In this paper, we study vacation time in multidetector networks designed to detect the presence of a source at an unknown location. We first determine the minimum achievable detection time Tcapand show the existence of randomized detection schemes that have detection times arbitrarily close to Tcapfor almost all configuration of detectors, provided the number of detectors n is sufficiently large. We call such schemes as capacity achieving and completely characterize all capacity achieving detection schemes.

Cognitive radio networks need to continuously monitor spectrum to detect the presence of the licensed users. In this paper, we have exploited spatial diversity in multiuser networks to improve the spectrum sensing capabilities of centralized cognitive radio (CR) networks. We develop a fixed and a variable relay sensing scheme. The fixed relay scheme employs a relay that has a fixed location to help the cognitive network base station detect the presence of the primary user. The variable relay sensing scheme employs cognitive users distributed at various locations as relays to sense data and to improve the detection capabilities. This effectively reduces the average detection time by exploiting spatial diversity inherent in multiuser networks. Finally, we study the network outage probabilities to compare the performances of the fixed and variable relay schemes.

In this paper, we present a simple reservation scheme for multicarrier networks employing channel aware Aloha. We consider the effect of channel measurement errors in the existing channel aware Aloha scheme with collision resolution and show that any uncertainty in channel conditions, however small it may be, leads to zero asymptotic throughput. We then describe a simple reservation scheme specially suited for multicarrier networks and show how multicarrier diversity can be effectively used to offset degradation in the throughput performance of channel aware Aloha operating with channel uncertainties.

Cognitive radio networks need to continuously monitor spectrum to detect the presence of the licensed users. In this paper, we have exploited spatial diversity in multiuser networks to improve the spectrum sensing capabilities of centralized cognitive radio (CR) networks. We develop a fixed and a variable relay sensing scheme. The fixed relay scheme employs a relay that is fixed in location to help the cognitive network base station detect the presence of the primary user. The variable relay sensing scheme employs cognitive users distributed at various locations as relays to sense data and to improve the detection capabilities. We theoretically prove that the proposed variable relay sensing scheme effectively reduces the average detection time which is also illustrated by an insightful example. Finally, we introduce a useful metric to measure the performance of fixed relay and variable relay schemes.

In this paper, the authors investigated the effect of multicarrier diversity on the stability region of channel aware Aloha. The authors show that the stability region of a two-user multicarrier network always includes and is much larger than that of a two-user single carrier network. Therefore, multicarrier diversity significantly improves stability of Aloha networks.

In this correspondence, we have studied the stability properties of multicarrier channel aware Aloha. For a simple two-user network, the stability region${\cal S}_2$under the classical collision model has been well-studied. In this correspondence, using multiple carriers, we show how to achieve any input rate vector in${\cal S}_2$with$1$-persistent Aloha. We then show how multicarrier diversity can be exploited to improve upon the existing stability region.

In cognitive radio networks, cognitive (unlicensed) users need to continuously monitor spectrum for the presence of primary (licensed) users. In this paper, we illustrate the benefits of cooperation in cognitive radio. We show that by allowing the cognitive users operating in the same band to cooperate we can reduce the detection time and thus increase the overall agility. We first consider a two-user cognitive radio network and show how the inherent asymmetry in the network can be exploited to increase the agility. We show that our cooperation scheme increases the agility of the cognitive users by as much as 35%. We then extend our cooperation scheme to multicarrier networks with two users per carrier and analyze asymptotic agility gain. In Part II of our paper [1], we investigate multiuser single carrier networks. We develop a decentralized cooperation protocol which ensures agility gain for arbitrarily large cognitive network population.

In cognitive networks, cognitive (unlicensed) users need to continuously monitor spectrum to detect the presence of primary (licensed) users. In part I, we have illustrated the benefits of cooperation in cognitive radio by considering a simple two-user network and showing improvement in agility. In part II, we investigate multiple cognitive user networks. We first consider multiuser single carrier networks and develop sufficient conditions for agility gain when the cognitive population is arbitrarily large. We then propose a practical algorithm which allows cooperation between cognitive users in random networks. Finally, we provide an example to illustrate the concepts developed in this paper.

Channel aware medium access protocols perform decisions primarily based on the instantaneous channel state information (CSI) available. In practice, channel measurements and quantization introduce distortion in the available CSI at the transmitter. In this paper, we investigate the effect of imperfect CSI on the asymptotic throughput of a particularly important medium access protocol - the channel aware Aloha. We show that any degree of uncertainty in the channel estimate leads to zero throughput asymptotically in the existing single carrier Aloha scheme; however, multicarrier diversity is effective in compensating for lack of perfect CSI.

In this paper, we illustrate the benefits of cooperation in cognitive radio. Cognitive (unlicensed) users need to continuously monitor spectrum for the presence of primary (licensed) users. We show that by allowing the cognitive radios operating in the same band to cooperate we can reduce the detection time and thus increase the overall agility. We first consider the case of two cognitive users and show how the inherent asymmetry in the network can be exploited to increase the agility. We then extend our protocol to study multi-user multi-carrier cognitive network. We compare our cooperation scheme with the non-cooperation scheme and derive expressions for agility gain. We show that our cooperation scheme reduces the detection time for the cognitive users by as much as 35%

In this paper, we highlight the advantage of multichannel random access schemes by jointly exploiting the inherent multiuser and multicarrier diversity. For a deeper understanding of network behavior, we define a parameter called the population density of the network and consider multichannel systems under different types of network density. We show that by exploiting the inherent diversity of the multicarrier networks, the asymptotic stable throughput (AST) of any single carrier medium access protocol can be increased. We have developed closed form expressions for the amount of gain that can be obtained using multicarrier diversity. We then take an example of the traditional Aloha systems to demonstrate the advantages of multicarrier systems over single carrier systems and show using simulation results that the maximum throughput gain achievable is as high as 50%.