CN1153499C - Method and apparatus for control of soft handoff usage in radiocommunication systems - Google Patents

Abstract

Radiocommunication systems having improved soft handoff capabilities are described. During soft handoff a mobile station may have several transmission sources (active set members) supplying it with substantially the same information. Mobile stations which are not moving may not require as many sources to provide adequate received signal quality. Therefore, mobile stations which are not moving are identified using locating techniques and members of the active set are compared with a higher threshold to remove weaker branches more rapidly than when using a threshold that is applied to mobile stations that are in motion.

Description

Method and apparatus for controlling soft handover usage in a wireless communication system

Field of Invention

The present invention relates generally to the field of communication systems with soft handover capability, and more particularly to reducing interference by limiting the application of soft handover under certain circumstances.

Background technique

A simplified configuration diagram of a cellular communication system is shown in FIG. 1 . Mobile stations M1-M10 communicate with the fixed portion of the Public Switched Telephone Network (PSTN) by sending and receiving radio signals to and from cellular base stations B1-B10. The cellular base stations B1-B10 are in turn connected to the PSTN via a Mobile Switching Center (MSC). Each base station B1-B10 transmits signals within a corresponding area or "cell" C1-C10. Within each cell, the base station transmits to the mobile unit on a downlink RF channel, and the mobile unit transmits information to the base station on an uplink RF channel.

Although cellular systems were originally designed to operate with a one-to-one correspondence between mobile stations and associated base stations covering geographic cells, it has been determined that by transmitting the same signal to mobile stations over more than one link, The effects of shadow occlusion and fading can be reduced. For example, two different base stations may transmit the same message to a mobile station on two different spatially offset links. The mobile station processes the signals from the two links by selecting or combining in some way (eg maximum ratio combining) the signals from the two links. This technique is called diversity. Conventional space diversity techniques employ two or more separate antennas in a single base station or in two or more base stations to communicate with mobile stations. However, diversity is not limited to spatially offset base stations or antennas (ie, multiple transmission paths). Diversity transmission can also be achieved by using one or more offsets in time, polarization, or frequency.

One occasion where macrodiversity is commonly implemented is during handover. In this case, the candidate base station (ie, the base station to which the mobile station is to be handed over to) starts simultaneously sending the same message to the mobile station before the current serving base station terminates its message information transmission. This use of macrodiversity is often referred to as soft handover.

Fig. 2 shows a soft handover arrangement in which a first original base station 202 and a second candidate base station 204 respectively send the same message 206 to a mobile station 208. The message 206 is sent to the mobile station 208 on different signal paths in the form of a first downlink 210 and a second downlink 212 . The first and second downlink signals 210 and 212 are recombined (or one of the received signals is selected) in the mobile station 208 to extract the message 206 . Mobile station 208 transmits to base stations 202 and 204 on first and second uplinks 214 and 216, respectively. At some point in time, the sending of message information from the first former base station 202 to the mobile station is terminated and the soft handover procedure is terminated.

Soft handover can also be accomplished using multiple transmissions from a single base station. Figure 3 shows a single base station macrodiversity arrangement in which the first and second directional beams 318 and 320 produced by the antenna array 304 respectively cover different areas. Mobile station 308 may switch from one beam to another in the manner described above. That is, the first directional beam 318 maintains the first diversity link including the first downlink 310 carrying the message 306 . A second directional beam 320 establishes a second diversity link including a second downlink 312 that also carries the message 306 . First and second uplinks 314 and 316 are transmitted from mobile station 308 to antenna array 304 within each beam 318 and 320, respectively. Again, at some point in time, transmission from the original beam will cease in order to complete the handover. The same approach can be used for base stations with directional or sectored antennas, where soft handover can be performed between sectored antennas, a technique sometimes referred to as "softer handover".

In a soft handover arrangement, the base stations and/or antennas that communicate with a particular mobile station are referred to as "active set" members. For example, referring back to FIG. 2, base stations 202 and 204 may be considered members of the active set. Those skilled in the art will appreciate that more than two base stations and/or antennas may be part of the active set. Membership of the active set may change when a mobile station enters or leaves a coverage area steered by base stations and/or antennas in the system.

Soft handover can be used in many different types of wireless communication systems, including those using Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). Soft handover increases robustness to achieve improved downlink quality and overcome fading. However, soft handover can sometimes have a negative impact on system capacity and network resources due to the use of additional transmission sources to send substantially the same information to receivers.

Conventional soft handover systems typically use the same total amount of downlink transmit power for each antenna in the active set. For example, in the IS-95 system, the same transmit power level is used for all downlinks in the active set. Adding and removing members from the active set requires careful consideration to minimize interference in unrelated links due to unwanted interference to other users. Therefore, one method for controlling interference is to limit the number of base stations and/or antennas in the active set. Such a technique is described, for example, in published International Patent Application (PCT) WO/95/12297 by Gilhousen et al., in which the reverse link signal strength of the sectors involved in a soft handover is monitored. If the signal strength of a transmission from a sector falls below a predetermined threshold within a predetermined time interval, the base station does not continue to transmit from that sector.

Another method used in conventional systems to reduce the impact of unwanted interference from soft handover/macrodiversity operation is power split control. In power split control, downlink power may be split equally among each active base station and/or antenna in the active set. That is, where there are three downlinks and a total transmit power P can be provided, each downlink has a transmit power level of P/3. However, even with such an allocation, an unnecessary amount of interference may still be introduced when the "weakest" downlink in the active set is operating at a power level of P/3. More specifically, a link may in fact provide only a small improvement in communication robustness, but may cause relatively large disruptions to surrounding communications by excessively introducing interference. Therefore, the C/I ratio of adjacent cells will be negatively affected, with only a minimal gain in communication efficiency.

Recently, applicants have recognized that certain traffic models suggest that the majority of calls (eg, around 70%) in wireless communication systems are initiated by mobile units in relatively stationary locations. This phenomenon may be attributable to the rapid reduction in size of mobile units in recent years, which now allows users to conveniently carry mobile units, in contrast to earlier mobile units which were very large and suitable for leaving them in a car. This may be the case if a large number of such stationary calls are made from locations within the cell that will trigger the system's soft handoff function, a large number of stationary mobile units will continue between one or more successive calls Time to maintain soft switching mode. That is, since the mobile units are stationary, they will continue to receive transmissions from multiple sources without switching. As mentioned above, this will have a negative impact on the overall system capacity. Also, a mobile unit will not always be able to gain much additional diversity gain from being in soft handoff mode while stationary.

Accordingly, it would be desirable to provide methods and systems for limiting the application of soft handoffs when a mobile unit is in a stationary situation in order to increase system capacity.

Contents of invention

The present invention solves the above-mentioned problems by identifying those mobile stations that are in a stationary or substantially stationary state, and by adjusting the threshold used to remove the weakest member from the active set or unconditionally removing the weakest member. question. In this way, additional system capacity can be obtained without adversely affecting the connection quality of stationary mobile stations.

In one embodiment of the present invention, a method for performing handover in a wireless communication system, comprising the steps of: sending information from a plurality of sources to a remote station; determining whether said remote station is substantially stationary; If the remote station is substantially stationary, then: estimate transmissions from each of the plurality of sources by using the result of the determining step against at least one threshold; and based on the result of the estimating step: Selectively terminating transmission from at least one of the plurality of sources. Other embodiments include the step of estimating further comprising the step of estimating the transmission against a first threshold if the remote station is substantially stationary, and estimating the transmission against a second threshold otherwise. to send. The plurality of sources includes a plurality of base stations. The plurality of sources includes a plurality of antenna elements associated with a base station. Said first threshold is higher than said second threshold, wherein said step of selectively terminating may terminate a transmission source associated with a connection to a substantially dormant remote station. The determining step further includes the steps of performing a positioning function within the wireless communication system to obtain a location of the remote station, and estimating successive locations to determine whether the remote station is substantially stationary. The determining step also includes the step of detecting the speed of the remote station. The determining step also includes the step of estimating at least one received signal quality parameter associated with the remote station. The information sent to the remote station from each of the plurality of sources is the same. The information sent to the remote station from each of the plurality of sources is different. The estimating step also includes estimating transmissions from each of the plurality of sources for a preselected parameter if the remote station is moving, and otherwise identifying a weakest source of the plurality of sources.

In another embodiment of the present invention, a wireless communication system includes: transmitting means for transmitting substantially the same information from a plurality of sources to a remote station; determining means for determining whether said remote station is substantially stationary; estimating means for estimating transmissions from each of said plurality of sources by using the output of said determining means when said determining means determines that the remote station is substantially stationary; and terminating means for and selectively terminating transmissions from at least one of said plurality of sources based on an output of said estimating means. Other embodiments include: said estimating means comparing a quantity associated with each transmission with a first threshold if said remote station is substantially stationary, and comparing the quantity with a second threshold otherwise . The plurality of sources includes a plurality of base stations. The plurality of sources includes a plurality of antenna elements associated with a base station. The first threshold is higher than the second threshold, wherein the means for selectively terminating may terminate transmissions associated with connections to substantially dormant remote stations. Said means for determining further comprises: performing and estimating means for performing a positioning function within said wireless communication system to obtain the position of said remote station, and to estimate successive positions in order to determine said remote station Whether the station is substantially stationary.

In yet another embodiment of the present invention, a system for adjusting the active set of sources sending a signal to a particular receiver includes: position determining means for determining whether said receiver is substantially stationary; and an estimating function an element for comparing the signal strength of each source in said active set to a threshold value, said threshold value being dependent on said receiver, when said position determining means has determined said receiver to be substantially stationary Varies whether to be substantially stationary; and removes a source from the active set based on the result of estimating the functional element. Other embodiments include the position determining means being located within the receiver. The position determining means is provided in a part of the system other than the receiver.

An exemplary embodiment of the present invention uses location information about a mobile station to determine whether it is stationary. Any known positioning technique based on the mobile station itself or on the system can be used. If the system recognizes that a particular connection is associated with a stationary mobile station, the system uses a higher threshold in order to keep the source in the active set for that connection, potentially reducing the number of members in the active set .

Description of drawings

These and other objects, features and advantages of the present invention can be more easily understood by reading the following detailed description in conjunction with the accompanying drawings, wherein:

Figure 1 shows a conventional cellular communication system in which the present invention can be utilized;

Figure 2 shows a soft handover scenario where two separate base stations communicate with one mobile station;

Figure 3 shows a macrodiversity scenario where a single base station communicates with a mobile station by using an array antenna or multiple sector antennas emitting separate beams, each beam steering a different coverage area;

Figure 4 shows a scenario according to an example of the invention, where a stationary mobile station and a moving mobile station are receiving signals from two transmission sources;

FIG. 5 is a flowchart illustrating a method for controlling soft handover/macrodiversity according to an exemplary embodiment of the present invention.

Detailed ways

The following exemplary embodiments are provided for a CDMA wireless communication system. However, those skilled in the art will appreciate that this access method is for illustration purposes only, and that the invention can be readily applied to other types of access methods, including Frequency Division Multiple Access (FDMA), Time Division Multiple Access Access (TDMA), Code Division Multiple Access (CDMA) and their hybrid schemes.

Consider the example situation shown in FIG. 4 . Here, the base station 400 supports wireless communication services in the first cell, and the base station 410 supports wireless communication services in the second cell. Although not shown, base stations 400 and 410 are in communication with a Radio Network Controller (RNC) through a Mobile Switching Center (MSC), which in turn is connected to the Public Switched Telephone Network (PSTN). For this exemplary embodiment, consider that the system shown in FIG. 4 operates using CDMA techniques with duplex downlink (ie, base station to mobile station direction) and uplink (ie, mobile station to base station direction) channels. For the CDMA system of this example, a physical channel is identified by its code (ie, short, long, or a combination thereof), frequency, and bandwidth.

This example also shows two mobile stations 420 and 430 positioned close to the cell border between base stations 400 and 410 . Of course, those skilled in the art will appreciate that base stations 400 and 410 typically support connections to many mobile stations simultaneously, however, the interconnection between the mobile stations and the network in these two examples is sufficient to illustrate soft handover in accordance with the present invention / macro diversity technology. In this example, mobile station 420 is stationary or substantially stationary relative to the two base stations, while mobile station 430 is moving away from base station 400 and generally toward base station 410 in the direction of the arrow shown. However, both mobile stations are currently receiving transmissions from both base stations, ie, they are in soft handoff mode.

As is well known in the art, a mobile station will monitor adjacent sources (eg, base stations or antenna elements) for emissions while it is connected to the system. Information about received signal quality and or strength is returned to the system and used to place these adjacent sources in the mobile station's candidate set. At some time, one or more sources in the candidate set may be appended to the active set. For example, in many systems, and especially in CDMA systems, when a mobile station approaches a cell boundary, the mobile station may enter an area where another transmission source (e.g., another base station and/or another transmission from the same base station) An antenna unit) initially transmits substantially the same information to the mobile station, which can then combine the multiple received signals to produce a composite signal whose quality is superior to that obtained by receiving information from only one source.

The network can recognize that a base station should be added or removed from the active set by monitoring the signal strength at which the mobile station receives information from its neighboring sources and comparing the monitored values to predetermined thresholds. For example, as in Hyoung-Goo Jeon et al., entitled "A Channel Assignment Scheme for Reducing Call Blocking rate in a DS-CDMA Cellular System (used in DS-CDMA system to reduce the channel assignment scheme of call blocking rate)" paper As described in , the system may have a predetermined parameter T_DROP such that if the measured signal strength is lower than T_DROP, the source is removed from the active set, and a predetermined parameter T_ADD such that if the measured signal strength If the intensity exceeds T_ADD, the corresponding emitter is added to the active group.

Jeon's paper proposes that when the system load is very high, the value of T_DROP can be increased to release the traffic channels used in soft handover, so that these traffic channels can be reused to establish new calls and prevent call blocking. However, applicants have discovered that instead of waiting until system capacity is reached to resolve these problems, it is possible to reduce the active set associated with particular calls by increasing T_DROP when those calls are directed to stationary mobile stations. In this way, system capacity can be optimized without detrimental effects on the quality associated with ongoing connections.

Returning to the example of FIG. 4, the present invention therefore controls the active set associated with mobile stations 420 and 430 differently. More specifically, as shown in the flowchart of FIG. 5, the system first determines at step 500 whether the mobile station is stationary or substantially stationary. This judgment can be made in many different ways. For example, some wireless communication systems provide mechanisms for determining the location of a mobile station in order to quickly answer emergency calls initiated by the user of the mobile station. Thus, a first method for conducting the test described by step 500 is to use successive location values to determine whether the mobile station is moving. A second method for identifying a stationary mobile station is to determine the speed of the mobile station. This can be done by measuring the Doppler effect of the received signal at the mobile station to determine the radial velocity of the mobile station relative to the base station. A third method for carrying out this check is to periodically report measurements concerning quality parameters associated with the signal received by the mobile station. For example, checking whether the received signal strength, bit error rate, and/or frame error rate from a connection is stable can provide an indication as to whether the mobile station is stationary.

If the first technique described above is used, location information can be determined in a number of ways. A mobile station can estimate its own position and send a message to the system with its coordinates. This can be accomplished by equipping the mobile station with a Global Positioning System (GPS) receiver and receiving location information from a network of GPS satellites.

Alternatively, a base station that transmits signals to and receives signals from the mobile station may be used to determine the location of the mobile station. Various techniques have been proposed to provide mobile unit location information, including the attenuation of the mobile station's signal, the angle of arrival, and the difference between the time of arrival (TDOA) of the mobile station's signal at different base stations. . See, for example, Louis A Stilp's paper entitled "Time Difference of Arrival Technology for Locating Narrowband Cellular Signals", SPIE Vol. 2602, pp. 134-144.

A third class of techniques for locating a mobile station in a wireless communication system involves providing an additional system, i.e. this may be a system that is completely unrelated to the wireless communication system, or may share various components (e.g. antennas) with the wireless communication system but A system that processes signals from the communication system separately. This is advantageous as a stopgap solution to provide mobile unit location without modifying the large number of existing base stations in the system. More details on the use of examples of add-on systems can be found in U.S. Patent No. 5,327,144 to Stilp et al., entitled "Cellular Telephone Location System," the contents of which are incorporated herein by reference, for reference.

Referring now to the flowchart of FIG. 5, the system may determine whether the mobile station is stationary or substantially stationary at step 500 using any of the methods noted above or other techniques. After making this decision, the system selects an appropriate threshold for removing (and/or adding) emitters from the active set. For example, after the step of determining that the mobile station 420 is relatively stationary, the system can use the first T_DROP value (step 510) to determine whether to remove the transmission source from the active set. This may involve comparing the received signal strength (RSS) associated with each emission source with a selected T_DROP value at step 516 and selectively removing those emission sources that drop below that value at step 518 . Otherwise, if the system determines that the mobile station is moving, eg, as in the case of mobile station 430 . The system can then use the second value of the T_DROP parameter, as shown in step 520 .

Typically, the second value of T_DROP will be lower than the first value of the T_DROP parameter, so that soft handoffs will last longer and more often for those mobile terminals that require additional received signals to maintain acceptable quality. Those skilled in the art will appreciate that the first and second values for the T_DROP parameter will likely vary from implementation to implementation based on operator network preferences. Also, as an alternative to providing a first higher threshold value, once a mobile station is identified as stationary, the system can evaluate all members of the active set and remove the weakest emission source therefrom.

After taking a call out of soft handover mode, T_DROP is incremented and the signal quality (or mobile station's rest quality) can be monitored continuously or periodically. If the quality starts to degrade and/or the mobile starts to move, the call can be pulled back into soft handoff mode again and T_DROP will be lowered.

In some cases, it may be desirable for the mobile station to be informed of the applicable thresholds for use in determining whether it is eligible for soft handoff mode. For example, in some cases T_ADD and T_DROP are cell-level parameters that are provided on a broadcast control channel to all mobile stations in the cell. When a mobile unit is identified as stationary, a message is sent on the traffic channel to the mobile station informing it of new T_DROP and/or T_ADD values without changing the T_ADD and T_DROP of all mobile stations in the cell.

Although the invention has been described in detail with reference to preferred embodiments only, it will be apparent to those skilled in the art that various modifications may be made without departing from the invention. For example, although only two thresholds are described above, those skilled in the art will appreciate that more than two thresholds may exist and be modified depending on whether the mobile station is stationary or not. Furthermore, the threshold value does not necessarily need to be changed to implement the concept of adjusting the active set for substantially stationary mobile stations. For example, when a stationary mobile station is identified, the base station with the lowest received signal strength may reduce its transmit power until it falls substantially below the unmodified T_DROP threshold. Accordingly, the invention is defined only by the following claims which are intended to include all equivalents of the present invention.

Claims (20)

1. A method for performing handover in a wireless communication system, comprising the following steps: sending information from multiple sources to remote stations; determining whether the remote station is substantially stationary; If the remote station is substantially stationary, then: Estimated by using the result of said determining step against at least one threshold value counting transmissions from each of said plurality of sources; and selectively terminating from said plurality of sources based on the results of said estimating step at least one source of the send. 2. The method of claim 1, wherein said step of estimating further comprises the step of: if said remote station is substantially stationary, estimating said transmission for a first threshold, otherwise, for Estimate the transmission at a second threshold. 3. The method of claim 1, wherein said plurality of sources comprises a plurality of base stations. 4. The method of claim 1, wherein said plurality of sources comprises a plurality of antenna elements associated with a base station. 5. The method of claim 2, wherein said first threshold is higher than said second threshold, wherein said step of selectively terminating may terminate with a substantially stationary remote station connection to the source of the emission. 6. The method of claim 1, wherein said determining step further comprises the steps of: Within the wireless communication system, a positioning function is performed to obtain the location of the remote station, and to estimate successive locations to determine whether the remote station is substantially stationary. 7. The method of claim 1, wherein said determining step further comprises the steps of: The speed of the remote station is detected. 8. The method of claim 1, wherein said determining step further comprises the steps of: At least one received signal quality parameter associated with said remote station is estimated. 9. The method of claim 1, wherein the information sent to the remote station from each of the plurality of sources is the same. 10. The method of claim 1, wherein the information sent to the remote station from each of the plurality of sources is different. 11. The method of claim 1, wherein said step of estimating further comprises estimating transmissions from each of said plurality of sources with respect to a preselected parameter if said remote station is moving, Otherwise, the weakest source of the plurality of sources is identified. 12. A wireless communication system comprising: sending means for sending substantially the same information from a plurality of sources to a remote station; determining means for determining whether said remote station is substantially stationary; estimating means for estimating transmissions from each of said plurality of sources by using the output of said determining means when said determining means has determined that the remote station is substantially stationary; and terminating means for selectively terminating transmissions from at least one of said plurality of sources based on the output of said estimating means. 13. The system of claim 12, wherein if said remote station is substantially stationary, said estimating means compares the parameter associated with each transmission with a first threshold value, otherwise the parameter Compare with the second threshold value. 14. The system of claim 12, wherein said plurality of sources comprises a plurality of base stations. 15. The system of claim 12, wherein said plurality of sources comprises a plurality of antenna elements associated with a base station. 16. The system of claim 13, wherein said first threshold is higher than said second threshold, wherein said means for selectively terminating can terminate a connection to a substantially stationary remote The emission source associated with the connection of the end station. 17. The system of claim 12, wherein said means for determining further comprises: performing and estimating means for performing positioning functions within said wireless communication system to obtain the position of said remote station, and estimating successive positions to determine whether said remote station is substantially stationary. 18. A system for adjusting the active set of sources sending a signal to a particular receiver, comprising: position determining means for determining whether said receiver is substantially stationary; and an estimation function for comparing the signal strength of each source in the active set to a threshold value when the position determining means has determined that the receiver is substantially stationary, the threshold value being dependent on the Whether the receiver is substantially stationary varies; and A source is removed from the active set based on the result of estimating the functional element. 19. The system of claim 18, wherein said position determining means is located within said receiver. 20. The system of claim 18, wherein said position determining means is located in a portion of said system other than said receiver.

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