132-Gb/s Photonics-Aided Single-Carrier Wireless Terahertz-Wave Signal Transmission at 450GHz Enabled by

64QAM Modulation and Probabilistic Shaping

Xinying Li1,2,*, Jianjun Yu1,4, Li Zhao1, Wen Zhou1, Kaihui Wang1, Miao Kong1, Gee-Kung Chang2, Ying Zhang3, Xiaolong Pan3, and Xiangjun Xin3

1Shanghai Institute for Advanced Communication and Data Science, Fudan University, Shanghai 200433, China 2Georgia Institute of Technology, Atlanta, GA 30332, USA (*xinying.li@ece.gatech.edu)

3Beijing University of Posts and Telecommunications, Beijing, China 4ZTE TX Inc., NJ, USA

Abstract: We experimentally demonstrate 132-Gb/s (12-Gbaud) photonics-aided single-carrier PDM-64QAM-PS5.5 THz-wave signal transmission at 450GHz over 20-km fiber-optics and 1.8-m wireless distance with BER under 4×10-2. The employment of probabilistic-constellation-shaping significantly improves transmission capacity and system performance. OCIS codes: (060.0060) Fiber optics and optical communications; (060.5625) Radio frequency photonics.

1. Introduction

On the one hand, the Terahertz-band (THz-band), ranging from 0.3THz to 10THz, inherently has huge available bandwidth, and therefore it is capable of accommodating extremely large mobile data capacity, with a simple system architecture and modulation format. Also, the THz-band antenna has a small and compact size, and therefore its monolithic integration with other front-end circuits is easy to implement. On the other hand, the THz-band has large atmospheric attenuation and the wireless Terahertz-wave (THz-wave) signal transmission over air space has very limited distance. Therefore, the research community is paying more attention to the application of the THz-band to indoor short-range WPANs and WLANs [1-5]. Compared to bandwidth-limiting all-electric technology, integrated photonics technology is more suitable for the generation, modulation, and detection of the high-carrier-frequency THz-wave signal carrying large-capacity mobile data [6]. Quite a few experimental demonstrations have verified that wireless MIMO can be seamlessly integrated with the technique of optical polarization multiplexing, to effectively double the wireless transmission capacity [1,7,8]. Also, compared to QPSK and 16QAM vector modulation, higher-level 64QAM vector modulation with a higher spectral efficiency can better boost the transmission capacity within a given signal bandwidth. The reported photonics-aided THz-wave signal systems, however, typically employ a SISO wireless transmission link [2-5] or lower-level QPSK/16QAM modulation [1-4]. Ref. 5 experimentally demonstrated a 64QAM-OFDM terahertz communication link, but with a low wireless transmission capacity of 59Gb/s and a very short wireless transmission distance of only 5cm. Recently, the technique of probabilistic constellation shaping (PS) has been intensively studied and well verified to be able to bring significant performance gains with extended distance or increased capacity [9]. Therefore, it is interesting to investigate the seamless integration of the aforementioned integrated photonics technology, wireless MIMO, high-level 64QAM modulation, and PS, to realize large-capacity THz-wave signal wireless transmission.

In this paper, employing integrated photonics technology, wireless 2×2 MIMO, high-level 64QAM modulation, and PS, we experimentally demonstrate 132-Gb/s single-carrier THz-wave signal transmission at 450GHz over 20-km SMF-28 and 1.8-m wireless distance, with a BER under the SD-FEC threshold of 4×10-2. The employment of PS significantly improves transmission capacity and system performance. To the best of our knowledge, this is the first time to realize >100-Gb/s single-carrier 64QAM modulated wireless THz-wave signal transmission.

2. Experimental setup

Fig. 1 gives the experimental setup of our demonstrated photonics-aided 2×2 MIMO wireless transmission system at THz-band. In our demonstrated system, 450-GHz THz-band carrier, carrying normal PDM-64QAM signal or PDM-64QAM-PS5.5 signal (5.5-bit/symbol/polarization), is generated by polarization-diversity photonic remote heterodyning of two CW lightwaves with 450-GHz frequency spacing. Two free-running ECLs, i.e., ECL1 at the optical transmitter end and ECL2 at the wireless transmitter end, are used to generate the two CW lightwaves.

At the optical transmitter end, we use a 64-GSa/s DAC to generate an electrical six-level signal, employing normal 64QAM or 64QAM-PS5.5 modulation, which is then boosted by two parallel EAs. Fig. 1(a) gives a schematic illustration for the PS of the 64QAM signal. The in-phase and quadrature components of the transmitted 64QAM signal can be considered as two independent PAM signals, and the levels of each PAM signal are distributed with non-equal probabilities following the Maxwell-Boltzmann distribution [10]. From the point of view

of the 64QAM signal constellation, the inner constellation points with lower energy are transmitted with a higher probability than the outer constellation points with higher energy. In our transmitter DSP, the PAM level distribution for 64QAM-PS5.5 is [0.41, 0.32, 0.19, 0.08]. Then, with the aid of an I/Q modulator cascaded with a polarization multiplexer, we use the boosted electrical six-level signal to modulate the CW lightwave generated from ECL1, to realize optical PDM-64QAM modulation. The I/Q modulator has 2.3-V half-wave voltage at 1GHz and 32-GHz 3-dB optical bandwidth. We insert a PM-EDFA between the I/Q modulator and polarization multiplexer to compensate for modulation and insertion losses. Then, we deliver the generated optical PDM-64QAM baseband signal over 20-km SMF-28, with 17-ps/km/nm CD at 1550nm.

At the wireless transmitter end, after passing through a polarization controller (PC), the received optical baseband signal is processed by optical polarization diversity operation based on an optical LO source (i.e., ECL2), a PBS, and three PM-OCs. Here, the PBS completely separates the X- and Y-polarization components of the received optical baseband signal. Then, the generated X- and Y-polarization optical THz-wave signals are boosted by two parallel EDFAs, passes through two parallel PCs, and finally converted by two parallel NTT Electronics antenna-integrated photomixer modules (AIPMs, IOD-PMAN-13001) into two electrical THz-wave signals, which can be considered as an electrical THz-wave signal employing PDM-64QAM modulation. Each AIPM, with a typical output power of -28dBm and an operating frequency range from 300GHz to 2500GHz, integrates a UTC-PD and a bow-tie or log-periodic antenna. Note that an ideal photomixer used to detect the optical polarization-multiplexing signal should be polarization insensitive since the optical polarization-multiplexing signal contains two signal components at orthogonal polarization (X- and Y-polarization). However, the fiber pigtail of the THz-band photomixer used in our experiment is polarization-maintaining fiber. In our experiment, we add a PC before each THz-band photomixer to adjust the polarization direction to get the maximal output from each photomixer.

LO

Signal

450GHz

(a) (b)

Port 1

Port 2

Port 3

Port 4

DSO

64GSa/s120GSa/s

ECL1

DAC(I) (Q)

I/Q MOD

EAEA PM-EDFA Pol.

Mux

EDFA2

X20km

SMF-28 LNA

HA

17.78cm

IMAMCAIPM

×36

RF

Y LNAHA

SAX

AIPM ×48

RF

Lens 1 Lens 3

X

YLens 2 Lens 4

2x2 MIMO at THz-band

17.78cm

EDFA3

Optical transmitter end Wireless transmitter end Wireless receiver end

PC

PC

PCECL2Y

XPBSPM-OC

PM-OC

PM-OC17.78cm

17.78cm

1.8m

Then, we deliver the 450-GHz electrical THz-wave signal over a 1.8-m 2×2 MIMO wireless THz-wave

transmission link. In our wireless transmission link, X- and Y-polarization wireless transmission links are parallel, and two pairs of lens are used to focus the wireless THz-wave signal to maximize the received wireless power by the wireless receiver end. The center of lens 1 and 3 is aligned with the X-polarization wireless transmission link, while that of lens 2 and 4 is aligned with the Y-polarization wireless transmission link. All lens are identical, and each of them has 10-cm diameter and 20-cm focal length. Each lens is separated from its corresponding horn antenna (HA) by 17.78cm. The insertion loss of each lens is smaller than 0.1dB.

At the wireless receiver end, we receive the wireless THz-wave signal with two parallel 26-dBi HAs, each operating within a THz-wave frequency range from 330GHz to 500GHz. For X-polarization signal, we utilize a VDI integrated mixer/amplifier/multiplier chain (IMAMC), driven by a 12.008-GHz sinusoidal LO source, to implement analog down conversion. The IMAMC, integrating a mixer, an amplifier, and a ×36 frequency multiplier, has an operating frequency range from 330GHz to 500GHz. Here, the LO frequency used to drive the mixer is therefore

Fig. 1. Experimental setup for 132-Gb/s single-carrier THz-wave signal transmission at 450GHz over 20-km SMF-28 and 1.8-m wireless distance. (a) Principle of probabilistic constellation shaping. (b) Measured X-polarization optical THz-wave signal spectrum (0.02-nm

resolution) after optical polarization diversity.

36×12.008=432.288GHz. Then, the down-converted 18-GHz X-polarization IF signal is boosted by a LNA with 40-dB gain, 14-dBm saturation output power, and 4~18-GHz operating frequency range. For Y-polarization signal, we utilize a VDI spectrum analyzer extender (SAX, WR2.2SAX), driven by a 9.006-GHz sinusoidal LO source, to implement analog down conversion. The SAX, integrating a mixer and a ×48 frequency multiplier, has an operating frequency range from 330GHz to 500GHz and an about 16-dB intrinsic mixer SSB conversion loss. Here, the LO frequency used to drive the mixer is therefore 48×9.006=432.288GHz, which is equal to the LO frequency used to drive the mixer at X-polarization. Then, the down-converted 18-GHz Y-polarization IF signal is boosted by a LNA with 50-dB gain, 15-dBm saturation output power, and 7~16-GHz operating frequency range. Here, Note that, due to the lack of available components, we use different analog down-converters and LNAs for X- and Y-polarization signals. Then, we use two 120-GSa/s ADC channels of a digital storage oscilloscope (DSO) to capture the X- and Y-polarization IF signals. Each 120-GSa/s ADC channel has 45-GHz electrical bandwidth. The subsequent offline DSP includes down conversion to baseband, CMMA equalization, carrier recovery, DD-LMS equalization, and BER calculation [9]. Fig. 1(b) gives the measured X-polarization optical THz-wave signal spectrum (0.02-nm resolution) after optical polarization diversity corresponding to 11-Gbaud normal 64QAM signal.

3. Experimental results

We measured the system BER performance versus the input power into each AIPM as given in Fig. 2(a). We can see that, compared to normal 64QAM, 64QAM-PS5.5 has a much better BER performance within a much larger OSNR range under identical bit rate. Up to 12-Gbaud (12×5.5×2=132-Gb/s) PDM-64QAM-PS5.5 signal at 450GHz can be delivered over 20-km SMF-28 and 1.8-m wireless distance with a BER under the SD-FEC threshold of 4×10-2. After removing 27% SD-FEC threshold, the total bit rate of 132Gb/s corresponds to a net bit rate of 103.9Gb/s.

4x10-2

(b)

(c)

(d)

(e)

Wireless transmitter end

Wireless receiver end

(a) (f)

1.8m

Figs. 2(b) and 2(c) give the captured 18-GHz IF signal spectrum and the recovered Y-polarization constellation

for 8-Gbaud normal PDM-64QAM signal transmission with 16-dBm input power into each AIPM and a BER of 2.6×10-2, while Figs. 2(d) and 2(e) give those for 9-Gbaud PDM-64QAM-PS5.5 signal transmission with 15.4-dBm input power and a BER of 1.2×10-2. Fig. 2(f) gives the photo of our 1.8-m wireless THz-wave transmission link.

4. Conclusions

We experimentally demonstrate a photonics-aided vector THz-wave signal wireless delivery system at 450GHz, which realizes 132-Gb/s (12-Gbaud) single-carrier PDM-64QAM-PS5.5 THz-wave signal delivery over 20-km fiber and 1.8-m wireless distance with a BER under 4×10-2. The employment of advanced DSP techniques, including probabilistic shaping, significantly improves transmission capacity and enhances air distance performance for 5G new radio mobile data communications. References [1] X. Li et al., “120Gb/s wireless Terahertz-wave signal delivery by 375GHz-500GHz multi-carrier in a 2×2 MIMO system,” OFC 2018, M4J.4. [2] S. Jia et al., “120 Gb/s multi-channel THz wireless transmission and THz receiver…,” Photon. Technol. Lett. 29, 310 (2017). [3] X. Pang et al., “Single channel 106 Gbit/s 16QAM wireless transmission in the 0.4 THz band,” OFC 2017, Tu3B.5. [4] P. T. Dat et al., “Millimeter- and terahertz-wave radio-over-fiber for 5G and beyond,” IEEE SUM, San Juan, Puerto Rico, 165 (2017). [5] M. F. Hermelo et al., “Spectral efficient 64-QAM-OFDM Terahertz communication link,” Opt. Express 25, 19360 (2017). [6] A. Nirmalathas et al., “Multi-gigabit indoor optical wireless networks-Feasibility…,” IEEE SUM, San Juan, Puerto Rico, 130 (2016). [7] J. Yu et al., “Faster than fiber: over 100-Gb/s signal delivery in fiber wireless integration system,” Optics Express 21, 22885 (2013). [8] R. Puerta et al., “Demonstration of 352 Gbit/s photonically-enabled D-band wireless delivery in one 2×2 MIMO …,” OFC 2017, Tu3B.3. [9] X. Li et al., “1-Tb/s Photonics-aided vector millimeter-wave signal wireless delivery at D-band,” OFC 2018, Th4D.1. [10] F. Buchali et al., “Rate adaptation and reach increase by probabilistically shaped 64- QAM…,” J. Lightw. Technol. 34, 1599-1609 (2016).

Fig. 2. (a) BER performance comparison between normal 64QAM and 64QAM-PS5.5 under identical bit rate. Captured 18-GHz IF signal spectrum and recovered Y-polarization constellation: (b) and (c) 8-Gbaud normal PDM-64QAM signal transmission; (d) and (e) 9-Gbaud

PDM-64QAM-PS5.5 signal transmission. (f) Photo of our 1.8-m wireless THz-wave transmission link.