1. Introduction

In order to realize the seamless integration of wireless and fiber-optic networks, the wireless links need to be developed to match the capacity of high-speed fiber-optic communication systems, while preserving transparency to bit rates and modulation formats [1–11]. Recently, the W-band (75-110GHz), with wider bandwidth and higher frequency, has attracted increasing interest as a candidate radio-frequency (RF) band to provide multi-gigabit wireless links for mobile data transmission. Meanwhile, the hybrid fiber-wireless link based on optically-modulated signal generation technique is expected to be suitable for W-band wireless transmissions as well as optical wireless links involving seamless integration of optical/wireless networks. Many fiber-wireless links in the W-band have been proposed and experimentally demonstrated in research community, and the bit rates of 40 and 100 Gb/s have been attained adopting spectral efficient modulation format and digital coherent detection [4–6]. A 100-Gb/s fiber-wireless link in the W-band was experimentally demonstrated adopting polarization-division-multiplexing 16-ary quadrature-amplitude-modulation (PDM-16QAM) with 1.2-m wireless delivery, but without wireline fiber transmission [6]. Furthermore, in the scheme, the X- and Y-polarization components from two transmitter antennas were not received simultaneously at the wireless receiver because there is only one receiver antenna in the system, and the net data rate is smaller than 100 Gb/s after removing the forward-error-correction (FEC) overhead as well. Because no polarization-diversity heterodyne coherent receiver is employed, the receiver in the scheme is polarization sensitive and one polarization tracking system will be needed in the real system. In fact, one polarization controller was employed in [6] to simulate polarization tracking system.

In this paper, we experimentally demonstrate the seamless integration of 57.2-Gb/s signal wireline transmission and 100-GHz wireless delivery adopting polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) modulation with 400-km single-mode fiber-28 (SMF-28) transmission and 1-m wireless delivery. The X- and Y-polarization components of the optical PDM-QPSK baseband signal are simultaneously up-converted to 100-GHz wireless carrier by optical polarization-diversity heterodyne beating, and then transmitted over a 2x2 multiple-input multiple-output (MIMO) wireless link. At the wireless receiver, a two-stage down conversion is firstly done in analog domain based on balanced mixer and sinusoidal RF signal, and then in digital domain based on digital signal processing (DSP). Polarization de-multiplexing is realized by constant modulus algorithm (CMA) based on DSP in heterodyne coherent detection. Our experimental results show that more taps are required for CMA when the X- and Y-polarization antennas have different wireless distance. The optimal CMA tap is longer than 23 when there is 10-cm difference on wireless distance between the X- and Y-polarization components. To our knowledge, the CMA tap for commercial 100G PDM-QPSK product is around 13, which means that more taps are required for this system if the X- and Y-polarization antennas have different distance.

2. Principle for the seamless integration of PDM signal wireline transmission and W-band wireless delivery over 2x2 MIMO wireless links

Figure 1 shows the architecture for the seamless integration of PDM signal wireline transmission and W-band wireless delivery over 2x2 MIMO wireless links, including central office (CO) to generate optically-modulated PDM baseband signal, remote antenna units (RAUs) to up-convert the optical PDM baseband signal into the W-band, and end users to down-convert the received W-band PDM signal into the baseband.

 

Fig. 1 The architecture for the seamless integration of PDM signal wireline transmission and W-band wireless delivery over 2x2 MIMO wireless links. Opt. Mod.: optical modulator, Pol. Mux: polarization multiplexer, SMF: single-mode fiber, OC: optical coupler, LO: local oscillator, PBS: polarization beam splitter, BPD: balanced photo detector, HA: horn antenna, Pow. Div.: power divider, CO: central office, RAU: remote antenna unit.

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At the optical baseband transmitter in the CO, the continuous wavelength (CW) lightwave from a laser is modulated by an optical modulator and then polarization-multiplexed by a polarization multiplexer to generate optical baseband signal. The optical modulator is driven by the transmitter data. After fiber-optic transmission, the baseband signal is respectively received by multiple RAUs. At the optical heterodyne up-converter in each RAU, there is a laser functioned as the local oscillator (LO), an optical 180° hybrid, two fast-response photo detectors (PDs) and two W-band transmitter horn antennas (HAs). Here, the LO is used as the carrier-frequency generating source. The frequency spacing between the LO and the laser in the CO is located in the W-band in order to generate the W-band central carrier frequency for the up-converted wireless signal. The optical 180° hybrid includes two polarization beam splitters (PBSs) and two optical couplers (OCs), and is used to implement polarization diversity of the received signal together with the LO in optical domain before heterodyne beating. Next, two fast-response PDs, functioned as two photo-mixers, directly up-convert the X- and Y-polarization components of the optical PDM signal into the W-band, respectively. It’s worth noting that X- or Y-polarization component of the PDM signal after polarization diversity does not mean that only X- or Y-polarization signal exists at each output port of PBSs. In fact, each output port contains both X- and Y-polarization signals. In this paper, we define one output port of each PBS as X-polarization component and the other as Y-polarization for simplification. The central frequency of the X- and Y-polarization up-converted components should be equal to the frequency spacing between the LO and the laser in the CO. Then, the X- and Y-polarization components, at the same time, are independently sent into free space by two transmitter HAs, and then received by two corresponding receiver HAs at the end user, which makes up a 2x2 MIMO wireless link based on microwave polarization multiplexing. At each end user, there is a two-stage down conversion [6]. In the first stage, the X- and Y-polarization components are respectively down-converted to a lower intermediate frequency (IF) in analog domain based on balanced mixer and sinusoidal RF signal, and subsequently implemented analog-to-digital conversion in a digital storage oscilloscope (OSC). Then, the second-stage down conversion and final data recovery is realized with DSP in digital domain.

3. Experimental setup

Figure 2 shows the experimental setup for the seamless integration of 57.2-Gb/s signal wireline transmission and 100-GHz wireless delivery adopting PDM-QPSK modulation with 400-km SMF-28 transmission and 1-m wireless delivery. At the optical baseband transmitter, there is an external cavity laser (ECL) with linewidth less than 100kHz and maximal output power of 14.5dBm. The CW lightwave at 1558.51nm from ECL is modulated by in-phase/quadrature (I/Q) modulator. I/Q modulator is driven by a 14.3-Gbaud electrical binary signal, which, with a pseudo-random binary sequence (PRBS) length of 2¹⁵-1, is generated from a pulse pattern generator (PPG). For optical QPSK generation, the two parallel Mach-Zehnder modulators (MZMs) in I/Q modulator are both biased at the null point and driven at the full swing to achieve zero-chirp 0- and π-phase modulation. The phase difference between the upper and the lower branches of I/Q modulator is controlled at π/2. The subsequent polarization multiplexing is realized by polarization multiplexer, comprising a PBS to halve the signal into two branches, an optical delay line (DL) to provide a 150-symbol delay, an optical attenuator to balance the power of two branches and a polarization beam combiner (PBC) to recombine the signal. The generated signal is launched into the straight line of five spans (the maximal distance) of 80-km SMF-28. Each span has 18-dB average loss and 17-ps/km/nm chromatic dispersion (CD) at 1550nm without optical dispersion compensation. Erbium-doped fiber amplifier (EDFA) is used to compensate the loss of each span. The total launched power (after EDFA) into each span is 0dBm.

 

Fig. 2 Experimental setup for the seamless integration of 57.2-Gb/s signal wireline transmission and 100-GHz wireless delivery. Inset (a) shows the X-polarization optical spectrum (0.01-nm resolution) after polarization-diversity splitting. EDFA: Erbium-doped fiber amplifier, SMF: single-mode fiber.

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At the optical up-converter, an ECL with linewidth less than 100 kHz is used as the LO at 1557.71nm, which has 100-GHz frequency offset (IF₁ = 100GHz) relative to the received signal. The principle of the up-converter is introduced in detail in Section 2. Each 100-GHz PD has 75-GHz 3-dB bandwidth and 7.5-dBm input power. The X- and Y-polarization up-converted components on 100-GHz wireless carrier independently pass through two 100-GHz narrowband electrical amplifiers (EAs) with 32-dB gain, and then, are simultaneously sent into a 2x2 MIMO wireless air link based on microwave polarization multiplexing. Each pair of transmitter and receiver HAs (transmitter HA1 and receiver HA1 as well as transmitter HA2 and receiver HA2) has a 0.5~1.5-m wireless distance, the X- and Y- polarization wireless links are parallel and two transmitter (receiver) HAs have a 40-cm distance. Each HA has a 25-dBi gain. Inset (a) shows the X-polarization optical spectrum after polarization-diversity splitting. Here, ch1 denotes the LO, while ch2 the received signal. The frequency spacing and power difference between ch1 and ch2 is 100GHz and 20dB, respectively.

At the wireless receiver, two-stage down conversion is implemented for the X- and Y-polarization received components. A 12-GHz sinusoidal RF signal firstly passes through an active frequency doubler (x2) and an EA in serial, and is then halved into two branches by a power divider. Next, each branch passes through a passive frequency tripler (x3) and an EA. As a result of this cascaded frequency doubling, an equivalent 72-GHz RF signal is provided for the corresponding balanced mixer. Therefore, the X- and Y-polarization components centered on 28GHz (IF₂ = 28GHz) are obtained after first-stage down conversion, as shown in Fig. 3(a) . Each band-pass low-noise amplifier (LNA) after the mixer is centered on 100GHz and has a 5-dB noise figure. The analog-to-digital conversion is realized in the real-time OSC with 120-GSa/s sampling rate and 45-GHz electrical bandwidth.

 

Fig. 3 (a) Electrical spectrum after first-stage down conversion; (b) DSP.

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Figure 3(b) shows the detailed DSP after analog-to-digital conversion. Firstly, the received signals are down-converted to the baseband by multiplying synchronous cosine and sine functions, which are generated from a digital LO for down conversion [12]. Secondly, a T/2-spaced time-domain finite impulse response (FIR) filter is used for CD compensation, where the filter coefficients are calculated from the known fiber CD transfer function using the frequency-domain truncation method. Fourthly, two complex-valued, 13~33-tap, T/2-spaced adaptive FIR filters, based on the classic CMA, are used to retrieve the modulus of the PDM-QPSK signal and realize polarization de-multiplexing. The subsequent step is carrier recovery, which includes frequency-offset estimation and carrier-phase estimation (CPE). The former is based on fast Fourier transform (FFT) method while the latter fourth-power Viterbi-Viterbi algorithm. Finally, differential decoding is used to eliminate the π/2 phase ambiguity before bit-error-ratio (BER) counting. In this experiment, the BER is counted over 12 × 10⁶ bits (12 data sets, and each data set contains 10⁶ bits).

4. Experimental results and discussions

Figure 4 gives the BER versus optical signal-to-noise ratio (OSNR) at 0.1-nm noise level after 1-m wireless delivery. The launched power into fiber is 0dBm. Here, back-to-back (BTB) denotes no fiber transmission. Compared to the BTB case, there is almost no OSNR penalty after 400-km SMF-28 transmission. Inset (a) and (b) show the X- and Y-polarization constellations after CPE over 400-km SMF-28 transmission, respectively. Figure 5 gives the X-polarization constellations after CMA and further CPE in the case of 13- and 23-tap CMA length, respectively. The Y-polarization constellations show the similar performance. There is 5-cm difference on wireless distance between the X- and Y-polarization components. It’s worth noting that transmitter HA1, receiver HA1 and receiver HA2 are all fixed, while the distance between transmitter HA2 and receiver HA2 is changed by moving transmitter HA2. We can see that the constellations for the 23-tap CMA length are much clearer than those for the 13-tap CMA length. Figure 6 gives the Y-polarization constellations after CPE for 13-, 23- and 33-tap CMA length, respectively. There is 10-cm difference on wireless distance between the X- and Y-polarization components. Similarly, the constellation becomes clearer as the length of CMA tap increases. Furthermore, more CMA taps will be required for the larger difference on wireless distance between the X- and Y-polarization components. The X-polarization constellations after CPE show the similar performance. To our knowledge, the CMA tap for commercial 100G PDM-QPSK product is around 13, which means that more taps are required for this system if X- and Y-polarization antennas have different distance.

 

Fig. 4 BER versus OSNR after 1-m wireless delivery. Inset (a) and (b) show the X- and Y-polarization constellations after CPE over 400-km SMF-28 transmission, respectively.

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Fig. 5 X-polarization constellations. (a) 13 tap, after CMA; (b) 13 tap, after CPE; (c) 23 tap, after CMA; (d) 23 tap, after CPE.

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Fig. 6 Y-polarization constellations after CPE for 13-, 23- and 33-tap CMA length.

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5. Conclusion

We first experimentally demonstrated a fiber-wireless system that is capable to deliver 57.2-Gb/s PDM-QPSK signal over 400-km SMF-28 and 1-m wireless delivery at 100GHz over a 2x2 MIMO wireless link based on microwave polarization multiplexing, adopting two-stage down conversion both in analog and digital domains. The optimal CMA tap is longer than 23 when there is 10-cm difference on wireless distance between the X- and Y-polarization components. To our knowledge, the CMA tap for commercial 100G PDM-QPSK product is around 13, which means that more taps are required for this system if X- and Y-polarization antennas have different distance.