Patent Description:
Wireless communication systems are rapidly growing in usage. Additionally, there exist numerous different wireless communication technologies and standards. Some examples of wireless communication standards include GSM, UMTS (associated with, for example, WCDMA or TD-SCDMA air interfaces), LTE, LTE Advanced (LTE-A), HSPA, 3GPP2 CDMA2000 (e.g., 1xRTT, 1xEV-DO, HRPD, eHRPD), IEEE <NUM> (WLAN or Wi-Fi), IEEE <NUM> (WiMAX), Bluetooth, and others. Many such wireless communication standards provide for the use of known signals (e.g., pilot or reference signals) for a variety of purposes, such as synchronization, measurements, equalization, control, etc..

<CIT> discloses a cell measurement method of a User Equipment.

<CIT> discloses methods and an apparatus for coordination of sending reference signals in wireless network.

<CIT> discloses techniques for sending information in a wireless network.

<CIT> discloses a transmitter apparatus, a receiver apparatus, a communication system and a communication method.

The invention is defined in the appended independent claims. The use of the word "embodiment" below in this description merely implies the illustration of examples or exemplary embodiments, if not otherwise defined by the appended claims. The scope of the invention is thus defined by the appended claims.

Embodiments are presented herein of, inter alia, methods for providing reference subframes in a cellular communication system, and of devices configured to implement the methods.

According to the techniques disclosed herein, base stations may coordinate to periodically provide a synchronized dedicated reference subframe which may be used by wireless devices for various synchronization and measurement purposes. Each base station's reference subframe may include control information regarding neighboring cells to facilitate neighbor cell measurement by UEs served by that base station.

In order to provide robustness against interference across cells (in particular given that the dedicated reference subframes of neighbor cells may be synchronized to occur at the same time), each base station may use reference signals which are orthogonal to the reference signals used by its neighbor base stations. For example, different cyclic shifts of Zadoff-Chu root sequences might be used by different base stations.

By providing such reference signals in a single dedicated subframe, the base stations may be freed from the need to include cell-specific reference symbols in other (e.g., data) subframes, at least in some instances. This may result in more efficient spectrum usage, and at least in some circumstances may also result in power consumption savings by network infrastructure equipment and/or user devices.

Note that the techniques described herein may be implemented in and/or used with a number of different types of devices, including but not limited to, base stations, access points, cellular phones, portable media players, tablet computers, wearable devices, and various other computing devices.

Accordingly, it will be appreciated that the above-described features are merely examples and should not be construed to narrow the scope described herein.

A better understanding of the present subject matter can be obtained when the following detailed description of the embodiments is considered in conjunction with the following drawings, in which:.

The following is a glossary of terms used in this disclosure:
Memory Medium - Any of various types of non-transitory memory devices or storage devices. The term "memory medium" is intended to include an installation medium, e.g., a CD-ROM, floppy disks, or tape device; a computer system memory or random access memory such as DRAM, DDR RAM, SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash, magnetic media, e.g., a hard drive, or optical storage; registers, or other similar types of memory elements, etc. The memory medium may include other types of non-transitory memory as well or combinations thereof. In addition, the memory medium may be located in a first computer system in which the programs are executed, or may be located in a second different computer system which connects to the first computer system over a network, such as the Internet. In the latter instance, the second computer system may provide program instructions to the first computer for execution. The term "memory medium" may include two or more memory mediums which may reside in different locations, e.g., in different computer systems that are connected over a network. The memory medium may store program instructions (e.g., embodied as computer programs) that may be executed by one or more processors.

Computer System - any of various types of computing or processing systems, including a personal computer system (PC), mainframe computer system, workstation, network appliance, Internet appliance, personal digital assistant (PDA), television system, grid computing system, or other device or combinations of devices. In general, the term "computer system" can be broadly defined to encompass any device (or combination of devices) having at least one processor that executes instructions from a memory medium.

User Equipment (UE) (or "UE Device") - any of various types of computer systems devices which are mobile or portable and which performs wireless communications. Examples of UE devices include mobile telephones or smart phones (e.g., iPhone™, Android™-based phones), portable gaming devices (e.g., Nintendo DS™, PlayStation Portable™, Gameboy Advance™, iPhone™), laptops, wearable devices (e.g., smart watch, smart glasses), PDAs, portable Internet devices, music players, data storage devices, or other handheld devices, etc. In general, the term "UE" or "UE device" can be broadly defined to encompass any electronic, computing, and/or telecommunications device (or combination of devices) which is easily transported by a user and capable of wireless communication.

Processing Element - refers to various elements or combinations of elements. Processing elements include, for example, circuits such as an ASIC (Application Specific Integrated Circuit), portions or circuits of individual processor cores, entire processor cores, individual processors, programmable hardware devices such as a field programmable gate array (FPGA), and/or larger portions of systems that include multiple processors.

<FIG> illustrates an exemplary (and simplified) wireless communication system, according to one embodiment. It is noted that the system of <FIG> is merely one example of a possible system, and embodiments may be implemented in any of various systems, as desired.

As shown, the exemplary wireless communication system includes a base station 102A which communicates over a transmission medium with one or more user devices 106A, 106B, etc., through 106N.

The base station 102A may be a base transceiver station (BTS) or cell site, and may include hardware that enables wireless communication with the UEs 106A through 106N. The base station 102A may also be equipped to communicate with a network <NUM> (e.g., a core network of a cellular service provider, a telecommunication network such as a public switched telephone network (PSTN), and/or the Internet, among various possibilities).

The communication area (or coverage area) of the base station may be referred to as a "cell. " The base station 102A and the UEs <NUM> may be configured to communicate over the transmission medium using any of various radio access technologies (RATs), also referred to as wireless communication technologies, or telecommunication standards, such as GSM, UMTS (WCDMA, TD-SCDMA), LTE, LTE-Advanced (LTE-A), HSPA, 3GPP2 CDMA2000 (e.g., 1xRTT, 1xEV-DO, HRPD, eHRPD), Wi-Fi, WiMAX etc..

102N) operating according to the same or a different cellular communication standard may thus be provided as a network of cells, which may provide continuous or nearly continuous overlapping service to UEs 106A-N and similar devices over a wide geographic area via one or more cellular communication standards.

Thus, while base station 102A may provide a "serving cell" for UEs 106A-N as illustrated in <FIG>, each UE <NUM> may also be capable of receiving signals from (and possibly within communication range of) one or more other cells (which might be provided by base stations 102B-N and/or any other base stations), which may be referred to as "neighboring cells".

Note that a UE <NUM> may be capable of communicating using multiple wireless communication standards. For example, a UE <NUM> may be configured to communicate using a wireless networking (e.g., Wi-Fi) and/or peer-to-peer wireless communication protocol (e.g., BT, Wi-Fi peer-to-peer, etc.) in addition to at least one cellular communication protocol (e.g., GSM, UMTS (WCDMA, TD-SCDMA), LTE, LTE-A, HSPA, 3GPP2 CDMA2000 (e.g., 1xRTT, 1xEV-DO, HRPD, eHRPD), etc.). The UE <NUM> may also or alternatively be configured to communicate using one or more global navigational satellite systems (GNSS, e.g., GPS or GLONASS), one or more mobile television broadcasting standards (e.g., ATSC-M/H or DVB-H), and/or any other wireless communication protocol, if desired. Other combinations of wireless communication standards (including more than two wireless communication standards) are also possible.

<FIG> illustrates user equipment <NUM> (e.g., one of the devices 106A through 106N) in communication with a base station <NUM> (e.g., one of the base stations 102A through 102N), according to one embodiment. The UE <NUM> may be a device with cellular communication capability such as a mobile phone, a hand-held device, a wearable device, a computer or a tablet, or virtually any type of wireless device.

The UE <NUM> may include one or more antennas for communicating using one or more wireless communication protocols or technologies. In one embodiment, the UE <NUM> might be configured to communicate using either of CDMA2000 (1xRTT / 1xEV-DO / HRPD / eHRPD) or LTE using a single shared radio and/or GSM or LTE using the single shared radio. The shared radio may couple to a single antenna, or may couple to multiple antennas (e.g., for MIMO) for performing wireless communications. In general, a radio may include any combination of a baseband processor, analog RF signal processing circuitry (e.g., including filters, mixers, oscillators, amplifiers, etc.), or digital processing circuitry (e.g., for digital modulation as well as other digital processing). Similarly, the radio may implement one or more receive and transmit chains using the aforementioned hardware. For example, the UE <NUM> may share one or more parts of a receive and/or transmit chain between multiple wireless communication technologies, such as those discussed above.

In some embodiments, the UE <NUM> may include separate (and possibly multiple) transmit and/or receive chains (e.g., including separate RF and/or digital radio components) for each wireless communication protocol with which it is configured to communicate. For example, the UE <NUM> might include a shared radio for communicating using either of LTE or 1xRTT (or LTE or GSM), and separate radios for communicating using each of Wi-Fi and Bluetooth.

<FIG> illustrates an exemplary block diagram of a UE <NUM>, according to one embodiment. As shown, the UE <NUM> may include a system on chip (SOC) <NUM>, which may include portions for various purposes. For example, as shown, the SOC <NUM> may include processor(s) <NUM> which may execute program instructions for the UE <NUM> and display circuitry <NUM> which may perform graphics processing and provide display signals to the display <NUM>. The processor(s) <NUM> may also be coupled to memory management unit (MMU) <NUM>, which may be configured to receive addresses from the processor(s) <NUM> and translate those addresses to locations in memory (e.g., memory <NUM>, read only memory (ROM) <NUM>, NAND flash memory <NUM>) and/or to other circuits or devices, such as the display circuitry <NUM>, wireless communication circuitry <NUM>, connector I/F <NUM>, and/or display <NUM>. The MMU <NUM> may be configured to perform memory protection and page table translation or set up. In some embodiments, the MMU <NUM> may be included as a portion of the processor(s) <NUM>.

As shown, the SOC <NUM> may be coupled to various other circuits of the UE <NUM>. For example, the UE <NUM> may include various types of memory (e.g., including NAND flash <NUM>), a connector interface <NUM> (e.g., for coupling to a computer system, dock, charging station, etc.), the display <NUM>, and wireless communication circuitry (e.g., radio) <NUM> (e.g., for LTE, Wi-Fi, GPS, etc.).

The UE device <NUM> may include at least one antenna (and possibly multiple antennas, e.g., for MIMO and/or for implementing different wireless communication technologies, among various possibilities), for performing wireless communication with base stations and/or other devices. For example, the UE device <NUM> may use antenna(s) <NUM> to perform the wireless communication. As noted above, the UE <NUM> may be configured to communicate wirelessly using multiple wireless communication standards in some embodiments.

As described further subsequently herein, the UE <NUM> may include hardware and software components for implementing features relating to the use of dedicated measurement / synchronization subframes in a cellular communication system, such as those described herein with reference to, inter alia, <FIG>. The processor <NUM> of the UE device <NUM> may be configured to implement part or all of the methods described herein, e.g., by executing program instructions stored on a memory medium (e.g., a non-transitory computer-readable memory medium). In other embodiments, processor <NUM> may be configured as a programmable hardware element, such as an FPGA (Field Programmable Gate Array), or as an ASIC (Application Specific Integrated Circuit). Alternatively (or in addition) the processor <NUM> of the UE device <NUM>, in conjunction with one or more of the other components <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to implement part or all of the features described herein, such as the features described herein with reference to, inter alia, <FIG>.

<FIG> illustrates an exemplary block diagram of a base station <NUM>, according to one embodiment.

The antenna(s) <NUM> may be configured to operate as a wireless transceiver and may be further configured to communicate with UE devices <NUM> via radio <NUM>. The radio <NUM> may be configured to communicate via various wireless telecommunication standards, including, but not limited to, LTE, LTE-A, UMTS, CDMA2000, Wi-Fi, etc..

The BS <NUM> may be configured to communicate wirelessly using multiple wireless communication standards. For example, as one possibility, the base station <NUM> may include an LTE radio for performing communication according to LTE as well as a Wi-Fi radio for performing communication according to Wi-Fi. In such a case, the base station <NUM> may be capable of operating as both an LTE base station and a Wi-Fi access point. As another possibility, the base station <NUM> may include a multi-mode radio, which is capable of performing communications according to any of multiple wireless communication technologies (e.g., LTE and Wi-Fi; LTE and UMTS; LTE and CDMA2000; UMTS and GSM; etc.).

As described further subsequently herein, the BS <NUM> may include hardware and software components for implementing features relating to the use of dedicated measurement / synchronization subframes in a cellular communication system, such as those described herein with reference to, inter alia, <FIG>. The processor <NUM> of the base station <NUM> may be configured to implement part or all of the methods described herein, e.g., by executing program instructions stored on a memory medium (e.g., a non-transitory computer-readable memory medium). Alternatively (or in addition) the processor <NUM> of the BS <NUM>, in conjunction with one or more of the other components <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to implement part or all of the features described herein, such as the features described herein with reference to, inter alia, <FIG>.

According to current implementations of LTE, cell specific reference symbols (CRS) are transmitted in every subframe, even if there is no UE allocation in the subframe. CRS may be provided, among various purposes, for UE tracking loops (such as time tracking loops, frequency tracking loops, Doppler and signal to interference plus noise (SINR) estimation loops, channel estimation, etc.), serving and neighboring cell measurements (e.g., RSRP/RSRQ measurements), demodulation of certain control channels (e.g., PHICH, PDCCH, PCFICH, PBCH), and certain legacy transmission modes for the physical downlink shared channel (PDSCH) that require CRS and less than <NUM> transmit antennas (e.g., transmission modes <NUM>-<NUM> ).

<FIG> illustrates an exemplary subframe reference symbol structure in which cell-specific reference symbols are provided, according to one embodiment. As shown in <FIG>, on each antenna port, certain resource elements of each subframe are used for transmission of cell-specific reference symbols, and certain resource elements of each subframe are left unused to avoid interference with transmission of cell-specific reference symbols on the other antenna port.

Use of CRS impacts LTE systems in a variety of ways. As one example, the current use of CRS by LTE cells in all subframes, including those in which no resources are allocated to communication with UEs by those cells, may cause significant interference for small cell (i.e., smaller than macro cell, e.g., micro cell, pico cell, femto cell) deployment scenarios. For example, since all such (small) cells may be system frame number (SFN) aligned, this may create interference, especially in the case of heterogeneous deployments where a Macro Cell is interfering with an adjacent small cell, since there may be a transmit power differential between the macro cell and the small cell. Furthermore, at least in some instances small cells may rely on almost blank subframe (ABS) scheduling and corresponding lack of interference from the macro cell in order to perform communications with its UEs, but since CRS may still be transmitted in an ABS subframe, interference from CRS in an ABS subframe may still impact decoding of the PDCCH in the small cell and possibly / eventually the PDSCH, even though the interference from the PDCCH and PDSCH in the macro cell may have been removed.

As another example, for an LTE cell provided on an unlicensed frequency band (e.g., an LTE-U deployment), if the cell transmits CRS signals on a particular frequency even if the cell is not scheduling any data for its UEs, this may create interference on any Wi-Fi access point that is trying to access that frequency. In contrast, if such a cell could go entirely silent during periods of time when no data communications are being performed by the cell on a given frequency, Wi-Fi devices may be able to more readily access that frequency such that Wi-Fi and LTE-U may be able to successfully coexist in that unlicensed frequency band.

Furthermore, the use of CRS has an impact on the total spectral efficiency of an LTE cell; each resource element on which a reference symbol is transmitted represents one less resource element, which can be allocated to data communication. Additionally, the use of CRS to support cell measurements and synchronization functions impacts UE power consumption, since in order for UEs to effectively use CRS for such functions, those CRS may need to be monitored on an ongoing basis.

Given such notable impacts on system function, it would be desirable to provide a way of achieving the functions of CRS while mitigating the drawbacks.

One such technique could include providing a periodic dedicated reference subframe. Such a subframe may contain sufficient reference information (e.g., as a sequence of reference symbols or other reference signal) to enable accurate tracking loop updates and RSRP/RSRQ measurements. Furthermore, such a reference subframe may be synchronized across all cells, e.g., in order to facilitate the possibility that a UE may perform measurements on both its serving cell and any neighboring cells, and may further include control information about some or all neighboring cells for such a purpose.

As a further consideration, in order to ensure robustness against interference across adjacent cells, reference signals or "measurement signals" used by different cells during the reference subframe may be designed to be orthogonal.

Overall, such a scheme may save spectral efficiency, reduce interference, and reduce UE power consumption. <FIG> is a communication / signal flow diagram illustrating such a scheme for providing coordinated reference subframes by cellular base stations, according to one embodiment. The scheme shown in <FIG> may be used in conjunction with any of the computer systems or devices shown in the above Figures, among other devices. Note that while the scheme shown in <FIG> may be used in conjunction with LTE systems (such as described with respect to <FIG>) as one possibility, it may also be possible to use such a scheme (or a variation thereon) in conjunction with any of various other cellular systems, as desired.

As shown, according to the scheme a "first" BS 102A and neighboring BSs 102B. 102N (e.g., such as illustrated in and described with respect to <FIG> and <FIG>) may each provide respective measurement signals in a time synchronized manner for measurement, synchronization, control and/or other purposes. A "first" UE <NUM> (e.g., such as illustrated in and described with respect to <FIG>) may perform serving and neighboring cell measurements, synchronization functions, and/or receive control information by way of the dedicated reference subframes.

Note that in various embodiments, some of the elements of the scheme shown may be performed concurrently, in a different order than shown, or may be omitted. Additional elements may also be performed as desired. As shown, the scheme may operate as follows.

In <NUM>, the first BS 102A transmits a first measurement signal during a reference subframe.

In <NUM>, each neighboring BS 102B-N of the first BS 102A transmit a respective measurement signal during the reference subframe. The first BS 102A and the neighboring BSs 102B-N coordinate their transmissions such that the reference subframe is time-synchronized between the base stations. In other words, steps <NUM> and <NUM> may be performed simultaneously.

Each of the measurement signals are orthogonal to each other. For example, each base station may utilize a different cyclical shift of a particular Zadoff-Chu root sequence for a given resource block. Any of various other mutually orthogonal signals may also or alternatively be used. This may ensure robustness against interference across adjacent cells and facilitate differentiation of the different transmissions by the different base stations.

In <NUM>, the first UE <NUM> performs serving cell measurements and synchronization functions as well as neighboring cell measurements on the first BS 102A and the neighboring BSs 102B-N based on the measurement signals provided by the first BS 102A and the neighboring BSs 102B-N during the reference subframe. Such synchronization functions and measurements may include signal strength and/or quality measurements (e.g., RSRP and RSRQ), updating timing loops, frequency loops, Doppler and/or SINR loops, performing channel estimation, etc..

Note that in some instances, control information regarding neighboring cells may be transmitted as part of the reference subframe. This may facilitate performing measurements of all neighboring cells of a given serving cell by a UE (e.g., the first UE <NUM>) during the reference subframe.

In some instances, the reference subframe is a dedicated synchronization / measurement subframe; for example, the first measurement signal transmitted by the first BS 102A may span the entire reference subframe, and respective measurement signals transmitted by neighboring BSs 102B-N may similarly span the entire reference subframe.

Reference subframes are provided by the BSs 102A-N at coordinated periodic intervals on an ongoing basis. For example, reference subframes may be scheduled for every <NUM>, or every <NUM>, or at any of a variety of other intervals. Between reference subframes the BSs 102A-N may also transmit data during data subframes, for example if any UEs (such as UE <NUM>) are scheduled for data communications with any of the BSs 102A-N. Alternatively or in addition, the BSs 102A-N may leave one or more subframes blank (i.e., not transmit) between the reference subframes, for example if no UEs are scheduled for data communications. Because the reference subframe may provide sufficient information for UEs to perform measurement and synchronization functions, it may be the case that during some or possibly all of the subframes between the reference subframes, cell specific reference symbols may not be used. As a result, subframe symbols of data subframes which might otherwise be set aside for cell-specific reference symbols may be used as data subcarriers, which may increase spectral efficiency of communications during those subframes. Additionally, subframe symbols of blank subframes which might otherwise be used for cell-specific reference symbols may also remain blank, which may reduce interference caused to other cells during those subframes.

In some implementations, information regarding the reference subframes may be provided by one or more of the BSs 102A-N, for example in broadcast system information (e.g., so as to be available to UEs which are not yet attached to the system and/or to UEs in RRC idle mode) and/or in RRC configuration messages (e.g., for UEs which are in RRC connected mode). In such a case, the first UE <NUM> and/or any other UEs in the communication system may utilize such information to determine when (e.g., in which system frame number (SFN) the reference subframe falls) and how (e.g., the nature / type / index number / etc. of the signals used by the base stations 102A-N) to perform the serving cell measurements and synchronization functions and/or the neighboring cell measurements.

Alternatively, the reference subframe structure (e.g., correspondence between cell IDs and particular reference signals, reference subframe timing, etc.) may be sufficiently defined in specification documents for the wireless communication technology being used in the communication system that information which is already provided (e.g., system frame number and cell id, such as might be obtained from system information blocks broadcast by each cell) may be sufficient to enable UEs to determine when and how to utilize the reference subframe(s).

The following information is provided as additional description of certain possible exemplary implementations in which the communication system operates according to LTE or LTE-A and considerations related thereto, and is not intended to be limiting to the disclosure as a whole. Numerous alternatives to and variations of the following details are also possible and should be considered within the scope of the present disclosure.

In at least some instances, (e.g., for cells that will support only new releases devices), it may be possible to minimize use of CRS and substantially implement such a scheme. However, note that even if such a scheme as illustrated in and described with respect to <FIG> is implemented, CRS may still be used in certain circumstances, if desired. For example, while the use of a periodic, dedicated reference subframe may enable a UE to perform all needed serving and neighboring cell measurements during that reference subframe, decoding of certain channels (e.g., CRS based pre-coded channels) may continue to utilize CRS, and so CRS may be transmitted for subframes (or possibly just specific resource blocks) that contain such channels (e.g., PBCH, PHICH, PCFICH). It may be the case in such an instance that CRS are not transmitted in a wideband manner (e.g., across all resource blocks / RBs) but are rather transmitted just for those RBs containing the desired control channel. For example, for the PBCH, CRS might be transmitted only in the six resource blocks in the center of the system bandwidth. Alternatively, or in addition, certain channels that previously relied on CRS based precoding may be replaced with channels that use UE-specific reference symbols; for example, the PDCCH may be replaced by an E-PDCCH that uses UE-specific reference symbols.

As a further possibility, some cells may be provided primarily or exclusively as secondary cells (secondary component carriers) in carrier aggregation schemes. For example, an LTE-U cell (a cell deployed in an unlicensed band, which might also be an example of a cell which supports only new releases devices) be deployed as a secondary component carrier ("Scell"); such a cell may be subject to cross carrier scheduling such that control communications may be performed on the primary component carrier ("Pcell"). In such a case, the LTE-U Scell may not have a PDCCH.

As a further consideration to avoid the need for CRS, the PDSCH may use transmission modes that require UE-specific reference symbols (i.e., and do not require CRS) such as LTE transmission modes <NUM>-<NUM>. Additionally, CQI/PMI/RI measurements may be based on LTE Release <NUM> defined Channel State Information-Reference Symbols (CSI-RS).

It may also be possible to implement dedicated reference subframes in a manner which co-exists substantially with the use of CRS, for example in order to continue providing support for legacy devices which are not configured to utilize such a reference frame and/or which require CRS for certain purpose. For example, if desired (e.g., in cells that will support both new releases devices and legacy devices), when no UEs are scheduled for data communications, it may be the case that CRS are not transmitted, and that reference subframes will be transmitted according to configured (e.g., as defined according to radio resource control (RRC) messages) characteristics. When UEs are scheduled for data communications, however, CRS may be used.

The reference subframe may be designed in any of a variety of ways. As a first example, the design may be based on Zadoff-Chu (ZC) sequences, such as provided according to the following equation: <MAT>, <NUM> ≤ n ≤ NZC - <NUM>, where u = physical root sequence index.

In this case, the orthogonality of measurement signals between cells may be ensured by the cyclic shifts of ZC sequences of the same root and by frequency domain division. For example, for a given resource block (RB), the same ZC root sequence is used for each cell, but each cell may be assigned a different cyclic shift of the root to ensure that the signals used by the different cells are orthogonal to each other. Different root sequences may be used for different resource blocks.

Note that such a reference sequence may span a whole subframe (two slots) or half of a subframe (one slot). In the case in which the reference sequence spans half of a subframe, each cell may be identified by two different sequences (e.g., one per slot); alternatively, the same sequence may be repeated in the time domain (e.g., for time diversity), and/or in a different resource block in the second slot (e.g., for frequency diversity) to form the measurement signal. Such "hopping" in the frequency domain may be static (e.g., known / defined in advance) or dynamic (e.g., signaled in RRC communications).

Note also that if the length of a reference sequence is not equal to (e.g., is greater or lesser than) the number of subcarriers available in a single RB, then the subcarrier spacing could be changed (e.g., shortened or lengthened) for the reference subframe, if desired.

Thus, in this example, a given cell's measurement signal may be defined and orthogonality with respect to other cells measurement signals ensured by the combination of the RB index, the ZC root sequence, and the root cyclic shift.

As an alternate example, reference symbol sequences as of 3GPP Release <NUM> could be used. Orthogonality may be ensured in such a case by using frequency domain division (FDM) in addition to code domain division (CDM). The concept may be similar to that used for CSI-RS. In this example, then, a given cell's measurement signal may be defined and orthogonality with respect to other cells measurement signals ensured by the combination of the reference symbol sequence, the CDM index and the RB index.

It should further be noted that, if desired (e.g., due to a finite set of orthogonal sequences and/or codes), a time domain offset for the transmission of the reference subframe could be used for each of various groups of cells. For example, a first group of cells could transmit in SFN k, while a second group could transmit in SFN j (e.g., where j and k are different numbers), etc. The groups of cells may be geographically coordinated such that in most cases, all neighboring cells may be synchronized (i.e., may transmit their reference subframes in the same SFN as each other), in order to provide UEs with the capability to perform all serving and neighboring cell measurements during the reference subframe.

As previously noted, a mapping between a given cell (e.g., having a particular cell ID) and the sequence/signal used for the reference subframe by that cell could be defined in a static manner (e.g., in the 3GPP specification), or provided dynamically using configuration messages.

In the case of dynamic configuration, the configuration information may be transmitted in an RRC message and/or in system information block (SIB) information (e.g., since it may be used in both idle and connected modes). Such configuration information might include a list of neighboring cells, their cell IDs, and a sequence/signal index (or other indicator) identifying the signal used for the reference subframe by each such cell, as one possibility. The configuration may also include one or more of the SFN starting time, the periodicity of reference subframes, and the subframe pattern in a cycle.

As a specific example, below is an exemplary RRC information element ("IE") "MeasSubframePattern" which may be used to indicate such information. Note that while the following example represents one possible configuration message, numerous alternatives to and variations of the following example are also possible.

For example some embodiments may be realized as a computer-implemented method, a computer-readable memory medium, or a computer system.

In some embodiments, a device (e.g., a UE <NUM>) may be configured to include a processor (or a set of processors) and a memory medium, where the memory medium stores program instructions, where the processor is configured to read and execute the program instructions from the memory medium, where the program instructions are executable to implement any of the various method embodiments described herein (or, any combination of the method embodiments described herein, or, any subset of any of the method embodiments described herein, or, any combination of such subsets). The device may be realized in any of various forms.

Claim 1:
A method, comprising:
transmitting (<NUM>), by a first base station (102A) when there is no data transmission by the first base station (102A), a plurality of subframes to wireless devices (<NUM>) in a coverage area of the first base station (102A), wherein the plurality of subframes comprises dedicated synchronization and measurement subframes and blank data subframes;
transmitting, by the first base station (102A), configuration information for the dedicated synchronization and measurement subframes, wherein the configuration information indicates the configured periodicity according to which the dedicated synchronization and measurement subframes are transmitted;
wherein transmitting (<NUM>) the plurality of subframes comprises transmitting the dedicated synchronization and measurement subframes periodically according to the configured periodicity; and
wherein each of the dedicated synchronization and measurement subframes comprise reference signals spanning the entire subframe or half the subframe.