In practical WLAN deployments, the capture effect has been shown to enhance the performance of stations residing close to the AP, while putting at disadvantage the distant nodes. In this paper, we introduce an analytical model to characterise the performance of 802.11 devices with heterogeneous capture probabilities and different network loads, and explore the interaction between the MAC operation and PHY capture. Unlike previous studies, we reveal that the throughput of stations experiencing low capture probabilities can also benefit from the capture effect when the stations retaining high capture probabilities are not saturated. Following these findings, we design a power-hopping scheme for 802.11 MAC that exploits the benefits of the capture effect to improve performance in dense deployments where nodes experience similar channel conditions. We investigate the potential gains of this mechanism by implementing a practical approximation using commercial off-the-shelf hardware and open-source drivers and, by conducting experiments in a real testbed, we show that our scheme can significantly outperform the standard 802.11 protocol in terms of throughput. The research leading to these results has received funding from the European Community's 7 th Framework Programme (FP7-ICT-2009-5) under grant agreement n. 257263 (FLAVIA project) and the Irish HEA PRTLI Cycle 4 FutureComm.

In this article, we introduce a power-hopping technique (PH-MAC) that, by alternating between different transmission power levels, aims to deliberately cause packet capture and thereby reduce the impact of collisions in 802.11 WLANs. We first devise an analytical model of the 802.11 protocol with heterogeneous capture probabilities, and show that, depending on the network load, the capture effect can enhance the throughput performance of all nodes. We base the design of PH-MAC on the findings following from this analysis and demonstrate that important performance improvements can be achieved by exploiting the interactions between the MAC and PHY layers to mitigate collisions. Finally, to understand the feasibility of this technique in practical deployments, we present a prototype implementation of PH-MAC which relies on commodity hardware and open-source drivers. We evaluate the performance of this implementation in an indoor testbed under different network conditions in terms of link qualities, network loads and traffic types. The experimental results obtained show that our scheme can provide significant gains over the default 802.11 mechanism in terms of throughput, fairness and delay.

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Abstract-Irregular LDPC codes can have capacityapproaching performance with an iterative BP decoder under AWGN channel with BPSK modulation. Generally in BPSK modulation, every bit within the block has the same power. In this paper, we investigate unequal power allocation (UPA) using BPSK modulation for irregular binary LDPC codes. We show that constant power modulation need not lead to the best performance. With UPA we see gains of up to 0.25dB, with larger gains possible if the receiver knows the details of the UPA scheme. Optimal power allocation is shown to depend strongly on the codes in use. Our work demonstrates the promise of the UPA scheme for some irregular LDPC codes.

While there have been considerable advances in the modelling of 802.11's MAC layer in recent years, 802.11 with finite buffer space is considered difficult to analyse. In this paper, we study the impact of finite buffers' effect on the 802.11 performance, in view of the requirements of interactive applications sensitive to delay and packet loss. Using both state-of-the art and simplified queueing models, we identify a surprising result. Specifically, we find that increased buffering throughout an 802.11 network will not only incur delay, but may actually increase the packet loss experienced by stations. By means of numerical analysis and simulations we show that this non-monotonic behaviour arises because of the contention-based nature of the medium access protocol, whose performance is closely related to the traffic load and the buffer size. Finally, we discuss on protocol and buffer tuning towards eliminating such undesirable effect.