MXPA00000957A - System and method for controlling antenna downtilt/uptilt in a wireless communication network - Google Patents

Abstract

The wireless communication system includes antennas having electrically controllable downtilt angles and downtilt controllers associated with each antenna. The downtilt controllers receive instructions from a main controller, and adjust the downtilt angles of the associated antennas in accordance with the received instructions.

Description

SYSTEM AND METHOD FOR CONTROLLING THE ASCENDENCE / DESCENDENCE OF AN ANTENNA IN A WIRELESS COMMUNICATION NETWORK FIELD OF THE INVENTION The present invention relates to a wireless communication system, as well as to a method for controlling the descent / descent of an antenna.

DESCRIPTION OF THE RELATED TECHNIQUE Conventional wireless communication systems include a plurality of cellular sites, each of which has a base station that sends and receives signals by one or more associated antennas or antenna modules. Usually, the antenna module includes at least one receiving and one transmitting antenna, but it can use a single antenna to perform both the transmitting and the receiving functions. The form of radiation (particularly, the main lobe) of, for example, a transmission antenna at a cellular site can be tilted from a horizontal reference of the antenna by a certain angle. This angle is known as REF .: 32598 the angle of descent or inclination of the antenna, and is measured to be positive from the horizontal reference of the antenna to the ground. Accordingly, an antenna with an angle of descent or inclination of 10 degrees tilts towards the earth more than an antenna with an angle of descent or inclination of 5 degrees.

Each antenna has a coverage area, which is a geographic area in which a mobile terminal has communication with a base station associated with the antenna. The extent of the coverage area of an antenna is affected by its angle of descent or inclination, and the angles of descent or inclination that surround it, but not necessarily adjacent antennas.

Conventionally, the angles of descent or inclination of the antennas in the wireless communication systems are placed at the same time as the installation of the system in accordance with predetermined angles of descent or inclination. The person who works in the installation climbs or climbs in each tower or antenna support (for example, a building), which supports the antennas in the system, and manually sets the descent or inclination angle of each antenna, conformity with the default values. If the descent or inclination angle needs to be changed after the network installation, the worker has to climb again on the antenna tower to manually adjust the descent angle or inclination of the antenna. While making adjustments to small portions of the system, in the wireless communication system can be practical in this way, making adjustments is cumbersome, time consuming, costly and potentially dangerous, since this requires a worker to climb the tower. the antenna to adjust the descent or inclination of the antenna. However, what is difficult, costly and complex, increases in relation to the number of antennas that require changes in their angles of descent or inclination. In addition, it is impractical to make adjustments in the descent or tilt angle based on strong events, such as changes in the time of a day (for example, traffic in the mobile terminal in a coverage area for a business complex). will be greater during busy hours), and longer time events (for example, a change in seasons where the foliage affects the signal to noise ratio).

Because the angles of descent or inclination of the antennas, in a wireless communication system directly affects the quality of the operation of the system, there is a need in a simple, easy and economical way to change the angles of descent or inclination of the antenna. the antennas in a wireless communication system, in order to improve its operation. Typically, operators monitor the quality of their systems by taking operational measures indicative of it. These operational measures include, but are not limited to, interference in the common channel (ie, interference between two signals using the same channel frequency), signal to noise plus the interference ratios within the coverage areas, bit error rates within of the coverage areas, call blocking rate (for example, the ratio of (1) the number of mobile terminals in a coverage area that has their call responses denied by the base station, this due to insufficient resources in the base station that are intended for (2) the antenna module for such coverage area for the number of mobile terminals that demand calls in the coverage area) within the coverage areas, etc. For example, the interference of the measured signal strength between two coverage areas may indicate a number of adjacent transmitted signals that overlap, and therefore, provide an indicator of the quality of the facilities between the coverage areas for these adjacent antennas. As another example, high call blocking rates may indicate unacceptable levels at which users are denied service (ie users of the mobile terminal) and / or an overload condition. Typically, when a call blocking rate or other load measurement in a base station is greater than a predetermined threshold, it is said that the base station serving the coverage area or the coverage area itself is overloaded. .

Some of the operational measures are carried out by means of one or more test receivers, which are located in known measurement locations within the wireless communication system, and operational measurements are made using the test receiver. Other operational measures, such as call blocking rates, are performed as part of the operation of the system. Changes in operational measures over time may reflect changes within the coverage area such as an increase in population, the addition of a new structure (eg, a building), etc. that affect the quality of system operation. Based on the operational measures, changes can be made to the wireless communication system in order to improve the quality of its operation.

For example, when a problem, such as a poor coverage (for example, a low signal-to-noise ratio for signals received in a coverage area), is indicated by the operational measures, the signal strength of the signals can be changed , transmitted by the antenna to the coverage area with the problem, and the signal strength of the signal transmitted by one or more antennas, for adjacent coverage areas can also be changed until the operational measures show an acceptable coverage.

However, there is a demand for greater freedom with regard to directing the quality of the system in terms of its operation. Specifically, there is a demand to improve, by alternative or additional methods, the direction of the quality of the system in terms of its operation. A simple, easy and inexpensive way to adjust the descent or tilt of antenna angles in a wireless communication system would facilitate the way to meet such demands.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a wireless communication system that includes antennas, which have electrically controllable descent or tilt angles, as well as descent or tilt controllers associated with each antenna. The descent or inclination controllers receive instructions from a main controller, and adjust the angles of descent or inclination of the associated antennas in accordance with the instructions received. From the main controller, an operator can make changes that vary from the wide changes in the descent or inclination of the antennas angles in the system, to change the angle of descent or inclination of a single antenna. From the base station associated with an antenna, a ground operator can make changes in the descent angle or inclination of the associated antenna. By making changes in the descent or tilt angle, either from the base station or from the main controller, using the present invention avoids the costly and dangerous process of scaling a tower or other supporting structure to manually adjust the descent angle or inclination of an antenna.

By virtue of the fact that the process of changing the descent or inclination of the angles is very simple with the help of the present invention, the present invention allows an adaptive control of the descent or inclination of the angles for the direction of the points referring to the quality of the system, such as a quality in the facility (eg, signal overlap), denial of service (eg, load) interference with the common channel plus interference ratios, bit error rate, etc., even during the operation of the system. Furthermore, the system according to the present invention allows the automation of the process or portions of the process for the management of these points concerning quality.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention becomes more understandable from the detailed description given hereinafter and from the accompanying drawings which are given by way of illustration only, wherein the same reference numerals designate like parts in the various drawings, and where : Figure 1 shows a wireless communication system in accordance with the present invention; Figure 2 shows a diagram for a cell site in the system of Figure 1;Figure 3 shows a block diagram of a receiving antenna module, which is used in the system of Figure 1; and Figure 4 shows a block diagram of a transmission antenna module used in the system of Figure 1.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Initially, the structure of a wireless communication system in accordance with the present invention will be described with reference to Figures 1-4. Further, the operation of a wireless communication system with reference to drawings 1-4 will be described in more detail. Next, an exemplary application of a wireless communication system in accordance with the present invention is provided.

WIRELESS COMMUNICATION SYSTEM Figure 1 illustrates a wireless communication system in accordance with the present invention. As shown, a plurality of cells 1, Cell 2, ... each of which includes a cell site CS1, CS2, ... respectively. A mobile switching center (MSC) 200 communicates with each cellular site CSl, CS2, .... and with a local exchange network 6. The local exchange network 6 represents networks over which the voice is communicated and / or data such as the Public Switched Telephone Network, the Integrated Service Digital Network, the internet (interconnected networks), another internet protocol network, etc. The MSC 200 is any known MSC except for the addition of a master descent control unit (MDCU) 202. However, the MDCU 202 need not form part of the MSC 200, on the contrary, it can be formed separately and still located remotely from the MSC 200. The MDCU 202 is a data processing system programmed to operate as described in more detail below, and, when it is part of the MSC 200, uses the memory and user interfaces provided by the user. MSC 200. When provided separately from the MSC 200, the MDCU 202 includes a memory, an interface with the user as well as an interface to the MSC 200.

Figure 2 shows an exemplary block diagram for each cell site CSl, CS2, .... in the wireless communication system in accordance with the present invention. As shown there, each cell site CSl, CS2, .... includes at least one electrically controllable antenna module 100, a support unit 102, a base station BS, and at least one DC descent or inclination controller. The controllable antenna module 100 is mounted on the support unit 102. The base station BS has communication of radio frequency (RF) signals to and from the antenna module 100. And has communication with the MSC 200. The controller of descent or tilt DC has communication with the base station and with the MSC 200 (more particularly, the MDCU 202), and controls the descent or tilt angle of the antenna module 100. The antenna module 100 includes one or more receiving antennas and / or electrically controllable transmission. Such controllable antennas can be of any type, such as, antennas of electrically controlled phase arrays, mechanically controlled motor phase-controlled antennas, antennas that are mechanically tilted by motor, etc. These antennas can be configured as omnidirectional antennas (360-degree azimuthal angle), three-sector antennas (120-degree azimuth angle), six-sector antennas (60-degree azimuth angle), or any other type of multi-sector antennae.

If a multi-sector antenna is used in each cellular site CSl, CS2, each of the sites CSl, CS2, ... has an antenna module 100 and a DC descent or inclination controller corresponding to each sector. For example, each antenna system of three sectors of a cell site employs three antenna modules, each with its own coverage area, and three controllers of descent or inclination DCs. A single base station BS maintains the communication of RF signals to and from the antenna modules 100, but the resources of the base station BS are divided among the three antenna modules 100.

The support unit 102 may be an antenna tower or any other support unit known in the art serving to support the antenna module 100 above the ground. The base station BS is known in the art for the transmission, reception and monitoring of wireless communications of, for example, mobile telephone calls, paging messages, etc., through the antenna module 100.

Figure 3 shows a block diagram of a controllable receiving antenna module 100a, which can be used as the antenna module 100 in Figure 2, in accordance with the present invention. Antenna module 100a is a controllable voltage phase array antenna. Antenna module 100a is a voltage controlled phase array antenna.

As shown in Figure 3, the receiving antenna module 100a includes a plurality of antenna elements 20? -20n, a plurality of filters 22? -22n, connected to the antenna elements 20? ~ 20n, a plurality of preamplifiers 24? -24n, connected to the filters 22? -22p, a plurality of phase changers 26? -26n, connected to the preamplifiers 24? -24n, a combiner 28 connected to the phase changers 26? -26n and a controller of phase change 29 connected to phase changers 26? -26n.

The antenna elements 20? -20n receive RF signals from external sources, for example, a mobile terminal. The filters 22? ~ 22n filter the RF signals received by the antenna elements 20? -20n and the preamplifiers 24? ~ 24n amplify the filtered RF signals. The phases of the RF signals leave the preamplifiers 24? -24p and are changed with the phase changers 261-26n. The combiner 28 combines the outputs of the phase changers 26? -26n and the output to the combined signal to a receiver, for example, the base station BS. The phase change controller 29 receives a control signal from the descent or DC inclination controller which indicates the desired descent or inclination angle or desired changes in the descent or inclination angle, and outputs the corresponding control signals to control the phases of the phase changers 26? -26n. Specifically, in this phase array antenna module 100a, the descent or tilt angle of the antenna module 100a is changed by varying the phases of the phase shifters 26? -26n, to obtain the desired descent or tilt angle or changes desired in the descent or inclination angle.

Figure 4 shows an exemplary block diagram of a controllable transmit antenna module 100b, which can be used as the antenna module 100 in Figure 2, in accordance with the present invention. The antenna module 100b is a voltage-controlled phase array antenna.

As shown in Figure 4, the transmission antenna module 100b includes a plurality of antenna elements 30? -30n, a plurality of filters 31? -32n, a plurality of power amplifiers 34? -34n connected to the filters 32? ? 32n, a plurality of phase changers 36? -36n connected to the power amplifiers 34? -34n, a splitter 38 connected to the '36 phase? ~ 35n changers, and a phase change controller 39 connected to the phase changers 36? -36n. The signals from a transmitter (eg, a base station BS) are divided into a plurality of transmission signals by the divider 38. The phase of each transmission signal is changed by a corresponding phase changer 36? -36n, and it is amplified by means of a corresponding power amplifier 34? -34n. The filters 32? ~ 32n filter the outputs of the power amplifiers 34? ~ 34n and the signals coming out of the filters 32? -32n are transmitted by the antenna elements 30? -30n. The phase change controller 39 receives a control signal from the descent or DC inclination controller, which indicates the desired descent or inclination angle or desired changes in the descent or inclination angle, and controls the phases of the changers of phase 36? -36n based on the same.

Several modifications are possible for both modules, reception 100a and transmission 100b, respectively. For example, with respect to the transmit antenna module 100b, the plurality of power amplifiers 36? -36n can be replaced by a single power amplifier that is placed before the splitter 38.

The reception and transmission antenna modules 100a and 100b in Figures 3 and 4 can be replaced or used in conjunction with any other type of antenna module to form the antenna module 100 in Figure 2, in accordance with this invention. In addition, the reception and transmission antenna modules 100a and 100b can be integrated into an antenna module as is known in the art, such that the antenna module 100 can be a transmission antenna module, a transmission module. reception antenna, or a reception and transmission antenna module.

OPERATION OF THE WIRELESS COMMUNICATION SYSTEM The operation of the wireless communication system in accordance with the present invention is now described. When an operator in the MSC 200 enters a desired descent or tilt angle or desired change in descent or tilt for an antenna module 100, the MDCU 202 outputs a control signal to the descent or tilt controller DC for the antenna module 100. The control signal provides the descent or DC tilt controller the desired descent or slope angle or desired change in the descent or tilt angle. In response to the received control signal, the DC descent or tilt controller generates and outputs a control signal to the antenna module 100 such that the desired descent or tilt angle or desired change in angle is obtained. by the antenna module 100. In this form, an operator located in the MSC 200 can remotely control the descent or tilt angle of the antenna module 100.

However, the operation of the wireless communication system is not limited to controlling the descent or tilt that originates from the MSC 200, to controlling the descent or tilt of a single antenna module 100, or to the intervention of the operator in the operation of control of offspring or inclination.

Instead of controlling the descent or tilt from the MSC 200, an operator in a base station BS, introduces a desired descent or slope angle or change in the descent or tilt angle for an antenna module 100 associated with the BS station. This information is supplied by the base station BS to the descent or DC tilt controller for the antenna module 100, and in response to this information, the DC tilt or descent controller generates and outputs a control signal to the module. antenna 100 so that the desired descent or tilt angle or desired change in the descent or tilt angle is obtained by the antenna module 100. Accordingly, the control of the offspring or inclination from both sides, from the MSC 200 and from the base station BS, eliminates the need to perform the costly and dangerous process of climbing the tower supporting the antenna module 100, so as to adjust the descent or inclination of the antenna module 100.

Instead of controlling the descent or tilt angle of a single antenna, an operator in the MSC 200 enters the angles of descent or inclination or desired changes in descent or tilt angles for various antenna module 100 as desired. The MDCU 202 then outputs the control signals to the descent or tilt controllers DCs for the antenna modules 100. Each control signal received by a descent or tilt controller indicates the desired descent or tilt angle, or change in the descent angle or inclination for the antenna module 100 associated with it. In accordance, the controllers of descent or inclination DCs perform the descent or inclination control of the antenna modules 100, in the same way as discussed previously. As a result, an operator in the MSC 200 can make substantially simultaneous changes in the angles of descent or inclination of the multiple antenna modules 100. Therefore, make extensive changes in the system in the angles of descent or inclination of the modules. antenna 100, in a wireless communication system in accordance with the present invention, is simple and easy.

The wireless communication system, in accordance with the present invention, can be used to easily and simply position the descent or inclination angles of the antenna modules 100 during installation. However, the system in accordance with the present invention also simplifies making changes in the angles of descent or inclination of the antenna modules 100, as part of the effort to improve the quality of the system to improve the process of facilitating a call, the denial of service, etc. As described in the background section of the invention, the operational measures, such as interference with the common channel, the ratios of the signal with the noise plus the interference within a coverage area, the bit error rates Within a coverage area, as well as measures of signal strength between two coverage areas, indicative of the quality of the system are typically made using a test receiver, and improvements in these operational measures are obtained through the experiment and the error. In accordance with the present invention, the descent or inclination angle of one or more antenna modules 100 can be changed, as well as the operational measurements taken after each change, until the operational measures indicate acceptable quality levels.

In addition, the present invention allows these processes to become automatic; thus eliminating the involvement of an operator. Especially, in one embodiment, the MDCU 202 is programmed to make timed changes in descent or tilt angles to compensate for time dependent changes in the load (e.g., change by stations or switching times). As a result, overloading or unacceptable levels of denial of service can be avoided. In another embodiment, the MDCU 202 receives the operational measures from the MSC 200, and is programmed to determine the angles of descent or inclination for the antenna modules 100; in accordance with any known or future development method for determining angles of descent or inclination, based on operational measures such as call blocking rates. Using the predetermined descent or inclination angles, the MDCU 202 then outputs the control signals to the appropriate descent or inclination controllers DCs. Accordingly, the wireless communication system in accordance with this modality allows the control of descent or adaptive inclination based equally on short term events. Then, an application of this modality of the wireless communication system is described in detail.

APPLICATION OF THE WIRELESS COMMUNICATION SYSTEM TO AVOID OVERLOADING IN AN AUTOMATED WAY As discussed above, each antenna module 100 has a coverage area that depends on its descent or tilt angle. The largest coverage area, most of the mobile terminals that can be located within the coverage area, require limited resources of the base station BS dedicated to the coverage area of the antenna module 100. When the resources of the base station BS are exceeded by the demand for those resources, it is said that the base station BS and / or the coverage area are overloaded. There are numerous criteria for measuring the load in a BS base station or coverage area to judge whether the base station BS and / or the coverage area are overloaded. For the purposes of the discussion, the remaining description will use the call blocking rate as the criterion for measuring the load, but the present invention is not limited to the use of this criterion. The call blocking rate ^ can also be defined in several ways, but again, for the purpose of the discussion, the call blocking rate as used in this description, is the ratio of (1), the number of terminals mobiles in a coverage area that has its call responses denied by the base station, this due to insufficient resources in a base station that is dedicated to the antenna module for such a coverage area, with (2) the number of mobile terminals who request calls in the coverage area.

When the call blocking rate exceeds a first predetermined threshold value, the base station BS is considered as overloaded. The call blocking rate as a measure of the load has been chosen because it exists conventionally, and the MSCs measure the call blocking rates for each coverage area of the base stations associated with them, and the supply of the MSC with the call blocking rates measured. Accordingly, it does not describe how call blocking rates are determined.

The present invention provides an easy and simple way to handle both; short-term events and long-term events, such as the increase in load during peak switching times. For example, when a base station is in the overloaded state, the MDCU 202 determines which coverage areas adjacent to the coverage area of the overloaded base station are available to handle the overload. For example, if the call blocking rate of a base station serving an adjacent coverage area is less than a second predetermined value, which is less than the first predetermined value, the base station and the adjacent coverage area are available.

The system according to the present invention then allows the MDCU 202 to easily reduce the coverage area served by the overloaded base station, increasing the descent angle or inclination of the antenna module to the coverage area and / or increasing one or more areas of coverage served by the available stations, decreasing the descent or inclination of the angles of the antenna modules for the available coverage areas. This changes the boundary between the coverage area of the overloaded base station and the coverage areas adjacent to it to transfer the load from the overloaded base station.

While the adaptive and automated application of the present invention has been described with respect to the elimination of overload, and thereby to the reduction of denial of service, the present invention also applies to the improvement of other aspects of a wireless communication system such as the quality of the facilitation.

The wireless communication system in accordance with the present invention is applicable to any system such as a time division multiple access system, a code division multiple access system, an analog system, etc. It is noted that in relation to this date, the best method known to the applicant, to implement said invention is that which is clear from the manufacture of the objects to which it refers.

Claims (26)

1. A wireless communication system, characterized in that it comprises: a first antenna having an electrically controllable descent or tilt angle; a first descent or tilt controller that outputs a control signal to a first antenna to control said descent or tilt angle of said first antenna.

2. The system of claim 1, characterized in that said control signals indicate a desired descent or inclination angle for said first antenna, and a desired change in said descent or inclination angle of said first antenna.

3. The system of claim 1, characterized in that it further comprises: a base station that transmits and receives signals via said first antenna, and outputs a control signal to said first descent or inclination controller; and wherein said descent or tilt controller controls said descent or inclination angle of said first antenna in response to said control signal from said base station.

4. The system of claim 1, characterized in that said first antenna and said first descent or inclination controller form part of a first cellular site.

5. The system of claim 1, characterized in that it further comprises: a second antenna having an electrically controllable descent or tilt angle; a second descent or tilt controller that gives it. output to a control signal towards said second antenna, to control the descent angle or inclination of said second antenna.

6. The system of claim 5, characterized in that it further comprises: a main controller that outputs a main control signal to at least one of the first and second controllers to adjust a relationship between said first and second antenna.

7. The system of claim 6, characterized in that said main controller, generates and outputs said main control signal based on at least one operational measurement of the wireless communication system.

8. The system of claim 7, characterized in that said operational measurement is at least one of load, signal strength of the signals received from said first and second antennas, interference between the signals received from said first and second antennas, a signal ratio with noise of the signals received from the first antenna, and a bit error rate of the signals received from said first antenna.

9. The system of claim 8, characterized in that the wireless communication system is a time division multiple access system, a code division multiple access system and an analog system.

10. The system of claim 6, characterized in that said main controller generates and outputs said main control signal, based on the input of the user.

11. The system of claim 6, characterized in that said relationship is the common channel interference, between the signals received from said first and second antennas.

12. The system of claim 6, characterized in that said relationship is a boundary between the coverage areas of said first and second antennas.

13. The system of claim 6, characterized in that said ratio is an amount by which such signals received from said first antenna overlap with the signals received from said second antenna.

14. The system of claim 6, characterized in that said relationship is relative to the charges of said first and second antennas.

15. The system of claim 1, characterized in that the wireless communication system is a time division multiple access system, a code division multiple access system and an analog system.

16. A wireless communication system, characterized in that it comprises: a plurality of cellular sites, each cellular site includes an antenna having an electrically controllable descent angle or inclination, which controls an angle of descent or inclination of said associated antenna; a master controller that outputs a control signal to at least one descendant or incline controller, to adjust said descent or inclination angle of said associated antenna;

17. The system of claim 16, characterized in that said main controller outputs control signals to adjust said angles of descent or inclination of more than one antenna at the same time.

18. The system of claim 16, characterized in that said main controller generates and outputs said main control signals, based on an operational measurement of said wireless communication system.

19. The system of claim 18, characterized in that said operational measurement is at least one of load, signal strength of the signals received from said two antennas, interference between the signals received from said first and second antennas, a signal-to-noise ratio received from one of said antennas, and a bit error rate of the signals received from one of said antennas.

20. A wireless communication system, characterized in that it comprises: a first cellular site that includes a primary antenna having an electrically controllable descent or tilt angle, and a primary tilt or descent controller for controlling said descent or tilt angle of said primary antenna, said primary antenna has a primary coverage area based on said descent angle or inclination thereof; at least one secondary cell site that includes a secondary antenna having an electrically controllable descent or tilt angle, and a secondary tilt or descent controller that controls said descent or tilt angle of said secondary antenna, said secondary antenna has an area of secondary coverage based on said descent angle or inclination thereof, said secondary coverage area is adjacent to said primary coverage area; and a main controller to determine if said primary coverage area is overloaded, and outputs the control signals to at least one of said primary and secondary controllers, to adjust a boundary between said primary and secondary coverage areas, respectively changing said descent or inclination angle of at least one of said primary and secondary antennas, when said primary coverage area is overloaded.

21. The system of claim 20, characterized in that said main controller determines whether said primary coverage area is overloaded by monitoring a primary load in said primary coverage area, and determining when said primary load exceeds a first predetermined threshold.

22. The system of claim 20, characterized in that said main controller monitors the secondary load in said secondary coverage area, and outputs said control signals based on said secondary load in said secondary coverage area, when said primary coverage area It is overloaded.

23. The system of claim 22, characterized in that said main co-controller determines whether said secondary coverage area is available to handle the load based on said secondary load, and outputs said control signals to adjust the boundary between said secondary coverage area. , when said primary coverage area is overloaded and said second coverage area is available.

24. The system of claim 23, characterized in that said main controller determines whether said second coverage area is available when said secondary load is less than a second predetermined threshold.

25. The system of claim 20, characterized in that said main controller outputs control signals to said primary descent or inclination controller, to increase said descent or inclination angle of said primary antenna when said primary coverage area is overloaded.

26. The system of claim 25, characterized in that said main controller outputs control signals to said secondary descent or inclination controller, to decrease said descent or inclination angle of said second antenna when said primary coverage area is overloaded.