Browsing by Author "Oberli, Christian"

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    Delay Probability Distributions of Acknowledged CSMA/CA With Finite Re-Transmissions Over Fading Channels
    (2024) Madariaga, Nicolas; Oberli, Christian; Estela, Maria Constanza
    The delay distribution observed between the dll of a pair of neighbouring nodes in wireless mesh networks is studied. Specifically, we model the delay for unslotted csma with acknowledgments and multiple transmission retries under given channel access and block-fading statistics. This mac mechanism is used, for instance, by the IEEE 802.15.4 Standard. It has three different possible outcomes: fts (acknowledgement received), caf and ftf (no acknowledgement received, all transmission attempts used). We model these outcomes in terms of the maximum allowed number of channel access attempts, the maximum number of transmission attempts, the probability of busy channel and the frame-loss probability. All these parameters are local to each node and hence our model allows for modeling link delays from the perspective of each individual node. This lies in contrast with prior work, based mostly on Markov chain approaches, which tend to be computationally intensive, often mathematically intractable and require taking a global view over the network. We validate our results with Monte-Carlo simulations and show that the proposed model for frame transfer success and frame transfer failure are accurate in terms of the delay distribution and mean value. In the case of channel access failure, the model is less precise but can be used as an upper bound for the expected delay.
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    Modulation and SNR Optimization for Achieving Energy-Efficient Communications over Short-Range Fading Channels
    (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2012) Rosas, Fernando; Oberli, Christian
    It is commonly assumed that the energy consumption of wireless communications is minimized when low-order modulations such as BPSK are used. Nevertheless, the literature provides some evidence that low-order modulations are suboptimal for short transmission distances. No complete analysis on how the modulation size and transmission power must be chosen in order to achieve energy-efficient communications over fading channels has been reported so far. In this paper we provide this analysis by presenting a model that determines the energy consumed per payload bit transferred without error over fading channels of various statistics.
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    On Maximizing the Probability of Achieving Deadlines in Communication Networks
    (2024) Becker, Benjamin; Oberli, Christian; Meuser, Tobias; Steinmetz, Ralf
    We consider the problem of meeting deadline constraints in wireless communication networks. Fulfilling deadlines depends heavily on the routing algorithm used. We study this dependence generically for a broad class of routing algorithms. For analyzing the impact of routing decisions on deadline fulfillment, we adopt a stochastic model from operations research to capture the source-to-destination delay distribution and the corresponding probability of successfully delivering data before a given deadline. Based on this model, we propose a decentralized algorithm that operates locally at each node and exchanges information solely with direct neighbors in order to determine the probabilities of achieving deadlines. A modified version of the algorithm also improves routing tables iteratively to progressively increase the deadline achievement probabilities. This modified algorithm is shown to deliver routing tables that maximize the deadline achievement probabilities for all nodes in a given network. We tested the approach by simulation and compared it with routing strategies based on established metrics, specifically the average delay, minimum hop count, and expected transmission count. Our evaluations encompass different channel quality and small-scale fading conditions, as well as various traffic load scenarios. Notably, our solution consistently outperforms the other approaches in all tested scenarios.
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    On the Deadline Miss Probability of Various Routing Policies in Wireless Sensor Networks
    (2021) Sepulveda, Matias; Oberli, Christian; Becker, Benjamin; Lieser, Patrick
    Moving data across communication networks is often subject to deadline requirements. An example is early warning of disasters of natural origin, where sensor measurements at the disaster location must be communicated across a network within a predefined maximum delay in order for a consequent warning to be timely. In this work, we present a probabilistic model that allows for characterizing the delay experienced by sensor measurements in a wireless sensor network from source to sink depending upon the routing metric used for forwarding the data through the network. Using link delay probability distributions and the probabilities of following different paths to the sink, source-to-sink delay distributions are found for routing policies based on minimum hop-count, minimum mean delay and the Joint Latency (JLAT) protocol. An algorithm for calculating the end-to-end source to sink delay probability density function (PDF) is presented for the general case of networks that use routing tables whose input for routing decisions is the remaining time-to-deadline. The work provides a general tool for routing delay analysis, allowing for comparison of the deadline miss probability between different routing policies. An improved form of JLAT is proposed. Its deadline miss probability is found using the presented algorithm and compared to the ones determined for minimum hop-count, minimum mean delay and JLAT by means of an example.
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    On using transmission overhead efficiently for channel estimation in OFDM
    (IEEE, 2010) Oberli, Christian; Estela Zamora, María Constanza; Rios Ojeda, Miguel Félix
    The limited time available for acquiring the channel state in mobile broadband wireless communication systems makes it crucial to count with channel estimation methods that attain the highest accuracy with a given preamble length. For the family of preambles that probe channels repeatedly at equispaced frequencies, we find that the shortest preamble that attains a given mean square error is a sequence that undersamples the channel spectrum, thereby allowing for a better mitigation of white Gaussian noise by averaging a higher number of observations.