# RFID Technology for Management and Tracking: e-Health Applications
## 2 System Architecture
### Radio Frequency Identification (RFID) Hardware and Considerations about Signal Propagation
Brown arrows show non-desired interrogation/response due to multipath affecting RFID antenna #1. Red arrows represent non-desired interrogation/response due to multipath affecting RFID antenna #2 (in this case, interrogation and response RFID signals are partially attenuated when passing through the wall). An example is presented in Figure 2b. Let us suppose that the UHF RFID antenna reading range reaches \(R=5\) m at its pattern maximum (green dot in Figure 2b). At an angle of 40\({}^{\circ}\), the radiation pattern decays 3 dB. Thus, compensable attenuation for this pattern level is given by (2):  
\[Comp.Att(\text{dB})=Att(R_{1})-Att(R_{2}) \tag{2}\]  
with \(R_{1}=5\) m and \(Comp.Att\) (dB) = 3 dB. Applying Equation (1), \(Att(R_{1}=5\) m) = 53.8 dB, so that \(Att(R_{2})=50.8\) dB, yielding \(R_{2}\sim\)4 m.  
Equation (2) can be used to predict the angle at which the same signal level as at the pattern maximum is reached given the distance from the antenna. For example, for \(R_{2}=3.3\) m, \(Att(R_{2})=47.8\) dB, yielding \(Comp.Att\) (dB) = 6 dB. From Figure 2a, this corresponds to an angle of approximately 60\({}^{\circ}\). Repeating this procedure for different angles and/or distances, an estimate of the UHF RFID antenna coverage area in the scenario to be deployed can be obtained, allowing users to decide whether this kind of antenna fulfills the coverage requirements or not. In addition to this, the power level of the RFID reader can be adjusted in most of commercial UHF RFID readers, so it can be tuned to modify the maximum reading distance (e.g., 5 m in the case of Figure 2b example). Figure 2: **(a)** Radiation pattern of a directive UHF RFID. Main cuts are depicted, highlighting the \(-3\) dB beamwidth. **(b)** Representation of the coverage area of the RFID antenna: red line represents all the points that receive the same signal strength taking into account indoor propagation model losses and the radiation pattern. A better way to assess RFID coverage in the scenario where the system is going to be deployed is, of course, by means of measurements. In the case of RFID antennas, coverage can be evaluated empirically by simply placing one or several RFID tags at those positions of interest, recording the values of received signal strength (RSS) and then plotting the RSS values over a plane of the scenario [30]. An application example is shown in Figure 3, where RSS measurements, conducted in the scenario shown in Figure 3d, were compared to theoretical RSS values calculated considering an indoor propagation losses model [30] and taking into account the radiation pattern of the UHF RFID antenna [30]. The effect of multipath can be clearly observed in the measurements (Figure 3a): signal strength does not monotonically decay with distance. Of course, more accurate methodologies for coverage evaluation (such as the proposed in [24]) can be considered depending on the system requirements. In general, RFID tags exhibit a dipole-like radiation pattern [31] with linear polarization. Because of this, most UHF RFID antennas are circularly polarized, so that the received signal strength is quite independent from RFID tag orientation with respect to the UHF RFID antenna. Another issue is the performance of RFID tags depending on the surface where they are placed. Low-cost UHF RFID tags do not perform well when placed on metallic containers or boxes, or recipients containing liquids, thus requiring a special kind of RFID tag (whose cost is typically 2-50 times higher). In the case of hospitals, most of the drugs are presented in cardboard or plastic containers, allowing the use of low-cost, conventional RFID tags. Coaxial cables are used to connect UHF antennas to the readers. In order to reuse existing hospital infrastructure, and thus minimize the cost of the RFID system deployment, connection between antennas and the RFID reader could be done using the radio and television distribution network based on 75 ohm coaxial cable.