0. Brief Introduction

Transcription

1 The IEEE standard specifies that a radio's PHY layer must provide an 8-bit integer value as an estimate of received signal power. This value is commonly known as Received Signal Strength Indication () in wireless sensor networks (WSN) community. While many protocols directly use measurements that radios provide, standard only requires that reported values should be linear and within ±6 db of actual values. However, ±6 db is a wide error margin. For example, Packet Reception Ratio (PRR) decrease from 100% to 0% with a 2 or 3 db difference in received signal strength. We examined two compliant radios, widely used TI CC2420 and Atmel AT86RF230, and discovered that y indeed introduce systematic errors when providing readings. Therefore, we also developed a calibration scheme to effectively minimize adverse effects associated with inaccurate values. 0. Brief Introduction This page is dedicated to illustrate calibration technique proposed in our EWSN'10 paper. The paper lists results of applying calibration technique in several areas such as PRR-SNR modeling, PRR-SINR modeling, WSN simulation, radio propagation model parameters estimation and RF-based localization. Click here to download data that we used to draw CC2420 reference curve. In txt file, first column is calibrated and second column is reported by CC2420 radio. They correspond to x-axis and y-axis of reference curve respectively. The slides are available here. 1. Response Curves 1/8

2 A response curve is derived by recording values output by radio chip at various input radio powers. In specific, X-axis shows actual radio power and Y-axis shows corresponding readings output by radio chip. The CC2420 datasheet includes a coarse response curve as follows. The datasheet concludes that readings are very linear. We tested response curves for 12 Tmote Sky motes and 2 MICAz motes. All curves are very similar and we derived a single reference curve for CC2420 radio which is shown below. This curve gives mapping from raw values (Y-axis) to calibrated values (X-axis). Click here to download data that we used to draw reference curve. Specifically, txt file contains two columns. The first column is calibrated and second column is reported by CC2420 radio. They correspond to x-axis and y-axis on figure respectively. 2/8

3 2. Calibrating The tells CC2420 us Next, reference value is curve directly be used calibrate values. Suppose CC2420 b, n we aprovide, point (0,b below. we point on find X-axis, correponding say ( plot point onto reference ) on curve, and as shown in turn infind figure,corresponding Now, calibrated is a.0). In this particular example, = -50dBm. When X-axis using value. data that we one doy-axis, interpolation to compute 3/8

4 The problem reference value, regions. re curve. could withas bea multiple above result, approach when didates. we is trythat to The find re following a corresponding are two graph non-monotonic labels X-axis se value regions twofor non-monotonic aon given y 4/8

5 In(a) order multipoint to find calibration true X-axis process value, as shown receiver below now would needs be sender cooperate, such 1) due Sender to 8receiving transmission several transmission power levels. power info in a following packet. 3at bits are needed when using 2) sender, After receiver start packets calibrating that were in point transmitted three different steps. power levels from CC2420 same given Plot power 8puts level. on graph where each represents apossible. packet thattowas transmitted atthat a 5/8

6 those (b) Put unchanged. 8along Note ony-axis that reference direction raw graph, on both values while graphs. ofkeeping those packets relative determine locations among locations those of 6/8

7 (c) calibrated Move along X-axis for those topackets. find best fit, and corresponding X-axis values would be 7/8

8 3. Brief Discussion Therefore, regions, The such multipoint itisis as accuracy. not AT86RF230. necessary calibration for Neverless, ison needed or radio due chips to Wireless that non-monotonic calibration do not have such regions still non-monotonic be ofused CC2420. to increase A would minimum lead of to7th 2 higher accuracy. should bemultipoint sufficient to do multipoint calibration. More -calibration readings. an 8-bit integer, but weusually get sub-integer value by averaging multiple 4. Reference Yin Chen, Andreas Terzis, Measurements for Radios Mechanisms and Effects of proceedings of European Conference on Sensor Networks.Calibrating Appeared (EWSN in 2010). PDF Slides BibTex 8/8