as standardized orthogonal frequency-division multiplexing (“OFDM”) signals. In one aspect, a processing system can identify multiple carriers in an OFDM signal communicated by a repeater. The carriers include carriers from frequency guard band other than those used for communicating payload data or pilot data. The processing system can generate dynamic characteristic data to be sent via the carriers. The processing system can generate modulate the carriers with the dynamic characteristic data. The modulated power level of the carriers is less than a maximum level of noise filtered by a device receiving the OFDM signal.