Ad hoc wireless networks: a bottom-up approach

Ad hoc wireless networks promise ubiquitous connectivity between wireless users (nodes). However, they differ substantially from fixed wired networks (e.g., fiber optical networks). While in the latter case it is possible to assume negligible bit error rate (BER) over a hop and simply view a hop as a logical link, in the case of ad hoc wireless networks this assumption does not hold. In this talk, we introduce a novel bottom-up approach to the study and design of ad hoc wireless networks, where the impact of physical layer on the higher layers is investigated. In particular, we consider multi-hop ad hoc wireless networks with perfectly uniform node distribution---although perfect uniformity in the node distribution is clearly unrealistic, this simple approach allows to gain significant insights into the characteristics of ad hoc wireless networks. Assuming that simultaneous communication routes are disjoint justifies absence of retransmission mechanisms, so that one can ignore the presence of buffers at the nodes (queuing theory need not be preliminary considered). The derivation of a simple expression for the link BER allows to analytically take into account many aspects of the network communication scenario: e.g., the modulation format, channel coding, the presence of inter-node interference (INI), the medium access control (MAC) protocol. We will introduce the concept of minimum spatial energy density, which quantifies the intuitive idea that there must be a minimum amount of energy floating in the network to guarantee connectivity. We will also introduce the concept of effective transport capacity, which quantifies the actual flow of information in the network, taking into account the connectivity level. Evaluating the effective transport capacity for various modulation formats clearly shows the existence of a trade-off between connectivity and spectral efficiency. High-order modulations allow to support larger transport capacities, but for relatively low traffic load connectivity is lost and the effective transport capacity rapidly drops to zero. This work is in progress, and many extensions are currently under investigation. This is a joint work with Prof. Ozan K. Tonguz, Carnegie Mellon University, USA.