Radio over fiber (RoF) technology has been extensively studied due to its numerous advantages, such as broad bandwidth, low attenuation, and flexibility for wireless links in recent years. RoF combined with optical remote heterodyne technology[
Chinese Optics Letters, Volume. 17, Issue 1, 010605(2019)
Application of Kramers–Kronig receiver in SSB-OFDM-RoF link
We propose and investigate the use of a Kramers–Kronig (KK) receiver in a single sideband orthogonal frequency division multiplexing radio over fiber (SSB-OFDM-RoF) link based on an optical remote heterodyne solution. This scheme is effective in eliminating the signal-to-signal beating interference introduced by square-law detection of a photo-detector in an SSB-OFDM-RoF link. We extensively study the influences of different carrier-to-signal power ratios (CSPRs), laser linewidths, and transmission distances on our proposed scheme. It is proved that the KK-based receiver can reduce optimal CSPR by more than 5 dB and provide about 1.1 dB gain over the conventional mixer-based receiver scheme with CSPR of 11 dB after 75 km fiber transmission.
Radio over fiber (RoF) technology has been extensively studied due to its numerous advantages, such as broad bandwidth, low attenuation, and flexibility for wireless links in recent years. RoF combined with optical remote heterodyne technology[
In OFDM-RoF systems, as is well known, heterodyne detection has a simpler structure and lower cost than coherent detection based on a balanced photo-detector (PD) because it requires only one single-ended PD. However, the signal-to-signal beating interference (SSBI) will be introduced by square-law detection of the PD. Without SSBI cancellation processing, the SSBI may deteriorate the system performance considerably. Generally, there are two common methods to obtain intermediate frequency or baseband from the RF signal; one is using a power detector, such as a Schottky diode[
In this Letter, we propose and investigate the application of the KK receiver in an SSB-OFDM-RoF link based on optical remote heterodyne solution. This scheme can accurately reconstruct the complex baseband signal in the digital domain from the RF signal, while effectively removing the SSBI term. Since the perturbation to the signal introduced by the nonlinear SSBI term is eliminated by KK operation, further signal processing, like electrical chromatic dispersion compensation (ECDC), can be performed at the receiver. The key advantages of the proposed KK receiver-based SSB-OFDM-RoF scheme are listed in the following: (1) simplify the system structure and reduce the complexity of transceiver, since some hardware devices, such as power detector, electric mixer, and microwave LO, are avoided; (2) significantly improve the system performance and effectively eliminate the SSBI term introduced by square-law detection of the PD.
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The schematic of the proposed optical remote heterodyne SSB-OFDM-RoF link based on the KK receiver is shown in Fig.
Figure 1.Schematic of the proposed optical remote heterodyne SSB-OFDM-RoF link based on the KK receiver. S/P, serial-to-parallel; P/S, parallel-to-serial; DAC, digital-to-analog converter; PC, polarization controller; OC, optical coupler; OS, optical splitter; EDFA, erbium-doped optical fiber amplifier; ADC, analog-to-digital converter.
In order to reach the minimum phase condition for the OFDM signal, the CSPR should be larger than the peak-to-average power ratio (PAPR) of the information-bearing signal[
After OCS modulation and coupling with an optical carrier, the output optical SSB-OFDM signal is given by
After fiber transmission and square-law detection, the photocurrent signal can be written as
Afterwards, the photocurrent signal is amplified and fed to a pair of antennas. This process can be modeled as band-pass filtering (BPF), where the DC and part of the SSBI outside the operating band of antennas are removed [as depicted in Fig.
Figure
Figure 2.Signal processing flow of the KK receiver. The baseband signal can be extracted from the RF signal by the KK receiver; meanwhile, the SSBI is effectively eliminated.
In theory, the KK receiver can accurately recover the baseband signal from the RF signal, and the SSBI introduced by the PD’s square-law detection, as mentioned above, can be effectively alleviated.
In the following, we evaluate the performance of the proposed optical remote heterodyne SSB-OFDM-RoF link base on the KK receiver, utilizing co-simulation through industry standard VPI Transmission Maker (VPI TM 9.1) and MATLAB. The simulation setup is the same as Fig.
By setting the peak threshold to be six times that of the average power of an unclipped OFDM signal, the PAPR reduces from the original 11.2 dB to 7.8 dB after CFR clipping. With an optimized optical modulation index of 0.11 and launch power of 0 dBm, the signal-to-noise ratio (SNR) versus different CSPRs is evaluated for back-to-back (B2B) transmission, as shown in Fig.
Figure 3.SNR versus the CSPR, where the launch power remains constant and the ROP is fixed at 0 dBm for B2B transmission.
We further investigate the effect of laser linewidths on the bit error rate (BER) versus received optical power (ROP) for 16-QAM and 64-QAM signals after 75 km SSMF transmission using the optimized CSPR. As shown in Fig.
Figure 4.BER versus ROP with different laser linewidths for 16-QAM and 64-QAM signals after 75 km SSMF transmission.
Figure
Figure 5.BER versus ROP curves with different SSMF lengths with and without CDC for 16-QAM signal.
Lastly, we compare the BER performance of a 94 Gb/s SSB-OFDM signal between the KK scheme and the conventional mixer-based receiver scheme for different CSPRs after 75 km SSMF transmission. Since the 16-QAM OFDM subcarriers closer to the optical carrier are deteriorated by the SSBI, the benefit of the KK receiver scheme is notably superior than that of the mixer-based scheme, as depicted in Fig.
Figure 6.BER performance comparison of the KK scheme and mixer-based scheme with 94 Gb/s 16-QAM SSB-OFDM signal. Constellation diagrams for the mixer-based scheme with CSPR of (a) 7 dB, (c) 11 dB, (e) 15 dB, and KK scheme with CSPR of (b) 7 dB, (d) 11 dB, (f) 15 dB at ROP of
The effective number of bit (ENOB) of the digital-to-analog converter (DAC) and analog-to-digital converter (ADC) is an important parameter to consider in practice. Moreover, the performance of the KK scheme is sensitive to the ENOBs, so we further study the impact of the ENOBs of the DAC and ADC with the results given in Fig.
Figure 7.BER versus ROP with different ENOBs of DAC and ADC.
We have investigated and demonstrated an SSB-OFDM-RoF link employing the optical remote heterodyne technique with a KK receiver. The obtained results show that the CSPR, laser linewidths, ENOBs of the ADC and DAC, and resampling rate before the KK algorithm play important roles on the signal’s transmission performance. With the help of post-CDC after the KK algorithm, no power penalty is observed after even 100 km SSMF transmission in our proposed SSB-OFDM-RoF scheme. In addition, the benefit of the KK receiver scheme is notably superior than that of the conventional mixer-based scheme, attributing to the effective elimination of SSBI by the KK algorithm.
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Yuancheng Cai, Yun Ling, Xiang Gao, Bo Xu, Kun Qiu, "Application of Kramers–Kronig receiver in SSB-OFDM-RoF link," Chin. Opt. Lett. 17, 010605 (2019)
Category: Fiber Optics and Optical Communications
Received: Sep. 21, 2018
Accepted: Nov. 23, 2018
Posted: Dec. 3, 2018
Published Online: Jan. 17, 2019
The Author Email: Yun Ling (yling@uestc.edu.cn)