C.2.2 Network Protocols
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The current work considers random access communications within a single cell and provides
an algorithmic suggestion for the almost optimal coordination of user actions.
In the scenario considered, an access effort is successful if (a) the signal is
detected at the receiver and (b) no collision occurs. The first event is controlled by the user transmission power and the second by the
choice of access (back-off) probability. These constitute the user action pair.
The algorithm aims at exploiting information from measurements and user reports, in order to estimate current
values of the system situation. Based on these, two optimization problems can be formulated and solved
for the so called contention level and transmission power level at the
base station side. The methodology to find the optimal values is based on minimization of a drift function.
The two values are broadcast in order to help the users update their actions ``almost optimally``.
In this way the wireless cell can achieve self-optimization, without outside coordination, by relying on such intelligent information exchange
and parameter estimation. Numerical results illustrate the great benefits of the
suggested algorithm, compared to scenarios where the actions remain fixed, at a very low or even zero cost in power expenditure and delay.
\newcommand{\chordsharp}{Chord$^\##$} Data lookup is a fundamental problem in peer-to-peer systems: Given a key, find the node that stores the associated object. Chord and other P2P algorithms use distributed hash tables (DHTs) to distribute the keys and nodes evenly across a logical ring. Using an efficient routing strategy, DHTs provide a routing performance of $O (\log N)$ in networks of $N$ nodes. While the routing performance has been shown to be optimal, the uniform key distribution makes it impossible for DHTs to support range queries. For range queries, consecutive keys must be stored on lo gically neighboring nodes. In this paper, we present an enhancement of Chord that eliminates the hash function while keeping the same routing performance. The resulting algorithm, named \chordsharp{}, provides a richer function ality while maintaining the same complexity. In addition to Chord, \chordsharp{} adapts to load imbalance.