Charles Chien

Low power TDMA in large wireless sensor networks

In this paper we present a novel TDMA scheme for a large population of sensors interconnected by a wireless multihop network. Applications of this wireless sensor network include battlefield surveillance, space exploration and condition based monitoring. Key characteristics of this system are the large number of sensor nodes and the need to rely on battery operation for a long period of time. Since communication is a major consumer of energy, low power communication protocols play a critical role in wireless sensor networks and have significant impact on the overall energy dissipation of these networked systems. This paper focuses on an energy efficient TDMA protocol, power aware clustered TDMA (PACT), that adapts the duly cycle to the user traffic. In other words, the radio is powered off if the network is inactive. Moreover, we apply passive clustering to take advantage of the redundant dense topology and prolong the lifetime of the entire network even further. At a given time, only a subset of network nodes (i.e., cluster heads and gateways) participates in the communication. The role of cluster heads and gateways is rotated according to their energy levels. The clustering requires no explicit control messages and therefore incurs negligible energy overhead. Using the proposed protocol, simulation and analytical results show significant improvement in energy saving and network lifetime.

Adaptive radio for multimedia wireless links

The quality of wireless links suffers from time-varying channel degradations such as interference, flat-fading, and frequency-selective fading. Current radios are limited in their ability to adapt to these channel variations because they are designed with fixed values for most system parameters such as frame length, error control, and processing gain. The values for these parameters are usually a compromise between the requirements for worst-case channel conditions and the need for low implementation cost. Therefore, in benign channel conditions these commercial radios can consume more battery energy than needed to maintain a desired link quality, while in a severely degraded channel they can consume energy without providing any quality-of-service (QoS). While techniques for adapting radio parameters to channel variations have been studied to improve link performance, in this paper they are applied to minimize battery energy. Specifically, an adaptive radio is being designed that adapts the frame length, error control, processing gain, and equalization to different channel conditions, while minimizing battery energy consumption. Experimental measurements and simulation results are presented to illustrate the adaptive radios energy savings.

Low-power direct-sequence spread-spectrum modem architecture for distributed wireless sensor networks

Emerging CMOS and MEMS technologies enable the implementation of a large number of wireless distributed microsensors that can be easily and rapidly deployed to form highly redundant, self-configuring, and ad hoc sensor networks. To facilitate ease of deployment, these sensors should operate on battery for extended periods of time. A particular challenge in maintaining extended battery lifetime lies in achieving communications with low power. This paper presents a direct-sequence spread-spectrum modem architecture that provides robust communications for wireless sensor networks while dissipating very low power. The modem architecture has been verified in an FPGA implementation that dissipates only 33 mW for both transmission and reception. The implementation can be easily mapped to an ASIC technology with an estimated power performance of less than 1 row.

Microelectromechanical system radio frequency switches in a picosatellite mission

Rockwell Science Center (RSC) has designed and implemented a microelectromechanical-system- (MEMS-) based radio frequency switch experiment in a miniature satellite format (picosat) as an initial demonstration of MEMS for space applications. This effort is supported by DARPA-MTO, and the mission was conducted with Aerospace Corporation and Stanford University as partners. MEMS surface-micromachined metal contacting switches were manufactured and used in a simple, yet informative, experiment aboard the miniature satellites to study the device behavior in space, and its feasibility for space applications in general. Communication links between multiple miniature satellites, as well as between the satellites and ground, were also achieved using communications circuits constructed and provided by RSC. Details of both the MEMS and radio communications and networking efforts will be discussed in this paper.

A low energy architecture for fast PN acquisition

Spread spectrum systems are being widely deployed today and are becoming more prevalent as most next-generation wireless systems are adopting it for their common air interface. These systems include the digital cellular IS-95 A/B/C, IEEE 802.11 wireless local area networks, as well as third-generation wideband code-division multiple access systems. In spread-spectrum systems, the receiver must synchronize on to the transmitted pseudo-noise (PN) code to obtain the improvement performance achieved through spreading. Since PN acquisition must process the spread-spectrum signal at a speed much faster than the transmitted data rate, its energy consumption can become significant and should be minimized for portable applications. Typically, either matched filters or serial correlators are used to acquire the PN code timing. This paper describes a hybrid PN acquisition architecture which employs both matched filters and serial correlators to achieve a lower energy consumption and fast acquisition time as compared to the traditional approaches of using either matched filters or serial correlators alone. The hybrid architecture has been implemented in RTL VHDL and synthesized down to gate level in a 0.5 micron CMOS library. Synthesis results show a factor of four reduction in energy for the hybrid scheme as compared to the matched filters architecture and a factor of two reduction in energy as compared to the serial architecture.

Robust adaptive error control

This paper presents a robust adaptive type-I hybrid ARQ scheme that can adapt to a slowly-varying wireless channel. The proposed system can improve system throughput substantially compared to a conventional type-I hybrid ARQ scheme. Most previous research on adaptive ARQ has assumed that the return channel used for acknowledgements is error free. This simplifying assumption is often necessary for initial performance evaluation but is unrealistic in practice. In this paper, we extend an existing adaptive ARQ scheme by making it robust to channel errors in both the forward and feedback channels. We also propose a solution to maintain code synchronization in the presence of packet errors and describe an implementation that combines the adaptive ARQ scheme with a SACK protocol that allows bi-directional transmission and piggybacked acknowledgements. In addition we have built a wireless test platform for mobile radio in an attempt to accurately measure the adaptive error control performance not only as a standalone entity, but also as a part of a complete protocol stack.

A PN-acquisition ASIC for wireless CDMA systems

CDMA spread-spectrum systems require PN-acquisition to synchronize the transmitted signal at the receiver. Fast acquisition minimizes the amount of synchronization overhead required in the communication link for maximum system throughput. Yet, the fast acquisition should be done with low energy for portable applications. Conventional acquisition techniques using matched filters or serial correlators alone provide either fast pseudo-noise (PN) acquisition for CDMA or low power dissipation but not both. This paper presents an ASIC which implements a hybrid PN acquisition architecture that achieves both fast acquisition and up to 50% reduction in energy dissipation compared to conventional techniques. This ASIC has been fabricated using 0.5-/spl mu/m CMOS technology with an area of 23 mm. It operates at 20 MHz with a 3.3 V supply and dissipates 50 mW per acquisition, or less than 1.5 mW per 50 byte packet.

Simulation and synthesis of VLSI communication systems

This paper describes CAD tools for communication system design. The tools allow for rapid algorithm development using a functional model library and scripting procedures that automate iterative optimization of algorithm parameters. Implementation tools are linked into the algorithm design environment to allow efficiency in generating hardware designs from algorithm descriptions. Examples from channel coding and from modern design illustrate how the data flow simulation in the CAD environment can be exploited for simulating and designing each of these components.