In LTE Rel-11 (Long Term Evolution, Release 11) a new enhanced downlink control channel has been introduced, the EPDCCH (Enhanced Physical Downlink Control CHannel). The EPDCCH may be used for heterogeneous network operation, where a UE (also referred to as a wireless terminal or user equipment node) with a large cell selection bias is connected to a lower power node (e.g., a pico base station), and high interference from a nearby high power node (e.g., a macro base station) can be reduced/avoided by frequency domain intercell interference coordination (f-ICIC) where the high power node avoids transmitting (or transmits with reduced power) the shared data channel in those resources (i.e., PRB or Physical Resource Block pairs) used by EPDCCH transmissions in the lower power node.
3GPP (3rd Generation Partnership Project) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNBs or enhanced nodeBs) to mobile stations (referred to as user equipment (UE) or wireless terminals) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a resource block (RB), which in its most common configuration includes 12 subcarriers and 7 OFDM symbols (one slot). A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE) as shown in FIG. 1.
Thus, an RB (resource block) may consist of 84 REs (i.e., with 7 OFDM symbols for each of 12 subcarriers). An LTE radio subframe may consist of multiple resource blocks in frequency, with the number of RBs determining the bandwidth of the system and two slots in time as shown in FIG. 2.
Furthermore, the two RBs in a subframe that are adjacent in time may be referred to as an RB pair (resource block pair).
In the time domain, LTE downlink transmissions may be organized into radio frames of 10 ms (milliseconds), with each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms (millisecond).
The signal transmitted by the eNB (base station or macro base station) in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel may distort the transmitted signals from the multiple antenna ports. To demodulate any transmissions on the downlink, a UE may rely on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their position(s) in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
Enhanced Control Signaling in LTE
Messages transmitted over the radio link to UEs or wireless terminals can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages may include commands to control functions such as the transmitted power from a UE, signaling of RBs within which data is to be received by the UE and/or transmitted from the UE, etc.
In Rel-8 (Release 8), the first one to four OFDM symbols, depending on the configuration, in a subframe are reserved to provide such control information, as shown above in FIG. 2. Furthermore, in Rel-11 (Release 11), an enhanced control channel was introduced (EPDCCH), in which PRB (Physical Resource Block) pairs are reserved to exclusively contain EPDCCH transmissions, although excluding from the PRB pair the one to four first symbols that may contain control information to UEs of releases earlier than Rel-11, as shown in FIG. 3.
FIG. 3 illustrates a downlink subframe showing 10 RB pairs and configuration of three EPDCCH regions (i.e., bottom, middle, and upper more darkly shaded regions) of size 1 PBR pair each. The remaining PRB pairs can be used for PDSCH transmissions.
Accordingly, the EPDCCH is frequency multiplexed with PDSCH (Physical Downlink Shared Channel) transmissions contrary to PDCCH (Physical Downlink Control CHannel) which is time multiplexed with PDSCH transmissions. Note also that multiplexing of PDSCH and any EPDCCH transmission within a PRB pair is not supported in LIE Rel-11.
Furthermore, two modes of EPDCCH transmission may be supported, localized EPDCCH transmission and distributed EPDCCH transmission. In distributed transmission, an EPDCCH is mapped to resource elements in up to D (also represented by the variable N) PRB pairs, where D=2, 4, or 8 (the value of D=16 is also being considered in 3GPP). In this way frequency diversity can be achieved for the EPDCCH message as shown in FIG. 4.
In FIG. 4, a downlink subframe shows 4 parts, or enhanced resource element groups (eREG), belonging to an EPDCCH that is mapped to multiple of the enhanced control regions (known as PRB pairs) to provide distributed transmission and frequency diversity.
In localized transmission, an EPDCCH is mapped to one PRB pair only, if the space allows (which is always possible for aggregation level one and two and for normal subframes and normal CP or cyclic prefix length also for level four). If the aggregation level of the EPDCCH is too large, a second PRB pair may be used as well, and so on, using more PRB pairs, until all eCCEs (enhanced Control Channel Elements) belonging to the EPDCCH have been mapped. The number of eCCEs that fit into one PRB pair is given by FIG. 5 illustrating localized transmission.
In FIG. 5, a downlink subframe shows 4 eCCEs belonging to an EPDCCH mapped to one of the enhanced control regions, to achieve localized transmission.
To facilitate the mapping of eCCEs to physical resources, each PRB pair is divided into 16 enhanced resource element groups (eREGs) and each eCCE is split into L=4 or L=8 eREGs for normal and extended cyclic prefix, respectively. An EPDCCH is consequently mapped to a multiple of four or eight eREGs depending on the aggregation level.
The eREGs belonging to an EPDCCH reside in either a single PRB pair (as may be typical for localized transmission) or a multiple of PRB pairs (as may be typical for distributed transmission). An exact division of a PRB pair into eREGs has not yet been decided in 3GPP. One example of a division of a PRB pair into eREGs is illustrated in FIG. 6. Furthermore, it is not yet agreed in 3GPP how L=4 or L=8 eREGs respectively are grouped into the eCCEs. It is also an open question as to how the encoded and modulated symbols of an EPDCCH message are mapped to the REs within the resources reserved by its associated eREGs.
FIG. 6 illustrates a PRB pair of normal cyclic prefix configuration in a normal subframe. Each tile is a resource element where the number corresponds to the eREG it is grouped within. REs with lighter shading correspond to REs belonging to the same eREG indexed with 0.
Allocation of EPDCCH Resources
EPDCCH resources are UE specifically configured in terms of EPDCCH sets. An EPDCCH set is a collection of N (also represented by the variable D) PRB pairs containing 16N/L eCCE, where agreed possible values of N=2, 4, 8. A UE can be configured with K sets simultaneously, and the value N can be different for each of the K sets. A maximum possible value of K has yet to be determined in 3GPP, but a typical value is K=2. Each set is also configured to be of either a localized or distributed type. For example, a UE may be configured with K=2 and N1=4 and N2=8, where the first set is used for localized transmission and the second set is used for distributed transmission. The total number of blind decodes (32 in the case that uplink MIMO is not configured) is split between the K sets. How this split is done has not been decided yet in 3GPP, but one potential solution is to split them as equal as possible between the sets. Hence, a UE will monitor Bi EPDCCH candidates in EPDCCH set i. An illustration for the case of K=3 sets with N=4 PRB pairs each is shown in FIG. 7.
FIG. 7 illustrates the definition of sets and clusters where the number of sets equals the RBG size. The number of RBGs per cluster is in this example is set to four which corresponds to four PRB pairs per set. A distributed EPDCCH transmission is mapped within one set.
Mapping of EPDCCH to RE
Each EPDCCH consists of AL (Aggregation Level) eCCEs where AL is the aggregation level of the message. Each eCCE in turn consists of L eREGs where L=4 or L=8. An eREG is a group of REs which will be defined in 3GPP specification TS 36.211. In each PRB pair, there are 16 eREGs. When EPDCCH collides in mapping with other signals (such as own cell CRS or own cell legacy control region), the other signals have priority, and EPDCCH is mapped around these occupied REs and code chain rate matching is applied. This means that an effective number of available RE per eREG is usually less than the 9 RE, but there may be no/little interference from these colliding signals introduced in the decoding since the EPDCCH is mapped around those.
DMRS for EPDCCH
It has been agreed in 3GPP RAN WG1 that each eCCE in a PRB pair of a localized EPDCCH set is associated with a DMRS (De-Modulation Reference Signal) antenna port (or AP) by specification, as shown by way of example in FIG. 8. Furthermore, it is agreed that in case an EPDCCH message occupies more than one eCCE of a PRB pair (i.e., for higher aggregation levels of localized EPDCCH messages), one of the associated ports is used for its demodulation. The port to use may be, for example, implicitly determined by RNTI (Radio-Network Temporary Identifier) or configured via RRC.
FIG. 8 illustrates an example of eCCE and DMRS port association for a PRB pair within a localized EPDCCH set.
For distributed transmission, it has been agreed that two DMRS ports are used to achieve spatial diversity, and an example is shown in FIG. 9. Each RE of the used eREG is in an alternating manner mapped to either of the two antenna ports to provide spatial diversity of the EPDCCH transmission.
FIG. 9 illustrates an example of an antenna port association for a PRB pair within a distributed EPDCCH set.
It is also agreed that the same scrambling sequence generator as is used for the PDSCH DMRS will be used for the EPDCCH DMRS. It is a working assumption that the generator is initialized by:cinit=([ns/2]+1)·(2X+1)·216+nSCID,where ns is the slot number in a radio frame. Values of X and nSCID are not yet decided. By initializing this generator in different ways, different pseudo-random sequences are obtained which is desirable from an interference perspective when the same DMRS port is transmitted on interfering radio resources (e.g., in neighboring cells). The randomness of the interfering sequence enables interference suppression in the channel estimation process by filtering or averaging the estimates. Configuration of this initialization has not been decided.
Enhanced Control Signaling for Heterogeneous Networks
The same enhanced control region (see for example FIG. 5) can be used in different transmission points within a cell or belonging to different cells, that are not highly interfering with each other. Such inter-deployment-layer interference may be reduced by various interference coordination techniques such as enhanced Inter-cell interference coordination (eICIC), or by Coordinated Multi Point (CoMP) operation introduced in LTE Rel-11.
To increase the UE pick-up area of a low-power node (i.e., the area in which a UE would connect a pico node or pico base station rather than a high power macro node or macro base station), cell range expansion (CRE) can be a powerful tool where a UE is prevented from making a handover to the macro layer (i.e., switching communications from a pico base station to a macro base station) unless the received power from the macro base station exceeds the received low-power node by a configured CRE margin. This effectively increases the “coverage area” of a low-power node. However, for UEs in the so-called cell-range expansion area (i.e., the area where UEs connect to the low-power node, but the macro node is received with a stronger power), it may be useful/essential that macro node reduces/minimizes the interfering signals in the subframes where the network communicates with these UEs.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.