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Abstract-We investigate the benefits of channel-aware (opportunistic) scheduling of transmissions in ad hoc networks. The key challenge in optimizing the performance of such systems is finding a good compromise among three interdependent quantities: 1) the density of scheduled transmitters; 2) the quality of transmissions; and 3) the long term fairness among nodes. We propose two new channel-aware slotted CSMA protocols opportunistic CSMA and quantile-based CSMA (QT-CSMA) and develop new stochastic geometric models to quantify their performance in terms of spatial reuse and spatial fairness. When properly optimized, these protocols offer substantial improvements in performance relative to CSMA-particularly, when the density of nodes is moderate to high. In addition, we show that a simple version of QT-CSMA can achieve robust performance gains without requiring careful parameter optimization. The quantitative results in this paper suggest that channel-aware scheduling in ad hoc networks can provide substantial benefits which might far outweigh the associated implementation overheads.Index Terms-Ad hoc networks, ALOHA, CSMA, O-CSMA, opportunistic scheduling, quantile scheduling, QT-CSMA, spatial fairness, spatial reuse.

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Abstract-We propose an analytical framework that optimizes the performance of various Hybrid Automatic Repeat reQuest (HARQ) schemes in wireless packet data systems. The optimization framework lends itself to maximizing the user throughput without assuming a specific HARQ scheme, a target packet error rate, and a mapping of signal-to-interference-noise ratios (SINR) into modulation and coding scheme (MCS) sets. Thus, a user selects an optimal amount of redundancy bits used for retransmissions, an optimal packet error rate, and an optimal SINR-MCS mapping such that the user throughput would be maximized. We show that there exists an optimal redundancy block size of incremental redundancy (IR) scheme when the number of retransmissions is limited, while as small as possible redundancy size is optimal when the number of retransmissions is unlimited. Our analytical results are verified through extensive simulations of the IEEE 802.16e system.