An OFDM-Aware Reservation Random Access Protocol for Interference Mitigation in OFDMA Femtocells

An efficient random access protocol utilizing the frequency diversity of OFDMA is proposed for mitigating femtocell interference in a decentralized cellular network. The proposed OFDM reservation random access (OFDM-RR) protocol builds upon a generalized version of the frequency-domain backoff scheme, which was originally proposed for WiFi networks. In this protocol, each node chooses one of K subchannels randomly and transmits a short reservation request on the corresponding subchannel. At the end of the slot, the node whose subchannel index is the highest wins the contention and immediately transmits on the full channel. A tie on the highest index is treated as a collision and is resolved by repeating the process with only those nodes involved in the collision. The throughput of this protocol is derived and compared with that of the classic reservation Aloha (R-Aloha) with the same total channel bandwidth. Although channelization increases the transmission time of the reservation packet in OFDM-RR (due to the smaller subchannel bandwidth), the collision resolution mechanism, which reduces the collision rate sharply, helps improve the throughput, particularly in the high-load region. For example, while R-Aloha almost collapses (near zero throughput) at an average load of ten requests/slot, the proposed protocol can provide a throughput of more than 55% at the same load using only two subchannels.