Patent Description:
The present disclosure relates generally to communication systems. More specifically, the present disclosure relates to systems and methods for synchronization signal and discovery signal transmission for licensed-assisted access (LAA) long term evolution (LTE).

Wireless communication devices have become smaller and more powerful in order to meet consumer needs and to improve portability and convenience. Consumers have become dependent upon wireless communication devices and have come to expect reliable service, expanded areas of coverage and increased functionality. A wireless communication system may provide communication for a number of wireless communication devices, each of which may be serviced by a base station. A base station may be a device that communicates with wireless communication devices.

As wireless communication devices have advanced, improvements in communication capacity, speed, flexibility and/or efficiency have been sought. However, improving communication capacity, speed, flexibility and/or efficiency may present certain problems.

For example, wireless communication devices may communicate with one or more devices using a communication structure. However, the communication structure used may only offer limited flexibility and/or efficiency. As illustrated by this discussion, systems and methods that improve communication flexibility and/or efficiency may be beneficial. <CIT>describes a method and an apparatus for transmitting synchronization signal in carrier aggregation system. Provided is a method and an apparatus for transmitting a synchronization signal in a carrier aggregation system. The method comprises: transmitting via a first serving cell a synchronization signal setting information with regard to a second serving cell; and transmitting the synchronization signal via the second serving cell, wherein the synchronization signal is variably set by means of the synchronization signal setting information.

The contribution "initial discussion on solutions for identified LAA functionalities", R1-<NUM> discusses potential solutions for LAA DL functionalities.

A user equipment (UE) is described. The UE includes a processor and memory in electronic communication with the processor. Instructions stored in the memory are executable to receive a configuration of a licensed-assisted access (LAA) for a serving cell from an evolved node B (eNB). The instructions are also executable to receive a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) of the serving cell. The PSS and the SSS are mapped according to a frame structure of frequency-division duplexing (FDD).

The LAA may be applicable to both of downlink transmissions only and both downlink and uplink transmissions.

The synchronization signal structure of the serving cell may be determined by the PSS and SSS structure and relative location of a duplexing method of a licensed primary cell.

Alternatively, the synchronization signal structure of the serving cell may be determined by whether the serving cell supports downlink (DL) and uplink (UL) transmissions. If the serving cell supports only DL transmissions, the synchronization signal structure of the serving cell may be determined by the PSS and SSS structure and relative location of a FDD serving cell. If the serving cell supports both DL and UL transmissions, the synchronization signal structure of the serving cell may be determined by the PSS and SSS structure and relative location of a TDD serving cell.

The synchronization signal structure of the serving cell may be configured by the eNB.

If the synchronization signal structure of the serving cell is determined by the PSS and SSS structure of a TDD serving cell, a relative position of the PSS and the SSS of the TDD serving cell may be maintained. A location of the PSS and the SSS may be shifted so that the PSS and the SSS are in the same subframe.

The UE may receive the PSS and the SSS of the serving cell in a fixed subframe location in a radio frame. The UE may receive the PSS and the SSS of the serving cell in a fixed subframe location in a burst of subframe transmissions. The PSS and the SSS of the serving cell may be in a first subframe in the burst of subframe transmissions. The PSS and the SSS of the serving cell may be in a fixed subframe index within the LAA set or burst of subframe transmissions.

A method by a UE is also described. The method includes receiving a configuration of a LAA for a serving cell from an eNB. The method also includes receiving a PSS and a SSS of the serving cell. The PSS and the SSS are mapped according to a frame structure of FDD.

An eNB is also described. The eNB includes a processor and memory in electronic communication with the processor. Instructions stored in the memory are executable to configure a LAA for a serving cell for one or more UEs. The instructions are also executable to transmit a PSS and a SSS of the serving cell. The PSS and the SSS are mapped according to a frame structure of FDD.

A method by an eNB is also described. The method includes configuring a LAA for a serving cell for one or more UEs. The method also includes transmitting a PSS and a SSS of the serving cell. The PSS and the SSS are mapped according to a frame structure of frequency-division duplexing (FDD).

A UE for receiving discovery reference signals (DRS) in a LAA serving cell is also described. The UE includes a processor and memory in electronic communication with the processor. Instructions stored in the memory are executable to receive a cell configuration of an unlicensed LAA serving cell from an eNB on a licensed LTE cell. The instructions are also executable to determine a DRS configuration. The instructions are further executable to detect and measure DRS on a configured unlicensed carrier based on the DRS configuration.

The UE may detect and measure the DRS of a LAA serving cell periodically in a fixed subframe location. The UE may detect and measure the DRS of a LAA serving cell in a fixed subframe location in a LAA set or burst of subframe transmissions.

The DRS of the LAA serving cell may be in each LAA set or burst of subframe transmissions. The DRS of the LAA serving cell may be in a first LAA set or burst of subframe transmissions within a DRS measurement timing configuration (DMTC) period.

A method for receiving DRS in an LAA serving cell by a UE is also described. The method includes receiving a cell configuration of an unlicensed LAA serving cell from an eNB on a licensed LTE cell. The method also includes determining a DRS configuration. The method further includes detecting and measuring DRS on a configured unlicensed carrier based on the DRS configuration.

An eNB for transmitting DRS in an LAA serving cell is also described. The eNB includes a processor and memory in electronic communication with the processor. Instructions stored in the memory are executable to configure an unlicensed LAA serving cell for one or more UEs. The instructions are also executable to determine a DRS configuration. The instructions are further executable to transmit DRS on a configured unlicensed carrier based on the DRS configuration.

A method for transmitting DRS in an LAA serving cell by an eNB is also described. The method includes configuring an unlicensed LAA serving cell for one or more UEs. The method also includes determining a DRS configuration. The method further includes transmitting DRS on a configured unlicensed carrier based on the DRS configuration.

The 3rd Generation Partnership Project, also referred to as "3GPP," is a collaboration agreement that aims to define globally applicable technical specifications and technical reports for third and fourth generation wireless communication systems. The 3GPP may define specifications for next generation mobile networks, systems and devices.

At least some aspects of the systems and methods disclosed herein may be described in relation to the 3GPP LTE, LTE-Advanced (LTE-A) and other standards (e.g., 3GPP Releases <NUM>, <NUM>, <NUM>, <NUM> and/or <NUM>). However, the scope of the present disclosure should not be limited in this regard. At least some aspects of the systems and methods disclosed herein may be utilized in other types of wireless communication systems.

A wireless communication device may be an electronic device used to communicate voice and/or data to a base station, which in turn may communicate with a network of devices (e.g., public switched telephone network (PSTN), the Internet, etc.). In describing systems and methods herein, a wireless communication device may alternatively be referred to as a mobile station, a UE, an access terminal, a subscriber station, a mobile terminal, a remote station, a user terminal, a terminal, a subscriber unit, a mobile device, etc. Examples of wireless communication devices include cellular phones, smart phones, personal digital assistants (PDAs), laptop computers, netbooks, e-readers, wireless modems, etc. In 3GPP specifications, a wireless communication device is typically referred to as a UE. However, as the scope of the present disclosure should not be limited to the 3GPP standards, the terms "UE" and "wireless communication device" may be used interchangeably herein to mean the more general term "wireless communication device.

In 3GPP specifications, a base station is typically referred to as a Node B, an eNB, a home enhanced or evolved Node B (HeNB) or some other similar terminology. As the scope of the disclosure should not be limited to 3GPP standards, the terms "base station," "Node B," "eNB," and "HeNB" may be used interchangeably herein to mean the more general term "base station. " Furthermore, the term "base station" may be used to denote an access point. An access point may be an electronic device that provides access to a network (e.g., Local Area Network (LAN), the Internet, etc.) for wireless communication devices. The term "communication device" may be used to denote both a wireless communication device and/or a base station.

It should be noted that as used herein, a "cell" may refer to any set of communication channels over which the protocols for communication between a UE and eNB that may be specified by standardization or governed by regulatory bodies to be used for International Mobile Telecommunications-Advanced (IMT-Advanced) or its extensions and all of it or a subset of it may be adopted by 3GPP as licensed bands (e.g., frequency bands) to be used for communication between an eNB and a UE. "Configured cells" are those cells of which the UE is aware and is allowed by an eNB to transmit or receive information. "Configured cell(s)" may be serving cell(s). The UE may receive system information and perform the required measurements on all configured cells. "Activated cells" are those configured cells on which the UE is transmitting and receiving. That is, activated cells are those cells for which the UE monitors the physical downlink control channel (PDCCH) and in the case of a downlink transmission, those cells for which the UE decodes a physical downlink shared channel (PDSCH). "Deactivated cells" are those configured cells that the UE is not monitoring the transmission PDCCH. It should be noted that a "cell" may be described in terms of differing dimensions. For example, a "cell" may have temporal, spatial (e.g., geographical) and frequency characteristics.

The systems and methods disclosed may involve carrier aggregation. Carrier aggregation refers to the concurrent utilization of more than one carrier. In carrier aggregation, more than one cell may be aggregated to a UE. In one example, carrier aggregation may be used to increase the effective bandwidth available to a UE. The same TDD uplink-downlink (UL/DL) configuration has to be used for TDD carrier aggregation (CA) in Release-<NUM>, and for intra-band CA in Release-<NUM>. In Release-<NUM>, inter-band TDD CA with different TDD UL/DL configurations is supported. The inter-band TDD CA with different TDD UL/DL configurations may provide the flexibility of a TDD network in CA deployment. Furthermore, enhanced interference management with traffic adaptation (eIMTA) (also referred to as dynamic UL/DL reconfiguration) may allow flexible TDD UL/DL reconfiguration based on the network traffic load.

It should be noted that the term "concurrent" and variations thereof as used herein may denote that two or more events may overlap each other in time and/or may occur near in time to each other. Additionally, "concurrent" and variations thereof may or may not mean that two or more events occur at precisely the same time.

An FDD cell requires spectrum (e.g., radio communication frequencies or channels) in which contiguous subsets of the spectrum are entirely allocated to either UL or DL but not both. Accordingly, FDD may have carrier frequencies that are paired (e.g., paired DL and UL carrier frequencies). However, TDD does not require paired channels. Instead, TDD may allocate UL and DL resources on the same carrier frequency. Therefore, TDD may provide more flexibility on spectrum usage. With the increase in wireless network traffic, and as spectrum resources become very precious, new allocated spectrum tends to be fragmented and has smaller bandwidth, which is more suitable for TDD and/or small cell deployment. Furthermore, TDD may provide flexible channel usage through traffic adaptation with different TDD UL/DL configurations and dynamic UL/DL re-configuration.

Synchronization signals may be used to perform time and frequency synchronization of a serving cell carrier. The synchronization signals may include a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). In a licensed LTE cell, the PSS and SSS broadcast periodically in fixed subframe indexes in the central <NUM> subcarriers of the carrier.

Licensed-assisted access (LAA) may support LTE in unlicensed spectrum. In a LAA network, the DL transmission may be scheduled in an opportunistic manner. For fairness utilization, an LAA eNB may perform functions such as clear channel assessment (CCA), listen before talk (LBT) and dynamic frequency selection (DFS). Thus, a LAA transmission may not guarantee a DL transmission in the fixed subframe location that contains the synchronization signals.

The broadcast of synchronization signals in a LAA cell may present different issues. One issue is what PSS and SSS structure should be used in a LAA cell. Another issue is which subframe should be used to carry the PSS and SSS.

Besides the PSS and SSS, the discovery signals of a serving cell may include other signals such as a channel state information-reference signal (CSI-RS) and a cell specific reference signal (CRS). The CSI-RS may be configured by upper layer signaling with a resource position and periodicity. CRS may be transmitted in a configured discovery subframe. However, in a LAA serving cell, the eNB cannot guarantee that the configured subframe can be transmitted due to listen-before-talk requirements. Thus, the same issues exist for other discovery signals as for PSS/SSS transmissions in a LAA network.

Various examples of the systems and methods disclosed herein are now described with reference to the Figures, where like reference numbers may indicate functionally similar elements. The systems and methods as generally described and illustrated in the Figures herein could be arranged and designed in a wide variety of different implementations. Thus, the following more detailed description of several implementations, as represented in the Figures, is not intended to limit scope, as claimed, but is merely representative of the systems and methods.

<FIG> is a block diagram illustrating one implementation of one or more eNBs <NUM> and one or more UEs <NUM> in which systems and methods for synchronization signal and discovery signal transmission may be implemented. The one or more UEs <NUM> communicate with one or more eNBs <NUM> using one or more antennas 122a-n. For example, a UE <NUM> transmits electromagnetic signals to the eNB <NUM> and receives electromagnetic signals from the eNB <NUM> using the one or more antennas 122a-n. The eNB <NUM> communicates with the UE <NUM> using one or more antennas 180a-n.

The UE <NUM> and the eNB <NUM> may use one or more channels <NUM>, <NUM> to communicate with each other. For example, a UE <NUM> may transmit information or data to the eNB <NUM> using one or more uplink channels <NUM>. Examples of uplink channels <NUM> include a PUCCH and a PUSCH, etc. The one or more eNBs <NUM> may also transmit information or data to the one or more UEs <NUM> using one or more downlink channels <NUM>, for instance. Examples of downlink channels <NUM> include a PDCCH, a PDSCH, etc. Other kinds of channels may be used.

Each of the one or more UEs <NUM> may include one or more transceivers <NUM>, one or more demodulators <NUM>, one or more decoders <NUM>, one or more encoders <NUM>, one or more modulators <NUM>, a data buffer <NUM> and a UE operations module <NUM>. For example, one or more reception and/or transmission paths may be implemented in the UE <NUM>. For convenience, only a single transceiver <NUM>, decoder <NUM>, demodulator <NUM>, encoder <NUM> and modulator <NUM> are illustrated in the UE <NUM>, though multiple parallel elements (e.g., transceivers <NUM>, decoders <NUM>, demodulators <NUM>, encoders <NUM> and modulators <NUM>) may be implemented.

The transceiver <NUM> may include one or more receivers <NUM> and one or more transmitters <NUM>. The one or more receivers <NUM> may receive signals from the eNB <NUM> using one or more antennas 122a-n. For example, the receiver <NUM> may receive and downconvert signals to produce one or more received signals <NUM>. The one or more received signals <NUM> may be provided to a demodulator <NUM>. The one or more transmitters <NUM> may transmit signals to the eNB <NUM> using one or more antennas 122a-n. For example, the one or more transmitters <NUM> may upconvert and transmit one or more modulated signals <NUM>.

The demodulator <NUM> may demodulate the one or more received signals <NUM> to produce one or more demodulated signals <NUM>. The one or more demodulated signals <NUM> may be provided to the decoder <NUM>. The UE <NUM> may use the decoder <NUM> to decode signals. The decoder <NUM> may produce one or more decoded signals <NUM>, <NUM>. For example, a first UE-decoded signal <NUM> may comprise received payload data, which may be stored in a data buffer <NUM>. A second UE-decoded signal <NUM> may comprise overhead data and/or control data. For example, the second UE-decoded signal <NUM> may provide data that may be used by the UE operations module <NUM> to perform one or more operations.

As used herein, the term "module" may mean that a particular element or component may be implemented in hardware, software or a combination of hardware and software. However, it should be noted that any element denoted as a "module" herein may alternatively be implemented in hardware. For example, the UE operations module <NUM> may be implemented in hardware, software or a combination of both.

In general, the UE operations module <NUM> may enable the UE <NUM> to communicate with the one or more eNBs <NUM>. The UE operations module <NUM> may include one or more of a UE cell configuration module <NUM> synchronization signals receiving module <NUM> and a discovery reference signals (DRS) receiving module <NUM>.

The UE cell configuration module <NUM> receives a cell configuration of an unlicensed LAA serving cell from an eNB <NUM>. The licensed-assisted access (LAA) in unlicensed band for LTE (also referred to as LTE unlicensed or unlicensed LTE) allows opportunistic usage of an unlicensed carrier for LTE transmissions. In one implementation, only DL LAA is performed. However, in another implementation, both UL and DL transmission may be performed. The LAA transmission is assisted with a licensed band. The UE cell configuration module <NUM> may receive the cell configuration for the LAA serving cell on an LTE cell that is a PCell. The LAA serving cell may be a SCell.

Carrier aggregation (CA) is one operation that may be performed with an unlicensed LAA cell operating with a licensed LTE cell. With CA, the radio frame (e.g., the system frame number (SFN)) may be synchronized across all serving cells. Furthermore, the subframe indexes may also be synchronized. In a CA case, the maximum time alignment (TA) differences among serving cells is <NUM> microseconds.

The synchronization signals are the first signals a UE <NUM> receives from a serving cell to identify the cell. The synchronization signals may include a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). These synchronization signals may be used to achieve radio frame, subframe, slot and symbol synchronization in the time domain, and identify the center of the channel bandwidth in the frequency domain. Thus, the synchronization signals may provide frequency synchronization for reference signals and physical channel resources.

In a licensed LTE serving cell, the synchronization signals (PSS/SSS) may be broadcast within every <NUM> millisecond (ms) radio frame in a fixed subframe and symbol location depending on the frame structure of the serving cell. The frame structure of the serving cell may be FDD or TDD.

In an LAA network, the DL transmission is scheduled in an opportunistic manner. For fairness utilization, a LAA eNB <NUM> is required to perform functions such as clear channel assessment (CCA), listen before talk (LBT) and dynamic frequency selection (DFS). Thus, a LAA transmission cannot guarantee a DL transmission in the fixed subframe location that contains the synchronization signals.

The described systems and methods provide different approaches for synchronization signal transmission and reception in a LAA serving cell. The synchronization signals receiving module <NUM> may determine a synchronization signal structure. The structure of PSS and SSS should follow existing LTE methods as much as possible. Therefore, the PSS/SSS may reuse existing PSS/SSS signals. However, the location of the PSS/SSS is not fixed based on the subframe index as in a LTE serving cell.

In a first approach, the synchronization signals receiving module <NUM> may determine the synchronization signal structure based on the PSS and SSS structure of a FDD serving cell. Therefore, the relative locations of the PSS and the SSS are based on the FDD serving cell.

In a second approach, the synchronization signals receiving module <NUM> may determine the synchronization signal structure based on the PSS and SSS structure and relative location of a duplexing method of a licensed primary cell. In this approach, the relative location of the PSS and the SSS may be based on the licensed PCell frame structure.

In a third approach, the synchronization signals receiving module <NUM> may determine the synchronization signal structure based on whether the LAA serving cell supports downlink (DL) and uplink (UL) transmissions. In this approach, if the LAA serving cell supports only DL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a FDD serving cell.

If the LAA serving cell supports both DL and UL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a TDD serving cell. For a TDD serving cell, a relative position of the PSS and the SSS of the TDD serving cell may be maintained, but the location of the PSS and the SSS may be shifted (compared to LTE systems) so that the PSS and the SSS are in the same subframe. Therefore, the PSS and the SSS locations for the LAA serving cell may be in different subframes or symbols as compared to the LTE systems.

In a fourth approach, the synchronization signal structure of the LAA serving cell is configured by the eNB <NUM>. In this approach, the PSS/SSS relative location is configured by the eNB <NUM>.

The synchronization signals receiving module <NUM> may search for, detect and decode the synchronization signals on a configured unlicensed carrier based on the synchronization signal structure. The synchronization signals receiving module <NUM> may receive the PSS and the SSS of the LAA serving cell on the configured unlicensed carrier.

Different approaches can be considered for synchronization signal transmission in a LAA cell. In a first approach, the LAA cell may broadcast the PSS and the SSS in a fixed subframe location in a radio frame. The PSS/SSS may be allocated in the first LAA DL subframe of each occurrence of a LAA DL transmission. Therefore, the synchronization signals receiving module <NUM> may detect and measure the synchronization signals of the LAA serving cell in a fixed subframe location in a radio frame.

In a second approach for synchronization signal transmission in a LAA cell, the LAA cell may broadcast the PSS and the SSS in a fixed subframe location in a LAA set or burst of subframe transmissions. Therefore, the synchronization signals receiving module <NUM> may detect and measure the PSS and the SSS of the LAA serving cell in a fixed subframe location in a LAA set or a burst of subframe transmissions. The PSS and the SSS of the LAA serving cell may be in the first subframe in the LAA set or burst of subframe transmissions. Alternatively, the PSS and the SSS of the LAA serving cell may be in a fixed subframe index within the LAA set or burst of subframe transmissions.

The synchronization signals receiving module <NUM> may perform subframe synchronization, slot synchronization and frequency synchronization for the LAA serving cell based on the detected PSS and SSS. In a LAA cell, the PSS/SSS may be used to provide subframe, slot synchronization and frequency synchronization. The radio frame synchronization may be provided by the licensed serving cell.

Besides PSS and SSS, the discovery signals of a serving cell may include other signals such as CSI-RS and CRS. The CSI-RS may be configured by upper layer signaling with a resource position and periodicity. CRS may be transmitted in a configured discovery subframe. However, in a LAA serving cell, the eNB <NUM> cannot guarantee that the configured subframe can be transmitted due to listen-before-talk requirements. Thus, the same issue exists for other discovery signals as for PSS/SSS transmissions in a LAA network, and similar methods can be applied to other discovery signals as for PSS/SSS.

The DRS receiving module <NUM> may determine a DRS configuration. A LAA cell may be suitable to be configured as a secondary small cell. The DRS may be applied to a LAA cell with a DRS measurement timing configuration (DMTC) configuration. A DRS measurement timing configuration DMTC may be configured for each frequency carrier. The DMTC may have a periodicity and offset.

The DRS receiving module <NUM> may detect and measure discovery reference signals on a configured unlicensed carrier based on the DRS configuration. It should be noted that the PSS and SSS may be included as a part of the DRS.

In a first approach, the LAA cell may transmit DRS according to the configuration as in a licensed cell. In this approach, the DRS receiving module <NUM> may detect and measure the discovery reference signals of the LAA serving cell periodically in a fixed subframe location. If the subframe is not occupied by LAA transmission following CCA and LBT procedures, the DRS may be broadcast as in regular LTE subframes. However, if the subframe is occupied by other unlicensed transmissions, the LAA cell may follow the CCA and LBT procedures and, thus, should not transmit a LAA subframe. Therefore, in one implementation, the DRS may be dropped if the LAA cell senses that the channel is busy. In another implementation, to keep the DRS broadcasting, only configured DRS may be transmitted, and no signals should be transmitted in other areas of the LAA subframe.

In a second approach for DRS transmissions in a LAA cell, the DRS receiving module <NUM> may detect and measure the DRS of the LAA serving cell in a fixed subframe location in a LAA set or burst of subframe transmissions. The LAA cell may broadcast DRS in a fixed subframe location in a LAA set or burst of subframe transmissions. In one implementation, the DRS of the LAA serving cell may be transmitted in the first several subframes of each LAA set or burst of subframe transmissions. In another implementation, with reduced DRS density, the DRS of the LAA serving cell may be always transmitted in the first LAA set or burst of subframe transmissions within a DMTC period. The DRS occasion may be in the range of <NUM> to <NUM> subframes.

The UE operations module <NUM> may provide information <NUM> to the one or more receivers <NUM>. For example, the UE operations module <NUM> may inform the receiver(s) <NUM> when to receive retransmissions.

The UE operations module <NUM> may provide information <NUM> to the demodulator <NUM>. For example, the UE operations module <NUM> may inform the demodulator <NUM> of a modulation pattern anticipated for transmissions from the eNB <NUM>.

The UE operations module <NUM> may provide information <NUM> to the decoder <NUM>. For example, the UE operations module <NUM> may inform the decoder <NUM> of an anticipated encoding for transmissions from the eNB <NUM>.

The UE operations module <NUM> may provide information <NUM> to the encoder <NUM>. The information <NUM> may include data to be encoded and/or instructions for encoding. For example, the UE operations module <NUM> may instruct the encoder <NUM> to encode transmission data <NUM> and/or other information <NUM>. The other information <NUM> may include PDSCH HARQ-ACK information.

The UE operations module <NUM> may provide information <NUM> to the modulator <NUM>. For example, the UE operations module <NUM> may inform the modulator <NUM> of a modulation type (e.g., constellation mapping) to be used for transmissions to the eNB <NUM>. The modulator <NUM> may modulate the encoded data <NUM> to provide one or more modulated signals <NUM> to the one or more transmitters <NUM>.

The UE operations module <NUM> may provide information <NUM> to the one or more transmitters <NUM>. This information <NUM> may include instructions for the one or more transmitters <NUM>. For example, the UE operations module <NUM> may instruct the one or more transmitters <NUM> when to transmit a signal to the eNB <NUM>. For instance, the one or more transmitters <NUM> may transmit during a UL subframe. The one or more transmitters <NUM> may upconvert and transmit the modulated signal(s) <NUM> to one or more eNBs <NUM>.

The eNB <NUM> may include one or more transceivers <NUM>, one or more demodulators <NUM>, one or more decoders <NUM>, one or more encoders <NUM>, one or more modulators <NUM>, a data buffer <NUM> and an eNB operations module <NUM>. For example, one or more reception and/or transmission paths may be implemented in an eNB <NUM>. For convenience, only a single transceiver <NUM>, decoder <NUM>, demodulator <NUM>, encoder <NUM> and modulator <NUM> are illustrated in the eNB <NUM>, though multiple parallel elements (e.g., transceivers <NUM>, decoders <NUM>, demodulators <NUM>, encoders <NUM> and modulators <NUM>) may be implemented.

The transceiver <NUM> may include one or more receivers <NUM> and one or more transmitters <NUM>. The one or more receivers <NUM> may receive signals from the UE <NUM> using one or more antennas 180a-n. For example, the receiver <NUM> may receive and downconvert signals to produce one or more received signals <NUM>. The one or more received signals <NUM> may be provided to a demodulator <NUM>. The one or more transmitters <NUM> may transmit signals to the UE <NUM> using one or more antennas 180a-n. For example, the one or more transmitters <NUM> may upconvert and transmit one or more modulated signals <NUM>.

The demodulator <NUM> may demodulate the one or more received signals <NUM> to produce one or more demodulated signals <NUM>. The one or more demodulated signals <NUM> may be provided to the decoder <NUM>. The eNB <NUM> may use the decoder <NUM> to decode signals. The decoder <NUM> may produce one or more decoded signals <NUM>, <NUM>. For example, a first eNB-decoded signal <NUM> may comprise received payload data, which may be stored in a data buffer <NUM>. A second eNB-decoded signal <NUM> may comprise overhead data and/or control data. For example, the second eNB-decoded signal <NUM> may provide data (e.g., PDSCH HARQ-ACK information) that may be used by the eNB operations module <NUM> to perform one or more operations.

In general, the eNB operations module <NUM> may enable the eNB <NUM> to communicate with the one or more UEs <NUM>. The eNB operations module <NUM> may include one or more of an eNB cell configuration module <NUM>, an eNB synchronization signals module <NUM> and an eNB DRS module <NUM>.

The eNB cell configuration module <NUM> may configure an unlicensed LAA serving cell for one or more UEs <NUM>. As described above, a LAA serving cell allows opportunistic usage of unlicensed carrier for LTE transmissions. The eNB cell configuration module <NUM> may transmit the cell configuration for the LAA serving cell on an LTE cell that is a PCell. The LAA serving cell may be an SCell.

The eNB synchronization signals module <NUM> may determine a synchronization signal structure. As described above, the synchronization signals may include the PSS and the SSS.

In a first approach, the eNB synchronization signals module <NUM> may determine the synchronization signal structure based on the PSS and SSS structure of a FDD serving cell. FDD PSS/SSS relative location may be used in a LAA serving cell.

In a second approach, the eNB synchronization signals module <NUM> may determine the synchronization signal structure based on the PSS and SSS structure and relative location of a duplexing method of a licensed primary cell. In this approach, the PSS/SSS relative location may be determined by the licensed PCell frame structure.

In a third approach, the eNB synchronization signals module <NUM> may determine the synchronization signal structure based on whether the LAA serving cell supports downlink (DL) and uplink (UL) transmissions. In this approach, if the LAA serving cell supports only DL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a FDD serving cell.

If the LAA serving cell supports both DL and UL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a TDD serving cell. For a TDD serving cell, a relative position of the PSS and the SSS of the TDD serving cell may be maintained, but the location of the PSS and the SSS may be shifted (compared to LTE systems) so that the PSS and the SSS are in the same subframe.

In a fourth approach, the synchronization signal structure of the LAA serving cell is configured by the eNB <NUM>. In this approach, the PSS/SSS relative location may be configured by the eNB synchronization signals module <NUM>.

The eNB synchronization signals module <NUM> may transmit the PSS and the SSS on a configured unlicensed carrier based on the synchronization signal structure. The eNB synchronization signals module <NUM> may transmit the PSS and the SSS of LAA serving cell on the configured unlicensed carrier.

Different approaches can be considered for synchronization signal transmission in a LAA cell. In a first approach, the LAA cell may broadcast the PSS and the SSS in a fixed subframe location in a radio frame. Therefore, the eNB synchronization signals module <NUM> may transmit the PSS and the SSS of the LAA serving cell in a fixed subframe location in a radio frame.

In a second approach for synchronization signal transmission in a LAA cell, the LAA cell may broadcast PSS and SSS in a fixed subframe location in a LAA set or burst of subframe transmissions. Therefore, the eNB synchronization signals module <NUM> may transmit the PSS and the SSS of the LAA serving cell in a fixed subframe location in a LAA set or a burst of subframe transmissions. The PSS and the SSS of the LAA serving cell may be in the first subframe in the LAA set or burst of subframe transmissions. Alternatively, the PSS and the SSS of the LAA serving cell may be in a fixed subframe index within the LAA set or burst of subframe transmissions.

As with the PSS and SSS, the eNB <NUM> may transmit DRS in a LAA serving cell. The eNB DRS module <NUM> may determine a DRS configuration. Discovery signals may be used for a LAA serving cell. The DRS may be applied to a LAA cell with a DMTC configuration. Besides PSS and SSS discussed above, the discovery signals of a LAA serving cell may include other signals such as CSI-RS and CRS.

The eNB DRS module <NUM> may transmit discovery reference signals on a configured unlicensed carrier based on the DRS configuration. In a first approach, the LAA cell may transmit DRS according to the configuration as in a licensed cell. In this approach, the eNB DRS module <NUM> may transmit the discovery reference signals of the LAA serving cell periodically in a fixed subframe location, as described above.

In a second approach for DRS transmissions in a LAA cell, the eNB DRS module <NUM> may transmit the discovery reference signals of the LAA serving cell in a fixed subframe location in a LAA set or burst of subframe transmissions. In this approach, the LAA cell may broadcast DRS in a fixed subframe location in a LAA set or burst of subframe transmissions. In one implementation, the eNB DRS module <NUM> may transmit the DRS of the LAA serving cell in the first several subframes of each LAA set or burst of subframe transmissions. In another implementation, with reduced DRS density, the eNB DRS module <NUM> may always transmit the DRS of the LAA serving cell in the first LAA set or burst of subframe transmissions within a DMTC period.

The eNB operations module <NUM> may provide information <NUM> to the one or more receivers <NUM>. For example, the eNB operations module <NUM> may inform the receiver(s) <NUM> when or when not to receive information based on the PSS and SSS.

The eNB operations module <NUM> may provide information <NUM> to the demodulator <NUM>. For example, the eNB operations module <NUM> may inform the demodulator <NUM> of a modulation pattern anticipated for transmissions from the UE(s) <NUM>.

The eNB operations module <NUM> may provide information <NUM> to the decoder <NUM>. For example, the eNB operations module <NUM> may inform the decoder <NUM> of an anticipated encoding for transmissions from the UE(s) <NUM>.

The eNB operations module <NUM> may provide information <NUM> to the encoder <NUM>. The information <NUM> may include data to be encoded and/or instructions for encoding. For example, the eNB operations module <NUM> may instruct the encoder <NUM> to encode transmission data <NUM> and/or other information <NUM>.

The encoder <NUM> may encode transmission data <NUM> and/or other information <NUM> provided by the eNB operations module <NUM>. The transmission data <NUM> may include network data to be relayed to the UE <NUM>.

The eNB operations module <NUM> may provide information <NUM> to the modulator <NUM>. This information <NUM> may include instructions for the modulator <NUM>. For example, the eNB operations module <NUM> may inform the modulator <NUM> of a modulation type (e.g., constellation mapping) to be used for transmissions to the UE(s) <NUM>. The modulator <NUM> may modulate the encoded data <NUM> to provide one or more modulated signals <NUM> to the one or more transmitters <NUM>.

The eNB operations module <NUM> may provide information <NUM> to the one or more transmitters <NUM>. This information <NUM> may include instructions for the one or more transmitters <NUM>. For example, the eNB operations module <NUM> may instruct the one or more transmitters <NUM> when to (or when not to) transmit a signal to the UE(s) <NUM>. In some implementations, this may be based on the PSS and SSS. The one or more transmitters <NUM> may upconvert and transmit the modulated signal(s) <NUM> to one or more UEs <NUM>.

It should be noted that a DL subframe may be transmitted from the eNB <NUM> to one or more UEs <NUM> and that a UL subframe may be transmitted from one or more UEs <NUM> to the eNB <NUM>. Furthermore, both the eNB <NUM> and the one or more UEs <NUM> may transmit data in a standard special subframe.

It should also be noted that one or more of the elements or parts thereof included in the eNB(s) <NUM> and UE(s) <NUM> may be implemented in hardware. For example, one or more of these elements or parts thereof may be implemented as a chip, circuitry or hardware components, etc. It should also be noted that one or more of the functions or methods described herein may be implemented in and/or performed using hardware. For example, one or more of the methods described herein may be implemented in and/or realized using a chipset, an application-specific integrated circuit (ASIC), a large-scale integrated circuit (LSI) or integrated circuit, etc..

<FIG> is a flow diagram illustrating one implementation of a method <NUM> for receiving synchronization signals in a LAA serving cell. The method <NUM> may be implemented by a UE <NUM>. The UE <NUM> may communicate with one or more eNBs <NUM> in a wireless communication network. In one implementation, the wireless communication network may include an LTE network.

The UE <NUM> may receive <NUM> a cell configuration of an unlicensed LAA serving cell from an eNB <NUM> on a licensed LTE cell. When a UE <NUM> is powered-on, the UE <NUM> may attempt to find a suitable cell to camp-on. However, in-order to camp on a particular cell, the UE <NUM> may perform a number of activities. For example, the UE <NUM> may perform a frequency search. The UE <NUM> may also perform cell synchronization. The UE <NUM> may further determine a physical cell ID. The UE <NUM> may additionally read a master information block (MIB).

When the UE <NUM> is switched on, it may scan and tune its radio to a frequency depending on which band it is supporting. If the UE <NUM> is tuned to a particular frequency channel, it will try finding the PSS and SSS. In one approach, the PSS may be transmitted in the last OFDM symbol of first time slot of the first and sixth sub-frame of a radio frame. From the PSS, the UE <NUM> may obtain a cell identity in group ranges from <NUM> to <NUM>.

Once the UE <NUM> obtains the PSS, the UE <NUM> may next obtain the SSS. The SSS signals may be transmitted in the same sub-frame as the PSS but in the symbol just before PSS. From the SSS, the UE <NUM> may obtain physical layer cell identity group ranges from <NUM> to <NUM>.

Using the cell identity in the group and physical cell identity group number, the UE <NUM> may calculate a physical cell ID (PCI) for a cell. This may be accomplished according to PCI = <NUM>*(Physical Cell Identity Group) + (Cell Identity In Group). Using this equation for the PCI, <NUM> unique physical cell identities can be created. Using this PCI, the UE <NUM> may detect the cell specific reference signal (CRS) that is used in channel estimation and cell selection.

A frequency search and cell synchronization procedure may be summarized as follows. The UE <NUM> may be switched on. The UE <NUM> may find a frequency and tune its radio to the frequency. The UE <NUM> may find the PSS and the SSS to determine the physical cell ID. Using this physical cell ID, the UE <NUM> may find the reference signal for channel estimation and cell selection. After obtaining the physical cell ID and reference signal location, the UE <NUM> will be able to read the MIB.

Upon reading the MIB, the UE <NUM> may proceed with reading other system information blocks (SIBs) and may perform cell selection. As demonstrated by this discussion, if the cell synchronization procedure fails and UE <NUM> is unable to determine the physical cell ID, camping on a cell may not succeed.

For a secondary serving cell (SCell), the cell configuration may be provided by the primary cell (PCell) radio resource control (RRC) configuration. A UE <NUM> may not be required to monitor the physical broadcast channel (PBCH) on a secondary cell (SCell). However, the PSS and SSS synchronization signals are still needed to perform time and frequency synchronization of the SCell. Therefore, the UE <NUM> receives the cell configuration for the LAA serving cell that is an SCell from an eNB <NUM> on an LTE cell that is the PCell.

The UE <NUM> may determine <NUM> a synchronization signal structure. There may be both FDD and TDD versions of LTE broadcast synchronization signals in the downlink (DL) direction. As described above, these synchronization signals may include the PSS and the SSS. The synchronization signals may be broadcast within every <NUM> radio frame. The UE <NUM> may use the synchronization signals to achieve radio frame, subframe, slot and symbol synchronization in the time domain. The UE <NUM> may also use the synchronization signals to identify the center of the channel bandwidth in the frequency domain. The UE <NUM> may further use the synchronization signals to deduce the PCI.

Detecting the synchronization signals may be a prerequisite to measuring the cell-specific reference signals and decoding the MIB on the PBCH. In one implementation, the PSS is broadcast twice during every radio frame and both transmissions are identical. The SSS may also be broadcast twice within every radio frame. The two transmissions of the SSS are different so the UE <NUM> can detect which is the first and which is the second transmission. It should be noted that the PSS cannot be used to achieve radio frame synchronization because both transmissions within the radio frame are identical and equally spaced in time.

The PSS may be used to achieve subframe, slot and symbol synchronization in the time domain. The PSS may also be used to identify the center of the channel bandwidth in the frequency domain. The PSS may further be used to deduce a pointer towards one of <NUM> PCI. PCI may be organized into <NUM> groups of <NUM>. Therefore, the PSS identifies the position of the PCI within the group but does not identify the group itself.

The SSS may be used to achieve radio frame synchronization. The SSS may also be used to deduce a pointer towards one of <NUM> PCI groups. The SSS may also allow the PCI to be deduced when combined with the pointer from the PSS.

In the case of FDD, the PSS may be broadcast using the central <NUM> subcarriers belonging to the last symbol of time slots <NUM> and <NUM>. Furthermore, in the case of FDD, the SSS may be broadcast using the central <NUM> subcarriers belonging to the second to last symbol of time slots <NUM> and <NUM>. An example of PSS and SSS timing for FDD is described in connection with <FIG>.

In the case of TDD, the PSS is broadcast using the central <NUM> subcarriers belonging to the third symbol of time slot <NUM> (e.g., subframe <NUM>) and the third symbol of time slot <NUM> (e.g., subframe <NUM>). Furthermore, in the case of TDD, the SSS is broadcast using the central <NUM> subcarriers belonging to the last symbol of time slot <NUM> (subframe <NUM>) and the last symbol of time slot <NUM> (subframe <NUM>). An example of PSS and SSS timing for TDD is described in connection with <FIG>.

The set of resource elements allocated to the synchronization signals may be independent of the channel bandwidth. The UE <NUM> may not require any knowledge of the channel bandwidth prior to detecting the synchronization signals. The downlink channel bandwidth may be subsequently read from the MIB on the PBCH.

In a LAA network, DL transmission is scheduled in an opportunistic manner. For co-existence with other networks on the same carrier, such as WiFi or LAA of the same or a different operator, a LAA eNB <NUM> may perform some functions to minimize interference. These functions may include clear channel assessment (CCA), listen before talk (LBT) and dynamic frequency selection (DFS). Thus, a LAA transmission may not guarantee a DL transmission in the fixed subframe location that contains the synchronization signals.

Therefore, a first LAA subframe transmission may need to perform carrier sensing, and if there is no ongoing transmission, the LAA subframe may be transmitted. Otherwise, the LAA cell should defer the transmission and perform a clear channel assessment again at the next subframe boundary.

In LAA, the serving cell should be synchronized with a licensed cell with a maximum timing advance difference of <NUM> microseconds. The time used for carrier sensing and CCA will be removed from the first LAA subframe transmission. Thus, the first LAA subframe may reserve several OFDM symbols for CCA (e.g., <NUM> or <NUM> or <NUM> OFDM symbols can be used for carrier sensing). If the channel is idle in the reserved period, a LAA subframe can be transmitted. The first LAA subframe may be a reduced LTE subframe with fewer OFDM symbols by removing the reserved length for carrier sensing.

To provide fairness to other networks on the same unlicensed carrier, the eNB <NUM> may configure a maximum number of continuous subframe transmissions k in a LAA cell (e.g., a set of LAA subframes or a burst of LAA subframes). The maximum transmission time in an unlicensed carrier may be different in different regions and/or countries based on the regulatory requirements. For example, the maximum transmission time during an unlicensed transmission may be approximately <NUM> in Japan; the maximum transmission time during unlicensed transmission is <NUM> in Europe. In one approach, the maximum number of continuous subframe transmissions k may be implicitly determined by the region/country regulator requirement. In another approach, the maximum number of continuous subframe transmissions k may be explicitly configured by higher layer signaling. An example of a LAA subframe burst transmission is described in connection with <FIG>. An example of LAA transmissions with coexistence of other unlicensed transmissions is described in connection with <FIG>.

As described above, the synchronization signals may be the first signals that a UE <NUM> needs to search for in order to obtain the cell ID, time and frequency synchronization. The synchronization signals may be in fixed locations in a licensed LTE cell. However, in a LAA serving cell, because of potential transmissions from other unlicensed networks (e.g., WiFi or other LAA cell), the eNB <NUM> may not guarantee that the synchronization signal can be transmitted in fixed subframe indexes, as shown in <FIG>.

Different approaches may be used for transmission of synchronization signals in LAA cells. To reuse proven LTE technologies, in a LAA cell, the PSS and SSS sequences of LTE should be reused. The PSS and SSS should be broadcast using the central <NUM> subcarriers. Furthermore, the structure and relative positions between PSS and SSS should be preserved.

The structure of PSS and SSS may follow existing LTE technologies as much as possible. However, with this approach, an issue is which PSS/SSS structure and relative location should be used: a FDD or a TDD type.

A benefit of FDD PSS/SSS is that the SSS and PSS are continuous in time. Thus, in a LAA cell with PSS/SSS structure following FDD cell (i.e., a frame structure type <NUM>), the SSS may be broadcast using the central <NUM> subcarriers belonging to the second to last symbol of time slots <NUM> of a LAA subframe carrying synchronization signal. The PSS may be broadcast using the central <NUM> subcarriers belonging to the last symbol of time slots <NUM> of a LAA subframe carrying synchronization signal. An example of a LAA synchronization signal structure that follows a FDD type is described in connection with <FIG>.

The TDD type synchronization signals may be located in two subframes. The second subframe may be a DL subframe or a special subframe. The TDD type synchronization signals may be separated by two OFDM symbols, as described above. A LAA cell may use the same PSS and SSS location as in a licensed serving cell. If the same TDD PSS and SSS structure is used, the PSS and SSS will be transmitted in two consecutive subframes.

However, for a LAA cell, it is not suitable to distribute the PSS/SSS into two subframes for fairness with other unlicensed transmissions. If there is no data to be transmitted, a LAA cell should not occupy another subframe just to send the synchronization signals. Therefore, if the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure of a TDD serving cell (e.g., if the TDD type synchronization signals are used), the relative position of PSS and SSS can be maintained, but the location of PSS and SSS should be shifted so that the PSS and SSS are in the same subframe. An example of a LAA synchronization signal structure that follows a TDD type is described in connection with <FIG>.

There are several approaches that can be considered to determine the PSS/SSS structure and relative location in a LAA cell. In a first approach, the UE <NUM> may determine <NUM> the synchronization signal structure based on the PSS and SSS structure of a FDD serving cell. The FDD PSS/SSS relative location may be used in a LAA serving cell. A benefit of the FDD PSS/SSS approach is that the SSS and PSS are continuous in time. For a LAA cell that supports DL only, the LAA cell can be regarded as a FDD DL carrier, thus FDD PSS/SSS is a better fit. For a LAA cell that supports both UL and DL transmissions, the FDD PSS/SSS relative location can also be used for its simplicity and continuous transmission.

In a second approach, the UE <NUM> may determine <NUM> the synchronization signal structure based on the PSS and SSS structure and relative location of a duplexing method of a licensed primary cell. In this approach, the PSS/SSS relative location may be determined by the licensed PCell frame structure. If the licensed PCell has FDD structure (i.e., subframe frame structure type <NUM>), the FDD PSS/SSS structure and relative position should be used. If the licensed PCell has a TDD structure (i.e., a subframe frame structure type <NUM>), the TDD PSS/SSS structure and relative position should be used.

In a third approach, the UE <NUM> may determine <NUM> the synchronization signal structure based on whether the LAA serving cell supports downlink (DL) and uplink (UL) transmissions. In this approach, the PSS/SSS relative location may be determined by whether the LAA cell supports both DL and UL transmissions. If a LAA cell is configured with only DL transmissions, the FDD PSS/SSS structure and relative position should be used. Therefore, if the LAA serving cell supports only DL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a FDD serving cell.

If a LAA cell is configured with both UL and DL transmissions, the TDD PSS/SSS structure and relative position should be used. Therefore, if the LAA serving cell supports both DL and UL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a TDD serving cell.

In a fourth approach, the synchronization signal structure of the LAA serving cell is configured by the eNB <NUM>. In this approach, the PSS/SSS relative location is configured by the eNB <NUM>. This approach may provide more flexibility.

If the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure of a TDD serving cell (e.g., if the TDD type synchronization signals are used), the relative position of PSS and SSS can be maintained, but the location of PSS and SSS may be shifted so that the PSS and SSS are in the same subframe. Alternatively, when PSS and SSS following TDD structure are transmitted, at least two consecutive LAA subframes should be transmitted.

The UE <NUM> detects and decodes <NUM> the PSS and the SSS on a configured unlicensed carrier based on the synchronization signal structure. The LAA serving cell transmits the PSS and the SSS on the configured unlicensed carrier. Because of carrier sensing and deferred transmission, how to determine the transmission location of the PSS and the SSS in a LAA cell may need to be defined.

Different approaches can be considered for synchronization signal transmission in a LAA cell. In a first approach, the LAA cell may broadcast the PSS and the SSS in a fixed subframe location in a radio frame. Therefore, the UE <NUM> may detect and decode <NUM> the synchronization signals of the LAA serving cell in a fixed subframe location in a radio frame. This approach may be similar to a licensed cell.

If the subframe is not occupied by other transmissions, the LAA transmission may follow CCA and LBT procedures, and the PSS and SSS may be broadcast as in regular LTE subframes. However, if the subframe is occupied by other unlicensed transmissions (e.g. another LAA or WiFi transmission), the LAA cell may follow the CCA and LBT procedures, but should not transmit a LAA subframe. Two approaches may be considered for this case.

In a first approach for the case when a subframe is occupied by other unlicensed transmissions, the LAA cell may defer the transmission in the subframe, and the corresponding PSS/SSS may be dropped. Thus, the UE <NUM> may detect PSS/SSS in a fixed location, but may expect the PSS/SSS is not transmitted. If the PSS/SSS is not detected in the fixed location, the subframe should also be dropped.

In a second approach for the case when a subframe is occupied by other unlicensed transmissions, to keep the PSS and SSS broadcasting, only PSS and SSS may be transmitted, and no other signals are transmitted in other areas of the LAA subframe. Although this approach may violate the listen before talk principle, this approach also has several benefits. For example, it is backward compatible with licensed LTE synchronization and cell search methods. Furthermore, the PSS and SSS only occupy the central <NUM> subcarriers of two OFDM symbols in the subframe, and may not cause a significant interference to other transmissions on the same unlicensed band.

In a second approach for synchronization signal transmission in a LAA cell, the LAA cell may broadcast PSS and SSS in a fixed subframe location in a LAA set or burst of subframe transmissions. Therefore, the UE <NUM> may detect and decode <NUM> the PSS and the SSS of the LAA serving cell in a fixed subframe location in a LAA set or a burst of subframe transmissions. The synchronization signals may always be transmitted in the first subframe of each occurrence of a LAA set or burst of subframe transmissions. Therefore, the PSS and the SSS of the LAA serving cell may be in the first subframe in the LAA set or burst of subframe transmissions. Alternatively, the PSS and the SSS of the LAA serving cell may be in a fixed subframe index within the LAA set or burst of subframe transmissions. An example of a FDD PSS/SSS structure in a burst of LAA subframe transmissions is described in connection with <FIG>.

The UE <NUM> performs <NUM> subframe synchronization, slot synchronization and frequency synchronization for the LAA serving cell based on the detected PSS and SSS. In a LAA cell, the PSS/SSS may be used to provide subframe, slot synchronization and frequency synchronization. The radio frame synchronization is provided by the licensed serving cell.

<FIG> is a flow diagram illustrating on implementation of a method <NUM> for transmitting synchronization signals in a LAA serving cell. The method <NUM> may be implemented by an eNB <NUM>. The eNB <NUM> may communicate with one or more UEs <NUM> in a wireless communication network. In one implementation, the wireless communication network may include an LTE network.

The eNB <NUM> configures <NUM> an unlicensed LAA serving cell for one or more UEs <NUM>. As described above, a LAA serving cell allows opportunistic usage of unlicensed carrier for LTE transmissions. The eNB <NUM> transmits the cell configuration for the LAA serving cell on an LTE cell that is a PCell. The LAA serving cell may be an SCell.

The eNB <NUM> determines <NUM> a synchronization signal structure. This may be accomplished as described above in connection with <FIG>. As described above, the synchronization signals include the PSS and the SSS.

In a first approach, the eNB <NUM> may determine <NUM> the synchronization signal structure based on the PSS and SSS structure of a FDD serving cell. An FDD PSS/SSS relative location may be used in a LAA serving cell.

In a second approach, the eNB <NUM> may determine <NUM> the synchronization signal structure based on the PSS and SSS structure and relative location of a duplexing method of a licensed primary cell. In this approach, the PSS/SSS relative location may be determined by the licensed PCell frame structure.

In a third approach, the eNB <NUM> may determine <NUM> the synchronization signal structure based on whether the LAA serving cell supports downlink (DL) and uplink (UL) transmissions. In this approach, if the LAA serving cell supports only DL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a FDD serving cell. If the LAA serving cell supports both DL and UL transmissions, the synchronization signal structure of the LAA serving cell is determined by the PSS and SSS structure and relative location of a TDD serving cell.

The eNB <NUM> transmits <NUM> the PSS and the SSS on a configured unlicensed carrier based on the synchronization signal structure. The eNB <NUM> transmits the PSS and the SSS of LAA serving cell on the configured unlicensed carrier.

Different approaches can be considered for synchronization signal transmission in a LAA cell. In a first approach, the LAA cell may broadcast the PSS and the SSS in a fixed subframe location in a radio frame. Therefore, the eNB <NUM> may transmit <NUM> the PSS and the SSS of the LAA serving cell in a fixed subframe location in a radio frame.

In a second approach for synchronization signal transmission in a LAA cell, the LAA cell may broadcast PSS and SSS in a fixed subframe location in a LAA set or burst of subframe transmissions. Therefore, the eNB <NUM> may transmit <NUM> the PSS and the SSS of the LAA serving cell in a fixed subframe location in a LAA set or a burst of subframe transmissions. The PSS and the SSS of the LAA serving cell may be in the first subframe in the LAA set or burst of subframe transmissions. Alternatively, the PSS and the SSS of the LAA serving cell may be in a fixed subframe index within the LAA set or burst of subframe transmissions.

<FIG> illustrates one example of timing of synchronization signals for FDD. The synchronization signals may include a PSS <NUM> and an SSS <NUM>. <FIG> illustrates a <NUM> radio frame <NUM> that includes ten subframes <NUM>. Each subframe <NUM> may be divided into time slots <NUM>.

As described above, the PSS <NUM> may be broadcast twice during every radio frame <NUM> and both transmissions are identical. In the case of FDD, the PSS <NUM> may be broadcast using the central <NUM> subcarriers belonging to the last symbol <NUM> of time slots <NUM> and <NUM>. In this example, one PSS 429a is broadcast in symbol <NUM> of time slot <NUM>, and another PSS 429b is broadcast in symbol <NUM> of time slot <NUM>.

The SSS <NUM> is broadcast twice within every radio frame <NUM>. The two transmissions of the SSS <NUM> are different so the UE <NUM> can detect which is the first and which is the second transmission. In the case of FDD, the SSS <NUM> is broadcast using the central <NUM> subcarriers belonging to the second to last symbol <NUM> of time slots <NUM> and <NUM>. In this example, one SSS 431a is broadcast in symbol <NUM> of time slot <NUM>, and another SSS 431b is broadcast in symbol <NUM> of time slot <NUM>.

This example assumes a normal cyclic prefix because there are <NUM> symbols <NUM> within each time slot <NUM>. An extended cyclic prefix may follow a similar pattern, except there are only <NUM> symbols <NUM> within the time slot <NUM> (e.g., the SSS <NUM> and PSS <NUM> may remain within the last two symbols <NUM> of the time slot <NUM>).

<FIG> illustrates one example of timing of synchronization signals for TDD. The synchronization signals may include a PSS <NUM> and an SSS <NUM>. <FIG> illustrates a <NUM> radio frame <NUM> that includes ten subframes <NUM>. Each subframe <NUM> may be divided into time slots <NUM>.

In the case of TDD, the PSS <NUM> is broadcast using the central <NUM> subcarriers belonging to the third symbol <NUM> of time slot <NUM> (e.g., subframe <NUM>) and the third symbol <NUM> of time slot <NUM> (e.g., subframe <NUM>). <FIG> shows the symbols 527a of subframe <NUM> and subframe <NUM> and the symbols 527b of subframe <NUM> and subframe <NUM>. The PSS 529a may be sent in the third symbol <NUM> of time slot <NUM>. The PSS 529b may be sent in the third symbol <NUM> of time slot <NUM>.

Subframe <NUM> may be a special subframe <NUM> so the PSS 529a is sent as part of the downlink pilot time slot (DwPTS). Subframe <NUM> may or may not be a special subframe <NUM>, depending upon the uplink-downlink subframe configuration. It is a special subframe <NUM> for configurations <NUM>, <NUM>, <NUM> and <NUM>. Otherwise it is a normal downlink subframe <NUM>.

The SSS <NUM> is broadcast using the central <NUM> subcarriers belonging to the last symbol <NUM> of time slot <NUM> (subframe <NUM>) and the last symbol <NUM> of time slot <NUM> (subframe <NUM>). Both time slots <NUM> and <NUM> may be within normal downlink subframes <NUM>.

In the case of TDD, the SSS <NUM> and PSS <NUM> are not in adjacent symbols <NUM>. The first two symbols <NUM> within time slots <NUM> and <NUM> are left available for the Physical Control Format Indicator Channel (PCFICH), the Physical Hybrid-ARQ Indicator Channel (PHICH) and Physical Downlink Control Channel (PDCCH).

This example assumes the normal cyclic prefix, uplink-downlink subframe configuration <NUM> and special subframe configuration <NUM>. The extended cyclic prefix follows a similar pattern except there are only <NUM> symbols <NUM> within the time slot <NUM>. For the extended cyclic prefix, the SSS <NUM> may remain within the last symbol <NUM> of time slots <NUM> and <NUM>, while the PSS <NUM> remains within the third symbol <NUM> of time slots <NUM> and <NUM>.

<FIG> illustrates an example of a LAA subframe burst <NUM> transmission. This transmission may also be referred to as a LAA subframe set transmission. To provide fairness to other networks on the same unlicensed carrier, the eNB <NUM> may configure a maximum number of continuous subframe transmissions k in a LAA cell (e.g., a set of LAA subframes or a burst of LAA subframes). The maximum transmission time in an unlicensed carrier may be different in different regions and/or countries based on the regulatory requirements.

In this example, the subframe is configured with normal cyclic prefix. The first two OFDM symbol length is reserved for carrier sensing. Thus, subframe <NUM> in a set of LAA subframes is a subframe with a reduced number of symbols. No sensing is necessary for continuous LAA subframe transmission after the first LAA subframe. The regular LTE subframe structure may be applied on consecutive subframes in a LAA subframe set.

It should be noted that the subframe index number in <FIG> refers to the index in a LAA subframe burst, instead of the subframe index in a radio frame as in legacy LTE cells.

<FIG> illustrates an example of LAA coexistence with other unlicensed transmissions. A licensed serving cell <NUM> is shown with a <NUM> radio frame <NUM>. A LAA serving cell <NUM> has LAA serving cell transmissions and other unlicensed transmissions (e.g., Wi-Fi or other LAA cells). Due to carrier sensing and deferred transmissions, the starting of a LAA transmission may be any subframe index in the radio frame <NUM> of the licensed frame structure.

<FIG> illustrates a LAA synchronization signal structure that follows FDD. An LAA subframe <NUM> is shown with synchronization symbols (e.g., PSS <NUM> and SSS <NUM>). In this example, the PSS/SSS in a LAA cell follows an FDD structure with normal prefix. The extended cyclic prefix follows a similar pattern except there are only <NUM> symbols <NUM> within the time slot <NUM>. The SSS <NUM> and PSS <NUM> may remain within the last two symbols <NUM> of the time slot <NUM>.

<FIG> illustrates a LAA synchronization signal structure that follows TDD with shifted PSS and SSS locations within one subframe. An LAA subframe <NUM> is shown with synchronization symbols (e.g., PSS <NUM> and SSS <NUM>). <FIG> shows two examples of a PSS/SSS structure following a TDD cell (e.g., frame structure type <NUM>) in a LAA cell with normal cyclic prefix. The extended cyclic prefix may follow a similar pattern except there are only <NUM> symbols <NUM> within the time slot <NUM>.

In one example (illustrated by symbols 927a), the PSS 929a is broadcast using the central <NUM> subcarriers belonging to the third symbol <NUM> of time slot <NUM> of the LAA subframe <NUM> with synchronization signal. The SSS 931a is broadcast using the central <NUM> subcarriers belonging to the last symbol <NUM> of time slot <NUM> of the LAA subframe <NUM> with synchronization signal.

In another example (illustrated by symbols 927b), the PSS 929b is broadcast using the central <NUM> subcarriers belonging to the last symbol <NUM> of time slot <NUM> of the LAA subframe <NUM> with synchronization signal. The SSS 931b is broadcast using the central <NUM> subcarriers belonging to the fourth to last symbol <NUM> of time slot <NUM> of the LAA subframe <NUM> with synchronization signal.

<FIG> illustrates an example of PSS/SSS transmissions in a LAA cell. Subframes <NUM> are illustrated in a burst of LAA subframe <NUM> transmissions. The LAA cell may broadcast PSS <NUM> and SSS <NUM> in a fixed subframe location in a LAA set or burst of subframe <NUM> transmissions. The synchronization signals (e.g., PSS <NUM> and SSS <NUM>) may be transmitted in the first subframe of each occurrence of a LAA set or burst of subframe <NUM> transmissions.

There are several benefits to broadcast the PSS and SSS in the first subframe of a burst of LAA subframe transmissions. The beginning of a LAA transmission can be detected based on PSS/SSS detection. The synchronization of each burst of LAA subframes <NUM> is synchronized with the latest set of synchronization signals, which may improve accuracy.

Similarly, if the LAA cell is configured with a maximum number of LAA subframes <NUM> in a burst transmission that is greater than <NUM>, the synchronization signals may be transmitted in a fixed subframe index within each occurrence of a LAA set or burst of subframe <NUM> transmissions. For example, the synchronization signals may be transmitted in subframe index <NUM> of each LAA burst of subframe <NUM> transmissions. If the maximum number of LAA subframes <NUM> in a burst transmission is greater than <NUM>, the synchronization signals may be transmitted in two fixed subframe indexes in the LAA burst transmissions. For example, if the maximum number of LAA subframes in a burst transmission is <NUM>, the PSS <NUM> and SSS <NUM> can be allocated in subframe indexes <NUM> and <NUM> in the LAA subframe burst transmissions.

<FIG> is a flow diagram illustrating one implementation of a method <NUM> for receiving discovery reference signals (DRS) in a LAA serving cell. The method <NUM> may be implemented by a UE <NUM>. The UE <NUM> may communicate with one or more eNBs <NUM> in a wireless communication network. In one implementation, the wireless communication network may include an LTE network.

The UE <NUM> may receive <NUM> a cell configuration of an unlicensed LAA serving cell from an eNB <NUM> on a licensed LTE cell. This may be accomplished as described above in connection with <FIG>. The eNB <NUM> may transmit the cell configuration for the LAA serving cell on an LTE cell that is a PCell. The LAA serving cell may be an SCell.

The UE <NUM> may determine <NUM> a DRS configuration. In one implementation, discovery signals may be used for small cells. DRS may be defined for small cell enhancements. Besides PSS and SSS, the discovery signals of a serving cell may include other signals such as a cell specific reference signal (CRS) and a channel state information-reference signal (CSI-RS). The discovery signals may be configured by a higher layer. CRS should be transmitted in a configured discovery subframe. The CSI-RS may be configured by upper layer signaling with a resource position and periodicity. Additionally, CSI-RS is assumed in the DRS for measurement if configured by higher layers. The UE <NUM> may use the DRS to obtain a transmit point identification (TPID).

A DRS measurement timing configuration (DMTC) may be configured for each frequency carrier. The DMTC may have a periodicity and offset. The DMTC periodicity may be configurable at least to <NUM>, <NUM>, or <NUM>. The duration of DMTC may be fixed to <NUM>.

The maximum duration of a DRS occasion may be <NUM> subframes and may be signaled per frequency to UEs <NUM>. The duration of the DRS occasion may be in the range of <NUM> and <NUM> subframes for FDD and in the range of <NUM> and <NUM> subframes for TDD, and is the same for all cells on one frequency. The SSS may occur in the first subframe of a DRS occasion. But the DRS occasion offset and duration may not be signaled. Once configured, a UE <NUM> may detect the periodic DRS from a small cell for cell synchronization and identification.

In another implementation, discovery signals may be used for a LAA serving cell. A LAA cell may be most suitable to be configured as a secondary small cell. Thus, the DRS may also be applied to a LAA cell with a DMTC configuration. Besides PSS and SSS discussed above, the discovery signals of a LAA serving cell may include other signals such as CSI-RS and CRS.

However, in a LAA serving cell, the eNB <NUM> may not guarantee that the configured DRS subframe can be transmitted due to listen-before-talk requirements. Thus, a similar issue exists for other discovery signals as for PSS/SSS transmissions in a LAA network. Similar methods can be applied to other discovery signals as for PSS/SSS.

The UE <NUM> may detect and measure <NUM> discovery reference signals on a configured unlicensed carrier based on the DRS configuration. Several approaches can be considered for DRS transmissions in a LAA cell. It should be noted that the PSS and SSS may be included as a part of the DRS.

In a first approach, the LAA cell may transmit DRS according to the configuration as in a licensed cell. In this approach, the UE <NUM> may detect and measure <NUM> the discovery reference signals of the LAA serving cell periodically in a fixed subframe location. If the subframe is not occupied by LAA transmission following CCA and LBT procedures, the DRS may be broadcast as in regular LTE subframes. However, if the subframe is occupied by other unlicensed transmissions, the LAA cell may follow the CCA and LBT procedures and, thus, should not transmit a LAA subframe. Therefore, in one implementation, the DRS may be dropped if the LAA cell senses that the channel is busy. In another implementation, to keep the DRS broadcasting, only configured DRS may be transmitted, and no signals should be transmitted in other areas of the LAA subframe.

In a second approach for DRS transmissions in a LAA cell, the UE <NUM> may detect and measure <NUM> the discovery reference signals of the LAA serving cell in a fixed subframe location in a LAA set or burst of subframe transmissions. The LAA cell may broadcast DRS in a fixed subframe location in a LAA set or burst of subframe transmissions. In one implementation, the DRS of the LAA serving cell may be transmitted in the first several subframes of each LAA set or burst of subframe transmissions. In another implementation, with reduced DRS density, the DRS of the LAA serving cell may always be transmitted in the first LAA set or burst of subframe transmissions within a DMTC period. The DRS occasion may be in the range of <NUM> to <NUM> subframes. An example of DRS transmissions in a LAA cell is described in connection with <FIG>.

<FIG> is a flow diagram illustrating on implementation of a method <NUM> for transmitting DRS in a LAA serving cell. The method <NUM> may be implemented by an eNB <NUM>. The eNB <NUM> may communicate with one or more UEs <NUM> in a wireless communication network. In one implementation, the wireless communication network may include an LTE network.

The eNB <NUM> may configure <NUM> an unlicensed LAA serving cell for one or more UEs <NUM>. As described above, a LAA serving cell allows opportunistic usage of unlicensed carrier for LTE transmissions. The eNB <NUM> may transmit the cell configuration for the LAA serving cell on an LTE cell that is a PCell. The LAA serving cell may be an SCell.

The eNB <NUM> may determine <NUM> a DRS configuration. In one implementation, discovery signals may be used for small cells. Discovery signals may be used for a LAA serving cell. The DRS may be applied to a LAA cell with a DMTC configuration. Besides PSS and SSS discussed above, the discovery signals of a LAA serving cell may include other signals such as CSI-RS and CRS. The discovery signals may be configured by a higher layer. CRS should be transmitted in a configured discovery subframe. The CSI-RS may be configured by upper layer signaling with a resource position and periodicity. Additionally, CSI-RS is assumed in the DRS for measurement if configured by higher layers.

The eNB <NUM> may transmit <NUM> discovery reference signals on a configured unlicensed carrier based on the DRS configuration. In a first approach, the LAA cell may transmit DRS according to the configuration as in a licensed cell. In this approach, the eNB <NUM> may transmit <NUM> the discovery reference signals of the LAA serving cell periodically in a fixed subframe location. If the subframe is not occupied by LAA transmission following CCA and LBT procedures, the DRS may be broadcast as in regular LTE subframes. However, if the subframe is occupied by other unlicensed transmissions, the LAA cell may follow the CCA and LBT procedures and, thus, should not transmit a LAA subframe. Therefore, in one implementation, the eNB <NUM> may drop the DRS if the LAA cell senses that the channel is busy. In another implementation, to keep the DRS broadcasting, the eNB <NUM> may transmit only configured DRS, and no signals are transmitted in other areas of the LAA subframe.

In a second approach for DRS transmissions in a LAA cell, the eNB <NUM> may transmit <NUM> the discovery reference signals of the LAA serving cell in a fixed subframe location in a LAA set or burst of subframe transmissions. The LAA cell may broadcast DRS in a fixed subframe location in a LAA set or burst of subframe transmissions. In one implementation, the eNB <NUM> may transmit <NUM> the DRS of the LAA serving cell in the first several subframes of each LAA set or burst of subframe transmissions. In another implementation, with reduced DRS density, the eNB <NUM> may always transmit <NUM> the DRS of the LAA serving cell in the first LAA set or burst of subframe transmissions within a DMTC period. The DRS occasion may be in the range of <NUM> to <NUM> subframes. An example of DRS transmissions in a LAA cell is described in connection with <FIG>.

<FIG> illustrates an example of DRS transmission in a LAA serving cell <NUM>. In this example, DRS are present in the first two subframes in the first occasion of LAA burst <NUM> transmissions within a DMTC period <NUM> of a licensed serving cell <NUM>. The DMTC period <NUM> in this example is <NUM>. The PSS <NUM> and SSS <NUM> are located in the first subframe of the first LAA burst <NUM> transmission. The CSI-RS <NUM> is configured in the second subframe of the first LAA burst <NUM> transmission. The CRS <NUM> may be present in both subframes as a part of DRS.

<FIG> illustrates various components that may be utilized in a UE <NUM>. The UE <NUM> described in connection with <FIG> may be implemented in accordance with the UE <NUM> described in connection with <FIG>. The UE <NUM> includes a processor <NUM> that controls operation of the UE <NUM>. The processor <NUM> may also be referred to as a central processing unit (CPU). Memory <NUM>, which may include read-only memory (ROM), random access memory (RAM), a combination of the two or any type of device that may store information, provides instructions 1457a and data 1459a to the processor <NUM>. A portion of the memory <NUM> may also include non-volatile random access memory (NVRAM). Instructions 1457b and data 1459b may also reside in the processor <NUM>. Instructions 1457b and/or data 1459b loaded into the processor <NUM> may also include instructions 1457a and/or data 1459a from memory <NUM> that were loaded for execution or processing by the processor <NUM>. The instructions 1457b may be executed by the processor <NUM> to implement one or more of the method <NUM> and <NUM> described above.

The UE <NUM> may also include a housing that contains one or more transmitters <NUM> and one or more receivers <NUM> to allow transmission and reception of data. The transmitter(s) <NUM> and receiver(s) <NUM> may be combined into one or more transceivers <NUM>. One or more antennas 1422a-n are attached to the housing and electrically coupled to the transceiver <NUM>.

<FIG> illustrates various components that may be utilized in an eNB <NUM>. The eNB <NUM> described in connection with <FIG> may be implemented in accordance with the eNB <NUM> described in connection with <FIG>. The eNB <NUM> includes a processor <NUM> that controls operation of the eNB <NUM>. The processor <NUM> may also be referred to as a central processing unit (CPU). Memory <NUM>, which may include read-only memory (ROM), random access memory (RAM), a combination of the two or any type of device that may store information, provides instructions 1557a and data 1559a to the processor <NUM>. A portion of the memory <NUM> may also include non-volatile random access memory (NVRAM). Instructions 1557b and data 1559b may also reside in the processor <NUM>. Instructions 1557b and/or data 1559b loaded into the processor <NUM> may also include instructions 1557a and/or data 1559a from memory <NUM> that were loaded for execution or processing by the processor <NUM>. The instructions 1557b may be executed by the processor <NUM> to implement one or more of the method <NUM> and <NUM> described above.

The eNB <NUM> may also include a housing that contains one or more transmitters <NUM> and one or more receivers <NUM> to allow transmission and reception of data. The transmitter(s) <NUM> and receiver(s) <NUM> may be combined into one or more transceivers <NUM>. One or more antennas 1580a-n are attached to the housing and electrically coupled to the transceiver <NUM>.

The various components of the eNB <NUM> are coupled together by a bus system <NUM>, which may include a power bus, a control signal bus and a status signal bus, in addition to a data bus. The eNB <NUM> may also include a digital signal processor (DSP) <NUM> for use in processing signals. The eNB <NUM> may also include a communications interface <NUM> that provides user access to the functions of the eNB <NUM>. The eNB <NUM> illustrated in <FIG> is a functional block diagram rather than a listing of specific components.

<FIG> is a block diagram illustrating one implementation of a UE <NUM> in which systems and methods for performing carrier aggregation may be implemented. The UE <NUM> includes transmit means <NUM>, receive means <NUM> and control means <NUM>. The transmit means <NUM>, receive means <NUM> and control means <NUM> may be configured to perform one or more of the functions described in connection with <FIG> and <FIG> above. <FIG> above illustrates one example of a concrete apparatus structure of <FIG>. Other various structures may be implemented to realize one or more of the functions of <FIG> and <FIG>. For example, a DSP may be realized by software.

<FIG> is a block diagram illustrating one implementation of an eNB <NUM> in which systems and methods for performing carrier aggregation may be implemented. The eNB <NUM> includes transmit means <NUM>, receive means <NUM> and control means <NUM>. The transmit means <NUM>, receive means <NUM> and control means <NUM> may be configured to perform one or more of the functions described in connection with <FIG> and <FIG> above. <FIG> above illustrates one example of a concrete apparatus structure of <FIG>. Other various structures may be implemented to realize one or more of the functions of <FIG> and <FIG>. For example, a DSP may be realized by software.

The term "computer-readable medium" refers to any available medium that can be accessed by a computer or a processor. The term "computer-readable medium," as used herein, may denote a computer- and/or processor-readable medium that is non-transitory and tangible. By way of example, and not limitation, a computer-readable or processor-readable medium may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer or processor. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray® disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers.

It should be noted that one or more of the methods described herein may be implemented in and/or performed using hardware. For example, one or more of the methods described herein may be implemented in and/or realized using a chipset, an application-specific integrated circuit (ASIC), a large-scale integrated circuit (LSI) or integrated circuit, etc..

Claim 1:
A user equipment, UE (<NUM>, <NUM>), comprising:
a processor (<NUM>); and
a memory (<NUM>) in electronic communication with the processor (<NUM>), wherein instructions (1457a, 1457b) stored in the memory are executable to:
receive a radio resource control, RRC, configuration including a configuration of a licensed-assisted access, LAA, for an LAA secondary serving cell, SCell, from an evolved node B, eNB (<NUM>, <NUM>);
determine a synchronization signal structure of the LAA SCell based on a primary synchronization signal, PSS, and a secondary synchronization signal, SSS, structure and relative PSS/SSS location of a frequency-division duplexing, FDD, serving cell; and
receive a PSS and a SSS of the LAA SCell, based on the synchronization signal structure and the configuration of an LAA for an LAA SCell, wherein the PSS and the SSS are mapped according to a frame structure of FDD, wherein
the LAA is applicable to downlink only transmissions, and
the LAA is performed in unlicensed band; and
perform subframe synchronization, slot synchronization and frequency synchronization for the LAA SCell based on the received PSS and SSS.