Cross-layer Rateless Codes In IEEE 802.11n Long-Distance Wireless Links

Long-distance IEEE 802.11 n wireless mesh networks(Long-Distance wireless Mesh networks, LDmesh) consist of routers and clients which are usually equipped with high-gain directional antennae and high-power wireless cards. In LDmesh networks, a single hop could be hundreds of kilometers long so that feedback-based solutions exhibit poor performance, such as automatic repeat-request. Improving channel utilization is still a challenge in protocol design.Through the analysis of large amounts of data, we sum up the features of failed frames: among all the frames which are not successfully received, about 50% of them are failed because of CRC(Cyclic Redundancy Check, CRC) checksum failure. Bit error ratio is usually below 10% as well as the bit flipping is randomly distributed. These features make the use of FEC(Forward Error Correction, FEC) schemes more feasible.This paper analyze the relationship between RSSI(Received Signal Strength Indication, RSSI) and PDR(Packet Delivery Ratio, PDR) for each MCS(Modulation and Coding Scheme, MCS), and proposes a MAC(Media Access Control, MAC) layer rateless codes algorithm AutoRate. Auto Rate targets at situations when the packet loss rate ranges from 10% to 80% and avoids retransmission by FEC. The sender uses RS-precoded(Reed-Solomon, RS) LT(Luby Transform, LT) interframe code to encode the original frames at MAC layer to counter bit error and packet loss.The paper provides a Matlab simulation for AutoRate and the simulation results show that AutoRate gets 100% of delivery rate without retransmission and the coding rate stands at above 0.6. The effective throughput can reach the 64% of that in theory.This paper also presents a rate selection algorithm based on AutoRate. By estimating RSSI, we calculate the loss rate for MCS candicates, as well as coding rate and effective throughput. We select MCS to maximize goodput, and at the same time, coding rate is determined. Simulation results indicate that this algorithm has higher effective throughput than that of minstrel_ht, which is the original rate selection algorithm of Atheros drivers. The loss rate of minstrel_ht ranges between 0% to 20% and this algorithm keeps at zero.