Authorized licensed use limited to: Huazhong University of Science and Technology. Downloaded on August Hybrid Digital-Analog Radio-over-Fiber (DA-RoF) Modulation and Demodulation Achieving a SNR Gain over Analog RoF of >10 dB at Halved Spectral Efficiency Xiang Liu Futurewei Technologies Bridgewater USA xiang.liu@futurewei.com 2021 30 2023

We propose and experimentally demonstrate a hybrid digital-analog radio-over-fiber technique based on cascaded digital PCS-121-QAM modulation and analog pulse code modulation for each radio waveform sample, achieving 12.8 dB SNR gain over A-RoF and 1.38% EVM. © 2021 The Author(s) OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation; (060.4230) Multiplexing. Digital radio-over-fiber (D-RoF) based on the common public radio interface (CPRI) is a well-established technique to support mobile fronthaul in cloud radio access network (C-RAN) [1-3]. The original D-RoF transmission using binary modulation is highly bandwidth-inefficient as compared to analog radio-over-fiber (A-RoF) techniques [4-8]. On the other hand, the error vector magnitude (EVM) performance of A-RoF needs to be much improved, especially for high-speed RoF, to support 5G wireless signals with 256-QAM and 1024-QAM formats [9]. Recently, much effort has been made to improve the EVM performance of A-RoF by trading its bandwidth efficiency, e.g. via various phase modulation (PM) induced bandwidth expansion techniques [10-13]. Digital SNR adaptation of A-RoF carrying up to 1048576-QAM signals was demonstrated with PM, achieving a scaling of 6 dB SNR gain for each doubling of bandwidth, or halving of spectral efficiency (SE) [13]. In parallel, it was recently found that the theoretical SE of DRoF can approach that of A-RoF when Shannon capacity approaching techniques are applied [14]. Moreover, the recovered SNR in D-RoF increases exponentially with the link bandwidth, offering a scaling of doubled SNR in dB for each halving of SE. It was experimentally demonstrated that at a halved SE, D-RoF achieved a SNR gain of ~8.2 dB over A-RoF and an output EVM of 3.13% for a CPRI-equivalent data rate of about 60 Gb/s [14]. In this paper, we propose a novel hybrid digital-analog radio-over-fiber (DA-RoF) technique based on cascaded digital probabilistic constellation shaped (PCS) n-QAM modulation and analog pulse code modulation (PCM), where the digital PCS-n-QAM signal provides a natural approximation of the wireless signal waveform and the PCM provides the analog representation of the approximation error with an appropriate magnification to effectively increase the SNR of the received RoF signal. We further experimentally demonstrate this DA-RoF technique in an 8-Gbaud single-carrier transmission experiment using a cost-effective 10-GHz-bandwidth intensity-modulation and directdetection (IM-DD) system, achieving a SNR gain of 12.8 dB over A-RoF and an output EVM of 1.38% for a CPRIequivalent data rate of 160 Gb/s. Figure 1 shows the fronthaul architecture based on CPRI and the modulation schemes for CPRI-based D-RoF, A-RoF, and the proposed DA-RoF. In D-RoF, digital modulation is used for the I/Q bits of all the antenna-carriers (AxCs)

 1. Introduction 2. Principle

with all the bits treated with equal importance. The CPRI control word (CW) bits are also digitally modulated. In the CPRI-compatible A-RoF [7], PCM is used to provide an analog representation of the wireless signal from each AxC with the I/Q bits sorted from the most significant bit (MSB) to the least significant bit (LSB), while the CW bits are digitally modulated. In the proposed CPRI-compatible DA-RoF, a given analog RoF signal, S, is represented as S=W1+W2, where W1 is a digital n-QAM signal for a natural approximation of S, and W2 is an analog signal representing the approximation error. In this paper, we use 121-QAM for W1, which is expressed as W1 =round(5S/Emax)Emax/5, where round() is a rounding function that rounds a complex number to its nearest Gaussian integer and Emax is the maximum amplitude of S (set by a suitable clipping). For a typical wireless signal with OFDM modulation, its time domain samples are complex Gaussian distributed, so its n-QAM approximation is naturally a PCS constellation. The analog signal W2 is magnified before being modulated via PCM to effectively increase its SNR. The CW bits are digitally modulated onto a 16-QAM control signal (CS). Fig. 2(a) shows the DSP flow diagram for the DA-RoF modulation. Proper normalization of the incoming RoF waveform is done to realize PCS-121-QAM with an appropriate entropy for W1. The analog signal W2 is magnified by a factor of (c2/c1) with respect to W1 before being modulated via PCM to effectively increase its SNR. Fig. 2(b) shows the DSP flow diagram for the DA-RoF demodulation. After RoF transmission, the W1 portion of the received RoF waveform is demodulated back to the original PCM-121-QAM, while the magnified W2 portion is shrunk by the same factor of c2 to its original amplitude. Then, the analog signal S is reconstructed by summing the recovered W1 and W2. The other DSP processes are similar to those described in Ref. [7].

(a) CPRI-compatible DA-RoF modulation (b) CPRI-compatible DA-RoF de-modulation

Channel #1 (IQ and CW) Channel #N (IQ and CW)

… … I/Q f itraaono andCW tdaa CW epS IQ I/Q f itraaono andCW tdaa CW epS IQ

S=I+jQ n o it a z li a m r o n d n a g ippn CW a M )( d n u o R -16AQM .doM W1 W2 CS 1 W S 1 c 2 c 3 c c1W1 c2W2 c3CS ) M D T ( g n i x e lilt p u m n i a m o d e m i T DA-RoF Signal c1W1 ) c n (yS c2W2 n o it a z i n o r h ycnS c3CS ) Q E ( n o it a z il a u q E 1 c  3 c  2 W2 c  CS ) W1 ( d n u o R +WW12S =

S . CW AM od -Q -M 16 eD I/Q CW g n i p p a m e D I/Q CW iton CW inba nad oCm IfoQ iton CW inba nad oCm IfoQ

 Channel #1 (IQ and CW)

… …

 Channel #N (IQ and CW) EQ update

3. Experimental setup and results Figure 3 shows the experimental setup, which is similar to that in Ref. [7]. The key difference is that the DA-RoF modulation and de-modulation are now used in the transmitter and receiver signal processing. The digital PCS-121QAM has an entropy of 5.12 bits/symbol. The original and recovered constellation diagrams of the RoF waveform, its digital PCS-121-QAM part, and its analog PCM part are added as insets to illustrate the DA-RoF modulation and de-modulation. The recovered DA-RoF signal spectra before and after fiber transmission (with 17 ps/nm dispersion) at -8 dBm received optical power (RoP) are shown in inset (d). Representative recovered constellation diagrams of the 16-QAM CW signal and the 64-QAM wireless signal after fiber transmission are shown in insets (h) and (i), respectively. The EVM of the wireless signal is much reduced compared to that in A-RoF. Fig. 4 (a) shows the SNR of the recovered wireless signal as a function of the RoP. At a RoP between -8 dBm and -2 dBm, DA-RoF provides a SNR gain between 10.5 dB and 12.8 dB, respectively. Fig. 4 (b) shows the EVM performance of the recovered wireless signal. At a RoP between -8 dBm and -2 dBm, the EVM for the DA-RoF case is reduced substantially to 2.28% and 1.38%, respectively. Representative constellation diagrams of the recovered wireless signals with 64-QAM and 1024QAM subcarrier modulations are shown as insets in Fig. 4 to indicate the dramatic performance improvements enabled by DA-RoF. The largest 5G wireless signal constellations used so far are 256-QAM and 1024-QAM, which require an EVM of below 3.5% and 2.5%, respectively [9,13]. Thus, the DA-RoF technique enables the support of these large constellations with additional margin for signal degradations occurred outside the fronthaul segment.

It is worth noting that the SNR gain provided by DA-RoF is at the expense of reduced SE. In the case of CPRIcompatible DA-RoF, the CPRI-equivalent data rate for the 8-Gaud signal is 160 Gb/s, which is 62.5% of that of CPRIcompatible A-RoF (256 Gb/s) [7]. In the absence of the CS, the SE of DA-RoF is 50% of that of A-RoF, because each complex sample of an A-RoF waveform is represented by two samples in DA-RoF, one in W1 and the other in W2. Thus, DA-RoF is capable of achieving a SNR gain over A-RoF of >10 dB at halved SE. As compared to the capacityapproaching D-RoF [14], the DA-RoF shows superior performance and does not require computation-intensive FEC, so energy-efficient and low-latency RoF transmission can be readily supported. Moreover, SNR adaptation can be flexibly achieved by adjusting the PCS-n-QAM entropy and the PCM magnification. Furthermore, more digital modulations can be cascaded to further trade SE for signal fidelity.

 4. Conclusion

We have proposed a DA-RoF technique based on cascaded digital PCS-n-QAM for naturally approximating the RoF waveform and analog PCM for representing the approximation error. We have further experimentally demonstrated this DA-RoF technique in an 8-Gaud IM/DD system, achieving a SNR gain of 12.8 dB over A-RoF and an output EVM of 1.38% for a CPRI-equivalent data rate of 160 Gb/s. With high-bandwidth dual-polarization coherent modulation/detection, this DA-RoF technique may enable the transmission of multi-Tb/s CPRI equivalent data rate per wavelength with high fidelity, low power consumption, and low latency for future 5G-and-beyond applications. 5. References

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