GB2574851A

GB2574851A - Cell measurements

Info

Publication number: GB2574851A
Application number: GB201810118A
Authority: GB
Inventors: Magadi Rangaiah Raghavendra
Original assignee: ARM Ltd; Advanced Risc Machines Ltd
Current assignee: ARM Ltd
Priority date: 2018-06-20
Filing date: 2018-06-20
Publication date: 2019-12-25
Anticipated expiration: 2038-06-20
Also published as: GB2574851B; GB201810118D0

Abstract

A method 400 in a terminal device (130 figure 2) configured to operate in a radio access network (110 figure 2), e.g. a Narrowband Internet-of-Things (NB-IoT) network, at different coverage levels, e.g. normal coverage (NC) level (210 figure 2) and enhanced coverage (EC) level (220 figure 2), in an idle mode. Measurements of a serving cell, such as signal reception level or signal quality level of a received signal, are performed 410 by the terminal device while operating at first and second coverage levels, evaluations are made 420, 430 at the first and second coverage levels to determine first and second cell selection results, and a determination is made 440 that a predetermined number of consecutive first and second cell selection results do not satisfy cell selection criterion. In response, measurement of a neighbouring cell may be initiated or an out-of-service timer for the serving cell may be started.

Description

CELL MEASUREMENTS

Background

A terminal device may operate in a radio access network in a connected mode and an idle mode. Specifically, the terminal device may connect to a cell of the radio access network in a connected mode to actively transmit and receive data. The cell on which the terminal device is connected may be referred to as a serving cell. If the terminal device stops transmitting and receiving data, the terminal device may transition from a connected mode to an idle mode. In an idle mode, the terminal device may receive control information and system information associated with the serving cell. When the terminal device is operating in an idle mode on a cell, the terminal device may be referred to as being camped on the serving cell. Control information and system information may be kept up-to-date in a terminal device while in idle mode in order, for example, to have the latest parameters available when transitioning from idle mode to connected mode to transmit and receive data.

The decision to camp on a particular cell may depend on an assessment made by the terminal device on whether the cell is a suitable cell. A suitable cell may be a cell which has various characteristics. One characteristic may be that the cell belongs to a preferred public land mobile network (PLMN). The preferred PLMN may be associated with a particular service provider, with whom the user of the terminal device may have a subscription. Another characteristic of a suitable cell is that the terminal device is permitted to camp on the cell i.e. the cell is not barred, reserved for other terminal devices, or restricted to other terminal devices. A further characteristic of a suitable cell is that the cell fulfils a cell selection criterion, which defines the required strength and quality of the signal reception at the terminal device. By only camping on suitable cells, the terminal device can minimise disruptions and provide a reliable service.

Sometimes a terminal device may experience poor radio conditions, whereby the strength and quality of the signal reception of the cell at the terminal device fails the cell selection criteria. In some examples, the cell may be considered unsuitable only after a certain number of cell selection criteria failures.

A terminal device may be configured to camp on a serving cell at different coverage levels. The coverage level of the terminal device may affect the maximum number of repetitions of a control channel or data channel transmission that the terminal device is configured to receive from the cell. In an enhanced coverage level, the number of repetitions of a transmission may be higher, in order to increase the probability of a terminal device successfully receiving the transmission. The coverage level of the terminal device may be determined by a coverage selection criterion, which defines the required strength and quality of a reference signal at the terminal device. Based on the outcome of the coverage selection criterion, a particular coverage level may be assigned to the terminal device.

The maximum number of times a terminal device may fail the cell selection criterion before neighbour cell measurements are initiated may depend on the coverage level of the terminal device. For example, a terminal device in a normal coverage level may fail the cell selection criterion twice before neighbour cell measurements are initiated. Whereas, a terminal device in an enhanced coverage level may fail the cell selection criterion four times before neighbour cell measurements are initiated.

In poor radio conditions at the edge of the serving cell, the terminal device may fluctuate between a first coverage level and a second coverage level, and at the same time may fail one or more cell selection criteria before initiating measurements of neighbouring cells to find a more suitable cell on which to camp. This fluctuation between coverage levels is time consuming and can also represent a drain on the battery, time and processing resources of the terminal device. There is thus a need for an optimised procedure to initiate neighbour cell measurements, with reduced requirements in terms of time, processing resources and/or power resources.

Summary

According to a first aspect of the present disclosure, there is provided a method in a terminal device configured to operate in a radio access network in a first coverage level and a second coverage level in idle mode. The method includes performing one or more measurements of a serving cell of the radio access network while operating at the first coverage level and while operating at the second coverage level. The method further includes evaluating the one or more measurements at the first coverage level against a cell selection criterion for the serving cell to determine one or more first cell selection results and evaluating the one or more measurements at the second coverage level against the cell selection criterion for the serving cell to determine one or more second cell selection results. The method includes determining that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion.

Performing a cell measurement procedure which determines that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion may provide a procedure for initiating neighbour cell measurements and/or starting an out-of-service timer for the serving cell. Performing the cell measurement procedure, regardless of the terminal device operating in a first coverage level or a second coverage level, may initiate neighbour cell measurements earlier when the terminal device experiences poor radio conditions on the serving cell. Initiating neighbour cell measurements earlier may reduce the power consumption of the terminal device. Similarly, starting an out-of-service timer earlier for a terminal device in poor radio conditions may initiate a cell search procedure earlier upon expiry of the out-of-service timer. Initiating a cell selection procedure may also reduce the power consumption of the terminal device by identifying a suitable cell sooner. Such savings are particularly important for terminal devices that are low-power devices which may have very limited power resources.

According to a second aspect of the present disclosure, there is provided a terminal device configured to operate in a radio access network in a first coverage level and a second coverage level in idle mode. The terminal device is configured to perform one or more measurements of a serving cell of the radio access network while operating at the first coverage level and while operating at the second coverage level. The terminal device is further configured to evaluate the one or more measurements at the first coverage level against a cell selection criterion for the serving cell to determine one or more first cell selection results and evaluate the one or more measurements at the second coverage level against the cell selection criterion for the serving cell to determine one or more second cell selection results. The terminal device is configured to determine that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion.

According to a third aspect of the present disclosure, there is provided a computer program product comprising instructions which, when executed by a computer, cause the computer to operate in a radio access network in a first coverage level and a second coverage level in idle mode. The computer program product is configured to perform one or more measurements of a serving cell of the radio access network while operating at the first coverage level and while operating at the second coverage level. The computer program product is further configured to evaluate the one or more measurements at the first coverage level against a cell selection criterion for the serving cell to determine one or more first cell selection results and evaluate the one or more measurements at the second coverage level against the cell selection criterion for the serving cell to determine one or more second cell selection results. The computer program product is configured to determine that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion.

Further features and advantages of the present disclosure will become apparent from the following description of preferred embodiments of the disclosure, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic representation of a radio access network.

Figure 2 is a schematic representation of a radio access network with terminal devices operating at different coverage levels.

Figure 3 is a flowchart of a procedure in a terminal device for performing and evaluating cell measurements at different coverage levels.

Figure 4 is a diagram of a procedure in a terminal device operating in idle mode.

Figure 5 is a flowchart of a procedure for performing cell measurements in a terminal device in idle mode.

Figure 6 is a chart showing cell measurements performed by a terminal device over time at different coverage levels.

Figure 7 is a schematic representation of a terminal device configured to operate in a radio access network.

Figure 8 is a schematic diagram of a processor and computer-readable storage instructions for performing a cell measurement procedure.

Detailed Description

In general, the Internet of Things (loT) describes a set of technologies that enable the connectivity of a large variety of objects to different services via the internet. Such objects are typically connected through a radio interface embedded in or otherwise associated with the object, which provides connectivity via a radio access network. Such objects may be referred to as loT devices or more generally terminal devices. The objects have a wide variety of uses and can take a large variety of shapes. Examples include static devices such as gas meters and household appliances, and mobile devices such as vehicles, tracking devices and wearable medical devices. Due to the nature of their functions, many loT devices have limited power and processing resources, and are often battery powered. In many examples, the lifetime of the loT device is limited by the lifetime of the battery. Thus, development of power-efficient processes is important for extending the lifetime of an loT device, or for reducing the cost of maintenance.

loT systems are characterised by having a large number of terminal devices, each of which requires a relatively small uplink and/or downlink data rate. To meet the requirements for low data transmission, low power consumption, low cost and network expansion capabilities, low power wide area networks (LPWAN) have been developed. Prominent within the LPWAN technologies is the 3GPP LTE Narrowband Internet of Things (NB-IoT) technology standard. Based on the 3GPP LTE standard, but optimised for the requirements of loT technology, the NB-IoT standard provides enhanced coverage for terminal devices, in order to provide a reliable service in difficult to reach locations.

When a terminal device experiences poor radio conditions, the terminal device may move from one coverage level to another coverage level, which in turn enhances the coverage of the serving cell. While operating in an idle mode, the terminal device may perform measurements of the serving cell to check that the cell provides a suitable level of radio performance for a given coverage level. The terminal device may follow a procedure to determine when the radio performance of the cell is not suitable. The procedure may rely on a defined cell selection criterion which can be satisfied or not satisfied depending on cell measurements made by the terminal device. If the procedure determines that the serving cell is unsuitable then the terminal device can take action to improve the radio performance, for example, by searching for a suitable neighbour cell on which to camp. The terminal device can also start an out of service timer. If this timer expires before a suitable neighbour cell is identified, then the terminal device can assume it is out of service and may begin an initial cell search procedure.

At each coverage level, the terminal device may fail a cell selection criterion a predetermined number of times before the cell is considered unsuitable and measurements of neighbouring cells are initiated. At the edge of coverage of a cell, the terminal device may fluctuate between coverage levels. This fluctuation may cause a delay in the terminal device determining that a serving cell is unsuitable. This in turn can cause a delay in starting neighbour cell measurements or starting an out of service timer.

In the following examples, procedures for performing and evaluating cell measurements are described. In certain examples, the terminal device may operate in more than one coverage level. In particular, the terminal device can determine that a serving cell is unsuitable and initiate steps to recover the radio coverage provided by the radio access network. These steps may include, for example, making neighbour cell measurements and/or starting an out of service timer. In the case that neighbour cell measurements are initiated, the terminal device may reselect to a new serving cell after a suitable neighbour cell is identified. In the case that an out of service timer is initiated, the terminal device may begin a cell search procedure after expiry of the out of service timer or may power down to conserve battery power. The terminal device may perform neighbour cell measurements concurrently while the out of service timer is running. In this case, measurements may stop when a suitable cell is found or when the timer expires, and the timer may stop when a suitable cell is found or when the timer expires.

Figure 1 is a schematic representation of a radio access network 100, such as a 3GPP LTE NB-IoT network, in accordance with an example. The radio access network 100 is a cellular communication network whereby the network 100 is distributed over a geographical land area using multiple cells. Each cell 120 of the network is provided by operation of a fixed-location base station 110, which may be referred to as a cell tower, comprising antennae and electronic communication equipment such as transceivers, digital signal processors and control electronics. The area of coverage of the cell!20 is indicated by the dashed line 125 in Figure 1. Multiple cells may be joined together to provide network coverage over a wider geographic area. Terminal devices 130, 140, 150, 160, 170, 180 operating in the cell 120 may communicate with each other (or with other terminal devices operating in other cells) via the base station 110.

A terminal device in an NB-IoT network 100 may be for example a mobile phone 130, a smart meter 140 such as a gas meter, a lightbulb 150, a household appliance 160, a wearable medical device 170, or a personal computer 180. A terminal device, for example a mobile phone 130 or a smart meter 140, may be referred to as connected to the base station 110 or to the corresponding cell 120.

A terminal device may be referred to as being in a connected mode when it is actively receiving and transmitting data between the terminal device and the network. A terminal device may be referred to as being in an idle mode when it is not actively connected to the network but is receiving control information and system information associated with the cell which is broadcast from the radio access network. In this scenario, the terminal device may be referred to as being camped on the cell.

Static terminal devices, such as smart meters 140, lightbulbs 150, household appliances 160, or personal computers 180, may often be located in environments where transmission of control information and system information by the radio access network to the terminal device may be obstructed or inhibited. For example, terminal devices such as smart meters 140 and other household appliances 160 may be located underground, in basements or deep inside buildings. Similarly, mobile terminal devices such as mobile phones 130 or wearable medical devices 170, may frequently be located in remote or rural areas. When the location of the terminal device makes the successful reception of necessary information difficult or impossible, extending the coverage of the cell increases the depth of radio coverage to enable terminal devices to operate in locations that may otherwise not be possible.

Figure 2 is a schematic representation of a radio access network 200, such as a 3GPP LTENB-IoT network, with terminal devices 130, 140, 150 operating at different coverage levels 210, 220. The radio access network 200 comprises a base station 110 that provides the network 200 with different areas of coverage 211, 221 corresponding to the different coverage levels 210, 220. A first or normal coverage (NC) level 210 may be associated with a first area of coverage 211. A second or enhanced coverage (EC) level 220 may be associated with a second wider area of coverage 221. Although the areas of coverage 211, 221 associated with the coverage levels 210, 220 differ in size, they are both associated with a single cell of the network 200. Terminal devices 130, 140, 150 may be referred to as being camped on a cell in a coverage level 210, 220. For the avoidance of doubt, while the present disclosure is described in relation to a first and second coverage level, a third, fourth, fifth etc. coverage level may be envisaged.

The network 200 may operate in one or more coverage levels 210, 220. In a first or normal coverage level 210, the base station 110 may transmit certain control information and system information with a predetermined power level for a predetermined number of repetitions. The coverage area associated with the normal coverage level 210 of the cell is illustrated by a dashed line 211 in Figure 1. In a second or enhanced coverage level 220, the base station may transmit certain control information and system information with a higher predetermined power level and/or for a higher predetermined number of repetitions. The depth or extent of radio coverage of the cell is thus increased. The coverage area associated with the enhanced coverage level 220 of the cell is illustrated by a dot-dashed line 221 in Figure 1.

A terminal device 130 may be in a first or normal coverage level 210 if a reference signal measurement received by the terminal device 130 passes a first coverage criterion. In some examples, the reference signal measurement may comprise a measurement of a narrowband reference signal and/or a narrowband synchronisation signal. The reference signal measurements may comprise measurements performed at one or more antenna ports of the terminal device. In some cases, the terminal device may perform reference signal measurements periodically e.g. every discontinuous reception cycle. The first coverage criterion may define a minimum strength value and a minimum quality value for the reference signal measurement. When the strength value and the quality value of the measurement exceeds the minimum requirements of the first coverage criterion, the terminal device 130 may camp on the cell in a normal coverage level 210.

A terminal device 150 may be in a second or enhanced coverage level 220 if a reference signal measurement received by the terminal device 150 passes a second coverage criterion. The second coverage criterion requirement may define a second, lower, minimum strength value and a second, lower, minimum quality value for the reference signal measurement. When the strength value and the quality value of the measurement exceeds the lower minimum requirements of the second coverage criterion, the terminal device 150 may camp on the cell in an enhanced coverage level 220. In some examples, when a terminal device is camped on a cell in an enhanced coverage level, it is possible that a neighbouring cell may provide a cell with better radio performance for the terminal device to camp on.

A terminal device 140 may be located in such an area where the strength and quality of the received reference signal measurement may sometimes pass the first coverage criterion and may sometimes pass the second coverage criterion. The terminal device 140 may be considered to be on the boundary between a normal coverage level 210 and an enhanced coverage level 220. The terminal device 140 may fluctuate between being camped on the cell in a normal coverage level 210 and camped on the cell in an enhanced coverage level 220. In some examples, when a terminal device is camped on a cell in fluctuating coverage levels, a neighbouring cell may provide a cell with better radio performance for the terminal device to camp on.

Aside from the coverage levels, a suitable cell may be required to fulfil a cell selection criterion, which defines a required reception level and quality level of a signal received from the cell by a terminal device. In some examples, the cell selection criterion may be fulfilled when the signal reception level and the signal quality level received from the cell exceed a particular value, for example a value greater than zero. In further examples, the signal reception level may comprise a measured cell reception level value and the signal quality level may comprise a measured cell quality value. The cell selection criterion may be referred to as the S criterion.

A positive cell selection result may occur for a cell satisfying the cell selection criterion. A negative cell selection result may occur for a cell not satisfying (i.e. meeting or fulfilling) the cell selection criterion.

Figure 3 is a flowchart 300 of a procedure in a terminal device for performing and evaluating cell measurements at different coverage levels 210, 220. The procedure describes a typical example of a system that may cause a delay in initiating measurements of neighbouring cells.

In step 310 of the flowchart 300, a counter Nnc and a counter Nec are initialised. The counter Nnc counts the number of cell selection results for a serving cell in normal coverage level 210. More specifically, Nnc counts the number of consecutive negative first cell selection results. The counter Nec counts the number of cell selection results for a serving cell in an enhanced coverage level 220. More specifically, Nec counts the number of consecutive negative second cell selection results.

After initialisation of the counters Nnc and a counter Nec, step 310 is followed by step 320. In step 320 of the flowchart 300, one or more measurements of the serving cell of a radio access network are performed. The measurements may comprise serving cell measurements. Serving cell measurements may be required to determine the fulfilment of the cell selection criterion. The measurements may further comprise reference signal measurements. Radio conditions measurements may be required to determine the fulfilment of the coverage criteria.

In step 330 of the flowchart 300, the coverage level of the serving cell is determined. If the reference signal measurements pass a first coverage criterion, the terminal device is determined to be in a first or normal coverage (NC) level 210. If the radio condition fails the first coverage criterion but passes the second coverage criterion, the terminal device is determined to be in a second or enhanced coverage (EC) level 220.

If the coverage level is determined to be a normal coverage (NC) level 210, the procedure follows a path to step 340 of the flowchart 300. In step 340, the procedure determines if the terminal device was previously in an enhanced coverage (EC) level 220 i.e. the terminal device has changed coverage levels from an enhanced coverage level 220 to a normal coverage level 210 since a cell selection criterion was last evaluated.

If the coverage level has changed, meaning the coverage level has changed from an enhanced coverage level 220 to a normal coverage level 210, then the procedure follows a path to step 350 of the flowchart 300. In step 350 of the flowchart 300, the counter Nnc is reset to zero. This ensures that the counter Nnc counts consecutive cell selection results in a normal coverage level 210. The procedure then follows a path to step 360 of the flowchart.

If the coverage level has not changed, the terminal device is still in a normal coverage level 210 and the counter Nnc is not reset i.e. the counter Nnc may continue counting. The procedure then follows a path to step 360, bypassing step 350 and thus does not reset the counter Nnc.

In step 360 of flowchart 300, a cell selection criterion of the serving cell in a normal coverage level 210 is evaluated. The cell selection criterion evaluates one or more serving cell measurements in normal coverage, producing a first cell selection result. Fulfilling the cell selection criterion in a normal coverage level 210 means that the serving cell is a suitable cell for the terminal device to camp on in a normal coverage level 210. Fulfilling the cell selection criterion in a normal coverage level 210 may be described as a positive first cell selection result. Not fulfilling the cell selection criterion in a normal coverage level 210 means that the serving cell may not be a suitable cell for the terminal device to camp on in a normal coverage level 210. Not fulfilling the cell selection criterion in a normal coverage level 210 may be described as a negative first cell selection result i.e. the cell selection criterion is not satisfied.

If the cell selection criterion produces a positive first cell selection result, the procedure follows a path to step 361 of the flowchart 300. In step 361, the counter Nnc is reset to zero. After resetting the counter Nnc, the procedure then follows a path that returns to step 320 of the flowchart 300. From step 320, the process of performing one or more measurements of the serving cell may be repeated.

If the cell selection criterion produces a negative first cell selection result, the procedure follows a path to step 370 of the flowchart 300. In step 370 of the flowchart 300, the value of the counter Nnc is increased by one. Increasing the value of Nnc comprises counting the number of consecutive negative first cell selection results. After counting the number of consecutive negative first cell selection results, the procedure then follows a path to step 380 of the flowchart 300.

In step 380 of the flowchart, it is determined if a preconfigured number of consecutive negative first cell selection results has been reached. The preconfigured number of consecutive negative first cell selection results may be defined as Nnc-max. In step 380 of the flowchart, the value of Nnc is compared with the preconfigured number Nnc-μαχ. In a typical example, Nnc-μαχ may comprise an integer number of discontinuous reception cycles. In specific examples, Nnc-max may be 2 discontinuous reception cycles.

If the value of Nnc is less than Nnc-max, the predetermined number of consecutive negative first cell selection results has not been reached. Thus, the procedure follows a path that returns to step 320 of the flowchart 300, whereby the process of performing one or more measurements of the serving cell can be repeated.

If the value of Nnc is not less than Nnc-max, the predetermined number of consecutive negative first cell selection results has been reached. Thus, the procedure follows a path to step 390 of the flowchart 300, to initiate measurements of neighbouring cells and/or start an out of service timer.

Before describing the process of initiating measurements of neighbouring cells, the description of the flowchart 300 returns to step 330. If the reference signal measurements pass the second coverage criterion but not the first coverage criterion, the terminal device is determined to be in a second or enhanced coverage (EC) level 220.

If the coverage level is determined to be an enhanced coverage (EC) level 220 in step 330, the procedure follows a path to step 345 of the flowchart 300. In step 345, the procedure determines if the terminal device was previously in a normal coverage (NC) level 210 i.e. the terminal device has changed coverage levels from a normal coverage level 210 to an enhanced coverage level 220 since a cell selection criterion was last evaluated.

If the coverage level has changed, meaning the coverage level has changed from a normal coverage level 210 to an enhanced coverage level 220, then the procedure follows a path to step 355 of the flowchart 300. In step 355 of the flowchart 300, the counter Nec is reset to zero. This ensures that the counter Nec counts consecutive cell selection results in an enhanced coverage level 220. The procedure then follows a path to step 365 of the flowchart.

If the coverage level has not changed, the terminal device is still in an enhanced coverage level 220 and the counter Nec is not reset i.e. the counter Nec may continue counting. The procedure then follows a path to step 365, bypassing step 355 and thus does not reset the counter Nec.

In step 365 of flowchart 300, a cell selection criterion of the serving cell in an enhanced coverage level 220 is evaluated. The cell selection criterion evaluates one or more serving cell measurements in enhanced coverage, producing a second cell selection result. Fulfilling the cell selection criterion in an enhanced coverage level 220 means that the serving cell is a suitable cell for the terminal device to camp on in an enhanced coverage level 220. Fulfilling the cell selection criterion in an enhanced coverage level 220 may be described as a positive second cell selection result. Not fulfilling the cell selection criterion in an enhanced coverage level 220 means that the serving cell may not be a suitable cell for the terminal device to camp on in an enhanced coverage level 220. Not fulfilling the cell selection criterion in an enhanced coverage level 220 may be described as a negative second cell selection result.

If the cell selection criterion produces a positive second cell selection result, the procedure follows a path to step 366 of the flowchart 300. In step 366, the counter Nec is reset to zero. After resetting the counter Nec, the procedure then follows a path that returns to step 320 of the flowchart 300. From step 320, the process of performing one or more measurements of the serving cell may be repeated.

If the cell selection criterion produces a negative second cell selection result, the procedure follows a path to step 375 of the flowchart 300. In step 375 of the flowchart 300, the value of the counter Nec is increased by one. Increasing the value of Nec comprises counting the number of consecutive negative second cell selection results. After counting the number of consecutive negative second cell selection results, the procedure then follows a path to step 385 of the flowchart 300.

In step 385 of the flowchart, it is determined if a preconfigured number of consecutive negative second cell selection results has been reached. The preconfigured number of consecutive negative second cell selection results may be defined as Necmax. In step 385 of the flowchart, the value of Nec is compared with the preconfigured number Nec-max. In a typical example, Nec-max may comprise an integer number of discontinuous reception cycles. In specific examples, Nec-max may be 4 discontinuous reception cycles.

If the value of Nec is less than Nec-max, the predetermined number of consecutive negative second cell selection results has not been reached. Thus, the procedure follows a path that returns to step 320 of the flowchart 300, whereby the process of performing one or more measurements of the serving cell can be repeated.

If the value of Nec is not less than Nec-max, the predetermined number of consecutive negative second cell selection results has been reached. Thus, the procedure follows a path to step 390 of the flowchart 300, to initiate measurements of neighbouring cells.

In step 390, one or more measurements of one or more neighbouring cell may be initiated and/or an out-of-service timer for the serving cell may be started.

In some examples, initiation of neighbour cell measurements may be performed when the counter Nnc has reached the predetermined number Nnc-max in a normal coverage level 210 or the counter Nec has reached the predetermined number Nec-max in an enhanced coverage level 220.

Neighbour cell measurements may comprise measuring a signal reception level and a signal quality level of a signal received from one or more neighbouring cells. The neighbouring cells may comprise intra-frequency neighbouring cells and/or interfrequency neighbouring cells. In some examples, the neighbouring cells may be automatically detected by the terminal device. In other examples, the radio access network may provide a list of neighbouring cells in system information associated with the serving cell that may be broadcast by the network. After performing neighbour cell measurements, the serving cell and the neighbouring cells are ranked based on the signal reception level and signal quality level of the received signals for each cell. In examples where the rank of a neighbouring cell is higher than the rank of the serving cell, the neighbouring cell is selected as the new cell on which to camp i.e. a cell reselection occurs. In examples where multiple neighbouring cells are ranked higher than the serving cell, the highest ranked neighbouring cell is selected. In examples where no neighbouring cell is ranked higher than the serving cell, the terminal device continues to camp on the serving cell.

In other examples, an out-of-service timer for the serving cell may be started, whereby the terminal device may begin a cell search procedure after expiry of the outof-service timer or may power down to conserve battery power. The terminal device may perform neighbour cell measurements concurrently while the out of service timer is running. In such examples, neighbour cell measurements may stop when a suitable cell is found or when the timer expires, and the timer may stop when a suitable cell is found or when the timer expires. The out-of-service timer may expire after 40 seconds for a terminal device in a normal coverage level 210 or after 80 seconds for a terminal device in an enhanced coverage level 220.

The procedure described above describes a typical example of a system that determines a coverage level of the terminal device and counts consecutive negative first cell selection results in a first coverage level and consecutive negative second cell selection results in a second coverage level. In the typical example, the system may reset the number of negative cell selection results when the terminal device moves from one level of coverage to another level of coverage. Such a procedure may delay initiating measurements of neighbouring cells and/or starting an out-of-service timer as the terminal device fluctuates (i.e. oscillates or alternates) between a normal coverage and an enhanced coverage.

Examples of methods and procedures of the present disclosure, in addition to apparatus and computer-readable storage media, will now be described. The present disclosure describes a cell measurement procedure in a terminal device operating in idle mode that may provide a time efficient and power efficient process of selecting a suitable cell to camp on.

The example procedures can help to reach an accurate determination that a serving cell is unsuitable whilst reducing possible delays and processing required to reach that determination. This in turn can provide a reduction in the overall power consumption for the terminal device since recovery procedures can begin sooner to identify alternative serving cells or to power down the device. A reduction in overall power consumption is especially beneficial in battery powered devices, for example, internet of things devices that may operate according to the NB-IoT technology standard. In certain cases, the lifetime of the terminal device may be extended by conserving battery power.

Figure 4 is a diagram of a procedure 400 in a terminal device operating in idle mode.

The procedure 400 comprises a block 410 where one or more measurements of the serving cell of a radio access network are performed in both a first coverage level (e.g. normal coverage level) and a second coverage level (e.g. enhanced coverage level). In some examples, performing serving cell measurements may comprise measuring a signal reception level and a signal quality level of a signal received from the serving cell.

In some examples, the measurements of the serving cell may be periodically performed. In further examples, the period of the serving cell measurements is related to the length of time taken to complete a discontinuous reception cycle for the serving cell.

In these examples, the serving cell measurements may be periodically evaluated. Similarly, the period of the evaluating of the serving cell measurements may be related to the length of the discontinuous reception cycle of the serving cell.

In some examples, evaluating the serving cell measurements may comprise evaluating a filtered set of measurements including a current measurement. A filtered set of measurements may use at least two measurements. In one example, at least two measurements in the filtered set are spaced by at least half of the length of time taken to complete the discontinuous reception cycle of the serving cell. The filtered set thus contains a current measurement and at least one earlier measurement, whereby at least two of the measurements are spaced by half the length of time taken for the serving cell to complete a discontinuous reception cycle. The filtering process therefore provides a means to minimise the effect of anomalous measurements.

In block 420, the terminal device evaluates one or more measurements at a first coverage level 210 against a cell selection criterion for the serving cell. The cell selection criterion may comprise one or more thresholds, wherein one or more thresholds may be a signal level threshold and a signal quality threshold. The signal level threshold may comprise a reception level value of the serving cell measured by the terminal device. The signal quality threshold may comprise a reception quality value of the serving cell measured by the terminal device.

In one instance of an evaluation, a single measurement or a filtered set of measurements at the first coverage level 210 may provide a positive first cell selection result when the single or filtered measurement exceeds the respective threshold. In another instance, a single measurement or a filtered set of measurements at the first coverage level 210 may provide a negative first cell selection result when the single or filtered measurement does not exceed the respective threshold. Hence, in one instance of an evaluation at the first coverage level 210, a positive or negative first cell selection result may arise. Further evaluations in subsequent periods may be based on a different single measurement or filtered set of measurements, and different first cell selection results may follow.

In block 430, the terminal device evaluates one or more measurements at a second coverage level 220 against the cell selection criterion for the serving cell. In some examples, the single or filtered measurements at the second coverage level 220 may provide a positive second cell selection result. In other examples, the single or filtered measurements at the second coverage level 220 may provide a negative cell selection result. Further evaluations in subsequent periods may be based on a different single measurement or filtered set of measurements, and different second cell selection results may follow.

Based on the results of the evaluations of the cell selection criterion at a first coverage level 210 and a second coverage level 220, the procedure 400 may comprise counting the first and second cell selection results. The first and second cell selection results may comprise positive first and second cell selection results and negative first and second cell selection results.

In further examples, counting the first and second cell selection results may comprise using a counter N to count the number of consecutive negative first and second cell selection results for the serving cell. The cell selection results are counted regardless of the level of coverage of the terminal device i.e. cell selection results are counted when the terminal device is in a first level of coverage and a second level of coverage. In some examples, the counter N is reset to zero when the cell selection result is positive. Therefore, the counter N counts the number of consecutive negative cell selection results.

In some examples, a predetermined number Nmax of consecutive cell selection results may be determined to be negative. The predetermined number Nmax may comprise a predetermined number stored in the memory of the terminal device.

In other examples, the predetermined number Nmax may be configured by the terminal device based on the system information received by the terminal device. The system information associated with the serving cell may be broadcast by the radio access network and received by the terminal device. The terminal device may then configure the predetermined number Nmax based on the system information received. Modification of the system information broadcast by the radio access network may result in the terminal device configuring a modified predetermined number Nmax to compare to the counter N. In some examples, determining that the counter N has reached the predetermined number Nmax of consecutive negative first and second cell selection results comprises comparing the counter N with the predetermined number Nmax.

If the counter N reaches the predetermined number Nmax, the procedure 400 determines that the predetermined number Nmax of consecutive cell selection results do not satisfy the cell selection results. In an alternative example, the counter can count down from the predetermined number Nmax until a value of zero is reached whereby a predetermined number of consecutive negative first and second cell selection results will be reached.

In block 440, the terminal device determines that a predetermined number of consecutive cell selection results do not satisfy the cell selection criterion. The determination may be based on the counting technique described above. The determination that a predetermined number of consecutive cell selection results do not satisfy the cell selection criterion may mean that the cell is an unsuitable cell to camp on and some form of recovery may be initiated.

In some examples, the procedure 400 may comprise initiating measurements of neighbouring cells upon determining that the predetermined number Nmax of consecutive cell selection results do not satisfy the cell selection criterion. When a suitable neighbour cell is found, the neighbour cell measurements may be stopped, and the suitable neighbour cell may be selected as a new serving cell for the terminal device.

In other examples, the procedure 400 may comprise starting an out-of-service timer for the serving cell upon determining that the predetermined number Nmax of consecutive cell selection results do not satisfy the cell selection criterion. Upon expiry of the out-of-service timer, the terminal device may begin a cell search procedure.

In other examples, the procedure 400 may comprise performing neighbour cell measurements concurrently while the out-of-service timer is running. The out-ofservice timer may then be stopped when a suitable neighbour cell is identified. Upon expiry of the timer before a suitable neighbour cell is identified, the terminal device may perform a cell search procedure or power down to conserve battery power.

Figure 5 is a flowchart 500 of a procedure for performing cell measurements in a terminal device in idle mode.

In step 510 of the flowchart 500, a counter N is initialised, wherein the counter N counts the number of cell selection results for a serving cell.

In step 520 of the flowchart 500, a measurement of the serving cell of a radio access network is performed. The serving cell measurement is carried out in order to determine the fulfilment of the cell selection criterion. In some examples, the measurement may be combined with an earlier measurement or earlier measurements to form a filtered measurement in step 520.

In step 530 of the flowchart 500, the cell selection criterion of the serving cell is evaluated based on the measurement output from step 520. In some examples, the serving cell measurement fulfils the cell selection criterion 530 and the serving cell is evaluated to be a suitable cell for the terminal device to camp on. Fulfilling or satisfying the cell selection criterion may be referred to as a positive cell selection result. Not fulfilling or satisfying the cell selection criterion may be referred to as a negative cell selection result.

The step 530 of evaluating the cell selection criterion is performed regardless of coverage level of the terminal device. When the terminal device is in a first coverage level, the cell selection criterion produces a cell selection result for the first coverage level (e.g. a normal coverage level). When the terminal device is in a second coverage level, the cell selection criterion produces a second cell selection result for the second coverage level (e.g. an enhanced coverage level).

If step 530 produces a positive cell selection result, the procedure follows a path to step 540 of the flowchart 500. The positive cell selection result may be a positive cell selection result for the normal coverage level or a positive cell selection result for the enhanced coverage level. In step 540 of the flowchart, the counter N is reset to zero. Upon completion of step 540, the procedure returns to step 520 of the flowchart, whereby a further measurement of the serving cell is performed. The further measurement may be performed after a discontinuous reception period.

If step 530 produced a negative cell selection result, the procedure follows a path to step 550 of the flowchart 500. The negative cell selection results may comprise a negative cell selection result for the normal coverage level or a negative cell selection result for the enhanced coverage level. In step 550 of the flowchart, the negative cell selection result is counted. In this example, the value of the counter N is increased by

1.

In step 560 of the flowchart 500, the value of the counter N is compared to the value of a predetermined number of consecutive negative cell selection results, Nmax. In a specific example, Nmax may be configured to be an integer value, for example 4 (which may correspond to 4 discontinuous reception cycles). The predetermined number Nmax may be stored in the memory of the terminal device. The predetermined number Nmax may be configured by a suitable control or broadcast message from the base station of the radio access network. According to the example, the predetermined number Nmax is independent of the coverage level of the cell.

If the value of N is less than Nmax, the procedure follows a path to step 520 to perform further measurements of the serving cell. The procedure comprising steps 520, 530, 550 and 560 may be repeated until the value of N reaches Nmax.

If the value of N reaches Nmax, the procedure follows a path to step 580 to initiate measurements of neighbouring cells and/or start an out-of-service timer for the serving cell. The serving cell may be deemed an unsuitable cell for the terminal device to camp on and thus neighbour cell measurements are initiated to find a suitable cell on which to camp. The out-of-service timer may be started when neighbour cell measurements are initiated. The terminal device may then stop neighbour cell measurements and reselect to a new serving cell when a suitable neighbour cell is identified or proceed to a cell search procedure if the out-of-service timer expires before a suitable neighbour cell is identified.

The procedure described above is an example of a system according to a method of the present disclosure that determines a coverage level of the terminal device and counts negative cell selection results from a first coverage level and a second coverage level. The negative cell selection results are counted regardless of coverage level of the terminal device.

In an example where further coverage levels exist in addition to the first and second coverage levels, the count N of negative cell selection results may include a count of negative cell selection results from the further coverage levels in addition to the first and the second coverage levels. In this example, the negative cell selection results are again counted regardless of the various coverage levels of the terminal device. For example, if there are 3 coverage levels then the count N may be performed over all 3 of the coverage levels. Likewise, if there are 4 coverage levels then the count N may be performed over all 4 of the coverage levels.

Figure 6 is a chart showing cell measurements 620 performed by a terminal device over time at different coverage levels 610. In some examples, one or more serving cell measurements are evaluated periodically in order to determine if the cell selection criterion is fulfilled. Within each measurement evaluation period, which may be described as a measurement cycle 620, the coverage level 610 of the terminal device is determined. For some measurement cycles 620, the coverage level may be determined to be a normal coverage level 210. For other measurement cycles 620, the coverage level may be determined to be an enhanced coverage level 220. In some examples, the coverage level is evaluated every measurement cycle.

Figure 6a is a chart showing cell measurements 620 performed by a terminal device over time at different coverage levels 610 according to the procedure described above in relation to Figure 3. In this example, the number of negative cell selection results 631, 632, 633, 634 in a first or normal coverage (NC) level 210 are counted with the counter Nnc 630. The number Nnc 630 of negative first cell selection results 631, 632, 633, 643 is reset each time the coverage level 610 moves from a normal coverage level 210 to an enhanced coverage level 220 in consecutive measurement cycles 620. In this example, the counter Nnc 630 counts the number of consecutive negative first cell selection results in a normal coverage level 210.

For example, the counter Nnc 630 counts one 631 in the first measurement cycle 620. In the second measurement cycle 620, when the coverage level has changed to an enhanced coverage level 220, the counter Nnc 630 is reset to Nnc = 0. Thus, when measurement cycle 620 reaches the fourth cycle (labelled ‘4’), the coverage returns to a normal coverage level 210, and the counter Nnc counts one 632 again, not two.

In this example, the number of negative cell selection results 641, 642, 642 in a second or enhanced coverage (EC) level 220 are also counted using the variable Nec 640. The number of negative second cell selection results 641, 642, 643 is reset each time the coverage level 610 moves from an enhanced coverage level 220 to a normal coverage level 210 in consecutive measurement cycles 620. In this example, the counter

Nec 640 counts the number of consecutive negative first cell selection results in an enhanced coverage level 220.

For example, the counter Nec 640 counts to two 642 when the measurement cycle 620 reaches the third cycle (labelled ‘3’). However, when the measurement cycle 620 reaches the fourth cycle (labelled ‘4’), the coverage level has changed to a normal coverage level 210, and the counter Nec 640 is reset to Nec = 0. Furthermore, when the measurement cycle 620 reaches the fifth cycle (labelled ‘5’), the coverage returns to an enhanced coverage level 220, and the counter Nec counts one 643 again, not three.

According to this example, measurements of neighbouring cells may only occur when Nnc 630 reaches a preconfigured value Nnc-max for a terminal device in a normal coverage level 210. In some specific examples, Nnc-μαχ may be two measurements cycles 620 or discontinuous reception cycles i.e. Nnc counts two consecutive negative cell selection results in a normal coverage level 210 before initiating 670 neighbour cell measurements and/or starting an out-of-service (OOS) timer. Similarly, measurements of neighbouring cells may only occur when Nec 640 reaches a preconfigured value Necmax for a terminal device in an enhanced coverage level 220. In some specific examples, Nec-max may be four measurements cycles 620 or discontinuous reception cycles i.e. Nec counts four consecutive negative cell selection results in an enhanced coverage level 220 before initiating neighbour cell measurements and/or starting an out-ofservice (OOS) timer.

Figure 6a shows a typical example where, in the seventh cycle of the measurement cycle 620, the value 634 for Nnc 630 reaches the preconfigured value Nnc-μαχ of two 634 i.e. Nnc = Nnc-μαχ. Finally, neighbour cell measurements may be initiated and/or an out-of-service timer started 650.

According to this example, the counter Nnc 630 in a normal coverage level 210 and the counter Nec 640 in an enhanced coverage level 220 may be reset every time the coverage level 210, 220 changes. Therefore, a terminal device 140 at a boundary of a normal coverage level 210 and an enhanced coverage level 220 may delay initiating measurements of neighbouring cells if the coverage level changes often as in Figure 6a. Nnc may not reach Nnc-μαχ in a normal coverage level 210, and Nec may not reach Nec-max in an enhanced coverage level 220 until many multiple measurement cycles 620 have been performed. The delay in initiating neighbour cell measurements and/or starting the out-of-service timer may result in unnecessary battery consumption for the terminal device.

Figure 6b is a chart showing cell measurements 620 performed by a terminal device over time at different coverage levels 610 in an example according to the procedure of Figure 5. In this example, the number of negative cell selection results 661, 662, 663, 664 in a first or normal coverage level 210 and a second or enhanced coverage level 220 are counted with the counter N 660. The number of negative first cell selection results 661, 662, 663, 664 is not reset each time the coverage level 610 moves from a normal coverage level 210 to an enhanced coverage level 220 in consecutive measurement cycles 620. Similarly, the number of negative first cell selection results 661, 662, 663, 664 is not reset each time the coverage level 610 moves from an enhanced coverage level 220 to a normal coverage level 210 in consecutive measurement cycles 620.

For example, the coverage level is a normal coverage level 210 in the first cycle ‘ Γ of the measurement cycle 620 and moves to an enhanced coverage level 220 in the second cycle ‘2’ of the measurement cycle 620. The negative cell selection results counter N 660 counts one 661 in the first cycle ‘ Γ of the measurement cycle and counts to two 662 in the second cycle ‘2’ of the measurement cycle. The counter N 660 continues to count, regardless of coverage level 210, 220.

According to examples of the present disclosure, measurements of neighbouring cells and/or starting an out-of-service timer (OOS) timer 670 may only occur when the counter N 660 reaches a predetermined value Nmax for a terminal device which can occur when the terminal device is in either a normal coverage level 210 or an enhanced coverage level 220. In some examples, Nmax may be four measurements cycles 620 or discontinuous reception cycles i.e. N counts four consecutive negative cell selection results, regardless of the coverage level 210, 220 before initiating neighbour cell measurements and/or starting an out-of-service timer 670.

Therefore, in the example of Figure 6b, a terminal device in poor radio conditions may initiate measurements of neighbouring cells earlier compared to the procedure in Figure 6a, for example. This may save processing resources and power resources as the terminal device may spend less time in poor radio conditions before initiating measurements to find a more suitable neighbour cell to camp on.

Figure 7 is a schematic representation of a terminal device 700 configured to operate in a radio access network, for example as described above. The terminal device 700 comprises a receiver 710, a memory' 720 and a cell selection results counter module 730. These components are communicatively coupled. In some examples, the terminal device 700 is aNB-IoT terminal device. In such examples, the radio access network is a NB-IoT network.

The memory 720 is configured to store the predetermined number of consecutive negative cell selection results Nmax. In some examples, the memory is a non-volatile memory such as flash memory or a hard drive. In other examples, the memory is a volatile memory such as a primary storage random access memory.

The receiver 710 is configured to perform serving cell measurements of a serving cell of the radio access network which may be used to evaluate a cell selection criterion.

The receiver 710 is configured to perform reference signal measurements for the terminal device on the serving cell which may be used to evaluate coverage criteria.

The cell measurement procedure module 730 is configured to receive serving cell measurements, evaluate a cell selection criterion for a first coverage level and evaluate the cell selection criterion for a second coverage level. The module is further configured to determine that a predetermined number of consecutive cell selection results do not satisfy the cell selection criterion.

Figure 8 is a schematic diagram 800 of a processor 810 and computer-readable instructions 820 for performing a cell measurement procedure which, when executed by at least one processor 810, cause the at least one processor 810 to perform a procedure according to examples described herein. The computer readable instructions 820 may be retrieved from a machine-readable media, for example any media that can contain, store, or maintain programs and data for use by or in connection with an instruction execution system. In this case, machine-readable media can comprise any one of many physical media such as, for example, electronic, magnetic, optical, electromagnetic, or semiconductor media. More specific examples of suitable machinereadable media include, but are not limited to, a hard disk drive, a random-access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory, or a portable disc.

At block 821 the instructions cause the processor to perform one or more measurements of a serving cell of a radio access network while operating at a first coverage level and while operating at a second coverage level. At block 822 the instructions cause the processor to evaluate the one or more measurements at the first coverage level against a cell selection criterion for the serving cell to determine one or more first cell selection results. At block 823 the instructions cause the processor to evaluate the one or more measurements at the second coverage level against the cell selection criterion for the serving cell to determine one or more second cell selection results. At block 824 the instructions cause the processor to determine that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion. In response to the determining step, neighbour cell measurements may be initiated and/or an out-of-service timer for the serving cell may be started.

The above embodiments are to be understood as illustrative examples of the present disclosure. Further embodiments of the disclosure are envisaged. For example, the method may be implemented in a cellular telephone. The method may further be implemented in any telecommunications system in which performing a cell measurement procedure resulting in initiating measurements of neighbouring cells and/or starting an out-of-service timer for a serving cell may be required.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the present disclosure, which is defined in the accompanying claims.

Claims

1. A method in a terminal device configured to operate in a radio access network in a first coverage level and a second coverage level in idle mode, the method comprising:

performing one or more measurements of a serving cell of the radio access network while operating at the first coverage level and while operating at the second coverage level;

evaluating the one or more measurements at the first coverage level against a cell selection criterion for the serving cell to determine one or more first cell selection results;

evaluating the one or more measurements at the second coverage level against the cell selection criterion for the serving cell to determine one or more second cell selection results; and determining that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion.

2. A method according to claim 1, comprising initiating measurement of a neighbouring cell of the radio access network in response to the determining step.

3. A method according to claim 1 or claim 2, comprising starting an out-of-service timer for the serving cell in response to the determining step.

4. A method according to claim 3, comprising performing a cell search procedure in response to expiry of the out-of-service timer.

5. A method according to any one of claims 1 to 4, wherein performing measurements of the serving cell comprises periodically measuring the serving cell.

6. A method according to any one of claims 1 to 5, wherein performing measurements of the serving cell comprises measuring at least one of:

a signal reception level of a signal received from the serving cell; and a signal quality level of a signal received from the serving cell.

7. A method according to any one of claims 1 to 6, wherein evaluating the measurements comprises periodically evaluating the measurements.

8. A method according to claim 7, wherein evaluating the measurements comprises periodically evaluating a filtered set of measurements including a current measurement.

9. A method according to any preceding claim, wherein the cell selection criterion comprises one or more thresholds, and evaluating the measurements comprises comparing the measurements with the one or more thresholds to determine whether or not the one or more thresholds are exceeded.

10. A method according to claim 9, wherein the one or more thresholds comprises a signal level threshold and a signal quality threshold.

11. A method according to claim 9 or claim 10, wherein the cell selection result does not satisfy the cell selection criterion when a threshold is not exceeded.

12. A method according to any preceding claim, wherein determining that a predetermined number of consecutive first and second cell selection criterion results do not satisfy the cell selection criterion comprises counting the number of consecutive instances where cell selection criterion results do not satisfy the cell selection criterion.

13. A method according to claim 12, wherein determining that a predetermined number of consecutive criterion results do not satisfy the cell selection criterion comprises:

recalling the predetermined number from a memory of the terminal device; and determining that the count of the number of consecutive criterion results that do not satisfy the cell selection criterion has reached or exceeded the predetermined number.

14. A method according to claim 12, wherein the predetermined number is configured by the terminal device based on system information of the serving cell broadcast by the radio access network.

5

15. A method according to any preceding claim, wherein the radio access network is a narrowband internet-of-things, NB-IoT, network.

16. A method according to any preceding claim, wherein the terminal device is a narrowband internet-of-things terminal device.

17. A terminal device configured to perform the method of any of claims 1 to 16.

18. A computer program product comprising instructions which, when executed by a computer, cause the computer to carry out the method of any of claims 1 to 16.