Chih-Min Chao

Code placement and replacement strategies for wideband CDMA OVSF code tree management

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.

A clock synchronization algorithm for multi-hop wireless ad hoc networks

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. In this paper, we propose an automatic self-time-correcting procedure (ASP) to achieve clock synchronization in a multihop environment. Our ASP has two features. First, a faster host has higher priority to send its timing information out than a slower one. Second, 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.

Design of structure-free and energy-balanced data aggregation in wireless sensor networks

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.

A Quorum-Based Energy-Saving MAC Protocol Design for Wireless Sensor Networks

Wireless sensor networks are mainly designed for environment surveillance, wherein wireless sensor nodes cooperate to get their job done. Generally, wireless sensors are battery powered; therefore, it is crucial for them to efficiently use their battery resources. Most of the existing power-saving protocols achieve power savings by periodically putting sensor nodes to sleep. Such a regular sleep/awake mechanism fails to adjust a sensor nodes sleep duration based on its traffic load, thus causing either lower power efficiency or higher latency. Furthermore, sensors may be deployed in hostile environments and may thus unexpectedly fail. Most power-saving protocols do not promptly react to such link breakage, resulting in long transmission delays. In this paper, we propose a quorum-based medium access control (QMAC) 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, i.e., the next-hop group, is also proposed to reduce transmission latency. Simulation results verify that the proposed QMAC saves more energy and keeps the transmission latency low.

Energy-conserving grid routing protocol in mobile ad hoc networks

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

Dynamic channel allocation with location awareness for multi-hop mobile ad hoc networks

The wireless mobile ad hoc network (MANET) has received a lot of attention recently. This paper considers the channel assignment problem in a MANET which has access to multiple channels. Although a MANET does not have the infrastructure of base stations, interestingly its channel assignment can be conducted efficiently in a way very similar to that in cellular systems (such as GSM). In this paper, we propose a new location-aware channel assignment protocol called GRID-B (read as GRID with Channel Borrowing), which is a sequel of our earlier GRID protocol [Location-aware channel assignment for a multi-channel mobile ad hoc network, Technical Report NCU-HSCCL-2000-02, 2000]. The protocol assigns channels to mobile hosts based on the location information of mobile hosts that might be available from the positioning device (such as GPS) attached to each host. According to our knowledge, no location-aware channel assignment protocol has been proposed before for MANETs. Several channel borrowing strategies are proposed to dynamically assign channels to mobile hosts so as to exploit channel reuse and resolve the unbalance of traffic loads among different areas (such as hot and cold spots). We then propose a multi-channel MAC protocol, which integrates GRID-B. Extensive simulation results are presented to show the advantage of the new GRID-B protocol.

A new channel hopping MAC protocol for mobile ad hoc networks

Although multiple channels are supported in the physical layer, the IEEE 802.11 MAC layer mechanism is designed for using a single channel. Exploiting multiple channels enhances spatial reuse and reduces transmission collisions and thus improves network throughput. Designing a multi-channel MAC protocol is much more difficult than designing a singlechannel one. New challenges, such as the channel allocation problem and the missing receiver problem, must be overcome. Existing multi-channel MAC protocols suffer from either higher hardware cost (because of applying multiple transceivers) or lower channel utilization (due to limited transmission opportunity). In this paper, a fully distributed channel hopping solution, the Cyclic-Quorum-based Multi-channel (CQM) MAC protocol, is proposed. We use the cyclic quorum in a novel way and the proposed protocol has several attractive features. First, only a single transceiver is needed for each node. Second, any sender is guaranteed to meet its receiver in a short time. Third, each nodes channel hopping sequence is derived from its node ID. This avoids exchanging control messages, such as each nodes hopping sequence or available channel list. Fourth, multiple transmission pairs can accomplish handshaking simultaneously. The proposed protocol is simple and efficient. Simulations results verify that our mechanism is a promising multi-channel MAC protocol for mobile ad hoc networks.

Design of Structure-Free and Energy-Balanced Data Aggregation in Wireless Sensor Networks

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.

A Fast Rendezvous-Guarantee Channel Hopping Protocol for Cognitive Radio Networks

In a cognitive radio network (CRN), two nodes have a rendezvous when they tune to the same channel simultaneously. Providing rendezvous guarantee between any pair of nodes is essential because a rendezvous is necessary for communication. Some existing works claim that rendezvous guarantee can be provided by using a dedicated common control channel to negotiate the channel being used. A serious problem of these mechanisms is that a globally available channel may not exist. Some channel hopping protocols try to provide a rendezvous guarantee without using a common control channel. However, these solutions may suffer from a poor number of rendezvous or uneven channel utilization. In this paper, we propose a novel distributed channel hopping protocol: Quorum and Latin square channel hopping (QLCH). The QLCH scheme efficiently provides rendezvous guarantee by utilizing the concept of quorum systems and Latin squares. The former is utilized to guarantee balanced rendezvous among nodes, whereas the latter is adopted to share the rendezvous among channels. QLCH efficiently provides fast rendezvous guarantee and can cooperate with other channel hopping protocols to enhance network throughput. Analytical and simulation results verify the improvement of QLCH. For example, when there are 30 flows using 11 channels in a CRN, the achieved time to rendezvous of QLCH is 9.84 times lower than that of the existing protocol SYNC-ETCH, whereas the achieved network throughput of QLCH is 362% higher than that of the existing protocol, i.e., Quorum-based channel hopping (L-QCH).

Providing complete rendezvous guarantee for cognitive radio networks by quorum systems and Latin Squares

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.