We experimentally demonstrate a record 400G optical wireless integration system simultaneously delivering 2 × 112 Gb/s two-channel polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) signal at 37.5 GHz wireless carrier and 2 × 108 Gb/s two-channel PDM quadrature phase shift keying (PDM-QPSK) signal at 100 GHz wireless carrier, adopting two millimeter-wave (mm-wave) frequency bands, two orthogonal antenna polarizations, multiple-input multiple-output (MIMO), photonic mm-wave generation and advanced digital signal processing (DSP). In the case of no fiber transmission, the bit error ratios (BERs) for both the 112 Gb/s PDM-16QAM signal after 1.5 m wireless delivery at 37.5 GHz and the 108 Gb/s PDM-QPSK signal after 0.7 m wireless delivery at 100 GHz are below the pre-forward-error-correction (pre-FEC) threshold of 3.8 × 10−3. To our knowledge, this is the first demonstration of a 400G optical wireless integration system in mm-wave frequency bands and also a capacity record of wireless delivery.
©2013 Optical Society of America
High-speed wireless communication technology is important for future defense and commercial applications. Defense Advanced Research Projects Agency (DARPA) is currently soliciting proposals to demonstrate 100 Gb/s wireless delivery at 100 GHz with wireless transmission distances up to hundreds of kilometers . A 100 Gb/s and beyond millimeter-wave (mm-wave) system at W-band (75-110 GHz) is adopted by a national project in Japan to provide the emergency service when large-capacity (>100 Gb/s) long-haul optical cables are broken during the earthquake and tsunami. At the same time, with the rapid increase of consumer demand, bit rates up to 50 Gb/s per channel will be required for smart mobile and fixed terminals with very high definition (HD) cameras such as 8k video (compared with current 1k). For example, some companies have introduced 8 and 4k super-HD (SHD) video cameras in smart phones that require transmission speed for uncompressed SHD video images of 60 and 30 Gb/s per channel, respectively. Hence for a few channels, 400G capacity will be required. On the other hand, due to wider bandwidths and higher frequencies, wireless delivery in mm-wave frequency bands is expected to provide multi-gigabit mobile data transmission, and has been intensively studied in the research community [2–20]. Furthermore, 100 Gb/s and beyond wireless mm-wave signal can be achieved based on photonic mm-wave technique, which further promotes the seamless integration of wireless and fiber-optics networks [18–20]. Very recently, we have experimentally demonstrated 108 Gb/s polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) data transmission over fiber and 2 × 2 multiple-input multiple-output (MIMO) wireless link at 100 GHz  as well as 112 Gb/s polarization-division-multiplexing 16-ary quadrature-amplitude-modulation (PDM-16QAM) data transmission over fiber and 2 × 2 MIMO wireless link at 35 GHz .
In this paper, we experimentally demonstrate an optical wireless integration system simultaneously delivering 2 × 112 Gb/s two-channel PDM-16QAM wireless signal at 37.5 GHz and 2 × 108 Gb/s two-channel PDM-QPSK wireless signal at 100 GHz. Compared to our previous work [19, 20], two orthogonal antenna polarizations and two mm-wave frequency bands are adopted to realize the simultaneous transmission of two PDM-QPSK channels and two PDM-16QAM channels. In the case of no fiber transmission, the bit error ratios (BERs) for both the 112 Gb/s PDM-16QAM signal after 1.5 m wireless delivery at 37.5 GHz and the 108 Gb/s PDM-QPSK signal after 0.7 m wireless delivery at 100 GHz are below the pre-forward-error-correction (pre-FEC) threshold of 3.8 × 10−3 . To our knowledge, this is the first demonstration of a 400G optical wireless integration system in mm-wave frequency bands and also a capacity record of wireless delivery.
2. Experimental setup for the 400G optical wireless integration delivery system
Figure 1 shows the experimental setup for the optical mm-wave generator for 2 × 112 Gb/s two-channel PDM-16QAM wireless signal at Q-band (33-50 GHz) and 2 × 108 Gb/s two-channel PDM-QPSK wireless signal at W-band.
At the optical baseband transmitter, there are four external cavity lasers (ECLs) with linewidth less than 100 kHz and output power of 14.5 dBm. For optical PDM-QPSK modulation, the two continuous-wavelength (CW) lightwaves from ECL1 at 1553.22 nm and ECL2 at 1553.82 nm, with 75 GHz frequency spacing, are first combined together by a 3-dB optical coupler (OC), and then modulated by an in-phase/quadrature (I/Q) modulator. The I/Q modulator is driven by a 27 Gbaud electrical binary signal, which, with a pseudo-random binary sequence (PRBS) length of 215-1, is generated from a pulse pattern generator (PPG). The two parallel Mach-Zehnder modulators (MZMs) in the I/Q modulator are both biased at the null point and driven at the full swing. The phase difference between the upper and lower branches of the I/Q modulator is controlled at π/2.
The subsequent polarization multiplexing is realized by a polarization multiplexer, comprising a polarization-maintaining OC to split the signal into two branches, an optical delay line (DL) in one arm to provide a 150-symbol delay, an optical attenuator in the other arm to balance the power of two branches and a polarization beam combiner (PBC) to recombine the signals. Thus, the 2 × 108 Gb/s two wavelength channels PDM-QPSK optical baseband signal is generated with 75 GHz channel spacing.
For optical PDM-16QAM modulation, similarly, the two CW lightwaves from ECL3 at 1554.43 nm and ECL4 at 1554.83 nm, with 50 GHz frequency spacing, are combined together by a 3-dB OC, then, modulated by an I/Q modulator and polarization-multiplexed by a polarization multiplexer. What is different from optical PDM-QPSK modulation is that the I/Q modulator is driven by a 14 Gbaud electrical four-level signal, which, with a PRBS length of 215-1, is generated from an arbitrary waveform generator (AWG). Note that an electrical amplifier (EA) is used at each output of the AWG to amplify the power of the electrical four-level signal, and electrical pre-equalization is adopted to compensate for the nonlinear effects caused by the EAs and I/Q modulator [22, 23]. Thus, the 2 × 112 Gb/s two wavelength channels PDM-16QAM optical baseband signal is generated with 50 GHz channel spacing.
The generated 2 × 108 Gb/s two wavelength channels PDM-QPSK and 2 × 112 Gb/s two wavelength channels PDM-16QAM optical signals are combined together by a 3-dB OC, amplified by an erbium-doped fiber amplifier (EDFA), and then launched into a 80-km single-mode fiber-28 (SMF-28). The 80-km SMF-28 has an 18-dB average fiber loss and a chromatic dispersion (CD) of 17 ps/km/nm at 1550 nm without optical dispersion compensation. The second EDFA is used to compensate for the fiber loss. The total optical power after the first and second EDFAs is 15 dBm and 18 dBm, respectively. Each EDFA has a noise figure of 5 dB. In the following part, ch1, ch2, ch3 and ch4 are used to denote the four channels at 1553.22 nm, 1553.82 nm, 1554.43 nm and 1554.83 nm, respectively.
For optical up-conversion, a 1 × 4 programmable wavelength selective switch (WSS) is first used to separate ch1-ch4 as well as suppress the amplified spontaneous emission (ASE) noise from the EDFAs. Another four ECLs with linewidth less than 100 kHz are used as local oscillators (LOs). LO1 and LO2 have 100 GHz frequency offset (FO) relative to ch1 and ch2, respectively. LO3 and LO4 have 37.5 GHz FO relative to ch3 and ch4, respectively. Here, 5-m SMF is used to de-correlate ch1 and ch2 or ch3 and ch4.
As shown in Fig. 2(a), for the coherent detection of ch1 or ch2, two polarization beam splitters (PBSs) and two OCs are used to implement polarization diversity of the received PDM-QPSK optical signal and LO in optical domain before heterodyne beating. Two photo detectors (PDs), each with 90-GHz 3-dB bandwidth and 7.5-dBm input power, directly up-convert the PDM-QPSK optical signal into the PDM-QPSK wireless signal at 100 GHz.
The coherent detection of ch3 or ch4, as shown in Fig. 2(b), is quite similar to that of ch1 or ch2. The main difference is that two balanced photo detectors (BPDs), instead of two PDs, are adopted to directly convert the received PDM-16QAM optical signal into the PDM-16QAM wireless signal at 37.5 GHz. Each BPD has a 10-dBm input power. It is worth noting that the polarization of the light in front of the PBS in Fig. 2 is arbitrary due to the fiber transmission. Thus, the X- or Y-polarization component at the output port of the PBS contains a mix of the data which is simultaneously encoded on the X- and Y-polarization at the transmitter.
Figure 3(a) and 3(b) show the optical spectra (0.1 nm resolution) after the first and second EDFAs, respectively. Figure 3(c)-3(f) show the optical spectra (0.02 nm resolution) after polarization diversity corresponding to ch1-ch4, respectively. Note that ch1 and ch2 have a larger signal bandwidth than the filtering bandwidth of the WSS and are thus spectrally shaped by the WSS. The introduced impairment due to the filtering effect can be compensated by advanced digital signal processing (DSP) algorithm at the receiver end .
Figure 4(a) shows the horn antenna (HA) system including a W-band HA array and a Q-band HA array. Each HA array includes four transmitter HAs and four receiver HAs.
The two pairs of transmitter and receiver HAs in the middle of the W-band HA array are in horizontal-polarization (H-polarization) state, while the other two pairs are in vertical-polarization (V-polarization). The 2 × 108 Gb/s two-channel PDM-QPSK wireless signal at 100 GHz is delivered by the W-band HA array, with ch1 and ch2 corresponding to the H- and V-polarization HA arrays, respectively. For the W-band HA array, each receiver HA can only get the wireless power from corresponding transmitter HA as shown by the black dashed lines, which is because the distance between the HAs is much larger with respect to a wavelength of only 3 mm for the 100 GHz wireless mm-wave signal. Thus, there hardly exists crosstalk at the same polarization (H or V) for the W-band HA array. Moreover, the crosstalk, even if exists, can be compensated by a long-tap constant modulus algorithm (CMA) .
The two pairs of transmitter and receiver HAs in the middle of the Q-band HA array are vertically polarized, while the other two pairs are horizontally polarized. The 2 × 112 Gb/s two-channel PDM-16QAM wireless signal at 37.5 GHz is delivered by the Q-band HA array, with ch3 and ch4 corresponding to the H- and V-polarization HA arrays, respectively. For the Q-band array, RX2 or RX3 can get the same wireless power from TX2 and TX3, while RX1 or RX4 can get the same wireless power from TX1 and TX4, as shown by the blue dashed lines. Thus, there exists large crosstalk at the same polarization (H or V) for the Q-band HA array. The crosstalk depends on the beam width of the 37.5 GHz mm-wave signal, the relative inclination of the HAs as well as the relative distance between the HAs.
The wireless distance is 0.7 m for the wireless transmission links at W-band, while 1.5 m at Q-band. Each HA has a 25-dBi gain. Each Q-band HA is connected with a 60-GHz EA with 30-dB gain and 24-dBm saturation output power, while each W-band transmitter HA connected with a 100-GHz EA with 30-dB gain and 10-dBm saturation output power. The 3-dB beam width at the input of each receiver HA is about 10° × 10°. The transmitted and received radio-frequency (RF) power is about 9 dBm and −10 dBm, respectively. The isolation between H- and V-polarization HA array is 33 dB. Thus, the crosstalk between H- and V-polarization signals can be ignored and other arrangements apart from H-V-V-H and V-H-H-V are also feasible for the HA system.
As shown in Fig. 4(b), for the received 100 GHz PDM-QPSK wireless signal (corresponding to ch3 or ch4), two-stage down conversion is firstly implemented at the wireless receiver. A 12-GHz sinusoidal RF signal firstly passes through an active frequency doubler ( × 2) and an EA in serial, and is then split into two branches by a power divider. Next, each branch passes through a passive frequency tripler ( × 3) and an EA. As a result of this cascaded frequency doubling, an equivalent 72-GHz RF signal is provided for the corresponding commercial balanced mixer. The 28-GHz intermediate-frequency (IF) signals after first-stage analog down conversion are amplified by two 40-GHz EAs, and then sent to the real-time oscilloscope (OSC) with 120-GSa/s sampling rate and 45-GHz electrical bandwidth. The subsequent digital DSP includes IF down conversion, CD compensation, CMA equalization, carrier recovery, differential decoding and BER counting [25, 26]. CMA equalization with 53 T/2 taps is used to realize polarization de-multiplexing and remove the crosstalk due to wireless delivery .
Analog down conversion is unnecessary for the received 37.5-GHz PDM-16QAM wireless signal (corresponding to ch1 or ch2), which is amplified by 60-GHz EAs and then directly sent to the real-time OSC with 120-GSa/s sampling rate and 45-GHz electrical bandwidth. The subsequent DSP includes IF down conversion, CD compensation, cascaded multi-modulus algorithm (CMMA) equalization, carrier recovery, differential decoding and BER counting [27, 28]. CMMA equalization is used to realize multi-modulus recovery and polarization de-multiplexing. Figure 5(a)-5(d) give the photos for the Q- and W-band wireless system, the transmitter HA array, the receiver HA array and 60-GHz EA array, respectively.
3. Experimental results and discussions
Figure 6 gives BER versus optical signal-to-noise ratio (OSNR) for the 108 Gb/s PDM-QPSK signal at ch2 after 0.7-m wireless delivery. Inset (a) in Fig. 6 shows the X-polarization QPSK constellation after 80-km SMF-28 transmission when the OSNR is 36 dB and the CMA-tap number is 53. BTB denotes no fiber transmission. The BTB BER is below 3.8 × 10−3 when the OSNR is above 32 dB. The BER after 80-km SMF-28 transmission is below 3.8 × 10−3 when the OSNR is 36 dB. 80-km SMF-28 transmission causes no penalty for the PDM-QPSK signal. It is because the nonlinear fiber transmission effect is very small, while all the linear fiber transmission effects, such as fiber CD and polarization mode dispersion (PMD), and the filtering effects can be completely compensated by the DSP algorithms at the wireless receiver [24–26]. The receiver sensitivity for the PDM-QPSK signal will be increased by 1 dB when the wireless transmission is removed. We have verified ch1 and ch2 have the same performance.
Figure 7 gives BER versus OSNR for the 112 Gb/s PDM-16QAM signal at ch4 after 1.5-m wireless delivery. The BTB BER is below 3.8 × 10−3 when the OSNR is above 24 dB, while the X- and Y-polarization constellations at 28 dB OSNR are shown as insets (a) and (b). Compared to the PDM-16QAM signal, an extra 8-dB OSNR is needed for the PDM-QPSK signal to achieve the same BER of 3.8 × 10−3. It is because the bandwidth-limited W-band devices as well as the real-time OSC adopted in our experiment largely degrade the performance of the large-bandwidth PDM-QPSK signal at W-band and lead to a penalty of at least 5 dB. The balanced mixer used at W-band in our experiment has a large insertion loss of ~10 dB, and can also introduce a large penalty for the PDM-QPSK signal. For the PDM-16QAM signal, however, there is no bandwidth limitation and also no balanced mixer needed, and thus the receiver sensitivity is higher. The receiver sensitivity for the PDM-16QAM signal will be increased by 1.6 dB when the wireless transmission is removed. Insets (c) and (d) show the X- and Y-polarization 16QAM constellations after 80-km SMF-28 transmission and 1.5-m wireless delivery at the BER of 9 × 10−4 and 33-dB OSNR. A penalty is introduced by 80-km SMF-28 transmission for the PDM-16QAM signal, which is because the PDM-16QAM modulation is more sensitive to fiber linear and nonlinear transmission impairments. We have verified ch3 and ch4 have the same performance.
For the first time, we experimentally demonstrate a record 400G optical wireless integration system, adopting two mm-wave frequency bands, two orthogonal antenna polarizations, MIMO, photonic mm-wave generation and advanced DSP. We believe that our novel system design and demonstration can usher in a new era of ultra-high bit rate (>400 Gb/s) optical wireless integration communications at mm-wave frequencies.
This work was partially supported by NNSF of China (61107064, 61177071, 60837004, 61250018), NHTRDP of China (2011AA010302, 2012AA011302), NKTR&DP of China (2012BAH18B00) and ICPSSTA (12510705600).
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