Research On Mitigating Multiple-User Inteference Of Downlink Based On OCDMA In VLC Network

With the universal popularization of multimedia services and mobile internet devices,the limited wireless electromagnetic spectrum occupied by traditional radio frequency wireless communication is not able to meet the requirements of rapidly increasing date-rate and quality-of-service any more.In order to solve this problem,researchers are developing optical wireless communication technologies which employ much higher frequency.One of optical wireless communication technologies,indoor visible light communication(VLC)based on light-emitting diode(LED)has become a hot research area,because of its rich spectrum resources,high power,large bandwidth,confidentiality,low price,and ability of providing services of green data communication and illumination without radiation at the same time.In indoor VLC network,when multiple users managed by the same LED illumination/communication terminal try to receive respective signals through downlink in the same moment,there is multiple-user interference existing,which seriously reduces the communication quality of indoor VLC network.Therefore,it is necessary to use channel multiplex technologies to separate signals from distinct users.Firstly,the application background and research progress of VLC are introduced in the thesis,after which some basic theories and research hotpots of indoor VLC network and OCDMA are also introduced.Secondly,a new construction of two dimensional(2-D)optical codes which employ controlling sequences generated by polynomials over galois field for frequency hopping and optical orthogonal codes for time spreading is proposed.The proposed 2-D optical codes have the advantage that code length and the number of available wavelengths are two independent parameters,which means larger code cardinality can be obtained by only increasing the number of available wavelengths without enlarging code length.Comparing with some other 2-D optical codes,the proposed 2-D optical codes are able to reach its largest code cardinality,even though the number of available wavelengths is prime.And under the same condition,the proposed 2-D optical codes have a larger code cardinality.In addition,the results of numerical analysis show that the proposed 2-D optical codes perform better in error probability.Morever,in order to en/decode codewords of the proposed 2-D optical codes with multiple pulses per wavelength,en/decoder supporting wavelength reuse is proposed.Finally,by combining the grouping feature of proposed 2-D optical codes and limited coverage of LED illumination/communication terminal,a new method of distributing codewords of proposed 2-D optical codes is presented.The results of numerical analysis show that the proposed distributing method is able to further reduce error probability,while supporting larger number of users.