Research On Flexibile And Efficient Orthogonal Frequency Division Multiplexing Passive Optical Networks

Orthogonal frequency division multiplxing (OFDM) technology is now widelyconsidered to be one of the most promising candidate of the next generation opticalaccess networks——passive optical network (PON) for its advantages of high spectraefficiency, compatibility with all kinds of baseband modulation formats, and convenientapplication of digital signal processing. While in the current OFDM-PON systems,OFDM is just used as a transmission method, how to improve the transmissionefficiency and flexibility based on the characteristics of OFDM technology and PONsystem becomes the key issues.To improve the transmission efficiency of OFDM-PON systems, proposals aregiven on further increasing the spectra efficiency and reducing the redundancy of thesystem. In order to further increase the spectra efficiency of OFDM-PON system, a newsignal generation and demodulation method based on fast Fourier transformation (FFT)for fast-OFDM is proposed. As a result, fast-OFDM realizes signal-sideband signalgeneration and shares the same system signal modulation and demodulation processwith the ordinary OFDM system， making it possible to utilize the new fast-OFDM tofurther increase the spectra efficiency of OFDM-PON system. To reduce thetransmission redundancy of the system, considering the characteristics of the shorttransmission distance, fast transmission link switch and relative stable channelcharacteristic of the access network, a new transmission chromatic dispersioncompensation method is proposed. The consteallation of every single subcarrier is usedto compensate the chromatic dispersion originated from transmission on the conditionthat only one training symbol is added before the data stream. Not only the transmissionredundancy because of frequent training symbol addition is greatly reduced, and theresponse of channel switch is improved as well.To increase the flexibility of OFDM-PON system, efforts are devoted to therealization of colorless optical network unit (ONU) and transmission capacity allocationbased on subcarrier selectively receiving. The fantastic correspondence between thespectrum and the baseband data of OFDM signal is utilized and a special OFDM signalis intentionally generated, where there is a freuquency gap between the optical carrier and the OFDM signal band. As a result, the optical carrier of the downstream OFDMsignal can be easily filtered out and reused for upstream transmission. Based on this, acolorless ONU scheme with periodical optical narrow band pass filter is proposed, andsymmetric transmission of both downstream and upstream transmission is also realized.For flexible system capacity allocation, the focus is shifted from the bandwidth to thereal transmission capacity of the ONUs. A flexible transmission capacity allocationstrategy is proposed by bit loading and power loading method under the premise ofkeeping the bandwidth of every ONU fixed. To receive signal more efficiently, thelimitation of the OFDM band is broken and a new selectively receiving scheme withcoherent detection and bandwidth-limited receiver as well as the corresponding signalprocessing algorithm is proposed, making it possible for the ONUs to only receive anyof their interested subcarriers instead of the whole OFDM band. With the proposedselectively receiving based transmission capacity allocation scheme, the bandwidthrequirement of the receiver in the ONUs is greatly relaxed, but the expense of thesystem is also tremendously reduced.