**5. Opto-VLSI-based tunable beamformer for microwave phased-array antenna**

The growing demand for broadband mobile telephone, Internet and multimedia services, combined with the limited radio frequency (RF) spectrum availability, will require a substantial increase in the channel capacity of wireless and mobile communication systems. However, in current wireless network systems, the base station has no information on the position of each mobile user and radiates the RF signal in all directions within a cell in order to provide radio coverage. This results in inefficient utilization of the radiated power during the transmission and causes interference to adjacent cells (co-channel) that use the same frequency. In addition, the antenna receiver detects signals from all directions including noise and interference signals, making the processing of the desired signals complicated, thus limiting the transmission speed and the number of users.

Phased array antennas are rapidly becoming the new frontier of wireless communications because of their ability to overcome user-interference effects effectively and increase channel capacity substantially, together with a more efficient use of transmitted energy. Every phased array antenna requires a key element, "the beamformer", which locates and tracks the various users associated to a base station and adapts the relative amplitude and phase shift at each antenna element so that the main lobe (highest gain) of the antenna is directed toward an active user, while nulls (i.e., where the directivity is zero) are adapted towards co-channel interferers. Phased array antennas refer to antenna arrays need adaptive signal processing capability to maximize the wanted signals and also minimize the interfering signals. Therefore, spatially-separated users can be served in the same base-station sector using the same frequency/time slot, which is called space division multiple access (SDMA) system.

The requirements for future phased array antenna beamformers include (i) the ability to perform wideband signal processing, (ii) short reaction time, (iii) small footprint (small size and light weight), (iv) increased coverage range and increased resolution, (v) reliability and maintainability, and (vi) low cost.

Currently, two beamforming approaches are mainly deployed in all-electronic phased array antennas. The first approach is based on the use of analogue RF phase shifters to adapt the antenna's directivity pattern (Sorensen et al., 2004), while the second approach is based on digitising the antenna signals using analogue-to-digital converters and using digital signal processing (DSP) to control the mainlobe as well as the nulls of the antenna (Jian et al., 2008). Both analogue and digital beamforming approaches have the advantages of flexibility; however, both are inherently narrowband because of the limited instantaneous bandwidths of RF phase shifters and analogue-to-digital converters.

Photonics-based broadband phased-array antenna beamformers have been extensively investigated over the last decade for applications ranging from modern microwave radar to wireless communication systems. A broadband phased-array antenna requires the generation of variable true-time delays at each antenna element to realize beam or null steering. Several approaches have been adopted to realise tunable true-time delay units, including the use of in-

This experiment demonstrated that arbitrary single or multiple true-time delays could be synthesized by slicing an RF-modulated broadband optical source and routing arbitrary sliced wavebands, through upload a phase hologram onto an Opto-VLSI processor, to a high-dispersion fiber where they experience RF delays that depend on their centre

**5. Opto-VLSI-based tunable beamformer for microwave phased-array antenna**  The growing demand for broadband mobile telephone, Internet and multimedia services, combined with the limited radio frequency (RF) spectrum availability, will require a substantial increase in the channel capacity of wireless and mobile communication systems. However, in current wireless network systems, the base station has no information on the position of each mobile user and radiates the RF signal in all directions within a cell in order to provide radio coverage. This results in inefficient utilization of the radiated power during the transmission and causes interference to adjacent cells (co-channel) that use the same frequency. In addition, the antenna receiver detects signals from all directions including noise and interference signals, making the processing of the desired signals complicated,

Phased array antennas are rapidly becoming the new frontier of wireless communications because of their ability to overcome user-interference effects effectively and increase channel capacity substantially, together with a more efficient use of transmitted energy. Every phased array antenna requires a key element, "the beamformer", which locates and tracks the various users associated to a base station and adapts the relative amplitude and phase shift at each antenna element so that the main lobe (highest gain) of the antenna is directed toward an active user, while nulls (i.e., where the directivity is zero) are adapted towards co-channel interferers. Phased array antennas refer to antenna arrays need adaptive signal processing capability to maximize the wanted signals and also minimize the interfering signals. Therefore, spatially-separated users can be served in the same base-station sector using the same frequency/time slot, which is called space division multiple access (SDMA)

The requirements for future phased array antenna beamformers include (i) the ability to perform wideband signal processing, (ii) short reaction time, (iii) small footprint (small size and light weight), (iv) increased coverage range and increased resolution, (v) reliability and

Currently, two beamforming approaches are mainly deployed in all-electronic phased array antennas. The first approach is based on the use of analogue RF phase shifters to adapt the antenna's directivity pattern (Sorensen et al., 2004), while the second approach is based on digitising the antenna signals using analogue-to-digital converters and using digital signal processing (DSP) to control the mainlobe as well as the nulls of the antenna (Jian et al., 2008). Both analogue and digital beamforming approaches have the advantages of flexibility; however, both are inherently narrowband because of the limited instantaneous bandwidths

Photonics-based broadband phased-array antenna beamformers have been extensively investigated over the last decade for applications ranging from modern microwave radar to wireless communication systems. A broadband phased-array antenna requires the generation of variable true-time delays at each antenna element to realize beam or null steering. Several approaches have been adopted to realise tunable true-time delay units, including the use of in-

thus limiting the transmission speed and the number of users.

wavelengths.

system.

maintainability, and (vi) low cost.

of RF phase shifters and analogue-to-digital converters.

fibre chirped Bragg gratings (FBGs) (Italia *et al.*, 2005a), free-space in conjunction with white cells (Mital et al., 2006a), integrated optical waveguides (Flamand et al., 2000), opticallyswitched fibre delay structures (Tong&Wu, 1998). However, none of these reported photonicsbased true-time delay units has the flexibility to either tune the true-time delay continuously or generate multiple tunable true-time delays for each antenna element simultaneously. Furthermore, the limited flexibility, reconfigurability, and tunability of current photonic beamformers make them impractical for realising broadband null steering.

Broadband null-steering beamformers are much more difficult to realise than beam-steering beamformers. Theoretical analysis of broadband null steering of phased-array antennas shows multiple variable true-time delays are needed for each antenna element, while only one variable true-time delay for an antenna element is required for broadband beam steering. An N-element smart antenna can synthesise (N–1) nulls only, and this requires the beamformer to simultaneously generate (2N-1–1) delayed versions of the RF signal received by the antenna.

Fig. 10(a) shows a typical N-element phased-array antenna architecture, whose array factor (or directional response) is given by (Zmuda *et al.*, 1998):

$$AF\_N(\theta) = \prod\_{n=1}^{N-1} \left( \mathbf{x} - \mathbf{x}\_n \right) = \sum\_{m=0}^{N-1} V\mathcal{V}\_m \mathbf{x}^m \tag{9}$$

where *x jkd* = exp sin ( ) θ , d is the antenna element spacing, k = wave number = ω/c, and xn = x (θn) is a zero of the polynomial AFN corresponding to an antenna null at the angular coordinate θn. Note that a change of even one zero affects all the weights, Wm. Note also that with N antenna elements, the phased-array antenna can synthesize only (N–1) nulls, as evident from Equation (9).

Without loss of generality, considering a 4-element phased array antenna, with its main lobe at an angle θ and nulls located along angular coordinates, θ1, θ2, and θ3, the array factor takes the form (Zmuda *et al.*, 2000):

$$AF\_4(\theta) = \sum\_{m=0}^{3} W\_m e^{j m l d \sin(\theta)} = \left( e^{j l d \sin(\theta)} - e^{j l d \sin(\theta\_1)} \right) \left( e^{j l d \sin(\theta)} - e^{j l d \sin(\theta\_2)} \right) \left( e^{j l d \sin(\theta)} - e^{j l d \sin(\theta\_3)} \right) \tag{10}$$

By expanding Eq. (10), we obtain

$$AF\_4(\theta) = \mathbf{x}^3 - \mathbf{x}^2 \left( e^{j a \sigma\_{21}} + e^{j a \sigma\_{22}} + e^{j a \sigma\_{23}} \right) + \mathbf{x} \left( e^{j a \sigma\_{11}} + e^{j a \sigma\_{12}} + e^{j a \sigma\_{13}} \right) - e^{j a \sigma\_{01}} \tag{11}$$

From Eq. (11), it can be observed that for a 4-element phased array antennas, 24-1–1 = 7 delay taps need to be generated by the true time delay unit in order to synthesis three nulls, and that the time delays required to be synthesized are:

$$\begin{aligned} \tau\_{21} &= \frac{d}{c} \sin(\theta\_1), \tau\_{22} = \frac{d}{c} \sin(\theta\_2), \tau\_{23} = \frac{d}{c} \sin(\theta\_3) \\ \tau\_{11} &= \frac{d}{c} [\sin(\theta\_1) + \sin(\theta\_2)], \tau\_{12} = \frac{d}{c} [\sin(\theta\_1) + \sin(\theta\_3)], \tau\_{13} = \frac{d}{c} [\sin(\theta\_2) + \sin(\theta\_3)] \\ \tau\_{01} &= \frac{d}{c} [\sin(\theta\_1) + \sin(\theta\_2) + \sin(\theta\_3)] \end{aligned} \tag{12}$$

Photonic Microwave Signal Processing Based on Opto-VLSI Technology 377

diameter. An optical lens (Lens 1) with 10 cm focal length was used between the collimator array and a diffraction grating plate to focus the collimated ASE beams onto a small spot onto the grating plate. The grating plate, having 1200 lines/mm and a blazed angle of 70° at 1530 nm, spatially de-multiplexed the ASE beams, and spread the ASE spectra into different directions. Another optical lens (Lens 2) with the same focal length, located in the middle position between the grating plate and the Opto-VLSI processor, was used to collimate the dispersed optical beams and map them onto the surface of the 2-D Opto-VLSI processor, which was partitioned into 4 rectangular pixel blocks by software. Each pixel block was assigned to a tunable laser and used to efficiently couple back any part of the ASE spectrum illuminating this pixel block along the incident path into the corresponding collimator port. The Opto-VLSI processor can arbitrarily select any wavebands that are mapped onto its surface using the principle of beam steering described above. The selected wavebands were coupled back into the corresponding fiber collimator port, and then routed back to the gain medium via the corresponding circulator, thus forming an optical loop for single-mode laser

A Labview software was developed to generate and upload the optimized digital phase holograms that simultaneously steer the desired wavebands for each channel and couple back into the corresponding collimators for subsequent recirculation in the fiber loops. Four different wavelengths can independently be selected for lasing within the different fiber loops by uploading appropriate phase holograms (blazed grating) that drive all the pixel blocks of the Opto-VLSI processor. Therefore, this structure enables generation of multiple tunable fiber laser sources, each of which can be independently tuned and output from a specific output port, which offers excellent flexibility to synthesis beam and

Each RF signal received by the element at the front-end of the phase-array antennas was used to intensity modulate the wavelength channels using an Electro-Optic Modulator (EOM). All the RF-modulated optical signals were coupled into a single fiber and routed into an EDFA for amplification, and then launched into a 10-km Corning LEAF non-zero dispersion shifted optical fiber with dispersion coefficient about 4.2 ps/nm/km and insertion loss of 0.2 dB/km at 1550 nm. Each RF-modulated optical signal experienced a true-time delay that depended on their centre wavelengths, before they were finally detected by a photo-diode that produced the sum of the delayed RF signals. In this way, the TTD between adjacent antenna elements were generated by controlling the wavelength spacing between the various wavelength channels. One of the attractive features of the proposed phased array antenna architecture shown in Fig. 1 is its ability to simultaneously generate all the tunable RF true-time delays for the smart antenna beamformer through optimised phase holograms uploaded onto the Opto-VLSI processor. Various phase holograms, which were synthesized and optimized for specific beam steering scenarios, were stored to enable beamsteering scenarios to be recalled through

Experiments were conducted using the setup illustrated in Fig. 11 to evaluate the performance of a 4-element rectangular patch type smart antenna system for flexible beam steering. The antenna elements were separated at a distance of 99 mm, corresponding to half of the RF operating wavelength thus alleviating the effect of side lobes. By refreshing phase holograms generated for the four pixel blocks, the lasing wavelengths can be tuned in terms of their wavelength separations needed to generate the TTD required for RF beam steering.

generation.

null steering.

software.

Generally, for an N-element broadband phased array, the synthesis of (N–1) broadband nulls can be achieved if the beamformer of the antenna can adaptively generate and combine (2N-1 –1) delayed versions of the RF signals received by the antenna elements, as illustrated in Fig. 10(b).

Fig. 10. (a) Typical phased-array antenna architecture. (b) Phased array antenna architecture for broadband null steering.

Recently a novel holographic-based broadband beamformer was proposed and demonstrated, employing an Opto-VLSI processor, a broadband light source, and high dispersion fibres to simultaneously generate arbitrary multiple true-time delays for each antenna element. This beamformer enables the realisation of adaptive multi-element antennas that significantly increase the capacity of next-generation wireless systems. The proposed beamformer has a number of novel features. First, it can adaptively achieve broadband beam- and null-steering through software; second, it incorporates microelectronics and photonics (Opto-VLSI) for RF signal processing, thus adding the flexibility, tunability, accuracy, and reconfigurability of microelectronics to the broadband capability of photonics; and third, it provides a cost-effective and compressed-hardware solution to multi-element antenna beamforming in next-generation wireless systems.

Figure 11 shows the proposed Opto-VLSI-based RF phased-array antennas architecture as well as experimental setup for synthesis of broadband beam steering. The structure was set to tune four fiber lasers, all controlled by a single 2-D Opto-VLSI processor with 8-bit phase level, 512 × 512 pixels of a pixel size of 15μm. Each tunable fiber laser employed an Erbiumdoped fiber amplifier (EDFA) operating in C-band, an optical coupler, a polarization controller (PC), and a circulator. The broadband amplified spontaneous emission (ASE) noise resulting from the optical amplifier was split by the optical coupler with a 5/95 power splitting ratio, where 5% of ASE power was used to extract the output of the tunable fiber laser while the remaining 95% was re-circulated in the fiber ring cavity to generate lasing. The polarization controller was used to optimize the diffraction efficiency of the grating plate and to enforce single-polarization lasing. All broadband ASE signals directed to the corresponding collimator array ports via optical circulators were collimated at about 0.5 mm

Generally, for an N-element broadband phased array, the synthesis of (N–1) broadband nulls can be achieved if the beamformer of the antenna can adaptively generate and combine (2N-1 –1) delayed versions of the RF signals received by the antenna elements, as

> Antenna Elements

(a) (b)

Fig. 10. (a) Typical phased-array antenna architecture. (b) Phased array antenna architecture

Recently a novel holographic-based broadband beamformer was proposed and demonstrated, employing an Opto-VLSI processor, a broadband light source, and high dispersion fibres to simultaneously generate arbitrary multiple true-time delays for each antenna element. This beamformer enables the realisation of adaptive multi-element antennas that significantly increase the capacity of next-generation wireless systems. The proposed beamformer has a number of novel features. First, it can adaptively achieve broadband beam- and null-steering through software; second, it incorporates microelectronics and photonics (Opto-VLSI) for RF signal processing, thus adding the flexibility, tunability, accuracy, and reconfigurability of microelectronics to the broadband capability of photonics; and third, it provides a cost-effective and compressed-hardware

solution to multi-element antenna beamforming in next-generation wireless systems.

Figure 11 shows the proposed Opto-VLSI-based RF phased-array antennas architecture as well as experimental setup for synthesis of broadband beam steering. The structure was set to tune four fiber lasers, all controlled by a single 2-D Opto-VLSI processor with 8-bit phase level, 512 × 512 pixels of a pixel size of 15μm. Each tunable fiber laser employed an Erbiumdoped fiber amplifier (EDFA) operating in C-band, an optical coupler, a polarization controller (PC), and a circulator. The broadband amplified spontaneous emission (ASE) noise resulting from the optical amplifier was split by the optical coupler with a 5/95 power splitting ratio, where 5% of ASE power was used to extract the output of the tunable fiber laser while the remaining 95% was re-circulated in the fiber ring cavity to generate lasing. The polarization controller was used to optimize the diffraction efficiency of the grating plate and to enforce single-polarization lasing. All broadband ASE signals directed to the corresponding collimator array ports via optical circulators were collimated at about 0.5 mm

τ τ τ 1:M Splitter

RFin

Combiner

θN-1

θi

<sup>θ</sup><sup>1</sup> <sup>θ</sup><sup>i</sup>

… τ τ … τ τ τ … τ

1:M Splitter 1:M Splitter

Combiner Combiner

……..

1 2 N

RFout

<sup>Σ</sup> <sup>+</sup> - - = Direction of ith null

Radiation Pattern

illustrated in Fig. 10(b).

RFin

for broadband null steering.

Array Elements

Combiner RFout

W0 W1 W2 WN-2 WN-1

diameter. An optical lens (Lens 1) with 10 cm focal length was used between the collimator array and a diffraction grating plate to focus the collimated ASE beams onto a small spot onto the grating plate. The grating plate, having 1200 lines/mm and a blazed angle of 70° at 1530 nm, spatially de-multiplexed the ASE beams, and spread the ASE spectra into different directions. Another optical lens (Lens 2) with the same focal length, located in the middle position between the grating plate and the Opto-VLSI processor, was used to collimate the dispersed optical beams and map them onto the surface of the 2-D Opto-VLSI processor, which was partitioned into 4 rectangular pixel blocks by software. Each pixel block was assigned to a tunable laser and used to efficiently couple back any part of the ASE spectrum illuminating this pixel block along the incident path into the corresponding collimator port. The Opto-VLSI processor can arbitrarily select any wavebands that are mapped onto its surface using the principle of beam steering described above. The selected wavebands were coupled back into the corresponding fiber collimator port, and then routed back to the gain medium via the corresponding circulator, thus forming an optical loop for single-mode laser generation.

A Labview software was developed to generate and upload the optimized digital phase holograms that simultaneously steer the desired wavebands for each channel and couple back into the corresponding collimators for subsequent recirculation in the fiber loops. Four different wavelengths can independently be selected for lasing within the different fiber loops by uploading appropriate phase holograms (blazed grating) that drive all the pixel blocks of the Opto-VLSI processor. Therefore, this structure enables generation of multiple tunable fiber laser sources, each of which can be independently tuned and output from a specific output port, which offers excellent flexibility to synthesis beam and null steering.

Each RF signal received by the element at the front-end of the phase-array antennas was used to intensity modulate the wavelength channels using an Electro-Optic Modulator (EOM). All the RF-modulated optical signals were coupled into a single fiber and routed into an EDFA for amplification, and then launched into a 10-km Corning LEAF non-zero dispersion shifted optical fiber with dispersion coefficient about 4.2 ps/nm/km and insertion loss of 0.2 dB/km at 1550 nm. Each RF-modulated optical signal experienced a true-time delay that depended on their centre wavelengths, before they were finally detected by a photo-diode that produced the sum of the delayed RF signals. In this way, the TTD between adjacent antenna elements were generated by controlling the wavelength spacing between the various wavelength channels. One of the attractive features of the proposed phased array antenna architecture shown in Fig. 1 is its ability to simultaneously generate all the tunable RF true-time delays for the smart antenna beamformer through optimised phase holograms uploaded onto the Opto-VLSI processor. Various phase holograms, which were synthesized and optimized for specific beam steering scenarios, were stored to enable beamsteering scenarios to be recalled through software.

Experiments were conducted using the setup illustrated in Fig. 11 to evaluate the performance of a 4-element rectangular patch type smart antenna system for flexible beam steering. The antenna elements were separated at a distance of 99 mm, corresponding to half of the RF operating wavelength thus alleviating the effect of side lobes. By refreshing phase holograms generated for the four pixel blocks, the lasing wavelengths can be tuned in terms of their wavelength separations needed to generate the TTD required for RF beam steering.

Photonic Microwave Signal Processing Based on Opto-VLSI Technology 379

The experiments demonstrated an Opto-VLSI-based tunable beamformer for adaptively steering the radiation patter of RF phased array antennas. By using phase holograms implemented by a single Opto-VLSI processor 4 independent optical wavebands can be generated and their spectral tuned, leading to 4 independent tunable fiber lasers. The RFmodulated lasers are used to generate TTD in 4-element rectangular patch arrayed antennas. Experimental results show the capability of the proposed structure to perform RF beam

(a)

7º

15º

22º

30º

(b)

(c)

(d)

Fig. 12. Opto-VLSI-based tunable laser generations (left), and the corresponding RF beam

steering between 0°-30°.

1 nm

2 nm

3 nm

4 nm

steering profile measured in the experiments (right).

Fig. 11. Opto-VLSI-based phased array antenna architecture for broadband beam steering.

For each beammsteering scenario, the RF signal produced after the photodetection of the delayed RF-modulated optical signals was monitored by an RF power meter. Figures 12 (ad) show several scenarios of measured antenna radiation patterns (right) corresponding to different equi-spaced wavelength channels (left) generated by the Opto-VLSI processor. For example, in Fig. 12 (a), the Opto-VLSI processor generated a wavelength spacing of 1 nm, which corresponds to 42-ps delay between adjacent antenna elements. The measured beamsteering angle was about 7°, which is in excellent agreement with the theoretical prediction of 7.2°. When the spacing between the laser channels was increased from 1 nm to 4 nm, the measured main lobe was steered from 7° to around 30º, as illustrated in Fig. 12. These experimental results agree very well with the theoretical predictions, demonstrating the excellent capability of the Opto-VLSI processor to realize a phased-array antenna beamformer.

Note that this structure has the capability of null steering if each laser channel is able to generate multiple laser wavelengths. Single or multiple arbitrary wavelengths for each laser channel can be generated and the amplitude of each laser of different wavelength can also be controlled simply by uploading the appropriate steering phase holograms onto the Opto-VLSI processor that can not only select arbitrary multiple wavelengths to lase but also control their cavity losses thus their output powers. Each pixel blocks steers a waveband along its initial path or slightly off-track so that variable optical attenuation (and hence RF attenuation) is achieved for all delayed RF signal simultaneously.

Lens 1 Fiber Collimator

Array

OA N

Optical Couplers

Fig. 11. Opto-VLSI-based phased array antenna architecture for broadband beam steering. For each beammsteering scenario, the RF signal produced after the photodetection of the delayed RF-modulated optical signals was monitored by an RF power meter. Figures 12 (ad) show several scenarios of measured antenna radiation patterns (right) corresponding to different equi-spaced wavelength channels (left) generated by the Opto-VLSI processor. For example, in Fig. 12 (a), the Opto-VLSI processor generated a wavelength spacing of 1 nm, which corresponds to 42-ps delay between adjacent antenna elements. The measured beamsteering angle was about 7°, which is in excellent agreement with the theoretical prediction of 7.2°. When the spacing between the laser channels was increased from 1 nm to 4 nm, the measured main lobe was steered from 7° to around 30º, as illustrated in Fig. 12. These experimental results agree very well with the theoretical predictions, demonstrating the excellent capability of the Opto-VLSI processor to realize a phased-array antenna

Note that this structure has the capability of null steering if each laser channel is able to generate multiple laser wavelengths. Single or multiple arbitrary wavelengths for each laser channel can be generated and the amplitude of each laser of different wavelength can also be controlled simply by uploading the appropriate steering phase holograms onto the Opto-VLSI processor that can not only select arbitrary multiple wavelengths to lase but also control their cavity losses thus their output powers. Each pixel blocks steers a waveband along its initial path or slightly off-track so that variable optical attenuation (and hence RF

95% 5%

**N**

**RF Amp**

attenuation) is achieved for all delayed RF signal simultaneously.

Lens 2

Steering Holograms

> Opto-VLSI Processor

OA 2

OA 1

PC

PC

PC

RFin

**HDF**

**OA**

beamformer.

RFout

Circulator

**1**

EOM **PD**

**Optical Coupler**

**2**

λ1

Controller

λM

Grating Plate

The experiments demonstrated an Opto-VLSI-based tunable beamformer for adaptively steering the radiation patter of RF phased array antennas. By using phase holograms implemented by a single Opto-VLSI processor 4 independent optical wavebands can be generated and their spectral tuned, leading to 4 independent tunable fiber lasers. The RFmodulated lasers are used to generate TTD in 4-element rectangular patch arrayed antennas. Experimental results show the capability of the proposed structure to perform RF beam steering between 0°-30°.

Fig. 12. Opto-VLSI-based tunable laser generations (left), and the corresponding RF beam steering profile measured in the experiments (right).

Photonic Microwave Signal Processing Based on Opto-VLSI Technology 381

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