WO2012138283A2

WO2012138283A2 - Methods and devices for handling inter rat mobility

Info

Publication number: WO2012138283A2
Application number: PCT/SE2012/050350
Authority: WO
Inventors: Lena MELIN; Sofia BRISMAR; Håkan Palm
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2011-04-07
Filing date: 2012-03-29
Publication date: 2012-10-11
Also published as: WO2012138283A3

Abstract

Methods for mobility from a 2G/3G network such as UTRAN to LTE are described. The methods are designed to improve mobility for LTE UEs. In accordance with embodiments of the invention there is no need to configure compressed mode or for UE measurement reporting. This is obtained by re-use of the RRC Release with Redirect procedure, with optimizations and adaptations for the typical behavior of the LTE UEs.

Description

Methods and devices for handling inter RAT mobility

TECHNICAL FIELD

The present invention relates to methods and devices for controlling a user equipment (UE) moving between different Radio Access Technologies (RATs). In particular the invention relates to methods and devices for controlling the behavior of a Long Term Evolution (LTE) enabled UE.

BACKGROUND

Radio access networks (RAN) of second and third generation (2G/3G generation) are widely deployed and service is offered in most geographical areas and countries in the world. The infrastructure required is a result of large investments and the RANs are continually developed and expanded. On top of the 2G/3G networks Long term evolution (LTE) networks are now being deployed, although still - and for a number of years ahead - the radio coverage is far more limited than for 2G/3G. An LTE subscriber that requires seamless service offering will therefore need to use a multi-RAT (Radio Access Technology) capable User Equipment (UE) where 2G/3G RANs are used as supplement when LTE radio coverage fails.

In Fig. 1 it is depicted how a UE can access a network via a 2G/3G site represented by the radio base station NodeB (or a Base Transceiver Station BTS) or a LTE radio base station eNB (evolved NodeB) .Fig. 1 further depicts various central nodes of the different radio networks and how they can be interconnected. Thus the 2G/3G radio base stations are connected to a central node being a Base Station Controller (BSC) or Radio Network Controller (RNC), respectively. Similarly the eNB is connected to a Mobility Management Entity (MME). Fig. 1 further depicts other networks node and interfaces between the different network nodes well-known to the person familiar with cellular radio networks. LTE technology is optimized for packet data transfer and traditional Circuit Switched (CS)-domain services like voice, UDI video and Short Message Service (SMS), are a few among many potential media streams that cannot be transferred natively over LTE. For access to these services the CS Fallback mechanism, standardized in 3GPP TS 23.272, Circuit Switched (CS) fallback in Evolved Packet System (EPS) can be utilized to give an LTE subscriber CS service access. This is done by transferring the UE to a 2G/3G Radio Access Network (RAN) at the time when the CS service is being requested. After the CS service ends the UE is expected to quickly return to LTE.

Mechanisms for Inter Radio Access Technology (IRAT) mobility are required. The LTE subscriber typically wants, in any place and at any time, to be connected through a radio access technology (RAT) that, in the best way possible, fulfils the service needs. A number of different mechanisms have been specified. However, as will be discussed below, LTE UEs can show a behavior that makes the currently specified mobility mechanisms inadequate.

As said, a number of Inter-RAT mobility mechanisms are specified by third generation Partnership Project (3GPP) some of which can be found in the following standard specifications: 3GPP TS 23.272, Circuit Switched (CS) fallback in Evolved Packet System (EPS); 3GPP TS 25.304, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode; 3GPP TS 25.331 , Radio Resource Control (RRC); Protocol specification (UTRAN); 3GPP TS 36.331 , Radio Resource Control (RRC), Protocol specification (E-UTRAN); 3GPP TS 25.413, UTRAN lu interface Radio Access Network Application Part (RANAP) signaling; 3GPP TS 36.413, (E-UTRAN) S1 Application Protocol (S1AP); 3GPP TS 25.133, and 3GPP TS 23.216, Single Radio Voice Call Continuity (SRVCC).

Further, Annex E of 3GPP TS 25.331 , Radio Resource Control (RRC); Protocol specification (UTRAN) contains a list of IRAT mobility mechanisms (UTRAN to LTE) from the UE point of view including: A. Measurements and cell reselection in idle

B. Measurements and cell reselection in CELL/URA_PCH

C. RRC release with redirection

D. RRC reject with redirection

E. E-UTRAN measurements/reporting in connected mode

F. Handover procedure in connected mode

These mobility mechanisms A- F are illustrated in Fig. 2.

The mobility mechanisms A, C and D are mandatory from Release-8. Mobility mechanism F is an optional User Equipment (UE) capability. Mobility mechanisms B and E are mandatory from Release-9.

The mobility mechanisms in C and F can be applied with or without prior measurements in connected mode.

The above mobility mechanisms are associated with different problems. For mobility mechanisms A and B, i.e. cell reselection, the cell reselection in Idle and URA_PCH/CELL_PCH are based on the absolute priority based cell reselection, specified in 3GPP TS 25.304, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode. This mechanism is configured to use System Information Block (SIB) 19 to broadcast information to the UE. The UE itself performs measurements and initiates cell reselection. The intention with this mechanism is that a UE shall, on its own initiative, be able to move freely to LTE when in idle mode or URA_PCH/CELL_PCH. Cell reselection in state CELL_FACH is not specified.

A problem recently observed is the extremely chatty behavior of LTE UEs. Thus, an LTE enabled UE is very seldom silent. This behavior can result in that the RAN may never be able to switch the LTE UE to idle. Moreover, when switched to URA/CELL_PCH the UE only spend very short time, before its next activity period of uplink/downlink data transfer, which means UTRAN need to switch them to CELL_FACH/CELL_DCH.

In multi-RAT networks the consequence of this behavior is that he time periods the UE spend in a state where cell reselection is supported are not long enough. The UE has not enough time to measure on LTE frequencies, for the purpose of cell reselection, before it is switched up again. This means that cell reselection does not take place. The end result is that the UE cannot move to LTE, even if there is LTE coverage - i.e. the cell reselection mechanism fails to move the UE to LTE.

For mobility mechanism C release with redirection is specified in Release-6 of 3GPP TS 25.331 , Radio Resource Control (RRC); Protocol specification (UTRAN)and for redirection to E-UTRA in release-8. Basically, the RRC Connection Release procedure, as specified in 8.1 .4 in 3GPP TS 25.331 , Radio Resource Control (RRC); Protocol specification (UTRAN) is used. A UE shall receive and act on an RRC Connection Release message in states CELL_DCH, CELL_FACH and CELL_PCH. Any ongoing procedure in the UE might be interrupted. At reception of RRC Connection Release the UE shall reply with RRC Connection Release Complete and go to idle and perform what is stated in section 8.5.2 of 3GPP TS 25.331 , Radio Resource Control (RRC); Protocol specification (UTRAN): When IE 'Redirection info' is included the following is stated:

- if the IE "E-UTRA target info" is present, attempt to camp on any of the frequencies for the indicated RAT included in the RRC CONNECTION RELEASE message, excluding any cell indicated in the list of not allowed cells for that RAT (i.e. the "blacklisted cells per freq list" for E-UTRA), if present. If no suitable cell on the indicated frequencies for that RAT is found within 10s, attempt to camp on any suitable cell on any frequencies of that RAT; or - if no suitable cell is found on the indicated UTRA carrier or RAT camp on any suitable cell.

There are drawbacks of using this mobility mechanism C for basic IRAT mobility from UTRAN to LTE. Some of these are:

-at reception of 'RRC Connection Release with Redirect' in the UE the UE is required to search for specified frequencies during 10s. If the UE is not within LTE coverage it will not find any LTE frequency. During this time the UE behavior is not specified by standard. For example, it is unspecified whether the UE shall listen for pages during this time. I.e. the UE is unreachable.

-when the time period of 10s is passed the UE is required to search and attempt to camp on any suitable cell on any frequencies of the RAT it has been redirected to, i.e. any available E-UTRA frequency. This means that, in case of 'UTRAN Shared networks' being deployed the UE will have to search also for not allowed networks. This consumes time.

-after the UE has failed to find any E-UTRA frequencies its behavior is even more vaguely specified. The UE shall "camp on any suitable cell", which means that it is not for sure that the UE returns to the original cell, or even the original network, or even the original RAT. Where the UE will turn up again will depend on UE and operator implementation on UE and SIM. If there is no LTE coverage, the UE turns up again in UTRAN. The risk with this scenario is the UTRAN repeatedly could try to make the UE reselect to LTE even if the UE is stationary.

The above listed drawbacks will for the end-user imply long time periods where the UE is not reachable by any RAT, which is undesired.

For mobility mechanisms C plus E a combination of Radio Resource Control (RRC) Release with Redirect and Measurements in Connected mode is possible. This will comprise sending RRC Measurement Control to the UE to configure E-UTRA measurements when in Cell_DCH. When certain events/thresholds are fulfilled UE reports its measurement results.

A combined solution of C plus E mean that UTRAN has the knowledge not to send RRC Release with redirect to the UE when it is not within LTE coverage and the drawbacks for mobility mechanism C as described above are avoided. However there are other drawbacks when applying mobility mechanism C plus E such problems may be:

-E-UTRA measurements are in competition with inter-frequency measurements and measurements on other RATs

-Measurements are not possible in CELL_FACH but are only performed in CELL_DCH. However since RRC Release with Redirect interrupts all ongoing data transfer it is best sent when the UE has had no activity for a while. I.e. the measurements are done in the UE in another time - and possible in another place - than when the Release with redirect is sent. I.e. LTE coverage is not guaranteed.

Hence there exist a need for new methods and devices providing improved performance in systems with Inter RAT mobility where the UE can move between a 2G/3G network and LTE and in particular between UTRAN and LTE.

SUMMARY

It is an object of the present invention to provide improved methods and devices to address the problems as outlined above.

This object and others are obtained by the methods and devices as set out in the appended claims. In accordance with embodiments described herein methods for mobility from a 2G/3G network such as UTRAN to LTE are described. The methods are designed to improve mobility for LTE UEs and also without the need to configure compressed mode or for UE measurement reporting.

In accordance with one embodiment a method in a user equipment is provided where the user equipment is enabled to connect to an E-UTRA network and also to a 2G/3G network. The UE receives a message, such as a RRC Connection Release message, from the radio access network when connected to the 2G/3G network, the message comprises redirection information and information regarding target E-UTRA frequencies. In response to the received message the UE starts a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell, wherein the timer is set to a variable time. The timer value can be set in response to information received in a message from a network.

In accordance with one embodiment the variable time is set dependent upon the number of E-UTRA frequencies listed in an Information Element of the RRC connection release message.

In accordance with one embodiment the time is set proportional to the number of E- UTRA frequencies listed in said Information Element.

In accordance with one embodiment the time is set to the number of E-UTRA frequencies listed in said Information Element multiplied by the time determined for the UE to measure and find a suitable cell on one E-UTRA frequency.

In accordance with one embodiment a method in a user equipment where the user equipment is enabled to connect to an E-UTRA network and also to an 2G/3G network is provided. The UE receives a message, such as a RRC Connection Release message, from the radio access network when connected to the 2G/3G network, the message comprises redirection information and information regarding target E-UTRA frequencies, the message further comprises information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell. The message can be divided into one or more messages. In response to the received message the UE starts the timer with a time set in accordance with said information for controlling a timer.

In accordance with one embodiment the information for controlling a timer determining the time that the UE is set to measure and search for a suitable E- UTRA cell is comprised in an Information Element of an RRC Connection Release message.

In accordance with one embodiment if no suitable E-UTRA cell has been found within the time that the UE is set to measure and search for a suitable E-UTRA cell, the UE reverts to the frequency used for connecting to the 2G/3G network.

In accordance with one embodiment a method in a user equipment where the user equipment is enabled to connect to an E-UTRA network and also to an 2G/3G network is provided. The UE receives a message, such as an RRC Connection Release message, from the radio access network when connected to the 2G/3G network, the message comprises redirection information and information regarding target E-UTRA frequencies, the message further comprises E-UTRA frequency control information for controlling if the UE shall continue to scan all available E- UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers EARFCNs. The message can be divided into one or more messages. The UE then scans available E-UTRA frequencies in response to said E-UTRA frequency control information.

In accordance with one embodiment said E-UTRA frequency control information is comprised in an Information Element of an RRC Connection Release message. In accordance with one embodiment a method in a user equipment where the user equipment is enabled to connect to an E-UTRA network and also to an 2G/3G network is provided. The UE receives a message, such as an RRC Connection Release message, from the 2G/3G network when operating in connected mode in the 2G/3G network. The message comprises redirection information and information regarding target E-UTRA frequencies. The UE scans for E-UTRA frequencies while remembering Radio Access Bearers and Radio Bearers configured for the user equipment in the 2G/3G network. The UE reverts to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within a specified time.

In accordance with one embodiment, the UE leaves connected mode in the 2G/3G network and connects to the E-UTRA network if the UE finds a suitable E-UTRA cell within the specified time.

In accordance with one embodiment, the UE receives information for controlling whether the user equipment shall revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time. According to this embodiment, the user equipment reverting to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time is conditioned on the received information indicating that the user equipment shall revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time.

In accordance with one embodiment, said information for controlling whether the user equipment shall revert to connected mode in the 2G/3G network is comprised in an information element of the RRC Connection Release message.

In accordance with one embodiment a method in a user equipment where the user equipment is enabled to connect to an E-UTRA network and also to an 2G/3G network is provided. The UE receives a message, such as an RRC Connection Release message, from the radio access network when connected to the 2G/3G network, the message comprises redirection information and information regarding target E-UTRA frequencies. The message further comprises a threshold level indicating a minimum Reference Signal Received Power, RSRP, and/or a minimum Reference Signal Received Quality, RSRQ, of an E-UTRA cell to be selected by the user equipment. The message can be divided into one or more messages. In response to the received message the UE selects an E-UTRA cell to camp on only when said threshold level indicating minimum Reference Signal Received Power, RSRP, and/or Reference Signal Received Quality, RSRQ is exceeded for the E- UTRA cell.

The threshold level indicating minimum Reference Signal Received Power, RSRP, and/or Reference Signal Received Quality, RSRQ can also be received in other types of messages such as a system information message. The threshold level can also be pre-stored or generated in the UE.

In accordance with one embodiment said threshold level indicating a minimum Reference Signal Received Power, RSRP, and/or a minimum Reference Signal Received Quality, RSRQ is comprised in an Information Element of an RRC Connection Release message.

In accordance with one embodiment a method in a user equipment where the user equipment is enabled to connect to an E-UTRA network and also to a 2G/3G network is provided. The UE receives a message, such as an RRC Connection Release message, from the 2G/3G network when operating in connected mode in the 2G/3G network. The message comprises redirection information and information regarding target E-UTRA frequencies. The UE scans E-UTRA frequencies while staying in connected mode in the 2G/3G network. If no suitable E-UTRA cell can be found within a specified time, the UE continues operating in connected mode in the 2G/3G network. In accordance with one embodiment, the UE leaves connected mode in the 2G/3G network and connects to the E-UTRA network if the UE finds a suitable E-UTRA cell within the specified time.

In accordance with one embodiment the UE receives information whether the UE shall stay in connected mode in the 2G/3G network while scanning for E-UTRA frequencies. According to this embodiment, staying in connected mode in the 2G/3G network while scanning for E-UTRA frequencies and continue operation in connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time are conditioned on the received information indicating that the UE shall stay in connected mode while scanning for E-UTRA frequencies.

In accordance with one embodiment a method in a central node of a cellular radio system. A message for a UE connected to a 2G/3G network is generated. The message comprises redirection information and information regarding target E- UTRA frequencies. The message further comprises information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell.

In accordance with one embodiment a method in central node of a cellular radio system. A message for a UE connected to a 2G/3G network is generated. The message comprises redirection information and information regarding target E- UTRA frequencies. The message further comprises E-UTRA frequency control information for controlling if the UE shall continue to scan all available E-UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers EARFCNs.

In accordance with one embodiment a method in a central node of a cellular radio system. A message for a UE connected to a 2G/3G network is generated. The message comprises redirection information and information regarding target E- UTRA frequencies. The message further comprises information for controlling whether the UE shall revert to WCDMA connected mode if no suitable E-UTRA cell can be found within a specified time.

In accordance with one embodiment a method in a central node of a cellular radio system. A message for a UE connected to a 2G/3G network is generated. The message comprises redirection information and information regarding target E- UTRA frequencies The message further comprises a threshold level indicating a minimum Reference Signal Received Power, RSRP, and/or a minimum Reference Signal Received Quality, RSRQ, of a E-UTRA cell to be selected by a user equipment.

Using embodiments as described herein will provide a mechanism that makes it possible for UTRAN to define (or alternatively know) for how long time UE is searching for E-UTRA and also to set a threshold for a minimum RSRP/RSRQ level on E-UTRA cell. It also makes it possible to control whether the UE shall continue searching for all available E-UTRA frequencies if a suitable cell is not found among the ones in 'redirection info'. Also a mechanism that enables for the UE, that has tried to reselect to LTE, but that finds itself outside LTE coverage, to return to the source UTRAN frequency is provided. Also, It is possible to apply as a fast track solution - without any changes in the standardized RRC messages, but with only impact on the nodes. However the methods get more sophisticated with a few additional information exchanges. An information exchange is performed to assure that both the UE and the network agrees on whether a new behavior will apply.

Further in some embodiments a mechanism is provided that makes it possible for UTRAN store the current UE configuration and to preserve the UE context and the lu connection toward the core network, supplemented by a mechanism in the UE that keeps the RAB/RB configuration during the time it searches for E-UTRA cells. This can e.g. be achieved by having the UE remain in connected mode during the time it searches for E-UTRA cells. Hereby, the UE is enabled to return to the original 2G/3G network connection in connected mode (CELL_FACH) by using the Cell Update procedure. This reduces signaling load and decrease disturbance for the end-user, and enables network control. The return to UTRAN connected mode is quick.

In accordance with some embodiments a central node in UTRAN is configured to keep track on how many times the UE is ordered to revert back to search for E- UTRA. Based on this information UTRAN can avoid repeated attempts to redirect the UE to E-UTRA if the radio coverage is bad.

The methods as described herein can be based upon already existing functionality. In existing, 3GPP specified, mobility mechanisms are all associated with different problems as discussed in the background section. Cell reselection mechanism, as specified, does not work for the chatty UEs. Handover requires support in the UE, the source and the target RAN. Release with redirect, as specified presently by 3GPP, cannot be used without disturbance for end user. Even combined with preceding measurements on E-UTRA the method becomes cumbersome. Measurements needs to co-exist with intra-frequency and (other) inter-RAT measurements, they consumes resources unnecessarily since the UE needs to be kept in CELL_DCH in order to perform the measurements.

The above methods provide a simple, quick and efficient mechanism for mobility from UTRAN to LTE. It solves the problem with chatty UEs getting stuck in WCDMA. Compared with other possible methods to solve that problem this method also has advantages for the UE vendors, the subscribers and the network operators. For example the following advantages:

^* It optimizes performance for the subscriber since there is no need to order the UE to measure on E-UTRA when it is in CELL_DCH. E-UTRA measurements increase RRC signaling and decreases UE efficiency. ^* It minimizes disturbance for subscriber since release with redirect can be used at points in time when it disturbs the subscriber the least (UE in CELL_FACH or CELL_PCH)

^* It optimizes performance for subscriber and network since measurements on E- UTRA do not need to co-exist with intra-frequency measurements and other inter- RAT measurements

^* It is easy to implement for the UE vendors since it is based on a combination of already existing functionality. It combines existing release with redirect functionality with something similar to the existing call re-establishment functionality.

^* Using the methods there is no need to standardize cell reselection in CELL_FACH or to increase time requirements for measurements in idle or URA_PCH or CELL_PCH state.

^* Since the UE returns to the source network in connected mode the network can easily keep track of the individual UE. Thereby the network can avoid repeated attempts to move the UE to E-UTRA if the UE is stationary in an area with too bad LTE radio coverage.

The invention also extends to a user equipment and to a central node in a radio access network arranged to perform the methods described above. The user equipment and the central node radio can be provided with a controller/controller circuitry for performing the above methods. The controller(s) can be implemented using suitable hardware and or software. The hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media. The processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawing, in which:

- Fig. 1 is a view of a cellular radio system,

- Fig. 2 is a view illustrating different IRAT mobility mechanisms (UTRAN to LTE),

- Fig. 3 is a signaling diagram illustrating signaling in accordance with a first embodiment,

- Fig. 4 is a signaling diagram illustrating signaling in accordance with a second embodiment,

- Fig. 5 is a signaling diagram illustrating signaling in accordance with a third embodiment,

- Fig. 6 is a general view of a user equipment,

- Fig. 7 is a general view of a radio network controller, and

- Fig. 8 is a signaling diagram illustrating signaling in accordance with a fourth embodiment.

DETAILED DESCRIPTION

While the below description focuses on a user equipment connected to a UTRAN network it is understood that similar principles can be applied for other networks such as GERAN. Also it is to be understood that the embodiments and parts thereof can be combined to form additional embodiments. Further while some information is described as transmitted in a single message, it is to be understood that the word message also includes the transmission of several messages such that different information is transmitted in different messages.

In accordance with one embodiment a method providing release with redirect is provided. The method is illustrated in Fig. 3. First, in a step 1 (S1 ), a UE is connected to the network via a 2G/3G connection, such as via a Wideband Code Division Multiple Access connection. Next, in a step 2 (S2), the UE receives a RRC Connection Release message from the radio access network generated by a central node in the network. For UTRAN the central node is a Radio Network Controller (RNC). The message comprises redirection information and information regarding the target E-UTRA cell as specified in existing standard. The message comprises an Information Element (IE) Έ-UTRA target info' that lists a number of E- UTRA frequencies. In response to the message received in step 2 the UE transmits a RRC Connection Release Complete message in a step 3 (S3). Then in a step 4 (S4) a timer is started. The timer determines the time that the UE is set to measure and search for a suitable E-UTRA cell. The timer is set to a variable value that can be dependent upon the number of E-UTRA frequencies listed in IE Έ-UTRA target info'. The timer value can in accordance with one embodiment be set proportional to the number of E-UTRA frequencies listed in IE Έ-UTRA target info. In accordance with one embodiment, a variable time Time_to_measure' during which the UE measures is determined as:

Time_to_measure = number of E-UTRA frequencies listed in IE Έ-UTRA target info' ^* T_search

Where T_search is the time it takes for the UE to measure and find a suitable cell on one E-UTRA frequency.

The timer value can also be set in accordance with other information received in a message from a network, in particular the 2G/3G network, for example system information received from the network.

The UE then in a step 5 (S5) enters idle mode and starts to search for a suitable cell among the E-UTRA frequencies listed in the redirection info. The UE then, in a step 6 (S6), continues to search for all available E-UTRA frequencies. If the UE finds a suitable E-UTRA cell that fulfils cell selection criteria it attempts to register in that cell according to existing procedures.

However, if there is no LTE coverage and the UE is not able to find a suitable E- UTRA cell within Time_to_measure' seconds set by the timer as determined in a step 7 (S7) the UE can be configured to revert to the original 2G/3G network and select a suitable cell on the original frequency, e.g. the old WCDMA frequency (UTRAN frequency). Once a suitable cell has been found on the old WCDMA frequency the UE can be configured to camp on this cell and continue to evaluate the cell reselection criteria as specified in 3GPP TS 25.304, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode.

In accordance with some embodiments the radio access network is set to control the timer used to control the time a UE search for E-UTRA frequencies before it reverts to WCDMA. To achieve this, a message is transmitted from the system to the UE for controlling the Timer. In accordance with some embodiments the RRC Connection Release message is extended with a new optional IE. This new IE can be termed IE Time to measure'. When IE Time to measure' is signaled by RAN to a UE, the time indicated in the IE is set to replace the value of the timer as otherwise set by the formula above or by a default value. In accordance with other embodiments the timer is controlled directly or indirectly via a system information message or another message from the network.

In accordance with some embodiments a central node, such as an RNC of the radio access network is configured to control if the UE shall continue to scan all available E-UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers (EARFCNs). To achieve this, the RRC Connection Release message is in some embodiments supplemented with information for controlling the behavior of the UE. To control the UE the RRC Connection Release message can in some embodiments be extended with a new optional IE. This new IE can be termed IE 'Measure E-UTRA target frequencies'. For example if termed IE 'Measure E-UTRA target frequencies' is set by a central node such a radio network controller of the RAN, the UE shall only measure the E- UTRA frequencies listed in the E-UTRA target info. If the indicator is not set, the UE shall apply the legacy behavior, that is, continue to scan all available E-UTRA frequencies if no suitable cell is found among the frequencies listed in the E-UTRA target info. In some embodiments the control signaling if the UE shall continue to scan all available E-UTRA frequencies or not is transmitted in a system information message or another message from the network.

In accordance with some embodiments the network (i.e. the central control node of the network) is configured to control the lowest level of the RSRP/RSRQ of the E- UTRA target cell. A threshold level of the E-UTRA cell in terms of RSRP (Reference Signal Received Power) or RSRQ (Reference Signal Received Quality) is then in accordance with some embodiments conveyed in the RRC Connection Release message This can be done in a new optional IE that can be termed 'RSRP/RSRQ threshold'. In some embodiments the threshold level is transmitted in a system information message or another message from the network.

The RSRP/RSRQ level of the target E-UTRA cell must be above this threshold in order for the UE to be allowed camp in the E_UTRA target cell. If the threshold is not met in a particular cell the UE needs to look for another cell or stay in the UTRAN cell. Hereby, the risk of having the UE bounce back and forth between UTRAN and E-UTRA is reduced. As a result a ping pong effect in areas where LTE has low coverage can be eliminated.

In Fig. 4 a flowchart illustrating some procedural steps that can be performed when controlling the UE in accordance with the above exemplary embodiments. First, in a step 1 (S1 ), a UE is connected to the network via a 2G/3G connection, such as via a Wideband Code Division Multiple Access connection. Next in a step 2a (S2a) the UE receives a RRC Connection Release message from the radio access network. The message optionally comprises redirection information and information regarding the target E-UTRA cell as specified in existing standard. In addition the message received in step 2a can comprise the lEs Time to measure' and/or 'Measure E-UTRA target frequencies' and/or E-UTRA RSRP/RSRQ threshold as described above. In response to the message received in step 2a the UE transmits a RRC Connection Release Complete message in a step 3 (S3). Then in a step 4 (S4) a timer is started. The timer determines the time that the UE is set to measure and search for a suitable E-UTRA cell. The timer is set as indicated in the IE Time to measure' if such an IE is received. The UE then in a step 5 (S5) enters idle mode and starts to search for a suitable cell among the E-UTRA frequencies listed in the redirection info. If Έ-UTRA RSRP/RSRQ threshold' is present in the RRC Connection Release message or the UE is configured to remember a RSRP/RSRQ threshold, for example the SIB19 parameter Thresh_x_low' if SIB 19 was stored in the WCDMA cell prior to the release, the RSRP/RSRQ levels of the target E-UTRA cell must be above the signaled thresholds to be suitable. The UE then, in a step 6a (S6a), continues to search for all available E-UTRA frequencies only if the IE "Measure E-UTRA target frequencies' is not present in the RRC Connection Release message. If Έ-UTRA RSRP/RSRQ threshold' is present in the RRC Connection Release message the RSRP/RSRQ levels of the target E-UTRA cell must be above the signaled. If the UE finds a suitable E-UTRA cell that fulfils the defined RSRP/RSRQ levels and cell selection criteria it attempts to register in that cell according to existing procedures.

When the UE has been configured to use received an E-UTRA RSRP/RSRQ threshold, this threshold will also be taken into account when determining if a suitable E-UTRA cell can be found. Once a suitable cell has been found on the old connection on the WCDMA frequency the UE can be configured to camp on this cell and continue to evaluate the cell reselection criteria as specified in 3GPP TS 25.304, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode. However, if there is no LTE coverage and the UE is not able to find a suitable E-UTRA cell within Time_to_measure' seconds set by the timer as determined in a step 7 (S7) the UE can be configured to revert to the original 2G/3G network and select a suitable cell on the original frequency, e.g. the old WCDMA frequency.

In accordance with yet another embodiment the RAN is configured to control whether the UE shall revert to WCDMA connected mode (connected mode in UTRAN) if no suitable E-UTRA cell can be found within a specified time, such as Time to measure' seconds. The UE remembers the Radio Access Bearers (RABs) and Radio Bearers (RBs) previously configured in UTRAN while the E-UTRA frequencies are scanned. In the same way the RAN will keep the resources allocated for the UE (including the lu connection) in case the UE will revert to the old WCDMA/UTRAN frequency.

To achieve this, the RRC Connection Release message in some embodiments comprises an indicator that controls the UE to remember the RABs and RBs configured in UTRAN while searching for a suitable E-UTRA cell. In accordance with one embodiment the RRC Connection Release message is extended with a new optional IE. This new IE can be termed IE 'Revert to WCDMA connected mode'. If this indicator is set by RAN, the UE shall remember the RABs and RBs configured in UTRAN while searching for a suitable E-UTRA cell. If the IE 'Revert to WCDMA connected mode' is not included the UE is configured to behave as described above in conjunction with Figs. 3 and 4. Also, In some embodiments the indicator that controls the UE to remember the RABs and RBs configured in UTRAN while searching for a suitable E-UTRA cell is transmitted in a system information message or another message from the network.

As soon as the UE finds a suitable E-UTRA cell it will camp on this cell and clear the previously stored information about RABs and RBs configured in UTRAN. The UE performs a Tracking Area Update in the new E-UTRA cell and the MME and source SGSN interact to relocate the UE context according to legacy mechanisms for cell reselection.

If the UE is not able to find a suitable E-UTRA cell within 'Time to measure' seconds it can be configured to revert to the old UTRAN frequency and perform a cell selection. When a suitable cell has been found on the old UTRAN frequency the UE restores the UTRAN RAB/RB configuration, enters state CELL_FACH and sends a Cell Update message on Common Control Channel (CCCH) in the cell to indicate that it is back. The cause value is in some embodiments set to "re-enter service area".

The RAN can determine a new target configuration for the UE which is signaled in the Cell Update Confirm message. Depending on the target configuration the UE can be configured to respond with a RB Reconfiguration Complete, Transport Channel Reconfiguration Complete, Physical Channel Complete or UTRAN Mobility Information Complete message. The RAN, e.g. a central node therein, can keep track on how many times the UE reverts back after being redirected to E-UTRA. Based on this information UTRAN can avoid repeated attempts to move the UE to E-UTRA if the UE is stationary in an area with bad E-UTRA radio coverage.

In Fig. 5 a flowchart illustrating some procedural steps that can be performed when controlling the UE in accordance with the above. For ease of understanding some procedural steps already described above in conjunction with Figs. 3 and 4 have been omitted, but it is understood that the steps described in conjunction with Figs. 3 and 4 can also be performed in the context of the exemplary embodiment described in Fig. 5. First, in a step 1 (S1 ), a UE is connected to the network via a 2G/3G connection, such as via a Wideband Code Division Multiple Access connection. Next in a step 2b (S2b) the UE receives a RRC Connection Release message from the radio access network. The message comprises redirection information and information regarding the target E-UTRA cell as specified in existing standard. In addition the message received in step 2b can comprise the lEs Time to measure' and 'Measure E-UTRA target frequencies' and Έ-UTRA RSRP/RSRQ and also an IE termed IE 'Revert to WCDMA as described above. In response to the message received in step 2b the UE transmits a RRC Connection Release Complete message in a step 3 (S3).

If the message received in step 2b comprises an IE 'Revert to WCDMA the UE can be configured to remember the UTRAN RAB/RB configuration and start to search for a suitable cell among the E-UTRA frequencies listed in the redirection info in a step 8a (S8a). On the network side the RAN can be configured to store the current UE configuration and to keep the UE context and the lu connection towards the core network and start an internal supervision timer T in a step 8b (S8b). If the UE finds a suitable E-UTRA cell that fulfils cell selection criteria it attempts to register in that cell according to existing procedures.

However, if no suitable E-UTRA cell has been found within Time to measure' seconds, the UE continues to find a suitable UTRAN cell on the old frequency in a step 9 (S9). The UE restores the UTRAN RAB/RB configuration and enters CELL_FACH state and transmits a Cell Update message to the RAN indicating reentering service area in a step 10 (S10). The RAN then receives the message in step 10. In response to receiving the message (S10) the internal timer T is stopped in the network (the central node of the network controlling the network timer) and the network determines a new target configuration in a step 1 1 (S1 1 ). The network then returns a cell update confirmation message to the UE including the new target configuration in a step 12 (S12) and the UE acknowledges the receipt thereof in a complete message in a step 13 (S13). The above steps performed in the network can in some embodiments be performed by a central node of the network such an RNC.

According to the embodiment described above, the UE reverting to connected mode in the 2G/3G network (e.g. UTRAN) if no suitable E-UTRA cell can be found within the specified time is conditioned on the UE receiving information (e.g. 'Revert to WCDMA connected mode') indicating that the UE shall revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found. In alternative embodiments no such information need to be received by the UE in order for the UE to revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found.

In accordance with still another embodiment the UE stays in connected mode in the UTRAN while searching for a suitable E-UTRA cell and if no suitable E-UTRA cell can be found within a specified time, such as Time to measure' seconds, the UE continues operating in connected mode in the UTRAN. This way the UE keeps its UTRAN configuration, including e.g. the Radio Access Bearers (RABs) and Radio Bearers (RBs), previously configured in UTRAN while the E-UTRA frequencies are scanned. In the same way the RAN will keep the resources allocated for the UE (including the lu connection) in case the UE will continue operating in connected mode in the UTRAN.

To achieve this, the RRC Connection Release messages may in some embodiments comprise an indicator that controls whether the UE should stay in connected mode in the UTRAN while searching for a suitable E-UTRA cell. In accordance with one embodiment the RRC Connection Release message is extended with a new optional IE. This new IE can e.g. be termed IE 'Stay in connected mode'. If this indicator is set by the UTRAN, the UE shall stay in connected mode in the UTRAN while searching for a suitable E-UTRA cell. If the IE 'Stay in connected mode' is not included, the UE is configured to behave as described above in conjunction with Figs. 3 and 4. Also, In some embodiments the indicator that controls whether the UE should stay in connected mode in the UTRAN while searching for a suitable E-UTRA cell is transmitted in a system information message or another message from the network.

As soon as the UE finds a suitable E-UTRA cell it will leave connected mode in the UTRAN (clear its UTRAN configuration) and camp on this E-UTRAN cell. The UE performs a Tracking Area Update in the new E-UTRA cell and the MME and source SGSN interact to relocate the UE context according to legacy mechanisms for cell reselection.

If the UE is not able to find a suitable E-UTRA cell within 'Time to measure' seconds it can be configured to revert to the old UTRAN frequency and perform a cell selection. When a suitable cell has been found on the old UTRAN frequency the UE continues its connected mode operation and sends a Cell Update message on Common Control Channel (CCCH) in the cell to indicate that it is continuing its connected mode operation. The cause value is in some embodiments set to "reenter service area".

The RAN can determine a new target configuration for the UE which is signaled in the Cell Update Confirm message. Depending on the target configuration the UE can be configured to respond with a RB Reconfiguration Complete, Transport Channel Reconfiguration Complete, Physical Channel Complete or UTRAN Mobility Information Complete message. The RAN, e.g. a central node therein, can keep track on how many times the UE continues connected mode operation in the UTRAN after being redirected to E-UTRA. Based on this information UTRAN can avoid repeated attempts to move the UE to E-UTRA if the UE is stationary in an area with bad E-UTRA radio coverage.

In Fig. 8 a flowchart illustrating some procedural steps that can be performed when controlling the UE in accordance with the above. For ease of understanding some procedural steps already described above in conjunction with Figs. 3 and 4 have been omitted, but it is understood that the steps described in conjunction with Figs. 3 and 4 can also be performed in the context of the exemplary embodiment described in Fig. 8. First, in a step 1 (S1 ), a UE is connected to the network via a 2G/3G connection, such as via a Wideband Code Division Multiple Access connection. Next in a step 2b (S2b) the UE receives a RRC Connection Release message from the radio access network. The message comprises redirection information and information regarding the target E-UTRA cell as specified in existing standard. In addition the message received in step 2b can comprise the lEs Time to measure' and 'Measure E-UTRA target frequencies' and Έ-UTRA RSRP/RSRQ and also an IE termed 'Stay in WCDMA connected mode' as described above. In response to the message received in step 2b the UE transmits a RRC Connection Release Complete message in a step 3 (S3). If the message received in step 2b comprises an IE 'Stay in WCDMA connected mode' the UE can be configured to stay in connected mode in the 2G/3G network (e.g. UTRAN/WCDMA) while searching for a suitable cell among the E-UTRA frequencies listed in the redirection info in a step 8a (S8a). On the network side the RAN can be configured to store the current UE configuration and to keep the UE context and the lu connection towards the core network and start an internal supervision timer T in a step 8b (S8b). If the UE finds a suitable E-UTRA cell that fulfils cell selection criteria it leaves connected mode in the 2G/3G network and attempts to register in that cell according to existing procedures.

However, if no suitable E-UTRA cell has been found within Time to measure' seconds, the UE continues to find a suitable WCDMA cell on the old frequency in a step 9 (S9). The UE continues operating in connected mode in the 2G/3G network and transmits a Cell Update message to the RAN indicating re-entering service area in a step 10 (S10). The RAN then receives the message in step 10. In response to receiving the message (S10) the internal timer T is stopped in the network (the central node of the network controlling the network timer) and the network determines a new target configuration in a step 1 1 (S1 1 ). The network then returns a cell update confirmation message to the UE including the new target configuration in a step 12 (S12) and the UE acknowledges the receipt thereof in a complete message in a step 13 (S13). The above steps performed in the network can in some embodiments be performed by a central node of the network such an RNC.

According to the embodiment described above, the UE staying in connected mode in the 2G/3G network while scanning for E-UTRA frequencies and continue operation in connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time are conditioned on the UE receiving information indicating that the UE shall stay in connected mode while scanning for E-UTRA frequencies. In alternative embodiments no such information need to be received by the UE in order for the UE to stay in connected mode while scanning for E-UTRA frequencies and continue operation in connected mode in the 2G/3G network if no suitable E-UTRA cell can be found.

Further in Fig. 6 a UE 1500 is schematically depicted. The UE 1500 comprises controller circuitry 1501 for performing all the procedures performed by the UE as described herein. The controller circuitry 1501 can be implemented using suitable hardware and or software. The hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media. The processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, a processor may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media. In addition the UE 1500 comprises an input/output device 1503 for receiving/transmitting data to the radio base station.

Further, in Fig.7 a central node 1600 of a cellular radio system is schematically depicted. The central node can for example be a radio network controller. The central node 1600 comprises controller circuitry 1601 for performing all the procedures performed by the central node on the network side as described herein. The controller circuitry 1601 can be implemented using suitable hardware and or software. The hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media. The processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media. In addition the central node 1600 comprises an input/output device 1603 for receiving/transmitting data to a UE (via a designated radio base station). Using the methods and devices as described herein will enable a LTE UEs to return to LTE in a quick and simple way. The disturbance for the subscriber and the service delay is minimized. It will also reduce the Main Processor (MP) load in the RNC. The methods do not require prior UE measurements in dedicated mode.

In the case there is no E-UTRA coverage the following advantages can be achieved: UTRAN has control of the mobility: UTRAN has control over the maximum time the UE is unreachable due to E-UTRA measurements. UTRAN can control whether the UE shall search on only a few E-UTRA frequencies or continue with a broad search on E-UTRA. In a shared network this saves time since the UE will not search and try to camp on an E-UTRA network where it cannot get service. Also, the UE context in UTRAN can be preserved and the UE can return to an already established connection. This offloads the MP resources in RNC since it reduces signaling. It also enables a quicker return to active mode in UTRAN which reduce the disturbance for user data sending.

Claims

1 . A method in a user equipment, the user equipment being enabled to connect to an E-UTRA network and also to a 2G/3G network, the method comprising the steps of:

- receiving a RRC Connection Release message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies,

- in response to receiving the RRC Connection Release message starting a timer the timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell, wherein the timer is set to a variable time.

2. The method according to claim 1 , wherein the variable time is set in response to information received in a message form a network.

3. The method according to claim 2, wherein the variable time is set dependent upon the number of E-UTRA frequencies listed in an Information Element of the RRC connection release message.

4. The method according to claim 3, wherein the time is set proportional to the number of E-UTRA frequencies listed in said Information Element.

5. The method according to claim 4, wherein the time is set to the number of E- UTRA frequencies listed in said Information Element multiplied by the time determined for the UE to measure and find a suitable cell on one E-UTRA frequency.

6. The method according to any of claims 1 - 5, wherein if no suitable E-UTRA cell has been found within the time that the UE is set to measure and search for a suitable E-UTRA cell, the UE reverts to the frequency used for connecting to the 2G/3G network.

7. The method according to any of claims 1 - 6, wherein the 2G/3G network is a UTRAN network.

8. The method according to claim 7, further comprising the UE restoring a Wideband Code Division Multiple Access ,WCDMA, Radio Access Bearer/ Radio Bearer RAB/RB configuration and entering CELL_FACH state when reverting to the connection on the frequency used for connecting to the 2G/3G network.

9. A method in a user equipment, the user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the method comprising the steps of:

- receiving a message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell, and

- starting the timer with a time set in accordance with said information for controlling a timer.

10. The method according to claim 9, wherein the 2G/3G network is a UTRAN network.

1 1 . The method according to claim 9 or 10, wherein the message is a RRC Connection Release message.

12. The method according to claim 1 1 , wherein information for controlling a timer determining the time that the UE is set to measure and search for a suitable E- UTRA cell is comprised in an Information Element of the RRC Connection Release message.

13. A method in a user equipment, the user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the method comprising the steps of:

- receiving a message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising E-UTRA frequency control information for controlling if the UE shall continue to scan all available E- UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers EARFCNs, and

- scanning available E-UTRA frequencies in response to said E-UTRA frequency control information.

14. The method according to claim 13, wherein the 2G/3G network is a UTRAN network.

15. The method according to any of claims 13 or 14, wherein the message is a RRC Connection Release message.

16. The method according to claim 15, wherein said E-UTRA frequency control information is comprised in an Information Element of the RRC Connection Release message.

17. A method in a user equipment, the user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the method comprising the steps of:

- receiving a message from the 2G/3G network when operating in connected mode in the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies,

- scanning E-UTRA frequencies while remembering Radio Access Bearers and Radio Bearers configured for the user equipment in the 2G/3G network, and - reverting to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within a specified time.

18. The method according to claim 17, wherein if the user equipment finds a suitable E-UTRA cell within the specified time, the user equipment leaves connected mode in the 2G/3G network and connects to the E-UTRA network.

19. The method according to any one of claims 17-18, wherein the method further comprises receiving information for controlling whether the user equipment shall revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time and wherein performing said reverting step is conditioned on the received information indicating that the user equipment shall revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time.

20. The method according any one of claims 17 - 19, wherein the 2G/3G network is an UTRAN network.

21 . The method according to claim 20, further comprising the user equipment restoring the Radio Access Bearers and Radio Bearers configured for the user equipment and entering CELL_FACH state when reverting to the frequency used for connecting to the 2G/3G network.

22. A method in central node of a cellular radio system, the central control node being adapted to generate a message for a UE, the method comprising the steps of:

- generating a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising information for controlling a timer determining the time that the UE is set to measure and search for a suitable E- UTRA cell.

23. The method according to claim 22, wherein the message is a RRC Connection Release message.

24. The method according to claim 22 or 23, wherein the 2G/3G network is a UTRAN network.

25. The method according to any of claims 23 - 24, wherein information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell is comprised in an Information Element of the RRC Connection Release message.

26. The method according to any of claims 22 - 25, wherein the central node is a radio network controller.

27. A method in central node of a cellular radio system, the central control node being adapted to generate a message for a UE, the method comprising the steps of:

- generating a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising E-UTRA frequency control information for controlling if the UE shall continue to scan all available E-UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers EARFCNs.

28. The method according to claim 27, wherein the 2G/3G network is a UTRAN network.

29. The method according to any of claims 27 - 28, wherein the message is a RRC Connection Release message.

30. The method according to claim 29, wherein said E-UTRA frequency control is comprised in an Information Element of the RRC Connection Release message.

31 . The method according to any of claims 27 - 30, wherein the central node is a radio network controller.

32. A method in central node of a cellular radio system, the central control node being adapted to generate a message, the method comprising the steps of:

- generating a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising information for controlling whether the UE shall revert to WCDMA connected mode if no suitable E-UTRA cell can be found within a specified time.

33. The method according to claim 32, wherein the 2G/3G network is an UTRAN network.

34. The method according to claim 32 or 33, wherein the message is a RRC Connection Release message.

35. The method according to claims 34, wherein said information for controlling whether the UE shall revert to WCDMA connected mode is comprised in an Information Element of the RRC Connection Release message.

36. The method according to any of claims 32 - 35, wherein the central node is a radio network controller.

37. A user equipment enabled to connect to an E-UTRA network and also to a 2G/3G network, the user equipment comprising:

- an input/output device adapted to receive a RRC Connection Release message from the radio access network when connected to the 2G/3G network, the RRC Connection Release message comprising redirection information and information regarding target E-UTRA frequencies,

- controller circuitry adapted to start a timer the timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell, wherein the timer is set to a variable time.

38. The user equipment according to claim 37, wherein the user equipment is adapted to set the variable time dependent upon information received in a message form a network.

39. The user equipment according to claim 38, wherein user equipment is adapted to set the variable time dependent upon the number of E-UTRA frequencies listed in an Information Element of the RRC connection release message.

40. The user equipment according to claim 39, wherein the user equipment is adapted to set the time proportional to the number of E-UTRA frequencies listed in said Information Element.

41 . The user equipment according to claim 40, wherein the user equipment is adapted to set the time to the number of E-UTRA frequencies listed in said Information Element multiplied by the time determined for the UE to measure and find a suitable cell on one E-UTRA frequency.

42. The user equipment according to any of claims 37 - 41 , wherein the UE is adapted to revert to the frequency used for connecting to the 2G/3G network if no suitable E-UTRA cell has been found within the time that the UE is set to measure and search for a suitable E-UTRA cell.

43. The user equipment according to any of claims 37 - 42, wherein the 2G/3G network is a UTRAN network.

44. The user equipment according to claim 43, wherein the UE is adapted to restore a Wideband Code Division Multiple Access ,WCDMA, Radio Access Bearer/ Radio Bearer RAB/RB configuration and entering CELL_FACH state when reverting to the frequency used for connecting to the 2G/3G network.

45. A user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the user equipment comprising:

- an input/output device adapted to receive a message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell, and

- controller circuitry adapted to start the timer with a time set in accordance with said information for controlling a timer.

46. The user equipment according to claim 45, wherein the message is a RRC Connection Release message.

47. The user equipment according to any of claims 45 or 46, wherein the information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell is comprised in an Information Element of the RRC Connection Release message.

48. The user equipment according to any of claims 45 - 47, wherein the 2G/3G network is a UTRAN network.

49. A user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the user equipment comprising:

- an input/output device adapted to receive a message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising E-UTRA frequency control information for controlling if the UE shall continue to scan all available E-UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers EARFCNs, and

- controller circuitry adapted to scan available E-UTRA frequencies in response to said E-UTRA frequency control information.

50. The user equipment according to claim 49, wherein the message is a RRC Connection Release message.

51 . The user equipment according to claim 50, wherein said E-UTRA frequency control information is comprised in an Information Element of the RRC Connection Release message.

52. The user equipment according to any of claims 49 - 51 , wherein the 2G/3G network is a UTRAN network.

53. A user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the user equipment adapted to:

- receive a message from the 2G/3G network when operating in connected mode in the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies,

- scan E-UTRA frequencies while remembering Radio Access Bearers and Radio Bearers configured for the user equipment in the 2G/3G network, and

- revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within a specified time.

54. The user equipment according to claim 53, wherein the user equipment is adapted to leave connected mode in the 2G/3G network and connect to the E- UTRA network if a suitable E-UTRA cell can be found within the specified time.

55. The user equipment according to any one of claims 53 - 54, wherein the user equipment is further adapted to receive information for controlling whether the user equipment shall revert to connected mode in the 2G/3G network if no suitable E- UTRA cell can be found within the specified time and wherein the user equipment is adapted to perform said reverting operation if the received information indicates that the user equipment shall revert to connected mode in the 2G/3G network if no suitable E-UTRA cell can be found within the specified time.

56. The user equipment according to any one of claims 53-55, wherein the 2G/3G network is an UTRAN network.

57. The user equipment according to claim 56, wherein the user equipment is adapted to restore the Radio Access Bearers and Radio Bearers configured for the user equipment and entering CELL_FACH state when reverting to the frequency used for connecting to the 2G/3G network.

58. A central node of a cellular radio system, the central control node being adapted to generate a message, the central node comprising:

- controller circuitry adapted to generate a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell.

59. The central node according to claim 58, wherein the 2G/3G network is a UTRAN network.

60. The central node according to claim 58 or 59, wherein the message is a RRC Connection Release message.

61 . The central node according to claim 60, wherein information for controlling a timer determining the time that the UE is set to measure and search for a suitable E-UTRA cell is comprised in an Information Element of the RRC Connection Release message.

62. The central node according to any of claims 58 - 61 , wherein the central node is a radio network controller.

63. A central node of a cellular radio system, the central control node being adapted to generate a message, the central node comprising:

- controller circuitry adapted to generate a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising E-UTRA frequency control information for controlling if the UE shall continue to scan all available E- UTRA frequencies or not if no suitable cell can be found on the specified E-UTRA Absolute Radio Frequency Channel Numbers EARFCNs.

64. The central node according to claim 63, wherein the 2G/3G network is a UTRAN network.

65. The central node according to claim 63 or 64, wherein the message is a RRC Connection Release message.

66. The central node according to claim 65, wherein said E-UTRA frequency control is comprised in an Information Element of the RRC Connection Release message.

67. The central node according to any of claims 63 - 66, wherein the central node is a radio network controller.

68. A central node of a cellular radio system, the central control node being adapted to generate a message, the central node comprising: - controller circuitry adapted to generate a message for a UE message connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising information for controlling whether the UE shall revert to WCDMA connected mode if no suitable E-UTRA cell can be found within a specified time.

69. The central node according to claim 68, wherein the 2G/3G network is an UTRAN network.

70. The central node according to claim 68 or 69, wherein the message is a RRC Connection Release message.

71 . The central node according to claim 70, wherein said information for controlling whether the UE shall revert to WCDMA connected mode is comprised in an Information Element of the RRC Connection Release message.

72. The central node according to any of claims 68 - 71 , wherein the central node is a radio network controller.

73. A method in a user equipment, the user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the method comprising the steps of:

- receiving a message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, and

- selecting a E-UTRA cell to camp on only when a threshold level indicating minimum Reference Signal Received Power, RSRP, and/or Reference Signal Received Quality, RSRQ is exceeded for the E-UTRA cell.

74. The method according to claim 73, wherein the a threshold level indicating a minimum Reference Signal Received Power, RSRP, or a minimum Reference Signal Received Quality, RSRQ, of a E-UTRA cell to be selected by the user equipment is comprised in a message received from the radio access network.

75. The method according to claim 74, wherein the 2G/3G network is a UTRAN network.

76. The method according to claim 74 or 75, wherein the message is a RRC Connection Release message.

77. The method according to claim 76, wherein said E-UTRA frequency control information is comprised in an Information Element of the RRC Connection Release message.

78. A method in central node of a cellular radio system, the central control node being adapted to generate a message, the method comprising the steps of:

- generating a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising a threshold level indicating a minimum Reference Signal Received Power, RSRP, and/or a minimum Reference Signal Received Quality, RSRQ, of a E-UTRA cell to be selected by a user equipment.

79. The method according to claim 78, wherein the 2G/3G network is a UTRAN network.

80. The method according to claim 78 or 79, wherein the message is a RRC Connection Release message.

81 . The method according to claim 80, wherein said E-UTRA frequency control is comprised in an Information Element of the RRC Connection Release message.

82. The method according to any of claims 78 - 81 , wherein the central node is a radio network controller.

83. A user equipment enabled to connect to an E-UTRA network and also to a 2G/3G network, the user equipment comprising:

- an input/output device adapted to receive a message from the radio access network when connected to the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, , and

- controller circuitry adapted to select a E-UTRA cell to camp on only when a threshold level indicating minimum Reference Signal Received Power, RSRP, or Reference Signal Received Quality, RSRQ is exceeded for the E-UTRA cell.

84. The user equipment according to claim 83, wherein the user equipment is adapted to receive the a threshold level indicating a minimum Reference Signal Received Power, RSRP, or a minimum Reference Signal Received Quality, RSRQ, of a E-UTRA cell to be selected by the user equipment in a message from the radio access network.

85. The user equipment according to claim 83 or 84, wherein the 2G/3G network is a UTRAN network.

86. The user equipment according to any of claims 83 - 85, wherein the message is a RRC Connection Release message.

87. The user equipment according to claim 86, wherein said E-UTRA frequency control information is comprised in an Information Element of the RRC Connection Release message.

88. A central node of a cellular radio system, the central control node being adapted to generate a message, the central control node comprising: - controller circuitry adapted to generate a message for a UE connected to a 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies, the message further comprising a threshold level indicating a minimum Reference Signal Received Power, RSRP, and/or a minimum Reference Signal Received Quality, RSRQ, of a E-UTRA cell to be selected by a user equipment.

89. The central control node according to claim 88, wherein the 2G/3G network is a UTRAN network.

90. The central control node according to any of claims 88 or 89, wherein the message is a RRC Connection Release message.

91 . The central control node according to claim 90, wherein said E-UTRA frequency control is comprised in an Information Element of the RRC Connection Release message.

92. The central control node according to any of claims 88 - 91 , wherein the central node is a radio network controller.

93. A method in a user equipment, the user equipment being enabled to connect to an E-UTRA network and also to an 2G/3G network, the method comprising the steps of:

- scanning E-UTRA frequencies while staying in connected mode in the 2G/3G network, and

- continue operating in connected mode in the 2G/3G network if no suitable E- UTRA cell can be found within a specified time.

94. The method according to claim 93, wherein if the user equipment finds a suitable E-UTRA cell within the specified time, the user equipment leaves connected mode in the 2G/3G network and connects to the E-UTRA network.

95. The method according to any one of claims claim 93-94, wherein the method further comprises receiving information whether the user equipment shall stay in connected mode in the 2G/3G network while scanning for E-UTRA frequencies and wherein performing said scanning and continued operation steps are conditioned on the received information indicating that the user equipment shall stay in connected mode while scanning for E-UTRA frequencies.

96. The method according to any one of claims 93-95, wherein the 2G/3G network is an UTRAN network.

97. The method according to any of claims 93 - 96, wherein the message is a RRC Connection Release message.

98. The method according to claim 97, wherein said information for controlling whether the user equipment shall stay in connected mode in the 2G/3G network while scanning for E-UTRA frequencies is comprised in an Information Element of the RRC Connection Release message.

99. A user equipment being enabled to connect to an E-UTRA network and also to a 2G/3G network, the user equipment adapted to:

- receive a message from the 2G/3G network, the message comprising redirection information and information regarding target E-UTRA frequencies,

- scan E-UTRA frequencies while staying in connected mode in the 2G/3G network, and

100. The user equipment according to claim 99, wherein the user equipment is adapted to leave connected mode in the 2G/3G network and connect to the E- UTRA network if a suitable E-UTRA cell can be found within the specified time.

101 . The user equipment according to any one of claims claim 99-100, wherein the user equipment is further adapted to receive information whether the user equipment shall stay in connected mode in the 2G/3G network while scanning for E-UTRA frequencies and wherein the user equipment is adapted to perform said scanning and continued operation in connected mode if the received information indicates that the user equipment shall stay in connected mode while scanning for E-UTRA frequencies.

102. The user equipment according to any one of claims 99-101 , wherein the 2G/3G network is an UTRAN network.

103. The user equipment according to any of claims 99 - 102, wherein the message is a RRC Connection Release message.

104. The user equipment according to claim 103, wherein said information for controlling whether the user equipment shall stay in connected mode in the 2G/3G network while scanning for E-UTRA frequencies is comprised in an Information Element of the RRC Connection Release message.

105. The method according to any of claims 17 - 21 , wherein the message is a RRC Connection Release message.

106. The method according to claim 105, wherein said information for controlling whether the UE shall revert to connected mode in the 2G/3G network is comprised in an Information Element of the RRC Connection Release message.

107. The user equipment according to any of claims 53 - 57, wherein the message is a RRC Connection Release message.

108. The user equipment according to claim 107, wherein said information for controlling whether the UE shall revert to connected mode in the 2G/3G network is comprised in an Information Element of the RRC Connection Release message.