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OFC 2014 © OSA 2014
Microwave Beam Steering with Tunable Spectral Filtering Using Cyclic Additional Optical True Time Delay Z. Cao1, Q. Wang1, R. Lu1,3, A. C. F. Reniers2, H.P.A. van den Boom1, E. Tangdiongga1, and A.M.J. Koonen1 1 COBRA Research Institute, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands Centre for Wireless Technology, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands 3 School of Optoelectronic Information, State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China Author e-mail address:
[email protected] 2
Abstract: A novel broadband microwave beam steering scheme for high capacity wireless communication is proposed with tunable spectral filtering using cyclic additional optical true time delay. The experimental results match well with the theoretical analysis. OCIS codes: (060.5625) Radio frequency photonics; (070.1170) Analog optical signal processing
2. Operation Principle The principle of MBS based on CAO-TTD is depicted in Fig.1. The optical carrier with an RF (microwave) signal modulated feeds the optical delay network as shown in Fig. 1 (a). The optical signal is converted back to the duplicated RF signals via photo-diodes (PDs). The optical delays are then transformed into equivalent microwave phase shifts (PSs) which will shape and steer the microwave beam. For a uniform linear array shown in Fig. 1(a), the array factor (AF) with OTTD can be generally expressed as: 2 fnd c AF ( , f ) I n exp[ j ( sin )] c d n 0 N 1
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1. Introduction Spatial division multiplexing (SDM) is of interest for wireless communications since it can drastically increase data rates by exploiting the capacity of spatial domain. Multiple-input multiple-output (MIMO) technology has been proposed and widely investigated with spatial cross-talks suppression assisted by digital signal processes (DSPs). However, in a MIMO system, the wireless signal power is diverged in all directions, which largely limits its energy efficiency. Apart from MIMO, microwave beam steering (MBS) can largely reduce the energy consumption by steering the wireless power to a specified direction. As a SDM technology, MBS can also boost the capacity but with much less DSP complexity. Phased antenna arrays (PAAs) are widely considered as the best candidate for MBS due to its fast steering capability and compactness [1]. A severe limitation is often caused by the use of phase shifters to steer the beam, which results in beam deformations ("squint") in the measured antenna pattern when high bandwidths are used. The use of optical true time delay (OTTD) technology potentially eliminates this bandwidth restriction since it provides a frequency independent time delay for each array elements with low loss, low complexity and compactness compared with traditional electrical true time delay (TTD) technology [2-6]. Moreover, OTTD can be integrated into radio-over-fiber (RoF) systems with low additional cost. In Ref. [6], we have reported our in-home radio-overfiber system with OTTD-MBS. A significant sensitivity improvement has been observed with 1-Gbps transmission. Nowadays, wireless services operated at different frequency bands co-exist. Meanwhile the dynamic spectrum allocations in wireless communications are highly desired. Motivated by these facts, we propose a novel broadband microwave beam steering scheme with tunable microwave filtering based on cyclic additional optical true time delay (CAO-TTD). Compared with traditional OTTD, CAO-TTD introduces negligible additional complexity. By including this low loss extra optical delay, spectral filtering of the RF signal can be achieved.
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Fig. 1 Principle of microwave phase antenna array based on cyclic additional optical true time delay (CAO-TTD)
where θ is the observing angle as shown in Fig. 1 (a), f the frequency of microwave signal, N the number of element antennas, d and Td the distance and delay between two adjacent antenna elements, and In the element weight. The total delay for each element antenna includes the free space travelling time (such as ‘Ttra1’ and ‘Ttra2’ shown in Fig.1) and the time caused by optical delays (‘0’ and ‘τ+mT’ in Fig.1). τ is the propagation delay between two adjacent element antennas as shown in Fig. 1(a). From Eq.1, we can see that the maximum value of AF is achieved when –sinθ+c×τ/d is equal to zero. In the other word, the main lobe of the microwave beam directs to the angle θ when τ is equal to d×sinθ/c. Now, we explore the properties of the
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TH3D.2.pdf
OFC 2014 © OSA 2014
proposed CAO-TTD. When the delay is introduced with integer multiple periods of the microwave carrier, the AF can be rewritten as: N 1 c 2 fnd AF ( , f ) I n exp j ( sin ) exp( j 2 fnmT ) c d n 0
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where T is the period of the microwave carrier, and m is the number of integer multiple. Note that the side lobes of the microwave beam are determined by the coefficient 2πfd/c. Therefore such feature will not be changed by the delay involved. When τ is equal to d×sinθ/c, the expression in the square bracket of Eq.2 will be zero. This means that the value of AF is only affected by the second exponential function which is essentially a microwave photonics filter [6]. Now we can see that the spectral filtering of the received microwave signal in the θ direction can be controlled by tuning the integer multiple m. This operation of microwave photonics filtering is illustrated in Fig. 1(b). When the main lobe is obtained, the RF signals from different element antennas can spatially be combined with m×T time difference. The resulting spectral filtering is schematically shown as the inset (i) of Fig. 1(b), with the free spectral range (FSR) equal to 1/(m×T). The optical operation of the additional delays introduces negligible power degradation since the optical loss can be very low. The CAO-TTD can be either a path-switch based or dispersion based schemes and the additional delays for spectral filtering will not introduce significant additional complexity. 3. Experimental Setup and Results VNA Fig. 2 shows the proof-of-concept experimental setup of the CAO-TTD scheme. The optical Bias PD1 EA1 AnTx1 ODL1 carrier is generated from a DFB laser at DFB 1550.016nm with 3dBm power. After a Splitter Wireless Links AnRx PC MZM polarization controller (PC), it is fed into a PD2 EA2 AnTx2 ODL2 Mach-Zenhder modulator (MZM) with 20Wireless Links GHz 3-dB bandwidth (BW). The MZM is MZM PD+EA biased at the quadruple point to obtain the AnRx AnTx AnTx1 AnRx maximum linear dynamic range in case of ODLs intensity modulation. The stimulus microwave sinuous signal from a vector network analyzer AnTx2 (VNA) drives the MZM to modulate the Inset (i) Inset (ii) optical carrier. The stimulus signal is swept from 7.5GHz to 12.5GHz. The modulated Fig. 2 The proof-of-concept experimental setup of cyclic additional optical true time delay scheme. optical signal is then split into two branches cascaded with tunable optical delay lines (ODL1/2). The ODL1 is used to compensate the offset delay between the two branches while the ODL2 is used to introduce the optical true time delays for both MBS and spectral filtering. The optical signals alongside the two branches are then fed into two 40-GHz 3-dB BW photo-diodes (PD1/2) for the optical to electrical conversion. For this experiment, in principle, the 3-dB BW of 12.5GHz can be acceptable. The converted microwave signals are then boosted into two element antennas (AnTx1/2) of the transmitter PAA via two broadband amplifiers with 12.5-GHz 3-dB BW. AnTx1/2 are identically broadband 10-GHz aperture antennas with 5-GHz 3-dB BW. A photo of whole experimental setup is shown as inset (i) in Fig.2. The antennas are placed on metal mounts to reduce the reflection from the optical table. The wireless links are shown as inset (ii) in Fig.2. AnTx1/2 and AnRx are at the same height and in a plane (working plane) parallel to the optical table. AnTx1/2 are laid close to each other with 41.5mm center-to-center distance. The radiation plane of AnTx1/2 are parallel to the one of AnRx as shown in inset (ii) of Fig.2. The mutual coupling between AnTx1 and AnTx2 is less than 20-dB. The microwave beam is steered in the plane parallel to the optical table. Before we investigate the properties of CAO-TTD, the wireless links from AnTx1/2 to AnRx are characterized as shown in Fig. 3(a). The wireless link connecting AnTx1 and AnRx is named ‘Link1’ as shown in Fig.2, and the other is named ‘Link-2’. The transmission curve (S12) is measured as shown in Fig. 3(a) and A 5GHz pass-band is observed with the out-of-band frequency components sharply reduced. A 4-dB received power difference of Link-1 and Link-2 is clearly shown in Fig. 3(b). The 4-dB power imbalance (PoIm) will limit the power suppression ratio (PSR) around -12.9-dB. This imperfect PSR will be observed in following. The 2-D far field pattern (FFA) of the element antenna in the working plane is shown in Fig. 3(c). The 3-dB angle width of the 2-D FFA is 70˚ (-35˚ - 35˚). The MBS and spectral filtering properties of CAO-TTD scheme are characterized. The normalized received power of 10-GHz microwave versus optical delays is shown in Fig. 4(a). The peaks with 0, 30, 60-mm optical length, and two minimum points with 15 and 45-mm optical lengths are observed. It is clear that the received power is periodic versus the optical delay with a 30mm periodic length (the wavelength of 10GHz microwave). Therefore the cyclic additional integer multiple of 30mm optical delay will not affect the beam profile at the carrier frequency. Meanwhile, the 30-mm optical delay can cover the whole MBS space. The detailed microwave beam steering investigation for such kind of systems can be found in [6]. Now we investigate the CAO-TTD induced spectral filtering.
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As shown in Fig. 4(b)-(c), the transmission curves (S12) for different CAO-TTDs are measured and compared with their corresponding simulated results. The measured curves have been calibrated to eliminate the frequency ripples of two wireless links. The measured and simulated curves are then normalized at their max values. Since the residual ripples of the measured curves exist, for some comparison sets, the measured curves are a little lower than the simulated ones. In general, good matches between the measured and simulated results are obtained. The PSRs for both Fig. 4(b) and (c) are around -13-dB, which exhibits a very good agreement with our previous analysis. The band-pass filtering with different optical delays (thus with different FSRs) are shown in Fig. 4(b). As the optical delay increases, the pass-band and FSR are both suppressed as indicated in Eq.2. This exhibits good configurability for spectral allocations. Three sets of CAO-TTDs with 29, 59, 87-mm optical delay are employed. These delays are chosen close to the integer multiple of the microwave carrier wavelength (30mm). And therefore the power of microwave carrier (10-GHz) is not affected by the cyclic additional optical delay as shown in Fig. 4(b). Such periodic feature of CAO-TTD is also demonstrated for the band-stop filtering shown in Fig. 4(c). As shown in Fig. 4 (a), 15-mm optical delay introduces a minimum point for microwave carrier (10-GHz). With 60-mm cyclic additional delay, the minimum point is observed at 10-GHz again as shown in Fig. 4(c). 4. Conclusion A novel broadband microwave beam steering with tunable spectral filtering using cyclic additional optical true time delay (CAOTTD) is proposed and experimentally investigated. With high energy efficiency, and tunable spectral filtering, we believe that the proposed CAO-TTD is attractive for future wireless communications, especially in the context of RoF networks. This work has been supported by The Netherlands Organization for Scientific Research (NWO) under the project grant Smart Optical-Wireless In-Home Communication Infrastructure (SOWICI). References [1] R. Mailloux, "Phased Arrya Antenna Handbook", Artech House, 2005. [2] W. Ng, et al, "The first demonstration of an optically steered microwave phased array antenna using true-time-delay," Lightwave Technology, Journal of, vol. 9, pp. 1124-1131, 1991. [3]O. Raz, et al, "Submicrosecond Scan-Angle Switching Photonic Beamformer With Flat RF Response in the C and X Bands," Lightwave Technology, Journal of, vol. 26, pp. 2774-2781, 2008. [4]I. Frigyes et al, "Optically generated true-time delay in phased-array antennas," Microwave Theory and Techniques, IEEE Transactions on, vol. 43, pp. 2378-2386, 1995.
[5] B. Vidal, et al, "Optical delay line employing an arrayed waveguide grating in fold-back configuration," Microwave and Wireless Components Letters, IEEE, vol. 13, pp. 238-240, 2003. [6] Z. Cao, et al, “Optical True-Time-Delay Microwave Beam-Steering with 1 Gb/s Wireless Transmission for In-Building Networks”, Proceedings of the 39th European Conference on Optical Communication (ECOC 2013), London, UK, Tu.1.F.2, 2013. [7] J. Capmany, et al, "A tutorial on microwave photonic filters," Lightwave Technology, Journal of, vol. 24, pp. 201-229, 2006.
