WO2013164026A1

WO2013164026A1 - Method for transmission of control channel signals

Info

Publication number: WO2013164026A1
Application number: PCT/EP2012/058100
Authority: WO
Inventors: Weijun Sun; Anahid Robert Safavi
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2012-05-03
Filing date: 2012-05-03
Publication date: 2013-11-07

Abstract

The present invention relates to a method in a network control node for transmission of control channel signals in a wireless communication system, said wireless communication system employing scheduling of physical resource blocks (PRBs) used for each new transmission of a control channel signal, and wherein each control channel signal is related to one or more control channel elements (CCEs) transmitted over one or more antenna ports; said method comprising the step of: transmitting control channel elements (CCEs) related to a control channel signal over two or more antenna ports, wherein each of said control channel elements (CCEs) is transmitted over only one associated antenna port of said two or more antenna ports independent of the number of control channel elements (CCEs) related to said control channel signal. Furthermore, the invention also relates to a network control node device, a computer program, and a computer program product thereof.

Description

METHOD FOR TRANSMISSION OF CONTROL CHANNEL SIGNALS

Technical Field

The present invention relates to a method in a network control node for transmission of control channel signals. Furthermore, the invention also relates to a network control node device, a computer program, and a computer program product thereof.

Background of the Invention

In a cellular wireless communication system, the downlink (DL) denotes the transmission of synchronization signals and information from a base station (eNB) to a mobile user equipment (UE). On the uplink (UL) the transmission direction is the opposite.

The DL of LTE cellular communication system is based on Orthogonal Frequency Division Multiplex (OFDM) transmission, using both time and frequency resource units for information transmission. The OFDM signal consists of a set of complex sinusoids, called subcarriers, whose frequencies are consecutive integer multiples of the basic (the lowest nonzero) subcarrier frequency, wherein each complex sinusoid is weighted by a modulation symbol conveying certain number of information bits. In the time domain an OFDM symbol period consists of an active part and a cyclic prefix part. The duration of active part is the inverse of the basic subcarrier frequency. A cyclic prefix (CP) is a signal appended at the beginning of each OFDM symbol, and it consists of a last portion of active OFDM symbol waveform.

The smallest time-frequency resource unit for DL LTE information transmission is called resource element (RE), occupying a single complex sinusoid frequency in an OFDM symbol. For the purpose of scheduling transmissions to the different UEs, the resource elements are grouped into larger units called physical resource blocks (PRB). A PRB occupies a half subframe (called "slot"), i.e. N_s^ _b = 7 (with normal cyclic prefix length) consecutive OFDM symbol intervals in time domain, and = 12consecutive subcarrier frequencies in frequency domain (occupying in total 180 KHz). Two PRBs in a subframe occupying the same subcarriers form a so called PRB pair. Each PRB is labelled by a unique PRB number, which is an index denoting the position of the subband that the PRB occupies within a given bandwidth. The PRBs are numbered from 0 to Λ½ -1 within a given bandwidth. Thus, the maximum LTE bandwidth (20 MHz) contains the maximum number of PRBs (i.e. 110), which is in LTE standard denoted by N™^^X'^DL = 110. The relation between the PRB number k

«_PRB in the frequency domain and resource elements (k, l) in a slot is given by «_PRB =—— .

The physical downlink control channel (PDCCH) is defined as a control channel signal containing information needed to receive and demodulate user-specific information transmitted from the eNB to a UE through another signal, called physical downlink shared channel (PDSCH). The PDCCH is transmitted in the control channel region occupying a few OFDM symbols and total downlink bandwidth at the beginning of a downlink subframe, which is the minimum time resource that can be allocated to a single UE. The number of OFDM symbols in each control channel region ranges from 1 to 4 as indicated by physical control format indicator channel (PCFICH) in each DL subframe. Downlink control information (DCI) conveyed by PDCCH includes information necessary to demodulate related PDSCH or physical uplink shared channel (PUSCH), such as time- frequency resource allocation, used modulation and coding scheme (MCS), etc. Error detection on DCI transmissions is provided through the Cyclic Redundancy Check (CRC). The CRC bits, calculated from the DCI information bits, are attached to the DCI.

In order to allow for a simple processing of control channels in the receiver and simple multiplexing at the transmitter, the mapping of PDCCHs to resource element is subject to a certain structure. The structure is based on so-called control channel elements (CCE). CCE is a name for a set of 36 useful resource elements. The mapping of CCEs, which are logical units, to resource elements, which are physical units, is a function of cell-id. The number of CCEs, one, two, four or eight required for a certain PDCCH depends on the payload size of downlink control information and the channel coding rate. The number of CCEs used for a PDCCH is also referred to as the aggregation level. In practice blind decoding of control channel is done on search spaces considering CCE as the processing unit. More precisely, a search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to blindly decode. The functional block diagram of a blind PDCCH detection attempt is shown in Fig. 1. Here, the detection complexity of a blind PDCCH detection attempt is dominated by the convolutional code (CC) decoder, linear detector and channel estimator. In a blind PDCCH detection attempt, the reconstructed CRC bits calculated by the UE from the demodulated control channel information bits will be compared with the demodulated CRC bits. If the demodulated and the reconstructed CRC bits are the same, the PDCCH is considered to be found and successfully decoded. To demodulate the PDCCH signal the UE needs the estimate of the propagation channel. The channel estimate is obtained from reference signals (RS) transmitted through specially allocated REs. The RSs are also used to define so-called antenna ports (APs). An AP is the baseband input into the corresponding separate antenna system. An antenna system consists of an RF chain connected to one or multiple antenna elements that should together produce a desired electro-magnetic radiation pattern. If there is more than one transmit antenna port, and more than one receive antenna port, the transmission is usually classified as Multiple Input Multiple Output (MIMO) transmission. The corresponding propagation paths between each transmit antenna port and each receive antenna port jointly define a MIMO propagation channel. On the LTE DL the different RSs are transmitted on different antenna ports, and thus can serve at the UE to identify separate propagation paths in MIMO propagation channel. In this way each RS defines a unique AP.

There are three types of DL reference signals in LTE:

• common reference signals (CRS) which are broadcasted by a base station to all UEs; · UE-specific channel state indication reference signals (CSI-RS); and

• UE-specific demodulation reference signals (DMRS).

The first two types of RSs are used by the UE to perform measurements of DL channel state. Measurements are fed back to the base station following different reporting types. Feedback contains CQI, PMI and rank information to be used for the subsequent transmission for desired UE. CQI contains channel quality indicator for the related bandwidth. PMI (Precoding Matrix Indicator) denotes the precoding matrix which is supposed to achieve the best throughput corresponding to the reported bandwidth. Bandwidths used for CQI and PMI might be different and are defined following the different reporting types. The third kind of RS, the DMRS, is used to demodulate the data transmitted on PDSCH in the same PRB as that of DMRS. Note however that in some transmission modes of PDSCH the DMRSs are not transmitted, so only the CRS is used for the demodulation of PDSCH in these transmission modes. Besides, the CRSs are the only reference signals used for the demodulation of the PDCCH signals.

Up to eight DMRS antenna ports {7, 8, 9, 10, 11, 12, 13, 14} are defined to support up to eight spatial layers of PDSCH transmission in LTE Rel-10. PDSCH is directly mapped onto the antenna ports defined by DMRS as illustrated in Fig. 2, showing the case of rank 2 transmission via AP 7 and AP 8. In Fig. 2 the mapping between APs and physical antennas which includes precoding vectors as well depends on the implementation and thus is not specified in the standard. All of the RSs are characterized by a unique combination of the particular time-frequency pattern of their REs and the modulation sequence whose elements modulate these REs. There are two possible time-frequency patterns of DMRSs within a PRB pair, as shown in Fig. 3.

It has been widely recognized that, in the future development scenarios, the control channel region for LTE PDCCH might suffer much serious interference coming from neighbouring cells. Moreover, the control channel region of PDCCH might not be sufficient to support a significant increase of the number of users.

Therefore, enhanced PDCCH (ePDCCH) has been introduced. Enhanced control channel is supposed to be scheduled in data region as PDSCH with granularity of one PRB. Scheduling denotes dynamic or semi-static allocation of PRBs by a NodeB (base station). In the case of dynamic allocation, feedback information received from a UE indicates the suitable PRBs to use for sending related control information. More precisely, the ePDCCH is sent over a selected PRB and the selected PRB can change from one subframe to other based on received feedback information from the UE. In the case of semi-static allocation, selected PRB remains the same for one or more consecutive subframes and is changed for example when there is a handover or a newly received allocation. This is similar to scheduling of PDSCH in the downlink. ePDCCH uses DMRSs for demodulation contrary to legacy PDCCH, and there are two types of ePDCCH: localized ePDCCH and distributed ePDCCH. Localized ePDCCH is transmitted over only one PRB pair scheduled by an associated eNB based on the information obtained from the CQI feedback thus providing frequency selective scheduling. Distributed ePDCCH is transmitted over more than one PRB pairs to achieve frequency diversity. It is assumed that distributed ePDCCH is used when there is no feedback available or the available feedback is not reliable. However, using distributed ePDCCH might lead to allocation of more PRBs compared to the localized case resulting less resources for PDSCH transmission.

Enhanced control channel element (eCCE) is the basic structure of ePDCCH and is used for multiplexing and blind decoding of ePDCCH. For the time being there is no clear definition of eCCE. One eCCE might contain several useful resource elements. The number of resource elements in different eCCEs might be different. When ePDCCH is transmitted, one, two or four eCCEs can be aggregated together based on the payload size and coding rate of the transmitted ePDCCH creating aggregation level of one, two or four. Therefore, one PRB pair can contain one or more eCCE depending on the eCCE size and the mapping rule used to map ePDCCH to the PRB pair.

It has been decided to use up to four DMRS antenna ports {7, 8, 9, 10} for demodulation of ePDCCH. When the used antenna ports are known to the UE there is no need to perform blind decoding to detect antenna port for a given ePDCCH therefore the detection complexity is reduced. One way to indicate the used antenna ports to the UE is to associate implicitly antenna ports to the useful eCCEs when transmitting ePDCCH.

If PMI feedback is available for a given UE, its ePDCCH can be spatially precoded with the appropriate precoding vector which will also be used to precode the associated DMRSs. It is always desirable to achieve the so-called precoding gain for ePDCCH based on PMI feedback. However, if PMI feedback is unavailable or unreliable for a given UE, distributed ePDCCH may be used. Distributed ePDCCH will occupy several PRB pairs and might lead to un- efficient allocation of resources. Using localized ePDCCH and trying to take advantage of existing precoders as much as possible is another alternative.

For a given ePDCCH, it is still an open issue how to associate eCCE(s) and AP(s) with respect to different aggregation levels. One important criterion to derive the desired association (mapping) between eCCEs and antenna ports is to keep the number of channel estimation and data detection operations as small as possible to speed up the ePDCCH decoding and reducing the complexity. Thus, it is an open problem how to transmit localized ePDCCH with or without reliable PMI available while reducing the computational complexity.

According to a first prior art solution, a localized ePDCCH is proposed to be transmitted only over one AP, with possible associations between eCCEs and APs as given in Fig.4. In this scheme, the used antenna port depends on the aggregation level and the used eCCE. If the association between antenna ports and eCCEs is known to the UE and if up to all four antenna ports are used no additional signalling is needed. One disadvantage of this scheme is that if PMI feedback is unavailable or unreliable for a given UE, its associated localized ePDCCH cannot be transmitted using the appropriate precoder over multiple antennas. In this case, the localized ePDCCH is precoded with one randomly selected precoding vector which might fail ePDCCH transmission. Another disadvantage of this scheme is about ePDCCH blind detection complexity. As in this scheme eCCEs corresponding to different aggregation levels use different antenna ports, ePDCCH detection and channel estimation on the desired eCCEs for different aggregation level has to be repeated considering all possible antenna ports. For example, for aggregation levels of 1, 2 and 4 eCCEs, the 2^nd eCCE will require up to 2 channel estimation operations and 2 data detection operation over AP 7 and 8 separately based on Fig. 4. Meanwhile, the 4^th eCCE will require up to 3 channel estimation operations and 3 data detection operations over AP 7, 9 and 10 respectively. For a localized ePDCCH with associations between eCCEs and APs as shown in Fig.4, the maximum numbers of different operations in one PRB pair are listed in Table 1. Here, we assume that 4, 2 and 1 detection attempts in one PRB pair are separately needed for blind detection of a given ePDCCH corresponding to aggregation levels of 1, 2 and 4 eCCEs.

The maximum number of operations for blind detection of a given ePDCCH

Channel estimation operations 8

(1 for eCCE 1, 2 for eCCE 2 and 3, 3 for eCCE 4)

ePDCCH linear detection operations 8 (1 for eCCE 1, 2 for eCCE 2 and 3, 3 for

eCCE 4)

Convolutional Code decoder decoding 7

attempts (4 for 1 eCCE, 2 for 2 eCCEs

aggregated, 1 for 4 eCCEs aggregated)

Table 1 : maximum numbers of different operations in one PRB pair for a given ePDCCH using up to four antenna ports.

According to a second prior art solution a localized ePDCCH is also proposed to be transmitted only over one AP with possible associations between eCCEs and APs as given in Fig.5. In this scheme, the used antenna port depends on the aggregation level and the used eCCE. It is assumed that one of the total two antenna ports is used to transmit eCCE. It is possible to configure two other antenna ports to be used to transmit ePDCCH. In this case some signalling is needed to indicate the exact association between antenna ports and eCCEs. The disadvantages of this scheme are similar to those in the first prior art solution, except that it can have a slightly better detection complexity for a given ePDCCH. For example, for different aggregation levels, only one ePDCCH detection and one channel estimation operation are needed for eCCE 1 and 2 in one PRB pair. However, for eCCE 3 and 4, 2 channel estimation operations and 2 ePDCCH detection over AP 7 and 9 respectively are still needed for different aggregation levels.

Summary of the Invention

An object of the present invention is to provide a solution which mitigates or solves the drawbacks and problems of prior art solutions.

According to a first aspect of the invention, the above mentioned objects are achieved by a method in a network control node for transmission of control channel signals in a wireless communication system, said wireless communication system employing scheduling of physical resource blocks (PRBs) used for each new transmission of a control channel signal, and wherein each control channel signal is related to one or more control channel elements (CCEs) transmitted over one or more antenna ports; said method comprising the step of: - transmitting control channel elements (CCEs) related to a control channel signal over two or more antenna ports, wherein each of said control channel elements (CCEs) is transmitted over only one associated antenna port of said two or more antenna ports independent of the number of control channel elements (CCEs) related to said control channel signal.

Embodiments of the above method are defined in the appended dependent claims. The invention also relates to a computer program and a computer program product.

According to a second aspect of the invention the above mentioned objects are achieved with a network control node device arranged for transmission of control channel signals in a wireless communication system, said wireless communication system employing scheduling of physical resource blocks (PRBs) used for each new transmission of a control channel signal, and wherein each control channel signal is related to one or more control channel elements (CCEs) transmitted over one or more antenna ports; said network control node device comprising at least two antenna ports and being further arranged to:

- transmit control channel elements (CCEs) related to a control channel signal over two or more antenna ports, wherein each of said control channel elements (CCEs) is transmitted over only one associated antenna port of said two or more antenna ports independent of the number of control channel elements (CCEs) related to said control channel signal.

The present invention provides a solution to transmit localized control channel signals (ePDCCH) in a robust way when PMI feedback is not available or is not reliable. Robustness is obtained by keeping the possibility of using different spatial precoders through different allocated antenna ports.

Moreover, by using the present invention the number of operations for channel estimation or control channel signal detection is reduced. Reducing detection complexity for control channel signal is important to achieve fast decoding of control channels and guarantee a timely transmission of related data channels. Another advantage of the present invention is the flexibility to perform mobile user specific configuration and therefore guarantee simultaneous transmission of control channel signal to different mobile users by the usage of different orthogonal antenna ports. Further applications and advantages of the invention will be apparent from the following detailed description.

Brief Description of the Drawings

The appended drawings are intended to clarify and explain different embodiments of the present invention in which:

Fig. 1 shows block diagram of a blind PDCCH detection attempt;

Fig. 2 illustrates PDSCH mapping onto antenna ports;

- Fig. 3 illustrates RS patterns of AP7 to API 4 in a PRB pair;

- Fig. 4 illustrates association between eCCEs and AP for different aggregation levels using up to four antenna ports according to prior art;

Fig. 5 illustrates association between eCCEs and APs for different aggregation levels using up to two antenna ports according to prior art;

Fig. 6 illustrates association between eCCEs and APs for different aggregation levels using up to four antenna ports according to an embodiment of the invention; and

Fig. 7 illustrates association between eCCEs and APs for different aggregation levels using up to two antenna ports according to an embodiment of the invention. Detailed Description of the Invention

To achieve the aforementioned and other objects, the present invention relates to a method in a network control node of a wireless communication system which employs scheduling of PRBs used for each new transmission of a control channel signal. Furthermore, each control channel signal is related to one or more CCEs which are transmitted over one or more antenna ports (e.g. the number of antenna ports used in LTE systems dedicated to demodulation reference signals).

The present method comprises the step of transmitting CCEs related to a control channel signal over two or more antenna ports of the network control device, wherein each transmitted CCE is transmitted over only one associated antenna port independent of the number of CCEs related to the control channel signal. This means that a specific CCE cannot to be transmitted on two different antenna ports. There is thus a mapping between the CCEs and antenna ports with the above limitation of having a fixed relation between the CCEs and antenna ports. It should be recalled that if only one antenna port is used for ePDCCH transmission with unreliable PMI feedback, it is possible to fail an ePDCCH (control channel signal) transmission. If several antenna ports are used to transmit ePDCCH where each antenna port has a different precoding vector there is less probability to fail ePDCCH transmission.

Therefore, to transmit a localized ePDCCH robustly, one possible method is to use multiple APs for a given ePDCCH. However, if PMI feedback is available and reliable for a given ePDCCH, it can also achieve the so-called precoding gain with multiple APs sharing the same selected precoding vector. So, with multiple APs allocated for a given ePDCCH transparency between transmission using multiple precoding vectors and using single precoding vector can be achieved. This means that an eNB has great flexibility to transmit a localized ePDCCH to a given UE using single or multiple precoding vectors transparently. This means that the UE can use the same detection method to decode ePDCCH transparently without knowing the number of used precoding vectors.

Hence, in order to reduce the detection complexity for a given ePDCCH, the solution according to the invention is that each of the eCCEs (e.g. in one PRB pair) should always be associated with one fixed AP for all the possible aggregation levels. That is, for a given UE the associated ePDCCH symbols on one of the eCCEs should always be transmitted only by the associated AP no matter which aggregation level is actually used. Therefore, channel estimation and ePDCCH detection on the resource elements in any one of the eCCEs are always implemented only on the associated fixed antenna port. That is, the UE doesn't need to repeat ePDCCH detection and channel estimation on the same eCCE over different APs when blindly detecting an ePDCCH with different possible aggregation levels. Then, for blind detection of an ePDCCH, those detected symbols on each of eCCEs will be aggregated according to the assumed aggregation levels and are then fed to the channel decoder.

For example, with 4 APs available for a specific UE, the associations between eCCEs and APs for a given ePDCCH are shown in Fig. 6. Here, eCCEs 1 to 4 are always associated with AP 7 to AP 10, respectively, independent of the aggregation level. For an ePDCCH of 1 eCCE, it might be transmitted on any one of the eCCEs over the associated AP. An ePDCCH aggregated by 2 eCCEs might use eCCE 1 on AP 7 and eCCE 2 on AP 8, or eCCE 3 and 4 on AP 9 and 10, respectively, which can use up to two different precoding vectors. An ePDCCH aggregated by 4 eCCEs might be transmitted by up to four different precoding vectors.

Since eCCEs 1 to 4 are always one-to-one associated (mapped) to AP 7 to AP 10 independent of the aggregation level, the UE doesn't need to repeat ePDCCH detection and channel estimation on the same eCCE over different APs for different aggregation levels. As shown in Table 2, the maximum numbers of operations in one PRB pair for a given ePDCCH with 4 APs can be reduced compared with prior art solutions. Here, only 4 ePDCCH detection operation and 4 channel estimation operation in total are needed instead of 8 ePDCCH detection operation and 8 channel estimation operation in Table 1. If only one unique association between eCCEs and antenna ports is considered creating one single configuration there is no need to signal this association to the UE.

According to another embodiment with 2 APs available for a specific UE, the associations between eCCEs and APs for a given ePDCCH are as shown in Fig. 7. In this case, eCCE 1 and 2 are always associated with AP 7, while eCCE 3 and 4 are always associated with AP 9. For an ePDCCH of 1 eCCE, it might be transmitted on any one of the eCCEs over the associated AP. An ePDCCH aggregated by 2 eCCEs might use eCCE 1 and 2 on AP 7, or use eCCE 3 and 4 on AP 9. An ePDCCH aggregated by 4 eCCEs might use up to two different precoding vectors. In Fig.7, since each eCCE is always associated with one of AP 7 and 9 independent of the aggregation level, the UE doesn't need to repeat ePDCCH detection and channel estimation on eCCE 3 and 4 over different APs for different aggregation levels as in prior art. All the possible configurations regarding different associations (mapping) between eCCEs and antenna ports can be specified e.g. in a wireless communication system standard. For a given ePDCCH, one configuration is used which can be signalled via PDCCH or as a function of the UE identification number in the network. For example, one possible configuration related to Fig.7 consists of associating first and second eCCEs to antenna port 7 and third and forth eCCEs to antenna port 9. Usage of antenna port 8 and 10 can be also considered which denotes another configuration. These are only two examples and different associations using one, two or four antenna ports are also possible. More precisely, the association between eCCEs and the antenna ports can be UE- specifically configured. According to this embodiment of the invention two different UEs can be configured with two different configurations using different orthogonal antenna ports. Therefore, these two UEs can receive simultaneously their own control channel information. It means that they can be paired in Multiple User MIMO (MU-MIMO) transmission. It is realised that this embodiment is not limited to two different UEs but may concern a plurality of UEs having their own user specific configuration.

Regarding the mapping between CCEs and antenna ports, two different embodiments have been discussed. According to one embodiment only one CCE is transmitted over one specific antenna port, which implies a one-to-one mapping between CCEs and antenna ports. This embodiment is illustrated in figure 6. The advantage of this scheme resides in the number of used precoding vectors. As several precoding vectors are used, if for some reason PMI feedback is not available or is unreliable, there is less probability to fail ePDCCH transmissions compared to prior art.

However, according to another embodiment two or more CCEs are transmitted over one specific antenna port, and this embodiment is illustrated in figure 7. The advantage with this embodiment is that ePDCCHs for different users can be transmitted simultaneously by using different configured antenna ports.

Preferably, the wireless communication system is a 3 GPP cellular wireless communication system; and the control channel signal corresponds to an ePDCCH and the CCEs are eCCEs. Further, the network control node is preferably a base station or a relay node or any other device having the suitable capabilities (functions) for downlink transmission of control channel signals in PRBs which are scheduled for each new transmission of control channel signals.

Furthermore, as understood by the person skilled in the art, any method according to the present invention may also be implemented in a computer program, having code means, which when run by processing means causes the processing means 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 comprises of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive. Moreover, the present invention also relates to a corresponding network control node device, such as a base station device or a relay node device or a similar device. The network control node device according to the invention is arranged to transmit CCEs related to a control channel signal over two or more antenna ports, and each of the CCEs is transmitted over only one associated antenna port of the two or more antenna ports independent of the number of CCEs related to the control channel signal.

The network control node device can be modified, mutatis mutandis, according to the different embodiments of the corresponding methods above. Finally, it should be understood that the present invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.

Claims

1. Method in a network control node for transmission of control channel signals in a wireless communication system, said wireless communication system employing scheduling of physical resource blocks (PRBs) used for each new transmission of a control channel signal, and wherein each control channel signal is related to one or more control channel elements (CCEs) transmitted over one or more antenna ports; said method comprising the step of:

- transmitting control channel elements (CCEs) related to a control channel signal over two or more antenna ports, wherein each of said control channel elements (CCEs) is transmitted over only one associated antenna port of said two or more antenna ports independent of the number of control channel elements (CCEs) related to said control channel signal.

2. Method according to claim 1, wherein said control channel elements (CCE) are distributed over one physical resource block (PRB) pair.

3. Method according to claim 1, wherein only one control channel element (CCE) is transmitted over one specific antenna port, i.e. one-to-one mapping between control channel elements (CCEs) and antenna ports.

4. Method according to claim 1, wherein two or more control channel elements (CCEs) are transmitted over one specific antenna port.

5. Method according to claim 1, wherein said two or more antenna ports are user- specific.

6. Method according to claim 1, wherein the association between antenna ports and control channel elements (CCEs) is user-specific.

7. Method according to claim any of the preceding claims, wherein said wireless communication system is a 3 GPP cellular wireless communication system.

8. Method according to claim 7, wherein said control channel signal corresponds to an enhanced physical downlink control channel (ePDCCH) and said control channel elements (CCEs) are enhanced control channel elements (eCCEs).

9. Method according to claim 1, wherein said network control node is a base station or a relay node.

10. Computer program, characterised in code means, which when run by processing means causes said processing means to execute said method according to any of claims 1-9.

11. Computer program product comprising a computer readable medium and a computer program according to claim 10, wherein said computer program is included in the computer readable medium, and comprises of one or more from the group: ROM (Read-Only Memory), PROM (Programmable ROM), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically EPROM) and hard disk drive.

12. Network control node device arranged for transmission of control channel signals in a wireless communication system, said wireless communication system employing scheduling of physical resource blocks (PRBs) used for each new transmission of a control channel signal, and wherein each control channel signal is related to one or more control channel elements (CCEs) transmitted over one or more antenna ports; said network control node device comprising at least two antenna ports and being further arranged to: