1. Introduction

Recent digital coherent communication techniques with higher order multi-level modulation such as 16-ary quadrature amplitude modulation (QAM) are possible candidates for a high-capacity transmission link with high spectral efficiency. The development of such digital coherent communication techniques not only rapidly increases link speeds to greater than 1 Tb/s but also enables the total capacity of the transmission to exceed 100 Tb/s in a single optical fiber [1, 2]. However, the capacity of the commercial wireless link technology will not permit us to achieve capacities of more than 10 Gb/s in spite of the usage of a millimeter-wave band of 60 GHz [3]. The capacity mismatch between the optical (wired) and the wireless link technologies could cause a bottleneck in future optical/wireless seamless networks. The link speed depends on the frequency and the bandwidth of the wireless carrier. Thus, a high carrier frequency with a large bandwidth is suitable for the realization of a high capacity wireless link. In fact, some wireless link experiments with data rates greater than 10 Gb/s have been performed at a frequency greater than 60 GHz [4–7]. Basically, in the high frequency region, a broadband transmitter with data rates greater than 10 Gb/s cannot be fabricated by using only electronic devices.

The generation of wireless signals, based on the radio-over-fiber (RoF) technology, is expected to be suitable for high-frequency wireless transmissions as well as in applications involving an optical/wireless seamless network. Generally, the RoF signal is a combination of a RoF local oscillator (LO) carrier component and an optically modulated baseband component. It is easy to convert the optical RoF signal to a wireless signal with a frequency up-conversion technique, which is the so-called direct optical up-conversion technique that utilizes high-speed photodetectors [8]. This technique will provide a pure signal without spurious tones as compared to any optical to electrical (OE) signal conversion techniques. This is because the generated signal is directly obtained from the incident optical signals by photomixing; therefore, an additional device is not used for OE conversion. On the other hand, it is possible that the photodetector, whose frequency is less than the separation frequency between the RoF-LO component and the baseband component, can detect only the baseband components, as in the case as the conventional optical communication scheme. Therefore, the generated RoF signal would be available for the dual purpose of both optical and wireless communications.

In this study, we show a W-band (75–110 GHz) wireless link with an optically generated RoF signal and a digital wireless receiver. The generation of a 40-Gb/s wireless signal and its subsequent digital demodulation after its transmission over a distance of 30 mm in free space were achieved by using a 16-QAM signal generator synthesized by coherent polarization synthesis using a dual-polarization quadrature phase-shift keying (QPSK) modulator [9]. In this system, a RoF signal generation strategy employing the direct optical up-conversion technique is used to form W-band signals on the optical receiver side using a high-speed uni-travelling-carrier photodiode (UTC-PD) [10]. The optical 16-QAM baseband component in the generated RoF signal is detectable with an optical coherent receiver. Therefore, the generated RoF signal is used for the optical baseband transmission system as well as for wireless transmission. On the wireless receiver side, the digital receiver consists of W-band waveguide components for heterodyne detection using a broadband mixer, and it enables digital signal processing in a manner similar to the optical digital coherent detection scheme.

2. Coherent optical/wireless seamless network

Figure 1 shows a schematic of our concept of coherent optical and wireless seamless network. This system is based on the RoF technology applicable to both the transmitter and the receiver. Both the optical and wireless signals are optically generated in the same transmitter. However, some optical bandwidth shaping must be implemented owing to the limitation of the assigned bandwidth of the wireless signal. The optically generated RoF signal is transmitted over the optical fiber. The transmitted signals will be received by an optical/wireless shared signal receiver. As mentioned above, for baseband (optical) signal detection, the bandwidth of the photodetector should be equal to or slightly less than that of the baseband signal. A digitally aided transmission technique such as optical coherent detection is one of the key features of the network. This is because the digital signal processing technique is a powerful tool for bandwidth optimization, for enhancement of the spectral efficiency of the signal, as well as for the compensation of fluctuations in the transmission media such as optical fibers and free space environments.

 

Fig. 1 Concept schematic of coherent optical and wireless seamless network.

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In general, the RoF converter with a high-speed photodiode has a bandwidth that is comparable to the bandwidth of the RoF signal and can generate a wireless signal from the received RoF signal. The detailed diagram of our concept of the transmitter, the RoF converter and the receiver is shown in Fig. 2. The transmitted wireless signal from the transmitter antenna is received by the receiver antenna unit. For the purpose as a high-speed signal repeater with the wireless signal using the RoF converters, the received wireless signals are modulated directly by the optical carrier component, and then the optical baseband signal is regenerated by optical single-sideband (SSB) modulation [5]. This RoF converter pair helps to realize a broadband backup transmission link when the optical fiber link cannot be established at the time of disaster as well as an application for a high-speed radio back-hauling connections between base stations of mobile wireless devices. In Ref. [5], the received wireless signal directly modulated the optical single sideband modulator to generate the optical baseband signals.

 

Fig. 2 Block diagram of concept of Fig. 1.

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For the direct detection of the transmitted wireless signal, a high-speed wireless receiver will be required at the end of the wireless link (the grayed boxes in Fig. 2). This receiver unit can be located and set up by the end user; however, the repeater unit mentioned above must be assembled and configured by professional engineers owing to the usage of an optical fiber link. In this case, the wireless receiver unit, rather than the RoF converter pairs, must be highly robust. Thus, the receiver should be constructed with only electronic devices. Moreover, the cost of the receiver is expected to be much lower than that of a conventional wireless receiver for fixed-wireless access communication because the receiver unit will be available in many areas. To realize a low-cost and highly robust receiver, a large number of the currently available analog high-speed components are not suitable. In addition, the high-speed optical signals that have a bandwidth greater than 10 GHz cannot be detected by the commercially available wireless components.

We propose a digital receiver with a high-speed analog-to-digital converter (ADC) diverted from the optical digital coherent receiver such as a 100 GbE (Gigabit Ethernet). A sampling rate greater than 56 GSa/s is suitable for not only the conventional method of intermediate frequency (IF) component sampling but also the direct sampling of the radio-frequency (RF) component, i.e., for the full digital sampling of the wireless signals [11]. The cost will decrease with the commercialization of the digital coherent technique. In this paper, we outline a demonstration of the capabilities of a digital wireless receiver with a RoF converter transmitter, as shown in Fig. 1.

3. Experimental setup for W-band RoF transmission

For the demonstration of wireless transmission with optically generated signals in the proposed optical/wireless seamless network, we tried to implement a high-bit-rate wireless transmission link greater than 20 Gb/s in the W-band (75–110 GHz). A bandwidth of 35 GHz is suitable for a high rate of symbol modulation. It should be noted that this band might not be assigned to any wireless data links. The demonstration was carried out in an electrically shielded room.

Figure 3 shows our experimental setup of the RoF transmitter and the wireless receiver. Our transmitter consisted of a two-tone optical signal source and an optical 16-QAM generator based on a DP-QPSK modulator. Details of the two-tone optical signal generator and an overview of the wireless receiver have already been reported [12–14]. The frequency separation between the two tones was 92.5 GHz. The two-tone signal was split by an optical arrayed waveguide grating (AWG) through an Er-doped fiber amplifier (EDFA). The upper frequency component was used as the optical reference signal of the RoF signal for the direct optical up-conversion. The lower frequency was used by the DP-QPSK modulator that is connected to four channels of a 10 Gb/s pulse pattern generator (PPG). We used a pseudo random bit stream with a length of 2¹⁵–1. A polarization coherent synthesis method was used for the 16-QAM, where two optical QPSK signals were combined within a polarizer that is placed behind the modulator [9]. The advantage of this method for the generation of a 16-QAM signal is the ease of control of the bias voltage of the modulator. The method used to control the bias voltage must be considered for such nested Mach-Zehnder modulators, because of the large number of electrodes. In this DP-QPSK scheme, only two independent QPSK bias controllers were required to generate two QPSK signals that are orthogonally polarized with respect to each other; such polarization of two coherent signals will generate the 16-QAM signal.

 

Fig. 3 Experimental setup.

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The reference signal and the generated 16-QAM signal, combined by a 3-dB optical coupler, were fed to a uni-traveling carrier photodiode (UTC-PD) working as a W-band photomixer [10]. A W-band horn antenna with an antenna gain of approximately 20 dBi would, when directly connected to the UTC-PD, transmit a W-band 16-QAM signal whose central frequency should be equal to that of the frequency separation of the two-tone signal. The RoF signal consisting of the reference and the 16-QAM signal can be also used for digital baseband transmission. When the signal is fed to a photodetector with a bandwidth that is less than the W-band frequency, an optical coherent receiver acts as a baseband optical receiver. To demonstrate this dual-purpose capability, we used the coherent receiver with a 1-nm band-pass filter.

On the receiver side, we used a combination of a W-band heterodyne and digital frequency down-conversion with IQ separation [12, 13]. The received signal was down-converted to an IF signal by the broadband W-band mixer. The IF band extended from 0 to 35 GHz when the W-band signal bandwidth increased from 75 to 110 GHz. Thus, the received IF signal was centered at 17.5 GHz, with a bandwidth of 20 GHz. Amplified IF signals were observed using a real-time digital oscilloscope with a bandwidth of 30 GHz and a sampling rate of 80 GSa/s. For phase detection, the digitized and carrier-recovered signal was multiplied with a complex sinusoidal signal, so that the in-phase (I) and the quadrature phase (Q) components could be separated. Processing schemes such as frequency domain equalization and symbol decision can be applied for both the I and the Q components in the same manner as an optical digital coherent detection technique. We set the receiver horn antenna at a distance of 30 mm from the transmitter antenna.

4. Results

The optical spectrum measured at the UTC-PD input is shown in Fig. 4(a). The optical reference component at a wavelength of 1551.9 nm and the 10-Gbaud modulated signals at a wavelength of around 1552.7 nm were clearly observed with a marked separation of approximately 0.8 nm, which corresponds to an optical frequency of 92.5 GHz. The IF electric spectrum was obtained by performing a fast Fourier transformation of the temporal IF signal detected by the oscilloscope (Fig. 4(b)). The observed main lobe of the 10-Gbaud modulated signal centered at 17.5 GHz had some parasitic peaks, which originated from the noise caused by the oscilloscope and the W-band components owing to the absence of the corresponding peak in the optical spectrum. Some spectrum distortions were reflected in the frequency response of the W-band components. The periodic structure in the main lobe is attributed to the interference between the transmitted wireless signals between the antennas. Although the optical signal-to-noise ratio (SNR) was greater than 30 dB, the electrical SNR was less than 20 dB.

 

Fig. 4 Spectra of (a) optical RoF signal and (b) FFT-transformed IF signal of the receiver mixer

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The constellation diagrams observed using the optical coherent receiver and the digital wireless receiver are shown in Fig. 5. For an optical signal, the bit error rate (BER) was measured to be 4.22 × 10⁻⁴. The difference in symbol separation in the optical and wireless signals and the distortions of the constellation of the wireless signal could be caused by noise generated in the W-band electrical components, as well as the loss attributed to transmission in free space.

 

Fig. 5 Constellation diagrams of (a) optical 16-QAM signal and (b) received wireless signal.

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The BERs of the transmitted wireless signals are shown in Fig. 6. This figure shows the BERs of not only 40-Gb/s 16-QAM signals but also 20-Gb/s QPSK signals measured with the same setup. Adaptive modulation by adjusting the polarizer in the transmitter was also successfully demonstrated. For the QPSK signal, the observed BERs were much less than the FEC limit of 2×10⁻³, with the clear constellation shown in the inset of the figure. In the case of the 16-QAM, the BER was calculated to be 1.90 ×10⁻³ when the radio power of the output port of the UTC-PD was −8 dBm. It should be noted that the differences between the QPSK and QAM results might depend on signal processing. Simple algorithms for the reduction of phase noise, such as the so-called M-th power algorithm, can be effective for QPSK, but cannot be easily applied for 16-QAM, because the QAM signal includes phase changes as well as intensity changes. The optimization of the algorithm and reduction of noise owing to the electronic components in the receiver can help improve the received signal quality.

 

Fig. 6 Bit error rates dependence on received wireless power for 16-QAM and QPSK [12].

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The results of this experiment showed that the transmission distance of 30 mm was in the intermediate region between the near-field and the far-field region, because the wavelength of the wireless signal was approximately 3.24 mm. In order to extend the transmission distance to demonstrate the concept shown in Fig. 1, it is necessary to increase the transmitter output power. It has been reported that 120-GHz RoF and wireless transmission experiments were performed with 10-Gb/s on-off keying modulation using high-power millimeter-wave integrated circuits at a distance of several kilometers [6, 15]. In addition, high-power amplifiers for the 90-GHz-band with an output power of 5 W have been developed [16]. Atmospheric attenuation is an important issue that must be considered for extending the transmission distance. The estimated and observed values of attenuation in the W-band are less than 1 dB/km and are approximately 10 times less than those in the 60 GHz band [17]. Therefore, an extension of the distance to several 100 m will be possible with these high-power electrical devices for a demonstration of the proof of our proposed concept.

5. Conclusion

We have successfully demonstrated a 40-Gb/s W-band RoF signal generation and its demodulation over a distance of 30 mm using the W-band electric receiver. These transmitter and receiver configurations are suitable for our proposed coherent optical/wireless seamless network, which has a capacity greater than 40 Gb/s. The generated RoF signal can also be demodulated by an optical coherent receiver designed for optical baseband transmission, thus making the generated signal suitable for the dual purposes of optical and wireless communication.