Backscatter radio is proposed for sensor networks. In that way, the transmitter for each sensor is simplified to a transistor connected to an antenna and therefore, the cost for each sensor's communicator becomes negligible, while energy used for wireless communication per sensor is minimized. A software-defined transceiver is built to transmit a carrier, receive the reflections from various sensors and extract their transmitted messages. This work presents a thorough model of the backscatter radio link, the system architecture and a set of data extraction techniques for each sensor's information, testing in practice a sensor communicating through backscatter at a range of approximately 15 meters indoors, with 5 milliwatt transmission power at 10 bits per second. This work highlights the idiosyncrasies of the backscatter channel and provides a new communication perspective in the fertile area of scalable sensor networks, especially when low bit-rate, ultra-low cost sensors are required.

We provide an overview of our experimental system, testing in practice a sensor communicating through backscatter at a range of approximately 15 meters indoors, with 5 mW transmission power at 10 bits per second. Our system is designed for simultaneous reception of signals continuously radio-backscattered from several ultra low-cost sensors. This work highlights the idiosyncracies of the backscatter channel and presents a proof- of-concept demonstration of backscatter radio for wireless sensor networks, especially when low bit-rate, ultra low-cost sensors are required.

Commercial motion-capture systems produce excellent in-studio reconstructions, but offer no comparable solution for acquisition in everyday environments. We present a system for acquiring motions almost anywhere. This wearable system gathers ultrasonic time-of-flight and inertial measurements with a set of inexpensive miniature sensors worn on the garment. After recording, the information is combined using an Extended Kalman Filter to reconstruct joint configurations of a body. Experimental results show that even motions that are traditionally difficult to acquire are recorded with ease within their natural settings. Although our prototype does not reliably recover the global transformation, we show that the resulting motions are visually similar to the original ones, and that the combined acoustic and intertial system reduces the drift commonly observed in purely inertial systems. Our final results suggest that this system could become a versatile input device for a variety of augmented-reality applications.

Networks consisting of low-cost wireless sensors have been the subject of extensive research, with applications ranging from environmental monitoring to building control systems, to name a few. In this paper we consider networks intended to provide wide area coverage of one or more physical, chemical or biological parameters. We envision a target average data rate per sensor of 1-5 bps and transmission ranges of up to 100 m. Unlike conventional networks, the goal of wide-area parameter characterization can be achieved even with less than 100% node reliability, a feature that allows us to propose a novel ultra-low-cost architecture based on redundant sensors. To achieve the low-cost goal, sensor nodes include no receivers, and they transmit their information to a central reader using radio backscatter. This communication modality makes it possible to achieve a few years of battery life with small and inexpensive batteries. Performance impact due to absence of individual reception acknowledgement in this unidirectional communication channel is offset mainly by transmitter spatial diversity achieved through deployment of additional nodes. We describe the design of the system and its components, and analyze its performance characteristics.

Current interest in indoor radio applications for wireless communications spans a wide frequency range: from the 902-928 MHz ISM band, to the 18 GHz band allowed by the FCC's recent rulemaking for low-power high-speed data and voice communications. The authors report measurements of indoor radio propagation in these two bands, in four indoor environments. They also examine the effects of the statistical variations in the instantaneous received signal level on system outage. They pay special attention to the feasibility of frequency reuse with and without antenna diversity. A plot of average attenuation as a function of distance is found to be well correlated with each site's expected propagation characteristics. These expectations were derived from measurements of the radio transparency of various building materials and careful surveying of each site. Furthermore, the effects of multipath fading are characterized by calculating the outage probability of a hypothetical communications system employing selection antenna diversity. They find that, in general, attenuation increases steeply as a function of distance, so that the probability of interference between two users on the same frequency is small unless they are very close to one another. This suggests the possibility of a high degree of frequency re-use, and particularly so for the case when antenna diversity is used; the possibility is confirmed by the outage calculations. They also find that performance variations from site to site can be large due to different methods of construction.>

The main result is the finding of the minimum distance, d/sub min/, for m-ary signals over a class of partial responses of interest in the field of digital magnetic recording. A previously published set of results (see H.K. Thapar and A.M. Patel, 1987) on d/sub min/ for a related class of partial responses relevant to the coaxial cable channel is extended. A simple, powerful algorithm for finding d/sub min/ is employed. A secondary result is given in a table, which extends a previously published table (see G.J. Foschini, 1975), reporting a solution to the corresponding problem for coaxial cable partial responses.>

Consideration is given to the case of an optical pulse containing phase noise which is passed through either an optical filter or (following heterodyne lightwave detection) an electrical filter. Because of the phase noise, the envelope of the filter output at any instant is a random variable. An analytical method is developed for estimating the probability density function (pdf) of this envelope for different kinds of filter responses and for realistic combinations of phase noise severity and filter bandwidth. Obvious applications are to detection analyses of coherent lightwave systems, wherein finite laser linewidths constitute an important source of impairment. For each of the several types of filters considered, the envelope PDF can be accurately fitted by an exponential function approximation, where the decay constant is related in a simple way to known system parameters.>

The decision of on-off keying (OOK) and frequency-shift keying (FSK) is considered for coherent lightwave communications in which the optical pulses are corrupted by the phase noise associated with nonzero laser linewidths. Heterodyne lightwave detection is assumed, followed by intermediate frequency filtering, envelope detection, and postdetection low-pass filtering. Using analytical results published previously, theoretical minimum values of the required average power are obtained in photons/bit, for achieving a bit error rate of 10/sup -9/. Numerical results are given as functions of laser linewidth-to-bit-rate ratio for OOK, binary FSK (or 2-FSK), and 8-FSK. It is shown that heterodyne detection for lightwave signals of this kind can be made highly robust to phase noise, in contrast to the case of binary phase-shift keying.>

An experimental realization of direct-sequence spreading and despreading of the optical spectrum using off-the-shelf electrooptic modulators is reported. This capability is required to implement a fiber-optic network originally devised by G. Foschini and G. Vannucci (1983). The network utilizes spread-spectrum techniques on the optical spectrum, together with coherent signal detection to accommodate a very large number of simultaneous users, for a total throughput measured in hundreds of Gb/s. The results obtained provide confidence that a network built around this idea would perform according to theoretical calculations. Coherent spectral spreading and despreading is thus established as a promising communication technique for fiber-optic systems.>

Waves are treated that modulate by either on-off keying (OOK) or binary frequency-shift keying (FSK) and are further impaired by additive Gaussian noise. Heterodyne detection of such a waveform produces an electronic bandpass signal, which, to ease demodulation in the presence of phase noise, is noncoherently demodulated to extract the baseband pulse stream. The treatment goes beyond previous bit error rate (BER) analyses of optical heterodyne receivers for OOK and FSK. First, there is full adherence to the standard (Brownian motion) model of phase noise. Also, the receiver structure is formulated in such a way that the probability density function of the receiver output samples can be accurately determined. This permits calculations of the additive noise and phase noise tolerable when achieving bit error rates as small as 10/sup -9/. Finally, the study is comprehensive regarding the range of parameters explored. Filtering at an intermediate frequency (IF) alone, as well as IF filtering plus postdetection low-pass filtering, is considered. When the receiver parameters decision threshold (for OOK) and IF filter bandwidth are optimized, large amounts of phase noise can be accommodated with only minor increases in required signal-to-noise ratio. This is especially important when the bit rate is moderate compared to the laser linewidth.>

The phase noise associated with single-mode semiconductor lasers must be accounted for in performance studies of lightwave communication systems. The standard phase noise model is a Brownian-motion stochastic process. Although many analyses of lightwave communication systems have been published, none, to the authors knowledge, has fully adhered to the standard model. The reason is that a proper characterization of filtered lightwave signal had not been achieved. Such a characterization, along with theoretical approaches to obtaining it, is detailed. The authors show, for example, how to generate probability density functions (PDFs) of the magnitude of a filtered laser tone (with special attention to the tail region) and how to analytically represent the characteristic function of the PDF in closed form in the small-phase-noise realm. With the characterization in place, the stage is now set for determining the bit-error rate performance of advanced detection techniques which seek to mitigate the phase noise impairment.>