Massimiliano Comisso

Throughput and Delay Analysis of 802.11-Based Wireless Networks Using Smart and Directional Antennas

This paper proposes a mathematical framework for comparing the performance of wireless networks based on the IEEE 802.11 standard, employing switched-beam, directional and adaptive antennas. The analysis adopts detailed models for the antenna system behavior and the propagation environment, taking into account multipath and interference. The purpose of this study is to determine the number of simultaneous communications that may be sustained by adopting advanced antenna systems, and to investigate, by analysis and simulation, the resulting performance improvements, in terms of throughput and packet delay. Both saturated and non-saturated traffic conditions are treated. Suitable 802.11 Medium Access Control (MAC) layer parameter settings, able to exploit smart and directional antenna capabilities for maximizing the throughput, are also discussed.

Theoretical analysis of asynchronous multi-packet reception in 802.11 networks

This paper proposes a mathematical framework for evaluating the throughput of an 802.11 distributed wireless network when the system allows multi-packet reception in asynchronous operating mode. Differently from previous studies on multi-packet reception, which assume a certain synchronization between the different sources at the beginning of the respective transmissions, this work theoretically analyzes a fully distributed scenario, where each node can access to the medium in a completely asynchronous way. More precisely, a new transmission can begin in a channel where other nodes are already communicating as long as the current channel load is within a given threshold. The proposed framework consists of a single source, 802.11 specific model, which generalizes a previously presented analytical model, and a novel network model, which is adopted to evaluate the theoretical performance in terms of throughput. The novel model, which includes the synchronous scenario as a particular case, is employed to investigate the influence of the number of allowed communications, of the load threshold, and of the minimum contention window on the network performance. Furthermore, the theoretical analysis is used to evaluate the throughput in presence of Rayleigh fading and noise when error correcting codes are adopted.

Fast Iterative Method of Power Synthesis for Antenna Arrays

A fast and simple method of power synthesis for antenna arrays of arbitrary geometry is presented. The method adopts an auxiliary function having the desired amplitude pattern and a generic phase pattern, and determines in closed form the array pattern approximating the auxiliary function in both amplitude and phase. A cost function involving the auxiliary function and the approximating pattern is minimized by modifying the phase pattern of the auxiliary function, using a simple and fast iterative technique. Once the optimal phase pattern has been found, the array pattern approximating the corresponding auxiliary function is the synthesized pattern. The proposed method allows to satisfy stringent requirements on the amplitude pattern, including multibeam synthesis and radiation suppression within large angular regions. Numerical results show the effectiveness of the algorithm and are compared to those obtained using other existing methods. In order to achieve a more significant comparison such methods have been properly modified to improve their performances. Such comparison shows that the presented algorithm allows a significant reduction of CPU time with respect to the other considered methods, as well as a weaker dependence on the starting point. Finally, the presented method is extended in such a way as to reduce the dynamic range ratio (DRR) of the array excitations.

Interference Mitigation on WLANs Using Smart Antennas

This paper investigates the performance of a fixed Wireless LAN in which nodes are equipped with fully adaptive smart antennas. The considered smart antenna system is a uniform circular array of microstrip patch elements in which the weights are updated using the unconstrained LMS algorithm. The behavior of a new directional MAC protocol for spatial multiplexing is analyzed and compared with IEEE 802.11 DCF. The paper purpose is the evaluation of interference and multipath effects on SDMA, using realistic models for the channel and for the smart antenna system. The link model takes into account path loss, cochannel interference and multipath, using a modified Jakes model. Results show that the performance of the WLAN are strictly dependent on the interferer characteristics and on the angular spread of the channel.

Fast Co-Polar and Cross-Polar 3D Pattern Synthesis With Dynamic Range Ratio Reduction for Conformal Antenna Arrays

This paper presents a fast method for conformal antenna arrays that enables the synthesis of 3D co-polar and cross-polar patterns, simultaneously reducing the dynamic range ratio (DRR) of the array excitations. The power synthesis problem is reduced to a field synthesis one by introducing two auxiliary phase patterns: one for the co-polar pattern and the other for the cross-polar pattern. The problem is then iteratively solved with respect to the two auxiliary phase patterns and to the array excitations. A modified version of the method is also proposed, for the particular case where the DRR reduction is not required, obtaining a further strong reduction of the computational time.

ON THE USE OF DIMENSION AND LACUNARITY FOR COMPARING THE RESONANT BEHAVIOR OF CONVOLUTED WIRE ANTENNAS

This paper analyzes the possibility to use dimension and lacunarity for comparing the resonant behavior of difierent convoluted wire antennas, including prefractal dipoles. Since previous studies have proved that the Hausdorfi fractal dimension is not suitable for antenna comparison purposes, this work proposes the adoption of a difierent approach for evaluating the dimension by using the measurement at scale -, which is more suitable for analyzing real phenomena. The results provided by this measure are compared to those obtained by using the average lacunarity. The objective is to verify if, given two convoluted wire dipoles, the dimension and average lacunarity provide su-cient information to infer which dipole exhibits the lower resonances. y

Multi-Antenna Techniques for Wireless Mesh Networks in an Outdoor Environment

This paper investigates the behavior of a wireless mesh network in which the nodes are equipped with a multiple antenna system. The objective of this study is the comparison of the low-cost switched-beam antennas with the more expensive fully adaptive smart antennas. The analysis and the simulations are performed together with some considerations regarding the advantages and the practical possibilities of employing multi-antenna systems on mesh routers. The analytical and the numerical results are derived employing realistic antenna radiation patterns and a proper spatial channel model.

3D Power Synthesis with Reduction of Near-Field and Dynamic Range Ratio for Conformal Antenna Arrays

A simple method of power synthesis for 3D radiation patterns of conformal antenna arrays is presented. The method enables to synthesize a desired far-field pattern simultaneously controlling the dynamic range ratio (DRR) of the array excitations and the electric field amplitude in a given region of interest close to the antenna. The power synthesis problem is reduced to a field synthesis one by introducing an auxiliary phase pattern, which is incorporated in a cost function together with the desired far-field pattern, the array pattern, the near electric field amplitude, and the array excitations. Such cost function is minimized by iteratively modifying the auxiliary phase pattern and the array excitations. Applications of the proposed method to conformal arrays of different geometries show that accurate results are obtained also in presence of stringent requirements and within acceptable CPU times, even when hundreds of elements are involved.

Performance Evaluation of Distributed Wireless Networks Using Smart Antennas in Low-Rank Channel

This paper investigates the performance of a distributed wireless network in which the nodes are equipped with fully adaptive smart antennas. The main goal is the evaluation of the influence that the geometrical configuration of the antenna array has on network throughput and packet delay. Analysis and simulations are performed to study the relationship between the geometry of the antenna array and the network topology. In this paper, the performance degradation due to both the small-channel angular spreads and the speed of mobile nodes is investigated. The results, which hold for the low-rank channel model only, extend to this specific case previous results obtained in ideal channel conditions. The study adopts an access scheme, previously proposed for ad-hoc networks, which is able to exploit the interferer suppression capabilities of smart antennas, together with realistic models for the antenna and the channel behaviors.

Multi-Packet Communication in 802.11 Networks: A MAC/PHY Backward Compatible Solution

This paper presents a novel Medium Access Control (MAC) protocol for 802.11 networks in which the nodes can adopt smart antenna systems. The presented protocol is designed to satisfy two fundamental requirements: the exploitation of adaptive arrays to increase the network throughput and the backward compatibility of the novel scheme with the 802.11 MAC layer. The developed solution allows the simultaneous coexistence of multiple communications, while maintaining, for the legacy nodes, the same throughput levels achievable in the 802.11 standard. The performance and the backward compatibility of the proposed MAC protocol are tested in multipath-fading environment and considering heterogeneous scenarios, including 802.11 legacy stations together with nodes implementing the novel scheme and equipped with different antenna systems.