Abbas Jamalipour

Performance of an integrated voice/data system in nonuniform traffic low Earth-orbit satellite communication systems

In some recent studies, the use of low Earth-orbit satellites in various applications is considered. In all of these studies, uniform distribution of traffic load is assumed. In this paper, the performance of a low Earth-orbit satellite communication system which is designed to service to two kinds of users; i.e., voice users and data users is estimated. The distribution of population of these users is assumed to be nonuniform. According to the simulation results, it is shown that the nonuniformity in traffic affects the performance of the system by decreasing the signal quality at the satellite which has to service to populated areas and increasing it superfluously at the satellite with not so populated service area, in a given period of time. By modeling the satellites during their movements, the change in signal quality while experiencing a peak of traffic load in their route is also determined. A modified power control method based on the amount of traffic load of each satellite is also examined and is shown that this method can make some performance improvements in signal quality, which is limited by special features of low Earth-orbit satellite systems. >

Traffic characteristics of LEOS-based global personal communications networks

The authors present important traffic issues in a low earth orbit satellite-based global personal communications network. The article gives an overview of the characteristics of tomorrows personal communications networks from a subscriber traffic viewpoint. Selection of multiple access scheme as well as traffic modeling in such systems is discussed, and effects of natural, unbalanced global communications traffic on the performance of the system are distinguished.

Transmit permission control on spread ALOHA packets in LEO satellite systems

A transmit permission control method for improving the throughput characteristics of a low Earth orbit (LEO) satellite communication system employing spread-slotted ALOHA multiple-access scheme is proposed. Both nonfading and fading satellite links are considered. The basic idea of the proposed scheme is to decrease the level of interference at each satellite and, hence, to increase the probability of packet success, by prohibiting the packet transmission from the users with relatively high propagation loss to their connecting satellites. It is shown that the method has the ability to improve the throughput performance in heavy traffic loads and the peak value of the throughput, significantly. It is also shown that the average delay performance of the system employing the proposed scheme is superior to that of the conventional system at heavy traffic loads.

Signal-to-interference ratio of CDMA in low Earth-orbital satellite communication systems with nonuniform traffic distribution

The performance of a satellite communication system in which the satellites are in non-geostationary orbits, with nonuniform geographical distribution of the traffic load is estimated through analysis. It is shown that when CDMA is applied to the reverse link, the traffic nonuniformity results in a large difference in the signal qualities. The signal above a dense traffic area has a low signal-to-interference ratio (SIR), while the signal over a sparse traffic area has an excessive SIR. In addition, a traffic assignment scheme, which makes the load of the satellite over the dense traffic area smaller, is proposed and its performance improvement is estimated.

Packet admission control in a direct-sequence spread-spectrum LEO satellite communications network

The effect of multiple-access interference on the throughput performance of a direct-sequence spread-spectrum low-Earth-orbiting satellite communications network is discussed. To recognize the effect of interference when their sources are either inside or outside the service area of a satellite, we develop a stochastic model for the location of users. We show that the effect of interference on the performance degradation from users with large propagation distance to their connecting satellites is the dominant factor. Hence, to improve the performance of the system, we propose a method in which the transmissions of packets are controlled according to their distances to connecting satellites as well as the traffic distribution.

Communication protocols and algorithms for the smart grid [Guest Editorial]

Electric power regulation and privatization is creating new challenges on high voltage transmission and energy distribution systems. The existing electrical infrastructures must be updated in order to meet the needs of the digital society. Smart Grid describes a next-generation electrical power system that creates an increased use of communications and information technology in generation, delivery and consumption of electrical energy.

A modified power control scheme for remedying the effects of traffic nonuniformity in LEO satellite communications systems

It has been shown in our previous studies that the geographical traffic nonuniformity considerably affects the performance of the low earth orbit satellite communications systems. In this paper, a new scheme for improving the throughput characteristics of these systems at nonuniform traffic distribution is proposed. In this method, some parts of the users under the satellite which is flying over the area with high traffic load are assigned to its neighbor satellites with lower transmitting power levels. It is shown that the method equalizes the traffic loads of the satellites to some degree and, hence, can improve the throughput characteristics of the system.

Self-organizing networks [Message from the Editor-in-Chief]

Wireless sensor networks and mobile ad hoc networks are considered as new communication environments where in general there is no central control on the activity of their users. In particular, users are free to move anywhere in the network, establish a connection, or move out of the network. There is no special binding such as one would see in a cellular mobile network, and as a consequence, the mobility pattern and modeling could be significantly different from those considered in legacy mobile networks. Localization techniques therefore become an important topic of research for these networks. Finding possible locations or future movement patterns of a user in these networks could result in much better resource allocation, packet delivery, and routing than when such information is not available. Typical mobility modeling, such as random walk, can be used in these network, but often they do not provide much intelligence on the operation of the network. Sophisticated mobility models specifically designed for ad hoc and sensor networks are necessary, while they have to place a minimum burden on nodes already low in resources. As an example, vehicular communications are constrained by road and traffic rules; therefore, directional mobility models would better simulate their mobility pattern. The self-organization in vehicular ad hoc networks would be better accomplished and transferred if some parameter-based mobility model is added to the traditional random mobility modeling.

Spread-slotted ALOHA throughput in nonuniform traffic situation for LEO satellite communication systems

An analytical framework to study the nonuniformity in geographical distribution of the traffic load in low Earth-orbital satellite communication systems is presented. In these systems, because of their small coverage area compared with conventional geostationary satellites, the nonuniformity of the traffic affects the performance of the system considerably. The throughput characteristics of a system with direct-sequence packet spread-slotted ALOHA multiple-access technique are evaluated and it is shown that that the traffic nonuniformity makes the characteristics of the system significantly different from the results of the uniform traffic case and that the performance of each user varies according to the location. Moreover, the interference from the users of adjacent satellites is shown to be one of the main factors that limits the performance of system.<<ETX>>

Throughput performance improvement of spread-slotted ALOHA in mobile satellite communications

Application of spread spectrum techniques in conventional slotted ALOHA, namely spread-slotted ALOHA, offers relatively high capacity multiple simultaneously packet transmissions. Two adaptive versions of the transmit permission control (TPC) scheme applicable in multi-cell communication systems are proposed. In the basic TPC scheme, by prohibiting the transmissions from the users with relatively high propagation loss to their connecting hub stations, the level of multiple access interference is decreased, and hence the throughput characteristics are improved. In the proposed schemes, the prohibition condition becomes more intelligent by considering the level of the offered traffic loads to the hub stations. It is shown that the adaptive schemes exhibits significantly improved characteristics at all offered traffic loads.