Energy-Limited vs. Interference-Limited Ad Hoc Network Capacity

In a multi-user system in which interference is treated as noise, increasing the power of all transmissions eventually makes thermal noise negligible and causes the network to be interference-limited. This paper attempts to determine the power level at which a random-access ad hoc network becomes interference limited. Furthermore, when the network is not interference-limited (i.e., when signal power does not completely overwhelm noise), the relationship between power and area spectral efficiency is quantified. It is shown that the key quantity is the energy per information bit, commonly referred to as E_b/N_o. Roughly speaking, a network becomes interference limited for E_b/N_o- values above approximately 15 dB; increasing E_b/N_o- leads to E_b/N_omiddot a negligible capacity increase, but decreasing E_b/N_o below this value does lead to a non-negligible capacity decrease. Furthermore, as E_b/N_o approaches the Shannon limit of -1.59 dB, network capacity is seen to be extremely sensitive to the value of J^.