Research On High-Performance Real-Time Optical Orthogonal Frequency-Division Multiplexing Communication System And Its Related Algorithms

To meet the increasing end-user's demands for broadband services,optical orthogonal frequency division multiplexing?OOFDM?has been regarded as one of the most promising technologies for future high-speed optical networks.In the recent years,OOFDM technique has attracted lots of attention and been extensively and thoroughly studied in academia,which lays a solid theoretical foundation for future applications of this technology.However,most of the related results are obtained by using off-line digital signal processing approaches.It usually does not take into account a series of issues in the process of implementing an OFDM base-band transceiver,suchasimplementationformandcomplexityofalgorithms,computational precision,operating frequency of systems and on-chip resources requirement,etc.Meanwhile,the traditional sophisticated real OFDM communication systems can be realized by means of series data processing scheme due to its relatively low bit rate when compared with high-speed optics fiber transmission systems,so it is not feasible to directly transplant the related DSP algorithms to optical fiber communication systems.In this paper,some key DSP algorithms suitable for hardware-efficient implementation are studied after theoretical analysis of several impairments existing in direct detection OOFDM?DDO-OFDM?,and then several real-time base-band OFDM transceivers are designed and implemented successfully.Finally,the transmission performance of the real-time DDO-OFDM systems is also experimentally investigated.As a result,the research work of this paper is expected to promote the standardization and practicalization of optic fiber OFDM technique.The main contents of this paper and innovative research efforts are summarized as follows:Firstly,a simplified one training sequence?TS?-based symbol timing synchronization and channel estimation algorithm is proposed to reduce the complexity of hardware implementation and overhead,and its performance was also verified in real-time DDO-OFDM systems.First of all,symbol timing synchronization was achieved by calculating the cross-correlation between the sign bits of received samples and that of local TS.The timing metric is obtained by just performing simple logical calculations and bit summations instead of complex multiplication and addition operations.In addition,the threshold-decision method is used to determine the starting position of the received OFDM samples.In this way,the proposed TS-based algorithm has a lower computational complexity when compared with the traditional TS-based symbol timing synchronization algorithms.So it can be implemented more effectively in hardware.Once the symbol timing synchronization is done,the data on the odd-subcarriers after time-domain to frequency-domain conversion are first used to estimate channel responses on odd-subcarrers by utilizing least square method.And after that the channel estimations on the even-subcarriers areperformedbyusingasimplelinearinterpolation.Therefore,the subcarrier-to-subcarrier beating interference?SSBI?in DDO-OFDM systems can be effectively avoided by adopting this estimation method,as a result,the estimate accuracy will be improved without SSBI.Meanwhile,the introduced overhead due to the TS can be neglected through reasonable design of the structure of OFDM frame.Secondly,a method consists of a pilot-aided SCFO estimation and compensation algorithm,and our proposed sampling clock frequency synchronization scheme based on this algorithm has been studied and utilized to solve the degraded receiver performance problem,which caused by existing a wide range of SCFO between transmitter and receiver.For small values of SCFO,this algorithm can effectively compensate the residual phase after channel equalization due to the SCFO.When SCFO values are large enough,the proposed scheme can be used to protect the received signal from severely inter-channel interference?ICI?.Meanwhile,it can combine with the SCFO estimation and compensation algorithm to relax the accuracy of sampling clock frequency synchronization.The theoretical analysis and experimental results show that this method including the algorithm and scheme can improve the transmission performance of DDO-OFDM with a low-complexity of hardware implementation.It also can reduce the requirements for precision and stability of the voltage control oscillators?VCOs?,which is used to realize sampling clock frequency synchronization.Thus,the system cost can be saved to some extent.Thirdly,to increase the data transmission rate per wavelength of passive optical networks?PONs?,high-level quadrature amplitude modulation?QAM?-encoded base-band OFDM transceivers are successfully implemented on field programmable gate array?FPGA?chips incorporating with high-speed digital-to-analog converters?DACs?and analog-to-digital converter?ADCs?,and its performance is also investigated and optimized in fiber-optic transmission systems with direct detection.First of all,256/64/16-QAM encoded OFDM transceiver and OFDM receiver for OFDM-PON with a net rate of 6.3 Gb/s and 10.4 Gb/s,respectively,are successfully implemented by using adaptively modulation technique within a electrical bandwidth of 1.1 GHz and 2.15 GHz,respectively.Then,to further improve the system spectral efficiency?SE?,1024-QAM-encoded OFDM transceiver is successfully developed and implemented by means of making ADC over-sampling combined with digital low-pass filter at the OFDM receiver.In addition,the direct current?DC?bias voltage applied for Mach-Zehnder modulator?MZM?and launch power of real-time optical OFDM signal are dynamically optimized by real-time monitoring bit error rate?BER?performance of the system.Moreover,the optimal clipping ratio?CR?over resolutions of DAC and ADC is analyzed by numerical simulation,and then the receiver sensitivity can be optimized by choosing the optimal CR,i.e.,to obtain the best balance between the digital signal clipping and quantization noise.For all of high-level QAM-encoded real-time OFDM signals,the experimental results show that the BERs after 20 km standard single mode fiber?SSMF?transmission are below than7%forward error correction?FEC?with a BER threshold of 3.8?10^-3.Finally,high-definition video signals transmission system based on OFDM modulation and demodulation technique are developed by using Xilinx high-speed series interface gigabit transceiver?GTX?to further verify the feasible of the developed base-band OFDM transceiver and the reliability of the established real-time DDO-OFDM transmission system.In such a system,either 1.485-Gb/s 720p or2.97-Gb/s 1080p high-definition video signal can be captured and modulated to obtain double-side-band optical OFDM signal by the real-time optical OFDM transmitter,and then successfully transmit over 20-km SSMF.After fiber link transmission,the received OFDM signals are demodulated in the real-time receiver.The recovered video streams are generated by the GTX and sent to a liquid crystal display?LCD?.The experimental results show that the recovered video signals can be well reproduced on the LCD almost without distortion.