3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNB or eNodeB) in LTE. A UE may more generally be referred to as a wireless device or a wireless terminal. An RBS is a general term for a radio network node capable of transmitting radio signals to the wireless device and receiving signals transmitted by the wireless device. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.
FIG. 1 illustrates a part of an LTE system. In the radio access network an eNodeB 101a serves a UE 103 located within the eNodeB's area of service or the cell 105a. The eNodeB 101a is connected via an X2 interface to a neighboring eNodeB 101b serving another cell 105b. The two eNodeBs 101a and 101b are connected to a core network node called Mobility Management Entity (MME). The core network in LTE is sometimes referred to as Evolved Packet Core (EPC), and the MME is one of the core network nodes in EPC. Together, the E-UTRAN, the EPC and potentially other entities too, such as service related entities, are referred to as the Evolved Packet System (EPS). S1 Application Protocol (AP) provides the signaling service between E-UTRAN and the EPC. The Non-Access Stratum (NAS) protocol is used for the control signaling between the UE and the MME.
In a current vision of the future development of the communication in cellular networks, huge numbers of mostly small autonomous wireless devices become increasingly important. Such autonomous wireless devices will typically, more or less infrequently—e.g. once per week to once per minute—transmit and receive only small amounts of data. These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers within or outside the cellular network. The application servers configure the devices and receive data from them. Hence, this type of communication is often referred to as machine-to-machine (M2M) communication and the devices may be denoted machine devices (MDs). In the 3GPP standardization the corresponding alternative terms are machine type communication (MTC) and MTC devices. The MTC devices are a subset of the more general term UE.
With the nature of MTC devices and their assumed typical usage follow that they will often have to be very energy efficient, since external power supplies will often not be available and since it is neither practically nor economically feasible to frequently replace or recharge their batteries. In some scenarios the MTC devices may not even be battery powered, but may instead rely on energy harvesting, i.e. gathering energy from the environment, and opportunistically utilizing the often very limited energy that may be tapped from e.g. sun light, temperature gradients, and vibrations.
A mechanism that has been introduced in 3GPP networks to conserve UE energy is Discontinuous Reception (DRX), which has been specified for both idle and connected mode. This mechanism allows a UE to spend most of the time in an energy efficient low power mode, often called sleep mode, while waking up to listen for certain downlink transmissions only on specific occasions. A UE applying DRX and being in idle mode—hereinafter referred to as idle mode DRX—wakes up to listen for pages, and a UE applying DRX and being in connected mode—hereinafter referred to as connected mode DRX—wakes up to listen for downlink resource assignments, i.e. downlink transmissions.
A DRX cycle essentially consists of a sleep period followed by an active period, although the occasions when the UE listens for pages in idle mode DRX are usually not referred to as active periods but rather as paging occasions. This DRX cycle is repeated over and over again until the wireless device is detached from the network, switches in either direction between idle mode and connected mode, or—for a device in connected mode DRX—is reconfigured. Typically, but not necessarily, the sleep period is longer than the active period. A DRX cycle may have a more complex structure than described above.
For connected mode DRX, for instance, the active period may end in a sequence of short cycles of sleep periods and active periods, but for the purpose of this disclosure the somewhat simplified view of a sleep period followed by an active period suffices. Chapter 5.7 of 3GPP TS 36.321 V11.3.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification (Release 11)”, June 2013, describes details of the connected mode DRX in LTE.
Currently the maximum DRX cycle length is 2.56 seconds, i.e. 256 radio frames of 10 milliseconds or 2560 subframes of 1 millisecond each, for both idle mode and connected mode DRX. However, in order to make the DRX mechanism even more effective for energy deprived MTC devices, 3GPP is working on extending the maximum DRX cycle length, and thus the sleep period, both for idle mode DRX and connected mode DRX, leveraging the delay tolerance and infrequent communication need of many MTC applications.
As the term Discontinuous Reception (DRX) implies, it concerns only the downlink, whereas a UE may initiate communication in the uplink at any time, irrespective of the DRX cycle.
The idle mode DRX cycle, i.e., the paging cycle, is configured in the UE through parameters in the system information (SI) that is broadcast in each cell, in combination with UE specific parameters. Alternatively, it is also possible to configure a UE specific paging cycle. The connected mode DRX cycle and other DRX parameters that may be used are configured in the UE through optional parameters. The parameters are typically provided in the RRCConnectionReconfiguration message of the Radio Resource Control (RRC)protocol, in conjunction with the idle to connected mode transition or at any other time when the UE is in connected mode.
A UE, e.g. an MTC device, applying extended connected mode DRX may lose its connection with the network, e.g. because it goes out of radio coverage or because its battery is discharged. Due to the long sleep periods and long periods without communication events, this may happen without the network detecting it. Consequently network resources and other resources in the RBS such as the eNodeB may be tied up in vain for extended periods of time.
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