Terminal device, base station, wireless telecommunications system and methods for transitioning between two modes of operation

A method of operating a terminal device in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation in which the terminal device does not communicate with the wireless telecommunications system and a second mode of operation in which the terminal device does communicate with the wireless telecommunications system, the method including: transitioning from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT filing PCT/EP2015/060379 filed May 11, 2015, and claims priority to European Patent Application 14 168 361.5, filed in the European Patent Office on May 14, 2014, the entire contents of each of which being incorporated herein by reference.

BACKGROUND

Field of the Disclosure

The present invention relates to a method, terminal device, base station, wireless telecommunications system and method therefor

Description of the Related Art

Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architectures, are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems.

For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and the coverage areas for these networks is expected to increase rapidly.

The anticipated widespread deployment of third and fourth generation networks has led to the parallel development of devices and applications which, rather than taking advantage of the high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of the coverage area. Examples include so-called machine type communication (MTC) applications, which are typified by semi-autonomous or autonomous wireless communication devices (i.e. MTC devices or MTC UEs) communicating small amounts of data on a relatively infrequent basis. Examples include so-called smart meters which, for example, might be located in a customer's house and periodically transmit information back to a central MTC server relating to the customer's consumption of a utility, such as gas, water, electricity and so on. Further information on characteristics of MTC-type devices can be found, for example, in the 30 corresponding standards, such as ETSI TS 122 368 V10.530 (2011-07)/3GPP TS 22.368 version 10.5.0 Release 10) [1].

Some typical characteristics of MTC type terminal devices/MTC type data might include, for example, characteristics such as low mobility, high delay tolerance, small data transmissions, a level of predictability for traffic usage and timing (i.e. traffic profile), relatively infrequent transmissions and group-based features, policing and addressing.

Unlike a conventional third or fourth generation terminal device (such as a smartphone), an MTC-type terminal is preferably relatively simple and inexpensive and able to operate with relatively low power consumption. For example, it may often be the case that an MTC-type terminal is required to operate for an extended period of time without an external source of power. However, whilst it can be convenient for an MTC-type terminal to take advantage of the wide coverage area and robust communications interface provided by third or fourth generation mobile telecommunication networks, there are aspects of these networks which are not well suited to simple and inexpensive devices. This is because such networks are generally optimised for use by devices that require high data rates and low latency. Although power usage is an important consideration for such devices, it is to some extent of secondary concern to issues of data rates and latency. The type of functions performed by a typical MTC-type terminal on the other hand (for instance collecting and reporting back data on a relatively infrequent basis) do not typically require high data rates furthermore are typically not time-critical.

The inventors have recognised a desire to allow certain types of terminal device to operate within a mobile telecommunications network with lower power consumption than other conventional terminal devices operating within the network.

It is an aim of the present disclosure to alleviate this problem.

SUMMARY

According to a first aspect, there is provided a method of operating a terminal device in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the terminal device does not communicate with the wireless telecommunications system and a second mode of operation where the terminal device does communicate with the wireless telecommunications system, the method comprising:

transitioning from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

According to a second aspect, there is provided

According to a second aspect, there is provided a method of operating a base station in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the base station does not communicate with the terminal device and a second mode of operation where the base station does communicate with the terminal device, the method comprising: transitioning from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

According to a third aspect, there is provided a terminal device for use in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the terminal device does not communicate with the wireless telecommunications system and a second mode of operation where the terminal device does communicate with the wireless telecommunications system, the terminal device comprising: a transceiver unit configured to communicate with the wireless telecommunications system and a processor unit configured to control the transceiver unit to transition from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

According to a fourth aspect, there is provided a base station for use in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the base station does not communicate with a terminal device and a second mode of operation where the base station does communicate with the terminal device, the base station comprising: a transceiver unit configured to communicate with the terminal device and a processor unit configured to control the transceiver unit to transition from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1provides a schematic diagram illustrating some basic functionality of a wireless telecommunications network/system operating in accordance with LTE principles. Various elements ofFIG. 1and their respective modes of operation are well-known and defined in the relevant standards administered by the 3GPP® body and also described in many books on the subject, for example, Holma, H. and Toskala, A. [2].

The network includes a plurality of base stations101connected to a core network102. In this regard, features that take place in the wireless telecommunications network could therefore take place in any one of the base station, core network or any other part of the network or any combination of parts of the wireless telecommunications network. Each base station provides a coverage area103(i.e. a cell) within which data can be communicated to and from terminal devices104. Data are transmitted from base stations101to terminal devices104within their respective coverage areas103via a radio downlink. Data are transmitted from terminal devices104to the base stations101via a radio uplink. The core network102routes data to and from the terminal devices104via the respective base stations101and provides functions such as authentication, mobility management, charging and so on. Terminal devices may also be referred to as mobile stations, user equipment (UE), user terminal, mobile radio, in this context MTC UE and so forth. Base stations may also be referred to as transceiver stations/nodeBs/e-NodeBs/eNBs, and so forth.

Mobile telecommunications systems such as those arranged in accordance with the 3GPP defined Long Term Evolution (LTE) architecture use an orthogonal frequency division multiplex (OFDM) based interface for the radio downlink (so-called OFDMA) and a single carrier frequency division multiplex based interface for the radio uplink (so-called SC-FDMA).FIG. 2shows a schematic diagram illustrating an OFDM based LTE downlink radio frame201. The LTE downlink radio frame is transmitted from an LTE base station (known as an enhanced Node B) and lasts 10 ms. The downlink radio frame comprises ten subframes, each subframe20lasting 1 ms. A primary synchronisation signal (PSS) and a secondary synchronisation signal (SSS) are transmitted in the first and sixth subframes of the LTE frame. A physical broadcast channel (PBCH) is transmitted in the first subframe of the LTE frame.

FIG. 3is a schematic diagram of a grid which illustrates the structure of an example conventional downlink LTE subframe (corresponding in this example to the first, i.e. left-most, subframe in the frame ofFIG. 2). The subframe comprises a predetermined number of symbols which are transmitted over a 1 ms period. Each symbol comprises a predetermined number of orthogonal sub-carriers distributed across the bandwidth of the downlink radio carrier.

The example subframe shown inFIG. 3comprises 14 symbols and 1200 sub-carriers spread 30 across a 20 MHz bandwidth. The smallest allocation of user data for transmission in LTE is a resource block comprising twelve sub-carriers transmitted over one slot (0.5 subframe). For clarity, inFIG. 3, each individual resource element (a resource element comprises a single symbol on a single subcarrier) is not shown, instead each individual box in the subframe grid corresponds to twelve sub-carriers transmitted on one symbol.

FIG. 3shows resource allocations for four LTE terminals340,341,342,343. For example, the resource allocation342for a first LTE terminal (UE1) extends over five blocks of twelve sub-carriers (i.e. 60 sub-carriers), the resource allocation343for a second LTE terminal (UE2) extends over six blocks of twelve sub-carriers and so on.

Control channel data are transmitted in a control region300(indicated by dotted-shading inFIG. 3) of the subframe comprising the first n symbols of the subframe where n can vary between one and three symbols for channel bandwidths of 3 MHz or greater and where n can vary between two and four symbols for channel bandwidths of 1.4 MHz. For the sake of providing a concrete example, the following description relates to carriers with a channel bandwidth of 3 MHz or greater so the maximum value of n will be 3. The data transmitted in the control region300includes data transmitted on the physical downlink control channel (PDCCH), the physical control format indicator channel (PCFICH) and the physical HARQ indicator channel (PHICH).

PDCCH contains control data indicating which sub-carriers on which symbols of the subframe have been allocated to specific LTE terminals. Thus, the PDCCH data transmitted in the control region300of the subframe shown inFIG. 3would indicate that UE1has been allocated the block of resources identified by reference numeral342, that UE2has been allocated the block of resources identified by reference numeral343, and so on.

PCFICH contains control data indicating the size of the control region (i.e. between one and three symbols).

PHICH contains HARQ (Hybrid Automatic Request) data indicating whether or not previously transmitted uplink data has been successfully received by the network.

Symbols in a central band310of the time-frequency resource grid are used for the transmission of information including the primary synchronisation signal (PSS), the secondary synchronisation signal (SSS) and the physical broadcast channel (PBCH). This central band310is typically 72 sub-carriers wide (corresponding to a transmission bandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals that once detected allow an LTE terminal device to achieve frame synchronisation and determine the cell identity of the enhanced Node B transmitting the downlink signal. The PBCH carries information about the cell, comprising a master information block (MIB) that includes parameters that LTE terminals use to properly access the cell. Data transmitted to individual LTE terminals on the physical downlink shared channel (PDSCH) can be transmitted in other resource elements of the subframe.

FIG. 3also shows a region of PDSCH containing system information and extending over a bandwidth of R344. A conventional LTE frame will also include reference signals which are not shown inFIG. 3in the interests of clarity.

The number of sub-carriers in an LTE channel can vary depending on the configuration of the transmission network. Typically this variation is from 72 sub carriers contained within a 1.4 MHz channel bandwidth to 1200 sub-carriers contained within a 20 MHz channel bandwidth (as schematically shown inFIG. 3). Data transmitted on the PDCCH, PCFICH and PHICH is typically distributed on the sub-carriers across the entire bandwidth of the subframe to provide for frequency diversity.

A terminal device in radio resource control (RRC) connected mode and RRC Idle mode receives and decodes PDCCH in subframes to identify if there are any transmission resource allocations (resource grants) for the terminal device in the subframe. A terminal device is thus required to receive and decode PDCCH for all subframes in which the terminal device might potentially be allocated transmission resources, even though in many of these subframes there might not be any data for the terminal device. Resources used in receiving and decoding PDCCH in subframes for which there is no data for the terminal device are in effect wasted. With this in mind, a known technique for lowering power consumption in LTE-type terminals is to restrict the number of subframes for which a terminal device should monitor PDCCH using discontinuous reception, DRX, techniques. DRX techniques involve a terminal device and a base station in effect agreeing times (e.g. particular subframes) during which the terminal device will be monitoring downlink physical channels and the base station can expect the terminal device to receive transmissions sent to it. The terminal device thus knows that outside these agreed times there are subframes when it will not receive transmissions from the base station, and the terminal device may conserve power during these subframes by not receiving and decoding PDCCH.

Thus, a DRX mode comprises alternating periods during which a terminal device could potentially receive data from the base station (and hence should monitor PDCCH) and periods during which the terminal device will not receive data (and hence need not monitor PDCCH to save power). The subframes in which the terminal device could receive data from the base station may be referred to as DRX inactive periods and the subframes in which the terminal device should not receive data from the base station may be referred to a DRX active periods.

In a conventional LTE network the timings of DRX inactive periods and DRX active periods for a given terminal device in RRC Connected mode are defined by various parameters (which may be defined in terms of numbers of subframes). There are six basic DRX parameters that define the pattern of DRX inactive and DRX active periods in LTE. These are:

(ii) On Duration Timer

(iii) DRX Short Cycle

(iv) DRX Short Cycle Timer

FIGS. 4 to 6are schematic diagrams showing how the above-identified DRX parameters are defined on a representative time axis t. (The timings in these figures are represented for clarity of explanation and are not necessarily shown to scale.)

FIG. 4schematically represents the basic underlying DRX cycle with periods when the terminal device receiver circuitry is active and monitoring PDCCH (DRX inactive) schematically represented by diagonally shaded blocks on the time axis t. This aspect of the LTE DRX mode may be referred to herein as the “normal” or “basic” DRX cycle/mode. The timings relating to this normal DRX cycle are set by the parameters DRX Cycle and On Duration Timer as schematically represented in the figure. Thus, in the normal DRX mode a terminal device activates its receiver circuitry and monitors PDCCH for a period corresponding to On Duration Timer once every DRX Cycle.

A relatively long basic DRX cycle allows for more power to be conserved. However, a long basic DRX cycle also results in increased latency because there are longer periods of time during which the terminal device is not monitoring PDCCH (and hence cannot be contacted). To address this LTE provides for two durations of DRX cycle, namely the basic/normal DRX cycle represented inFIG. 4, and a shorter DRX cycle. The short DRX cycle is broadly similar to the normal DRX cycle in overall structure in that it also comprises a regular pattern of DRX inactive and DRX active periods. However, the short DRX cycle adopts a shorter repeat period. The operation of the short DRX cycle is governed by the parameters DRX Short Cycle and DRX Short Cycle Timer. DRX Short Cycle is the repeat period for the short DRX cycle (DRX Cycle is an integer multiple of DRX Short Cycle in LTE). DRX Short Cycle timer defines the number of short DRX cycle periods before the normal DRX cycle is entered. (In LTE the On Duration Timer applies for both short and normal DRX cycles.)

Thus a terminal device which has concluded communicating with a network initially enters the short DRX cycle mode before entering the longer/normal DRX cycle mode (assuming no communications are made during the period established by DRX Short cycle Timer). The principle underlying this approach in LTE is a recognition that a terminal device is more likely to need to re-communicate with a network relatively soon after a previous communication, and so a shorter DRX cycle can be used to reduce latency for a period after a recent communication. If, however, the terminal device does not re-communicate with the base station during this period, the terminal device may then drop into the longer normal DRX cycle.

FIG. 5schematically represents some aspects of the short DRX cycle in LTE.FIG. 5is similar to, and will be understood from,FIG. 4, except the left-most DRX cycle inFIG. 4is replaced inFIG. 5with a section of short DRX cycle mode. In the example ofFIG. 5the DRX Short Cycle is one-half the normal DRX Cycle. The DRX Short Cycle Timer in this particular timing example is taken to expire at the end of the second DRX Short Cycle represented inFIG. 5such that the normal (longer) DRX cycle, as represented inFIG. 4, picks up from this point.

In summary, in the absence of any transmissions to the terminal device or uplink scheduling requests, the DRX mode comprises a number of short cycles followed by a longer DRX opportunity until the next DRX cycle begins.

However, in addition to the regular and repeating DRX inactive periods during which a terminal device monitors PDCCH as represented inFIGS. 4 and 5, LTE defines various non-repeating/irregular DRX inactive periods during which a terminal device is required to monitor PDCCH, and these are schematically represented inFIG. 6.

The upper part ofFIG. 6is a timeline representing various periods during which a terminal device receiver is active while the lower part ofFIG. 6is a corresponding timeline representing periods during which the terminal device transmitter is active.

As withFIGS. 4 and 5, the upper part ofFIG. 6uses blocks to identify times at which the terminal device is required to monitor PDCCH.

Here it is assumed for the period of time prior to that represented inFIG. 6the terminal device is in the normal DRX mode such as represented inFIG. 4, but in the left-most DRX inactive period represented inFIG. 6, the terminal device receives a downlink communication on PDSCH. This may be any conventional downlink communication.

In LTE, the receipt of a downlink communication initiates a timer during which a terminal device is required to continue monitoring PDCCH, even if the On Duration Timer associated with the normal regular and repeating DRX cycle expires. This timer is set by the DRX Inactivity Timer parameter. Thus, the DRX Inactivity Timer causes the DRX inactive period during which the terminal device must monitor PDCCH to be extended beyond the “normal” DRX inactive period if a downlink communication is received during the “normal” inactive period. This is schematically represented by the grid shading inFIG. 6for the leftmost DRX inactive period. If any further communications are received by the terminal device during the extended DRX inactive period, the DRX Inactivity Timer is reset, thereby extending the DRX inactive period further still. Only once the DRX Inactivity Timer expires can the terminal device re-enter DRX active mode.

Although the expiration of the DRX Inactivity Timer is one mechanism to allow the UE to enter the DRX opportunity, the network may also send a DRX command to allow the terminal device to enter the DRX opportunity. Specifically, the network may set the logic channel ID (LCID) in the MAC sub-header as “11110” [3],[4]. Once the UE receives the command, the UE will go to sleep.

In response to the PDSCH allocation represented in the left-most DRX inactive period in the upper part ofFIG. 6, the terminal device will, in accordance with conventional techniques, transmit uplink acknowledgement signalling (ACK/NACK signalling) for the (schematically represented in the lower part ofFIG. 6by the chequer-board shaded block). In LTE the terminal device sends its acknowledgement signalling four subframes after the subframe containing the relevant PDSCH allocation. If the terminal device is unable to properly decode the PDSCH allocation it will transmit negative acknowledgement (NACK) signalling. In response to this the base station schedules a retransmission of the information comprising the PDCCH allocation. In LTE the base station has some flexibility with regards to rescheduling the retransmission. The base station cannot reschedule the transmission before a time set by HARQ RTT Timer (e.g. eight subframes) after the initial PDSCH allocation has expired, but the base station does not need to schedule the retransmission in the subframe immediately after HARQ RTT Timer expires.

Accordingly, if a terminal device cannot properly decode a PDSCH allocation and transmits corresponding negative acknowledgement signalling, the terminal device must reactivate its receiver circuitry when HARQ RTT Timer expires in the expectation that the base station will at some stage after HARQ RTT Timer expires schedule a retransmission of the information sent in the previous PDSCH allocation. The parameter DRX Retransmission Timer specifies the amount of time the terminal device must remain active after expiry of HARQ RTT Timer to monitor PDCCH for a resource allocation for a retransmission of the earlier PDSCH allocation that was negatively acknowledged. This period of time during which the terminal device cannot remain in DRX active mode is schematically represented inFIG. 6by the block with dotted shading. Although not shown inFIG. 6for the purposes of clarity, a retransmission of a previous negatively-acknowledged PDSCH allocation may be expected to occur during the period corresponding to the DRX Retransmission Timer, and this will require the terminal device to remain in an active mode monitoring PDCCH waiting for the retransmission to be received on PDSCH or for the DRX Retransmission Timer to expire.

The additional periods during which the terminal device must monitor PDCCH under the DRX Inactivity Timer (grid shading inFIG. 6) and DRX Retransmission Timer (dot shading inFIG. 6) are over and above the regular short cycle and normal cycle DRX periods. The periods associated with the regular are repeating DRX cycles therefore remains, as indicated by the diagonal shaded blocks inFIG. 6(with the short DRX cycle mode being triggered by the PDSCH allocation).

Thus, the left-hand half ofFIG. 6represents how the repeating and regular pattern of active and inactive DRX periods ofFIGS. 4 and 5becomes disrupted when a terminal device receives downlink communications and how this result in additional periods of time during which the terminal device must monitor PDCCH.

The right-hand half ofFIG. 6represents another situation which results in a terminal device needing to monitor PDCCH outside the repeating and regular pattern of active and inactive DRX periods such as represented inFIGS. 4 and 5. This is triggered by the terminal device making a scheduling request (SR) with an uplink transmission on the physical uplink control channel (PUCCH). A terminal device will typically do this when it wishes to request uplink resources because the terminal device has data it needs to communicate to the network. The PDCCH SR is schematically represented in the lower part ofFIG. 6by the brick-shaded block.

When a terminal device transmits a SR on PUCCH it can expect to receive a response from the base station on PDSCH. In order to receive the response, the terminal device must therefore monitor PDCCH for the PDSCH allocation message. That is to say, on sending the PUCCH SR, the terminal device must exit DRX active mode. This is schematically represented inFIG. 6by the by the block with zigzag shading. Once the terminal device receives the PDSCH allocation in response to the PUCCH SR, the DRX Inactivity Timer is restarted as discussed above, and as schematically represented in the right-hand part of the upper timeline inFIG. 6.

Thus, the right-hand half ofFIG. 6represents how the repeating and regular pattern of active and inactive DRX periods ofFIGS. 4 and 5also becomes disrupted when a terminal device requests uplink resources and how this again results in additional periods of time during which the terminal device must monitor PDCCH.

The parameters DRX Cycle, On Duration Timer, DRX Short Cycle, DRX Short Cycle Timer, DRX Inactivity Timer, and DRX Retransmission Timer which define the DRX timings are shared between the base station and terminal device through RRC signalling in accordance with conventional techniques. The starting point of the DRX cycle (i.e. what might be termed its phase relative to the system frame numbering) is determined by DRX Start Offset which is communicated through RRC signalling. Thus both the terminal device and the network can determine from the system frame number the particular subframes when the terminal device receiver should be active and listening to PDCCH. This allows the base station to schedule transmissions to the terminal device at the appropriate times and the terminal device to activate its receiver circuitry to receive any such transmissions at the appropriate times.

Further information on conventional DRX operation in LTE-type networks can be found in the relevant standards. See, for example, ETSI TS 136 331 V11.3.0 (2013-04)/3GPP TS 36.331 version 11.3.0 Release 11 [5], and ETSI TS 136 321 V11.2.0 (2013-04)/3GPP TS 36.321 version 11.2.0 Release 11 [6].

The current RRC protocol and DRX operation are designed to target terminal devices with high traffic demands, small latency and high levels of mobility. The current RRC protocol and DRX operation does not therefore consider the unique properties of MTC type terminal devices. Typically, these devices have low traffic volume, infrequent and intermittent data bursts and a lack of mobility. These inherent incompatibilities of the RRC and DRX protocol design and the low energy consumption of MTC devices are to be addressed by the inventors in this disclosure.

FIG. 6describes the DRX operation of a terminal device when operating in an RRC Connected mode. However, the terminal device may operate in an RRC Idle mode. In this mode, the terminal device has already registered with the network but is not connected and there is no radio link established between it and the network. In the RRC Idle mode the terminal device monitors a paging channel to detect incoming calls, acquires system information and performs neighbouring cell measurement and cell reselection. The upper layers may configure the terminal device with a terminal device specific DRX. The DRX cycle can be from 32 to 256 radio frames and the on duration is 1-4 subframes. The terminal device controls its own mobility and its location is only known at Tracking Area level.

FIG. 7shows a timing diagram explaining the RRC state transition from the RRC Idle mode to the RRC Connected mode. The RRC state transition is explained below.

1. During the paging occasions in RRC idle mode, the terminal device wakes up to monitor PDCCH in order to search for the presence of a paging message. Once the terminal device finds the Paging Radio Network Temporary Identifier (P-RNTI) then it proceeds to decode the paging message located in PDSCH which is indicated by PDCCH.
2. After decoding the paging message, if the terminal device does not find its own terminal device identity then it returns to DRX operation in the RRC Idle mode. This is shown in the first two paging occasions inFIG. 7.
3. If the terminal device finds its identity in the message it triggers the Random Access Procedure (RAP) followed by the establishment of the RRC Connection, i.e. moving from the RRC Idle mode to the RRC Connected mode.
4. RAP: the UE sends a random access preamble to the base station and the base station confirms by sending a random access response (RAR).
5. RAP/RRC connection establishment: after the UE receives the RAR, a Layer 2/Layer 3 message is scheduled for uplink transmission to the base station on the PUSCH [7]. It conveys the RRC connection request.
6. RRC connection establishment: the base station sends the RRC connection setup message to the terminal device. The DRX operation parameters can be carried in this message.
7. RRC connection establishment: the terminal device sends back a message indicating the completion of RRC connection setup.
8. RRC connection establishment: upon reception of this message, the base station can also transmit to the terminal device the RRC messages including security mode command and RRC connection reconfiguration.
9. RRC connection establishment: the RRC state transition is complete once the terminal device sends back reconfiguration completion message. After this message exchange the RRC connection is established and the terminal device and network enter the RRC connected mode.

After the network moves the terminal device into the RRC connected mode, a terminal device inactivity timer is started immediately. It is a vendor-specific implementation choice and indicates the duration after the base station has cleared its Transmission buffer and does not detect any uplink data from the terminal device. In most LTE radio access networks (RAN), it is configured to approximately 10 s. After the RRC connection is established the UE may transmit and receive continuously or it may enter DRX mode. All the DRX operation parameters have been informed by the network via RRC setup or RRC reconfiguration message and listed in Table 1 below.

TABLE 1DRX ParameterDescriptionDRX CycleThe duration of one ‘ON time’ + one ‘OFF time’.(This value does not explicitely specified in RRCmessages. This is calculated by the subframetime and longdrx- CycleStartOffset)onDurationTimerThe duration of ‘ON time’ within one DRX cycledrx-Inactivity timerSpecify how long UE should remain ‘ON’ afterthe reception of a PDCCH. When this timeris on UE remains in ‘ON state’ which mayextend UE ON period into the period whichis ‘OFF’ period otherwise.drx-RetransmissionSpecifies the maximum number of consecutivetimerPDCCH subframes the UE should remain activeto wait an incoming retransmission after thefirst available retransmission timeshortDRX-CycleDRX cycle which can be implemented withinthe ‘OFF’ period of a long DRX Cycle.drxShortCycleTimerThe consecutive number of subframes the UE shallfollow the short DRX cycle after the DRX InactivityTimer has expired

When the network detects that the terminal device is not transmitting or receiving, the terminal device inactivity timer is activated. After the terminal device inactivity timer expires, an RRC connection release message is sent from the network and the terminal device transitions to the RRC idle mode to save radio resources and battery. This release procedure is shown inFIG. 8.

Referring toFIG. 8, after the terminal device inactivity timer expires at the base station101, a terminal device release request is sent to the Mobile Management Entity (MME)102A. The MME102A sends a Release Access Bearers request to the Serving GateWay (S-GW)102B. The S-GW102B sends a Release Access Bearers response back to the MME102A. A terminal device Context release command is sent from the MME to the base station101. An RRC connection release is established between the base station101and the terminal device104. The base station101sends a terminal device context release complete signal to the MME102A.

There are several issues with the current transition procedure between RRC Idle and RRC Connected modes and between the RRC Connected and the RRC Idle modes. Firstly, data traffic in MTC type terminal devices is far more infrequent compared with other traffic such as smartphone traffic. Typically, traffic for MTC type terminal devices may occur from once or twice a minute to once or twice a day. This means that many of the paging occasions are unnecessary and drain the battery of the terminal device.

Additionally, the MTC traffic is typically very short in length compared with other traffic. Therefore, in combination with the long period of time between consecutive occurrences of traffic for an MTC type terminal device, the transition between the RRC Connection mode and the RRC Idle mode is almost always repeated (with the corresponding signalling requirements) for a short MTC device packet.

Finally, many MTC type terminal devices, such as smartmeters and the like have little or no mobility. This is not considered in the current RRC state transition procedure.

A flow chart900describing, in general, embodiments of the present disclosure is shown inFIG. 9. The flow chart900starts at step905. A first stage is conducted in step910. Specifically, a measurement stage is conducted at step910. The measurement stage will be described with reference toFIGS. 10A, 10B and 11.

After the measurement stage is conducted, a second stage is conducted in step915. Specifically, a semi-persistent RRC state transition (SPRST) stage is carried out in step915. The SPRST stage will be described with reference toFIGS. 12A, 12B, 13A, 13B, 14, 15, 16A, 16B and 17.

The flow chart then finishes at step920.

Measurement Stage

During the measurement stage the inter-arrival times are determined from key traffic parameters. During the first stage with a measuring time duration Tms, the base station101measures the key traffic parameters based on network memory. The key traffic parameters in this case are traffic parameters that indicate a downlink transmission to the UE. Examples of this may include a paging message having the particular UE identity such as the IMSI (International Mobile Subscriber Identity). Although the above mentions the base station101performing the measurement of inter-arrival times, the disclosure is no way limited to this. For example, the core network could measure the key traffic parameters and this the inter-arrival times. In this case, the result of the measurement stage could then be passed to the base station by the core network.

The duration of Tmsis determined by the base station101. Assuming a Poisson process for the MTC type terminal device [8], the traffic parameter to be measured is, in embodiments, the mean inter-arrival time Tint. It should be noted that other measures of inter-arrival time are envisaged such as the median inter-arrival time, or any kind of inter-arrival time. As different terminal devices receive downlink transmissions with different periodicity, different terminal devices will likely have different Tint. Therefore, a common Tmsfor all terminal devices will result in different measuring occasions. In this regard, Tmsshould be terminal device-specific in the sense that a common number of measuring occasions is defined as Nmsand based on different Tint. Thus, Tmswill be different for different terminal devices. Until the base station101has determined the traffic parameters, the terminal device104follows normal procedures in DRX and RRC mode changes. In other words, the terminal device104continues reading PDCCH in each DRX cycle and only leaves the RRC Connected mode once it is released by RRC signalling.

Of course, although the above mentions the base station101as collecting the key traffic parameters and measuring the traffic parameters, the disclosure is not so limited. As noted above, for example, the core network (CN)102could collect and measure the traffic parameters for each UE. This could take place, for example, by the core network providing statistics on inter-arrival times for all the radio bearers of a UE.

Once Tintis obtained, the next step is to choose appropriate time at which the terminal device and network performs the transition from RRC idle to RRC connected mode. This is defined as Ttransand is subject to one or more (in any order) of three defined service requirements: delay tolerance, false transmission probability and signalling overhead ratio.

In SPRST mode, it is expected that the transmission endures certain level of delay because as long as the system is in RRC idle mode, the arrived packets are buffered at the base station101and waits for the RRC state transition. The delay, denoted as Td, is a random variable and the longer the Ttrans, the larger the delay is envisaged. There are three different ways to demonstrate the delay caused by the SPRST scheme: maximal delay Tmax, average delay Tave, and probability delay TPwhich indicates that Prob{Td≤TP}=P. Apparently, Tmaxis equal to Ttrans. In order to obtain Taveand TP, we need to derive the probability density function (pdf) of the random variable Td.

Assuming Poisson process with mean arrival rate λ=1/Tini, we consider the general case where N packets are assumed to arrive between time (l−1)Ttransand lTtranswhere l is a generic counter which is used to depict the interval between two occasions when the terminal device wakes up from power saving. This is diagrammatically shown inFIG. 10Athat depicts the terminal device entering power saving mode at (l−1) Ttransand wakes up again after one Ttransinterval at lTtrans. Whilst the 1stto Nth packet have been received at the base station buffer.

The n-th packet arrives at time

sn=(l-1)⁢Ttrans+∑i=1n⁢⁢xi,(1)
where xiis the inter-arrival time between packet (i−1) and i except x1. Since all the packets arrived during time (l−1)Ttrans, lTtrans) are buffered at the eNB101and sent until time lTtrans, the delay of the n-th packet is given as

For the first packet, the inter-arrival time between itself and the previous packet is z1. Clearly, z1and x2to xNfollow the exponential distribution and are independent with each other. If l=1, z1=x1; otherwise z1≥x1. However, according to [9], x1also follows the exponential distribution and is independent with x2to xN. This means that dnis irrelevant with l so that we can consider a simplified case as shown inFIG. 10Aand we have

Referring toFIG. 10B, firstly consider the case N=1, i.e. there is only one packet arrived during time (0, Ttrans). The joint density for X1and S2is
fx1s2(x1,s2)=fx1(x1)fx2(s2−x1).  (4)

The marginal density of S2can be obtained from integrating x1out from the joint density, which takes the form:
fx1s2(x1,s2)=λ2exp(−λx1)exp(−λ(s2−x1))=λ2exp(−λs2), for 0≤x1≤s2(5)

The joint density does not contain x1. Thus, for a fixed s2, the conditional density of X1given S2=s2is uniform over 0≤x1≤s2. Considering N=1, it implies that Ttrans≤s2so that the conditional density of X1is also uniform over 0≤x1≤Ttrans. It is easy to see that the delay dnalso follows the uniform distribution over [0, Ttrans]

For the more general, the same behaviour is observed here as
fs1sN+1(s1, . . . ,sN,sN+1)=λ2exp(−sN+1) for 0≤s1≤ . . . sN≤sN+1.  (6)

The interpretation here is the same as with S2. The joint density does not contain any arrival time other than sn, except for the ordering constraint 0≤s1≤ . . . ≤sN+1, and thus this joint density is constant over all choices of arrival times satisfying the ordering constraint. If any snis uniformly distributed, the delay dnis also uniformly distributed and the probability density function (pdf) and cumulative distribution function (CDF) are, respectively,

This equation reveals a very important conclusion that the distribution of the delay is solely determined by Ttransand irrelevant with respect to the mean inter-arrival time, i.e. the feature of the Poisson process.

FIG. 11shows the cumulative distribution function (CDF) of the delay for Ttrans=1, 5, 10, and 20 s for different mean arrival time (1, 5 and 10 s). The aforementioned conclusion is confirmed by the perfect match between the curves from Monte-Carlo simulation and theoretical derivation. The distribution of the delay is irrelevant with the distribution of the inter-arrival time but only determined by Ttrans. Then we have
Tave=Ttrans/2,TP=P*Ttrans.  (8)

Even though the machine type traffic is usually delay tolerant, it does not mean the machine type device can stand infinite delay and there could be an upper limit Tupfor it. This upper limit set the first constraint for the value of Ttrans. As aforementioned, there are three constraints on the delay, in order to satisfy all the constraints, Ttransare given:
Ttrans≤min{Tmax,2Tave,TP/P}.(9)
False Transition Probability Pfal

Pfalis defined as the probability that no data packet arrives during Ttransso that although the RRC transition is conducted, no data is transmitted after RRC connection is established. Thus, the energy consumed as well as the signalling exchanged is wasted. It has been established by the inventors that the shorter the value of Ttrans, the larger the possibility of no data packet arriving is. Pfalcan be obtained based on the distribution of the Poisson process as
Pfal=exp(−λTtrans).  (10)

Where λ is a parameter used to represent the expected number of events in a time interval. Given a target Pfaland assuming its corresponding Tmin, we should have
Ttrans≥−Tint*lnPfal.  (11)
Signalling Overhead Ratio

Another factor to consider is the signalling overhead ratio, i.e. signalling overhead per packet. As explained above, since the MTC traffic is normally intermittent bursty data packets, the relative amount of signalling over user plane data is usually very large because the RRC connection establishment and release requires a few hundred bytes at a time for very small amount of data to be transmitted. In this regard, it is more efficient that some packets are buffered at the base station101and transmitted to the terminal device in one occasion of RRC connection establishment and release. This is particularly relevant to MTC type data where latency is not normally critical.

Assuming that at least K packets are transmitted with one RRC transition (and thus providing a signalling overhead ratio 1/K) with a probability of PK, it can be expressed as

Then the constraint on Ttransis
Ttrans≥f−1(Pk).  (13)

Combining three constraints, we have
max{f−1(Pk),−Tint*lnPfal}≤Ttrans≤min{Tmax,2Tave,TP/P}.(14)

As an example, assuming the MTC traffic follows Poisson process with mean inter-arrival time of 30 seconds, in order to choose Ttrans, the following constraints are assumed:Maximal delay=250 s;Average delay≤100 s;99 percent of delay should be smaller than 200 s;False transition probability Pfal≤0.05;Once the RRC connection is established, the probability of transmitting at least 2 packets is more than 0.9.

Of course, other or different constraints may be provided and these may be set by the Mobile Network Operator (MNO). Based on equations (8)-(14), we have
max{f−1(0.9)=160,−Tint*ln 0.05=89.87}≤Ttrans≤min{Tmax=250,2Tave=200,TP/P=202.2}.  (15)

Thus Ttransshould be chosen in the range of [160,200].

Once the range of Ttransis found, there are two possibilities to choose Ttrans. It is possible to represent the length of Ttransitself in seconds. However, in this example (where an average value of Ttransis 180 ms), 8 bits would be required. Therefore to reduce signalling load, in embodiments of the disclosure, two options are provided that represent the length of Ttranswith fewer bits which saves signalling load:

Option 1: Let Ttrans′=(Tmin+Tmax)/2 and N=floor(Ttrans′/Δt), where Δt is the maximal DRX cycle. As would be appreciated, the maximal DRX interval is a known figure which at the time of writing is 2.56 seconds. Therefore, Ttrans=N*Δt. Then use L bits to indicate N. In other words, with option 1, Ttrans′ is the mean average of the minimum and maximum value in the range of Ttransderived from equation (15). In this case, Ttrans′ is 180. N depicts how many maximal DRX cycles would fit into Ttrans, which for a maximal DRX of 2.56 seconds, means N is 70. Therefore, using option 1, the value of 70 can be depicted with fewer bits than the value of 180.

Option 2: Let S={1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096} (i.e. a sequence of numbers that are powers of two, 2N) and N′=floor(Ttrans′/Δt). N can only be chosen from S and N=argmin |N′−N|. Ttrans=N*Δt. In this case, N′ is an intermediate value and in this option, the value of N′ is also 70. Instead of signalling N′, however, a value from the list S that is closest to N′ is selected as shown by N=argmin|N′−N| which equals the value of N that minimises |N′−N|. As will be appreciated, the values of S are all binary maximum numbers. In other words, select a value from the list S that is closest to N′ and in this example, N would be 64. With option 2, N=64 can be depicted with fewer bits than either depicting the length of Ttransitself or option 1.

Both option 1 and option 2 have advantages. The advantage of the first option is the accuracy of Ttransand the advantage of the second option is that less bits are used to indicate N compared with the first option.

Mechanism to Determine Ttransin the Measurement Stage

In order to determine Ttrans, the following steps are carried out:

Step 1—For the intended terminal device, the base station101stores the IMSI of the terminal device and sets the measuring time Tmsand measuring time counter, nmsto 0.

Step 2—On the nms-th measuring occasion, once the base station101observes a paging message sent to the terminal device, the base station101saves the inter-arrival time Tn,msin association with the IMSI of the terminal device. The value nmsis set as nms=nms+1. If nms<Nms, then step 2 is repeated. If not, step 3 is commenced.

Step 3—For the terminal device,

Step 4—Using equation 14 and the assumed constraints, the range of Ttransis determined.

Step 5—Use option 1 or 2 to determine Ttrans=N*Δt and the binary expression of N.

As would be appreciated, the base station101must know whether a particular terminal device104that is being paged is under its cell. This is to ensure that the base station101can connect the paging message frequency (to a particular IMSI) with RRC signalling to the terminal device104in the cell. This allows the network to move to the SPRST stage. Due to the low mobility (or static) nature of MTC type terminal devices, it is unlikely that the terminal device will be associated with a different base station during the whole Ttransmeasurement stage.

However, in the event that the terminal device104does move to operate under a different101, the previous base station101can send the value of Ttrans(if calculated) or the values of nmsand measuring time Tmswith the handover message to the different base station. Alternatively, when moving from communicating with one base station to another base station, the terminal device may be released from operating in the SPRST stage and wait until the network re-instructs the terminal device to operate in the SPRST stage again.

As a further alternative, if the core network (or RAN) made the appropriate measurements, the core network (or RAN) could pass the value of Ttransto the new base station.

After the measurement stage has taken place and the value of Ttranshas been calculated, the terminal device104enters the SPRST stage where the terminal device104enters a state with timers guiding its transfer between the RRC Idle mode and the RRC Connected mode.

As an outline, the SPRST stage can be described as follows:

The duration of the SPRST stage is determined by a timer Tmodeat the base station101. Once the measurement stage is complete, the base station101will instruct the terminal device104to enter the SPRST mode by sending a positive flag and the RRC transition timer Ttransto the terminal device and the timer Tmodestarts. Once Tmodeexpires, the base station101sends a negative flag to notify the terminal device104to go back to normal RRC operation.

In the SPRST mode, the RRC transition timer Ttransis held at both the base station101and the terminal device. The timer at the base station starts at Tsprst=0 and the timer at the terminal device starts at Tsprst=Δ, where Δ is determined by the propagation and processing delays (caused by decoding the PDSCH (or msg2) that carries Ttransand the resetting of the RRC transition timer. This delay is negligible for the purposes of managing the simultaneous entry to and exit from SPRST mode for the base station and the terminal device. If a packet arrives at the base station101from the MME when Ttranshas not expired, the packet is buffered at the base station101. At the terminal device side, the terminal device104keeps silent without being activated to checking paging message periodically when Ttranshas not expired. Once Ttransexpires, the terminal device104starts the random access procedure (RAP) by sending the random access (RA) preamble to the base station101followed by a RRC establishment procedure and Ttransis reset immediately. In other words, every time Ttransexpires, the network automatically performs RRC transition from the RRC idle mode to the RRC connected mode, i.e. the RRC transition happens periodically depending on Ttransbut not on the paging occasions. Thus the energy consumed by periodically checking paging message in the RRC idle mode is saved.

FIG. 12Ashows a flow chart explaining the process for the SPRST stage from the RAN side andFIG. 12Bshows a flow chart explaining the process for the SPRST stage from the terminal device side.

Referring toFIG. 12A, the flow chart1200starts at step1202. In step1204, a time indicating the length of time of the SPRST is determined. This time is deemed Tmode. Tmodeis the duration of time that the Semi Persistent RRC State Transition (SPRST) is in force. It is a parameter decided by the network when it decides how long to allow a terminal device to stay in a mode where it cannot be reached by the base station apart from every Ttranswhen the terminal device wakes up. At the beginning of the process, the base station sets a timer with starting value of Tmode, and when that timer expires the base station commands the terminal device to return to normal RRC Connected mode with only DRX cycle to provide it power savings.

In step1206, the RAN (via the base station101) sends a positive flag to the terminal device104instructing the terminal device104to enter the SPRST mode. This flag may be sent over the PDSCH. Of course, the flag may be sent in any appropriate packet or may be sent as a separate packet, but in embodiments, the flag may be sent in either the Random Access Response (msg2) from the base station or in the RRC Connection Setup Message. In the RRC Connection Setup Message, there is room for non-critical extensions which allow adding of further bit fields into the fields of that message. The base station101starts timer TSPRSTat 0 when the base station101enters the SPRST mode (the SPRST mode expiring when the value in timer TSPRSTequals Tmode). Of course, the timer TSPRSTmay start at Tmodeand count down to 0.

The value of Tmodeis the duration of the SPRST mode. This is a parameter value that is fed into the timer TSPRST.

In step1208, the RAN (via the base station101) sends the calculated Ttransvalue to the terminal device. This value is sent during the RRC: Connection setup message, which is carried on PDSCH, signalling between the base station101and the terminal device104.

During the period Ttransthe terminal device104does not check the paging message. Any packets which are destined for the terminal device104during the period Ttransare stored at the base station101. In step1210, the RAN and the terminal device104establish RRC Connected mode at the expiration of the time Ttrans. As the RAN and terminal device104establish the RRC Connected mode, the packets stored at the base station101are transmitted to the terminal device104after the RRC Connected mode has been established.

If, during the period of Ttrans, an adjustment needs to be made to the value of Ttransin subsequent Ttransperiods, a new value of Ttransmay be included in the packets stored at the base station101. When the packets are then sent to the terminal device104during the RRC_Connected mode set at the expiration of the Ttransperiod, the terminal device104will receive the updated Ttransand will use the new value of Ttranssubsequently. This is step1212inFIG. 12A.

The value of Ttransmay be updated in response to a change in the value of Ttransdetermined during the measurement stage. In other words, although a value of Ttransis set initially during the measurement stage, the measurement stage does not end when a value of Ttransis determined. In fact, the measurement stage continues whenever the terminal device104is operating in the SPRST mode. Therefore, the value of Ttransmay be periodically updated.

Although the calculated value of Ttransmay vary during the measurement stage, the terminal device and RAN may only operate on a new value of Ttranswhen the measured value of Ttransvaries from the operational value of Ttransby a predetermined amount such as 2% or by some other threshold set either by the Standard or by an MNO. Additionally, or alternatively, the value of Ttransmay be changed in dependence on the stored packets. In this case, if when the terminal device104and the base station101operate using the RRC_Connected mode at the expiration of Ttransthere are no stored packets, then the value of Ttransmay be too short. This is especially the case if there are no stored packets for consecutive expired periods of Ttrans. This is because at the expiry of Ttrans, the terminal device104downloads the stored packets which are destined for the terminal device104. Therefore, if there are no stored packets, this means no packets are destined for the terminal device104during this period and so the period is too short.

Conversely, if data packets which would have been sent to the terminal device104on two or more different occasions during a single Ttransperiod are stored, then the Ttransmay be too long. In this instance, however, if the data packets have a high level of delay tolerance (i.e. are not time critical data packets), then having data packets that would have been sent to the terminal device104on two or more different occasions during one Ttransperiod may not be problematic.

It is desirable to have a suitable value of Ttransbecause a Ttransthat is too short involves unnecessary RRC_Connected mode transitions which is wasteful of power and having a Ttransvalue that is too long can increase latency of the data packets which is unsuitable for time critical packets.

After the expiration of Tmodethe RAN (via the base station101) informs the terminal device104of the end of the SPRST mode. This is achieved by the RAN sending a negative flag (via the base station101) to the terminal device104to notify the terminal device104to return to normal RRC operation. This is step1214but will be explained in more detail later.

The process ends at step1216.

Referring toFIG. 12B, the flow chart1250starts at step1252. After the terminal device104receives the positive flag from the base station101(sent in step1206ofFIG. 12A), the terminal device104starts operating in the SPSRT mode. This is step1254. As noted above, the timer TSPRSTstarts at 0 in the base station101. However, to ensure that the base station101and the terminal device104are synchronised, the TSPRSTtimer at the terminal devices starts as soon as it receives and decodes the message carrying Ttransfrom the base station. In practice, the UE timer starts at Δ, where Δ is determined by the propagation and processing delays as explained above. This delay is negligible for the purposes of synchronous operation over SPRST mode between the base station and the terminal device.

In step1256the terminal device104remains silent in the RRC Idle mode and is not activated to check paging messages periodically when the value of Ttranshas not expired. In step1258, timer Ttransexpires and the terminal device104starts the random access procedure by sending the random access preamble to the base station101followed by an RRC establishment procedure. This transitions the terminal device104from the RRC Idle mode to the RRC Connected mode. The value of Ttransin the timer is reset and the stored data packets are then communicated over the air between the base station101and the terminal device104.

After expiration of time Tmode, the terminal device104receives the negative flag from the base station101(step1214ofFIG. 12A) and the terminal device104leaves the SPSRT mode in step1260.

The process ends in step1262.

In step1206ofFIG. 12Aand step1254ofFIG. 12B, it is noted that the base station101and the terminal device104must enter the SPRST mode. Two alternative mechanisms for the base station101and terminal device104to enter the SPRST mode is described with reference toFIGS. 13A and 13B.

Alternative1is shown inFIG. 13Awhere a terminal device104communicates with a base station101.

In step1302, normal DRX is conducted at terminal device side and the terminal device104is activated to check paging message every TDPX_idleseconds, where TDPX_idleis the DRX cycle in RRC_idle mode.

In paging occasion1303, the base station101receives a paging message from MME (via the base station104) and sends the paging message to the terminal device104and with the paging message, a positive flag (1 bit) indicating the start of SPRST mode is sent as well. This is step1305and starts Tmode.

In step1307A the terminal device104sends RA preamble and in step1307B, the base station101sends back RAR.

In step1309A, the terminal device104sends RRC connection request and in step1309B, the base station101sends back the RRC connection setup message. Within the message, the DRX timers and the chosen value of Ttransare included.

In step1313the terminal device104sends back RRC connection complete message to the base station101.

In step1315, the base station101sends security mode and RRC reconfiguration message and the terminal device104responds.

The terminal device104and the base station101are now operating in RRC Connected mode.

Alternative2is shown inFIG. 13Bwhere a terminal device104communicates with a base station101.

In step1352, normal DRX is conducted at the base station side and the terminal device104is activated to check paging message every TDRX_idleseconds, where TDRX_idleis the DRX cycle in RRC idle mode.

In paging occasion1353, the base station101receives a paging message from MME (via the base station104) and sends the paging message to the terminal device104.

In step1357A the terminal device104sends RA preamble and in step1357B, the base station101sends back RAR.

In step1359A, the terminal device104sends RRC connection request and in step1359B, the base station101sends back the RRC connection setup message. Within the message, a positive flag (1 bit) indicating the start of SPRST mode is sent as well as the DRX timers and the chosen value of Ttransare included. This starts Tmode.

In step1363the terminal device104sends back RRC connection complete message to the base station101.

In step1365, the base station101sends security mode and RRC reconfiguration message and the terminal device104responds.

The terminal device104and the base station101are now operating in RRC Connected mode.

Once the terminal device104enters the RRC Connected mode, the terminal device104starts to transmit and receive data. In existing LTE systems, a terminal device inactivity timer is required to instruct the terminal device104to return to the RRC Idle mode. However, in embodiments of this disclosure, the terminal device inactivity timer is not required. Once transmission is completed, the base station101sends a RRC release message and the terminal device104enters the DRX mode and waits for the RRC release message. Without the terminal device activity timer, the terminal device104is expected to stay in DRX mode of the RRC Connected mode for a very short time before returning to the RRC Idle mode. The proposed RRC release procedure is shown inFIG. 14.

Referring toFIG. 14, a flow chart1400of the release mechanism is shown. In step1402, the base station101sends the RRC release message to the terminal device104. The terminal device104then transits to the RRC Idle mode as instructed by the base station101.

After the first RRC establish and release procedure explained above in relation toFIGS. 13A, 13B and 14, the terminal device104will not be activated periodically to check the paging information because the data transmission for a given terminal device104only happens at a given time when Ttransexpires. Therefore, the RRC connection establishment procedure is illustrated asFIG. 15.

Referring toFIG. 15, a flow chart1500describing the RRC connection establishment procedure is shown. As the terminal device104is operating in the SPRST mode, the terminal device104remains silent and does not check the paging message1502. Therefore, if the base station101receives packets from the MME before Ttransexpires, the packet goes to the buffer in the base station101for storage therein. Of course, the packets may be stored at the CN or RAN level if the CN or RAN control the SPRST mode. The terminal device104remains silent.

Once Ttransexpires at the terminal device104, the terminal device104sends the RA preamble and the base station101sends back the RAR. This is step1504.

The terminal device104sends the RRC connection request and the base station101sends back the RRC connection setup message. This is step1506. Within the message, Ttranscan be included.

The terminal device104sends back the RRC connection complete message to the base station101in step1508.

The base station101sends security mode and RRC reconfiguration message to the terminal device104in step1510. Ttransmay also be transmitted here.

In step1512, the terminal device104sends responses to the base station101. In order to release the RRC connection, the same process as described with reference toFIG. 14is followed.

Once Tmodeexpires at the base station101, the base station101knows the SPRST mode should be switched back to normal RRC procedure. In order to achieve this, the base station101notifies the terminal device104to do so by sending a negative flag. Similar to the first RRC connection establishment procedure, there are two options: the negative flag may be carried by RAR response or may be carried in the RRC connection setup request message. The last RRC connection establishment procedure is shown inFIG. 16AandFIG. 16B. Specifically, the first option is shown inFIG. 16Aand the second option is shown inFIG. 16B.

Referring toFIG. 16A, a flow chart1600describing the first option for the last RRC connection establishment procedure is shown. As the terminal device104is operating in the SPRST mode, the terminal device104remains silent and does not check the paging message1602. Therefore, if the base station101receives packets from the MME before Ttransexpires, the packet goes to the buffer in the base station101for storage therein. The MTC UE remains silent.

Once Ttransexpires at the terminal device104, the terminal device104sends the RA preamble and the base station101sends back the RAR. In the RAR, however, in this case a 1 bit negative flag is included that indicates that this is the last RRC connection establishment procedure. This is step1604.

The terminal device104sends the RRC connection request and the base station101sends back the RRC connection setup message. This is step1606. Within the message, Ttranscan be included.

The terminal device104sends back the RRC connection complete message to the base station101in step1608.

The base station101sends security mode and RRC reconfiguration message to the terminal device104in step1610. Ttransmay also be transmitted here.

In step1612, the terminal device104sends responses to the base station101.

Referring toFIG. 16B, a flow chart1650describing the second option for the last RRC connection establishment procedure is shown. As the terminal device104is operating in the SPRST mode, the terminal device104remains silent and does not check the paging message1652. Therefore, if the base station101receives packets from the MME102before Ttransexpires, the packet goes to the buffer in the base station101for storage therein. The terminal device104remains silent.

Once Ttransexpires at the terminal device104, the terminal device104sends the RA preamble and the base station101sends back the RAR. This is step1654.

The terminal device104sends the RRC connection request and the base station101sends back the RRC connection setup message. This is step1656. Within the RRC connection setup message, Ttranscan be included. Further, within the RRC connection setup message, a 1 bit negative flag is included that indicates that this is the last RRC connection establishment procedure.

The terminal device104sends back the RRC connection complete message to the base station101in step1658.

The base station101sends security mode and RRC reconfiguration message to the terminal device104in step1660. Ttransmay also be transmitted here.

In step1662, the terminal device104sends responses to the base station101.

After execution of the steps shown in eitherFIG. 16A or 16B, the terminal device104will operate in the RRC Connected mode. In order to transition from the RRC Connected mode to the RRC Idle mode, a release message is sent from the base station101to the terminal device104. This is shown inFIG. 17.

FIG. 17shows a flowchart1700explaining the issuance of a release message from the base station101to the terminal device104. The base station101sends a release message to the terminal device104in step1702. In response, the terminal device104transitions to the RRC Idle mode in step1704. The terminal device104then enters the DRX state and checks the paging message every TDPX_Idleseconds in step1706.

With the above describe SPRST procedure, monitoring the paging occasions in the RRC idle mode is not required because for each terminal device104, the RRC transition time is fixed and known to itself. The terminal device104can just wake up at given time to receive the data directed to it.

Since the base station101(in embodiments) acts as the main node controlling the SPRST scheme and determining the parameters, it can perform the measurement or estimation (based on CN information) of traffic simultaneously and adaptively adjust the parameters based on the measurement results (or information provided by CN) to achieve a subtle balance between latency and energy consumption. Of course, similar comments apply if the CN or RAN act as the controlling node for the SPRST scheme as is envisaged.

Further, it is noted that conventionally most data traffic pattern information is not relevant as conventional terminal devices react to human interaction. In other words, conventionally, a terminal device will become operational when a user interacts with the terminal device or when a notification (such as a call or text message) is sent from another user to the terminal device. Thus, past data traffic patterns in a conventional sense provide no real indication of future data traffic patterns. However, the inventors have departed from this convention and have set the transition from an idle state to a connected state based on data traffic patterns, thus cutting down on signalling and so saving energy.

If large latency is allowed, the packets can be aggregated and sent in one RRC Connected opportunity to reduce the overall RRC signalling load.

DRX is basically a MAC layer operation targeting to the terminal devices with high traffic demand and normal mobility. Other optimization schemes, such as baseband procedure optimization and power amplifier optimization, focus on physical layer and hardware. The proposed scheme happens in the RRC layer.

FIG. 18schematically shows a telecommunications system500according to an embodiment of the present disclosure. The telecommunications system500in this example is based broadly around an LTE-type architecture. As such many aspects of the operation of the telecommunications system500are known and understood and are not described here in detail in the interest of brevity. Operational aspects of the telecommunications system500which are not specifically described herein may be implemented in accordance with any known techniques, for example according to the current LTE-standards.

The telecommunications system500comprises a core network part (evolved packet core)502coupled to a radio network part. The radio network part comprises a base station (evolved-nodeB)504coupled to a plurality of terminal devices. In this example, two terminal devices are shown, namely a first terminal device506and a second terminal device508. It will of course be appreciated that in practice the radio network part may comprise a plurality of base stations30serving a larger number of terminal devices across various communication cells. However, only a single base station and two terminal devices are shown inFIG. 18in the interests of simplicity.

As with a conventional mobile radio network, the terminal devices506,508are arranged to communicate data to and from the base station (transceiver station)504. The base station is in 35 turn communicatively connected to a serving gateway, S-GW, (not shown) in the core network part which is arranged to perform routing and management of mobile communications services to the terminal devices in the telecommunications system500via the base station504. In order to maintain mobility management and connectivity, the core network part502also includes a mobility management entity (not shown) which manages the enhanced packet service, EPS, connections with the terminal devices506,508operating in the communications system based on subscriber information stored in a home subscriber server, HSS. Other network components in the core network (also not shown for simplicity) include a policy charging and resource function, PCRF, and a packet data network gateway, PDN-GW, which provides a connection from the core network part502to an external packet data network, for example the Internet. As noted above, the operation of the various elements of the communications system500shown inFIG. 7may be broadly conventional apart from where modified to provide functionality in accordance with embodiments of the present disclosure as discussed herein.

In this example, it is assumed the first terminal device506is a conventional smartphone type terminal device communicating with the base station504in a conventional manner. This conventional terminal device506comprises a transceiver unit506afor transmission and reception of wireless signals and a processor unit506bconfigured to control the device506. The processor unit506bmay comprise a processor unit which is suitably configured/programmed to provide the desired functionality using conventional programming/configuration techniques for equipment in wireless telecommunications systems. The transceiver unit506a20and the processor unit506bare schematically shown inFIG. 7as separate elements.

However, it will be appreciated that the functionality of these units can be provided in various different ways, for example using a single suitably programmed general purpose computer, or suitably configured application-specific integrated circuit(s)/circuitry. As will be appreciated the conventional terminal device506will in general comprise various other elements associated with its operating functionality.

In this example, it is assumed the second terminal device508is a machine-type communication (MTC) terminal device504adapted to support operation in accordance with embodiments of the present disclosure when communicating with the base station504. As discussed above, machine-type communication terminal devices can in some cases be typically characterised as semi-autonomous or autonomous wireless communication devices communicating small amounts of data. Examples include so-called smart meters which, for example, may be located in a customer's house and periodically transmit information back to a central MTC server data relating to the customer's consumption of a utility such as gas, water, electricity and so on. MTC devices may in some respects be seen as devices which can be supported by relatively low bandwidth communication channels having relatively low quality of service (QoS), for example in terms of latency. It is assumed here the MTC terminal device508inFIG. 18is such a device.

The MTC device508comprises a transceiver unit508afor transmission and reception of wireless signals and a processor unit508bconfigured to control the MTC device508. The processor unit508bmay comprise various sub-units, for example a DRX control unit, for providing functionality in accordance with embodiments of the present disclosure as explained further herein. These sub-units may be implemented as discrete hardware elements or as appropriately configured functions of the processor unit. Thus the processor unit508bmay comprise a processor unit which is suitably configured/programmed to provide the desired functionality described herein using conventional programming/configuration techniques for equipment in wireless telecommunications systems. The transceiver unit508aand the processor unit508bare schematically shown inFIG. 18as separate elements for ease of representation. However, it will be appreciated that the functionality of these units can be provided in various different ways, for example using a single suitably programmed general purpose computer, or suitably configured application-specific integrated circuit(s)/circuitry. It will be appreciated the MTC device508will in general comprise various other elements associated with its operating functionality.

The base station504comprises a transceiver unit504afor transmission and reception of wireless signals and a processor unit504bconfigured to control the base station504to operate in accordance with embodiments of the present disclosure as described herein. The processor unit506bmay again comprise various sub-units, such as a scheduling unit, for providing functionality in accordance with embodiments of the present disclosure as explained further below. These sub-units may be implemented as discrete hardware elements or as appropriately configured functions of the processor unit. Thus, the processor unit504bmay comprise a processor unit which is suitably configured/programmed to provide the desired functionality described herein using conventional programming/configuration techniques for equipment in wireless telecommunications systems. The transceiver unit504aand the processor unit504bare schematically shown inFIG. 18as separate elements for ease of representation. However, it will be appreciated that the functionality of these units can be provided in various different ways, for example using a single suitably programmed general purpose computer, or suitably configured application-specific integrated circuit(s)/circuitry. It will be appreciated the base station504will in general comprise various other elements associated with its operating functionality.

Thus, the base station504is configured to communicate data with both the conventional terminal device506and the terminal device508according to an embodiment of the disclosure over respective communication links510,512. The base station504is configured to communicate with the conventional terminal device506over the associated radio communication link510following the established principles of LTE-based communications, and in particular using conventional DRX and RRC procedures. However, communications between the base station504and the MTC terminal device508operate using modified DRX and RRC procedures in accordance with certain embodiments of the present disclosure as described herein. Thus, one aspect of certain embodiments of the disclosure is that the base station is configured to operate by communicating with different classes of terminal device (e.g. a first class of terminal device, for example comprising conventional LTE terminal devices, such as smartphones, and a second class of terminal device, for example comprising MTC-type terminal devices) using different discontinuous reception procedures/modes and using the SPRST mode. That is to say, a base station may operate to communicate with a first class (group/type) of terminal device in accordance with a first DRX mode associated first DRX mode timings and to communicate with a second class (group/type) of terminal device in accordance with a second DRX and RRC mode associated second DRX and RRC mode timings, the rules governing the DRX mode timings of the second DRX and RRC modes being different from those of the first DRX and RRC modes. Whether or not a particular terminal device or base station supports modified DRX procedures in accordance with embodiments of the present disclosure may be established in accordance with conventional techniques for sharing terminal device and base station capability information in wireless telecommunications network, for example based on signalling exchange during a RRC connection establishment procedure.

Some respective features of the present disclosure are defined by the following numbered paragraphs.

1. A method of operating a terminal device in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the terminal device does not communicate with the wireless telecommunications system and a second mode of operation where the terminal device does communicate with the wireless telecommunications system, the method comprising:transitioning from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

2. A method according to paragraph 1, comprising storing in the wireless telecommunications system, during the first mode of operation, data packets destined for the terminal device; and receiving from the wireless telecommunication system those stored data packets during the second mode of operation.

3. A method according to paragraph 2 wherein the data packets are stored in one of a base station, a network core or a radio access unit of the wireless telecommunications system.

4. A method according to any one of paragraphs 1 to 3 wherein, prior to entering the mode transition state, the time period is defined in the wireless telecommunications system by a method comprising the steps of:determining a number of measuring occasions at which a paging message is sent from the wireless telecommunication system to the terminal device, the paging message being sent to indicate a data packet to send to the terminal device;measuring, at each measuring occasion, the inter-arrival time of the data packet; anddetermining a range of time period values based on the inter-arrival time over the measuring occasions.

5. A method according to paragraph 4, wherein the range for the time period is defined in accordance with a delay tolerance in communicating a data packet between the terminal device and the wireless telecommunications system such that the range of the time period complies with the constraint
Ttrans≤min{Tmax,2Tave,TP/P}
Where Ttransis the time period, Tmaxis the maximum delay allowed in communicating the data packet, Taveis the average delay in communicating the data packet, and TPis an upper bound with probability P for the delay in communicating the data packet.

6. A method according to paragraph 4 or 5, wherein the range for the time period is defined in accordance with a probability that no data packet arrives during the first mode of operation such that the range of the time period complies with the constraint
Ttrans≥−Tint*lnPfal
Ttransis the time period, Tintis the inter-arrival time and Pfalis the probability that no data packet arrives during the first mode of operation.

7. A method according to paragraph 4, 5 or 6, wherein the range for the time period is defined in accordance with a ratio of signalling per data packet that complies with the constraint
Ttrans≥f−1(Pk)
Where Ttransis the time period, and

PK=1-∑k=0K-1⁢⁢exp⁡(-λ⁢⁢Ttrans)⁢(λ⁢⁢Ttrans)kk!=f⁡(Ttrans);
where K is the minimum number of packets transmitted during the second mode of operation, and λ represents the expected number of events in a time interval in a Poisson process.

8. A method according to any one of paragraphs 4 to 7, comprising receiving a representation of a selected value for the time period, the representation being the closest integer number of maximal DRX durations in the selected value of the time period.

9. A method according to any one of paragraphs 4 to 7, comprising receiving a representation of a selected value for the time period, the representation being selected from a sequence of numbers that are powers of two, wherein the selection is closest to the integer number of maximal DRX durations in the selected value of the time period.

10. A method according to any preceding paragraph wherein the time period is calculated within one of the base station, a network core or a radio access unit.

11. A method according to any preceding paragraph, prior to operating in the mode transition state, further comprising the steps of:receiving from the wireless telecommunications system a flag and the time period value; and in response to the flag, the method further comprises entering the mode transition state and operating in the second mode of the mode transition state.

12. A method according to paragraph 11, wherein the system flag and the time period value is sent with a paging message from the wireless telecommunication network.

13. A method according to paragraph 11, comprising the steps of: receiving a paging message from the wireless telecommunication network; sending a random access, RA, message to the wireless telecommunication network; receiving an RA response message from the wireless telecommunication network; sending a radio allocation control, RRC, connection request message and receiving an RRC setup message from the wireless telecommunication network, wherein the RRC setup message includes the system flag and the time period value.

14. A method according to any one of paragraphs 11, 12 or 13 comprising the steps of receiving, from the wireless telecommunication network, an RRC release message and in response to the RRC release message, the method comprises transitioning to the first mode of operation.

15. A method according to any one of paragraphs 11 to 14, wherein when the terminal device is operating in the second mode of the mode transition state, the method further comprises receiving, from the wireless telecommunications network, a second flag indicating that in response to the next RRC release message, the terminal device will leave the mode transition state.

16. A method according to paragraph 15 comprising sending a random access message and receiving from the wireless telecommunication system a random access response message that includes the second flag.

17. A method according to paragraph 15 comprising sending a random access message; receiving from the wireless telecommunication system a random access response message; sending a radio allocation control, RRC, connection request message and receiving from the wireless telecommunications system an RRC connection setup message that includes the second flag.

18. A method of operating a base station in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the base station does not communicate with the terminal device and a second mode of operation where the base station does communicate with the terminal device, the method comprising:transitioning from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

19. A method according to paragraph 18, comprising storing in the wireless telecommunications system, during the first mode of operation, data packets destined for the terminal device; and transmitting to the terminal device those stored data packets during the second mode of operation.

20. A method according to paragraph 19 wherein the data packets are stored in one of a base station, a network core or a radio access unit of the wireless telecommunications system.

21. A method according to any one of paragraphs 18 to 20 wherein, prior to entering the mode transition state, the time period is defined in the wireless telecommunications system by a method comprising the steps of:determining a number of measuring occasions at which a paging message is sent from the wireless telecommunication system to the terminal device, the paging message being sent to indicate a data packet to send to the terminal device;measuring, at each measuring occasion, the inter-arrival time of the data packet; anddetermining a range of time period values based on the inter-arrival time over the measuring occasions.

22. A method according to paragraph 22, wherein the range for the time period is defined in accordance with a delay tolerance in communicating a data packet between the terminal device and the wireless telecommunications system such that the range of the time period complies with the constraint
Ttrans≤min{Tmax,2Tave,TP/P}
Where Ttransis the time period, Tmaxis the maximum delay allowed in communicating the data packet, Taveis the average delay in communicating the data packet, and TPis an upper bound with probability P for the delay in communicating the data packet.

23. A method according to paragraph 21 or 22, wherein the range for the time period is defined in accordance with a probability that no data packet arrives during the first mode of operation such that the range of the time period complies with the constraint
Ttrans≥−Tint*lnPfal
Ttransis the time period, Tintis the inter-arrival time and Pfalis the probability that no data packet arrives during the first mode of operation.

24. A method according to paragraph 21, 22 or 23, wherein the range for the time period is defined in accordance with a ratio of signalling per data packet that complies with the constraint
Ttrans≥f−1(Pk)
Where Ttransis the time period, and

PK=1-∑k=0K-1⁢⁢exp⁡(-λ⁢⁢Ttrans)⁢(λ⁢⁢Ttrans)kk!=f⁡(Ttrans);
where K is the minimum number of packets transmitted during the second mode of operation, and λ represents the expected number of events in a time interval in a Poisson process.

25. A method according to any one of paragraphs 22 to 24, comprising transmitting a representation of a selected value for the time period, the representation being the closest integer number of maximal DRX durations in the selected value of the time period.

26. A method according to any one of paragraphs 22 to 24, comprising transmitting a representation of a selected value for the time period, the representation being selected from a sequence of numbers that are powers of two, wherein the selection is closest to the integer number of maximal DRX durations in the selected value of the time period.

27. A method according to any one of paragraphs 18 to 26 wherein the time period is calculated within one of the base station, a network core or a radio access unit.

28. A method according to any preceding paragraph, prior to operating in the mode transition state, further comprising the steps of:transmitting to the terminal device a flag and the time period value; and in response to transmitting the flag, the method further comprises entering the mode transition state and operating in the second mode of the mode transition state.

29. A method according to paragraph 28, wherein the system flag and the time period value is sent with a paging message from the wireless telecommunication network.

30. A method according to paragraph 28, comprising the steps of: transmitting a paging message to the terminal device; receiving a random access, RA, message from the terminal device; transmitting an RA response message to the terminal device; receiving a radio allocation control, RRC, connection request message and transmitting an RRC setup message to the terminal device, wherein the RRC setup message includes the system flag and the time period value.

31. A method according to any one of paragraphs 28, 29 or 30 comprising the steps of transmitting, to the terminal device, an RRC release message and in response to the transmission of the RRC release message, the method comprises transitioning to the first mode of operation.

32. A method according to any one of paragraphs 28 to 31, wherein when the base station is operating in the second mode of the mode transition state, the method further comprises transmitting, to the terminal device, a second flag indicating that in response to the next RRC release message, the base station will leave the mode transition state.

33. A method according to paragraph 32 comprising receiving a random access message and transmitting to the terminal device a random access response message that includes the second flag.

34. A method according to paragraph 33 comprising receiving a random access message; transmitting to the terminal device a random access response message; receiving a radio allocation control, RRC, connection request message and transmitting to the terminal device an RRC connection setup message that includes the second flag.

35. A terminal device for use in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the terminal device does not communicate with the wireless telecommunications system and a second mode of operation where the terminal device does communicate with the wireless telecommunications system, the terminal device comprising:a transceiver unit configured to communicate with the wireless telecommunications system and a processor unit configured to control the transceiver unit to transition from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

36. A device according to paragraph 35, wherein during the first mode of operation, data packets destined for the terminal device are stored within the wireless telecommunications system and the transceiver unit is configured to receive the stored data packets during the second mode of operation.

37. A device according to paragraph 36 wherein the data packets are stored in one of a base station, a network core or a radio access unit of the wireless telecommunications system.

38. A device according to any one of paragraphs 35 to 37 wherein, prior to entering the mode transition state, the time period using which the processor unit is configured to control the transceiver unit is defined in the wireless telecommunications system by:determining a number of measuring occasions at which a paging message is sent from the wireless telecommunication system to the terminal device, the paging message being sent to indicate a data packet to send to the terminal device;measuring, at each measuring occasion, the inter-arrival time of the data packet; anddetermining a range of time period values based on the inter-arrival time over the measuring occasions.

39. A device according to paragraph 38, wherein the range for the time period is defined in accordance with a delay tolerance in communicating a data packet between the terminal device and the wireless telecommunications system such that the range of the time period complies with the constraint
Ttrans≤min{Tmax,2Tave,TP/P}
Where Ttransis the time period, Tmaxis the maximum delay allowed in communicating the data packet, Taveis the average delay in communicating the data packet, and TPis an upper bound with probability P for the delay in communicating the data packet.

40. A device according to paragraph 38 or 39, wherein the range for the time period is defined in accordance with a probability that no data packet arrives during the first mode of operation such that the range of the time period complies with the constraint
Ttrans≥−Tint*lnPfal
Ttransis the time period, Tintis the inter-arrival time and Pfalis the probability that no data packet arrives during the first mode of operation.

41. A device according to paragraph 38, 39 or 40, wherein the range for the time period is defined in accordance with a ratio of signalling per data packet that complies with the constraint
Ttrans≥f−1(Pk)
Where Ttransis the time period, and

PK=1-∑k=0K-1⁢⁢exp⁡(-λ⁢⁢Ttrans)⁢(λ⁢⁢Ttrans)kk!=f⁡(Ttrans);
where K is the minimum number of packets transmitted during the second mode of operation, and λ represents the expected number of events in a time interval in a Poisson process.

42. A device according to any one of paragraphs 38 to 41, wherein the transceiver unit is configured to receive a representation of a selected value for the time period, the representation being the closest integer number of maximal DRX durations in the selected value of the time period.

43. A device according to any one of paragraphs 38 to 41, wherein the transceiver unit is configured to receive a representation of a selected value for the time period, the representation being selected from a sequence of numbers that are powers of two, wherein the selection is closest to the integer number of maximal DRX durations in the selected value of the time period.

44. A device according to any one of paragraphs 34 to 43 wherein the time period is calculated within one of the base station, a network core or a radio access unit.

45. A device according to any one of paragraphs 34 to 44, prior to operating in the mode transition state, the transceiver unit is configured to:receive from the wireless telecommunications system a flag and the time period value; and in response to the flag, the processor unit is configured to enter the mode transition state and to control the transceiver unit to operate in the second mode of the mode transition state.

46. A device according to paragraph 45, wherein the transceiver unit is configured to receive a system flag and the time period value with a paging message from the wireless telecommunication network.

47. A device according to paragraph 46, wherein the transceiver unit is configured to receive a paging message from the wireless telecommunication network; send a random access, RA, message to the wireless telecommunication network; receive an RA response message from the wireless telecommunication network; send a radio allocation control, RRC, connection request message and receive an RRC setup message from the wireless telecommunication network, wherein the RRC setup message includes the system flag and the time period value.

48. A device according to any one of paragraphs 45, 46 or 47, wherein the transceiver unit is configured to receive, from the wireless telecommunication network, an RRC release message and in response to the RRC release message, the processor unit is configured to transition to the first mode of operation.

49. A device according to any one of paragraphs 45 to 48, wherein when the terminal device is operating in the second mode of the mode transition state, the transceiver unit is configured to receive, from the wireless telecommunications network, a second flag indicating that in response to the next RRC release message, the processing unit will be configured to leave the mode transition state.

50. A device according to paragraph 49 wherein the transceiver unit is configured to send a random access message and receive from the wireless telecommunication system a random access response message that includes the second flag.

51. A device according to paragraph 49 wherein the transceiver unit is configured to send a random access message; receive from the wireless telecommunication system a random access response message; send a radio allocation control, RRC, connection request message and receive from the wireless telecommunications system an RRC connection setup message that includes the second flag.

52. A base station for use in a wireless telecommunications system which, during a mode transition state, supports a first mode of operation where the base station does not communicate with a terminal device and a second mode of operation where the base station does communicate with the terminal device, the base station comprising:a transceiver unit configured to communicate with the terminal device and a processor unit configured to control the transceiver unit to transition from the first mode of operation to the second mode of operation at the expiration of a time period whereby the time period is defined by the data traffic pattern to the terminal device.

53. A base station according to paragraph 52, wherein during the first mode of operation, data packets destined for the terminal device are stored within the wireless telecommunications system and the transceiver unit is configured to transmit to the terminal device those stored data packets during the second mode of operation.

54. A base station according to paragraph 53 wherein the data packets are stored in one of the base station in a storage unit, a network core or a radio access unit of the wireless telecommunications system.

55. A base station according to any one of paragraphs 52 to 54 wherein, prior to entering the mode transition state, the time period using which the processor unit is configured to control the transceiver unit is defined in the wireless telecommunications system by:determining a number of measuring occasions at which a paging message is sent from the wireless telecommunication system to the terminal device, the paging message being sent to indicate a data packet to send to the terminal device;measuring, at each measuring occasion, the inter-arrival time of the data packet; anddetermining a range of time period values based on the inter-arrival time over the measuring occasions.

56. A base station according to paragraph 55, wherein the range for the time period is defined in accordance with a delay tolerance in communicating a data packet between the terminal device and the wireless telecommunications system such that the range of the time period complies with the constraint
Ttrans≤min{Tmax,2Tave,TP/P}
Where Ttransis the time period, Tmaxis the maximum delay allowed in communicating the data packet, Taveis the average delay in communicating the data packet, and TPis an upper bound with probability P for the delay in communicating the data packet.

57. A base station according to paragraph 55 or 56, wherein the range for the time period is defined in accordance with a probability that no data packet arrives during the first mode of operation such that the range of the time period complies with the constraint
Ttrans≥−Tint*lnPfal
Ttransis the time period, Tintis the inter-arrival time and Pfalis the probability that no data packet arrives during the first mode of operation.

58. A base station according to paragraph 55, 56 or 57, wherein the range for the time period is defined in accordance with a ratio of signalling per data packet that complies with the constraint
Ttrans≥f−1(Pk)
Where Ttransis the time period, and

PK=1-∑k=0K-1⁢⁢exp⁡(-λ⁢⁢Ttrans)⁢(λ⁢⁢Ttrans)kk!=f⁡(Ttrans);
where K is the minimum number of packets transmitted during the second mode of operation, and λ represents the expected number of events in a time interval in a Poisson process.

59. A base station according to any one of paragraphs 55 to 58, wherein the transceiver unit is configured to transmit a representation of a selected value for the time period, the representation being the closest integer number of maximal DRX durations in the selected value of the time period.

60. A base station according to any one of paragraphs 55 to 58, wherein the transceiver unit is configured to transmit a representation of a selected value for the time period, the representation being selected from a sequence of numbers that are powers of two, wherein the selection is closest to the integer number of maximal DRX durations in the selected value of the time period.

61. A base station according to any one of paragraphs 55 to 60 wherein the time period is calculated within one of the base station, a network core or a radio access unit.

62. A base station according to any one of paragraphs 52 to 61, prior to operating in the mode transition state, the transceiver unit is configured to:transmit to the terminal device a flag and the time period value; and in response to transmitting the flag, the processing unit is configured to enter the mode transition state and to control the transceiver unit to operate in the second mode of the mode transition state.

63. A base station according to paragraph 62, wherein the transceiver unit is configured to transmit a system flag and the time period value with a paging message to the terminal device.

64. A base station according to paragraph 63, wherein the transceiver unit is configured to: transmit a paging message to the terminal device; receive a random access, RA, message from the terminal device; transmit an RA response message to the terminal device; receive a radio allocation control, RRC, connection request message and transmit an RRC setup message to the terminal device, wherein the RRC setup message includes the system flag and the time period value.

65. A base station according to any one of paragraphs 62, 63 or 64 wherein the transceiver unit is configured to transmit, to the terminal device, an RRC release message and in response to the transmission of the RRC release message, the processing unit is configured to transition to the first mode of operation.

66. A base station according to any one of paragraphs 62 to 65, wherein when the base station is operating in the second mode of the mode transition state, the transceiver unit is configured to transmit, to the terminal device, a second flag indicating that in response to the next RRC release message, processing unit is configured to leave the mode transition state.

67. A base station according to paragraph 66 wherein the transceiver unit is configured to receive a random access message and to transmit to the terminal device a random access response message that includes the second flag.

68. A base station according to paragraph 67 wherein the transceiver unit is configured to receive a random access message; transmit to the terminal device a random access response message; receive a radio allocation control, RRC, connection request message and transmit to the terminal device an RRC connection setup message that includes the second flag.

69. A wireless telecommunication system comprising the terminal device according to any one of paragraphs 35 to 51 and a base station according to any one of paragraphs 52 to 68.

REFERENCES