4. Coverage area synthesis for RFID-based ETC system

After the description of a general optimization procedure for a pattern defined on a planar surface (which can be used for the synthesis of a high data-rate service area), in this section we will consider the coverage area synthesis problem from the ETC application point of view. As briefly described in Section 1, the objective of a coverage area synthesis in this context should be the maximization of the communication area length in the travel direction and not the synthesis of a specific pattern geometry. For this case, an optimization procedure similar to the one described in Section 3 might also be derived. Nonetheless, channel phenomena, for example, fading [21], are known and, together with other possible implementation tolerances, might lead to suboptimal solutions in spite of the synthesis effort.

For this reason, a simple iterative methodology for synthesizing a planar antenna array with both the aim of stretching the coverage area toward the longitudinal direction and confining it within a roadlane is described. This method has the advantage of providing acceptable results with a reduced number of antenna elements with respect to the optimization presented in Section 3.

#### 4.1 RFID-based DSRC system

RFID technology is usually employed for the implementation of DSRC because of its well-known advantages of excellent accuracy and the possibility to be read at high speed [22]. A RFID-based DSRC system is basically realized by means of a RSU beacon reader, raised installed in order to guarantee sufficient visibility, and some OBU transponders. Moreover, antennas are constrained to radiate with circular polarization (CP) for two main reasons: CP reduces polarization mismatch due to reciprocal rotation between RSU and OBU devices and improves immunity to multipath effect [23]. The latter is a fundamental characteristic which guarantees the validity of a free space propagation loss model [21].

The coverage area definition is based on the threshold power Pbound which, in the case of a monostatic backscatter [22] RFID-based system, can be interpreted as the tag sensitivity threshold Ptag,th and the reader sensitivity Preader,th. Therefore,

area center position y<sup>0</sup> has been progressively increased to be the points on the coverage area plane which corresponds to the broadside direction, that is,

, <sup>y</sup><sup>0</sup> <sup>¼</sup> <sup>5</sup>:5 m with <sup>θ</sup><sup>A</sup> <sup>¼</sup> 45°

order to confirm the steering direction choice discussed in Section 3.1.

4. Coverage area synthesis for RFID-based ETC system

ments with respect to the optimization presented in Section 3.

It is of interest to observe that a decrease in mechanical tilt leads to a decrease in the required beamwidth and, consequently, an increase in the required array size.

The non-broadside case is also considered. In Figure 7(b) the performance of the broadside optimization and the non-broadside optimization is compared in

It is clear from Figure 7(b) that the choice of the steering direction affects the sidelobe level outside the coverage area. In fact, a 1° decrease in the steering direction with respect to the broadside, that is, the steering direction which corresponds to the coverage area center point, yields a sidelobe-level improvement of 22.5 dB. On the other hand, an increase of 1° leads to a sidelobe

After the description of a general optimization procedure for a pattern defined

For this reason, a simple iterative methodology for synthesizing a planar antenna array with both the aim of stretching the coverage area toward the longitudinal direction and confining it within a roadlane is described. This method has the advantage of providing acceptable results with a reduced number of antenna ele-

RFID technology is usually employed for the implementation of DSRC because of its well-known advantages of excellent accuracy and the possibility to be read at high speed [22]. A RFID-based DSRC system is basically realized by means of a RSU beacon reader, raised installed in order to guarantee suffi-

constrained to radiate with circular polarization (CP) for two main reasons: CP reduces polarization mismatch due to reciprocal rotation between RSU and OBU devices and improves immunity to multipath effect [23]. The latter is a fundamental characteristic which guarantees the validity of a free space propagation

The coverage area definition is based on the threshold power Pbound which, in the case of a monostatic backscatter [22] RFID-based system, can be interpreted as the tag sensitivity threshold Ptag,th and the reader sensitivity Preader,th. Therefore,

cient visibility, and some OBU transponders. Moreover, antennas are

on a planar surface (which can be used for the synthesis of a high data-rate service area), in this section we will consider the coverage area synthesis problem from the ETC application point of view. As briefly described in Section 1, the objective of a coverage area synthesis in this context should be the maximization of the communication area length in the travel direction and not the synthesis of a specific pattern geometry. For this case, an optimization procedure similar to the one described in Section 3 might also be derived. Nonetheless, channel phenomena, for example, fading [21], are known and, together with other possible implementation tolerances, might lead to suboptimal solutions in spite of the

and that the array is assumed to be of minimum size.

The achieved sidelobe levels are larger than 20 dB.

, and <sup>y</sup><sup>0</sup> <sup>¼</sup> <sup>9</sup>:5 m with <sup>θ</sup><sup>A</sup> <sup>¼</sup> 30°

,

<sup>y</sup><sup>0</sup> <sup>¼</sup> <sup>3</sup>:25 m with <sup>θ</sup><sup>A</sup> <sup>¼</sup> <sup>60</sup>°

Array Pattern Optimization

deterioration.

synthesis effort.

loss model [21].

40

4.1 RFID-based DSRC system

according to the free space propagation model, the communication area is defined as the set of coordinates xR and yR in the reference plane zR ¼ htag in which

$$\begin{cases} \begin{aligned} P\_{\text{forward}}(\mathbf{x}\_{\mathcal{R}}, \boldsymbol{y}\_{\mathcal{R}}) &= \boldsymbol{P}\_{t} + \boldsymbol{G}\_{t}(\boldsymbol{\phi}, \boldsymbol{\theta}) + \boldsymbol{G}\_{r} + 20 \log\_{10} \left( \frac{\lambda\_{0}}{4\pi r} \right) \geq P\_{\text{tag,th}} \\\\ P\_{\text{back}}(\mathbf{x}\_{\mathcal{R}}, \boldsymbol{y}\_{\mathcal{R}}) &= P\_{\text{forward}}(\mathbf{x}\_{\mathcal{R}}, \boldsymbol{y}\_{\mathcal{R}}) + \boldsymbol{G}\_{t}(\boldsymbol{\phi}, \boldsymbol{\theta}) + \boldsymbol{G}\_{r} + 20 \log\_{10} \left( \frac{\dot{\lambda}\_{0}}{4\pi r} \right) + 10 \log\_{10} \boldsymbol{M} \geq P\_{\text{reader,th}} \end{aligned} \end{cases} \tag{13}$$

where Pt is the transmitted power, Gtð Þ¼ ϕ; θ Gt,max þ 20 log <sup>10</sup>Fð Þ ϕ; θ represents the antenna array gain pattern (which includes the normalized synthesis function), Gr is the tag gain, and M is the modulation factor (for a passive tag, M ¼ 0:25 [22]).

## 4.2 Antenna array synthesis with iterative method

Let us consider the normalized synthesis function in (6) for a rectangular planar array of Nx � Ny elements with uniform interelement distances dx and dy which can be rewritten as

$$F(\phi,\theta) = \frac{r\_0}{r} \frac{f(\phi,\theta)}{f(\phi\_0,\theta\_0)} \sum\_{n=1}^{N\_x} \sum\_{m=1}^{N\_{\mathcal{I}}} w\_{n,m} e^{j\left[k\_x(n-1)d\_x + k\_{\mathcal{I}}(m-1)d\_{\mathcal{I}}\right]} \tag{14}$$

The synthesis problem is basically the definition of:


A simple iterative method to synthesize the coverage area with the objective of stretching its length toward the travel direction is described [16]. In this case, complex coefficients wn,m are taken as in (12), that is,

wn,m <sup>¼</sup> an,me j k ½ � ð Þ <sup>x</sup>�kx,<sup>0</sup> ð Þ <sup>n</sup>�<sup>1</sup> dxþð Þ ky�ky,<sup>0</sup> ð Þ <sup>m</sup>�<sup>1</sup> dy , with an,m based on Tschebyscheff coefficients and Rx and Ry the Tschebyscheff design sidelobe level [2].

Then, the synthesis process can be performed according to the following steps:


6.Choose the best steering elevation θ<sup>0</sup> in the sense of maximizing the length of the coverage area along with the coordinate yR (with starting coordinate yR ¼ 0).

Each step is iteratively executed to compare the Tschebyscheff synthesis results and verify the conditions in (13) and then determine the coefficients an,m.

### 4.3 Synthesis example and experimental results

A coverage area synthesis is herein described for the case of a reader height hA ¼ 5:5 m with mechanical tilt θ<sup>A</sup> ¼ 45°. and OBU height htag ¼ 1:5 m. System parameters are chosen according to the standard EPC Gen2 for UHF RFID [24] for the carrier frequency 920 MHz which limits the effective isotropic radiated power to the value PEIRP ¼ Pt þ Gt,max ¼ 36 dBm. After that, the other parameters are Gr ¼ 5 dBi, Ptag,th ¼ �20 dBm (the sensitivity of the commercial product Impinj Monza R6 [25]), and Preader,tag ¼ �84 dBm (the sensitivity of the commercial product Impinj Indy R2000 [26]).

employed. A compact CP UHF antenna with gain Gr ¼ 4 dBi has been used as tag

(a) 4 x 4 CP microstrip patch antenna array fabricated prototype and (b) experimental setup for the ETC

position of the tag device in the road plane xR; yR

Pt,min which activates the tag, that is, Pforward xR; yR

<sup>¼</sup> <sup>P</sup>tag,th, under the condition that <sup>P</sup>back xR; yR

similar way to what has been described in [29], the power Pforward xR; yR

limited PEIRP ¼ 36 dBm is applied and a specific Ptag,th ¼ �20 dBm is chosen has

been compensated during the power evaluation). Experimental results are shown in

Good correspondence among simulations (SR), antenna measurement projection on the road plane (AM), and experimental results (ER) is visible, and only few discrepancies arise. These are mainly due to the 1 dB tag antenna gain reduction with respect to the design parameter, the tag antenna radiation pattern (not taken into account), and other possible errors in fixing the antenna

Comparison of synthesis results obtained by projecting the measurement results of the antenna to the road plane (AM), simulation (SR), and experimental results (ER). (a) Received power contour plot at htag ¼ 1:5 m and

(b) received power profile along yR coordinate as function of htag .

The transmitted power Pt has been regulated in the range 5 ÷ 30 dBm for each

to determine the minimum value

<sup>¼</sup> Pt,min <sup>þ</sup> Gtð Þþ <sup>ϕ</sup>; <sup>θ</sup> Gr <sup>þ</sup> <sup>20</sup>

<sup>¼</sup> <sup>P</sup>tag,th <sup>þ</sup> <sup>P</sup>EIRP � Pt,min � Gt,max (cable losses have

≥Preader,th. After that, in a

when a

antenna.

Figure 9.

log <sup>10</sup>

λ0 4πr

been inferred as Pforward xR; yR

antenna array system measurement.

Array Pattern Synthesis for ETC Applications DOI: http://dx.doi.org/10.5772/intechopen.80525

Figure 10(a) and (b).

mechanical tilt θA.

Figure 10.

43

Following the synthesis process described above, the optimized coverage area for a 6 m road width is achieved as depicted in Figure 8(a), with the following synthesis parameters: Nx ¼ 4, Ny ¼ 4, dx ¼ 0:45λ0, dy ¼ 0:48λ<sup>0</sup> (with λ<sup>0</sup> evaluated at 920 MHz), θ<sup>0</sup> ¼ �5°, Rx ¼ 30 dB, Ry ¼ 25 dB, and the coefficients as in [27].

The achieved coverage area is 8 m long, covers the required transversal direction width, and presents very low lateral sidelobes. Figure 8(b) also presents the achieved coverage area at htag ¼ 2:5 m (it could represent the tag height of a truck) and htag ¼ 1 m (that can represent the tag height of a motorcycle) along with the coordinate yR, and it shows that the higher the tag height htag , the shorter the coverage area. This is acceptable because the speed of a truck is usually lower than the speed of a common vehicle, so the available transaction time will be longer.

In order to confirm the simulation results, the synthesized antenna array has been designed and manufactured, as shown in Figure 9(a). The design process of the 4 � 4 CP microstrip patch antenna array is described in [27]. Furthermore, the 12 dBi RHCP gain antenna prototype has been fixed at the height hA ¼ 5:5 m with a metallic scaffolding and used for collecting experimental results, as depicted in Figure 9(b). A commercial Impinj Speedway R420 UHF reader [28] (Preader,th ¼ �84 dBm) and a tag device with Ptag,th ¼ �32 dBm have been

#### Figure 8.

Synthesis example of the coverage area at 920 MHz. (a) Received power contour plot at htag ¼ 1:5 m and (b) received power profile along yR coordinate as function of the tag height htag .

Figure 9.

6.Choose the best steering elevation θ<sup>0</sup> in the sense of maximizing the length of the coverage area along with the coordinate yR (with starting coordinate

Each step is iteratively executed to compare the Tschebyscheff synthesis results

A coverage area synthesis is herein described for the case of a reader height hA ¼ 5:5 m with mechanical tilt θ<sup>A</sup> ¼ 45°. and OBU height htag ¼ 1:5 m. System parameters are chosen according to the standard EPC Gen2 for UHF RFID [24] for the carrier frequency 920 MHz which limits the effective isotropic radiated power to the value PEIRP ¼ Pt þ Gt,max ¼ 36 dBm. After that, the other parameters are Gr ¼ 5 dBi, Ptag,th ¼ �20 dBm (the sensitivity of the commercial product Impinj Monza R6 [25]), and Preader,tag ¼ �84 dBm (the sensitivity of the commercial prod-

Following the synthesis process described above, the optimized coverage area for a 6 m road width is achieved as depicted in Figure 8(a), with the following synthesis parameters: Nx ¼ 4, Ny ¼ 4, dx ¼ 0:45λ0, dy ¼ 0:48λ<sup>0</sup> (with λ<sup>0</sup> evaluated at 920 MHz), θ<sup>0</sup> ¼ �5°, Rx ¼ 30 dB, Ry ¼ 25 dB, and the coefficients as in [27]. The achieved coverage area is 8 m long, covers the required transversal direction

width, and presents very low lateral sidelobes. Figure 8(b) also presents the achieved coverage area at htag ¼ 2:5 m (it could represent the tag height of a truck) and htag ¼ 1 m (that can represent the tag height of a motorcycle) along with the coordinate yR, and it shows that the higher the tag height htag , the shorter the coverage area. This is acceptable because the speed of a truck is usually lower than the speed of a common vehicle, so the available transaction time will be longer. In order to confirm the simulation results, the synthesized antenna array has been designed and manufactured, as shown in Figure 9(a). The design process of the 4 � 4 CP microstrip patch antenna array is described in [27]. Furthermore, the 12 dBi RHCP gain antenna prototype has been fixed at the height hA ¼ 5:5 m with a metallic scaffolding and used for collecting experimental results, as depicted in

Figure 9(b). A commercial Impinj Speedway R420 UHF reader [28] (Preader,th ¼ �84 dBm) and a tag device with Ptag,th ¼ �32 dBm have been

Synthesis example of the coverage area at 920 MHz. (a) Received power contour plot at htag ¼ 1:5 m and (b)

received power profile along yR coordinate as function of the tag height htag .

and verify the conditions in (13) and then determine the coefficients an,m.

4.3 Synthesis example and experimental results

yR ¼ 0).

Array Pattern Optimization

uct Impinj Indy R2000 [26]).

Figure 8.

42

(a) 4 x 4 CP microstrip patch antenna array fabricated prototype and (b) experimental setup for the ETC antenna array system measurement.

employed. A compact CP UHF antenna with gain Gr ¼ 4 dBi has been used as tag antenna.

The transmitted power Pt has been regulated in the range 5 ÷ 30 dBm for each position of the tag device in the road plane xR; yR to determine the minimum value Pt,min which activates the tag, that is, Pforward xR; yR <sup>¼</sup> Pt,min <sup>þ</sup> Gtð Þþ <sup>ϕ</sup>; <sup>θ</sup> Gr <sup>þ</sup> <sup>20</sup> log <sup>10</sup> λ0 4πr <sup>¼</sup> <sup>P</sup>tag,th, under the condition that <sup>P</sup>back xR; yR ≥Preader,th. After that, in a similar way to what has been described in [29], the power Pforward xR; yR when a limited PEIRP ¼ 36 dBm is applied and a specific Ptag,th ¼ �20 dBm is chosen has been inferred as Pforward xR; yR <sup>¼</sup> <sup>P</sup>tag,th <sup>þ</sup> <sup>P</sup>EIRP � Pt,min � Gt,max (cable losses have been compensated during the power evaluation). Experimental results are shown in Figure 10(a) and (b).

Good correspondence among simulations (SR), antenna measurement projection on the road plane (AM), and experimental results (ER) is visible, and only few discrepancies arise. These are mainly due to the 1 dB tag antenna gain reduction with respect to the design parameter, the tag antenna radiation pattern (not taken into account), and other possible errors in fixing the antenna mechanical tilt θA.

#### Figure 10.

Comparison of synthesis results obtained by projecting the measurement results of the antenna to the road plane (AM), simulation (SR), and experimental results (ER). (a) Received power contour plot at htag ¼ 1:5 m and (b) received power profile along yR coordinate as function of htag .
