WO2011160285A1

WO2011160285A1 - Control channel assignment and reception

Info

Publication number: WO2011160285A1
Application number: PCT/CN2010/074182
Authority: WO
Inventors: Jianghua Liu; Wennstrom Mattias
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2010-06-21
Filing date: 2010-06-21
Publication date: 2011-12-29

Abstract

An improved control channel assignment and reception is disclosed. In a radio communication system, a remote node performs blind detection of a downlink control channel being scheduled by and transmitted from a network node. This blind detection is performed by detecting at least two candidates downlink control channels, wherein each one of said at least two candidate downlink control channels has one control channel element, or has more than one control channel elements, and these at least two candidate downlink control channels can have at least partly overlapping search spaces. According to the invention, the remote node detects the at least two candidate downlink control channels, whereby each correctly detected one of the at least two candidate downlink control channels is determined to have one control channel element, or to have more than one control channel elements. Then, if at least one correctly detected candidate downlink control channels has more than one control channel elements, the remote node determines the downlink control channel to include more than one control channel elements.

Description

CONTROL CHANNEL ASSIGNMENT AND RECEPTION

Field of the invention

The present invention relates to a method as defined in the preamble of claim 1, i.e. a method in a radio communication system, wherein a remote node performs blind detection of a downlink control channel being scheduled by and transmitted from a network node by detecting at least two candidates downlink control channels, each one of said at least two candidate downlink control channels having one control channel element, or having more than one control channel elements, wherein said at least two candidate downlink control channels can have at least partly overlapping search spaces.

The present invention also relates to a remote node as defined in the preamble of claim 17, i.e. a remote node in a radio communication system, said remote node being arranged for performing blind detection of a downlink control channel being scheduled by and transmitted from a network node by said remote node being arranged for detecting at least two

candidates downlink control channels, each one of said at least two candidate downlink control channels having one control channel element, or having more than one control channel elements, wherein said at least two candidate downlink control channels can have at least partly overlapping search spaces . The present invention also relates to a network node as defined in the preamble of claim 18, i.e. a network node in a radio communication system, said network node being arranged for communicating with a remote node being arranged for performing blind detection of a downlink control channel being scheduled by and transmitted from a network node by said remote node being arranged for detecting at least two candidates downlink control channels, each one of said at least two candidate downlink control channels having one

control channel element, or having more than one control

channel elements, wherein said at least two candidate downlink control channels can have at least partly overlapping search spaces .

The present invention also relates to a computer program and a computer program product .

Related art and background of the invention The invention can be implemented in essentially any wireless radio communication system including a network node, such as a Base Station (BS) , a NodeB (NB) , or an eNodeB (eNB) , and a remote node, such as a User Equipment (UE) , a Mobile Station (MS) , or any other device communicating with a network node over a radio interface. However, for pedagogic reasons, in this document, the invention will often be exemplified and explained in terms of a 3rd Generation Partnership Project

Long Term Evolution (LTE) system and/or an LTE-Advanced system, which is one possible implementation for the invention. Thus, when e.g. an eNB, a UE, and specific control channels of the

LTE and/or LTE-A systems are used for explaining the invention, these terms should be understood as referring to a network node, a remote node, and general control channels,

respectively. In a radio communication system, such as LTE, one kind of

Physical Downlink Control Channel (PDCCH) is transmitted by an eNB to inform a User Equipment (UE) of allocated resources, a transport block size, Multiple Input Multiple Output (MIMO) related parameters, such as a Precoding Matrix Indicator (PMI) and/or a rank, and the like, that are being used for downlink communication. Each PDCCH is UE specific, i.e. it is meant for a certain UE, and is distinguished by a UE specific Cyclic

Redundancy Check (CRC) mask, which is used to scramble the CRC of PDCCH.

In any given downlink sub- frame, the PDCCHs for all the

scheduled UEs are transmitted, and each UE then performs blind detection to search for a specific PDCCH, which is intended for that specific UE by using the UE specific CRC mask. At the eNB side, CRC calculation, CRC scrambling, channel coding, and modulation are performed individually for each PDCCH. Each PDCCH belongs to a limited set of Downlink Control

Information (DCI) formats, which is pre-configured semi- statically for each UE . This means that each UE knows at each PDCCH decoding instance how many information bits it can

expect to encounter in the PDCCH, i.e. the UE knows the

payload size of the PDCCH.

Hence, when the UE attempts to detect a PDCCH from a number of possible transmitted PDCCHs, i.e. a number of candidate PDCCHs, the UE assumes that the known DCI format is used, and utilizes this DCI format when performing demodulation, channel decoding, CRC descrambling by using the UE specific CRC mask, and CRC calculation. If the CRC calculation is correct, the UE will assume that the PDCCH is correctly detected. Hence, this

detection of the PDCCH is denoted blind detection since the UE needs to decode multiple hypotheses to find its control

message.

Normally, one PDCCH comprises building blocks of Control

Channel Elements (CCEs) , wherein a PDCCH can consist of 1, 2, 4, or 8 CCEs, i.e. L=l, 2, 4, 8 CCEs, where L is the

corresponding aggregation size/level of the PDCCH. The

aggregation size used for transmission of the PDCCH depends on the DCI information payload, and on the path loss to the UE, since a relatively larger aggregation size provides a lower code rate and better protection, which can be useful for a relatively higher path loss.

Thus, the eNB needs to assign the PDCCHs for the scheduled UEs, and these scheduled PDCCHs might possibly have differing

numbers of CCEs per PDCCH. The eNB can as a maximum use a total of N (n=0, 1,...N-1) number of available CCEs in each subframe .

To reduce the number of blind decodings a UE needs to perform when performing the blind detection, a principle of CCE

assignment is used in LTE . According to this principle, if a number of L consecutive CCEs is allocated for a PDCCH, then the index n of the first CCE of these allocated CCEs shall fulfill the equation n mod L=0. Figure 1 shows an exemplary illustration of possible CCE

assignments, wherein PDCCHs for four UEs (UE 1,2,3,4) comprise 1, 2, 8 and 4 CCEs, respectively, and there is a total of N=20 CCEs available. The equation n mod L=0 is here fulfilled for each one of the PDCCHs for the UEs. For example, the PDCCH for UE 2 has aggregation level L=2 and consequently maps the PDCCH onto two CCEs where the first CCE has number 2 and thus has an index fulfilling n mod 2=0. Also, as an other example 16 mod

4=0 holds for the PDCCH for UE 4, since it has aggregation level L=4 and its first CCE has number 16, as can be seen in figure 1.

After the blind detection has been performed, when the UE decodes a DCI in the corresponding PDCCH, it will receive the Physical Downlink Shared Data Channel (PDSCH) based on the information included in the PDCCH. Thereby, the UE can detect the received PDSCH. Depending on the detection result, i.e.

depending on if the UE can receive the PDSCH, the UE feeds back an acknowledgement (ACK) or a negative acknowledgement (NACK) to the eNB. Based on these ACKs and/or NACKs, the eNB then makes a decision for retransmission or new data

transmission at the next scheduling instance, according to the principle of Hybrid Automatic Request (H-ARQ) .

The ACK or NACK for a received PDSCH is transmitted by the UE on a Physical Uplink Control Channel (PUCCH) . In LTE , the PUCCH format la/lb is used for ACK/NACK transmission and

ACKs/NACKs from UEs are multiplexed in the form of Code

Division Multiplexing (CDM) . For each UE, the CDM code

sequence used for the ACK/NACK transmission is implicitly decided by the index of the first CCE of the corresponding PDCCH. Thus, n^n>

puccH, where - rices is the index of the first CCE of the PDCCH that carried the PDSCH assignment,

- N^n> PUCCH is a higher layer configured parameter, and

- n^(1> PUCCH, being calculated, is then used for deciding the resource or sequence to be used for the ACK/NACK transmission on the PUCCH.

For example, UE 2 in figure 1 can detect that the PDCCH has two CCEs and that the index of the first CCE is 2. Therefore, UE 2 can deduce that n^(1> _PUCCH=2+N^(1> _PUCCH.

LTE-Advanced (LTE-A) is the evolution of LTE, in which the PUCCH format la/lb will still be used and in which multiple transmit antennas are supported for uplink transmission. Also, in LTE-A, transmit diversity is proposed for PUCCH format la/lb for improving the uplink performance. Spatial Orthogonal Resource Transmit Diversity (SORTD) has been chosen as the transmit diversity scheme. SORTD transmission requires two orthogonal sequences, and one sequence per antenna.

When utilizing the principle of sequence allocation used in LTE also for LTE-A, the two sequences supporting SORTD

transmission can be implicitly decided by the indices of two CCEs of the PDCCH.

The choice of the first sequence is based on the same implicit decision as used in LTE, i.e. it depends to the first CCE index n_CCE- The choice of the second sequence depends on the second CCE index n_CCE+l- Thus:

■Π PUCCH-Antennal= lcCE+N^¹^PUCCH, and ■Π PUCCH-Antenna2= lcCE+l+N^¹^PUCCH ·

This sequence allocation scheme requires that there are at least two CCEs for the used PDCCH. However, in some cases, for example if the payload size of the PDCCH is small and one CCE is sufficient, the PDCCH only has one CCE.

If the UE detects that the PDCCH has only one CCE, the UE can fall back to single antenna port transmission for the ACK/NACK transmission. Hence, if one UE is configured to be in SORTD mode, the ACK/NACK transmission scheme will use either SORTD transmission or single antenna port transmission, and the choice between SORTD transmission and single antenna port transmission will be based on the detected number of CCEs. In other words, the choice between SORTD transmission and single antenna port transmission is based on the aggregation level L of the PDCCH.

The PDCCH carrying the Downlink Control Information (DCI) is comprised of a number of CCEs, the number and location of which being unknown to the UE . Hence, blind detection must be performed by the UE . For obtaining the DCI and for being able to decide on a transmission scheme for the subsequent PUCCH transmission, the UE has to gain knowledge of the DCI message as well as the aggregation level L of the PDCCH. However, under certain circumstances, a PDCCH can by the UE be detected correctly, even though the UE has assumed an

incorrect CCE aggregation level to have been used for that PDCCH. Thus, under some circumstances, the UE can with high probability correctly detect a PDCCH as having a different CCE aggregation level than what was actually used in the PDCCH transmission .

Since the CCE aggregation level of the PDCCH implicitly determines the transmission mode to be used for the PUCCH, this faulty CCE aggregation level assumption may lead to a wrong decision in the UE for the PUCCH transmission scheme to be used. The eNB will of course know which aggregation level it has used for transmitting the PDCCH, and when the UE make such a wrong decision, the eNB and the UE will have different understandings on the transmission scheme to be used. Thereby, the transmission and reception performance will be degraded due to the mismatched transmitter and receiver, i.e. to the mismatched remote node and network node.

Aim and most important features of the invention

It is an object of the present invention to provide a control channel assignment and reception that solves the above stated problem .

The object is achieved by the above mentioned method according to the characterizing portion of claim 1, i.e. by said remote node performing the steps of:

- detecting said at least two candidate downlink control channels, whereby each correctly detected one of said at least two candidate downlink control channels is determined to have one control channel element, or to have more than one control channel elements; and

- determining, if at least one correctly detected candidate downlink control channels has more than one control channel elements, said downlink control channel to include more than one control channel elements.

The object is also achieved by the above mentioned remote node according to the characterizing portion of claim 17, i.e. the remote node comprising

- detection means, arranged for detecting said at least two candidate downlink control channels,

- candidate determination means, arranged for determining each correctly detected one of said at least two candidate downlink control channels to have one control channel element, or to have more than one control channel elements; and

- determination means, arranged for determining, if at least one correctly detected candidate downlink control channels has more than one control channel elements, said downlink control channel to include more than one control channel elements.

The object is also achieved by the above mentioned network node according to the characterizing portion of claim 18, i.e. the network node comprising:

schedulation means, arranged for scheduling a downlink control channel as including either one control channel element, or more than one control channel elements, wherein said

schedulation means is arranged for scheduling a downlink control channel having one control channel element in:

- a control channel element having an even control channel element index number, only if a subsequent control channel element having an odd subsequent control channel element index number is utilized for transmission of a further downlink control channel, said further downlink control channel being different from said downlink control channel; or otherwise in - a control channel element having an odd control channel element index number.

The method, the remote node, and the network node according to the present invention are characterized in that the control channel is assigned by the network node, and the remote node detects candidate downlink control channels and determines the number of CCEs in the downlink control channel, such that a correct determination of the number of CCEs in the downlink control channel can be established. Thus, the remote node can, by utilization of the invention, determine the aggregation level of the downlink control channel, such that the network node and the remote node both have the same opinion about the aggregation level used.

According to an embodiment of the invention, the remote node selects a transmission mode to be used in an uplink control channel. Specifically, the remote node chooses between a single antenna transmission mode and a SORTD transmission mode based on the determined aggregation level. Thus, by a correct determination of the aggregation level, by utilization of the method of the invention, also a correct selection of the transmission mode to use can be achieved, which solves the transmission mode confusion problem in the system.

Thus, by using the invention in the network node, such as an eNB, and in the remote node, such as a UE, the PDCCH

aggregation level confusion problem can be avoided and the UE can correctly determine the transmission scheme to be used with high probability. Furthermore, the eNB and UE will have the same understanding of the transmission scheme to be used for the ACK/NACK transmission. The present invention can be applied in essentially any wireless communication system utilizing multiple antenna transmission.

Detailed exemplary embodiments and advantages of the method, remote node, and network node according to the invention will now be described with reference to the appended drawings illustrating some preferred embodiments.

Brief description of the drawings

Fig. 1 shows an exemplary illustration of a CCE assignment. Fig. 2 shows an exemplary illustration of search spaces.

Fig. 3 shows an exemplary illustration of UE-specific search spaces .

Fig. 4 shows an exemplary illustration of Case 1.

Fig. 5 shows an exemplary illustration of Case 2-1. Fig. 6 shows an exemplary illustration of Case 2-2.

Fig. 7 shows a flow chart diagram for a method of the

invention .

Fig. 8 schematically shows a radio communication system according to the invention. Detailed description of preferred embodiments

In principle, when the UE blindly searches in its search space to detect its own PDCCH within the N available CCEs, the UE needs to monitor all candidate PDCCHs under the hypothesis of all different CCE aggregation levels L, since the UE does not know the aggregation level of its own PDCCH until it has been detected. This extensive monitoring leads to a significant computation burden on the UE . Therefore, a side constraint has been inserted in the LTE specification in order to simplify the blind search for the UE . The side constraint was mentioned above and states that a PDCCH consisting of L {L=l,2,4, 8) consecutive CCEs should start on a CCE n (where n=0, 1 , ...N-1) fulfilling n mod L=0.

A set of PDCCH candidates for the UE to monitor at a certain aggregation level L is defined in terms of a corresponding search space S^L . The concept of a search space is illustrated in figure 2. It can be observed that the maximum number of PDCCH candidates to be monitored in this example is 37, divided into 20 candidates with 1 CCE, 10 candidates with 2 CCEs, 5 candidates with 4 CCEs, and 2 candidates with 8 CCEs, respectively. If there are more CCEs available for the PDCCH, i.e. the number IT is higher than 20, also the number of PDCCH

candidates to be monitored by the UE will be higher. In order to reduce the PDCCH blind detection complexity and to save UE power, the number of PDCCH candidate in each search space is restricted.

Furthermore, as illustrated I figure 3, in the current LTE specification, the search space is defined to be UE-specific, and the number of PDCCH candidates in the UE-specific search space S^L are 6 candidates with 1 CCE, 6 candidates with 2 CCEs, 2 candidates with 4 CCEs, and 2 candidates with 8 CCEs, respectively.

The UE-specific search space S^L is determined by a UE ID, a sub-frame index, and a CCE aggregation level. An example of a UE-specific search space with a restricted number of PDCCH candidates is illustrated in Figure 3. For the UE-specific search space S^L, it is possible that two or more different CCE aggregation level search spaces overlap each other, i.e. two or more candidate PDCCHs can overlap each other in the search spaces as is shown in figure 3. When the overlap is present and the PDCCH is assigned in the

overlapping region, the PDCCH will by the UE be blindly detected, in which blind detection the UE assumes usage of two or more different aggregation levels corresponding to the two or more at least partly overlapping candidate PDCCH search spaces, respectively. Therefore, if more than one of the PDCCH candidates are correctly detected, the blind detection may result in a network node and UE confusion problem being related to the number of CCEs used in the PDCCH. Thus, the PDCCH can be correctly detected under the assumption of two or more different aggregation levels, which might lead to

mismatch of the view the network node and the UE has on the number of CCEs in the PDCCH.

For instance, one PDCCH with aggregation level 1, i.e. L=2, can be transmitted by a network node on CCE 2 in figure 3. CCE 2 is located both in the PDCCH candidate of UE-specific search spaces S¹ and S². The candidate PDCCH, being in S¹ and

including CCE 2, should be correctly detected, since it is the one being transmitted. But, when the UE blindly detects the second PDCCH candidate, being in S² and including CCE 2 and 3, this second candidate PDCCH may also be correctly detected. If this second candidate PDCCH is correctly detected by the UE, the UE will assume that the PDCCH it is to receive comprises two CCEs (CCE 2 and CCE 3) . Here, the network node will know that the PDCCH includes only CCE 2, but the UE will believe that the PDCCH includes both CCE 2 and CCE 3.

As was described above, for LTE-A, if the UE believes that the PDCCH includes two or more CCEs, two CDM code sequences will be implicitly determined by the two or more CCEs. These two code sequences are then used by the UE for SORTD transmission of the ACK/NACK. For the ACK/NACK transmission scheme in the uplink, either SORTD or single antenna port transmission is utilized depending on the number of detected CCEs. Thus, in this example, the network node will expect a single antenna port transmission, since it knows that the PDCCH includes only one CCE . But, the UE will utilize SORTD transmission utilizing two antennas, since it believes that two CCEs were included in the PDCCH. This confusion will cause poor performance for the ACK/NACK transmission in the uplink, and it is thus desirable to solve the CCE detection confusion problem.

Thus, if UE-specific search spaces are overlapping, there are two cases of CCE confusion; case 1 and case 2 described below. In case 1, the UE believes that the PDCCH includes 1 CCE (L=l) although two or more CCEs (L>2) are included in the

transmitted PDCCH, and the UE also succeeds in correctly detecting a candidate PDCCH having two or more CCEs under the incorrect assumption that it includes only one CCe . This first case, wherein L>2 is detected as L=2, may occur when the PDCCH payload size is very small and all the information bits are transmitted in the first CCE, and the bits transmitted on the subsequent CCEs are parity bits due to the circular buffer rate matching. Hence, under the assumption of L=2, the

information bits only in the first CCE alone, of the PDCCH including two or more CCEs, may be correctly detected. Case 1 is illustrated in figure 4 for the confusion, in which L=2 is detected as L=2 by the UE .

Case 1 is related to the implementation of the PDCCH blind detection. There are two possible blind detection schemes which are here denoted Case 1-1 and Case 1-2. In Case 1-1, the UE monitors all the PDCCH candidates within different search spaces, and then decides which one PDCCH of the candidate PDCCH is valid. This decision is made according to the blind detection results. In Case 1-2, the UE blindly monitors all the PDCCH candidates in each search space according to a predefined detection order of CCE aggregation level . Once one PDCCH candidate in a certain search space is correctly detected, the UE will stop the detection of further aggregation level PDCCH candidates. Thus, the following blind detections, i.e. the blind

detections that would have been performed if no PDCCH had been correctly detected, are not performed.

For Case 1-1, when the PDCCH candidates in both S¹ and S^L, where L=2 (or 4 or 8) , are correctly detected, UE needs to make a decision on the correct aggregation level L. I.e., the UE needs to decide how many CCEs that are included in the PDCCH .

For example, as shown in figure 4, two CCEs (CCE 4 and CCE 5) can be assigned for the PDCCH of a certain UE, and the two CCEs are in the overlapping region of the UE specific search spaces of S¹ and S². When the UE blindly detects the PDCCH candidates in the UE- specific search spaces, the PDCCH

candidate comprised of CCE 4 and CCE 5 and the PDCCH candidate comprised of only CCE 4 are correctly detected under the assumption of L=2, and L=2 respectively. I.e., if the UE would assume two CCEs and one CCE being included in the PDCCH, respectively, both the candidate PDCCH including the two CCEs (CCE 4 and CCE 5) , and the candidate PDCCH including only CCE 4 would be correctly detected, respectively. This causes confusion related to the aggregation level, i.e. to the number of CCEs being included in the PDCCH. According to an aspect of the invention, this problem is

solved by said remote node detecting the at least two

candidate downlink control channels, i.e. the at least two candidate PDCCHs, and determining each correctly detected one of the at least two candidate downlink control channels to have one control channel element, or to have more than one control channel elements. Thus, the aggregation level is

determined for each one of the correctly detected ones of the at least two candidate downlink control channels. Also, if at least one of those at least one correctly detected candidate downlink control channels has more than one control channel elements, the remote node determines the downlink control channel to include more than one control channel

elements . Thus, if one or more of the correctly detected candidate

PDCCHs are determined to have more than one CCE, then the

PDCCH sent is determined to have more than one CCE. In other words, according to the invention, the remote node, such as a UE, assumes that the aggregation level of the PDCCH is L>2 if it can determine that at least one correctly detected

candidate PDCCH includes more than one CCE. By this assumption, the aggregation level confusion of case 1-1 is mitigated.

Further, according to an embodiment of the invention, the remote node is arranged to detect the candidate downlink

control channels in a certain order, such that a candidate downlink control channel having more than one control channel elements is detected before a candidate downlink control

channel having only one control channel element .

Thus, for Case 1-2, in order to avoid the confusion from

L=2 (or 4, 8) to L=2, i.e. the confusion of that the UE

believes that the PDCCH includes one CCE (L=2) although two or more CCEs (L>2) are included in the PDCCH, the monitoring of search space S² is performed prior to monitoring of search space S¹. This is also is a reasonable assumption for SORTD configured UEs, since SORTD is normally configured for UEs with poor channel conditions, or for UEs being at the cell edge. Due to the poor channel conditions, two or more CCEs are then normally assigned for the PDCCH to improve the

performance over the bad channel. Hence, the assumption of detection of search space S² prior to detection of search space S¹ , according to this embodiment of the invention, can also enable a reduction of the number of blind detections being performed, which lowers the computational complexity and thus also saves power.

The aforementioned CCE detection confusion problem can cause the UE to make a wrong decision when choosing the transmission scheme to be used for ACK/NACK PUCCH transmission, and the transmission resource (s) used for this ACK/NACK PUCCH

transmission. Since the PUCCH transmission is a response to the scheduled PDSCH, which is pointed out by the PDCCH in the case of dynamic scheduling, the PUCCH shall be associated with the PDCCH.

According to an embodiment of the invention, in order to solve this problem, the remote node selects a transmission mode to be used for an uplink control channel, wherein this selection is based on the determination of the number of CCEs being included in the downlink control channel.

According to an embodiment of the invention, if the downlink control channel includes one control channel element, the selected transmission mode includes transmission utilizing one single antenna port. I.e., single port antenna transmission is then used for transmission of the ACK/NACK PUCCH. When one single antenna port is used for the transmission, the code sequence being used for this transmission by the remote node is determined by the remote node based on the one single control channel element n_CCE · Thus, the code sequence

transmission resource can implicitly be determined from the

According to another embodiment of the invention, if the downlink control channel includes more than one control channel element, the selected transmission mode includes transmission utilizing two antenna ports for transmission of the ACK/NACK PUCCH.

When two antenna ports are used for the transmission, these two antenna ports are utilized for achieving Spatial

Orthogonal Resource Transmit Diversity (SORTD) transmission. Also, the two code sequences being used for this SORTD

transmission by the remote node are determined by the remote node based on the more than one control channel elements, i.e. on n_CcE and n_CCE+l · Thus, the code sequences transmission resources can implicitly be determined from the more than one CCEs n_CCE and n_CCE+l .

Thus, if the aggregation level is determined to be higher than 1, i.e. if it is determined that the PDCCH includes more than one CCEs, then the UE can further decide that SORTD will be used for the ACK/NACK transmission, and the two CDM code resources to be used for SORTD are implicitly determined by the two CCEs of the PDCCH. Thereby, the confusion of the network node and remote node opinion of the transmission scheme used is solved, since both the remote node and the network node will both use SORTD transmission. Further, according to an embodiment of the invention, the network node only schedules a downlink control channel having one control channel element in a control channel element having an odd control channel element index number.

This embodiment solves the problem occurring in case 2, wherein the UE believes that the transmitted PDCCH includes two or more CCEs {L>1, i.e. L=2, L=4 , or L=8) although only one CCE (L=2) is included in the PDCCH, and the UE also succeeds in correctly detecting a candidate PDCCH having only one CCE (L=l) under the incorrect assumption that it has two or more CCEs. Case 2 may occur when the UE-specific search spaces S¹ and S^L (L>1) overlap and the PDCCH with L=l is assigned on an n_CcE with even CCE index numbering in the UE- specific search space overlapping region. Thus, by this embodiment of the invention, this confusion problem is solved.

Further, according to an embodiment of the invention, the network node only schedules a downlink control channel having one control channel element in a control channel element having an even control channel element index number if a subsequent control channel element having an odd subsequent control channel element index number is utilized for

transmission of a further downlink control channel, wherein further downlink control channel is different from the

downlink control channel. For example, this further downlink control channel can be a downlink control channel for another remote node, or e.g. a channel uplink grant PDCCH for the same remote node.

As is shown in figures 5 and 6, depending on if the subsequent CCE index numbering n_CcE+l is empty or assigned to a further PDCCH, e.g. a PDCCH of another UE, there are two corresponding sub-cases related to case 2. In subcase 2-1, the subsequent CCE index numbering n_CcE+l is occupied by a PDCCH with L=2 of another UE . In subcase 2-2, the subsequent CCE index numbering riccE+1 is empty. As an example, and without loss of generality, the cases where L=l is detected as L=2 are hereafter discussed in relation to the case illustrations in figure 5 and figure 6.

For the case 2-1, being illustrated in figure 5, the

probability that a PDCCH including only 1 CCE is correctly detected as having two CCEs is marginal, and can be ignored because of the interference from the subsequent CCE , having index number n_CCE+l . Thus, since the subsequent CCE includes a PDCCH for intended for another UE, or includes e.g. an uplink grant PDCCH for the same UE, detection of this subsequent CCE will cause so much interference that the UE rather easily can deduce that this detection is not correct.

For the case 2-2, being illustrated in figure 6, wherein the subsequent CCE (having CCE index n_CCE+l) is empty and can be regarded as white Gaussian noise, the PDCCH candidate

including two CCEs in search space S², i.e. CCE 4 and CCE 5 in figure 6, can be correctly detected by the UE assuming that it includes only one CCE, i.e. as being in search space S¹ , with high probability, if the payload size of the PDCCH is small and has a low coding rate, or has a high SNR.

Thus, according to this embodiment of the invention, to avoid the occurrence of case 2-2, the PDCCH with one CCE can not be assigned on n_CCE in the search space S¹ when n_CCE is an even number and n_CCE +1 is empty or <NIL> .

Thus, according to different embodiments of the invention, the PDCCH including one CCE can be assigned to the n_CcE with an odd CCE index numbering; or the PDCCH can be assigned to the n_CcE with even CCE index numbering when the n_CcE+l with the

subsequent odd CCE index numbering is assigned for another

PDCCH, e.g. a PDCCH for another UE or an uplink grant PDCCH for the same UE . Particularly, the PDCCH with one CCE can not be assigned on n_CCE in the search space overlapping region while rices is an even CCE index number and n_CcE +1 is empty or <NIL> . Especially, according to an embodiment, since the n_CcE +1 is empty, the PDCCH can easily be shifted to the empty CCE TICCE +1 if n_CCE +1 also belongs to the search space of S¹. Also, another UE's PDCCH with one CCE can easily be moved to n_CcE +1.

These embodiments of the invention solve the confusion problem and are also very easily implemented, and there is no impact on the complexity and scheduling flexibility of the system.

Further, the different steps of the method of the invention described above can be combined or performed in any suitable order. A condition for this of course, is that the

requirements of a step, to be used in conjunction with another step of the method of the invention, in terms in terms of available measures, such as PDCCHs, determined number of CCEs etc., must be fulfilled.

Figure 7 shows a flow chart diagram of the method of the invention. In a first step, the remote node detects at least two candidate downlink control channels. For each correctly detected one of these at least two candidate downlink control channels, it is determined if they have one control channel element, or if they have more than one control channel

elements, respectively. In a second step, if at least one of the correctly detected candidate downlink control channels has more than one control channel elements, the remote node determines the downlink control channel to include more than one control channel elements . According to an aspect of the invention, the remote node being arranged for performing the blind detection of the downlink control channel comprises detecting means. This detecting means is arranged for detecting the at least two candidate downlink control channels. The remote node further comprises candidate determination means, which is arranged for

determining, for each one of these at least two candidate downlink control channels being correctly detected, if it has one control channel element, or more than one control channel elements. The remote node also comprises determination means, which is arranged for determining the downlink control channel to include more than one control channel elements if at least one of the correctly detected candidate downlink control

channels has more than one control channel elements. Further, according to another aspect of the invention, the network node transmitting the downlink control channel

comprises schedulation means, where this schedulation means is arranged for scheduling a downlink control channel having one control channel element in a control channel element having an even control channel element index number only if a subsequent control channel element having an odd subsequent control

channel element index number is utilized for transmission of a further downlink control channel. This further downlink

control channel should then be different from said downlink control channel, and can e.g. be meant for another remote node.

Otherwise, if the subsequent control channel element having an odd subsequent control channel element index number is empty, the schedulation means is arranged for scheduling a downlink control channel having one control channel element in a

control channel element having an odd control channel element index number . Both the remote node and the network node of the invention can be adapted to include means for performing any of the steps of the method of the invention. A trivial requirement is, of course, that such a step does involve the remote node and the network node, respectively.

Those skilled in the art should understand that that the

foregoing embodiments or part of the procedures may be

implemented through programs instructing related hardware means, and that the program can be stored on a computer

readable storage media. Thus, the method of the invention can be implemented by a computer program, having code means, which when run in a computer causes the computer to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product . The computer readable medium may consist of essentially any memory, such as a ROM (Read-Only Memory) , a PROM (Programmable Readonly Memory) , an EPROM (Erasable PROM) , a Flash memory, an

EEPROM (Electrically Erasable PROM), or a hard disk drive.

Figure 8 schematically illustrates a radio communication

system 800 according to the invention. The radio communication system 800 includes at least one network node 810 and at least one remote node 820 communicating with each other over a radio interface 830.

In the network node 810, the hardware means 811, being a

computer, a processor, a DSP (Digital Signal Processor) , an

ASIC (application Specific Integrated Circuit) or the like, is connected to an antenna 813 receiving and transmitting signals over the radio interface 830. The hardware means 811 is, when being e.g. a processor, a DSP, a computer or the like,

connected to a computer readable storage media 812. The

computer readable storage media 812 includes ROM/RAM, soft discs, Compact Disk, etc., and is arranged for providing the hardware means 811 with instructions needed for performing the method of the invention, i.e. for arranging the network node with the schedulation means as described above. Correspondingly, in the remote node 820, the hardware means 821, being a computer, a processor, a DSP (Digital Signal

Processor) , an ASIC (application Specific Integrated Circuit) or the like, is connected to an antenna 823 receiving and transmitting signals over the radio interface 830. The

hardware means 821 is, when being e.g. a processor, a DSP, a computer or the like, connected to the computer readable

storage media 822. The computer readable storage media 822 includes ROM/RAM, soft discs, Compact Disk, etc., and is

arranged for providing the hardware means 821 with

instructions needed for performing the method of the invention, i.e. for arranging the remote node as having detection means, candidate determination means, and determination means as described above.

The method, the remote node, and the network node according to the invention may be modified by those skilled in the art, as compared to the exemplary embodiments described above.

As is obvious for a skilled person, a number of other

implementations, modifications, variations and/or additions can be made to the above described exemplary embodiments. It is to be understood that the invention includes all such other implementations, modifications, variations and/or additions which fall within the scope of the claims.

Claims

1. Method in a radio communication system, wherein a remote node performs blind detection of a downlink control channel being scheduled by and transmitted from a network node by detecting at least two candidates downlink control channels, each one of said at least two candidate downlink control channels having one control channel element, or having more than one control channel elements, wherein said at least two candidate downlink control channels can have at least partly overlapping search spaces,

characterized by said remote node performing the steps of:

- determining, if at least one correctly detected candidate downlink control channel has more than one control channel elements, said downlink control channel to include more than one control channel elements.

2. Method as claimed in claim 1, wherein a candidate downlink control channel having more than one control channel elements is detected before a candidate downlink control channel having one control channel element.

3. Method as claimed in claim 1, wherein said at least two candidate downlink control channels have at least partly overlapping search spaces.

4. Method as claimed in claim 1, wherein said remote node selects a transmission mode to be used for an uplink control channel, the selection being based on the determination

whether said downlink control channel includes one control channel element, or more than one control channel elements.

5. Method as claimed in claim 4, wherein, when the

determination results in that said downlink control channel includes one control channel element, said transmission mode includes transmission utilizing one single antenna port.

6. Method as claimed in claim 4, wherein, when the

determination results in said downlink control channel

including more than one control channel element, said

transmission mode includes transmission utilizing two antenna ports .

7. Method as claimed in claim 6, wherein said two antenna ports are utilized for achieving Spatial Orthogonal Resource Transmit Diversity (SORTD) transmission.

8. Method as claimed in claim 7, wherein two code sequences being used for said SORTD transmission are determined based on said more than one control channel elements.

9. Method as claimed in claim 4, wherein said uplink control channel transmits at least acknowledgements (ACKs) and/or negative acknowledgements (NACKs) .

10. Method as claimed in claim 1, wherein said network node schedules a downlink control channel having one control channel element in a control channel element having an odd control channel element index number.

11. Method as claimed in claim 1, wherein said network node schedules a downlink control channel having one control channel element in a control channel element having an even control channel element index number only if a subsequent control channel element having an odd subsequent control channel element index number is utilized for transmission of a further downlink control channel, said further downlink

control channel being different from said downlink control channel .

12. Method as claimed in claim 11, wherein said at least two candidate downlink control channels have overlapping search spaces for said control channel element having said even control channel element index.

13. Method as claimed in claim 1, wherein said radio

communication system is a Long Term Evolution Advanced (LTE- Advanced) system comprising said network node being an eNodeB (eNB) , said remote node being a User Equipment (UE) , said downlink control channel being a Physical Downlink Control Channel (PDCCH) , and an uplink control channel being a

Physical Uplink Control Channel (PUCCH) .

14. Method as claimed in claim 1, wherein said remote node is configured to be in a Spatial Orthogonal Resource Transmit Diversity (SORTD) mode.

15. Computer program, characterized in code means, which when run in a computer causes the computer to execute the method according to any of the claims 1-14.

16. Computer program product including a computer readable medium and a computer program according to claim 15, wherein said computer program is included in the computer readable medium .

17. A remote node in a radio communication system, said remote node being arranged for performing blind detection of a downlink control channel being scheduled by and transmitted from a network node, by said remote node being arranged for detecting at least two candidates downlink control channels, each one of said at least two candidate downlink control channels having one control channel element, or having more than one control channel elements, wherein said at least two candidate downlink control channels can have at least partly overlapping search spaces,

characterized by

18. A network node in a radio communication system, said network node being arranged for communicating with a remote node being arranged for performing blind detection of a downlink control channel being scheduled by and transmitted from a network node by detecting at least two candidates downlink control channels, each one of said at least two candidate downlink control channels having one control channel element, or having more than one control channel elements, wherein said at least two candidate downlink control channels can have at least partly overlapping search spaces,

characterized by

- a control channel element having an even control channel element index number, only if a subsequent control channel element having an odd subsequent control channel element index number is utilized for transmission of a further downlink control channel, said further downlink control channel being different from said downlink control channel; or otherwise in

- a control channel element having an odd control channel element index number.

19. The network node as claimed in claim 18, wherein said at least two candidate downlink control channels have overlapping search spaces for said control channel element having said even control channel element index.