TABLE OF CONTENTS We now examine techniques based on detailed information. These include triangulation using distance estimates, pattern matching, and sequence decoding. Although used in the large-scale GPS, basic time-of-flight techniques using RF signals are not capable of providing precise distance estimates over short ranges typical of WSN because of synchronization limitations. Therefore other techniques such as radio signal strength (RSS) measurements and time difference of arrival (TDoA) must be used for distance-estimation.
To a first-order approximation, mean radio signal strengths diminish with distance according to a power law. One model that is used for wireless radio propagation is the following [171]:
| (3.2) |  |
where P r , dB( d) is the received power at distance d and P( d 0) is the received power at some reference distance d 0, ? the path-loss exponent, and X ?,dB a log-normal random variable with variance ? 2 that accounts for fading effects. So, in theory, if the path-loss exponent for a given environment is known the received signal strength can be used to estimate the distance. However, the fading term often has a large variance, which can significantly impact the quality of the range estimates. This is the reason RF-RSS-based ranging techniques may offer location accuracy only on the order of meters or more [154]. RSS-based ranging may perform much better in situations where the fading effects can be combatted by diversity techniques...
