WO2010031205A1

WO2010031205A1 - Soft handover in a mobile communication network

Info

Publication number: WO2010031205A1
Application number: PCT/CN2008/001630
Authority: WO
Inventors: Lei Xiao; Bin Xu; Huaisong Zhu
Original assignee: Telefonaktiebolaget L M Ericcson (Publ)
Priority date: 2008-09-19
Filing date: 2008-09-19
Publication date: 2010-03-25

Abstract

A method and apparatus for performing soft handover on a user equipment (211) located in a first cell in a mobile communication network is provided. The method comprises the steps of: determining among adjacent cells (#1, #2) if there is a second cell (#1) that is synchronized to the user equipment's uplink signal (S306, S406); if it exists, putting the second cell (#1) into an active set of the user equipment (211) (S310, S410); performing Multi-Cell Joint Detection for the user equipment (211 ) with respect to the first cell (#3) and the second cell (#1) (S312, S412); and combining the uplink detection results (S314, S414).

Description

SOFT HANDOVER IN A MOBILE COMMUNICATION NETWORK

TECHNICAL FIELD

The present invention generally relates to soft handover in a mobile communication network. The invention more particularly relates to a method, a communication device and a mobile communication network for supporting soft handover.

BACKGROUND Soft Handover or soft HandOff (soft HO) is a basic feature used by traditional Code Division Multiple Access (CDMA) standards, where a cell phone is simultaneously connected to two or more cells (or cell sectors) during a call. During soft handover, cell phones continuously make power measurements of a list of neighboring cells (base stations), and determine whether or not to request or end soft handover with the cells on the list. As is known, soft HO may improve HO performance and network performance.

Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) standard has been adopted by 3GPP since Rel-4, known as "UTRA TDD 1.28Mcps Option". TD-SCDMA uses Time Division Duplex (TDD), in contrast to the Frequency Division Duplex (FDD) scheme used by Wideband Code Division Multiple Access (WCDMA). By dynamically adjusting the number of timeslots used for downlink and uplink, the system can more easily accommodate asymmetric traffic with different data rate requirements on downlink and uplink than FDD schemes. Since it does not require paired spectrum for downlink and uplink, spectrum allocation flexibility is also increased. Also, using the same carrier frequency for uplink and downlink means that the channel condition is the same on both directions, and the base station can deduce the downlink channel information from uplink channel estimates, which is helpful to the application of beamforming techniques. TD-SCDMA also uses TDMA in addition to the CDMA used in WCDMA. This reduces the number of users in each timeslot, which reduces the implementation complexity of multiuser detection and beamforming schemes. However, traditional soft HO can not be used in a TD-SCDMA system due to the two following reasons.

The first reason is that TD-SCDMA is a synchronous system so it is hard to maintain more than one radio link of a User Equipment (UE) which are synchronized to many Radio Base Stations (RBS). More specifically, TD-SCDMA is a TDD system and its uplink and downlink signals follow specific timing rules as shown in Figure 1. That is, the uplink or downlink signals must arrive at the RBS or UE at a specific time to avoid interference. So it is hard either for the RBS or UE to setup more radio links and be synchronized. Another reason is that TD-SCDMA is a code limited system and it can not afford duplicate code allocation in several RBSs to serve one UE. More specifically, soft HO needs several RBSs to serve one UE at the same time which requires resource allocation in each RBS. However, code resource, instead of interference as in other CDMA systems, is a bottleneck for system capacity in a TD-SCDMA system. So traditional soft HO may considerably degrade system capacity for TD-SCDMA.

Thus, in the current TD-SCDMA system there is only hard HO and so called Baton HO, which in principle also is hard HO. Without soft HO, the HO success rate, call quality/data throughput etc. during HO will all be impacted. More detailed description about the advantages of soft HO over hard HO will not be provided here since it is well known in the field of CDMA.

Moreover, intra-frequency interference is the most critical problem for TD-SCDMA. Especially during HO, it has been shown that intra-frequency interference may lead to serious system performance degradation. SUMMARY

Therefore, it is an object of the present invention to address the problem outlined above by providing a method and apparatus for supporting soft handover and soft combination in a TD-SCDMA system and other synchronous systems.

According to an aspect of the invention, a method for performing soft handover on a user equipment located in a first cell in a mobile communication network is provided. The method comprises the steps of: determining among adjacent cells whether there is a second cell that is synchronized to the user equipment's uplink signal, if it exists, putting the second cell into an active set of the user equipment, performing Multi-Cell Joint Detection for the user equipment with respect to the first cell and the second cell, and combining the uplink detection results.

The mobile communication network may be a TD-SCDMA network. The method may further comprise the steps of measuring in the adjacent cells intensity of the uplink signal from the user equipment, and transmitting a downlink signal from the second cell which is the same as that from the first cell to the user equipment.

In an embodiment, the first cell and the second cell are served by one same base station and the combination is a soft combination performed in the radio base station. In another embodiment, the first cell and the second cell are served by two different radio base stations and the combination is a selective combination performed in a radio network controller controlling the two radio base stations.

The method may further comprise the step of the user equipment measuring intensity of downlink signals from the first cell and adjacent cells, and wherein the step of determining may further comprises determining whether the downlink signal of the second cell is strong.

The method may further comprise the user equipment soft combining the downlink signals from the first cell and the second cell. The downlink signals from the first cell and the second cell may be transmitted via same downlink code resources or transmitted via different downlink code resources. According to another aspect of the invention, a radio base station may perform soft handover on a user equipment located in a first cell of a mobile communication network. The radio base station comprises means for determining among adjacent cells whether there is a second cell that is synchronized to the user equipment's uplink signal, means for putting the second cell into an active set of the user equipment if such a cell exists, means for performing Multi-Cell Joint Detection for the user equipment with respect to the first cell and the second cell; and means for combining the uplink detection results. The mobile communication network may be a TD-SCDMA network.

The radio base station may further comprise means for measuring in the adjacent cells intensity of the uplink signal from the user equipment, and means for transmitting a downlink signal from the second cell which is the same as that from the first cell to the user equipment.

In an embodiment, the first cell and the second cell are served by the radio base station and the combination is a soft combination performed in the radio base station. The means for determining may further comprise means for determining whether the downlink signal of the second cell is strong. The downlink signals from the first cell and the second cell may be transmitted via same downlink code resources or via different downlink code resources. According to a further aspect of the invention, a radio network controller for performing soft handover on a user equipment located in a first cell of a mobile communication network is provided. The radio network controller comprises means for determining among adjacent cells whether there is a second cell that can detect the user equipment's uplink signal, based on measurement results from radio base stations serving the adjacent cells; means for putting the second cell into an active set of the user equipment if it exists; and means for combining uplink detection results from Multi-Cell Joint Detection for the user equipment with respect to the first cell and the second cell.

The mobile communication network may be a TD-SCDMA network.

The radio network controller may further comprise means for transmitting a downlink signal from the second cell which is the same as that from the first cell to the user equipment.

In an embodiment, the combination is a selective combination performed in the radio network controller.

The means for determining may further comprise means for determining whether the downlink signal of the second cell is strong.

The downlink signals from the first cell and the second cell may be transmitted via same downlink code resources or via different downlink code resources.

In a still further aspect of the invention, a mobile communication network comprising a user equipment, at least one above-mentioned radio base stations and an above-mentioned radio network controller is provided.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, will be best understood by reference to the following description taken together with the accompanying drawings, in which:

Fig. 1 is a diagram illustrating a TD-SCDMA frame structure in time, frequency and code axes; Fig. 2 is a schematic diagram illustrating a communications network according to the invention;

Fig. 3 is a schematic flowchart illustrating a procedure of soft HO according to an embodiment of the invention; and Fig. 4 is a schematic flowchart illustrating a procedure of soft

HO handover according to another embodiment of the invention.

DETAILED DESCRIPTION

Throughout the drawings, the same reference characters will be used for corresponding or similar elements.

Before describing various embodiments in detail, it is to be understood that this invention is not limited to the particular component parts of the devices described or process steps of the methods described as such devices and methods may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the singular forms "a", "an", "another" and "the" may also encompass plural referents unless the context clearly dictates otherwise. Thus, for example, the term "a device" may refer to one or more devices, and the like.

Briefly described, a method and an arrangement are provided for supporting soft HO in a TD-SCDMA system. The term "User Equipment" (UE) used herein may mean a mobile terminal, e.g. a mobile or cellular phone or a mobile TV client, but it may also mean some other type of terminal possible to connect to a communication network and perform soft HO. The term "cell" used herein may mean an area that is covered by a radio base station therein or by a sector of the radio base station. It should be understood that the term "handover" is used in a generic manner. That is, it does not necessarily indicate that a UE has to move from one cell into another cell. As long as there is a change in the active set of the UE (even if the UE does not actually reach in another cell or does not release the link with the original cell), the procedure may be called handover. The term soft handover may not only indicate handover between different radio base stations (inter-RBS), but intra-RBS handover which is also called as softer handover.

Although the embodiments of the present invention are illustrated in context of a TD-SCDMA system, the teaching of the present invention can also be applied to other communication systems which have a principle similar to TD-SCDMA. In the embodiments only three cells and one UE are shown, however, the number of the cells and UEs involved in the soft HO should not be limited to this. Besides, the signalling as shown in the figures is for illustrative purpose only. The procedure of the invention does not necessarily have to follow the signalling as described in connection with the figures. Instead, the signaling steps may be combined in any suitable order, or part of them may be omitted, or new signaling steps may be added if necessary.

For a better understanding of the invention it may be useful to begin with a brief system overview. Fig. 2 schematically shows a TD-SCDMA communications network. The communication network comprises a core network (CN) and a radio area network (RAN). As shown in Fig. 2, the RAN comprises a radio network controller (RNC) 205, a user equipment (UE) 21 1 and three radio base stations (RBS) 201 , 202 and 203 serving three cells # 1 , #2 and #3 respectively. Although not shown, it is also possible that the three cells # 1 , #2 and #3 are served by a single RBS, i.e. the three cells # 1 , #2 and #3 are covered by up to three sectors of the RBS. The UE 21 1 may be a cellular telephone, a PDA or a portable computer or the like. When the UE 21 1 is located in one of the cells, it may communicate with the corresponding RBS. The RNC 205 is connected to and controls the three RBSs 201 , 202 and 203. As shown in Fig. 2, the UE 21 1 is located in cell #3, but its uplink signal will also arrive at cell # 1 and cell #2, especially when it is in HO area (the overlapped area). The uplink signal propagated in cell #1 and cell #2 will cause interference and impact other users' detection in cell #1 and cell #2.

To alleviate such interference to cell # 1 and cell #2, currently there is a solution called Multi-Cell Joint Detection (MCJD) which is for example described in WO 200603221 IAl . That is, the cell # 1 and cell #2 will consider the UE 21 1 's uplink signal as a valid or useful signal rather than an interference as long as it is strong enough, and detects it in conjunction with other real users in the cell # 1 and cell #2. The details of the MCJD technique will not be further described hereinbelow since it is well known in the prior art.

In the present invention, MCJD is used in the TD-SCDMA system to improve the performance of the system. One problem in the current MCJD solution is that the UE 21 1 's detections in cell # 1 and cell #2 are only limited in chip level and the purpose is only to enhance detections of the real users in the cell # 1 and cell #2.

However, in the present invention, the inventor conceives that such detection results can also be used to enhance detection of the UE 21 1 , similar to the soft HO case in other CDMA systems. That is, cell # 1 and cell #2 can decode UE 21 1 's uplink signal and send its symbols to the RNC to form selective combination together with symbols from the cell #3, or form soft combination in the RBS if the cells # 1 , #2 and #3 are located in the same RBS.

Thus, UE 21 1 ' s uplink signal detection can be more reliable due to the above combination, which is similar to the soft HO case in other CDMA systems.

With the introduction of MCJD, the cells # 1 , #2, and #3 can receive UE 21 1 's uplink signal at proper timing. Since the cells # 1 ,

#2 and #3 are synchronized in downlink (which is a basic feature of

TD-SCDMA), it also means the downlink signals of the cells # 1 , #2 and #3 can also arrive at UE 21 1 at proper timing if the cells # 1 , #2 and #3 send the same downlink signals to UE 21 1. There shall be no timing problem as mentioned in the first reason which limits usage of soft HO in TD-SCDMA, and the problem is thereby alleviated. As for the limitation on code resources, in the invention, the cells # 1 and #2 will not use their own code resources, instead, they will use the same code resource as cell #3, including scrambling code, midamble, spreading code, etc. That is, three downlink signals which are the same will be sent from the cells# l , #2 and #3 to the UE 21 1. Thereby chip level combination, which is different from the soft HO in other CDMA systems, can be used for these three downlink signals. Since TD-SCDMA is a code limited system, normally the cells # 1 and #2 should have enough TX power to send additional downlink signals for the UE 21 1. Moreover, smart antenna is widely used in the TD-SCDMA system, which also means the TX power in each antenna is not exactly the same to form downlink beamforming. So even though sometimes the cells # 1 and #2 do not have power left for a specific antenna, there should always be another antenna that can transmit to the UE 21 1. Thus, the two limitations to use soft HO in TD-SCDMA can be overcome by the solution of the present invention, but this new type of soft HO is also different from normal soft HO. This new technique will be applied whenever another cell, such as the cell # 1 , can detect the UE 21 1 ' s uplink signal (either during a normal call holding or during the moving into another cell).

Now with reference to Fig. 3 and Fig. 4, a procedure of soft HO according to embodiments of the invention will be described.

Fig. 3 shows a procedure of soft HO which is performed in one RBS (intra-RBS case). In the embodiment of the invention, the cells # 1 , #2 and #3 are served by one RBS 203 , i.e. the three cells are covered by up to three sectors of the RBS 203 respectively. The UE 21 1 is initially located in cell #3, and both its uplink and downlink resources are allocated by the cell #3 (S302). The RBS 203 measures in the cells # 1 , #2 and #3 the intensity of the uplink signal from the UE 21 1 by in-prior information such as information about allocated codes and carriers from the RNC 205 (S304). Then the RBS 203 determines whether there is another cell that can detect the uplink signal of the UE 21 1 based on the measurement results (S306). In the embodiment, the UE 21 1 ' s uplink signal can be detected by the cell #1 , while the UE 21 1 's uplink signal can not be detected by the cell #2 since it is not synchronized. The UE 21 1 also measures the intensity of the downlink signals from all the cells, for example, by measuring pilots or other criteria (S308). In the conventional communication systems, only the cells from which the downlink signals are strong (e. g. greater than a predetermined threshold) can be put into an active set of the UE 21 1. However, in the present embodiment, in order to be added to the active set of the UE 21 1 , a cell should both have a strong downlink signal to the UE 21 1 and can detect the uplink signal of UE 21 1 (i.e. it is synchronized with the uplink signal of UE 21 1 ). In this case, by the invention, cell # 1 is put to UE 21 l ' s active set by the RBS 203 or the RNC 205 (S310); the cell #2 will not be put in the active set so as not to serve the UE 21 1. The RBS 203 will make MCJD for the UE 21 1 with respect to the cell # 1 and cell #3 together with their other users (S312), and the uplink detection results by the cell # 1 and 3 will be soft combined by the RBS 203 to enhance the UE 21 l 's detection (S3 14). In this case, the cell # 1 may get downlink information from the cell #3 and send it to UE 21 1 via the same code resources as cell #3 (S3 16). That is, the downlink signal from cell # 1 is the same as that from the cell #3. The UE 21 1 also soft combines the downlink signals from the cell # 1 and cell #3 to obtain a diversity gain (S31 8). As an alternative embodiment, cell #2 may be served by an RBS other than the RBS 203. In this case, the detection of uplink signal of UE 21 1 in the cell #2 will be performed by the other RBS, and the active set of the UE 21 1 may be maintained by the RNC 205 only. In the above embodiment, the cell # 1 is added into the active set of the UE 21 1. However, as is apparent to those skilled in the art, it is possible to continuously add other cells into the active set, and the MCJD may be performed with respect to all the cells in the active set.

Fig. 4 shows a procedure of soft HO which is performed between different RBSs (inter-RBS case). In the embodiment of the invention, the cells #1 , #2 and #3 are served by RBSs 201 , 202 and 203 respectively. The UE 21 1 is initially located in cell #3, and both its uplink and downlink resources are allocated by the cell #3 (step S402). The RBSs 201 , 202 and 203 respectively measure the intensity of the uplink signal from the UE 21 1 by in-prior information such as information about allocated codes and carriers from the RNC 205 (step S404). Then the RNC 205 determines whether there is another cell that can detect the uplink signal of the UE 21 1 based on the measurement results from the RBS 201 , 202 and 203 (S406). In the embodiment, the UE 21 1 's uplink signal can be detected by the cell # 1 ; while the UE 21 1 's uplink signal can not be detected by the cell #2 since it is not synchronized. The UE 21 1 also measures the intensity of the downlink signals from all the cells, for example, by measuring pilots or other criteria (step S408). In order to be put into the active set of the UE 21 1 , a cell should both have a strong downlink signal to the UE 21 1 and be able to detect the uplink signal of the UE 21 1. In this case, by the invention, cell # 1 is put to UE 21 1 ' s active set by the RNC 205 (S410); the cell #2 will not be put in the active set so as not to serve the UE 21 1. The RBSs 201 , 202 and 203 will make MCJD for the UE 21 1 with respect to the cell # 1 and cell #3 together with their other users (S412), and the uplink detection results will be sent to the RNC 205 to form selective combination to enhance the UE 21 1 's detection (S414). In this case, cell # 1 may get downlink information from cell #3 and send it to the UE 21 1 via the same code resources as the cell #3 (S416). That is, the downlink signal from the cell # 1 is the same as that from the cell #3. The UE 21 1 also soft combines the downlink signals from cell # 1 and cell #3 to obtain a diversity gain (S418). In this case, the RNC 205 needs to send the UE 21 1 ' downlink information to the cell # 1 which in turn sends the same signal as the cell #3 to UE 21 1.

For the procedure of soft HO as described above with reference to figures 3 and 4, since the uplink signal can be detected, which means that the uplink signal is synchronized with both the cell # 1 and cell #3, the same downlink signals from the cell #1 and cell #3 are also synchronized to UE 21 1. The detection in both uplink and downlink are feasible.

In the above embodiments, the downlink signal from the cell newly added in the active set such as cell # 1 is transmitted via the same downlink code resources as the original downlink code resources as used in the cell #3. However, in another embodiment, the cell # 1 may use its own downlink code resources for transmitting downlink signal, instead of the original downlink code resources as used in cell #3. This solution will be very close to the traditional soft HO solution. By this invention, the new soft HO can be applied in a

TD-SCDMA system to enhance uplink/downlink performance. Moreover, the intra-frequency interference, the most critical problem in TD-SCDMA, will be alleviated. Instead, the interference will be used as a valid signal for detecting, decoding and combining. While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt to a particular situation and the teaching of the present invention without departing from its central scope.

Claims

1. A method for performing soft handover on a user equipment (21 1 ) located in a first cell (#3) in a mobile communication network, comprising the steps of: determining among adjacent cells (# 1 , #2) whether there is a second cell (# 1 ) that is synchronized to the user equipment' s uplink signal (S306, S406), based on measurement of the uplink signal from the user equipment (21 1 ); if it exists, putting the second cell (# 1 ) into an active set of the user equipment (21 1 ) (S310, S410); performing Multi-Cell Joint Detection for the user equipment (21 1 ) with respect to the first cell (#3) and the second cell (# 1) (S3 12, S412); and combining the uplink detection results (S314, S414).

2. The method of claim 1 , wherein the mobile communication network is a Time Division-Synchronous Code Division Multiple Access network.

3. The method of claim 1 , further comprising the steps of: measuring in the adj acent cells (# 1 , #2) intensity of the uplink signal from the user equipment (21 1 ) (S304, S404); and transmitting a downlink signal from the second cell (# 1 ) which is the same as that from the first cell (#3) to the user equipment (S3 16, S416).

4. The method of any one of claims 1 to 3, wherein the first cell

(#3) and the second cell (# 1 ) are served by one same radio base station and the combination is a soft combination performed in the radio base station.

5. The method of any one of claims 1 to 3 , wherein the first cell (#3) and the second cell (# 1 ) are served by two different radio base stations and the combination is a selective combination performed in a radio network controller controlling the two radio base stations.

6. The method of any one of claims 1 to 3, further comprising the step of the user equipment (211) measuring intensity of downlink signals from the first cell (#3) and the adjacent cells (#1, #2), and wherein the step of determining (S306, S406) further comprises determining whether the downlink signal of the second cell (#1) is strong.

7. The method of claim 3, further comprising the user equipment (211) soft combines the downlink signals from the first cell (#3) and the second cell (#1).

8. The method of claim 3, wherein the downlink signals from the first cell (#3) and the second cell (#1) are transmitted via same downlink code resources.

9. The method of claim 3, wherein the downlink signals from the first cell (#3) and the second cell (#1) are transmitted via different downlink code resources.

10. A radio base station (203) for performing soft handover on a user equipment located in a first cell (#3) of a mobile communication network, comprising: means (S306) for determining among adjacent cells (#1,#2) whether there is a second cell (#1) that is synchronized to the user equipment's uplink signal, based on measurements of the uplink signal from the user equipment (211); means (S310) for putting the second cell (#1) into an active set of the user equipment (211) if it exists; means (S312) for performing Multi-Cell Joint Detection for the user equipment (211) with respect to the first cell (#3) and the second cell (#1); and means (S314) for combining the uplink detection results.

11. The radio base station (203) of claim 10, wherein the mobile communication network is a Time Division-Synchronous Code

Division Multiple Access network.

12. The radio base station (203) of claim 10, further comprising: means (S304) for measuring in the adjacent cells (# 1 , #2) intensity of the uplink signal from the user equipment (21 1 ); and means (S3 16) for transmitting a downlink signal from the second cell (# 1 ) which is the same as that from the first cell (#3) to the user equipment.

13. The radio base station (203) of any one of claims 10 to 12, wherein the first cell (#3) and the second cell (# 1 ) are served by the radio base station (203) and the combination is a soft combination performed in the radio base station (203).

14. The radio base station (203) of any one of claim 10 to 12, wherein the means (S306) for determining further comprises means for determining whether the downlink signal of the second cell (# 1 ) is strong.

15. The radio base station (203) of claim 12, wherein the downlink signals from the first cell (#3) and the second cell (# 1) are transmitted via same downlink code resources.

16. The base station (203) of claim 12, wherein the downlink signals from the first cell (#3) and the second cell (# 1 ) are transmitted via different downlink code resources.

17. A radio network controller (205) for performing soft handover on a user equipment (21 1 ) located in a first cell (#3) of a mobile communication network, comprising: means (S406) for determining among adjacent cells (# 1 , #2) whether there is a second cell (# 1 ) that is synchronized to the user equipment' s uplink signal, based on measurements from radio base stations serving the adjacent cells (# 1 , #2); means (S410) for putting the second cell (# 1 ) into an active set of the user equipment (21 1 ) if it exists; and means (S414) for combining uplink detection results from

Multi-Cell Joint Detection for the user equipment (21 1 ) with respect to the first cell (#3) and the second cell (# 1 ).

18. The radio network controller (205) of claim 17, wherein the mobile communication network is a Time Division-Synchronous Code Division Multiple Access network.

19. The radio network controller (205) of claim 17, further comprising means (S416) for transmitting a downlink signal from the second cell (# 1 ) which is the same as that from the first cell (#3) to the user equipment.

20. The radio network controller (205) of any one of claims 17 to 19, wherein the combination is a selective combination performed in radio network controller (205).

21. The radio network controller (205) of any one of claims 17 to 19, wherein the means (S406) for determining further comprises means for determining whether the downlink signal of the second cell (# 1 ) is strong.

22. The radio network controller (205) of claim 19, wherein the downlink signals from the first cell (#3) and the second cell (# 1) are transmitted via same downlink code resources.

23. The radio network controller (205) of claim 19, wherein the downlink signals from the first cell (#3) and the second cell (# 1) are transmitted via different downlink code resources.

24. A mobile communication network, comprising a user equipment, at least one radio base station (203) according to any one of claims 10 to 16 and a radio network controller (205) according to any one of claims 17 to 23.