GB2581953A - Synchronisation signals - Google Patents

Abstract

A UE in a cellular communications network connected using a first Radio Access Technology (RAT) may utilise synchronisation signals from a second RAT to re-synchronise with the first radio access technology. Base stations for the first and second radio access technologies may be co-located, and synchronised in time and frequency. Information to enable decoding of the synchronisation signal may be transmitted via the first or second radio access technology. Preferably, the first RAT is Narrowband Internet of Things (NB-IoT), and the second RAT is Cat-M. A ReSynchroisation Signal (RSS) has been defined in the standards to assist Cat-M devices in quickly synchronizing with a serving cell. However, implementing an RSS signal in NB-IoT consumes too many radio resources to be practical. An embodiment describes an NB-IoT base station transmitting parameters to enable the UE to receive an RSS signal transmitted by a Cat-M base station. Full details of the Cat-M transmission may not be required, but only selected details.

Description

Synchronisation Signals

Technical Field

[1] The following disclosure relates to processes for synchronising a mobile device with a cellular network, and in particular to the synchronisation of Internet of Things devices.

Background

[2] Wireless communication systems, such as the third-generation (3G) of mobile telephone standards and technology are well known. Such 3G standards and technology have been developed by the Third Generation Partnership Project (3GPP). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Communication systems and networks have developed towards a broadband and mobile system.

[3] In cellular wireless communication systems User Equipment (UE) is connected by a wireless link to a Radio Access Network (RAN). The RAN comprises a set of base stations which provide wireless links to the UEs located in cells covered by the base station, and an interface to a Core Network (CN) which provides overall network control. As will be appreciated the RAN and CN each conduct respective functions in relation to the overall network. For convenience the term cellular network will be used to refer to the combined RAN & CN, and it will be understood that the term is used to refer to the respective system for performing the disclosed function.

[4] The 3rd Generation Partnership Project has developed the so-called Long Term Evolution (LTE) system, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network where one or more macro-cells are supported by a base station known as an eNodeB or eNB (evolved NodeB). More recently, LTE is evolving further towards the so-called 5G or NR (new radio) systems where one or more cells are supported by a base station known as a gNB. NR is proposed to utilise an Orthogonal Frequency Division Multiplexed (OFDM) physical transmission format.

[5] Various protocols have arisen for Internet of Things (loT) device, typically aligned with different cellular standards. For example, Cat-M is a category of devices operating within the LTE cellular standards, and NB-loT is a category operating within the 5G/NR standards. Cat-M and NB-loT may be referred to as different Radio Access Technologies (RATs).

[6] loT devices benefit from DRX operation such that they can sleep for long periods of time when they do not need to be active. However, waking up after an extended sleep may require resynchronisafion with the cellular network.

[7] A Wake Up Signal (WUS) has been provided in the relevant standards for both Cat-M and NB-loT devices. The WUS may be used to wake up devices when desired by the cellular network. However, the WUS cannot be used for resynchronisafion after long sleep periods as it is transmitted only if a device is paged. A ReSynchronisation Signal (RSS) has been defined in the standards for Cat-M devices to assist a Cat-M device in quickly synchronising with the serving cell thus reducing energy wastage when waking up.

[8] However, implementing an RSS signal in NB-loT consumes too many radio resources and is hence impractical. RSS uses 2 PRBs and is repeated 8, 16, 32 or 40 times with a periodicity of 160, 320, 640, or 1280 ms. This means in a typical RSS signal 16, 32, 64, 80 Physical Resource Blocks (PRBs) are consumed every 160/320/640/1280ms (depending on the RSS configuration used by the eNb).

[9] The Cat-M standards provide 50 PRBs every ms for a 10MHz bandwidth. An RSS using 32 PRBs every 160ms only consumes 0.4% of the bandwidth, or 0.05% if transmitted every 1280ms. RSS is therefore practical for a Cat-M system. In contrast, NB-loT only has one RB available per ms per carrier. Hence 20% of the available bandwidth would be needed for 32 PRBs every 160 ms.

[10] There is therefore a requirement for a system to allow efficient wake-up and re-synchronisation by NB-IoT devices.

Summary

[11] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[12] There is provided a method for resynchronising a UE, the UE being connected to a first base station utilising a first radio access technology, the method comprising the steps of receiving at the UE information to permit the UE to receive and decode a synchronisation signal transmitted by a second base using a second radio access technology, wherein the first and second base stations are synchronised in time and frequency; receiving and decoding the synchronisation signal of the second radio access technology at the UE; and utilising the content of the synchronisation signal at the UE to synchronise the UE to the base station utilising the first radio access technology.

[13] The information may be received in a message transmitted by the first base station using the first radio access technology.

[14] The information may be received in a message transmitted by the second base station using the second radio access technology.

[15] The message may be a System Information message.

[16] The first radio access technology may be NB-loT.

[17] The second radio access technology may be Cat-M.

[18] The step of synchronising may be performed after the UE wakes from sleeping.

[19] The information may comprise the identity of the second base station.

[20] The identity may be the PCI of the second base station.

[21] The information may comprise at least one of an indication of time of transmission of the synchronisation signal, an indication of transmission power of the synchronisation signal, and the frequency location of the synchronisation signal.

[22] The time of transmission may be indicated relative to a SFN boundary of the first or second radio access technology.

[23] The indication of transmission power may be indicated relative to CRS.

[24] The frequency location may be indicated relative to the downlink carrier frequency of the second radio access technology.

[25] The information may comprise information on the downlink carrier configuration of the second radio access technology.

[26] There is also provided a UE configured to perform the methods described herein.

[27] There is also provided a method for resynchronising a UE, the UE being connected to a first base station utilising a first radio access technology, the method comprising the steps of transmitting from the first base station to the UE, using the first radio access technology, information to permit the UE to receive and decode a synchronisation signal transmitted by a second base using a second radio access technology, wherein the first and second base stations are synchronised in time and frequency.

[28] The first radio access technology may be NB-IoT.

[29] The second radio access technology may be Cat-M.

[30] The information may comprise the identity of the second base station.

[31] The identity may be the PCI of the second base station.

[32] The information may comprise at least one of an indication of time of transmission of the synchronisation signal, an indication of transmission power of the synchronisation signal, and the frequency location of the synchronisation signal.

[33] The time of transmission may be indicated relative to a SFN boundary of the first or second radio access technology.

[34] The indication of transmission power may be indicated relative to CRS.

[35] The frequency location may be indicated relative to the downlink carrier frequency of the second radio access technology.

[36] The non-transitory computer readable medium may comprise at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory and a Flash memory.

Brief description of the drawings

[37] Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Like reference numerals have been included in the respective drawings to ease understanding.

[38] Figure 1 shows a schematic diagram of selected components of a cellular wireless communications network; [39] Figure 2 shows a method of sharing synchronisation signals; and [40] Figures 3 & 4 show a set of comparative use cases. Detailed description of the preferred embodiments [41] Those skilled in the art will recognise and appreciate that the specifics of the examples described are merely illustrative of some embodiments and that the teachings set forth herein are applicable in a variety of alternative settings.

[42] Figure 1 shows a schematic diagram of three base stations (for example, eNB or gNBs depending on the particular cellular standard and terminology) forming a cellular network. Typically, each of the base stations will be deployed by one cellular network operator to provide geographic coverage for UEs in the area. The base stations form a Radio Area Network (RAN). Each base station provides wireless coverage for UEs in its area or cell. The base stations are interconnected via the X2 interface and are connected to the core network via the Si interface. As will be appreciated only basic details are shown for the purposes of exemplifying the key features of a cellular network. The particular interface names may vary between standards, but the principles and general assignment of functions remains the same.

[43] The base stations each comprise hardware and software to implement the RAN's functionality, including communications with the core network and other base stations, carriage of control and data signals between the core network and UEs, and maintaining wireless communications with UEs associated with each base station. The core network comprises hardware and software to implement the network functionality, such as overall network management and control, and routing of calls and data.

[44] As set out above, an RSS signal is not defined in the NB-IoT RAT and hence the wake-up process for a UE may be inefficient. However, defining an RSS within the NB-IoT does not appear practical due to the transmission resources required. The disclosure below sets out methods and systems to allow re-use of an RSS transmitted by a different RAT to assist synchronisation of a UE connected via NB-IoT.

[45] Cells for different RATs may be co-located and synchronised and hence UEs positioned in the cell of one RAT may be able to receive signals transmitted from a base station on a different RAT. Normally a UE operating according to one RAT cannot decode and interpret signals from another RAT as the UE does not have the information required to decode the signals. The following disclosure therefore transmits the required information to a UE to enable the UE to utilise signals from another RAT. In a particular example, an NB-IoT UE receives information regarding Cat-M transmissions such that the UE can receive the Cat-M RSS.

[46] Figure 2 shows a flowchart of a method to enable a UE operating on a first RAT to receive synchronisation signals transmitted on a second RAT.

[47] At step 200 an NB-IoT base station transmits in its system information (SI) broadcast details regarding a co-located Cat-M cell which UEs connected to the NB-IoT cell may be able to receive and utilise. In general terms, a base station of a first RAT transmits details of a second RAT which the UE may be able to receive.

[48] Specifically, the NB-IoT base station transmits parameters to enable the UE to receive an RSS signal transmitted by the Cat-M base station. Full details of the Cat-M transmissions may not be required, but only selected details. The details may be transmitted as additional content in an existing NB-SIB, or a new NB-SIB may be defined.

[49] The details may include the Cat-M RSS configuration, for example:-a. The time offset of the RSS. This may be relative to the NB-IoT SFN boundary, or relative to the Cat-M SFN boundary. If relative to the Cat-M boundary the relationship between timing of the NB-IoT and Cat-M signals may also be required or is set to a predefined value.

b. RSS transmission power, powerBoost. This may be relative to CRS and nrs-CRSPowerOffset provided (it is already provided in samePCI case only), or instead a nrs-RSS-PowerOffset may be provided.

c. The frequency location of the RSS. This may be related to the DL Cat-M carrier frequency, which can be provided (or be implicitly provided for inband samePCI or inband different PCI case).

[50] The identity of the Cat-M cell may be included, for example indicated by the PCI. This may not be required if the Cat-M cell and NB-IoT cell share a PCI.

[51] The carrier configuration (dICarrierConfig) of the Cat-M cell may be included:-a. This parameter is optional but it needs to be added in scenarios where the NB-IoT cell is used close to an LTE cell (synchronized with the NB-IoT cell) or other scenarios different from samePCI, for which the LTE carrier is not known. This is the case for instance in guardband or standalone deployments. This field serves the same purpose as the field eutra-CRS-Sequencelnfo or "indexToMidPrb" used in samePCI (to specify the reference PRB or frequency of the LTE cell).

b. In an option, this Cat-M RSS config/PCI of Cat-M cell is added to the inter-RAT assistance information providing the frequency identifiers of neighbouring eMTC carriers, on a per eMTC carrier basis, in which case the dlCarrierConfig is already provided by the frequency identifier.

[52] The broadcast data may be indicated in the language of the standards as:-NB-SIB RSS or NB SIB2-extension RSS-Config Duration fregLocation-inbandGuardband (0..98) OPTIONAL oeriodicity powerBoost timeOffset categoryMpci OPTIONAL dlCarrierConfig OPTIONAL [53] Not all possible parameter values may be relevant for NB-IoT, and so the full value range for each parameter possible in the Cat-M standards may not be supported in the broadcast format to reduce broadcast overhead.

[54] At step 201 the UE receives the information and sets an appropriate configuration to enable receipt of the RSS upon waking up. As will be appreciated steps 200 & 201 are generally performed at initial connection to a cell, or at intervals if new data is needed, but are not necessarily performed at the same time as, or as a deliberate part of, the method described herein.

[55] At step 202 when the UE wakes up it receives the Cat-M RSS signal and utilises the content to re-synchronise with the network. Once synchronised the UE can receive the Wake Up Signal (WUS) at step 203, and subsequently communicate with the network.

[56] In an alternative example a UE may be a multi-mode device which can support both Cat-M and NB-IoT RATs. Such devices are capable of receiving Cat-M signals directly, and hence can acquire the Cat-M base station's NB-SIB signal in order to acquire details of the RSS. Such a multi-mode device may thus use its Cat-M compatibility to acquire the RSS information, which may then be utilised when operating in NB-IoT mode to resynchronise when waking up.

Typically, NB-SIB is received only once and the power saving from having access to the RSS will quickly compensate for the power consumed received NB-SIB from the Cat-M base station.

[57] As will be appreciated, cells of different radio access technology may share parts of a base station with other radio access technology. For example, co-located NB-IoT and Cat-M base stations may share some or all hardware components. As will be appreciated references to an "NB-loT base station" or "Cat-M base station" are therefore references to the logical and functional entity rather than implying an independent hardware unit.

[58] In addition to the features discussed above, other information regarding the Cat-M cell may be broadcast by the NB-IoT cell. For example, an explicit indication may be transmitted to indicate that a UE may use Cat-M cell signals, and/or the identity of suitable cells. The ability to use RSS information may be specified as standard, or indicated implicitly. For example, use of RSS may be allowed if the operation mode is inband-SamePCI, and/or inband-DifferentPCI. This may be a standard configuration or could be indicated in broadcast or UE-specific signalling.

[59] In the above example the RSS information is transmitted in a broadcast message. However, the information could also be included in a UE-specific or group-specific message, albeit such mechanisms may incur a greater signalling overhead.

[60] In the case of the operation mode being inband-SamePCI no additional information may be needed for the UE to be able to receive the RSS, since synchronisation and co-location can be assumed. In contrast, in operation mode inband-differentPCI the UE has knowledge of CRS location, but no other assumptions can be made about a Cat-M cell, since even a co-located cell is not necessarily synchronised. Hence, additional information is required, which may implicitly indicate that the UE Is permitted to use the RSS signal.

[61] As set out above, the information may be broadcast in a new SIB, or an extension of an existing NB-SIB. For example, the information could be added to the inter-RAT assistance information providing frequency identifiers of neighbouring eMTC carriers, on a per eMTC carrier basis. In addition to the frequency identifier of a neighbouring eMTC carrier, the network could indicate the above information providing knowledge on whether there is a Cat-M cell which can be used for time/frequency synchronization to the serving NB-IoT cell on that carrier.

[62] The UE is likely to detect a change in (Cat-M) RSS configuration by failing to detect or synchronise from the Cat-M signal. The UE can then read the Cat-M SI to obtain the updated configuration. Alternatively, the network may indicate a change in configuration via an NB-loT message (for SI broadcast).

[63] The impact of RSS on power consumption during wake up can be approximately considered by comparing the periods the receiver is on for. The estimated "on" times, and hence power consumption, are summarised in the following table for an example system. The actual figures will vary depending on various aspects of the configuration and these values are provided by way of example only.

[64] As set out in the following points, the use of RSS may bring a range of advantages over utilising PSS/SSS:- * RSS is a continuous signal of at least 8ms and it thus possible to absorb an uncertainty at wake up by waking up in the middle of the RSS. As opposed to the NPSS or NSSS which are sparse, it is not necessarily to be waken up before the NPSS/NSSS start * RSS is a contiguous signal which is easier to process than NPSS/NSSS and hence the number of accumulations M using RSS will be smaller than the number of accumulations N used for NSSS [65] This is summarized in the following table in which:- * B is the boot time in ms (implementation dependent, may be very small) * X is the uncertainty at wake up (implementation dependent and it also depends on the sleep duration * M is the number of accumulations to decode RSS / WUS * N is the number of accumulations to decode NPSS/NSSS (N > M) Without RSS With RSS Small uncertainty at wake up -low SNR Uncertainty of X subframes (X<10)-high SNR Uncertainty of X subframes (X<10)-low SNR Uncertainty of X subframes (X>10)-low SNR Very large uncertainty -SFN unknown B + N + M B + 2 M (M<N) B + (2X+1) + 1 B + 1 + 1 B + N (2X+1) + M B + 10 N + M B + 10 N + 640 + M B + 2 M (M<N) B + 2 M B + 160 + M [66] Figure 3 shows a selection of use-cases without RSS, and Figure 4 shows a comparative set of examples with RSS.

[67] As part of Cat-M enhancements it may be possible to use RRS for RRM measurements. It may also be beneficial to allow this for NB-IoT using RRS. This would enable the UE to avoid additional reception of NRS/NPSS/NSSS for the purpose of RRM, thereby limiting power consumption. Possible usage authorization of RRS for RRM may be broadcast as well.

[68] In order for a device connected to a first RAT to receive signals from a second RAT, the two networks must be synchronised in time and frequency otherwise the UE may not be able to ascertain the location of the other RAT's signals. Time synchronisation is in terms of SFN. Typically, the time synchronisation will be at least 7 SFN LSB bits between a Cat-M and NB-IoT cell due to the maximum RSS period being 1280ms. Preferably the two RATs will be synchronised by frame as well as SFN, and optionally, H-SFN.

[69] Although not shown in detail any of the devices or apparatus that form part of the network may include at least a processor, a storage unit and a communications interface, wherein the processor unit, storage unit, and communications interface are configured to perform the method of any aspect of the present invention. Further options and choices are described below.

[70] The signal processing functionality of the embodiments of the invention especially the gNB and the UE may be achieved using computing systems or architectures known to those who are skilled in the relevant art. Computing systems such as, a desktop, laptop or notebook computer, hand-held computing device (PDA, cell phone, palmtop, etc.), mainframe, server, client, or any other type of special or general purpose computing device as may be desirable or appropriate for a given application or environment can be used. The computing system can include one or more processors which can be implemented using a general or special-purpose processing engine such as, for example, a microprocessor, microcontroller or other control module.

[71] The computing system can also include a main memory, such as random access memory (RAM) or other dynamic memory, for storing information and instructions to be executed by a processor. Such a main memory also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor. The computing system may likewise include a read only memory (ROM) or other static storage device for storing static information and instructions for a processor.

[72] The computing system may also include an information storage system which may include, for example, a media drive and a removable storage interface. The media drive may include a drive or other mechanism to support fixed or removable storage media, such as a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a compact disc (CD) or digital video drive (DVD) read or write drive (R or RVV), or other removable or fixed media drive. Storage media may include, for example, a hard disk, floppy disk, magnetic tape, optical disk, CD or DVD, or other fixed or removable medium that is read by and written to by media drive. The storage media may include a computer-readable storage medium having particular computer software or data stored therein.

[73] In alternative embodiments, an information storage system may include other similar components for allowing computer programs or other instructions or data to be loaded into the computing system. Such components may include, for example, a removable storage unit and an interface, such as a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, and other removable storage units and interfaces that allow software and data to be transferred from the removable storage unit to computing system.

[74] The computing system can also include a communications interface. Such a communications interface can be used to allow software and data to be transferred between a computing system and external devices. Examples of communications interfaces can include a modem, a network interface (such as an Ethernet or other NIC card), a communications port (such as for example, a universal serial bus (USB) port), a PCMCIA slot and card, etc. Software and data transferred via a communications interface are in the form of signals which can be electronic, electromagnetic, and optical or other signals capable of being received by a communications interface medium.

[75] In this document, the terms 'computer program product', 'computer-readable medium' and the like may be used generally to refer to tangible media such as, for example, a memory, storage device, or storage unit. These and other forms of computer-readable media may store one or more instructions for use by the processor comprising the computer system to cause the processor to perform specified operations. Such instructions, generally 45 referred to as 'computer program code' (which may be grouped in the form of computer programs or other groupings), when executed, enable the computing system to perform functions of embodiments of the present invention. Note that the code may directly cause a processor to perform specified operations, be compiled to do so, and/or be combined with other software, hardware, and/or firmware elements (e.g., libraries for performing standard functions) to do so.

[76] The non-transitory computer readable medium may comprise at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory and a Flash memory. In an embodiment where the elements are implemented using software, the software may be stored in a computer-readable medium and loaded into computing system using, for example, removable storage drive. A control module On this example, software instructions or executable computer program code), when executed by the processor in the computer system, causes a processor to perform the functions of the invention as described herein.

[77] Furthermore, the inventive concept can be applied to any circuit for performing signal processing functionality within a network element. It is further envisaged that, for example, a semiconductor manufacturer may employ the inventive concept in a design of a stand-alone device, such as a microcontroller of a digital signal processor (DSP), or application-specific integrated circuit (ASIC) and/or any other sub-system element.

[78] It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to a single processing logic. However, the inventive concept may equally be implemented by way of a plurality of different functional units and processors to provide the signal processing functionality. Thus, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organisation.

[79] Aspects of the invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented, at least partly, as computer software running on one or more data processors and/or digital signal processors or configurable module components such as FPGA devices.

[80] Thus, the elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term 'comprising' does not exclude the presence of other elements or steps.

[81] Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by, for example, a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate.

[82] Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to 'a', 'an', 'first', 'second', etc. do not preclude a plurality.

[83] Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term 'comprising' or "including" does not exclude the presence of other elements.