Research On Pilots Based Channel Estimation Technology For Compatible Single-sideband Modulation OFDM Transmission With Direct Detection

Orthogonal Frequency Division Multiplexing technology is famous for its highspectral efficiency and flexible resources allocation. Moreover, as its excellentperformance on anti-chromatic dispersion, it has been developed rapidly in opticalcommunication systems. A direct-detection optical OFDM (DDO-OFDM) system hasa more wide application for its simple implementation, stable performance and lowcost. Traditional intensity-modulation based double sideband DDO-OFDM requires aguard band between optical carrier and data subcarriers to avoid optical carrier tosubcarrier mixing interference. However, such method will decrease the spectralefficiency immensely. Therefore, to avoid frequency fading by eliminating doublesideband modulation and stay high spectral efficiency, single sideband modulation(SSB) is applied to DDO-OFDM system. Compatible SSB (CSSB) modulation is thebest of them all to implement an optical system for its simplicity and spectralefficiency. The presented work aims on the pilots based channel estimation technologyfor CSSB DDO-OFDM systems. The article accomplishes its works on trainingsequence, pilots and interpolation respectively, and validates our theory throughsimulations with matlab and optisystem.1. The article analyzes system model of CSSB DDO-OFDM firstly. Weimplement our CSSB DDO-OFDM system with a dual-electrode Mach-ZehnderModulator (MZM) and also we give the mathematical model of this implementation.We set up this implementation in optisystem for simulation.2. The article optimizes training sequence for the system. Training sequenceplays an important role in symbol synchronization and rough estimation. Theoptimization of training sequence made here is based on power. We use the parameterTDPPR (TS-to-Data Peak Power Ratio) to investigate the performance of systemunder different training sequence power. The simulation results show the TDPPR ofvalue1.4gives the best performance.3. The article introduces the pilots based channel estimation tech nology to CSSBDDO-OFDM system for the first time and proposes a pilots interval allocationoptimization scheme and a pilots power allocation optimization scheme. The mainimpairment of a CSSB DDO-OFDM system is the frequency selecting fading of highfrequency area. Mixing interference of low frequency area also reduces the performance of the system. According to system impairments we design the pilotpattern on interval allocation and power allocation. The simulation results show thatthe Increased Interval Allocation and the First Decreased Then Increased PowerAllocation give the best performance.4. The article analyzes four interpolation algorithms which use for recoveringchannel response and concludes with a best interpolation algorithm for CSSBDDO-OFDM system. Pilots based channel estimation first recovers the channelresponse on pilot subcarriers with LS (Least Square) algorithm and then recovers thewhole channel response by interpolation. The simulation results show the CubicSpline interpolation method has the best performance and stability in CSSBDDO-OFDM system as it doesn’t require the evenly distribution of pilots.