Josep Cotrina

Outdoor IEEE 802.11g cellular network performance

Most studies about the performance of IEEE 802.11 are limited to a single cell environment. Nevertheless, the idea of designing an outdoor cellular network based on WLAN IEEE 802.11 is very attractive, due to the several advantages that this technology presents: the low cost of the equipment, its operation in unlicensed spectrum and its higher data rates. In this paper, we study the possibility of designing an outdoor cellular network based on the IEEE 802.11g standard. We present its performance under different load conditions and compare this behavior with the results obtained in an isolated single cell environment, without co-channel interference. Finally, going a step further, this paper explores the IEEE 802.11g cellular network performance for different cluster sizes, as a method to reduce the interference influence on networks performance.

Wireless technology applied to GIS

At present, there is a growing interest in wireless applications, due to the fact that the technology begins to support them at reasonable costs. In this paper, we present the technology currently available for use in wireless environments, focusing on Geographic Information Systems. As an example, we present a newly developed platform for the commercialization of advanced geographical information services for use in portable devices. This platform uses available mobile telephone networks and wireless local area networks, but it is completely scalable to new technologies such as third generation mobile networks. Users access the service using a vector map player that runs on a Personal Digital Assistant with wireless access facilities and a Global Positioning System receiver. Before accessing the information, the player will request authorization from the server and download the requested map from it, if necessary. The platform also includes a system for improving Global Positioning System localization with the Real Time Differential Global Positioning System, which uses short GSM messages as the transmission medium.

Enhancement proposal for WLAN IEEE 802.11e: desynchronization of its working procedure

Up to the present, several studies have been performed in order to provide prioritization of stations or classes of service for WLAN IEEE 802.11. The IEEE 802.11e draft specification aims to extend the original IEEE 802.11 MAC protocol to support QoS. One of the mechanisms for prioritizing traffic is the assignment of different AIFS times to each priority level. Nevertheless, the AIFSs employed are separated by values that are multiples of the slot time. Therefore, due to the fact that the backoff time counter is slotted, the different priority levels can attempt for transmission simultaneously. In this paper, we evaluate the performance of the IEEE 802.11e when its working procedure is desynchronized: we avoid that stations belonging to different priority levels attempt for transmission at the same time. We assign AIFS times that are separated by values that are not multiples of the slot time, in order to avoid collisions between the different priority levels. We present a mathematical model and simulation results for analyzing the performance of the differentiation mechanism proposed. The results show that it solves the strangulation of low priority traffic, fact that occurs in IEEE 802.11e EDCA. Moreover, this proposal leads to a significant increase in the performance of the system as a whole

IEEE 802.11g performance in presence of beacon control frames

Most studies of the performance of IEEE 802.11 consider scenarios of ad-hoc topology networks and do not contemplate the network broadcast information contained in beacon frames. The paper presents a study of the performance of a WLAN whose infrastructures topology is such that the access point is in charge of broadcasting the beacon frames. Thus, it is more realistic than previous studies, because beacon frames are usually transmitted in order to announce control information and network identity. Furthermore, in the coverage area, user stations are likely to be working at different data rates, depending on their signal quality. Because beacon frames must be received by all stations, they are transmitted at the lowest data rate operating in the coverage area. The article introduces a mathematical method to calculate the influence of beacon frames on the total throughput, collision probability and delays of the IEEE 802.11g protocol. The model is validated by simulation analysis.

Propagation delay influence in IEEE 802.11 outdoor networks

Originally, Wireless Local Area Networks served only small indoor areas. Nevertheless, the idea of employing IEEE 802.11 networks in large outdoor environments is a very attractive possibility. IEEE 802.11 technology offers several advantages: the low cost of equipment, its operation in the unlicensed spectrum and its higher data rates. Since the advent of the first IEEE 802.11 standard, a great deal of research has been carried out. So-called Wifi-based Long Distance networks are currently being deployed. In this paper, we study the suitability of employing IEEE 802.11 networks in large outdoor environments without modifying the standard working procedure. In such scenarios, IEEE 802.11 networks should offer coverage ranges of several kilometer, which leads to high propagation delay values. Thus, we analyze the influence of increasing propagation delay in the IEEE 802.11 MAC protocol. To carry out our analysis we present a mathematical model and simulation results. We provide an operating range in which IEEE 802.11 performance is feasible and establish a throughput threshold according to the propagation delay.

Tracing illegal redistribution using errors-anderasures and side information decoding algorithms

In a fingerprinting scheme, a distributor places marks in each copy of a digital object. Placing different marks in different copies uniquely identifies the recipient of each copy, and therefore allows tracing of the source of an unauthorised redistribution. A widely used approach to the fingerprinting problem is the use of error correcting codes with a suitable large minimum distance. With this approach, the set of embedded marks in a given copy is precisely a code word of the error correcting code. We present two different approaches that use side information for the tracing process. The first approach deals uses the Guruswami–Sudan errors-and-erasures list decoding algorithm whereas the second approach shows the use of a full side information matrix.

A note about the traceability properties of linear codes

We characterize the traceability properties of linear codes. It is well known that any code of length n and minimum distance d is a c-TA code if c2 < n/(n-d). In this paper, we show that a less restrictive condition can be derived. In other words, there exists a value ZC, with n - d ≤ ZC ≤ c(n - d), such that any linear code is c-TA if c ≤ n/ZC. We also prove that in many cases this condition is also necessary. These results are applied to cyclic and Reed-Solomon codes.

Fingerprinting schemes. identifying the guilty sources using side information

In a fingerprinting scheme a distributor places marks in each copy of a digital object. Placing different marks in different copies, uniquely identifies the recipient of each copy, and therefore allows to trace the source of an unauthorized redistribution. A widely used approach to the fingerprinting problem is the use of error correcting codes with a suitable minimum distance. With this approach, the set of embedded marks in a given copy is precisely a codeword of the error correcting code. We present two different approaches that use side information for the tracing process. The first one uses the Guruswami-Sudan soft-decision list decoding algorithm and the second one a modified version of the Viterbi algorithm.

A note about the identifier parent property in Reed-Solomon codes

Codes with traceability properties are used in schemes where the identification of users that illegally redistribute content is required. For any code with traceability properties, the Identifiable Parent Property (c-IPP) seems to be less restrictive than the Traceability (c-TA) property. In this paper, we show that for Reed-Solomon codes both properties are in many cases equivalent. More precisely, we show that for an [n, k, d] Reed-Solomon code, defined over a field that contains the n -d roots of unity, both properties are equivalent. Also, we show how the strategy we propose can be applied to other cases by proving the equivalence of both properties for a particular code of characteristic 2. This answers a question posted by Silverberg et al. (2001, 2003), for a large family of Reed-Solomon codes.

On the IPP Properties of Reed-Solomon Codes

Codes with traceability properties are used in schemes where the identification of users that illegally redistribute content is required. For any code with traceability properties, the Identifiable Parent Property (c-IPP) seems to be less restrictive than the Traceability (c-TA) property. In this paper, we show that for Reed-Solomon codes both properties are in many cases equivalent. More precisely, we show that for an [n,k,d] Reed-Solomon code, defined over a field that contains the n − d roots of unity, both properties are equivalent. This answers a question posted by Silverberg et al. in [10,11], for a large family of Reed-Solomon codes.