Abstract:
A base station tunes a transceiver to listen to accesses made by mobile terminals on the base stations own control channel. Once the functionality of the transceiver is verified by correlating the known accesses to the received accesses, the base station then tunes the transceiver to control channels of nearby cells to receive accesses directed to those nearby cells. From these accesses, the MSC can determine carrier-to-interference ratios between the cells.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a base station equipped with an interference measuring device, and more particularly, to a method of testing the measuring device to determine if the interference measuring device is working properly.  
           [0002]    In cellular communication systems, frequency reuse plans allow the same frequency to be used more than once within the system. Rather than use a single high power transmitter to cover a large geographic area, cellular systems employ a large number of low-power transmitters that broadcast signals in relatively small geographic areas referred to as cells. Each cell may be only a few miles across, and theoretically could be as small as a few city blocks. By reducing the coverage area of the transmitter and creating a large number of cells, it is possible to reuse the same frequency in different cells. Thus, a single frequency may be used multiple times throughout the entire cellular system to increase system capacity.  
           [0003]    To avoid co-channel interference, cells operating on the same frequency are spatially separated so that a mobile terminal operating within a cell receives desired signals at a higher level than any potential interfering signals from co-channel cells. Cells operating at different frequencies are placed between any two co-channel cells. In general, the power of any interfering signal diminishes with increasing distance between interfering users. A carrier frequency can be reused if the interference level is reduced sufficiently by separation between the co-channel cells. The interference level is measured by the carrier power to interference power ratio, C/I.  
           [0004]    Interference is one of the major limiting factors in the performance of a mobile communication network. Ideally, interference between cells should be minimized. However, there are presently few techniques by which interference between cells can be measured. One technique currently being used involves sending a technician to the field to make phone calls and evaluate signal quality. Another technique, proposed by the assignee of the present invention, involves the base stations listening to access requests made by mobile terminals on an access channel in neighboring cells. A base station using this method tunes one of its receivers to an access channel of a neighboring cell and performs measurements on the access requests made by mobile terminals in the neighboring cell. This functionality is sometimes referred to as a Listening Control Channel (LICC). With enough measurements from various base stations within a measurement area, a mobile switching center can estimate a carrier-to-interference ratio.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a method through which the functionality of a listening transceiver used to estimate the C/I ratio can be verified before performing signal strength measurements. The listening transceiver tunes to the control channel for the home base station and listens for an access request on the control channel. A control channel transceiver at the home base station also receives access requests on the control channel. If the control channel transceiver receives an access request that is not received by the listening transceiver, the base station assumes that the listening transceiver is dysfunctional and generates an alarm. If the listening transceiver receives an access request, the signal strength of the access request measured by the listening transceiver is compared to the signal strength measured by the control channel transceiver to verify that the listening transceiver is functioning properly. If the signal strength measurements are within predetermined limits, the listening transceiver is determined to be operating properly. If, however, there is a discrepancy, the base station generates an alarm and transmits the alarm to a MSC or other entity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic diagram illustrating an exemplary cellular network, such as may use the present invention;  
         [0007]    [0007]FIG. 2 is a functional block diagram illustrating an exemplary base station with a listening transceiver;  
         [0008]    [0008]FIG. 3 is a flow diagram illustrating a procedure used by the base station for performing signal strength measurements in remote cells; and  
         [0009]    [0009]FIG. 4 is a flow diagram illustrating a testing procedure used by the base station to test the functionality of its listening transceiver. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    The backbone of present day mobile communications is the cellular network. An exemplary cellular network  10  is illustrated in FIG. 1. The communications network  10  illustrated in FIG. 1 is based on the standards published by the Telecommunications Industry Association (TIA) and the Electronics Industry Association (EIA) known as TIA/EIA-136. FIG. 1 shows the logical architecture of a communication network and is not meant to imply any particular physical implementation. The following description is intended to describe how to implement the present invention in an exemplary mobile communication system. Those skilled in the art will recognize that the present invention can be adapted for use in mobile communication systems employing different standards, including the standards known as Global System for Mobile Communication (GSM), Digital Advance Mobile Phone Service (D-AMPS), IS-95, and IS-2000.  
         [0011]    The cellular network shown in FIG. 1 comprises a plurality of base stations  14  which are connected by one or more mobile services switching centers (MSC)  12  to a terrestrial communications network, such as the Public Switched Telephone Network (PSTN). Each base station  14  is located in and provides service to a geographic region referred to as a cell  15 . In general, there is one base station  14  for each cell  15  within a given system. Within each cell  15 , there may be a plurality of mobile terminals that communicate via radio link with the base station  14 . The base station  14  allows the user of the mobile terminal to communicate with other mobile terminals, or with users connected to the PSTN. The MSC  12  routes calls to and from the mobile terminals through the appropriate base station  14 . Cellular network  10  may employ any of a number of well established standards such as TIA/EIA-136.  
         [0012]    An exemplary base station  14  is illustrated in FIG. 2. The base station  14  comprises a Control and Radio Interface (CRI)  18 , and a transceiver array  20 . CRI  18  connects to the MSC  12  via a T 1  or E 1  line. Transceiver array  20  comprises a plurality of transceivers  24 ,  26 ,  28 , and  30  that communicate with mobile terminals. Signals transmitted by mobile terminals are received by antennas  32 , passed through a low noise amplifier (LNA)  34 , and processed by the appropriate transceiver  24 ,  26 ,  28 , or  30 . Signals transmitted from the base station  14  are sent from the appropriate transceiver  24 ,  26 ,  28 , or  30  to a transmit filter  36  and broadcast from antenna  38 .  
         [0013]    Transceiver  24  transmits and receives on the control channel frequency assigned to the base station  14  and is referred to herein as the control channel transceiver. Transceivers  26 , referred to herein as traffic channel transceivers, transmit and receive on frequencies that bear user traffic. Transceiver  28  is a verification transceiver, and handles hand offs between base stations. Transceiver  28  may serve at times as a traffic channel transceiver  26 . Transceiver  30 , referred to herein as the listening transceiver, implements the LICC functionality described in U.S. Pat. No. 6,112,082, which is hereby incorporated by reference in its entirety. Listening transceiver  30  may also handle user traffic when it is not implementing LICC functionality.  
         [0014]    [0014]FIG. 3 is a flow diagram illustrating a signal measurement procedure used by a base station  14  to perform signal strength measurements of access requests in remote cells. The signal strength measurements may be used, for example, to compute C/I ratios as is well-known in the art. The procedure begins at block  50  when the MSC  12  instructs the base station  14  to start the LICC functionality. For the sake of clarity, an example using the cells of FIG. 1 will be used. Suppose MSC  12  wishes to determine how much cell  155 H interferes with the other cells  15  within the measurement area  16 . MSC  12  determines the DCCH of cell  155 H. This corresponds to determining the channel number (CHNR) of the control channel (e.g., DCCH) for the cell to be measured, which in this example is cell  15 H. Having determined that the DCCH of cell  15 H is CHNR X, MSC  12  instructs the base stations  14  in neighboring cells to measure the signal strength of access requests made on the DCCH in cell  15 H. The base stations  14  tune their respective listening transceivers  30  to CHNR X (block  52 ) and listen for access requests on the DCCH for a predetermined period of time. The length of the test period is not important to the present invention, but is typically in the order of ten minutes. However, the test period could be for a longer or shorter period of time. When an access request is received, the base station  14  measures the signal strength of the access request and may also determine a bit error rate (BER). The measurements may then be stored temporarily in the base station  14  or MSC  12  until the test period ends. A signal strength measurement and BER determination is made each time an access request is received until the test period expires. At the end of the test period, the base station  14  determines whether at least one access request was received (block  56 ). The test period may be a fixed period of time. Alternatively, the test period may terminate after the first burst that is successfully demodulated. If so, the base station  14  then forwards the signal strength measurements and BER measurements to the MSC  12  (block  58 ). If the base station  14  does not receive an access request in the predetermined time period, it notifies the MSC  12  that no access request was received (block  62 ) and the procedure ends (block  64 ). In the past, this procedure could result in false alarms as, e.g., cell  15 B may not receive any measurements from mobile terminals in cell  15 H due to path loss, terrain, or other factors. While this actually represents the fact that the C/I ratio for the cells  15 B and  15 H is low, a base station  14  could interpret this event as a failure of the listening transceiver  30  and generates an alarm.  
         [0015]    The present invention avoids false alarms by performing a self-test at each base station  14  to check the functionality of the listening transceiver  30  before beginning signal strength measurements. If the listening transceiver is determined to be functional, the base station  14  using the present invention may still send a notification to the MSC  12  indicating that no access requests were received, but would not generate an alarm.  
         [0016]    [0016]FIG. 4 illustrates a test procedure according to the present invention to verify that the listening transceiver  30  is functional prior to beginning the signal strength measurements to determine the C/I ratio. The test procedure starts at block  100  with MSC  12  instructing the base stations  14  to perform LICC testing. The base station  14  tunes its listening receiver  30  to the control channel (e.g., DCCH) of the home base station  14  (block  102 ) and waits a predetermined period of time for an access request (blocks  104 ,  106 , and  118 ). When an access request is received by the control channel (DCCH) transceiver  24  (block  104 ), the base station  14  or MSC  12  determines whether the access request was also received by the listening (LICC) transceiver  30  (block  106 ). If not, the base station  14  generates an alarm (block  122 ) to notify the MSC  12  or other appropriate entity that the listening transceiver  30  is not working properly. If the listening receiver  30  receives the access request, the base station  14  compares the signal strength of the access request measured by the listening receiver to the signal strength measured by the DCCH receiver (blocks  108 ,  110 ,  112 ). If the signal strength measurement is within predetermined limits, the listening receiver  30  is determined to be operating properly (block  114 ). The base station  14  may send a notification to the MSC  12  to notify the MSC  12  that the listening transceiver  30  is functioning. The base station  14  then begins measurement reporting for remote cells (block  116 ). Measurement reporting may be carried out substantially as shown in FIG. 3. Returning to block  112 , if the signal strength of the access request measured by the listening transceiver  30  is not within predetermined limits of the signal strength measured by the DCCH transceiver, an alarm is generated  122  and sent to the MSC  12  or other appropriate entity.  
         [0017]    The listening transceiver  30  remains tuned to the DCCH of the home base station until an access request is received on the DCCH, or until a predetermined test period has expired (block  118 ). The test period will typically be less than one minute, and may be less than  10  seconds.  
         [0018]    However, longer test periods may also be used. If an access request is not received on the DCCH within the test period, the base station  14  notifies the MSC  12  that it is not able to determine the status of the listening transceiver  30  (block  120 ). The base station  12  may then proceed with signal strength measurements (block  116 ) on channels in the remote cells, or may take other remedial actions as needed or desired, such as wait longer.  
         [0019]    The provision of the diagnostic test allows the MSC  12  to verify that the listening transceiver  30  is functioning properly. It should be appreciated that while the present invention has described the invention as being controlled by MSC  12 , other elements within the cellular network  10  may also have this control function. Likewise, this could be implemented in a pico-network or other Wireless Office solution that does not have an MSC  12  per se. Such adaptations are considered within the skill of one of ordinary skill in the art and are not detailed explicitly.  
         [0020]    The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.