WO2012134535A1

WO2012134535A1 - Enhanced node b and method of transmitting physical-downlink control channels (pdcchs) in a lte-a system

Info

Publication number: WO2012134535A1
Application number: PCT/US2011/054502
Authority: WO
Inventors: Xiaogang Chen; Yuan Zhu; Yi Hsuan; Huaning Niu; Qingua LI; Jong-Kae Fwu
Original assignee: Intel Corp
Current assignee: Intel Corp
Priority date: 2011-04-01
Filing date: 2011-10-01
Publication date: 2012-10-04
Anticipated expiration: 2013-10-01
Also published as: CN103563319B; EP2695345B1; US20140044070A1; CN103563319A; EP2695345A1; US10178605B2; EP2695345A4

Abstract

Embodiments of an eNB and method of transmitting PDCCHs are generally described herein. In some embodiments, the eNB may be configured to transmit PDCCHs that are based on a UE-specific reference signal (i.e., a UE-RS). In these embodiments, the UEs may be able to demodulate their PDCCH using a demodulation reference signal (DM-RS) that is specific to the UE (e.g., instead of a common reference signal). The PDCCHs disclosed herein may be suitable the Uu interface of an LTE-A system.

Description

ENHANCED NODE B AND METHOD OF TRANSMITTING PHYSICAL-DOWNLINK CONTROL CHANNELS (PDCCHS) IN A LTE-A SYSTEM

PRIORITY CLAIM

This application claims the benefit of priority under 35 USC 119(e) to United States Provisional Patent Application Serial No. 61/471,042, filed April 1, 2011, [reference number P37663Z], which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

Embodiments pertain to wireless communications. Some embodiments relate to the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) standard of the third- generation partnership project (3 GPP), Releases 10 and later, known as Long Term Evolution (LTE) Advanced and referred to as LTE-A. Some embodiments relate to the transmission of downlink control channels in LTE-A systems.

BACKGROUND

In a 3 GPP LTE network, user equipment (UE) are provided control information from an enhanced or evolved Node B (eNB) over a downlink control channel, such as a physical- downlink control channel (PDCCH). The current configuration of these control channels utilize excess bandwidth, limit scheduling flexibility, and use a common reference signal for demodulation. This configuration limits coverage and capacity of the downlink control channel.

Thus, there are general needs for an eNB and methods for transmitting downlink control channels that utilize less bandwidth, increase scheduling flexibility, and do not require the use of a common reference signal for demodulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is illustrates an eNB and a plurality of UEs in accordance with some embodiments; FIG. 2 illustrates spatially-multiplexed PDCCHs in accordance with some embodiments; FIG. 3 illustrates PDCCHs for subset-based demodulation in accordance with some embodiments; FIG. 4 illustrates PDCCHs with multi-user downlink control information (DCI) in accordance with some embodiments;

FIG. 5 is a table illustrating the number of control-channel elements (CCEs) and idle resource element per resource block; and

FIG. 6 illustrates a blind decoding search space region in accordance with some embodiments.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims.

FIG. 1 is illustrates an eNB and a plurality of UEs in accordance with some embodiments. The eNB 102 and UE 104 may operate as part of an LTE-A network, such as radio-access network (RAN) 106 and may communicate with each other using a plurality of channels. The eNB 102 may communicate with a plurality of UEs 104 using an orthogonal frequency division multiple access (OFDMA) technique in the downlink. The UEs 104 may communicate with the eNB 102 using a single-carrier frequency division multiple access (SC-FDMA) technique in the uplink although this is not a requirement. Downlink channels may include a physical downlink shared channel (PDSCH), and a physical broadcast control channel (PBCCH), and the PDCCH, among others. Uplink channels may include a random access channel (RACH), a physical uplink control channel (PUCCH) and a physical uplink shared channel (PUSCH), among others. The PUSCH is generally shared by several UEs for transmitting information data to the eNB 102 in accordance with the SC-FDMA technique. A PDCCH may be transmitted to each UE 104 to provide control information to a particular UE 104 as discussed in more detail below.

In accordance with some embodiments, the eNB 102 may be configured to transmit PDCCHs that are based on a UE-specific reference signal (i.e., a UE-RS). In these embodiments, UEs 104 may be able to demodulate their PDCCH using a demodulation reference signal (DM- RS) that is specific to the UE (e.g., instead of using a common reference signal). The PDCCHs disclosed herein may be suitable for the Uu interface in an LTE-A system. The Uu interface is the radio interface between a UE 104 and the RAN 106. In some embodiments, methods of transmitting various PDCCHs are provided. In some other embodiments methods of decoding various PDCCHs are provided.

In accordance with some embodiments, the eNB 102 may be configured to transmit spatially-multiplexed (SM) PDCCHs in a single resource block (RB) to a plurality of UEs 104. In these embodiments, a plurality of spatial-division multiplexed (SDM) layers may be transmitted within the RB and each layer may include a different DM-RS transmitted in non- orthogonal resource elements. Each layer may also include a different PDCCH transmitted on orthogonal resource elements of the RB. In these embodiments, the DM-RSs and the PDCCHs are precoded for an associated layer based on precoding-matrix indicators (PMIs) received from the UEs 104. These embodiments are described in more detail below in reference to FIG. 2.

In accordance with some other embodiments, the eNB 102 may be configured to transmit subset-based PDCCHs in a single RB to a plurality of UEs 104. In these embodiments, a plurality of PDCCHs may be transmitted within a single RB such that each PDCCH is transmitted on a different subset of resource elements of the RB. For each PDCCH, transmitting a DM-RS may be transmitted within the RB such that each DM-RS is transmitted on an orthogonal subset of resource elements of the RB. These embodiments are described in more detail below in reference to FIG. 3.

In accordance with some other embodiments, the eNB 102 may be configured to transmit PDCCHs with multi-user downlink control information (i.e., a MU-DCI) in a single RB to a plurality of UEs 104. In these embodiments, a plurality of SDM layers may be transmitted within a RB. At least a first of the layers may include at least one PDCCH transmitted in orthogonal resource elements of the RB. A second of the SDM layers may include at least one other PDCCH transmitted in the same resource elements as one of the PDCCHs of the first of the layers. A different DM-RS for each PDCCH may be transmitted. Each DM-RS may be transmitted on an orthogonal subset of resource elements of the RB within corresponding symbol-duration times. In an example embodiment, a first and a second PDCCH in the first layer may be spatially multiplexed to construct a MU-DCI. A third PDCCH transmitted in the second layer may be a single user DCI (SU-DCI), although this is not a requirement. In this example, both a MU-DCI and a SU-DCI can exist in the same RB. These embodiments are described in more detail below in reference to FIG. 5.

In accordance with some embodiments, the eNB 102 may utilize two or more antennas and the UEs 104 may utilize two or more antennas to allow for multiple-input multiple output (MIMO) communications. In some embodiments, the eNB 102 may utilize up to eight or more antennas and may be configured for multi-user (MU) MIMO (MU-MIMO) communications. In these embodiments, symbols for each UE 104 may be specifically precoded prior to downlink transmission on the PDSCH.

The eNB 102 and the UEs 104 may include several separate functional elements, one or more of the functional elements may be combined and may be implemented by combinations of software-configured elements, such as processing elements including digital signal processors (DSPs), and/or other hardware elements. For example, some elements may comprise one or more microprocessors, DSPs, application specific integrated circuits (ASICs), radio-frequency integrated circuits (RFICs) and combinations of various hardware and logic circuitry for performing at least the functions described herein. In some embodiments, the functional elements may include one or more processes operating on one or more processing elements. In addition, the eNB 102 and the UEs 104 may each include physical-layer circuitry for transmitting and receiving radio-frequency signals and media-access control (MAC) layer circuitry for controlling access to the wireless medium.

FIG. 2 illustrates spatially-multiplexed PDCCHs in accordance with some embodiments. In FIG. 2, different layers 211, 212, and 213 of RB 207 are illustrated. A first PDCCH 221 is transmitted at a first layer 211 on a first set of resource elements 205, a second PDCCH 222 is transmitted at a second layer 212 on a second set of resource elements 205, and a third PDCCH 223 is transmitted at a third layer 213 on a third set of resource elements. Accordingly, the resource elements of each PDCCH 221 , 222 and 223 are orthogonal in both time and frequency, as well as separated spatially (i.e., by precoding) since they are transmitted at different layers. Although FIG. 2 illustrates only three PDCCHs for transmission to three UEs 104 (FIG. 1), the scope of the embodiments is not limited in this respect and is applicable to the transmission of many PDCCHs to many UEs.

The use of spatial-domain multiplexing or spatial-division multiplexing for PDCCH transmissions provides for an increase in system capacity providing for the allocation of resources of more than one UE in a single RB. In these embodiments, both the eNB 102 (FIG. 1) and the UEs 104 may utilize two or more antennas to implement spatial multiplexing. In some embodiments, (i.e., 8Tx embodiments) the eNB 102 may utilize up to eight or more antennas.

In the example illustrated in FIG. 2, each resource element (RE) 205 may comprise one subcarrier for a duration of one OFDM symbol. Each RB 207 may span several (e.g., up to twelve or more) subcarriers in the frequency direction 217 and may have a duration of one or two slots (e.g., up to fourteen or more OFDM symbols) in the time direction 219. In the illustrated embodiments, each RB 207 may comprise two slots (shown as slot 0 and slot 1) in which each slot has a duration of seven OFDM symbols ( shown as 1=0 through 1=6). A LTE- configured radio frame (e.g., a downlink subframe), for example, may comprise a set of many RBs 207 that are orthogonal in time and in frequency.

In accordance with some embodiments, the eNB 102 may be configured to transmit spatially-multiplexed PDCCHs 221, 222 and 223 in a single RB 207 to a plurality of UEs 104. In these embodiments, the eNB 102 may transmit within the RB 207 a plurality of SDM layers 21 1, 212, and 213. Each layer 211, 212, and 213 may include a different DM-RS 231 232 or 233 that is transmitted in non-orthogonal resource elements 205 of the RB 207. Each layer 21 1, 212, and 213 may include a different PDCCH 221, 222 or 223 transmitted on orthogonal resource elements 205 of the RB 207. In these embodiments, each DM-RS 231, 232 and 233 and each PDCCH 221, 222 and 223 may be precoded for an associated layer 21 1, 212, or 213 based on PMIs received from the UEs 104.

In some embodiments, the DM-RSs 231, 232 or 233 are used by the UEs 104 to demodulate downlink control information (DCI) carried by their PDCCH 221, 222 and 223. The DCI may include, among other things, resource assignments and control information for the associated UEs 104. In these embodiments, each UE 104 may have a dedicated DM-RS. The resource assignments in each PDCCH 221, 222 and 223 may include a resource allocation (RA) for one of the UEs 104. The RA may be an allocation of resources on a shared data channel (i.e., a scheduling grant on the PDCCH) for subsequent resources on the PDSCH.

In some embodiments, each PDCCH may be transmitted by one or more control channel elements (CCEs). The number of CCEs used for transmission of a particular PDCCH may be determined by the eNB 102 and may be based on channel conditions. Each CCE may correspond to physical REs of a resource element group.

In some embodiments, legacy control information 201, a common reference signal (CRS) 202, and a channel-state information reference signal (CSI-RS) 203 may be transmitted in orthogonal resource elements 205 of the RB 207 across the plurality of layers 211, 212, 213 without precoding. The legacy control information 201, CRS 202, and the CSI-RS 203 are transmitted without precoding, while the DM-RSs and the PDCCHs are precoded. In some embodiments, the legacy control information 201, CRS 202, and the CSI-RS 203 may be transmitted on the particular REs of a RB 207 as illustrated in FIG. 2. Each the DM-RS 231, 232 or 233 may also be transmitted on the particular REs 205 of a RB 207 as illustrated in FIG. 2.

In some embodiments, each DM-RS 231, 232 and 233 may comprise a UE-specific reference signal (UE-RS). In these embodiments, each UE 104 has a unique DM-RS that is used for demodulating its control information (e.g., the DCI). Prior to transmitting the PDCCHs, the eNB 102 may be configured to map a first PDCCH 221 to a first set of resource elements at the first layer 211, map a second PDCCH 222 to a second set of resource elements at the second layer 212, and map a third PDCCH 223 to a third set of resource elements at the third layer 213. The first, second and third sets of resource elements may comprise resource elements that are orthogonal in both time and frequency. In these embodiments, PDCCH information bits may be mapped to physical resource elements on a set of OFDM symbols, as indicated by a physical control format indicator channel (i.e., the PCFICH). In these embodiments, nothing is transmitted in the first set of resource elements at the second and third layers, nothing is transmitted in the second set of resource elements at the first and third layers, and nothing is transmitted in the third set of resource elements at the first and second layers. This may reduce interference between each PDCCH.

In some embodiments, the mapping may include excluding REs at all layers that are reserved for the DM-RSs 231, 232 or 233 and excluding REs at all layers reserved for the legacy control information 201, the CRS 202 and the CSI-RS 203. In some LTE-A embodiments, the single RB comprises twelve subcarriers for a duration of up to fourteen OFDM symbols of two slots, although the scope of the embodiments is not limited in this respect as embodiments are applicable to other RB configurations.

FIG. 3 illustrates PDCCHs for subset-based demodulation in accordance with some embodiments. In these embodiments, the eNB 102 (FIG. 1) may be configured for transmission of subset-based PDCCHs in a single RB 307 to a plurality of UEs 104 (FIG. 1). The eNB 102 may transmit a plurality of PDCCHs 321, 322 and 323 within single RB 307 such that each PDCCH is transmitted on a different subset of resource elements 305 of the RB 307. For each PDCCH, the eNB 102 may transmit a DM-RS 331, 332 or 333 within the RB 307. Each DM-RS may be transmitted on an orthogonal subset of resource elements 305 of the RB 307 as illustrated.

In these embodiments, each DM-RS 331, 332 and 333 is transmitted on a different subcarrier frequency 351, 352 or 353 within corresponding symbol-duration times. The different subsets of resource elements 305 that are used for the transmission of the PDCCH are orthogonal in both frequency and time.

In these embodiments, each PDCCH 321, 322 and 323 is transmitted by one or more CCEs depending on the aggregation level. For an aggregation level of one, each PDCCH 321, 322 and 323 may be transmitted by one CCE. For an aggregation level of two, each PDCCH 321, 322 and 323 may be transmitted by two CCEs, etc. In some embodiments in which a UE 104 provides wideband PMI feedback, the eNB 102 may be configured to precode the PDCCH with the wideband PMI. In case when more than one layer is used for PDSCH transmission, the eNB 102 may be configured to use a precoding vector in the first layer of the precoder indicated by the PMI as the precoder for PDCCH.

In embodiments in which a UE 104 provides subband PMI feedback, if the PDSCH overlaps with the PDCCH, the precoder used for the PDCCH may be configured to align with the precoder for the PDSCH in the same virtual resource block (VRB). If the PDSCH is transmitted by more than one layer, the same scheme used in wideband feedback may be applied. If the PDSCH does not overlap with the PDCCH, the eNB 102 may determine the

implementation. In these embodiments, the same precoder may be used if the PDSCH overlaps with the PDCCH. In some embodiments, the PDCCH may use the precoder for the first layer if the PDSCH is transmitted with more than one layer.

FIG. 4 illustrates PDCCHs with multi-user DCI in accordance with some embodiments. In these embodiments, eNB 102 (FIG. 1) may be configured to transmit PDCCHs 421 and 423 with a multi-user DCI in a single RB 407 to a plurality of UEs 104. In these embodiments, the eNB 102 may transmit within the RB 407 a plurality of SDM layers 411 and 412. At least a first of the layers 411 may include two PDCCHs 421 , 423 transmitted in orthogonal resource elements of the RB 407. A second of the SDM layers 412 may include at least one other PDCCH 422 transmitted in the same resource elements as one of the PDCCHs of the first of the layers 41 1. The eNB 102 may transmit a different DM-RS 431, 432 and 433 for each PDCCH within the RB 407. Each DM-RS 431, 432 and 433 may be transmitted on an orthogonal subset of resource elements of the RB 407 within corresponding symbol-duration times. In other words, each DM- RS 431, 432 and 433 may be transmitted during the same symbol times (shown horizontally in FIG. 4) but on different subcarriers (shown vertically in FIG. 4). As illustrated in FIG. 4, DM-RS 431, 432 and 433 are transmitted during symbols five and six of slot one and symbols five and six of slot two.

In some embodiments, the PDCCHs 421 and 422 of the first layer may be precoded with a same precoding vector, and the PDCCH 423 of the second layer may be precoded 412 with a different precoding vector. The DM-RSs 431, 432 and 433 may be precoded by the

corresponding precoder. In these example embodiments, DM-RS 431 and DM-RS 432 may both be precoded with a first precoder and DM-RS 433 may be precoded with a second precoder. In these example embodiments, the first PDCCH 421 and the second PDCCH 422 in the first layer 411 may be spatially multiplexed to construct a MU-DCI, and the third PDCCH 423 in the second layer 412 may provide a single user DCI (SU-DCI). In this way, both a MU-DCI and a SU-DCI can exist in the same RB 407.

The use of a multi-user DCI may be used to further increase the capacity of the PDCCHs. In some embodiments, through the use of DM-RSs, the implementation of MU-DCI may be similar to an implementation of MU-MIMO. As illustrated in FIG. 4, three PDCCHs may be scheduled in one VRB, and two of the PDCCHs may have an aggregation level of two and the other PDCCH may have an aggregation level of one. As an example, the PDCCHs may be paired as illustrated in FIG. 4. In these embodiments, DM-RS 431 (set 1) and DM-RS 432 (set 2) may be used to demodulate PDCCH 421 and PDCCH 422 respectively with an aggregation level of two and DM-RS 433 (set 3) may be used to demodulate PDCCH 423. These specific embodiments may be extended to a more general case, where the PDCCHs with different aggregation levels are paired together. Because the power for each PDCCH is decreased a half due to MU pairing, there may be are some performance loss compared with single user (SU) SU- DCI, but better performance than that of the PDCCH configuration of 3 GPP LTE Rel.10 may be achieved.

FIG. 5 is a table illustrating the number of CCEs and idle resource element per RB. For each PDCCH configuration 501, the number of CCEs per RB is illustrated in column 507 and the number of available or idle resource elements per RB is illustrated in column 508. Embodiments of the present invention take advantage of these available resource elements to make more efficient use of the available bandwidth.

FIG. 6 illustrates a blind decoding search space region in accordance with some embodiments. Search space region 600 includes legacy control region 601, PDCCH group 1 region 602, PDCCH group 2 region 603, and PDCCH group 3 region 604. The PDCCH regions may include a PDCCH 61 1 for aggregation level 1, a PDCCH 612 for aggregation level 2, a PDCCH 614 for aggregation level 4, a PDCCH 618 for aggregation level 8, and regions 608 for the UE-RS. In these embodiments, blind decoding may be kept to an acceptable number for the Uu interface. If a UE 104 (FIG. 1) that is configured to operate in accordance with 3 GPP LTE Rel. l 1 should search both the legacy control region 601 and the UE-RS based PDCCH region 604 for UE specific DCI. The DCI may be provided in both of the two regions, although this is not a requirement. In these embodiments, semi-statistic high layer signaling may be provided to indicate to a UEs 104 which PDCCH region (e.g., legacy or UE-RS) should be searched for blind decoding.

In high-mobility situations, transmit diversity may also be used for DCI transmission in the legacy PDCCH region 601. Open loop precoding may also be used for PDCCH transmission based on the UE specific reference signal. As shown in FIG. 8, one PDCCH is distributed across multiple PDCCH groups and precoded by different precoders that are predefined and configurable for each PDCCH group. ^° to are code words may be selected, for example, from Table 6.3.4.2.3-2 or Table 6.3.4.2.3-3 of 3GPP T.S. 36.211 aOO according to different configurations of transmit antenna ports. This embodiment may favor a UEs 104 with higher mobility through frequency diversity and may improve the performance of PDCCH decoding.

Furthermore, a predefined precoder for different PDCCH groups may be cycled as a function of the subframe to provide time domain diversity gain. For example,

In these embodiments, UE-RS based open loop for PDCCH transmission may be used for high mobility UEs.

In summary, embodiments of an eNB 102 and method of transmitting PDCCHs in an LTE- A system are generally described herein. In some embodiments, the eNB 102 may be configured to transmit PDCCHs that are based on a UE-specific reference signal. In these embodiments, the UEs 104 may be able to demodulate their PDCCH using DM-RS that is specific to the UE (e.g., instead of a common reference signal). The PDCCHs disclosed herein may be suitable the Uu interface of an LTE-A system.

Embodiments may be implemented in one or a combination of hardware, firmware and software. Embodiments may also be implemented as instructions stored on a computer-readable storage device, which may be read and executed by at least one processor to perform the operations described herein. A computer-readable storage device may include any non-transitory mechanism for storing information in a form readable by a machine (e.g., a computer). For example, a computer-readable storage device may include read-only memory (ROM), random- access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and other storage devices and media. In some embodiments, the eNB 102 and the UEs 104 may include one or more processors and may be configured with instructions stored on a non-tangible computer-readable storage device.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.

Claims

CLAIMS What is claimed is:

1. A method of transmitting spatially-multiplexed physical-downlink control channels (PDCCHs) in a single resource block (RB) from an eNB to a plurality of UEs, the method comprising:

transmitting within the RB a plurality of spatial-division multiplexed (SDM) layers, wherein each layer includes a different demodulation reference signal (DM-RS) transmitted in non-orthogonal resource elements of the RB,

wherein each layer further includes a different PDCCH transmitted on orthogonal resource elements of the RB, and

wherein transmitting comprises precoding each DM-RS and each PDCCH for an associated layer based on precoding-matrix indicators (PMIs) received from the UEs.

2. The method of claim 1 wherein the DM-RSs are used by the UEs to demodulate downlink control information (DCI) carried by their PDCCH, the DCI including resource assignments and control information for the associated UEs.

3. The method of claim 2 wherein each PDCCH is transmitted by one or more control channel elements (CCEs).

4. The method of claim 3 further comprising transmitting legacy control information, a common reference signal (CRS), and a channel-state information reference signal (CSI-RS) in orthogonal resource elements of the RB across the plurality of layers without precoding.

5. The method of claim 4 wherein each the DM-RS comprises a UE-specific reference signal (UE-RS).

6. The method of claim 4 wherein prior to transmitting, the method includes:

mapping a first PDCCH to a first set of resource elements at the first layer;

mapping a second PDCCH to a second set of resource elements at the second layer; and mapping a third PDCCH to a third set of resource elements at the third layer (213), wherein the first, second and third sets of resource elements comprise REs that are orthogonal in both time and frequency.

7. The method of claim 6 wherein the mapping includes:

excluding resource elements reserved for the DM-RSs; and

excluding resource elements reserved for the legacy control information, the CRS and the CSI-RS.

8. The method of claim 1 wherein the single resource block comprises twelve subcarriers for a duration of up to fourteen OFDM symbols of two slots.

9. An enhanced node B (eNB) configured to transmit spatially-multiplexed physical- downlink control channels (PDCCHs) in a single resource block (RB) from an eNB to a plurality of UEs, the eNB configured to:

transmit within the RB a plurality of spatial-division multiplexed (SDM) layers, each layer including a different demodulation reference signal (DM-RS) transmitted in non- orthogonal resource elements of the RB, each layer further including a different PDCCH transmitted on orthogonal resource elements of the RB; and

precode each DM-RS and each PDCCH for an associated layer based on precoding- matrix indicators (PMIs) received from the UEs.

10. The eNB of claim 9 wherein the DM-RSs are used by the UEs to demodulate downlink control information (DCI) carried by their PDCCH, the DCI including resource assignments and control information for the associated UEs, and

wherein the eNB is further configured to transmit legacy control information, a common reference signal (CRS), and a channel-state information reference signal (CSI-RS) in orthogonal resource elements of the RB across the plurality of layers without precoding.

11. The eNB of claim 10 further configured to:

map a first PDCCH to a first set of resource elements at the first layer;

map a second PDCCH to a second set of resource elements at the second layer; and map a third PDCCH to a third set of resource elements at the third layer, wherein the first, second and third sets of resource elements comprise REs that are orthogonal in both time and frequency.

12. A method of transmitting subset-based physical-downlink control channels

(PDCCHs) in a single resource block (RB) from an eNB to a plurality of UEs, the method comprising:

transmitting a plurality of PDCCHs within a single RB, wherein each PDCCH is transmitted on a different subset of resource elements of the RB; and

for each PDCCH, transmitting a DM-RS within the RB, each DM-RS being transmitted on an orthogonal subset of resource elements of the RB.

13. The method of claim 12 wherein each DM-RS is transmitted on a different subcarrier frequency within corresponding symbol-duration times, and

wherein the different subsets of resource elements used for the transmission of the PDCCH are orthogonal in both frequency and time.

14. The method of claim 13 wherein each PDCCH is transmitted by one or more control channel elements (CCEs) depending on the aggregation level.

15. The method of claim 14 wherein the DM-RSs are used by the UEs to demodulate downlink control information (DCI) carried by their PDCCH, the DCI including resource assignments and control information for the associated UEs, and

16. A method of transmitting physical-downlink control channels (PDCCHs) with a multi-user downlink control information (MU-DCI) in a single resource block (RB) from an eNB to a plurality of UEs, the method comprising:

transmitting within the RB a plurality of spatial-division multiplexed (SDM) layers, wherein at least a first of the layers includes one PDCCHs transmitted in orthogonal resource elements of the RB, and

wherein a second of the SDM layers includes at least one other PDCCH transmitted in the same resource elements as one of the PDCCHs of the first of the layers, transmitting different demodulation reference signals (DM-RS) for each PDCCH within the RB, each DM-RS being transmitted on an orthogonal subset of resource elements of the RB within corresponding symbol-duration times.

17. The method of claim 16 wherein transmitting comprising precoding the PDCCH of the first layer with a same precoding vector, and

wherein the method further includes precoding the PDCCH of the second layer with a different precoding vector.

18. The method of claim 17 wherein the DM-RSs are used by the UEs to demodulate their DCI, the DCI including resource assignments and control information for the associated UEs.

19. An evolved node B (eNB) configured to transmit spatially-multiplexed physical- downlink control channels (PDCCHs) in a single resource block (RB) from an eNB to a plurality of UEs, the eNB comprising physical-layer circuitry configured to:

transmit within the RB a plurality of spatial-division multiplexed (SDM) layers, each layer including a different demodulation reference signal (DM-RS) transmitted in non- orthogonal resource elements of the RB and including a different PDCCH transmitted on orthogonal resource elements of the RB; and

precode each DM-RS and each PDCCH for an associated layer based on precoding- matrix indicators (PMIs) received from the UEs,

wherein each the DM-RS comprises a UE-specific reference signal (UE-RS).

20. The eNB of claim 19 wherein the physical-layer circuitry is further configured to transmit legacy control information, a common reference signal (CRS), and a channel-state information reference signal (CSI-RS) in orthogonal resource elements of the RB across the plurality of layers without precoding.

21. The eNB of claim 20 wherein the physical-layer circuitry is further configured to: map a first PDCCH to a first set of resource elements at the first layer;

map a second PDCCH to a second set of resource elements at the second layer;

map a third PDCCH to a third set of resource elements at the third layer (213);

exclude resource elements that are reserved for the DM-RSs; and exclude resource elements reserved for the legacy control information, the CRS and the CSI-RS,

wherein the first, second and third sets of resource elements comprise REs that are orthogonal in both time and frequency.