Coverage and density of a low power, low data rate, spread spectrum wireless sensor network for agricultural monitoring

A physical layer specification for a low power, low complexity, low data rate sensor network suitable for agricultural monitoring is investigated. Code division multiple access (CDMA) with high processing gain is used to facilitate transmission powers which comply with the Ultra Wide Band (UWB) spectral mask, and this permits physically small nodes with limited energy storage capacity. The interference arising from each node is calculated, and it is shown that for the investigated scenario and specification, an aggregate data rate of 2 bytes per minute and a node population of approximately 1000 can be supported at distances up to a few kilometres from the central node, with less than 0.2% chance of failure due to multiple access interference

Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies

Aims. To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular cores, whose asymmetries trace infall and expansion motions. Methods. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN. Results. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity (HCN to H13CN). This is most evident in the HCN 12–11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field changes from infall in the outer part to expansion in the inner part. Conclusions. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important

Herschel observations of EXtra-Ordinary Sources (HEXOS): detecting spiral arm clouds by CH absorption lines

We have observed CH absorption lines (J = 3/2, N = 1 ← J = 1/2, N = 1) against the continuum source Sgr B2(M) using the Herschel/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show “spiral arm” cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr B2 envelope. The observations show that each “spiral arm” harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H2 column densities found at lower AV by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds

Spatial reuse of the radio channel in CDMA-enabled, ad-hoc wireless sensor networks

To date, random access protocols like Carrier Sense Multiple Access (CSMA) have been the preferred means of sharing the radio channel in SpeckNets and other ad hoc wireless sensor networks. This paper considers Code Division Multiple Access (CDMA) as an alternative multiple access method, and evaluates the introduction of exclusion zones around each node as a low cost means of managing the Near Far problem. It is shown that, if appropriately sized zones are established, improved spatial reuse of the radio channel is possible (i.e. more concurrent transmissions can take place in proximity), potentially leading to energy savings in the MAC protoco

Coarse angle rotation mode CORDIC based single processing element QR-RLS processor

Over the last 30 years Digital Signal Processing algorithm implementation has been driven by the continued progress and availability of high speed ASIC circuit technology. The classic method of CORDIC (Coordinate Rotation by Digital Computer) arithmetic has been widely implemented as part of the computational requirements of the well known QR-RLS (recursive least squares) algorithm. In this paper we propose a new modified version of the CORDIC that features a single processor element that is easily pipelinable and can be used to implement both the Givens generations and Givens rotations associated with the QR update. Using a Xilinx FPGA for implementaton results show that this proposed structure requires less resources and produces a more regular and therefore lower cost structure than other equivalent methods recently presented

A low power, digital transceiver for wireless sensor networks

This presentation discusses a low-power, digital transceiver for wireless sensor networks. We introduce the concept of speckled computing and look at the challenges that it presents for a DSP physical layer design. A first generation transceiver implementation is discussed; in particular a method of simplifying symbol synchronisation and the use of pulse-shaping to increase transmission efficiency

Symbol synchronisation implementation for low-power RF communication in wireless sensor networks

Speckled Computing is a novel vision of a wireless sensor network consisting of small nodes which can sense, compute and network wirelessly. The nodes will individually have limited power and processing resources, but together will form a powerful processing system. Electrical power resources at such a volume are severely restricted, and as such design decisions are made with low-power as the first priority. This work examines the use of Manchester encoding in the digital transceiver to reduce the complexity of symbol synchronisation. A Manchester decoder has been implemented which has the useful property of being tolerant to oscillator inaccuracies, allowing a cheap and low-power clock source to be employed. A realistic implementation of the decoder using rectangular pulseshaping and an oversampling ratio of 8 allows an on-chip oscillator tolerance of more than 11%