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We experimentally demonstrated full-duplex bidirectional transmission of 10-Gb/s millimeter-wave (mm-wave) quadrature phase shift keying (QPSK) signal in E-band (71–76 GHz and 81–86 GHz) optical wireless link. Single-mode fibers (SMF) are connected at both sides of the antenna for uplink and downlink which realize 40-km SMF and 2-m wireless link for bidirectional transmission simultaneously. We utilized multi-level modulation format and coherent detection in such E-band optical wireless link for the first time. Mm-wave QPSK signal is generated by photonic technique to increase spectrum efficiency and received signal is coherently detected to improve receiver sensitivity. After the coherent detection, digital signal processing is utilized to compensate impairments of devices and transmission link.
©2014 Optical Society of America
1. Introduction
With the ever increasing demands for delivering high-data-rate services, current microwave wireless systems confront bottleneck in terms of limited available bandwidth resources and severe congestion. To overcome these problems, researchers are now concentrating on utilizing the mm-wave frequency bands for wireless communication. Mm-wave frequency is attracting for wireless communication due to the advantages of broad bandwidth, reduced interference, good isolation and improved security [1–3]. E-band, ranging from 71- to 76-GHz and 81- to 86-GHz in the extremely high frequency bands of radio spectrum, is sufficient to support the transmission of up to 5-GHz full-duplex bidirectional transmission. Research efforts have been contributed to E-band wireless communication in various aspects including opto-electronic devices and transmission links [4,5]. Meanwhile, the mm-wave optical wireless integration technique is capable of realizing seamless integration of flexible wireless link and high-data-rate fiber-optic networks. With the merits of both optical and wireless communication and transparency to signal bit rates and modulation formats, the mm-wave optical wireless integration technique has been considered one of the best candidates for future mobile communication system [6–9]. Mm-wave optical wireless link can also play an important role in emergency communication when optical fiber cut occurs due to natural disaster [10]. In previous paper, we have demonstrated bi-direction transmission in E-band optical wireless link [11]. However, in that paper, no transmission fiber is connected and envelop detector which restricts the adoption of multi-level modulation formats enabling high capacity rather than coherent receiver is employed.
In this paper, we experimentally demonstrated full-duplex bidirectional transmission of 10-Gb/s mm-wave QPSK signal in E-band optical wireless link. Transmission fibers are connected at both sides of the antenna for uplink and downlink which realize 40-km SMF and 2-m wireless link for bidirectional transmission simultaneously. We utilized multi-level modulation format and coherent detection in such E-band optical wireless link for the first time. Millimeter-wave QPSK signal is generated by photonic technique to increase spectrum efficiency and received signal is coherently detected to improve receiver sensitivity. After the coherent detection, digital signal processing is utilized to compensate the filtering effect and other linear effects. The experimental results show that 10-Gb/s signal can get error free by FEC after transmission for both downlink and uplink. The proposed E-band bidirectional transmission scheme is one promising solution for future high-data-rate optical wireless integration applications, especially for emergency communication where optical fiber cut probably occurs.
2. Principle of full-duplex bidirectional transmission in E-band optical wireless link
Figure 1 illustrates the principle of full-duplex bidirectional transmission in E-Band optical wireless link. E-band is separated into two windows by the amateur radio (76- to 81-GHz), also known as 4-millimeter band. In the proposed scheme, we use 71- to 76-GHz millimeter-wave band for downlink and 81- to 86-GHz millimeter-wave band for uplink.
Fig. 1 Principle of full-duplex bidirectional transmission in E-band optical wireless link. RAU: remote access unit, HA: horn antenna, Tx: transmitter, Rx: receiver, O/E: optical to electrical, E/O: electrical to optical.
For downlink direction, the signal carried by optical carrier is transmitted over fiber. The optical signal then undergoes optical-electrical (O/E) conversion by high-bandwidth photo diode (PD) and upconverted to the E-band. After the upconversion, the E-band signal is amplified by a mm-wave electrical amplifier (EA) and then passes through diplexer which is connected with a horn antenna (HA). The E-band signal is received by HA after wireless link transmission. The received electrical signal then passes through another diplexer and another mm-wave EA to compensate the loss. Then the received E-band signal is modulated on optical carrier by high-bandwidth optical modulator. The modulated optical signal is injected into fiber and finally received by the coherent receiver which enables various modulation formats. The coherent detection can provide high receiver sensitivity and make the receiver more tolerant to electrical-optical (E/O) conversion loss introduced by optical modulator. Digital signal processing is implemented after coherent detection to compensate various impairments.
For uplink direction, the configuration is similar to that of downlink direction except that the center frequency of E-band signal is between 81- and 86-GHz. The uplink transmission also includes 2 spans of fiber transmission, wireless link transmission, O/E conversion and E/O conversion. However, the same HA pair employed at downlink can be shared for uplink by two diplexers. It should be noted that the transmission fiber for downlink and uplink can be the same one if 3-port optical circulator is employed to combine the optical signal.
3. Experimental setup
The experimental setup of full-duplex bidirectional transmission of 10-Gb/s mm-wave QPSK signal in E-band optical wireless link is shown in Fig. 2. The proposed scheme is able to support full-duplex bidirectional transmission over 40-km SMF and 2-m wireless link simultaneously. The carrier frequency of E-band signal for downlink and uplink is 73.4-GHz and 82.8-GHz respectively.
Fig. 2 The experimental setup of full-duplex bidirectional transmission of 10-Gb/s mm-wave QPSK signal in E-band optical wireless link. Inset (a), (b) and (c) show the photos of RAU1, wireless link and RAU2 respectively. Inset (d) and (e) give the electrical spectrum (3-MHz resolution) after O/E conversion in downlink and uplink respectively. ECL: external cavity laser, EDFA: erbium-doped fiber amplifier, PM-OC: polarization maintaining optical coupler, Att.: attenuator, SMF: single-mode fiber, PD: photo diode, EA: electrical amplifier, IM: intensity modulator.
For downlink direction, two wavelength fully tunable external cavity lasers (ECL) with 100-kHz linewidth and 13-dBm output power are employed. The continuous wave (CW) lightwave with emitting frequency at 193.4906-THz from ECL1 is utilized as optical signal carrier. Optical in-phase/quadrature (I/Q) modulator with the 3-dB bandwidth of 31-GHz and insertion loss of 6-dB is employed to modulate the optical QPSK signal. The modulated signal is power-amplified by erbium-doped fiber amplifier (EDFA) and then combined with the CW lightwave from ECL2 by a polarization-maintaining optical couple (PM-OC). ECL2, with same linewidth and output power as ECL1, is employed as carrier-frequency generating source and its emitting frequency is set at 193.5640-THz to ensure adequate frequency spacing between ECL1 and ECL2, which is able to generate the desired 73.4-GHz mm-wave carrier frequency. The combined optical signal is injected into 20-km SMF with 5-dB loss and 17-ps/nm/km dispersion. After fiber transmission, optical signal is directly detected by a 100-GHz PD (U2t XPDV4120R) with 3-dB bandwidth of 90-GHz to realize O/E conversion. The following mm-wave EA with 10-dBm output power is employed to amplify the E-band electrical signal. The EA we utilized in experiment is low-noise amplifier with noise figure of 5.5-dB and average gain of 25-dB.The amplified electrical signal then passes through the diplexer1 connected to HA1 with high gain of 25-dBi. After the 2-m wireless link transmission, E-band electrical signal is received by HA2 and diplexer2. It should be noted that the insertion loss for both diplexers is less than 2-dB.Then an intensity modulator (IM) with 3-dB bandwidth of 39-GHz and insertion loss of 6dB is employed to realize the E/O conversion. The frequency response of IM is still good around 80-GHz according to our measurement.By adjusting the DC bias on the modulator, the output power from IM1 reaches minimum when RF signal is disconnected. The following EDFA2 is used to compensate the insertion loss of IM1. After the power amplification, optical signal is injected into another span of 20-km SMF. At last, the signal is received by coherent receiver and digital storage oscilloscope (OSC) with 20-GHz bandwidth and 80GSa/s sampling rate is utilized to obtain the data. The data is offline processed by DSP to recover the original signal. For uplink direction, configurations identical with downlink direction are adopted except that E-band carrier frequency is 82.8-GHz. It is worth noting that we employ ECL1 as optical carrier of QPSK signal for both directions so as to simplify the experimental setup.
Inset (a), (b) and (c) show the photos of RAU1, wireless link and RAU2 respectively. Inset (d) and (e) give the electrical spectrum (3-MHz resolution) after O/E conversion in downlink and uplink respectively. The spectrum resolution is 3-MHz while the spectrum span for (d) and (e) is 10-GHz and 14-GHz respectively. For both situation, we find the electrical power at E-band carrier frequency is less than −50-dBm and the signal-to-noise ratio is about 15-dB. We can found frequency shift in insets (d) and (e) in Fig. 2 which mainly result from the frequency stability of the ECLs we employed.
4. Results and discussion
Figure 3 gives the optical spectra at point a, b, c and d in Fig. 2 and resolutions of all spectra are 0.02-nm. Figures 3(a) and 3(b) are optical spectra of the combined optical signal after PM-OC for downlink and uplink respectively. The frequency spacing is just 73.4-GHz and 82.8-GHz which can be observed. Figures 3(c) and 3(d) are optical spectra after E/O conversion over only 0.5-m wireless link without transmission fiber. For Fig. 3(c), the center wavelength of ECL3 appears in the middle while the modulated signal wavelength after IM1 can be found at left and right side with both 73.4-GHz spacing. The wavelength of local oscillator at following coherent receiver matches the wavelength on the right to detect the signal. For Fig. 3(d), we find the center wavelength of ECL5 in the middle of the spectrum and the frequency spacing of both sides are 82.8-GHz. There exists a sideband close to the modulated signal band and will lead to degradation of bit error ratio (BER) performance in coherent receiver. This can be explained by the imperfection of frequency response of devices including 100-GHz EA. Since 82.8-GHz is closer to the border of the frequency response, signal on this carrier frequency suffers more than that on 73.4-GHz which leads to distortion in spectrum. The different optical signal-to-noise ratio (OSNR) in these spectra is worth noting, since it can explain why modulated signal in Figs. 3(a) and 3(b) is narrower than the modulated sidebands in Figs. 3(c) and 3(d).
Fig. 3 Optical spectra (0.02-nm resolution) in full-duplex bidirectional E-band optical wireless link. (a) After PM-OC 1. (b) After PM-OC 2. (c) After EDFA 2. (d) After EDFA 3. LO: local oscillator, EDFA: erbium-doped fiber amplifier, PM-OC: polarization maintaining optical coupler
Figure 4(a) shows the measured BER versus OSNR for 8-Gb/s downlink QPSK signal at 73.4-GHz with different transmission distance. We find almost identical BER performance for BTB, 2-m wireless link and 40-km SMF with 2-m wireless link. That is to say, there is almost no penalty after transmission over fiber and wireless link. OSNR about 10-dB is necessary to reach the BER of 10−3 Since the data rate is no very high, the signal distortion introduced by chromatic dispersion is really limited. The attenuator before transmission fiber decreases the influence of fiber nonlinearity. Fading in wireless link also does not affect the signal in 2-m distance. All these reasons explain why no penalty is observed. The measured BER versus OSNR for 8-Gb/s uplink QPSK signal at 82.8-GHz with different transmission distance is shown in Fig. 4(b). Similar to the downlink situation, there is almost no penalty after fiber and wireless link transmission. Compared with downlink signal at 73.4-GHz, the OSNR requirement of uplink signal at 82.8-GHz is 2-dB higher at BER of 10−3. The results correspond with sideband found in Fig. 3(d). In the experimental setup we propose, the BER performance is mainly restricted by signal rate and opto-electronic devices bandwidth instead of transmission distance.
Fig. 4 BER versus OSNR for 8-Gb/s QPSK signal at E-band with different transmission distance. (a) Downlink at 73.4-GHz. (b) Uplink at 82.8-GHz.
Figure 5(a) gives the BER versus OSNR curves for 5-Gb/s, 8-Gb/s and 10-Gb/s downlink QPSK signal at 73.4-GHz with transmission over 40-km SMF and 2-m wireless link. For the same OSNR, BER performance decreases as the signal rate gets higher. Higher signal rate is more sensitive to filtering effect of diplexer and other impairments when OSNR is low. This explains why BER of 10-Gb/s signal degrades much faster than that of 8- and 5-Gb/s signal and can be observed by the gradient of the curves. The necessary OSNR requirement for the BER of 10−3 is 5-dB, 10-dB and 18-dB for 5-Gb/s, 8-Gb/s and 10-Gb/s respectively. Similar to Figs. 5(a) and 5(b) shows the BER versus OSNR curves for 5-Gb/s, 8-Gb/s and 10-Gb/s uplink QPSK signal at 82.8-GHz with transmission over 40-km SMF and 2-m wireless link. The BER performance for different signal rates at 82.8-GHz all deteriorate compared with 73.4-GHz downlink situation, as analyzed in the last paragraph. However, we find the 10-Gb/s curve (blue triangle) does not change for both downlink and uplink. This means the signal rate becomes the dominant factor for BER performance when it reaches 10-Gb/s. From Figs. 5(a) and 5(b), we find bidirectional 10-Gb/ signal can get error free after FEC.
Fig. 5 BER versus OSNR for QPSK signal at E-band over 40-km SMF and 2-m wireless link with different bit rate. (a) Downlink at 73.4-GHz. (b) Uplink at 82.8-GHz.
Figure 6 shows the constellations of received QPSK signal at different signal bit rate. 20-Gb/s (10Gbaud) QPSK signal cannot be recovered since four constellation points are not separated which can be observed in Fig. 6(a). The filtering effect of diplexer is considered an important factor for signal bit rate up to 20-Gb/s. In such case, the BER cannot be measured because of the strong filtering effect of diplexer. The constellation of 5- and 4-Gbaud QPSK signal is shown in Figs. 6(b) and 6(c), respectively. In Fig. 6(b), the 5-Gbaud signal is not well converged onto four constellation points and some scattered points will result in degradation of BER performance. The constellation of 4-Gbaud QPSK signal is separated and converged to four points well after DSP, which can be found in Fig. 6(c). The constellation of 2.5-Gbaud QPSK signal shows the best performance which can be found in Fig. 6(d).
Fig. 6 Constellations of received QPSK signal at (a) 10-Gbaud (20-Gb/s). (b) 5-Gbaud. (c) 4-Gbaud. (d) 2.5-Gbaud.
5. Conclusion
We experimentally demonstrated full-duplex bidirectional transmission of 10-Gb/s mm-wave QPSK signal in E-band optical wireless link. Transmission fibers are connected at both sides of the antenna for uplink and downlink which realize 40-km SMF and 2-m wireless link for bidirectional transmission simultaneously. We utilized multi-level modulation format and coherent detection in such E-band optical wireless link for the first time. Mm-wave QPSK signal is generated by photonic technique to increase spectrum efficiency and received signal is coherently detected to improve receiver sensitivity. After the coherent detection, digital signal processing is utilized to compensate impairments of devices and transmission link. The experimental results show that 10-Gb/s signal can get error free by FEC after transmission for both downlink and uplink. The proposed E-band bidirectional transmission scheme is one promising solution for future high-data-rate optical wireless integration applications, especially for emergency communication where optical fiber cut probably occurs.
Acknowledgments
This work was partially supported by the NNSF of China (No. 61325002, No. 61250018, No. 61177071), NHTRDP (863 Program) of China (2012AA011302), The National Key Technology R&D Program (2012BAH18B00), Key Program of Shanghai Science and Technology Association (12dz114300, 12510705600 and 13JC1400700).
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