WO2008124943A1

WO2008124943A1 - A diversity system for antenna sharing deployment

Info

Publication number: WO2008124943A1
Application number: PCT/CA2008/000709
Authority: WO
Inventors: Stuart Dean; Graham Belcher
Original assignee: Tenxc Wireless Inc.
Priority date: 2007-04-16
Filing date: 2008-04-16
Publication date: 2008-10-23
Also published as: CA2582866A1

Abstract

A wireless communications system is disclosed in which first and second base transceiver stations are connected to an antenna having first and second diversity characteristics and channels. First and second inputs of a first 90° or 180° combiner are fed the first characteristic of the first channel and a characteristic of the second channel and first and second inputs of the second combiner are fed the second characteristic of the first channel and a characteristic of the second channel opposite to that of the second input of the first combiner. The summation outputs of the first and second combiners are fed to the main ports of the first and second stations and the difference outputs thereof are fed to the diversity ports of the second and first stations. If the characteristic of the second input of a combiner is opposite to the characteristic of the first input, amplitude nulls may be significantly reduced.

Description

A DIVERSITY SYSTEM FOR ANTENNA SHARING DEPLOYMENT

FIELD OF THE INVENTION

The present invention relates to wireless communications systems and in particular to a tower-sharing arrangement in which multiple wireless operators may share a common antenna.

BACKGROUND TO THE INVENTION

Wireless communications systems are becoming increasingly prevalent around the world. While historically, such systems as cellular telephone networks have been primarily deployed in industrialized nations, the technology has become sufficiently mature that developing nations are now deploying extensive wireless networks. Indeed, many such nations view the rapid deployment of wireless technology as a key means to accelerate their industrial development and their entry into the top economic tier.

However, as with many such initiatives, the economies of developing nations often cannot support, much less justify the significant up front expenditures uncovered in creating the infrastructure to support such emerging technologies.

Additionally, even if the infrastructure funding were made available, other physical resources might be lacking. For example, many developing nations have a very high population density, so that acquisition of real estate to house cellular base stations and antennas to service such dense populations is often prohibitively expensive and may in fact not be available at any price. Indeed, the spacing between such base stations and antennas is likely to be much less than for industrialized nations because of the increased population density to be serviced.

One approach that has been proposed to overcome such financial and other limitations is for multiple network operators to share physical resources. For example, a plurality of operators may decide to share a common antenna tower, thus significantly reducing each operator' s infrastructure costs of establishing the network.

While such tower-sharing arrangements resolve some significant stumbling blocks to wireless deployment strategies, conventional arrangements are fraught with difficulties.

For example, conventional tower-sharing arrangements, such as is shown in prior art Figure 1, may consist of tower-sharing without diversity, in which a first operator (Operator A) uses a first polarization, for example, the +45° polarization port 110 of a multiple (typically dual) polarized antenna 100, which is connected to the main input port 121 of Operator A' s associated base transceiver station (BTS 1) 120, and other operators (Operator B) use other available polarizations, which are preferably mutually orthogonal, for example, the -45° polarization port 115 of the common antenna 100, which is connected to the main input port 131 of Operator B' s associated base transceiver station (BTS 2) 130.

Those having ordinary skill in this art will readily appreciate that it is beneficial to have diversity capability, so that in a decorrelated fading environment, one may choose the stronger of the two diverse signals or else combine the signals in some fashion to optimize the system performance.

One significant disadvantage of conventional tower-sharing approaches is that known schemes for providing diversity reception capability are limited.

Those having ordinary skill in this art will appreciate that it is possible to split the two signals emanating from output ports 110, 115 through the introduction of power splitters to achieve diversity capability

Conventional rules of thumb suggest that splitting a signal in two in this way will introduce a signal loss of approximately 3 dB into the path in both the transmit and receive directions, which loss may not be acceptable. Tapping the output or adding external low- noise amplifiers LNAs would have significant cost consequences, which also may not be acceptable.

As seen in prior art Figure 2, two antennas, designated A 210 and B 220 respectively, in a so-called quad pole package 200 are used. A quad pole antenna consists of a pair of antennas mounted in a single casing giving the appearance of a single physical antenna, and which may therefore alleviate some of the problems with site fees. Indeed, often lease costs for antenna base stations are predicated on the number of antenna towers, so the use of quad pole antennas may halve the real estate budget.

Whatever the real estate cost implications, for purposes of providing diversity capability, the prior art discloses a configuration of such a quad pole antenna to provide diversity capability to two operators.

In such a configuration, both polarity ports (eg. +45° 230 and -45° 235) of antenna A 210 are respectively connected to the main 241 and diversity 242 inputs of base transceiver station BTS 1 120 associated with Operator A and both polarity ports 245 and 240 of antenna B 220 are respectively connected to the main 251 and diversity 252 inputs of base transceiver station BTS 2 130 associated with Operator B. Thus, diversity performance for two operators may be obtained from an antenna pair that outwardly appears to be a single antenna. This configuration is reasonably effective, but has a limited growth path toward high capacity systems in that, when a large number of frequency channels are used, the transmit power combining reduces the signal power by a minimum of 1/n, where n is the number of independent signal channels. While additional antennae could be provided to overcome such power limitations, appropriate separation requires the use of additional tower space. Lack of spectrum may also limit the growth because of interference considerations, a well known problem in the art.

Those having ordinary skill in this art will readily appreciate that the antenna 100 with polarization diversity may be equally substituted by a pair of spatially- diverse antennas.

Accordingly, it is desirable to provide a true tower-sharing antenna diversity arrangement.

Moreover, it is desirable to provide a tower- sharing antenna diversity arrangement that dispenses with any requirement to have internal access to the base transceiver station of either network operator or that, from the point of view of one network operator, is relatively otherwise impervious to network faults of the other network operator.

Furthermore, it is desirable to provide a tower- sharing antenna diversity arrangement that does not suffer from any significant degradation in diversity performance through the tower-sharing arrangement.

SUMMARY OF THE INVENTION

The present invention accomplishes these aims by providing a tower-sharing antenna diversity arrangement in which the two signals provided to each base transceiver station correspond to different channels and to different polarizations .

The inventive arrangement makes use only of passive equipment outside the respective base transceiver stations, so that operator independence is provided.

The first and second inputs / outputs of a first 90° Or 180° power combiner / divider are fed a signal with the second diversity characteristic of the first channel and a characteristic of the second channel, and first and second inputs / outputs of the second combiner are fed the first diversity characteristic of the first channel and the diversity characteristic of the second channel that is opposite to that of the second diversity input / output of the first combiner. The summation ports of the second and first combiners are fed to the main input / output ports of the first and second base transceiver stations (BTS 1 and BTS 2), and the difference ports thereof are fed to the diversity input ports of the first and second stations respectively.

If the diversity characteristic of both inputs to a combiner are the same (e.g. same polarizations) there may be amplitude nulls in the antenna gain pattern under certain circumstances. If the diversity characteristic of the second input of a combiner is opposite to the diversity characteristic of the first input, amplitude nulls in the antenna gain pattern may be significantly reduced.

According to a first broad aspect of an embodiment of the present invention, there is disclosed a diversity antenna sharing wireless communications system having a first base transceiver station having main and diversity input ports and a second base transceiver station having main and diversity input ports associated with a common antenna,

the common antenna being adapted to provide a first and second diversity characteristic for each of a first and second channel; the system comprising first and second combiners each having first and second input ports accepting respective first and second input signals and first and second output ports,

wherein the first output port of each combiner generates a summation signal comprising a sum of the first and second input signals and the second output port of each combiner generates a difference signal comprising a difference of the first and second input signals,

wherein, for the first combiner, the first input port is connected to the second diversity characteristic provided for the first channel of the antenna, the second input port is adapted to receive a signal containing the second channel of the antenna, the first output port is connected to the main input port of the second base transceiver station and the second output port is connected to the diversity input port of the first base transceiver station; and

wherein, for the second combiner, the first input port is connected to the first diversity characteristic provided to the first channel of the antenna, the second input port is adapted to receive a signal containing the second channel of the antenna, the first output port is connected to the main input port of the first base transceiver station and the second output port is connected to the diversity input port of the second base transceiver station,

whereby each of the main and diversity input ports of each of the first and second base transceiver stations receive signals that contain components of each of the first and second channels and components of each of the first and second diversity characteristics;

whereby the diversity characteristic provided to the component of each channel provided to the main input port of each of the first and second base transceiver stations is different from the diversity characteristic provided to the component of the corresponding channel provided to the diversity input port thereof; and

whereby the diversity characteristic provided to the component of each channel provided to the first base transceiver station at each of the main and diversity input ports thereof is different from the diversity characteristic provided to the component of the corresponding channel provided to the corresponding input port of the second base transceiver station.

According to the second broad aspect of an embodiment of the present invention, there is disclosed a method of sharing a common antenna, adapted to provide a first and second diversity characteristic for each of a first and second channel, between first and second base transceiver stations each having a first and second input ports in a wireless communications system, comprising the steps of:

connecting a sum of the second diversity characteristic provided for the first channel of the antenna and a signal containing the second channel of the antenna to the first input port of the second base transceiver station and a difference of the second diversity characteristic provided for the first channel of the antenna and the signal containing the second channel of the antenna to the second input port of the first base transceiver station; and

connecting a sum of the first diversity characteristic provided to the first channel of the antenna and a signal containing the second channel of the antenna to the first input port of the first base transceiver station and a difference of the first diversity characteristic provided to the first channel of the antenna and a signal containing the second channel of the antenna to the second input port of the second base transceiver station,

whereby each of the first and second input ports of each of the first and second base transceiver stations receive signals that contain components of each of the first and second channels and components of each of the first and second diversity characteristics;

whereby the diversity characteristic provided to the component of each channel provided to the first input port of each of the first and second base transceiver stations is different from the diversity characteristic provided to the component of the corresponding channel provided to the second input port thereof; and

whereby the diversity characteristic provided to the component of each channel provided to the first base transceiver station at each of the first and second input ports thereof is different from the diversity characteristic provided to the component of the corresponding channel provided to the corresponding input port of the second base transceiver station. BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which:

Figure 1 is a prior art block diagram of a non- diversity tower-sharing arrangement using a dual polarized antenna;

Figure 2 is a prior art block diagram of a diversity tower-sharing arrangement using a quad-pole antenna package;

Figure 3 is a block diagram of a diversity tower- sharing arrangement using a split sector antenna array, according to a first embodiment of the present invention;

Figure 4 is a plot of the antenna gain as a function of azimuthal angle for the embodiment of Figure 3;

Figure 5 is a block diagram of a diversity tower- sharing arrangement using the split sector antenna array of Figure 3, according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to Figure 3, there is shown a tower-sharing diversity antenna configuration, shown generally at 300, which comprises a split sector array

^■ antenna 310, first and second combiners Combiner 1 320 and Combiner 2 330, as well as a pair of conventional diversity base transceiver stations BTS 1 120 and BTS 2 130.

A split sector array antenna, such as is shown at 310, is an antenna designed to provide coverage not only for one of the conventional tri-sector array configurations typically used by cellular operators, but for a configuration in which such a sector is replaced with two complementary and slightly overlapping sub-sectors so that there will be coverage from up to six sub-sectors for high capacity systems. In such configurations, optimized sector patterns have the capability of providing even greater subscriber capacity than conventional tri-sector antenna arrays .

Such split sector arrays comprise a single antenna 310 having 4 ports 311-314, comprising two sub- sectors or channels, conventionally denoted left (L) and right (R) , each having dual polarization, typically at +45° and -45°. Each sub-sector or channel, when used in a conventional sector to provide two sub-sectors for increased capacity purposes, covers approximately half, typically denoted the left or right side (viewed from above) of a conventional sector.

The combiners Combiner 1 320 and Combiner 2 330 maybe a 180°or a 90° combiner accept two input signals at inputs designated A 321, 331 and B 322, 332 respectively, and generate summation signals at outputs Σ 323, 333 respectively and difference signals at outputs Δ 324, 334, respectively, where

Σ = A +B and A = A-B (1) for a 180° combiner, or

Σ = A + jB and A = A-JB (?)

for a 90° combiner

Cables from the split sector array 310 to the combiner inputs 321, 322, 331 and 332 have a defined length such that the phases of the signal outputs are substantially the same .

In this first embodiment, the L and R channels of a certain polarization are fed into the inputs of the same combiner, so that, for example, Combiner 1 320 has its A input 321 connected to the L -45° output 312 of the split sector antenna 310 and its B input 322 connected to the R - 45° output 314, while Combiner 2 330 has its A input 331 connected to the L +45° output 311 of the split sector antenna 310 and its B input 332 connected to the R +45° output 313.

Then, the summation signals of the two combiners 323, 333 are fed into the main inputs of the base transceiver stations, while the difference signals are fed into the diversity inputs of the other base transceiver station. That is, as shown in Figure 3, Combiner 1 320 has its summation output 323 connected to the main input 251 of base transceiver station BTS 2 130, and its difference output 324 connected to the diversity input 242 of base transceiver station BTS 1 120, while Combiner 2 330 has its summation output 333 connected to the main input 241 of base transceiver station BTS 1 120, and its difference output 334 connected to the diversity input 252 of base transceiver station BTS 2 130. Preferably the combiners 320, 330 are located in the feed system, and still more preferably, at the masthead.

By this configuration, polarization diversity is re-introduced into the tower-sharing arrangement, since the main and diversity inputs of a base station are driven by different combiners, and thus of signals of differing polarity. Additionally, operator interdependence is avoided, as is any issue relating to internal access to either base transceiver station.

The cost of so doing is minimal, involving simply a pair of combiners, which are purely passive devices and thus, relatively maintenance free.

Again, by this configuration, the failure of one base transceiver station will have no operational impact on the other base transceiver station.

Furthermore, the embodiment of Figure 3 substantially avoids any issue relating to amplitude ripple, in that as a mobile subscriber for a given network operator moves across the coverage area of the sector, there are appropriately phased signal components from each channel .

An exemplary overall power distribution of the antenna system of Figure 3 is shown in Figure 4. Four distributions are shown. Plot 410 corresponds to a power distribution for a conventional sector antenna system. Plot 420 corresponds to a power distribution for the right beam of the antenna system of Figure 3. Plot 430 corresponds to a power distribution for the left beam of the antenna system of Figure 3. Finally, plot 440 corresponds to a composite power distribution of distributions 420 and 430, the power distribution that would be experienced by each operator in the embodiment shown in Figure 3.

With this embodiment, the main and diversity channels for each base transceiver station remain relatively decorrelated. This permits the maintenance of effective, albeit somewhat reduced polarization diversity in the cross-over regions at the boresight, but overall the diversity performance is satisfactory across the entire sector coverage area.

By the use of 180° combiners, any pattern nulls in the summation path will be minimized and are dependent on the separation of the phase centres of both the left and right sub-arrays. On the other hand, there will be a pattern null for the difference path. Because this signal is fed into the diversity channel of the base transceiver station, it will provide some degradation of the diversity signal.

Multipath effects will likely offer some mitigation as the angle of arrival of the signals is spread over an arc thus reducing the impact of any deep pattern null. Finally, those having ordinary skill in this art will appreciate that the amplitude null lies in the diversity channel and not in the main channel.

In the case of the use of 90° combiners, shallow nulls are produced in both the transmit and receive paths. The depth and position of these nulls are dictated by the positions of the beam phase centres, column spacing and beam weights.

Nevertheless, minor adjustments to the controlling parameters may reduce the depth and control the position of such amplitude nulls. Referring now to Figure 5, a second embodiment is shown. The components in this embodiment are identical to the components in the embodiment shown in Figure 3. The only difference is that whereas in Figure 3, the L and R channels of a certain polarization are fed into the inputs of the same combiner, in the embodiment of Figure 5, they are fed into the inputs of different combiners, so that the summation and difference signals generated by each combiner contains components of both a L and R channel, as well as of a +45° and -45° polarization component.

In Figure 5, for example, Combiner 1 320 has its A input 321 connected to the L -45° output 312 of the split sector antenna 310 and its B input 322 connected to the R +45° output 313, while Combiner 2 330 has its A input 331 connected to the L +45° output 311 of the split sector antenna 310 and its B input 332 connected to the R -45° output 314.

By making this change, the performance degradation discussed in respect of the embodiment of Figure 3 may be substantially mitigated, while the power distribution of the primary pattern will remain largely as set out in Figure 4.

In this second embodiment, the combined azimuth patterns, if measured using a single polarized antenna in a vertical orientation, would also be largely as shown 440 in Figure 4. However, for the 180° combiner case, the null at the pattern cross-over region (boresight) is polarization dependent so that the deep null a practitioner of the art might expect would only occur for a vertically polarized mobile signal.

A similar effect also occurs when 90° combiners are employed.

Because the antenna polarisation in the region is horizontal for the difference channel and vertical for the summation channel, polarisation orthogonality and hence optimal diversity performance is maintained.

Nevertheless, irrespective of whether 90° or 180° combiners are used, orthogonal polarization is maintained between the main and the diversity channels of each base transceiver station.

Those having ordinary skill in this art will appreciate that the split-sector array antenna 310 could be replaced by a pair of dual-polarization antennas in a quad- pole package. As well, concurrently, a spatially diverse antenna system could also be employed in the present invention. In such a case, each beam would be generated by a different antenna, in each of two polarizations. However, by definition, this would not mean antenna sharing arrangement

The present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combination thereof. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and methods actions can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language.

Suitable processors include, by way of example, both general and specific microprocessors. Generally, a processor will receive instructions and data from a readonly memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto- optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of volatile and non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; CD-ROM disks; and buffer circuits such as latches and/or flip flops. Any of the foregoing can be supplemented by, or incorporated in ASICs (application-specific integrated circuits), FPGAs (field- programmable gate arrays) or DSPs (digital signal processors) .

It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present invention, without departing from the spirit and scope of the present invention.

Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein.

Accordingly, the specification and the embodiments are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the following claims.

Claims

CLAIMS :

1. A diversity antenna sharing wireless communications system having a first base transceiver station having main and diversity input ports and a second base transceiver station having main and diversity input ports associated with a common antenna,

the common antenna being adapted to provide a first and second diversity characteristic for each of a first and second channel;

the system comprising first and second combiners each having first and second input ports accepting respective first and second input signals and first and second output ports,

wherein, for the first combiner, the first input port is connected to the second diversity characteristic provided for the first channel of the antenna, the second input port is adapted to receive a signal containing the second channel of the antenna, the first output port is connected to the main input port of the second base transceiver station and the second output port is connected to the diversity input port of the first base transceiver station; and wherein, for the second combiner, the first input port is connected to the first diversity characteristic provided to the first channel of the antenna, the second input port is adapted to receive a signal containing the second channel of the antenna, the first output port is connected to the main input port of the first base transceiver station and the second output port is connected to the diversity input port of the second base transceiver station,

2. A diversity antenna sharing wireless communications system according to claim 1, wherein for the first combiner, the second input port is connected to the first diversity characteristic provided for the second channel of the antenna, and

for the second combiner, the second input port is connected to the second diversity characteristic provided for the second channel of the antenna.

3. A diversity antenna sharing wireless communications system according to claim 1, wherein

for the first combiner, the second input port is connected to the second diversity characteristic provided for the second channel of the antenna, and

for the second combiner, the second input port is connected to the first diversity characteristic provided for the second channel of the antenna.

4. A diversity antenna sharing wireless communications system according to claim 1, wherein the first and second combiners are 180° combiners.

5. A diversity antenna sharing wireless communications system according to claim 1, wherein first and second combiners are 90° combiners.

6. A diversity antenna sharing wireless communications system according to claim 1, wherein the antenna is a split sector antenna array.

7. Adversity antenna sharing wireless communications system according to claim 1, wherein the antenna is a dual- antenna quad pole package.

8. A diversity antenna sharing wireless communications system according to claim 1, wherein the first and second diversity characteristics are polarization diversities.

9. A diversity antenna sharing wireless communications system according to claim 8, wherein the first diversity characteristic is +45°polarization.

10. A diversity antenna sharing wireless communications system according to claim 9, wherein the second diversity characteristic is -45° polarization.

11. A diversity antenna sharing wireless communications system according to claim 1, wherein the first channel is a left (1) channel.

12. A diversity antenna sharing wireless communications system according to claim 11, wherein the second channel is a right (R) channel.

13. A method of sharing a common antenna, adapted to provide a first and second diversity characteristic for each of a first and second channel, between first and second base transceiver stations each having a first and second input port in a wireless communications system, comprising the steps of:

whereby the diversity characteristic provided to the component of each channel provided to the first base transceiver station at each of the first and second input ports thereof is different from the diversity characteristic provided to the component of the corresponding channel provided to the corresponding input port of the second base transceiver station.

14. A method of sharing a common antenna, according to claim 13, wherein the signal containing the second channel that is summed with the first diversity characteristic provided to the first channel for forwarding to the first input port of the second base transceiver station and differenced with the first diversity characteristic provided to the first channel for forwarding to the second input port of the first base transceiver station, has the second diversity characteristic, and the signal containing the second channel that is summed with the second diversity characteristic provided to the first channel for forwarding to the first input port of the first base transceiver station and differened with the second diversity characteristic provided to the first channel for forwarding to the second input port of the second base transceiver station, has the first diversity characteristic.

15. A method of sharing a common antenna, according to claim 13, wherein: the signal containing the second channel that is summed with the first diversity characteristic provided to the first channel for forwarding to the first input port of the second base transceiver station and differenced with the first diversity characteristic provided to the first channel for forwarding to the second input port of the first base transceiver station, has the first diversity characteristic, and the signal containing the second channel that is summed with the second diversity characteristic provided to the first channel for forwarding to the first input port of the first base transceiver station and differened with the second diversity characteristic provided to the first channel for forwarding to the second input port of the second base transceiver station, has the second diversity characteristic.