Research On Channel Estimation Technologies Based On Pilots In Optical OFDM System

The orthogonal frequency division multiplexing (OFDM) technology hasreceived a lot of attentions and actively investigated in optical communications due toits high spectrum efficiency, flexible spectrum allocation and anti-interference ability.The OOFDM system based on the direct detection (DDO-OFDM) is very easy to beimplemented and has the merits of low cost and stability operation. This paper focuseson the optimization of pilot structure and selection of interpolation algorithms in thechannel estimation technology based on pilots in DDO-OFDM system. The mainresearch results include:1. Optimize OFDM frame structure for DDO-OFDM system. The channelresponses are two-dimensional stochastic signals. Inserting pilots can be taken astwo-dimensional sampling. The number of pilots in the frequency and time domainshould be given. The optimized OFDM frame structure for the DDO-OFDM systemwas built by numerical simulation. The results show that the frame structure with300OFDM symbols including one pilot symbol and290data symbols has the bestperformance, where the pilot symbol consists of8pilot subcarriers and192datasubcarriers.2. Investigate optimal schemes of power allocation for pilot and datasubcarriers. In the DDO-OFDM system, when the total transmitted power is constant,increasing the power of pilots can improve channel estimation accuracy but alsoincrease the interference between them. Therefore, in order to obtain the bestperformance, the optimal ratio of the pilot-to-total power (PTPR) should be given.The performance of systems with different PTPRs is investigated by numericalsimulation. When the PTPR is1/10, in another word, when the average pilot power toaverage data power ratio is8/3, the system has the best channel estimationperformance and the requirement for receiving optical signal-to-noise ratio is thelowest.3. Propose an optimal pilot power allocation and pilot interval design. Theperformance of the DDO-OFDM system without frequency guardband will bedegraded by the frequency fading and subcarriers to subcarriers mixing interference.And different OFDM subcarriers at different frequencies may suffer from varying distortions.The experimental results show that allocating more power on the pilots at higher frequencies and place more pilots at lower frequencies can improve the channel estimation in the mostdegree and improve the robustness to interference.4. Select a suitable interpolation algorithm for the DDO-OFDM system. Fourcommon interpolation algorithms are compared theoretically and experimentally. Thetheoretical analysis and experimental results both show that the channel estimation based oncubic smoothing spline interpolation has the best performance and stability and does notrequire for evenly distributed pilots.