WO2000065742A2

WO2000065742A2 - Tailored coverage area for adaptive antennas

Info

Publication number: WO2000065742A2
Application number: PCT/SE2000/000722
Authority: WO
Inventors: Tomas ÖSTMAN; Bo Hagerman; Gunnar Monell
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1999-04-27
Filing date: 2000-04-25
Publication date: 2000-11-02
Also published as: WO2000065742A3; AU4632300A

Abstract

A method and system for tailoring the coverage area of one or more cells in a radiocommunication system is disclosed. Cell coverage is tailored through the definition of a cell profile which may include a power profile and/or a hysteresis profile. Use of a cell power profile permits the system operator the adjust the antenna radiation patterns of appropriate base stations throughout the network. Use of a cell hysteresis power profile further permits the system operator to make additional adjustments to the borders of cells, for the purpose of hand-off, beyond the cell border adjustments implemented by the cell power profile. Definition of power and/or hysteresis profiles advantageously allows the system operator to tailor the borders of each cell in the network without necessarily having to add or relocate base stations.

Description

TAILORED COVERAGE AREA FOR ADAPTIVE ANTENNAS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent

Application No. 60/131,239 filed on April 27, 1999, the entire content of which is hereby incorporated by reference. This application is related to co-pending U.S. Patent

Application No. (Attorney Docket No. 040010-391) "Tailored Power Levels at Handoff and Call Set-up," the entire content of which is hereby expressly incorporated by reference.

BACKGROUND

The present invention pertains to radiocommunication systems and, more particularly, to customizing cell coverage areas in radiocommunication systems. The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.

Figure 1 illustrates an example of a conventional cellular radio communication system 100. The radio communication system 100 includes a plurality of radio base stations 170a-n connected to a plurality of corresponding antennas 130a-D. The radio base stations 170a-n in conjunction with the antennas 130a-n communicate with a plurality of mobile terminals (e.g. terminals 120a, 120b and 120m) within a plurality of cells llOa-n. Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink. The base stations are connected to a mobile telephone switching office (MSO) 150. Among other tasks, the MSO coordinates the activities of the base stations, such as during the handoff of a mobile terminal from one cell to another. The MSO, in turn, can be connected to a public switched telephone network 160, which services various communication devices 180a and 180b.

The system base stations 170a-170n are generally located, and their associated antennas and transmitters are configured, so as to provide optimal coverage of the mobile phone service area. To ensure optimal coverage, the system operator conducts surveys of the network for the purpose of determining areas of inadequate cell coverage or areas where system resources are being allocated inefficiently. An example of a cell in which resources are allocated inefficiently is illustrated in Fig. 2, where the base station's 200 antenna and transmitters (not shown) are configured in such a way that an excessive amount of energy is being directed toward an area within the cell 210 where the need for cellular service is minimal (e.g, an area that includes a mountain 220). This inefficient allocation of resources further creates an area 230 of poor radio coverage within cell 210. Another example where cellular network resources are allocated inefficiently is shown in Fig. 3. An obstacle 300, within the transmission path of a first base station 330, causes radio reflection 310 into a neighboring cell 320. This reflection can create undesirable interference with the transmissions from the base station 340 of the neighboring cell 320. If it is determined that the coverage area associated with a cell is inadequate, or that system resources are being allocated inefficiently, such problems may be solved by reducing the size of the cell so the cell coverage area no longer encompasses the physical obstacle causing the problem. Reducing the size of the cell may be accomplished by reducing base station transmit power. Reducing base station transmit power, however, may leave "gaps" in system cell coverage. These "gaps" would thus necessitate relocation of existing base stations or the placement of additional base stations (e.g., microcells) to complete the system coverage. Adding or relocating base stations, however, would require additional hardware and increase system operating costs. Failure to optimize the coverage area associated with a cell may further cause an inefficient usage of system resources during mobile station hand-off. As a mobile station travels from one cell to the next, it undergoes a process known as hand-off. Hand-off helps to ensure that the connection between the mobile station and the target base station is established before the connection between the mobile station and the serving base station is terminated, so as to prevent "dropping" of the call. Hand-off is typically initiated based on the signal quality of the connection between the mobile station and the serving base station and the signal quality of the connection between the mobile station and the target base station. For instance, if the signal quality associated with a connection between the mobile station and the target base station exceeds the signal quality associated with the connection between the mobile station and the serving base station, the mobile station may initiate a handoff from the serving base station to the target base station. However, under certain circumstances, the signal quality associated with the connection between the mobile station and the target base station, and the signal quality associated with the connection between the mobile station and the serving base station may fluctuate with respect to each other such that the mobile station initiates a handoff from the serving base station to the target base, and soon thereafter, from the target base station back to the serving base station. This phenomenon is often referred to as the "ping-pong" effect, and when it occurs, it results in an inefficient utilization of resources as one skilled in the art will readily appreciate.

Most conventional systems employ a handoff hysteresis value to minimize inadvertent hand-offs due to fluctuations in signal quality, for example, when the mobile station is operating near the serving cell border. The hysteresis value thus serves to minimize a "ping-pong" hand-off effect between adjacent base stations. When handoff hysteresis is employed, a mobile station will not initiate handoff unless the signal quality associated with the connection between the mobile station and the target base station exceeds the signal quality associated with the connection between the mobile station and the serving base station, plus the hysteresis value. Nevertheless, mobile station traffic patterns at or near the borders of cells may lead to the occurrence of an excessive number of hand-offs where each hand-off causes a load on system resources. Again, cell base stations may be repositioned, or additional base stations may be added to help alleviate the problem. However, as previously stated such solutions require additional hardware and increase operating costs.

SUMMARY These, and other problems, drawbacks, and limitations of conventional cellular systems are overcome by the present invention through the customization of the coverage area of one or more cells in a cellular network. In general, tailoring the coverage of a given cell is achieved by defining a cell profile for the cell. The cell profile in turn may be defined as a function of a power profile and/or a hysteresis profile. Use of a power profile permits the system operator to adjust the antenna radiation patterns of appropriate base stations throughout the network. Use of a hysteresis profile further permits the system operator to make additional adjustments to the borders of cells, for the purpose of hand-off, beyond the cell border adjustments implemented by the power profile.

Definition of power and/or hysteresis profiles advantageously allows the system operator to tailor the borders of each cell in the network without necessarily having to add or relocate base stations.

In one exemplary embodiment of the present invention, a method of defining the coverage area of a cell in a cellular radiocommunication system is provided. This exemplary embodiment comprises the steps of: defining a power profile of said cell; and adjusting a maximum output power of an antenna associated with said cell using said power profile.

In a further exemplary embodiment, a system for defining the coverage area of a cell in a cellular radiocommunication system is provided. This exemplary embodiment comprises: a database containing a first set of data defining a power profile of said cell; a management node, wherein said management node updates said first set of data in accordance with inputs received at said management node; and an antenna associated with said cell, wherein a maximum output power of said antenna is adjusted using said first set of data.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects, features and advantages of the present invention, as well as other features, will be more readily understood upon reading the following detailed description in conjunction with the drawings in which:

Figure 1 shows a conventional radio communication system including plural base stations and a mobile telephone switching office; Figure 2 shows a conventional cell encompassing a physical obstacle;

Figure 3 shows reflections from a first conventional cell interfering with the transmissions of a second convention cell;

Figure 4 illustrates an angle θ associated with a cell base station in accordance with exemplary embodiments of the invention; Figure 5 illustrates a Power profile look-up table in accordance with one exemplary embodiment of the invention;

Figure 6 illustrates a Power profile look-up table in accordance with another exemplary embodiment of the invention; and

Figure 7 illustrates a cell profile in accordance with exemplary embodiments of the invention.

DETAILED DESCRIPTION

The present invention involves customizing the coverage area of cells in a cellular network through the use of adaptive antennas. In general, the coverage area of a cell may be customized by defining a cell profile for the cell, where the cell profile defines the effective border of the cell. The cell profile, in turn, may be defined in terms of (or as a function of) the transmission pattern and the maximum radiated output power output associated with cell's base station antenna(s), herein referred to as the cell's power profile A. The cell profile may also be defined as a function of handoff hysteresis values associated with the cell, wherein the handoff hysteresis values associated with the cell are herein referred to as the hysteresis profile H. A cell profile R can thus be defined as a function of the cell's power profile A and or the cell's hysteresis profile H. Accordingly, a desired cell profile P may be achieved by adjusting the cell's power profile A and/or the cell's hysteresis profile H. Therefore, using the cell profile R in accordance with exemplary embodiments of the invention, the system operator advantageously has the ability to tailor the coverage area of each cell in the network in order to account for specific environmental and geographical considerations (e.g., natural and man-made obstacles) affecting each cell, without having to add or relocate network resources. In one exemplary embodiment of the present invention, the cell profile P may be defined by a power profile A which determines the cell base station's maximum permitted output power. The power profile A, in turn, may further be defined by the following function:

A =AΘ,T,S) Eqn. (l)

As indicated in Eqn. (1), the power profile A is a function of a number of parameters. The first parameter θ represents the angle between a first radial projecting from the cell base station, for example, the radial 400 projecting from the base station 410 in Fig. 4, and the radial 420 projecting from the base station 410. The second parameter T represents the topology of the cell coverage area. The topology parameter Tmay take into account the geography of the cell coverage area as well as and natural and/or man-made objects which may effect the transmission of and/or reception of signals at the given angle θ. A third parameter S represents the network or system configuration surrounding the cell. The parameter S may take into account such factors as the radiated power associated with the base stations in adjacent cells.

One skilled in the art will recognize that the specific values assigned to each of the parameters θ, T and S can be determined through system modeling or through actual measurements. For example, a model of the cell's topology at a given angle 0may indicate that signal transmissions will experience some degree of interference and or attenuation due to naturally occurring and/or man-made obstacles. Accordingly, a specific value for the topology parameter T corresponding to a given angle #may be selected for the purpose of modifying the power profile A, wherein the modification of the power profile A due to the change in value of the topology parameter T at angle # results in a corresponding modification to the cell profile P which is a function of the power profile A. The modified cell profile, in turn, results in less interference for mobile station's operating at or near the cell border, along a radial that approximates angle θ. One skilled in the art will further recognize that variables other than θ, T, and S may be used to define the power profile A. For example, a parameter which relates to the traffic capacity of the cell may be an additional parameter used in defining the power profile A. Such a parameter might be desirable where traffic capacity changes dramatically over given periods of time.

The present invention, in accordance with the exemplary embodiments described above, may be implemented using a look-up table, an algorithm, or both. Fig. 3 illustrates the use of a look-up table (LUT) 500 where each LUT entry defines one of a number of discrete transmission power level values associated with the power profile A. For example, a first LUT entry 510 might define the transmit power level for the power profile A for angles θ in the interval 0-5 degrees, wherein a value of 0 dB for the topology parameter T and a value of 0 dB for the system configuration parameter S is shown to correspond with a maximum output power reference level of X dBm. A second LUT entry 520 might define the transmit power level for the power profile A for angles in the interval of 6-15 °, wherein a value of -5 dB for the Topology parameter T and a value of 0 dB for the System Configuration parameter S is shown to correspond with a transmit power level of X-5 dBm. A third LUT entry 530 might define the transmit power level at angles in the interval of 16-45 °, wherein a value of 0 dB for the Topology parameter T and a value of +10 dB for the System Configuration parameter S is shown to correspond with a transmit power level of X+10 dBm. The present invention might also be implemented through the use of an LUT 500 shown in Fig. 3, combined with an algorithm, where the algorithm is, more specifically, used to make small adjustments to the power profile values.

If the parameter values in the LUT are set remotely by a management node, the LUT 500, or data derived from an algorithm, can be stored in a database at any appropriate point in the network, including the MSO or the Central Operations Center. Additionally, the management node, which can access the data stored in the database and also receive inputs which can be used to alter the algorithm or update values stored in the database, can be located at any appropriate point in the network. This location could include, among other points in the network, the Mobile Switching Center or the Central Operations Center.

The transmit power associated with the power profile A, which is a function of the parameters θ, T, and S, can be used to modify the radiated power output corresponding to one or more antenna beams of the cell's base station adaptive antenna array. In accordance with exemplary embodiments of the present invention, adaptive phase array antennas such as those disclosed in U.S. Patent No. 5,924,020, the disclosure of which is hereby incorporated by reference, are used in conjunction with the power profile A. As one skilled in the art will readily appreciate, the antenna beams associated with an adaptive antenna may be generated at any of a number of power levels and in any of a number of directions. Therefore, through the use of an adaptive antenna, the maximum power level and direction of each of the radiated beams defining a sector of a cell can be individually adjusted such that the cumulative area covered by the plurality of antenna beams defines a desired cell coverage area. The cell coverage area defined by the cell profile P, which, in turn, is defined by the power profile A, may be achieved by adjusting the adaptive antenna so that it produces an antenna pattern that reflects the cell profile P. Accordingly, a LUT 600 of another exemplary embodiment is shown in Fig. 6. In this embodiment, each LUT entry defines the maximum output power for each antenna beam as a function of parameters T and S. For example, a first LUT entry 610 might define the maximum transmit power level for the power profile A of a first beam (beam,) of the antenna array, wherein a value of 0 dB for the topology parameter T and a value of 0 dB for the system configuration parameter S is shown to correspond with a maximum output power reference level of X dBm. A second LUT entry 620 might define the maximum transmit power level for the power profile A of a second beam (beam₂) of the antenna array, wherein a value of -5 dB for the Topology parameter Tand a value of 0 dB for the System Configuration parameter S is shown to correspond with a transmit power level of X-5 dBm. A third LUT entry 630 might define the maximum transmit power level of a third beam (beam₃) of the antenna array, wherein a value of 0 dB for the Topology parameter and a value of +10 dB for the System Configuration parameter S is shown to correspond with a transmit power level of X+10 dBm. In another exemplary embodiment of the present invention, the cell profile P may be defined by a hysteresis profile H. The hysteresis profile Hmay itself be further defined using parameters similar to those described above with respect to the power profile A. For example, the hysteresis profile may be defined by the following relationship:

H=/θ,r,S) Eqn. (2)

where θ, T, S are defined similarly to Eqn. (1) above. In addition, the hysteresis profile Hmay be implemented using a LUT that is similar to either LUT 500 or LUT 600 illustrated in Figs 5 and 6. Accordingly, the hysteresis profile H can vary as a function of the direction (e.g., θ) a mobile terminal is heading in a cell.

The hysteresis profile thus gives the possibility not only to have different hysteresis values depending on which neighboring cell the mobile terminal is traveling into, but also to have different hysteresis values depending on the position of the mobile terminal along the border of two neighboring cells. In still another exemplary embodiment, the cell profile P may be defined as a function of the power profile A and the hysteresis profile H:

P= _A(β,T,S),H(β,T,S)) Eqn. (3) As illustrated in Fig. 7, the cell profile P would determine the antenna patterns 720, which defines the radiated power boundary 700 of the cell, and the hysteresis profile 710, which specifies the cell boundary for the purpose of hand-off.

Through use of the cell profile P, the exemplary embodiments of the invention described above permit the configuration of cell borders throughout at least a portion of a network so as to maintain a desired level of cell coverage. Use of the cell profile P advantageously allows the system operator to make fine adjustments the transmit power and hysteresis for any angle and/or sector of a cell and thus ensures an appropriate amount of cell coverage that can account for obstacles and reduce interference and account for levels of increased traffic capacity at any specific location within a cell. One skilled in the art will appreciate that the exemplary embodiments of the present invention are applicable with any radio communication system access strategy, including TDMA, FDMA, and CDMA.

The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.

Claims

What is Claimed is;

1. A method of defining the coverage area of a cell in a cellular radiocommunication system comprising the steps of: defining a power profile of said cell; and adjusting a maximum output power of an antenna associated with said cell using said power profile.

2. The method of claim 1 , further comprising the step of: defining a hysteresis profile of said cell.

3. The method of claim 1 , wherein said power profile is a function of a plurality of parameters.

4. The method of claim 3, wherein said plurality of parameters includes an angle from a fixed point associated with a base station located in the cell.

5. The method of claim 3, wherein said plurality of parameters includes a parameter relating to a topology of the cell area.

6. The method of claim 3, wherein said plurality of parameters includes a parameter relating to a configuration of the cellular system.

7. The method of claim 2, wherein said hysteresis profile is a function of a plurality of parameters.

8. The method of claim 2, wherein said hysteresis profile assumes different values on performing handoff from said cell to a neighboring cell depending on the position along a border between the cells of a mobile station needing handoff assistance.

9. The method of claim 7, wherein said plurality of parameters includes an angle from a fixed point associated with a base station located in the cell.

10. The method of claim 7, wherein said plurality of parameters includes a parameter relating to a topology of the cell area.

11. The method of claim 7, wherein said plurality of parameters includes a parameter relating to a configuration of the cellular system.

12. The method of claim 1 , wherein said antenna is an antenna array and wherein said output power is adjusted for each beam associated with the antenna array.

13. The method of claim 12, wherein the output power for each beam associated with the antenna array is individually selectable to a plurality of power levels.

14. The method of claim 13 , wherein output power of at least one beam associated with the antenna array is different than output power of another beam associated with the array.

15. The method of claim 1 , wherein the first set of data is derived using an algorithm.

16. The method of claim 1 , wherein an algorithm operates on said first set of data before the data is used to adjust the output power of said antenna.

17. A system for defining the coverage area of a cell in a cellular radiocommunication system comprising: a database containing a first set of data defining a power profile of said cell; a management node, wherein said management node updates said first set of data in accordance with inputs received at said management node; and an antenna associated with said cell, wherein a maximum output power of said antenna is adjusted using said first set of data.

18. The system of claim 17, wherein said database contains a second set of data defining a hysteresis profile of said cell.

19. The system of claim 17, wherein said power profile is a function of a plurality of parameters.

20. The system of claim 19, wherein said plurality of parameters includes an angle from a fixed point associated with a base station located in the cell.

21. The system of claim 19, wherein said plurality of parameters includes a parameter relating to a topology of the cell area.

22. The system of claim 19, wherein said plurality of parameters includes a parameter relating to a configuration of the cellular system.

23. The system of claim 18, wherein said hysteresis profile is a function of one or more parameters.

24. The method of claim 18, wherein said hysteresis profile assumes different values on performing handoff from said cell to a neighboring cell depending on the position along a border between the cells of a mobile station needing handoff assistance.

25. The system of claim 18, wherein said plurality of parameters includes an angle from a fixed point associated with a base station located in the cell.

26. The system of claim 18, wherein said plurality of parameters includes a parameter relating to a topology of the cell area.

27. The system of claim 18, wherein said plurality of parameters includes a parameter relating to a configuration of the cellular system.

28. The system of claim 17, wherein said antenna is an antenna array and wherein said output power is adjusted for each beam associated with the antenna array.

29. The system of claim 28, wherein the output power for each beam associated with the antenna array is individually selectable to a plurality of power levels.

30. The system of claim 29, wherein output power of at least one beam associated with the antenna array is different than output power of another beam associated with the array.

31. The system of claim 17, wherein the first set of data is derived using an algorithm.

32. The system of claim 17, wherein an algorithm operates on said first set of data before the data is used to adjust the output power of said antenna.