The case for random access in OFDMA femtocells

Data services in current cellular networks are growing exponentially, due to the proliferation of high-end mobile devices (smartphones, tablets, ereaders) supporting ever increasing multimedia applications. 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 per second per Hertz per area). 4G network operators are moving toward a heterogeneous architecture consisting of overlaid cells of various sizes. A variety of low-power, small-cell base stations and relays are being deployed to increase capacity around local, typically indoor hotspots. The primary concern in such scenarios is understanding the nature and impact of inter-cell interference in dense deployments of such unmanaged small cells, and arriving at solutions for its mitigation. Resource allocation in traditional wide-area cellular networks usually assume a fully loaded network (i.e. many simultaneous active users in a cell) where the system performance is insensitive to the activity of a single user. This assumption is not suitable for femtocells which are designed to serve very few (indoor) 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.