Digital predistortion of a full-duplex Radio-over-Fiber mobile fronthaul link with feedback loop

Abstract

Radio over Fiber (RoF) fronthaul system linearization has become much more important due to the distortions produced by the optical link between the Baseband Unit (BBU) and the Remote Radio Head (RRH) in a Cloud Radio Access Network (C-RAN) architecture. Its main disadvantage is that in a real scenario the output signal may be far a few kilometers. Along the literature, digital predistortion (DPD) on RoF mobile fronthaul architectures considers an ideal feedback link, but in a real scenario the captured output signals at the BBU side are affected by the RoF feedback effects. In a first place, we evaluate the effects of this feedback loop on the downlink RoF predistorter. Then, we propose and experimentally demonstrate the use of a postdistorter on the feedback RoF in order to compensate its effects. This feedback RoF link can be accomplished in the same optical link by means of wavelength division-multiplexing (WDM). 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 feedback RoF postdistortion it is possible to achieve Adjacent Channel Power Ratio (ACPR) and Error Vector Magnitude (EVM) values in the downlink RoF system close to the results with an ideal feedback link.