WO2002032164A2

WO2002032164A2 - A system and method for applying reciprocity for downlink interference identification

Info

Publication number: WO2002032164A2
Application number: PCT/US2001/031446
Authority: WO
Inventors: Houston Mccauley; Ali R. Shah
Original assignee: Ericsson, Inc.
Priority date: 2000-10-11
Filing date: 2001-10-09
Publication date: 2002-04-18
Also published as: JP2004512715A; CN1480006A; GB2386301B; GB2386301A; AU2002211522A1; GB0308835D0; WO2002032164A3; DE10196772T5

Abstract

A system and method of identifying sources of downlink interference among cells in a telecommunications network (10). Initially, call events (80) occuring in offending cells (72) are recorded. In conjunction, the disturbance events (78) occuring in a cell of the telecommunications network (10) are recorded. The recorded call events (80) with recorded disturbance events (82) are then correlated. At least one cell experiencing disturbance (78) on the downlink (134) is then identified. Next, the uplink disturbance source offending a cell is determined utilizing Call Event Recording (CER) (80) and Radio Disturbance Recording (RDR) (82) correlations. Finally, reciprocity is applied in order to identify downlink sources of interference within a cell causing disturbances (78) in other cells.

Description

A SYSTEM AND METHOD FOR APPLYING RECIPROCITY FOR DOWNLINK INTERFERENCE IDENTIFICATION

CROSS-REFERENCE TO RELATED APPLICATION The application is commonly assigned and related to U.S. Patent

Application Serial No. 09/426,139 entitled "System and Method For Identification of Uplink/Downlink Interference Sources," by Ali R. Shah and Hossam H'mirny, filed October 22, 1999 (the "Related Application"), the entirety of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates in general to wireless telecommunications networks and applications and, in particular, to a method and system of identifying sources of downlink interference among cells in a wireless telecommunications network. More particularly, the present invention applies to the utilization of call event and disturbance event correlations and the rule of reciprocity to locate sources of downlink interference in the network.

BACKGROUND OF THE INVENTION As previously discussed in U.S. Patent Application Serial No. 09/426,139, calls by mobile subscribers can be affected by interference which can cause radio disturbance events. This, in turn, limits the efficiency of the network. As such, it is important to identify those cells within the network which are sources of and subject to radio disturbance events. Interference itself can be either external or internal to the radio network and may effect either the downlink (i.e. from the base station to the mobile) or the uplink channel. A particular cell may be offending another cell in the service area by creating interference that limits radio activities in the disturbed cell. A radio disturbance event typically occurs during a cellular call, either on the downlink (from a base station to a mobile station) or on the uplink (from a mobile station to a base station).

Various methods and systems currently exist for determining when a cell has been disturbed. Typically, a comparison of Signal Strength (SS) versus a measurement of speech quality can be employed to determine the Bit Error Rate (BER) of the transmission channel. For example, if the SS is high and the BER is low, this results in "good" or acceptable speech quality. Ideally, the SS would be low and the BER low, so that good speech quality results from a low amplitude signal with few errors. If, on the other hand, the SS is low and the BER is low, it is assumed that the coverage is poor and that either service must be handed off to another station in the network or discontinued.

If the SS is high and the BER is high, coverage is good, and the resulting high error rate can be blamed on high levels of interference. Thus, when sufficient signal strength is correlated with degraded speech quality for an extended period of time (usually measured in seconds), that cell can be considered "disturbed." Failure to identify and analyze sources of such disturbances could result in poor channel quality and the sealing of devices, which means they are unavailable for use in handling calls, as described in the Related Application. An obvious reason for interference can be the presence of one or more

"killer" cells in the service area. Non-mature networks include killer cell sites which are caused by base stations extending to a greater height than surrounding base stations. Due to their relatively great height, these base stations cause a type of interference easily identifiable by the drive test. Mature networks, on the other hand, comprise base stations which are all similar in height. As a result, sources of interference are not easily identified by drive test observation.

One method for identifying interference in a telecommunications network involves the use of downlink interference prediction tools, or prediction methods which use model-based prediction algorithms. Such tools predict where interference may exist within a given network coverage area. The prediction results are then utilized for frequency and cell planning, particularly in initial network designs. The validity of such predictions is dependent on a number of factors, including the accuracy of the propagation model utilized and the resolution of the terrain data. While such tools may be used in identifying cells that are causing downlink interference, they are often inaccurate because of the dependence on predictions. That is, such prediction tools do not always account for "real-life" sources of interferences in the coverage area as determined through more empirical measurement methods. Another method utilized to identify disturbed and offending cells involves drive testing by field personnel. The drive test can be performed by turning off all adjacent/co-channel transmitters for a particular cell and then keying up each transmitter individually. A drive test team then drives the area in a motorized vehicle to observe and measure interference within the drive area. A disadvantage of the drive test method is that it is inherently labor intensive and costly since it requires continuous measurement by field personnel. In addition, the drive test approach, while sometimes useful, does not take into account variations in mobile station types.

The Related Application discloses a method of identifying and analyzing sources of interference that utilizes available qualitative call record/disturbance data about the network. The techniques of the Related Application are most useful in identifying sources of uplink interference since they rely on finding correlations between call activity in one or more offending cell(s) and recorded disturbances in a disturbed cell under consideration as measured in the uplink channel. While certain assumptions about the downlink can be derived from the correlation techniques of the Related Application, they do not accurately qualify conditions on the downlink.

In short, the prior art methods of analyzing and identifying downlink interference in the network coverage area are generally unsuitable for today's modem wireless network. Accordingly, a need exists for a method of analyzing interference on the downlink channel.

SUMMARY OF THE INVENTION The present invention provides a method and system for identifying sources of downlink interference among cells in a wireless telecommunications network. With the present invention, the network operator can identify sources of interference and use this information in designing the network or improving performance. Disclosed in one embodiment is a method of identifying sources of downlink interference among cells in a telecommunications network. The method comprises the step of determining the uplink disturbance source offending a cell utilizing Call Event Recording (CER) and Radio Disturbance Recording (RDR) correlations between a disturbed cell and a plurality of offending cells. Initially, at least one cell experiencing disturbance on the downlink is identified. Call events occurring in the offending cells, as well as disturbance events occurring in the disturbed cell of the telecommunications network are recorded. The recorded call events are then correlated with the recorded disturbance events. The method further comprises the step of computing the signal strength statistics of the disturbance on the uplink of a cell. This is followed by the step of calculating the statistics of the mobile transmit power for the offender using Radio Environment Statistic (RES) measurements. Using RES measurements of the disturbance and the mobile transmit power, the path loss on the uplink is then computed since the path loss on the uplink is approximately equal to the path loss on the downlink.

The method also comprises the step of applying reciprocity to identify downlink sources of interference within the cell causing disturbances in other cells. The rule of reciprocity dictates that if the mobiles of the offending cells create interference on the uplink for the disturbed cell, then the disturbed cell also creates interference on the downlink. Therefore, the cells that are disturbed on the uplink are potential candidates to consider as those who disturb their offending cells, on the downlink. Hereinafter, "offender" and "offending cells" may be used interchangeably. Also, "interfered cell" and "disturbed cell" may be substituted for one another.

The method further comprises the step of computing the Received Signal Strength (RSS) on the downlink of the offender using the base station transmit power. The RSS is then compared with a predetermined threshold in order to determine if significant downlink interference is indicated. The threshold for the comparison can be chosen by the network engineer.

Technical advantages of the present invention include identification of downlink sources of interference in the shortest amount of time and by use of disturbance data already contained in the network.

Other technical advantages include more accurate identification and analysis of downlink interference sources that assist the network operation in designing the network to improve performance and increase capacity. The method and system of the present invention utilizes empirical measurements based on recorded disturbance events, rather than predictions. As such, the interference analysis takes into account the behavior and activity of all mobiles within the network rather than a particular mobile event.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, including its features and advantages, reference is made to the following detailed description of the invention, taken in conjunction with the accompanying drawings of which:

FIGURE 1 depicts a telecommunications network in which a preferred embodiment of the present invention may be implemented;

FIGURE 2a illustrates a non-mature network having at least one "killer" cell site; FIGURE 2b illustrates a mature network in which a preferred embodiment of the present invention may be implemented;

FIGURE 3 illustrates downlink adjacent channel interference in a telecommunications network; FIGURE 4 illustrates Call Event Recording (CER) and Radio Disturbance

Recordings (RDR) correlations, in accordance with the Related Application;

FIGURE 5 is a high-level logic flow diagram illustrating process steps for implementing the method and system of the Related Application;

FIGURE 6 is a diagram illustrating the time correlation of call events with radio disturbance events;

FIGURE 7 is a table showing Signal Strength (SS) and Bit Error Rate (BER) varied at high and low, respectively;

FIGURE 8 illustrates the step of verifying definite sources of disturbance;

FIGURE 9 illustrates the concept of reciprocity as applied to the present invention; and

FIGURE 10 is a high-level flow diagram illustrating steps for implementing the method and system of the present invention, in accordance with a preferred embodiment.

Corresponding numerals and symbols in the figures refer to corresponding parts in the detailed description unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention.

To better understand the invention, reference is made to Figure 1, wherein a diagram of a telecommunications network, denoted generally as 10, in which a preferred embodiment of the present invention may be implemented is shown. Telecommunications network 10 includes a Switching System (SS) 22 and a Base Station System (BSS) 40. Each of these systems contain a number of functional units which process information and carry out operations of a functional telecommunications network 10. The functional units themselves may be implemented utilizing various telecommunications hardware devices.

The SS 22 includes a Visitor Location Register (VLR) 30, a Home Location Register (HLR) 28, an Authentication Center (AUC) 24, an Equipment Identity Register (EIR) 26, and a Mobile Switching Center (MSC) 27. The BSS 40 comprises a Base Station Controller (BSC) 46 and a Base Transceiver Station (BTS) 44. An Operations and Maintenance Center (OMC) 48 is connected to equipment present within SS 22 and to BSC 46. The dashed lines in Figure 1 represent information transmission, while solid lines represent both call connections and information transmission. Telecommunications network 10 illustrated in Figure 1 may be realized as a network of neighboring radio cells, which together provide complete coverage for a service area. The service area is the geographic area served by a given telecommunications supplier and the area in which the supplier stands ready to provide its service. Each cell contains a BTS 44 operating on a set of radio channels. These channels differ from the channels utilized by neighboring cells in order to avoid interference.

Each BSC 46 controls a group of BTSs 44. The BSC 46 controls well- known telecommunication functions, such as "Handover" and power control. A number of BSCs (e.g., BSC 46) are served by a MSC 27, which controls calls to and from a Public Switched Telephone Network (PSTN) 18 as well as other fixed networks. MSC 27 also controls calls to and from an Integrated Services Digital Network (ISDN) 12, a Public Land Mobile Network (PLMN) 20, a Circuit Switched Public Data Network (CSPDN) 16, and also, various private networks, such as a Packet Switched Public Data Network (PSPDN) 14. Each unit is actively involved in carrying speech connections between the Mobile Station (MS) 42 and, for example, a subscriber in a fixed network, such as PSTN 18. Because of the extreme difficulties involved in completing MS 42 terminated telephone call, a number of databases located within the telecommunications network 10 keep track of the MS 42. The most important of these databases is the HLR 28. When a user subscribes to a wireless telecommunications network, such as the telecommunications network 10 depicted in Figure 1 , the user is registered within the HLR 28. The HLR 28 contains subscriber information, such as supplementary services and authentication parameters.

Data describing the location of the MS 42, such as the area (i.e., the MSC area) in which the MS 42 presently resides, is contained within the HLR 28. The MSC area represents that portion of the telecommunications network 10 covered by a single MSC 27. In order to route a call to a mobile subscriber within a telecommunications network, such as the telecommunications network 10 depicted in Figure 1 , the path through the network links to the MSC 27 in the MSC area where the subscriber is currently located. Data describing the location of the MS 42 is thus actively altered as the MS 42 moves from cell to cell within the telecommunications network 10. MS 42 sends location information, via MSC 27 and VLR 30, to an associated HLR 28, which permits MS 42 to receive calls. The AUC 24 is connected to HLR 28, and provides HLR 28 with authentication parameters and ciphering keys utilized for security purposes.

Furthermore, VLR 30 is a database that contains information regarding all mobile stations currently located in the MSC area. When MS 42 roams in a new MSC area, the VLR 30 connected to the MSC 27 in that particular area requests data about the MS 42 from HLR 28. Simultaneously, HLR 28 is provided with the location of the MSC area in which MS 42 resides. If it is later desired to make a call from MS 42, VLR 30 will have at its disposal all the information necessary for call set-up, without being forced to interrogate HLR 28 each time a call is made. The VLR 30 thus functions as a distributed HLR 28. As such, VLR 30 also contains precise information about the location of the MS 42 in the MSC area.

If an individual subscriber within the PSTN 18 desires to make a call to a subscriber, an exchange within PSTN 18 connects the call to MSC 27 equipped with a function commonly known as a "gateway" function. In the telecommunications industry, MSC 27 having a "gateway" function is commonly referred to as a Gateway MSC (GMSC). Thus, the MSC 27 in telecommunications network 10 of Figure 1 may be implemented as a GMSC. That is, most MSC's within GSM telecommunications networks function as a GMSC. The GMSC must find the location of the searched MS 42, which can be accomplished by interrogating the HLR 28 where the MS 42 is registered. The HLR 28 then replies with the address of the current MSC area. Thereafter, the GMSC can re-route the call to the correct MSC 27. When the call reaches that MSC 27, the VLR 30 will have additional information regarding the precise location of the MS 42. The call can then be switched through to completion.

The telecommunications network 10 depicted in Figure 1 may be implemented as a GSM-type network. Those skilled in the art can appreciate that although the present invention is described and illustrated in the context of a GSM network standard, the present invention may also be implemented in accordance with other standards and networks, including AMPS/TDMA utilized in North and South America. The GSM network standard, as discussed herein, is merely presented for illustrative purposes only and is not a limiting feature of the present invention. With reference to Figures 2a and 2b, a non-mature network 33 and a mature network 35 are illustrated, respectively. A non-mature network 33 typically comprises at least one "killer" cell site which may be caused by a base station 44a which is greater in height when compared to base stations in neighboring cells. For example, the base station 44a may extend to a height of 90m while base station 44 extends to a height of 40m. As previously discussed, one method utilized to identify disturbed and offending cells involves drive testing by field personnel. The drive test can be performed by turning off all adjacent/co-channel transmitters for a particular disturbed cell and then keying up each transmitter individually. Meanwhile, a drive test team drives the area (e.g., non-mature networks 33 and mature networks 35) in a motorized vehicle to observe and measure interference within the drive area. In a non-mature network 33, the source of interference is obvious. That is, the base station 44a at a height of 90m can be easily observed during a drive test and identified as a source of interference. In this case, base station 44a does not allow frequency reuse due to its height being greater that the surrounding base stations, such as base station 44, which creates interference within the network coverage area.

As a result, the capacity of the network is decreased and performance is poor.

In response to decreased capacity, such a non-mature network can be converted to mature network, such as mature network 35, by creating a network with a plurality of base stations, such as base station 44, that are all the same height (e.g., 40m). The mature network 35 allows for better frequency reuse, and thus, increased capacity. However, sources of interference in a mature network 35 are not easily identified using drive test observation.

Figure 3 is a block diagram, denoted generally as 50, illustrating downlink adjacent channel interference in a telecommunications network 10. According to the method and system described in the Related Application (U.S. Patent Application Serial No. 09/426,139), sources of downlink interference (i.e., offending cells) are identified along with disturbed cells within network 10, such as that depicted and described in Figure 1. The call events occurring in offending cells of the telecommunications network 10 and the disturbance events occurring in the disturbed cell of the telecommunications network 10 are first recorded in conjunction with each other. The analysis is then based on correlating call events and thereafter correlating these events with disturbance events. The time correlation generates a list of "offending cells" and "disturbed cells". This correlation then permits the perceived interference in an adjacent/ co-channel cell to be associated with a possible source of disturbance. Statistical methods, as opposed to prediction tools, are employed thereafter to determine definite sources of disturbance. This allows for a more accurate method and system for identifying and analyzing interference in a telecommunications network, such as telecommunications network 10.

Figure 3 thus illustrates the existence of downlink adjacent channel interference in a network 50 consisting of 2 cells (C1 , C2), 4 base stations (52, 54, 56 and 58) and mobile stations 60 and 62. Those skilled in the art can appreciate that each of the mobile stations 60, 62 of Figure 3 are analogous to mobile station 42 of Figure 1. In the example depicted in Figure 3, base stations 52, 58, and 54 are transmitting at frequencies G1, G2, and G3, respectively. Thus, carrier on the channel utilized by mobile station 60 is used by the channel utilized by base station 56 to the mobile station 62. Thus, base station 56 transmits on G1 and is therefore co-channel to G1 of base station 52. In this arrangement, base station 56 creates a certain amount of interference (I) disturbing the downlink channel between the base station 52 and mobile station 60 in cell C1. The present invention provides a method of analyzing the downlink interference to permit the network operation to identify its source and minimize its effect. The radio base station contributing to the interference is termed the

"offender," and the cell in which it resides is referred to as the "offending cell."

Mobile station 60, which is affected by adjacent channel interference, is referred to in the parlance utilized herein as "disturbed" and belongs to the "disturbed cell." Thus, Figure 3 illustrates an example of downlink interference where the base station 56 disturbs a mobile station 60. The Related Application applies mostly to the analysis of uplink interference when mobiles of an offending cell disturb radio base station receivers on the uplink.

Sources of disturbance are not, however, confined to co-channel radio base stations. Another source of disturbance can be found in adjacent channel base stations, which are still considered as internal disturbance sources. The invention described herein functions in a manner that identifies all possible sources of interference within a telecommunications network such as telecommunications network 10. Those skilled in the art will appreciate that the terms "disturbance" and "interference" can be utilized interchangeably and such terms are utilized interchangeably herein.

A "radio disturbance" or "disturbance event" on a downlink channel is also detected for calls when sufficient Signal Strength (SS) is correlated in time with degraded speech quality for a period greater than _ seconds. The length of time is dependent on the hardware and measurement method utilized. Speech quality is measurable as C/l (i.e., Carrier to Interference ratio) or BER (i.e., Bit Error Rate).

In accordance with the Related Application, sources of interference can be analyzed in the following manner. Initially, possible sources of disturbance can be detected by correlating call event recordings with disturbance event recordings. Definite sources of disturbance can then be verified utilizing propagation considerations. Thereafter, a test can be performed to verify if all disturbance events have been correlated with call events. If all disturbance events have been successfully correlated with call events, then a disturbance distribution is computed for each disturbed cell. Improvements are then recommended by balancing the amount of coverage area with acceptable levels of interference. Long term improvements to the system can also be recommended based on the disturbance distribution. If, however, all disturbance events are not successfully correlated with call events, then external sources of interference are identified. An alternative approach involves identifying the disturbed cells and then concentrating on those identified disturbed cells, rather than the entire telecommunications network 10.

Figure 4 illustrates a technique for detecting possible sources of interference in accordance with the Related Application. The Call Event (Traffic) Recording (CER) 80, also referred to as a "cell event recording" present in the offending cells 72 and the Radio Disturbance Recordings (RDR) 82 present in the disturbed cell 76 of the telecommunications network 10 run in conjunction with each other. RDR 82 is a feature designed to monitor radio disturbance events 78 that affect speech quality in the telecommunications network. The degree of radio channel interference in the network is measured by the RDR 82. Network interference from both adjacent channel sites and external radio interference, which in most cases come from sources outside the telecommunications network, can be measured. The data recorded will be useful to the network operator in locating and correcting the cause of the disturbance events 78. While the RDR 82 measures the disturbance 78 experienced by device

84, the CER 80 identifies the call start and stop times for possible offending cells 72. The time stamps of the call start attempts (voice channel seizure events) in CER 80 for possible adjacent channel (or co-channel) mobile stations 42. The recorded call events 80 are then correlated with the recorded disturbance events 82. A distribution of disturbed cell 76 and offending cells 72 within the telecommunications network 10 is then computed as a function of time to obtain a statistical correlation of call events 80 in offending cells 72 and subsequent disturbance events 78 resulting in the disturbed cell 76. This is done to identify the possible source of disturbance events 78 within the disturbed cell 76. One of the many data components to collect from the method of analyzing interference is the start time of sealing. Sealing of a device 84 occurs due to an adjacent channel mobile station 42 in an offending cell 72 which transmits to the BTS 44 in the disturbed cell 74. As such, the mobile station signal in the offending cell 72 is interfering with the BTS 44 in the disturbed cell 74 because the signal can reach this particular disturbed cell 74, even though it is farther away. Therefore, when the signal is above a certain threshold, any calls placed on that co-channel to the device will be unsuccessful and the device 84 will seal at a particular time.

The interference created from the adjacent/co-channel interference signal reduces the Carrier-to-interference (C/l) ratio to an unacceptable level . The C/l ratio, as a function of the equipment in the telecommunications network, goes below some predetermined acceptable level for the network. As a result, it is a better solution to seal the device 84, rather than have an unacceptable call due to interference. However, this results in minimal use of network resources. Therefore, the Related Application provides a method and system for identifying sources of radio disturbances resulting from, for example, adjacent channel interference that permit the network operator to determine where the sources of such disturbances are located within the network coverage area in order to plan accordingly. Figure 5 is a high-level process flow diagram 88 illustrating in steps a process for identifying uplink/downlink interference in a telecommunications network. As depicted at step 90 of Figure 5, call events (or cell traffic events) in the telecommunications network are identified. At step 92, disturbance events within the telecommunications network are also identified. Thereafter, at step 94, possible sources of disturbance are detected by correlating call event recordings with disturbance event recordings (i.e., identified disturbance events) performed as a function of time. At step 96, definite sources of disturbance are verified utilizing propagation considerations or propagation models. The verifying step is performed using free space path loss considerations. Furthermore, all adjacent channel cells for which calculated signal attenuation indicates that they are too far away to be able to generate perceived disturbance events are excluded from further analysis. Thereafter, at step 98, a test is performed to determine whether or not all disturbance events have been correlated with call events. If all disturbance events are not correlated, then at step 100, an attempt is made to identify possible sources (adjacent channels or co-channels) of external interference. Those skilled in the art will appreciate that disturbance events may not all be attributable to mobile stations associated with the offending cell. External sources can also be the cause of such disturbance events. If all sources of disturbance are not correlated, then the possible sources of external interference must be investigated, as indicated at step 100. Thereafter, at step 102, internal sources of interference are identified on the downlink using reciprocity. The rule of reciprocity dictates that if the mobile stations of the offending cells create interference on the uplink for the disturbed cell, then the disturbed cell may disturb the mobile stations of the offending cells on the downlink. Therefore, the cells that are disturbed on the uplink are potential candidates for consideration as those who disturb their offending cells on the downlink. Finally, at step 104, a disturbance distribution is created. A distribution of disturbed and offending cells within the telecommunications network 10 as a function of time is computed to obtain a statistical correlation of call events in offending cells and subsequent disturbances resulting in the disturbed cell. Such computation and distribution is used to identify the possible sources of disturbance that caused the sealing of at least one device within the disturbed cell. If, however, all disturbance events are correlated, as indicated at step 98, then process flow is directed to 102, wherein sources of interference are identified, thereby omitting implementation of the operation described at step 100. Following implementation of the function indicated at step 102, the operation described at step 104 occurs (i.e., disturbance distribution created).

Figure 6 is a graphic 110 illustrating time correlation of call events with radio disturbance events. In the initial operations associated with the process of Figure 5, possible sources of disturbance are identified. This search process correlates voice channel seizure events from call event recordings with the start and stop time of disturbance event recordings. This correlation is accomplished utilizing disturbance events and voice channel seizure time stamps. In addition, adjacent channel information is considered at this stage. In Figure 6, a disturbance event is described by a high BER with sufficient Signal Strength (SS). The BER and SS thresholds can be varied in accordance with network requirements. With reference to Figure 7, the correlation between signal strength, Bit Error Rate and the resulting radio network environment is shown. In determining when a cell has been disturbed, a comparison of Signal Strength (SS) versus a measurement of speech quality can be employed to determine the Bit Error Rate (BER) of the transmission channel. For example, under the conditions of block 112 where the SS is high and the BER is high, the radio environment is "good" as it is expected that a low BER would be measured when a high SS is transmitted. A more ideal situation occurs on block 114 where both the SS and BER are low. Under the conditions of block 116, if the SS is low and the BER is high meaning that coverage is poor and another base station, for example, may be needed in the service area. The present invention has application to radio environments experiencing the conditions of block 118 give a high SS results in a high BER. In this scenario, it is assumed that interference is significantly affecting the network. To verify sources of disturbance, a more ideal situation occurs in block

114 where both the SS and the BER are low, propagation considerations (i.e., propagation model) are utilized. All adjacent channel cells for which calculated signal attenuation indicates that the cells are too far away to be able to generate perceived disturbances are excluded from further analysis. The propagation model takes into consideration, for example, the geographical position, antenna direction, antenna height and Effective Radiated Power (ERP).

A correlation matrix can thus be utilized to identify possible cells that are potential offenders for each disturbed cell. See Table 1 below:

Table 1 is a list of disturbed cells 76 and possible offending cells 72 in an arrangement of cells similar to that shown and denoted generally as 120 in Figure 8. Originally, the list created is done with respect to a disturbed cell such as disturbed cell 76. The correlation and verification process then allows us to reverse the list and create another matrix ranked by offending cells such as offending cells 72 rather than the disturbed cell 76. The definition of each column in Table 1 is in Table 2 shown below:

Table 2

The present invention involves identifying cells with interference problems by searching for those cells that have a high BER (i.e. greater than 1 %) for good SS. Less than 5% of samples typically have SSs less than -100 dBm. As a result of identifying those cells that have a high BER, a list of such cells is compiled. These cells are identified via an associated MSC 42. A determination is then made as to which devices within these cells also have interference problems by observing the cells contained within the compiled cell list. In addition, start and stop times for all calls having a BER greater than 1 % and a SS greater than -100 dBm are also identified.

If too many telecommunications events (e.g., start and stop times) are identified, a re-correlation can be performed utilizing a greater delta value. A list of devices having high BER in each of the disturbed cells 76 is then provided, in addition to a listing of disturbance events, including stop and start times, and a BER distribution and SS distribution. Thereafter, channel numbers associated with devices having high BER are identified. A list of channel numbers, along with data describing the distribution of such channel numbers, is also determined, including the cells within which such channel numbers are located. Next, adjacent/co-channel disturbance analysis is performed for the start and stop times identified earlier for the disturbed devices. Given the channel numbers identified earlier, adjacent channels are identified in which a call in progress is completely overlapped with a disturbance event. As a result of this calculation, a table of disturbed cells 76 and offending cells 72 is created, including a determination of how many correlations were found for each disturbed and offending pair. A disturbance distribution created by the possible offending cells 72 is also created. This adjacent channel disturbance analysis can be repeated for adjacent channels, and a table with adjacent channel offenders can also be created, similar to the table of disturbed cells 76 and offending cells 72.

As a result of these calculations, possible offending cells 72 can be identified having the highest number of correlations. In addition, a determination can also be performed as to whether the offender's signal can possibly interfere with the disturbed cells 76. Thus, a list of possible offenders on the downlink can be compiled, and this information can be utilized for short and long term recommendations.

With reference to Figufe 9, the concept of reciprocity is illustrated and denoted generally as 130. The internal sources of interference on the downlink 134 are also identified on this basis using the notion of reciprocity. That is, if the mobile station 42 of the offending cell 72 creates interference, or disturbance events on the uplink 132 for the disturbed cell 76, then by the rule of reciprocity, the base station 44 of the disturbed cell 76 may disturb the mobile station 42 of the offending cells 72 on the downlink 134. Therefore, the cells that are disturbed 76 on the uplink 132 are potential candidates to consider as those who disturb their offending cells 72 on the downlink 134. Those skilled in the art can appreciate that the offending cells 72 on the uplink become the disturbed cells via mobile stations on the downlink, and the disturbed cell 76 on the uplink becomes the offending cell via a base station on the downlink.

Furthermore, the disturbance events 78 may not all be attributable to mobile station 42 of the offending cells 72. External sources could also be the culprits of such interference. As such, if all sources of disturbance are not correlated, then the possibility of external interference will not be considered.

Figure 10 is a high-level logic flow diagram 140 illustrating process steps implementing the method and system of the present invention, in accordance with a preferred embodiment of the present invention. It can be appreciated by those skilled in the art that Figure 10, as illustrated and described herein, presents a self-consistent sequence of steps leading to a desired result. The steps are those requiring the physical manipulation of physical quantities. Usually, although not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated.

It has proven convenient at times by those skilled in the art, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities.

Further, the manipulations performed are often referred to in terms, such as "designating," "delivering" or "conveying", which are commonly associated with mental operations performed by a human operator. No such capability of a human operator is necessary or desirable in most cases of the operations described herein, which form part of the present invention. As indicated herein, these operations are primarily machine operations. Useful machines for performing operations of a preferred embodiment of the present invention include data-processing systems, such as a general-purpose digital computer or other similar devices. In all cases the distinction between the method of operations in operating a computer and the method of computation itself should be borne in mind.

The present invention relates to method steps for processing electrical or other (e.g. mechanical, chemical) physical signals to generate other desired physical signals, and can be implemented via a computer or microcomputer. However, it is not necessary to maintain within a computer memory of a mobile station, such as MS 42, or cellular telephone subscriber unit, instructions implementing these method steps. Such instructions can be maintained within a computer memory location of a wireless telephone base station, such as BS 44, or at a central broadcasting center from which such base stations receive instructions. Implementation of the method described herein is left to the discretion of a particular wireless telephone system designer, whether cellular- based or otherwise.

It can be appreciated by those skilled in the art that the methods described herein can be implemented as a program product (e.g., a control program residing in a computer memory). The program product contains instructions that when executed on a CPU, carry out the operations depicted in the logic flow diagram of Figure 10. While the present invention is described in the context of a fully functional telecommunications network 10, those skilled in the art will further appreciate that the present invention is capable of being distributed as a program product in a variety of forms. The present invention applies equally, regardless of the particular type of signal-bearing media utilized to actually carry out the distribution. Examples of signal-bearing media include recordable-type media, such as floppy disks, hard-disk drives and CD ROM's, and transmission-type media, such as digital and analog communication links.

Preferred implementations of the invention can include implementations to execute the method or methods described herein as a program product residing in a memory of a microcomputer. Alternatively, a preferred embodiment of the present invention can include a program product residing in a microcomputer memory located at an MSC (e.g., MSC 27 of Figure 1 herein). The MSC 27 controls system operations in wireless telecommunications networks, thereby managing calls, tracking billing information, and locating mobile station subscribers. The program product thus includes sets of instructions for executing the method and system described herein. Until required by a microcomputer, the set of instructions may be stored as a computer-program product in another computer memory. For example, the set of instructions may be stored as a computer-program product in a disk drive attached to a microcomputer (which may include a removable memory such as an optical disk or floppy disk for eventual use in the disk drive). The computer-program product can also be stored at another computer and transmitted, when desired, to a user's workstation by an internal or external network. Those skilled in the art will appreciate that the physical storage of the sets of instructions physically changes the medium upon which it is stored so that the medium carries computer-readable information. The change may be electrical, magnetic, chemical, or some other physical change. While it is convenient to describe the invention in terms of instructions, symbols, characters, or the like, the reader should remember that all of these and similar terms should be associated with the appropriate physical elements.

Thus, as depicted at step 142 of Figure 10, between a disturbed cell and a plurality of offending cells, the uplink disturbance source offending a cell is determined utilizing Call Event Recording (CER) and Radio Disturbance Recording (RDR) correlations. Initially, at least one cell experiencing disturbance on the downlink is identified. The call events occurring in the offending cells of the telecommunications network are then recorded, followed by the recording of disturbance events occurring in the disturbed cell of the telecommunications network. The recorded call events are then correlated with the recorded disturbance events, as disclosed in the invention of U.S. Patent Application Serial No. 09/426,139.

Once the uplink disturbance source has been determined, the signal strength statistics of the disturbance on the uplink of the cell is computed. The statistics of the mobile transmit power for the offender are then calculated using Radio Environment Statistic (RES) measurements. RES measurements provide BER, SS, disturbance, and Uplink and Downlink data distribution. Using the statistics of the disturbance and the mobile transmit power, the Path Loss (PL) on the uplink is computed with the formula:

PL_ui = P_Ms(avg) - RSS_uWist(avg)

where RSS_ui-_di_st(avg) is the average Received Signal Strength (RSS) of the disturbance on the uplink and Pιws(avg) is the average mobile output power of the offender. As such, the path loss on the uplink (PL_Uι) is equal to the path loss on the downlink.

Using the base station transmit power, the RSS on the downlink of the offender is then computed at step 144. That is, the computed power on the downlink of the offender due to reciprocity would be:

RSSdi-of avg) = P_RBS - PL, dl

where RSS_dι-_0ff(avg) is the average Received Signal Strength (RSS) of the offender on the downlink, P_RBs is the Radio Base Station (RBS) power and PL_dι is the path loss on the downlink.

The Received Signal Strength (RSS_dι_-0ff(avg)) of step 144 is then compared with a predetermined threshold at step 146, in order to determine if significant downlink interference is indicated. The threshold for the comparison, in one embodiment, can be chosen by the network engineer. If the RSS_d..off(avg) is not greater than the predetermined threshold, then the next cell is considered at step 148. If, however, the RSS_dι_-0ff(avg) is greater than the predetermined threshold at step 146, then the disturbance is found to be significant. Thus, the uplink source of disturbance is identified and quantified at step 150 as a downlink source of disturbance based on reciprocity. For example, in one embodiment, suppose the RSS(avg) on the uplink of the disturbed cell is -106dBm. Also, the mobile station power is 24dBm. Then the uplink path loss on average is 130dB. Suppose the downlink power is 20dBm. In this case, the average received signal on the downlink of the offender is -110dBm, which may not constitute a disturbance. On the other hand, if the downlink power is 30dBm, then the average RSS of the offender on the downlink is -lOOdBm. Thus, we can see that disturbance in this case is significant.

Those skilled in the art can thus appreciate that the invention described herein explains a method and system for applying reciprocity for the identification of downlink interference sources. The method and system described herein is based on Radio Environment Statistic (RES) measurements via computing signal strengths and comparing them against a threshold. Path balance is also utilized in the computation. As such, RES measurements provide an advantage over prediction tools. In short, utilizing the method and system described herein allows for quantifying the accuracy of the traffic/disturbance "correlation" method to identify the sources of interference on the downlink, and thus, results in improvements in the performance of a telecommunications network due to more accurate network planning. While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Claims

WHAT IS CLAIMED IS:

1. A method of identifying sources of downlink interference among cells in a telecommunications network comprising the steps of: between a disturbed cell and a plurality of offending cells, determining the uplink disturbance source offending a cell utilizing Call Event Recording (CER) and Radio Disturbance Recording (RDR) correlations; and applying reciprocity to identify downlink sources of interference within said cell causing disturbances in other cells.

2. The method according to Claim 1 wherein said determining step is preceded by the step of identifying at least one cell experiencing disturbance on the downlink.

3. The method according to Claim 2 wherein said identifying step further includes the step of recording the call events occurring in the offending cells of the telecommunications network.

4. The method according to Claim 2 wherein said identifying step further includes the step of recording disturbance events occurring in the disturbed cell of the telecommunications network.

5. The method according to Claim 4 wherein said recording step is followed by the step of correlating the recorded call events with the recorded disturbance events.

6. The method according to Claim 1 wherein said determining step is followed by the step of computing the signal strength statistics of the disturbance on the uplink of a cell.

7. The method according to Claim 6 wherein said computing step is followed by the step of calculating the statistics of the mobile transmit power for the offender using Radio Environment Statistic (RES) measurements.

8. The method according to Claim 7 wherein said calculating step is followed by the step of computing the path loss on the uplink using the statistics of the disturbance and the mobile transmit power.

9. The method according to Claim 1 wherein said applying step further includes the step of computing the Received Signal Strength (RSS) on the downlink of the offender using the base station transmit power.

10. The method according to Claim 9 wherein said computing step is followed by the step of comparing the Received Signal Strength (RSS) with a predetermined threshold in order to determine if significant downlink interference is indicated.

11. A system for identifying sources of downlink interference among cells in a telecommunications network comprising: between a disturbed cell and a plurality of offending cells, a means for determining the uplink disturbance source offending a cell utilizing Call Event Recording (CER) and Radio Disturbance Recording (RDR) correlations; and means for applying reciprocity to identify downlink sources of interference within said cell causing disturbances in other cells.

12. The system according to Claim 11 further comprising a means for identifying at least one cell experiencing disturbance on the downlink.

13. The system according to Claim 12 wherein said means for identifying further includes a means for recording the call events occurring in the offending cells of the telecommunications network.

14. The system according to Claim 12 wherein said means for identifying further comprises a means for recording disturbance events occurring in the disturbed cell of the telecommunications network.

15. The system according to Claim 14 wherein said means for recording further includes a means for correlating the recorded call events with the recorded disturbance events.

16. The system according to Claim 11 wherein said means for determining further comprises a means for computing the signal strength statistics of the disturbance on the uplink of a cell.

17. The system according to Claim 16 wherein said means for computing further includes a means for calculating the statistics of the mobile transmit power for the offender using Radio Environment Statistic (RES) measurements.

18. The system according to Claim 17 wherein said means for calculating further comprises a means for computing the path loss on the uplink using the statistics of the disturbance and the mobile transmit power.

19. The system according to Claim 18 wherein said path loss on the uplink is equal to the path loss on the downlink.

20. The system according to Claim 11 wherein said means for applying further includes a means for computing the Received Signal Strength on the downlink of the offender using the base station transmit power.

21. The system according to Claim 20 wherein said means for computing further comprises a means for comparing the Received Signal Strength with a predetermined threshold in order to determine if significant downlink interference is indicated.

22. The system according to Claim 21 wherein said threshold for the comparison is chosen by the network engineer.

23. A program product for identifying sources of downlink interference among cells in a telecommunications network comprising: between a disturbed cell and a plurality of offending cells, instruction means residing in a computer for determining the uplink disturbance source offending a cell utilizing Call Event Recording (CER) and Radio Disturbance Recording (RDR) correlations; and instruction means residing in a computer for applying reciprocity to identify downlink sources of interference within a cell causing disturbances in other cells.

24. The program product according to Claim 23 wherein said program product further comprises instruction means residing in a computer for identifying at least one cell experiencing disturbance on the downlink.

25. The program product according to Claim 24 wherein said instruction means residing in a computer for identifying further includes instruction means residing in a computer for recording the call events occurring in offending cells of the telecommunications network.

26. The program product according to Claim 24 wherein said instruction means residing in a computer for identifying further includes instruction means residing in a computer for recording disturbance events occurring in a cell of the telecommunications network.

27. The program product according to Claim 26 wherein said instruction means residing in a computer for recording further comprises instruction means residing in a computer for correlating the recorded call events with the recorded disturbance events.

28. The program product according to Claim 23 wherein said instruction means residing in a computer for determining further comprises instruction means residing in a computer for computing the signal strength statistics of the disturbance on the uplink of a cell.

29. The program product according to Claim 28 wherein said instruction means residing in a computer for computing further comprises instruction means residing in a computer for calculating the statistics of the mobile transmit power for the offender using Radio Environment Statistic (RES) measurements.

30. The program product according to Claim 29 wherein said instruction means residing in a computer for calculating further comprises instruction means residing in a computer for computing the path loss on the uplink using the statistics of the disturbance and the mobile transmit power.

31. The program product according to Claim 23 wherein said instruction means residing in a computer for applying further includes instruction means residing in a computer for computing the Received Signal Strength (RSS) on the downlink of the offender using the base station transmit power.

32. The program product according to Claim 31 wherein said instruction means residing in a computer for computing further comprises instruction means residing in a computer for comparing the Received Signal Strength with a predetermined threshold in order to determine if significant downlink interference is indicated.