Abstract:
A method and apparatus for reducing interference caused by an interfering signal are disclosed for use in an access node, such as a base station, communicating with at least one mobile terminal in a wireless network, the method comprising the following steps: receiving information related to at least one data packet that is to be transmitted by a second access node to at least a second mobile terminal, receiving time-frequency information about the point in time when the at least one data packet will be transmitted from the second access node to the second mobile terminal, estimating the interfering signal based on the at least one data packet, cancelling interference based on the estimated interference signal at the point in time. This facilitates interference cancellation, especially in a network using distributed RRM.

Description:
TECHNICAL FIELD 
     The present invention relates to an interference reduction device as defined in the preamble of claim  1 , an access node as defined in the preamble of claim  10  and a method as defined in the preamble of claim  12 . 
     BACKGROUND AND PRIOR ART 
     In wireless communication networks, uplink and downlink traffic must be duplexed in such a way that they can be separated from each other. The most common methods of doing this are Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), i.e. separating the uplink and downlink signals in frequency and time, respectively. Each of these methods has advantages and disadvantages over the other. 
     TDD has the advantage over FDD that required resources in the uplink and downlink can be assigned in a more flexible manner. Further filters, which are costly and potentially space-requiring, are not needed, and channel reciprocity may be exploited for channel and path gain estimations. 
     To exploit the resources of the wireless network as flexibly as possible, it should preferably be possible to vary the time used for uplink and downlink traffic, respectively, in order to accommodate varying traffic needs in up and downlink over time. This means that at any given time, it is possible that some cells will use uplink resources and some cells will use downlink resources. This leads to increased interference between cells, both between base stations and between mobile terminals. 
     In particular, the interference between base stations is a problem, since the path loss between base stations is often very low in comparison with the path loss between mobile terminals. Downlink traffic transmitted from one base station will interfere with uplink traffic received in another base station, since the former will typically have much higher power than the latter. 
     Traditionally, methods of avoiding such interference have involved resource allocation or scheduling. 
     One such method to avoid the interference between base stations is obviously to assign uplink and downlink resources in any two cells such that they never overlap. Such a solution is described in Lindström, M.: “Resource Allocation for Asymmetric Traffic in Time Division Duplexing Mode Cellular Networks” (2003, Licentiate thesis, 2003. In a system with many cells this may be difficult to achieve, since the resource assignments in the cells are interconnected. Also, the flexibility of assigning different data rates in adjacent cells depending on the needs is lost. 
     Another solution, disclosed in U.S. Pat. No. 6,334,057, that allows overlaps of uplink and downlink traffic in two adjacent cells, is to schedule mobile terminals that are close to a base station, instead of mobile terminals farther away, during any potential uplink-downlink overlap between any two cells. 
     Tardy, Grøndalen, Vezzani: “Interference in TDD based LMDS systems” IST Mobile and Wireless Communications Summit, Thessaloniki, 17-19 Jun. 2002, describes a different approach based on interference cancellation techniques. The base station that experiences interference can obtain a copy of the data transmitted by the interfering base station and use this copy to cancel interference. No concrete suggestions are given for how to achieve this in a transport efficient manner, or for a cellular system with at least partially distributed Radio Resource Management (RRM). 
     OBJECT OF THE INVENTION 
     It is an object of the invention to provide interference cancellation, especially in a system using distributed radio resource management. 
     SUMMARY OF THE INVENTION 
     This object is achieved according to the present invention by an interference reduction device for reducing interference caused by an interfering signal, in an access node, communicating with at least one mobile terminal in a wireless network, said interference reduction device being characterized in that it comprises 
     a first receiving unit arranged to receive information related to at least data packet that is to be transmitted by a second access node to at least a second mobile terminal, 
     a second receiving unit arranged to receive timing-frequency information about at least one point in time when the at least one data packet will be transmitted from the second access node to the second mobile terminal, 
     an estimating unit arranged to estimate the interfering signal based on the at least one data packet, 
     a cancelling unit arranged to cancel interference based on the estimated interference signal at the point in time. 
     The object is also achieved by a method of reducing interference caused by an interfering signal, in an access node, such as a base station, communicating with at least one mobile terminal in a wireless network, comprising the following steps: 
     receiving information related to at least one data packet that is to be transmitted by a second access node to at least a second mobile terminal, 
     receiving timing information about the point in time when the at least one data packet will be transmitted from the second access node to the second mobile terminal, 
     estimating the interfering signal based on the at least one data packet, 
     cancelling interference based on the estimated interference signal at the point in time. 
     The access node may be any type of node providing access to mobile terminals in a wireless network, such as a base station. While this is not strictly true for a relay station, the term access node is intended in this document to cover relay stations and similar nodes as well. 
     Hence, the invention enables flexible use of uplink/downlink resources in TDD networks by removing a priori known interference with a focus on distributed Radio Resource Management (RRM) operation and improved transport efficiency. 
     The solution according to the invention takes into account the situation where each base station handles its own scheduling, modulation and coding selection and to some extent, power control. Such a situation exists in several systems today, and is likely to be more common in the future. Also, the data packets that are to be used for interference cancellation can be transmitted at an arbitrary point in time rather than exactly when they are transmitted to the mobile terminal. 
     The interference reduction means preferably uses format information, such as modulation and coding information when estimating the interfering signal. This format information can be obtained in different ways. In one embodiment the interference reduction device further comprises a third receiving unit for receiving format information about a signalling format used by the second base station when transmitting the at least one data packet, wherein the estimating means is arranged to estimate the interfering signal based on the at least one data packet and the format information. This information may be received from the interfering base station. Alternatively, the modulation and coding scheme used may be known in advance in the interference reduction device. It would also be possible to let the interference reduction means guess or estimate the modulation and coding scheme. The estimating unit may then be arranged to estimate the interfering signal based on the at least one data packet and format information about a predetermined format. 
     The estimating unit may further be arranged to estimate at least a first and a second part of the interfering signal using at least a first and a second predetermined format, respectively, and said interference reduction device is arranged to determine which of the at least first and second predetermined format provides the best interference reduction. This allows for a certain amount of trial and error when selecting the format information to be used. 
     In a preferred embodiment said first and second receiving means are arranged to receive said at least one data packet and/or said timing information from the second access node. This solution is preferable from a distributed RRM point of view. 
     Alternatively, said first and second receiving means may be arranged to receive said at least one data packet and/or said timing information from a control node in the core network. 
     Preferably, the cancelling means is arranged to cancel interference only when the first access node is receiving information in the uplink while the second access node is transmitting information in the downlink. This embodiment minimizes the amount of additional data that must be sent and the processing requirements for interference calculation, by only performing interference cancellation when it is really needed. 
     The object stated above is also achieved by an access node for use in a wireless network for connecting at least one terminal to the network, characterized in that it comprises at least one interference reduction device as defined above. 
     The access node may further comprise communication means arranged to transmit to another access node in the network at least one data packet that is to be transmitted to the other access node and timing information about when the at least one data packet is to be transmitted. 
     The invention also relates to a wireless Communication network, characterized in that it comprises at least one access node according to the above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail in the following, by way of example and with reference to the appended drawings in which: 
         FIGS. 1A and 1B  illustrate communication in a wireless network using TDD 
         FIG. 2  illustrates a part of a wireless communications network using the inventive method for interference reduction 
         FIG. 3  illustrates an interference reduction device according to the invention 
         FIG. 4  illustrates use of the inventive method in a wireless communications network using relay stations. 
         FIG. 5  illustrates a method according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1A  illustrates, schematically, a first and a second cell of a wireless communications network.  FIG. 1B  illustrates, schematically, the assignment of resources in the first and the second cell respectively. Time slots in which downlink resources are used are denoted DL and time slots in which uplink resources are used are denoted UL. 
     In each cell there is a base station  1 ,  3  communicating with a mobile terminal  5 ,  7 , respectively. As can be seen in  FIG. 1B , there is a period of three time slots, between the vertical dashed lines, in which uplink resources are used in the first cell and downlink resources are used in the second cell.  FIG. 1A  illustrates the situation in this period, which will be referred to as an overlap period. The signals between a sender and an intended receiver are shown as solid lines and the signals between a sender and an unintended receiver, which may cause interference, are shown as dashed lines. In  FIG. 1A , the first mobile terminal  5  is transmitting an uplink signal intended for the first base station  1 . The second base station  3  is transmitting a downlink signal intended for the second mobile terminal  7 . The downlink signal transmitted from the second base station  3  will be received as an interfering signal in the first base station with a relatively high power compared to the uplink signal received from the first mobile terminal  5 . Similarly, the uplink signal transmitted from the first mobile terminal  5  will be received in the second mobile terminal  7  as an interfering signal, interfering with the downlink signal received from the second base station  3 . Typically, this interference between mobile terminals is much less than the aforementioned interference between base station, due to differences in propagation characteristics. Hence interference between mobile terminals will not be further considered. 
       FIG. 2  illustrates two cells of a wireless network in which the invention has been implemented. As in  FIG. 1   a  a first  11  and a second  13  base station serve as access nodes in a wireless network (not shown) for a first  15  and a second  17  mobile terminal, respectively. A Radio Network Controller (RNC)  19 , or equivalent, controls the base stations  11 ,  13  as is common in the art. 
     Each base station  11 ,  13  comprises an interference reduction device  21 ,  23  arranged to cancel out interference as will be discussed in the following with reference to  FIG. 3 , in which the interference reduction device  21  of the first base station is shown. The description is also valid for the interference reduction device  23  of the second base station. 
     The interference reduction device  21  comprises a first receiving unit  25  arranged to receive from the second base station  13  data packets that are to be transmitted from the second base station to one or more mobile terminals. The second base station may transmit all data packets or only the ones that may be transmitted during a time of overlap as discussed in connection with  FIG. 1B . The interference reduction device  21  also comprises a second receiving unit  26  arranged to receive timing, frequency or timing-frequency information from the second base station, related to when the data packets will be transmitted, on which frequency, or a combination of both. Preferably, the interference reduction device  21  also comprises a third receiving unit  27  for receiving information about the signal format that will be used by the second base station when transmitting the data packets. 
     An estimating unit  28  is arranged in the interference reduction device to estimate (in the sense of reconstructing) an interference signal that will be caused in the first base station by the data transmitted from the second base station, based on the data packets received in the first receiving unit  25 , and, if present, the format information received in the third receiving unit  27 . If no format information is received, a predetermined format may be used in estimating the signal, or the estimating unit may try to guess what the format may be. It would be possible to let the estimating unit try different formats during the interference cancellation. The interference reduction device  21  could then determine which format gives the best result and instruct the estimating unit to select this format for further use. 
     A cancelling unit  29  receives the estimated signal. The cancelling unit  29  also receives the desired signal, that is, the signal intended for the base station served by the interference reduction device, which is subject to interference. The cancelling unit then uses the estimated signal to cancel, or reduce, the interference signal from the desired signal at the right time, as determined by means of the timing information received by the second receiving unit  26 . Note that the format information received by the third receiving unit  27  may be forwarded to the estimating unit  28  instead of the cancelling unit  29 , in which case the estimating unit  28  yields a created interference signal at the right time and frequency as input to the cancelling unit  29 . 
     The estimated signal must be weighted (i.e. appropriately scaled in phase and amplitude) before it is used for interference reduction, in dependence of how strongly the interfering signal interferes with the desired signal. This may be done in the cancelling unit  29  on the basis of the received signal. For example, the weight factor or vectors can be selected such that a metric representative of interference reduction of the interfered signal of interest is optimized. Alternatively, the estimating unit  28  may adjust the estimated signal according to appropriate weight factors before forwarding it to the cancelling unit  29 . In the latter case the estimating unit needs to receive a channel estimate signal which can be used to estimate the level of interference. This channel estimate signal is indicated in  FIG. 3  as a dashed line entering the estimating unit  28 . Note that the weights may be frequency dependent, or as in OFDM or OFDMA, a different weight factor is, or may be, selected for each OFDM or OFDMA subcarrier. 
     Of course, preferably, both base stations comprise interference reduction devices and both base stations are arranged to inform other base stations about the data that it is to transmit and also provide them with timing information and format information. 
     The data packets and other information transmitted from the second base station to the interference reduction device of the first base station could be transmitted through the air interface, or through the core network. Alternatively, instead of transmitting the data packets from the second base station to the interference reduction device of the first base station, the data packets could be transmitted to both base stations from the network, represented by the RNC  19 . If the first base station already has the data packets, the second base station would only have to transmit information such as the sequence numbers identifying the data packets that were to be transmitted, instead of the packets themselves. 
     A base station may experience interference from more than one other base station. In this case the interference reduction device should be arranged to receive information about the data packets transmitted from each of the other base stations, and the format used, if applicable, so that it can perform interference cancellation for each of the interfering signals. All or some of the interfering signals may be cancelled. Of course, it is possible to perform interference cancellation in the first base station only when the first base station is receiving data in the uplink, to cancel out downlink signals transmitted from one or more of the other base stations at the same time. 
       FIG. 4  illustrates use of the inventive method in a relaying system. In this system a first  31  and a second  33  relay station communicating with a first  35  and a second  37  mobile terminal, respectively, are receiving data from a base station  36 . Depending on the transmission conditions, each relay station  31 ,  33  potentially overhears data intended for the other relay station. For the specific example in  FIG. 4  where the second relay station  33  transmits in DL it is sufficient if the first relay station  31  overhears the information sent from the base station  36  to the second relay station  33 . 
     As in  FIG. 1 , the signals between a sender and an intended receiver are shown as solid lines and the signals between a sender and an unintended receiver, which may cause interference, are shown as dashed lines. In  FIG. 4 , the first mobile terminal  35  is transmitting an uplink signal intended for the first relay station  31 . The second relay station  33  is transmitting a downlink signal intended for the second mobile terminal  37 . The downlink signal transmitted from the second base station  33  will be received as an interfering signal in the first base station  31 . Similarly, the uplink signal transmitted from the first mobile terminal  35  will be received in the second mobile terminal  37  as an interfering signal, but this interfering signal is normally considered to be negligible. 
     Each relay station  31 ,  33  therefore comprises an interference reduction device  21  as described in connection with  FIG. 3 . The function of the interference reduction device will be discussed in connection with  FIG. 5 . Since the first relay station  31  has previously overheard the data that are to be transmitted from the relay station  33 , these data do not have to be transmitted again from the second relay station. To perform interference cancellation the first relay station  31  must have information about the format used by the second relay station  33  when transmitting downlink. Such format information may be transmitted directly from the second relay station  33  to the first relay station  31 . It may also be received from the base station  36 . Alternatively, the interference reduction device  21  may estimate or guess the format, as discussed in connection with  FIG. 3 . Hence, the interference reduction device  21  already has the data needed to generate the estimated interference signal. The timing information should still be transmitted from the second relay station to the first relay station. 
     The invention can be used not only between base stations, but between any kind of access nodes in a wireless network, such as relay stations. Of course, interference between a base station and a relay station, or vice versa, can be reduced in the same way using the same type of interference reduction device. In the discussion of  FIG. 5 , below, therefore, the term access node is used to refer to a base station or a relay station. 
       FIG. 5  illustrates a basic embodiment of the method according to the invention. In step S 1 , which is an optional step, the second access node selects the data packets that may cause interference in the first access node when they are transmitted from the second access node to the second mobile terminal. 
     In step S 2  the selected data packets from step S 1  are transmitted to the first access node. Alternatively, all data packets that will be transmitted from the second access node in a certain period of time are transmitted to the first access node. Also, timing information is transmitted, indicating when the packets will be transmitted from the second access node to the second mobile terminal. Also if different information is sent concurrently on different frequencies, as in OFDMA, this information is forwarded with the timing information. Format information indicating, for example, the modulation and coding scheme that will be used may also be included. 
     In step S 3  the data transmitted in step S 2  are received in the interference reduction device of the first access node. As mentioned above, instead of the actual data packets themselves, the identity of the data packets may be transmitted, if the first access node has already received the actual data packets. 
     In step S 4  the timing information is received in the interference reduction device of the first access node. (If needed, as in OFDMA, the resource allocation in the frequency domain is received) 
     In step S 5  the format information (if included) is received in the interference reduction device of the first access node. 
     In step S 6  the estimating unit of the interference reduction device uses the information received to estimate the interference signal that will be caused by the packets when they are transmitted from the second access node to the second mobile terminal. 
     In step S 7  the estimated interference signal is used to cancel, or reduce, the interference signal at the time when the packets are actually transmitted from the second access node to the second mobile terminal.