The openness of underwater wireless sensor networks (UWSNs) exposes them to potential eavesdropping attacks, enabling attackers to trace back and identify the source nodes of packet flows. This poses a significant threat to the confidentiality of sensitive applications, known as the Source Location Privacy (SLP) problem. Conventional packet encryption methods are ineffective in defending against SLP attacks since attackers do not need to know the content of the packets. A commonly used method to address the SLP problem is to establish fake transmission paths, making attackers follow fake paths and thus extending the time required to trace back to the source node. Existing SLP solutions that use fake transmission paths only consider individual source nodes, where the fake paths constructed for different source nodes are independent and cannot cooperate to resist attacks. In this paper, a Fake Path Co-Construction source location privacy protection protocol (FPCC) suitable for UWSNs is proposed. FPCC combines the existing transmission paths and creates co-constructed fake paths to simultaneously protect two source nodes. Simulation results confirm that FPCC, when compared with existing well-performed SLP protection protocols, extends safety time without increasing the number of nodes involved in transmitting fake packets.

In wireless sensor networks, sensor nodes are usually battery-powered and thus how to conserve energy is a critical issue. Allowing sensor nodes to enter sleep mode periodically to reduce power consumption is a common mechanism to conserve energy. However, existing power-saving protocols cannot generate arbitrary duty cycle based on a sensor node's traffic load. This implies that improvement on energy efficiency is still needed. In this paper, we propose a power-saving protocol, E-MAC, which is capable of generating any duty cycle based on its traffic load. Simulation results verify that E-MAC outperforms existing power-saving protocols (such as Green-MAC) in terms of power consumption, network throughput, and network lifetime.

In cognitive radio networks (CRNs), a rendezvous between two nodes exists when they tune to the same channel simultaneously. Rendezvous guarantee between any pair of nodes is essential because a rendezvous is a necessary condition for a communication. Several existing works claim that such a guarantee can be provided by using a dedicated common control channel for channel negotiation. A serious problem of these mechanisms is that a globally available channel may not exist. Some channel hopping protocols try to provide rendezvous guarantee without using a common control channel. However, these solutions may suffer from low number of rendezvous and uneven channel utilization. In this paper, we propose a novel distributed channel hopping protocol called Quorum and Latin Squares Channel Hopping, QLCH. QLCH efficiently provides rendezvous guarantee by utilizing the concept of quorum systems and latin squares. The former is utilized to guarantee rendezvous while the latter is adopted to share the rendezvous among channels. Simulation results verify that the proposed QLCH protocol performs better in terms of network throughput and time to rendezvous when compared to existing protocols, L-QCH, ACH, and SYNC-ETCH.

Compared with traditional terrestrial radio transmissions in wireless sensor networks, the challenges of transmissions in underwater sensor networks (UWSNs) include lower transmission rate, longer delay time, and higher power consumption. In such a circumstance, the negative effects of transmission collisions deteriorate. Most of the existing UWSN medium access control (MAC) protocols handle the collision problem in a single-hop or light-loaded environment. They fail to function effectively in a multihop network consisting of more sensor nodes with heavier traffic loads. Using the concept of cyclic quorum systems, we propose a distributed multiple-rendezvous multichannel MAC protocol, MM-MAC, in this paper to reduce collision probability. The advantages of the proposed protocol are threefold: 1) Only one modem is needed for each node to solve the missing receiver problem which is often encountered in multichannel protocols; 2) multiple sensor node pairs can complete their channel negotiations on different channels simultaneously; and 3) data packets will not be collided by control packets and vice versa. Simulation results verify that our protocol can reduce collision probability significantly which enhances the network performance in a multihop UWSN.

Power saving is always a critical issue in wireless sensor networks (WSNs). To reduce power consumption, utilizing duty cycling, i.e., sensor nodes switching to sleeping mode for most of the time, is commonly used in WSNs. However, sensor nodes may not be able to stay awake simultaneously to communicate with each other if their active periods are not properly scheduled. That is, a sensor node may not have a rendezvous with another node. To solve this problem, we propose a new scheduling mechanism, the Staggered Scheduling protocol (SS), in this paper. By adjusting each node's instantaneous duty cycle in a staggered way, the proposed SS scheme can i) ensure each pair of nodes to rendezvous with each other in a limited time span, ii) produce a lower rendezvous variance when compared with existing mechanisms, and iii) allow sensor nodes to choose their target duty cycles independently. Theoretical analysis and extensive simulations have verified the superiority and feasibility of the proposed SS scheme.

Prolonging network lifetime is always a critical issue in wireless sensor networks (WSNs). To reduce energy consumption, utilizing duty cycling, i.e., sensor nodes switching to sleeping mode for most of the time, is a common strategy in WSNs. However, without properly scheduled, sensor nodes may not be able to stay awake simultaneously to communicate with each other. That is, nodes may not have a rendezvous to each other. To solve this problem, in this paper, we propose a new scheduling mechanism, Staggered Scheduling (SS). By adjusting each node's instantaneous duty cycle in a staggered way, the SS scheme can i) ensure each pair of nodes a rendezvous in a limited time span, ii) produce more rendezvous when compared with existing mechanisms, and iii) allow each sensor node to choose its target duty cycle independently. Extensive simulation results verify the superiority and feasibility of the proposed SS scheme.

Compared with traditional terrestrial radio transmissions in Wireless Sensor Networks (WSNs), the challenges of transmissions in UnderWater Sensor Networks (UWSNs) include lower transmission rate, longer delay time, and higher power consumption. In such a circumstance, the negative effects of transmission collisions deteriorate. Most of the existing UWSN MAC protocols handle the collision problem in a single hop or light-loaded environment. They fail to function effectively in a multi-hop network consisting of more sensor nodes with heavier traffic loads. In this paper, using the concept of cyclic quorum systems, we propose a distributed multiple rendezvous multichannel MAC protocol, MM-MAC, to reduce collision probability. The advantages of the proposed protocol are threefold: 1) Only one modem is needed for each node to solve the missing receiver problem which is often encountered in multichannel protocols. 2) Multiple sensor node pairs can complete their channel negotiations on different channels simultaneously. 3) Data packets will not be collided by control packets, and vice versa. Simulation results verify that our protocol can reduce collision probability significantly which enhances the network performance in a multi-hop UWSN.

Since sensor nodes are energy-constrained, energy saving is a critical issue in wireless sensor networks. By reducing the number of transmissions, data aggregation is an effective approach to save energy. In the literature, most of data aggregation protocols rely on a structured architecture to accomplish the data gathering task. Such structure-based methods suffer from high maintenance overhead in a dynamic environment where sensor nodes may move or fail unexpectedly. In this paper, we propose a structure-free and energy-balanced data aggregation protocol (SFEB). The two-phase aggregation and dynamic aggregator selection of SFEB enable both efficient data gathering and balanced energy consumption. Extensive simulations verify the superiority of our SFEB.

Wireless sensor networks are mainly designed for environment surveillance where wireless sensor nodes cooperate to get their job done. Generally, wireless sensors are battery powered. Therefore, it is crucial to use sensor nodes' battery resources efficiently. Most of the existing power saving protocols achieve power saving by putting sensor nodes to sleep periodically. Such a uniform sleep/wake up mechanism fails to adjust a sensor node's sleep duration based on its traffic load, thus causing either lower power efficiency or higher latency. In this paper, we proposed a quorum-based MAC protocol that enables sensor nodes to sleep longer under light loads. Since traffic flows toward the sink node in wireless sensor networks, a new concept next hop group was also proposed to reduce transmission latency. Simulation results verified that the proposed QMAC saved more energy and kept the transmission latency low.

Abstract A contention‐based wireless ad hoc medium access control (MAC) protocol, such as carrier sense multiple access with collision avoidance (CSMA/CA), has excellent efficiency when the system is light loaded. The main drawback of such protocols is their inefficiency and unbounded delay when the system load is heavy. On the other hand, a contention‐free MAC protocol, such as token passing, has a better and fair throughput when the system is heavy loaded. The main drawback of such protocols is their inefficiency when only a small amount of users want to transmit. In this paper, we propose a new load awareness single‐hop wireless ad hoc MAC protocol (which is called theLAprotocol) that exploits the benefits of both contention‐based and contention‐free protocols. A contention‐based MAC protocol is used when the system is light loaded and a contention‐free one is used otherwise. OurLAprotocol, which operates in a distributed fashion and is fully compatible with the IEEE 802.11 wireless local area network (WLAN) standard, can switch smoothly between the contention‐based protocol and the contention‐free one. Simulation results show that our protocol indeed extracts the better part of two kinds of protocols. Copyright © 2006 John Wiley & Sons, Ltd.

In this paper, the authors propose a power-saving protocol, the fuzzy control quorum-based energy conserving protocol (FQEC), for IEEE 802.11 ad hoc networks. In the IEEE 802.11 power saving mode, hosts have to wake up every beacon interval to check if they have to remain awake in the remaining of that beacon interval. It is not an efficient power-saving solution since a host with little traffic still has to wake up every beacon interval. In this paper, the authors investigate the possibility to use a fuzzy control system to dynamically adjust the sleep duration in order to conserve more energy. In the fuzzy control system, the authors define the control rules according to the experienced latency of both incoming and outgoing packets. Such a definition exploits each host's traffic load to obtain the best sleep/awake frequency. Simulation results verify that our FQEC indeed achieves better energy conservation.

This paper handles the internal and external fragmentation problems of CDMA systems using orthogonal variable spreading factor (OVSF) codes. Internal fragmentation occurs when the allocated data rate is larger than what is requested while external fragmentation occurs when the OVSF code tree is too fragmented to support a call even if there are sufficient capacity remaining in the code tree. The key factor in solving these two problems is the OVSF code assignment strategy. Most works in the literature do not consider the time-varying and location-dependent channel conditions. In this paper, we formulate the fragmentation problem as a multiple knapsack problem where each OVSF code is considered as a knapsack. We propose single-code, time-shared strategies that consider channel conditions while solving these fragmentation problems. We show that internal fragmentation can be significantly reduced if we can properly select users to share OVSF codes. Simulation results verify that our strategies do use the precious wireless bandwidth efficiently

The lifetime of a mobile ad hoc network (MANET) depends on the durability of the mobile hosts' battery resources. In the IEEE 802.11 Power Saving Mode, a host must wake up at every beacon interval, to check if it should remain awake. Such a scheme fails to adjust a host's sleep duration according to its traffic, thereby reducing its power efficiency. This paper presents new MAC protocols for power saving in a single hop MANET. The essence of these protocols is a quorum-based sleep/wake-up mechanism, which conserves energy by allowing the host to sleep for more than one beacon interval, if few transmissions are involved. The proposed protocols are simple and energy-efficiency. Simulation results showed that our protocols conserved more energy and extended the lifetime of a MANET.

Packet scheduling in a WCDMA system poses a new challenge due to its nature of variable bit rates and location-dependent, time-varying channel conditions. In this work, three new downlink scheduling algorithms for a WCDMA base station are proposed to support multimedia transmissions. Using a credit management and a compensation mechanism, our algorithms provide rate guarantee and fair access to mobile terminals. In particular, we propose to allow a user to simultaneously use multiple OVSF codes in a time-sharing manner, which we call a multicode, shared model. Using multiple codes allows us to compensate those users suffering from bad communication quality or even errors. The proposed schemes can tolerate a multistate link condition (compared to the typically assumed two-state, or good-or-bad, link condition) by adjusting the number of OVSF codes and the spreading factor of each code. Simulation results show that the proposed schemes do achieve higher bandwidth utilization while keeping transmission delay low.

In the third-generation partnership project technical specification, orthogonal variable spreading factor (OVSF) codes are used as channelization codes. The use of OVSF codes can provide variable data rates to flexibly support applications with different bandwidth requirements. Most works in the literature assume that only one single OVSF code is used to support one connection. This may sometimes waste the scarce wireless bandwidth since the allocated bandwidth will increase exponentially as the spreading factor decreases, i.e., a user may be "overserved." In this paper, we consider the possibility of using multiple OVSF codes to support a connection. We show how using multiple codes can reduce the internal and external fragmentations of an OVSF code tree. The tradeoff between bandwidth utilization and hardware complexity of a multicode system is analyzed. The result shows that using two or three codes will be quite cost effective. Several multicode assignment and reassignment strategies, namely, random, leftmost, crowded-first-space, and crowded-first-code, are proposed based on such an environment. Our simulation results show significant increase in code tree utilization and significant reduction in code blocking probability by using the crowded-first-space and crowded-first-code schemes.

In multihop wireless ad hoc networks, it is important that all mobile hosts are synchronized. Synchronization is necessary for power management and for frequency hopping spread spectrum (FHSS) operations. IEEE 802.11 standards specify a clock synchronization protocol but this protocol suffers from the scalability problem due to its inefficiency contention mechanism. We propose an automatic self-time-correcting procedure (ASP) to achieve clock synchronization in a multihop environment. Our ASP has two features. Firstly, a faster host has higher priority to send its timing information out than a slower one. Secondly, after collecting enough timing information, a slower host can synchronize to the faster one by self-correcting its timer periodically (which makes it becoming a faster host). Simulation results show that our ASP decreases 60% the average maximum clock drift as compared to the IEEE 802.11 and reduces 99% the number of asynchronism in a large-scale multihop wireless ad hoc networks.

A contention-based wireless ad hoc medium access control (MAC) protocol, such as carrier sense multiple access with collision avoidance (CSMA/CA), has the excellence of simple and efficient when the system is light-loaded. The main drawback of such protocols is their inefficiency and unbounded delay when system load is heavy. On the other hand, a contention-free MAC protocol, such as token passing, has better and fair throughput when the system is heavy-loaded. The main drawback of such protocols is their inefficiency when only a small amount of users want to transmit. In this paper, we propose a new load awareness wireless ad hoc MAC protocol (which is called the LA protocol) that exploits the benefits of both contention-based and contention-free protocols. A contention-based MAC protocol is used when system is light-loaded and a contention-free one is used otherwise. Our LA protocol, which operates distributed and is fully compatible with IEEE 802.11 wireless local area network (WLAN) standard, can switch smoothly between the contention-based protocol and the contention-free one. Simulation results show that our protocol indeed extracts the better part of two kinds of protocols and performs well in all systems loads.

The lifetime of a mobile ad hoc network (MANET) depends on the durability of the battery resource of the mobile hosts. Earlier research has proposed several routing protocols specifically on MANET, but most studies have not focused on the limitations of battery resource. We propose a new energy-aware routing protocol, which can increase the durability of the energy resource and, therefore, the lifetime of the mobile hosts and the MANET. The proposed protocol can conserve energy by shortening the idle period of the mobile hosts without increasing the probability of packet loss or reducing routing fidelity. Simulation results indicate that this new energy-conserving protocol can extend the lifetime of a MANET.

Packet scheduling in a WCDMA system poses a new challenge due to its nature of variable bit rates and location-dependent, time-varying channel conditions. In this work, three new downlink scheduling algorithms for a WCDMA base station are proposed to support multimedia transmissions. Using a credit management and a compensation mechanism, our algorithms provide rate guarantee and fair access to mobile terminals. In particular, we propose to allow a user to simultaneously use multiple OVSF codes in a time-sharing manner, which we call a multicode, shared model. Using multiple codes allows us to compensate those users suffering from bad communication quality or even errors. The proposed schemes can tolerate a multi-state link condition (compared to the typically assumed twostate, or good-or-bad, link condition) by adjusting the number of OVSF codes

In the 3GPP technical specification, OVSF codes are used as channelization codes. The use of OVSF codes can provide variable data rates to flexibly support applications with different bandwidth requirements. Most works in the literature assume that only one signal OVSF code is used to support one connection. This may sometimes waste the scarce wireless bandwidth since the allocated bandwidth will increase exponentially as the spreading factor decreases, i.e., a user may be "over-served". In this paper, we consider the possibility of using multiple OVSF codes to support a connection. We show how using multiple codes can reduce internal fragmentation and external fragmentation of a OVSF code tree. The tradeoff between bandwidth utilization and hardware complexity of a multi-code system is analyzed. The result shows that using 2 or 3 codes will be quite cost-effective. Several multi-code assignment and reassignment strategies, namely random, left-most, crowded-first-space, and crowded-first-code, are proposed based on such environment. Our simulation results show significant increase in code tree utilization and significant reduction in code blocking probability by using the crowded-first-space and crowded-first-code schemes.

The use of OVSF codes in WCDMA systems has offered opportunities to provide variable data rates to flexibly support applications with different bandwidth requirements. Two important issues in such an environment are the code placement problem and code replacement problem. The former may have significant impact on code utilization and, thus, code blocking probability, while the latter may affect the code reassignment cost if dynamic code assignment is to be conducted. The general objective is to make the OVSF code tree as compact as possible so as to support more new calls by incurring less blocking probability and less reassignment costs. Earlier studies about these two problems either do not consider the structure of the OVSF code tree or cannot utilize the OVSF codes efficiently. To reduce the call blocking probability and the code reassignment cost, we propose two simple yet efficient strategies that can be adopted by both code placement and code replacement: leftmost and crowded-first. Numerical analyses on call blocking probability and bandwidth utilization of OVSF code trees when code reassignment is supported are provided. Our simulation results show that the crowded-first strategy can significantly reduce, for example, the code blocking probability by 77 percent and the number of reassignments by 81 percent, as opposed to the random strategy when the system is 80 percent fully loaded and the max SF = 256.

The use of OVSF codes in WCDMA systems has offered opportunities to provide variable data rates to flexibly support applications with different bandwidth requirements. Two rarely addressed issues in such environments are the code placement problem and code replacement problem. The former may have significant impact on code utilization and thus code blocking probability, while the latter may affect the code reassignment cost if dynamic code assignment is to be conducted. The general objective is to make the OVSF code tree as compact as possible in order to support new calls, either with less blocking or with less reassignment cost. This paper is perhaps the first one which addresses these issues in WCDMA. Three simple strategies, which can be adopted by both code placement and code replacement, are proposed: random, leftmost, and crowded-first. Among them the crowded-first strategy looks most promising, which is shown to be able to reduce, for example, the code blocking probability by 77% and the number of reassignments by 81% as opposed to the random strategy when the system is 80% fully loaded and the max SF=256.