CA2109508C - Directional handover control in digital mobile radio systems employing maho - Google Patents
Directional handover control in digital mobile radio systems employing mahoInfo
- Publication number
- CA2109508C CA2109508C CA002109508A CA2109508A CA2109508C CA 2109508 C CA2109508 C CA 2109508C CA 002109508 A CA002109508 A CA 002109508A CA 2109508 A CA2109508 A CA 2109508A CA 2109508 C CA2109508 C CA 2109508C
- Authority
- CA
- Canada
- Prior art keywords
- base site
- bts
- communication unit
- serving
- frequency list
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 238000004891 communication Methods 0.000 claims abstract description 28
- 238000000034 method Methods 0.000 claims abstract description 17
- 230000001413 cellular effect Effects 0.000 claims abstract description 7
- 150000002500 ions Chemical class 0.000 description 5
- 238000005259 measurement Methods 0.000 description 4
- 238000010586 diagram Methods 0.000 description 3
- 230000011664 signaling Effects 0.000 description 2
- VOXZDWNPVJITMN-ZBRFXRBCSA-N 17β-estradiol Chemical compound OC1=CC=C2[C@H]3CC[C@](C)([C@H](CC4)O)[C@@H]4[C@@H]3CCC2=C1 VOXZDWNPVJITMN-ZBRFXRBCSA-N 0.000 description 1
- 241000123815 Lutjanus apodus Species 0.000 description 1
- 101100296015 Mus musculus Ovos gene Proteins 0.000 description 1
- 230000010267 cellular communication Effects 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- VJYFKVYYMZPMAB-UHFFFAOYSA-N ethoprophos Chemical compound CCCSP(=O)(OCC)SCCC VJYFKVYYMZPMAB-UHFFFAOYSA-N 0.000 description 1
- 230000002452 interceptive effect Effects 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
Classifications
-
- 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
- H04W36/304—Reselection being triggered by specific parameters by measured or perceived connection quality data due to measured or perceived resources with higher communication quality
-
- 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
-
- 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
- H04W36/32—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data
-
- 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
- H04W36/32—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data
- H04W36/326—Reselection being triggered by specific parameters by location or mobility data, e.g. speed data by proximity to another entity
Abstract
A method of selecting handover targets for a communication unit (40) exchanging a communicated signal through a serving base site (10-19) in a digital cellular system. The method uses timing advance to determine a relative distance of a communication unit (40) to the serving base site (10-19). The method comprises the steps of comparing a timing advance value with a first threshold. The first threshold may be equal to a distance one and one-half times the radius of the serving cell. When the timing advance value exceeds the first threshold, a base site frequency list is modified to include base sites non-adjacent the serving base site. The identity of the base sites non-adjacent the serving base sites may be determined from the directionality of communication unit (40). The method further includes the step of selecting a handover target providing highest relative RRSI value to the communication unit (40) from the modified base site frequency list.
Description
WO 93/19560 210 9 5 0 8 PCr/US93/00750 DIRECTIONAL HANDOVER CONT~OL IN DIGITAL MOBILE
RADIO SY:~lt~.llS EMPLOYING MAHO
Field of the Invention The field of the invention relates to radio communication sysle"-s and, in specific, to digital radio communication systems.
Background of the Invention Digital cellular communication systems are known.
Such systems are, typically, compris~ of a number of cells, each having a service cover~ge area, and a number of cellular v telephones (communication units) (also sometimes referred to hereinafter as mobile stations or MSs). The service coverage areas of adjacent cells are typically arranged to partially ovarlap in such a manner as to provid~ a su~stantially continuous coverage area in which a communic-a~;on unit rec~i~in~ service from one cell may be handed off to an adjacent cell with no interruption in service.
The Groupe S~.ecial Mobile (GSM) Pan-European digital cellular system, as specified in GSM recomme.,Jalio,)s available from the European Telecommunications Sta"dar~s Institute (ETSI) is an example of just such a sy;,t~.n~
The GSM s~ste.,- is a TDM/TDMA syslem providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assign~ to a base-transceivar station (BTS) local6d at a base site within a cell, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots.
Exchanges of paging and setup control infofn,a~ion within GSM between MSs and BTSs typically occurs on the common control channel (CCCH) whkh occupies at least one slot of a primary channel of the BTS. Transl"illeJ by the BTS
WO g3/19562 1 ~) 9 ~ O ~ , , PCI/US93/00750 , on the CCCH are dislincti~e idenlificalion signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. CCCH information allows an MS to differonliate between primary and non-primary 5 channels.
Upon actiJdlion, an MS scans a pre-pro~rd",.n~d sra_t~um in search of CCCH idenlification signals lrans,)litted from nearby BTSs. Upon detecting a CCCH
identification signal, the communication unit measures a 10 signal quality factor (such as signal slr~ lh) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the spectrum, the MS generally -~el~cts the BTS pro~idi.)~ the largest relative signal quality factor as a 15 serving BTS. Upon identifying, and locking onto a suitably strong signal, the communication unit monitors the s'~16ct~;d CCCH for incoming calls. While monitoring the serving BTS, the MS receives an adjacent base site frequency list on the CCCH of the servin3 BTS. The set of frequencies identifies 20 the spectral location of primary channels of BTSs, adjacent the serving BTS. Li.,-itation of the set of frequencies to BTSs adjacent the serving BTS reduces the time period required to measure, and transfer to a serving BTS the signal str~n"~
values of BTSs that are presumably the best l,a"dov~r 25 candidates. Limitation of the set of frequencies to adjacent BTSs also reduces the possibility of ha"~over to a distant BTS
with a hi~h si~nal strength value G~se~l by signal propagation anomalies.
During normal operation (including during activ~ calls), 30 me MS.monitors for, Wentifies, and measures a received signal strength indication (RSSI) of primary channels of nearby BTSs. The MS detects and measures the RSSI of nearby BTSs by reference to the frequency list communicated to the MS by the selving BTS. Upon detection of a signal of a nearby 35 BTS, ~he MS also decodes an ID of me nearby BTS.
If ir.volv~d in an active call, the MS relays measurement information back to the base site on an ~cociqt~-~ signaling channel. Through such a process, it is possible for the MS to WO 93/19560 210 9 5 0 8 PC~/US93/00750 maintain an ~soci~ion with the most appropriate (proxi.,.at~) BTS. The process may entail an autonomous switching by the MS to a dir~erent BTS, causing perhaps a re-r~gislralio., by the MS with the system indicating that such a 5 switch has occurred. Alte---ali~ely, during an active communicdtion exchan~e, the MS may be commanded by the s~sle... to handover to a more appro,~riate BTS. Handover l ~ed upon information provided to the BTS by the MS is commonly referred to as a mobile assisted l-andover (MAHO).
Under GSM, a decision to handoff a communication unit to a target BTS may be based upon a power budget e~ pr~ssion (see GSM Recommc..Jaiion 5.08). The power budget expr~ssion provides a method of comparin.J a path loss between an MS and serving cell with a path loss between the MS and a pot~n~ial 15 handoff target cell.
Under GSM, ha..Jovw may also be desirable when the MS
exceeds a specified distance from a serving BTS. Handoff may be desirable in such case to minimize eflecti~a cell size and to insure that an MS is served from the nearest BTS. Other 20 har,dover r~Jses, as specified in GSM recom,.,E..Ja~ion 5.08, include handover for reason of RXQUAL (high bit-error-rate threshold), and har,dover for reason of RXLEV (down link threshold or uplink threshold).
Where the decision to lla.lderl is based on distance, the 25 parameter that may be used as an indication of distance is timin~ a~vanca. Timing advance is a parameter measured by a BTS based upon round-trip signal delay of a signal transmitted from the serving BTS to the MS and back to the BTS. The measured value is then used to adjust the timing of 30 the MS to ensure that transmissie,-s from an MS arrive at a BTS within the TDM slot assigned to the MS.
While the prior art handover al~orithm has worked well, problems may be experienced in target selection. Under GSM, potential targets are determined from RSSI values measured 35 by the MS. rotential targets, on the other hand, are limited to BTSs adjacent the serving BTS (as determined by the frequency list communiGAteJ to the MS). If handover is deferred because a target BTS is operating at capacity, then WO 9:~/19560 2 ~. 0 9 ~ 0 8 ~ PCI/US93/00750 the MS may move past adjacent BTSs into the service areas of non-adjacent BTSs. As the MS continues to rec~ive service through the original serving BTS the MS may not detect nearby non-adjacent BTSs (as in the case of a fast mobile moving 5 through micro cells). If the MS ."ovos too far from the serving BTS before a channel becomes available in an adjacent BTS then the MS may be dropped. Even if not dropped, an MS
oporatin~, relatively far from a serving BTS, presumably at full power, may create a source of i"te.~erence sufficient to 10 cause dropped calls in the case of other MSs.
When a channel becomes av~ hl~ in a target BTS
(adjacent the original se~ )g BTS) a har,Jover may take place to the target BTS. At that time, a new frequency list may be tra,.sf~r.~J to the MS. The MS may now measure an RSSI value 15 of a target BTS adjacent the new serving BTS, and a further l,&.,dol~r may take place that finally places the MS in the most proxi.~,ate cell. The result of all of this handovcr activity is a ratcheting of the MS from one cell to another;
from the original serving BTS, to an adjacent BTS, to a 20 proxi,a~e BTS. Ratcheting not only consumes valuable control resources of a communication system but also reduces the availability of traffic channels as intermediate BTSs alternately reserve, and then release, traffic channels as the MS ratchets to a proximate BTS.
Ratchetin~ may also occur as a result of signal anomalies where obstructions or other local signal propa~ation characteristics cause a MS to pass through adiacent BTS sen~ice areas without hanJo~ r. Re~Use of the problem ~ssociated with ratcheting, a need exists for a better ,--etl.GJ of ide.~tifyiny proximate BTSs by MSs.
WO 93/19560 2 ~ O ~ 5 0 ~3 PCI/US93/00750 Summary of the Invention A method of ssl~ting handover targets for a 5 communication unit ~xchanging a communicated signal through a servin~ base site in a digital cellular syste.,-. The method comprising the steps of comparing a timing advance value with a first threshold, and, when the timing advance value exceeds the first threshold, modifying a base site 10 frequency list to include base sites non-adjacent the serving base site. The method further includes the step of selecting a handover target providing a highest relative RSSI value to the communication unit from the modified base site frequency list.
Brief Description of the Drawing FIG. 1 depicts a cellular radiotelephone communication system including a number of cells and a communication unit, according to the invention.
FIG. 2 comprises a block diagram of a base transceiver station located at a base site under the invention.
FIG. 3 comprises a block diagram of a communication unit, according to the invention.
.
FIG.-4 depicts a communical;on unit passi.. g out of a serving~cell into an adjacent cell, in accordance with the invention.
Detailed Description of the rrvf~r-ed Embodiment The solution to the problem of identifying nearby base sites lies, conceptually, in dynamically modifying the list of - 2109 ~ 08 WO 93/19560 PCl'/US93/00750 base site frequencies l-ans,--illed from a serving base site to a communicalion unit, about to be tra"ifer,~d, based upon directionality and timing advance. Directionality in a sectored cell may be established by a sector ID.
Directionality in an omnidirectional cell may be established through an interactive process by first transmitting a list of frequencies containing BTSs in repre-e.)tali~a lec~t;ons and then updating the list of frequencies based on the ID of the represe.,ta~ d base site proJiJin~ a higher relative RSSI
1 0 value to the MS.
Shown in FIG. 1 is a communieAlien system (100), generally, under the invention. Included with the communication system (100) are a number of service coverage areas (10-19), each containing a base site (20-29) that may be c~nlrally leç~te I within the coverage area (10-19). Communication services within such a system (100) are typically available within service coverage areas (10-19) from a BTS lo~t~ l at each of the base sites (20-29).
Shown in FIG. 2, generally, is a block diagram of a BTS
(30), in acco~ance with the ifivenlion. Included within the BTS (30) is a resource controller (31), and transceivars (32-34).
A BTS (30), locAt.~d at a base site (26) (hereinafter referred to as ~BTS 26"), is constructed to transmit identification and timing signals on a control resource (slot 1) as generally provWed under GSM recomn.~.,da~ions. Each BTS (20-29) transmits a list of base site frequencies, to inactive MSs, for pu~-es of RSSI measurement of a~jacent BTSs (e.~. BTS 26 transmits a list of primary frequencies of BTSs 20-25) also as provided under GSM.
Shown in FIG. 3 is an MS (40), generally, in accordance with the im~enlion. Included within the MS (40) is a controller (41) and ~ssoci~d memory (42), a transcaiv~r (43), and a signal measurement device (44).
Inactive MSs (40) within the communication system (100) are constructed to monitor a proxi...~te BTS (20-29) and measure RSSI values of BTSs (20-29) identified to the MS by WO93/19560 ~ i08 PCI/US93/00750 _7_ the monitored BTS (20-29). Base site frequencies identified to the MS (40) by the proximate BTS (20-29) are seored within the memory (42) for subsequent RSSI measurement.
Durin~ the inactive state the MS (40) selects the BTS (20-29) 5 providing the highest relative RSSI value as the monitored BTS (20-29).
Call requests, caJI set-up, and call maintenance wi~hin the communication system (100) are handled as provided by GSM. During an active call the base site frequency list is 10 communicated to an active MS (40) on an ;~ssoci~t~ ~ signaliny channel (e.g. a slow ~ssoGi~t~d signaling channel (SMCH)).
RSSI values and IDs of as many as six BTSs (20-29) are retumed to a serving BTS (20-29), also, on the SACCH.
Upon call set-up an MS (40) is assigned to a traffic 15 channel (TCH) within a TDM slot (1-7) on a primary channel or 0-7 ôn a secondary channel). Tha signal from MS (40) is maintained within the slo~ by a timing advance value transmitted to the MS (40) by the serving BTS (e.g. 26).
The conte..t of the base site frequency list, in accordance with the invention, is generate~ under a suitable aborithm. Under one embodiment of the ir.vGntion the contents of the base site frequency list are dependent upon the results of a comparison of the timing advance value with a set of timin~ advance threshold values.
Under such an embodiment a first threshold value may be made equal to the timing advance value of an MS (40) located at a distance from the serving BTS (e.g. 1-1/2 times the radius of the serving cell (10-19). Where the timing advance value is less than the first threshold, the ser~in~ BTS
(e.g.- 26) t~a.-s,-lits a first set of base site frequencies including--BTSs (20-25) adjacent the serving BTS (26). Where the ffming advance value exceeds the first threshold, the base site frequency list is adjusted, in accordance with the invention, to include non-adjacent BTSs. Different frequency lists, in fact, may be generated based upon a series of threshold values representing distance, and direction, from a serving BTS.
WO 93/19~i60 2 ~ 0 8 P~/US93/00750 By way of example, MS 40 r~-estc access to and is granted a TCH through BTS 26. The base site fre~uency list communi~t~-~ to the MS (40) incl~des BTSs (20-25). As a communicated signal is exchan~ed by the MS (40) through the '.
serving BTS (26) the MS (40) may move into an adjacent cell (14, FIG. 4) and now be closer to a BTS (24). The MS (40) during such move continues to lf~ns~ RSSI values to the serving BTS (26). Upon entering the adjacent cell (14) the RSSI value from BTS 24 may indicate the need for a har,dov~r 1 0 from the original serving BTS (26) to the adjaeent BTS (24).
If a TCH is available at BTS (24) the original serving BTS (26) transmits control information to the MS (40) causing the MS
(40) to handover to the adjacent BTS (24).
If a TCH is not available at the adjacent BTS (24) then 1 5 the MS (40) continues to move across the adjacent cell (14) exchanging a commu"ic-~te-J signal through the original serving BTS (26). As the MS (40) continues to move away from the serving BTS (26), the timing advance value is incr~ J, by the original serving BTS (26) to acco".,-.G.lale the inc.~esed range between the MS (40) and the serving BTS
(26). As the MS (40) p~s~s the first lhresl-old, the serving BTS (26) modifies the base site frequency list. Upon determining that the MS (40) has passed the first threshold, the serving BTS (26) now determines, based upon relativ~
RSSI values, that the MS (40) is now proximate to BTS 24.
Based upon such proximity, the serving BTS (26) now l~ns",i~s a modified base sit~ frequency list that includes the proximat~ BTS (24) and BTSs (25, 23, 28, 27, and 29) that are adjacent the proximate BTS (24).
As the MS (40) continues to move across the adjacent cell (14);and RSSI values from BTS (24) continues to provide the highest relative value the serving BTS (26) will continue to seek ha,.~o~r to the proximate BTS (24). If, on the other hand, the RSSI value of a non-adjacent BTS (27) should provide a higher relative value than the other BTSs (24, 25, 23, 28, 27, and 29) within the base site frequency list then the serving BTS (26) would seek l.andwar to the non-adjacent BTS (27).
WO 93/19560 2 1 0 ~ ~ ~ 8 Pcr/usg3/oo7so g If, after enl~ri.,!a the adjacent cell (14), the MS (40) should then proceed to enter adjacent cell (15) then the serving BTS (26) would detect such movement by relative RSSI values. In the case where RSSI values from BTSs 24 and 5 25 were substantially equal, the serving BTS (26) would aller.,ately seek handover to either of the adjacent 8TSs (24 or 25). If the timing a~a.)ce value in such case should exceed the first threshold then the serving BTS (26) would add non-adjacent BTS 29 to the base site frequency list.
In another embodiment of the invention, timing advance may be used to reduce interference by precluding ha"Jo~rer in circumstances involving a MS (40) lo~e-l on an elavated structure (e.g. a hill, high-rise building, or bridge which provides an a~lv~--tageo ~s position to see many, or all, of the BTSs in system 100). In the case of an elevated structure, the MS (40) rac~iv;ng service from a serving BTS (26) may be sitl~t~ in a lin~of-site posi~ion to an adjacent BTS (24) and measure a relatively high RSSI value even though the MS (40) iS IQc~te I within the serving cell (16) and closer to the serving BTS (26) than the adjacent BTS (24). Moreover, se of local reuse conditions, a hanJwer of an MS (40) on an elevate~ structure may be a source of interference to BTSs (20-22).
In the case of elovat~ structures within a cell (10-19) a minimum distance threshold may be established. The minimum distance threshold may be a timing advance value equal to a cell radius. Under such an embodiment, I.al,Jover is deferred until a serving BTS (26) determines that the minimum distan¢e threshold has been exceeded.
- 30 -Note that it may be possible (under unusual RF
propagation conditions) for the highest signal strength that the MS (40) sees to indicate a different BTS than the proxi".ale BTS. If these conditions are possible in systen, 100, then it is a~a.)tageous to use timing advance plus the two strongest adjacent BTSs to det~rmine the new frequency list, etc. In the case where the two slfongGst adjacent BTSs cannot be correlated to the same direction from the serving WO 93/19560 2 1 0 9 ~ O ~ PCI'/US93/00750 call, the new frequency list will revert to th~ original list of adjacent cells in an effort to determino a new directionality.
RADIO SY:~lt~.llS EMPLOYING MAHO
Field of the Invention The field of the invention relates to radio communication sysle"-s and, in specific, to digital radio communication systems.
Background of the Invention Digital cellular communication systems are known.
Such systems are, typically, compris~ of a number of cells, each having a service cover~ge area, and a number of cellular v telephones (communication units) (also sometimes referred to hereinafter as mobile stations or MSs). The service coverage areas of adjacent cells are typically arranged to partially ovarlap in such a manner as to provid~ a su~stantially continuous coverage area in which a communic-a~;on unit rec~i~in~ service from one cell may be handed off to an adjacent cell with no interruption in service.
The Groupe S~.ecial Mobile (GSM) Pan-European digital cellular system, as specified in GSM recomme.,Jalio,)s available from the European Telecommunications Sta"dar~s Institute (ETSI) is an example of just such a sy;,t~.n~
The GSM s~ste.,- is a TDM/TDMA syslem providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assign~ to a base-transceivar station (BTS) local6d at a base site within a cell, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots.
Exchanges of paging and setup control infofn,a~ion within GSM between MSs and BTSs typically occurs on the common control channel (CCCH) whkh occupies at least one slot of a primary channel of the BTS. Transl"illeJ by the BTS
WO g3/19562 1 ~) 9 ~ O ~ , , PCI/US93/00750 , on the CCCH are dislincti~e idenlificalion signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. CCCH information allows an MS to differonliate between primary and non-primary 5 channels.
Upon actiJdlion, an MS scans a pre-pro~rd",.n~d sra_t~um in search of CCCH idenlification signals lrans,)litted from nearby BTSs. Upon detecting a CCCH
identification signal, the communication unit measures a 10 signal quality factor (such as signal slr~ lh) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the spectrum, the MS generally -~el~cts the BTS pro~idi.)~ the largest relative signal quality factor as a 15 serving BTS. Upon identifying, and locking onto a suitably strong signal, the communication unit monitors the s'~16ct~;d CCCH for incoming calls. While monitoring the serving BTS, the MS receives an adjacent base site frequency list on the CCCH of the servin3 BTS. The set of frequencies identifies 20 the spectral location of primary channels of BTSs, adjacent the serving BTS. Li.,-itation of the set of frequencies to BTSs adjacent the serving BTS reduces the time period required to measure, and transfer to a serving BTS the signal str~n"~
values of BTSs that are presumably the best l,a"dov~r 25 candidates. Limitation of the set of frequencies to adjacent BTSs also reduces the possibility of ha"~over to a distant BTS
with a hi~h si~nal strength value G~se~l by signal propagation anomalies.
During normal operation (including during activ~ calls), 30 me MS.monitors for, Wentifies, and measures a received signal strength indication (RSSI) of primary channels of nearby BTSs. The MS detects and measures the RSSI of nearby BTSs by reference to the frequency list communicated to the MS by the selving BTS. Upon detection of a signal of a nearby 35 BTS, ~he MS also decodes an ID of me nearby BTS.
If ir.volv~d in an active call, the MS relays measurement information back to the base site on an ~cociqt~-~ signaling channel. Through such a process, it is possible for the MS to WO 93/19560 210 9 5 0 8 PC~/US93/00750 maintain an ~soci~ion with the most appropriate (proxi.,.at~) BTS. The process may entail an autonomous switching by the MS to a dir~erent BTS, causing perhaps a re-r~gislralio., by the MS with the system indicating that such a 5 switch has occurred. Alte---ali~ely, during an active communicdtion exchan~e, the MS may be commanded by the s~sle... to handover to a more appro,~riate BTS. Handover l ~ed upon information provided to the BTS by the MS is commonly referred to as a mobile assisted l-andover (MAHO).
Under GSM, a decision to handoff a communication unit to a target BTS may be based upon a power budget e~ pr~ssion (see GSM Recommc..Jaiion 5.08). The power budget expr~ssion provides a method of comparin.J a path loss between an MS and serving cell with a path loss between the MS and a pot~n~ial 15 handoff target cell.
Under GSM, ha..Jovw may also be desirable when the MS
exceeds a specified distance from a serving BTS. Handoff may be desirable in such case to minimize eflecti~a cell size and to insure that an MS is served from the nearest BTS. Other 20 har,dover r~Jses, as specified in GSM recom,.,E..Ja~ion 5.08, include handover for reason of RXQUAL (high bit-error-rate threshold), and har,dover for reason of RXLEV (down link threshold or uplink threshold).
Where the decision to lla.lderl is based on distance, the 25 parameter that may be used as an indication of distance is timin~ a~vanca. Timing advance is a parameter measured by a BTS based upon round-trip signal delay of a signal transmitted from the serving BTS to the MS and back to the BTS. The measured value is then used to adjust the timing of 30 the MS to ensure that transmissie,-s from an MS arrive at a BTS within the TDM slot assigned to the MS.
While the prior art handover al~orithm has worked well, problems may be experienced in target selection. Under GSM, potential targets are determined from RSSI values measured 35 by the MS. rotential targets, on the other hand, are limited to BTSs adjacent the serving BTS (as determined by the frequency list communiGAteJ to the MS). If handover is deferred because a target BTS is operating at capacity, then WO 9:~/19560 2 ~. 0 9 ~ 0 8 ~ PCI/US93/00750 the MS may move past adjacent BTSs into the service areas of non-adjacent BTSs. As the MS continues to rec~ive service through the original serving BTS the MS may not detect nearby non-adjacent BTSs (as in the case of a fast mobile moving 5 through micro cells). If the MS ."ovos too far from the serving BTS before a channel becomes available in an adjacent BTS then the MS may be dropped. Even if not dropped, an MS
oporatin~, relatively far from a serving BTS, presumably at full power, may create a source of i"te.~erence sufficient to 10 cause dropped calls in the case of other MSs.
When a channel becomes av~ hl~ in a target BTS
(adjacent the original se~ )g BTS) a har,Jover may take place to the target BTS. At that time, a new frequency list may be tra,.sf~r.~J to the MS. The MS may now measure an RSSI value 15 of a target BTS adjacent the new serving BTS, and a further l,&.,dol~r may take place that finally places the MS in the most proxi.~,ate cell. The result of all of this handovcr activity is a ratcheting of the MS from one cell to another;
from the original serving BTS, to an adjacent BTS, to a 20 proxi,a~e BTS. Ratcheting not only consumes valuable control resources of a communication system but also reduces the availability of traffic channels as intermediate BTSs alternately reserve, and then release, traffic channels as the MS ratchets to a proximate BTS.
Ratchetin~ may also occur as a result of signal anomalies where obstructions or other local signal propa~ation characteristics cause a MS to pass through adiacent BTS sen~ice areas without hanJo~ r. Re~Use of the problem ~ssociated with ratcheting, a need exists for a better ,--etl.GJ of ide.~tifyiny proximate BTSs by MSs.
WO 93/19560 2 ~ O ~ 5 0 ~3 PCI/US93/00750 Summary of the Invention A method of ssl~ting handover targets for a 5 communication unit ~xchanging a communicated signal through a servin~ base site in a digital cellular syste.,-. The method comprising the steps of comparing a timing advance value with a first threshold, and, when the timing advance value exceeds the first threshold, modifying a base site 10 frequency list to include base sites non-adjacent the serving base site. The method further includes the step of selecting a handover target providing a highest relative RSSI value to the communication unit from the modified base site frequency list.
Brief Description of the Drawing FIG. 1 depicts a cellular radiotelephone communication system including a number of cells and a communication unit, according to the invention.
FIG. 2 comprises a block diagram of a base transceiver station located at a base site under the invention.
FIG. 3 comprises a block diagram of a communication unit, according to the invention.
.
FIG.-4 depicts a communical;on unit passi.. g out of a serving~cell into an adjacent cell, in accordance with the invention.
Detailed Description of the rrvf~r-ed Embodiment The solution to the problem of identifying nearby base sites lies, conceptually, in dynamically modifying the list of - 2109 ~ 08 WO 93/19560 PCl'/US93/00750 base site frequencies l-ans,--illed from a serving base site to a communicalion unit, about to be tra"ifer,~d, based upon directionality and timing advance. Directionality in a sectored cell may be established by a sector ID.
Directionality in an omnidirectional cell may be established through an interactive process by first transmitting a list of frequencies containing BTSs in repre-e.)tali~a lec~t;ons and then updating the list of frequencies based on the ID of the represe.,ta~ d base site proJiJin~ a higher relative RSSI
1 0 value to the MS.
Shown in FIG. 1 is a communieAlien system (100), generally, under the invention. Included with the communication system (100) are a number of service coverage areas (10-19), each containing a base site (20-29) that may be c~nlrally leç~te I within the coverage area (10-19). Communication services within such a system (100) are typically available within service coverage areas (10-19) from a BTS lo~t~ l at each of the base sites (20-29).
Shown in FIG. 2, generally, is a block diagram of a BTS
(30), in acco~ance with the ifivenlion. Included within the BTS (30) is a resource controller (31), and transceivars (32-34).
A BTS (30), locAt.~d at a base site (26) (hereinafter referred to as ~BTS 26"), is constructed to transmit identification and timing signals on a control resource (slot 1) as generally provWed under GSM recomn.~.,da~ions. Each BTS (20-29) transmits a list of base site frequencies, to inactive MSs, for pu~-es of RSSI measurement of a~jacent BTSs (e.~. BTS 26 transmits a list of primary frequencies of BTSs 20-25) also as provided under GSM.
Shown in FIG. 3 is an MS (40), generally, in accordance with the im~enlion. Included within the MS (40) is a controller (41) and ~ssoci~d memory (42), a transcaiv~r (43), and a signal measurement device (44).
Inactive MSs (40) within the communication system (100) are constructed to monitor a proxi...~te BTS (20-29) and measure RSSI values of BTSs (20-29) identified to the MS by WO93/19560 ~ i08 PCI/US93/00750 _7_ the monitored BTS (20-29). Base site frequencies identified to the MS (40) by the proximate BTS (20-29) are seored within the memory (42) for subsequent RSSI measurement.
Durin~ the inactive state the MS (40) selects the BTS (20-29) 5 providing the highest relative RSSI value as the monitored BTS (20-29).
Call requests, caJI set-up, and call maintenance wi~hin the communication system (100) are handled as provided by GSM. During an active call the base site frequency list is 10 communicated to an active MS (40) on an ;~ssoci~t~ ~ signaliny channel (e.g. a slow ~ssoGi~t~d signaling channel (SMCH)).
RSSI values and IDs of as many as six BTSs (20-29) are retumed to a serving BTS (20-29), also, on the SACCH.
Upon call set-up an MS (40) is assigned to a traffic 15 channel (TCH) within a TDM slot (1-7) on a primary channel or 0-7 ôn a secondary channel). Tha signal from MS (40) is maintained within the slo~ by a timing advance value transmitted to the MS (40) by the serving BTS (e.g. 26).
The conte..t of the base site frequency list, in accordance with the invention, is generate~ under a suitable aborithm. Under one embodiment of the ir.vGntion the contents of the base site frequency list are dependent upon the results of a comparison of the timing advance value with a set of timin~ advance threshold values.
Under such an embodiment a first threshold value may be made equal to the timing advance value of an MS (40) located at a distance from the serving BTS (e.g. 1-1/2 times the radius of the serving cell (10-19). Where the timing advance value is less than the first threshold, the ser~in~ BTS
(e.g.- 26) t~a.-s,-lits a first set of base site frequencies including--BTSs (20-25) adjacent the serving BTS (26). Where the ffming advance value exceeds the first threshold, the base site frequency list is adjusted, in accordance with the invention, to include non-adjacent BTSs. Different frequency lists, in fact, may be generated based upon a series of threshold values representing distance, and direction, from a serving BTS.
WO 93/19~i60 2 ~ 0 8 P~/US93/00750 By way of example, MS 40 r~-estc access to and is granted a TCH through BTS 26. The base site fre~uency list communi~t~-~ to the MS (40) incl~des BTSs (20-25). As a communicated signal is exchan~ed by the MS (40) through the '.
serving BTS (26) the MS (40) may move into an adjacent cell (14, FIG. 4) and now be closer to a BTS (24). The MS (40) during such move continues to lf~ns~ RSSI values to the serving BTS (26). Upon entering the adjacent cell (14) the RSSI value from BTS 24 may indicate the need for a har,dov~r 1 0 from the original serving BTS (26) to the adjaeent BTS (24).
If a TCH is available at BTS (24) the original serving BTS (26) transmits control information to the MS (40) causing the MS
(40) to handover to the adjacent BTS (24).
If a TCH is not available at the adjacent BTS (24) then 1 5 the MS (40) continues to move across the adjacent cell (14) exchanging a commu"ic-~te-J signal through the original serving BTS (26). As the MS (40) continues to move away from the serving BTS (26), the timing advance value is incr~ J, by the original serving BTS (26) to acco".,-.G.lale the inc.~esed range between the MS (40) and the serving BTS
(26). As the MS (40) p~s~s the first lhresl-old, the serving BTS (26) modifies the base site frequency list. Upon determining that the MS (40) has passed the first threshold, the serving BTS (26) now determines, based upon relativ~
RSSI values, that the MS (40) is now proximate to BTS 24.
Based upon such proximity, the serving BTS (26) now l~ns",i~s a modified base sit~ frequency list that includes the proximat~ BTS (24) and BTSs (25, 23, 28, 27, and 29) that are adjacent the proximate BTS (24).
As the MS (40) continues to move across the adjacent cell (14);and RSSI values from BTS (24) continues to provide the highest relative value the serving BTS (26) will continue to seek ha,.~o~r to the proximate BTS (24). If, on the other hand, the RSSI value of a non-adjacent BTS (27) should provide a higher relative value than the other BTSs (24, 25, 23, 28, 27, and 29) within the base site frequency list then the serving BTS (26) would seek l.andwar to the non-adjacent BTS (27).
WO 93/19560 2 1 0 ~ ~ ~ 8 Pcr/usg3/oo7so g If, after enl~ri.,!a the adjacent cell (14), the MS (40) should then proceed to enter adjacent cell (15) then the serving BTS (26) would detect such movement by relative RSSI values. In the case where RSSI values from BTSs 24 and 5 25 were substantially equal, the serving BTS (26) would aller.,ately seek handover to either of the adjacent 8TSs (24 or 25). If the timing a~a.)ce value in such case should exceed the first threshold then the serving BTS (26) would add non-adjacent BTS 29 to the base site frequency list.
In another embodiment of the invention, timing advance may be used to reduce interference by precluding ha"Jo~rer in circumstances involving a MS (40) lo~e-l on an elavated structure (e.g. a hill, high-rise building, or bridge which provides an a~lv~--tageo ~s position to see many, or all, of the BTSs in system 100). In the case of an elevated structure, the MS (40) rac~iv;ng service from a serving BTS (26) may be sitl~t~ in a lin~of-site posi~ion to an adjacent BTS (24) and measure a relatively high RSSI value even though the MS (40) iS IQc~te I within the serving cell (16) and closer to the serving BTS (26) than the adjacent BTS (24). Moreover, se of local reuse conditions, a hanJwer of an MS (40) on an elevate~ structure may be a source of interference to BTSs (20-22).
In the case of elovat~ structures within a cell (10-19) a minimum distance threshold may be established. The minimum distance threshold may be a timing advance value equal to a cell radius. Under such an embodiment, I.al,Jover is deferred until a serving BTS (26) determines that the minimum distan¢e threshold has been exceeded.
- 30 -Note that it may be possible (under unusual RF
propagation conditions) for the highest signal strength that the MS (40) sees to indicate a different BTS than the proxi".ale BTS. If these conditions are possible in systen, 100, then it is a~a.)tageous to use timing advance plus the two strongest adjacent BTSs to det~rmine the new frequency list, etc. In the case where the two slfongGst adjacent BTSs cannot be correlated to the same direction from the serving WO 93/19560 2 1 0 9 ~ O ~ PCI'/US93/00750 call, the new frequency list will revert to th~ original list of adjacent cells in an effort to determino a new directionality.
Claims (10)
1. A method of selecting handover targets for a communication unit exchanging a communicated signal through a serving base site in a digital cellular system, such method comprising the steps of:
A) comparing a timing advance value with a first threshold;
B) when the timing advance value exceeds the first threshold, modifying a base site frequency list to include base sites non-adjacent the serving base site; and, C) selecting a handover target providing a highest relative RSSI value to the communication unit from the modified base site frequency list.
A) comparing a timing advance value with a first threshold;
B) when the timing advance value exceeds the first threshold, modifying a base site frequency list to include base sites non-adjacent the serving base site; and, C) selecting a handover target providing a highest relative RSSI value to the communication unit from the modified base site frequency list.
2. The method as in claim 1 further comprising the step of communicating the modified base site frequency list to the communication unit.
3. The method as in claim 1 further comprising the step of measuring RSSI values of the modified base site frequency list by the communication unit.
4. The method as in claim 1 further comprising the step of communicating the measured RSSI values to the serving base site.
5. The method as in claim 1 further comprising the step of handing over to the selected handover target.
6. An apparatus for selecting handover targets for a communication unit exchanging a communicated signal through a serving base site in a digital cellular system, such apparatus comprising:
A) means for comparing a timing advance value with a first threshold;
B) means for modifying a base site frequency list to include base sites non-adjacent the serving base site; and, C) means for selecting a handover target providing a highest relative RSSI value to the communication unit from the modified base site frequency list.
A) means for comparing a timing advance value with a first threshold;
B) means for modifying a base site frequency list to include base sites non-adjacent the serving base site; and, C) means for selecting a handover target providing a highest relative RSSI value to the communication unit from the modified base site frequency list.
7. The apparatus as in claim 6 further comprising means for communicating the modified base site frequency list to the communication unit.
8. The apparatus as in claim 6 further comprising means for measuring RSSI values of the modified base site frequency list by the communication unit.
9. The apparatus as in claim 6 further comprising means for communicating the measured RSSI values to the serving base site.
10. The apparatus as in claim 6 further comprising means for handing over to the selected handover target.
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|---|---|---|---|
| US07/856,278 US5327575A (en) | 1992-03-23 | 1992-03-23 | Directional handover control in digital mobile radio systems employing MAHO |
| US856,278 | 1992-03-23 |
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|---|---|
| CA2109508A1 CA2109508A1 (en) | 1993-09-24 |
| CA2109508C true CA2109508C (en) | 1999-01-12 |
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| CA002109508A Expired - Lifetime CA2109508C (en) | 1992-03-23 | 1993-01-27 | Directional handover control in digital mobile radio systems employing maho |
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| EP (1) | EP0591478B1 (en) |
| JP (1) | JP3189278B2 (en) |
| KR (1) | KR970002762B1 (en) |
| CA (1) | CA2109508C (en) |
| DE (1) | DE69319821T2 (en) |
| SK (1) | SK131093A3 (en) |
| WO (1) | WO1993019560A1 (en) |
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| US5095500A (en) * | 1989-12-07 | 1992-03-10 | Motorola, Inc. | Cellular radiotelephone diagnostic system |
| US5038399A (en) * | 1990-05-21 | 1991-08-06 | Motorola, Inc. | Method for assigning channel reuse levels in a multi-level cellular system |
| US5179559A (en) * | 1990-07-27 | 1993-01-12 | Motorola, Inc. | Handoff method for a cellular system |
| US5119502A (en) * | 1990-08-30 | 1992-06-02 | Telefonaktiebolaget L M Ericsson | Periodic system ordered rescan in a cellular communication system |
-
1992
- 1992-03-23 US US07/856,278 patent/US5327575A/en not_active Expired - Lifetime
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1993
- 1993-01-27 JP JP51653093A patent/JP3189278B2/en not_active Expired - Lifetime
- 1993-01-27 SK SK1310-93A patent/SK131093A3/en unknown
- 1993-01-27 EP EP93903686A patent/EP0591478B1/en not_active Expired - Lifetime
- 1993-01-27 DE DE69319821T patent/DE69319821T2/en not_active Expired - Fee Related
- 1993-01-27 WO PCT/US1993/000750 patent/WO1993019560A1/en active IP Right Grant
- 1993-01-27 CA CA002109508A patent/CA2109508C/en not_active Expired - Lifetime
- 1993-01-27 KR KR1019930703337A patent/KR970002762B1/en not_active IP Right Cessation
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|---|---|
| CA2109508A1 (en) | 1993-09-24 |
| SK131093A3 (en) | 1994-08-10 |
| EP0591478A4 (en) | 1994-08-17 |
| JP3189278B2 (en) | 2001-07-16 |
| DE69319821T2 (en) | 1999-03-04 |
| DE69319821D1 (en) | 1998-08-27 |
| US5327575A (en) | 1994-07-05 |
| JPH06508497A (en) | 1994-09-22 |
| EP0591478A1 (en) | 1994-04-13 |
| KR970002762B1 (en) | 1997-03-10 |
| EP0591478B1 (en) | 1998-07-22 |
| WO1993019560A1 (en) | 1993-09-30 |
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