WO1998049858A1 - A cellular telephone system and a method of conducting an inter-system soft handoff - Google Patents
A cellular telephone system and a method of conducting an inter-system soft handoff Download PDFInfo
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- WO1998049858A1 WO1998049858A1 PCT/US1998/008368 US9808368W WO9849858A1 WO 1998049858 A1 WO1998049858 A1 WO 1998049858A1 US 9808368 W US9808368 W US 9808368W WO 9849858 A1 WO9849858 A1 WO 9849858A1
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- queue length
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- 230000001413 cellular effect Effects 0.000 title claims abstract description 72
- 238000000034 method Methods 0.000 title claims abstract description 33
- 238000012544 monitoring process Methods 0.000 claims description 10
- 230000000977 initiatory effect Effects 0.000 claims description 5
- 238000005259 measurement Methods 0.000 abstract description 7
- 238000004891 communication Methods 0.000 description 11
- 238000012545 processing Methods 0.000 description 10
- 230000011664 signaling Effects 0.000 description 9
- 230000005540 biological transmission Effects 0.000 description 6
- 238000010586 diagram Methods 0.000 description 4
- 230000008901 benefit Effects 0.000 description 3
- 230000003139 buffering effect Effects 0.000 description 2
- 230000010267 cellular communication Effects 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 230000004044 response Effects 0.000 description 2
- 230000007704 transition Effects 0.000 description 2
- 230000008859 change Effects 0.000 description 1
- 238000005315 distribution function Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000010295 mobile communication Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/16—Performing reselection for specific purposes
- H04W36/22—Performing reselection for specific purposes for handling the traffic
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/14—Reselecting a network or an air interface
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/16—Performing reselection for specific purposes
- H04W36/18—Performing reselection for specific purposes for allowing seamless reselection, e.g. soft reselection
Definitions
- the present invention relates to a cellular telephone system and a method of conducting an inter-system soft handoff. More specifically, the present invention relates to a novel and improved method and apparatus for conducting telephone calls or other communications between wireless telecommunications systems that supports inter-system soft handoff.
- FIG. 1 is a diagram of a cellular telephone system configured in accordance with the use of the IS-95 over-the-air interface standard.
- the IS- 95 CDMA over the air interface standard (IS-95) promulgated by the Telecommunications Industry Association (TIA) defines a protocol for providing wireless telephone service via code division multiple access (CDMA) modulated RF signals.
- IS-95 CDMA over the air interface standard
- TIA Telecommunications Industry Association
- CDMA code division multiple access
- IS-95 has many derivatives and related standards including IS-95-A, ANSI J-STD-008, IS-99 and IS-657, referred to herein collectively as "IS-95."
- a cellular telephone system configured substantially in accordance with the use of IS-95 is described in US patent 5,103,459 entitled “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System” assigned to the assignee of the present invention and incorporated herein by reference.
- subscriber units 10 (typically cellular telephones) conduct communications by interfacing with base stations 12 using CDMA modulated RF signals. From base stations 12 the communications are further conducted using base stations controllers (BSC's) 14 and mobile switching centers (MSC's) 16 with public switched telephone network (PSTN) or other subscriber units 10. BSC's 14 provide call mobility functionality, as described in greater detail below, and MSC's 16 provides call routing, billing and switch interface functionality. Additionally, the data exchanged between the various systems shown in FIG. 1, including subscriber units 10 and base stations 12, are processed in frames. This data can be either traffic data which includes voice information, or signaling which is used to configure and control the various systems shown.
- BSC's 14 base stations controllers
- MSC's mobile switching centers
- PSTN public switched telephone network
- PSTN public switched telephone network
- BSC's 14 provide call mobility functionality, as described in greater detail below
- MSC's 16 provides call routing, billing and switch interface functionality.
- CDMA technology allows adjacent base stations in an IS-95 system to conduct communications over the same RF bandwidth, which increases the frequency reuse factor of the cellular telephone system when compared to other types of cellular telephone technology.
- increasing the frequency reuse factor increases the efficiency with which the cellular telephone system uses the available RF bandwidth, which is one of the advantages of the IS-95 standard.
- Soft handoff can be used to transition a subscriber unit 10 from the coverage area of a first base station 12 to the coverage area of a second base station 12.
- Soft handoff involves simultaneously interfacing a subscriber unit 10 with two or more base stations as the subscriber unit transitions between the associated coverage areas of the two base stations 12.
- One of the subscriber units 10 shown in FIG. 1 is in soft handoff.
- Soft handoff can be contrasted with hard handoff, where the interface with the first base station 12 is terminated before the link with the second base station has been established. Maintaining at least one link at all times improves the reliability and quality of the communication being conducted.
- BSC's 14 perform various functions that allow mobile communications to be conducted including functions that facilitate soft handoff.
- BSC's 14 perform "frame selection” and "frame distribution" for the set of base stations 12 involved in the soft handoff call.
- Frame selection is the selection of one frame for further processing from the set of frames received from the set of base stations used during a soft handoff. Frame selection maintains the highest quality link by selecting the best frame available at all times.
- Frame distribution is the duplication and distribution of frames directed to the subscriber unit 10 to the set of base stations 12 involved in the call. Frame distribution allows each base station 12 to transmit a copy of the frame to the subscriber unit 10 increasing the probability of successful reception of at least one frame.
- subscriber unit 10A is interfacing with a single base stations 12A and is located near the coverage area of a second base station 12B. Base stations 12A and 12B, however, are coupled to different BSC's 14. This complicates the process of performing soft handoff because neither BSC 14 has access to both base stations 12A and 12B.
- Inter-system soft handoff Performing soft handoff between two base stations 12 that are controlled by two different BSC's 14 is referred to as "inter-system soft handoff.” Inter-system soft handoff can be contrasted with "intra-system soft handoff” in which a soft handoff is conducted between two base stations 12 controlled by the same BSC.
- inter-system soft handoff For inter-system soft handoff, frame selection cannot be performed as easily because neither BSC 14 has access to the set of frames that are produced by the two base stations 12. Additionally, frame distribution is also rendered more difficult because neither BSC can transmit the set of frames generated to both base stations. Nonetheless, it is necessary to use two or more BSC's 14 in many large metropolitan areas because the number of base stations required to provide adequate coverage exceeds the capabilities of a single cellular system formed using a single BSC 14, which in turn creates the need to conduct inter-system soft handoff.
- a system and method for performing an inter-system soft handoff is described in copending US patent application serial no. 08/649,959 entitled "Inter-system Calling Supporting Inter-System Soft Handoff" assigned to the assignee of the present invention and incorporated herein by reference.
- a channel between the two BSC's 14 is established via an interconnect for exchanging the data necessary to perform soft handoff.
- each BSC is required to either store or obtain information typically tracked within an MSC, and therefore must interact with an MSC in order to perform the soft handoff.
- the interconnect is comprised of a standard T-l or E-l link, which are well known and therefore relatively inexpensive to implement.
- signaling messages are exchanged between the two BSC's 14 involved in the call.
- the information contained in the signaling messages require each BSC 14 to track certain information including the number of calls being conducted over the interconnect, or interact with an MSC tracking that information, and to provide the other BSC 14 with certain predefined types of information.
- the number of calls on the interconnect are tracked to avoid overloading the interconnect.
- Requiring BSC's 14 to obtain, track or provide certain types of information is highly restrictive, and can be difficult for some manufacturers of BSC's to implement. Additionally, it requires a significant degree of coordination and cooperation between BSC and MSC manufactures, which is not always feasible. Thus, a system and method of performing inter-system soft handoff that requires less cooperation and coordination between BSC's 14 and MSC's 16 would be highly desirable.
- An object of the present invention is to provide a novel and improved method and apparatus for operating a cellular telephone system that supports performing an inter- system soft handoff.
- the invention provides a method for conducting an inter-system soft handoff of a call made by a subscriber unit between a first cellular system and a second cellular system, the method comprising a) monitoring a traffic level exchanged between the first cellular system and the second cellular system; and b) granting a soft handoff request based on said traffic level.
- the invention provides A cellular telephone system capable of conducting an inter-system soft handoff with a second cellular telephone system, the cellular telephone system comprising: a cellular system interface port for monitoring a traffic level exchanged with the second cellular telephone system; a selector for initiating an inter system soft handoff; and an admission control subsystem for admitting said inter- system soft handoff and rejecting said inter-system soft handoff based on said traffic level.
- the invention provides a controller for controlling hand off between two cellular telephone systems, the controller comprising means for monitoring data flow between the two systems and determining therefrom whether or not to allow hand off between the two systems.
- a subscriber unit generates a pilot strength measurement report when the pilot channel from a base station is detected.
- a base station controller receiving the pilot strength measurement report determines that the base station is part of a second cellular telephone system
- an inter-system soft handoff request is generated.
- An admission control subsystem receives the inter-system soft handoff request and either grants or denies the request based on the traffic level on the interconnect with the second cellular telephone system. For example, if the traffic level is too high, the inter-system soft handoff request is rejected.
- the traffic level is determined based on link load messages received periodically by the admission control subsystem that are generated by an interface port coupled to an interconnect between the first cellular telephone system and the second cellular telephone system.
- the link load message include peak queue length and average frame rate information.
- FIG. 1 is a block diagram of a set of cellular systems configured in accordance with the prior art
- FIG. 2 is block diagram of a set of cellular telephone systems configured in accordance with one embodiment of the invention
- FIG. 3 is a flow chart illustrating the steps performed in granting or declining an inter-system soft handoff request when performed in accordance with one embodiment of the invention.
- FIG. 2 is a block diagram of a portion of a set of cellular telephone systems configured in accordance with one embodiment of the invention.
- Base station controllers (BSC's) 24A and 24B are coupled to base stations 22 and to mobile switching center (MSC) 16 of FIG. 1 (MSC is not shown in FIG. 2).
- Subscriber unit 20 interfaces with base station 22 A via RF signals modulated in accordance with the IS-95 standard, and therefore using CDMA modulation techniques.
- Subscriber unit 20 is shown a similar distance from base stations 22A and 22B, and therefore entering the coverage area of base stations 22B.
- base station 22A is coupled to BSC 24A and base station 22B is coupled to BSC 24B.
- BSC 24A is shown in additional detail to illustrate a preferred configuration in accordance with one embodiment of the invention.
- - CDMA interconnect subsystem (CIS) 30 is coupled to base stations 22A and 22D, selector subsystem 40, call control processor (CCP) 42, admission control subsystem 44, and interconnect 46 via base station communication network (BCN) ports 32a - 32f.
- BCN ports 32a provides frame buffering by queuing both inbound and outbound frames.
- Selector 40 is coupled to MSC 16 of FIG. 1, and interconnect 46 is coupled to BSC 24B.
- Interconnect 46 is typically a Tl or El connection, the use of which is well known in the art.
- each network packet contains the address of the subsystem to which it is directed.
- Selector subsystem 42 performs the frame selection and frame distribution functions for soft handoff as described above.
- CCP 42 allocates and deallocates resources for processing the call including signal processing resources within base stations 22 and selector resources within selector subsystem 40.
- PSMR pilot strength measurement report
- the selector Upon receipt of the PSMR, the selector will determine that the pilot channel from base station 22B is being detected by subscriber unit 20. Using a pilot database (PDB) located within selector subsystem 40 (not shown) the selector subsystem 40 determines that the pilot channel is from a base station coupled to BSC 24B, rather than from a base station coupled to BSC 24A, and therefore not directly accessible by selector subsystem 40. In response to this determination, the selector subsystem transmits an inter-system soft handoff request to admission control subsystem 44 specifying the BSC 24 with which the inter-system soft handoff is to be conducted.
- PDB pilot database
- admission control subsystem 44 In addition to inter-system soft handoff requests, admission control subsystem 44 also receives traffic report messages from BCN port 32f, as well as any other BCN ports 32 coupled to other BSC's 24. (Typically, BSC 24A will be coupled to several other BSC's 24, although only one is shown in FIG. 2 to facilitate understanding of the invention.) Using the traffic reports, admission control subsystem 44 either accepts or rejects the inter-system soft handoff request and indicates such to selector subsystem 40 via a signaling message. If the inter-system soft handoff request is rejected, the transfer of subscriber unit 20 to the coverage area of base station 22B may be performed using a hard handoff via the MSC
- selector subsystem 40 transmits an inter-system soft handoff initiation message to BSC 24B, and begins to copy and transmit forward link frames directed to subscriber unit 20 to BSC 24B via interconnect 46.
- BSC 24B responds by instructing base station 22B to allocate signal processing resource for conducting the call, and by forwarding the forward link frames received to base station 22B which transmits the frames to subscriber unit 20 via CDMA modulated RF signals. Additionally, BSC 24B forwards reverse link frames received by base station 22B from subscriber unit 20 to BSC 24A where they are routed to selector subsystem 40 for frame selection.
- a steady state inter-system soft handoff has been established with a minimum of signal exchange and processing by BSC 24B.
- the ability of an inter-system soft handoff to be established by BSC 24A with a minimum amount of coordination and cooperation has been provided.
- BCN port 32f transmits link load messages to admission control subsystem 44.
- the link load messages are transmitted periodically at a period Ts a p ieLoad/ and indicate R ave / the average frame reception rate of BCN port, and Q peak the peak transmission queue length, in bytes, of BCN port 32f over the last period T Sample oad-
- R ave is the total number of good frames received by BCN port 32f from BSC 24B during the previous period T Samp ie Load divided by the duration of the period T Samp ieLoad-
- Q peak is the peak queue length, in bytes, of BCN part 32f over the last period T Samp ⁇ eL ⁇ ad-
- FIG. 3 is a flow chart illustrating the step performed by admission control subsystem 44 to determine whether to admit or reject an inter- system soft handoff request. The processing begins at step 50 and at step 52 the inter-system soft handoff request is received. At step 54 it is determined whether the average frame reception rate R ave is greater than a frame reception rate threshold R ⁇ , and if so the inter-system soft handoff request is rejected at step 56 and the procedure terminates at step 70.
- step 60 it is determined at step 60 whether the peak transmission queue length Q peak is greater than a queue length threshold Q ⁇ , and if so the inter-system soft handoff request is rejected at step 56 and the procedure terminates at step 70. If it is determined that the peak queue length Q peak s not greater than a peak queue length threshold Q ⁇ , the inter-system soft handoff is granted at step 62 and the procedure terminates at step 70.
- the thresholds QT and RT are set based the data capacity of interconnect 46 and the sample period T SampleLoad , with the preferred sample period T SampleLoad equal to two (2) seconds.
- the peak queue length threshold Q T is set based on the number of T-l or E-l connection that make up interconnect 46. If interconnect 46 is comprised of a set of M T T-l lines, as follows:
- QUEUE_CAPACITY is the depth of the FIFO queue within BCN poart 32f . If interconnect 46 is comprised of a set of M E E-l lines, the
- peak queue length threshold Q ⁇ is set as follows:
- the threshold on the peak transmission queue length is the number of bytes that can be queued while still allowing all queued data to be transmitted with a delay of 3 ms. Keeping the queue delay less than 3 ms keeps the delay introduced by transmission of traffic frames through BCN port 32f, interconnect 46 and BSC 24B sufficiently low that the quality of the communication remains acceptable to the end user. As is well known in the art, a delay of significantly more than 100 ms are noticeable to the user. Given the necessary delay for all the other processing that must be performed, an additional delay for buffering of no more than 3 ms is preferred. Of course, the queue length must also be kept less than the queue in order to avoid losing data. In a preferred embodiment of the invention, the QUEUE_CAPACITY of BCN port 32f is three kilo-bytes (3K).
- the frame reception rate threshold R ⁇ is set to as follows:
- N is the maximum number of network packets that can be transmitted by interconnect 46 per 20 millisecond (ms) time interval. N is described in greater detail below.
- the reduction to 90% (.9) is to allow for errors and changes in the frame rate due to the 2 second sample period T Samp ⁇ eLoad -
- the further reduction to 1/105% (1.05) is to account for signaling messages.
- the maximum number of network packets that can be transmitted by interconnect 46 N depends on the capacity of interconnect 46 and nature of data being transmitted.
- the IS-95 standard provides two quality levels of telephone service are provided, with each level or service having an associated set of traffic rates, or "rate sets.”
- rate sets facilitates the generation of voice data at variable rates in response to the changes in voice activity that occur as a natural part of speech.
- the frame sizes of the high quality level of service, or "rate set two,” service is provided in Table I, along with the probability of occurrence of each rate during a typical conversation.
- Table 1.14.4 Kbps Rate Set Frames Table II lists N (the maximum number of network packets that can be transmitted by interconnect 46) when interconnect 46 is comprised of various numbers of T-l and E-l links.
- a "remote" intra-system soft handoff is initiated between base stations 22B and 22C.
- an intra-system soft handoff is conducted between two base stations 22 coupled to the same BSC 24B, or within the same cellular system, and an inter-system soft handoff is between two base stations 22 that are coupled to different BSC's 24 and therefore part of different cellular systems.
- a remote intra-system soft handoff is one conducted between two base stations 22 that are part of the same cellular telephone system, but where the frame selection and frame distribution are performed at a remote BSC 24A.
- the remote intra-system soft handoff is performed in similar fashion to that perform during initiation of the inter-system soft handoff.
- selector subsystem 40 transmits an inter-system soft handoff request to admission telephone system, but where the frame selection and frame distribution are performed at a remote BSC 24A.
- PSMR pilot strength measurement report
- the remote intra-system soft handoff is performed in similar fashion to that perform during initiation of the inter-system soft handoff.
- selector subsystem 40 transmits an inter-system soft handoff request to admission control subsystem 44.
- Admission control subsystem 46 responds by performing the steps of FIG. 3 to either grant or deny the request.
- Performing the remote intra-system soft handoff as described above also increases the likelihood of proper and reliable call processing because, in order to conduct the soft handoff, selector subsystem 40 must transmit frames to both base stations 22B and 22C via interconnect 46. Additionally, selector subsystem 40 must receive frame from both base stations 22B and 22C in order to perform frame selection. The transmission of these additional frames adds to the traffic transmitted across interconnect 46, and thus performing the remote intra-system soft handoff as described above facilitates ensuring adequate capacity exist to carry such additional traffic.
- a method and apparatus for operating a cellular telephone system that allows inter-system soft handoff has been described.
- the exemplary embodiment provided above is to enable any person skilled in the art to make or use the present invention.
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Priority Applications (7)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
KR10-1999-7009810A KR100509407B1 (en) | 1997-04-29 | 1998-04-29 | Inter-system soft handoff |
EP98919899A EP0979584B1 (en) | 1997-04-29 | 1998-04-29 | A cellular telephone system and a method of conducting an inter-system soft handoff |
DE69831738T DE69831738T2 (en) | 1997-04-29 | 1998-04-29 | CELLULAR TELEPHONE SYSTEM AND METHOD FOR GENTLE INTERSYSTEM PASSING |
AT98919899T ATE305708T1 (en) | 1997-04-29 | 1998-04-29 | CELLULAR TELEPHONE SYSTEM AND METHOD FOR SOFT INTERSYSTEM HANDOVER |
AU72582/98A AU7258298A (en) | 1997-04-29 | 1998-04-29 | A cellular telephone system and a method of conducting an inter-system soft handoff |
JP54720998A JP4031054B2 (en) | 1997-04-29 | 1998-04-29 | Cellular telephone system and method for performing intersystem soft handoff |
HK00105875A HK1026798A1 (en) | 1997-04-29 | 2000-09-19 | A cellular telephone system and a method of conducting an inter-system soft handoff. |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US08/845,944 | 1997-04-29 | ||
US08/845,944 US6002933A (en) | 1997-04-29 | 1997-04-29 | Inter-system soft handoff |
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WO1998049858A1 true WO1998049858A1 (en) | 1998-11-05 |
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PCT/US1998/008368 WO1998049858A1 (en) | 1997-04-29 | 1998-04-29 | A cellular telephone system and a method of conducting an inter-system soft handoff |
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US (2) | US6002933A (en) |
EP (1) | EP0979584B1 (en) |
JP (1) | JP4031054B2 (en) |
KR (1) | KR100509407B1 (en) |
CN (3) | CN1311708C (en) |
AT (1) | ATE305708T1 (en) |
AU (1) | AU7258298A (en) |
DE (1) | DE69831738T2 (en) |
HK (1) | HK1026798A1 (en) |
WO (1) | WO1998049858A1 (en) |
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Also Published As
Publication number | Publication date |
---|---|
EP0979584B1 (en) | 2005-09-28 |
KR20010020222A (en) | 2001-03-15 |
CN1893727A (en) | 2007-01-10 |
DE69831738D1 (en) | 2005-11-03 |
CN1893727B (en) | 2011-01-19 |
ATE305708T1 (en) | 2005-10-15 |
US20020132622A1 (en) | 2002-09-19 |
US6002933A (en) | 1999-12-14 |
CN1510962A (en) | 2004-07-07 |
HK1026798A1 (en) | 2000-12-22 |
JP4031054B2 (en) | 2008-01-09 |
KR100509407B1 (en) | 2005-08-22 |
AU7258298A (en) | 1998-11-24 |
JP2001523418A (en) | 2001-11-20 |
CN1259262A (en) | 2000-07-05 |
CN1311708C (en) | 2007-04-18 |
EP0979584A1 (en) | 2000-02-16 |
DE69831738T2 (en) | 2006-07-06 |
CN1147196C (en) | 2004-04-21 |
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