Experimental demonstration of quasi-synchronous CDMA-VLC systems employing a new OZCZ code construction

In this paper, we propose a new construction of optical zero correlation zone (OZCZ) code set for quasi-synchronous code division multiple access (QS-CDMA) visible light communication (VLC) system. The proposed code set includes a pair of unipolar and bipolar code sets, which is more suitable for intensity modulation and direct detection (IM/DD) and maintains dimming values at 50% regardless of user numbers for CDMA-VLC system. A QS-CDMA-VLC system adopting the proposed code set is investigated by numerical simulation and experimental verification. The system bit error rate (BER) performance is evaluated with different user numbers, sample rates, delay user numbers and transmission distances. In a 32-user CDMA-VLC system, the 250 MS/s transmission at a distance of 1.5 m can be achieved successfully with the BER less than the forward-error-correction (FEC) threshold. And the BER varies slightly taking account of small delay time. The results indicate that the new OZCZ code construction can effectively overcome non-perfect synchronous problem when the time delay among users does not exceed the zero correlation zone length.