1. Field of the Invention
The present invention relates to a wireless cellular communication system with at least one enhanced Node B (eNB) and at least one User Equipment (UE). More particularly, the present invention relates to a wireless communication system in which multiple physical antennas are represented by logical antenna ports in reference signals.
Throughout the following description of exemplary embodiments of the present invention, the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8-10 is regarded as a legacy system whereas the in-development Release 11 and future releases are considered to be systems in which exemplary embodiments of the present invention can be implemented. However, this is not intended to be a limitation of the invention or its application and it is to be understood that the current invention can also be applied to other cellular systems where appropriate.
2. Description of the Related Art
Generally, mobile communication systems have been developed to provide a voice communication service to users on the move. As time has progressed, mobile communication systems have evolved to support data communication services as well as standard voice communication services, and can now also support high speed data communication services. However, there is a need for more sophisticated mobile communication systems to mitigate resource shortages and to meet the high-speed service requirements of users.
The LTE system is a next generation broadband communication technology developed by the 3GPP in order to meet such requirements. The LTE system is a technology for realizing high-speed packet-based communication at up to 100 Mbps. To achieve these requirements, discussions are being held on various aspects. For example, discussions are being held regarding one scheme for reducing the number of nodes located in a communication path by simplifying a configuration of the network, and another scheme for maximally approximating wireless protocols to wireless channels.
In the aforementioned LTE wireless communication system, at least two kinds of reference signals are defined.
The first kind of reference signal is referred to as a Common Reference Signal (CRS). CRS is cell specific, and all the UEs connecting to the eNB can use CRS for demodulation when CRS-based transmission is configured.
The second kind of reference signal is referred to as a DeModulation Reference Signal (DMRS). DMRS is UE specific. That is, the UE will use the DMRS within its allocated resources for demodulation of the said allocation resources, where the DMRS and the data are precoded with the same weights among antenna ports.
The control channel is usually transmitted in the beginning of a sub-frame in order that the UE can efficiently acquire the scheduling information as quickly as possible. Considering the 3GPP LTE as an example, the Physical Downlink Control CHannel (PDCCH) is configured to be transmitted in the first one to four Orthogonal Frequency Division Multiplexing (OFDM) symbols in a sub-frame.
FIG. 1 illustrates a subframe structure with 2-OFDM-symbol PDCCH and DMRS with ports 7˜14 configured according to the related art.
Referring to FIG. 1, the ports 7˜10 use a spreading factor of 2 to multiplex two DMRS ports on two consecutive Resource Elements (REs) in the time domain. Ports 11˜14 use the same resource as ports 7˜10 but use a spreading factor of 4 for to multiplex 4 DMRS ports on the four consecutive REs in a subcarrier.
To increase the capacity of the legacy PDCCH, the Enhanced Control CHannel (E-CCH) is proposed to be allocated in the legacy Physical Downlink Scheduling CHannel (PDSCH) region. DMRS based transmission should be supported for E-CCH since E-CCH should work for those special Multicast-Broadcast Single Frequency Network (MBSFN) subframes where CRS is absent. The E-CCH corresponds to Enhanced Physical Downlink Control CHannel (E-PDCCH) described in LTE standard specification.
In the legacy system, DMRS is used for decoding of PDSCH. The characteristics of the DMRS configurations, including the number of DMRS ports and scrambling sequence ID, are indicated to the UE using Downlink Control Information (DCI) in the PDCCH. However, if DMRS is used for E-CCH transmission, the DMRS configuration cannot be previously indicated by the control channel itself. Thus, a predefined configuration or implicit indication should be enabled for the UE to obtain DMRS configurations.
There are basically two kinds of E-CCH structures.                Interleaved mode: an Enhanced Control Channel Element (E-CCE) contains REs distributed in multiple Resource Blocks (RBs);        Localized mode: an E-CCE contains REs within one RB        
Exemplary embodiments of the present invention focus on the case of E-CCH with localized E-CCE distribution.
FIG. 2 illustrates an E-CCE localized structure in an RB, where 4 E-CCEs are allocated in one Physical Resource Block (PRB) in a subframe, according to the related art.
Referring to FIG. 2, a logical E-CCE can contain a set of either consecutive or distributed physical REs in the RB. FIG. 2 also illustrates a physical structure, where an E-CCE consists of REs in three distributed subcarriers, according to the related art. The allocated REs for E-CCH exclude those REs allocated for legacy PDCCH, and any type of reference signals when configured.
Therefore, a need exists for methods of allocation of DMRS resources and implicit indication of DMRS configuration for E-CCH transmission.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.