A 4-channel, upstream, digital filter multiple access (DFMA) passive optical network (PON) is experimentally demonstrated for the first time in a 26km SMF IMDD system employing real-time ONUs and OLT, which are dynamically reconfigurable in terms of channel spectral allocation and signal modulation parameters. To combat physical channel frequency response-induced cross channel interference (CCI) between spectrally-overlaid orthogonal channels, a real-time, reconfigurable CCI cancellation (CCIC) function implemented in the OLT receiver DSP, is demonstrated to significantly improve the DFMA PON performance and robustness in terms of; significant reductions in total channel BERs and individual subcarrier BERs, substantial increases in channel capacities, considerably reduced sensitivity to ONU synchronisation and excellent transparency to the interfering channel’s signal modulation format. The trade-off between the complexity of the CCIC function’s filter, in terms of tap count (multiplier elements) and the DFMA PONs performance is optimised to determine the optimum trade-off. It is shown that the CCIC filter only requires as few as 42 taps to achieve excellent CCIC performance. We also fully analyse the DSP complexity of the transmit and receive DFMA filters and the CCIC filters for varying DFMA channel counts and varying ratios of AD/DA converter sample rate to digital logic clock rate (parallelisation factor). It is shown that DFMA filter complexity is independent of channel count and furthermore that for channel counts ≥4 the CCIC filter only needs to process ~10 interference-inducing samples per wanted signal sample, resulting in only ~10 multipliers per parallel filter in the CCIC filter. We have therefore successfully demonstrated an ultra-low complexity, real-time CCIC function, which considerably enhances the performance and robustness of DFMA PONs.