Wireless ad hoc network is decentralized wireless network, which does not rely on a preexisting infrastructure, such as routers in wired networks or access points in managed (infrastructure) wireless networks. Instead, each node participates in routing by forwarding data for other nodes. The determination of which nodes forward data is made dynamically based on the network connectivity. Node density has a great impact on the performance and efficiency of wireless ad hoc networks by influencing some factors such as capacity, network contention, routing efficiency, delay, and connectivity.

On one hand, maintaining stable connectivity is a big challenge for sparsely deployed and highly dynamic ad hoc wireless network. Vehicle ad hoc network (VANET) which consists of highly mobile vehicles with wireless interfaces is one type of such network, especially in rural areas where vehicles traffic are very sparse. One of the most important applications built on top of VANET is the safety application. In VANET safety applications, source vehicles that observe accidents or some other unsafe conditions of the roads generate warning messages about the conditions, and propagate the warning messages to the following vehicles. In this way, the following drivers have the opportunity to do some necessary action before they reach the potential danger zone to avoid accident. The safety application requires timely and accurate warning message detection and delivery. However, recent researches have shown that sparse and highly dynamic vehicle traffic leads network fragmentation, which poses a crucial research challenge for VANET safety application.

On the other hand, reducing contention and thus maximizing the network throughput is also a big challenge for densely deployed ad hoc wireless network, especially when many devices are located in a small area and each device has heavy duty message to transmit. The WiFi interface perhaps is the most common interface found in mobile devices for data transfer as it provides good combination of throughout, range and power efficiency. However, the WiFi interface may have to consume a large amount of bandwidth and energy for contention and combating collision, especially when mobile devices located in a small area all have heavy traffic to transmit.

Meanwhile, ZigBee is an emerging wireless communication technology which supports low-cost, low-power and short-range wireless communication. Nowadays, it has been common for a mobile device, such as smart phone, PDA and laptop, to have both WiFi and Bluetooth interfaces. As the ZigBee technology becomes more and more mature, it will not be surprising to see the ZigBee interface commonly embedded in mobile devices together with WiFi and Bluetooth interfaces in the near future. The co-existence of the ZigBee and the WiFi interfaces in the same mobile device inspires us to develop new techniques to address the above two issues.

Specifically, this thesis presents two systems built based on ZigBee-assisted ad-hoc networking of multi-interface mobile devices. In order to achieve stable connectivity in a sparse and dynamic VANET, the first system integrates a network of static roadside sensors and highly mobile vehicles to improve driving safety. In order to reduce contention in a densely deployed ad hoc wireless network, the second system assists WiFi transmission with ZigBee interface for multi-interface mobile devices. Extensive implementations and experiments have been conducted to demonstrate the effectiveness of our proposed systems.