WO2012116725A1

WO2012116725A1 - An antenna arrangement

Info

Publication number: WO2012116725A1
Application number: PCT/EP2011/052938
Authority: WO
Inventors: Henrik Asplund; Jonas Medbo
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2011-02-28
Filing date: 2011-02-28
Publication date: 2012-09-07

Abstract

An antenna arrangement (33) comprising at least one elongated structure (36) for guiding an electromagnetic wave is disclosed. The elongated structure (36) comprises a plurality of through-going perforationsandeach said perforation is adapted to allow a small fraction of the total energy in the guided electromagnetic wave to be radiated out from the perforation. The antenna arrangement (33) is adapted to be used by a base station (31) in a cellular communication system and the elongated structure (36) is arranged to be oriented in a substantially vertical direction. Additionally, a base station site (30) comprising the antenna arrangement is described.

Description

AN ANTENNA ARRANGEMENT

TECHNICAL FIELD The present invention discloses a novel antenna arrangement and a base station site comprising said antenna arrangement.

BACKGROUND When deploying wireless communications systems such as, for example, cellular systems, in indoor environments in general, so called "leaky cables" are sometimes used, also sometimes referred to as leaky feeders or radiating cables. A leaky cable is a cable which is capable of conducting electromagnetic radio frequency energy, and which has been provided with apertures in order to make the cable radiate, i.e. to allow some of the energy to "leak" from the cable, thus enabling the cable to act as an antenna. Such an antenna, i.e. a leaky cable, will due to reciprocity be able to act equally well as a receiving as a transmitting antenna. Due to its nature of a cable, a "leaky cable antenna" will, as compared to a traditional antenna, act more like a line source than a point source, obtaining a more uniform coverage level compared to a point source antenna from which the radiated power falls off rapidly with distance, thus making it easier to obtain coverage in tunnels, along railways or where a high degree of "shadowing" occurs when using a point source antenna. An example of the latter is an indoor scenario, e.g. an office landscape.

A leaky feeder is typically designed as a coaxial cable or a waveguide where the outer conductor is perforated in order to create holes or slots through which some of the energy in the cable can escape and radiate into free space. Various designs exist for the slot geometry and separations. The slots can be uniformly distributed along the length of the cable or clustered in groups, thereby providing different radiating properties. Variations of the slot structure, shape, and density along the cable allow a cable designer to shape how much the cable is radiating from different sections and also in what directions. The latter property is realized through selecting on which side of the cable the slots are placed, as each slot will have directional radiation properties that essentially form a lobe or beam away from the cable.

It has been found through measurements and numerical simulations that a leaky feeder will have its radial radiation maximum in the direction that the slots are facing. More importantly, depending on the frequency and slot separation, the maximum radiation will be in a cone at a certain polar angle from the longitudinal axis. When the radiation has its maximum along the cable it is said to operate in the coupling mode, while when the maximum is more perpendicular to the cable it is said to operate in the radiating mode. Figure la illustrates the cone angle of radiation from a leaky cable in coupling mode and figure lb illustrates the cone angle of radiation from a leaky cable in radiating mode.

While the leaky cable is well suited to achieve good coverage in the vicinity of the cable such as in indoor or underground deployments, it can be difficult to use it to provide coverage over wider areas due to the very high directivity that the cable has in the far field. A conical beam may also not be well suited to the coverage area. Traditional prior art antennas which are more point source-like are preferably used in such scenarios, even though these antennas have limited degrees of freedom in shaping the radiation pattern due to the compact size. Prior art antennas also rely on good impedance and radiation resistance matching in order to be effective radiators. However, thereby they become sensitive to detuning due to e.g. objects or persons in the near field or in contact with the antenna. Furthermore, the field strength at short distances has strong fluctuations. Such fluctuations are a result of nulls below the main beam lobe and can be suppressed using null-filling and tapering techniques.

However, these techniques add complexity to the antenna arrangement design.

SUMMARY It is therefore an object of the present invention to address some of the problems and disadvantages outlined above and to provide a radio base station site and an antenna arrangement with a leaky cable which has improved properties as compared to the prior art. The above stated object is achieved by means of an antenna arrangement and a base station site according to the independent claims, and by the embodiments according to the dependent claims.

In accordance with one embodiment, an antenna arrangement comprising at least a first elongated structure for guiding an electromagnetic wave is provided. The elongated structure comprises a plurality of through-going perforations and each said perforation is adapted to allow a fraction of the total energy in the guided electromagnetic wave to be radiated out from the perforation. Furthermore, the antenna arrangement is adapted to be used by a base station in a cellular communication system. Moreover, the elongated structure is arranged to be oriented in a substantially vertical direction.

In accordance with another embodiment, a base station site comprising a base station and an antenna arrangement which comprises at least a first elongated structure for guiding an electromagnetic wave is provided. The elongated structure comprises a plurality of through-going perforations and each perforation is adapted to allow a fraction of the total energy in the guided electromagnetic wave to be radiated out from the perforation. The base station site is adapted to be used in a cellular communication system. Moreover, the elongated structure is arranged to be oriented in a substantially vertical direction.

An advantage of particular embodiments is that they provide additional degrees of freedom in synthesizing a suitable radiation pattern compared to antennas traditionally used by base stations in cellular communication systems. The additional degrees of freedom can be utilized to create higher and/ or more uniform antenna gain within an intended coverage area, while minimizing the antenna gain outside the same intended coverage area which will lead to reduced interference towards and from other neighbouring cells or services in the cellular communication system. Another advantage of particular embodiments is that they can easily be made to conform to a constructed or natural structure, such as the framework/ truss of a tower, a building wall or a cliff. This may be utilized to reduce the visual impact and in some cases the wind load compared to e.g. panel antennas which are commonly used in current cellular communication systems.

Yet another advantage of particular embodiments is the low radiated power per unit length and corresponding low field strengths near the antenna arrangement and the base station site. Comparing an embodiment comprising an leaky cable of 16 m with a i m long prior art antenna design, both radiating the same power, it is evident that the electric field strength near the antenna arrangement will be reduced by a factor 1 /4. This is very beneficial for achieving compliance with regulatory safety limits for radio frequency exposure.

Yet another advantage of particular embodiments is the fact that each through-going perforation is a rather poor radiator i.e. it has a rather poor impedance match to the intrinsic impedance of the elongated structure (usually 50 ohm). The benefit of this is that the presence of an object very near a part of the elongated structure only has a very limited detuning effect, in contrast the rather strong detuning that can be the result with a prior art antenna arrangement. Thus, the radiation efficiency of particular embodiments is more insensitive to disturbances from objects in the near field.

Further advantages and features of embodiments of the present invention will become apparent when reading the following detailed description in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, reference is made to the following drawings and preferred embodiments of the invention.

Figure la and lb illustrate the cone angle of radiation from a leaky cable in coupling mode and the cone angle of radiation from a leaky cable in radiating mode, respectively. Figure 2 shows a macrocell site with a base station connected to a prior art antenna arrangement in a cellular communication system.

Figure 3 shows an example of an embodiment of a base station site comprising an antenna arrangement according to an exemplary embodiment.

Figure 4 shows a diagram of the signal strength at ground level as a function of distance of a prior art antenna arrangement at two different tilt angles and the signal strength of an embodiment of the antenna arrangement of the present invention.

Figure 5 shows a diagram of the SIR of a prior art antenna arrangement and an exemplary embodiment assuming an identical site at a distance of 4 km from the embodiment.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular device configurations in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practised in other embodiments that depart from these specific details. In the drawings, like reference signs refer to like elements.

Moreover, those skilled in the art will appreciate that the means and functions explained herein below may be implemented using software functioning in conjunction with a programmed microprocessor or general purpose computer, and/or using an application specific integrated circuit (ASIC). It will also be appreciated that while the current invention is primarily described in the form of methods and devices, the invention may also be embodied in a computer program product as well as a system comprising a computer processor and a memory coupled to the processor, wherein the memory is encoded with one or more programs that may perform the functions disclosed herein. The invention will be described below with reference to the accompanying drawings, in which the structures for guiding an electromagnetic wave are shown as coaxial cables. It should however be pointed out that this is merely an example intended to enhance the reader's understanding of the invention and should not be seen as limiting the choice of structure, which can, for example, also comprise one or more of the following:

- waveguides,

- strip line arrangements,

- micro strip arrangements.

The operation of an elongated structure, such as a leaky cable, as an antenna arrangement can mathematically be described as follows. A total of a number, N , radiating slots are positioned along the cable, with coordinates f_n = x„x + y„ + z_n z . The complex excitation α_η οΐ each slot is a function of the electric and magnetic field inside the elongated structure at the position of the slot, as well as the properties of the slot itself. Assuming that each slot is an isotropic radiator, the magnitude of the electric field at an observation point r' = x'x + y'y + z'z can be expressed as the superposition of the complex field contribution from each slot as

E{ ) C where k = 2π/λ is the wave number.

The directive characteristics of each slot may of course be taken into account by making a_n - a_n (r_n - r ') ; even though the size of each slot in relation to the frequency is small, it provides the opportunity of optimizing the radiation pattern.

When the elongated structure is straight the symmetry dictates that the radiation pattern E(r') will be circularly symmetric around the longitudinal axis of the elongated structure. To illustrate, consider a design in which the slots are uniformly separated with a spacing of half a wavelength, and where they are excited with equal amplitude and a linear phase gradient according to a = a■ _e ^{mn sme} . The radiation maximum for this design will occur in a cone with polar angle Θ from the longitudinal axis. As previously mentioned with reference to figure la, the cable 10 operates in the coupling mode when the radiation 12 has its maximum along the cable, and the cable operates in the radiation mode when the radiation 12 has its maximum more perpendicular to the cable illustrated in figure lb.

The radiation slots are preferably elongated slots 1 1 which are through-going perforations and have a main direction of extension which makes the slots radiate. The main direction of extension which makes a slot radiate differs between different kinds of cables: in a coaxial cable the main direction of extension should not coincide with the cable's main length of extension. In a waveguide, or a micro strip or strip line structure, the main direction of extension of a slot can coincide with that of the structure or cable and still radiate. It should be mentioned that, the shape of the radiation elements can be chosen from a wide variety of different kinds of perforations in the outer conductor of the structure e.g. elongated rectangular or oval slots. It should however be pointed out that most shapes of perforations will give rise to a radiating effect. Also, with reference to other kinds of possible structures for guiding an electromagnetic wave, such as waveguides or strip line and micro strip structures, it can be pointed out that the perforations which form the radiation elements should be made in the conductor of such structures.

Figure 2 shows a typical prior art radio base station site 20 comprised in a cellular communications system. The radio base site 20 provides radio coverage in a macrocell in the cellular communication system.

The term macrocell is commonly used to describe the widest range of cell sizes in the cellular communication system. Antennas for macrocells are usually mounted on towers, ground-based masts, rooftops or other structures at a height that provides a clear view over the surrounding buildings and terrain. The base station site 20 includes a base station 21 located in a shed 22 on the ground. It further includes an antenna arrangement 23 mounted at the top of a tower 24. The base station 21 is connected to the antenna arrangement 23 with feeder cables 25. The antenna arrangement 23 is typical an omnidirectional or a directional antenna which radiates 26 equally in all directions or more in one direction that the other, respectively, providing radiation coverage in the macrocell over the whole cell coverage area.

In order to provide good coverage in the macrocell covered by the base station 21 in the cellular communication system, while at the same time minimizing the interference to neighboring cells, it is common to tilt the radiation pattern of the antenna arrangement by mechanical or electrical tilt. However, the optimal tilt angle for achieving optimal cell coverage is usually another angle than an optimal tilt angle achieving low interference between the cells. The former requires a radiation beam pointing towards the cell edge while the latter requires a radiation beam of null just beyond the cell edge. The degrees of freedom of a prior art antenna arrangement make these two requirements conflicting, as the optimal tilt angle achieving maximum gain and the optimal tilt angle achieving the radiation beam of null cannot be arbitrarily chosen. Furthermore, the field strength at short distances has strong fluctuations. Such fluctuations are a result of nulls below the main beam lobe and can be suppressed using null-filling and tapering techniques.

However, these techniques add complexity to the antenna arrangement design. Additional disadvantages with this deployment are the losses in the feeder cables, the cost of the tower that should support antenna arrangements with high wind load and weight, and the compliance zones necessary to ensure compliance with regulatory safety limits for radio frequency exposure.

A concept of the embodiments described hereinafter is to provide an antenna arrangement to be used by a base station comprised in a base station site in a cellular communication system. The antenna arrangement comprises at least a first elongated structure, such as a coaxial cable, a waveguide, a strip line arrangement or a micro strip arrangement, for guiding an electromagnetic wave. The structure comprises a plurality of through-going perforations, such as the previously mentioned slots. Each perforation allows a small fraction of the total energy in the guided electromagnetic wave to be radiated out from the elongated structure. The elongated structure is arranged to be to be oriented in a substantially vertical direction, thus utilizing the previously described conical radiation characteristic and thereby creating an omni-directional cell coverage area as is commonly used in cellular communication systems.

In figure 3 an exemplary embodiment of the invention is shown. A base station site 30 which is adapted to be used in a cellular communications system is illustrated. The base station site 30 comprises a base station 31. The base station 31 , e.g. a NodeB in UTRAN (Universal Terrestrial Radio Access Network) or a eNodeB in E-UTRAN (Evolved-UTRAN), communicates wirelessly with cellular user equipments (UEs) in a cell covered by the base station 31.

The base station 31 is located in a shed 32 on the ground. Furthermore, the base station 31 is connected to an antenna arrangement 33 via feeder cables

34. The antenna arrangement 33 is mounted in a tower 35. The antenna arrangement 33 comprises at least one elongated structure 36 for guiding an electromagnetic wave. In the exemplary embodiment illustrated in figure 3 two elongated structures 36 are depicted. Each structure 36 comprises a plurality of through-going perforations (not shown in figure) such as the slots previously described. Furthermore, each perforation is adapted to allow a small fraction of the total energy in the guided electromagnetic wave to be radiated out from the perforation. In figure 3 the radiation 37 of only some of the perforations are shown. The elongated structures 36 are arranged to be substantially vertically oriented in relation to the ground. Thus, the antenna arrangement is arranged to provide radiation coverage in the cell covered by the base station in the cellular communication system. In one exemplary embodiment the elongated structures 36 are controlled by the base station 31 to operate in the radiating mode and the radiation coverage is conical with respect to the substantially vertical direction. Moreover, in the cell covered by the base station the coverage at ground level is substantially equal over the whole area and at a distance exceeding the radius of the cell area the radiation coverage at ground level drops substantially sharply at an increase rate as a function of distance compared to inside the cell. In fig 4, this sharp drop of coverage is visible at distances beyond 2000 m for this embodiment.

In one exemplary embodiment, the elongated structures comprise subsections including radiation elements of substantially the same shape. However, in another exemplary embodiment, the elongated structures could comprise subsections including radiation elements of different shapes. Furthermore, the subsections could comprise radiation elements with a substantially equal slot separation or a substantially non-equal slot separation. Moreover, the subsections could be arranged to radiate with the substantially same characteristics or with different characteristics such as power or cone angle.

It should be pointed out that the elongated structure could be mounted on a constructed or any natural vertical structure. Examples of such structures are: a tower, mast, building wall, tree, flag pole or cliff etc.

In addition, the antenna arrangement may be used as an antenna for MIMO (Multiple Output Multiple Input) applications, wherein the antenna arrangement includes at least two elongated structures previously described. In MIMO applications, two different data streams Di and D₂ may be transmitted, one in each elongated structure 36 or both streams may be transmitted in both elongated structure, if the appropriate gain and/ or phase weighting of the data streams is applied. The embodiment comprising two elongated structures is highly suitable for MIMO applications, since the two elongated structures will have very similar radiation patterns, thereby reducing the likelihood of power imbalance in the MIMO channel which would otherwise result in reduced capacity.

Studies of the cell coverage of the antenna arrangement 33 indicate that the radiation coverage out to about 1000 m is much more uniform compared to prior art antenna arrangements. Another advantage is that the antenna arrangement 33 provides less unwanted radiation into other neighbouring cells compared to prior art antenna arrangements.

The difference in cell coverage between the antenna arrangement 33 and a prior art antenna arrangement such as the prior art antenna arrangement 23 described in connection to figure 2 is illustrated in a diagram shown in figure 4. The signal strength of the antenna arrangement 33 comprising the elongated structures i.e. leaky feeders is represented by the unbroken line 40 in the diagram. The signal strength of the prior art antenna arrangement 23 provided at a first tilt angle optimizing cell coverage at the cell edge is represented by a first broken line 41. Further, the signal strength of the prior art antenna arrangement 23 provided at a second tilt angle optimizing the signal-to-interference ratio (SIR) is represented by a second broken line 42. The antenna arrangement 33 is able to maintain almost uniform cell coverage out to 2000 m, which can be seen in the diagram.

In figure 5 a diagram showing the SIR for the same three cases is illustrated. However, a neighbouring cell is assumed, comprising a base station that uses an identical antenna arrangement at a distance 4000 m from the antenna arrangement. The SIR of the antenna arrangement 33 comprising the elongated structures i.e. leaky feeders is represented by the unbroken line 50 in the diagram. The signal strength of the prior art antenna arrangement 23 provided at a first tilt angle optimizing cell coverage at the cell edge is represented by a first broken line 51. Further, the signal strength of the prior art antenna arrangement 23 provided at a second tilt angle optimizing the signal-to-interference ratio (SIR) is represented by a second broken line 52.

When comparing the two diagrams it is evident that the antenna arrangement 33 comprising the elongated structures i.e. leaky feeders is able to provide a SIR which is on a par with the optimally tilted prior art antenna arrangement 23 represented with the broken lines 51 and 52 while at the same time providing a cell coverage which is more than 10 dB better on the cell edge. The SIR may be further improved using another exemplary embodiment in which the power radiated from the subsections of the elongated structure is tapered. One possible tapering is to smoothly lower the radiated power towards the ends of the elongated structure, which will result in a further suppression of side-lobes and as a consequence a further reduced radiation into neighboring cells.

It should be pointed out that the present invention is applicable in any cellular communication system. For example, Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS- 136/TDMA), and Integrated Digital Enhanced Network (iDEN), third generation of cellular wireless standards (3G) as well as fourth generation of cellular wireless standards (4G).

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive.

Claims

1. An antenna arrangement (33) comprising at least a first elongated structure (36) for guiding an electromagnetic wave, said structure comprising a plurality of through-going perforations, each said perforation adapted to allow a fraction of the total energy in the guided electromagnetic wave to be radiated out from the perforation, said antenna arrangement (33) is adapted to be used by a base station (31) in a cellular communication system, characterized in that said elongated structure (36) is arranged to be oriented in a substantially vertical direction.

2. The antenna arrangement (33) according to claim 1, wherein the antenna arrangement is arranged to provide radiation coverage in a cell covered by the base station in the cellular communication system and wherein the radiation coverage is conical with respect to the substantially vertical direction.

3. The antenna arrangement (33) according to claim 2, wherein the coverage at ground level is substantially equal over an area corresponding to a cell coverage area covered by the base station in the cellular communication system.

4. The antenna arrangement (33) according to claims 2 or 3, wherein the radiation coverage at ground level drops substantially sharply at a distance exceeding a radius of the cell coverage area covered by the base station in the cellular communication system.

5. The antenna arrangement (33) according to any of claims 1 to 4, wherein the at least first elongated structure comprises subsections comprising radiation elements of substantially the same shape.

6. The antenna arrangement (33) according to any of claim 1 to 4, wherein the at least first elongated structure comprises subsections comprising radiation elements of different shapes.

7. The antenna arrangement (33) according to any of claims 1 to 6, wherein the at least first elongated structure comprises subsections comprising radiation elements with a substantially equal slot separation.

8. The antenna arrangement (33) according to any of claims 1 to 6, wherein the at least first elongated structure comprises subsections comprising radiation elements with a substantially non-equal slot separation.

9. The antenna arrangement (33) according to any of claims 1 to 8, wherein the at least first elongated structure comprises subsections arranged to radiate with the substantially same characteristics such as power or cone angle.

10. The antenna arrangement (33) according to any of claims 1 to 8, wherein the at least first elongated structure comprises subsections arranged to radiate with different characteristics such as power or cone angle.

1 1. The antenna arrangement (33) according to any of claims 1 to 10, wherein the at least first elongated structure is mounted on a constructed or natural vertical structure such as a tower, mast, building wall, tree, flag pole or cliff.

12. The antenna arrangement (33) according to any of claims 1 to 1 1, wherein the elongated structure is one of the following: a coaxial cable, a waveguide, a strip line arrangement and a micro strip arrangement.

13. The antenna arrangement (33) according to any of claims 1 to 12, comprising at least two elongated structure, wherein the antenna arrangement is arranged to provide for diversity communications or Multiple Input Multiple Output, MIMO, communications.

14. A base station site (30) comprising a base station (31) and an antenna arrangement (33) which comprises at least a first elongated structure (36) for guiding an electromagnetic wave, said structure (36) comprising a plurality of through-going perforations, each said perforation is adapted to allow a fraction of the total energy in the guided electromagnetic wave to be radiated out from the perforation, said base station site (30) is adapted to be used in a cellular communication system, characterized in that said elongated structure (36) is arranged to be oriented in a substantially vertical direction.

15. The base station site (30) according to claim 14, wherein the antenna arrangement is arranged to provide radiation coverage in a cell covered by the base station in the cellular communication system and wherein the coverage is conical with respect to the substantially vertical direction.

16. The base station site (30) according to claim 15, wherein the coverage at ground level is substantially equal over an area corresponding to a cell coverage area covered by the base station in the cellular communication system.

17. The base station site (30) according to claims 14 or 15, wherein the radiation coverage at ground level drops substantially sharply at a distance exceeding a radius of the cell coverage area covered by the base station in the cellular communication system.

18. The base station site (30) according to any of claims 14 to 17, wherein the at least first elongated structure comprises subsections comprising radiation elements of substantially the same shape.

19. The base station site (30) according to any of claim 14 to 17, wherein the at least first elongated structure comprises subsections comprising radiation elements of different shapes.

20. The base station site (30) according to any of claims 14 to 19, wherein the at least first elongated structure comprises subsections comprising radiation elements with a substantially equal slot separation.

21. The base station site (30) according to any of claims 14 to 19, wherein the at least first elongated structure comprises subsections comprising radiation elements with substantially non-equal slot separations.

22. The base station site (30) according to any of claims 14 to 21 , wherein the at least first elongated structure comprises subsections arranged to radiate with the substantially same characteristics such as power or cone angle.

23. The base station site (30) according to any of claims 14 to 21 , wherein the at least first elongated structure comprises subsections arranged to radiate with different characteristics such as power or cone angle.

24. The base station site (30) according to any of claims 14 to 23, wherein the at least first elongated structure is mounted on a constructed or natural vertical structure such as a tower, mast, building wall, tree, flag pole or cliff.

25. The base station site (30) according to any of claims 14 to 24, wherein the elongated structure is one of the following: a coaxial cable, a waveguide, a strip line arrangement and a micro strip arrangement.

26. The base station site (30) according to any of claims 14 to 25, wherein the antenna arrangement comprising at least two elongated structure, wherein the antenna arrangement is arranged to provide for diversity communications or Multiple Input Multiple Output, MIMO, communications.