Radio-frequency-identification (RFID) systems have become widespread as reliable means of storing and remotely retrieving data through the use of compact RFID tags. In particular, the use of ultrahigh- frequency (UHF) passive RFID is appealing for many applications since it enables recognition from a reasonable distance. The technology is ideal for supply-chain management and several major firms, such as Wal-Mart and Tesco, are planning to mandate the use of UHF RFID in their supply chains.^1,2

The performance of a UHF RFID system is usually characterized by its interrogation range, which is defined as the maximum distance at which an RFID reader can recognize a tag. This can be divided into two categories: the forward-link interrogation range (FIR) and the reverse-link interrogation range (RIR). In UHF RFID systems, the forward-link refers to the communication link from a reader to a tag, whereas the reverse-link is that from the tag to the reader. FIR is defined as the maximum distance at which the tag receives such power as to turn on and back-scatter, and RIR is the maximum distance at which the reader can decode the data of the tag satisfying a SNR requirement. Since the actual interrogation range is determined by the smaller value of FIR and RIR, both values should be considered simultaneously when deploying UHF RFID systems.^3-6

Figure 1 shows UHF RFID link concepts compared to a typical wireless communication system such as code-division- multiple-access (CDMA) or Global System for Mobile Communications (GSM) cellular systems. In a typical wireless communication system, the forward link refers to the communications link from a base station (BS) to a mobile station (MS), whereas the reverse link is that from the MS to the BS. The noise levels at both the links are determined by thermal noise power, which is

P_{N, thermal} = 4kTB (1)

where
k = Boltzman’s constant,
T = the absolute temperature (in K), and
B = the bandwidth.

Generally, wireless communication systems exhibit link balance between the forward and reverse links, where the dynamic range between two links is almost the same. Therefore, the forward-link coverage corresponds closely with the reverse-link coverage, although the transmit powers for the forward and reverse links may differ.

In contrast, the communications link of a passive UHF RFID system has an imbalance between the forward and reverse links (Fig. 1). This is because the RFID tag has no internal power supply and must harvest energy from a continuous-wave (CW) signal transmitted by the RFID reader. Consequently, the FIR is mainly dependent on the threshold power necessary to power up the tag. Another major difference is that the phase noise of the transmitter (Tx) leakage at the reader’s circulator has greater influence on system noise than the thermal noise at the reader’s receiver (Rx). Therefore, there is the possibility that the RIR has a smaller value than FIR especially for poorly designed stationary readers or handheld readers.

In this examination of UHF RFID systems, it will be assumed that the RFID reader’s antenna employs polarization that matches that of the tag’s antenna. If r is used to denote the operational distance between an RFID tag and the reader operating in free space, then the power received by the RFID tag, P_Rx, can be found by applying the Friis electromagnetic (EM) wave propagation equation:

where
λ = the wavelength in free space,
P_Tx = the signal power feeding into the reader antenna by the transmitter,
G_R = the gain of the reader antenna,
G_T = the gain of the tag antenna,

One portion of the power P_Rxis absorbed by the tag for DC power generation and the other portion of P_Rxis backscattered for the reverse link. In order to ensure correct operation of the tag, the absorption power must be larger than the minimum operating power required for tag operation, P_TH} In the case of a tag with amplitude-shift-keying (ASK) modulation, the time-averaged absorption power of the tag is given by7:

where
m = the modulation depth.

Generally, P_TH is determined according to the tag chip design and antenna matching conditions. The FIR, T_forward, can then be derived using Eq. 4:

The FIR calculated by Eq. 4 has a value of 8 m as shown in Fig 2. In Eq. 4, the FIR is proportional to the square root of the transmitted effective isotropic radiated power (EIRP) P_TxG{T, and the tag antenna’s gain, GR, and is inversely proportional to the square root of the tag’s power threshold level, P_TH. From experience, it is known that the RF threshold power level required to turn on a tag ranges from 10 µ} W (-20 dBm) to 50 µW (-13 dBm).⁸ The modulation depth, m, is chosen to be an average value between 0.1 and 0.9.

In the reverse link, the backscattered signal from a tag should be strong enough that the reader’s demodulation output signal will meet the system’s signal-to-noise-ratio (SNR) requirement. To calculate the SNR of the demodulation output signal of the reader, consider the conventional reader architecture of Fig. 3. The RFID reader is composed of an LO, transmitter, receiver, and antenna with a circulator. The circulator is a nonreciprocal three-port device, where the signals travel from the transmitter port to the antenna port or from the antenna port to the receive port. In practice, the circulator cannot totally isolate the transmitter from the receiver due to the inherent leakage between its ports. Generally, the Tx/Rx isolation ranges from 20 to 50 dB.10 Therefore, the phase noise of the Tx leakage power is much stronger than the thermal noise, to a degree that the RIR mainly depends on the Tx/Rx isolation level. On the other hand, in a typical wireless communication system, the Tx leakage is normally not a major problem because duplexing techniques such as frequency division duplexing (FDD) and time division duplexing (TDD) are applied.

As shown in Fig 3, the LO provides two identical frequency signals: one for the transmitter and the other for the receiver. Neglecting the amplitude noise, the LO signal can be expressed as

where
A_LO = the amplitude of the LO signal,
ω = the angular frequency, and
θ _LO(t) = the phase noise of the LO signal.

The RFID system’s power amplifier (PA) boosts the level of the LO signal. This amplified signal feeds the reader antenna via the circulator and then is radiated into free space. Simultaneously, the reader antenna receives backscattered signals from the tag.

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