1. Field of Invention
The present invention relates to a multiple-beam antenna system having a plurality of element antennas disposed in the configuration of a circle, a cylinder or a straight line to form a plurality of beams at one time. For example, the present invention relates to a holographic multiple-beam antenna system capable of forming multiple beams by using complex video signals each including amplitude information and phase information obtained by a receiver connected to the element antennas.
2. Prior Art
FIG. 1A shows an example of a conventional array antenna system as disclosed in a book entitled "Antenna Engineering" written by Keiji Endo et al. and published by Nikkan Kogyo Shimbun-sha in 1969. In FIG. 1A, the reference signals A1, A2, . . . , AN each designate an element antenna arranged at regular intervals around a circle having the diameter D. As shown in this figure, assuming that the main beam arrives at an angle .PHI. and that the observation point is located at an angle .phi. relative to the direction of the main beam, each of N element antennas A1, A2, . . . , AN has a phase expressed by the following equation: ##EQU1## where .lambda. is the wavelength of a transmitted wave; k=1, 2, . . . , N in correspondence with element antennas A1, A2, . . . , AN. If the radiation pattern of the respective element antennas is uniform, that is, omnidirectional, and the amplitude of an exciting current applied to each of the element antennas is the same, the composite directional pattern E(.PHI.) is expressed as follows: ##EQU2## FIG. 1B shows a composite directional pattern E(.PHI.) when .PHI.=0, D=2.lambda., N=6, 8, 16 and .infin. are used in equation (2).
The above description concerns the case of transmission, but a similar discussion could be made for the case of reception.
Being constructed in the manner described above, the conventional array antenna system needs to steer the main beam by giving the element antennas a phase as expressed by equation (1), in order to allow a plurality of beams arriving at the antenna system from various directions to be received at one time. This means that the conventional array antenna system is incapable of receiving a plurality of beams of radio waves simultaneously.
Accordingly, there has been a desire for a multiple-beam antenna system which can form a plurality of main beams at one time so as to allow the antenna system to simultaneously receive a plurality of radio waves impinging thereto. For example, an algorithm has been proposed to allow a circular array antenna to form a plurality of beams at the same time. Such an algorithm will be explained with reference to FIGS. 2A-2C.
In FIG. 2A, N antennas are disposed at intervals represented by an equal angle .DELTA..theta. [=.pi./(N-1)] on a semicircle having the diameter .alpha.. Assuming that a radio wave comes to the antenna in the direction .psi., a signal received by the .gamma.-th element antenna is expressed as follows: ##EQU3## where .lambda. is the wavelength of the incoming radio wave, and .gamma.=0, 1, 2, . . . , N-1.
Then, a weighting function T.sub..gamma. shown below is multiplied by the signal received by the .gamma.-th element antenna and expressed in equation (3): ##EQU4## where .gamma.=0, 1, 2, . . . , M-1. M is the number of element antennas which contribute to the formation of the beams, and can be arbitrarily decided. Accordingly, EQU T.sub..gamma. =0
when .gamma. is larger than M. In equation (4), W.sub..gamma. is a window function such as a Hamming function and a Hunning function.
The antenna radiation pattern at this time E.sub.k (.psi.) is given by the following equation: ##EQU5## where k=0, 1, 2, . . . , Q-1; and Q.gtoreq.N+M-1. In equation (5) S.sub..gamma. =0 in a case where .gamma. is equal to or larger than N. Equation (5) takes a form of the convolution integration of S.sub..gamma. (.psi.) and T.sub.-.gamma., and thus the Fourier transforms X.sub.p and Y.sub.p of S.sub..gamma. (.psi.) and T.sub.-.gamma., respectively, are expressed as follows: ##EQU6## This means that antenna radiation pattern E.sub.k (.psi.) may be given by first obtaining the above-described X.sub.p and Y.sub.p and then performing inverse Fourier transform of X.sub.p and Y.sub.p. In other words, antenna radiation pattern E.sub.k (.psi.) is expressed by the following equation: ##EQU7## It is possible to speed up Fourier transform and inverse Fourier transform by using FFT (Fast Fourier Transform) or IFFT (Inverse Fast Fourier Transform).
FIG. 2B shows an example of beams formed at the same time in twelve equiangular directions when the window function is the Taylor 35 dB and N=42, M=11, .DELTA..theta.=.pi./21 and .alpha.=4.58 .lambda..
The above-described algorithm for forming multiple beams can be achieved by utilizing a flowchart such as that shown in FIG. 2C. In step S1, received signal S.sub..gamma. is Fourier-transformed to obtain X.sub.p by using equation (6). Then in step S2, weighting function T.sub..gamma. to be multiplied by the received signal is Fourier-transformed to obtain Y.sub.p by using equation (7). The thus obtained X.sub.p and Y.sub.p are multiplied in step S3 to obtain product Z.sub.p [equation (8)]. Step S4 performs inverse Fourier transform of Z.sub.p to obtain antenna radiation pattern E.sub.k (.psi.) as expressed by equation (9).
As for Fourier transform of antenna outputs, the article "Applying Superresolution to Circular Arrays" written by Ulrich Petri and Pedro de la Fuente and appearing at pages 882-885 in IEEE ISAP '87 discloses a Fourier transform technique applied to outputs from respective element antennas arranged on a circle.
A digital beam forming technique is also known. For example, U.S. Pat. No. 4,656,479 discloses that the outputs from receivers each connected to a corresponding one of a plurality of element antennas are converted to digital signals which in turn are used to form beams by digital calculation. Such a digital beam forming technique is used in a holographic multiple-beam antenna system, as referred to in the article "Digital Multiple Beamforming Techniques for Radars" written by Abraham Rubin and Leonard Weinberg and appearing at pages 152-163 in EASCON-78 (IEEE, EASCON-78).
The main portion of such an antenna system is shown schematically in FIG. 3A. In this figure, N element antennas 1 form an antenna array 2. Each element antenna 1 is connected to RF amplifier 3 which amplifies an RF signal received by a corresponding element antenna. The amplified RF signal is converted to an IF signal by mixer 4. The IF signal output from mixer 4 is amplified by IF amplifier 5 and supplied to phase detector 6 which includes a coherent oscillator capable of generating a coherent signal as a reference signal. Phase detector 6 operates to perform the phase detection of the IF signal while reserving the phase of the IF signal, and converts the IF signal to a baseband complex video signal composed of amplitude information and phase information. The I (in-phase) channel and Q (quadrature) channel outputs of phase detector 6 respectively pass through low pass filters 7 and are input to A/D converters 8 where the baseband complex video signal is converted to a digital signal. The digitized complex video signals are each input to output level adjusters 9 which are operable to weight the digital complex video signal in order to reduce the side lobe level during beamforming.
N receivers 10 each including RF amplifier 3, mixer 4, IF amplifier 5, phase detector 6, low pass filter 7, A/D converters 8 and output level adjusters 9 are provided in correspondence with the respective element antennas.
The signals output from receivers 10 connected to element antennas 1 are processed in a digital manner in digital multiple-beam forming means 11 to form a multiplicity of beams the number of which corresponds to the number of element antennas. Coherent integrating means 12 performs coherent integration of the respective beams formed for a predetermined period of time by digital multiple-beam forming means 11.
An operation of the holographic multiple-beam antenna system shown in FIG. 3A will be explained below. An RF signal received by each of N element antennas is input to a corresponding receiver 10, amplified by RF amplifier 3, converted to an IF signal by mixer 4 and amplified by IF amplifier 5. This IF signal is phase-detected by phase detector 6 and converted to a complex video signal comprising I and Q channel signals. The video signal in each channel is then limited in bandwidth by low pass filter 7, converted to a digital video signal by A/D converter 8, weighted so as to reduce the side lobe level in beamforming by output level adjuster 9 and then input to digital multiple-beam forming means 11.
Assume that N element antennas are arranged in the direction parallel to the X coordinate as shown in FIG. 3B, the angle between the direction of the incoming radio wave and the X coordinate being .alpha., the distance between the adjacent element antennas being d, and the wavelength being .lambda.. Then there is the phase difference 2.pi.(d cos .alpha.)/.lambda. between the signals received by the adjacent element antennas. The output of k-th receiver 10, .sigma.(k), is input to digital multiple-beam forming means 12 which performs the following calculation: ##EQU8## Such calculation allows the digital multiple-beam forming means 11 to form N beams which have the maximum gain in the direction .alpha..sub.r =cos.sup.-1 (r.lambda./Nd). In other words, one RF signal input makes beamforming output Br corresponding to N beams. In equation (10), W.sub.k is a weighting factor given in output level adjuster 9 in receiver 10.
In such a holographic multiple-beam antenna system, the irregularities in the respective element antennas, the components of the receivers and the length of the transmission lines connecting them would produce amplitude and phase differences between the outputs of the respective receivers. This would make it difficult to form accurate beams as expressed by equation (10). Further, a change in temperature would vary the characteristics of receivers and element antennas, which would also bring about amplitude and phase differences.