small scale and low cost => large numbers of sensor fields can be deployed
monitor phenomena “up close”.
Enables spatially and temporally dense monitoring
Nyquist Sampling – you must sample often enough (in time or space)
Inverse problems are very difficult, e.g., by sensing the temperature at a few places, determine the temperature everywhere (numerically unstable, i.e., errors are amplified). Instead, directly sense the temperature everywhere.
Wireless interface allow little infrastructure – easy deployment
Wireless interface allow cooperation and distributed computing
Instability of an inverse problem for T=1:.25:10 G=zeros(21,21); for i=-10:10 G(i+11,:) = normpdf([-10:10]-i,0,T); end u0=sin([-10:10]/10*pi)'; %rand(21,1); u2=G*u0; u0hat = inv(G)*u2; u0NoiseEst = inv(G)*(u2+rand(size(u2))*.01); figure(1) clf plot(u0) hold on plot(u2,'g') plot(u0hat,'r') legend('original','observed','inverse') axis([0,21,-1,1]); title(sprintf('Inverse problem at %.2f ',T)) T pause end Contaminate sources Air sensors Objective: use the sensors to determine the source of the contaminate. Assume that the density at (x,y) is x y Wind direction where U is the flow into the environment at the source Solution: put the sensor near to the source
Vision Embed numerous sensing nodes to monitor and interact with physical world Network these devices so that they can execute more complex tasks. Note that these two problems are very different. Different scales and different requirements. It is difficult to lump them into a single “sensors” group.
Smart dust: passive transmission High power laser emitted from BS for downlink and uplink communication. Downlink Laser Uplink CCD Corner-Cube Uplink DataIn Data Image Sensor Retroreflector DataIn Photo- Downlink DataOut detector Base-StationTransceiver DustMote Signal Selection and Processing Uplink Data ... Out N Out 1 Array UnmodulatedInterrogation ModulatedReflected Lens Lens ModulatedDownlinkDataor BeamforUplink BeamforUplink
The reader must use specialized components such as circulators or couplers to ensure that only reflected signals are received.
But there are many objects that will reflect the signal.
These reflections might be stronger than the tag.
Each reflection is a sine wave at the same frequency, but with different phase and amplitude,
Recall that the sum of different sine waves at the same frequency, but different phase and amplitude is again a sine wave with some phase and amplitude, where the resulting phase and amplitude depends on the phases and amplitudes of the different sine waves.
Thus, when the antenna is made a short circuit or not, the phase (and amplitude) will change.
By detecting changes in the phase, we can detect that the antenna was changed.
E.g., if the phase changes quickly (say 50 times a sec), then a zero is received. If the Phase changes slowly (25 times a sec), then a one is received.
E.g., nodes break, crash, run out of power (consider a deployment where each node will last only a week, but nodes come out of hibernation at random times so the network has a lifetime of several months (see HW on next page))