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In this paper, an improved channel estimation scheme based on time-domain orthogonal gray complementary training sequence (Golay TS) is proposed to resist subcarrier mutual beat interference in 16-quadrature amplitude modulation multiband orthogonal-frequency-division-multiplexing ultrawide band over fiber (16QAM MB-OFDM UWBoF) systems. The simulation results showed that the performance of system with the improved Golay TS channel estimation scheme was improved by about 1 dB compared with the traditional Golay TS channel estimation scheme, at a bit error rate (BER) of 3.8 × 10^{−3} after 70 km standard single mode fiber (SSMF) transmission. At the same time, the synchronization and channel estimation performance of Golay TS and the selected frequency-domain orthogonal training sequences (TSs) were compared. The simulation results showed that the performance of system with the TSs channel estimation was improved by about 2.7 dB compared with the improved Golay TS channel estimation.

As we all know, symbol timing synchronization is a key step that must be performed before signal demodulation at the receiving end of the signal. Therefore, the symbol timing synchronization step is very important [

Recent studies have shown that the more commonly used channel estimation schemes are blind estimation and data-assisted estimation [

This paper proposed joint symbol synchronization and channel estimation scheme based on the same training sequence and improved the channel estimation scheme for ISMI. Simulation results showed that the improved Golay TS channel estimation scheme improved receiver sensitivity by about 1 dB compared with the previous scheme, but TSs played the best channel estimation performance.

The MB-OFDM UWBoF signal is obtained from the baseband orthogonal-frequency-division-multiplexing (OFDM) signal after digital processing, and it is given by

After the PD at the receiving end is photoelectrically converted and filtered, the resulting electrical MB-OFDM UWB signal can be further simplified as

It can be seen from the above formula that the received signal is composed of three parts:

The Golay TS synchronization scheme adopted in this article refers to the method used by He [

Considering that the MB-OFDM UWB signal at the receiving end can introduce the effect of ISMI interference after PD conversion and in the case of a small signal, other odd-numbered high-order interferences can be ignored [

For TSs, it is a training sequence constructed to resist ISMI interference that is introduced by the device characteristics of MZM and PD. Its special structure determined the effect of suppressing ISMI interference. Given that the ISMI interference can be introduced after passing through the PD which only fell on even-numbered subcarriers, the channel obtained from the training sequence may estimate that its odd-numbered subcarriers will not be affected by ISMI interference. Therefore, the channel estimation based on the TSs can be easily obtained.

After symbol synchronization [

At that time, the channel response function based on TSs channel estimation can be expressed as

In general, the training sequence is accompanied by many MB-OFDM UWB data symbols, and the system overhead brought by training sequence can almost be negligible. Therefore, the scheme of joint synchronization and channel estimation based on a single training sequence has the advantages of low complexity and high accuracy, so it is suitable for such quasistatic channels of optical fiber systems.

The simulation setup of the MB-OFDM UWBoF transmission system based on the joint synchronization and channel estimation scheme of a single training sequence is shown in Figure

Simulation diagram of the MB-OFDM UWBoF transmission system based on a single training sequence scheme.

The main parameters of the transceiver based on a single training sequence scheme.

Parameter name | Parameter value |
---|---|

Single OFDM symbol duration | 312.5 ns |

FFT/IFFT size/number of subcarriers | 128 |

CP and ZPS | 37 (=16 + 21) |

Data/pilot/null subcarrier/guard interval | 100/12/6/10 |

Bandwidth (relative to one of the sub-bands) | 528 MHz |

Number of TS/OFDM symbols | 3/312 |

DAC and ADC sampling rate | 10.561GSps and 20GSps |

Modulation format | 16QAM |

The total number of bits per OFDM symbol | 16QAM: 100 ∗ 4 = 400 bits |

Original signal bit rate | 16QAM: log2 (16) ∗ (1/312.5) ∗ 100 = 1.28 Gbps |

Signal net bit rate | 400 ∗ 312/ (315 ∗ 165 ∗ 20/10.561) = 1.268 Gbps |

Optical spectrum efficiency | 1.268/ (100/128 ∗ 0.528) = 3.074 bit/s/Hz |

At the receiver, the captured signal was exported through the MATLAB output window for offline DSP processing. MB-OFDM UWB digital signals were digitally downconverted to baseband signals. Training sequences were used to achieve symbol synchronization, CP data removal, and OFDM signal demodulation. After that, according to the scenario of (

We compared the synchronization performance of Golay TS and TSs. As shown in Figure

(a) MB-OFDM UWB signal frame timing synchronization metrics based on Golay TS scheme. (b) TS-based timing synchronization metrics for MB-OFDM UWB signal frames.

In terms of channel estimation, for the 16QAM MB-OFDM UWBoF system, the BER performance of the channel estimation based on three training sequences was simulated after 70 km SSMF transmission. It can be seen from Figure ^{−3}, the receiver sensitivity of the signal was improved by about 1 dB compared with the original Golay TS auxiliary channel estimation to scheme by using the improved Golay TS auxiliary channel estimation scheme in the 16QAM MB-OFDM UWBoF system. Compared with the improved Golay TS auxiliary channel estimation scheme, the receiver sensitivity of the 16QAM MB-OFDM UWB signal was improved by about 2.7 dB when using the TSs auxiliary channel estimation scheme.

BER performance based on channel estimation of three training sequences.

After the simulation through 70 km SSMF transmission, the 16QAM MB-OFDM UWB signal was based on the constellation diagram of the three training sequence channel estimates when the received optical power was −16 dBm. Figure

Constellation diagram based on channel estimation of three training sequences.

The channel estimation scheme proposed in this paper adopted only odd-numbered subcarrier data. Then, interpolates were used to obtain the channel estimation scheme for all subcarriers. Since the ISMI interference was suppressed, the performance of channel estimation was significantly improved. The simulation results showed that the improved scheme could increase the receiver sensitivity of the signal by around 1 dB when using Golay TS-assisted channel estimation. The receiver sensitivity of TSs channel estimation was improved by about 2.7 dB compared with the improved Golay TS channel estimation.

No data used to support the findings of the study.

The authors declare that they have no conflicts of interest.

The authors really appreciate the support by the National Natural Science Foundation of China (61861019), the Natural Science Foundation of Hunan Province of China (2019JJ50483), the project of Hunan Provincial Department of Education (18B316 and 18C0588), and the Scientific Research Foundation of Hunan Province Higher Education Key Laboratory of Modeling and Monitoring on the Near-Earth Electromagnetic Environments (N201901).