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Full-band direct-conversion receiver with enhanced port isolation and I/Q phase balance using microwave photonic I/Q mixer (Invited Paper) Jianqiang Li (李建强)1, Jia Xiao (肖 佳)1, Xiaoxiong Song (宋骁雄)1, Yue Zheng (郑 月)1, Chunjing Yin (尹纯静)1, Qiang Lv (吕 强)2,3, Yuting Fan (樊宇婷)1, Feifei Yin (尹飞飞)1, Yitang Dai (戴一堂)1, and Kun Xu (徐 坤)1,* 1
State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China 2 CETC Key Laboratory of Aerospace Information Applications, Shijiazhuang 050081, China 3 The 54th Research Institute of China Electronics Technology Group Corporation, Shijiazhuang 050081 China *Corresponding author:
[email protected] Received September 18, 2016; accepted December 15, 2016; posted online January 4, 2017 A full-band direct-conversion receiver using a microwave photonic in-phase and quadrature (I/Q) mixer is proposed and experimentally evaluated in terms of radio frequency (RF) range, port isolation, phase imbalance, conversion gain, noise figure, spurious-free dynamic range, and error vector magnitude. The proposed microwave photonic I/Q mixer shows significant advantages in local oscillator leakage and I/Q phase imbalance over entire RF bands, which are recognized as major drawbacks of conventional direct-conversion receivers. OCIS codes: 060.5625, 070.1170, 250.4110. doi: 10.3788/COL201715.010014.
The growing use of the radio spectrum requires electronic systems to operate at extended radio-frequency (RF) bands and signal bandwidth, putting forward great challenges to the RF receiver design. Microwave photonic techniques are recently introduced to overcome these limitations due to several inherent advantages, such as potential full-band operation, large instantaneous bandwidth, high RF isolation, low-loss transmission, and electromagnetic interference immunity[1–3]. As for frequency conversion that is a crucial function in an RF receiver, several microwave photonic schemes have been studied. A photonic method for wideband tunable RF conversion was presented by Harris Corporation[4]. With different photodetection fashions, a reconfigurable photonic microwave mixer was proposed[5]. A further RF front-end design was done in Ref. [6], using an integrated ultra-high quality factor bandpass filter. In the above works, tunable narrow-band optical filters have to be used to reduce the spurs, raising the implementation complexity. In addition, the above works are mainly oriented to super-heterodyne receivers. Compared to the super-heterodyne, the directconversion architecture with zero intermediate frequency (IF) can circumvent image problems, be more suitable to a multi-standard software-defined radio, facilitate amplifier and filter design, and potentially decrease power consumption. More importantly, the high port isolation property of microwave photonic mixers determines the mitigation of self-mixing and local oscillator (LO) leakage, which are considered as major drawbacks of direct-conversion receivers. In this Letter, we propose using a microwave 1671-7694/2017/010014(4)
photonic in-phase and quadrature (I/Q) mixer to implement a full-band direct-conversion receiver. Optical I/Q modulators are employed instead of the conventional Mach–Zehnder modulators (MZMs) to achieve singlesideband carrier-suppressed (SSB-CS) modulation, saving the tunable narrow-band optical filters. The schematic diagram of the proposed direct-conversion receiver based on a microwave photonic I/Q mixer is shown in Fig. 1. A continuous wave (CW) laser at around 1550 nm is first generated and then split into two paths. The RF and LO signals at the same central frequency are, respectively, applied to the optical carriers on the two paths by two optical LiNbO3 I/Q modulators both working at the SSB-CS mode. Two microwave quadrature hybrids and automatic bias controllers (ABCs) enable SSB-CS modulation of the two optical I/Q modulators. The SSB-CS signals on the two paths are sent to a 90° optical hybrid, generating two differential optical inphase outputs (labeled as I1 and I2) and two differential optical quadrature outputs (labeled as Q1 and Q2). The I1 and I2 outputs are injected into a balanced photodetector (BPD), producing the in-phase component (I) of the down-converted RF signal. Q1 and Q2 outputs were injected into the other BPD, producing the quadrature component (Q). Note that the direct current (DC) offset in the I and Q baseband signals could be mitigated due to use of BPDs. Finally, the I and Q baseband signals are digitized and captured by a digital storage oscilloscope (DSO) at 6.25 GSa/s for subsequent digital signal processing (DSP).
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Fig. 1. Block diagram of the proposed full-band direct-conversion receiver based on a microwave photonic I/Q mixer.
A series of experiments was carried out to evaluate both the microwave and system performances of the proposed direct-conversion receiver based on a microwave photonic I/Q mixer. In the experiments, a microwave vector signal generator (VSG) was used to generate a single-carrier 16ary quadrature amplitude modulation (16QAM) RF signal with a 40 MHz bandwidth and a 0.35 roll-off factor for testing. A microwave source was used to provide the LO signal with a carrier frequency identical to the RF signal. The carrier frequencies of both the used VSG and microwave source can be tuned up to 20 GHz. The two 200 MHz BPDs were both embedded with ∼20 dB low-noise amplifiers (LNAs) to partially compensate for the conversion loss. The optical power output from the CW laser was about 18 dBm. Offline digital signal processing was done in Matlab[7,8]. After the I/Q imbalance was digitally compensated based on the Gram–Schmidt algorithm, the sample streams were resampled to two samples/symbol. The blind adaptive equalization was performed by a 17-tap T/ 2-spaced finite impulse-response (FIR) filter. The FIR filter was first adapted by the standard constant-modulus algorithm (CMA) for pre-convergence. The final adaptation was done by switching the CMA to a decisiondirected least-mean-square (DD-LMS). In the DD loop, the carrier recovery was done based on the blind phase search method. Finally, the error vector magnitude (EVM) was calculated. Note that several amounts of residual DC offset and second-order distortion may occur due to the device’s imperfection and the BPDs’ nonlinearity. The residual DC offset was further compensated for by DSP. The optical power input to the BPDs was kept in the linear region to achieve a low level of second-order distortions. The path mismatch between the two I/Q modulators might have effects on the phase noise of the final zero-IF signal. There are two ways to solve the problems. First, a narrow-linewidth laser is expected to be used to allow a larger amount of path mismatch. Second, the phase noise can be further compensated for in the digital domain. First, we experimentally evaluated the microwave photonic I/Q mixer as a four-port microwave component (i.e., Port RF, LO, I, and Q). By the same test methods used for
conventional microwave components, port isolation, conversion loss, noise figure, and spurious-free dynamic range (SFDR) as a function of the RF carrier frequency were all measured. The LO-to-RF port isolation was measured to always be larger than 100 dB over the entire frequency range, which effectively avoids LO leakage and selfmixing. Since the input optical power to the BPDs are around −10 dBm, the mixer exhibited a conversion loss that was 10–15 dB despite the presence of the embedded LNAs. The conversion loss can be further reduced by inserting optical amplifiers to compensate for the optical loss. The curves in the noise figure and the SFDR are plotted in Figs. 2 and 3, respectively, where two converted IFs are considered (i.e., 1 GHz and 100 MHz). Note that the used optical I/Q modulator from Fujitsu only has a 3 dB bandwidth of ∼22 GHz, which mainly limited the operating frequency range. It can be seen that the microwave performance holds within 20 GHz with insignificant degradation. Specifically, the noise figure is below 45 dB, and the SFDR stays beyond 105 dB·Hz2∕3 . The performance can be further improved by decreasing the optical loss, by using pre-LNA and broadband half-wave voltage (Vpi)
Fig. 2. Noise figure as a function of the RF carrier frequency.
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Fig. 3. SFDR as a function of the RF carrier frequency.
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Fig. 4. Phase imbalance as a function of the RF carrier frequency.
optical modulators. In addition, we also measured the amplitude and phase imbalance of the proposed I/Q mixer with the help of the digital signal processing. In the proposed I/Q mixer, the power and phase imbalance between the I/Q paths mainly originate from the optical 90° hybrid, the different responses of the two BPDs, and the power/phase mismatch among the paths after the optical 90° hybrid. The typical amplitude imbalance between the I and Q outputs is within 0.4 dB over the entire RF bands below 20 GHz. In practice, phase imbalance is more crucial than amplitude imbalance, which can be easily compensated. Therefore, the phase imbalance was evaluated as a function of the RF carrier frequency. As shown in Fig. 4, the phase deviation was within −1° ∼ þ3° over the entire RF bands below 20 GHz. In fact, it is quite tough for microwave I/Q mixers to achieve this level of phase balance across multiple octaves in frequency. For example, the typical phase deviation over 2–18 GHz is larger than 10° for the I/Q mixers with a model number of MLIQ0218 from Marki Microwave and a model number of IRM0226LC1Q from MITEQ. The consistence and high level of phase balance over whole RF bands is expected to be another essential advantage of microwave photonic mixers for full-band direct-conversion receivers. Next, the performance in the EVM was evaluated by directly converting 40 Mbaud single-carrier 16QAM RF signals. Given a fixed input RF power of 5 dBm, the EVM performance as a function of the RF carrier frequency is shown in Fig. 5, where a typical constellation is also inserted. It can be clearly seen that the EVM holds over the entire 20 GHz with less than 0.4% of the EVM variation. The EVM performance as a function of the input RF power is shown in Fig. 6 for two typical RF carrier frequencies. Given a 10% of the EVM threshold, the EVM performance holds over 45 dB variation of input RF power for both curves, indicating a large dynamic range of the proposed direct-conversion receiver. Note that the input RF power ends at 25 dBm due to our limited experimental instruments.
Fig. 5. EVM as a function of RF carrier frequency.
EVM (%)
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Fig. 6. EVM as a function of the input RF power.
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In conclusion, a full-band direct-conversion receiver using a universal microwave photonic I/Q mixer is proposed. The full-band operation is tested in terms of RF carrier frequency range, conversion gain, noise figure, SFDR, phase imbalance, and EVM. The performance holds in the entire 20 GHz range under test. The performance can be further improved by reducing the optical loss, using pre-LNA, and more advanced optical modulators. Besides the well-known full-band operation, large instantaneous bandwidth, and electromagnetic interference immunity, the proposed microwave photonic I/Q mixer also shows mitigated LO leakage and I/Q phase imbalance, which are recognized as major drawbacks of conventional direct-conversion receivers. Therefore, a microwave photonic mixer is suggested as needing more attention for direct-conversion receivers. This work was supported in part by the National 863 Program of China (No. 2015AA016903), the National Natural Science Foundation of China (No. 61431003, 61601049, and 61401411), the Innovation Foundation of
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China Electronics Technology Group Corporation (CETC), and the Innovation Foundation of Key Laboratory of Aerospace Information Applications at CETC. References 1. T. R. Clark and R. Waterhouse, IEEE Microwave Mag. 12, 87 (2011). 2. X. Han, S. Zhang, C. Tong, N. Shi, Y. Gu, and M. Zhao, Chin. Opt. Lett. 11, 050604 (2013). 3. Y. Jin, E. H. W. Chan, X. Feng, X. Wang, and B. Guan, Chin. Opt. Lett. 13, 050601 (2015). 4. A. Mast, C. Middleton, S. Meredith, and R. DeSalvo, in IEEE Aerospace Conference (2012). 5. Z. Tang and S. Pan, in International Topical Meeting on Microwave Photonics (MWP) and the 2014 9th Asia-Pacific Microwave Photonics Conference (APMP) (2014). 6. H. Yu, M. Chen, Q. Guo, M. Hoekman, H. Chen, A. Leinse, R. G. Heideman, S. Yang, and S. Xie, in 2015 Optical Fiber Communications Conference and Exhibition (OFC) (2015). 7. S. J. Savory, J. Lightwave Technol. 16, 1164 (2010). 8. J. Li, E. Tipsuwannakul, T. Eriksson, M. Karlsson, and P. A. Andrekson, J. Lightwave Technol. 30, 1664 (2012).
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