Opportunistic Communication based Medium Access Control (Manzur's PhD work)

Opportunistic communication techniques have shown to provide significant performance improvements in centralised random access wireless networks. The key mechanism of opportunistic communication is to send back-to-back data packets whenever the channel quality is ‘good’. Recently there have been attempts to introduce opportunistic communication techniques in distributed wireless networks such as wireless ad hoc networks.

 In line of this research we propose a new paradigm of medium access control (Channel MAC) based on the channel randomness and opportunistic communication principles. Scheduling in Channel MAC depends on the instance at which the channel quality improves beyond a threshold, while neighbouring nodes are deemed to be silent. Once a node starts transmitting, it will keep transmitting until the channel becomes ‘bad’. We derive an analytical throughput equation of the proposed MAC in a multiple access environment and validate it by simulations. It is observed that Channel MAC outperforms IEEE 802.11 for all probabilities of good channel condition and all number of nodes. For higher number of nodes, Channel MAC achieves higher throughput at lower probabilities of good channel condition increasing the operating range. Furthermore, the total throughput of the network grows with increasing number of nodes considering negligible propagation delay in the network. Please find the relevant papers about it:

However, we need to achieve a scalable channel prediction scheme to realise the practical Channel MAC. We come up with a mean-value based channel prediction scheme, which aids in developing the practical scheme for the Channel MAC. NS2 simulation result shows that the practical Channel MAC outperforms the IEEE 802.11 in throughput due to its channel diversity mechanism in spite of the prediction errors and packet collisions. Please find the relevant papers about it:

Next, we extend the Channel MAC protocol considering multi-rate supports. At present two prominent multi-rate mechanisms, Opportunistic Auto Rate (OAR) and Receiver Based Auto Rate (RBAR) are unable to adapt to short-term changes in channel conditions during transmission as well as to use optimum power and throughput during packet transmissions.  On the other hand considering perfect channel prediction, each transmitter-receiver pair can fully utilise its non-fade duration using a rate-adaptive transmission following the Channel MAC scheduling principle. So we combine both the scheduling and the rate adaptive transmission based on the channel state information at the node-pairs, to design a `Rate adaptive Channel MAC’. Therefore ideally the protocol considers a perfect channel prediction and a rate adaptation. Please find the relevant papers about it:

However to implement the Rate adaptive Channel MAC, we need to use (a) a channel prediction scheme to identify transmission opportunities and (b) auto rate adaptation during those times. For channel prediction, we apply the scheme proposed for the practical implementation of Channel MAC. We propose a `safety margin' technique to realise the auto-rate adaptation scheme. Simulation results reveal a significant performance gap between the proposed Rate adaptive Channel MAC and existing rate adaptive protocols- such as `Opportunistic Auto Rate' (OAR). The proposed scheme outperforms OAR for all types of network topologies and system parameters that we verified through NS2 simulations. Please find the relevant papers about it:

 http://www.oocities.org/manzur_bd2004/journal_rate_adaptive_channel_mac.pdf

(Draft paper)