WO2002041605A2 - Procede et appareil pour effectuer un sauvetage simultane de plusieurs connexions dans des systemes de telecommunication - Google Patents

Procede et appareil pour effectuer un sauvetage simultane de plusieurs connexions dans des systemes de telecommunication Download PDF

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Publication number
WO2002041605A2
WO2002041605A2 PCT/US2001/048334 US0148334W WO0241605A2 WO 2002041605 A2 WO2002041605 A2 WO 2002041605A2 US 0148334 W US0148334 W US 0148334W WO 0241605 A2 WO0241605 A2 WO 0241605A2
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WIPO (PCT)
Prior art keywords
rescue
mss
potentially failing
connection
failing
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PCT/US2001/048334
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English (en)
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WO2002041605A3 (fr
WO2002041605A9 (fr
Inventor
Jason F. Hunzinger
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Denso Corporation
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Publication date
Priority claimed from US09/978,974 external-priority patent/US7133675B2/en
Application filed by Denso Corporation filed Critical Denso Corporation
Priority to AU2002236623A priority Critical patent/AU2002236623A1/en
Priority to JP2002543205A priority patent/JP4089432B2/ja
Publication of WO2002041605A2 publication Critical patent/WO2002041605A2/fr
Publication of WO2002041605A3 publication Critical patent/WO2002041605A3/fr
Publication of WO2002041605A9 publication Critical patent/WO2002041605A9/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
    • H04W16/02—Resource partitioning among network components, e.g. reuse partitioning
    • H04W16/06—Hybrid resource partitioning, e.g. channel borrowing
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/24—Reselection being triggered by specific parameters
    • H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
    • H04W36/305—Handover due to radio link failure
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W24/00—Supervisory, monitoring or testing arrangements

Definitions

  • the present invention relates, generally, to communication network management and, in one embodiment, to methods and apparatus for simultaneously preventing loss of signal and dropped connections between multiple mobile stations, such as cellular or PCS telephones, and a communication infrastructure (network).
  • multiple mobile stations such as cellular or PCS telephones
  • a communication infrastructure network
  • FIG. 1 depicts an example of a mobile station (MS) 10 operated by a mobile user that roves through a geographic area served by a wireless infrastructure or network including a first base station (BS) 12 with wireless sectors A 14 and sector B 16, and a second BS 18, with a sector C 20.
  • MS 10 travels from position A to position B to position C and will, as a matter of course, experience variations in signal strength and signal quality of the forward link associated with the BS(s) that it is in contact with.
  • a connection as referred to herein includes, but is not limited to, voice, multimedia video or audio streaming, packet switched data and circuit switched data connections, short message sequences or data bursts, and paging.
  • Dropped connections can range from being a nuisance to devastating for cellular telephone users. For example, a dropped emergency 911 connection can be critical or even fatal. Dropped connections can create consumer frustration significant enough to cause the consumer to change service providers. Thus, the prevention of dropped connections is of major importance to cellular network providers.
  • FIG. 2 illustrates an exemplary communication link 22 between a MS 24 and a BS 26. Communications from the BS 26 to the MS 24 are called the forward link, and communications from the MS 24 to the BS 26 are called the reverse link.
  • a BS 26 is typically comprised of multiple sectors, usually three. Each sector includes a separate transmitter and antenna (transceiver) pointed in a different direction. Because the term BS is often used to generally identify a transceiver, it should be understood that the terms BS and sector are used herein somewhat interchangeably.
  • the forward and reverse links utilize a number of forward and reverse channels.
  • the BS 26 broadcasts on a plurality of forward channels. These forward channels may include, but are not limited to, one or more pilot channels, a sync channel, one or more paging channels, and multiple forward traffic channels.
  • the pilot, sync, and paging channels are referred to as common channels because the BS 26 communicates those channels to all MSs. Generally, these common channels are not used to carry data, but are used to broadcast and deliver common information.
  • the multiple forward traffic channels are referred to as dedicated channels, because each forward traffic channel is intended for a specific MS 24 and may carry data.
  • Each sector within BS 26 broadcasts a pilot channel that identifies that sector and is simple for a MS 24 to decode. Both sectors and pilot channels are distinguished by pseudo-noise (PN) offsets.
  • PN pseudo-noise
  • the word "pilot" can be used almost interchangeably with the term sector, because a pilot channel identifies a sector.
  • the pilot channel implicitly provides timing information to the MS, and is also used for coherent demodulation, but it otherwise typically does not contain any data.
  • a MS When a MS is first powered up, it begins searching for a pilot channel.
  • a MS acquires (is able to demodulate) a pilot channel, the timing information implicit in the pilot channel allows the MS to quickly and easily demodulate a sync channel being transmitted by the network.
  • the MS is then able to acquire a paging channel being transmitted by the same BS that is transmitting the pilot channel. That BS is known as the active BS.
  • a "page" is transmitted to that MS on the paging channel of that BS.
  • the MS may then monitor that paging channel while the MS is idle and waiting for incoming connections or an incoming message.
  • each BS may utilize one pilot channel, one sync channel and one paging channel that are common for all MSs to receive.
  • some BSs may employ multiple paging channels.
  • the reverse channels may include an access channel and one or more reverse traffic channels. After a MS receives an incoming page from a BS, the MS will initiate a connection setup using, in part, an access channel.
  • the previously described channels may employ different coding schemes.
  • time division multiple access multiple channels may be communicated at a particular frequency within a certain time window by sending them at different times within that window.
  • channel X may use one set of time slots while channel Y may use a different set of time slots.
  • frequency division multiple access FDMA
  • multiple channels may be communicated at a particular time within a certain frequency window by sending them at different frequencies within that window.
  • code division multiple access CDMA
  • channels are defined by codes such as Walsh codes or quasi-orthogonal functions (QOF) such that the channels have minimal interference with one another even though they may be transmitted in the same frequency band and during the same time.
  • the data from each channel is coded using Walsh codes or QOFs and then combined into a composite signal.
  • This composite signal is spread over a wide frequency range at a particular time.
  • the original data may be extracted.
  • Walsh codes and QOFs create coded data that, when combined, don't interfere with each other, so that the data can be separated out at a later point in time to recover the information on the various channels.
  • two coded sequences of data are added together to produce a third sequence, by correlating that third sequence with the original codes, the original sequences can be recovered.
  • knowledge of the other codes is not necessary.
  • CDMA wireless communication system is fully described by the following standards, all or which are published by the TELECOMMUNICATIONS INDUSTRY ASSOCIATION, Standards & Technology Department, 2500 Wilson Blvd., Arlington, VA 22201, and all of which are herein incorporated by reference: TIA/EIA-95B, published February 1, 1999; and TIA/EIA/IS-2000, Volumes 1-5, Release A, published March 1, 2000.
  • the Walsh codes or QOFs are used to code a particular channel.
  • the simple to decode pilot channel may be the all one coded Wo Walsh code.
  • the sync channel may use the alternating polarity W 32 Walsh code and again, these codes are fixed and known.
  • Each MS groups the channels into various sets, which may include, but is not limited to, an active set, a neighbor set, a candidate set, and a remaining set.
  • the MS active set contains the pilots or PN offset identifiers that a MS is utilizing at any point in time. Thus, when a MS is idle, but monitoring a single BS for pages and overhead updates, the active set for that MS will contain that BS pilot or PN offset identifier as its only member.
  • a MS in communication with sector “A” in BS “A” will begin to communicate with a sector “B” in BS “A” without first dropping sector “A,” and as a result both sector “A” and “B” will be in the active set.
  • a MS in communication with BS “A” will begin to communicate with a BS “B” only after first dropping BS “A,” and as a result either BS “A” or “B” will be in the active set at any one time, but not both.
  • the MS assigns rake receiver fingers to multiple channels from one or more sectors at the same time.
  • the MS should be receiving the same data from both of those BSs.
  • the rake receiver will therefore receive encoded data from different sectors on different channels, demodulate those sectors independently, and then combine the data.
  • the data from a strong channel may be weighted more heavily than data from a weak channel, which is likely to have more errors.
  • the data with a higher likelihood of being correct is given higher weight in generating the final result.
  • a neighbor set which includes BSs that are neighbors to the active BS is received by the MS on a common channel.
  • the neighbor set is updated on a traffic channel.
  • any other BSs in the network that are not in the active, neighbor, or candidate sets comprise the remaining set.
  • the network whether a MS is idle or active, the network repeatedly sends overhead messages 30, 32 and 34 to the MS. These overhead messages contain information about the configuration of the network. For example, the extended neighbor list overhead message 34 tells the MS what neighbors exist and where to look for them. These neighbor identifiers are stored, at least temporarily, within the memory of the MS.
  • the candidate set is a set of BSs that the MS has requested as part of its active set, but have not yet been promoted to the active set.
  • FIG. 4 depicts a generic structure of a wireless infrastructure 56.
  • a client
  • the MS 36 continually monitors the strength of pilot channels it is receiving from neighboring BSs, such as BS 38, and searches for a pilot that is sufficiently stronger than a "pilot add threshold value.”
  • the neighboring pilot channel information may be communicated to the MS through network infrastructure entities including BS controllers (BSC) 40 that may control a cell cluster 42, or a mobile switching center (MSC) 44.
  • BSC BS controllers
  • MSC mobile switching center
  • the MS and one or more of these network infrastructure entities contain one or more processors for controlling the functionality of the MS and the network.
  • the processors include memory and other peripheral devices well understood by those skilled in the art.
  • the MS 36 As the MS 36 moves from the region covered by one BS 38 to another, the MS 36 promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the BS 38 or BSs of the promotion of certain pilots from the Neighbor Set to the Candidate Set via a Pilot Strength Measurement Message (PSMM).
  • PSMM also contains information on the strength of the received pilot signals.
  • the BS 38 determines a BS or network Active Set according to the Pilot Strength Measurement Message, and may notify the MS 36 of the new Active Set via an HDM. It should be noted, however, that the new active set may not always exactly comply with the MS's request, because the network may have BS resource considerations to deal with.
  • the MS 36 may maintain communication with both the old BS 38 and the new BS so long as the pilots for each BS are stronger than a "pilot drop threshold value.” When one of the pilots weakens to less than the pilot drop threshold value, the MS 36 notifies the BSs of the change. The BSs may then determine a new Active Set, and notify the MS 36 of that new Active Set. Upon notification by the BSs, the MS 36 then demotes the weakened pilot to the Neighbor Set. This is one example of a handoff scenario. It is typical for a MS 36 to be starting a handoff or in the process of handoff when connections fail. This is expected because poor coverage or weak signal environments generally exist near cell boundaries, in areas of pilot pollution, or areas significantly affected by cell breathing, all which are well known in the art.
  • FIG. 5 shows a situation known in the art as a Layer 2 Acknowledgment Failure for a CDMA wireless network.
  • the MS is transmitting a PSMM 48 requiring an acknowledgment by the BS.
  • the BS may be receiving it correctly, but in the case shown in FIG. 5, the MS is not receiving the BS's acknowledgment (ACK) 46.
  • FIG. 6 shows a second situation for which recovery is possible using the current invention in a CDMA wireless network.
  • This situation is known in the art as a Forward Link Fade Failure.
  • a fade is a period of attenuation of the received signal power.
  • a channel 58 may be broken up into slots 60, or superframes, typically of 80 millisecond duration. Each slot may be divided into three phases 62. These phases are numbered: 0, 1 and 2. Overlapping on top of the phases are four frames 64. These four frames are aligned with the three phases at the superframe boundaries. Each frame 64 is therefore typically 20 milliseconds long. Within each frame 64 is a header area 66, some signaling information 68 and perhaps some data 70. It should be understood that the content of the frames 64 can differ. One frame may contain signaling and data, another may contain only signaling, and yet another may contain only data.
  • Each frame 64 may also have a different data rate, which can be changed on a frame-by-frame basis.
  • information may be transmitted at a one-eighth frame rate, which would be beneficial because less power or bandwidth is required to communicate information at a slower rate.
  • the BS may therefore continuously instruct the MS, through power control bits in a configuration message, to transmit at various power levels to maintain an error rate of approximately one percent as the MS moves around in a particular area, or other types of interferences begin or end.
  • the MS typically abides by the power levels that are being recommended to it by the BS.
  • the BS can also change its transmitter power for a particular channel.
  • both the BS and the MS may continuously provide each other feedback in order to change the other's power levels.
  • the BS may not necessarily change its transmitter power levels based on the feedback from the MS.
  • error rates may not be controllable to about one percent as a MS moves about in a cellular network and experiences variations in signal strength and signal quality due to physical impediments, interference from adjacent channels, and positions near the edges of sectors, and as the error rates rise to intolerable levels, dropped connections become a problem.
  • a connection as referred to herein includes, but is not limited to, voice, multimedia video and audio streaming, packet switched data and circuit switched data calls, short message sequences or data bursts, and paging.
  • the procedure which will be generally referred to herein as the Forward Rescue Procedure (FRP), allows systems to recover from failures at the MS or BS that would otherwise result in dropped connections.
  • FRP Forward Rescue Procedure
  • Examples of failure scenarios that can be overcome using the FRP include forward link Layer 2 (L2) acknowledgement failures and loss of forward link signal due to a fade that causes loss of signal for a period of time exceeding a threshold value.
  • L2 forward link Layer 2
  • a MS will autonomously add BS pilot channels to the active set of its rake receiver in order to rescue the connection in danger of dropping.
  • the network infrastructure will initiate transmission on alternative forward link channels that are likely to be monitored by the MS during an FRP. If the same channels are monitored by the MS and transmitted on by the infrastructure, the connection in danger of dropping can be rescued.
  • the general FRP includes a MS FRP, and may also include an infrastructure FRP. FIG.
  • FIG. 8 illustrates an example of the timeline of the MS FRP and infrastructure FRP in a typical connection rescue.
  • the MS FRP is central to any rescue, the infrastructure FRP, although recommended, is not strictly necessary. Triggering of the MS FRP depends upon the type of failure that occurs. In the case of a Layer 2 failure, the FRP is activated upon a number of failed retransmissions of a message requiring acknowledgments. In the case of a Forward Link Fade Failure, the FRP is activated if there exists a loss of signal for a period of time exceeding a threshold value (see reference character 72). The MS starts an FRP timer at the time the rescue attempt is started (see reference character 74).
  • the MS turns off its transmitter and selects a new active set (see reference character 74).
  • the MS effectively assumes a handoff direction based on the PSMM(s) that it has sent (whether or not the PSMM was actually sent, successfully sent, or acknowledged).
  • the MS then begins to cycle through this new Active set searching for a rescue channel.
  • rescue channel encompasses the various schemes for defining channels as utilized by the various communication protocols, for purposes of simplifying the disclosure, a rescue channel will herein be identified as an Assumed Code Channel (ACC) (see reference character 78).
  • ACC Assumed Code Channel
  • the infrastructure FRP although recommended, is not strictly necessary for every BS in the network. If the infrastructure FRP is implemented (see reference character 80), the infrastructure (network) selects sectors from which it will transmit the ACC.
  • null (blank) data is transmitted over the ACC during rescue.
  • data may be communicated over the ACC, although a MS would only hear this data if it actually finds and successfully demodulates that ACC.
  • the MS will find and demodulate N 3M good frames of the ACC (see reference character 82), turn on its transmitter, and begins to transmit back to the BS.
  • the rescue is completed (see reference character 84) and the BS may re-assign the MS to more permanent channels. Additionally, the network may re-assign the ACCs via overheads, for example.
  • the BSs may also re-assign the MS active set to clean up after the rescue by sending a Rescue Completion Handoff message 86 which can re-use any existing handoff messages such as General or Universal Handoff Direction messages.
  • a Rescue Completion Handoff message 86 can re-use any existing handoff messages such as General or Universal Handoff Direction messages.
  • Embodiments of the present invention provide an efficient and safe procedure to rescue communication connections from dropping when multiple connections are failing simultaneously.
  • the simultaneous rescue of connections is applicable to both forward and reverse-based rescue procedures.
  • the network can assign rescue codes to the MSs, and simultaneous rescues can thereafter be initiated using rescue channels defined by the rescue codes.
  • the network may attempt to ensure that the MSs in need of rescue use different rescue codes, to the extent possible. This may be accomplished by having many rescue codes, strategically assigning rescue codes, pseudo-randomly assigning rescue codes to MSs (e.g. using an ESN-based hash), and the like.
  • each time a MS fails the remaining MSs may be assigned an equal distribution of rescue codes not being used by a failing MS, until there was only one unused rescue code remaining, at which time all remaining MSs would be assigned to that one unused rescue code.
  • its assigned rescue code can be made available again, and the strategic assignment of rescue codes can be revised to account for this newly available resource.
  • each time a MS fails the MS next most likely to fail would be assigned a rescue code not being used by a failing MS, and the remaining MSs would be assigned an equal distribution of the remaining rescue codes not being used by a failing MS.
  • the network can sequentially rescue the connections in danger of being dropped using rescue slots.
  • the rescue slot approach the rescue of simultaneously occurring failing connections are sequenced so that simultaneous rescues are actually avoided.
  • Individual MSs choose, or are assigned, different rescue slots in which to attempt a rescue.
  • a rescue slot may be defined to equal to a typical rescue duration, so that each rescue slot should provide enough time to effect a rescue.
  • the rescue slot may be equal to the MS transmit duration during a rescue attempt, or less than the MS transmit duration.
  • the network system time can be divided into rescue cycles and rescue slots, wherein each MS will be assigned to a particular rescue slot within a rescue cycle.
  • the length of the rescue cycle and the number of rescue slots in the rescue cycle may be defined by a particular communication standard, or may be configurable using overhead messages. Every MS uses the same system time reference to calculate when its assigned rescue slot occurs, and rescue can begin. Note that multiple MSs may be assigned to the same rescue slot.
  • FIG. 1 illustrates a roving mobile station moving amongst different locations between sectors in a wireless communication system.
  • FIG. 2 illustrates an exemplary communication link between a mobile station and a base station in a wireless communication system.
  • FIG. 3 illustrates overhead messages communicated from a base station to a mobile station in a wireless communication system. .
  • FIG. 4 illustrates a wireless communication infrastructure in communication with a roving mobile station.
  • FIG. 5 is a message sequence between a mobile station and a base station resulting in a dropped connection due to Layer 2 Acknowledgement failure.
  • FIG. 6 is a timeline that is representative of a dropped connection resulting from fading of the forward link in a wireless telecommunications network.
  • FIG. 7 is a timeline of a superframe, divided into three phases and four frames, for use in a wireless telecommunications network.
  • FIG. 8 is a timeline of one embodiment of the Forward Rescue Procedure when it is activated.
  • FIG. 9 illustrates an example of a simultaneous code-based reverse link rescue according to an embodiment of the present invention.
  • FIG. 10 illustrates an example of a simultaneous code-based forward link rescue according to an embodiment of the present invention.
  • FIG. 11 illustrates an example of how location can be incorporated into the strategic assignment of rescue codes according to an embodiment of the present invention.
  • FIG. 12 illustrates an example of transmission overlap in a sequential reverse-based rescue that does not utilize rescue slots.
  • FIG. 13 illustrates the basic concepts of a slot-based simultaneous rescue scheme according to an embodiment of the present invention.
  • FIG. 14 illustrates a common time frame called system time that has certain points of reference that define modulation code timing, channel timing, paging slot timing, and the like according to an embodiment of the present invention.
  • FIG. 15 illustrates that in the time slot simultaneous rescue approach, a MS can be rescued only during its assigned rescue slot, independent of when the failure was detected, according to an embodiment of the invention.
  • FIG. 16 illustrates that in the time slot simultaneous rescue approach, each rescue slot may be less than the MS transmit duration, creating the possibility of overlapping transmissions, according to an embodiment of the present invention.
  • the BS may send multiple rescue codes to the MSs in advance of any failures, and simultaneous rescues of failing connections can thereafter occur using different rescue channels defined by the rescue codes.
  • These rescue codes may be strategically distributed to the MSs and dynamically changed to minimize the chance that multiple MSs may attempt a rescue using the same rescue channel.
  • Walsh codes or QOFs define the rescue codes.
  • Walsh codes, long or short codes one type of QOF can also be used to define the rescue codes. Long codes are generally used for encryption on the forward link, and are used for MS channelization on the reverse link (i.e., each MS has its own long code).
  • FIG. 9 illustrates an example of a simultaneous code-based reverse link rescue 88 according to an embodiment of the present invention.
  • MSI is instructed to use CODE1 as a rescue channel via an overhead message 90
  • MS2 is instructed to use CODE2 as a rescue channel via overhead message 92.
  • Any number of strategies may be employed to assign rescue codes and rescue channels to MSs.
  • code-based rescue procedures may attempt to ensure that the MSs in need of rescue use different rescue codes, to the extent possible. This may be accomplished by having many rescue codes, strategically assigning rescue codes, pseudo-randomly assigning rescue codes to MSs (e.g.
  • MSI When MSI detects a failing connection, it transmits a reverse rescue channel 94 to the BS using CODEl, and when MS2 detects a failing connection, it transmits a reverse rescue channel 96 to the BS using CODE2.
  • these transmissions can occur at the same time, because they use different rescue channels, as defined by rescue codes CODEl and CODE2.
  • the BS will find and demodulate a predetermined number of good frames of the rescue channels defined by CODEl and CODE2 and begin to transmit back to MSI and MS2. Once MSI, MS2 and the BS receive a predetermined number of good frames, the rescue is completed, the connection can be continued, and the BS may re-assign the MS to more permanent channels.
  • FIG. 10 illustrates an example of a simultaneous code-based forward link rescue 98 according to an embodiment of the present invention.
  • MSI prior to detecting any connections in danger of failing, MSI is instructed to use CODEl as a rescue channel via overhead message 100, and MS2 is instructed to use CODE2 as a rescue channel via overhead message 102.
  • any number of strategies may be employed to assign rescue codes and rescue channels to MSs.
  • the BS transmits a forward rescue channel 104 to MSI using CODEl
  • MS2 detects a failing connection
  • the BS transmits a forward rescue channel 106 to MS2 using CODE2.
  • FIG. 10 illustrates an example of a simultaneous code-based forward link rescue 98 according to an embodiment of the present invention.
  • embodiments of the present invention may attempt to ensure that the MSs in need of rescue use different rescue codes, to the extent possible.
  • MSI and MS2 will find and demodulate a predetermined number good frames of the rescue channels defined by CODEl and CODE2, turn on their transmitters, and begin to transmit back to the BS.
  • MSI, MS2 and the BS receive a predetermined number of good frames, the rescue is completed, the connection can be continued, and the BS may re-assign the MS to more permanent channels.
  • rescue codes may be strategically assigned to MSs in advance of the failure, to minimize the probability that multiple failing MSs will attempt to rescue using the same rescue channel.
  • the two rescue codes may be initially distributed randomly but evenly to all MSs. If one MS with an assigned rescue code of CODEl is detected as failing, all other MSs can be assigned CODE2.
  • CODE2 can be assigned to the MS next most likely to fail (described in greater detail below), and all other MSs would be assigned a 50/50 distribution of the two rescue codes. By doing so, if a second MS fails before the first failing MS is rescued, the second failing MS will likely use a different code from the first failing MS. In addition, this embodiment leaves all of the remaining MSs assigned equally to CODEl and CODE2, so that if more than two MSs simultaneously fail, the distribution of MSs trying to rescue using CODEl and CODE2 will likely be more evenly distributed.
  • This dynamic distribution of rescue codes can be extended to situations where more than two rescue codes are available, and to identifying and assigning rescue codes to a succession of MSs determined to be likely to fail.
  • the MS next most likely to fail would be assigned a rescue code not being used by a failing MS, and the remaining MSs would be assigned an equal distribution of the remaining rescue codes not being used by a failing MS.
  • all of the remaining MSs would be assigned an equal distribution of all rescue codes.
  • the embodiment described above also attempts to minimize the probability that multiple failing MSs will attempt to rescue using the same rescue channel, but additionally takes into account empirical evidence of MSs that may be likely to fail.
  • the network may assign a list of several rescue codes to each MS, and the MS will then perform a hashing function or other selection methodology to choose one of the rescue codes.
  • the network will perform the same hashing function or selection methodology to identify which rescue code was actually chosen by the MS.
  • the BS could identify the MS that is receiving the weakest signals, based on PSMMs received from the MSs. Alternatively, the BS could identify a MS reporting the same type or pattern of pilot energies as the failing MS in a PSMM received from that MS. Such MSs may be in the same location as the failing MS, or may be experiencing the same type of communication difficulties as the MS, and therefore may fail in the near future. In another embodiment, if the MSs have locational capability, MSs in the same area as the failing MS could be identified as the next most likely to fail.
  • the BS could detect that a number of bad frames were received from a particular MS, although not enough to trigger the start of a rescue procedure. Such a MS could be identified as the MS next most likely to fail. Alternatively, the BS could detect that no proper acknowledgement was received from a MS after a number of retransmissions of a message by the BS, although not enough to trigger the start of a rescue procedure.
  • a combination of factors could be used, such as identifying the MS that transmitted a number of bad frames (but not enough to trigger the start of a rescue procedure), and was in a particular location. It should be evident from the description of this hybrid embodiment that any number of variations of the above-described examples involving identifying the MS next most likely to fail, fall within the scope of the present invention.
  • the strategic assignment of rescue codes may also take into account MS location, if the MSs have locational capability. The rescue codes could be initially assigned evenly based on MS locations within the network. A further embodiment incorporating location into the strategic assignment of rescue codes is illustrated in FIG. 11, wherein only two rescue codes CODEl and CODE2 are available for purposes of simplicity of illustration only. In FIG.
  • the assignment of a MS to a rescue code may be based on the priority of the MS. For example, government employees or users who pay a higher service fee may be given higher priority for rescue codes, or may be given rescue codes not assigned to any other MS. Assignment may also be based on the type of connection (voice, data, packet data, etc.), the order in which MSs failed in the past, or the likelihood that a MS will have a successful rescue. In determining the likelihood that a MS will have a successful rescue, the network may consider the frequency and timing of past rescues and other indicators such as the receipt of bad frames that are suggestive of a poor connection.
  • the network can sequentially rescue the connections in danger of being dropped using rescue slots.
  • this rescue slot approach the rescue of simultaneously occurring failing connections is sequenced so that simultaneous rescues are actually avoided. Nevertheless, this approach will be referred to herein as a "simultaneous" rescue approach, because the failures occur at or near the same time.
  • FIG. 12 provides an overview of a sequential reverse-based rescue not employing rescue slots. In FIG. 12, MSI is detected as failing at time 112, and MS2 is detected as failing at time 114.
  • FIG. 12 illustrates an overlap 120 in which multiple MSs will try to transmit the same rescue channel at the same time, creating interference that may possibly prevent rescue. It should be understood that such a situation is not improbable, because simultaneous dropped connections can occur during peak usage time, or in the same location (in close proximity) because of multiple MSs experiencing similar conditions, and the same BS may also be needed to rescue to connection. This transmission overlap need not occur in forward based rescue procedures, because the network can sequence the transmission of the rescue channel such that only one MS is rescued at a time.
  • Embodiments of the present invention improve upon the sequential rescue scheme described above by using rescue slots.
  • individual MSs choose, or are assigned, different rescue slots in which to attempt a rescue.
  • FIG. 13 illustrates the basic concepts of a rescue slot-based simultaneous rescue scheme according to an embodiment of the present invention.
  • a rescue channel e.g. CHI
  • MSI e.g. MSI, MS2, etc.
  • rescue channel e.g. CHI
  • a plurality of rescue channels assigned to the MSs may be transmitted at each rescue slot.
  • the rescue slot approach requires that the MSs and the network operate in accordance with some predetermined time reference.
  • BSs and MSs operate on a common time frame called system time that has certain points of reference that define modulation code timing, channel timing, slot timing, and the like.
  • system time illustrated in FIG. 14, a paging channel 132 may be split into 80 ms superfra es 134, each superframe containing four 20 ms frames 136. The superframe is also split into three phases 138. Each superframe 134 represents a portion of a paging slot 140 within a paging cycle 142.
  • the actual assignment of a MS to a paging slot within a paging cycle may be defined by a hashing formula based on the MS's ESN and other parameters, and is therefore pseudo-random. Note that multiple MSs may be assigned to the same paging slot. As defined by a slot cycle index, a paging slot repeats at predictable intervals. During each paging slot, an idle MS assigned to that paging slot "wakes up" and looks for messages directed to it, such as a page.
  • system time can be similarly divided into rescue cycles and rescue slots, wherein each MS will be assigned to a particular rescue slot within a rescue cycle.
  • the length of the rescue cycle and the number of slots in the rescue cycle may be defined by a particular communication standard, or may be configurable using overhead messages for adapting to load conditions. Every MS uses the same system time reference to calculate when its assigned slot occurs, and rescue can begin.
  • the actual assignment of a MS to a rescue slot within a rescue cycle may be defined by a hashing formula based on the MS's ESN and other parameters, and is therefore pseudo-random. Note that multiple MSs may be assigned to the same rescue slot.
  • the assignment of a MS to a rescue slot may be based on location of the MS, similar to the assignment of a MS to a rescue code based on location as discussed with reference to FIG. 11.
  • the assignment of a MS to a rescue slot may be based on the priority of the MS. For example, government employees or users who pay a higher service fee may be given higher priority for rescue slots, or may be given rescue slots not assigned to any other MS. Assignment may also be based on the type of connection (voice, data, packet data, etc.), the order in which MSs failed in the past, or the likelihood that a MS will have a successful rescue. In determining the likelihood that a MS will have a successful rescue, the network may consider the frequency and timing of past rescues and other indicators such as the receipt of bad frames that are suggestive of a poor connection.
  • rescue does not necessarily begin at the time when an L2 Acknowledgement of a Forward Link Fade failure occurs, or when the FRP is activated.
  • a MS is assigned a rescue slot, that MS can be rescued only during its assigned rescue slot, independent of when the failure was detected. For example, as illustrated in FIG. 15, if a failure is detected at time 146, the MS must wait a time period 148 until the rescue can be completed at time 150. However, if a subsequent failure is detected at time 152, the MS must wait a time period 154, where time period 154 > time period 148 in the present example, until the rescue can be completed at time 156. Note that the delay should not be more than one complete rescue cycle 162 minus the slot time 164.
  • any number of slots may be made available for assignment to MSs.
  • the number of slots are not limited to slots within a superframe, or other predefined time boundaries in system time.
  • the rescue cycle can be tied to the paging channel frame for the paging channel cycle, which can be minutes in duration, and thus many slots would be available within such a rescue cycle.
  • a MS may have to wait too long for its turn to be rescued.
  • too few slots are available, the likelihood increases that multiple MSs in need of rescue will be assigned to the same slot.
  • further embodiments take into account the rescue cycle, number of MSs, the number of MSs likely to fail at any one time, and the like, and empirically determine the number of rescue slots which minimizes both the rescue wait time and the likelihood that multiple MSs in need of rescue will be assigned to the same slot.
  • a slot may be defined to equal to a typical rescue duration, so that each rescue slot should provide enough time to effect a rescue.
  • the rescue slot may be equal to the MS or BS transmit duration (depending on whether the rescue procedure is reverse or forward based) during a rescue attempt.
  • the MS or BS transmit duration is shorter than the duration of the rescue, and thus each rescue slot will not provide enough time for a rescue to be completed.
  • simultaneous transmissions will be avoided, thereby eliminating the interference problems associated with having multiple MSs transmitting at the same time.
  • each rescue slot 158 may be less than the MS transmit duration 162 during a rescue attempt. This creates the possibility of overlapping transmissions 164, but allows for more slots to exist within a rescue cycle. Even with a transmission overlap, the network still has a period of time 160 in which only one MS is likely to be transmitting for rescue, and in which the BS need only respond to that particular MS. In this embodiment, although transmission overlap and interference is possible, because there are more rescue slots 158, the chance of having two failing MSs assigned to the same or adjacent rescue slots and encountering this transmission overlap is lower. Conversely, as the number of rescue slots and overlap decreases, the chance of having two failing MSs assigned to the same or adjacent rescue slots, where interference is more likely, goes up.
  • PG SLOT is the paging slot assigned to that MS
  • CR_SLOTS is the total number of rescue slots.
  • the above equation assumes PG_SLOT > CR_SLOTS.
  • the network may set CR_SLOTS to the known total number of rescue slots, or to a value that indicates an entry in a lookup table or formula that provides the number of rescue slots indirectly.
  • CR_SLOT [t/Cl + C2] mod CR_SLOTS, where t is the system time at the assigned slot, Cl is the slot time, and C2 is an offset factor.
  • a "rescue delay timer" which delays the start of rescue, is set to an initial value CR_DELAY_TIME (in frames) and started when a rescue procedure is initiated. Rescue is prohibited until the rescue delay timer reaches its terminal count.
  • the total rescue waiting period for the MS may be set to the following initial value by the MS: (CR_DELAY_TIME x 80 + CR_SLOT_ALIGN x 20) ms, where CR_SLOT_ALIGN is the minimum number of frames that the MS must wait, independent of CR_DELAY_TIME, to arrive at the next occurrence of the MS's assigned slot at system time t.
  • CR_SLOT_ALIGN is the minimum number of frames that the MS must wait, independent of CR_DELAY_TIME, to arrive at the next occurrence of the MS's assigned slot at system time t.
  • the addition of CR_SLOT_ALIGN ensures that rescue transmission from MSs that simultaneously drop calls are offset in time with high probability.
  • the rescue code and rescue slot embodiments for simultaneous rescue are not mutually exclusive. In other embodiments of the present invention, these two schemes may be combined. For example, two failing MSs, assigned to the same rescue slot, may nevertheless be rescued simultaneously if the two MSs are assigned different rescue codes. Conversely, two failing MSs, assigned to the same rescue code, may nevertheless be rescued simultaneously if the two MSs are assigned to different rescue slots. It should be understood that various combinations of the features of code-based rescue and slot-based rescue, described above, fall within the scope of the present invention.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

L'invention concerne un procédé de sauvetage de connexions de communication risquant d'être perdues lors d'un échec simultané de plusieurs connexions. Le sauvetage simultané de connexions peut s'appliquer à la fois à des procédés de sauvetage aller et à des procédés de sauvetage retour. Dans un mode de réalisation, le réseau peut affecter des codes de sauvetage aux MS, puis on peut lancer des sauvetages simultanés au moyen de canaux de sauvetage définis par lesdits codes de sauvetage. Afin de minimiser les risques de collision, le réseau peut essayer de s'assurer que les MS nécessitant un sauvetage utilisent des codes de sauvetage différents, lorsque les circonstances le permettent. Dans un autre mode de réalisation non mutuellement exclusif, le réseau peut effectuer un sauvetage séquentiel des connexions risquant d'être perdues, au moyen de fentes de sauvetage. Dans le mode de réalisation comportant les fentes de sauvetage, des MS individuels choisissent différentes fentes de sauvetage, ou différentes fentes de sauvetage leur sont attribuées, en vue d'essayer d'effectuer un sauvetage dans lesdites fentes.
PCT/US2001/048334 2000-11-14 2001-11-06 Procede et appareil pour effectuer un sauvetage simultane de plusieurs connexions dans des systemes de telecommunication WO2002041605A2 (fr)

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AU2002236623A AU2002236623A1 (en) 2000-11-14 2001-11-06 System for rescuing multiple mobile station connections
JP2002543205A JP4089432B2 (ja) 2000-11-14 2001-11-06 遠距離通信システムにおける複数の接続の同時救済のための方法

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US24894700P 2000-11-14 2000-11-14
US60/248,947 2000-11-14
US09/978,974 US7133675B2 (en) 2000-10-17 2001-10-16 Forward link based rescue channel method and apparatus for telecommunication systems

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