Resource configuration for EPDCCH

A system and method for providing both localized and distributed transmission modes for EPDCCH is disclosed, where one EPDCCH comprises of one or multiple CCEs. Localized versus distributed transmission may be defined in terms of the EPDCCH to CCE resource mapping. In a localized transmission CCEs are restricted to be contained within one PRB. In a distributed transmission a CCE spans over multiple PRBs. A UE can be configured to either receive the EPDCCH only in localized or only in distributed transmissions. A UE can also be configured to expect EPDCCH transmissions in both localized and distributed transmissions. In each PRB configured by the higher layer as an EPDCCH resource, 24 REs that may be used for any DMRS transmission are always reserved and not used for EPDCCH transmission.

BACKGROUND

In LTE networks, downlink control information (DCI) is transmitted to user equipment (UE) in the Physical Downlink Control Channel (PDCCH). The LTE UE obtains the resource allocations for uplink and downlink transmissions from the PDCCH. The PDCCH onto which the DCI is mapped has different formats and depending on its size is transmitted in one or more control channel elements (CCE). A CCE corresponds to thirty-six resource elements (REs).

In LTE Release 8-10, legacy PDCCH is transmitted and demodulated based on cell-specific reference signals (CRS). A legacy PDCCH is precoded with transmit diversity on 1/2/4 CRS antenna ports, cross-interleaved with other PDCCHs, and then distributed over the entire system bandwidth in the legacy control region of a subframe. The legacy control region comprises of the first K orthogonal frequency division multiplexing (OFDM) symbols in the first slot of a subframe, where K=1, 2, 3 for system bandwidths greater than 10 Physical Resource Blocks (PRBs), and K=2, 3, 4 otherwise. The control region size K is signaled in the Physical Control Format Indicator Channel (PCFICH). Through CRS-based transmit diversity and cross-interleaving, legacy PDCCH exploits spatial and frequency diversity to achieve robustness and ensures its reliable reception and cell coverage.

In LTE Release 10, a new PDCCH transmission scheme is introduced for the relay backhaul link, called R-PDCCH. R-PDCCH inherits all the DCI formats of legacy LTE systems (i.e., DCI formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 4). R-PDCCH is transmitted in the Physical Downlink Shared Channel (PDSCH) region to exploit higher control channel capacity. R-PDCCH can be transmitted with both CRS-based and Demodulation Reference Signal (DMRS)-based transmission. For DMRS-based R-PDCCH, transmission is based on single-layer beamforming on DMRS antenna port7with a scrambling ID (SCID) of 0. R-PDCCH is transmitted in 1/2/4/8 Virtual Resource Blocks (VRBs), which are dynamically selected from a subset of NVRBR-PDCCHVRBs semi-statically configured by a higher layer.

An enhanced PDCCH (EPDCCH) is introduced in LTE Release 11 to expand on R-PDCCH. Similar to R-PDCCH, the EPDCCH is transmitted in the PDSCH region to achieve higher control channel capacity. The higher layer configures a set of PRBs as the EPDCCH resources. The PRBs provide a physical grouping of subcarriers and OFDM symbols. The EPDCCH reuses the concept of CCEs to map the EPDCCH onto the PRBs configured by the higher layer for EPDCCH transmission. Demodulation of EPDCCH is based on DMRS transmitted in the PRBs that are used for transmission of the enhanced control channel. EPDCCH is not demodulated by CRS such that it can be transmitted in non-backward-compatible carriers and Multicast-Broadcast Single Frequency Network (MBSFN) subframes. Antenna ports7-10or subsets thereof are used with EPDCCH.

SUMMARY OF THE INVENTION

A system and method for providing both localized and distributed transmission modes for EPDCCH is disclosed, where one EPDCCH comprises of one or multiple CCEs. Localized versus distributed transmission may be defined in terms of the EPDCCH to CCE resource mapping. In a localized transmission CCEs are restricted to be contained within one PRB. In a distributed transmission a CCE spans over multiple PRBs. A UE can be configured to either receive the EPDCCH only in localized or only in distributed transmissions. A UE can also be configured to expect EPDCCH transmissions in both localized and distributed transmissions. In each PRB configured by the higher layer as an EPDCCH resource, 24 REs that may be used for any DMRS transmission are always reserved and not used for EPDCCH transmission.

DETAILED DESCRIPTION

Both localized and distributed transmission modes are possible for EPDCCH. Localized transmission is primarily used to reap scheduling gain (e.g. in frequency-selective channels) and beamforming gain (e.g. in correlated antenna setup). Distributed transmission is mainly used to achieve diversity gain and to improve link robustness (e.g. in uncorrelated antenna setup and at high Doppler where CSI feedback is unreliable). The difference between “localized” and “distributed” transmissions may be defined in different ways.

In one definition, localized versus distributed transmission is defined in terms of the EPDCCH resource configuration. If the configured EPDCCH PRB resources are adjacent in the frequency domain, it is a localized transmission. When the PRB resources are not adjacent, it is a distributed transmission.

Since the EPDCCH PRB resources are semi-statically configured by a higher layer, it does not matter from the EPDCCH demodulation perspective whether the PRBs are adjacent in frequency or not. UE behavior would be exactly the same in both cases. Because the 3GPP specification is defined in terms of UE behavior, it is unclear if such a definition warrants two EPDCCH transmission schemes in the specification.

In another definition, localized vs. distributed is defined in terms of EPDCCH resource mapping. With distributed transmission, an EPDCCH for a particular user is transmitted across a set of PRBs even if a smaller set of PRBs would be sufficient for transmitting that EPDCCH. With localized transmission, an EPDCCH is transmitted in the smallest number of PRBs required for transmitting that EPDCCH.

FIG. 1illustrates localized versus distributed transmission of EPDCCH according to one embodiment. For localized transmission, EPDCCH101for a particular user is restricted to the minimum set of PRBs102(i.e., one PRB) required to transmit the control information intended for that user. With distributed transmission, EPDCCH103for a particular user is transmitted in pieces103a-nthat are spread across a set of PRBs104-107. EPDCCH103is divided across each of PRBs104-107even if a smaller set of PRB would be sufficient for transmitting that EPDCCH103(e.g., one PRB).

A direct result of this mapping is that one PRB may be shared by multiple EPDCCHs. In either the localized or the distributed case, each PRB may contain a part of multiple EPDCCHs. This second definition is used herein for defining localized and distributed modes.

To distribute an EPDCCH into multiple PRBs, the EPDCCH is divided into smaller resource units. An EPDCCH is divided into multiple control channel elements (CCE), each of which comprises of a set of resource elements (RE), e.g., thirty-six. Optionally, each CCE can be further divided into smaller-size resource element groups (REG) each comprising a fixed number of resource elements. As a result, distributed transmission may be achieved by distributing the EPDCCH CCEs/REGs into multiple PRBs. In order to achieve more frequency diversity, the CCEs/REGs can further be interleaved before being distributed to the PRBs.

One method for EPDCCH resource assignment is through semi-static radio resource control (RRC) configuration, where the set of CCE/REG resources to which an EPDCCH is mapped are higher-layer configured semi-statically and UE-specifically. On the other hand, it is possible for the set of CCE/REG resources on which an EPDCCH is mapped to be dynamically adapted across different subframes. That is, an eNodeB higher layer semi-statically signals a set of EPDCCH configuration parameters, while the exact CCE/REG resources for a particular EPDCCH in each subframe vary as a function of the semi-statically signaled configuration parameters and other dynamic parameters (e.g., subframe index, UE-ID, etc.). This further increases the EPDCCH diversity.

The CCE/REG resources in the configured EPDCCH PRB resources may be indexed using different methods as described below.

In one embodiment, the CCEs/REGs are continuously numbered within each PRB and across different PRBs. For example, assume each PRB has two CCEs. The CCEs in a first PRB are numbered “CCE1” and “CCE2,” and the CCEs in the next PRB are numbered as “CCE3” and “CCE4.”

In another embodiment, the CCE/REG resources corresponding to the same CCE/REG position in each PRB are continuously numbered. For example, assume two PRBs are configured as EPDCCH resources and each PRB has two CCEs. The CCEs in the first PRB are numbered as “CCE1” and “CCE3,” and the CCEs in the second PRB are numbered as “CCE2” and “CCE4.”

Similarly, the same principles may be used to map CCEs/REGs for a particular user/DCI to the available CCE/REG resources. In one embodiment, the CCEs/REGs in an EPDCCH are first mapped in the dimension of available CCE/REG resources within a PRB, and then mapped to additional PRBs if more control resources are needed. This may be suitable for localized transmission, for example. In another embodiment, the CCEs/REGs in an EPDCCH are first mapped in the frequency dimension along the configured PRBs resources, and then mapped in the dimension of CCE/REG resources. Such frequency-first mapping achieves more frequency diversity and re-uses existing PDSCH design principles.

The CCE/REG resources in each PRB may be calculated assuming that a 3-8 layer DMRS pattern (24 REs/PRB) is always reserved and not used for EPDCCH transmission. This is because a UE does not know the DMRS pattern for PDSCH transmission prior to receiving any EPDCCH. In other words, even if the actual EPDCCH transmission to one UE uses 2-layer DMRS, which incurs 12 REs/PRB DMRS overhead, EPDCCH transmission shall always exclude the 24REs/PRB DMRS pattern that may be used for 3-8 layer DMRS transmission to any UE.

Similarly, the available CCE/REG resources in each PRB should be calculated assuming that certain resources (such as, for example, channel state information reference signals (CSI-RS) and CRS) are reserved and not used for EPDCCH transmission. Also, if additional enhanced control channels (e.g., enhanced Physical Hybrid-ARQ Indicator Channel (EPHICH), enhanced Physical Control Format Indicator Channel (EPCFICH), or enhanced Physical Broadcast Channel (EPBCH)) are introduced, the resources occupied by such enhanced control signals should be reserved and not used for EPDCCH transmission.

For the sake of discussion herein, assume that one EPDCCH comprises of L CCEs where each CCE comprises some number of resource elements (e.g., 36 resource elements per CCE). L may typically take on values from the set {1, 2, 4, 8}, although other values of L are also possible. It is possible that the CCE formation is different for localized and distributed EPDCCH. For example, a CCE may be restricted to one PRB for localized EPDCCH transmission, but may be allowed to span over multiple PRBs for distributed EPDCCH transmission.

The resource elements available for EPDCCH transmission in each PRB are grouped into M “CCE candidates”. Each CCE candidate contains a set of one or more resource elements.

FIG. 2illustrates one PRB201consisting of two (M=2) CCE candidates202,203. PRB201consists of twelve tones having fourteen OFDM symbols per tone. Certain resource elements in PRB201are not available for EPDCCH transmission, such as resource elements being used for legacy PDCCH204, CRS205, or DMRS206amongst others. The blank resource elements (i.e., REs207) in each CCE candidate202,203may be used for EPDCCH transmission. Additionally, any blank resource elements207that are not used for EPDCCH transmission (i.e., orphan REs) may be used for transmitting other signals, such as PDSCH, EPHICH, EPCFICH, or EPBCH.

It will be understood that in other embodiments, PRB201may be divided into more than two CCE candidates. Additionally, other reference signals (e.g., CSI-RS) may be assigned to the blank resource elements in PRB201, which would need to be considered when mapping EPDCCH.

The LTE DMRS pattern contains two orthogonal code division multiplexed (CDM) groups. It is necessary for each CCE candidate to be associated with at least one set of DMRS, preferably CDM group1, so that demodulation is possible. It is also desirable for each CCE candidate to have the same DMRS symbol density on each DMRS antenna port so that channel estimation performance is balanced.

An EPDCCH of aggregation level L should be mapped to L CCE candidates in the EPDCCH control resources, either in one or multiple PRBs. The UE needs to know the position of the assigned CCE candidates in each PRB in order to decode EPDCCH. This is discussed below separately for localized transmission and distributed transmission.

Alternatively, orphan REs can be used to facilitate frequency domain intercell interference coordination (ICIC). For example, twenty-four REs may be reserved for DMRS within a PRB-pair that is used to transmit EPDCCH. If only twelve of the reserved REs are actually used to transmit DMRS, then the other twelve REs in that PRB-pair, which are not used for DMRS, EPDCCH, or any other signals or channels, remain unused. In one embodiment, one transmission point may configure twelve REs for antenna ports7/8in the PRB-pair (twelve REs corresponding to antenna ports9/10in that PRB-pair have zero-power). Assuming the two transmission points are synchronized in time and frequency, another transmission point may configure twelve REs for antenna ports9/10in the same PRB-pair (twelve REs corresponding to APs7/8in that PRB-pair have zero-power). Accordingly, due to the orthogonal zero-power orphan REs from each transmission point, the DMRS REs experience reduced intercell interference resulting in improved channel estimates and an overall improved system performance.

If all 24 DMRS REs are always reserved in a PRB-pair used for EPDCCH, ICIC can be configured for both localized and distributed transmissions. In one embodiment, one transmission point may use distributed transmission with transmit diversity on antenna ports7/8, and another transmission point may use localized transmission with M=2 CCE candidates associated with antenna ports9/10.

With localized transmission, an EPDCCH is mapped to as few PRBs as possible. For example, L CCEs of one EPDCCH are mapped to L CCE candidates in one EPDCCH PRB resource. As noted above, each PRB comprises M CCE candidates. The EPDCCH may be mapped as follows in one embodiment:

If L≦M, then EPDCCH is mapped to one PRB and occupies the first L available CCE candidates. Alternatively, it is also possible that the L CCE candidates on which the L CCEs are mapped to are configured by a higher layer.

If L>M, then EPDCCH is mapped to ┌L/M┐ PRB. In each of the first (┌L/M┐−1) PRBs, all M CCEs are used for EPDCCH mapping. In the last PRB, (L−(┌L/M┐−1)×M) CCEs are used for EPDCCH mapping.

It is also possible for CCE mapping to use a mechanism similar to LTE Release 8 PDCCH mapping. Assume that the EPDCCH control region consists of a set of CCE candidates numbered from 0 to NCCE−1, where NCCEis the total number of CCE candidates in the configured EPDCCH PRB resources. The set of EPDCCH candidates to monitor are defined in terms of search spaces, where a search space Sk(L)at aggregation level Lε{1, 2, 4, 8} is defined by a set of EPDCCH candidates. The CCE candidates corresponding to EPDCCH candidate M of the search space Sk(L)are given by
L{(Yk+m′)mod └NCCE/L┘}+i(Eq. 1)
where Ykis a hashing parameter as a function of higher-layer configuration, I=0, . . . , L−1, and m′=m, where m=0, . . . , M(L)1. M(L)is the number of EPDCCH candidates to monitor in the given search space. An example of M(L)is given Table 1.

With distributed transmission, one EPDCCH is distributed into multiple PRBs and occupies a set of resource elements in each PRB even though fewer PRBs (including just a single PRB) might suffice. The EPDCCH may be mapped to PRB using several different schemes.

In one embodiment, the EPDCCH for a UE is mapped to N CCE candidates in each PRB. The value of N may be fixed or semi-statically and UE-specifically configured by a higher layer. Hence one EPDCCH is mapped to ┌L/N┐ PRBs. It is possible for N to be dynamically adapted dependent on a subframes/PRB index. For each UE, the set of N CCE candidates in each PRB on which an EPDCCH is mapped semi-statically may be configured by higher-layer RRC signaling.

FIG. 3illustrates distributed mapping of EPDCCH for UEs301and302according to one embodiment. In this example, the N CCE candidates used for EPDCCH transmission may be fixed in each PRB in a UE-specific manner. For example, UE301may be configured to perform blind decoding of EPDCCH of aggregation level L, assuming EPDCCH is mapped to the first CCE candidate303in every PRB304. UE304may be configured to blind decode EPDCCH mapped to the second CCE305candidate in each PRB304. As noted above, instead of dividing the PRB into multiple CCE candidates, the PRB can be divided into other resource units, such as multiple REGs, in other embodiments. Accordingly, UEs301and302may also decode EPDCCH spread across REGs in every PRB.

To further improve the frequency diversity, the CCE candidate used for EPDCCH transmission may be dynamically adapted dependent on the PRB index or subframe index, for example, following a pre-determined hopping pattern or cycling pattern. Assuming EPDCCH of aggregation level L is mapped to L/2 PRBs and occupies N=2 CCE candidates in each PRB. The index of N CCE candidates used for EPDCCH transmission in the l-th PRB could be determined as
lm, . . . , lm+N−1  (Eq. 2)
where
lm=mod(l+└M/N┘×N,M).  (Eq. 3)

For example, referring toFIG. 4, a first UE401may be configured to perform blind decoding of EPDCCH mapped to the 1st/2nd/3rd/4th CCE or REG candidates (i.e.,403-406, respectively) in the 1st/2nd/3rd/4th PRBs (i.e.,407-410, respectively). A second UE402may be configured to blind decode EPDCCH mapped to 2nd/3rd/4th/1st CCE or REG candidates (i.e.,411-416, respectively) in the 1st/2nd/3rd/4th PRBs (407-410, respectively).

A UE may be semi-statically configured to monitor either localized EPDCCH or distributed EPDCCH. For each UE, the eNodeB can configure one of {0, 1, 2} sets of physical resources for EPDCCH transmission. Moreover, the eNodeB can configure a UE to either receive the EPDCCH only in localized or only in distributed transmissions. This is possible because the channel characteristics (e.g. antenna correlation, UE mobility) do not change quickly. Therefore, it is not necessary for a UE to monitor both localized and distributed EPDCCH simultaneously.

Alternatively, the eNodeB can configure a UE to expect EPDCCH transmissions in both localized and distributed transmissions, i.e., it is possible for a UE to monitor both localized and distributed ePDCCH, in different search spaces.

The EPDCCH can be transmitted in a subset of PRBs configured for EPDCCH transmission allowing for coordination among eNodeBs to reduce inter-cell interference. The EPDCCH harnesses multi-user diversity and beamforming gains to increase robustness and system performance.

FIG. 5is a block diagram of a wireless communication network500, which may be an LTE network. LTE utilizes OFDM on the downlink and single-carrier frequency division multiplexing (SC-FDM) on the uplink. LTE partitions system bandwidth into multiple orthogonal subcarriers, which may be referred to as tones or bins. Each subcarrier may be modulated with data, control or reference signals. The wireless network500includes a number of evolved Node Bs (eNBs)501and other network entities. The eNBs501communicate with user equipment devices (UEs)502. Each eNB501provides communication services for a particular geographic area or “cell”503, which may be a macro cell, a pico cell, a femto cell, and/or other types of cell. A network controller504may couple to a set of eNBs501and provide coordination and control for these eNBs501.

UEs502may be stationary or mobile and may be located throughout the wireless network500. UEs502may be referred to as a terminal, a mobile station, a subscriber unit, a station, such as a mobile telephone, a personal digital assistant (PDA), a wireless modem, a laptop or notebook computer, a tablet, and the like. A UE502communicates with an eNBs501serving the cell503in which the UE502is located.

FIG. 6is a high level block diagram of a system600that may be used as an eNB or UE, which may be, for example, an eNBs501or UEs502inFIG. 5. System600receives data to be transmitted from an interface601at transmit processor602. The data may include, for example, audio or video information or other data file information to be transmitted. The transmit processor602may also receive control information, such as EPDCCH, PCFICH, PHICH, or PDCCH, from a controller603. Transmit processor602processes (e.g., encode and symbol map) the data and control information to obtain data symbols and control symbols. The transmit processor602may also perform spatial processing or precoding on the data symbols and/or the control symbols. The output of the transmit processor602is provided to a modem604. Modem604processes the output symbol stream to obtain an output sample stream that is further processed by converting to analog, amplifying, and upconverting before being transmitted via antenna605. In other embodiments, multiple modems604may be used to support multiple-input multiple-output (MIMO) transmission on multiple antennas605.

Signals are also received at system600on antenna605from other devices. The received signals are provided to modem604for demodulation. Modem604processes the signals by filtering, amplifying, downconverting, and/or digitizing, for example, to obtain input samples. Modem604or a receive processor606may further process the input samples to obtain received symbols. Receive processor606then processes the symbols by demodulating, deinterleaving, and/or decoding, for example. Receive processor605then provides decoded data to interface601for use by the eNB or UE. Receive processor further provides decoded control information to controller603.

Controller603may direct the operation of system600in the eNB or UE, such as by mapping EPDCCH to PRBs (i.e., in an eNB) or monitoring localized and distributed EPDCCH in different search spaces (i.e., in a UE). A memory607may store data and program codes for controller603, transmit processor602, and/or receive processor606. Additional components, such as a scheduler608may schedule downlink and/or uplink data transmission by system600(e.g., in an eNB).

FIG. 7is a flowchart illustrating a method or process for mapping EPDCCH to physical resource blocks in a base station according to one embodiment. In step701, downlink control information to be transmitted to user equipment in an enhanced physical downlink control channel is identified. In step702, a set of physical resource blocks to be used as enhanced physical downlink control channel resources are configured by the base station. In step703, the enhanced physical downlink control channel is distributed across resource elements in a subset of the physical resource blocks in the set of physical resource blocks.

The downlink control information may be transmitted using two or more physical resource blocks even if a single physical resource block is sufficient for transmitting the downlink control information. In one embodiment, at least one PRB is shared by two or more EPDCCHs. In another embodiment, each of the set of PRBs contains a part of multiple EPDCCHs. The set of PRBs may be limited to a minimum number of PRBs required to transmit the DCI to the UE.

The resource elements may be grouped into two or more resource partitions in each PRB, and the EPDCCH is then distributed across the resource partitions in each of the physical resource blocks. The resource partitions may be CCEs or REGs, for example. The resource partitions may be restricted to one PRB for localized EPDCCH transmission, but may span over multiple PRBs for distributed EPDCCH transmission.

Resource elements in each PRB may be excluded from transmitting EPDCCH if they are used for demodulation reference signals or channel state information reference signals or cell-specific reference signals.

FIG. 8is a flowchart illustrating a method or process for receiving EPDCCH in PRBs at a UE according to one embodiment. In step801, transmissions are received from a base station. In step802, the enhanced physical downlink control channel information is extracted from resource elements in the transmissions. In step803, the EPDCCH information from multiple resource elements in one or more PRBs is combined to identifying DCI.

The user equipment may be configured to semi-statically monitor localized EPDCCH information, or may be semi-statically configured to monitor distributed EPDCCH information. The user equipment may also be configured to monitor both localized and distributed enhanced physical downlink control channel information in different search spaces.

Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions, and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.