Power-controlled packet relays in wireless data networks

We investigate the problem of packet relaying in power-controlled wireless data networks. For reasons of limited range or energy efficiency, packets are not transmitted from the source to the destination nodes directly, but in two hops via an intermediary relay node. The nodes are equipped with buffers to store the incoming packets while they take turns to transmit. The communication takes place over wireless links whose quality fluctuates due to fading and interference from other users. At each point in time, the system has to decide (i) which transmitter takes possession of the channel, and (ii) the power level at the transmitter. On adverse channel conditions, the system can even decide to refrain from all transmissions (back-off), waiting for the links to improve before resuming transmissions. Clearly, this dilemma induces the fundamental trade-off between packet delay and transmission power. In a delay-sensitive configuration, the transmitters prefer to remain always active so as to keep the packet backlogs at the queues low. On the contrary, a power-sensitive configuration favors the use of back-off, resulting in low transmission power levels at the cost of increased packet delay and high packet backlogs. Overall, the decision is taken based on (i) the state of the wireless links, and (ii) the backlog size of the queues. Toward designing efficient transmission schemes, we model the system as a controlled Markov chain and formulate the optimization problem in the context of dynamic programming. We construct a set of transmission schemes by analyzing the system dynamics, which are evaluated over standard approaches and shown to achieve significant performance gains.