CA2311727C - Diversity handling moveover for cdma mobile telecommunications - Google Patents

Abstract

In connection with a diversity handling moveover procedure for a cellular system having soft handover, allocation of a replacement diversity handling unit in a target node occurs only after a moveover decision has been made by a source node. Initially, an original or source diversity handling unit at the source node performs connection combining and connection splitting functions for legs of a mobile connection routed through plural base stations serving a mobile station. In accordance with movement of the mobile station, the source node makes the movement decision. The moveover decision can be based on factors such as base station utilization and/or actual and/or predicted directional movement of the mobile station. In some embodiments, the diversity handling moveover procedure involves selection among plural nodes for situs of the replacement or target diversity handling unit.

Description

DIVERSITY HANDLING MOVEOVER

FOR CDMA MOBILE TELECOMMUNICATIONS
BACKGROUND
1. FIELD OF THE INVENTION

The present invention pertains to telecommunications, and particularly to io any cellular/mobile telecommunications using diversity (soft) handover, such as occurs in spread spectrum or code division multiple access (CDMA) technology.

2. RELATED ART AND OTHER CONSIDERATIONS

In mobile telecommunications, a mobile station such as mobile telephone communicates over radio channels with base stations. Typically a plurality of base 15 stations are, in turn, ultimately connected to a mobile switching center.
The mobile switching center is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network.

In a code division multiple access (CDMA) mobile telecommunications 20 svstern, the information transmitted between a base station and a particular mobile station is modulated bv a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are utilizing the same radio frequency.
Thus, in CDMA. the individual radio links are discriminated on the basis of codes.
Various aspects of CDMA are set forth in Garg, Vijay K. et al., Applications of CDMA
in Wireless/Persona! Communications, Prentice Hall (1997).

In addition, in CDMA mobile communications, typically the same baseband signal with suitable spreading is sent from several base stations with overlapping coverage. The mobile terminal can thus receive and use signals from several base stations simultaneously. Moreover, since the radio environment changes io rapidly, a mobile station likely has radio channels to several base stations at the same moment, e.g., so that the mobile station can select the best channel and, if necessary, use signals directed to the mobile from various base stations in order to keep radio interference low and capacity high. This utilization of radio channels from multiple base stations by a mobile station in a CDMA scheme is termed "soft handover."

Fig. 1 shows a radio access network (RAN) 20 which comprises radio network controllers (RNC) 221 and 222 respectively connected to mobile switching centers (MSC) 241 and 242. Radio network controller (RNC) 221 is connected to base stations (BS) 261,1, 261,2, and 261,3; radio network controller (RNC) 222 is connected to base stations (BS) 262,1, 262 and 262,3. At the moment shown in Fig. 1, and for 2o reasons summarized above, mobile station MS is shown in Fig. I as having radio communication with two base stations, particularly base stations 261,2, and 261,3. The lines 281,2 and 281,3 each represents a communication path. Specifically, line 281,2 depicts both the radio channel from mobile station MS to base station BS 261,2 and the land line link channel from base station BS 261,2 to radio network controller (RNC) 221; line 281,3 depicts both the radio channel from mobile station MS to base station BS 261,3 and the land line link.channel from base station BS 261,2 to radio network controller (RNC) 221. In the case of both lines 281,2 and 281,3, the land line link is connected to a diversity handover unit (DHU) 301 of radio network controller (RNC) 221=

Thus, as depicted with reference to Fig. l, the mobile connection with mobile station MS potentially utilizes several "legs", each leg being represented by the lines 281,2 and 281.3 in the case of mobile station MS of Fig. 1. As the overall connection between mobile station MS and any other party is viewed, the diversity handover unit (DHU) 301 serves essentially both to combine and split the different legs utilized by a mobile station. The splitting occurs in the sense that information directed toward the mobile station is directed along the plural parallel legs to differing base stations. Information received from a base station may actually be obtained through several of the legs (e.g., from several base stations), in which sense the diversity handover unit (DHU) 301 serves a combining function.

Fig. 1 illustrates the simple case in which the different legs of the is connection, represented by lines 281,2 and 281,3, are for base stations BS
all of which are connected to radio network controller (RNC) 221. However, should the mobile station MS roam sufficiently to pick up signals from another base station controlled by another RNC, e.g., into or proximate a cell handled by base station BS 262,1, for example, a more complex situation occurs as shown in Fig. lA.

In the situation depicted in Fig. lA, the mobile connection involving mobile station MS employs base stations belonging to differing radio network controllers (RNC). Such situation involves a different type of handover -- an inter-RNC
soft handover. Inter-RNC soft-handovers are made between two or several RNCs.
In the particular situation shown in Fig. lA, an inter-RNC soft handover is made between radio network controller (RNC) 221, which is also known as the "Source" RNC, and radio network controllers (RNC) 222, which is also known as the "Target" RNC.
Radio network controller (RNC) 221 is the Source RNC since it has current control of the mobile radio connection. The Target RNC is an RNC, other than the Source RNC, that has, or has been decided to have, base stations utilized by the mobile radio connection.
To facilitate, e.g., inter-RNC soft-handovers, the radio network controllers (RNC) 221 and 222 are connected by an inter=RNC transport link 32. Inter-RNC
transport link 32 is utilized for the transport of control and data signals between Source RNC 221 and Target RNC 222, and can be either a direct link or a logical link as described, for example, in International Application Number PCTIUS94/12419 (International Publication Number WO 95/15665).

lo Thus, in Fig. IA, the mobile station MS communicates not only through the leg represented by line 281,3, but now also by the leg represented by line 282,1.
The leg represented by line 282,1 includes the radio link between mobile station MS
and base station BS 262, 1, as well as the information pertinent to the mobile connection which is carried over inter-RNC transport link 32.

As mobile station MS continues to move, it may eventually occur that all of the base stations utilized by the mobile station are served by the Target RNC 222, as is pictured in Fig. 1B. In such case, inter-RNC transport link 32 must carry both legs of the mobile connection, represented by lines 282,1 and 282,2, respectively.
Carrying multiple legs of the same mobile connection undesirably demands further resources from inter-RNC transport link 32. In Fig. lB, diversity handover unit 301 handles all combining and splitting operations, even though no base station owned by Source RNC
221 is employed by the mobile connection with mobile station MS.

For the situation shown in Fig. 1B, resources of inter-RNC transport link 32 can be preserved if the diversity handling operations are moved to Target RNC 222.
Utilization of inter-RNC transport link 32 would be reduced in that, for example, multiple packets destined in pacallel for base stations BS 26 2,1 and 262,2 need not be carried on link 32, but rather a diversity handover unit at Target RNC 222 could instead WO 99/27,741 PCT/SE98/02134 perform the splitting. A similar economy results in having a diversity handover unit at Target RNC 22-) combining the signals from mobile station MS as received via the base stations BS 261, 1 and 262,2, and forwarding a resultant signal to Source RNC
221.

5 The movement of diversity handling operations (diversity handling "moveover") to a Target RNC, such as Target RNC 222 of Fig. 1B, is a complex endeavor, and can potentially result in an interruption of the established mobile connection. The prior art approach to moving diversity handling operations is shown in International Application Number PCT/US94/12419 (International Publication Number io WO 95/ 15665). That approach. illustrated in Fig. 1 C. involves a two step process. The first step of the process is routing the mobile connection, in a bypass mode, through a diversity handling unit (DHU) 302 in the Target RNC 222 upon first utilization by mobile station MS of the Target RNC (e.g., when a base station served by Target RNC
is first invoked). In its bypass mode, diversity handling unit (DHU) 302 in the Target RNC 22-) performs no combining or splitting operations. Rather, all combining and splitting operations remain the province of diversity handling unit (DHU) 301 in Source RNC 222.

Only later, when (and if) all base stations utilized by mobile station are owned by the Target RNC 222, is the second step of the above-described approach implemented. In the second step, the combining and splitting functions are moved from diversity handover unit (DHU) 301 of Source RNC 221 to diversity handover unit (DHU) 302 of Target RNC 22,~, and diversity handover unit (DHU) 301 is bypassed as shown in Fig. 1C.

Prior art techniques of diversity handling moveover are problematic for several reasons. For example, at the time shown in Fig. 1B prior to actual transfer of combining/splitting operations to Target RNC 222, two transport connections (e.g., two legs of traffic for a single connection) are still wastefully used on inter-RNC transport link 32. Moreover, for each RNC that is used by the mobile radio connection, one diversity handover unit (DHU) 30 is necessary. The use of two diversity handover units (s) is a further waste of hardware.

s What is needed therefore, and an object of the invention, is an efficient and economical diversity handling moveover technique.

SUMMARY OF THE INVENTION

In connection with a diversity handling moveover procedure. allocation of a -o replacement diversity handling unit in a target node occurs only after a moveover decision has been made by a source node. Initially, an original or source diversity handling unit at the source node performs connection combining and connection splitting functions for legs of a mobile connection routed through plural base stations serving a mobile station. In accordance with movement of the mobile station, the 15 source node makes a decision to move the connection combining and connection splitting functions to the target node. The moveover decision can be based on factors such as base station utilization and/or actual and/or predicted directional movement of the mobile station, as well as transmission costs. In some embodiments, the diversity handling moveover procedure involves selection among plural nodes for situs of the 20 replacement or target diversity handling unit.

In one mode of the invention, an anchor node performs the actual switchover which causes moveover of the diversity handoff unit. In addition. a change of mobile switching center (MSC) can be implemented.

The present invention optimizes the transmission path between network nodes 25 and utilization of hardware in the nodes (e.g., diversity handover units and interfaces).
The moveover of the present invention is executed independently from radio interface handover, allowing separation of optimizing radio resources and f xed line transmission resources. In accordance with the present invention, the moveover decision can thus also be based on transmission utilization and delay.

According to an aspect of the present invention there Is provided a method of operating a telecommunications system, the method comprising:
using a first diversity handling unit to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
allocating a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and determining that the connection combining and connection splitting operations should be transferred in accordance with one of (i) addition, and (ii) disconnection of a base station involved in diversity handling of the mobile connection, wherein the added or disconnected base station remains operational in the telecommunications system after the addition or disconnection of the base station.

According to another aspect of the present invention there is provided a method of operating a telecommunications system, the method comprising:
using a first diversity handling unit to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;

allocating a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and determining that the connection combining and connection splitting operations should be transferred in accordance with one of (i) addition, and (ii) disconnection of a base station, 7a wherein it is determined that the connection combining and connection splitting operations should be transferred when a predetermined relationship exists between a first number of base stations served by a node whereat the first diversity handling unit resides and a second number base stations served by a node whereat the second diversity handling unit is to reside.

According to a further aspect of the present invention there is provided a method of operating a telecommunications system, the method comprising:
using a first diversity handling unit to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
allocating a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit;
wherein the first diversity handling unit is situated at a first node, the first node controlling a first set of base stations, and wherein a second node controls a second set of base stations and a third node controls a third set of base stations; and selecting between the first node and the second node as situs for the second diversity handling unit.

According to a further aspect of the present invention there is provided a method of operating a telecommunications system, the method comprising:
using a first diversity handling unit to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
allocating a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit;
performing a switchover operation at least partially at an anchor node after completion of the allocation in order to remove the first diversity handling unit from the mobile connection; and 7b wherein the first diversity handling unit is situated at a first node, wherein the second diversity handling unit is situated at a second node and wherein the anchor node is physically distinct from the first node and the second node.

According to a further aspect of the present invention there is provided a telecommunications system comprising:
plural nodes each controlling a corresponding set of base stations;
a first diversity handling unit residing at a first of the plural nodes, the first diversity handling unit performing connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
wherein the first node causes allocation of a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and wherein it is determined that the connection combining and connection splitting operations should be transferred when a predetermined relationship exists between a first number of base stations served by a node whereat the first diversity handling unit resides and a second number of base stations served by a node whereat the second diversity handling unit is to reside.
According to a further aspect of the present invention there is provided a telecommunications system comprising:
plural nodes each controlling a corresponding set of base stations;
a first diversity handling unit residing at a first of the plural nodes, the first diversity handling unit performing connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
wherein the first node causes allocation of a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit;
wherein a switchover operation is performed at an anchor node; and 7c wherein the first diversity handling unit is situated at a first node, wherein the second diversity handling unit is situated at a second node and wherein the anchor node is physically distinct from the first node and the second node.

According to a further aspect of the present invention there is provided a method of operating a telecommunications system, the method comprising:
using a first diversity handling unit to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations with a mobile station;
determining that the connection combining and connection splitting operations should be transferred to a second diversity handling unit ih accordance with whether any base station managed by a particular network controller is involved in a connection with the mobile station; and performing a switchover operation from the first diversity handling unit to the second diversity handling unit, wherein it is determined whether the connection combining and connection splitting operations should be transferred to a second diversity handling unit in accordance with whether a predetermined amount of time has elapsed after a determination that no base station managed by the network controller is involved in the connection with the mobile station.

According to a further aspect of the present invention there is provided a method of operating a telecommunications system, the method comprising:
using a first diversity handling unit to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations with a mobile station;
determining that the connection combining and connection splitting operations should be transferred to a second diversity handling unit in accordance with whether any base station managed by a particular network controller is involved in a connection with the mobile station; and performing a switchover operation from the first diversity handling unit to the second diversity handling unit, 7d wherein it is determined whether the connection combining and connection splitting operations should be transferred to a second diversity handling unit in accordance with a prediction of future movement direction of the mobile station.

According to a further aspect of the present invention there is provided a method of operating a telecommunications system, the method comprising:
providing first and second diversity handling units, respectively;
using the first diversity handling unit to perform connection combining and connection splitting operations for a connection maintained through a plurality of base stations with a mobile station;
predicting a future direction of movement of the mobile station;
determining whether to use the second diversity handling unit to perform the connection combining and connection splitting operations for the connection, based at least in part upon the predicted future direction of movement of the mobile station.

According to a further aspect of the present invention there is provided a method of operating a telecommunications system including two or more nodes each of which controlling an assigned set of base stations, said method comprising the steps of:
using a first diversity handling unit at a first node to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
determining a transfer of the connection combining splitting operation to a second diversity handling unit based on transmission costs;
allocating the second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and selecting between the first node and a second node as situs for the second diversity handling unit.
According to a further aspect of the present invention there is provided a telecommunications system comprising:
plural nodes each controlling a corresponding set of base stations;

7e a first diversity handling unit residing at a first of the plural nodes, said unit performing connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations and.said unit determining a transfer of the connection combining splitting operation to a second diversity handling unit based on transmission costs, the first node comprising means for allocation of a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and means for selecting between remaining ones of the plural nodes as situs for the second diversity handling unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the folfowing more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
Fig. l, Fig. lA, Fig. 1B, and Fig. IC are each diagrammatic views showing prior art management of a mobile connection for a mobile station moving from base stations controlled by a Source radio network controller to base stations owned by a Target radio network controller.

Fig. 2, Fig. 2A, and Fig. 2B are each diagrammatic views showing differing stages of management of a mobile connection according to a mode of the present invention, including a mode of a diversity handling moveover from a first radio network controller to a second radio network controller according to the present invention.

7f Fig. 3 is a schematic view of a Source radio network controller (RNC) according to an embodiment of the invention.

Fig. 4 is a schematic view of a diversity handover unit (DHU) included in the Source radio network controller (RNC) of Fig. 3.

Fig. 5, Fig. 5A, and Fig. 5B are schematic views of a Source radio network controller (RNC) according to an embodiment of the invention at points in time corresponding to Fig. 2, Fig. 2A, and Fig. 2B, respectively.

Fig. 6 is a schematic view of a diversity handover unit (DHU) included in the Target radio network controller (RNC) of Fig. 5.

Fig. 7 is a schematic view of a sequence of operations involved in a diversity handling moveover procedure according to a mode of the invention involving two radio network control nodes.

Fig. 8, Fig. 8A, Fig. 8B, and Fig. 8C are diagrammatic view of frames to according to an embodiment of the invention.

Fig. 9 and Fig. 9A are flowcharts showing different variations of logic for making a diversity handling moveover decision according to the invention.

Fig. 10 is diagrammatic view of a radio network area having more than two radio network controllers.

Fig. 11, Fig. 1 lA, Fig. 11B, and Fig. 11C are each diagrammatic views showing differing stages of management of the mobile connection of Fig. 2-Fig.
2B as diversity handoff units moveover returns from a second radio network controller to a first radio network controller, with the mobile connection utilizing a MSC
connected to the first radio network controller.

Fig. 12, Fig. 12A, Fig. 12B, and Fig. 12C are diagrammatic views similar to Fig. 11, Fig. 11A, Fig. 11B, and Fig. 11C, respectively, but with the mobile connection utilizing a MSC connected to the second radio network controller.

. , . , 9 Fig. 13 and Fig. 13A are diagrammatic views respectively showing before and after performance of a diversity handover unit moveover involving three distinct radio network control nodes.

Fig. 14 is a schematic view of a sequence of operations in a diversity handover moveover procedure according to a node of the invention involving three distinct radio network control nodes.

Fig. 15 is a flowchart showing operation involved in a plural DHU
moveover.

DETAILED DESCRIPTION OF THE DRAWINGS
In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention.
However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

Fig. 2 shows a portion of a radio access network 120 according to one mode of the present invention which comprises a first set of base stations (BS) 1261,1 , 1261,2, and 1261,3 connected by land lines to Source radio network controller (RNC) 1221, and a second set of base stations (BS) 1262,1, 1262,2, and 1262,3 connected by land lines to a second radio network controller (RNC) 1222 which serves as a Target radio network controller. Source RNC 1221 is connected to mobile switching center 1241 while Target RNC 1222 is connected to mobile switching center 1242.
Source RNC 1221 and Target RNC 122,) are connected by inter-RNC transport link 132.
Inter-RNC transport link 132 can be either a direct link or a logical link. In the case of a logical link, inter-RNC transport link 132 is physically connected to a transport network or a public switched telephone network (such -as ISDN or PSTN). Source RNC

5 and Target RNC 1221 are considered as control nodes of radio access network 120 in that, among other things, these RNCs control or manage the sets of base stations respectively connected thereto.

As described in more detail herein, for a mobile connection controlled by t o Source RNC 1221, in accordance with the present invention allocation of a diversity handover unit in Target RNC 1222 is not accomplished until Source RNC 1221 has made a decision to move diversity handling operations to Target RNC 1222. In this regard, Fig. 2 depicts radio access network 120 at a time when Source RNC 1221 makes a decision for diversity handling moveover; Fig. 2A shows radio network area 120 at a subsequent time during which diversity handover unit (DHU) 1302 is in the process of being allocated for Target RNC 1222; and Fig. 2B shows radio access network 120 at a yet subsequent time when diversity handling moveover has been completed and a switchover has occurred in Source RNC 1221.

Source RNC 1221 is shown in more detail in Fig. 3 as comprising a switch 1401. Switch 1401, which is controlled by RNC control unit 1421, has a plurality of ports, some of which are connected to diversity handover unit (DHU) 1301 and others of which are connected to various interfaces. Diversity handover unit (DHU) 1301 is connected to a timing unit 1411. The RNC control unit 1421 is connected to each element of RNC 1221.

For reasons explained hereinafter, Source RNC 1221 is connected to a signaling network represented by line 143. Source RNC 1221 is connected to signaling network 143 via a signaling interface 14311. Signaling interface 14311 is connected to RNC control unit 1421.

The interfaces connected to ports of switch 1401 include MSC interface unit 1441; RNC interface unit 1461; and base station interface unit 1481. MSC

interface unit 1441 is connected to mobile switching center 1241. RNC
interface unit 1461 is connected to inter-RNC transport link 132. Base station interface unit 1481 is connected to the first set of base stations (BS) served by Source RNC 1221.
Although base station interface unit 148, is shown logically as one unit serving all base stations, it should be understood that physically base station interface unit 1481 can be a separate io unit for each base station.

Diversity handover unit (DHU) 1301 of Source RNC 1221 is shown in more detail in Fig. 4. In particular, diversity handover unit (DHU) 1301 primarily comprises controller 1601; frame splitter 1621; a set of downlink buffers 1641-1641 _n; a frame selector 1661; and a set of uplink buffers 1681-1 - 1681 _n.
Both frame splitter 1621 and frame selector 1661 receive timing signals on line 1701 from timing unit 1411, and are also connected to controller 1601.

Frame splitter 1621 receives (via switch 1401) input in the form of frames on line 1721 from mobile switching center 1241, while frame selector sends output in the form of frames on line 1741 to mobile switching center 1241. Lines 1721 and 1741 are connected to particular ports of switch 1401 which are internally connectable through switch 1401 to mobile switching center 124 1.

Frame splitter 1621 outputs frames received from mobile switching center 1241 on line 1721 to each of one of the buffers in set 164 which correspond to a base station currently involved in the mobile connection for the mobile station.
Each of the buffers in set 164 is in turn connected to transmit the frames it receives to switch 1401 over lines 1761 _ 1- 1761-n. Lines 1761 _ 1- 1761-n are connected to respective input ports of switch 1401, which input ports are internally cross connectable (e.g., switchable) through switch 1401 to the downlink land lines for the respective base stations BS participating in the mobile connection.

Frame selector 1661 receives frames from various ones of the buffers in the set of up-link buffers 1681 - 168n. Base stations (BS) participating in a mobile connection send frames to their corresponding up-link buffers 1681 - 168n on corresponding ones of lines 1781 _ 1- 1781 _n. Each of lines 1781 _ 1- 1781 _n is connected to corresponding ports of switch 1401, with those ports being internally io switchable through switch 1401 to up-link land lines from the conr-sponding base stations (BS). Thus, when plural base stations are involved in a mobile connection, for plural frames representing the same up-linked information (e.g., frames having the same frame number) frame selector 1661 picks the best frames for transmission (via switch 1401) to mobile switching center 1241.

t 5 As indicated above, controller 1601 of diversity handover unit (DHU) 1301 is connected to frame splitter 1621 and frame selector 1661, for controlling operations thereof, as well as by line 1801 to RNC control unit 1421. RNC
control unit 1421 supervises operation of controller 1601 and coordinates operations of controller 1601 with other elements of Source RNC 1221.

20 Fig. 5 shows Target RNC 1222, according to an example embodiment of the invention, at a particular instance in time corresponding to the time of Fig. 2. Target RNC 1222 includes switch 1402; timing unit 1412; RNC control unit 1422; base station interface unit (BS I/F unit) 1482; MSC interface unit 1442; and diversity handover unit (DHU) 1302. For simplicity, some connections within Target RNC 1222 are not 25 shown, such as connections of all elements both to timing unit 1412 and RNC
control unit 142?.

At the time shown in Fig. 5, mobile station MS has just moved to a geographical location at which it is in radio communication with base stations (BS) 1262,1 and 1262,2, but no longer in radio communication with any base station owned by Source RNC 1221 (see Fig. 2). At the time shown in Fig. 5, no diversitv handover unit (DHU) in Target RNC 122-) has been allocated to the mobile connection for mobile MS. Rather, the legs of the connection are all routed through switch 1402 without being routed through diversity handover unit (DHU) 1302.

In the above regard, as shown iri Fig. 5, switch 1402 has, among its many other ports, base station downlink ports PBD 1-1, PBD 1-2, PBD2-1 and PBD2-2;
base io station uplink ports PBUI-1, PBU1-2, and PBU2-2; ports PRI and PR2 connected to downlink and uplink terminals respectively of RNC I/F unit 1462; and diversity handover unit ports PD I - PD6. As shown in Fig. 5, base station uplink ports and PBU1-2 and base station downlink ports PBDI-I and PBD1-2 are connected through base station I/F unit 1482 to base station (BS) 1262,1; while base station uplink is ports PBU2-1 and PBU2-2 and base station downlink ports PBD2-I and PBD2-2 are connected through base station I/F unit 1482 to base station (BS) 1262.2. At the time shown in Fig. 5, base station uplink ports PBU1 and PBU2 are connected (via switch 1402) to interface uplink port PR2; base station downlink ports PBD1 and PBD2 are connected (via switch 1402) to interface downlink port PR1.

20 Diversity handover unit (DHU) 1302 of Target RNC 1222 shown in Fig.
6, is understood to have essentially the same structure as does diversity handover unit (DHU) 1301 of Fig. 4, with one primary exception. That exception is that, in diversity handover unit (DHU) 1302, lines 1722 and 1742 will ultimately connected by switch 1402 to RNC I/F unit 1462 rather than to a mobile switching center.
Specifically, as 25 explained hereinafter. lines 172,) and 1742 are connectable by switch 1402 to switch ports PRI and PR2 for respective application to downlink and uplink terminals of RNC
I/F unit 1462 (see Fig. 5).

As mentioned above, at the time shown in Fig. 5, no diversity handover unit (DHU) in Target RNC 1222 has been allocated to the mobile connection for mobile MS. It should be realized that Target RNC 122-7 likely does have other diversity handover units connected to switch 1402, and-that those other diversity handover units s are handling mobile connections other than to the particular mobile station MS shown in Fig. 5. Such other diversity handover units would accordingly have routed there through frames involved in other mobile connections, e.g., frames routed from MSC 1242 to the other mobile station and frames routed from the other mobile station (and through various base stations) to MSC 124-). However, for the purposes of the -o mobile connection to mobile MS here under discussion, at the time of Fig. 5 no diversity handover unit of Target RNC 122-) is yet involved.

Fig. 7 shows a sequence of operations involved in a diversity handling moveover procedure according to an embodiment of the invention. Fig. 7 shows in three separate parallel vertical lines the operations conducted by each of Source RNC
15 1221, Target RNC 1222, and the base stations (BS) served by Target RNC 1222 and involved in the mobile communication with mobile station MS.

Operation 7-1 of Fig. 7 shows RNC control unit 1421 of Source RNC
1221 making a decision that diversity handover unit moveover (e.g., diversity handling moveover) should occur. In essence, according to one mode of the invention, RNC
20 control unit 1421 decides to make a diversity handling moveover when Source RNC
1221 no ionger has any of its base stations (BS) 1261 - 1263 involved in the mobile connection for mobile station MS. Logic of RNC control unit 1421 relating to the diversity handling moveover decision is discussed in more detail in connection with Fig. 9.

25 After RNC control unit 1421 has decided to make a diversity handling moveover, RNC control unit 1421 performs operation 7-2. Operation 7-2 involves RNC control unit 1421 allocating or setting up a connection on inter-RNC link 132. In WO, 99/27741 PCT/SE98/02134 connection with operation 7-2, RNC control unit 1421 allocates port of switch 1401 and a port of RNC interface unit 1461 to be used by the new connection on inter-RNC link 132 between Target RNC 1222 and Source RNC 1221. The allocation at operation 7-is for a new connection, i.e., a connection for mobile station MS, since connections for 5 other mobile stations may already exist.

Upon completion of the allocation of operation 7-2, at operation 7-3 RNC
control unit 1421 sends a signaling message including a diversity handling (DHU) moveover request to Target RNC 1222. Signaling messages between RNCs are transmitted over signaling network 143, and can be in accordance with signaling system -o no. 7, for example. Internally in Source RNC 1221 the diversity handover (DHU) moveover request is routed through switch 1401 and RNC interface unit 1461 for application on inter-RNC link 132. The diversity handover unit (DHU) moveover request instructs Target RNC 1222 to allocate a diversity handover unit (DHU) for the mobile connection for mobile station MS and to connect the allocated diversity 15 handover unit (DHU) to the base stations currently used by mobile station MS. The signaling message which includes the diversity handover unit (DHU) moveover request carries the following information: (1) the identity of the connection on the inter-RNC
link 132 to be associated with the connection between the newly allocated diversity handover unit (DHU) and the applicable MSC (e.g., MSC 1241); and (2) the identities of the base stations previously allocated by diversity handover unit (DHU) 1401 of Source RNC 1221 and currently in use for the mobile connection with mobile station MS.

The signaling message from Source RNC 1221 (which includes the diversity handover unit (DHU) moveover request) is routed through switch 1402 to RNC control unit 1422 of Target RNC 1222. The routing of the signaling message to RNC control unit 1422 may include routing through a signaling network. Upon receipt of the signaling message, RNC control unit 1422 performs operations 7-4, 7-5, and 7-6 as shown in Fig. 7. Performance of operations 7-4, 7-5, and 7-6 is reflected by Fig. 5A.

Operation 7-4 involves allocating a diversity handover unit, particularly diversity handover unit (DHU) 1302, to the connection for mobile station MS.
Allocating diversitv handover unit (DHU) 130-) to the connection for mobile station MS
includes connecting, via switch 140,), handover unit (DHU) 130-) to RNC
interface unit 1462. Such connection is accomplished by connecting port PD 1 to port PR2 and connecting port PR1 to port PD2 [see Fig. 5A]. In terms of diversity handover unit (DHU) 1302, such connection means that line 1742 (connected to port PD 1) is connected to port PR2, so that (upon completion of moveover) frames will be sent from frame selector 1662 to RNC I/F 1462, and ultimately via switch 1401 to mobile io switching center MSC 1241 [see Fig. 6]. In addition, line 172,) (connected to port PD2) is connected to port PR1, so that (upon completion of moveover) frames can be sent through switch 1401 and switch 1402 to frame splitter 162,) [see Fig. 6].

Operation 7-5 includes allocating new connections between Target RNC
1222 and the base stations controlled by Target RNC 1222 which are serving mobile station MS, particularly base station (BS) 1262,1 and base station (BS) 1262,2.

Allocating such new connections includes assigning ports PBU 1-2 and PBD 1-2 to the connection respecting base station (BS) 1262, 1, and assigning ports PBU2-2 and PBD2-2 to the connection respecting base station (BS) 1262,2. Further, ports PBU1-2 and PBU2-2 are connected to ports PD3 and PD4, respectively, while ports PBD1-2 and PBD2-2 are connected to ports PD5 and PD6, respectively. Ports PD3 and PD4 of switch 1402 are connected to uplink lines 178,) for eventually transmitting frames to frame selector 1662 [see Fig. 6]. Ports PD5 and PD6 of switch 1402 are connected to downlink lines 1762 for eventually obtaining frames from frame splitter 1622 [see Fig.
6].

Operation 7-6 shows RNC control unit 1422 of Target RNC 1222 sending a signaling message with an additional link setup request to each of the base stations owned by Target RNC 1222 which are serving mobile station MS. These signaling messages are sent over semipermanent connections established at start-up of the base 'V1W0'99127741 PCT/SE98/02134 stations. RNC control unit 1422 knows how to set up these signaling messages in view of the parameters received in the moveover request [see operation 7-3 ]. The signaling messages sent for operation 7-6 each include an identity of the transceiver and an identity of the allocated connection in the link between Target RNC 122-) and the respective base station. Although unillustrated, it should be understood that in sending the signaling messages that RNC control unit 1422 is connected internally through switch 1402 to ports PBD I and PBD2.

The additional link setup request sent to base station (BS) 1262,1 and base station (BS) 1262,2 at operation 7-6 instruct the base stations to connect their i o transceivers in parallel to Target RNC 1222 via the connections specified in the additional link setup request. The transceivers are already connected to Source RNC
1221, i.e., base station (BS) 1262,1 is already connected via ports PBU1-l and PBD1-1;
base station (BS) 1262,2 is already connected via ports PBU2-1 and PBD2-1. The new parallel connections for base station (BS) 1262,1 involve ports PBU2-1 and PBD2-1;

the new parallel connections for base station (BS) 1262,2 involve ports PBU2-2 and PBD2-2. The involved base stations. i.e., base station (BS) 1262,1 and base station (BS) 1262,2, each connect their transceiver to the connection and send a confirmatory response message to Target RNC 1221Target RNC 1222 as operation 7-7 [see Fig.
7].

Operation 7-8 of Fig. 7 is a synchronization procedure between the involved base stations, i.e., base station (BS) 1262,1 and base station (BS) 1262,2, and diversity handover unit (DHU) 130-). Aspects of the synchronization procedure are discussed subsequently.

At the time shown in Fig. 5A, base station (BS) 1262,1 and base station (BS) 1262,2 transmit information routed through Source diversity handover unit (DHU) 1301 to mobile station MS in the downlink. In the uplink, on the other hand, both base station (BS) 1262,1 and base station (BS) 1262,2 send the same received data (although perhaps of differing quality) and control information to both Source diversity handover unit (DHU) 1301 and Target diversity handover unit (DHU) 1302.

When all affected base stations have been synchronized. as operation 7-9 RNC control unit 1422 of Target RNC 1222 sends a signaling message to Source RNC
1221. The signaling message of operation 7-9 includes a switchover request.
Prior to switchover in Source RNC 1221, all traffic on inter-RNC link 132 involving all legs of the mobile connection with mobile station MS from/to RNC I/F unit 1461 were routed through switch 1401 and diversity handover unit (DHU) 1301, as indicated by the lines PRE-X in Fig. 3. Upon receipt of the switchover request, RNC control unit 1421 of Target RNC 1222 performs a switchover procedure as indicated by operation 7-10 [see Fig. 7]. The switchover procedure preferably occurs in between frame transmission in order to avoid frame lost, else synchronization/time alignment must previously be reached between Source RNC 1221 and Target RNC 1222.

In the switchover procedure of operation 7-10, RNC control unit 1421 operates switch 1401 to connect RNC I/F unit 1461 through switch 1401 to MSC
IF/unit 1441, as shown by the line POST-X in Fig. 3. As a result of the switchover, frames from MSC 1241 are routed through switch 1401 for application by RNC I/F
unit 1461 to inter-RNC link 132, and in Target RNC 122~ through switch 1402 and to diversity handover unit (DHU) 1302. The frames are split in diversity handover unit (DHU) 1302 for application to both of base station (BS) 1262,1 (through port PBDI-2) and base station (BS) 1262,2 (through port PBD2-2). Moreover, frames destined for MSC 1241 are routed through switch 1402 to diversity handover unit (DHU) 1302 where they are combined, with a resultant (e.g., best) frame being routed via switch 1402 and applied by RNC I/F unit 146-? to inter-RNC link 132. In Source RNC

the frames are routed through switch 1401 to MSC 1241, bypassing diversity handover unit (DHU) 1301.

As indicated by operation 7-11, upon completion of switchover Source RNC 1221 sends a signaling message to Target RNC 1222 with a switchover confirmation. Referring to operation 7-12, upon receipt of the switchover confirmation, RNC control unit 1422 of Target RNC 1222 operates switch 1402 to release ports PB.U I -l and PBD1-1 formerly used by base station (BS)1262,1, and ports PBD2-1 formerly used by base station (BS) 1262,2. The DHU at Source RNC 1221 thus then becomes an available resource. At this point, the mobile connection with mobile station MS involves only the legs which utilize the base station-connecting ports newly involved in connection with the diversity handling moveover. Fig. 5B shows the legs of the mobile connection for io mobile station MS within Target RNC 122.

In addition to signaling messages such as those described with reference to Fig. 7 (which can be similar to the MAP protocol employed in GSM), inter-RI~1C link 132 carries traffic or user frames as well as dummy frames and synchronization adjustment frames. Examples of traffic frame formats are shown in Fig. 8 and Fig. 8A.
Fig. 8 particularlv shows an uplink traffic frame which has a frame type field 8- 1; a frame number field 8-2; a quality indicator field 8-3; and, a user traffic data field 8-4.
Fig. 8A shows a downlink traffic frame which has a frame type field 8A-1; a frame number field 8A-2; and, a user traffic data field 8A-4.

An example of contents of the quality indicator fields 8-3 and 8-4 is a 2o received signal-to-noise ratio (SIR) for the frame. In addition, a checksum result for the frame could be included in the quality indicator fields 8-3 and 8-4. The quality indicator is used by the frame selector in the diversity handover unit (DHU) to select and forward (to the appropriate mobile switching center MSC) the user traffic data of the best frame. of all of the frames received with a given frame number from all base stations involved for the connection with mobile station MS. The frame splitter, on the other hand, copies the user traffic data to all base stations involved in the connection for mobile station MS. and attaches the frame number for field 8A-2 [see Fig. 8A].
The WA,99l,27741 PCT/SE98/02134 timing unit (e.g., timing unit 1411) supplies the diversity handover unit (DHU) with a frame number and with a frame number clock reference.

A dummy frame, an example of which is illustrated in Fig. 8B, can be used for establishment of communication and synchronization when no real traffic is to 5 be sent. The dummy frame is also used in the downlink from Target diversity handover unit (DHU) 1302 to the base stations before switchover to establish synchronization.
The example dummy frame of Fig. 8B includes a frame type field 8B-1, a frame number field 8B-2, and a dummy data field 8B-4.

A synchronization frame, such as that shown in Fig. 8C, is employed to io carry synchronization adjustment values. The example of the synchronization frame shown in Fig. 8C includes a frame type field 8C-1 and an adjustment value data field 8C-4. In one mode of the invention, the adjustment value data field 8C-4 contains a positive or negative time offset value.

As mentioned above, such as with reference to operation 7-1 of Fig. 7, for 15 example, RNC control unit 1422 of Source RNC 1221 determines when to initiate a diversity handover unit moveover procedure. In the preferred embodiment, such decision is based on optimization of transmission resources. One example of logic implemented in the course of the diversity handling moveover decision is reflected in Fig. 9.

20 The diversity handover unit moveover decision logic of Fig. 9 involves (after an initialization step 9-1) a periodic determination (at step 9-2) whether any base station managed by Source RNC 1221 is involved in the mobile connection with mobile station MS. If the determination at step 9-2 is negative, execution loops back to re-check the determination of step 9-2 at a future time. However, should the determination of step 9-2 be positive, i.e., no base station managed by Source RNC 1221 is involved with mobile station MS, a Timer Tl is set (step 9-3). As seen hereinafter, Timer T1 is employed to counteract a potential ping-pong effect. As indicated by steps 9-4 and 9-5, if a base station managed by Source RNC 1221 is added to the mobile connection for mobile station MS before Timer T1 expires, the potential DHU moveover is quelched and (at step 9-6) Timer T1 is stopped and reset prior to looping back to step 9-2. If Timer T 1 expires before a base station managed by Source RNC 1221 is added to the mobile connection for mobile station MS, steps 9-7 and 9-8 are performed. At step 9-7, another RNC is selected as the Target RNC. Once the Target RNC has been selected, the DHU moveover procedure as described in Fig. 7 is performed (as represented by step 9-8 of Fig. 9).

In connection with step 9-7 of Fig. 9 described above, it is possible that mobile station MS may be in radio contact with base stations, some of whom belonging to differing RNCs. For example, in the scenario depicted in Fig. 10, Source would have to select between Target RNC 1222 and Target RNC 1223 as candidates for the diversity handling moveover procedure. It will be appreciated that yet more RNCs may be in contention for receipt of the diversity handling function, the configurations shown herein being exemplary and not limiting.

Variations of the diversity handling moveover decision logic of Fig. 9 occur in differing embodiments. Whereas the example of Fig. 9 requires that no base station be managed by Source RNC 1221 before the diversity handling moveover is performed, other variations have differing moveover criteria relative to the number of base stations managed by the Source RNC 1221 and the Target RNC 1222. For example, moveover criteria may be satisfied in one variation in which Source RNC
1221 manages only one base station involved in the mobile connection, and Target RNC 1122 manages a predetermined number (greater than one) of base stations involved in the mobile connection.

As an example of another variation, information regarding directionality of movement of mobile station MS may be considered for determining when to perform the diversity handling moveover. In this regard, RNC control unit 1421 can administer a database wherein is stored a log of coordinates or other geographical indicative information ascertained from mobile station MS or actually reported by mobile station MS. By analysis of such log, RNC control unit 1421 can determine a vector of current movement for mobile station, or a prediction based on prior history of movement of mobile station, and use such vector or prediction for determining a point at which diversity handling moveover would optimize resources of the radio network area.

Information regarding directionality of movement of mobile station MS
may be used as a factor when selecting among plural RNCs for a new DHU situs is io required [see, for example, selection step 9-7 of Fig. 9].

Another variation of the diversity handling moveover decision logic of Fig. 9 is based on transmission cost. According to this variation, total transmission costs for an existing connection to mobile station MS (including all legs reaching all base stations) is first determined. Then, for each potential RNC to which diversity handling could be moved, a total cost is determined for the connection for mobile station (again including all legs reaching all base stations). Total cost is computed by an operator-defined or self-configured metric, and includes the cost of transmission between any pair of RNCs and the cost of transmission between an RNC and the base stations it manages. In accordance with this variation, the diversity handling moveover is performed when an RNC other than the current Source RNC is determined to provide lower transmission costs (including threshold margins).

Fig. 9A shows an example of diversity handing moveover decision-making which is based on such factors as transmission cost as summarized above. Step 9A-1 shows starting and initialization of the decision-making logic of Fig.
9A. At step 9A-2, it is determined whether a base station has been added or removed from the set of base stations involved in the soft handover servicing of the mobile connection. If there is no change in the membership of the set of base stations, the logic of Fig.
9A loops back to step 9A-1 so that the membership can be respectively monitored.

If the membership of the set does change, at step 9A-3 a pool of candidate RNC nodes is developed. The pool does not include the RNC node at which the DHU
currently resides. Then, at step 9A-4 the fictive transmission cost for each candidate RNC node in the pool is calculated. Similarly, at step 9A-5 the fictive transmission cost for the node where the DHU currently resides is calculated.

At step 9A-6, it is determined whether it is cost effective to have a DHU
moveover. The cost effectiveness is determined by (1) finding the candidate RNC node io with the least fictive transmission cost, (2) adding a marginal or threshold margin cost factor to the fictive transmission cost of the least expensive candidate RNC
node, and (3) comparing the same with the fictive transmission cost of the RNC node where the DHU currently resides. If the RNC node where the DHU currently resides remains the least expensive, no DHU moveover is necessary and the logic loops back to step 9A-1.

1s If it is determined at step 9A-6 that a DHU moveover is cost effective, at step 9A-7 the candidate RNC node with the least fictive transmission cost is selected to be the target RNC node. Then, at step 9A-8 the DHU moveover to the Target RNC
node (selected at step 9A-7) is initiated. If it is determined at step 9A-9 that the DHU
moveover was a success, the logic of Fig. 9A terminates as shown by step 9A-10.

20 Otherwise, the logic of Fig. 9A is re-started by a loop back to step 9A- 1.

The calculations of steps 9A-4 and 9A-5 to determine fictive transmission costs are now described for a representative node (RNC(i). It is assumed for sake of discussion that there are "j" number of base stations included in the set involved in soft handover for the mobile connection, and that each base station BS(I) is described as 25 being controlled by RNC(r(j)). Then, the fictive transmission cost C(i) for node RNC(i) is determined as follows:

; WO 99/27741 PCT/SE98/02134 c(i) cost(RNC(i) to RNC(r(j))) + cost(RNC(anchor) to RNC(i)).

The meaning of "anchor" will hereinafter be explained, but in the mode illustrated in Fig. 2. The anchor RNC can be presumed to be the Source RNC.
The cost for the various RNC to RNC connections can be set or input by operation command.

Alternately, information can be utilized from a routing protocol such as PNNI, specified by ATM-Forum. One possibility is to use the number of intermediate switching nodes as a metric. For example, the cost between two RNC nodes can be the number of intermediate switching nodes in the transmission path between the two RNC
nodes.

In certain situations, a mobile connection for mobile station MS may be to supported by two or more diversity handover units (DHUs) in parallel, e.g., for different logical channels/services). An example of such a situation is a mobile station having plural services (e.g. multimedia services), in which each service has one of a corresponding plurality of logical channels in parallel. These logical channels may have different requirements for bitrate, delay, and other characteristics. In this example, one DHU serves one logical channel. In the base station the logical channels for such a mobile connection are multiplexed with code channels). Thus, there is a base station-RNC(DHU)-MSC connection for each logical channel. Even if the logical channels are handled together over the radio interface (e.g., BS-MS), they can be treated separately after the base station. In such case, each DHU can be moved independently using the 2o DHU moveover procedure of the present invention. The DHU for the control channel signaling defines the role of the Source RNC and thus is moved last.

As employed herein, synchronization includes the principle that existing timing on the radio interface is maintained, e.g., frames are exchanged between the base stations and the mobile station at, e.g., 10 millisecond intervals in a certain phase. After connections have been set up between Target RNC 1222 and all involved base stations VKO-99/27,741 PCT/SE98/02134 (e.g., base station (BS) 1262,1 and base station (BS) 1262,2), a series of events occur between Source RNC 1221 and Target RNC 1222.

In the uplink between Source RNC 1221 and Target RNC 122-), a base station transmits frames received from mobile station MS to both Target diversity 5 handover unit (DHU) 1302 and Source diversity handover unit (DHU) 1301. The set of uplink buffers 168 of diversity handover unit (DHU) 1302 receive frames from different (e.g., both) base stations. For frames having the same frame number for the same connection, frame selector 1662 selects the frame with best quality indicator [see Fig.
8]. The uplink frames are forwarded to Source RNC 1221 (at least before the 10 switchover is requested from Target RN 1222). Target diversity handover unit (DHU) 1302 indicates to RNC control unit 1422 when synchronization is established.

In the downlink, diversity handover unit (DHU) 1302 determines an initial departure time for numbered downlink frames based on the received numbered uplink frames, and starts transmitting numbered downlink dummy frames [see Fig. 8B].

15 The base stations receive the numbered dummy frames and instruct diversity handover unit (DHU) 1302 to adjust its departure time if the frames arrive too early or too late.
Further, the base stations indicated to RNC control unit 1422 when synchronization is established.

When Target RNC 1222 determines that synchronization is established 20 for both uplink and downlink, Target RNC 1222 so informs Source RNC 1221 using the switchover request of operation 7-9 [see Fig. 9]. At that point, Source RNC

performs the switchover procedure 7-10 as described above. When valid frames start to arrive from Target RNC 1222 to each involved base station (e.g., base station (BS) 1262,1 and base station (BS) 1262,2 ), the base stations start transmitting (to mobile 25 station MS) the frames received from Target RNC 1222 rather than the frames received from Source RNC 1221.

If the total delay is increased using diversity handover unit (DHU) 1302 of Target RNC 1222, certain activities may occur. For example, if time alignment to the mobile switching center (MSC) is supported, diversity handover unit (DHU) can send commands to the mobile switching center (MSC) to adjust its timing.
During this adjustment process, however, some frames may be lost. If time alignment to the mobile switching center (MSC) is not supported, a frame slip may occur. To avoid this problem, an initial connection should be set up with sufficient delay margins (e.g., some extra buffering in diversity handover unit (DHU) 1301).

Fig. 2 - Fig. 2C illustrates diversity handling moveover, e.g. moveover io from DHU 1301 of RNC 1221 to DHU 1302 of RNC 1222, as mobile station MS
traveled to utilize only base station BS 1262,1 and 1262,2 controlled by RNC
1222.
Fig. 11 - Fig. 11C progressively show stages of a return diversity handling moveover as mobile station MS travels back to utilize only base station BS 1261,2 and 1261,3 controlled by RNC 1221.

The procedures and operations involved in the return of diversity handling moveover, especially as depicted in Fig. 1 lA - Fig. 11C are understood from analog to the procedures and operations aforedescribed with respect to Fig. 2 - Fig. 2B, respectively. However, since the DHU allocated in RNC 1221 upon return diversity handling moveover is, in fact, newly allocated and not necessarily the same as former 2o DHU 1301, the new DHU of RNC 1221 is referenced in Fig. I 1B - Fig. 11C as DHU
1301.

The return diversity handling moveover of Fig. 11-Fig. 11 C is conducted under the condition that DHU 1302 of RNC 1222 is connected via RNC 1221 to the MSC 1241 handling the mobile connection. Thus, until diversity handling moveover returns back to RNC 1221 as depicted in Fig. 11 C, the inter-RNC link 132 carries both (1) frames between MSC 1241 and DHU 1302, (2) frames between DHU 1302 and base WO I9%27741 PCT/SE98/02134 station BS 1261,2, and (3) frames between DHU 130-) and base station 1261,3 [see Fig.
1 lA]. MSC 1241 is connected to RNC 1221.

The situation depicted in Fig. 1 lA - Fig. 11C of having inter-RNC link 132 carrying so many frames can be obviated if, at an appropriate time (such as the time between the times shown by Fig. 2B and Fig. 11, for example) there is a change of MSC

handling the mobile connection. For example, traffic on inter-RNC link 132 can be reduced if the mobile connection is changed such that MSC 1242 is utilized rather than MSC 1241. When such a change of MSC is implemented, the return diversity handling moveover back to RNC 1221 is as shown in Fig. 12-Fig. 12C rather than as in Fig. 11-io Fig. I 1 C. At the time shown in Fig. 12C, consideration ought to be given whether the mobile connection should change back to using MSC 1241 rather than MSC 1242.
Modes of the invention previously described have involved a Source RNC

1221 and a Target RNC 1222, one of which is connected to the MSC handing the mobile connection without an intermediate RNC. Fig. 13 and Fig. 13A depict a different mode in which the mobile connection utilizes MSC 1240 and wherein RNC
1220 functions as an "anchor" RNC. An "anchor" RNC is an RNC most closely connected to the pertinent MSC for the mobile connection and is the RNC which performs the switchover of a diversity handling moveover to connect a new DHU.

In reality, all modes of the invention involve three logical RNCs for performing diversity handling moveover: (1) an "anchor" RNC which performs the switchover to connect a new DHU; (2) a "source" RNC which controls the old DHU
and which initiates the diversity handling moveover procedure; and (3) a "target" RNC
which controls the new DHU (and which becomes the source RNC after the diversity handling moveover procedure is completed). In the preceding mode of Fig. 2-Fig. 2B, RNC 1221 functioned both as the source RNC and the anchor RNC. In the modes of Fig. I l-Fig. 11C and Fig. 12-Fig. 12C, on the other hand, RNC 1222 functioned as the anchor RNC 1220.

In the mode of the invention shown in Fig. 13 and Fig. 13A, anchor RNC
1220 performs the switchover whereby the diversity handling is moved over from DHU
1301 of Source RNC 1221 to DHU 130-) of target RNC 122-). Fig. 13 shows the situation prior to diversity handling moveover; Fig. 13A shows the situation after completion of diversity handling moveover.

Fig. 14 shows a sequence of operations involved in a diversity handling moveover procedure according to the mode of Fig. 13-Fig. 13A (i.e., a mode in which an "anchor" RNC other than the source RNC or the target RNC perform the switchover). Operation 14-1 shows the Source RNC 1221 making a decision that a diversity handling moveover should occur. Such decision can be made according to any of the various criteria herein described, such as the criteria of Fig. 9 or Fig. 9A, for example. Once the diversity handling moveover decision has been made, set operation 14-2 source RNC 1221 sends a "diversity handling (DH) moveover initiation"
message to anchor RNC 1220. Then, at operation 14-3, anchor RNC 1220 sends a diversity moveover request message to Target RNC 1222. In response, operation 14-4 shows Target RNC 1222 allocating a diversity handling unit (e.g., DHU 1302) to be the recipient of the moveover. Details of DHU allocation of operations 14-4, like various other operations of Fig. 14, are understood with reference to analogous operations previously described in connection with Fig. 7. Upon completion of the DHU

2o allocation, at operation 14-5 target RNC 1222 sends anchor RNC 1220 a "DH
moveover proceed" message.

After Target RNC 1222 indicates set operation 14-5 that DH moveover may proceed, at operation 14-6 anchor RNC 1220 sends a"transmission path SETUP"
message to Target RNC 1222. The transmission path SETUP message of operation 6 serves to set up an inter-RNC transmission link connection between anchor RNC
1220 and Target RNC 1222. The message of operation 14-6 may be sent through intermediate switching nodes or via a signaling network. In some embodiments, the connection set up by operation 14-6 may be set up all the way from Anchor RNC

to each active base station. Upon establishment of the inter-RNC transmission link connection, at operation 14-7 Target RNC 1222 sends a "transmission path CONNECT" message to Anchor RNC 1220.

Operation 14-8 involves Target RNC 1222 sending an "additional link setup" message to each active base station served by Target RNC 1222 for the mobile connection. In response, each active base station returns an "additional link setup response" message to Target RNC 1222 (operation 14-9). Then, at operation 14-10, synchronization occurs between Target RNC 1222 and each active base station.

Upon obtaining synchronization, at operation 14-11, Target RNC 1222 to sends a "switchover request" message to Anchor RNC 1220. In response, at operation 14-12, the Anchor RNC 1220 performs the switchover procedure. After completion of the switchover, at operation 14-13, Anchor RNC 1220 sends a "DHU release"
message to Source RNC 1221. In response, at operation 14-14, Source RNC 1221 returns a "DHU release confirmation" message. Further, as indicated by operation 14-15.
Source RNC 1221 sends a "transmission path release" message to Source RNC 1221 as reflected by operation 14-16.

At operation 14-17, Source RNC 1221 sends a series of "transmission path release" messages to Target RNC 1222, one message for each active base station.
In response, at operation 14-18 the Target RNC 1222 returns a same number series (one for each base station) of "transmission path release complete" messages to Source RNC
1221.

At operation 14-19 Source RNC 1221 sends a "DH release complete"
message to Anchor RNC 1220. Anchor RNC 1220 responsively sends at "switchover confirmation" message to Target RNC 1222 at operation 14-20. Upon receipt of the "switchover confirmation" message, Target RNC 1222 sends, to each active base station, a "release link to old DHU" message (operation 14-21). In return, each active base station sends a "release link response'.' message to Target RNC 1222 (operation 14-22).

Various ones of the messages described in connection with Fig_ 14 may be sent via intermediate switching nodes. Moreover, various operations or 5 combinations of operations may be performed in parallel. For example, operations 8-11 may be performed in parallel with operations 6-7. Further operation 15-16 may be performed in parallel with operation 17-18. Message (e.g. signals) for transmission path set up, connect. release, and complete as used herein are similar to ISUP.

The mode of Fig. 13 and Fig. 13A shows a generalization of earlier to depicted modes. It should be accordingly realized that the present invention encompasses systems having any number of radio network control nodes.

The present invention also encompasses situations in which plural diversity handover units (DHUs) are moved together. Fig. 15 shows general operations performed in conjunction with a plural DHU moveover, in particular "N" number of 15 DHUs. At operation 15-1. the target RNC allocates N number of target DHUs.
Operation 15-2 shows N connections being set up between the Anchor RNC and the Target RNC. At operation 15-3, each active base station is connected to cell N
target DIIUs. Then, the anchor RNC performs N number of switchover operations (operation 15-4). As shown in operation 15-5, on each of the N connections, the base station 20 transmits data from Target DHU when it arrives, and before that data from the source DHU.

It should be understood, however, that the switchover from Source DHU
to Target DHU need not be simultaneous for all N connections. The switchover must not be done for all N connections to a mobile station.

In the embodiments described herein, for sake of simplicitv only a limited number of base stations (BS) have been shown as connected to each RNC. It should be understood that differing and greater numbers of base stations (including just one base station) can be connected to each RNC, and accordingly that the number of downlink and uplink buffers in each diversity handover unit (DHU) is dependent thereon.
Moreover, it should be understood that the radio access network, e.g. radio access network 120, can have RNCs and base stations other than and additional to those illustrated.

In the foregoing discussion, the MSC I/F units and the RNC I/F units have t o been illustrated as separate interfaces. It should be understood, however, that these interfaces can be combined into a single transport network interface unit.

In the embodiments herein illustrated, the connection between RNCs has been shown as a direct link. It should also be understood that such connection can instead be switched via any intermediate node. For example, in an embodiment in 1s which such connection is switched, the intermediate node can be an MSC, an RNC, or a node in the public switched telephone network (PSTN).

Moreover, it should be understood that an RNC most probably is connected to several other RNCs (e.g., as many as eight), and that plural inter-RNC
20 links may accordingly be provided. Thus, reference herein to connection to an inter-RNC transport links refers to an appropirate one of a potential plurality of such links.
The reader should appreciate that plural RNCs can be collocated to form "clusters", and that from outside the cluster the cluster is viewed as being a larger "logical" RNC. A physical inter-RNC connection can thus be within the bigger logical 25 RNC as well as between RNC clusters.

All scenarios and modes of diversity handling moveover described herein presume that handover (i.e., the adding and/or removing of base station legs) is inhibited until the diversity of handling moveover procedure is completed.

The present invention provides numerous advantages, including optimization of resources of the network area. For example, the duration of utilization of legs between base stations and diversity handover units (DHUs) is reduced.
Moreover, the present invention economizes transport resources on the inter-RNC links.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be to understood that the invention is not to be iimited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of operating a telecommunications system including two or more nodes each of which controlling an assigned set of base stations, said method comprising the steps of:
using a first diversity handling unit at a first node to perform connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations;
determining a transfer of the connection combining splitting operation to a second diversity handling unit based on transmission costs;
allocating the second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and selecting between the first node and a second node as situs for the second diversity handling unit.

2. The method according to claim 1, further comprising determining that the connection combining and connection splitting operations should be transferred in accordance with one of addition or disconnection of a base station.

3. The method according to claim 1, further comprising determining that the connection combining and connection splitting operations should be transferred in accordance with directional movement of a mobile station involved in the mobile connection.

4. The method of any one of claims 1 to 3, further comprising determining that the connection combining and connection splitting operations should be transferred when a predetermined relationship exists between a first number of base stations served by a mode whereat the first diversity handling unit resides and a second number of base stations served by a node whereat the second diversity handling unit is to reside.

5. The method of any one of claims 1 to 4, further comprising the step of performing at an anchor node that is physically distinct from the first and second node a switchover operation in order to remove the first diversity handling unit from the mobile connection.

6. The method according to claim 5, further comprising the step of routing uplink frames from a base station participating in the mobile connection to both the first diversity handling unit and the second diversity handling unit after the allocating and before the performing of the switching operation.

7. The method according to claim 5, further comprising the step of choosing a second mobile switching center to replace a first mobile switching center for handling the mobile connection.

8. A telecommunications system comprising:
plural nodes each controlling a corresponding set of base stations;
a first diversity handling unit residing at a first of the plural nodes, said unit performing connection combining and connection splitting operations for a mobile connection maintained through a plurality of base stations and said unit determining a transfer of the connection combining splitting operation to a second diversity handling unit based on transmission costs, the first node comprising means for allocation of a second diversity handling unit to perform the connection combining and connection splitting operations only after it is determined that the connection combining and connection splitting operations should be transferred from the first diversity handling unit; and means for selecting between remaining ones of the plural nodes as situs for the second diversity handling unit.

9. The telecommunications system according to claim 8, wherein the second diversity handling unit requests the first diversity handling unit to perform a switchover operation after completion of the allocating in order to remove the first diversity handling unit from the mobile connection.

10. The telecommunications system of claim 8 or 9, wherein during the allocation a base station participating in the mobile connection routes uplink frames to both the first diversity handling unit and the second diversity handling unit.