Random Access in OFDMA Femtocells

Cellular networks are confronted with an exponential increase in capacity requirements, due to the proliferation of high-end consumer devices (smartphones, tablets, e-readers) consuming ever increasing multimedia content. Clearly, availing of new spectrum (as with 4G allocations) is a necessary part of meeting this challenge, but so are improved network architectures that achieve enhanced spatial spectral efficiency (bits/s/Hz/area). Network operators are increasingly moving toward a heterogeneous architecture consisting of overlaid cells of various sizes; in particular, a variety of small, low-power pico/femto base stations and relays are being deployed to increase capacity around local hotspots. OFDMA femtocells are considered a key enabler in 3GPP LTE-Advanced networks of very high data rates for indoor users. However, the unmanaged nature of femtocells implies the need for careful modeling (and ultimately, managing) of inter-cell interference especially in dense deployments. So far, co-channel interference in femtocells has been investigated using traditional, cellular resource allocation approaches that usually assume a fully loaded network where the system is insensitive to the activity of a single user. This assumption is not suitable for femtocells which are designed to serve very few users and thus lacks the presumed traffic aggregation. This article provides a fresh look at the femtocell interference problem from a multiple access perspective. It first compares the multi-channel features inherent to OFDMA with traditional multi-channel MAC designs for wireless LANs and highlights how the ability to schedule users in both time and frequency as afforded by OFDMA, can be exploited to achieve more effective (distributed) random access. A simplified MAC model that hides the complexity of LTE protocol stack is proposed to facilitate the design and analysis of random access MAC algorithms in the context of OFDMA femtocells.