Abstract:
The present invention relates to method for communicating in a network comprising the step a) of a primary station transmitting to at least one secondary station on a downlink control channel an indication related to a resource allocated to said secondary station in accordance with a semi persistent schedule, wherein the step a) comprises the step of transmitting to the secondary station at least two messages including at least part of the indication in one subframe.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for communicating in a network comprising a primary station and at least one secondary station, and to such a secondary station. More specifically, this invention relates to a method for communicating in a mobile telecommunication network, like a GSM (Global System for Mobile communications) or a UMTS (Universal Mobile Telecommunications System) network. 
         [0002]    This invention is, for example, relevant for UMTS and UMTS Long Term Evolution, but as well to hubs which route connections from multiple terminals to base stations. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a mobile telecommunication network like a UMTS system, a primary station, for instance a Node B (or Base Station or eNB) communicates with at least one secondary station, for instance a User Equipment (UE or Mobile Station), by means of a plurality of channels. In order to transmit data to the primary station, or to receive data from the primary station, a secondary station needs to listen to signaling message on at least one control channel to know when to transmit or receive, and to know how to transmit or receive. Among the parameters used to determine how to communicate with the primary station, the secondary station needs for instance at least some of the following: 
         [0004]    For downlink data transmissions, each such control signalling message may typically include at least some or all of the following:
       Time-frequency resource allocation,   Number of MIMO (Multiple In Multiple Out) layers used,   Hybrid ARQ (Automatic ReQuest) process number,   Modulation and Coding Scheme for each layer,   New data indicator for each layer,   Redundancy version for each layer,   Precoding information for each layer,   UE identity,   Cyclic Redundancy Cycle.       
 
         [0014]    Usually, all these parameters are transmitted on a single control channel. However, for some applications, a semi persistent scheduling with a particular signaling is used. In accordance with semi persistent scheduling scheme, a resource is allocated for instance periodically (i.e. each subframe or each n th  subframe) for a large period of time. This is used for instance for communication where repeated resources will be used, like Voice over IP. By using this semi persistent allocation, it is possible to fix some parameters of the future transmissions, for instance by higher layer signaling, and the required amount of signaling to be transmitted on the control channel is reduced. The transmission on higher layer has the drawback of being not as quick as the normal signaling. However, this is well adapted for semi persistent scheduling. 
         [0015]    What still needs to be transmitted to the secondary station on the control channel is a message to activate or deactivate or modify a Semi Persistent Scheduling (SPS). 
         [0016]    However, if this SPS message is missed or wrongly decoded, or received in error, this can lead to several problems, like collisions with another communication or corruption, leading to delays and errors. 
       SUMMARY OF THE INVENTION 
       [0017]    It is an object of the invention to propose a method for alleviating these problems. 
         [0018]    Another object of the invention is to propose a method improving the reliability of transmission of the SPS messages. 
         [0019]    Still another object of the invention is to propose a method improving the robustness of SPS messages without causing more delay in the signalling. 
         [0020]    Still another object of the present invention is to improve the reliability of the SPS messages without increasing the amount of overhead or without changing the current transceiver implementation. 
         [0021]    To this end, in accordance with a first aspect of the invention, it is proposed a method for communicating in a network comprising the step a) of a primary station transmitting to at least one secondary station on a downlink control channel an indication related to a resource allocated to said secondary station in accordance with a semi persistent schedule, wherein the step a) comprises the step of transmitting to the secondary station at least two messages including at least part of the indication in one subframe. 
         [0022]    In accordance with a second aspect of the invention, a primary station is proposed, said primary station comprising means for communicating with at least one secondary station, wherein the primary station comprises a transmitter for transmitting the secondary station on a control channel an indication related to a resource allocated to said secondary station in accordance with a semi persistent schedule, wherein the transmitter is arranged to transmit to the secondary station at least two messages including at least part of the indication in one subframe. 
         [0023]    In accordance with a third aspect of the invention, a secondary station is proposed, said secondary station comprising means for communicating with at least one primary station, the secondary station comprising a receiver for receiving from the primary station on a control channel an indication related to a resource allocated to said secondary station in accordance with a semi persistent schedule, wherein the secondary station is arranged for receiving within one subframe at least two messages including each at least part of the indication. 
         [0024]    In accordance with a fourth aspect of the invention, a communication system is proposed, said communication system comprising a primary station communicating with at least one secondary station, wherein the primary station comprises a transmitter for transmitting to the at least one secondary station on a control channel an indication related to a resource allocated to said secondary station in accordance with a semi persistent schedule, wherein the transmitter is arranged to transmit within one subframe to the secondary station at least two messages including at least part of the indication. 
         [0025]    As a consequence, the likelihood of the message relating to a semi persistent schedule being received correctly is increased, and the likelihood of the message being received incorrectly is decreased. Moreover, since the secondary station tries to blind decoding all possible messages on the control channel to check whether one is addressed to itself, it is not required to change the implementation of the secondary stations or the size of the messages. Moreover, this permits to have a quick and reliable signalling compared to higher layer signalling which causes overhead and sometimes delays. It is to be noted that the fact that two messages are in the same subframe permits to reduce delays, when compared with a solution where a plurality of messages are sent over a plurality of subframes. 
         [0026]    It is to be noted that this solution comes into opposition with the current assumption for Long Term Evolution in 3GPP, according to which a secondary station would only receive one signalling message on the PDCCH per subframe. 
         [0027]    These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The present invention will now be described in more detail, by way of example, with reference to the accompanying drawing, wherein: 
           [0029]      FIG. 1  is a block diagram of a system in which is implemented the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    The present invention relates to a system of communication  300  as depicted on  FIG. 1 , comprising a primary station  100 , like a base station, and at least one secondary station  200  like a mobile station. 
         [0031]    The radio system  300  may comprise a plurality of the primary stations  100  and/or a plurality of secondary stations  200 . The primary station  100  comprises a transmitter means  110  and a receiving means  120 . An output of the transmitter means  110  and an input of the receiving means  120  are coupled to an antenna  130  by a coupling means  140 , which may be for example a circulator or a changeover switch. Coupled to the transmitter means  110  and receiving means  120  is a control means  150 , which may be for example a processor. The secondary station  200  comprises a transmitter means  210  and a receiving means  220 . An output of the transmitter means  210  and an input of the receiving means  220  are coupled to an antenna  230  by a coupling means  240 , which may be for example a circulator or a changeover switch. Coupled to the transmitter means  210  and receiving means  220  is a control means  250 , which may be for example a processor. Transmission from the primary radio station  100  to the secondary station  200  takes place on a downlink channel  160  and transmission from the secondary radio station  200  to the first radio station  100  takes place on an uplink channel  260 . 
         [0032]    In current versions of the specifications for the UMTS LTE, signalling is provided on a control channel (called PDCCH) transmitted from the primary station (eNB) to the secondary station (UE) to indicate the transmission resources used for data transmission between both of them. 
         [0033]    For example, for downlink data transmissions, each such control signalling message may typically include at least some or all of the following: Time-frequency resource allocation, Number of MIMO layers used, Hybrid ARQ process number, MCS for each layer, new data indicator for each layer, redundancy version for each layer, Precoding information for each layer, UE identity, CRC. 
         [0034]    In UMTS and also in LTE, usually one control signalling message is transmitted per secondary station per subframe. As a consequence, each secondary station tries to decode all the messages on the control channel to find a message which is addressed to this secondary station. 
         [0035]    One feature under discussion for UMTS LTE is semi-persistent scheduling (SPS). This may be applied in uplink or downlink or both. What is meant by semi persistent scheduling, is that, for instance, in the uplink, resources are granted for the secondary station transmission on a periodic basis (i.e. a given resource allocation is available every n subframes). A secondary station is configured to use SPS by higher layer signalling which may specify the period for instance or other parameters. Currently it is proposed to activate or deactivate the schedule using a message on the control channel PDCCH. An existing message format would be used. The same scheduling messages are also proposed for controlling a semi-persistent schedule (SPS). This control may be to activate, deactivate or modify a SPS. A SPS (for which some parameters may be configured by higher layers), grants resources on a periodic basis in order to match the needs of specific applications.(e.g. uplink transmission resources every 20 ms to support VoIP). 
         [0036]    However, there is a problem if an activation is falsely detected, since uplink transmissions would start erroneously causing interference. Currently, the SPS activation message have a different identifier (called SPS-RNTI for SPS Radio Network Temporary Identifier) encoded in the Cyclic redundancy check, and can therefore be distinguished from normal messages (with an identifier called C-RNTI for Cell Radio Network Temporary Identifier). 
         [0037]    If a message relating to SPS is received in error, or one is received when none was transmitted, then the following problems can occur until the SPS is terminated:
       For uplink SPS:       
 
         [0039]    Transmission using an uplink resource when none was expected by the eNodeB, potentially corrupting other uplink transmissions, and probably followed by one or more re-transmissions.
       For downlink SPS:—       
 
         [0041]    The UE attempts to decode a PDSCH packet not intended for it, followed by uplink transmission of an NACK (which the eNodeB is likely to either ignore or misinterpret as being from a different UE). 
         [0042]    Therefore methods to improve the reliability of this signalling are of interest, in particular to reduce the number of falsely detected PDCCHs (i.e. the secondary station receives a signal which it decodes and the CRC passes, even though no PDCCH was transmitted for that secondary station). 
         [0043]    Already proposed solutions to improve reliability of SPS grant messages are:
       Increase the CRC length. This may require modification of current transceiver implementation.   Fix some bits in the PDCCH message to effectively increase the length of the CRC. This has the disadvantage that the message contents are restricted.   Use RRC (higher layer) signalling. This implies more overhead.       
 
         [0047]    In accordance with the first aspect of the invention, the solution proposed is to use two PDCCH messages at the same time, i.e. in the same subframe. In one example of the invention, the messages should be identical. The only disadvantage is the increased overhead compared with a single PDCCH. It can also removes the need to encode SPS grants with a different ID (SPS-RNTI). This could be considered as a double length PDCCH with two copies of the data and CRC. In another example of the invention, the messages may be different. For the case when two messages are useful this avoids the increased overhead. 
         [0048]    In fact, this invention is based on the recognition that a solution based on the UE receiving two PDCCH&#39;s in the same subframe can achieve the desired result but without any extra delay. Currently the assumption for LTE in 3GPP is that the UE would only receive one PDCCH per subframe (or at least only one PDDCH for UL and one for DL). However, since the secondary station would typically try blind decoding all possible PDCCHs within a given search space, there is no practical difficulty in receiving more than one. 
         [0049]    In one embodiment based on LTE, a SPS message is indicated by reception of two identical PDCCH messages with the same SPS-RNTI encoded in the CRC. In this embodiment, if only one PDCCH message were received, the UE could interpret this as a potential SPS message but only transmit (or receive) one packet. 
         [0050]    In another embodiment based on LTE, a SPS message is indicated by reception of two identical PDCCH messages except that they have different RNTIs encoded in the CRC. One could be the C-RNTI for normal signalling and the other the SPS-RNTI. In this embodiment, if only one PDCCH message were received with SPS-RNTI, the UE could interpret this as a potential SPS message. In such a case, in order to limit interference, the UE could be required to only transmit (or receive) one packet if only the C-RNTI is received, and no packets if only the SPS-RNTI is received. One advantage of this embodiment is that the starting position in the control channel space of the PDCCH carrying the SPS message could be unambiguously identified (e.g. as corresponding to the PDCCH with the normal C-RNTI). This would be important if uplink resources (e.g. for ACK/NACK) are determined by the PDCCH starting position. 
         [0051]    In another embodiment based on LTE, the UE recognises an SPS message sent via PDCCH only if it also receives another PDCCH message sent at the same time. For example, an SPS message granting uplink resources (indicated by SPS-RNTI) would be sent at the same time as an ordinary message granting DL resources (indicated by C-RNTI). Since the UE has received two PDCCH messages, it can recognise the SPS message. As another example, an SPS message granting uplink resources would be sent at the same time as another SPS message granting downlink resources. Since the UE has received two PDCCH messages, it can recognise both as SPS messages. In case the eNodeB does not have two PDCCH messages to send, then two identical copies of one message could be sent, or the second message could have a null or zero resource allocation. 
         [0052]    It is to be noted that in variants of the invention, the allocated resource specified by the SPS is a downlink resource, i.e. the secondary station receives periodically data in this semi persistently scheduled resource. 
         [0053]    In other variants of the invention, the allocated resource specified by the SPS is an uplink resource, i.e. the secondary station transmits periodically data in this semi persistently scheduled resource. 
         [0054]    This invention may be implemented in mobile communication systems where communication devices utilize centralized scheduling, such as UMTS and LTE, WiMAX, or UWB. 
         [0055]    Moreover, this invention could as well be implemented for hubs which route connections from multiple terminals to base stations. Such devices would appear like a secondary station from the point of view of the network. 
         [0056]    In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed. 
         [0057]    The inclusion of reference signs in parentheses in the claims is intended to aid understanding and is not intended to be limiting. 
         [0058]    From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art of radio communication and the art of transmitter power control and which may be used instead of or in addition to features already described herein.