In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile station and/or user equipment units communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones or “smartphones”, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-comprised, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area, or cell, being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “NodeB” or “B node” and in other networks may be called Evolved NodeB (eNB) or Evolved Universal Terrestrial Radio Access Network (E-UTRAN) NodeB (eNB) and which in this document also is referred to as a base station. A base station may serve one or multiple cells, wherein a cell may also be referred to as a sector. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines and/or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The 3rd Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Long Term Evolution (LTE) together with Evolved Packet Core (EPC) is the newest addition to the 3GPP family.
In a currently popular vision of the future development of the communication in cellular networks huge numbers of (mostly) small autonomous devices become increasingly important. These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers (which configure the devices and receive data from them) within or outside the cellular network. 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 machine type communication devices (MTC devices) (with the latter being a subset of the more general term user equipment, UE, or wireless terminal). In terms of numbers MTC devices will dominate over human users, but since many of them will communicate very scarcely, their part of the traffic volume will be much smaller than their part of the “user” population.
With the nature of MTC devices and their assumed typical uses 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, for example, gathering energy from the environment, opportunistically utilizing (the often very limited) energy that may be tapped from sun light, temperature gradients, vibrations, etc. For such energy deprived devices the traffic is characterized by small infrequent transactions (often delay tolerant), which will result in a large signaling overhead. Hence, reducing the signaling overhead is an important means to facilitate for such devices to efficiently function, with a long battery lifetime, using a cellular network.
A mechanism that has been introduced in cellular networks in order to save energy in the wireless terminal is discontinuous reception (DRX), which allows a wireless terminal to remain in an energy-saving sleep state most of the time, while waking up to listen for pages (in idle mode DRX) or downlink resource assignments (i.e., downlink transmissions) (in connected mode DRX). Furthermore, in order to make the DRX mechanism even more effective for energy deprived MTC devices, the maximum DRX cycle length (and thus sleep period) will be extended, for example, denoted “long idle mode DRX” and “long connected mode DRX” or “extended idle mode DRX” and “extended connected mode DRX”. A DRX cycle thus comprises a sleep period followed by an active period and this cycle is repeated over and over.
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, but for the aim of the example embodiments described herein, the simplified DRX cycle description suffices. Benefits of the long connected mode DRX concept include both reduction of control plane overhead (by reducing the number of idle to connected mode transitions) and reduction of energy consumption in the wireless terminal.
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