Rethinking capacity per unit cost

Verdú's seminal work offers an elegant characterization of capacity per unit cost, but many results hold only when a cost-free input symbol is available. Furthermore, optimal transmission strategies under such scenarios typically result in infinitesimal spectral efficiency. We argue that the existence of a cost-free symbol is unrealistic and that the resulting low-rate transmission policies are not applicable to narrowband systems. We therefore examine capacity per cost when no cost-free symbol is available. In this case, the corresponding transmission rates are bounded away from zero. Specifically we propose a cost function for wireless channels that balances both energy and temporal costs. Using this cost function, we explore the capacity per cost regions of single- and multiple-user channels.