Abstract:
A coordinating node avoids or reduces interference between relay nodes by coordinating subframe allocation for the interfering relay nodes. The coordinating node identifies the interfering relay nodes that require the same subframe allocation and provides the necessary signaling so that the involved nodes get information concerning the subframe allocations. The system may be implemented using a centralized node (e.g., OAM) or distributed coordinating nodes.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 61/314,380, filed Mar. 16, 2010, which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    In LTE systems (3GPP LTE Rd-10), the use of relays has been proposed to improve the coverage and capacity of LTE networks. A relay node can be positioned between a donor eNB and a user terminal (UT) so that transmissions between the eNB, referred to as the donor eNB, and the UE are relayed by the relay node. Release 10 of LTE supports Type 1 relay nodes. A type 1 relay controls cells, each of which appears to a user terminal as a separate cell distinct from the donor cell. The cells controlled by the relay node have their own Physical Cell ID (as defined in LTE Rel-8) and transmit their own synchronization channels, reference symbols etc. In the context of single-cell operation, the user terminal receives scheduling information and HARQ (Hybrid Automatic Repeat-reQuest) feedback directly from the relay node and sends its control channels (SR/CQI/ACK) to the relay node. A type one relay is backward compatible and appears as a base station (eNodeB) to Release 8 user terminals. Thus, from the perspective of a user terminal, there is no difference being served by a base station or a Type 1 relay node. 
         [0003]    Transmissions between the relay node and the donor base station are over a radio interface called the Un interface. The Un interface is referred to herein as the backhaul link. Transmissions between user terminal and relay node are over a radio interface called the Uu interface. The Uu interface is referred to herein as the access link. The access link is the same as for direct communication between the user terminal and base station without a relay in between. If the transmissions on backhaul and access links are within the same frequency band, the relay node is referred to as inband relay node. In case the transmissions are on a separate frequency bands, the relay node is referred to as outband relay node. 
         [0004]    To enable inband relay nodes to be functional, the relay node cannot transmit and receive at the same time on the same frequency, since this could cause intolerable self-interference. For the downlink, certain subframes are configured as MBSFN subframes so that the relay node does not transmit anything in its own cell on the access interface. During an MBSFN subframe, the user terminals in the relay cell do not expect to receive any reference signals or downlink (DL) data from the relay node beyond what is transmitted in the first two OFDM symbols of the subframe. Instead, the relay node listens to the downlink transmissions on the backhaul link during the rest of these subframes (which are hence used for carrying downlink data from the donor eNB to the relay nodes). Similarly, in the uplink, the relay node cannot simultaneously listen to transmissions from the user terminal on the access link and transmit to its donor base station on the backhaul link. However, in the uplink, there is no problem with the relay node temporarily disregarding the access link. Thus, interference can be avoided by not scheduling any data on the relevant subframes. 
         [0005]    The performance of a relay-enhanced system is dependent on the subframe allocations. Alternative configurations can also achieve different objectives when it comes to capacity, coverage, peak rates etc. For example, if the backhaul link (the Un interface) is the bottleneck, it is beneficial to have as many subframes allocated to the backhaul as possible. This is likely to happen if there are many relay nodes served by the same donor base station or if the traffic in a certain relay cell is high. If the backhaul link (Un interface) is of good radio quality compared to the access link (Uu interface), it is better to have more subframes allocated to the access link since this is limiting. Generally, the load distribution within the donor station cell is an important factor for subframe allocation. The optimal subframe allocation is likely to depend on the relation between traffic served directly by the donor base station and the traffic served by relay nodes. The interference between relay nodes, as well as between relay nodes and base stations, may be considered in making subframe allocations. Hence, the optimal allocation may be different for different relay nodes. 
         [0006]    Configuring relay nodes in a system with different subframe allocations may lead to an interference problem between multiple relay nodes. The problem of interfering relay nodes can be solved by having the same subframe allocation in all relay nodes that risk interfering with each other. The drawback of this approach, however, is that it would be difficult to optimize the resource usage when all relay nodes are constrained to share the same subframe allocation. 
       SUMMARY 
       [0007]    The present invention provides a method and apparatus to avoid interference between relay nodes by coordinating subframes for interfering relay nodes. The present invention provides a mechanism to identify interfering relay nodes that require the same allocation and provides the necessary signaling so that the involved nodes get information concerning the restrictions of the subframe allocations. The system may be implemented using a centralized node (e.g., OAM) or distributed coordinating nodes. 
         [0008]    One exemplary embodiment of the invention comprises a method of configuring a relay node. The method can be performed manually, or by a management node. The method comprises identifying one or more neighboring relay nodes; determining interference information for the neighboring relay nodes; and configuring a subframe allocation based on the interference information for the neighboring relay nodes. In some embodiments, the relay node is assigned to a relay node pool based on the interference information so that all relay nodes in the same relay node pool use a common subframe allocation pattern. Relay nodes in the relay node pool may be served by the same or different base stations. 
         [0009]    Another exemplary embodiment comprises a coordinating node for configuring a relay node. The coordinating node comprises a signaling interface to receive interference information regarding one or more neighboring relay nodes; and a processor programmed to configure a subframe allocation for a relay node based on the interference from other relay nodes. 
         [0010]    The invention enables the best possible subframe allocation for relay nodes in a system, avoiding the intolerable interference that might occur without in the absence of coordination. It also helps in the deployment of relay nodes because there is less concern of inter-relay node interference. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of a communication system according to the present invention including a relay node. 
           [0012]      FIG. 2  is a schematic diagram showing one representative subframe allocation for a relay node. 
           [0013]      FIG. 3  illustrates interference between neighboring relay nodes. 
           [0014]      FIG. 4  illustrates an exemplary method of subframe allocation according to one embodiment. 
           [0015]      FIG. 5  illustrates the use of relay pools for relay nodes using a common subframe allocation pattern. 
           [0016]      FIG. 6  illustrates an exemplary coordinating node for determining subframe allocations. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Turning now to the drawings,  FIG. 1  illustrates an exemplary communication network  10  according to one exemplary embodiment of the present invention. The present invention is described in the context of a Long-Term Evolution (LIE) network, which is specified in Release 10 of the LTE standard. However, those skilled in the art will appreciate that the invention may be applied in networks using other communication standards. 
         [0018]    The communication network  10  comprises a plurality of base stations  20  providing radio coverage in respective cells  12  of the communication network. Only one base station  20  is shown in  FIG. 1 . In the exemplary communication network  10 , a relay node  30  relays signals between the base station  20  and one or more user terminals  50  in a relay cell  14 . Relay node  30  is a type-1 relay as defined in Release 10 of the LTE standard. 
         [0019]    For downlink communications, the relay node  30  receives signals from the base station  20  over the Un interface and transmits signals to the user terminals  50  over the Uu interface. For uplink communications, the relay node  30  receives signals from the user terminals  100  over the Uu interface and transmits signals to the base station over the Un interface. The Un interface is referred to herein as the backhaul link, and the Uu interface is referred to herein as the access link. 
         [0020]    In one exemplary embodiment, the relay node  30  is an inband relay that transmits and receives on the same frequency. For good performance, the relay node  30  cannot transmit and receive at the same time on the same frequency due to self-interference. Therefore, for either uplink or downlink communications, certain subframes are designated for transmissions between the base station  20  and the relay node  30 , and the remaining subframes are designated for transmissions between the relay node  30  and the user terminals  50 . 
         [0021]    For the downlink, certain subframes are designated for downlink transmissions from the base station  20  to the relay node  30  and the remaining subframes are designated for downlink transmissions from the relay node  30  to the user terminals  50 . In one exemplary embodiment, the subframes designated for downlink transmissions from the base station  20  to the relay node  30  are configured as MBSFN subframes. During an MBSFN subframe, the user terminals in the relay cell do not expect to receive any reference signals or downlink data from the relay node  30  beyond what is transmitted in the first two OFDM symbols of the subframe. Instead, the relay node  30  listens to the downlink transmissions on the backhaul link. 
         [0022]    Similarly, in the uplink, the relay node  30  cannot simultaneously listen to transmissions from the user terminals  50  on the access link and transmit to the serving base station  20  on the backhaul link. For uplink communications, interference can be avoided by not scheduling data transmissions from the user terminals  50  on subframes used to transmit data to the base station  20 . 
         [0023]      FIG. 2  illustrates an exemplary subframe allocation for downlink transmissions. A radio frame typically includes ten subframes, of which up to six subframes can be configured as MBSFN subframes. Subframes  0 ,  4 ,  5 , and  9  cannot be configured for MBSFN. Thus, as many as six out of ten subframes in a radio frame can be used for downlink transmissions from the base station  20  to the relay node  30 . The remaining subframes can be used for downlink transmissions from the relay node  30  to the user terminals  50 . Thus, downlink transmissions from the base station  20  to the relay node  30  are time-multiplexed with the downlink transmissions from the relay node  30  to the user terminals  50 . 
         [0024]    A subframe allocation similar to  FIG. 2  could also be made for uplink transmissions, however, it is not required that the same subframe allocations be used for uplink and downlink communications. 
         [0025]    The optimal subframe allocation for a relay node  30  depends to some extent on the number of user terminals  50  served by the relay node and the interference experienced by the relay node  30 . If the relay node  30  serves a large number of user terminals  50  so that the backhaul link is the bottleneck, it may be desirable to designate as many subframes as possible for downlink transmissions from the base station  20  to the relay node  30 . On the other hand, when the access link is limiting, it may be beneficial to designate more subframes for transmissions from the relay node  30  to the user terminals  50 . Other considerations in the subframe allocation include the quality of the backhaul link compared to the access link, and the ratio of the user terminals  50  served directly by the base station  20  to those served by relay nodes  30 . Thus, the “best” allocation for the different relay nodes  30  may be different. 
         [0026]    Configuring relay nodes with different subframe allocations may cause unwanted interference between the relay nodes.  FIG. 3  illustrates the interference problem where neighboring relay nodes  30  use different subframe allocations. As used herein, the term “neighboring relay nodes” refers to relay nodes sufficiently close from a radio propagation perspective so that the transmissions from or to one relay node may interfere with transmission from or to another relay node. Neighboring relay nodes may be controlled by the same base station or by different base stations. In this example, relay node A uses subframes  1 ,  2 ,  3 ,  6 ,  7 , and  8  to receive downlink transmissions from the base station  20 , while relay node B uses subframes  6 ,  7 , and  8 . Thus, relay node B may transmit on subframes  1 ,  2 , and  3  when relay node A is trying to receive data over the backhaul link. If the relay nodes  30  are not sufficiently separated from a radio propagation perspective, the interference from relay node B may become intolerable for relay node A, making it impossible for relay node A to receive transmissions from the base station  20  over the backhaul link. Thus, using different subframe allocations in different relay cells  14  makes deployment of relay nodes  30  more difficult, because greater care needs to be taken to make sure that the relay nodes  30  do not interfere with one another. 
         [0027]    Generally, interference between relay nodes  30  using the same subframe allocation is not a problem. In this case, all relay nodes  30  will transmit signals to the user terminals  50  at the same time. The simultaneous transmission from multiple relay nodes  30  will create interference at the user terminals  50  receiving transmissions from either relay node  30 , but that interference problem is the same as in any other re-use 1 system. Interference at the user terminal  50  can be handled by cell selection and hand-over. The problem arises when the relay nodes  30  have different subframe allocations because the transmissions from one relay node  30  may interfere with the reception of another relay node  30 . 
         [0028]    According to embodiments of the present invention, interference is avoided by coordinating the subframe allocation among different relay nodes  30 . More particularly, a mechanism is provided to identify relay nodes  30  having the potential to create intolerable interference and a signaling scheme is provided to coordinate subframe allocations. In general, the procedure for determining the subframe allocation for a given relay node  30  involves two phases. In the first phase, the interference situation of the relay node  30  is evaluated to identify neighboring relay nodes with the potential to create intolerable interference. In the second phase, the interference information is evaluated to determine a subframe allocation that reduces the interference between neighboring relay nodes  30 . The evaluation is typically performed by coordinating node  150  ( FIG. 6 ) as hereinafter described. 
         [0029]      FIG. 4  illustrates an exemplary procedure  100  for determining the subframe allocation for a given relay node  30 . The coordinating node  150  receives interference information characterizing the interference attributable to one or more relay nodes  30  (block  102 ). After receiving the interference information, the coordinating node  150  configures a subframe allocation for a target relay node based on the received interference information (block  104 ). The coordinating node  150  may be a centralized node in the communication network  10  responsible for subframe allocation for relay cells  14  in a radio access network. For example, an Operations and Maintenance server (OAM) in the core network may serve as a coordinating node  150 . Alternatively, the responsibility for subframe allocation in a radio access network may be distributed among two or more coordinating nodes  150 . In this case, the base station  20  may serve as coordinating nodes  150 . 
         [0030]    The interference information may, for example, comprise a list of interfering relay cells  14  with some indication of the interference level. For example, the received signal strength (RSRP) from the interfering cells may serve as one measure of the interference levels. In other embodiments, the interference information may comprise a list of interfering relay cells  14  on a per subframe basis, with or without an indication of the interference level. The interference information may be manually collected by a service technician using the relay node  30  or other equipment to perform measurements. The interference information can then be input to the coordinating node via a conventional user interface. For example, the service technician could input the interference information into an OAM functioning as a coordinating node  150 . Alternatively, measurements may be performed by the relay node  30  or other equipment connected to the relay node  30 . The measurements can be processed to generate interference information, which is then reported to the coordinating node  150  over a communications link. The manual approach is simpler to implement because it does not require any new signaling. However, the interference environment may change as new buildings are constructed or new relay cells are added to the network. The time and labor involved make the manual approach costly to implement on a frequent basis. Further, the manual approach is subject to human error. The automatic approach enables more frequent update of the subframe allocations as the interference environment changes. However, new signaling may be required to implement the automatic approach. 
         [0031]    There are several alternatives for signaling interference information between a node where the measurements are made and a coordinating node  150  where the decision on subframe allocation is made. In one exemplary embodiment, the relay nodes  30  may use a signaling to report interference information to an OAM functioning as the coordinating node  150  for all relay cells  14  in a radio access network. In another exemplary embodiment, the base station  20  may function as coordinating nodes  30  to determine the subframe allocations for the relay nodes  30  served by the base station  20 . In this case, the relay nodes  30  may signal interference information to the base station  20  using radio resource control (RRC) signaling, X 2 , or other signaling. The base stations  20  may use the X 2  signaling interface to share interference information reported by the relay nodes  30 . In either case, once the interference information is available to the coordinating node  150 , the coordinating node  150  may use the interference information to determine the subframe allocations. 
         [0032]    In one exemplary embodiment, the relay nodes  30  provide the coordinating node with a list of neighboring relay nodes  30  and the corresponding signal strength measurements (RSRPs) to provide an indication of the interference level. Subframe allocations may be based, at least in part, on comparison of the signal strength measurements to a threshold. The threshold can be a fixed value, or may vary dynamically, depending on operating conditions. For example, the threshold may depend on the backhaul link quality for the relay node  30  because it is the backhaul link that suffers interference from a neighboring relay node  30  with a different subframe configuration. The relay nodes  30  may report the received signal strength from the serving base station  20  in addition to the interference information. The threshold may be set at a point that is safely below the received signal strength from the serving base station  20  so that different subframe configurations may be allowed when the interference level is below the threshold. 
         [0033]    In one exemplary embodiment, one or more relay node pools may be defined where the relay nodes in the same relay node pool use the same subframe allocation pattern. In this case, determining the subframe allocation pattern reduces to determining the relay node pool for the relay node  30 . In general, the interference between relay nodes  30  in the same pool will be high, whereas the interference between relay nodes  30  in different pools will be low. 
         [0034]      FIG. 5  illustrates the concept of relay node pools.  FIG. 5  shows a relay node pool  16  comprise three relay nodes  30  designated as relay nodes A, B, and C. The relay nodes  30  in a relay node pool  16  may all be served by the same base station  20  or, as shown in  FIG. 5 , may be served by different base stations  20 . 
         [0035]    In systems where the subframe allocations are determined by a centralized coordinating node (e.g., OAM)  150 , the coordinating node  150  needs to signal the subframe allocation to each base station  20  and relay node  30 . In systems where the base stations  20  serve as coordinating nodes, the base stations  20  need to signal the subframe allocations to the relay nodes  30  under the control of the base station  20 . 
         [0036]    The base station  20  and relay nodes  30  may store a table of subframe allocation patterns in memory. In this case, the subframe allocation can be signaled by sending an index indicating which of the subframe allocations stored in memory has been selected. Coordinating node  150  may send an index to indicate a particular subframe allocation pattern. In some embodiments, the relay node  30  may be part of a relay node pool. In this case, the index may indicate the pool to which the relay node is assigned, which is an implicit indication of the subframe allocation pattern. In some embodiments, the relay node pool may be assigned multiple subframe allocations patterns. In this case, the index needs to indicate the pool and subframe allocation pattern (e.g., pool  1 , pattern  3 ). 
         [0037]    In embodiments where the decision making concerning subframe allocation patterns is centralized, coordination of the subframe allocation patterns for relay nodes served by different base stations is simplified. Referring to  FIG. 5 , relay nodes A and B are served by base station  1 , while relay node C is served by base station  2 . It is assumed in this example that relay nodes A, B, and C are creating interference with one another and should be placed in the same relay node pool  16 . If the subframe allocation is made by a centralized node, e.g., OAM, the coordinating node  150  may indicate to the serving base station  1  that relay nodes A and B should be assigned to the same relay node pool  16 . The OAM can also indicate to serving base station  2  that relay node C should be assigned to the same pool  16 . 
         [0038]    Where the decision making for subframe allocation is distributed between the serving base stations  20 , the serving base stations  20  will need to exchange interference information and negotiate the subframe allocation patterns. In this example, relay nodes A and B will report interference from relay node C to serving base station  1 . Similarly, relay node C will repot interference from relay nodes A and B to serving base station  2 . The interference information may be shared between the base stations  20 . The base stations  20  can then negotiate the subframe allocation for relays A, B, and C. For example, serving base station  1  may indicate a desired subframe allocation pattern for relays A, B, and C to base station  2 . Base station  2  can either accept the proposed subframe allocation, or reject the allocation and propose a different subframe allocation pattern. This process can be repeated until the base stations  20  have agreed upon the subframe allocation pattern. 
         [0039]      FIG. 6  illustrates the main functional components of a coordinating node  150  for coordinating the subframe allocation patterns. The coordination node  150  comprises an interface circuit  152  and a configuration processor  154 . The interface circuit  152  may comprise a user input interface to receive interference information from a system user. Alternatively, or in addition, the interface circuit  152  may comprise a network interface over which the interference information is received. The configuration processor  154  may comprise one or more microprocessors, hardware, firmware, or a combination thereof, for processing the interference information and making decisions on subframe allocation patterns. As previously noted, the interface circuit  152  and configuration processor  154  may be embodied in a centralized node such as an OAM. Alternatively, the interface circuit  152  and configuration processor  154  may be embodied in a base station  20   
         [0040]    The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.