Receiver Antenna Array Levente Dudás #1, Vilmos Rösner #2, Rudolf Seller #3, Károly Kazi #4, Nguyen Thi Ngoc Minh #5

#1,2,3 Department of Broadband Infocommunications and Electromagnetic Theory Budapest University of Technology and Economics Goldmann Square 3, Budapest, H-1111, Hungary

1 dudas@mht.bme.hu, 2 rosner@mht.bme.hu, 3 seller@mht.bme.hu #4 Bonn Hungary Electronics Ltd, Hungary

4kazik@bme-mw.eu#5 Academy of Military Science and Technology - Radar Institute, Vietnam

Abstract — Our aim is to develop a microwave receiver antenna system, which can form its radiation pattern (main lobe direction, null-point direction, structure of the side lobes), measure the direction of the RF source in milliseconds. In this paper we will show the theoretical background of the antenna system, the build-up of the realized system and some measurement results.

I. THEORETICAL BACKGROUND [1-5]

A. Digital beam forming We had developed a receiver antenna system, which is an

equidistant antenna row – Fig. 1.

Fig. 1. Equidistant antenna row with 4 elements

In this arrangement, the distance between antenna elements is d (in wavelength). The direction of the RF source is ϑ .

Fig. 2. The 4 channel receiver antenna system

Fig. 2 shows the simplified build-up of the antenna system. The received RF signal on each antenna element is zk. The received signals are modified by hk, which are complex numbers. After the modification, these RF input signals are summed. The output signal of this spatial finite impulse response filter is y (1).

hzyT

⋅= (1)

The analogy between the time-frequency and distance-angle domain are the following: time – distance, frequency – angle, sampling time – sampling distance, transfer function – radiation pattern.

If this complex weighting vector is used ( h ), the radiation pattern of the antenna system can be formed. If the direction of the RF source is ϑ , the phase difference between the received RF signal on two adjacent channels is (2).

ϑβ cos⋅⋅=ΔΦ d (2) where is the wavenumber. The output signal of the antenna system is the following (3).

−

=

−⋅==1

0

cos)(K

k

djkk ehFy ϑβϑ (3)

The received signal depends on the direction of the RF

signal direction and the complex weighting vector ( h ).If the antenna distance is half wavelength,

[ ]1111=T

h (left) - [ ]jjhT

−−= 11 (right), the radiation pattern of the antenna system is on Fig. 3 (in dB scale).

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90

180

270

analized radiation pattern in polar diagram

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90

180

270

analized radiation pattern in polar diagram

Fig. 3. Calculated radiation patterns with different weighting

The main direction, the null-point direction, the main lobe width and the side lobe distribution is controlled by the

weighting vector ( h ) – so the antenna pattern can be electronically controlled.

B. Direction estimation [1,2]

If the task is to estimate the direction of the RF source, the correlation matrix has to be measured and calculated

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according to the received RF signals (4), where N is the number of the received signal vector samples.

−

=

×=1

0

1 N

k

Hzz

NR (4)

The quality of R estimation depends on the multitude of N. In addition, it is necessary to create a scanning vector for

the direction estimation (5). [ ]ϑβϑβϑβϑ cos3cos2cos1)( djdjdj eees −−−= (5)

In case of antenna row, the ϑ is run from 0 to 180 degrees. Two base estimation methods are programmed. The first is

Bartlett-method (Fourier estimation) and the second is Capon-method.

The conventional (Fourier) direction estimation can be calculated with (6).

)()()( ϑϑϑH

sRsF ⋅⋅= (6) Versus ϑ , with help of the scanning vector, this algorithm

computes the correlation between the actual value of the scanning vector and the received signals.

If there are correlated RF sources around the antenna system (e.g. multi-path propagation) and/or there are some disturbing RF sources, this algorithm is in good order, but its performance is relatively low.

The Capon-method maximizes the signal-to-interference and noise ratio on the output of the antenna system. This maximalization can be calculated (in first approximation) to match null-direction into the direction of the disturbing RF sources (7).

⋅⋅= −

)()(

1lg10)( 1

ϑϑϑ

sRsC

H (7)

Thanks to the maximalization (disturbing signal level minimalization), the dynamic range of the Capon-method is much more higher than the Bartlett-method, but this method is sensitive to correlated RF sources (e.g. multi-path propagation, reflected sources).

In case of correlated sources, the correlation matrix can be singular, and the matrix inversion is not computable. This problem can be solved by correlation destroying algorithms.

II. ANTENNA SYSTEM REALIZATION [4,5]

This antenna system is working on X-band (9..10 GHz). The elementary antennas are microwave patch antennas, and the distance between antenna-elements is 0.83 wavelengths – see Fig. 4.

Fig. 4. Microwave antenna system with the calibration port

The block scheme of the realized antenna system is on Fig. 5.

Fig. 5. Build-up of the receiver antenna system

For the digital beam forming and the direction estimation, it is necessary to measure both the amplitude and the phase distribution of the incoming RF signal on the receiver antennas.

Because of this, we had developed a 4 channel coherent receiver. See Fig. 5, the incoming RF signal on each RF channels is amplified by microwave Low Noise Amplifier after band pass filtering. This RF signal is converted down to the intermediate frequency (IF) band with microwave dual-balanced-mixer and microwave PLL. The filtered and amplified IF signal is digitalized by 10 bits analogue-to-digital converter. This digitalized IF signal is converted down to the base-band with complex-numerical-controlled-oscillator. The low-pass-filtered digital base-band signal is processed by an FPGA and stored into a memory, from where it can be downloaded to the controller PC via serial port. The maximal bandwidth is 1 MHz (lower bandwidth can also be set).

Both the converting and the sampling are synchronized by reference-clock source and phase-locked-loop, because the phase coherence is essential for the perfect functioning of the antenna system.

III. ANTENNA SYSTEM CALIBRATION [3]

The practical realization causes transfer function difference between RF channels. So it is necessary to calibrate the transfer function of the receiver channels.

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The calibration process is the following: An RF signal source is connected to the calibration port of the microwave antenna array (see the left side of Fig. 4). This microwave network distributes the incoming calibration signal to 4 parts, which is coupled to the antennas with micro-strip transmission lines. These equal phase signals are received and downloaded to the PC. The controller program makes calibration vector according to these signals. On each channels this is a complex multiplication before the h weighting vector, which forms the radiation pattern of the antenna system.

IV. MEASUREMENT RESULTS

The operating antenna system (after calibration) in our laboratory is on Fig. 6.

Fig. 6. Microwave antenna system is in operation

On the monitor screen, the base-band received signals in time domain and its constellation are visible (Fig. 6) which are converted down from X-band.

A. Direction estimation The transmitter unit and its patch antenna were mounted

opposite the antenna system – see Fig. 7.

Fig. 7. Microwave transmitter

This was a sinusoidal microwave signal source (microwave synthesizer). Its transmitted power is up to 0 dBm. This was the first signal source (primary).

Another synthesizer was also mounted to disturb the reception, but the secondary source power was -20 dBm.

With these transmitters, we were able to analyse the direction estimation algorithms, its dynamic range, direction-sensitivity.

Fig. 8. Fourier estimation

The result of the direction estimation using Bartlett-method (6) is on Fig. 8. The primary and the secondary signal source are highly visible: the direction of the primary source is 90 degrees, and the secondary is about 65-75 degrees. This algorithm scans the equal amplitude distribution radiation pattern, and calculates the correlation between the received signal and the scanning vector. Both the primary and secondary signal source are received by the main lobe because the relatively low direction difference. This reception causes little dynamic and directivity (amplitude is in linear scale - V).

Fig. 9. Capon estimation

The result of the Capon-estimation is on Fig. 9. In this case, the amplitude is in logarithmic scale (dB). Both the primary and the secondary signal source are highly visible: 90 and 72 degrees.

When the scanning vector is analysing around 90 degrees, the influence of the secondary disturbing signal source is attenuated with the null-point direction matching.

Thanks to the interference cancellation method, the dynamic range of the Capon-method is much more higher than the Fourier-method, but this algorithm is sensitive for the correlated signals – correlation matrix singularity (e.g. multi path propagation – reflection from a near metal surface).

B. Radiation pattern forming If weighting vector (h) is used in the receiver antenna

system, the main lobe direction, the main lobe width, the null-point directions and the side lobe distribution can be controlled.

For example, let the main direction be 72 degrees with constant illumination function. The measured radiation pattern is on Fig. 10.

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-40

-35

-30

-25

-20

-15

-10

-5

0090

180

270

measured radiation pattern 2

Fig. 10. Measured radiation pattern

There are three radiation pattern measurement methods are programmed:

First is a conventional method, when motorized antenna rotator is used – receiver antenna system is mechanically rotated opposite the transmitter and level of the received signal is measured and stored to PC.

Second is also conventional method with manually rotator – the direction of the antenna system is adjusted, and measured the received signal point-by-point.

Third is the electronically rotation. In this case, the direction of the transmitter is fixed. This method uses rotator vector before weighting. The controller program analyse the received signal level versus the rotation direction. This measurement signal is on Fig. 10. The main lobe direction is 72 degrees with constant illumination – as visible, the maximal side lobe level is -13 dB see function sin(x)/x.

V. CONTROLLER SOFTWARE

The controller software front panel is on Fig. 11-12.

Fig. 11. Controller software 1

Fig. 12. Controller software 2

The base-band received I-Q signals in time domain (Fig. 11 left), the digital beam forming part (Fig. 11 right), the direction estimation part (Fig. 12 left) and the radiation pattern measurement part (Fig. 12 right) are shown.

All program set can be saved to binary data file, and measured data to Excel format file.

VI. CONCLUSION

We had developed an X-band 4 channel coherent receiver antenna system for digital beam forming and direction estimation application.

The measurement results show that this antenna system, which uses two built-in direction estimation methods, and three antenna radiation pattern measurement methods, is in right order.

In practice, using this antenna system, efficient interference cancellation (MSINR – Maximal Signal to Noise and Interference Ratio) with e.g. null-point matching, high speed direction estimation (in some milliseconds), high speed radiation pattern rotation, switch and matching without mechanical antenna rotator (electronically controlled antenna pattern) can be realizable.

ACKNOWLEDGMENT

This work was supported by Mobile Innovation Centre (Hungary) and Bonn Hungary Electronics Co. Ltd. (Hungary).

REFERENCES[1] Farina, A. Antenna-Based Signal Processing Techniques for Radar

Systems. Artech House, Norwood, 1992. [2] Litva, J., Kwok-Yeung Lo, T. Digital beamforming in wireless

communications. Artech House Publisher, 1996. [3] Rappaport, T. S. Smart Antennas. IEEE, 1998. [4] Ko, C. K., Murch R. D. Compact Integrated Diversity Antenna for

Wireless Communications. IEEE Transactions on Antenna and Propagation, 2001.

[5] Proakis J. G. Digital Communication. McGraw-Hill Higher Education, 2005.

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