The Research For The Key Technology Of DD-OOFDM System Based On The Single-sideband Modulation

With the continuous improvement of information technology, the demand for information transmission capacity of human society is growing at an alarming rate. The optical orthogonal frequency division multiplexing (OOFDM) system has the advantages of the high spectral efficiency, the dispersion tolerance, the easy dynamic channel estimation and compensation etc. It can dynamically change the number of sub carriers, resource allocation and modulation mode, realize the smooth upgrade of the transmission rate of optical communication system, greatly reduced optical communication system upgrades and expansion caused by the renewal of equipment and labor costs. So it is a very promising technique for high speed optical communication system for the future.Compared with the coherent optical OFDM (CO-OFDM) system,the direct detection OOFDM (DD-OOFDM) system has the advantages of the simple structure, the low cost, the insensitive frequency offset and phase noise, so it is more suitable for the optical access network and long distance optical fiber transmission system with the sensitive cost. The optical single sideband (OSSB) modulation technique can effectively overcome the traditional double sideband (DSB) caused by the system dispersion fading effect, so the SSB-OOFDM system becomes the more excellent scheme in the DD-OOFDM system with the more practical application value. At this time, the signal to signal beat interference(SSBI) becomes a major factor which influences the transmission performance of the DD-SSB-OOFDM system. Therefore, how to overcome the influence of SSBI effectively, reduce or even eliminate the guard band (GB) between the optical carrier and OOFDM sideband, to improve the system spectrum efficiency, and reduce the transmitter and receiver electronics device bandwidth burden and the cost of equipment,is the key to improve the performance of DD-OOFDM system. Taking the starting point of the DD-SSB-OOFDM system, this paper proposes the solutions of effective SSBI elimination, which will simplify the optical fiber network structure, reduce the laying of fiber network as well as the maintenance cost, and the actual design and application of optical access network system has the important reference significance.Firstly, this paper introduces the development history, the research significance as well as the advantages and disadvantages of DD-OOFDM technology in the optical communication. Then, we introduce the principle of modulation and demodulation of the baseband OFDM signal,analysis the basic principle of generating SSB-OOFDM signal based on a Mach-Zehnder modulator (MZM) and a polarization modulator (PolM),respectively, and the principle of generation and elimination solutions of SSBI in the DD-OOFDM system. Finally, on the basis of DD-OOFDM system, we propose the beat interference cancellation receiver (BICR)structure and system link scheme with the relatively simple structure, the low cost, and the good performance based on the two OSSB modulation formats. The detailed theoretical analysis and the deep link simulation system is conducted for the proposed scheme. This dissertation will mainly study the influence of SSBI on DD-OOFDM system from two aspects below:(1) We present a new method to implement BICR for the SSB-OOFDM signal, which improves the spectral efficiency by reducing the GB between the optical carrier and the OOFDM signal while mitigating the impact of SSBI. This BICR structure is relatively simple using only an optical interleaver (IL), a 3×3 optical coupler (OC) and three photodiodes (PDs). A system simulation for the 40Gbit/s 16-QAM SSB-OOFDM signal with the reduced GB is carried out to prove the feasibility of this new scheme. The simulation results show that our proposed BICR has a better performance to suppress SSBI and a higher spectral efficiency than the conventional direct-detection receiver (CDDR)according to the EVMs and the constellations.(2) We have proposed a novel OOFDMlink scheme with the colorless BICR structure for the SSB-OOFDM signal with orthogonally polarized optical carrier and sideband, which is generated by using a polarization modulator and an optical band-pass filter (OBPF). The BICR,employing only a polarization beam splitter (PBS) and a balanced photodiode pair (BPD), can colorlessly mitigate the SSBI induced by the square-law detection in a photodiode and thus the spectral efficiency is improved by reducing the GB between the optical carrier and OOFDM signal. A simulation link for the 40Gbit/s 16-QAM SSB-OOFDM signal with reduced GB is built to demonstrate the feasibility of our proposed scheme. The simulation results indicate that the link has a higher spectral efficiency compared to the conventional intensity modulation and direction detection (IM/DD) scheme and the BICR exhibits a better performance to suppress SSBI according to the error vector magnitude and the constellation diagrams.In summary, the two proposed BICR scheme both can effectively eliminate the SSBI to improve the system spectrum efficiency, and reduce the bandwidth requirements of electronic devices at the transmitter and receiver fronts by reducing the inserted GB. Moreover, in the case of not increasing the system complexity, the performance of DD-OOFDM system is improved effectively.