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Abstract
A novel RF front-end, which could simultaneously realize wideband RF signal self-interference cancellation (SIC), local oscillator (LO) generator based on optoelectronic oscillator (OEO) and frequency down-conversion has been proposed and experimentally demonstrated. In our microwave photonic RF front-end, only one single-polarization optical in-phase and quadrature-phase (IQ) modulator are required. The upper Mach-Zehnder modulator (MZM) of this optical IQ modulator works as a mixer; the lower MZM works as a reference arm; the parent Mach-Zehnder interferometer (MZI) is used to combine two output optical signals of these two child MZMs. In this way, not only self-interference signal is cancelled in optical domain but also frequency down-conversion is realized at the same time. On the other hand, the upper MZM is also shared to form an OEO by using a self-polarization-stabilization technique. By this means, no external LO signal for frequency down-conversion and electrical attenuator for SIC are needed in our scheme, contributing to compact structure and cost reduction. In our proof-of-concept experiment, a LO signal with central frequency of 10 GHz and phase noise of -108.66 dBc/Hz@10kHz is generated. By optimizing the bias points of the used optical IQ modulator, a 5×20MHz 64-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (64QAM-OFDM) LTE-A signal with central frequency of 12.6 GHz is down-converted to 2.6 GHz, and about 28 dB cancellation ratio is achieved. The proposed scheme is suitable for wideband, integrated co-frequency co-time full duplex 5G communication.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Co-frequency and co-time full duplex (CCFD) technique and millimeter-wave communication technique are two key techniques for the fifth generation (5G) mobile communication [1]. For a RF transceiver using CCFD technique, there will be a large self-interference (SI) to the signal of interest (SOI), because the transmitted signal is also received by its own receiving antenna. This interference signal is in-band signal and it cannot be removed by a bandpass filter (BPF). On the other hand, frequency down-conversion is necessary for 5G RF front-end, because the power fading induced by the dispersion of optical fibers between RF front-end and central office significantly degrades the transmission performance, especially for millimeter-wave communication. However, the electrical mixer and local oscillator (LO) for frequency down-conversion at such high frequency of up to 28 GHz are expensive and difficult to manufacture. Microwave photonic technique provides an alternative method to solve these problems [2–4]. For self-interference cancellation (SIC), a compact dual-drive Mach-Zehnder modulator (DD-MZM) is used to serve as a subtractor to exterminate the self-interference in [5]. By adjusting the electrical tunable delay line and tunable attenuator, the phase and amplitude of the reference signal and the self-interference signal could be matched. In [6], a SIC scheme based on an optical in-phase and quadrature-phase (IQ) modulator is reported, and only an extra electrical time delay line is needed to realize the phase-match. It is a good solution to use one single polarization optical IQ modulator for SIC, although the used bias point could be further optimized in this scheme to maximize the cancellation ratio. In our previous work [7], a modified scheme is proposed where the operating bias point gets optimized for SIC. On the other hand, frequency down-conversion using microwave photonic technique has been widely studied [8–11]. In [8], an optoelectronic oscillator (OEO) based on a phase modulator and a tunable bandpass filter is reported to simultaneously realize RF photonic downconverter and LO generation. A microwave IQ mixer is proposed in [9], in which not only a dual-parallel MZM and a wavelength division multiplexer but also an external LO are required. A microwave phase tunable mixer is also realized by using a dual-polarization DD-MZM in [10]. Moreover, a polarization division multiplexing dual-parallel MZM is used to realize fundamental IQ down-conversion and efficient sub-harmonic IQ down-conversion in [11]. In this scheme, two polarization beam splitters and two balanced photodiodes are required.
The schemes mentioned above could separately realize SIC and frequency down-conversion. However, to simultaneously cope with these two problems, more Mach-Zehnder interferometer (MZI)-type modulators are required, resulting in the increase of system cost and bias control complexity. For example, a dual-polarization optical IQ modulator is used to simultaneously realize self-interference cancellation and frequency down-conversion in [12], and external LO and an electrical attenuator are required in this scheme. For low-cost and low-power consumption consideration, a 5G RF front-end should be simple-structure and multi-functional. Therefore, an efficient method which could integrate not only SIC and frequency down-conversion but also LO generator by using least number of MZI-type modulators is highly desired.
In this paper, based on our previous work [13], we propose a novel method to simultaneously realize wideband SIC, LO generator based on OEO, and frequency down-conversion by using only one single-polarization optical IQ modulator. In our scheme, the upper MZM of this optical IQ modulator works as a mixer; the lower MZM works as a reference arm; the parent MZI is used to couple the two output signals of these two child MZMs. By optimizing the bias points of the used optical IQ modulator, not only self-interference signal is cancelled in optical domain, but also frequency down-conversion is realized at the same time. Moreover, the upper MZM is also shared to form an OEO based on the self-polarization-stabilization technique reported in our previous work [14]. Therefore, no external LO signal and electrical attenuator are needed for frequency down-conversion and SIC. In the proof-of-concept experiment, the results show that a 5×20 MHz 64-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (64QAM-OFDM) LTE-A signal [15] with central frequency of 12.6 GHz is down-converted to 2.6 GHz, and about 28 dB cancellation ratio is achieved. Furthermore, a LO signal with the phase noise of -108.66 dBc/Hz at 10 kHz away from the carrier frequency of 10 GHz is achieved in our test.
2. Principle
The principle of the proposed 5G RF front-end is shown in Fig. 1(a). A continuous-wave (CW) light from a distributed feedback laser (DFB) is injected into a single polarization optical IQ modulator. The modulated light is divided into two parts by an optical coupler (OC), and one part of the modulated lights goes through a self-polarization stabilization dual-loop structure which is reported in our previous work [14]. In each loop structure, the input light will retrace its path through a 45° Faraday rotator (FR1 and FR2) and standard single mode fiber (SSMF) with different length after reflecting off a 45° Faraday rotator mirror (FRM1 and FRM2). These output optical signals of this dual-loop structure are combined in a polarization beam splitter (PBS), and then they are captured by a photodiode (PD1). With the help of this two-loop structure, the output power of this PBS will be stable and not be affected by the mechanical vibration or temperature fluctuation [14]. After a BPF and an electrical amplifier (EA), the generated RF signal is used to drive the upper MZM (MZM1) of this optical IQ modulator to close the OEO loop. This RF signal acts as the LO signal for frequency down-conversion in our scheme.
Fig. 1. The principle of the proposed 5G RF front-end (a), and the schematic diagram of the optical spectra in RF front-end (b).
For SIC, the transmitted signal TX is divided into two parts by an electrical splitter (ES1). One part is launched by the transmitting antenna, and this signal is also captured by the receiving antenna as the SI signal. Due to the CCFD communication, the SOI occupies the same frequency band with the SI signal, and it is captured by the receiving antenna at the same time. The total received signal is called as RX. The other part of TX acts as the reference signal. The RX signal is combined with the generated LO signal by an electrical coupler (EC). Afterwards, this combined electrical signal is loaded to MZM1 which is biased at null point (${\varphi _I} = 180^\circ$). Meanwhile, the reference signal is delayed by a tunable electrical delay line (T-delay) and then loaded to the lower MZM (MZM2) to remove the SI signal. The parent MZI of this optical IQ modulator is biased at null point (${\varphi _P} = 180^\circ$). After that, the undesired optical sideband is filtered by an optical bandpass filter (OBPF), and the filtered optical signal is then captured by PD2. In this way, the wideband RF signal SIC and frequency down-conversion could be simultaneously realized as shown in the schematic diagram of the optical spectra in Fig. 1(b). It should be noticed that although the optical carrier in MZM1 is suppressed, the optical carrier in MZM2 still exists, so the OEO can maintain fundamental frequency oscillation.
To explain this principle clearly, we firstly calculate the output optical field ${E_{out}}$ of the optical IQ modulator. At MZM1, the driving signal consists of the RX signal and the LO signal, while the driving signal of MZM2 is the reference signal. Therefore, ${E_{out}}$ can be expressed as:
From Eq. (2), it can be observed that when ${\varphi _I} = 180^\circ$, the output signal of MZM1 has stronger 1st optical sideband, leading to higher power of down converted SOI, the beating product of the SOI and the LO signal in optical domain. Thus, MZM1 is set at null point in our test, which is different from the bias point used in [7]. Then the cancellation performance is explored in terms of ${\varphi _Q}$ and ${\varphi _P}$. We define output signal-interference-ratio (OSIR) as the power ratio of the SOI and the SI signal at IF, so the cancellation ratio of the proposed system is equivalent to OSIR. According to Eq. (2), OSIR could be written as:
Fig. 2. The contour map of OSIR versus ${\varphi _Q}$ and ${\varphi _P}$ (a), the enlarged contour map of the right dash box (b).
3. Experimental setup and results
The experimental setup of the proposed scheme is shown in Fig. 3. In our proof-of-concept experiment, the SOI is a 5×20MHz 16/64QAM-OFDM LTE-A signal with central frequency of 12.6 GHz, and it is generated by a vector signal generator (VSG, R&S SMW200A). An arbitrary wave generator (AWG, Tektronix AWG 7122C) is used to generate a 150/300 MHz 16/64QAM-OFDM signal with central frequency of 2.6 GHz, and it is then mixed with a 10 GHz LO produced by a microwave signal generator (MSG, R&S SMF100A). The mixed signal is then filtered by an electrical BPF with central frequency of 12 GHz and 3-dB bandwidth of 2 GHz, and the left sideband signal is eliminated to emulate the SI signal with central frequency of 12.6 GHz. In our experiment, the optical spectra of the optical signals are observed by an optical spectrum analyzer (OSA, YOKOGAWA Q6370C-20). The generated LO is measured by a phase noise analyzer (PNA, R&S FSV), and the down converted SOI is analyzed by an electrical spectrum analyzer (ESA, R&S FSWP). In our experiments, we use a commercial optical IQ modulator (Fujitsu FTM7961EX) with DC half voltage of 9 V, RF half voltage of 3.5 V and 3-dB bandwidth of 22 GHz.
3.1. The performance of the generated LO signal
Figures 4(a) and 4(b) show the measured power spectra of the generated LO signal after the filter and EA with different spans of 26.5 GHz and 5 MHz, respectively. The gain of the used EA is 28 dB and its noise figure is less than 5 dB. The electrical power of the generated LO signal is 14.5 dBm which is equivalent to an amplitude value of 1.679 V. The central frequency of this LO signal is 9.9996 GHz because the central frequency of the used BPF2 is about 10 GHz. The sidemode suppression ratio of 52.65 dB can be achieved as shown in Fig. 4(b). Figure 5 shows the single-sideband (SSB) phase noise of the generated LO signal. The phase noise of the generated LO signal at frequency offset of 10 kHz is -91.14 dBc/Hz, -111.04 dBc/Hz, and -108.66 dBc/Hz in single-loop structure with 200 m SSMF fiber, single-loop structure with 2 km SSMF fiber, and dual-loop structure with 100 m and 1 km SSMF fiber, respectively. Compared to the conventional single-loop scheme, the proposed dual-loop OEO has good phase noise performance at both low and high frequency offset, as well as better stability because it needs only half length of optical fiber [14].
Fig. 4. The measured power spectrum of the generated LO signal with span of 26.5 GHz (a) and 5 MHz (b).
Fig. 5. The measured SSB phase noise curves of the single-loop OEOs with 200 m and 2 km SSMF fiber, and the proposed dual-loop OEO with 100 m and 1 km SSMF fiber.
We also test the frequency and power stability of the generated LO signal. Figures 6(a) and 6(b) show the frequency drift and the power drift of the generated LO signal within one hour, respectively. The frequency drift of the generated LO signal is 4 ppm and the power drift is only 0.51 dB within one hour. In this test, the experiment is conducted at room temperature and there is no temperature control. We can say that the generated LO signal in our scheme has good performance in power stability due to the self-polarization stabilization dual loop structure. The frequency stability can be further improved by setting the OEO loop in an incubator using multicore fiber or dynamic feedback compensation technique [16,17].
Fig. 6. The experimental results of frequency drift (a) and power drift (b) of the generated LO signal at 10 GHz within one hour.
3.2. The performance of the wideband RF signal self-interference cancellation and frequency down-conversion
In our test, a 5×20 MHz LTE-A signal with central frequency of 12.6 GHz is down-converted to 2.6 GHz, and SI signals with different bandwidths of 150 MHz and 300 MHz are both tested. The electrical power of the LTE-A signal and the SI signal is 0 dBm which is equivalent to an amplitude value of 0.316 V. The optical spectra of optical signals before and after the OBPF are shown in Figs. 7(a) and 7(b), respectively. As shown in Fig. 7(a), there are many side modes appearing due to the large modulation depth of the LO signal, and nonlinear transfer function of the optical IQ modulator as well. We can also know that the even-order side modes are suppressed, because MZM1 is biased at null point. It could be clearly observed in Fig. 7(b) that only the -1st side mode optical signal which includes the SOI and LO signal is remained by using an OBPF with optical bandwidth of 6 GHz. After that, this -1st side mode optical signal will be captured by PD2 and the down-converted SOI can be acquired.
Figures 8(a) and 8(b) show the electrical spectra of the received down-converted signal with and without SIC when the bandwidths of the SI signals are 150 MHz and 300 MHz respectively. The achieved cancellation ratio is 28.8 dB and 27.27 dB when the bandwidths of the SI signals are 150 MHz and 300 MHz respectively. The measured error vector magnitude (EVM) performance of the down converted SOI with the SI signal bandwidth of 150 MHz and 300 MHz are tested and plotted in Figs. 9(a) and 9(b), respectively. In this test, the electrical power of the SI signal is set to 0 dBm, and the electrical power of the interested OFDM signal is swept from 0 dBm to -15 dBm. Thus, ESIR, which is defined as the power ratio of the interested OFDM signal and the SI signal in the Rx signal, varies from 0 dB to -15 dB. All 5 band 16QAM LTE-A are tested in our experiment, and EVM performance of the central band without SI is tested for comparison. It could be clearly observed in the insets of Figs. 9(a) and 9(b) that without the help of SIC, the constellation of all 5 band 16QAM LTE-A signal is absolute mess and cannot be demodulated correctly in both 150 MHz and 300 MHz SI signals cases, even when ESIR is set to 0 dB. However, when SIC is turned on, the transmission performance below the EVM limit of 12.5% could be achieved for all 5 bands [15]. Different band 16QAM-OFDM signal has a little different EVM performance, and this could be attributed to the uneven frequency response of the used electrical components. This phenomenon could also be observed in Figs. 8(a) and 8(b). The constellations of the down converted SOI signal are plotted in the insets of Figs. 9(a) and 9(b).
Fig. 8. The electrical spectra of the down converted SOI with and without SIC when the bandwidths of the SI signals are 150 MHz (a) and 300 MHz (b).
Fig. 9. The measured EVM performance of 16QAM-OFDM signal versus ESIR when the bandwidths of the SI signals are 150 MHz (a) and 300 MHz (b).
To verify the applicability of the proposed RF front-end, experimental investigation are performed by using 5×20MHz 64QAM-OFDM LTE-A signal with central frequency of 12.6 GHz as the SOI. The measured EVM performance of the down converted SOI in terms of ESIR is reported in Fig. 10, and in this case the SI signal bandwidth is 300 MHz. In this test, the electrical power of the SI signal is set to 0 dBm, and the electrical power of the interested OFDM signal is swept from 0 dBm to -15 dBm. Compared to Fig. 9(b), a higher ESIR is required to achieve the EVM value better than 8% for all band signals due to the more stringent EVM requirement for 64QAM [15], and this could be explained by that larger signal-to-noise ratio is required for higher-order modulation format. However, the transmission performance below the EVM limit of 8% could be achieved for all 5 bands signals when the proposed SIC is turned on. The constellations of the down converted SOI are plotted in the insets of Fig. 10.
Fig. 10. The measured EVM performance of 64QAM-OFDM signal versus ESIR when the bandwidth of the SI signal is 300 MHz.
4. Conclusions
We have experimentally demonstrated a novel RF front-end to use only one single-polarization optical IQ modulator to simultaneously realize wideband RF signal self-interference cancellation, OEO-based LO generator, and frequency down-conversion for the first time. In our scheme, the upper MZM of this optical IQ modulator is used to form a dual-loop OEO based on self-polarization stabilization technique, and no external LO signal is needed. The measured SSB phase noise of the generated 10 GHz LO signal is -108.66 dBc/Hz@10kHz, and the side mode suppression ratio of 52.65 dB is achieved. To make this RF front-end more compact, the upper MZM and the lower MZM are shared to realize SIC and frequency down-conversion at the same time. The experimental results show that a 5×20 MHz 64QAM-OFDM LTE-A signal with central frequency of 12.6 GHz is down converted to 2.6 GHz, and about 28 dB cancellation ratio is achieved. The transmission performance of the down converted SOI below the EVM limit could be achieved even when the SI signal bandwidth is 300 MHz. These results make the proposed method very suitable for wideband, integrated co-frequency co-time full duplex 5G communication.
Funding
National Key Research and Development Program of China (2018YFB1800904); National Natural Science Foundation of China (61675083); Fundamental Research Funds for the Central Universities (2019kfyXMBZ033); State Key Laboratory of Advanced Optical Communication Systems and Networks. (2019GZKF7).
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