phase shifters or optoelectronic delay line application to ... · digital signal processor with a...

Phase shifters or optoelectronic delay line application to sequential analysis of space in adaptive phased array antennas

Z. Bielecki1, W. Kołosowski2, M. Muszkowski2 & E. Sędek2 1Military University of Technology, Warsaw, Poland

2Telecommunication Research Institute, Warsaw, Poland

Abstract In the case of the increasing number of frequency users, more attention is paid to effective usage of allocated frequency bands by operators and special services. Adaptive space filtering is a great advance in phase array antenna application. The main advantage of this kind of phased array antenna is the flexible antenna characteristics shaped possibility by amplitude and phase changes of antenna supply signals. Dynamic beam shape control needs to apply digital phase shifters. The resolution of phase shifters depends on a number of control bits. The dependence of phase shifter resolution on directional antenna characteristics is presented in this paper. Because of the large cost of phase shifters and attenuators applied to adaptive phased array antennae, the optoelectronic approach has been presented. In this case, high resolution beam positioning is obtained, which is not possible in digital phase shifters application. Keywords: array antenna, adaptive phased arrays antenna.

1 Introduction

Application of adaptive phased arrays antennas provides detection of required signals and attenuation of noise (reference) signals. The main feature of this kind of antenna is flexible and continuous shaping of directional characteristics by the change of amplitude and phase relations between signals of different antenna elements. Practical applications of adaptive antennas need to solve difficult technological problems like selection of transmitter-receiver modules, phase shifters, power dividers and connection lines. A number of those components can exceed a few hundreds. Rapid development of optoelectronic components,

© 2005 WIT Press WIT Transactions on Modelling and Simulation, Vol 41, www.witpress.com, ISSN 1743-355X (on-line)

Computational Methods and Experimental Measurements XII 241

especially used for telecommunication, like elements dedicated for fibre optic signal transmission and optical signal processing, provide to build adaptive antennas based on advanced optoelectronic components. In this paper, adaptive antennas based on digital phase shifters and optoelectronic units for adaptive antenna control have been described.

2 Electronic antenna beam shaping

The adaptive phased arras antennas system with phase shifters consists of 8 channels, 8 weight units (5-bit digital phase shifters), signals summing unit and digital signal processor with a complex algorithm. This algorithm uses the information based only on the received signals and provides optimal weight values for each channel. The adaptive phased arrays antennas system is presented in fig. 1.

Figure 1: Adaptive phased arrays antennas system. Each channel of this system consists of liner antenna set, microwave directional coupler (splitter), low noise preamplifier, intermediate frequency block, IQ converter, and analogue to digital converter. Each part of the adaptive antenna system has a special function and it influences final antenna characteristics. The IQ converter separates in-phase portion and quadrature portion of the signal. The digital weight units have main influence on a beam shape of antenna. Those units can include only phase shifters, then a main beam position can be controlled only (fig. 2) or phase shifters with attenuators, then a beam position and full antenna characteristics control can be achieved (fig. 3). Main task of digital weight units is adaptive setting of main antenna beam on a direction of useful signal which is to be detected. When the algorithms are used determining also directions of interference signals, then disturbances from these directions are attenuated. For each of the described variants, it is necessary to control a phase or simultaneously phase and amplitude of currents supplying radiating elements of the antenna.


242 Computational Methods and Experimental Measurements XII

Figure 2: Antenna characteristics control based on phase shift distribution (dashed lines – radiation direction 00, solid lines – radiation direction 200).

Figure 3: Antenna characteristics control based on phase shift and amplitude distribution. (dashed lines – radiation direction 00, solid lines – useful signal 200 and interference signal 00).



3 Experimental results

Computer simulation provides weight values and phase shifts for 8-element linear antenna set. This set is based on microstrip technology using. 5-bit phase shifters (fig. 4). Eight-element antenna set characteristics for three different values of main lobe shifts is presented in fig. 5.

Figure 4: Eight-element linear antenna set with phase shifters units. a) experimental set-up b) linear antenna set view.

Figure 5: Measured antenna characteristics for different value of θ0.



Figure 5 presents the measured characteristics of antenna array of Fig. 4 for selected values of main lobe’s shift of antenna radiation characteristics. The first curve corresponds to 00 shift, the second one to 5.80 shift, and the third curve to 12.50 shift. To limit the errors connected with positioning of the main lobe of radiation characteristics, 8-element shifters or even the shifters of larger number of elements should be used. However, it causes increase in these devices price. In this case, optoelectronic methods can be used.

4 Phase errors in digital phase shifter application

Due to application of adaptive algorithm, it is possible to determine the weight values for any angles of main lobe of antenna setting. A number of bits that control a digital phase shifter is decisive for accuracy of phase shifts positioning. For 5-bit phase shifter, a resolution of phase positioning is 11.250. Errors of beam positioning for 8-element linear antenna are presented in table 1, for the required beam angle θ0 = 5.80.

Table 1.

Output number

1 2 3 4 5 6 7 8

Phase required 0 17.73 35.46 53.19 70.92 88.65 106.38 124.11 Phase

approximated 0 22.5 33.75 56.25 67.50 90 101.25 123.75

Phase Error 0 4.77 –1.71 3.06 –3.42 1.35 5.13 –0.36

5 Optoelectronic antenna beam shaping

Active phased arrays provide a flexible way to electronic beam scanning in wide angle range without a need of mechanical rotation of the antenna and easy spatial characteristics beam shaping achieved by independent control of antenna elements. Optoelectronic technology has been applied to microwave antenna beam scanning. The scheme of a control system for 16-element linear antenna is presented in fig. 6. It consists of the optical transmitter and receiver blocks, optical multiplexers and 16-channel fibre optic parametric unit. Optical carrier signal generated in main optical transmitter is modulated by a microwave pulse signal. Next, it is routed via optical multiplexer (1×2) to the input of a fibre optic parametric unit. Optical signal is divided into 16 channels, delayed and attenuated independently in each channel according to the control process. Sixteen optical signals are routed via optical multiplexers to optical receivers block, where the optic to electric conversion is performed and they are connected to transmitter part of the antenna. The reverse direction microwave signals from the received part of the antenna modulate optical carriers in block of 16 optical transmitters. Optical signals are routed via optical multiplexers to the fibre optic parametric unit and they are attenuated, delayed and splitted into a summation signal by an optical splitter. This signal is routed via the optical multiplexer to optical receiver where the optic to electric conversion is performed.



Figure 6: Antenna control system scheme.

Optical transmitter box is presented in fig. 7. The applied optical Mach-Zender modulator operates in a third optic window allowing to 10 GHz optical signal modulation. A specialised microwave amplifier has been applied as a modulator driver. An optical receiver box is presented in fig. 8. The optical receiver performs optic to electric signal conversion. It applies 8-GHz bandwidth optical photodiode. A microwave amplifier has been used, thus additional amplifying of 22 dB is achieved.

Figure 7: Optical transmitter box.

Figure 8: Optical receiver box.

Small board of optical receivers where applied in a set of 16 receivers (fig. 9). Detail measurements of the developed system are based on measurements of the output signals power distribution and microwave signals delay between a modulating signal and 16 output signals of the control system. The measurements have been done using microwave network analyser HP8720B. Power and phase of microwave signal have been measured. The phase was converted into time delay for signal frequency equal to 5 GHz. The antenna elevation characteristics have been calculated on the basis of the obtained results.



Antenna beam scanning in the range of 00÷450 has been achieved. Elevation characteristics of the antenna, calculated theoretically a) and based on measurements b) for the distribution of the output power like cos²x +0.4 are presented in fig. 10.

Figure 9: Optical receiver. Radiation characteristics of 16-element antenna row, performed for non-uniform signals power distribution at the control system outputs proves considerable compatibility to theoretical simulations. The developed system ensures opportunity of the beam propagation direction control with an excellent accuracy of 0.10. This allows for achieving of an ideal agreement of the antenna beam propagation angle according to the desired direction. For 16 antenna elements, the beam width reaches 6.3º for 0º propagation angle and 9º for 45º propagation angle. Side lobes level is –25 dB related to the main lobe in the simulation, while real characteristics side lobes level is about –17 dB. It can be reduced by application of high precision optical attenuators.

6 Conclusions

Experimental analysis and computer simulation prove that application of optical weigh units in phase array beam forming provide fully antenna characteristics control. Using digital microwave phase shifters, the calculated phase shift must be approximated according to a phase shifter resolution. Optimal technique depends on a number of antenna elements which implicates a beam width. For the very narrow beams, the best solution is application of optical delay lines in the weigh units providing high angle resolution and positioning accuracy.



Figure 10: Elevation characteristics of 16-element linear antenna, (A) – calculated in theory, (B) – calculations based on measurements.

References

[1] Mailloux R. J., Phased Array Antenna Handbook, Artech House Inc.1994, [2] Grimes G., Microwave Journal, Microwave Fiberoptic Delay Lines.

August 1992. [3] Zmuda H., Toughlian E. N., Photonic Aspect of Modern Radar, Artech

House, Inc. 1994 [4] Kumar A., Antenna Design With Fiber Optics, Artech House, Inc. 1996



[5] Seeds A. J., Application of Opto-Electronic Techniques in Phased Array Antenna Beamforming, Microwave Photonic, Technical Digest, MWP 1997

[6] Frankel M.Y., Esman R.D., Array Transmitter/Receiver Controlled by a True Time-Delay Fiber-Optic Beamformer, Parent M.G. IEEE Photonics Technology Letters Volume: 7 10 , Page(s): 1216 –1218, October 1995

[7] Soref R., Optical Dispersion Technique for Time-Delay Beam Steering, Appl. Opt. Vol. 31, No. 35, pp. 7395-7397, Dec. 1992

[8] Muszkowski M., Projekt koncepcyjny układu sterowania i dystrybucji sygnałów mikrofalowych do 16-elementowej płaskiej anteny jednowymiarowej z zastosowaniem włókien światłowodowych o wysokiej dyspersji, Arch. PIT, B12/177/2000, L.Dz. 18988/2000

[9] Dufrene R., Sędek E., Kołosowski W., Wnuk M., Lisowski J., Muszkowski M., Metody optyczne i elektroniczne kształtowania charakterystyki kierunkowości anteny ścianowej - zalety i wady, Prace PIT nr127 Warszawa 2001



phase shifters or optoelectronic delay line application to ... · digital signal processor with a...

Documents