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Photonic switched beamformer implementation for broadband wireless access in transmission and reception modes at 42.7 GHz
Optics Communications 249 (2005) 441–449 www.elsevier.com/locate/optcom
Photonic switched beamformer implementation for broadband wireless access in transmission and reception modes at 42.7 GHz Miguel A. Piqueras, Borja Vidal *, Javier Herrera, Valentı´n Polo, Juan. L. Corral, Javier Martı´ Nanophotonics Technology Center, Polytechnic University of Valenica, Camino de Vera, s/n, 46022 Valencia, Spain Received 18 November 2004; received in revised form 26 January 2005; accepted 28 January 2005
Abstract The performance of a 3-bit photonic switched beamformer for phased array antennas based on optical switches and dispersive media is experimentally demonstrated at 42.7 GHz. Both transmission and reception modes have been experimentally tested. Broadband data transmission and radiation pattern measurements are presented. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.90.+m; 42.81.Wg; 84.40.Ua Keywords: Microwave photonics; Optical signal processing; Fiber optics
1. Introduction Optical beamforming networks for phasedarray antennas [1–7] have many advantages over their electrical counterparts such as small size, low weight, no susceptibility to electro-magnetic interference and, especially, wide instantaneous
*
Corresponding author. Tel.: +34963879747; +34963877827. E-mail address: [email protected] (B. Vidal).
fax:
bandwidth and squint-free array steering (truetime delay, TTD). These two last features are very interesting for broadband wireless access (BWA) networks. In recent years, there has been a great increase in the research and deployment of BWA [8]. Several frequency bands have been allocated for it, mainly in the 28 GHz band (e.g., LMDS) and 40 GHz band (e.g., MVDS) [9]. However, at high frequencies with high data rates, multipath fading and cross-interference become serious problems which can be eased by using smart antenna technology (phased array with adaptive
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.01.053
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M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
capabilities). Smart antennas have been traditionally controlled employing digital or IF/RF processing. Nevertheless, electrical beamforming networks have severe drawbacks when high bit rate signals at millimeter-wave (mm-wave) bands are considered. These limitations can be overcome using optical beamforming techniques [6,7] in the base station (BS), not only in transmission but also in reception mode. This paper reports experimental results for a photonic beamformed beam-switched phasedarray antenna in transmission (downstream) and reception (upstream) modes intended for next generation BS smart antennas in fixed and mobile BWA in the mm-wave band. The paper is structured as follows. Section 2 describes the optical beamformer both in transmission and reception modes. In Section 3, experimental results for both modes are shown including direction of arrival (DOA) estimation measurements. Finally, conclusions are provided in Section 4.
2. Beamformer architecture description 2.1. Transmission mode The beamforming architecture in the transmission mode is depicted in Fig. 1. A set of optical carriers generated by individual laser sources is externally modulated using a elec-
tro-optical Mach–Zehnder modulator (MZM) driven by an mm-wave amplifier. The MZM output is launched to a TTD switching matrix comprised by four cascaded optical switches. Standard single mode fibre (SSMF) coils of lengths L, 2L, and 4L, are placed between the upper ports of consecutive switches. Therefore, the optical signal is routed through different aggregated lengths of fiber depending on the bias voltages of the switches, up to eight different possible combinations (3bit). Due to the wavelength-dependent dispersive behavior of the optical fiber, each modulated optical carrier suffers a different time delay when travels through the switching matrix and dispersive fibers, depending on the selected path as given by s ¼ D L Dk;
ð1Þ
where D is the dispersion parameter of the fiber, L is the length of the optical path and Dk is the wavelength spacing between optical carriers. This relative time delay between optical carriers corresponds to different beam steering angles according to hi ¼ arcsinð2 si fRF Þ:
ð2Þ
The WDM optical signal is amplified using an Erbium-doped fiber amplifier (EDFA), then each optical carrier is demultiplexed and launched to a high speed photo-receiver, where the signal is converted back to the electrical domain. After amplification using an mm-wave amplifier, the four modulated electrical carriers are radiated using a
Fig. 1. Photonic beam switched architecture for the transmission mode.
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
k/2 four-element patch antenna. The phase-front of the radiated signal can be varied among eight different positions depending on the chosen combination of switching voltages (i.e., total delays), according to (2). To avoid the carrier suppression effect, which can result in power degradation in the beamformer performance, single sideband (SSB) modulation can be used. In order to achieve this modulation the optical carriers have been shifted 0.2 nm from the nominal central bandpass frequencies of the demultiplexer and, in this way, the demultiplexer filters one of the sidebands of each optical carrier. 2.2. Reception mode The schematic of the 3-bit beamformer in reception mode is shown in Fig. 2. The 32-QAM modulating signal at 1 GHz was generated using broadband modem arbitrary board. This IF signal was upconverted to the mm-wave frequency band before being radiated. The received signal at the array antenna elements is translated to the optical domain using as many electro-optical MZM as antenna elements, i.e., four MZMs for a four element antenna. Each amplitude modulated optical carrier is proportionally delayed by the true-time delay unit which switches between eight different lengths of SSMF. Finally, the four optical modu-
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lated carriers are photodetected using a single photodiode. The detected signal is downconverted to IF and demodulated. Fig. 3 shows the 3-bit beamformer in receiving mode combined with the control electronics and the array antenna. 2.3. Time delay calibration Both in transmission and reception modes, the length differences between each demultiplexer/multiplexer outputs and the antenna elements have to be compensated to avoid undesired time delay differences among the RF signals. As a first solution, several patchcords of SSMF have been introduced to correct these differences but since the beamformer is intended to the mm-wave band, the time delay unbalance between each path has to be lower than 1 ps and therefore the patchcord length accuracy has to be better than 0.2 mm. It is really difficult to obtain such an accuracy in the fibre patchcord length and, therefore, one optical delay line (ODL) has been introduced before the photodiodes in the transmission scheme and after the MZMs in the reception architecture to allow fine adjustment of the time delay. These ODLs can change its absolute time delay up to 300 ps which is equivalent to 6 cm of SSMF with n = 1.467. In this calibration procedure, the first step is to determine the absolute time delay (path length
Fig. 2. Photonic beam switched architecture for the reception mode.
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Fig. 3. Photonic 3-bit beamformer in receiving mode combined with the control electronics and the array antenna. PC: personal computer used to control the beamformer. PCE: photonic beamformer control equipment (bias source used to bias several components).
difference) between each branch (from the demultiplexer output to the PD and from the MZM to the multiplexer output). The time delay can be measured using an optical network analyser which can obtain fiber lengths with centimeter accuracy and using a mechanical method for electro-optic devices (external modulators and photodiodes). After introducing patchcords to roughly compensate these time delays, the remaining time delay error has to be corrected. Small time delay errors of a few picoseconds appear in these measurements due to noise, accuracy of the vector network analyser (VNA) as well as differences between consecutive connections of the electrical cables. Since the time delay needed for beam-steering is in the order of picoseconds these errors have to be compensated. Therefore, a complementary procedure has been used to obtain fine time delay calibration. It is based on measuring the radiation pattern of pairs of antenna elements and, by means of the angle position of the radiation nulls, to derive the time delay between them. Three pairs are used: antenna elements 1–2, 2–3 and 3–4. Thus, three antenna arrays with interelement spacing of k/2 are obtained. Although this fine calibration is based on phase adjusting, thanks to the previous coarse
calibration, time delay can be calibrated without ambiguity. Finally, in order to steer the beams to positive and negative angles relative to the center of the sector (broadside), a fixed time shift of (15.7/2) ps to each adjacent element will be included. Thus, positive and negative steering angles can be obtained as can be seen in Table 1. Fig. 4 shows the antenna pattern measurement at 42.7 GHz used for the calibration of the time delay between antenna elements 3 and 4.
3. Experimental results 3.1. Transmission mode The setup used to measure the performance of the beamforming network in transmission mode is depicted in Fig. 5, where the power budged and the spectra of the data signal at the input and output of the beamformer can be seen. The 155 Mbit/s 32-QAM modulating signal at 1 GHz was generated using one modem arbitrary board. After filtering, the signal was upconverted to 42.7 GHz using an electrical upconverter. Then, the signal is amplified to obtain a power level of the modulated RF carrier equal to +7 dBm, being
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449 Table 1 Correspondence among switching voltages, overall delay and steering angle with respect to broadside Switching control
Total time delay (ps)
Steering angle (°)
0110 1010 0000 1100 0101 1001 0011 1111
0 2.2440 4.4880 6.7320 8.9760 11.2200 13.4640 15.7080
41.7683 28.3257 16.4301 5.2331 5.7640 16.9818 28.9277 42.4808
Switch control Ô0Õ corresponds to cross-state and Ô1Õ to bar state.
Fig. 4. Antenna pattern used for calibration of antenna elements 3–4: (solid) theory; (dotted) measured.
its signal to noise ratio (SNR) 32.5 dB. This signal is impinged into the MZM. The modulated WDM signal was launched to the 3-bit TTD switching matrix, controlled by a 4-outputs external tunable bias source. The lengths of the SSMF fiber coils used to implement the time delays were 170, 340 and 680 m corresponding to a binary delay line. Before each photoreceiver, an ODL is used for the reasons explained in the previous section. The detected RF signal is further amplified using an mm-wave amplifier of 36 dB and radiated using the four-element patch antenna. After propagation through a 1.4 m radio link (far field at 42.7 GHz), a 40 dB gain electrical downconverter, fed by a 40 GHz horn antenna, downconverts the 42.7 GHz signal back to 1 GHz. The output
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SNR is 31 dB for a signal power at 1 GHz of 2.22 dBm. The intermediate frequency signal is demodulated by a different modem board, which allows to measure signal quality parameters such as the bit error rate (BER). Fig. 6 shows the schematic of the computer controlled measurement platform used to obtain pattern diagrams. The downconverter is mounted in a post that can be moved along a 90° circumference arch using a computer controlled step-engine, with programmable speed, acceleration and angular step-size. Fig. 7 shows the measured radiation patterns corresponding to the delay and steering angles of Table 1. The experimental results agree quite well with theory as can be seen in the inset of Fig. 2 where the measured radiation pattern corresponding to the switching matrix state 0000 (340 m) is shown and compared with theory. The measured 3 dB beamwidth also agrees very well with theory. The feasibility of the proposed optically beamformed antenna in transmission mode was confirmed by radio transmission experiments in the 40 GHz band. A 32-QAM 155 Mb/s 1 GHz signal was generated using a modem arbitrary board. Fig. 8 shows the measured constellation diagram after detection. Error-free operation was achieved using 255/256 Reed–Solomon forward error correction codes. The experimental setup SNR was limited due to the 1 dB interception point of the upconverter, which imposed +7 dBm output power level for distortion-free operation. The SNR can be improved using a higher linearity upconverter nevertheless the optical beamformer is almost transparent, introducing a SNR degradation of 1.5 dB. 3.2. Reception mode The experimental setup of the 3-bit beamformer in the reception mode is shown in Fig. 9, which is similar to the setup in transmission where the input and output data spectrum can be seen in the inset. Using a modem arbitrary board, a 155 Mbit/s 32-QAM modulating signal at 1 GHz was generated. Then, this signal is upconverted to
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Fig. 6. Schematic of the measurements setup used to obtain radiation pattern diagrams. PSB: photonic switched beamformer.
42.7 GHz before being radiated. The received signal at the array antenna elements is translated to the optical domain using four MZM (four element antenna array). The lengths of the SSMF fiber coils are the same used in the transmission experi-
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Fig. 7. Measured radiation pattern at 40 GHz for the eight beam positions in transmission. Inset: (dashed) measured radiation pattern corresponding to the switching state 0000 (340 m), (solid) theoretical radiation pattern.
ment (170, 340 and 680 m). Finally, the four optical modulated carriers are photodetected using a single photodiode. The detected signal is downconverted to IF and demodulated.
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Fig. 8. Constellation of the 155 Mbps 32-QAM after detection (Tx mode).
Fig. 9. Experimental setup for the reception mode.
The beam patterns for the eight beam steering angles have been measured, as shown in Fig. 10, where theoretical predictions are not shown for the sake of clarity. Several modulation formats (QPSK, 8PSK, 16QAM and 32-QAM) have been evaluated to demonstrate the beamforming capability for high data
rate transmissions. Error free operation was obtained for all modulation formats except for 32QAM for which a BER of 106 was obtained using Reed–Solomon and convolutional codes (4/5) when a payload rate of 155 Mbps was used. Fig. 11 shows the measured constellation diagram after detection for 155 Mbps 32-QAM modulation.
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3.3. DOA estimation DOA estimation will be very interesting to provide advanced services in BSs, such as user location, as well as to choose the active beam pointing the mobile terminal. The DOA algorithm proposed in [10] has been experimentally demonstrated using the system depicted in Fig. 9 due to its suitability for beam switched beamformers. Fig. 12 shows the experimental performance of the DOA estima40
Fig. 10. Measured antenna radiation pattern over the 90° sector in receiving mode.
The measured SNR at output was 25 dB and was limited due to the 1 dB interception point of the upconverter, which imposed a low output power limit for distortion-free operation. From the measured output SNR it can be derived that the optical beamformer introduces a SNR reduction of around 4 dB. Since the SNR is mainly limited by the electrical subsystem, it can be improved using an upconverter with higher linearity.
estimated position (degrees)
30 20 10 0 -10 -20 -30 -40 -40
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20
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Fig. 12. Performance of the DOA algorithm proposed in [7]. The solid line corresponds with theory and the dotted one with measurements.
Fig. 11. Constellation of the 155 Mbps 32-QAM after detection (Rx mode).
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
tion algorithm. The dotted line corresponds to the measurements and the solid line to theory. As may be observed, a good accuracy of the estimation is achieved. However, in the extremes of the graph the error increases due to the absence of adjacent beams. The maximum error obtained in the estimation was 3.66° with an root mean square (RMS) error of just 0.77°.
4. Conclusion The performance of a photonic beamformed beam-switched phased array antenna based on optical switches and dispersive media has been experimentally demonstrated in the 40 GHz band both in transmission and reception modes. Errorfree 42 GHz transmission of 32-QAM 155 Mb/s data has been achieved using the proposed optically beamforming network in transmission and 106 was obtained in the receiving mode. In both cases, the optical beamformer is almost transparent introducing a small degradation over the SNR. DOA estimation has been also experimentally evaluated showing a good agreement between measurements and theory.
Acknowledgments This work has been partially funded by the European Commission through the Projects OBA-
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NET IST-2000-25390 and GANDALF IST-1507781-STP.
References [1] W. Ng, A. Walston, G. Tangonan, J. Newberg, J.J. Lee, N. Bernstein, Journal of Lightwave Technology 9 (1991) 1124. [2] R.D. Esman, M.Y. Frankel, J.L. Dexter, L. Goldberg, M.G. Parent, IEEE Photonics Technology Letters 5 (11) (1993) 1347. [3] D.T.K. Tong, M.C. Wu, IEEE Photonics Technology Letters 8 (6) (1996) 812. [4] J.L. Corral, J. Martı´, S. Regidor, J.M. Fuster, R. Laming, M.J. Cole, IEEE Transactions on Microwave Theory and Techniques 45 (8) (1997) 1531. [5] B. Vidal, J.L. Corral, M.A. Piqueras, J. Martı´, IEEE Journal of Selected Topics in Quantum Electronics 8 (6) (2002) 1202. [6] M.A. Piqueras, B. Vidal, J. Herrera, V. Polo, J.L. Corral, J. Martı´, in: 30th European Conference on Optical Communication (ECOC), Tu3.5.5, Stockholm (Sweden), 5–9 September 2004, p. 214. [7] B. Vidal, M.A. Piqueras, J. Herrera, V. Polo, J.L. Corral, J. Martı´, in: 2004 International Topical Meeting on Microwave Photonics (MWP2004), Ogunquit (USA), 4–6 October 2004, p. 309. [8] M. Peruyero, in: Broadband Fixed Wireless Access Workshop, 4th Concertation Meeting IST Area IV.5 Mobile/ Wireless/Satellite, Brussels (Belgium), March 2001. [9] IEEE Standard for Local and Metropolitan Area Networks. Part 16: Standard Air Interface for Fixed Broadband Wireless Access Systems. IEEE standard 802.16, 2001. [10] P. Sanchis, V. Polo, J. Herrera, J.L. Corral, M.-S. Choi, J. Martı´, Microwave and Optical Technology Letters 39 (2003) 199.
Photonic switched beamformer implementation for broadband wireless access in transmission and reception modes at 42.7 GHz Miguel A. Piqueras, Borja Vidal *, Javier Herrera, Valentı´n Polo, Juan. L. Corral, Javier Martı´ Nanophotonics Technology Center, Polytechnic University of Valenica, Camino de Vera, s/n, 46022 Valencia, Spain Received 18 November 2004; received in revised form 26 January 2005; accepted 28 January 2005
Abstract The performance of a 3-bit photonic switched beamformer for phased array antennas based on optical switches and dispersive media is experimentally demonstrated at 42.7 GHz. Both transmission and reception modes have been experimentally tested. Broadband data transmission and radiation pattern measurements are presented. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.90.+m; 42.81.Wg; 84.40.Ua Keywords: Microwave photonics; Optical signal processing; Fiber optics
1. Introduction Optical beamforming networks for phasedarray antennas [1–7] have many advantages over their electrical counterparts such as small size, low weight, no susceptibility to electro-magnetic interference and, especially, wide instantaneous
*
Corresponding author. Tel.: +34963879747; +34963877827. E-mail address: [email protected] (B. Vidal).
fax:
bandwidth and squint-free array steering (truetime delay, TTD). These two last features are very interesting for broadband wireless access (BWA) networks. In recent years, there has been a great increase in the research and deployment of BWA [8]. Several frequency bands have been allocated for it, mainly in the 28 GHz band (e.g., LMDS) and 40 GHz band (e.g., MVDS) [9]. However, at high frequencies with high data rates, multipath fading and cross-interference become serious problems which can be eased by using smart antenna technology (phased array with adaptive
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.01.053
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M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
capabilities). Smart antennas have been traditionally controlled employing digital or IF/RF processing. Nevertheless, electrical beamforming networks have severe drawbacks when high bit rate signals at millimeter-wave (mm-wave) bands are considered. These limitations can be overcome using optical beamforming techniques [6,7] in the base station (BS), not only in transmission but also in reception mode. This paper reports experimental results for a photonic beamformed beam-switched phasedarray antenna in transmission (downstream) and reception (upstream) modes intended for next generation BS smart antennas in fixed and mobile BWA in the mm-wave band. The paper is structured as follows. Section 2 describes the optical beamformer both in transmission and reception modes. In Section 3, experimental results for both modes are shown including direction of arrival (DOA) estimation measurements. Finally, conclusions are provided in Section 4.
2. Beamformer architecture description 2.1. Transmission mode The beamforming architecture in the transmission mode is depicted in Fig. 1. A set of optical carriers generated by individual laser sources is externally modulated using a elec-
tro-optical Mach–Zehnder modulator (MZM) driven by an mm-wave amplifier. The MZM output is launched to a TTD switching matrix comprised by four cascaded optical switches. Standard single mode fibre (SSMF) coils of lengths L, 2L, and 4L, are placed between the upper ports of consecutive switches. Therefore, the optical signal is routed through different aggregated lengths of fiber depending on the bias voltages of the switches, up to eight different possible combinations (3bit). Due to the wavelength-dependent dispersive behavior of the optical fiber, each modulated optical carrier suffers a different time delay when travels through the switching matrix and dispersive fibers, depending on the selected path as given by s ¼ D L Dk;
ð1Þ
where D is the dispersion parameter of the fiber, L is the length of the optical path and Dk is the wavelength spacing between optical carriers. This relative time delay between optical carriers corresponds to different beam steering angles according to hi ¼ arcsinð2 si fRF Þ:
ð2Þ
The WDM optical signal is amplified using an Erbium-doped fiber amplifier (EDFA), then each optical carrier is demultiplexed and launched to a high speed photo-receiver, where the signal is converted back to the electrical domain. After amplification using an mm-wave amplifier, the four modulated electrical carriers are radiated using a
Fig. 1. Photonic beam switched architecture for the transmission mode.
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
k/2 four-element patch antenna. The phase-front of the radiated signal can be varied among eight different positions depending on the chosen combination of switching voltages (i.e., total delays), according to (2). To avoid the carrier suppression effect, which can result in power degradation in the beamformer performance, single sideband (SSB) modulation can be used. In order to achieve this modulation the optical carriers have been shifted 0.2 nm from the nominal central bandpass frequencies of the demultiplexer and, in this way, the demultiplexer filters one of the sidebands of each optical carrier. 2.2. Reception mode The schematic of the 3-bit beamformer in reception mode is shown in Fig. 2. The 32-QAM modulating signal at 1 GHz was generated using broadband modem arbitrary board. This IF signal was upconverted to the mm-wave frequency band before being radiated. The received signal at the array antenna elements is translated to the optical domain using as many electro-optical MZM as antenna elements, i.e., four MZMs for a four element antenna. Each amplitude modulated optical carrier is proportionally delayed by the true-time delay unit which switches between eight different lengths of SSMF. Finally, the four optical modu-
443
lated carriers are photodetected using a single photodiode. The detected signal is downconverted to IF and demodulated. Fig. 3 shows the 3-bit beamformer in receiving mode combined with the control electronics and the array antenna. 2.3. Time delay calibration Both in transmission and reception modes, the length differences between each demultiplexer/multiplexer outputs and the antenna elements have to be compensated to avoid undesired time delay differences among the RF signals. As a first solution, several patchcords of SSMF have been introduced to correct these differences but since the beamformer is intended to the mm-wave band, the time delay unbalance between each path has to be lower than 1 ps and therefore the patchcord length accuracy has to be better than 0.2 mm. It is really difficult to obtain such an accuracy in the fibre patchcord length and, therefore, one optical delay line (ODL) has been introduced before the photodiodes in the transmission scheme and after the MZMs in the reception architecture to allow fine adjustment of the time delay. These ODLs can change its absolute time delay up to 300 ps which is equivalent to 6 cm of SSMF with n = 1.467. In this calibration procedure, the first step is to determine the absolute time delay (path length
Fig. 2. Photonic beam switched architecture for the reception mode.
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Fig. 3. Photonic 3-bit beamformer in receiving mode combined with the control electronics and the array antenna. PC: personal computer used to control the beamformer. PCE: photonic beamformer control equipment (bias source used to bias several components).
difference) between each branch (from the demultiplexer output to the PD and from the MZM to the multiplexer output). The time delay can be measured using an optical network analyser which can obtain fiber lengths with centimeter accuracy and using a mechanical method for electro-optic devices (external modulators and photodiodes). After introducing patchcords to roughly compensate these time delays, the remaining time delay error has to be corrected. Small time delay errors of a few picoseconds appear in these measurements due to noise, accuracy of the vector network analyser (VNA) as well as differences between consecutive connections of the electrical cables. Since the time delay needed for beam-steering is in the order of picoseconds these errors have to be compensated. Therefore, a complementary procedure has been used to obtain fine time delay calibration. It is based on measuring the radiation pattern of pairs of antenna elements and, by means of the angle position of the radiation nulls, to derive the time delay between them. Three pairs are used: antenna elements 1–2, 2–3 and 3–4. Thus, three antenna arrays with interelement spacing of k/2 are obtained. Although this fine calibration is based on phase adjusting, thanks to the previous coarse
calibration, time delay can be calibrated without ambiguity. Finally, in order to steer the beams to positive and negative angles relative to the center of the sector (broadside), a fixed time shift of (15.7/2) ps to each adjacent element will be included. Thus, positive and negative steering angles can be obtained as can be seen in Table 1. Fig. 4 shows the antenna pattern measurement at 42.7 GHz used for the calibration of the time delay between antenna elements 3 and 4.
3. Experimental results 3.1. Transmission mode The setup used to measure the performance of the beamforming network in transmission mode is depicted in Fig. 5, where the power budged and the spectra of the data signal at the input and output of the beamformer can be seen. The 155 Mbit/s 32-QAM modulating signal at 1 GHz was generated using one modem arbitrary board. After filtering, the signal was upconverted to 42.7 GHz using an electrical upconverter. Then, the signal is amplified to obtain a power level of the modulated RF carrier equal to +7 dBm, being
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449 Table 1 Correspondence among switching voltages, overall delay and steering angle with respect to broadside Switching control
Total time delay (ps)
Steering angle (°)
0110 1010 0000 1100 0101 1001 0011 1111
0 2.2440 4.4880 6.7320 8.9760 11.2200 13.4640 15.7080
41.7683 28.3257 16.4301 5.2331 5.7640 16.9818 28.9277 42.4808
Switch control Ô0Õ corresponds to cross-state and Ô1Õ to bar state.
Fig. 4. Antenna pattern used for calibration of antenna elements 3–4: (solid) theory; (dotted) measured.
its signal to noise ratio (SNR) 32.5 dB. This signal is impinged into the MZM. The modulated WDM signal was launched to the 3-bit TTD switching matrix, controlled by a 4-outputs external tunable bias source. The lengths of the SSMF fiber coils used to implement the time delays were 170, 340 and 680 m corresponding to a binary delay line. Before each photoreceiver, an ODL is used for the reasons explained in the previous section. The detected RF signal is further amplified using an mm-wave amplifier of 36 dB and radiated using the four-element patch antenna. After propagation through a 1.4 m radio link (far field at 42.7 GHz), a 40 dB gain electrical downconverter, fed by a 40 GHz horn antenna, downconverts the 42.7 GHz signal back to 1 GHz. The output
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SNR is 31 dB for a signal power at 1 GHz of 2.22 dBm. The intermediate frequency signal is demodulated by a different modem board, which allows to measure signal quality parameters such as the bit error rate (BER). Fig. 6 shows the schematic of the computer controlled measurement platform used to obtain pattern diagrams. The downconverter is mounted in a post that can be moved along a 90° circumference arch using a computer controlled step-engine, with programmable speed, acceleration and angular step-size. Fig. 7 shows the measured radiation patterns corresponding to the delay and steering angles of Table 1. The experimental results agree quite well with theory as can be seen in the inset of Fig. 2 where the measured radiation pattern corresponding to the switching matrix state 0000 (340 m) is shown and compared with theory. The measured 3 dB beamwidth also agrees very well with theory. The feasibility of the proposed optically beamformed antenna in transmission mode was confirmed by radio transmission experiments in the 40 GHz band. A 32-QAM 155 Mb/s 1 GHz signal was generated using a modem arbitrary board. Fig. 8 shows the measured constellation diagram after detection. Error-free operation was achieved using 255/256 Reed–Solomon forward error correction codes. The experimental setup SNR was limited due to the 1 dB interception point of the upconverter, which imposed +7 dBm output power level for distortion-free operation. The SNR can be improved using a higher linearity upconverter nevertheless the optical beamformer is almost transparent, introducing a SNR degradation of 1.5 dB. 3.2. Reception mode The experimental setup of the 3-bit beamformer in the reception mode is shown in Fig. 9, which is similar to the setup in transmission where the input and output data spectrum can be seen in the inset. Using a modem arbitrary board, a 155 Mbit/s 32-QAM modulating signal at 1 GHz was generated. Then, this signal is upconverted to
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-20 -30 -25 -35
MODEM
VC
-40 -45
-55 -55 -60
-60
-65 42.65 42.66 42.67 42.68 42.69 42.7 42.71 42.72 42.73 42.74 42.75 Frequency (GHz)
-65 0.95
OSICS
(Optical Source)
0.96
0.97
0.98
0.99 1 1.01 Frequency (GHz)
1.02
1.03
up conv
Σ
MZM
TTD
EDFA
L MZM
L TTD
GEDFA
3 dBm LΣ
1.04
1.05
MODEM
VC
-5 dBm -3 dBm
POL
Po = 6dBm
Pout = -2.22 dBm SNRout = 31.0 dB
-45 -50
POL
L POL
-40
-50
POL
POL
Amplitude (dBm)
-35
-13 dBm
DEMUX
PUP-C = 7 dBm SNRUP-C = 32.5 dB
Amplitude (dBm)
-30
ODL +FC
PD
τ
ODL +FC
PD
τ
1.4 m @ 42.7 GHz
ODL +FC
PD
τ
LLINK GAR
ODL +FC
PD
τ
LDEMUX LODL+FC
5 dBm
down conv
PPD-IN= -8 dBm
Ga
Gpos
L C1
LEDL
GAT
-35.22 dBm 0.78 dBm
PRF-OUT = -54.22 dBm -36.22 dBm SNR RF-OUT = 31.04 dB
Fig. 5. Experimental setup for the transmission mode.
45˚ 15˚
Amplitude (dB)
0
Angle
-5 -10 -15 -20 -25 -30 -35 -40 -40 -30 -20 -10 0 10 20 30 40 Azimuth (degrees)
-15˚
-30
Fig. 6. Schematic of the measurements setup used to obtain radiation pattern diagrams. PSB: photonic switched beamformer.
42.7 GHz before being radiated. The received signal at the array antenna elements is translated to the optical domain using four MZM (four element antenna array). The lengths of the SSMF fiber coils are the same used in the transmission experi-
-20
-10
0
-45˚
Normalised amplitude (dB)
Fig. 7. Measured radiation pattern at 40 GHz for the eight beam positions in transmission. Inset: (dashed) measured radiation pattern corresponding to the switching state 0000 (340 m), (solid) theoretical radiation pattern.
ment (170, 340 and 680 m). Finally, the four optical modulated carriers are photodetected using a single photodiode. The detected signal is downconverted to IF and demodulated.
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
447
Fig. 8. Constellation of the 155 Mbps 32-QAM after detection (Tx mode).
Fig. 9. Experimental setup for the reception mode.
The beam patterns for the eight beam steering angles have been measured, as shown in Fig. 10, where theoretical predictions are not shown for the sake of clarity. Several modulation formats (QPSK, 8PSK, 16QAM and 32-QAM) have been evaluated to demonstrate the beamforming capability for high data
rate transmissions. Error free operation was obtained for all modulation formats except for 32QAM for which a BER of 106 was obtained using Reed–Solomon and convolutional codes (4/5) when a payload rate of 155 Mbps was used. Fig. 11 shows the measured constellation diagram after detection for 155 Mbps 32-QAM modulation.
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M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
3.3. DOA estimation DOA estimation will be very interesting to provide advanced services in BSs, such as user location, as well as to choose the active beam pointing the mobile terminal. The DOA algorithm proposed in [10] has been experimentally demonstrated using the system depicted in Fig. 9 due to its suitability for beam switched beamformers. Fig. 12 shows the experimental performance of the DOA estima40
Fig. 10. Measured antenna radiation pattern over the 90° sector in receiving mode.
The measured SNR at output was 25 dB and was limited due to the 1 dB interception point of the upconverter, which imposed a low output power limit for distortion-free operation. From the measured output SNR it can be derived that the optical beamformer introduces a SNR reduction of around 4 dB. Since the SNR is mainly limited by the electrical subsystem, it can be improved using an upconverter with higher linearity.
estimated position (degrees)
30 20 10 0 -10 -20 -30 -40 -40
-30
-20
-10 0 10 angular position (degrees)
20
30
40
Fig. 12. Performance of the DOA algorithm proposed in [7]. The solid line corresponds with theory and the dotted one with measurements.
Fig. 11. Constellation of the 155 Mbps 32-QAM after detection (Rx mode).
M.A. Piqueras et al. / Optics Communications 249 (2005) 441–449
tion algorithm. The dotted line corresponds to the measurements and the solid line to theory. As may be observed, a good accuracy of the estimation is achieved. However, in the extremes of the graph the error increases due to the absence of adjacent beams. The maximum error obtained in the estimation was 3.66° with an root mean square (RMS) error of just 0.77°.
4. Conclusion The performance of a photonic beamformed beam-switched phased array antenna based on optical switches and dispersive media has been experimentally demonstrated in the 40 GHz band both in transmission and reception modes. Errorfree 42 GHz transmission of 32-QAM 155 Mb/s data has been achieved using the proposed optically beamforming network in transmission and 106 was obtained in the receiving mode. In both cases, the optical beamformer is almost transparent introducing a small degradation over the SNR. DOA estimation has been also experimentally evaluated showing a good agreement between measurements and theory.
Acknowledgments This work has been partially funded by the European Commission through the Projects OBA-
449
NET IST-2000-25390 and GANDALF IST-1507781-STP.
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