Evolved UTRAN (E-UTRAN), sometimes also referred to as LTE (Long Term Evolution), is a novel radio access technology being standardized by the 3rd Generation partnership Project (3GPP). Only the packet-switched (PS) domain will be supported in E-UTRAN, i.e. all services are to be supported in the PS domain. The standard will be based on OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and SC-FDMA (Single Carrier Frequency Domain Multiple Access) in the uplink.
In the time domain, one subframe of 1 ms duration is divided into 12 or 14 OFDM (or SC-FDMA) symbols, depending on the configuration. One OFDM (or SC-FDMA) symbol consists of a number of subcarriers in the frequency domain, depending on the channel bandwidth and configuration. One OFDM (or SC-FDMA) symbol on one subcarrier is referred to as a Resource Element (RE).
In E-UTRAN no dedicated data channels are used; instead, shared channel resources are used in both downlink and uplink. These shared resources, DL-SCH (Downlink Shared Channel) and UL-SCH (Uplink Shared Channel), are controlled by one or more schedulers that assign different parts of the downlink and uplink shared channels to the UEs for reception and transmission respectively.
The assignments for the DL-SCH and the UL-SCH are transmitted in a control region covering a few OFDM symbols in the beginning of each downlink subframe. The DL-SCH is transmitted in a data region covering the rest of the OFDM symbols in each downlink subframe. The size of the control region is either one, two, three or four OFDM symbols and is set per subframe.
Each assignment for DL-SCH or UL-SCH is transmitted on a physical channel named PDCCH (Physical Downlink Control Channel). There are typically multiple PDCCHs in each subframe and the UEs will be required to monitor the PDCCHs to be able to detect the assignments directed to them.
Groups of resource elements that can be used for the transmission of control channels are referred to as Control Channel Elements (CCEs), and a PDCCH is mapped to a number of CCEs. For example, a PDCCH consists of an aggregation of 1, 2, 4 or 8 CCEs. A PDCCH consisting of one CCE is referred to as a PDCCH at aggregation level 1, a PDCCH consisting of two CCEs is referred to as a PDCCH at aggregation level 2, and so on. Each CCE may only be utilized on one aggregation level at a time. The variable size achieved by the different aggregation levels is used to adapt the coding rate to the required block error rate (BLER) level for each UE. The total number of available CCEs in a subframe will vary depending on several parameters, such as the number of OFDM symbols used for the control region, the number of antennas, the system bandwidth, the PHICH (Physical HARQ Indicator Channel) size etc.
Each CCE consists of 36 REs. However, in order to achieve time and frequency diversity for the PDCCHs, each CCE and its REs are spread out, both in time over the OFDM symbols used for the control region, and in frequency over the configured bandwidth. This is achieved through a number of operations including interleaving, and cyclic shifts etc. These operations are however predefined, and are completely known to the UEs. That is, each UE knows which resource elements make up each CCE, and is therefore able to decode the relevant resource elements in order to decode any desired PDCCH.
The existing system has the disadvantage that, as UEs have no knowledge of where the PDCCHs directed specifically to them are located, each UE has to decode the entire set of possible PDCCHs, i.e. the entire PDCCH space. The entire PDCCH space includes all CCEs on all aggregation levels. This would mean that considerable UE resources are consumed in decoding a large number of PDCCHs, of which only a few were actually directed to them. This will waste the limited UE battery power and hence reduce the UE stand-by time.
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