High Throughput Transmissions in OFDM based Random Access Wireless Networks

In Random Access Wireless Networks it is common to occur packet collisions due to different users trying to access simultaneously to a given physical channel. The conventional approach is to discard all blocks involved in the collision and retransmit them again. To reduce the chances of multiple collisions each user transmits in the next available slot with a given probability. With this strategy, if two packets collide we need at least three time slots to complete the transmission (more if there are multiple collisions), which results in a throughput loss. To overcome this problem, a TA (Tree Algorithm) combined with a SIC (Successive Interference Cancellation) scheme was proposed in (Yu & Giannakis, 2005). Within that scheme, the signal associated to a collision is not discarded. Instead, if the packets of two users collide then, once we receive with success the packet of one of those users, we can subtract the corresponding signal from the signal with collision and recover the packet from the other user. With this strategy, a collision involving two packets requires only one additional time slot to complete the transmission, unless there are multiple collisions. However, the method has a setback since possible decision errors might lead to a deadlock. (Wang et al., 2005) Another problem with these techniques is that we do not take full advantage of the information in the collision. The ideal situation would be to use the signals associated to multiple collisions to separate the packets involved (in fact, solving collisions can be regarded as a multiuser detection problem). In (Tsatsanis et al., 2000) a multipacket detection technique was proposed where all users involved in a collision of