Sofie Pollin

Performance Analysis of Slotted Carrier Sense IEEE 802.15.4 Medium Access Layer

Advances in low-power and low-cost sensor networks have led to solutions mature enough for use in a broad range of applications varying from health monitoring to building surveillance. The development of those applications has been stimulated by the finalization of the IEEE 802.15.4 standard, which defines the medium access control (MAC) and physical layer for sensor networks. One of the MAC schemes proposed is slotted carrier sense multiple access with collision avoidance (CSMA/CA), and this paper analyzes whether this scheme meets the design constraints of those low-power and low-cost sensor networks. The paper provides a detailed analytical evaluation of its performance in a star topology network, for uplink and acknowledged uplink traffic. Both saturated and unsaturated periodic traffic scenarios are considered. The form of the analysis is similar to that of Bianchi for IEEE 802.11 DCF only in the use of a per user Markov model to capture the state of each user at each moment in time. The key assumptions to enable this important simplification and the coupling of the per user Markov models are however different, as a result of the very different designs of the 802.15.4 and 802.11 carrier sensing mechanisms. The performance predicted by the analytical model is very close to that obtained by simulation. Throughput and energy consumption analysis is then performed by using the model for a range of scenarios. Some design guidelines are derived to set the 802.15.4 parameters as function of the network requirements.

Harmful Coexistence Between 802.15.4 and 802.11: A Measurement-based Study

Due to recent advances in wireless technology, a broad range of standards catering to a diverse set of users are currently emerging. Interoperability and coexistence between these heterogeneous networks are becoming key issues, and proper mitigation of these issues requires a good understanding of how and why heterogeneous networks may harm each others p.erformance. In this paper, we focus on the coexistence of 802.11 (wireless LAN) and 802.15.4 (sensor networks) in the ISM band. These networks have very different transmission characteristics that result in asymmetric interaction patterns. Consequently, many studies assume that the impact of 802.15.4 on 802.11 is negligible. In this paper, we examine this assumption in detail and show that, in many cases, 802.15.4 significantly impacts 802.11 performance. Even when 802.15.4 is executing a listen-before-send, which should theoretically prevent interference, a significant 802.11 performance degradation frequently occurs due to disparate slot sizes between the two protocols. This is one of the first papers studying the listen-before-send performance for heterogeneous networks with substantial measured data. The results raise important coexistence issues for 802.15.4 and 802.11 in particular, but even more so for dynamic spectrum sharing between heterogeneous devices in general.

Performance improvement with predictive channel selection for cognitive radios

Prediction of future availability times of different channels based on history information helps a cognitive radio (CR) to select the best channels for control and data transmission. Different prediction rules apply to periodic and stochastic ON-OFF patterns. A CR can learn the patterns in different channels over time. We propose a simple classification and learning method to detect the pattern type and to gather the needed information for intelligent channel selection. Matlab simulations show that the proposed method outperforms opportunistic random channel selection both with stochastic and periodic channel patterns. The amount of channel switches needed over time reduces up to 55%, which reduces also the delay and increases the throughput.

State of the Art in Opportunistic Spectrum Access Medium Access Control Design

Opportunistic spectrum access (OSA) allows unlicensed secondary networks to share licensed spectrum in space and time, but only when licensed users are not using the spectrum. Thus this novel spectrum management technique started drawing the attention of researchers recently. Although many interesting approaches have been proposed, most techniques are early proposals that often only cover a subset of the problems related to OSA. In this paper, we give an extensive overview of medium access control design challenges specific to OSA, while discussing the main approaches proposed so far in the literature. We give an extensive survey of protocols proposed and discuss which features are not explored yet and which one need to be looked at more carefully.

A Revenue Enhancing Stackelberg Game for Owners in Opportunistic Spectrum Access

A Stackelberg game between three players; spectrum owner, primary users and secondary users is presented under the opportunistic spectrum access (OSA) model, where the secondary users share the channel with primary users in time and secondary user access is performed through a non- perfect listen-before-send scheme. It is shown through simulations that the spectrum owner can enhance her revenue by allowing OSA with a non-zero interference probability to the primary users. The subscription fee of the primary users is lowered in exchange of the non-zero interference probability. The revenue enhancement results from the subscription fees of the secondary users and better utilization of the spectrum. It is also shown through simulations that the enhancement is available for a large range of user preferences such as the value of primary service relative to the secondary, and optimal action of the spectrum owner is robust against estimation errors in these preferences.

Local Estimation of Probabilities of Direct and Staggered Collisions in 802.11 WLANs

Current 802.11 networks do not typically achieve the maximum potential throughput despite link adaptation and cross-layer optimization techniques designed to alleviate many causes of packet loss. A primary contributing factor is the difficulty in distinguishing between various causes of packet loss, including collisions caused by high network use, co-channel interference from neighboring networks, and errors due to poor channel conditions. In this paper, we propose a novel method for estimating various collision type probabilities locally at a given node of an 802.11 network. Our approach is based on combining locally observable quantities with information observed and broadcast by the access point (AP) in order to obtain partial spatial information about the network traffic. We provide a systematic assessment and definition of the different types of collision, and show how to approximate each of them using only local and AP information. Additionally, we show how to approximate the sensitivity of these probabilities to key related configuration parameters including carrier sense threshold and packet length. We verify our methods through NS-2 simulations, and characterize estimation accuracy of each of the considered collision types.

Identifying Spectrum Usage by Unknown Systems using Experiments in Machine Learning

We adopt a machine learning approach towards the problem of identifying wireless systems present in a dynamic radio environment with heterogeneous usage. To classify the wireless systems, we utilize two features that typify spectrum use--center frequency and the frequency spread--and cluster the measurement data in this space. Since the systems are unknown prior to clustering, we use an unsupervised clustering method that uses the Chinese restaurant process implemented using Gibbs sampling. The system identification is divided into two parts: training and online classification. In the training phase, we assign wireless systems present in the surrounding to the clusters while the online classification uses this trained data to perform classification. By means of an extensive measurement campaign, we show that the proposed machine learning process achieves up to 90% correctness in classifying the wireless systems considered here.

Pairwise Algorithm for Distributed Transmit Beamforming

In this paper, we present a novel algorithm for distributed transmit beamforming enabling multiple single-antenna nodes to simultaneously transmit a common message such that they constructively interfere at the receiver. In order to constructively interfere, the nodes iteratively estimate and adjust their carrier phases with the help of receiver feedback. The receivers feedback is based on composite channel estimates calculated from two simultaneous transmissions initiated by the nodes at each iteration. We prove that starting from arbitrary carrier phases, the algorithm converges to the maximum gain almost surely. The algorithm is compared to the 1-Bit Feedback algorithm presented in [3] through simulation, and is shown to outperform it by margins of 4:1 and 3:1 on metrics reflecting power consumption and convergence rate, respectively. We simulate the impact of phase estimation errors on the algorithms steady-state gain and show it to be robust to moderately large errors of plusmn 20deg. We analyze the causes of asynchrony in simultaneous message transmissions and simulate its contribution to phase estimation errors in the context of IEEE 802.15.4 packets. We show that the contribution falls within the acceptable plusmn 20deg range.

Performance Analysis of Double-Channel 802.11n Contending with Single-Channel 802.11

Due to their ease of deployment, 802.11 networks are widely used. To cope with increasing throughput requirements, and to take advantage of improvements in hardware performance, 802.11n has been introduced. One of the extra features is the 20/40 functionality, which allows 802.11n devices to operate using one or two 802.11 channels. In this paper, we study the contention rules for such 20/40 operation on top of the 802.11 Distributed Coordination Function (DCF). We then study the performance of double-channel users in presence of single-channel 802.11 users. Similar to the 802.11 model introduced by Bianchi, we introduce a Markov model for both the legacy 802.11 and double-channel 802.11n users. To couple the Markov models of both types of users, we have to understand how to relate virtual time slots of both types. Next, we introduce collision rules that capture traditional in-channel 802.11 collisions as well as collisions between the different types of users. This allows us to complete the analytical throughput model for 802.11 and double-channel 802.11n users when they coexist. The conclusions are important for understanding 802.11 networks in particular, but also for contention of heterogeneous devices in general.

Comparison of Opportunistic Spectrum Multichannel Medium Access Control Protocols

This work comprehensively compares four possible multichannel medium access control (MAC) approaches for opportunistic spectrum access (OSA). One of important conclusions to be drawn from the analysis is that multichannel OSA MACs that spread both control and data between all available channels, e.g., hopping MACs, perform best among possible OSA MAC implementations when the PU traffic has long ON/OFF periods compared to the time-scale of the SU channel access.