Future wireless communication systems are likely to comprise a large number of autonomous devices, which devices more or less infrequently transmit, receive, or are polled for small amounts of data. These devices are assumed to not necessarily be associated with humans but are rather sensors or actuators of different kinds, which communicate with application servers or similar network entities within or outside a cellular network.
This type of sporadic small data communication is often referred to as machine-to-machine, M2M, communication and the devices are often denoted machine devices, MDs. Examples of M2M applications are almost countless, e.g., in private cars for communicating service needs, in water or electricity meters for remote control and/or remote meter reading, in street-side vending machines for communicating when goods are out-of-stock or when enough coins are present to justify a visit for emptying, in taxi cars for validating credit cards, or in surveillance cameras for home or corporate security purposes. In the context of 3GPP standardization the corresponding alternative terms are Machine Type Communication, MTC, and Machine Type Communication, MTC, devices, the latter being a subset of the more general term user equipment, UE.
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, i.e., gathering energy from the environment, opportunistically utilizing the often very limited energy that may be tapped from sun light, temperature gradients, vibrations, and the like.
So far the MTC related work in 3GPP and in other standardization projects has focused on MTC devices directly connected to the cellular network via the radio interface of the cellular network. However, a scenario which is likely to be more prevalent is one where most MTC devices connect to the cellular network via a gateway. In such scenarios the gateway acts like a UE towards the cellular network while also maintaining a local network, typically based on a short range radio technology, towards the MTC devices. Thus, the gateways are often equipped with communication modules or units which support both the radio access technology of the cellular network and the access technology of the local network, e.g. a radio access technology. Such a local network, which extends the reach of the cellular network to other radios outside the cellular network, has been coined capillary network. The gateway connecting or linking the capillary network to the cellular network will be herein referred to as a Capillary Network Gateway, CGW.
Radio technologies that are expected to be common in capillary networks include e.g. IEEE 802.15.4, e.g. with 6LoWPAN or ZigBee as the higher layers, Bluetooth Low Energy or low energy versions of the IEEE 802.11 family, i.e. Wi-Fi. A capillary network may be single hop, i.e. all MDs have a direct link to the CGW, e.g. a Wi-Fi network with the CGW as the access point, or multi-hop, i.e. some MDs may have to communicate via one or more other MDs to reach the CGW, e.g. an IEEE 802.15.4+ZigBee network with the CGW being a controller for a personal area network, PAN. In multi-hop cases the Routing Protocol for Low-Power and Lossy Networks, RPL, may be used.
Presently, in cases where there is a choice between several CGWs for linking an MD with a cellular network, the MD commonly selects CGW based only on propagation conditions between the MD and the CGW. That is, a measure of signal-to-noise ratio, SNR, or the like, is used to decide on a suitable CGW. Since present selection mechanisms based only on propagation conditions between the MD and the CGW can result in sub-optimal network traffic processing and overall degradation of aspects such as network control, improvements in the selection mechanism of CGWs are desired.