WO2013127453A1

WO2013127453A1 - Control channels for wireless communication

Info

Publication number: WO2013127453A1
Application number: PCT/EP2012/053475
Authority: WO
Inventors: Timothy Moulsley; Milos Tesanovic; Matthew Webb
Original assignee: Fujitsu Limited
Priority date: 2012-02-29
Filing date: 2012-02-29
Publication date: 2013-09-06

Abstract

A scheme for the transmission and reception of downlink control channel messages in LTE which can be applied in association with random access transmission (RACH) in the uplink. Initially, the UE monitors a specific search space on the downlink control channel (PDCCH) for specific types of message (DCI messages) from the eNodeB following a RACH transmission. Subsequently (112 - 120) the UE is configured to monitor a different search space and/or control channel (e.g. E-PDCCH) for the same type of DCI message. The UE then proceeds (122) to monitor the different search space and/or control channel. Allowing DCI messages to be transmitted on the E-PDCCH rather than the PDCCH reduces the loading on PDCCH and improves control channel efficiency (in terms of resource use). In the case of RACH, this raises the limit on the rate at which random access transmissions can be processed. The potential increase in total number of blind decodings required to receive the E-PDCCH may be mitigated by means of limiting the duration of monitoring the E- PDCCH to subframes where the expected DCI messages may be transmitted by the network.

Description

Control Channels for Wireless Communication

Field of the Invention

The present invention relates to wireless communication systems, for example systems based on the 3GPP Long Term Evolution (LTE) and 3GPP LTE-A groups of standards.

Background of the Invention

Wireless communication systems are widely known in which base stations (BSs) form "cells" and communicate with terminals (called user equipments or UEs in LTE) within range of the BSs. In such a system, each BS divides its available bandwidth, i.e. frequency and time resources in a given cell, into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers of radio communication links between the base stations of adjacent cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, but in the simplest case it communicates with one "serving" or "primary" cell.

For assisting understanding of the inventive concepts to be described later, some outline will be given of some of the features of LTE which are of particular relevance to embodiments of the present invention. However, it is to be understood that the present invention is not restricted to use in LTE.

Basic LTE Network Topology The network topology in LTE is illustrated in Figure 1. As can be seen, each terminal or UE 12 connects over a wireless link via a Uu interface to a base station or eNodeB 11 , and the network of eNodeBs is referred to as the eUTRAN 10.

Each eNodeB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or "core network" entities, including a Serving Gateway (S-GW 22), and a Mobility Management Entity (MME 21 ) for managing the system and sending control signalling to other nodes, particularly eNodeBs, in the network. In addition, a PDN or Packet Data Network Gateway (P-GW) is present, separately or combined with the S-GW 22, to exchange data packets with any packet data network including the Internet. The core network 20 is called the EPC or Evolved Packet Core. Machine Type Communication (MTC) and Machine-to-Machine (M2M) Communication

Machine-to-Machine (M2M) communication, usually referred to in the context of LTE as Machine Type Communication (MTC), is a form of data communication which involves one or more entities that do not necessarily need human interaction; in other words the 'users' may be machines.

MTC is different from current communication models as it potentially involves very large number of communicating entities (MTC devices) with little traffic per device. Examples of such applications include: fleet management, smart metering, product tracking, home automation, e-health, etc.

MTC has great potential for being carried on wireless communication systems (also referred to here as mobile networks), owing to their ubiquitous coverage. However, for mobile networks to be competitive for mass machine-type applications, it is important to optimise their support for MTC. Current mobile networks are optimally designed for Human-to-

Human communications, but are less optimal for machine-to-machine, machine-to-human, or human-to-machine applications. It is also important to enable network operators to offer MTC services at a low cost level, to match the expectations of mass-market machine-type services and applications.

To fully support these service requirements, it is necessary to improve the ability of mobile networks to handle machine-type communications.

In the LTE network illustrated in Figure 2, a group of MTC devices 200 is served by an eNodeB 11 which also maintains connections with normal UEs 12. The eNodeB receives signalling from the MME 21 and data (for example, a request for a status report from a supervisor of the MTC devices) via the S-GW 22.

Thus, there is a MTCu interface analogous to the Uu interface, and the MTC devices will be served in a similar way to normal user equipments by the mobile networks. When a large number of MTC devices connect to the same cell of a UMTS RNS or an LTE eNodeB, each of the devices will need resources to be allocated to support the individual devices' applications even though each MTC device may have little data.

In the remainder of this specification, the term "UE" includes "MTC device" unless otherwise demanded by the context.

For assisting understanding of the inventive concepts to be described later, some outline will be given of some specific aspects or features of LTE which are of particular relevance in the present invention. Further details of the features outlined below are given by the following documents, hereby incorporated by reference:-

3GPP TS 36.212: "Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding"

3GPP TS 36.213: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures"

3GPP TS 36.321 : "Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification"

OFDMA and SC-FDMA In the downlink of an LTE system, in other words the direction of transmission from the base station (eNodeB) towards the user equipments (UEs), individual OFDM subcarriers or sets of subcarriers are assigned to different user equipments. The result is a multi-access system referred to as OFDMA (Orthogonal Frequency Division Multiple Access). By assigning distinct frequency/time resources to each user equipment in a cell, OFDMA can substantially avoid interference among the users served within a given cell.

The UEs are allocated a specific number of subcarriers for a predetermined amount of time. An amount of resource consisting of a set number of subcarriers and OFDM symbols is referred to as a physical resource block (PRB) in LTE. PRBs thus have both a time and frequency dimension. Allocation of RBs is handled by a scheduling function at the eNodeB.

The uplink in an LTE wireless communication system employs a variant of OFDMA called Single-Carrier FDMA (SC-FDMA). Essentially, SC-FDMA is a linearly precoded OFDMA scheme, involving an additional DFT step before the conventional OFDMA processing.

Access to the uplink by multiple UEs is enabled by assigning to each UE a distinct set of non-overlapping sub-carriers. This allows a single-carrier transmit signal, reducing the peak-to-average power ratio (PAPR) in comparison with OFDMA.

Frame Structure and Resource Blocks

In a wireless communication system such as LTE, data for transmission on the downlink is organised in OFDMA frames each divided into a number of sub-frames. Various frame types are possible and differ between frequency division duplex (FDD) and time division duplex (TDD) for example. In FDD, transmission occurs simultaneously on DL and UL using different carrier frequencies, whilst in TDD downlink and uplink transmissions occur on the same carrier frequency and are separated in time. An FDD frame consists of 10 uplink subframes and 10 downlink subframes occurring simultaneously. In TDD, various allocations of subframes to downlink and uplink are possible, depending on the load conditions. Subframes may consequently be referred to as uplink subframes or downlink subframes.

Figure 3 shows a generic frame structure for LTE, applicable to the downlink, in which the 10 ms frame is divided into 20 equally sized slots of 0.5 ms. A sub-frame SF consists of two consecutive slots, so one radio frame contains 10 sub-frames. The UEs are allocated, by a scheduling function at the eNodeB, a specific number of subcarriers for a predetermined amount of time. Such allocations typically apply to each subframe. Resources are allocated to UEs both for downlink and uplink transmission (i.e. for both downlink subframes and uplink subframes). The transmitted signal in each slot is described by a resource grid of sub-carriers and available OFDM symbols, as shown in Figure 4. Each element in the resource grid is called a resource element (RE) and each resource element corresponds to one symbol.

For each transmission time interval of 1 ms, a new scheduling decision is taken regarding which UEs are assigned to which time/frequency resources during this transmission time interval, the scheduling being made in units of PRBs. As shown in Figure 4, one physical resource block is usually defined as 7 consecutive OFDM symbols in the time domain and 12 consecutive sub-carriers in the frequency domain. Several resource blocks may be allocated to the same UE, and these resource blocks do not have to be contiguous with each other. Scheduling decisions are taken at the eNodeB, using a scheduling algorithm which takes into account the radio link quality situation of different UEs, the overall interference situation, Quality of Service requirements, service priorities, etc. Channels

In LTE, several channels for data and control signalling are defined at various levels of abstraction within the system. Figure 5 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them. For present purposes, the channels at the physical layer level are of most interest.

On the downlink, user data is carried on the Physical Downlink Shared Channel (PDSCH). There are various control channels on the downlink, which carry signalling for various purposes including so-called Radio Resource Control (RRC), a protocol used as part of radio resource management, RRM. In particular this signalling comprises the Physical Downlink Control Channel, PDCCH (see below). To facilitate measurements of the radio link properties by UEs, and reception of some transmission channels, reference signals are embedded in each downlink subframe. Thus, resource allocations for channels including PDCCH and PDSCH have to fit around the reference signals. The reference signals provide an amplitude and/or phase reference for allowing the UEs to correctly decode the remainder of the downlink transmission. Reference signals include a cell-specific (or common) reference signal (CRS), and a UE-specific demodulation reference signal DMRS.

Meanwhile, on the uplink, user data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH). By means of frequency hopping on PUSCH, frequency diversity effects can be exploited and interference averaged out. The control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel state information (CSI), as represented for example by channel quality indication (CQI) reports, and scheduling requests. Of particular interest for present purposes, there is also a Physical Random Access Control Channel, PRACH with at the transport layer level, a corresponding Random Access Channel, RACH (see below).

PDCCH and PCI

In LTE, both the DL and UL are fully scheduled since the DL and UL traffic channels are dynamically shared channels. This means that PDCCH must provide scheduling information to indicate which users should decode the physical DL shared channel (PDSCH) in each sub-frame and which users are allowed to transmit on the physical UL shared channel (PUSCH) in each sub-frame. PDCCH is used to carry scheduling information - called downlink control information, DCI - from eNodeBs to individual UEs. Conventionally, one PDCCH message contains one DCI format. This is often intended for one individual UE, but some messages are also broadcast (e.g. intended for multiple UEs within a cell). Thus PDCCH can also contain information intended for a group of UEs, such as Transmit Power Control (TPC) commands. In addition the PDCCH can be used to configure a semi- persistent schedule (SPS), where the same resources are available on a periodic basis. The motivation for SPS is to support applications. Moreover, and of significance for the invention to be described, PDCCH can be used to order a UE to perform a random access (RACH) procedure as explained more fully below, for example for uplink timing alignment.

PDCCH is transmitted on an aggregation of one or several consecutive control channel elements (CCEs), where a control channel element corresponds to 9 resource element groups (REG). Each REG in turn occupies four of the Resource Elements (REs) shown in Figure 4.

More particularly PDCCH contains: - the resource allocations for the downlink transport channel DL-SCH

- Transmit Power Control (TPC) commands for PUCCH and the uplink transport channel UL- SCH; these commands enable the UE to adjust its transmit power to save battery usage

- Hybrid-Automatic Repeat Request (HARQ) setup information

- MIMO precoding information.

A cyclic redundancy check (CRC) is used for error detection of the DCI. The entire PDCCH payload is used to calculate a set of CRC parity bits, which are then appended to the end of the PDCCH payload.

As multiple PDCCHs relevant to different UEs can be present in one sub-frame, the CRC is also used to specify which UE a PDCCH is relevant to. This is done by scrambling the CRC parity bits with a Radio Network Temporary Identifier (RNTI) of the UE. The RNTI is thus associated with the PDCCH and the DCI. Various kinds or RNTI are defined, as explained in more detail below. The size of the DCI depends on a number of factors, and thus it is necessary that the UE is aware of the size of the DCI, either by RRC configuration or by another means to signal the number of symbols occupied by PDCCH. Depending on the purpose of the DCI message, different DCI formats are defined. The DCI formats include:

^• Format 0 for transmission of uplink shared channel (UL-SCH) allocation

^• Format 1 for transmission of DL-SCH allocation for Single Input Multiple Output (SIMO) operation

· Format 1 A for compact transmission of DL-SCH allocation for SIMO operation or allocating a dedicated preamble signature to a UE for theRACH procedure

^■ Format 3 and format 3A for transmission of TPC command for an uplink channel.

DCI Formats 3 and 3A carry multiple power control bits representing multiple power control commands, each power control command being intended for a different UE. The main application of interest for Formats 3 and 3A is to support SPS in the uplink (since UE specific PDCCH DCI formats to carry power control commands are not then required). Since, as already mentioned, multiple UEs can be scheduled within the same sub-frame,

conventionally therefore multiple DCI messages are sent using multiple PDCCHs.

The format to be used depends on the purpose of the control message. For example, DCI Format 1 is used for the assignment of a downlink shared channel resource when no spatial multiplexing is used (i.e. the scheduling information is provided for one code word transmitted using one spatial layer only). The information provided enables the UE to identify the resources, where to receive the PDSCH in that sub-frame, and how to decode it.

Besides the resource block assignment, this also includes information on the modulation and coding scheme and on the hybrid ARQ protocol used to manage retransmission of non- received data. A UE needs to check all possible combinations of PDCCH locations, PDCCH formats, and DCI formats and act on those message with correct CRCs (taking into account that the CRC is scrambled with a RNTI). This is called "blind decoding". To reduce the required amount of blind decoding of all the possible combinations, for each UE a limited set of CCE locations is defined where a PDCCH may be placed. The set of CCE locations in which the UE may find its PDCCH is called the "search space". In LTE, separate UE-specific search spaces (UESSS) and common search spaces (CSS) are defined, where a dedicated search space is configured for each UE individually, while all UEs are informed of the extent of the common search space CSS. The RACH Procedure

The Physical Random Access Channel PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. If a scheduling request (SR) is triggered at the UE, for example by arrival of data for transmission on PUSCH, when no PUSCH resources have been allocated to the UE, the SR is transmitted on a dedicated resource for this purpose. If no such resources have been allocated to the UE, the RACH procedure is initiated. The transmission of SR is effectively a request for uplink radio resource on the PUSCH for data transmission. Thus, RACH is provided to enable UEs to transmit signals in the uplink without having any dedicated resources available, such that more than one terminal can transmit in the same PRACH resources simultaneously. The term "Random Access" (RA) is used because (except in the case of contention-free RACH, described below) the identity of the UE (or UEs) using the resources at any given time is not known in advance by the network

(incidentally, in this specification the terms "system" and "network" are used

interchangeably). Preambles (which when transmitted, produce a signal with a signatures which can be identified by the eNodeB) are employed by the UEs to allow the eNodeB to distinguish between different sources of transmission. RACH can be used by the UEs in either of contention-based and contention-free modes. In contention-based RA, UEs select any preamble at random, at the risk of "collision" at the eNodeB if two or more UEs accidentally select the same preamble. Contention-free RA avoids collision by the eNodeB informing each UE which preambles may be used. Referring to Figure 3, the Physical Random Access Channel PRACH typically operates as follows (for contention based access):-

(i) The UE10 receives the downlink broadcast channel for the cell of interest (serving cell).

(ii) The network, represented in Figure 3 by eNodeB 20, indicates cell specific information including the following: resources available for PRACH

preambles available (up to 64)

preambles corresponding to small and large message sizes. (iii) The UE selects a PRACH preamble according to those available for contention based access and the intended message size.

(iv) The UE 10 transmits the PRACH preamble (also called "Message 1", indicated by (1 ) in the Figure) on the uplink of the serving cell. The network (more particularly the eNodeB of the serving cell) receives Message 1 and estimates the transmission timing of the UE.

(v) The UE 10 monitors a specified downlink channel PDSCH, signalled via CSS on PDCCH, for a response from the network (in other words from the eNodeB). That is, in response to the UE's transmission of Message 1 , the UE 10 receives both a Random Access Response or RAR ("Message 2" indicated by (2) in Figure 3) on PDSCH and (not shown) a notification on PDCCH of the resource to be used for this. The RAR contains an UL grant for transmission on PUSCH and a Timing Advance (TA) command for the UE to adjust its transmission timing. (vi) In response to receiving Message 2 from the network, the UE 10 transmits on PUSCH ("Message 3", shown at (3) in the Figure) using the UL grant and TA information contained in Message 2.

(vii) As indicated at (4), a contention resolution message may be sent from the network (in this case from eNodeB 20) in the event that the eNodeB 20 received the same preamble simultaneously from more than one UE, and more than one of these UEs transmitted Message 3.

If the UE does not receive any response from the eNodeB within a predefined time window, the UE selects a new preamble and sends a new transmission in a RACH subframe after a random back-off time.

For contention-free RA, the procedure is simpler: (i) The eNodeB configures the UE with a preamble from those available for contention- free access. (ii) The UE transmits the preamble (Message 1 ) on the uplink of the serving cell.

(iii) The UE receives the RAR (Message 2) via PDSCH from the network, which contains an UL grant for transmission on PUSCH. The resource to be used for RAR is again signalled on PDCCH using CSS.

In both contention-based and contention-free RACH procedures, the RAR contains a Cell Radio Network Temporary Identifier (C-RNTI) which identifies the UE. In the contention- based procedure, the UE transmits this C-RNTI back to the eNodeB in Message 3 and, if more than one UE does so there will be a collision at the eNodeB which may then initiate the contention resolution procedure.

Situations where the RACH process is used include:

- Initial access from RRCJDLE

RRC connection re-establishment

Handover

- DL data arrival in RRC_CONNECTED (when non-synchronised)

- UL data arrival in RRC_CONNECTED (when non-synchronised, or no SR resources are available)

Positioning (based on Timing Advance)

The RACH procedure can be triggered in response to a PDCCH order (e.g. for DL data arrival, or positioning). Contention free RA is only applicable for handover, DL data arrival and positioning.

RNTIs

RNTIs or Radio Network Temporary Identifiers, mentioned earlier, are used by the eNodeB to scramble the CRC applied to the PDCCH payload. Types of RNTI currently defined in LTE include the following.

P-RNTI (Paging RNTI):

To receive paging messages from E-UTRAN, UEs in an idle mode monitor the PDCCH channel for a P-RNTI value used to indicate paging. If the terminal detects a group identity used for paging (the P-RNTI) when it wakes up, it will process the corresponding downlink paging message transmitted on the PCH.

SI-RNTI (System Information RNTI):

The presence of system information on DL-SCH in a sub-frame is indicated by the transmission of a corresponding PDCCH marked with a special System Information RNTI (SI-RNTI). This PDCCH message indicates the transport format and physical resources (set of resource blocks) allocated for system-information transmission. M-RNTI (MBMS RNTI):

This is used in Multimedia Broadcast Multicast Services (MBMS), a point-to-multipoint transmission scheme available in LTE. RA-RNTI (Random Access RNTI):

The RA-RNTI is used on the PDCCH when Random Access Response (RAR) messages are transmitted, to identify which time-frequency resource was utilized by the UE to transmit a Random Access preamble. In the event of a collision when multiple UEs select the same signature in the same preamble time-frequency resource, they each receive the RAR message.

C-RNTI (Cell RNTI):

The C-RNTI is used by a given UE while it is in a particular cell, after it has successfully joined the network by performing a network entry process with the eNodeB of that cell. The C-RNTI is used for normal scheduling of downlink resources for the UE, also called dynamic scheduling as opposed to semi-persistent scheduling (see below). TC-RNTI:

If a UE does not have an allocated C-RNTI, then a Temporary C-RNTI (TC-RNTI) is used for further communication between the terminal and the network. Once the UE has completed the network entry process, the TC-RNTI is changed to a C-RNTI. SPS-C-RNTI (Semi-Persistent Scheduling C-RNTI):

This form of RNTI is used in SPS (see below). For the configuration or reconfiguration of a persistent schedule, RRC signalling indicates the resource allocation interval at which the radio resources are periodically assigned to a specific UE. Specific transmission resource allocations in the frequency domain, and transmission attributes such as the modulation and coding scheme, are signalled using the PDCCH. The actual transmission timing of the PDCCH messages is used as the reference timing to which the resource allocation interval applies. When the PDCCH is used to configure or reconfigure a persistent schedule, it is necessary to distinguish the scheduling messages which apply to a persistent schedule from those used for dynamic scheduling. For this purpose, a special identity is used, known as the Semi-Persistent Scheduling C-RNTI (SPS-C-RNTI), which for each UE is different from the C-RNTI used for dynamic scheduling messages.

TPC-PUCCH-RNTI (Transmit Power Control-Physical Uplink Control Channel-RNTI) and TPC-PUSCH-RNTI (Transmit Power Control-Physical Uplink Shared Channel-RNTI):

The power-control message is directed to a group of terminals using an RNTI specific for that group. Each terminal can be allocated two power-control RNTIs, one for PUCCH power control and the other for PUSCH power control. Although the power control RNTIs are common to a group of terminals, each terminal is informed through RRC signalling which bit(s) in the DCI message it should follow.

SPS

Semi-Persistent Scheduling, SPS, schedules resources for UEs on an ongoing basis and thereby reduces control channel overhead for applications that require persistent radio resource allocations such as VoIP (Voice over Internet Protocol). In LTE, both the DL and UL are fully scheduled as already mentioned so that without SPS, every DL or UL physical resource block (PRB) allocation must be granted via a PDCCH message. Note that although retransmissions on PUSCH can be made autonomously without an explicit UL grant, the first transmission would require a grant. This works well with large packet sizes and only a few users to be scheduled each sub-frame. However, for applications that require persistent allocations of small packets, the control channel overhead due to scheduling information can be greatly reduced with SPS. In SPS, the eNodeB defines a persistent resource allocation that a user should expect on the DL or can transmit on the UL. This can also be highly beneficial for MTC for example, where the MTC devices may be expected to transmit a small amount of data at fixed intervals.

E-PDCCH

A new control channel design (E-PDCCH) is under discussion in 3GPP for LTE. This will transmit DCI messages in the same resources as currently reserved for downlink data (PDSCH). The E-PDCCH will support a UESSS, but it is open whether a CSS will be specified for E-PDCCH.

A possible motivation for using a CSS on E-PDCCH is to reduce congestion on PDCCH, for example if there are more urgent DCI messages to be sent than can be accommodated within one subframe, then these could be sent on E-PDCCH, and by using CSS any UE can be addressed.

A PDCCH transmission typically contains a payload of around 50 bits (including CRC), with additional channel coding to improve robustness to transmission errors. For some applications only small data packets are required, so the PDCCH payload may represent a significant overhead. This may be even more significant for some configurations of TDD, with a limited proportion of subframes allocated for DL transmission. In addition, there is a limit on the maximum number of PDCCH messages that can be transmitted at the same time (i.e. within the same subframe), which may be insufficient to support a large number of active UEs transmitting or receiving only small data packets.

One scenario where such control channel limitations may be significant is for Machine-To- Machine (M2M) Communication or Machine Type Communication (MTC). As a particular example, a sensor application may require small data packets (e.g. temperature readings) to be sent at short intervals from a large number of devices within one cell.

One situation where E-PDCCH might be used for DCI messages currently sent using CSS on PDCCH is in connection with the RACH procedure. The UE may send a RACH preamble for initial access, when an SR (scheduling request) is triggered or on reception of a PDCCH order. Currently, as already mentioned, the reply in the form of a random access response (RAR) from the eNodeB is sent on the PDSCH, and signalled via the CSS on PDCCH, with CRC scrambled by RA-RNTI.

Any means of supporting the RAR using E-PDCCH would reduce the loading on PDCCH, and are therefore of significant interest.

Note that the loading due to RACH access may become more significant in future LTE scenarios. For example, MTC devices may make significant use of RACH and may be more numerous in future.

It is likely that the eventual specification will allow that E-PDCCH may be transmitted in either a frequency-localized or a frequency-distributed manner depending on the

requirements of the system.

Summary of the Invention

According to a first aspect of the present invention, there is provided a wireless

communication method in which a terminal receives control channel messages from a wireless network, the method comprising:

arranging the terminal to monitor control channel messages, having a given format and associated with a first identifier, in accordance with a first configuration including a first search space and a first control channel; and

arranging the terminal to monitor control channel messages, having the same format and associated with a second identifier, in accordance with a second configuration including a second search space and a second control channel;

wherein at least one of said first and second identifiers, said first and second search spaces, and said first and second control channels are non-identical. "Arranging" the terminal as specified above may involve configuring the terminal during operation for example by higher-level signalling, or may be done within the specification of the network. "Non-identical" includes the possibility that one of the first and second identifiers, search spaces and/or control channels may overlap with or include the other. In some, but not all embodiments the first and second control channels have different structures. In some, but not all embodiments the first and second search spaces are different from one another.

In some, but not all embodiments the first and second identifiers are different from one another.

In any case, preferably, the first and second configurations further include respective sets of timings for monitoring by the terminal. In a frame-based wireless communication system in which frames and subframes are defined (such as LTE), these sets of timings may be distinct sets of subframes within one or more frames. Thus, preferably, the respective sets of timings are also non-identical for the first and second control channels.

Preferred embodiments of the present invention are applied to an LTE network; in this case, the control channel messages may be (or may include) downlink control information DCI, at least the first control channel may be the physical downlink control channel PDCCH, at least the first search space may be the common search space CSS, and at least the first identifier may be a random access radio network temporary identifier RA-RNTI. Also, the above- mentioned sets of timings may be sets of subframes within an LTE frame. In a first preferred embodiment of the present invention, the second control channel is E- PDCCH. More generally, either or both of E-PDCCH and PDCCH may be monitored according to the configuration.

In a second preferred embodiment of the present invention, the second search space is UESSS. Also in this embodiment, preferably the second identifier is C-RNTI. In other words, the first and second configurations may differ such that in the first configuration the control channel messages are sent for example associated with the identifier RA-RNTI in the CSS and on PDCCH, whereas in the second configuration the control channel messages are sent associated with the identifier C-RNTI in the UESSS, either on PDCCH or E- PDCCH.

In any of the above methods, it is possible that the terminal communicates with the network via both a primary cell and at least one secondary cell; in this case, control channel messages may be transmitted on the second control channel via at least one the cells in response to a request transmitted to the network via the secondary cell. A PRACH preamble is one example of such a request, in which case the control channel messages would include scheduling information for a random access response RAR. In the third embodiment of the present invention, as a specific example of this case, the terminal transmits a PRACH preamble on the uplink of the secondary cell and monitors for the scheduling information of the RAR on E-PDCCH, which may be transmitted on either or both of the cells.

In a fourth embodiment, when the method is applied to a network employing a RACH-type procedure such as in LTE, the method may further comprise the terminal notifying the network, by its choice of PRACH preamble when performing a RACH procedure, of its capability for monitoring control channel messages in accordance with the second configuration, the second configuration being already known to both the terminal and the network.

It is possible for the same mobile terminal to use both configurations simultaneously. Thus, within a predetermined time interval defined in the network (e.g. in the same frame), the terminal may monitor and receive control channel messages in accordance with both the first and second configurations.

A fifth embodiment uses this principle to improve the probability of correct reception of the control channel messages by the terminal. That is, the same control channel messages are transmitted in both configurations, such that the location of the message in the first search space is linked to the location of the message in the second search space. Linking the locations avoids an increase in the number of blind decoding attempts required at the terminal. In other embodiments, the two configurations are employed as alternatives. In particular, the first configuration may be one in accordance with existing network specifications and suitable for legacy terminals, whilst the second configuration may be intended for terminals with more advanced capabilities. The feature of the fourth embodiment may be employed to allow the terminal to signal to the network its capability of accepting the second

configuration. In such a case, the terminal ceases monitoring control channel messages in accordance with the first configuration, when monitoring control channel messages in accordance with the second configuration. In particular, this principle may be applied to the RACH procedure in LTE, in which a control channel message (DCI) follows transmission of a PRACH preamble from the terminal to the network. In a sixth embodiment, the meaning of the control channel messages depends upon the control channel and/or search space on which the control channel messages are received. Thus, the terminal can infer the meaning based on the channel/search space concerned. According to a second aspect of the present invention, there is provided base station equipment for use in any of the methods as defined above and arranged for transmitting control signals in accordance with said first and second configurations.

According to a third aspect of the present invention, there is provided a terminal for use in any of the methods as defined above, arranged for monitoring control channel messages in accordance with said first and second configurations.

The method may be a machine-to-machine, M2M, or machine type communication, MTC, method wherein the terminal may be an autonomous machine (e.g., MTC device).

Further aspects of the present invention may provide a RRM entity in a wireless communication network for configuring base station equipment and terminals for performing any of the methods as defined above. A further aspect relates to software for allowing transceiver equipment equipped with a processor to provide base station equipment or a terminal as defined above. Such software may be recorded on a computer-readable medium.

In general, and unless there is a clear intention to the contrary, features described with respect to one aspect of the invention may be applied equally and in any combination to any other aspect, even if such a combination is not explicitly mentioned or described herein.

As is evident from the foregoing, the present invention involves signal transmissions between a network and terminals in a wireless communication system. In a wireless communication system, typically, wireless access to the network is provided by one or more base stations or access points. Such a base station may take any form suitable for transmitting and receiving such signals. It is envisaged that the base stations will typically take the form proposed for implementation in the 3GPP LTE and 3GPP LTE-A groups of standards, and may therefore be described as an eNodeB (eNB) (which term also embraces Home eNodeB or HeNB) as appropriate in different situations. However, subject to the functional requirements of the invention, some or all base stations may take any other form suitable for transmitting and receiving signals from user equipments. Similarly, in the present invention, each terminal may take any form suitable for transmitting and receiving signals from base stations. For example, the terminal may be referred to as a subscriber station (SS), or user equipment (UE), and may take any suitable fixed-position or movable form. For the purpose of visualising the invention, it may be convenient to imagine the terminal as a mobile handset (and in many instances at least some of the user equipments will comprise mobile handsets), however no limitation whatsoever is to be implied from this. In particular the terminals may be MTC devices. In the detailed description which follows, in which embodiments of the present invention are described with respect to LTE, the terminal is referred to as a UE in accordance with usual LTE terminology.

Thus, embodiments of the present invention may provide a scheme for the transmission and reception of downlink control channel messages in LTE which can be applied in association with random access transmission (RACH) in the uplink. A feature of embodiments of the present invention is that the initially, the UE monitors a specific search space on the downlink control channel (PDCCH) for specific types of message (DCI messages) from the eNodeB following a RACH transmission. Subsequently the UE monitors a different search space and/or control channel (e.g. E-PDCCH) for the same type of DCI message. However, it is to be noted that the RACH procedure is not the only application of the present invention.

An advantage of embodiments of the present invention is that DCI messages can be transmitted on the E-PDCCH rather than the PDCCH, which reduces the loading on PDCCH and improves control channel efficiency (in terms of resources use). In the case of RACH this raises limit on the rate at which random access transmissions can be processed. The potential increase in total number of blind decodings required to receive the E-PDCCH may be mitigated by means of limiting the duration of monitoring the E-PDCCH to subframes where the expected DCI messages may be transmitted by the network.

Brief Description of the Drawings

Reference is made, by way of example only, to the accompanying drawings in which:

Figure 1 schematically illustrates a basic LTE network topology;

Figure 2 schematically illustrate a network topology for a wireless communication system with MTC devices;

Figure 3 illustrates a generic frame structure employed for the downlink in an LTE wireless communication system;

Figure 4 illustrates resource allocation within a frame;

Figure 5 shows relationships between various channels defined in LTE;

Figure 6 shows a conventional RACH procedure;

Figure 7 shows a proposed resource configuration for PDCCH and E-PDCCH;

Figures 8 and 9 show two examples of resources allocated for PDCCH and E-PDCCH within a physical resource block;

Figures 10 to 14 show examples of possible allocation of resources to PDCCH and E- PDCCH taking into account reference signals DMRS and CRS;

Figure 15 is a flowchart illustrating steps in a method in accordance with a first embodiment of the present invention; and

Figure 16 is a flowchart illustrating steps in a method in accordance with a fourth embodiment of the present invention.

Detailed Description

Before describing embodiments of the present invention, some further discussion will be given in relation to the control channel PDCCH and the proposed channel E-PDCCH in LTE. However, it is to be noted that the present invention is not restricted to application to

PDCCH, or to LTE.

Following on from the discussion given in the introduction, some specific points of relevance are as follows.

PDCCH may occupy the first 1 ,2,3 or 4 OFDM symbols in a subframe (4 is a special case for small system bandwidths). Consequently, the available bandwidth for PDCCH is rather limited. For convenience, a PDCCH message in accordance with a DCI format is referred to below simply as "a DCI format".

Existing DCI formats indicate data transmission in either DL or UL, but not both. However, some DCI formats for DL resource scheduling may also trigger some kind of transmission on the UL (e.g. a random access preamble for RACH). A given PDCCH may be transmitted in any one of a number of given locations (which is a search space comprising a pre-determined subset of all the possible locations). The UE attempts blind decoding of the PDCCH in each location within the search space. The UE is required to blind decode only a limited number of PDCCH candidates. The common search space CSS is defined for all UEs. Also, the UE-specific search spaces UESSS are defined based on particular identities (RNTIs).

As already mentioned, a given PDCCH may be transmitted using identities (RNTIs) such as:

C-RNTI: UE identity for normal operation

SPS C-RNTI: UE identity for activating/modifying/deactivating SPS

transmission on an individual UE basis

- TPC-PUCCH-RNTI: Group identity for Power control of PUCCH

TPC-PUSCH-RNTI: Group identity for Power control of PUSCH.

As already mentioned, the RNTI is used to scramble the 16-bit CRC attached to the payload. This allows the UE to both identify whether the message has been decoded correctly, and confirm the RNTI value.

Each different type of message is conveyed using a different DCI format. Many PDCCH messages are intended to be received by only one UE, others are intended for more than one UE. In particular DCI formats 3 and 3A carry multiple power control commands, each power control command intended for a different UE.

Turning to E-PDCCH, this will occupy defined parts of the system bandwidth, for example a small set of PRBs which may change from subframe to subframe. An example of resources allocated for PDCCH and E-PDCCH is shown in Figure 7. In Fig. 7, the vertical axis represents the frequency domain and the horizontal axis, the time domain (two subframes and a fraction of a third are shown). The light shading shows an example of resource allocation for PDCCH and the darker shading shows how E-PDCCH might be

accommodated. DCI messages, characterised by a DCI format, may be transmitted in a subset of the resources defined for PDCCH. As already mentioned, the set of locations in which the UE checks for the presence of a given DCI format is termed the search space, and search spaces are also defined for E-PDCCH. Using E-PDCCH, multiple DCI messages may be transmitted within one PRB. One example of how different DCI messages for different users may be multiplexed together in the same PRB is shown in Figure 8 for a single subframe. Each rectangle in Figure 8 corresponds to a Resource Element (RE). Some of the REs are reserved for PDCCH, as shown by the shading in the first three symbols in the subframe. The E-PDCCH resources for the respective users occupy distinct frequency bands on the remaining symbols, in this example. In order to improve robustness, more resources can be allocated for a DCI message (in both PDCCH and E-PDCCH). The expansion of resources compared with the minimum is called the aggregation level, which may be {1 , 2, 4, 8}. Figure 9 shows an example of E-PDCCH transmissions for two users with aggregation level 2.

Both localised and distributed transmission will be supported by E-PDCCH. For localised transmission a given DCI message would be transmitted within a one PRB, or perhaps a small number (e.g. 2) of adjacent PRBs. For distributed transmission a DCI message would be transmitted within a small number (e.g. 4) of PRBs distributed across the frequency domain.

Allocation of resources to E-PDCCH is complicated in practice by the presence of reference signals like CRS and DMRS. The inventors have further considered this point as follows.

The design of PDCCH makes use of groups of 4 REs to make a Resource Element Group (REG). This concept can be considered for E-PDCCH. In addition, the possible presence of common reference symbols (CRS) and demodulation reference symbols (DMRS) should be take into account. This leads to a possible arrangement for multiplexing E-PDCCH resources such as shown in Figure 10. This is compatible with likely design requirement that 4 DCI messages can fit in one PRB.

In Figures 10 to 14, the shading denotes those resources already needed for the existing purposes of PDCCH and reference signals. Resources allocated to E-PDCCH are denoted by numbers in the corresponding REs. In Figure 10, certain REs are identified as "Orphan REs". This would be a consequence of supporting transmit diversity based on SFBC (Space-Frequency Block Code). Ideally, a block of symbols for which SFBC is applied should be located close together in the frequency domain. For LTE Release 8 the specification allows SFBC to be applied if the frequency domain separation of symbols within the block is no more than one sub-carrier. For the arrangement of Figure 10, it is assumed that REs which do not satisfy this criterion cannot be part of a REG. In PDCCH the minimum resource allocation for one DCI message is one CCE, which consists of 9 REGs, each of 4 REs, (i.e. a total of 36 REs). In E-PDCCH the corresponding eCCE may have a size which depends on the scenario. In the example of Figure 10, each resource allocation (eCCE) consists of either 6 or 7 REGs.

Figure 11 shows a further example, with 24 REs reserved for DMRS. Under the assumption that this arrangement would correspond to localised transmission and that SFBC would not be required for this case, no restriction is necessary on the separation between REs of the same REG. In this case, no orphan REs are needed.

Some examples are given in Figures 12 to 14 for the case where a REG consists of 2 REs.

Turning now to the invention itself, a feature of certain embodiments as applied to LTE, and starting with a default configuration, is to configure the UE to expect the RAR on the E- PDCCH, or more generally, to configure the UE to monitor for a given DCI format on a different search space and/or a different control channel. Although the invention and embodiments are aimed mainly at the RACH procedure, this is not the only application.

In a first embodiment based on LTE, the network operates using FDD and comprises one or more eNodeBs, each controlling at least one downlink cell, each downlink cell having a corresponding uplink cell. Each DL cell may serve one or more terminals (UEs) which may receive and decode signals transmitted in that cell.

As already mentioned, in order to schedule the appropriate use of transmission resources in time, frequency and spatial domains for transmission to and from the UEs, the eNodeBs send control channel messages (PDCCH) to the UEs. A DCI message carried on PDCCH typically indicates whether the data transmission will be in the uplink (using PUSCH) or downlink (using PDSCH), it also indicates the transmission resources, and other information such as transmission mode and data rate.

As determined by higher layers, the UE physical layer performs blind decoding for a number of possible PDCCH message types (DCI formats) over defined search spaces (CSS and UESSS) on the downlink primary cell (Pcell). The embodiments below typically assume that the UE has a pre-existing configuration whereby it monitors for DCI messages in at least one search space (e.g. CSS) on a control channel (e.g. PDCCH). In this first embodiment, the UE is configured by the network to monitor the CSS on a new control channel (E-PDCCH) for DCI messages. The configuration may be via higher layer signalling (e.g. RRC in LTE). The configuration may define one or more of the following:-

- The specific RNTI or RNTIs which may be used for scrambling the CRC of the DCI message

- Specific subframes during which the CSS on E-PDCCH should be monitored

- Specific subframes during which the CSS on PDCCH need not be monitored (in order to avoid increasing the total number of blind decodings) Figure 15 shows a flowchart of the procedure in this embodiment. First (step 112) the network decides to use the new control channel for use in transmitting DCI messages, in this case E-PDCCH as distinct from PDCCH. Next (step 114) the search space which the terminal should use to find DCI messages on the new channel, is determined. Then (116) the network decides which kind of RNTI to use with the new control channel, more precisely, to use to scramble the CRC of each message.

The next step (118) as already mentioned is to determine a set of subframes within each frame within which the UE should monitor for possible DCI messages. As noted above, this may also involve specifying subframes which no longer need to be monitored for messages on the existing control channel (PDCCH).

Incidentally, steps 112 - 118 may be carried out together or in any order. The UE is then configured (step 120) with all of this information to enable it to monitor and receive control channel messages on the new control channel, and it begins to do so (122).

In a preferred version of the first embodiment, the UE is configured to monitor DCI messages with CRC scrambled by RA-RNTI on CSS on E-PDCCH in subframes where the UE expects to receive a RAR from the network. The UE can compute the appropriate RA- RNTI from the subframe and PRACH resources in which the PRACH preamble was transmitted. As an extension of this embodiment, in the same subframes as monitoring CSS on E-PDCCH, the UE is not required to monitor CSS on PDCCH.

In a more general version of the first embodiment, the UE is configured to monitor a new search space which may be (but is not necessarily) on a new control channel. Since space in CSS is limited, defining a new search space can free resources in CSS regardless of whether or not a new control channel is employed for this purpose. A second embodiment is applicable to those cases where the network is aware of the UE identity (i.e. the UE is configured with C-RNTI), and the network is also aware of which UE has sent the PRACH preamble. This applies in the case of contention free RACH. For such cases the UE is configured to monitor DCI messages with CRC scrambled by C- RNTI on UESSS on PDCCH in subframes where the UE expects to receive a RAR from the network. As already mentioned, the RAR itself is transmitted on PDSCH, but the resource allocation to be used for this is notified separately on PDCCH. This means that instead of monitoring for the RAR with RA-RNTI on CSS, the UE monitors for the RAR notification with C-RNTI on UESSS. Note that in this embodiment it is not necessary to employ E-PDCCH; however, in an alternative version of this embodiment the UE monitors for the scheduling of the RAR on UESSS on E-PDCCH. As an extension of this alternative version, in the same subframes, the UE is not required to monitor CSS on PDCCH.

A third embodiment is like the second embodiment except that the PRACH preamble is sent on an Scell uplink. That is, a UE already in communication with the network via a primary cell may wish to perform a RACH procedure with another cell; this would be required if it is necessary to establish the correct timing for the Scell uplink. According to the existing LTE specifications, the RAR could be sent on PDCCH on Scell or Pcell. In this embodiment the UE is configured to monitor for the scheduling of the RAR on E-PDCCH (which could be E- PDCCH of the Scell, Pcell or both). In variations of this embodiment, the RAR can be sent via one of UESSS or CSS on E-PDDCH respectively.

In a fourth embodiment (which may be combined with the others) the PRACH preambles are partitioned. One partition is used by legacy UEs. A second partition is used by UEs implementing the invention. For the UEs transmitting a preamble in the second partition, the notification of the resource used for the RAR is not expected on the CSS on PDCCH as is used conventionally, but rather according to the particular version of the embodiment may be on CSS on E-PDCCH, UESSS on E-PDCCH or UESSS on PDCCH.

As already mentioned, the UE allows a certain time window for reception of the RAR. As a variation of this embodiment, if the RAR, according to this invention, is expected on a different search space/control channel, the time window for reception of the RAR could be different (e.g. made longer, for example for applications which are delay tolerant). As further a variation of this embodiment the time window for reception of the RAR could depend on the RACH preamble selected for transmission by the UE. Since the time window may be as long as several subframes, one way to vary the time window is to change the number of subframes allowed for reception of the RAR. Figure 16 is a simplified flowchart for the process flow in this embodiment. In advance of normal operation of the network, for example within the system specifications, the available preambles are partitioned (step 142) in the manner noted above. During operation, and having already established communication with the network via one or more eNodeB, the UE signals its ability to use the new control channel by its selection of preamble in a RACH procedure (step 144). That is, by selecting a preamble from the second partition rather than one from the first partition, the UE indicates to the network its readiness to monitor the new control channel.

If necessary (not shown), the eNodeB transmits to the UE the configuration information to enable the UE to monitor E-PDCCH. Then (146), the eNodeB transmits a DCI message to the UE; this may include the notification of the resource it will use for the RAR in the RACH procedure, this being transmitted separately on PDSCH. The UE monitors E-PDCCH and receives the DCI message (148). In a fifth embodiment, two versions of the same DCI message are sent on different control channel and/or search spaces: for example, one on PDCCH and one on E-PDCCH. The location (in terms of both time and frequency domain) of the DCI message in the search space on PDCCH is linked to the location of the DCI message in the search space on E- PDCCH. For example the DCI message transmitted on PDCCH may be transmitted in particular frequency resources within a subframe which are such as to imply other particular frequency resources in which the version of the DCI message carried on E-PDCCH is being transmitted. The respective DCI messages may be located in the same subframe using one of the example resource allocations shown in Figures 10 to 14. In this case a single blind decoding attempt can make use of the received signals corresponding to both PDCCH and E-PDCCH to improve the reliability of the decoding process. A legacy UE would receive only the signal on PDCCH, while a UE operating according to this embodiment would also receive the signal on E-PDCCH and could combine the two signals together to improve probability of correct reception. In a preferred version of this embodiment, the aggregation levels of the two versions of the DCI message are the same. In a sixth embodiment, the UE is configured to search a new search space (which may be on a new control channel) and the meaning of a DCI message depends on which control channel and/or search space it is received by the UE. For example, DCI messages currently transmitted using SPS-C-RNTI on PDCCH, which intended for control of semi-persistent scheduling, could instead be indicated by C-RNTI on E-PDCCH.

Various modifications are possible within the scope of the present invention.

The UE may be configured during operation of the system in particular for the new search space/control channel, but at least the above-mentioned pre-existing configuration may be defined as part of the system configuration. The term "configure" is thus to be interpreted broadly.

Any of the embodiments and variations mentioned above may be combined in the same system. The same eNodeB may operate in accordance with more than one of the embodiments simultaneously, and one UE may likewise operate in accordance with more than one of the embodiments simultaneously. Whilst the above description has been made with respect to LTE and LTE-A, the present invention may have application to other kinds of wireless communication system also. Accordingly, references in the claims to "user equipment" are intended to cover any kind of subscriber station, MTC device and the like and are not restricted to the UE of LTE.

In any of the aspects or embodiments of the invention described above, the various features may be implemented in hardware, or as software modules running on one or more processors. Features of one aspect may be applied to any of the other aspects.

The invention also provides a computer program or a computer program product for carrying out any of the methods described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein.

A computer program embodying the invention may be stored on a computer-readable medium, or it may, for example, be in the form of a signal such as a downloadable data signal provided from an Internet website, or it may be in any other form.

It is to be understood that various changes and/or modifications may be made to the particular embodiments just described without departing from the scope of the claims. To summarise, embodiments of the present invention provide a scheme for the transmission and reception of downlink control channel messages in LTE which can be applied in association with random access transmission (RACH) in the uplink. A feature of

embodiments is that initially, the UE monitors a specific search space on the downlink control channel (PDCCH) for specific types of message (DCI messages) from the eNodeB following a RACH transmission. Subsequently the UE is configured to monitor a different search space and/or control channel (e.g. E-PDCCH) for the same type of DCI message.

Industrial Applicability

Allowing DCI messages to be transmitted on the E-PDCCH rather than the PDCCH reduces the loading on PDCCH and improves control channel efficiency (in terms of resources use). In the case of RACH this raises the limit on the rate at which random access transmissions can be processed. The potential increase in total number of blind decodings required to receive the E-PDCCH may be mitigated by means of limiting the duration of monitoring the E-PDCCH to subframes where the expected DCI messages may be transmitted by the network.

Claims

1. A wireless communication method in which a terminal receives control channel messages from a wireless network, the method comprising:

wherein at least one of said first and second identifiers, said first and second search spaces, and said first and second control channels are non-identical.

2. The method according to claim 1 wherein the first and second control channels have different structures.

3. The method according to claim 1 or 2 wherein the first and second search spaces are different from one another.

4. The method according to claim 1 , 2 or 3 wherein the first and second identifiers are different from one another.

5. The method according to any preceding claim wherein the first and second configurations further include respective sets of timings for monitoring by the terminal.

6. The method according to any preceding claim wherein the wireless network is an LTE network, and the control channel messages are downlink control information DCI, at least the first control channel is the physical downlink control channel PDCCH, at least the first search space is the common search space CSS, and at least the first identifier is a random access radio temporary network identifier RA-RNTI.

7. The method according to claim 6 wherein the second control channel is E-PDCCH.

8. The method according to claim 6 wherein the second search space is UESSS.

9. The method according to claim 6 wherein the second identifier is C-RNTI.

10. The method according to any preceding claim wherein the terminal communicates with the network via both a primary cell and at least one secondary cell, and wherein control channel messages are transmitted on the second control channel via at least one of the cells in response to a request which the terminal transmits to the network via the secondary cell.

11. The method according to claim 6 further comprising the terminal notifying the network, by its choice of PRACH preamble when performing a RACH procedure, of its capability for monitoring control channel messages in accordance with the second configuration.

12. The method according to any preceding claim wherein within a predetermined time interval defined in the network, the terminal monitors control channel messages in accordance with both the first and second configurations.

13. The method according to claim 10 wherein the same control channel messages are transmitted in both configurations, such that the location of the message in the first search space is linked to the location of the message in the second search space.

14. The method according to any of claims 1 to 9 wherein the terminal ceases monitoring control channel messages in accordance with the first configuration, when monitoring control channel messages in accordance with the second configuration.

15. The method according to any preceding claim wherein the meaning of the control channel messages depends upon the control channel and/or search space on which the control channel messages are received.

16. Base station equipment for use in the wireless communication method according to any preceding claim and arranged for transmitting control signals in accordance with said first and second configurations.

17. Terminal for use in the wireless communication method according to any of claims 1 to 15, arranged for monitoring control channel messages in accordance with said first and second configurations.