Linearization of a Radio-over-Fiber mobile fronthaul with feedback loop

The main disadvantage of Radio over Fiber (RoF) links digital predistortion (DPD) is that it is necessary to feedback the output signals at the Remote Radio Head (RRH) in a real scenario. Along the literature digital predistortion on RoF mobile fronthaul architectures considers an ideal feedback system, so it cannot be deployed in real systems. We propose the use of a RoF link to feedback these signals. This RoF link can be accomplished by an independent RoF link, in which the feedback signals are sent from the RRH to the Baseband Unit (BBU). Experiments are carried out with a 20MHz-LTE downlink signal (16QAM) at 2.65 GHz within the Band 7 of the LTE standard. The DPD performance has been tested in an intensity modulation/direct detection (IM/DD) RoF mobile fronthaul system. Experimental results reveal that with a properly postdistorter modeling of the feedback RoF link it is possible to achieve Adjacent Channel Power Ratio (ACPR) and Error Vector Magnitude (EVM) values close to the results with an ideal feedback link.