WO2000019741A1

WO2000019741A1 - Method for improving performance in a communication system

Info

Publication number: WO2000019741A1
Application number: PCT/US1999/019215
Authority: WO
Inventors: Gary John Pregont; Jeffrey D. Bonta
Original assignee: Motorola Inc.
Priority date: 1998-09-30
Filing date: 1999-08-24
Publication date: 2000-04-06

Abstract

The present invention provides a method for improving perfomance in a communication system (100) that includes a first base transceiver station (101) having a first channel (112) a second base transceiver station (102) having a second channel (113). The method comprises determining that there is interference between the first channel (112) and the second channel (113). The first channel (112) and the second channel (113) are identified as an interfering channel pairing, and the assignment of the first channel (112) or the second channel (113) when the other channel is in use is deprioritized, based at least in part upon the interfering channel pairing. By deprioritizing the assignment of a channel that might interfere with a channel already in use, improved communication within the communication system (100) is provided.

Description

METHOD FOR IMPROVING PERFORMANCE IN A COMMUNICATION SYSTEM

Field of the Invention

The present invention relates generally to communication systems, and more particularly to a method for improving performance in a communication system that has interference between base transceiver stations.

Background of the Invention

Cellular system performance has improved over time via the introduction of new planning tools and new handover algorithms and the like. These improvements have been focused in two main areas.

First, improvements have been made in the system planning area. Tools exist to develop an optimum, or near optimum, frequency plan, also referred to as a site location plan. The problem with this approach is that output of many of these tools is limited to the quality of the data that is input. So while the frequency plan or neighborhood list is as good as can be developed, it may not be the best, based on real-world conditions that are difficult to simulate. In addition, the predictions that these tools make may be based on exact cell locations. In reality, site acquisition is getting more difficult and compromises must be made in cell placement. Also, many times the tools are not used properly by the system, or field engineers, and more error is introduced. Therefore, the system that is deployed is rarely as good as the system looks on paper. Then, when the system grows, the tools are often not used again to optimize the system. The second area where improvements have been concentrated is m performance enhancing algorithms. This would include, inter alia, new handover algorithms and search routines. Many of these are based on the principle of detecting a problem, such as call quality degradation, and performing some corrective action for the proolem. The actions taken are typically a handover to a better cell. The problem with this solution is that some level of bad performance must occur before an action is taken. Many times, by the time the bad performance is detected, the call has dropped or the customer has become irritated by the reduced quality of the call . In addition, there is typically nothing in place to prevent this same condition from affecting a future call that experiences the same αifflculties . Consequently, a need exists for a method for improved communication m a communication system, and more particularly for improved communication m communication systems that have interference between base transceiver stations.

Brief Description of the Drawings

FIG. 1 depicts a communication system in accordance with the preferred embodiment of the present invention;

.->. FIG. 2 depicts a flow diagram for improving performance m a communication system m accordance with the preferred embodiment of the present invention;

FIG. 3 depicts a flow chart for determining if there is interference between a first channel and a second channel in accordance with the preferred embodiment of the present invention;

FIG. 4 depicts a flow diagram for determining if there is a possible interfering channel pair m accordance with the preferred embodiment of the present invention;

FIG. 5 depicts a coverage diagram for a first and second base transceiver station m accordance with an alternate embodiment of the present invention; and FIG. 6 depicts a flow chart for determining if there are partially interfering channel pairs m accordance with the alternate embodiment of the present invention.

Detailed Description of a Preferred Embodiment

The present invention provides a method for improving performance in a communication system that includes a first base transceiver station having a first channel and a second base transceiver station having a second channel. The method comprises determining that there is interference between the first channel and the second channel. The first channel and the second channel are identified as an interfering channel pairing, and the assignment of the first or second channel when the other channel is m use is deprioritized, based at least in part upon the interfering channel pairing. By deprioritizmq the assignment of a channel that mignt interfere with a cnannel already in use, improved communication within a communication system is provided. The present invention can be better understood with reference to FIGs. 1-6. Referring now to FIG. 1, a Global System for Mobile communications (GSM) communication system 100 in accordance with the preferred embodiment of the present invention is depicted. Communication system 100 is preferably a GSM system, but could alternately be PDC, Advanced Mobile Phone Service (AMPS) , United States Time Division Multiple Access (US- TDMA) , Code Division Multiple Access (CDMA) , or any other suitable communication system. Referring to FIG. 1, acronyms are used for convenience. The following is a list of the acronyms used m FIG. 1:

BTS Base Transceiver Station

BSC Base Station Controller

GSM Global System for Mobile communications

HLR Home Location Register

ISDN Integrated Services Digital Network

MS Mobile Station

MSC Mobile Switching Center

OMCR Operations and Maintenance Center - Radio

OMCS Operations and Maintenance Center - Switch

PSTN Public Switched Telephone Network

VLR Visitor Location Register

XCDR Transcoder

A first base transceiver station 101 is located m a first coverage area 104 and communicates witn a mobile station 105. Mobile station 105 can be a mobile unit, a remote unit, a fixed wireless terminal, or any other unit capable of sending or receiving RF communications to or from first BTS 101. Communication is via a first digital radio channel 112 that contains data information compatible with a GSM communication system. Mobile station 106 communicates with second BTS 102 that is located m second coverage area 107. Communication is via a second digital radio channel 113 that contains data information compatible with a GSM communication system. Base transceiver stations 101 and 102 can include multiple channels.

Also included m FIG. 1 is base transceiver station 102 located within coverage area 107. BTSs 101 and 102 are each coupled to a base station controller (BSC) 114 which includes an XCDR 110. As shown in FIG. 1, the dotted lines within BSC 114 represent control information paths while the solid lines within BSC 114 represent speech information paths. As is clear from FIG. 1, XCDR 110 routes the speech information to and from the base transceiver stations 101 and 102. The speech information is routed to an MSC 115 which provides switching functions for access to PSTN 120 or ISDN 122. HLR 116 and VLR 117 provide location and billing services for the entire system as is well known m the art, while OMCS 124 and OMCR 112 provide diagnostics and maintenance services for the entire system, as is also well known m the art. Mobile stations 105 and 106 each preferably include a receiver, a transmitter, a processor, and memory. The receiver receives information from BTSs 101 and 102, respectively, via first channel 112 and second channel

113, respectively. The transmitters transmit information to first base station 101 and second base station 102, respectively . The processors update variables pertaining to mobile stations 105 and 106 based at least in part upon the information received from BTSs 101 and 102. The memory stores information related to mobile stations 105 and 106.

As stated above, problems can occur when second mobile station 106 attempts to make a call when second channel 113 has a history of interfering with first channel 112. Second channel 113 may determine, prior to being assigned, whether any other channel currently interferes with second channel 113. However, in the present invention, BTS 102 determines whether second channel 113 interferes with any surrounding channels. This process is an improvement over prior art systems and leads to improved communication in communication systems by not only checking to see if any other channel interferes with the channel that is about to be assigned to a call, but also by checking to see if the channel that is about to be assigned to a call interferes with any channel currently in use.

In the preferred embodiment of the present invention, second BTS 102 will not assign channel 113 if second BTS 102 determines that second channel 113 has previously interfered with first channel 112. When it is determined that first channel 112 interferes with second channel 113 or that second channel 113 interferes with first channel 112, first channel 112 and second channel 113 are identified as an interfering channel pairing. In accordance with the preferred embodiment of the present invention, when one channel that has an interfering channel pairing is assigned to a call, a channel that is associated with the assigned channel as an interfering channel pairing will be depπoritized for call assignment purposes .

In this manner, the channel will be lowered such that, if other channels are available for the call and do not have a history of interfering with the assigned channel, they will be assigned prior to the interfering channel. The presence of an interfering channel pairing can alternately be used as one of many factors used during the assignment of a channel to a call. For example, other factors, such as signal strength, direction of movement of a mobile station, velocity of a mobile station, and capabilities of a channel, can also be used and weighted to determine the optimal channel to be assigned to the call.

FIG. 2 depicts a flow diagram for improving performance in a communication system. In the preferred embodiment of the present invention, a centralized controller, such as a BSC, determines (201) whether there is interference between a first channel and a second channel. Althougn the preferred embodiment indicates that the BSC is performing this processing, the processing can be done at other portions of the system, including the BTSs or the MSC. The first channel and the second channel are preferably channels within different base transceiver stations at different cellular sites that are transmitting at the same or similar frequencies. The process of determining if there is interference between a first and a second channel is described in FIG. 3. If there is no interference between the first channel and the second channel, the process ends (299) . If there is interference between the first channel and the second channel, the first channel and the second channel are identified (203) as an interfering channel pair. In the preferred embodiment, the interfering channel pairing is stored m a database that is accessible by the centralized controller.

The centralized controller then deprioritizes (205) the assignment of the first channel when the second channel is m use. In the preferred embodiment of the present invention, when a first channel and a second channel are identified as an interfering channel pair, the assignment of one while the other is currently involved in a call is deprioπtized. This deprioπtization is preferably such that the channel is still m service, but will only be assigned as a last resort when all non-interfermg channels have been assigned. Alternately, when a channel that is an interfering channel pair with another channel is assigned, the other channel can be taken out of service temporarily while the call on the first channel is continuing. Upon termination of the call on the first channel, the second channel can then be assigned to a call. When identified as an interfering channel pair, the second channel will preferably not be assigned to a call while the first channel is currently assigned to a call, and the first channel will preferably not be assigned while the second channel is assigned to a call unless all other channels are currently assigned. In this manner, interference between adjacent channels is minimized. FIG. 3 depicts a flow diagram for depicting the preferred process for determining if there is interference between the first channel and the second channel, as determined at box 201 in FIG. 2. The centralized controller determines (301) the current quality of a signal. The determination can be made relating to a signal transmitted from a communication unit, such as a mobile station, or a base transceiver station. The centralized controller then determines (303) the average quality of the signal. Data is contained in a message report. The sampling is done once every message reporting period, which is preferably about every 0.5 seconds. In the preferred embodiment, five samples are adequate to determine the average quality of the signal. The centralized controller then determines (305) the level of the signal, which is preferably determined by measuring the Radio Signal Strength Indicator (RSSI) .

The centralized controller then determines (307) whether the current quality of the signal is significantly less than the average quality of the signal. If the current quality is significantly less than the average quality, this may indicate interference between two channels. As used herein, the current quality of the signal is significantly less than the average quality of the signal if the current quality falls below a predetermined threshold. In the preferred embodiment, the quality of a signal is measured on an eight point scale, with 0 being the best quality and 7 being tne lowest quality. In the preferred embodiment, a difference of three or more between the current quality and the average quality indicates that the current quality is significantly less than the average quality. The base transceiver stations receive a series of quality metrics from the mobile stations. In the preferred embodiment, these quality metrics have a value from 0 to 7, with 0 representing the best quality metric and 7 representing the worst metric, i.e. highest Bit Error Rate (BER) . For example, if the BTS receives a series of two values from the mobile station, and then suddenly receives a five value, the BTS will infer that at the time the quality metric went from 2 to 5 that some change in call quality happened. The most probable event that caused such a change in call quality is interference from an adjacent base transceiver station transmitting at the same or similar frequency.

The centralized controller then determines (312) if there is a possible interfering channel pair. If the current quality is significantly less than the average quality, as determined at step 307, this is a strong indication that there is interference between channels. Step 312, which is depicted in further detail below in reference to FIG. 4, determines if there are channels that have the possibility of interfering with each other. If two interfering calls begin at approximately the same time, the current quality of the signal will not be significantly less than the average quality of the signal. Consequently, even though there is an interfering channel pair, box 307 will not indicate a Yes value. This case can be determined by the centralized controller by looking for a condition of poor quality in a strong signal condition. If tne current quality is not significantly less than the average quality, the centralized controller determines (309) if the current quality is less than a first threshold. Poor quality is adjustable, but quality is typically considered poor when a quality rating of 5, 6, or 7 on the eight point scale is determined. Consequently, the first threshold is preferably set to 4. If the current quality is not less than the first threshold, the communication system indicates (313) that there is not interference between the two channels. The quality is typically a measure of the Bit Error Rate.

If the current quality is less than the first threshold, the centralized controller determines (311) if the current level of the signal is greater than a second threshold. The second threshold is adjustable and will vary from cell to cell. The second threshold is preferably set to a value that would typically provide a good quality call for the particular cell. The level is typically measured via the RSSI. If the current quality is less than the first threshold and the current level of the signal is greater than a second threshold, this indicates that two calls are interfering with each other. Consequently, the centralized controller determines (312) if there is a possible interfering channel pair that might be accounting for this condition, as described in more detail in FIG. 4.

Turning now to FIG. 4, which details the preferred embodiment for determining if there is a possible interfering channel pair as depicted at step 312 of FIG. 3, the centralized controller determines (401) the coverage area of the first oase station. The centralized controller then determines (403) the location of the second base station. The locations are preferably determined as a set of coordinates that uniquely identify the physical location of the BTSs, and are preferably determined during system set up.

The centralized controller then determines (407) if the location of the second base station is in the coverage area of the first base station. In the preferred embodiment of the present invention, to determine if the location of the second base station is m the coverage area of the first base station, the centralized controller determines if the second base station is in the beam width of the first base station antenna. The centralized controller then determines if the second base station is the closest base station to the first base station of all base stations in the beam width of the first base station antenna. The centralized controller also determines if the second base station uses the same or similar frequency as the first base station. The centralized controller also determines if the distance of the first oase station from the second base station exceeds a predetermined distance. In the preferred embodiment, the predetermined distance is about 2,500 meters. If all of these conditions are met, the centralized controller determines that the second base station is in the coverage area of the first base station .

If the second base station is in the coverage area of the first base station, this indicates that there may be interference between channels having the same or similar frequencies at the two base stations. If the second base station is m the coverage area of the first base station, the centralized controller determines (409) if the frequencies of the first channel and the second channel match. As used herein, the frequencies of the channels match if they are the same frequency, commonly referred to as co-channel interference, or if they are adjacent channels. If the frequencies of the first channel and the second channel match, the centralized controller indicates (411) that the first channel and the second channel are a possible interfering pair.

Consequently, a Yes value " C" is returned in FIG. 3 after decision box 312.

If the second base station is not m the coverage area of the first base station as determined at box 407 or the frequencies of the first and second channels do not match as determined at box 409, the centralized controller will indicate (413) that the first and second channels are not a possible interference channel pair. The process will then return a No value " D" in FIG. 3 after decision box 312.

It should be understood that the flowchart of FIG. 4 car represent the detection of possible interfering channel pairs for the uplink scenario and the downlink scenario. As used herein, downlink refers to signals generated from a piece of cellular infrastructure equipment, such as a base transceiver station, and transmitted for reception at a remote unit. As used herein, uplink refers to signals generated by a remote unit that are transmitted and intended for reception by cellular infrastructure equipment, such as base stations. Although FIG. 4 depicts the preferred embodiment of the present invention and depicts the downlink scenario, it snould De understood that the uplink scenario is covered by the flowchart of FIG. 4 as well. For example, in the downlink scenario, a second base station is located m the coverage area of a first base station that is transmitting a signal that interferes with a signal transmitted from the second base station. In the uplink scenario, an uplink signal sent from a second remote unit to the second base station interferes with a signal sent from a first remote unit to the first base station. Similar processing would occur to identify that first and second base transceiver stations as interfering channel pairs for the uplink case as m the downlink case described above.

FIG. 5 depicts a coverage diagram for a first and second base transceiver station m accordance with an alternate embodiment of the present invention. A first base transceiver station 501 includes a unidirectional antenna that emits a signal m one direction. The signal covers a coverage area 511. A second base transceiver station 502 also includes a unidirectional antenna that transmits a signal over a coverage area 512. In many communication systems, coverage areas overlap, which provides for increased possibility of call continuity due to the ability to transfer processing from a first base transceiver station to a second base transceiver station. However, this overlapping is not desirable when the two base stations are using the same channel.

First coverage area 511 and second coverage area 512 overlap in coverage area 513. In accordance with an alternate embodiment of the present invention, since coverage area 513 is an overlap coverage area of coverage areas 511 and 512, a call associated w th a remote unit 507 that is m coverage area 513 might interfere with a call associated with a remote unit 505 or a call associated with a remote unit 506. As an example, if remote unit 507 is communicating with first BTS 501, and remote unit 506 attempts to make a call with second BTS 502, if the two channels are approximately the same frequency, the call associated with remote unit 506 will interfere with the current call of remote unit 507. Similarly, if remote unit 507 is communicating with second BTS 502, a call associated with remote unit 505 might interfere with a call associated with remote unit 507 if remote unit 505 is assigned a channel that has approximately the same frequency as that being used by second BTS 502.

In accordance with the alternate embodiment of the present invention, a method is achieved that allows for communications in adjacent coverage areas that are identified as being partially interfering. In the scenario of partially interfering coverage areas as depicted in FIG. 5, a centralized controller will allow two communications to continue if neither is currently m the overlapping coverage area. For example, the centralized controller would allow remote unit 506 to be assigned while m coverage area 512 while remote unit 505 is communicating with BTS 501 while in coverage area 511.

However, if remote unit 507 is located m overlapping coverage area 513 and is communicating with first BTS 501, the centralized controller will not allow remote unit 506 to start a communication with second BTS 502, as tnis communication would interfere with the communication of remote unit 507. In the same manner, if remote unit 507 is in overlapping coverage area 513 wnile communicating with second BTS 502, the centralized controller will not allow remote unit 505 to start a communication with first BTS 501, since this communication would interfere with the communication of remote unit 507. In similar fashion, if either remote unit 505 is in communication with first BTS 501 or remote unit 506 is in communication with second BTS 502, the centralized controller would not allow remote unit 507 to begin a communication with the other BTS while it is located in the partially overlapping coverage area 513.

FIG. 6 depicts a flow chart for determining if there are partially interfering channel pairs m accordance with the alternate embodiment of the present invention. A centralized controller determines (601) the coverage area of a first base transceiver station. The centralized controller determines (603) the coverage area of a second base transceiver station. The centralized controller then determines (605) if there is an overlapping coverage area between the first coverage area and the second coverage area.

The determination of partially overlapping coverage areas is preferably performed at step 312 of FIG. 3 while determining if there is a possible interfering channel pair. In the alternate embodiment, a partial overlapping coverage area may or may not be detectable using base station locations and antenna beam width information as described with regard to the preferred embodiment. Instead, a possible partially interfering channel pair is considered when a call associated with a remote unit on a first base station is interfering with a call associated w tn a remote unit on a second base station, and the first base station and the second base station share the same channel frequency.

If the interference is detected in the downlink, the location of the remote unit being interfered with is determined. For example, m FIG. 5, if remote unit 507 is being interfered with in the downlink, then the location of remote unit 507 is determined. If the interference is detected m the uplink, the location of a remote unit associated with a call at a second base station that shares the same channel frequency and timeslot is determined. For example, if remote unit 505 associated with a call on base station 501 is being interfered with on the uplink, the closest base stations that share the same channel frequency and timeslot are examined for the existence of an active call. In this case, second base station 502 shares the same channel frequency and timeslot as remote unit 505 and first base station 501. Remote unit 507 is an active call that is using this channel frequency and timeslot. In this manner the location of remote unit 507 is determined.

The location determination can be accomplished using techniques known in the art, such as by using GPS receivers, TDOA, TOA, AOA, or by using mobile reported signal strength vectors. The location or signal strength vectors associated with each identified overlapping coverage area are stored with the interfering channel pairing information as a condition for channel deprioπtization . The channel pair is now considered a partially interfering channel pair. Channel assignment of a partially interfering channel in accordance with the alternate embodiment has tne additional requirement to compare the location of the remote unit with the stored location associated with the partially interfering channel pair. If the current location of the remote unit and the stored location are substantially similar or within a threshold, then the channel assignment at a first base station can not be made if the other channel m the partially interfering channel pair at the second base station is m use by another remote unit. The threshold value is dependent upon the location method used.

If there is not an overlapping coverage area, the centralized controller determines (611) that the first and second channels are not a partially interfering channel pair. If there is an overlapping coverage area, the centralized controller determines (607) that the first and second channels are a partially interfering channel pair.

It should be understood that the alternate embodiment of the present invention works best m conjunction with the preferred embodiment of the present invention. For example, using the alternate embodiment with the preferred embodiment would allow the communication system to determine overlapping areas of interference and only deprioritizes calls when one of the remote units is within the overlapping coverage area. In this manner, not only is interference between adjacent channels decreased, but the coverage areas and overall coverage of the communication system is not greatly diminished. Thus, the present invention provides a method for improving performance in a communication system. By identifying two channels as interfering channel pairings, the assignment of either cnannel wnen the other cnannel is currently m service is deprioritized. Consequently, a channel that is in service will have a reduced likelihood that interference will be introduced by a second channel coming into service.

The present invention provides for improved performance by looking at whether the channel to be assigned will potentially interfere with a channel in service, in addition to determining if any channel currently in service may interfere with the channel potentially coming into service. Consequently, since channels that are currently being used for communication will have a lower chance of being interfered with by a channel being assigned to a call, the performance of the communication system is improved.

By incorporating the alternate embodiment of the present invention with the preferred embodiment, a method is provided that allows for calls to be assigned m areas that are identified as interfering channels pairs if the location of the remote unit is outside of the overlapping coverage area of the two base transceiver stations. In this manner, calls are allowed to be processed, while deprioritizing the assignment of a call if it is in an area that might interfere with a call at an adjacent base transceiver station.

While this invention has been described in terms of certain examples tnereof, it is not intended that it be limited to the above description, but rather only to the extent set forth in the claims that follow.

Claims

Claims We claim:

1. A method for improving performance in a communication system, the communication system including a first base transceiver station having a first channel and a second base transceiver station having a second channel, the method comprising the steps of: determining that there is interference between the first channel at the first base transceiver station and the second channel at the second base transceiver station; identifying the first channel and the second channel as an interfering channel pairing; and deprioritizing the assignment of the first channel when the second channel is n use based at least in part upon the interfering channel pairing.

2. A method for improving performance in a communication system m accordance with claim 1, wherein the step of determining that there is interference comprises the steps of: determining the current quality of a signal transmitted from a communication unit in the communication system; determining the average quality of the signal; and comparing the current quality to the average quality.

3. A method for improving performance m a communication system m accordance with claim 1, wherein the step of determining that there is interference comprises the steps of: determining the current quality of a signal transmitted from the first base transceiver station; determining the average quality of the signal; and comparing the current quality to the average quality.

4. A method for improving performance m a communication system m accordance with claim 1, wherein the step of determining that there is interference comprises the steps of: determining the current quality of a signal transmitted from the second base transceiver station; determining the average quality of the signal; and comparing the current quality to the average quality.

5. A method for improving performance in a communication system in accordance with claim 1, wherein the step of determining that there is interference comprises the steps of: determining that the current quality of a signal transmitted from a communication unit in the communication system is below a first threshold; and determining that the current level of the signal exceeds a second threshold.

6. P method for improving performance in a communication system in accordance with claim 1, wherein the step of determining that there is interference comprises the steps of: determining that the current quality of a signal transmitted from the first base transceiver station is below a first threshold; and determining that the current level of the signal exceeds a second threshold.

7. A method for improving performance m a communication system m accordance with claim 1, wherein the step of determining that there is interference comprises the steps of: determining that the current quality of a signal transmitted from the second base transceiver station is below a first threshold; and determining that the current level of the signal exceeds a second threshold.

8. A method for improving performance in a communication system m accordance with claim 1, wherein the first base transceiver has a first location and the second base transceiver has a second location, and wherein the first base transceiver station transmits signals over a first region and the second base transceiver station transmits signals over a second region, wherein the step of determining that there is interference comprises the step of determining that the first location is within the second region.

9. A methoα for improving performance in a communication system, the communication system including a first base transceiver station having a first channel and a second base transceiver station having a second channel, the method comprising the steps of: determining that the first channel at the first base transceiver station interferes with the second channel at the second base transceiver station; identifying the first channel and the second channel as an interfering channel pairing; processing a call at the first channel at the first base transceiver station; and deprioritizing the assignment of the second channel based at least m part upon the interfering channel pairing.

10. A method for improving performance in a communication system, the communication system including a first base transceiver station having a first channel and a second base transceiver station having a second channel, the method comprising the steps of: determining that there is interference between the first channel at the first base transceiver station and the second channel at the second oase transceiver station; determining a first coverage area associated with the first base transceiver station; determining a second coverage area of the second base transceiver station; determining if there is an overlapping coverage area, the overlapping coverage area comprising an area that is covered by the first coverage area and the second coverage area; identifying, if there is an overlapping coverage area, the first channel and the second channel as a partially interfering channel pairing; and deprioritizing the assignment of the first channel when the second channel is in use by a remote unit that is located in the overlapping coverage area.