Collision Detection Using a Multiple Symbol Noncoherent Soft Output Detector
Systems and methods for detecting collisions in radio frequency tags in accordance with embodiments of the invention are disclosed. In one embodiment, a receiver system includes a receiver configured to receive and sample a phase modulated input signal, and a multiple symbol noncoherent soft output detector configured to receive the sampled input signal and to generate a soft metric indicative of the reliability of a detected symbol based upon observations over multiple symbols, a collision detector configured to calculate a decision metric from a set of soft metrics generated by the multiple symbol noncoherent soft output detector and detect a collision when the decision metric satisfies a predetermined criterion.
1 . A receiver system, comprising:
a receiver configured to receive and sample a phase modulated input signal; and
a multiple symbol noncoherent soft output detector configured to receive the sampled input signal and to generate a soft metric indicative of the reliability of a detected symbol based upon observations over multiple symbols;
a collision detector configured to calculate a decision metric from a set of soft metrics generated by the multiple symbol noncoherent soft output detector and detect a collision when the decision metric satisfies a predetermined criterion.
2 . The receiver system of claim 1 , wherein the soft metric is the Log Likelihood Ratio of the detected symbol based upon observations over multiple symbols.
3 . The receiver system of claim 2 , wherein the observations include observations over a two symbol sequence.
4 . The receiver system of claim 2 , wherein the observations include observations over a three symbol sequence.
5 . The receiver system of claim 1 , wherein the set of soft metrics generated by the multiple symbol noncoherent soft output detector are generated based upon observations of a unique sequence of symbols identifying an RFID tag.
6 . The receiver system of claim 5 , wherein the unique sequence of symbols identifying an RFID tag is an RN16 transmission.
7 . The receiver system of claim 1 , wherein the decision metric is based upon a central moment of the distribution of the set of soft metrics generated by the multiple symbol noncoherent soft output detector.
8 . The receiver system of claim 7 , wherein the decision metric is normalized over a power of the first moment of the distribution of the set of soft metrics generated by the multiple symbol noncoherent soft output detector.
9 . The receiver system of claim 7 , wherein the predetermined criterion is the decision metric exceeding a threshold.
10 . The receiver system of claim 9 , wherein the phase modulated input signal includes a preamble sequence and the threshold is based upon the preamble correlation normalized by the number of one half symbols used in the preamble.
11 . The receiver system of claim 1 , wherein the decision metric is a count of the number of soft metrics in the set of soft metrics generated by the multiple symbol noncoherent soft output detector.
12 . The receiver system of claim 1 , wherein:
the phase modulated input signal comprises data that is phase modulated on a carrier; and
the multiple symbol differential detector assumes that carrier phase of the input signal is constant over the time duration of the observations.
13 . The receiver system of claim 12 , wherein the phase modulated input signal is a binary phase modulated signal.
14 . The receiver system of claim 12 , wherein the phase modulated input signal is an FM0 modulated signal.
15 . The receiver system of claim 12 , wherein the phase modulated input signal is a Multiple-Phase-Shift Keying modulated signal.
16 . The receiver system of claim 1 , wherein:
the multiple symbol differential detector comprises a plurality of matched filters having different numbers of samples configured to integrate the samples during each half-symbol period; and
the multiple symbol differential detector is configured to use the outputs of each of the plurality of matched filters to determine the most likely symbol duration.
17 . The receiver system of claim 1 , further comprising an antenna configured to receive a phase modulated signal comprising symbols transmitted by an RFID tag.
18 . A method for detecting collisions in received radio frequency transmissions, the method comprising:
receiving and sampling a phase modulated input signal to produce symbol samples;
combining symbol samples to produce symbol observations;
generating a soft metric indicative of the reliability of a detected symbol based upon symbol observations over multiple symbols;
calculating a decision metric from a set of generated soft metrics; and
detecting a collision in received radio frequency transmissions when the calculated decision metric satisfies a predetermined criterion.
19 . The method of claim 18 , wherein the soft metric is the Log Likelihood Ratio of the detected symbol based upon observations over multiple symbols.
20 . The method of claim 19 , wherein the observations include observations over a two symbol sequence.