Research On Medium Access Control In Wireless Ad Hoc Networks

With the rapid advancement in wireless communications and electronics, WirelessAd Hoc Network is gaining more popularity for various applications. Wireless ad hocnetwork is a wireless system without a fixed infrastructure and it operates in a self-organized and distributed manner to offer more robustness. As a chief component ofwireless ad hoc network protocols, Medium Access Control (MAC) protocol directlydetermines both packet reception and delivery and is desisive for system performance.Due to its distributed operation manner, ad hoc network lacks a centrilized controlscheme to arbitrate contention among active stations. It has to exchange control signalsbefore data communication to reserve channel, and then adopts backoff schemes tocoordinate medium access and allocates system resources reasonablely, which enablesits packet collision resolution and a preferred communication order maintenance.This thesis focuses on MAC protocol in wireless ad hoc network and investigatesits inffuence on system performance imposed by signal handshake schemes and backoffalgorithms as shown below:Hidden Terminal Problem is pervasive in ad hoc network. It not only degradessystem throughput and channel utilization, but also increases packet delivery la-tency. Traditional MAC protocols utilize RTS/CTS/DATA/ACK access mode tosolve this problem, however, due to the limitation of coordination scope, HiddenReceiver Problem, which is a common instance of hidden terminal problem, cannot be totally resolved under single channel scenario.MAC protocol adopts contention window scheme to resolve collisions. Actually,different packet collisions should be dealt with different manners. Existed backoffalgorithms did not take further investigation into the difference of collisions andtreated them in the same way, which impacts the system performance.Collision not only wastes scarce bandwidth and exhausts limited node energy, butalso decreases network throughput while increasing packet latency. Traditionalbackoff algorithms attempt to change contention window size to enhance random-ness of backoff process, which in turn contributes to its resolution for collisions.However, such scheme can only resolve collisions to some extent and more specif-ically, the collisions can not be totally removed from network.Based on such observations, this thesis addresses medium access control schemein wireless ad hoc network. The research is carried out from four issues: (1) Researchon hidden receiver problem; (2) Analysis and modeling on collisions in contention-based wireless ad hoc network; (3) Research on backoff algorithms based on collision classification model. This work has been supported by the National Science Foundationof China"Information Processing and Transmission on Reconstructable MIMO-basedWireless Sensor Networks"(No.60572049) and"Research on Key Issues on CognitiveRadio Networks"(No.60602029) respectively.The contributions of this thesis include:1. Solution to Hidden Receiver Problem: This thesis analyzes hidden receiverproblem in detail and proposes Explicit Blocking Notification (EBN) scheme toaddress it. EBN modifies traditional RTS/CTS/DATA/ACK access model toadd a new phase BN (Blocking Notification) as RTS/CTS/BN/DATA/ACK toexplicitly notify the potential sender about the blocked state of hidden terminal. Itadopts Query/Wait mode to eliminate blindly RTS retransmission to solve hiddenreceiver problem.2. Collision Classification Model: Traditional backoff algorithms only concen-trate on adjusting contention window size to achieve collision resolution, while in-goring the discrepancy of different packet collisions. Accroding to different backoffstages of collision involvers, this thesis puts forward for the first time the CollisionClassification Model, which classifies collisions in contention-based wireless ad hocnetwork into Cross Collision and Intra Collision. It proposes clearly that the twocollisions should be dealt with different policies. It then proposes Sequential Dis-crete Window Distribution (SDWD) scheme to resolve dominant cross collisions innetworks, while the intra collisions are resolved with an appropriate distributionwindow size to achieve a tradeoff between intra collision probability and packetlatency.3. Backoff Algorithms based on Collision Classification Model: This thesisproposes two backoff algorithms based on collision classification model: CollisionClassification Resolution (CCR) algorithm and Collision Free CCR (CF-CCR)algorithm. The two algorithms are both based on SDWD scheme and are differentin their resolutions to succeeded nodes. CCR distributes succeeded senders incurrent contention window while they are allocated a independent distributionwindow in CF-CCR, which enables its convergence to a collision-free state finally.These two algorithms provide comprehensive improvement to IEEE 802.11 DCFand exhibit respective advantages in different scenarios.