GB2576203A

GB2576203A - Method and apparatus for signalling for group handover or cell re-selection in non-terrestrial networks

Info

Publication number: GB2576203A
Application number: GB1812978.3A
Authority: GB
Inventors: Hunukumbure Mythri; Qi Yinan
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2018-08-09
Filing date: 2018-08-09
Publication date: 2020-02-12
Anticipated expiration: 2038-08-09
Also published as: GB2576203B; GB201812978D0

Abstract

A wireless communication system comprises a first non-terrestrial network, NTN, airborne base station 310 (a drone, Unmanned Aerial Vehicle, UAV), a second replacement NTN airborne base station (BS) 312 and a plurality of remote wireless communication units 325 (User Equipment, UE). The second replacement airborne BS arranged to: establish a wireless backhaul or front-haul communication link 330 (an X2 link) with the first airborne BS; and broadcast a transmission (comprises at least one cell identifier parameter and synchronization signal) to the plurality of UEs at a second frequency that is different to a first frequency used by the first airborne BS, substantially within a coverage area of the first airborne BS. The first airborne BS arranged to: broadcast an inter-frequency measurement report request to the plurality of UEs served by the first airborne BS, wherein the measurement report request includes measurements for the second frequency. The first BS initiates at least one/multiple simultaneous (group) handover procedures of the plurality of UEs from the first BS to the second replacement BS for UE in RRC-connected state. UE in RRC-inactive state nay initiate a cell re-selection procedure. The first BS and second BS may be remote radio heads, RRHs.

Description

METHOD AND APPARATUS FOR SIGNALLING FOR GROUP HANDOVER OR CELL RE-SELECTION IN ΝΟΝ-TERRESTRIAL NETWORKS

Technical Field [0001] This invention relates generally to implementing signalling techniques in a nonterrestrial network (NTN), and in particular, but not limited to, supporting handover of a group of active users in NTNs.

Background [0002] In recent years, third generation (3G) wireless communications have evolved to the long term evolution (LTE) cellular communication standard, sometimes referred to as 4^thgeneration (4G) wireless communications. Both 3G and 4G technologies are compliant with third generation partnership project (3GPP™) standards. 4G networks and phones were designed to support mobile internet and higher speeds for activities, such as video streaming and gaming. The 3GPP™ standards are now developing a fifth generation (5G) of mobile wireless communications, which is set to initiate a step change in the delivery of better communications, for example powering businesses, improving communications within homes and spearheading advances such as driverless cars.

[0003] Referring to FIG. 1, a known simplified 5G architecture diagram 100 illustrates a first terrestrial 5G base station 102 supporting communications within a coverage area 104, including communication support for a wireless communication unit, sometimes referred to as a terminal device, such as a user equipment UE 106. In 5G, the UE 106 is able to support traditional Human Type Communications (HTC) or the new emerging Machine Type Communications (MTC). The UE 106 is considered to be active when communicating and in the operational state technically known as radio resource control (RRC) connected, or the UE is considered to be in the new 5G-new radio (NR) defined ‘RRC-inactive’ state. The known simplified architecture diagram 100 comprises a second terrestrial 5G base station 112 supporting communications within a coverage area 114, including communication support for

- 1 the UE 106 as it transitions to the second terrestrial 5G base station 112 coverage area 114. A backhaul connection 122, generally an Xn (based on X2) interface that specifies some additional procedures for coordinating aspects relevant for handover, connects the first terrestrial 5G base station 102 and second terrestrial 5G base station 112 via a gateway 120. For the RRC-inactive state UEs, a cell re-selection process is warranted, if the signal strength from the current serving cell deteriorates as the UE transitions from 5G base station 102 to 112.

[0004] Currently, a handover between base stations is performed when a UE 106 approaches a cell edge, i.e. at the edge of the coverage area 104 from first terrestrial 5G base station 102. Handover of the active communication from the first terrestrial 5G base station 102 to the second terrestrial 5G base station 112 happens when the measured signal strength (as measured by the UE 106) of the second base station 112 becomes a Δ dB higher than the signal strength of serving base station 102, to avoid an instantaneous decision that would have to be reversed soon thereafter based on similar signal strength measurements. The effect of a communication being handed over between two base stations too frequently is typically referred to as a ‘ping-pong’ handover effect.

[0005] Similarly a cell re-selection process is performed for ‘RRC inactive’ state UEs when the UE nears the cell edge of the current 5G base station 102 that it is camped on, and is able to receive a signal from the neighbour base station 112. This known 5G cell reselection process is also driven by signal strength measurements of the base stations carried out by the UE. Again, a A dB threshold is enacted to avoid repeated cell re-selections in a ‘ping-pong’ effect.

[0006] Thus, the 3 GPP NR (and LTE) handover and cell re-selection procedures are governed by the measurement reports that the UE 106 is providing of signals received from serving base station 102 and one or more neighbour base station(s) 112, sometimes referred to as fifth generation Node Bs (gNB) or eNBs. These measurement reports can be periodic or instructed by the serving gNB 102 and the UE 106 can report a number of neighbour gNB (or eNB) signal strengths that is/are above a threshold. The threshold A dB is set to avoid ping-pong type handover and cell re-selection exchanges near the cell border, as the instantaneous signal strengths can very dynamically. The serving gNB 102 can instruct each individual UE 106 to provide these measurement reports and the measurement frequency can be adapted depending on whether a particular UE is nearing a cell edge, for example. The serving gNB 102 will initiate

-2the handover process on an individual UE basis, if the serving gNB signal strength is below (with threshold considered) a reported neighbour gNB signal strength. In this regard, unicast messages are sent from the serving gNB 102 to each UE 106 to initiate a normal, individual handover operation. Each of these respective handovers is based on the individual UE 106 measurement reports identifying the signal strength of transmissions from gNBs that it can ‘see’. Also, the cell re-selection processes happen on an individual UE basis, based on the measurements of the current camped-on and neighbour base stations (gNBs). In the ‘RRCinactive’ state, the current camped-on base station (gNB) 102 is able to instruct the UEs 106 on an individual basis in order to carry out these measurements and the UE 106 themselves will initiate and carry out the cell re-selection process.

[0007] Another instance where a wireless communication unit/UE 106 may need to transition to being supported by a neighbour gNB is when its serving gNB fails. In this case, the wireless communication unit/UE 106 must undergo re-registration process with the (new) neighbour gNB in order to re-start communications.

[0008] Non-Terrestrial Networks (NTNs) is an emerging area in 3GPP™ discussions for 5G, where a study item recently concluded in 5GRAN1, and a 5GRAN2 study item is proposed to start in August 2018. Non-Terrestrial Networks aim to provide 5G cellular coverage using space borne and/or air borne platforms, where traditional ground based networks have difficulty in providing coverage and/or capacity. FIG. 2 illustrates the link configuration 200 currently envisioned for 5G air-borne communications. As illustrated, an air-borne platform (5G base station 220 employs a service link 215 to a handheld or Internet of Things (loT- similar to MTC) device 210, whilst facilitating communications via a feeder link 225 to a 5G gateway 230, and thereafter the 5G core network 240 and a public data network 250, such as the Internet.

[0009] Particularly with air-borne platforms (such as High Altitude Platforms (HAPs) or drones), there will be a need to replace a single active airship or drone with another, for example due to battery power draining or maintenance reasons. In this scenario, the group of active users that the current airship or drone is supporting will need to be handed over to the replacement airship or drone in a quick and efficient manner. The current 5G new radio (NR) and LTE standards do not have provisions for such an accelerated group handover or cell re-selection procedures.

-3[0010] Hence, the inventors have identified a problem with extrapolating the above mentioned ‘terrestrial’ handover and cell re-selection approaches, when considering a nonterrestrial network implementation, in that a NTN base station will need to be wholly replaced whilst actively handling many active users, and likely more frequently as airborne battery supplies will run down and/or for maintenance reasons. Furthermore, hot switch-over techniques for a fully-functioning terrestrial base station to be switched in, or gradually switched over to, is unsuitable in an NTN scenario.

Summary of the Invention [0011] In a first aspect of the invention, a wireless communication system includes a first base station, for example a wireless non-terrestrial network base station, supporting communications with a plurality of wireless communication units, and a second base station, for example a replacement wireless non-terrestrial network base station. The second base station includes a transceiver and a processor operably coupled to the transceiver and arranged to establish a wireless backhaul or front-haul communication link with the first base station and broadcast a transmission to the plurality of remote wireless communication units at a second frequency that is different to a first frequency used by the first base station, substantially within a coverage area of the first base station, wherein the broadcast transmission comprises at least one cell identifier parameter and at least one synchronization signal. The first base station includes a transceiver and a processor operably coupled to the transceiver and arranged to broadcast an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes measurements for the second frequency.

[0012] In this manner, a serving base station is able to obtain inter-frequency measurement reports from its served remote wireless communication units (e.g. UEs) and, say, initiate a handover operation when they are in a radio resource controlled (RRC)-connected state. Thus, in some examples of the invention, a mechanism for provision of signalling procedures can support a group handover scenario in an airborne NTN base station (or gNB) for the RRC-connected state UEs.

-4[0013] In an optional example, the first base station and second base station may be nonterrestrial network, NTN, airborne base stations. In some examples, a new broadcast signalling message may be configured as part of the System Information Block (SIB). In some examples, this new SIB message may be configured in ways that only the NTN connected UEs (and devices) would be able to read. For example, this new SIB message may be configured with a binary header bit, and when active can indicate to the NTN connected UE’s to read-on for the full message.

[0014] In an optional example, the inter-frequency measurement report request may include a measurement threshold configured to trigger the remote wireless communication units performing and reporting the measurement where a signal strength from the second NTN base station is AdB higher than a measured signal strength of the first NTN base station. In this manner, this will prompt the remote wireless communication units (such as UEs) to conduct the measurements of the neighbour base station that is now physically very near to the serving/camped-on base station.

[0015] In some examples, a mechanism for broadcast signalling from the serving base station or gNB is proposed, through which the gNB requests the RRC-connected UEs to provide inter-frequency signal strength measurements of the neighbour gNBs. In some examples, the threshold Δ dB for the measurement reporting may be minimized, so that the UE will report neighbour gNB signal strengths even if they are similar to the serving gNB signal strengths.

[0016] In an optional example of the invention, it is envisaged that the remote wireless communication units (e.g. UEs) may be in one of the following operational states when the broadcast message is received: a RRC-connected state: where the UEs are actively communicating data with another UE or a PDN; a RRC-inactive state: where the UEs have minimal signalling with the base station and can transmit sporadic, short packets, for example as defined for a 5G system; a RRC-idle state: where the UEs are not performing any data communication.

[0017] In some examples, the RRC-inactive UEs may be configured to carry out cell reselection procedures, after conducting the inter-frequency (neighbour cell) measurements. In some examples, the cell re-selection may be carried out even when the neighbour gNB signal

-5strengths are of a similar value, as the Δ dB thresholds are now minimized. In some examples, the cell re-selection procedures may be configured to happen according to the 5GNR/LTE standards, initiated by the aforementioned broadcast signalling procedure.

[0018] In an optional example of the invention, the NTN base stations may be implemented as simple remote radio heads (RRH) carrying out only the transceiver radio frequency (RF) functions, whilst the higher layer RAN operations may be carried out in a centralized processing unit, preferably based on the ground. This option, termed a fully centralized RAN, much simplifies the design and reduces power consumption of the air-borne (NTN) part of the network, thereby allowing longer operational times before replacement due to battery drain or maintenance reasons.

[0019] In some examples, the communications between the RRHs and the centralized processing unit may occur through front-haul links. In this configuration the handover and/or cell re-selection options may be physically carried out in the centralized processing unit, but the signalling procedures between RRH and the UEs may remain the same as current 5G operation.

[0020] In an optional example of the invention, the processor centralization may be carried out partially, to any intermediate step below a fully centralized configuration. Again, in this example embodiment, the signalling procedures remain unchanged.

[0021] In some examples, the serving base station or gNB may be configured to carry out the handover procedures for the RRC-connected UEs that report the signal strengths of neighbour gNB(s). In some examples, the handovers may be carried out even when the neighbour gNB signal strengths are of similar value, as the Δ dB thresholds are now minimized. In some examples, the handover procedures may be configured to happen separately for each UE, as and when they report back the requested measurements. In some examples, multiple handover procedures may happen simultaneously, as multiple RRC connected UEs will report back the signal strengths following the inter-frequency measurements. In some examples, the handover procedures may be configured to happen according to the 5G NR/LTE standards, initiated by the aforementioned broadcast signalling procedure.

[0022] In a second aspect of the invention, a first base station is described that communicates with a plurality of remote wireless communication units. The first base station

-6includes a processor, operably coupled to a transceiver and arranged to establish a wireless backhaul or front-haul communication link with a second replacement base station, operate at a first frequency that is different to a second frequency used by the second replacement base station and broadcast an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes a request for signal strength measurements for the second frequency.

[0023] In a third aspect of the invention, a remote wireless communication unit is described that is configured to communicate with a first base station on a first frequency and a replacement second base station on a second frequency, wherein the remote wireless communication unit includes a transceiver; and a processor, operably coupled to the transceiver and arranged to receive a broadcast a transmission at the second frequency that is different to a first frequency used by the first base station, substantially within a coverage area of the first base station, wherein the broadcast transmission comprises at least one cell identifier parameter; and at least one synchronization signal, receive a broadcast inter-frequency measurement report request on the first frequency, wherein the measurement report request includes measurements for a second frequency, different to the first frequency, and used by a second replacement base station, and transmit an inter-frequency measurement report to the first base station in response thereto.

[0024] In a fourth aspect of the invention, a method for a first base station that communicates with a plurality of remote wireless communication units is described. The method includes establishing a wireless backhaul or front-haul communication link with a second replacement base station; operating at a first frequency that is different to a second frequency used by the second replacement base station; and broadcasting an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes a request for signal strength measurements for the second frequency.

[0025] In a fifth aspect of the invention a method for a remote wireless communication unit configured to communicate with a first base station on a first frequency and a replacement second base station on a second frequency is described. The method includes: receiving from the second replacement base station a broadcast transmission on a second frequency, wherein the

-7 second replacement base station operates substantially within a coverage area of the first base station, and wherein the broadcast transmission comprises at least one cell identifier parameter; and at least one synchronization signal of the second replacement base station; being served by the first base station; receiving from the first base station a broadcast inter-frequency measurement report request on the first frequency, wherein the measurement report request includes a request for measurements on the second frequency that is different to the first frequency; and transmitting an inter-frequency measurement report to the first base station in response thereto.

Brief Description of the Drawings [0026] Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, similar reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

[0027] FIG. 1 illustrates a known simplified 5G architecture configured to handle handover of individual communications.

[0028] FIG. 2 illustrates a proposed 5G link configuration for NTN air-borne communications.

[0029] FIG. 3 illustrates a 3GPP™ 5G communication system with NTN base stations adapted in accordance with some example embodiments of the present invention.

[0030] FIG. 4 illustrates a block diagram of an NTN base station communicating with a UE, adapted in accordance with some example embodiments of the invention.

[0031] FIG. 5 illustrates a simplified flowchart of an NTN base station group handover operation, in accordance with some example embodiments of the invention.

[0032] FIG. 6 illustrates a simplified flowchart of an NTN base station group cell reselection operation, in accordance with some example embodiments of the invention.

-8[0033] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.

Detailed Description [0034] Examples of the invention describe a wireless communication system that includes a serving base station and a replacement base station, which in some instances may be a first serving non-terrestrial network, NTN, airborne base station and a second replacement NTN airborne base station, and a plurality of remote wireless communication units, such as UEs or MTC devices. Examples of the invention describe a mechanism whereby UEs or MTC devices are able to transition from being served by the first serving base station to the second replacement base station. In examples of the invention, the second replacement base station includes a transceiver and a processor arranged to establish a wireless backhaul or front-haul communication link with the first base station. The processor of the second replacement base station is configured to broadcast a transmission to the plurality of remote wireless communication units on a second frequency that is different to a first frequency used by the first base station, and the second replacement base station operates substantially within a coverage area of the first base station. The broadcast transmission from the second replacement base station includes at least one cell identifier parameter; and at least one synchronization signal. In

-9examples of the invention, the first base station includes a transceiver and a processor arranged to broadcast an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes measurements for the second frequency.

[0035] In this manner, a serving base station is able to obtain inter-frequency measurement reports from its served remote wireless communication units (e.g. UEs) and, say, initiate a handover operation when they are in a radio resource controlled (RRC)-connected state. Thus, in some examples of the invention, a mechanism for provision of signalling procedures can support a group handover scenario in an airborne NTN base station (or gNB) for the RRC-connected state UEs.

[0036] In some optional examples, for example in a 5G system, the remote wireless communication units (e.g. UEs), when they are in a RRC-inactive state, may be able to initiate a cell re-selection procedure in response to the conducted measurements.

[0037] In some optional examples, for example in a 5G system, the replacement gNB is able to initiate its transmissions and establish a backhaul or front-haul communication link with the serving gNB, say, an X2 link, using current known procedures, prior to the serving gNB initiating the proposed signalling mechanism that includes a broadcast inter-frequency measurement request, in order to expedite a group handover.

[0038] Although example embodiments of the invention are described with reference to handover of a group of UEs in a 5G architecture that included NTN base stations, it is envisaged that some aspects of the invention are not so constrained/limited. For example, it is envisaged that the description may be enacted for a long Term Evolved (LTE™) system, for example operating a set of NTN base stations and UEs. For example, it is also envisaged that the description may be enacted for an LTE or 5G-NR ground based special system, where the active base station or gNB needs to replaced, whilst retaining the RRC-active state UEs (for example a cell on wheels (CoW) system that are employed at disaster and emergency situations.

[0039] Thus, examples of the invention also describe a mechanism for provision of cell reselection in an airborne NTN base station (or gNB) for the RRC-inactive state UEs. Examples

-10of the invention describe how a camped-on gNB is configured to prompt a cell re-selection process by use of a broadcast message.

[0040] Example embodiments are described with respect to a range of potential remote wireless communication units, such as user equipment (UE), Internet of Things (loT) devices, Human Type Communications (HTC), the new emerging Machine Type Communications (MTC), etc., with the expression remote wireless communication units encompassing all such device implementations and applications. For completeness, the description may interchangeably use such terminology.

[0041] Example embodiments are described with reference to radio access networks, which term encompasses and is considered to be equivalent to and interchangeable with communication cells, namely the facilitation of communications within a cell that may access other parts of the communication system as a whole.

[0042] Referring now to FIG. 3, part of a wireless communication system 300 is shown in outline, in accordance with one example embodiment of the invention. In this example embodiment, the wireless communication system 300 is compliant with, and contains network elements capable of operating over, a 5^th generation (5G) wireless communication system, which is currently under discussion in the third Generation Partnership Project (3GPP™) specification for 5G, based around Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink (DL) and DFT-spread OFDM (DFT-s-OFDM) in the uplink (UL), as described in the 3GPP™ TS 38.211 series of specifications.

[0043] The wireless communication system 300 architecture consists of radio access network (RAN) and core network (CN) elements (not shown), with the core network elements being coupled to external networks (named Packet Data Networks (PDNs)), such as the Internet or a corporate network.

[0044] As illustrated, the CN is operably connected to two gNodeBs (gNBs) 310, 312, with a respective, substantially-identical, coverage area or cell 385. A plurality of wireless communication units 325 communicate with a serving gNB 310. In accordance with example

-11 embodiments of the present invention, at least one gNB 310 and at least one UE 325 (amongst other elements) have been adapted to support the concepts hereinafter described.

[0045] In this example of the invention, the main component of the RAN is an NTN airborne gNB 310, which performs many standard base station functions and is connected to the CN via an SI interface/feeder link and to the wireless communication units 325 via a Uu interface. A wireless communication system will typically have a large number of such infrastructure elements where, for clarity purposes, only a limited number are shown in FIG. 3. The gNB 310 control and manage the radio resource related functions for a plurality of remote wireless communication units 325. Each of the wireless communication units 325 comprise a transceiver unit 327 operably coupled to signal processing logic 308 (with one wireless communication unit illustrated in such detail for clarity purposes only). The system comprises many other remote wireless communication units 325 and gNBs 310, which for clarity purposes are not shown.

[0046] Notably, in accordance with some example embodiments, the system determines that the NTN airborne gNB 310 needs to be replaced, e g. for regular maintenance, or due to battery power, etc. In this regard, an NTN airborne replacement gNB 312 needs to be positioned physically close to the serving NTN airborne gNB 310, in order to serve the same set of UEs 325 in the same cell 385. Thus, in examples of the invention, the NTN airborne replacement gNB 312 is configured to operate at a second frequency that is a different frequency than a first frequency that the current serving NTN airborne gNB 310 uses, and the handover(s) that follow are configured to be inter-frequency handovers.

[0047] After the NTN airborne replacement gNB 312 positions itself close to the serving NTN airborne gNB 310, the NTN airborne replacement gNB 312 is configured first to establish the backhaul link to the core network. In examples of the invention, the NTN airborne replacement gNB 312 will be granted a cell ID and operational parameters, such as the carrier frequency and serving bandwidth by the core network. Then, the NTN airborne replacement gNB 312 is configured to establish the X2 (wireless) link 330 with the current serving NTN airborne gNB 310. Once these set-up procedures are complete, the NTN airborne replacement gNB 312 is configured to start to broadcast its parameters and synchronisation signals within the

-12coverage area 385 of its new cell (which is nearly identical to the existing served cell by the other gNB).

[0048] The NTN airborne replacement gNB 312 includes a transceiver and a processor that is arranged to establish a wireless backhaul or front-haul communication link with the first base station, i.e. the serving NTN airborne gNB 310. The NTN airborne replacement gNB 312 is configured to broadcast a transmission to the plurality of remote wireless communication units, e.g. UEs 325, on the second frequency that is different to a first frequency (used by the first base station , i.e. the serving NTN airborne gNB 310. The broadcast transmission from the NTN airborne replacement gNB 312 includes at least one cell identifier parameter; and at least one synchronization signal, so that each of the UEs 325 are able to recognise the existence of the NTN airborne replacement gNB 312. As the NTN airborne replacement gNB 312 is positioned close to the serving NTN airborne gNB 310, the broadcast transmissions of each cover substantially the same coverage area 385, as shown.

[0049] The first base station, i.e. the serving NTN airborne gNB 310, also includes a transceiver and a processor, which is arranged to broadcast an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units, served by the first base station. Notably, the measurement report request includes measurements for the second frequency used by the NTN airborne replacement gNB 312 that the UEs now recognise.

[0050] In this manner, the serving NTN airborne gNB 310 is able to obtain inter-frequency measurement reports from its served remote wireless communication units (e.g. UEs 325) and, say, initiate a handover operation when they are in a radio resource controlled (RRC)-connected state. Thus, in some examples of the invention, a mechanism for provision of signalling procedures can support a group handover scenario in an airborne NTN base station (or gNB) for the RRC-connected state UEs.

[0051] In examples of the invention, the handover process may encompass the serving gNB 310 issuing a handover command to the UE 325, once the target gNB 312 agrees to the handover. The serving gNB 310 will then start to push all of the unacknowledged packets to the target gNB 312 through the X2 link 330. The UE 325 performs uplink synchronisation to the

-13 target gNB 312 and may also obtain resource access through, say, a contention free random access channel (RACH) procedure. In some examples, the UE 325 may also send a handover complete message to the target gNB 312, effectively making it the new serving gNB.

[0052] At this point, the new serving eNB will inform the core network of the handover complete and a late path switch occurs, so that the all packets (concerning the active UE 325) are now routed to the new serving gNB, i.e. NTN airborne replacement gNB 312 in this example.

[0053] In some optional examples, for example in the illustrated 5G system, the remote wireless communication units (e.g. UEs 325), when they are in a RRC-inactive state, may be able to initiate a cell re-selection procedure in response to the conducted measurements.

[0054] In some examples, it is envisaged that the new broadcast signalling message transmitted by the serving NTN airborne gNB 310 may be configured as part of the System Information Block (SIB). In some examples, this new SIB message may be configured in ways that only the NTN connected UEs 325 would be able to read. For example, this new SIB message may be configured with a binary header bit, and when active can indicate to the NTN connected UE’s 325 to read-on for the full message.

[0055] In an alternative example of the invention, a front-haul link may be formed, for example in a case where a centralized BBU processing network 350 (sometimes referred to as a fully centralized radio access network (RAN)) is used. Here, in this example, the NTN airborne gNB 310 and the NTN airborne replacement gNB 312 the may be configured as simple remote radio heads (RRH) carrying out only the transceiver RF functions, while the higher layer RAN operations can be carried out in a centralized processing unit 358 operably and wirelessly connected to the RRHs via a respective transceiver 357. In this example, the centralized BBU processing network 350 is preferably based on the ground. This optional implementation much simplifies the design and reduces power consumption of the air-borne (NTN) part of the network, thereby allowing longer operational times before replacement due to battery drain or maintenance reasons. The communications between the RRHs and the centralized processing unit happens through the front-haul links 352. In this configuration the handover and cell reselection options are physically carried out in the centralized processing unit 358, in response to

-14the signalling procedures between RRH and the UEs based on the broadcast inter-frequency message report request and reports sent in response thereto.

[0056] In a further envisaged optional example of the invention, the processor centralization may be carried out partially in a centralized BBU processing network 350 and partially in the NTN airborne gNB 310 and the NTN airborne replacement gNB 312. Thus, operations and any intermediate steps (as described below) may be distributed there between.

[0057] Referring now to FIG. 4, more detailed block diagrams of a gNB wireless base station 310 (equivalent in functionality details to gNB wireless base station 312 in FIG. 3) and a remote wireless communication unit (such as a UE 325) are illustrated, where the respective communications units have been adapted in accordance with some example embodiments of the invention.

[0058] The gNB wireless base station 310 contains an antenna 402, for receiving transmissions, coupled to an antenna switch or duplexer 404 that provides isolation between receive and transmit chains within the gNB wireless base station 310. One or more receiver chains, as known in the art, include receiver front-end circuitry 406 (effectively providing reception, filtering and intermediate or base-band frequency conversion). The receiver front-end circuitry 406 is coupled to a signal processing module 408 (generally realized by a digital signal processor (DSP)). A skilled artisan will appreciate that the level of integration of receiver circuits or components may be, in some instances, implementation-dependent.

[0059] The controller 414 maintains overall operational control of the gNB wireless base station 310. The controller 414 is also coupled to the receiver front-end circuitry 406 and the signal processing module 408. In some examples, the controller 414 is also coupled to a frequency generation circuit 417 and a memory device 416 that selectively stores operating regimes, such as decoding/encoding functions, synchronization patterns, code sequences, and the like. A timer 418 is operably coupled to the controller 414 to control the timing of operations (e.g. transmission or reception of time-dependent signals) within the gNB wireless base station 310.

-15 [0060] As regards the transmit chain, this essentially includes an input module 420, coupled in series through transmitter/modulation circuitry 422 and a power amplifier 424 to the antenna 402, antenna array, or plurality of antennas. The transmitter/ modulation circuitry 422 and the power amplifier 424 are operationally responsive to the controller 414. The signal processor module 408 in the transmit chain may be implemented as distinct from the signal processor in the receive chain. Alternatively, a single processor may be used to implement a processing of both transmit and receive signals, as shown in FIG. 4. Clearly, the various components within the gNB wireless base station 310 can be realized in discrete or integrated component form, with an ultimate structure therefore being an application-specific or design selection.

[0061] The processor 408 and transceiver (e.g. transmitter/modulation circuitry 422 and receiver front-end circuitry 406) of the gNB wireless base station 310 establish a wireless backhaul or front-haul communication link with a second replacement base station, e.g. gNB wireless base station 312 in FIG. 3). The gNB wireless base station 310 operates at a first frequency that is set by frequency generation circuit 417 and that is different to a second frequency used by the second replacement base station. The processor 408 and transmitter/modulation circuitry 422 are configured to broadcast an inter-frequency measurement report request on the first frequency to a plurality of remote wireless communication units 325 served by the gNB wireless base station 310, wherein the measurement report request includes a request for signal strength measurements for the second frequency. The receiver front-end circuitry 406 and processor 408 then receive the reported measurement reports from the served UEs 325.

[0062] In some examples, the inter-frequency measurement report request includes a measurement threshold configured to trigger the remote wireless communication units performing and reporting the measurement in response to a signal strength from the second base station (e.g. gNB wireless base station 312) being AdB higher than a measured signal strength of the first base station. In some examples, the measurement threshold may be configured to be a minimum AdB threshold between signal strengths from the gNB wireless base station 310 and the second base station (e.g. gNB wireless base station 312).

[0063] In some examples, for a handover process, once the measurements are provided to the processor 408, and if the signal from the neighbour gNB (e.g. gNB wireless base station 312)

-16has sufficient signal strength (for example as determined by the threshold Δ dB) the serving gNB initiates the handover, for example in the known manner. In some examples, as the UEs may be responding in an RRC-connected state (i.e. the UEs are actively communicating data with another UE or a packet data network (PDN)), the gNB wireless base station 310 may initiate a substantially simultaneous handover of a group of UEs. In this example, the substantially simultaneous handover of a group of UEs is dependent on the respective measurement reports, as the handovers are governed by the measurements the UE 325 makes of the serving/camped-on and neighbour cells.

[0064] FIG. 4 also shows a high level block diagram of the wireless communication unit 325 contains an antenna 452, for receiving transmissions, coupled to an antenna switch or duplexer 454 that provides isolation between receive and transmit chains within the wireless communication unit 325. One or more receiver chains, as known in the art, include receiver front-end circuitry 456 (effectively providing reception, filtering and intermediate or base-band frequency conversion). The receiver front-end circuitry 456 is coupled to a signal processing module 458 (generally realized by a digital signal processor (DSP)). A skilled artisan will appreciate that the level of integration of receiver circuits or components may be, in some instances, implementation-dependent.

[0065] The controller 464 maintains overall operational control of the wireless communication unit 325. The controller 464 is also coupled to the receiver front-end circuitry 456 and the signal processing module 458. In some examples, the controller 464 is also coupled to a frequency generation circuit 467 and a memory device 466 that selectively stores operating regimes, such as decoding/encoding functions, synchronization patterns, code sequences, and the like. A timer 468 is operably coupled to the controller 464 to control the timing of operations (e.g. transmission or reception of time-dependent signals) within the wireless communication unit 325.

[0066] As regards the transmit chain, this essentially includes an input module 470, coupled in series through transmitter/modulation circuitry 472 and a power amplifier 474 to the antenna 452, antenna array, or plurality of antennas. The transmitter/ modulation circuitry 472 and the power amplifier 474 are operationally responsive to the controller 464.

-17[0067] The signal processor module 458 in the transmit chain may be implemented as distinct from the signal processor in the receive chain. Alternatively, a single processor may be used to implement a processing of both transmit and receive signals, as shown in FIG. 4. Clearly, the various components within the wireless communication unit 325 can be realized in discrete or integrated component form, with an ultimate structure therefore being an application-specific or design selection.

[0068] The processor 458 and transceiver (e.g. transmitter/modulation circuitry 472 and receiver front-end circuitry 456) of the UE 325 are configured to communicate with the gNB wireless base station 310 on a first frequency that is set by frequency generation circuit 467. In accordance with examples of the invention, the frequency generation circuit 467 may also configure the UE 325 to operate on a second frequency so that it can receive at least one cell identifier parameter and at least one synchronization signal, from the replacement gNB 312, which may be an airborne gNB, to enable the UE 325 to recognise the existence of the NTN airborne replacement gNB 312 as a prospective candidate base station. The second frequency is different to the first frequency used by the serving base station (i.e. gNB wireless base station 310). The processor 458 and receiver front-end circuitry 406 are configured to receive a broadcast inter-frequency measurement report request on the first frequency from the gNB wireless base station 310, wherein the measurement report request includes a request for signal strength measurements for the second frequency that the NTN airborne replacement gNB 312 is using. Once signal strength measurements on the requested frequencies have been performed by the UE 325, the transmitter/modulation circuitry 422 and processor 408 then transmit the reported measurement reports to the gNB wireless base station 310.

[0069] In some examples, when the UE 325 is in a radio resource controlled (RRC)-inactive state (e.g. where the UEs have minimal signalling with the base station and can transmit sporadic, short packets), the processor 458 may be configured to initiate a cell re-selection procedure to the second replacement base station in response to the conducted measurements. Again, in this example, the cell re-selection procedure is governed by the measurements that the UE 325 makes of the serving/ camped-on and neighbour cells.

[0070] In some examples, for example in an RRC-idle state (e.g. where the UEs are not performing any data communication), the UE 325 is able to conduct these measurements

-18periodically at a rate that it has configured or the serving/ camped-on gNB can instruct the UEs to provide these measurements. For this example cell re-selection procedure, once the measurements are conducted by the UE 325 and if the neighbour gNB 321 has sufficient signal strength (as determined by the threshold Δ dB), the UE 325 may itself initiate the process.

[0071] In a further aspect of the invention, it is noted that the standardization of a 5G new radio (NR) has introduced a new radio resource control (RRC) state, termed RRC-inactive, which is targeted towards reducing the energy consumption (through reduced signalling) and to reduce the delays in short packet transmissions. This new RRC state will lie between the traditional RRC-connected (active UEs, which have been described above within the context of a group handover) and the RRC-idle states. This new RRC state is designed mainly to support short and infrequent transmissions in MTC type applications. The inventors of the present invention believe that the new RRC state may be ideally suited to the air-borne platforms, particularly in supporting many such remote MTC applications in future, due to better coverage and infrequent use.

[0072] It is envisaged that some of the MTC devices communicating with such an airborne NTN gNB will engage in sporadic short packet transmissions and, thus, stay in the RRC-inactive state for a long time. Within this RRC-inactive state, the serving gNB will have minimum signalling with such devices. When the serving gNB needs to be replaced by a new airborne gNB, it is envisaged in some examples that all such RRC-inactive devices may be configured to carry out a cell re-selection process.

[0073] Normally the cell re-selection process is enacted by the periodic measurements the remote device is making. Typically, if the remote device finds that the current cell on which it is camping (i.e. its host) has a lower signal strength (e.g. lower by a Δ dB threshold) than a neighbour cell, the remote device will try to camp on the neighbour cell. However, it is envisaged that if the serving gNB (e.g. host cell) has sufficient signal strength, these periodic, infrequent measurements may be carried out with much less frequency.

[0074] Thus, in some examples of the invention, a mechanism is described to alert all these remote MTC devices in the RRC-inactive state, in order to carry out these neighbour cell measurements and subsequently initiate the cell re-selection process.

-19[0075] For these MTC remote devices, it is envisaged that a group ‘cell re-selection’ process may be activated, once the replacement NTN gNB is in place and has broadcasted its cell ID etc. In this context, in accordance with examples of the invention, the serving NTN gNB may be configured to prompt the cell re-selection process for the remote MTC devices by sending a broadcast message as previously described. Although considered outside of the scope of the present invention, in some implementations there may be changes in other RAN operations (such as RACH pre-amble configuration) for the NTN connected UEs and devices. Hence, in some examples, it is reasonable to assume that the UE/device will be made aware when it connects to an NTN cell. In some examples, the SIB message itself can instruct RRC-inactive UE’s to carry out inter-frequency neighbour measurements (with no threshold offset). Once these measurements are conducted, this will be followed by the cell re-selection procedure. Examples of the invention may be configured as a part of RRC configuration, and thus may be software configurable.

[0076] Referring now to FTG. 5 a simplified flowchart 500 of an NTN base station group handover operation is illustrated, in accordance with some example embodiments of the invention. The flowchart starts at 502 and, at 504, a serving NTN gNB is configured to broadcast an inter-frequency measurement request. In some examples, the inter-frequency measurement request is broadcast with a reduced or minimized Δ dB reporting threshold so that the handover operation can be triggered proactively for a group of UEs. An extreme case is when Δ=0 dB where all UEs will perform handover. At 506, the active UEs and loT devices provide the interfrequency measurement reports in response to the broadcast request. At 508, the serving gNB then issues multiple handover commands for all UEs and loT devices that meet the criteria, e.g., the measured signal strength difference between serving gNB and target gNB is higher than A dB reporting threshold, for the reported neighbour signal strengths. At 510, the handover processes are then completed in the default manner.

[0077] In some examples, although the handover operation is performed on an indivisual UE (e.g. remote wireless communication unit) basis, it is envisaged that the handover may be triggered for a group of UEs to perform a simultaneous handover operation. At 512, a determination is made as to whether the allocated time for performing all handovers has finished.

-20If the allocated time for performing all handovers in 512 has finished, then the process ends at 516. However, if the allocated time for performing all handovers has not finished in 512, a determination is made at 514 as to whether all active UEs (and loT devices) have been handed over to the replacement NTN gNB. If all active UEs (and loT devices) have been handed over to the replacement NTN gNB in 514, the process ends at 516. However, if all active UEs (and loT devices) have not been handed over to the replacement NTN gNB in 514, the flowchart loops back to 504.

[0078] Referring now to FIG. 6 a simplified flowchart 600 of an NTN base station group cell re-selection operation is illustrated, in accordance with some example embodiments of the invention. The flowchart starts at 602 and, at 604, a serving NTN gNB is configured to broadcast an SIB message for a request for inter-frequency measurement reports. In some examples, the inter-frequency measurement request is broadcast with a reduced or minimized Δ dB reporting threshold. At 606, RRC-inactive devices are configured to conduct the inter-frequency measurements and report back to the serving NTN gNB. In accordance with examples of the invention, at 606, the RRC-inactive devices are also configured to initiate a cell re-selection process. {How is this forced operation achieved ... check in the submission.} At 608, one or more other cell re-selection procedures are performed in parallel, thereby transferring the camping cell (serving NTN gNB) for the RRC-inactive devices to the replacement NTN gNB. Once the RRC-inactive devices have been transferred to the replacement NTN gNB, and a suitable time gap has elapsed, the current (now previous) serving gNB will be placed out of service at 610. The flowchart then ends at 612.

[0079] In particular, it is envisaged that the aforementioned inventive concept can be applied by a semiconductor manufacturer to any integrated circuit comprising a signal processor configured to perform any of the aforementioned operations. Furthermore, the inventive concept can be applied to any circuit that is able to configure, process, encode and/or decode signals for wireless distribution. It is further envisaged that, for example, a semiconductor manufacturer

-21 may employ the inventive concept in a design of a stand-alone device, such as a digital signal processor, or application-specific integrated circuit (ASIC) and/or any other sub-system element. [0080] It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors, for example with respect to the signal processor may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controller. Hence, 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 organization.

[0081] 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. 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.

[0082] 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 recognize 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.

[0083] 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

-22not 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.

[0084] Thus, communication units such as NTN airborne gNBs and terminal devices, a communication system and methods relating to group handover for terminal devices such as UEs or loT devices have been described, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated. Furthermore, communication units such as NTN airborne gNBs and terminal devices, a communication system and methods relating to cell reselection for RRC inactive terminal devices such as UEs or loT devices have been described, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated

Claims

-23 Claims:

1. A wireless communication system comprises a first base station, a second replacement base station and a plurality of remote wireless communication units, wherein the second replacement base station comprises:

a transceiver; and a processor, operably coupled to the transceiver and arranged to:

establish a wireless backhaul or front-haul communication link with the first base station;

broadcast a transmission to the plurality of remote wireless communication units at a second frequency that is different to a first frequency used by the first base station, substantially within a coverage area of the first base station, wherein the broadcast transmission comprises at least one cell identifier parameter; and at least one synchronization signal;

wherein the first base station comprises:

broadcast an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes measurements for the second frequency.

2. The wireless communication system of Claim 1, where the first base station and second base station are non-terrestrial network, NTN, airborne base stations.

3. The wireless communication system of Claim 1 or Claim 2, wherein the broadcast message is contained within a part of a System Identification Block (SIB) message.

4. The wireless communication system of Claim 3 when dependent upon Claim 2, wherein the SIB message is configured to be only readable by NTN served remote wireless communication units.

5. The wireless communication system of Claim 4, wherein the SIB message comprises a binary header bit that, when active, indicates to the connected remote wireless communication units to read a full message.

6. The wireless communication system of any preceding Claim where the inter-frequency measurement report request comprises a measurement threshold configured to trigger the remote wireless communication units performing and reporting the measurement in response to a signal strength from the second base station being AdB higher than a measured signal strength of the first base station.

7. The wireless communication system of Claim 6 wherein the measurement threshold configured to trigger the remote wireless communication units to perform and report the measurement is configured to be a minimum AdB threshold between signal strengths from the first base station and the second base station.

8. The wireless communication system of any preceding Claim, wherein the remote wireless communication units are in a radio resource controlled (RRC)-connected state and wherein the first base station is configured to initiate at least one handover procedure of at least one remote wireless communication unit from the first base station to the second replacement base station in response to a received measurement report.

9. The wireless communication system of Claim 8, wherein the first base station initiates multiple simultaneous handover procedures of the plurality of remote wireless communication units from the first base station to the second replacement base station in response to multiple RRC connected UE measurement reports.

10. The wireless communication system of any of preceding Claims 1 to 7, wherein the remote wireless communication units are in a radio resource controlled (RRC)-inactive state and in response to the conducted measurements at least one remote wireless communication unit initiates a cell re-selection procedure.

11. The wireless communication system of any of preceding Claims 1 to 7, wherein the remote wireless communication units are in a radio resource controlled (RRC)-idle state such that the remote wireless communication units perform measurements in their current periodicity and perform cell re-selection at their own timing.

12. The wireless communication system of any preceding Claim, wherein the broadcast wireless backhaul or front-haul communication link is an X2 communication link.

13. The wireless communication system of any preceding Claim wherein the first base station and second base station are remote radio heads, RRHs, configured to perform only the transceiver radio frequency functions and the wireless communication system further comprises a ground based centralized processing unit, wirelessly connected to the RRHs and configured to perform higher layer radio access network operations.

14. A first base station that communicates with a plurality of remote wireless communication units, wherein the first base station comprises a processor, operably coupled to a transceiver and arranged to:

establish a wireless backhaul or front-haul communication link with a second replacement base station;

operate at a first frequency that is different to a second frequency used by the second replacement base station; and broadcast an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes a request for signal strength measurements for the second frequency.

15. The first base station of Claim 14 wherein the first base station is a non-terrestrial network, NTN, airborne base station comprising a receiver arranged to receive measurement reports from the remote wireless communication units in a radio resource controlled (RRC)connected state, and in response thereto initiate at least one handover procedure of at least one

-26remote wireless communication unit from the first NTN base station to the second replacement NTN airborne base station.

16. The first base station of Claim 14 or Claim 15 wherein the broadcast message is contained within a part of a System Identification Block (SIB) message

17. A remote wireless communication unit configured to communicate with a first base station on a first frequency and a replacement second base station on a second frequency, wherein the remote wireless communication unit comprises:

receive a broadcast a transmission at the second frequency that is different to a first frequency used by the first base station, substantially within a coverage area of the first base station, wherein the broadcast transmission comprises at least one cell identifier parameter; and at least one synchronization signal;

receive a broadcast inter-frequency measurement report request on the first frequency, wherein the measurement report request includes measurements for a second frequency, different to the first frequency, and used by a second replacement base station; and transmit an inter-frequency measurement report to the first base station in response thereto.

18. The remote wireless communication unit of Claim 17 wherein the first base station and second base station are non-terrestrial network, NTN, airborne base stations.

19. The remote wireless communication unit of Claim 17 or Claim 18, wherein the remote wireless communication unit is in a radio resource controlled (RRC)-connected state and the processor is configured to be handed over from the first base station to the second replacement base station in response to the transmitted measurement report.

20. The remote wireless communication unit of Claim 17 or Claim 18, wherein the remote wireless communication unit is in a radio resource controlled (RRC)-inactive state and in response to the conducted measurements the remote wireless communication unit is configured to initiate a cell re-selection procedure.

21. The remote wireless communication unit of Claim 17 or Claim 18, wherein the remote wireless communication unit is in a radio resource controlled (RRC)-idle state, such that the processor performs measurements in the current periodicity of the remote wireless communication unit and performs cell re-selection at its own timing.

22. A method for first base station that communicates with a plurality of remote wireless communication units, the method comprising:

establishing a wireless backhaul or front-haul communication link with a second replacement base station;

operating at a first frequency that is different to a second frequency used by the second replacement base station;

broadcasting an inter-frequency measurement report request on the first frequency to the plurality of remote wireless communication units served by the first base station, wherein the measurement report request includes a request for signal strength measurements for the second frequency.

23. A method for a remote wireless communication unit configured to communicate with a first base station on a first frequency and a replacement second base station on a second frequency, wherein the method comprises:

receiving from the second replacement base station a broadcast transmission on a second frequency, wherein the second replacement base station operates substantially within a coverage area of the first base station, and wherein the broadcast transmission comprises at least one cell identifier parameter; and at least one synchronization signal of the second replacement base station;

being served by the first base station;

-28receiving from the first base station a broadcast inter-frequency measurement report request on the first frequency, wherein the measurement report request includes a request for measurements on the second frequency that is different to the first frequency; and transmitting an inter-frequency measurement report to the first base station in response thereto.