Method for Energy-Efficient Unicast and Multicast Transmission in a Wireless Communication System

Abstract

A method is provided for time multiplexing subframes on a serving cell to a user equipment (UE). [Solution] The method involves an eNodeB communicating by bitmap indication using a first bit (bit "0") of a set of subframes (501, 502, 503) operating using a legacy Long Term Evolution (LTE) transmission format and using a second bit (bit "1") of another set of subframes (504) operating using an evolved transmission NCT (New Carrier Type) format including reduced density cell-specific reference signal (CRS) transmission without a physical downlink control channel (PDCCH) control region.

Description

Technical Field

[0001] This application relates to wireless communication, and more particularly, to user equipment connected to a base station.

Background Art

[0002] A cellular communication network incorporates a number of wireless terminal devices and a number of base stations to provide communication services such as telephone communication, data, video, messaging, chat, and broadcast. A number of wireless terminals can be connected to a serving cell controlled by a base station (BS). Typical access methods used in widely deployed cellular networks include frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), or combinations thereof. A base station (BS) may also be referred to as a NodeB in UMTS (Universal Mobile Telecommunications System), or an evolved NodeB (eNB) in Long Term Evolution specified by 3GPP (Third Generation Partnership Project), a base transceiver system (BTS), an access point (AP), or some other equivalent term.

[0003] When configured, eNodeB hardware is generally fixed and stationary, but may be mobile in some cases, such as when deployed in a vehicle. In contrast to the eNodeB, a wireless terminal device can be portable hardware. Wireless terminal devices are typically referred to as user equipment (UE), mobile stations, cell phones, personal digital assistants (PDA), wireless modem cards, and the like. Uplink (UL) communication refers to communication from a fixed or mobile UE to the eNodeB, while downlink (DL) communication refers to communication from the eNodeB to a fixed or mobile UE. Each eNodeB includes radio frequency transmitters and receivers used to communicate directly with mobile terminals, and the mobile terminals can move freely around or be in a fixed location. Similarly, each UE includes radio frequency transmitters and receivers used to communicate directly with the eNodeB.

[0004] FIG. 1 shows an exemplary wireless telecommunications network 100. This exemplary telecommunications network includes base stations 101, 102, and 103, but in operation, a telecommunications network necessarily includes more base stations. Each of the base stations 101, 102, and 103 (eNB) is operable over corresponding coverage areas 104, 105, and 106. The coverage area of each base station is further divided into cells. In the illustrated network, the coverage area of each base station is divided into three cells. A handset or other user equipment (UE) 109 is shown in cell A 108. Cell A 108 is within the coverage area 104 of base station 101. Base station 101 transmits transmissions to UE 109 and receives transmissions from UE 109. As UE 109 moves out of cell A 108 and into cell B 107, UE 109 is handed over to base station 102. Since UE 109 is synchronized with base station 101, UE 109 can use unsynchronized random access to initiate a handover to base station 102.

[0005] Figure 2 shows the relationship between an E-UTRAN (Evolved Universal Terrestrial Radio Access) 200, such as that illustrated in Figure 1, and a core network (CN) 210 in an LTE wireless network. eNodeBs 203 and 204 communicate with an MME (Mobility Management Entity) 211 and a serving gateway 212 via an S1 signaling interface 205. UEs 201 and 202 communicate with eNodeBs 203 and 204, respectively, via an air interface. This illustration shows two eNodeBs, but there are more eNodeBs connected to the same MME in the deployed network, and one eNodeB can be connected to several MMEs. In the E-UTRAN, eNodeBs can communicate with each other via an X2 interface 206.

[0006] <Description of the LTE System> The LTE wireless network, also known as E-UTRAN (Evolved Universal Terrestrial Radio Access), is standardized by the 3GPP working group (WG). OFDMA and SC-FDMA access schemes are used for the downlink (DL) and uplink (UL) of E-UTRAN, respectively, as part of E-UTRA (Evolved Universal Terrestrial Radio Access). Referring to Figure 3 here, a legacy LTE DL transmission format is illustrated, showing the time-frequency resource mapping of the physical channel at a 1 millisecond (ms) transmission time interval (TTI), also known as a subframe. Downlink control-plane and user-plane data are scheduled by the physical downlink control channel (PDCCH) or the enhanced physical downlink control channel (EPDCCH), and the actual data is transmitted on the physical downlink shared channel (PDSCH). The minimum granularity for resource allocation for the PDSCH and EPDCCH is a pair of physical resource blocks (PRBs). The control region 301 includes DL control signaling including the PDCCH, PHICH (Physical Hybrid Automatic Repeat reQuest Indicator Channel), and PCFICH (Physical Control Format Indicator Channel). Common and dedicated control information is transmitted on the PDCCH, while dedicated control information is transmitted on the EPDCCHs 305, 306 if present. Cell-specific reference signals (CRS) are transmitted on one or more antenna ports and can be used for radio resource management (RRM) and radio link monitoring (RLM) functions, and for demodulation of control information on the PDCCH and data transmission on the PDSCH. Alternatively, the UE is configured to demodulate the PDSCH (302, 303, 304) and / or EPDCCH (305, 306) using dedicated demodulation reference signals (DMRS) transmitted only within the PRBs containing data or control information.Important cell information necessary for initial access by the UE is transmitted on the Physical Broadcast Channel (PBCH), and other system information and paging information are transmitted on the PDSCH. The EPDCCH and PDSCH are frequency multiplexed across the system bandwidth, and FIG. 3 illustrates the partitioning into three PDSCH regions 302, 303, 304, and two EPDCCH regions 305 and 306. Additional signals such as Channel State Information Reference Signals (CSI-RS) or Positioning Reference Signals (PRS) may also be transmitted in the subframe.

[0007] Unicast and multicast data can be transmitted over the same carrier. Unicast consists of a bi-directional point-to-point or point-to-multipoint transmission between the network and each of the UEs, with dedicated connections to each UE. Multicast data transmission to a group of UEs is supported by the evolved multimedia broadcast multicast service (E-MBMS) feature and consists of a downlink-only multipoint-to-multipoint connection. Only UEs that have subscribed to an MBMS service receive the content. Time sharing of unicast and multicast data is achieved by defining a subset of subframes to support MBMS single frequency network (MBSFN) transmission. Figure 4 shows an MBSFN subframe 400. The non-MBSFN region 401 contains the PDCCH, PHICH, and PCFICH, and the PDCCH can be used to schedule uplink data transmission and to signal power control commands for a group of UEs. The CRS is transmitted in the non-MBSFN region 401 for demodulation of the aforementioned channels. The MBSFN region 402 is used to transmit multicast data on the physical multicast channel (PMCH) for UEs that have subscribed to one or more MBMS services. In an MBSFN area, a set of non-synchronized eNodeBs can jointly transmit multicast data in an MBSFN subframe, thereby improving the reception quality. Therefore, the MBSFN reference signal (MBSFN-RS) is used in the MBSFN region 402 for demodulation of the PMCH. SUMMARY OF THE INVENTION

[0008] Current and future trends in cellular networks predict a sharp growth in data traffic, partly due to the rapid adoption of mobile Internet devices and the associated data-demanding applications. The increasing traffic demand will drive the need for energy conservation, especially as the number of cells deployed within the network increases. Unfortunately, current base stations typically transmit control channels and cell-specific reference signals regardless of whether any user equipment (UE) actually exists in the cell being served.

Brief Description of the Drawings

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Best Mode for Carrying Out the Invention

[0019] As cellular networks evolve to cope with this rapid growth in cellular data traffic, it has been found that impairments are introduced because much of the traffic becomes localized in hotspots in both indoor and outdoor scenarios. Heterogeneous networks are becoming increasingly common, in which small cells controlled by low-power base stations are deployed to increase capacity in hotspots and / or improve cellular coverage. In the 3GPP (Third Generation Partnership Project) Long Term Evolution (LTE) system, the base station, also known as the evolved NodeB (eNB), always transmits the cell-specific reference signal (CRS) and the time-multiplexed physical downlink control channel (PDCCH). However, always transmitting the cell-specific reference signal and the time-multiplexed physical downlink control channel (PDCCH) becomes a problem as traffic and demand increase.

[0020] <Description of the evolved transmission format> The 3GPP Radio Access Network (RAN) standardization body is taking various measures to address energy-efficient transmission involved in the evolution from the "always-on" DL transmission mode to the "on-demand" mode. One such approach is the introduction of the evolved DL transmission format, which is characterized by the absence of legacy downlink cell-specific reference signals and control channels, including PDCCH, PHICH, and PCFICH that rely on CRS for demodulation. Referring to Figure 3A here, PDSCH 312, 313, 314 and EPDCCH 315, 316 span the entire subframe. CRS is transmitted at a reduced density in the time domain and, optionally, in the frequency domain. For example, CRS can be transmitted with a single antenna port with a 5 ms periodicity and can occupy either the full system bandwidth or a reduced bandwidth. By removing the legacy control signals and CRS, significant energy savings can be provided for lightly loaded or unloaded cells. This results in up to 80% energy savings when one out of five subframes is used for transmission when there is no UE in that cell. Also, by removing the legacy control signals and CRS, resources that can be used for data transmission are released, thus increasing the spectral efficiency and peak data rate. The evolved transmission format may also be known as NCT (New Carrier Type). However, a major drawback is that UEs of previous LTE releases may not be able to attach to cells operating using the NCT structure.

[0021] The evolved transmission format or NCT can be configured for a secondary cell (SCell) in carrier aggregation (CA), or can be configured for single cell operation (standalone mode). In SCell operation, all the system information required for the NCT-SCell can be provided to the UE by dedicated signaling. A natural question is whether the advantages of NCT can also be obtained when NCT is deployed in standalone mode. This means that the UE can belong to the NCT as a primary cell either at initial access or via handover from a different cell. This includes synchronization, broadcast and system information transmission, and mobility control, and may require a new approach to access the cell. In both CA-based or standalone operation, new approaches are also required to multiplex reference signals, synchronization signals, and physical channels on the OFDM time-frequency resource grid.

[0022] Towards the goal of improving energy conservation in a wireless network, it is desirable to limit the base station transmission to the user equipment for a certain period of time. For example, a small cell deployed inside an office building can operate at full power during the day, but at night, this small cell should operate at reduced power or be completely turned off when there is no one in the building. Also, reduced downlink (DL) signaling also reduces DL inter-cell interference, which has become a bottleneck for capacity improvement as the number of cells added to the cellular topology increases.

[0023] Energy-efficient transmission can be made possible by constructing NCT as an SCell in carrier aggregation. One embodiment of the present invention describes a method for constructing an evolved transmission format for SCell operation, small cell operation in a heterogeneous network (HetNet), or a combination thereof.

[0024] In the case of CA, the UE is provided with all the system information necessary to receive data and control information regarding the SCell via dedicated radio resource control (RRC) signaling. The eNodeB configures the UE for PDSCH reception in a secondary serving cell (SCell) configured as an NCT. The eNodeB provides all the system information to the UE by means of RRC signaling including the system information contained in the Master Information Block and the SIB (System Information Block). System information that cannot be signaled to the UE, such as the SFN (System Frame Numver) which changes every 10 ms, is assumed by the UE to be the same for both the NCT SCell and the cell from which the UE receives the system information via dedicated signaling. Therefore, the PBCH may not be transmitted in the NCT. The eNodeB also provides an indication via RRC signaling as to whether the SCell is operating using the legacy LTE transmission format or the evolved transmission format. Alternatively, the UE can identify whether the SCell is operating using the legacy LTE transmission format or the evolved transmission format via the presence or absence of existing or new signals. In one embodiment, the discovery signal can identify the SCell as operating using the evolved transmission format. The NCT indication determines the location of the DMRS, which depends on the duplex mode (FDD or TDD), the cyclic prefix, or the subframe type (normal or special subframe). The NCT indication may also determine whether the PBCH is transmitted in the NCT, in which case the UE does not rate match to the resource elements reserved in other ways for PBCH transmission. Alternatively, in addition to the aforementioned NCT indication, additional RRC signaling may indicate whether the PBCH is transmitted, i.e., whether to rate match to the resource elements reserved for PBCH transmission.

[0025] When the SCell is an NCT, similar to the case of legacy LTE transmission formats, the UE performs cell search by detecting the legacy primary and secondary synchronization signals (PSS, SSS). Alternatively, the UE may use the discovery signal for cell detection. The PSS / SSS are transmitted at the same time-frequency locations as in the case of legacy transmission formats. In different embodiments, the locations of the PSS / SSS are different. In yet another embodiment, for example, the PSS / SSS may not be transmitted in each radio frame for improved energy efficiency and inter-cell interference reduction, and the UE may blindly detect the presence of the PSS / SSS or, alternatively, may be notified by the network via explicit or implicit signaling about the subframes for searching the PSS / SSS.

[0026] Other variations of SCell operation are not excluded either. For example, the SCell may be controlled by a second eNodeB different from the eNodeB controlling the primary serving cell. Both NodeBs coordinate their scheduling and RRM decisions with respect to the UE backhaul connection.

[0027] <Evolved Transmission Format in Single Cell Operation> In the stand-alone operation of evolved transmission formats, various new techniques are required to achieve the goals of energy efficiency, reduced inter-cell interference, and increased spectral efficiency.

[0028] One such approach is to use a demodulation reference signal (DMRS) to transmit common control information on a shared downlink or broadcast channel. When a reduced density of CRS is used only for tracking and not for data demodulation, a new approach is needed for DMRS-based transmission of common control information, including system information, paging notifications, and UL power control commands. Such an approach uses the common control channel specifications, in which all UEs, or a group of UEs, can monitor control message scheduling common system information, paging information, and group power control commands. Another approach is to use DMRS-based transmission to transmit broadcast information that a UE needs to belong to a cell without prior reception of a handover command, such as DL bandwidth and system timing reference (system frame number). Yet another approach is to use the transmission of UE-specific reference signals for radio link monitoring (RLM) including new RLM procedures and measurements for radio resource management (RRM) including new mobility procedures and measurements, and for channel state information (CSI) feedback including new measurements and procedures based on, for example, CSI-RS.

[0029] Yet another approach is needed for cell identification and accessibility, and an approach is needed to prevent access of UEs of previous releases to cells using the NCT format. In the previous release, a UE could obtain the MIB and at least SIB Type1 to determine whether a cell was blocked. If this legacy procedure is followed for NCT, UEs of previous releases can consume a significant amount of energy during cell search and intra-frequency and / or inter-frequency measurements, especially in highly dense small cell deployments. Thus, the energy savings on the network side are offset by the increased energy consumption on the terminal side.

[0030] Therefore, the following method is proposed to enable access to a cell that operates using an evolved transmission format for a UE.

[0031] In one embodiment, when the UE is in the RRC_IDLE state, cell selection follows the conventional LTE procedures of Releases 8, 9, 10, and 11, for example. Since the UE assumes a legacy transmission format, it may not be able to camp on or attach to this cell. On the other hand, if an RRC_IDLE UE discovers an NCT for which it had an RRC connection within the last N hours (where N is a fixed value), the UE may perform an initial cell attachment to the NCT without handover via a cell operating using a legacy transmission format. Otherwise, the UE searches for and attaches to a cell operating using a legacy transmission format. The UE may choose to camp on this cell in the RRC_IDLE state or may choose to transition to the RRC_CONNECTED state.

[0032] In another embodiment, the UE is in the RRC_CONNECTED mode and is handed over to a cell operating using the NCT. The handover is network-controlled but UE-assisted. That is, the network may rely on the UE to find and report cells operating in the evolved transmission mode. The eNodeB may assist the UE by providing a list of cells to which the UE may report radio resource management (RRM) measurements. For each cell in this list, the eNodeB indicates the transmission format, legacy or NCT. In yet another embodiment, a set of physical cell IDs (PCIs) is divided into several ranges, and the UE may infer from the provided list of PCIs which transmission format is used by each cell. For example, if the range of valid PCIs is divided into two ranges, each of which contains consecutive cell IDs in increasing order, the set containing PCI 0 is associated with cells operating using a legacy transmission format, while the set not containing PCI 0 is associated with the NCT structure.

[0033] In one embodiment, RRM measurements including measurements of reference signal received power (RSRP) and reference signal received quality (RSRQ) are performed with channel state information reference signal (CSI-RS). The CSI-RS configuration is configured by RRC signaling from the eNodeB to the UE. In one embodiment, in a list of cells in which the UE can perform RRM measurements, the eNodeB provides the corresponding CSI-RS configuration. In different embodiments, the RRM measurements are performed on CRSs with reduced density. In still another different embodiment, the network provides additional assist information for the RRM measurements performed on CRSs with reduced density, such as in the time and / or frequency resources for performing the measurements. In still another embodiment, the UE is configured with some such configurations, sometimes called CSI processes, regardless of whether the measurements are performed using CRSs with reduced density, CSI-RS, or any other discovery signal.

[0034] In another embodiment, the eNodeB may configure the UE with report criteria for RRM measurement purposes including periodic, event-triggered, or event-triggered / periodic measurements. The eNodeB may further configure the UE to report only the PCI for which the report was triggered. In another embodiment, the eNodeB configures the UE to report not only the PCI but also the associated RRM measurement results. Also, the report criteria may include an offset such that the UE reports only the cell ID and the associated measurements when the cell measurement is better than a certain threshold than the serving cell. The eNodeB may then use the NRT (Neighbor Relation Table) to look up whether the reported cell ID is an NCT. The eNodeB may also configure the UE with the system bandwidth for each neighboring cell included in the aforementioned list of PCI. This facilitates accurate RSRP or RSRQ measurements. Alternatively, the NodeB indicates to the UE the measurement bandwidth for each neighboring cell included in the aforementioned list of PCI.

[0035] In yet another embodiment, the source eNodeB initiates a handover to the target eNodeB. If the target eNodeB successfully acknowledges the handover request, the target eNodeB provides the UE with the following handover information, namely, RRC reconfiguration information including all system information regarding the target cell, and an indication of the transmission format in the target cell, which is relayed via the source eNodeB, and the UE performs synchronization with respect to the target cell according to the transmission type.

[0036] In the current LTE specifications up to Release 11, the system frame number (SFN) is not exchanged between eNodeBs via the X2 interface. Therefore, the SFN is not part of the mobility information sent to the UE during handover preparation. When the PBCH is not transmitted for the NCT format, there is no means for the target cell to use the NCT to indicate the SFN to the incoming UE during or after the handover procedure. A new approach is needed to facilitate handover from a cell operating using a legacy transmission format to a cell operating using the NCT format.

[0037] To solve this problem, the RRC information element indicating which cell's SFN can be used as the reference SFN for the target cell is included in the mobility control information. In other words, the UE signals the proxy cell whose SFN determined by decoding the PBCH of the proxy cell is used as the reference SFN of the target cell. The RRC information element may include the PCI of the proxy cell operating using the legacy transmission format. The mobility control information is prepared by the target eNB using NCT. The target eNB can determine the appropriate PCI through the maintenance operation function (OAM) information or via its NRT (Neighbor Relation Table). In another embodiment, the reference cell for the SFN of the target cell is explicitly provided by the source eNB. That is, the SFN broadcast in the PBCH of the source cell is the reference SFN for the target cell using NCT. In yet another embodiment, the RRC information element included in the mobility control information indicates the actual SFN of the target cell, or alternatively, an offset that the UE can apply to the SFN of the proxy cell.

[0038] The mobility method that provides a reference SFN for a target cell is also advantageous when the target eNB uses a legacy carrier but operates in cell range expansion (CRE). In such a scenario, when the UE is handed over, the UE may experience heavy interference and may not be able to reliably detect the PBCH of the target cell. In particular, this is the case when the UE does not have a PBCH interference cancellation receiver. This mobility method can cause the network to configure a larger CRE bias, thereby promoting a larger offload gain especially when the UE without an advanced receiver moves from a heavily loaded cell to a lightly loaded cell. Therefore, regardless of whether the target cell uses legacy or NCT transmission format, the SFN reference can be included in the mobility control information. More generally, this proposed mobility technique can always be configured when the UE cannot reliably detect the PBCH of the target secondary serving cell, where the cell is controlled by the serving eNodeB or a different eNodeB. Also, the cell can operate using a legacy transmission mode, but the PBCH is not transmitted by the eNodeB that controls such a cell. For example, the UE can be configured to transmit data to and receive data from multiple eNodeBs. In such a case, only one eNodeB can transmit the broadcast channel (i.e., the master eNodeB), and the other eNodeBs (i.e., secondary eNodeBs) transmit only data or dedicated control information. In such an arrangement, the proposed method is used to obtain the SFN for the cell controlled by the secondary eNodeB from the PBCH transmitted by the master eNodeB.

[0039] <Time Multiplexing of Hybrid Subframes for Both Legacy and Evolved Transmission Formats> As described above, the NCT format is not backward compatible. Therefore, a cellular operator may not use this transmission format in a cell where all connected UEs support this feature. Also, it seems to be more suitable for small cells rather than macro cells, and since they provide coverage and ensure a minimum quality of service to all UEs, they may not be turned off. Also, in the absence of PDCCH transmission, a new method is required to send common control signaling, including broadcast, system information, and paging.

[0040] One solution to address these drawbacks of NCT is through time-domain multiplexing of subframes that operate both in legacy and evolved transmission formats. The eNodeB may indicate to the UE a set of subframes that operate using either the legacy or evolved transmission structure. This subframe indication is communicated to the UE via RRC signaling. In one embodiment, the subframe indication is in the form of a bitmap. A bit value of “1” indicates that the subframe is of the NCT format, while a bit value of “0” indicates that the subframe is of the legacy transmission format. That is, the subframe includes either a time-multiplexed control region and a unicast or multicast data region. Referring now to FIG. 5, an exemplary bitmap of length 10 is shown for one radio frame. Other bitmap sizes are possible. Subframes 501, 502, 503 use the legacy transmission format, where the UE monitors the PDCCH for common and dedicated control signaling and CRS may be used for demodulation of some of the channels transmitted in this subframe. The UE may also use these subframes for radio link monitoring (RLM) or radio resource management (RRM) measurements. The UE may also be configured to monitor the EPDCCH in subframes 501, 502, and 503. Other subframes, such as 504, indicate the NCT format. Thus, in these subframes, the UE monitors only the EPDCCH for downlink control information as the PDCCH is not transmitted. In different embodiments of the present invention, the UE is signaled a bitmap indicating the set of subframes in which the UE monitors the PDCCH. The UE may be configured to monitor both DL assignments, UL grants, and group power control commands, or only UL grants and group power control commands. This subframe indication concept is applied in different ways for energy-efficient multiplexing of (a) unicast and multicast data, and (b) transmission using legacy and evolved transmission formats in a cell.

[0041] <Monitoring of Control Channel and PBCH> Subframes operating using the legacy transmission structure are composed of the PDCCH region and include CRS transmission on one, two, or four antenna ports. These legacy subframes are used to transmit common control information in the common search space of the PDCCH. The set of legacy subframes includes at least subframe 0 of each radio frame. Therefore, the PBCH can be transmitted and detected by UEs of all LTE releases. The PDSCH is demodulated using DMRS in all subframes. Alternatively, the UE is configured for data demodulation using CRS in legacy subframes and DMRS only in NCT subframes. In yet another embodiment, the downlink control information (DO) format received on the control channel indicates to the UE which reference signal to use for demodulation. However, the UE demodulates the PBCH and PDCCH using 1-, 2-, or 4-port CRS.

[0042] In different embodiments, the UE may demodulate the PBCH and PDCCH using 1-port CRS. That is, at most 1-port CRS is transmitted in the mixed subframe transmission format. RRM, RLM, and / or CSI measurements are performed in subframes carrying 1-port CRS.

[0043] <Multiplexing of PMCH, PDSCH, EPDCCH, and PDCCH> It is possible to operate using the NCT format in one cell and the legacy transmission format in different cells when both cells are deployed at the same carrier frequency. The NCT format is used exclusively in all subframes in a small cell or only in subframes configured for transmission using the evolved transmission format. In such a scenario, an MBMS service can be provided in an MBSFN area composed of cells operating using different transmission formats. For example, in a heterogeneous network, a macro cell and a small cell can each operate using the legacy and evolved transmission formats at a shared carrier frequency. Therefore, when eNBs of both the macro cell and the small cell participate in MBSFN transmission, at least for subframes carrying PMCH in the small cell, the PMCH transmission should be synchronized with the PMCH transmission in the macro cell. As a result, since the small cell does not include a control region, the first one or two OFDM symbols in the small cell layer are wasted to align MBSFN transmission across the macro and small cell layers.

[0044] To address this waste, the eNB of the small cell may transmit the PDCCH in the one or two OFDM symbols. This enables scheduling of uplink data transmission on the PUSCH in future subframes. A bitmap is signaled to the UE to indicate which subframes are reserved for PMCH transmission, regardless of whether the UE subscribes to the MBMS service in a cell operating using the evolved transmission structure. In the subframe where the PMCH is indicated, the UE monitors the PDCCH as in the legacy transmission structure. As a further example for reducing UE PDCCH processing, the UE may be configured to monitor only the PDCCH in the PMCH subframe for either a UL grant or a group power control command. In yet another example, in the subframe where the PMCH is indicated, the UE monitors the PDCCH as in the legacy transmission structure for downlink control information that schedules DMRS-based PDSCH transmission. The starting OFDM symbol of such PDSCH transmission is indicated by the PCFICH received in the legacy transmission structure of such subframe, or is configured by a higher layer. This enables the network to schedule unicast transmission in subframes where the PMCH is indicated but not scheduled, or where the PMCH is scheduled but no data is received by the eNodeB due to congestion in the backhaul connection. In the subframe where the PMCH is not indicated, the UE monitors the EPDCCH.

[0045] <Multiplexing of Unicast / Multicast Data in the Evolved Transmission Format> In an MBSFN area, a set of synchronized eNodeBs jointly transmits multicast data in an MBSFN subframe. Since the eNodeBs are geographically scattered, an extended cyclic prefix (CP) with a length of 16.67 microseconds is defined in the LTE standard to support a large delay spread seen by the UE in the MBSFN area. By enabling an even longer CP, an increase in the MBSFN area can also be supported. This increases the MBSFN combination gain at the UE since more unsynchronized eNBs can participate in the multicast transmission. For example, the CP length can be doubled up to 33.33 μs, and the symbol length can also be doubled to maintain the same CP overhead. This approach is not backward compatible. That is, the MBMS service for the extended MBSFN area cannot be enjoyed by UEs of previous LTE releases. Since potential revenue may not offset the required capital investment, this reduces the incentive for cellular operators to provide such features. On the other hand, this is beneficial for operators for whom backward compatibility is not an issue. For example, the NCT format is used in frequency bands that are not currently supported by UEs of previous releases or require special permission for access. Therefore, the applicant proposes the following approach to support the multiplexing of unicast and multicast data in cells operating using the evolved transmission format.

[0046] In one embodiment, MBMS and unicast data are time multiplexed in the NCT transmission format. There are two types of subframes. Namely, a normal subframe for carrying unicast data (PDSCH and EPDCCH), and an MBSFN subframe for multicast data. For both normal and MBSFN subframes, PDSCH and PMCH transmissions start at symbol 0. Subframes 0 and 5 may not be configured as MBSFN subframes. That is, the MBSFN subframe utilization rate has an upper limit of 80%.

[0047] In another embodiment, the evolved physical broadcast channel (EPBCH) is transmitted in the second slot of the first subframe of the radio frame. Alternatively, the EPBCH may be transmitted in the first slot of the same subframe. In yet another embodiment, the EPBCH may span both slots in a PRB pair. The EPBCH is demodulated by group-specific reference signals on one or two antenna ports. In one embodiment, the group-specific RS may use the same waveform and random number generator as the UE-specific RS of LTE Release 11 on antenna ports {7}, {8}, or {7, 8}. Other combinations such as {7, 9} or {9, 10} are not excluded either. The position of this group-specific RS in the time-frequency resource grid of LTE is the same as that of the existing UE-specific RS. In different embodiments, a new set of EPBCH RSs may be transmitted on one or two antenna ports, and the RSs may be included in the PRBs containing the OFDM symbols and the EPBCH.

[0048] In yet another embodiment, the primary and secondary synchronization signals (PSS / SSS) are transmitted in subframes 0 and 5 for both FDD and TDD, in subframes 0 and 5 for SSS, and in 1 and 6 for PSS. In yet yet another embodiment, PMCH is transmitted in a subset of subframes. For FDD, a subset of MBSFN subframes is taken from {1, 2, 3, 4, 6, 7, 8, 9}, and for TDD, a subset of MBSFN subframes is taken from {3, 4, 7, 8, 9}. This is because subframe 2 is always UL and subframe 6 is for carrying PSS in the existing TDD UL-DL configuration. Alternatively, in TDD, PSS and SSS can be carried in subframes 0 and 5 for TDD, freeing up subframes 1 and 6 as potential MBSFN subframes. FIG. 6 shows the mapping for exemplary FDD and TDD. FIG. 7 illustrates the exemplary mapping of EPBCH, PSS / SSS, and PMCH for a certain PRB out of six PRBs at the center of the system bandwidth. (a) of FIG. 7 shows the time multiplexing scheme of mixed unicast and multicast data in NCT. PSS and SSS are moved to symbols 1 and 2 respectively to avoid collision with group-specific (demodulation) RS on symbols 5 and 6. (b) of FIG. 7 shows an alternative embodiment where the demodulation RS for antenna ports 7 and 8 is shifted from symbol 5 / 6 of slot 0 to symbols 1 and 2 of slot 1 in subframe 0. This maintains the same positions of PSS and SSS as in the legacy transmission format. With this mapping design, 8 resource elements per PRB (2 for 1-port CRS and 4 for group-specific RS) are used for RS, enabling the same coding rate for EPBCH as for the legacy PBCH. Note that EPBCH may not require a PHICH configuration. By reducing the number of spare bits by up to 5, the payload for the MIB (Master Information Block) can be reduced to up to 2 octets (16 bits).This provides a lower coding rate of (16 + 16) / 480 = 1 / 60 (with 16 CRC bits) compared to the 1 / 40 coding rate for legacy PBCH.

[0049] In one embodiment, the length of the cyclic prefix for PMCH transmission is extended to support a larger delay spread. In another embodiment, the CP is set to 33.33 microseconds.

[0050] In another embodiment, control information for supporting the reception of multicast traffic and control channels (e.g., in the case of MBSFN subframe configuration and MCCH scheduling) is provided via the broadcast of system information in a unicast subframe scheduled by a common search space on an extended physical downlink control channel (EPDCCH). Specifically, the EPDCCH is a physical control channel that supports multicast transmission similar to the PDCCH in the previous LTE release.

[0051] In yet another embodiment, a new approach is needed to support the scheduling of UL grants in MBSFN subframes for FDD with paired UL carriers or for TDD with one or more UL subframes. Two alternatives are Option 1 and Option 2. Option 1 is the frequency at which EPDCCH and PMCH are multiplexed in an MBSFN subframe. The bandwidth for PMCH transmission JPEG0007709419000001.jpg1318 is defined in the system information broadcast, where JPEG0007709419000002.jpg1036, which is the DL system bandwidth. As shown in Figure 8, EPDCCH is configured in a subset of PRBs that is not part of the PMCH bandwidth. In a subframe containing PMCH transmission, the eNodeB can schedule PDSCH transmission, for example, in the PRBs reserved for EPDCCH according to the procedure for PDSCH transmission whose resource allocation does not overlap with that of the scheduling EPDCCH, using the existing transmission modes TM9 and 10. In other words, unicast and PMCH transmissions are multiplexed in the frequency domain in the MBSFN subframe. Option 2 is that the scheduling of UL grants occurs only in subframes of the legacy transmission format. For example, if the MBSFN subframe configuration for a radio frame in FDD is {1, 2, 3, 4, 6, 7, 8, 9}, the UL subframes for the paired carriers are scheduled as follows. The UL grants for subframes 0 to 4 are transmitted in subframe 0, and the UL grants for subframes 6 to 9 are transmitted in subframe 5.

[0052] <Multiplexing of EPDCCH, PRS, and PMCH in the evolved transmission format> EPDCCH and PMCH can be transmitted in the same subframe when the system bandwidth is divided into M sets, with M - 1 sets assigned for the transmission of EPDCCH and 1 set assigned for PMCH. These M - 1 sets can partially overlap but are mutually exclusive, and the Mth set used for PMCH transmission is not assigned for EPDCCH transmission. If the M sets configured for PMCH and EPDCCH transmission do not cover the entire system bandwidth, EPDCCH can schedule PDSCH in the PRBs not covered by the M sets. Also, the PDSCH transmission in the subframe configured for PMCH transmission can overlap with some of the PRBs that are part of the M - 1 sets assigned for the transmission of EPDCCH.

[0053] In one embodiment, the union of the M-1 sets is signaled to the UE via a bitmap of length {6, 15, 25, 50, 75, 100}, where a bit value of "1" indicates that the corresponding PRB pair in the frequency domain is used for EPDCCH, and a bit value of "0" indicates that the corresponding PRB pair is assigned to PMCH. Other mappings are not excluded, and the main concept is that the bitmap indicates which PRB pairs are used for EPDCCH and PMCH. Alternatively, the bit value "0" may indicate that the corresponding PRB in the frequency domain is used for EPDCCH.

[0054] In another embodiment, a combination index is used to indicate to the UE the PRBs in the frequency domain used for PMCH.

[0055] In the frequency domain, multiplexing EPDCCH and PMCH requires that since only PMCH is identified due to the extended CP, when the two channels coexist in an OFDM symbol, EPDCCH is always transmitted with the extended cyclic prefix (CP). Since the eNB may not know whether the UE is currently subscribed to MBMS and thus receiving PMCH, and since the UE needs to know the CP for demodulation, a way is needed to eliminate any ambiguity between the eNB and the UE regarding the CP used for a given OFDM symbol. The eNB can transmit the EPDCCH with the normal or extended cyclic prefix (CP) according to a bitmap known to the UE via a configuration by a higher layer. When PDSCH transmission is scheduled in a subframe configured for PMCH transmission, the CP of the PDSCH also follows the aforementioned bitmap configured by a higher layer. Alternatively, the CP of the PDSCH can follow the CP of the scheduling EPDCCH. However, this may require special processing when the scheduling cell is different from the serving cell (cross-carrier scheduling). In other words, the EPDCCH scheduling PDSCH in the subframe configured for PMCH can be transmitted using the normal CP or the extended CP according to the subframe on its component carrier, while the PDSCH follows the bitmap provided by a higher layer.

[0056] The length of such a bitmap can be 6, 10, 24, 40, or any other integer value. In one embodiment, a "1" indicates that extended CP is used in the associated subframe. In another embodiment, a "0" indicates that extended CP is used in the associated subframe. The subframes in which extended CP is used for EPDCCH transmission may include at least all the subframes in which PMCH is transmitted. To avoid puncturing of EPDCCH by the positioning reference signal (PRS), when frequency resources are partitioned such that PMCH and EPDCCH are multiplexed, PRS is transmitted only in PRBs that are part of the PMCH section in a subframe configured for PRS transmission by a higher layer and also configured for PRS transmission. Alternatively, PRS is transmitted only in PRBs that are part of the PMCH section in a subframe configured for PRS transmission by a higher layer and also shown for EPDCCH transmission with extended CP.

[0057] In an MBSFN area, the participating eNBs need to transmit PMCH with the same bandwidth. Therefore, the first eNB may transmit a message to the second eNB via a backhaul connection, notifying the second eNB of which PRBs it intends to use for PMCH transmission. In one embodiment, this message is a bitmap of length {6, 15, 25, 50, 75, 100}, where a "1" indicates that the associated PRB in the frequency domain is used for PMCH. Alternatively, a "0" may indicate that the associated PRB in the frequency domain is used for PMCH. In another embodiment, a combination index is used in such a message to indicate the PRBs in the frequency domain used for PMCH. Also, the first eNB can transmit a request for PMCH information to the second eNB via a backhaul connection, in which case the second eNB responds with a message notifying the first eNB of which PRBs it intends to use for PMCH transmission.

[0058] <Location of PSS / SSS / DMRS for the evolved transmission format> In the current LTE release, DMRS is not transmitted in the six central PRBs of the system bandwidth if it collides with PSS and SSS. Since the reduced CRS may not be appropriate or sufficient for PDSCH demodulation at the UEs, a new solution is needed to enable PDSCH demodulation at the UEs. The locations of PSS and SSS are changed for cells operating with the evolved transmission format. In one embodiment, the relative locations of PSS and SSS are changed compared to the legacy transmission format to enable easy differentiation between transmission formats and duplex modes. FIG. 9 illustrates an exemplary mapping, in which for FDD, SSS precedes PSS by two symbols, and for TDD, PSS precedes SSS by three symbols. A further advantage of this mapping for TDD is that it provides the possibility that the reduced CRS is transmitted in the first symbol of the special subframe. Different embodiments are shown in FIG. 10. For FDD, SSS is mapped to the last symbol of subframes 0 and 5, while PSS is mapped to the symbol preceding SSS, i.e., the 13th symbol for normal CP and the 11th symbol for extended CP. Yet another embodiment for FDD is shown in FIG. 11, where PSS and SSS are shifted to OFDM symbols 1 and 2, respectively.

Claims

1. A method comprising: configuring a user equipment (UE) to receive data on a secondary serving cell, the secondary serving cell operating using a transmission format that includes transmission of cell-specific reference signals (SRS) transmitted at a period of 5 milliseconds (ms), the transmission format not including a physical downlink control channel (PDCCH) control region; receiving transmissions from the secondary serving cell according to the transmission format; wherein, when a physical broadcast channel (PBCH) is not present on the transmission from the secondary serving cell, the UE does not rate match to resource elements reserved for PBCH transmission. A method.

2. The method according to claim 1, wherein the UE is provided with all system information required to receive data and control information.

3. The method according to claim 1, wherein the UE is provided with an indication of whether the secondary serving cell operates using a legacy transmission format or an evolved transmission format.

4. The method according to claim 1, wherein the UE is provided with information as to which serving cell's sequence frame number should be used as a reference for the secondary serving cell instead of decoding a primary broadcast channel on the secondary serving cell.

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