In indoor optical wireless communication (OWC) systems, ambient lights in particular fluorescent lamps introduce a periodic interference signal, which has the potential to severely degrade link performance. One of the simplest techniques often used to mitigate the interference from artificial sources of ambient light is electrical high-pass filtering (HPF). However, HPF introduces baseline wander (BLW), which is more severe for modulation techniques that contain a significant amount of power located at DC and low frequencies. In this paper the effects of BLW on systems employing digital pulse interval modulation (DPIM) are extensively investigated. Also investigated are the effects of cut-on frequency of the HPF on the optical power requirement and power penalty for different bit rates, where results are compared with the more established techniques of on-off keying (OOK) with a non-return to zero signalling and pulse position modulation (PPM). The optimum HPF cut-on frequency, which minimises the overall power penalty, is estimated. It is shown that at high data bit rates (100 Mbps), DPIM power requirement is considerably lower (~5 dB) than OOK and similar to that of PPM.