Medium access for narrowband wireless ad-hoc networks - requirements and initial approaches

The initial requirements for a tactical military mobile ad-hoc network are discussed and implications on the design of the link layer for a narrowband system have been investigated. The main focus is on medium access control (MAC) protocols suitable for time division multiple access (TDMA) based mobile VHF and UHF combat networks carrying both data and voice traffic. The work is carried out as part of a NATO SC6 attempt to define a CNR network enabling standardised communications between nations employed in joint operations. In this initial time slotted design the narrowband system is assumed to occupy 25 kHz of spectrum, thus efficient transfer of information and reasonably low overhead ensuring scalability is required to support networks of various sizes. Support for quality of service classes, prioritisation and pre-emption is required. The access to the shared radio channel is managed by the link layer MAC protocol. Terrain obstacles, interference, jamming and potentially long distances may lead to multiple hops, and the distributed resource allocation should handle both hidden and exposed nodes in a time dynamic network topology. The MAC protocols should, in cooperation with the network layer, offer uni- and multicast as well as for example regular transmission of position updates of nearby friendly forces. A literature review has been performed and interesting MAC concepts identified. Furthermore, network timing approaches are studied to some extent. Network splitting and merging as well as radio based combat identification (RBCI) are examples of topics briefly discussed. Dynamic time division multiple access (D-TDMA) and soft reservation schemes such as collision avoidance time allocation (CATA) are identified as potential solutions fulfilling most of the above mentioned requirements. The main challenge for both approaches is to limit overhead due to signalling of control messages while at the same time fulfilling the requirements. We have developed an initial link layer design for two candidate approaches. Initial performance assessments are presented and the characteristics of the two alternatives are compared. For a slowly varying network topology and relatively long traffic flows, as experienced for combat networks operating at VHF frequencies, D-TDMA seems to be the most promising approach with respect to the available traffic capacity. With increasing node mobility, higher operating frequency or a more bursty traffic pattern, the soft reservation approaches may become viable alternatives. In such cases reservation signalling would for example occupy more of the available radio resource when utilising D-TDMA. Examples of possible signalling approaches are given for both uni- and multicasted traffic over one or more hops. For both approaches instability of the contention mechanism(s) during heavy load requires a form of connection admission control to ensure successful outcome of the process for start-up of new traffic flows.