GB2574852A

GB2574852A - Random access procedures

Info

Publication number: GB2574852A
Application number: GB201810120A
Authority: GB
Inventors: Magadi Rangaiah Raghavendra; Malik Akshitij; Burhan Abeezar; Kumar P Soman Pradeep; Bharatkumar Sherdiwala Bhaumik
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: GB2574852B; GB201810120D0

Abstract

When a terminal device is in connected mode it may no longer receive system information broadcast by a cell and therefore if the system information has been modified since the terminal device transitioned from idle mode to connected mode, said system information, and more particularly random access parameters from System Information Block Type 2, may be out-of-date. Disclosed is a method in a terminal device such as an NB‑IoT (narrowband Internet-of-Things) terminal device (140) configured to operate in a radio access network (110) in a connected mode. Stored random access parameters of a cell may be updated and used when the terminal device is required to perform a random access procedure 320 in connected mode, such as after a Radio Link Failure (RLF). The terminal device in connected mode reads system information of the cell broadcast by the network. A new change identifier in a SystemInfoValueTagList is read from newly received System Information Block Type 1 and compared 330 with a previously stored 310 change identifier. If the change identifiers match then System Information Block Type 2 information, and thus the random access parameters, are considered valid, otherwise the random access parameters are considered invalid.

Description

RANDOM ACCESS PROCEDURES

Background

A terminal device may operate in a radio access network in connected mode and idle mode. When the terminal device is operating in idle mode in a cell, the terminal device may be referred to as being camped on the cell. The cell may be referred to as a serving cell. A terminal device camped on a serving cell in idle mode receives and stores system information broadcast by the cell which informs the terminal device about operating parameters of the cell and the radio access network. The terminal device may connect to a serving cell of the radio access network and enter into connected mode to actively transmit and receive data. In particular, when the terminal device has data to transmit, the terminal device can transition from idle mode to connected mode on the serving cell. The connection establishment procedure involves performing a random access procedure as well as re-establishing the higher-layer (NAS) connections. This involves re-authenticating the terminal device, re-establishing security contexts and potentially completing a new attach procedure with the core network. While in connected mode, the terminal device may not receive system information broadcast by the serving cell to which it is connected. Accordingly, if system information is modified by the cell while the terminal device is in connected mode, the system information previously stored by the terminal device may become out of date or invalid.

In some situations, a terminal device in connected mode may be required to perform a procedure to re-establish connection with the radio access network. For example, a connection re-establishment procedure may be necessary when the terminal device experiences radio link failure with the radio access network. The connection reestablishment procedure involves performing a random access procedure on an uplink random access channel of the cell. The random access procedure uses system information of the serving cell broadcast by the radio access network.

In another example, the random access procedure may be required when a terminal device is in connected mode and has uplink data to transmit but no uplink resources allocated to it.

When the terminal device is required to perform a random access procedure in connected mode, the system information that the terminal device has previously stored may be out of date. This will occur, for example, when the system information is modified after the terminal device transitioned to connected mode. In this situation, the random access parameters used by the terminal device may not match the random access parameters used by the cell, and as a consequence, the random access procedure may fail. Failure of the random access procedure may cause failure in the connection re-establishment procedure performed by the terminal device. Upon failure of the connection re-establishment procedure, the terminal device may suffer radio link failure, lose connection with the radio access network and transition to idle mode. Transitioning to idle mode of the terminal device will interrupt data reception and transmission. In examples where a terminal device is a mobile device making a telephone call, the user will experience a telephone call failure.

If the terminal device has transitioned to idle mode, the terminal device may acquire up to date system information and perform another connection establishment procedure to reconnect to the cell.

Failure of the random access procedure and the subsequent connection establishment procedure is time consuming and can also represent a significant drain on the power resources and processing resources of the terminal device.

Summary

According to a first aspect of the present disclosure, there is provided a method in a terminal device configured to operate in a connected mode. The method includes retaining stored system information comprising random access parameters of a cell of the radio access network, determining that the terminal device is required to perform a random access procedure in connected mode, and checking the validity of the stored system information. The method further includes updating the stored system information with new system information if the stored system information is found to be invalid and performing the random access procedure in connected mode using the random access parameters in the stored system information. The stored system information may be updated with new system information if the stored system information pertaining to the random access procedure is found to be invalid.

Checking the validity of the stored system information and updating the stored system information with new system information if the stored system information is found to be invalid may provide a saving in terms of time, processing requirements and power resources. Determining the system information, in particular the random access parameters in the system information, is valid before initiating a connection reestablishment procedure may provide a time saving and power saving. Such savings are a result of avoiding performing a connection re-establishment procedure using invalid random access parameters, which results in a failed procedure and potentially unnecessary interference to other terminal devices within the coverage of the serving cell. Instead, the random access parameters are updated if the random access parameters are determined to be invalid, and thus a successful connection re-establishment procedure may be more likely. Such savings are particularly important for terminal devices that are low-power devices which may have very limited computational resources.

According to a second aspect of the present disclosure, there is provided a terminal device configured to retain stored system information comprising random access parameters of a cell of the radio access network, determine that the terminal device is required to perform a random access procedure in connected mode, and check the validity of the stored system information. The terminal device is further configured to update the stored system information with new system information if the stored system information is found to be invalid and perform the random access procedure in connected mode using the random access parameters in the stored system information.

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 retain stored system information comprising random access parameters of a cell of the radio access network, determine that the terminal device is required to perform a random access procedure in connected mode, and check the validity of the stored system information. The computer programme product is further configured to update the stored system information with new system information if the stored system information is found to be invalid and perform the random access procedure in connected mode using the random access parameters in the stored system information.

Further features and advantages of the present disclosure will become apparent from the following description of preferred embodiments of the invention, 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 system diagram illustrating a typical example of a process in a terminal device to re-establish connection in a connected mode.

Figure 3 is a diagram of a method in a terminal device operating in a connected mode.

Figure 4 is a flowchart of a method for triggering a random access procedure in a terminal device.

Figure 5 is a flowchart of another method for triggering a random access procedure in a terminal device.

Figure 6 is a flowchart of a method in a terminal device for checking the validity of stored system information.

Figure 7 is a schematic diagram showing examples of system information in System Information Block Type 1.

Figure 8 is a schematic diagram showing an example of change identifiers contained in system information.

Figure 9 is a flowchart of a method in a terminal device for checking the validity of stored system information for a System Information Block Type 1.

Figure 10 is a schematic diagram showing examples of system information in a Master Information Block.

Figure 11 is a flowchart of a method in a terminal device for checking the validity of stored system information for a Master Information Block.

Figure 12 is a system diagram illustrating an example of a process in a terminal device to re-establish connection in a connected mode.

Figure 13 is a schematic diagram of a terminal device configured to operate in a radio access network.

Figure 14 is a block diagram showing an example of a computer system.

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, providing connectivity via radio access networks. 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 their nature, many loT devices have limited power and processing resources, and are typically 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.

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, NB-IoT provides enhanced coverage for terminal devices, in order to provide a reliable service in difficult to reach locations. An NB-IoT terminal device may be required to repeat transmissions multiple times to reestablish connectivity to the radio access network. Such transmissions may represent a significant drain on the battery of the terminal device if the transmissions from the terminal device to the network are unsuccessful, resulting in a connection reestablishment failure.

Reduction in the time and processing required to re-establish connectivity to the cell of the radio access network can provide a reduction in power consumption for the terminal device, and thus may extend its lifetime. Furthermore, improving the probability of a successful connection re-establishment procedure may provide a more reliable service to the user of the terminal device and result in lower interference levels within the serving cell (or radio access network).

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 comprising multiple cells distributed over a relatively wide geographic area. Figure 1 shows a single cell 120 of the network 100 served by a fixed-location base station 110, which may be referred to as a cell tower. The base station 110 comprises antennae and electronic communication equipment such as transceivers, digital signal processors and control electronics. Multiple cells may be joined together to provide network coverage over a wide geographic area. Terminal devices 130, 140, 150, 160, 170, 180 within the cell 120 may communicate with each other (or with other terminal devices connected to other cells) via the base station 110.

The terminal devices in the 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 alternatively as operating with the base station 110 or with the corresponding cell 120.

The NB-IoT terminal device, for example smart meter 140, may be in an idle mode when it is not actively connected to the network but is nevertheless receiving NBloT system information of a cell of the radio access network broadcast by the base station 110. In this scenario, the terminal device 140 may be referred to as being camped on the serving cell.

The NB-IoT terminal device 140 may be in a connected mode when it is actively receiving and transmitting data via a radio link with the network. In connected mode, the terminal device 140 may not receive NB-IoT system information broadcast from the cell that it is connected to. The terminal device 140 may only receive NB-IoT system information broadcast from the cell when the terminal device is in idle mode.

The terminal device 140 may move or transition from an idle mode to a connected mode when data is required to be transmitted. While in connected mode, a radio link failure may occur in the terminal device 140. In the event of the radio link failure in connected mode, the NB-IoT terminal device 140 performs a connection reestablishment procedure to re-establish radio link connectivity.

Figure 2 is a system diagram illustrating an example of a procedure 200 in the terminal device 140 to re-establish connection in a connected mode.

The procedure 200 in Figure 2 begins with the terminal device 140 operating in idle mode 201. In step 210 of the procedure 200, the terminal device 130 reads NB-IoT system information 210 broadcast by the cell 120 of the radio access network and stores that system information. If system information has been stored earlier, then the newly received system information may replace the system information stored earlier. The system information may comprise one or more of a Master Information Block 211 (MIB), a System Information Block Type 1212 (SIB 1) and a System Information Block Type 2 213 (SIB2). The System Information Block Type 2 213 may comprise random access parameters.

In step 220 of the procedure 200, the terminal device establishes a connection with the radio access network. The terminal device in connected mode may be referred to as being RRC Connected. Establishing an RRC Connection 220 involves performing a random access procedure 221, 222. The random access procedure may use random access parameters from System Information Block Type 2 213.

The random access procedure comprises transmitting a selected random access preamble 221 from the terminal device to the radio access network on an uplink random access channel.

The network responds by sending a random access response to the terminal device on a downlink shared channel. The random access response may comprise an uplink resource grant. The terminal device may then send an RRC Connection Request message to the network using the uplink resource grant. The network may then respond by sending the terminal device a contention resolution message 222. The contention resolution message comprises information necessary for further transmissions between the terminal device and the network. Successful reception of the contention resolution message 222 by the terminal device completes the RRC Connection establishment procedure 220.

In step 230 of the procedure 200, the terminal device transitions to connected mode 202. The terminal device may be referred to as being RRC Connected. When the terminal device is in connected mode 202, the terminal device no longer receives system information broadcast by the cell. However, when in connected mode 202, the terminal device may now transmit and receive data.

After establishing an RRC Connection, the terminal device in connected mode 202 may experience a radio link failure with the radio access network. Radio link failure may occur at any point when the terminal device is in connected mode 202 if the location of the terminal device changes or if the radio link is otherwise compromised between the terminal device and the base station.

In step 240 of the procedure 200, the terminal device determines that it has undergone a radio link failure with the radio access network. The criteria for determining radio link failure is dependent on the configuration of the terminal device. Radio link failure may be declared if the measured reception level and/or the measured quality level of a signal received by the terminal device falls below a predetermined threshold. Radio link failure may also be declared if the terminal device fails to decode information contained in a physical downlink control channel and/or a physical downlink shared channel due to the low quality of the signal. In an example according to Figure 2, radio link failure may be declared for example when uplink synchronisation has been lost between the terminal device and the network for a predetermined period of time.

In step 250 of the procedure 200, the terminal device in connected mode 202 initiates a connection re-establishment procedure. The connection re-establishment procedure comprises a random access procedure, as described in procedure 220. The random access procedure involves sending a random access preamble 251 using random access parameters from System Information Block Type 2 210.

In a typical example, the system information of the cell of the radio access network may have been modified since the terminal device transitioned from idle mode 201 to connected mode 202. Thus, the system information, and more particularly the random access parameters from System Information Block Type 2 213, may be out-ofdate. In such a situation, the random access procedure, specifically the random access preamble transmission 251 will fail. Upon failure of the random access preamble transmission, the terminal device will repeat the transmissions a predetermined number of attempts.

The number of repeat transmissions per attempt is determined by the level of coverage assigned to the terminal device. A terminal device in an enhanced coverage level may be configured to repeat more transmissions than a terminal device in a normal coverage level. The number of repeat transmissions that the terminal device may attempt is configured in System Information Block Type 2 213. In particular, the number may be an integer value between 0 and 128, allocated to the parameter numRepetitionsPerPreambleAttempt. In Figure 2, the random access preamble transmission 251 is determined to have failed after the failure of the fourth random access preamble transmission attempt.

The failure of the random access preamble is determined by the terminal device by failure to receive a random access response from the radio access network. When the random access preamble has failed a predetermined number of attempts, the terminal device determines a re-establishment failure 252. The terminal device then transitions from connected mode 202 to idle mode 201. If the terminal device is, for example, a mobile telephone, the mobile telephone may experience a disruption in service and the telephone call may be disconnected.

An loT system may be expected to have tens of thousands of terminal devices connected to a single serving cell. Of the terminal devices connected to the serving cell, it is possible that approximately 25-30% of the terminal devices may be in poor coverage conditions. In such a scenario, if even a small percentage of, for example, 25% of the terminal devices suffer re-establishment failure, that may result in approximately 50 to 150 terminal devices experiencing connection re-establishment problems when the system information is modified. These terminal devices may experience service disruptions in addition to creating interference for other terminal devices in their immediate vicinity.

In a typical example, the connection re-establishment procedure 250 may be a power intensive process. The terminal device may send the random access preamble transmissions 251 with increasing power levels in order to improve the probability of a successful transmission. Thus, the connection re-establishment procedure 250 using out-of-date random access parameters is considered a waste of precious power resources 203 for the terminal device.

In step 260 of the procedure 200, the terminal device reads the system information 260 of the cell broadcast by the network. The system information may comprise one or more Master Information Block 261 (MIB), System Information Block Type 1 262 (SIB1) and System Information Block Type 2 263 (SIB2). As described above, the system information was modified since the last time the terminal device read the system information 210. Upon reading the system information again 260, the terminal device is now in possession of up-to-date system information, specifically random access parameters from System Information Block Type 2 263.

In step 270 of the procedure 200, the terminal device establishes an RRC connection to the radio access network. Establishing an RRC Connection 270 involves performing a random access procedure 271, 272. The random access procedure may use random access parameters from System Information Block Type 2 263. As the terminal device is in possession of up-to-date system information, the random access procedure, specifically the random access preamble transmission may succeed. Upon successful transmission of the random access preamble, the network may transmit a random access response to the terminal device. The terminal device may then send an RRC Connection Request message to the network and the network may respond with a contention resolution message 272. Upon successful completion of the establish RRC connection procedure 270, the terminal device may transition to connected mode 202.

In step 280 of the procedure, the terminal device re-establishes a higher layer non-access stratum (NAS) connection. Connection via the NAS layer allows for mobility management of the terminal device by the mobility management entity (MME). Establishment of the NAS connection involves an authentication procedure 281 performed by the network. Once the authentication procedure 281 is complete, the network may then initiate a NAS security mode control procedure 282. Upon establishment of the security mode control procedure 282, the terminal device performs an attach procedure 283, resulting in management of the terminal device by the MME. Once the terminal device has re-established the NAS connection, which consumes power resources 203 of the terminal device, data transfer by the terminal device can be resumed.

From the point in time when a radio link failure has been determined 240 in the terminal device to completion of the NAS connection re-establishment 280, the terminal device may have consumed approximately 0.034 mWh of power when operating in a region of normal coverage. In contrast, in a region of extremely poor coverage, the terminal device may have consumed approximately 1.218 mWh of power during the same period.

In step 290 of the procedure 200, data transfer by the terminal device with the radio access network is resumed. If the terminal device is for example a mobile telephone, the mobile telephone will only now be able to continue the telephone call.

Figure 3 is a diagram of a method 300 in a terminal device operating in a connected mode.

The method 300 comprises a step 310 of retaining stored system information of a cell of the radio access network while operating in a connected mode. The stored system information may be stored in memory of the terminal device. In some examples, the memory is a non-volatile memory such as flash memory or a hard disk drive. This allows the stored system information of the cell to be maintained if the terminal device is powered off. In other examples, the memory is a volatile memory such as a primary storage random access memory.

In some examples, the step 310 of retaining stored system information may comprise retaining the system information in a memory of the terminal device. In other examples, the method comprises a step of receiving and storing the system information of the cell which is broadcast from the radio access network while in idle mode. In such examples, the receiving and storing of the system information while in idle mode is performed prior to retaining the stored system information while in connected mode.

The method 300 comprises a further step 320 of determining that a random access procedure is required by the terminal device. Such a requirement may occur, for example, when the terminal device determines that a radio link failure has occurred or when the terminal device determines that uplink data needs to be transmitted while in connected mode.

In some examples, determining that a random access procedure is required comprises determining a radio link failure in the terminal device, wherein determining a radio link failure comprises determining that an out-of-synchronisation timer has expired. A detailed description of determining a radio link failure is described in relation to Figure 4.

In other examples, determining that a random access procedure is required in connected mode comprises determining that uplink data needs to be transmitted in connected mode. Similarly, a detailed description of determining uplink data needs to be transmitted in connected mode is described in relation to Figure 5.

The method 300 comprises a next step 330 of checking the validity of stored system information of the cell. Checking the validity of the stored system information comprises comparing the stored system information with new system information broadcast from the cell. The stored system information is determined to be valid if the new system information is the same as the stored system information. The stored system information is determined to be invalid if the new system information is not the same as the stored system information.

In some examples, the stored system information can expire and thus become invalid. For example, if the system information has not been updated after a predetermined length of time, the terminal device may presume the system information has expired and is no longer valid. In such examples, checking the expiry of the stored system information may comprise checking the expiration of a timer in the stored system information. Upon expiry of the timer, the stored system information is determined to be invalid. In examples where the terminal device is an NB-IoT device, the system information may be considered to be invalid after 24 hours. The validity of the stored system information may then be checked by comparing the stored system information with new system information broadcast from the cell, as described by step 330.

If the stored system information is determined to be valid, the method 300 bypasses several steps and transitions to step 350 of the method 300. Step 350 comprises performing the random access procedure using the stored system information. Valid stored system information signifies that the stored system information of the cell is up to date. In an example, the terminal device may perform the random access procedure using the random access parameters in the stored system information.

If the stored system information is determined to be invalid, the method 300 transitions to step 340 of the method 300. Step 340 comprises updating the stored system information with the new system information. System information may comprise blocks of system information, wherein a block of system information may comprise random access parameters. In some examples, the further block of system information comprises System Information Block Type 2 information which comprises random access parameters. Valid random access parameters are necessary for a random access procedure to be successful. Invalid stored system information signifies that the stored system information of the cell is no longer up to date. Updating the stored system information comprises selectively replacing the invalid stored system information (comprising invalid random access parameters) with new system information (comprising valid random access parameters).

In such examples, updating the random access parameters that are determined to be invalid may increase the likelihood of a successful random access procedure. Additionally, using valid random access parameters may reduce the risk of a radio link failure by the terminal device. Ensuring that the random access parameters are valid before performing a random access procedure may avoid the terminal device sending multiple random access transmissions that results in a connection re-establishment failure, which is a significant drain of the battery of the terminal device. Furthermore, using valid random access parameters reduces the time and processing resources required to re-establish connection and resume data transfer, and provides a more reliable service to the user of the terminal device.

Upon completion of selectively replacing the invalid stored system information with new system information, the method 300 comprises a final step 350 of performing the random access procedure using the stored system information and new system information. In some examples, the stored system information may be completely replaced by the new system information and the terminal device may perform the random access procedure using the new random access parameters in the new system information. In other examples, the stored system information may only be partly replaced by the new system information and the terminal device may perform the random access procedure using the stored random access parameters in the stored system information and the new system information. In such examples, selectively replacing only the system information that is determined to be invalid reduces the required processing resources of the terminal device. Reduced processing may allow the terminal device to re-establish connection earlier, thus saving time and power resources.

Figure 4 is a flowchart of a method 400 for triggering a random access procedure in a terminal device. In some examples, a random access procedure is required upon a terminal device determining that a radio link failure has occurred.

In step 410 of the method 400, an out-of-synchronisation timer in system information is checked. System information associated with a cell of the radio access network may be broadcast by the network. The out-of-synchronisation timer may be started after receiving a pre-determined number of consecutive out-of-synchronisation indications from lower layers of a radio protocol architecture. The lower layers comprise the physical layer which transmits and receives control information, system information and data over the air interface by the terminal device to the radio access network. Upon checking the out-of-synchronisation timer, the routine follows a path to step 420.

In step 420 of the method 400, the out-of-synchronisation timer has expired. In some examples, the out-of-synchronisation timer is a predetermined length of time. After the predetermined length of time has passed since the out-of-synchronisation timer started, the out-of-synchronisation timer is determined to have expired. Upon expiry of the out-of-synchronisation timer, the routine follows a path to step 430.

In step 430 of the method 400, a random access procedure is required to be performed. In some examples, upon expiry of the out-of-synchronisation timer, a connection re-establishment procedure is initiated by the terminal device. The connection re-establishment procedure comprises at least a random access procedure.

Figure 5 is a flowchart of another method 500 for triggering a random access procedure in a terminal device. In another example, a random access procedure may be required when a terminal device in connected mode determines that uplink data is required to be transmitted.

In step 510 of the method 500, a scheduling request has been triggered. A scheduling request may be initiated when the terminal device has uplink data to transmit to the radio access network. A scheduling request may be transmitted on an uplink control channel. Upon triggering the scheduling request, the routine follows a path to step 520.

In step 520 of the method 500, it is determined that there is an absence of a physical uplink control channel. Absence of a physical uplink control channel results in the scheduling request being unable to grant resources in the uplink to the terminal device. Upon determination of this absence, the routine follows a path to step 530.

In step 530 of the method 500, a random access procedure is required to be performed to establish synchronization with uplink and downlink channels, and to be granted uplink channel resources. Synchronisation of the uplink and downlink channels and allocation of uplink resources allows a scheduling request to be transmitted on an uplink control channel. Upon successful completion of the random access procedure, the terminal device may now be able to perform a scheduling request in order to transmit uplink data.

Figure 6 is a flowchart of a method 600 in a terminal device for checking the validity of stored system information.

In step 610 of the method 600, stored system information of the cell is recalled in the terminal device. The stored system information may comprise random access parameters. The stored system information of Figure 6 is the system information of the network that may be used by the terminal device in connected mode or in idle mode and is stored in the memory of the terminal device. The stored system information includes a change identifier.

In step 610, the stored change identifier in the stored system information is read from the memory of the terminal device. Once the stored system information is read, the procedure follows a path to step 650 of the method 600.

In step 630 of the method 600, new system information of the cell is received by the terminal device. The new system information may comprise random access parameters. The new system information in Figure 6 is broadcast by the network within the cell so that it can be received by the terminal device. The new system information includes a change identifier. The change identifier is updated by the network if the system information has been modified since the system information was last broadcast by the network.

In step 640 of the method 600, the new change identifier in the new system information broadcast by the network is read by the terminal device. In some examples, it may be possible to receive a subset of the new system information which may include the new change identifier. In this case, the terminal device may read only the new change identifier in the new system information, instead of all the new system information, which may reduce the time required to identify the invalid stored system information. Receiving and/or reading further system information may only be performed if needed. Once the new system information is read, the routine follows a path to step 650 of the method 600.

In step 650, the stored change identifier is compared with the new change identifier. If the stored change identifier and the new change identifier are the same, the routine follows a path to step 660 of the method 600, whereby the stored system information is considered to be valid. If the stored change identifier and the new change identifier are not the same, the routine follows a path to step 670 of the method 600, whereby the stored system information is considered to be invalid (which may include all the stored system information being invalid or a subset of the stored system information being invalid).

In an example, there is at least one stored change identifier in the stored system information and there is at least one new change identifier in the new system information. Each stored change identifier has a corresponding new change identifier.

In an example, the system information comprises blocks of system information. If the stored change identifier and the new change identifier are the same, a further block of stored system information is determined to be valid 660. If the stored change identifier and the new change identifier are not the same, the further block of stored system information is determined to be invalid 670.

In an example, a block of system information comprises random access parameters. If the stored change identifier and the new change identifier are the same, the random access parameters are determined to be valid 660. If the stored change identifier and the new change identifier are not the same, the random access parameters are determined to be invalid 670.

Figure 7 is a schematic diagram showing examples of system information in System Information Block Type 1. System information may comprise multiple blocks of system information 700, 710, 720, 730, 740, 750. In some examples, the system information comprises System Information Block Type 1 information 700. System Information Block Type 1 information 700 may comprise one of more of a SystemlnfoValueTagList 710 and a SchedulinglnfoList 720. In some examples, the system information may comprise further system information blocks 730, 740, 750.

In examples according to the present disclosure, the system information comprises a further system information block, wherein the further system information block comprises System Information Block Type 2 information 730. In such examples, System Information Block Type 2 information 730 comprises random access parameters necessary to complete a random access procedure. Further system information may also comprise System Information Block Type 22 information 750, wherein System Information Block Type 22 information 750 comprises random access parameters for performing a random access procedure on an additional carrier e g. a non-anchor carrier.

Therefore, in some examples, a modification in SystemlnfoValueTagList 710 by the network indicates a modification in a further System Information Block 730, 740, 750. In some examples, the modified further information block may be a System Information Block Type 2 730, which comprises random access parameters. Therefore, if a change identifier in the System Information Block Type 1 700 is modified, the random access parameters may be considered invalid.

Figure 7a is a schematic diagram showing an example of System Information Block Type 1 information 700. System Information Block Type 1 information 700 may comprise a plurality of change identifiers 710. The plurality of change identifiers may comprise a SystemlnfoValueTagList 710, wherein each change identifier 711, 712, 713 is an element in the SystemlnfoValueTagList 710. System Information Block Type 1 information 710 may also comprise a plurality of scheduling information 720. The plurality of scheduling information may comprise a SchedulinglnfoList 720, wherein each element 721, 722, 723 contains scheduling information. Each change identifier 711, 712, 713 in the SystemlnfoValueTagList 710 corresponds to an element of scheduling information 721, 722, 723 in the SchedulinglnfoList 720. The number of elements in the SystemlnfoValueTagList 710 is equal to the number of elements in the SchedulinglnfoList 620.

Figure 7b is a schematic diagram showing an example of a SystemlnfoValueTagList 710. In some examples, the change identifiers 711, 712, 713 in the SystemlnfoValueTagList 710 comprise integer values. In such examples, when the system information 700, 730, 740, 750 has been modified since the system information was last broadcast by the network, the integer value 711, 712, 713 is increased. In some examples, the integer value 711, 712, 713 follows a sequence of integer numbers from 0 to 3 (which may be represented by 2 binary bits). When the integer value is 3 and the system information is modified, the integer value of the change identifier is reset or cycled to 0.

In some examples, a change identifier 711, 712, 713 in the SystemlnfoValueTagList 710 may correspond to a further block of system information, such as a System Information Block 730, 740, 750. In such an example, modification of a change identifier 711, 712, 713 in SystemlnfoValueTagList 710 signifies that the corresponding System Information Block 730, 740, 750 has been modified.

In examples according to the present disclosure, the change identifier 711 in the SystemlnfoValueTagList 710 corresponds to a further block of system information, System Information Block Type 2 information 730. In such an example, modification of a change identifier 711 in SystemlnfoValueTagList 710 by the network signifies to a terminal device that the System Information Block Type 2 information 730 has been modified. The System Information Block Type 2 information 730 comprises random access parameters. Thus, the change identifier 711 in SystemlnfoValueTagList 710 signifies to a terminal device that random access parameters have been modified.

In other examples, a change identifier 711 in the SystemlnfoValueTagList 710 may correspond to a system information container which may comprise one or more System Information Blocks 730, 740, 750. In these examples, when the change identifier 711 is modified, all System Information Blocks 730, 740, 750 of the system information container are acquired by the terminal device. The change identifier 711 in the SystemlnfoValueTagList 710 may be referred to as a SystemlnfoValueTagSI.

Figure 7c is a schematic diagram showing an example of a SchedulinglnfoList 720. In some examples, the elements 721, 722, 723 in the SchedulinglnfoList 720 comprise scheduling information for the System Information Block 730, 740, 750. For example, the scheduling information elements may provide information on the periodicity, a timing offset, a repetition pattern and/or mapping information relating to when and how frequently the network sends the corresponding System Information Block. In such an example, a terminal device may acquire or read the SchedulinglnfoList 720 when a modification of a change identifier 711, 712, 713 in SystemlnfoValueTagList 710 has been detected in order to know when the relevant system information is being broadcast. Acquiring or reading the corresponding scheduling element 721, 722, 723 allows the terminal device to correctly acquire the modified System Information Block 730, 740, 750.

Specifically, modification of a change identifier 711 in SystemlnfoValueTagList 710 indicates to a terminal device that the corresponding element of scheduling information 721 (for System Information Block Type 2 730) in the SchedulinglnfoList 720 should be acquired or read. Acquiring the corresponding scheduling element 721 allows the modified System Information Block Type 2 730, and thus the random access parameters, to be acquired efficiently.

As described in relation to Figure 7b, a change identifier 711 or a SystemlnfoValueTagSI in the SystemlnfoValueTagList 710 may correspond to a system information container comprising one or more System Information Blocks 730, 740, 750. In these examples, acquiring the corresponding scheduling element 721 in SchedulinglnfoList 720 allows the system information container to be acquired. Thus, all System Information Blocks of the system information container are acquired by the terminal device.

Acquisition of the SchedulinglnfoList 720 by the terminal device may occur concurrently with acquisition of the SystemlnfoValueTagList 710 by acquiring the whole of the System Information Block Type 1 700.

In some examples, the first change identifier 711 in the SystemlnfoValueTagList 710 and the first element of scheduling information 721 in the SchedulinglnfoList 720 corresponds to System Information Block Type 2 information 730.

Figure 8 is a schematic diagram showing an example of change identifiers 810, 820. In an example, a stored SystemlnfoValueTagList 810 contains a plurality of change identifiers 811, 812, 813 and a new SystemlnfoValueTagList 820 contains a plurality of change identifiers 821, 822, 823. The position of the change identifiers 811, 812, 813 and change identifiers 821, 822, 823 corresponds to the position of the change identifiers 711, 712, 713 in Figure 7 i.e. change identifiers 811, 821, and 711 all relate to SIB2. Each change identifier 811, 812, 813, 821, 822, 823 may comprise an integer value.

In the example of Figure 8, each stored change identifier 811, 812, 813 in the stored SystemlnfoValueTagList 810 is compared to the corresponding new change identifier 821, 822, 823 in the new SystemlnfoValueTagList 820. If the stored integer value 811, 812, 813 and the new integer value 821, 822, 823 are the same, the terminal device may consider that the corresponding System Information Block 730, 740, 750 has not been modified, and the corresponding stored System Information Block 730, 740, 750 is determined to be valid. If the stored integer value 811, 812, 813 and the new integer value 821, 822, 823 are not the same, the terminal device may consider that the corresponding System Information Block 730, 740, 750 has been modified, and the corresponding stored System Information Block 730, 740, 750 is determined to be invalid.

In other examples, the modification of a system information container comprising System Information Block Type 2 may be indicated by a change identifier. The change identifier may be referred to as a systemlnfoValueTagSI. A stored change identifier 811, 812, 813 may be compared to a new change identifier 821, 822, 823. If the stored change identifier 811, 812, 813 and the new change identifier 821, 822, 823 are the same, the terminal device may consider the corresponding system information container (comprising System Information Block Type 2) valid. If the stored change identifier 811, 812, 813 and the new change identifier 821, 822, 823 are not the same, the terminal device can consider the corresponding system information container (comprising System Information Block Type 2) invalid.

In the case where the System Information Block Type 2 information 730 comprises random access parameters, the first stored change identifier 811 in the stored SystemlnfoValueTagList 810 is compared to the first new change identifier 821 in the new SystemlnfoValueTagList 820. If the stored integer value 811 and the new integer value 821 are the same, the System Information Block Type 2 information 730 has not been modified by the network, and the random access parameters are determined by the terminal device to be valid. If the first stored integer value 811 and the first new integer value 821 are not the same, the corresponding System Information Block Type 2 information 730 has been modified, and the random access parameters are determined by the terminal device to be invalid.

In the example shown in Figure 8, the first change identifier 811 in the stored SystemlnfoValueTagList 810 equals 0 and the first change identifier 821 in the new SystemlnfoValueTagList 820 equals 1. Therefore, the terminal device will notice that the first change identifier has incremented by 1 and will consider the corresponding stored System Information Block 730 (in this example, the System Information Block Type 2) to be invalid.

In case the stored second change identifier 812 is compared, the value is equal to 3 which is the same as the value of the corresponding new second change identifier 822 in the new SystemlnfoValueTagList 820. Therefore, the terminal device will notice that the stored and the new change identifiers are the same, and will consider the corresponding stored System Information Block 740 (in this example, the System Information Block Type 3) to be valid.

Figure 9 is a flowchart of a method 900 in a terminal device for checking the validity of stored system information for System Information Block Type 1. In an example according to a method of the present disclosure, the stored system information comprises System Information Block Type 1 700 and a further block of system information comprising System Information Block Type 2 730. The System Information Block Type 1 700 comprises at least a SystemlnfoValueTagList 710, 810 and a SchedulinglnfoList 720, 820. The System Information Block Type 2 information 730 comprises random access parameters necessary for performing a random access procedure.

In step 910 of the method 900, a new change identifier 821 in a new SystemlnfoValueTagList 820 is read from a newly received System Information Block Type 1. The new change identifier is a change identifier for the System Information Block Type 2 information 730. In some examples, the new change identifier is the first change identifier in the new SystemlnfoValueTagList 820.

In step 920, a stored change identifier 811 in the stored SystemlnfoValueTagList 810 is recalled. The stored change identifier is a change identifier for the System Information Block Type 2 information 730. The stored change identifier may also be the first change identifier in the stored SystemlnfoValueTagList 810. The stored SystemlnfoValueTagList 810 may be stored in the memory of the terminal device.

In step 930, the stored change identifier 811 is compared with the new change identifier 821. If the stored change identifier 811 is the same as the new change identifier 821, then the System Information Block Type 2 information 730, and thus the random access parameters, are considered valid. Then the method 900 bypasses several steps to reach a step 970 of performing a random access procedure using the stored random access parameters in the System Information Block Type 2 information 730.

If the stored change identifier 811 is not the same as the new change identifier 821, then the System Information Block Type 2 information 730, and thus the random access parameters, are considered invalid. Then the method 900 follows a path to step 940, whereby the stored SystemlnfoValueTagList 810 is updated with the new change identifier 821 in the new SystemlnfoValueTagList 820.

In step 950, the new SchedulinglnfoList 720 in the newly received System Information Block Type 1 700 is read, which may involve reading the first new scheduling information element 721 in the new SchedulinglnfoList 720. Reading the scheduling information element 721 provides the terminal device with timing information for when and how frequently the network broadcasts the System Information Block Type 2 information 730. In some examples, scheduling information providing the terminal device with timing information for when and how frequently the network broadcasts System Information Block Type 22 information 750 may also be acquired.

In step 960, the System Information Block Type 2 information 730 is acquired by the terminal device using the scheduling information from step 950. Hence, the system information, comprising the System Information Block Type 1 700 and a further block of System Information Block Type 2 information 730, is updated.

In such examples, the processing requirements are limited to acquiring the System Information Block Type 2 information 730 only when the System Information Blocks Type 2 information 730, and thus the random access parameters, are determined to be invalid and need to be updated.

In some examples, a further block of System Information Block Type 22 information 750 may be acquired and updated by the terminal device using the scheduling information from step 950. Acquisition of System Information Block Type 22 information 750 may be performed by a terminal device to obtain system information, specifically random access parameters, for additional carriers e.g. nonanchor carriers.

In step 970, a random access procedure using the stored system information and/or the new system information is performed. In examples according to the present disclosure, the system information 700, 730 may comprise the stored System Information Block Type 1 700 and/or new System Information Block Type 1 700, in addition to a stored further block of System Information Block Type 2 information 730 or a new further block of System Information Blocks Type 2 information 730. In such examples, System Information Block Type 2 information 730 comprises random access parameters. Hence, the system information 700, 730 used for the random access procedure may comprise stored random access parameters or new random access parameters.

Figure 10 is a schematic diagram showing examples of system information in a Master Information Block. System information may comprise a Master Information Block 1000.

Figure 10a is a schematic diagram showing an example of a Master Information Block 1000. The Master Information Block 1000 may comprise a change identifier 1010, wherein the change identifier may be a SystemlnfoValueTag 1010.

In some examples, a modification of SystemlnfoValueTag 1010 (in the Master Information Block 1000) indicates a modification in SystemlnfoValueTagList 710 (in System Information Block Type 1 700). As described earlier, the modification in SystemlnfoValueTagList 710 may indicate a modification in further System Information Blocks 730, 740, 750. In some examples, the modified further system information block may be a System Information Block Type 2 730, which comprises random access parameters. Therefore, if a change identifier in the Master Information Block 1000 is modified by the network, then the earlier stored random access parameters may be considered invalid by the terminal device.

Figure 10b is a schematic diagram showing an example of a SystemlnfoValueTag 1010. In some examples, the change identifier 1011 of the SystemlnfoValueTag 1010 comprises an integer value. In such examples, when the system information 700 has been modified since the system information 700 was last broadcast by the network, the integer value 1011 of the SystemlnfoValueTag 1010 is increased. In some examples, the integer value follows a sequence of integer numbers from 0 to 31 (which may be represented by 5 binary bits). When the change identifier is 31 and the system information is modified, the integer value of the change identifier is reset or cycled back to 0.

In some examples, a change identifier 1011 of the SystemlnfoValueTag 1010 may correspond to a further block of system information 700, such as System Information Block Type 1. In such an example, modification of a change identifier 1011 of the SystemlnfoValueTag 1010 signifies to a terminal device that the System Information Block Type 1 700 has been modified.

Figure 10c is a schematic diagram showing an example of two change identifiers. In this example, a stored SystemlnfoValueTag 1020 and a new SystemlnfoValueTag 1030 may each contain a change identifier 1021, 1031. Each change identifier 1021, 1031 may comprise an integer value. In an example according to a method of the present disclosure, the stored change identifier 1021 of the stored SystemlnfoValueTag 1020 is compared to the new change identifier 1031 of the new SystemlnfoValueTag 1030. If the stored integer value 1021 and the new integer value 1031 are the same, the terminal device may consider that the corresponding System Information Block Type 1 700 has not been modified, and an earlier received and stored version of the System Information Block Type 1 is determined to be valid. If the stored integer value 1021 and the new integer value 1031 are not the same, the terminal device may consider that the corresponding System Information Block Type 1 700 has been modified by the network, and an earlier received and stored version of the System Information Block Type 1 is determined to be invalid.

In a specific example, if the stored change identifier 1021 of the stored SystemlnfoValueTag 1020 equals 14 and the new change identifier 1031 of the new SystemlnfoValueTag 1030 equals 15, then the corresponding System Information Block Type 1 700 is determined to be invalid.

In such examples, the processing requirements are limited to acquiring the System Information Block Type 1 700 only when the SystemlnfoValueTag 1010 identifies the System Information Block Type 1 700 information as invalid.

Figure 11 is a flowchart of a method 1100 in a terminal device for checking the validity of stored system information for a Master Information Block. In an example according to a method of the present disclosure, the stored system information comprises Master Information Block information 1000 and a further block of system information comprising System Information Block Type 1 700. The Master Information Block 1000 comprises at least a SystemlnfoValueTag 1010, 1020, 1030.

In step 1110 of the method 1100, a new change identifier 1031 in a new SystemlnfoValueTag 1030 is read from a newly received Master Information Block.

In step 1120 of the method 1100, a stored change identifier 1021 in the stored SystemlnfoValueTag 1020 is recalled. The stored SystemlnfoValueTag 1020 may be stored in the memory of the terminal device.

In step 1130 of the method 1100, the stored change identifier 1021 is compared with the new change identifier 1031. If the stored change identifier 1021 is the same as the new change identifier 1031, then the routine bypasses several steps to reach a step 1160 of camping on the cell, as described later.

If the stored change identifier 1021 is not the same as the new change identifier 1031, then the routine follows a path to step 1140, whereby the stored change identifier 1021 of the stored SystemlnfoValueTag 1020 is updated with the new change identifier 1031 of the new SystemlnfoValueTag 1030.

In step 1150 of the method 1100, the System Information Block Type 1 700 is acquired. As described above in relation to Figure 6, the System Information Block Type 1 700 may also comprise a change identifier 710, wherein the change identifier 710 comprises a SystemlnfoValueTagList 710 and may be used to check the validity of a further block of System Information Block Type 2 information 730. In such an example, the method to check the validity of stored system information i.e. System Information Block Type 1 700 and System Information Block Type 2 730, may be performed as described above in relation to Figure 9.

In examples where the SystemlnfoValueTagList 710 is not present in System Information Block Type 1 700, the terminal device may consider any stored further blocks of system information (except System Information Block Type 14) to be invalid. The stored further blocks of system information may then be acquired and updated as described in relation to Figure 9.

In step 1160 of the method 1100, a random access procedure using stored system information and/or new system information is performed.

In the example of Figure 11, the method may further comprise replacing the stored Master Information Block 900 with the new Master Information Block 900.

Following a check of the validity of stored System Information Block Type 1 700 in the example of Figure 11, the resulting SIB1 information in the system information may comprise the stored System Information Block Type 1 or the newly acquired System Information Block Type 1. If the resulting SIB 1 information comprises the stored System Information Block Type 1 then the SIB2 information will also comprise stored System Information Block Type 2.

In the case where System Information Block Type 1 is newly acquired, the resulting SIB2 information in the system information may comprise the stored System Information Block Type 2 or a newly acquired System Information Block Type 2.

In examples according to the present disclosure, the system information may comprise System Information Block Type 2 730, which in turn may comprise random access parameters. Therefore, in step 1160, the random access procedure may be performed using stored random access parameters or new random access parameters.

In the examples of system information described above in connection with Figures 7 to 11, the radio access network may be a narrow band internet of things (NBloT) network, and the terminal device may be a narrow band internet of things (NBloT) terminal device. In this case, the system information may be NB-IoT system information (which may be defined in the LTE NB-IoT technology standard with an NB” suffix). In a network which includes NB-IoT co-existing in the same carrier space as non-NB-IoT LTE, the NB-IoT system information may be different and separate from the LTE system information, and may be sent in a different physical carrier from the LTE system.

Figure 12 is a system diagram illustrating an example of a procedure 1200 in the terminal device 140 to re-establish connection in a connected mode.

The procedure 1200 in Figure 12 begins with the terminal device 140 operating in idle mode 1201. In step 1210 of the procedure 1200, the terminal device 140 reads NB-IoT system information 1210 broadcast by the cell 120 of the radio access network and stores that system information. If system information has been stored earlier, then the newly received system information may replace the system information stored earlier. The system information may comprise one or more of a Master Information Block 1211 (MIB), a System Information Block Type 11 212 (SIB1) and a System Information Block Type 2 1213 (SIB2). The System Information Block Type 2 1213 may comprise random access parameters.

In step 1220 of the procedure 1200, the terminal device establishes a connection with the radio access network. The terminal device in connected mode may be referred to as being RRC Connected. Establishing an RRC Connection 1220 involves performing a random access procedure 1221, 1222. The random access procedure may use random access parameters from System Information Block Type 2 1213.

The random access procedure comprises transmitting a selected random access preamble 1221 from the terminal device to the radio access network on an uplink random access channel.

The network responds sending a random access response to the terminal device on a downlink shared channel. The random access response may comprise an uplink grant resource. The terminal device may then send an RRC Connection Request message to the network using the uplink grant resource. The network may then respond by sending the terminal device a contention resolution message 1222. The contention resolution message comprises information necessary for further transmissions between the terminal device and the network. Successful reception of the contention resolution message 1222 by the terminal device completes the RRC Connection establishment procedure 1220.

In step 1230 of the procedure 1200, the terminal device transitions to connected mode 1202. The terminal device may be referred to as being RRC Connected. When the terminal device is in connected mode 1202, it no longer receives system information broadcast by the cell. However, when in connected mode 1202, the terminal device may now transmit and receive data.

In some examples according to the present disclosure, the system information of the cell of the radio access network may have been modified since the terminal device transitioned from idle mode 1201 to connected mode 1202. Thus, the system information, and more particularly the random access parameters from System Information Block Type 2 1213, may be out-of-date.

In step 1240 of the procedure 1200, the terminal device determines that it has undergone a radio link failure with the radio access network. The criteria for determining radio link failure is dependent on the configuration of the terminal device. Radio link failure may be declared if the measured reception level and/or the measured quality level of a signal received by the terminal device falls below a predetermined threshold. Radio link failure may also be declared if the terminal device fails to decode information contained in a physical downlink control channel or a physical downlink shared channel due to the low quality of the signal. In an example according to Figure 12, radio link failure may be declared for example when uplink synchronisation has been lost between the terminal device and the network for a predetermined period of time.

In step 1250 of the procedure 1200, the terminal device in connected mode 1202 reads system information 1250 of the cell broadcast by the network. The system information may comprise one or more of a Master Information Block 1251 (MIB), System Information Block Type 1 1252 (SIB1) and System Information Block Type 2 1253 (SIB2). The system information may be modified since the last time the terminal device read the system information 1210. As described above, the terminal device may selectively acquire and read the new system information block by block using value tags contained in the system information. Hence, the terminal device can quickly and efficiently determine from acquired blocks whether further blocks need to be acquired and read. Upon reading the necessary system information again 1250, the terminal device is now in possession of valid up-to-date system information, specifically random access parameters from System Information Block Type 2 1253.

In step 1260 of the procedure 1200, the terminal device establishes an RRC connection to the radio access network. Establishing an RRC Connection 1220 involves performing a random access procedure 1261, 1262. The random access procedure may use random access parameters from System Information Block Type 2 1253. As the terminal device is in possession of up-to-date system information, the random access procedure, specifically the random access preamble transmission by the terminal device and reception by the radio access network may have a high likelihood of success. Upon successful transmission of the random access preamble, the network may transmit a random access response to the terminal device. The terminal device may then send an RRC Connection Request message to the network and the network may respond with a contention resolution message. Upon successful completion of the establish RRC connection procedure 1260, the terminal device continues to be in connected mode 1202.

In step 1270 of the procedure 1200, the terminal device resumes the RRC Connection with the radio access network. If the terminal device is for example in the middle of data transmission session, then the session may continue without disruption that may otherwise occur if the terminal device transitioned to idle mode.

From determination that the terminal device has undergone a radio link failure to connection re-establishment in order to resume data transfer, the terminal device may consume approximately 0.017 mWh of power when the terminal device is operating in a region of normal coverage. For a terminal device operating in a region of extremely poor coverage, the terminal device may consume approximately 0.330 mWh of power to re-establish connection to resume data transfer.

As such, for a terminal device operating in normal coverage, approximately 50% of the power consumed by following the procedure illustrated in Figure 2 may be saved by following the procedure of Figure 12. For a terminal device operating in a region of extremely poor coverage, up to 73% of the power consumed by following the procedure of Figure 2 may be saved by following the procedure of Figure 12.

Figure 13 is a schematic diagram of a terminal device 1300 configured to operate in a radio access network, for example as described above. The terminal device 1300 comprises a receiver 1310, a memory 1320 and a random access procedure module 1330. These components are communicatively coupled. In some examples, the terminal device 1300 is a NB-IoT terminal device. In such examples, the radio access network is a NB-IoT network.

The memory 1320 is configured to retain stored system information 1000, 700, 730 of a cell of a radio access network. In some examples, the system information may comprise one or more of a Master Information Block 1000, System Information Block Type 1 700 and System Information Block Type 2 730. In such examples, the System Information Block Type 2 730 comprises random access parameters. In some examples, the memory is a non-volatile memory such as flash memory or a hard drive. This allows the stored system information to be maintained if the terminal device is powered off. In other examples, the memory is a volatile memory such as a primary storage random access memory.

The receiver 1310 is configured to receive a transmission that determines that a random access procedure is required to be performed by the terminal device in connected mode.

The receiver 1310 is further configured to receive new system information 1000, 700, 730 comprising random access parameters of the cell broadcast by the radio access network.

The random access procedure module 1330 is configured to check the validity of the stored system information 1000, 700, 730, update the stored system information with the new system information if the stored system information is found to be invalid, and perform the random access procedure in connected mode using the random access parameters in the stored system information. In some examples, checking the validity of the stored system information 1000, 700, 730 may comprise comparing stored change identifiers 1020, 810 in the stored system information 1000, 700, 730 with new change identifiers 1030, 820 in the new system information 1000, 700, 730 broadcast from the cell, for example as described above. In some examples, updating the stored system information 1000, 700, 730 may comprise selectively replacing the stored system information 1000, 700, 730 that is found to be invalid with the new system information 1000, 700, 730, for example as described above. In some examples, performing the random access procedure in connected mode comprises using the stored system information 1000, 700, 730 and/or the new system information 1000, 700, 730, for example as described above.

Figure 14 is a block diagram showing an example of a computer system 1400 comprising a processor 1410 and a computer readable storage 1420. The processor 1410 may be implemented as a single processor or multiple processors. The computer readable storage 1420 may be a non-transitory computer readable storage medium comprising a set of computer readable instructions 1450 which, when executed by the processor 1410, cause the processor 1410 to perform a method according to examples described herein. The computer readable instructions 1450 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 machine-readable 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 1421 the instructions cause the processor to retain stored system information of a cell of a radio access network in the memory of a terminal device. At block 1422 the instructions cause the processor to determine that the terminal device is required to perform a random access procedure. At block 1423 the instructions cause the processor to check the validity of the system information stored in the memory of the terminal device. Upon determination of the validity of the stored system information, at block 1425 the instructions cause the processor to perform the random access procedure using the stored system information. Upon determination of the invalidity of the stored system information, at block 1424 the instructions cause the processor to selectively replace the stored system information with new system information broadcast from the cell. At block 1425 the instructions cause the processor to perform the random access procedure using the random access parameters in the stored system information.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention 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 a random access procedure in connected mode is 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 invention, 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 connected mode, the method comprising:

retaining stored system information comprising random access parameters of a cell of the radio access network;

determining that the terminal device is required to perform a random access procedure in connected mode;

checking the validity of the stored system information;

updating the stored system information with new system information if the stored system information is found to be invalid; and performing the random access procedure in connected mode using the random access parameters in the stored system information.

2. A method according to claim 1, wherein checking the validity of the stored system information comprises checking the validity of the stored system information using a stored change identifier in the stored system information.

3. A method according to claim 2, wherein checking the validity of the stored system information comprises:

receiving new system information broadcast from the cell containing a new change identifier; and comparing the stored change identifier in the stored system information with the new change identifier in the new system information.

4. A method according to claim 3, wherein checking the validity of the stored system information comprises:

determining the stored system information is valid if the stored change identifier is the same as the new change identifier; and determining the stored system information is invalid if the stored change identifier is not the same as the new change identifier.

5. A method according to any preceding claim, wherein the system information comprises multiple blocks of system information.

6. A method according to claim 5, wherein a block of system information comprises a master information block.

7. A method according to claim 5 or claim 6, wherein a block of system information comprises a system information block.

8. A method according to claim 5 dependent on claim 3, wherein the new change identifier is received in a first block of the system information, and the stored change identifier is stored in a corresponding first block of the stored system information, and checking the validity of the stored system information comprises determining the validity of a second block of stored system information based on the comparing step.

9. A method according to any preceding claim, wherein retaining stored system information comprises retaining the system information in a memory of the terminal device.

10. A method according to any preceding claim, comprising receiving and storing system information of the cell broadcast from the radio access network while in idle mode prior to retaining the stored system information while in connected mode.

11. A method according to any preceding claim, wherein updating the stored system information comprises selectively replacing the stored system information determined to be invalid with new system information.

12. A method according to any preceding claim, wherein determining that the random access procedure needs to be performed comprises determining a radio link failure in the terminal device.

13. A method according to claim 12, wherein determining a radio link failure comprises determining that an out-of-synchronisation timer has expired.

14. A method according to any preceding claim, wherein determining the terminal device is required to perform a random access procedure in connected mode comprises determining that uplink data needs to be transmitted while in connected mode.

15. A method according to any preceding claim, wherein performing the random access procedure comprises sending a random access preamble.

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

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

18. A terminal device configured to perform the method of any claim 1 to 17.

19. A computer program product comprising instructions which, when executed by a computer, cause the computer to carry out the method of any claims 1 to 17.