WO2009038528A2

WO2009038528A2 - Reduced interference in an mbms enabled system

Info

Publication number: WO2009038528A2
Application number: PCT/SE2008/051023
Authority: WO
Inventors: Lei Xiao; Jingyue Nie; Bin Xu
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2007-09-18
Filing date: 2008-09-12
Publication date: 2009-03-26
Also published as: CN101785208A; WO2009038528A3

Abstract

The invention discloses a method for a cellular system (100) which uses TD- SCDMA, Time Division Synchronous Code Division Multiple Access. According to the method, MBMS services, Multi media Broadcast Multicast Service, are enabled in a first (120) and a second (160) cell in the system, so that there can be both MBMS and non-MBMS users (130,170; 140; 180) in said first and second cells at one and the same time. According to the method, traffic to non-MBMS users (140, 180) in the first and second cells is sent using one and the same basic midamble code per cell which is shifted between the different non-MBMS users in each cell, and the traffic to MBMS users (130, 170) in both of said first and second cells who use the same MBMS services is varied between the cells with respect to the basic midamble code or/and the midamble shift.

Description

REDUCED INTERFERENCE IN AN MBMS ENABLED SYSTEM

TECHNICAL FIELD

The present invention discloses a method for use in a TD-SCDMA system in which MBMS services are enabled.

BACKGROUND

In cellular wireless TD-SCDMA systems, i.e. systems which use Time Division Synchronous Code Division Multiple Access, different so called scramble codes and basic midamble codes are used for neighbouring cells in the system in order to decrease the inter-cell interference.

In addition to this, user specific so called midamble shifts are used in order to enable different users to identify the traffic which is intended for them, and to enable the users to perform so called channel estimation. However, for systems which use so called MBMS services, Multi-media Broadcast Multicast, which is a feature which is or will be standardized by the CCSA, Chinese Communication Standardization Association, one and the same basic midamble code and one and the same midamble shift are used for all cells in the system which carry the same MBMS service in the same timeslot and frequency. In this way, MBMS users in the system can perform so called "soft combination" of signals received from more than one cell in order to improve the SNR, the signal to noise ratio of the MBMS signal.

In the case of MBSFN timeslots, i.e. MBMS over a single frequency, there is only a single midamble and this is derived from a single basic midamble code which is not necessarily cell-specific.

European Patent Application EP 1 727 297 discloses a method for reducing inter-cell interference in a communications system. In order to use the method disclosed in this document, a user terminal needs to be equipped with at least two antennas for receiving signals.

European Patent Application EP 1 793 518 discloses a method for multi cell joint detection.

SUMMARY

It is a purpose of the current invention to offer an improvement of the TD

SCDMA MBMS technology described above.

This purpose is achieved in that the present invention discloses a method for use in a cellular communications system which uses the TD-SCDMA principle, Time Division Synchronous Code Division Multiple Access, and in which system MBMS services, Multi media Broadcast Multicast Service, are enabled in at least a first and a second cell in the system.

Thus, in a system in which the invention is applied, there can be both MBMS and non-MBMS users in the first and second cells at one and the same time.

According to the invention, traffic to non-MBMS users in the first and second cells is sent using one and the same basic midamble code per cell which is shifted between the different non-MBMS users in each cell, while traffic to MBMS users in both of said first and second cells who use the same MBMS services is varied between the cells with respect to the basic midamble code or/and the midamble shift.

In this way, an MBMS user in a cell can still receive the same MBMS signals from its "own" cell as well as from adjacent cells, but will be able to distinguish between the signals from the different cells. This will enable users to obtain a higher so called "combination gain" when combining the signals from the different cells by means of so called "soft combining". Suitably, in one embodiment of the invention, one and the same basic midamble code but a different midamble shift is used for signals in different cells which carry the same MBMS signal in the same timeslot and frequency, i.e. in this embodiment, it is the midamble shift which is varied while the midamble is kept the same.

Also, suitably, the midamble shift is such that the midamble is shifted cyclically between the cells.

These and other embodiments of the invention will be described in more detail in the detailed description below.

The invention also discloses a controlling station for a cell in a cellular TD SCDMA MBMS system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows an overview of a system in which the invention can be applied, and

Fig 2 shows a problem to be solved by the invention, and Fig 3 shows a flowchart of a method of the invention, and Fig 4 shows a block diagram of a transceiver of the invention.

DETAILED DESCRIPTION

Fig 1 shows a schematic view of a system 100 in which the invention can be applied. The system 100 is a system in which the so called TD SCDMA principle is used, Time Division Synchronous Code Division Multiple Access. In addition, the system is enabled for so called MBMS services, Multimedia Broadcast Multicast Service, which is a principle which at present is mainly used in systems which operate according to principles established by CCSA, Chinese Communication Standardization Association.

As indicated in fig 1 , the system 100 can comprise a number of cells, two of which, 120 and 160, are shown in fig 1. Each cell 120,160, can comprise a number of users, UEs, ("User Equipment"), and fig 1 shows two UEs 130,140; 170,180, in each of the cells 120,160. For each cell, there is a controlling node, a so called NodeB, which serves to control the traffic to and from the users in the cell.

The fact that MBMS services are enabled in the system is indicated in fig 1 by means of the text MBMS or NMNBS (non MBMS) next to the UEs in the cells.

In the system 100, various so called midamble codes and shifts of these midamble codes, i.e. "midamble shifts", may be used by the NodeBs in the system. At present, for MBMS services, the same midamble code with the same midamble shift is used by more than one NodeB in the system

A problem in present day systems is shown in fig 2: as shown, the signals from the two NodeBs shown in fig 1 , 110 and 150, may reach one and the same UE, for example the UE 130 of cell 120, and the signals may in fact reach the UE in a plurality of ways due to multipath propagation. Some possible examples of multipath propagation paths are shown in fig 2 as P1 - P4.

As can be seen, if P2 and P4 are of essentially the same length, the UE 130 will not be able to differentiate between the signals from the two NodeBs, since the same midamble code and the same midamble shift are used by both of the NodeBs. This is detrimental, if, for example, a UE wants to use so called CIR, Channel Impulse Response, or if a UE wishes to use so called "soft combining" of the signals received from multiple NodeBs, since "soft combining" makes it necessary to know which signal that originates from which NodeB. This problem is particularly noticeable for MBMS users such as the UE 130.

The CIR is an estimation of the receiving channel by means of detecting a training sequence or, in a TD system, a midamble. From the channel estimation, the receiver can find out the phase and amplitude information of the received signal and use this in order to better detect data.

The problem of inter-cell interference, particularly that caused by multipath propagation, as shown in fig 2, is addressed by the present invention by proposing that traffic to MBMS users in different cell of the system but who use the same MBMS services is varied between the cells with respect to either the midamble or the midamble shift. If there is such a variation, the UE 130 will still be able to receive the signals from both NodeBs, 130, 150, but will be able to distinguish between the signals, and will thus be able to perform CIR and/or soft combining.

Preferably, the variation of the MBMS signal is with respect to the midamble shift, i.e. so that the same midamble is used for traffic to the users in the different cells who use the same MBMS services, but with the midamble being shifted between the different cells, i.e. between the NodeBs of the cells. However, it is also perfectly possible to vary the midamble as such between the cells. When the MBMS is configured to each cell in the system, the basic midamble and its shift should be configured to the NodeBs in such a way that the same shift is not used by two NodeBs, at least not within

"reception distance" from each other.

In a preferred embodiment, one and the same midamble is used, but with a different midamble shift for different cells which carry the same MBMS signal in the same timeslot and frequency. The notion of the same MBMS signal can be illustrated by the following example: assume that NodeB number 1 , 2, 3 and 4 broadcast TV channel 1 , and NodeB number 5, 6, 7, and 8 broadcast TV channel 2; there should then be a variation in midamble or midamble shift between those NodeBs which broadcast channel 1 and those which broadcast channel 2.

Preferably, if the principle which is used is midamble shift, then the shift is carried out cyclically between the cells.

Above, it has been shown how traffic to users of the same MBMS services in the first and second cells is sent. Regarding the issue of non-MBMS users in the cells of the system in which the invention is applied, the following can be said: traffic to non-MBMS users in the first and second cells is sent using one and the same basic midamble code for each cell, which is shifted between the different non-MBMS users in the cell.

Thus, by means of the present invention, users will be able to receive one and the same MBMS signal both from their "own" cell and from at least one surrounding cells and to make soft combination of those signals. In addition, the MBMS signals from the surrounding cells will be distinguishable by virtue of the different midamble shifts, which will make the user's channel estimation more reliable.

Fig 3 shows a flowchart of a method 300 of the invention. Steps which are options or alternatives are shown with dashed lines in fig 3.

As has been explained above, the method 300 is intended for use in a cellular communications system which uses, step 305, the TD-SCDMA principle, Time Division Synchronous Code Division Multiple Access. According to the method 300, MBMS services, Multi media Broadcast Multicast Service, are enabled, step 310, in at least a first and a second cell in the system, so that, step 315, there can be both MBMS and non-MBMS users in the first and second cells at one and the same time.

Also according to the method 300, traffic to non-MBMS users in the first and second cells is sent, step 320, using one and the same basic midamble code per cell which is shifted between the different non-MBMS users in each cell, and, step 325, the traffic to MBMS users in both of said first and second cells who use the same MBMS services is varied between the cells with respect to the basic midamble code or/and the midamble shift.

In one embodiment, as shown in step 330, the same basic midamble but a different midamble shift is used for signals in different cells which carry the same MBMS signal in the same timeslot and frequency.

In another embodiment, step 335, the basic midamble code is shifted cyclically between the cells.

The invention also discloses a cellular communications system which uses the TD-SCDM principle, Time Division Synchronous Code Division Multiple Access.

Fig 4 shows a schematic view of such a system 400. The numbering of the components in the system 400 in fig 4 corresponds to that of fig 1.

Thus, the system 400 is a cellular communications system which uses the TD-SCDMA principle, Time Division Synchronous Code Division Multiple Access, and the system 400 comprises at least a first, 120, and a second, 160, cell. Each of the cells 120, 160, in the system 400 is associated with a NodeB 110; 150, and MBMS services, Multi media Broadcast Multicast Service, are enabled in both the first and the second cell. The word "associated" with is here used to describe the relationship between the cells and the NodeBs, to indicate that the NodeB of the two cells may in some applications be one and the same physical unit, although it serves two different cells.

As indicated in fig 4, in the system 400, traffic to non-MBMS users, 140, 180, in the first and second cells is sent using one and the same basic midamble code per cell which is shifted between the different non-MBMS users in each cell.

According to the invention, in the system 400, the traffic to MBMS users, 130, 170, in both of said first and second cells who use the same MBMS services is varied between the cells with respect to the basic midamble code or/and the midamble shift. This is indicated by means of the "MBMS arrow" in the cell 160 being described by means of the text MBMS', as opposed to the text MBMS in the cell 120.

In one version of the system 400, the same basic midamble code but a different midamble shift is used for signals in the first 120 and the second 160 cells which carry the same MBMS signal in the same timeslot and frequency.

In one embodiment of the system 400, the basic midamble code is shifted cyclically between the first and the second cells.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims.

Claims

1. A method (300) for use in a cellular communications system (100) which uses (305) the TD-SCDMA principle, Time Division Synchronous Code Division Multiple Access, according to which method MBMS services, Multi media Broadcast Multicast Service, are enabled (310) in at least a first (120) and a second (160) cell in the system, so that (315) there can be both MBMS and non-MBMS users (130,170; 140; 180) in said first and second cells at one and the same time, and according to which method traffic to non-MBMS users (140, 180) in the first and second cells is sent (320) using one and the same basic midamble code per cell which is shifted between the different non-MBMS users in each cell, the method being characterized in that (325) the traffic to MBMS users (130, 170) in both of said first and second cells who use the same MBMS services is varied between the cells with respect to the basic midamble code or/and the midamble shift.

2. The method (300, 330) of claim 1 , according to which the same basic midamble code but a different midamble shift is used for signals in different cells which carry the same MBMS signal in the same timeslot and frequency.

3. The method (300, 335) of claim 1 or 2, according to which the basic midamble code is shifted cyclically between the cells.

4. A cellular communications system (400) which uses the TD-SCDMA principle, Time Division Synchronous Code Division Multiple Access, which system comprises at least a first (120) and a second (160) cell, each of which cell is associated with a NodeB (110; 150), and in which system MBMS services, Multi media Broadcast Multicast Service, are enabled in both of said first (120) and a second (160) cells, in which system traffic to non-MBMS users (140, 180) in the first and second cells is sent using one and the same basic midamble code per cell which is shifted between the different non- MBMS users in each cell, the system (400) being characterized in that (325) the traffic to MBMS users (130, 170) in both of said first and second cells who use the same MBMS services is varied between the cells with respect to the basic midamble code or/and the midamble shift.

5. The system (400) of claim 4, in which the same basic midamble code but a different midamble shift is used for signals in the first (120) and second (160) cells which carry the same MBMS signal in the same timeslot and frequency.

6. The system (400) of claim 4 or 5, in which the basic midamble code is shifted cyclically between the first and the second cell.