CROSS REFERENCE TO RELATED APPLICATION
This application is a national phase application based on PCT/EP2006/005202, filed May 31, 2006.
FIELD OF THE INVENTION
The present invention refers to delay lines, and more particularly it concerns a tunable waveguide delay line in which delay tuning is obtained by varying the position of a dielectric member within the waveguide.
Preferably, but not exclusively, the present invention has been developed in view of its use in transmitting apparatus in wireless communication systems exploiting the so-called Dynamic Delay Diversity (DDD) technique.
BACKGROUND OF THE INVENTION
A currently used technique for improving performance of wireless communication systems, in particular in downlink direction, adds a delay diversity to the space and/or polarization diversity provided by transmitting antenna arrays. In other words, different elements in the array transmit differently delayed replicas of a same signal. In case of DDD technique, the different replicas undergo time-varying delays. At a receiver, the differently delayed replicas give rise to alternate constructive and destructive combinations.
A wireless communication system exploiting the DDD technique is disclosed for instance in WO 2006/037364 A.
Use of the DDD technique entails the provision of time-varying or tunable delay lines in the signal paths towards different antenna elements.
Assuming for sake of simplicity that the signals can be considered single-frequency signals, so that applying a time delay is equivalent to applying a phase shift, a delay line with length L introduces a phase shift φ=β·L, or a delay τ=dβ/dω, on the signal propagating through it, β being the propagation constant of the line and ω being the angular frequency. Thus, in order to vary the phase shift (or the delay), either β or L is to be varied. The most commonly used solution relies on a variation of β.
Several variable phase shifters based on the variation of β are known in the art, such lines generally relying upon the variation of the position of a dielectric member relative to a transmission line.
Variable phase shifters using microstrip transmission lines perturbed by dielectric elements are for instance illustrated in U.S. Pat. No. 6,075,424 A and U.S. Pat. No. 6,504,450 B2.
U.S. Pat. No. 6,075,424 discloses a phase shifter in which a dielectric slab is movable in the space between a transmission line and a ground plane. The slab has a width or a thickness or a dielectric constant that is variable from a leading edge to a trailing edge with reference to the direction of displacement, so that different relative positions of the slab and the line result in different values of the effective dielectric constant of the line and hence in different propagation velocities of the signal.
U.S. Pat. No. 6,504,450 discloses a phase shifter acting on a plurality of input signals. The shifter has a plurality of microstrip transmission lines shaped as concentric arcs of circumferences, and a semicircular dielectric member rotatable about an axis perpendicular to the plane of the transmission lines. The dielectric member, while rotating, covers increasing portions of each transmission line, thereby varying the phase shift induced by each of them.
In U.S. Patent Publication No. 2003/0042997 A1 and JP patent document 2001/068901 A are illustrated variable phase shifters implemented in rectangular waveguides.
U.S. Patent Publication No. 2003/0042997 Al discloses a phase shifter having an air-dielectric sandwich structure placed in a conventional rectangular waveguide. There, the dielectric constant of the structure, and hence the phase shift or the delay, is varied by varying the width of the air gap between a perturbing dielectric member and the waveguide walls.
JP patent document 2001/068901 A also discloses a phase shifter comprising a rectangular waveguide and a dielectric or metallic member partly inserted within the waveguide and movable with respect to the waveguide so that its insertion depth is changed.
SUMMARY OF THE INVENTION
The Applicant has observed that, even if the device disclosed in U.S. Pat. No. 6,075,424 A is suitable for operating in the frequency range used for wireless communications (from about 0.5 to about 5 GHz), it cannot provide the important phase (and time delay) variations required by the DDD technique when applied to mobile communication systems, such as UMTS systems. Moreover, the structure with a suspended transmission line is not suitable for the relative high powers used for instance in base stations or repeaters of a mobile communication system (typically, up to some ten watts).
As regards the device disclosed in document U.S. Pat. No. 6,504,450, use of the microstrip technology results in a very compact device, yet it renders the device unsuitable for the application to DDD, since a microstrip cannot bear the relatively high powers involved in the preferred application.
Additionally, the Applicant has also observed that in such devices implemented in conventional rectangular waveguides, even if they can tolerate the powers involved, the cut-off frequency for operation at the frequencies of interest for mobile communications is obtained only with considerable transversal sizes of the waveguide. Such considerable sizes make the device unsuitable for applications exploiting antenna diversity, where several delay lines might have to be installed in a same equipment.
Thus, the need exists for a tunable delay line which allows attaining relatively important delay variations, is capable of tolerating high signal powers and has reduced size, so that it is suitable for applications, like DDD, where a plurality of delay lines are to be used within a same apparatus.
According to a first aspect of the invention, there is provided a continuously tunable delay line, including a waveguide and a dielectric perturbing member movable within the waveguide for varying the propagation characteristics thereof and hence the delay imparted by the line, wherein the waveguide is a ridge waveguide with a longitudinally extending ridge, and the perturbing member is longitudinally arranged within the waveguide and is movable so as to vary its position relative to a longitudinal end surface of the ridge.
In a preferred embodiment of the invention, the perturbing member is displaceable parallel to itself in a longitudinal axial plane of the guide towards and away from the end surface, so as to vary the width of an air gap between the ridge and the perturbing member. The perturbing member can move through a slot formed in a waveguide wall portion facing the free end surface, or it can be mounted onto a support connected to rods movable through openings formed in the wall portion.
In another preferred embodiment of the invention, the perturbing member is displaceable parallel to itself in a direction transversal to the longitudinal axial plane of the guide, so as to vary the facing areas of opposite surfaces in the ridge and the perturbing member.
In another preferred embodiment of the invention, the perturbing member is displaceable parallel to itself in a direction transversal to said longitudinal axial plane of the guide, so as to vary the facing areas of opposite surfaces in the ridge and the perturbing member.
Use of a ridge guide allows lowering the cut-off frequency of the fundamental mode of propagation, resulting in a linear delay-versus-frequency behaviour in a range of interest and in a reduction of the size of the devices. Moreover, a ridge guide exhibits a high mechanical strength, is compatible with the relative high signal powers encountered in the preferred application and minimises ohmic loss.
In a second aspect, the invention also provides an apparatus for transmitting a signal to a plurality of users of a wireless communication system via diversity antennas, said apparatus including, along a signal path towards the diversity antennas, at least one tunable delay line generating at least one variably-delayed replica of the signal and consisting of a ridge waveguide delay line according to the invention.
In a further aspect, the invention also provides a wireless communication system including the above transmitting apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects, characteristics and advantages of the invention will become apparent from the following description of preferred embodiments, given by way of non-limiting examples and illustrated in the accompanying drawings, in which:
FIG. 1 is a schematic cross-sectional view explaining the basic principles of a tunable delay line according to the invention;
FIGS. 2A and 2B are representations of the electric field distribution in a ridge waveguide and a conventional waveguide, respectively;
FIG. 3 are graphs of the propagation constant and the characteristic impedance of a delay line according to the invention versus the distance between the ridge and the perturbing member;
FIG. 4 is a graph of the loss of a delay line according to the invention versus the distance between the ridge and the perturbing member;
FIG. 5 is a longitudinal cross-sectional view of a first embodiment of the invention;
FIG. 6 is a longitudinal cross-sectional view of a second embodiment of the invention;
FIGS. 7, 8, 9 are graphs of the phase shift, the return loss and the insertion loss, respectively, versus frequency, for different relative positions of the ridge and dielectric member in the delay line of FIG. 6;
FIG. 10 is a longitudinal cross-sectional view of part of a variant of the delay line shown in FIG. 6;
FIG. 11 is a schematic cross-sectional view of a third embodiment of the invention;
FIG. 12 is a schematic cross-sectional view of a fourth embodiment of the invention; and
FIG. 13 is a schematic block diagram of a transmitting apparatus of a wireless communication system with dynamic delay diversity, using delay lines according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, there is schematically shown in cross sectional view the structure of a tunable delay line according to the invention, generally denoted by 1. The physical support for the delay line is a ridge guide 2, which consists of a conductive, typically metallic, waveguide with rectangular cross section having a longitudinal partition or ridge 3 extending from one wall to short distance from the opposite wall. By way of non-limiting example, the drawings show a ridge 3 vertically projecting from the upper wall or ceiling 2a of the guide. However, the ridge could also project from the bottom wall or from a side wall, if the guide is vertically arranged.
As known, and as shown in FIG. 2A, the presence of a conductive ridge 3 acts so that the electric field is essentially concentrated in the region below ridge 3, instead of being distributed over substantially the whole width of the guide, as is the case for a conventional rectangular waveguide 20 (see FIG. 2B).
Taking this into account, as depicted in FIG. 1, the propagation characteristics of a ridge guide like guide 2 can be varied by introducing a dielectric perturbing member 4 in the region below ridge 3. In order to obtain a delay line whose delay can be varied in continuous and periodic manner as designated by the arrow direction herein as well as in FIGS. 5, 6 and 10 in time, perturbing member 4 must be displaceable relative to the ridge in continuous and periodic manner. In a preferred embodiment the perturbing member 4 is displaceable in a main axial plane of the ridge 3, towards and away from the end surface 3a of the ridge. Moving perturbing member 4 closer to or farther from ridge 3 results in a delay increase or decrease, respectively. A typical displacement frequency for perturbing member 4 could be 50 Hz. Displacement can be motor driven, or it may be obtained by piezoelectric transducers, or yet by voice coils, if important displacements are to be achieved. The means controlling the displacement are substantially conventional and are not shown in the drawings.
Perturbing member 4 is made of a dielectric material capable of resisting the signal powers envisaged in the desired application, for instance a tantalate, a niobate, alumina (Al2O3), lanthanum aluminate (LaAlO3), titanium oxide (TiO2), a titanate, etc. Such materials exhibit dielectric constants ∈r from about 10 to about 300. Titanium oxide and titanates are preferred in that they are relatively inexpensive and exhibit high dielectric constants, so that they the desired overall delay variation can be achieved with limited displacements of perturbing member 4. This assists in making compact devices. By way of example, hereinafter reference will be made to a dielectric member made of TiO2, which has a dielectric constant ∈r=104.
A ridge guide produces a significant lowering of the cut-off frequency of the fundamental mode of propagation, resulting in an approximately constant delay-versus-frequency behaviour in the range of interest. Lowering the cut-off frequency intrinsically implies a reduction of the size of the devices. Moreover, for a given cut-off frequency, a ridge guide has a greatly reduced cross sectional size with respect to a conventional rectangular waveguide, as it can be appreciated from FIGS. 2A and 2B which show, on the same scale, a ridge waveguide 2 and a conventional rectangular guide 20 for a cut-off frequency of 1.5 GHz.
Moreover, a ridge guide exhibits a high mechanical strength, is compatible with the relatively high signal powers encountered in the preferred use in base stations and repeaters of a mobile communication system and minimises ohmic loss.
Coming back to FIG. 1, in a first embodiment of the invention, perturbing member 4 is vertically displaceable between an uppermost position, in which it can be substantially in contact with bottom surface 3a of ridge 3, and a lowermost position in which it is spaced apart from that bottom surface 3a. Thus, an air gap 5 with periodically variable width exists between perturbing member 4 and bottom surface 3a of ridge 3. The variation of the width of air gap 5 determines the variation in the delay imparted by delay line 1. The vertical displacement of perturbing member 4 is permitted by a slot 6 formed in floor 2b of waveguide 2. As the currents on the waveguide wall propagate longitudinally, slot 6 does not significantly perturb the field lines inside the guide and hence it does not degrade the electrical performance.
FIGS. 3 and 4 are graphs showing the influence of the displacement of perturbing member 4 on the propagation constant β, the characteristic impedance and the waveguide loss in case of a dielectric member made of TiO2. The influence of the inputs and outputs has not been taken into account.
The solid line and the dotted line in FIG. 3 show respectively the behavior of propagation constant β in radiants/mm, left scale on the axis of the ordinates and the characteristic impedance Z0 in ohms (Ω), right scale on the axis of the ordinates, respectively, versus the air gap width in mm on the axis of the abscissa. The graphs have been plotted under the assumption that perturbing member 4 can be displaced from 0 to 1 mm from the bottom of ridge 3. The graphs show that the propagation constant decreases as the air gap width increases, the variation being almost negligible for great air gap widths and becoming very sharp as the air gap width approaches 0 mm. On the contrary, the characteristic impedance increases in an almost linear manner as the air gap width increases, this linearity being maintained for the major part of the displacement range considered, except for air gap widths close to 0.
FIG. 4 is a graph of the losses of the delay line. The graph has been plotted considering a dielectric loss tangent tgδ=0.00025 for perturbing member 4 and a conductivity of 5.8×107 Siemens/m for ridge 3. The losses increase as dielectric member 4 approaches ridge 3, since the region below ridge 3 becomes more and more filled with dielectric material. The graph is plotted only for the air gap displacement range 0 to 0.05 mm from the bottom of ridge 3, where the loss variation is detectable. In any case, the maximum loss is lower than 1.1 dBm, such a value being considered as acceptable.
The graphs of FIGS. 3 and 4 allow an evaluation of the length required of delay line 1. A signal propagating through delay line 1 is to be delayed by a time varying delay τ(t) that is to range from 0 to the carrier period T=1/f, f being the central frequency of the signal spectrum. Such delay corresponds, for the central frequency, to a phase shift φ(t) ranging from 0 to 2π. It is recalled that any delay imparted to a signal with a bandwidth Δf corresponds to a phase shift linearly varying with the frequency. Denoting by L the length of delay line 1, Δβ the difference between the values of propagation constant β in two different positions of dielectric member 4 and Δφ the corresponding differential phase shift, the following relation exists:
Δφ=−Δβ*L
By setting Δφ=−2π, value L=2π/Δβ is obtained for the length of delay line 1. For instance, the curves for β and Z0 show that a displacement of dielectric member 4 by only 0.05 mm from its uppermost position (in substantial contact with the bottom of ridge 3) results in a variation of β equal to about 0.145 rad/mm, so that a delay of one period is obtained by a length of about 43 mm only, considering a waveguide having internal dimensions of 36×18 mm with a ridge having a width of 4 mm and an height of 17 mm. Such variation of β corresponds to a variation of Z0 of about 7 ohm.
Thus, as a conclusion, FIGS. 3 and 4 show that displacement ranges much shorter than 1 mm from the edge of ridge 3, and even much shorter than 0.5 mm, can be used for perturbing member 4. However, care should be taken in choosing the uppermost position of perturbing member 4. Actually, on the one hand a displacement range closer to the ridge allows a required delay variation to be obtained with shorter displacements of perturbing member 4, which assists in obtaining compact structures; on the other hand this would result in a stronger variation of the characteristic impedance and in a loss increase.
FIG. 5 is a longitudinal cross-section of a first practical construction of a delay line 1 with a perturbing member 4 vertically displaceable through a slot 6. The same reference numerals as in FIG. 1 are used to indicate like parts. Movable perturbing member 4 is the central part of a dielectric body 7 that, in order to obtain a good matching, extends over the whole ridge length. End portions 7a, 7b of dielectric body 7 are stationary and, in correspondence with such stationary portions 7a, 7b, vertically extending connectors 8 are provided for connection of coaxial cables forming the input/output ports of the guide. Connectors 8 are so constructed that, in each cable, the central conductor is directly connected to ridge 3 and the outer conductor is electrically connected to the structure of guide 2. In such a construction, the attainable delay is in first approximation proportional to length Lpert of perturbing member 4.
Such a configuration ensures an optimum mechanical robustness and low electric losses.
FIG. 6 shows a delay line 101 that differs from delay line 1 shown in FIG. 5 with respect to of the construction of the perturbing member and of the transition between the guide and the input/output coaxial cables. Elements corresponding to those shown in FIG. 5 are denoted by like references, in a series beginning with reference numeral 101 In FIG. 6, perturbing member 104 is still vertically displaceable through a slot 106 in guide floor 102b, but it has almost the same length as ridge 103 and forms the whole of the dielectric body mounted in waveguide 102. Ridge 103 has, in its bottom surface 103a, a longitudinal recess 109 defined by two downward-extending projections 110 and receiving perturbing member 104 during at least the upper part of its displacement. Furthermore, the input/output coaxial cables are connected to waveguide 102 through connectors 108 that longitudinally project from waveguide 102. Connectors 108 are still constructed so that the central conductor of the respective coaxial cable can be directly connected to ridge 103 and the external conductor can be electrically connected to the waveguide structure. The inner cavity 108a of each connector 108 ends at a corresponding projection 110 of ridge 103.
The construction still affords the advantages of mechanical robustness and has the advantage of being simpler than that shown in FIG. 5. A further advantage is that the longitudinal connection of the coaxial cables eliminates the need for the stationary portions 7a, 7b (FIG. 5) where connectors 8 are mounted, so that perturbing member 104 (FIG. 6) can extend over almost the whole length of the ridge: thus, either a reduced displacement range is necessary for attaining a desired maximum delay or a higher delay can be obtained for a same overall displacement.
FIGS. 7 to 9 are graphs of the performance of delay line 101 of FIG. 6, for different positions of perturbing member 104. The graphs have been plotted assuming a length of the perturbing member Lpert=40 mm and a displacement range of 0.2 mm for perturbing member 104, between uppermost and lowermost positions spaced by 0.3 mm and 0.5 mm, respectively, from bottom surface 103a of ridge 103. More particularly, solid line curve a refers to the lowermost position of perturbing member 104, dashed line curve b refers to a spacing of 0.45 mm between perturbing member 104 and ridge 103, dash-and-dot line curve c to a spacing of 0.4 mm, dotted line curve d to a spacing of 0.35 mm and dash-and-double dot line curve e to the uppermost position of perturbing member 104.
FIG. 7 shows the differential phase shift (in degrees) between input and output ports 108 versus frequency (in GHz). The curves show a substantially linear behavior of the phase shift versus frequency. As shown, an overall displacement of 0.2 mm allows attaining a differential phase shift of about 90° (i.e. a delay tuning by about T/4) over the whole downlink band of the UMTS system (about 2.11 to about 2.17 GHz). Wider tuning ranges for the delay can be obtained by increasing either the displacement range for perturbing member 104 or the input-output distance (i.e. the line length). Some considerations in this respect will be made further on.
FIG. 8 shows that the return loss is strongly dependent on the position of perturbing member 104. In any case, with the considered displacement range, it can be seen that the return loss in dB vs. frequency in GHz is better than about 15 dB over the whole downlink band of the UMTS system.
In FIG. 9, the insertion loss in dB vs. frequency in GHz has been calculated by taking into account the loss of the dielectric (TiO2) and of the waveguide metal (copper). FIG. 9 shows that the insertion loss is less than 0.2 dB over the whole downlink band of the UMTS system and has a limited dependence, in such band, on the position of perturbing member 104.
As noted above, to obtain greater delays than those considered in the above discussion, either the displacement range of perturbing member 104 or the length of delay line 101 (substantially coinciding with that of perturbing member 104) should be increased. Yet, an increase of the overall displacement range results in greater distances from a position of perturbing member 104 for which the line parameters have been optimized and thus in greater mismatch. Increasing the delay line length of course affects the compactness of the device.
In the variant delay line 101′ shown in FIG. 10, ridge 103′ of waveguide 102′ has no projection like projections 110 of FIG. 6, and hence no recess is formed in bottom surface 103a′ of ridge 103′. Hence, perturbing member 104′ has actually the same length as ridge 103′. The advantages of a greater length of the perturbing member are further enhanced.
In the embodiment shown in FIG. 11, where elements corresponding to those shown in the previous Figures are denoted by like references, in a series starting with 201, waveguide 202 has an upper wall or ceiling 202a. Delay line 201 still has a vertically movable dielectric perturbing member 204 separated from ridge 203 by air gap 205. Member 204 however, instead of being movable through a slot in guide floor 202b, is supported by a metal body 211 connected to a pair of rods 212 (only one being shown in the drawing) that are connected to the displacement control members and are vertically displaceable through respective openings 213 in guide floor 202b. That solution minimizes the overall area of the passages formed in guide floor 202b and consequently current interruption. That solution can be adopted for both the construction with vertical connectors 8 (FIG. 5) and that with longitudinal connectors 108, 108′ (FIGS. 6, 10), respectively. A single rod could even be used.
In the embodiment shown in FIG. 12, delay line 301 includes a dielectric perturbing member 304 that is horizontally displaceable in a direction transverse to the longitudinal extension of ridge 303, so that the different delays correspond to different relative positions of dielectric member 304 relative to longitudinal axial plane A-A of ridge guide 302. Dielectric member 304 is secured (e.g. glued) to dielectric rods 314, 315 made of a material with lower permittivity than dielectric member 304 and low loss. The rods 314, 315 are connected to the displacement control members and are horizontally displaceable through respective openings 316, 317 in longitudinal side walls 302c, 302d of guide 302. This embodiment also minimizes the area of passages formed in guide 302. A single rod could even be used.
FIG. 13 schematically shows a transmitter of a wireless communication system using dynamic delay diversity, like the system disclosed in the above mentioned WO 2006/037364 A. The transmitter can be employed in base stations, repeaters or even mobile stations of the system. Here, an input signal IN is fed to a base-band block 50 that outputs a base-band version of signal IN. The base-band signal is fed to an intermediate-frequency/radio-frequency block 55 connected to a signal splitter 60, which creates two or more signal replicas by sharing the power of the signal outgoing from block 55 among two or more paths leading, possibly through suitable amplifiers 65a, 65b . . . 65n, to respective antenna elements 70a, 70b . . . 70n. The first path is shown as an undelayed path, whereas respective tunable delay lines 75b . . . 75n according to the invention are arranged along the other paths, each line 75b . . . 75n delaying the respective signal replica by a time varying delay τb(t) . . . τn(t). The delay variation law may be different for each line. A delay line could be provided also along the first path. The tuning control members are included within the delay lines for sake of simplicity of the drawing.
It is clear that the above description has been given by way of non-limiting example and that the skilled in the art can make changes and modifications without departing from the scope of the invention.
Patent Grant
8076997
