1. Introduction

High-speed optical wireless integration system in millimeter-wave (mm-wave) bands is being developed to provide multi-megabit ~multi-gigabit mobile applications by making use of the enormous bandwidth of the fiber and the flexibility via wireless delivery. It has been intensively researched due to its advantages of extremely large bandwidth, low transmission loss, availability of optical amplifiers and broad coverage during the past few years [1–14]. Usually, an optical wireless system consists of three modules: central office, base station (BS) and wireless receiver. In order to realize such a system, the generation of high-speed mm-wave wireless signals is very important to match large capacity of the fiber transmission system. Photonic generation of wireless radio frequency (RF) signals has been proposed to integrate wireless and fiber-optic network seamlessly, so that the limitation of capacity by spectrally inefficient on-off keying (OOK) modulation format or complex electronic arbitrary waveform generators could be eliminated. Recently, we have experimentally demonstrated a 100G and a 400G optical wireless integration system by utilizing the technologies of polarization division multiplexing (PDM), photonic mm-wave generation and multiple-input multiple-output (MIMO) [8–10]. In both systems, heterodyning coherent detection has also been introduced into these systems to process the down conversion of an optical signal to a millimeter-wave wireless signal, so advanced digital signal processing (DSP) could be used to reduce bit error ratio (BER) at the receiver. However, in such schemes, the wireless receiver demodulated the high-speed mm-wave signals in the electrical domain directly, and the transmission distance has been limited because there is severe attenuation for atmospheric propagation of the mm-wave signals. Moreover, the demand on electrical devices for demodulation would be higher with the increasing capacity and frequency of the RF signals, making the wireless receiver more complicated [13,14]. has proposed a 100 GHz optical-wireless-over fiber system which is RF transparent, to adopt coherent detection and advanced DSP for mm-wave signals. In this case, the transmission distance could be increased without any relay BSs and only twice baud-rate electrical device is needed for wireless signal demodulation. Furthermore, it is expected to provide high-speed communication under the condition of natural disasters such as earthquake and tsunamis which would cause extensive damage to communication buildings and equipment [12]. However, there is no wireless and long distance fiber transmission, and the demonstrated system is not for bi-directional transmission in [14].

In this paper, we experimentally demonstrate a full-duplex bi-directional transmission optical wireless integration system at W-band (75-100 GHz) with the speed up to 15 Gb/s for both 95.4 GHz link and 88.6 GHz link. In the experiment, the mm-wave wireless signals are generated by heterodyne mixing of an optical quadrature-phase-shift-keying (QPSK) signal with a free-running lightwave at different wavelengths, and demodulation of RF signal is based on coherent detection and advanced DSP. Compared to [14], 2 m wireless delivery and (20 + 20) km single-mode fiber-28 (SMF-28) transmission are deployed in the full-duplex system. And they cause no optical signal-to-noise ratio (OSNR) penalty. With the OSNR of 15 dB, the BERs for 10 Gb/s signal transmission at 95.4 GHz and 88.6 GHz are below the forward-error-correction (FEC) threshold of 3.8 × 10⁻³ whether the post filter is used or not, while the BER for 15 Gb/s QPSK signal employing post filter in the link of 95.4 GHz is 2.9 × 10⁻³. And the BER for 15 Gb/s bi-directional links is below the FEC threshold when the OSNR is above 13.8 dB in the link of 95.4 GHz and 15.8 dB in the link of 88.6 GHz. To our knowledge, this is the first demonstration of a full-duplex bi-directional optical wireless link at W-band and also the first investigation for a bi-directional optical wireless-over 20 km fiber system.

2. Experiment for system frequency response measurement

Figure 1 shows the experimental setup for system frequency response measurement in the full duplex bi-directional optical wireless link. In the experiment, the laser sources are fully tunable C-band external cavity lasers (ECLs) with line-width less than 100 kHz and output power of 13 dBm. At the transmitter central station (CS), the continuous-wavelength (CW) lightwave at λ₁ from ECL₁ is first fed into an in-phase/quadrature (I/Q) modulator to generate 4 Gb/s optical QPSK signal, and then heterodyne beat with the free-running CW lightwave at λ₂ from ECL₂ by employing a polarization maintaining optical coupler (PM-OC), enabling the produced mm-wave at a frequency of $f_{RF}=c|1/{\lambda }_1-1/{\lambda }_2|$(c is the velocity of light). Here, we fix the wavelength of ECL₁ and change the wavelength of ECL₂ to change the mm-wave frequency to simulate optical wireless-over fiber transmission. To realize bi-directional delivery link, there is another ECL (ECL₃) functioning as ECL₂ at the transmitter CS. Therefore, in our experiment, λ₁ is set unchanged, while, λ₂ or λ₃ is set at different index for downlink and uplink, so the mm-waves at the frequency of 95.4 GHz or 88.6 GHz are obtained to carry high-speed signals for downlink and uplink transmission, respectively.

 

Fig. 1 Experimental setup for system frequency response measurement.

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At the transmitter BS, a 100G PD (6 dBm input power) is employed to detect the optical QPSK signal realizing W-band signal conversion. And a W-band electrical amplifier (EA₁) is followed. Then through a horn antenna (HA₁) with a gain of 25 dBi, the 4 Gb/s (2 Gbaud) QPSK data is transmitted over 0.7 m wireless link. After 0.7 m wireless delivery, receiver BS is deployed to modulate the wireless QPSK signal into an optical carrier at wavelength λ₄ emitted from ECL₄. The optical outputs from the intensity modulator (IM) have a central carrier at λ₄ and two sidebands at ${\lambda }_4\pm {\lambda }_{RF}$(${\lambda }_{RF}=c/f_{RF}$), as shown in Figs. 2(a) and 2(b). Note that the EA₂ is a W-band amplifier for the link of 95.4 GHz, and E-band (60~90 GHz) for the link of 88.6 GHz.

 

Fig. 2 Optical spectra (0.1 nm resolution) after the IM: (a) for the link of 95.4 GHz, and (b) for the link of 88.6 GHz. optical spectrum response: (c) for the link of 95.4 GHz, and (d) for the link of 88.6 GHz.

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For simplification, neither fiber transmission nor receiver CS is used for system frequency response measurement. With all parameters of optical and electrical components unchanged, we measure the OSNR (0.1 nm resolution) of the lower sideband (${\lambda }_3-{\lambda }_{RF}$) of the optical signal after the IM. Figures 2(c) and 2(d) show the optical spectrum response for the link of 95.4 GHz and 88.6 GHz, respectively. For the 95.4 GHz link, the measured results show that the spectral response of the whole system is not good; especially when the frequency is higher than 100 GHz, the response is weak. On the other hand, the E-band amplifier is employed in the 88.6 GHz link, so the gain below 60 GHz and higher than 90 GHz in the optical spectrum is small. Thus, the overall system frequency response is dependent on the bandwidth of IMs and EAs, and atmospheric propagation attenuation of the mm-wave signals. To reduce effect of the spectrum response of the whole system, the post filter could be used to suppress the spectrum of the signal to improve the system performance [15].

3. Experimental setup for the full duplex bi-directional optical wireless-over fiber link

Figure 3 shows the schematic of the full-duplex bi-directional transmission optical wireless-over fiber system at W-band which can achieve the capacity up to 15 Gb/s for both 88.6 GHz link and 95.4 GHz link.

 

Fig. 3 Experimental setup for the full-duplex bi-directional transmission optical wireless-over fiber system.

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To reduce the cost and the complexity of the full-duplex system, only one generator of optical QPSK signal at the transmitter CS is employed for both links. At the transmitter CS, the lightwave from ECL₁ is for QPSK modulation, while the free-running light from ECL₂ and ECL₃ is for heterodyne beating. The frequencies of ECL₁, ECL₂ and ECL₃ are at 193.4900 THz, 193.5786 THz and 193.3946 THz, respectively, so the frequency spacing $f_{RF1}$ (between ECL₁ and ECL₂) is 88.6 GHz, and $f_{RF2}$ (between ECL₁ and ECL₃) is 95.4 GHz, realizing bi-directional transmission based on different mm-wave frequencies. The I/Q modulator with 3dB bandwidth of 31 GHz and insertion loss of 6 dB, composed of two parallel Mach-Zehnder modulators (MZMs), is used to generate up to 15 Gb/s optical QPSK signals, which together with the output of ECL₂ or ECL₃ acts as the input of the PM-OC after power amplification by an erbium-doped fiber amplifier (EDFA). Then the coupled signal is power attenuated and launched into 20 km SMF-28 which has 5 dB fiber loss. The input power into the fiber is 8 dBm. Figures 4(a) and 4(d) show the optical spectra before the first 20 km SMF-28 transmission of both links.

 

Fig. 4 Optical spectra (0.02 nm resolution) for the link of 88.6 GHz: (a) before the first 20 km fiber transmission, (b) after the IM, (c) LO for coherent detection. Optical spectra (0.02 nm resolution) for the link of 95.4 GHz: (d) before the first 20 km SMF-28 transmission, (e) after the IM, (f) LO for coherent detection.

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The transmitter BS consists of a PD, a W-band EA and a HA. 3 dB bandwidth of the PD is 90 GHz and it is used to realize photoelectric conversion of the QPSK signal from optical baseband into W-band (the mm-wave frequency is 95.4 GHz and 88.6 GHz respectively for bi-directional links). And the subsequent EA with output power of ~10 dBm and gain more than 25 dB is utilized so that the mm-wave signal has enough power for wireless transmission. Then, the generated mm-wave signals are delivered over 2 m bi-directional wireless link through two pairs of transmitter and receiver HAs. Each pair of HAs is for 2 m unidirectional wireless distance transmission and each HA is of a gain of 25dBi.

At the receiver BS, the wireless QPSK signal is amplified by an EA (EA₂ is an E-band amplifier for the link of 88.6 GHz, and EA₄ is a W-band amplifier for the link of 95.4 GHz). The optical carrier at wavelength λ₄ (1550.15 nm) emitted from ECL₄ or λ₅ (1548.21 nm) emitted from ECL₅ is carrier-suppression modulated by an IM which has 3 dB bandwidth of ~35 GHz and insertion loss of 6 dB. It should be addressed that each IM is direct-current (DC) -biased at the minimal output under the condition of that there is no mm-wave signal driving the modulators so the carrier can be suppressed. Figures 4(b) and 4(e) show the optical spectra after the IMs. The optical output from IMs has a central carrier at λ₄ or λ₅ and two sidebands at ${\lambda }_4\pm {\lambda }_{RF1}$(${\lambda }_{RF1}=c/f_{RF1}$) or ${\lambda }_5\pm {\lambda }_{RF2}$(${\lambda }_{RF2}=c/f_{RF2}$). However, large sidebands can’t be generated because the response of the IMs is weak with the signal at ~100 GHz. And due to the limited DC extinction ratio, the center carrier cannot be fully suppressed. Therefore, there is a higher center carrier related to the sidebands.

Before launched into another 20 km fiber, the regenerated optical QPSK signal is first amplified by an EDFA. Then, a tunable optical filter (TOF) is used to extract the upper sideband (${\lambda }_4+{\lambda }_{RF1}$or${\lambda }_5+{\lambda }_{RF2}$) to homodyne beat with the local oscillator (LO).

At the receiver CS, the LO with the wavelength at ${\lambda }_4+{\lambda }_{RF1}$or ${\lambda }_5+{\lambda }_{RF2}$(as shown in Figs. 4(c) and 4(f)), together with the extracted corresponding pure sideband, is mixed in an optical 90° hybrid and photo-detected in the coherent receiver for QPSK demodulation. Then, the obtained I and Q signals are sampled by a real-time digital storage oscilloscope (OSC) which has a sampling rate of 80 GS/s and 20 GHz electrical bandwidth, and off-line processed by adopting advanced DSP algorithm. Firstly, the clock is extracted, and CD compensation is processed. Then, frequency-offset estimation (FOE) and carrier phase estimation (CPE) is used for carrier recovery. Besides the typical steps aforementioned, it is worth noting that for the detailed DSP after analog-to-digital conversion, a post filter is utilized to improve the BER performance by suppressing the spectrum. Figures 5(a)–5(f) show the constellation before clock extraction, after clock extraction, after CD compensation, after FOE, after CPE and after post filter, respectively. Since the post filter converts the binary signal to the duo-binary one in both I and Q components of the QPSK signal, the conventional 4-point QPSK signal is transformed into 9-point quadrature amplitude modulation (9QAM) one [15].

 

Fig. 5 Constellations after each step of off-line DSP: (a) before clock extraction, (b) after clock extraction, (c) after CD compensation, (d) after FOE, (e) after CPE, (f) after post filter.

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4. Results and discussions

Figures 6(a) and 6(b) give the BER versus OSNR for 10 Gb/s QPSK signal under different transmission conditions for the links of 88.6 GHz and 95.4 GHz, respectively. The results show that the system has the same BER performance whether wireless link and fiber transmission are used or not. It means that there is no OSNR penalty for both 2 m wireless delivery and (20 + 20) km SMF-28 transmission. The reason is that the transmission fiber and wireless distance is short. Nonlinear effect of fiber and wireless transmission is very small, while other linear effects of the system such as CD and filtering effect can be compensated by advanced DSP at the receiver CS. If very long fiber such as over 1000 km or over 10 m wireless is used, the BER curves versus OSNR will be different.

 

Fig. 6 BER versus OSNR for 10 Gb/s QPSK signal: (a) for the link of 88.6 GHz, (b) for the link of 95.4 GHz.

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Figure 7(a) denotes the BER performance for QPSK signal transmission over the optical wireless-over fiber link at 95.4 GHz with 2 m wireless delivery and (20 + 20) km SMF-28 transmission. The BER is below 3.8 × 10⁻³ with the OSNR above 6.7 dB when 10 Gb/s signal has been transmitted without post filter processing at the receiver CS. And in the cases of 15 Gb/s QPSK transmission without and with post filter, the BER is below 3.8 × 10⁻³ when the OSNR are above 15.1 dB and 13.8 dB, respectively. It clearly shows that the lower speed the link has, the better BER performance there is. In addition, we can see from the results that the post filter processing at the receiver CS could improve the BER performance of the system. This is because the spectrum response of the link is not good, and the post filter suppresses the spectrum of the signal so the effect of the response of the whole system could be reduced [15]. With the OSNR of 15 dB, the BER of 2.9 × 10⁻³ can be achieved for 15 Gb/s signal transmission with post filter, while the BER without post filter processing is 4 × 10⁻³ (above the FEC threshold). Figure 7(b) shows the BER performance for QPSK signal transmission at 88.6 GHz link. The 88.6 GHz link performs the different BER from the link of 95.4 GHz because of the obviously different frequency response. For the 15 Gb/s QPSK transmission with post filter at the receiver CS, the BER is below 3.8 × 10⁻³ when the OSNR is above 15.8 dB. The requirement of OSNR has been increased by 2 dB compared to the link of 95.4 GHz. For both links, due to the limited bandwidth of the optical and electrical components, especially IM, the bit rate of each link has to be smaller than 20 Gb/s so that the transmitted signal can be recovered at the receiver CS.

 

Fig. 7 BERs versus OSNR for 15 Gb/s QPSK signal transmission: (a) 95.4 GHz, (b) 88.6 GHz.

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Figures 8(a)–8(c) give the electrical spectra after 10 Gb/s, 15 Gb/s and 16 Gb/s QPSK signal transmission at 95.4 GHz, respectively. In order to obtain the electrical spectrum, heterodyne detection is employed at the receiver CS. The LO is about 12.5 GHz away from the measured signal, so we can see that there is 12.5 GHz spectrum shift. And since the IM modulator has bad frequency response over 80 GHz, the measured electrical spectra are unsymmetrical, which is caused by the weak response at higher frequency.

 

Fig. 8 Electrical spectra after: (a) 10 Gb/s, (b) 15 Gb/s, and (c) 16 Gb/s QPSK signal transmission at 95.4 GHz link.

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5. Conclusion

We have demonstrated a full-duplex bi-directional signal transmission at W-band over 40 km (20 km + 20 km) fiber and 2 m wireless distance with the capacity up to 30 Gb/s for the first time. In our proposed experiment, mm-wave QPSK signal is generated by photonic technique and coherent detection is used to improve receiver sensitivity and advanced DSP is used to improve the BER performance of the system. Due to the bandwidth limitation of the optical and electrical components, the bit rate of the signal transmission is limited to be smaller than 20 Gb/s.