METHOD AND SYSTEM FOR HANDOVER OF A SHARED CHANNEL IN A CDMA CELLULAR MOBILE RADIO SYSTEM

Abstract: The present invention relates to cellular mobile radio system, and more especially it relates to Code Division Multiple Access, CDMA, cellular mobile radio systems, particularly to handover of a downlink shared or common channel in such systems. A method and system for hard handover of a shared channel within a dynamic Active Set is proposed.

Description: Method and system for handover of a shared channel in a CDMA cellular mobile radio system TECHNICAL FIELD OF THE INVENTION The present invention relates to cellular mobile radio systems, and more especially to handover of a common or shared channel in such a system, particularly in a Universal Mobile Telecommunications System, UMTS. BACKGROUND AND DESCRIPTION OF RELATED ART To conduct handover from one radio channel, connecting a mobile station to a first radio base station, to another radio channel, connecting the mobile station to a second radio base station, as the mobile station moves in a cellular mobile radio system is well known. When each radio base station covers an area, a cell, a radio channel is handed over as the mobile station moves from one cell to another. Use of shared or common channels, particularly for signaling purposes, is known from prior art. A shared channel or common channel is shared between or common to several users as opposed to a dedicated channel, dedicated to a particu- lar user or a particular purpose. Either the same channel is used for dedicated use and shared purposes (inbound signaling) or different channels are used for these services (outbound signaling) , generally both services are handed over simultaneously with the change of base stations, according to prior art. From prior art it is also known to hand over dedicated channels from one sector to another associated with one single base station in so-called sectorized cells. In this case it is also known to use shared channels that may be common to the various sectors and consequently does not need to be handed over as the mobile station moves between the sectors associated with the base station. International patent application WO0031988 describes a method of selecting a master cell from a set of cells, a so-called Active Set. The international patent application describes a rule where the cell, which has been in the Active Set for the longest time, is selected as the master cell. Upon removal from the Active Set a new master cell, chosen in accordance with the rule, replaces the existing master cell. International patent application WO0035206 discusses the use of a Downlink Shared Channel, DSCH, and its synchronization to a Broadcast Control Channel, BCCH, and related difficulties of using soft handover of a DSCH. It suggests synchronization of the DSCH to a Dedicated Channel, DCH, in place of the BCCH. International patent application W09914972 considers the load of real-time data and non real-time data in the process of selecting a base station during handover. It also incorporates the total transmission power used by the base station as a parameter. U. S . Patent US5781861 reveals that traffic density and power allocation are used as handover parameters. European patent application EP0876008 discusses the inter- relationship between spreading factor and data rate. SUMMARY OF THE INVENTION To only use base station selection criteria based on power transmitted to/from one single user equipment, UE, is inadequate in several respects. An Active Set is a Set of radio links simultaneously involved in a specific communication service between a User Equipment and a UTRAN access point. Within an Active Set several base stations may perform almost equally well as regards received signal strength. To simply select the base station yielding the strongest received signal for hard handover of a shared channel will then cause an excessive number of handovers. Consequently, an object of this patent application is to present a selection process for selection of a new base station for hard handover in relation to the cells associated with an Active Set. It is also an object to achieve relevant decision criteria for handover that are related to traffic load/congestion and to the total power consumption at a radio base station, including the power transmitted to all mobile stations communicating with the RBS, particularly on a shared channel. These objects are met by using a dynamic Active Set oriented cell selection for handover of a common downlink packet data channel, such as a DSCH, the selection being traffic related, power related or link related. Preferred embodiments are grouped into two fundamental embodiments. According to a first fundamental embodiment of the invention, the common packet data channel of a serving base station is not handed over to another base station, until the radio link between the User Equipment and the serving base station is scheduled, in accordance with a specified criterion, to be excluded from the Active Set. When the handover is completed, the base station to which the common packet data channel is handed over acts as the serving base station. According to a second fundamental embodiment of the invention, the best cell, in accordance with a specified criterion, is selected within the Active Set as a serving cell of the common packet data channel. Preferred embodiments further reveals related criteria for handover. Preferred embodiments of the invention, by way of examples, are described with reference to the accompanying drawings below. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 displays an exemplary code tree and corresponding spreading factors, according to prior art. Figure 2 illustrates schematically a radio communication system and a first Active Set according to the invention. Figure 3 illustrates schematically a radio communication system and a second Active Set according to the invention. Figure 4 illustrates schematically a radio communication system and a third Active Set according to the invention. Figure 5 illustrates schematically a radio communication system and a fourth Active Set according to the invention. DESCRIPTION OF PREFERRED EMBODIMENTS A shared or common channel, such as a Downlink Shared Channel, DSCH, need not be time aligned between cells. As a consequence, soft handover of a shared or common channel is not immediately available for this channel. A DSCH is paired with a Dedicated Physical Channel, DPCH. The Dedicated Physical Channels of various cells are time aligned and accessible for soft handover. The different time alignment of associated channels raises problems when it comes to handover of channels not being time aligned. Hard handover and soft handover as such are well known. Using the terminology of figures 2-5, hard handover is a category of handover procedures where all the old radio links in the User Equipment UE are abandoned before new ra- dio links are established whereas soft handover is a category of handover procedures where the radio links are added and abandoned in such manner that the User Equipment UE always keeps at least one radio link to a Base Station (BS1-BS6) . Similar measures, such as energy to noise ra- tio, E c /No, or energy to interference ratio on a Primary Common Pilot Channel, PCPICH, Received Signal Code Power, RSCP, on the PCPICH or downlink path loss, form a basis of a performance criterion used for soft and hard handover. However, hard handover, according to prior art, of a common or shared channel will result in inferior throughput. The proposed hard handover within a dynamic Active Set is characterized by criteria to be used within the dynamic Active Set such as instantaneous link quality; average link quality; persistent link quality; total downlink power; us- age of Physical Downlink Shared Channel, PDSCH; maximal PDSCH power; real time to non-real time traffic ratio or spreading factor as will be explained in detail below. Handover is not restricted only to handover between base stations. However, for reasons of linguistic simplicity the specification describes the handover process according to the invention as a handover between base stations . It should be clear that handover between antenna sectors using separate shared channels is included as well. In accordance with this specification each such sector will be treated as a base station. Also included is interfrequency handove . Interfrequency handover is performed between separate carrier frequencies or frequency bands . Each such carrier frequency or frequency band is treated as a base station below. In Code Division Multiple Access, respective digital user signals are separated by individual spreading codes . Each code represents a spreading factor. Sub-channels or lower rate channels are separated by sub-codes with a greater spreading factor. Figure 1 illustrates the interrelations between data rates/sub-channels and sub-codes/spreading factors . It displays a binary code tree where a spreading factor, SF, equal to one occupies the entire code tree, i.e. all levels indicated by SF=2 , SF=4, SF=8 and SF=16. This spreading factor translates into the highest user data rate. A sub-code with a spreading factor equal to two occupies only half the tree, the leftmost sub-tree or the rightmost sub-tree, in each case the sub-tree occupying the levels indicated by SF=4, SF=8 and SF=16 preventing other users from using the corresponding codes. Similarly, if a user is assigned a sub-code corresponding to a spreading factor equal to four, he occupies a sub-tree (and corre- sponding sub-channels) originating from the level indicated by SF=4 down to SF=16. Spreading factor four represents a medium user data rate. A user allocated a channel of spreading factor 16 occupies the lowest data rate in this exemplary figure. Spreading codes for SF=1, SF=2 and SF=4 are denoted at the root of the corresponding sub-tree. This code tree is only an example not limiting the scope of the invention. Figure 2 illustrates schematically a radio communication system. User Equipment UE is subject to soft handover in- volving radio links associated with Base Stations BS3 , BS4 and BS5 in the Active Set. One of the BSs is in charge of communication on the DSCH. Figure 2 also illustrates Base Stations BS1, BS2 and BS3 connected to a Serving Radio Met- work Controller, Serving RNC or SRNC. The Serving RNC controls the connection between a Core Network and the UE. The Serving RNC is connected to the Core Network over a so- called Iu interface, Iu. The Serving RNC is also connected to another Radio Network Controller, RNC, called Drift RNC or DRNC. In the figure the Drift RNC controls Base Stations BS4, BS5 and BS6. The Serving RNC and the Drift RNC are connected to the controlled base stations over a so- called Iub interface. The RNCs (SRNC, DRNC) are intercon- nected over an Iur interface. As the User Equipment UE dissociates from BS2 , it will finally be excluded from the Active Set when path loss increases and channel quality decreases, as schematically shown in figure 3 where the Active Set comprises links associated with BS3 and BS4. In figure 4, in comparison to figure 3, two more links associated with Base Stations BS5 and BS6 have been included in the Active Set . In figures 2 , 3 and 4 the Serving RNC remains the same . In figure 5, the link associated with BS3 is excluded from the Active Set due to channel and traffic situation. In figure 5 SRNC maintains the connection to the core network. However, as all Base Stations (BS4, BS5, BS6) associated with links involved in the Active Set are controlled by one RNC, the DRNC in figure 5, this RNC could be SRNC (inter- changing the roles of the two RNCs) in figure 5 if SRNC-re- location were used. Preferred embodiments are grouped into two fundamental embodiments. In a first fundamental embodiment of the invention, the common packet data channel of the serving cell is not handed over until its associated radio link is scheduled, in accordance with a specified criterion, to be excluded from the Active Set. Considering a process where base stations are excluded and added as in the sequence of figures 2-5 and where initially BS3 is the serving base station of the common packet data channel, it will not be handed over until figure 5 where the radio link associated with BS3 is excluded from the Active Set. Criteria for selection of a serving cell associated with a radio link within the Active Set, comprising Base Stations BS4, BS5 and BS6 is set out below. According to a second fundamental embodiment of the inven- tion, the serving cell is handed over whenever another cell with an associated radio link within the Active Set shows superior performance according to some specified criteria as set out hereinafter. In a time-varying environment with rapidly changing channel quality, measures need to be un- dertaken in order not to increase the number of handovers as further explained in the proceeding . In WCDMA/UMTS a Universal Terrestrial Radio Access Network, UTRAN, chooses the serving base station routing a DSCH to the UE. Rapid time variations can be averaged out by long term filtering. An alternative is to prescribe that any change of a parameter forming a basis for handover shall persist a specified amount of time for a handover to be initiated. If, in one embodiment in accordance with the second fundamental embodiment, a performance criterion in- dicates that a base station does not have the best channel to/from the UE within the Active Set, a handover is initiated only if this condition persists a specified amount of time . For embodiments in accordance with the second fundamental embodiment it can also be advantageous to use a hysteresis, handing over a DSCH to another base station only if the performance measure of the candidate base station supersedes the performance measure of the presently serving base station by a specified amount, i.e. introduc- ing a threshold. Of vital importance to the invention is that the criteria are only applied within the Active Set. Consequently, a base station not included in the Active Set is never a candidate base station for a handover of the shared channel. For the purpose of illustration, BS3 in figure 5 is not a candidate serving base station, as it is not associated with a link within the Active Set. In figure 5 only BS4, BS5 and BS6 may be handover candidates for the shared channel as they are the only base stations with associated radio links included in the Active Set. When it comes to possible choices of parameter selections using one single parameter, common in prior art, introduces problems regarding throughput. Preferred embodiments in accordance with the first and second fundamental embodi- ments implement some particular parameter selections advantageously used for cell selection from a dynamic Active Set in order to increase throughput. The following handover parameters, one or more of which to be combined with at least one channel criterion described above, for comparing handover candidate base stations associated with links within the Active Set are proposed, - total transmitted downlink power from the base station or antenna sector, - PDSCH occupancy, - lowest spreading factor available for PDSCH, - maximum power available for PDSCH, and - ratio of real-time to non real-time traffic. Total transmitted downlink power reflects the margin to congestion. Occupancy is measured as the fraction of time the PDSCH is occupied or used, as opposed to idle. Balancing PDSCH occupancy or usage within an Active Set will also increase fairness between users in different cells associ- ated with links of the Active Set. A small spreading factor corresponds to a high bit rate. The smaller the ' spreading factor available the larger the sub-tree of a code tree available. The error rate depends on received bit energy. Everything else being kept equal for a power- limited channel, a bit rate increase implies a requirement for minimum transmission power to increase, in order not to increase the error rate. Consequently, with a high maximum power available for PDSCH the data rate on the channel can be increased without increasing the error rate more than allowed for. If the ratio of real-time traffic to non real-time traffic is high it is generally beneficiary to select another cell with a lower ratio . The requirement on the real-time traffic ratio is motivated by e.g. - it protects real-time traffic, and - it enables free capacity to be used by PDSCH. The invention is not intended to be limited only to the embodiments described in detail above. Changes and modifications may be made without departing from the invention. It covers all modifications within the scope of the following claims .

Claims: CLAIMS 1 A method of handover of a shared channel from a first radio entity to a second radio entity, the method c h a r a c t e r i z e d i n that only one or more radio entities associated with one or more radio links in a dy- namic active set of a user equipment are considered candidates for handover of the shared channel. 2. The method according to claim 1 c h a r a c t e r i z e d i n that the radio entities considered handover candidates are base stations . 3. The method according to claim 1 c h a r a c t e r i z e d i n that the radio entities considered handover candidates are antenna sectors . . The method according to claim 1 c h a r a c t e r i z e d i n that the radio entities considered handover candidates are carrier frequencies. 5. The method according to any of claims 1-4 c h a r a c t e r i z e d i n that the shared channel is handed over not until the radio link of the first radio entity is scheduled to be excluded from the dynamic active set, the exclusion being based upon at least a first parameter and the handover candidate being selected based upon at least a second parameter. 6. The method according to any of claims 1-4 c h a r a c t e r i z e d i n that one of the one or more hand- over candidates is selected for handover based upon at least two parameters, the first parameter being processed according to one of the processes of long term filtering, persistency checking, hysteresis thresholding. 7. The method according to claim 5 or 6 c h a r a c t e r i z e d i n that the second parameter is one of total transmitted downlink power from the radio entity, PDSCH occupancy, lowest spreading factor available for PDSCH and ratio of real-time to non real-time traffic. 8. The method according to claim 5 or 6 c h a r a c t e r i z e d i n that the first parameter is one of received signal strength, received bit energy, interference level, noise level, downlink path loss, error rate or a combination thereof. 9. The method according to claim 5 or 6 c h a r a c t e r i z e d i n that the first parameter is one of energy to interference ratio on a Primary Common Pilot Channel and received signal code power on a Primary Common Pilot Channel. 10. A radio communication system c h a r a c t e r i z e d b y means for carrying out the method in any of claims 1- .

Inventors: ANDERSSON ANDREAS, DENKERT NIKLAS, KARLSSON PATRIK, ANDERSSON, ANDREAS, DENKERT, NIKLAS, KARLSSON, PATRIK
Applicants: ANDERSSON ANDREAS, DENKERT NIKLAS, ERICSSON TELEFON AB L M, KARLSSON PATRIK, ANDERSSON, ANDREAS, DENKERT, NIKLAS, KARLSSON, PATRIK, TELEFONAKTIEBOLAGET L M ERICSSON (PUBL)
Application year: 2001
Main classification: H04Q7
IPC codes: H04W4, H04W36