Portfolio selection is a vital research field in modern finance. Multi-objective portfolio optimization problem is the portfolio selection process that results in the highest expected return rate and the lowest identified risk among the various financial assets. This paper proposes a model that can efficiently suggest a portfolio that is worth investing. First, a cluster analysis model is introduced in order to categorize a huge amount of stock data into several groups based on their associated return rate and the risk. Several validity indexes are used to select the optimal number of clusters/stocks to be included in the portfolio. Finally, the multi-objective genetic algorithm is used to build portfolio optimization with highest return rate and lowest risk. The proposed model is tested on the data obtained from the Stock Exchange of Thailand.

Vehicular ad hoc wireless network (VANET) exhibits a bipolar behavior in terms of network topology: fully connected topology with high traffic volume or sparsely connected topology when traffic volume is low. In this work, we develop a statistical traffic model based on the data collected on 1-80 freeway in California in order to study key performance metrics of interest in disconnected VANETs, such as average re-healing time (or the network restoration time). Our results show that, depending on the sparsity of vehicles, the network re-healing time can vary from a few seconds to several minutes. This suggests that, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as dynamic source routing (DSR) and ad hoc on-demand distance vector routing (AODV) will not work with such long re-healing times.

A vehicular ad hoc network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on 1-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time). Our results show that, depending on the sparsity of vehicles or the market penetration rate of cars using Dedicated Short Range Communication (DSRC) technology, the network re-healing time can vary from a few seconds to several minutes. This suggests that, for vehicular safety applications, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. In addition, the developed analytical framework and its predictions provide valuable insights into the VANET routing performance in the disconnected network regime.

This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes

Routing protocols developed for ad hoc wireless networks use broadcast transmission to either discover a route or disseminate information. More specifically, reactive routing protocols has to flood the network with a route request (RREQ) message in order to find an optimal route to the destination. Several applications developed for vehicular ad hoc wireless networks (VANET), which is a subset of MANET, rely on broadcast to propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem. In this paper, we explore how serious the broadcast storm problem is in both MANET and VANET by examining how broadcast packets propagate in a 2-dimensional open area and on a straight road or highway scenarios. In addition, we propose three novel distributed broadcast suppression techniques; i.e., weighted p-persistence, slotted 1-persistence, and slotted p- persistence schemes. Our simulation results show that the proposed schemes can achieve up to 90% reduction in packet loss rate while keeping the end-to-end delay at acceptable levels for most VANET applications. They can also be used together with the route discovery process to guide the routing protocols to select routes with fewer hop counts.

In this paper, we compare the performance of two possible switching techniques for ad hoc wireless networks: namely, reservation-based (RB) and non-reservation-based (NRB) switching. Fundamental network performance metrics, such as throughput and delay, are studied to understand the tradeoffs between the two switching schemes. In particular, we identify the circumstances where each of these two schemes is better than the other.

In this paper, the performance of two possible switching schemes for ad hoc wireless networks, namely reservation-based (RB) and non-reservation-based (NRB) is investigated. It is shown that route reservation would be the scheme of choice in a scenario where delay is a constraint, the network is dense, and/or nodes move with moderate speed. However, a non-reservation scheme is preferable in a scenario where the network is sparse, there is heavy traffic load, nodes are static, and/or delay is not a major concern.

In this paper, we propose a simple semi-analytical approach for the evaluation of the impact of mobility on the bit error rate (BER) performance of multi-hop ad hoc wireless networks. Analytical expressions, relating the BER at the end of a multi-hop route with the mobility characteristics of the nodes and the routing strategy, are derived. Two node mobility models are considered: direction-persistent (DP) and direction-nonpersistent (DNP). In particular, two network switching scenarios are analyzed: (i) opportunistic non-reservation-based switching (ONRBS), where a message flows from source to destination by opportunistically choosing the available shortest consecutive links; and (ii) reservation-based switching (RBS), where, after the creation of a multi-hop route from source to destination, the message is "forced" to flow over the reserved links, regardless of their actual lengths. The network performance is evaluated in ideal (without inter-node interference, INI) and realistic (with INI) cases. The improved robustness against mobility offered by ONRBS, with respect to RBS, is analyzed and quantified.

It is now well known that Internet traffic exhibits self-similarity, which cannot be described by traditional Markovian models such as the Poisson process. In this work, we simulate a simple network with a full implementation of TCP-Reno. We also assume Poisson arrivals at the application layer specifically to determine whether TCP can cause self-similarity even when input traffic does not exhibit long-range dependence. Our study shows that, at some loads TCP can induce the appearance of self-similarity. In particular, when load is low and loss is rare, traffic looks Poisson. When load is high and the network is overloaded, TCP congestion control can smooth out the burstiness of the aggregate stream so that traffic at the bottleneck tends to Poisson. However, when load is intermediate and the network is prone to occasional bouts of congestion, as is typical of many networks, traffic can become self-similar. Moreover, factors such as round trip time and number of streams passing through the bottleneck can cause the network to become congested at different loads, and consequently affect the range of load over which self-similarity can be observed. The impact of one self-similar TCP stream has also been observed. In particular, our study show that if one or more streams passing through the bottleneck is self-similar and the aggregate flow does not exceed the capacity, traffic observed at the bottleneck will also be self-similar.