Effects of mm-wave propagation channels on technology choices for 5G on-frequency repeaters

Wireless systems at mm-wavelengths have poor in building coverage, which is encouraging the study of on-frequency relays and repeaters. Such devices suffer from loop-back interference (LI) that can increase noise and distortion and cause device instability. Cancelling loops can be used to null the strongest multipath LI components. We conduct LI measurements to ascertain the reduction of LI vs the number of cancelled paths. The required number of cancelled paths appears to increase with the square root of the channel bandwidth as more paths are resolved. DSP cancelling appears to be the the most practical solution because of the difficulty of implementing wideband delays and the absence of a significant direct path leakage between the repeater's transmit and receive antennas.