Feasibility of backscatter RFID systems on the human body
© Grosinger; licensee Springer. 2013
Received: 30 December 2012
Accepted: 25 February 2013
Published: 20 March 2013
In this contribution, the author examines the feasibility of on-body backscatter radio frequency identification (RFID) systems in the ultra high frequency range. Four different on-body RFID systems are investigated operating monopoles or patch antennas at 900 MHz or 2.45 GHz. The systems’ feasibility is analyzed by means of on-body channel measurements in a realistic test environment. The measured channel transfer functions allow to evaluate if enough power is available for a reliable backscatter communication. This evaluation is done with the aid of outage probabilities in the forward link and the backward link of the systems. Using these probabilities, the on-body systems prove feasible when using state-of-the-art reader and transponder chips. In particular, the use of semi-passive RFID transponder chips leads to a reliable performance in the systems’ forward links. The robust performance of the systems’ backward links is clearly shown for the 900 MHz monopole antenna configuration, while the limitations in the backward links of the other systems have to be overcome by the use of a second reader unit on the person’s back. The novel feasibility analysis presented here allows to examine each system parameter individually and thus leads to reliable and robust backscatter RFID systems.
Wireless body area networks (WBANs) enable many new promising applications in the field of remote health monitoring, therapy support at home, wellness, and fitness. Therefore, the attention of the industry and the scientific community is highly drawn to WBANs [1, 2]. WBANs connect sensor nodes situated in clothes, on the body, or under the skin of a person through a wireless communication channel.
A promising communication technology for WBANs is backscatter radio frequency identification (RFID) in the ultra high frequency (UHF) range. Backscatter RFID relies on the radio communication between an RFID reader, acting as a control unit, and a multitude of passive or semi-passive RFID transponders (tags), acting as sensor nodes. The principle of communication for transmitting information from the tag to the reader relies on a modulated backscatter signal. All power for the transmission of the sensor data is drawn from the electromagnetic field radiated by the reader. Hence, their low-power consumption makes backscatter tags appropriate for WBANs that require small, light-weight, and low-maintenance sensor nodes. In addition, research efforts are ongoing to integrate sensing capabilities in backscatter tags without further enhancing their power consumption [3–5]. Such sensor tags can then be beneficially used to monitor the physiological parameters of a person (e.g., blood pressure, temperature, heartbeat, or body motion).
In backscatter RFID systems, it is vital to ensure a reliable power transmission to the backscatter tags and to realize a robust wireless communication between the reader and the tags . To assure this, it is advisable to investigate the on-body radio propagation channel including the effects of the antennas and to study the wireless power transmission and communication in realistic operating environments. These characteristics are investigated in this contribution which studies the feasibility of four different backscatter RFID systems in a WBAN.
Previous studies on UHF RFID-based WBANs have focused on in-body [7–9] and off-body [10–12] communication systems. So far, the investigation of backscatter communication systems on the human body has received less attention in the literature. A first feasibility analysis of an on-body backscatter RFID system is presented in  and is based on indoor backscatter measurements at 870 MHz. The investigated RFID system consisted of an on-body reader with a patch antenna and five on-body tags composed of custom-built wearable felt antennas.
This contribution provides a feasibility analysis of four different on-body RFID systems. The feasibility analysis is based on outage probabilities derived from on-body channel measurements in a realistic test environment at 900 MHz and 2.45 GHz. In comparison to backscatter measurements, the analysis based on channel measurements allows to examine each system parameter individually and thus gives a deeper insight in the wireless power transmission and communication of backscatter RFID systems. The investigated RFID systems are composed of custom-built monopole or patch antennas operating at 900 MHz or 2.45 GHz. Each antenna acts as both reader and tag antennas.
The article is organized as follows. Section 2 describes the investigated on-body RFID systems and defines outage probabilities for a backscatter RFID system. The outage probabilities of the investigated RFID systems are then found by means of on-body channel measurements in Section 3. Section 4 analyzes the feasibility of the different on-body RFID systems based on these measurements.
2 On-body RFID systems
Practically, monopole antennas are not suitable for WBAN applications because they are not low profile. However, monopoles show the best performance in on-body systems [18, 19] and are used as a best-case reference in this study to define an upper bound for the performance of practical system implementations. Conversely, low profile patch antennas are especially suitable for on-body applications . In this study, the less efficient patch antennas provide an insight in the performance of practical system implementations .
2.1 RFID outage probabilities
where τ is the power transmission coefficient between the tag’s antenna and chip  and PTag is the power received by the tag’s antenna. S21 is the channel transfer function between the reader antenna and the tag antenna, |S21|2 defines the channel gain in the forward link of the system.
and defines the outage probability of the system’s forward link, more precisely the probability that the backscatter system operates at its limit. This outage probability can be found by plotting the cumulative distribution function (CDF) of the measured channel gain in the forward link, |S21|2.
where η is the modulation efficiency of the tag . S12 is the channel transfer function between the tag antenna and the reader antenna. |S12|2 defines the channel gain in the backward link.
This probability can be found by plotting the CDF of the product of the measured channel gain in the forward link and the backward link, |S21|2|S12|2.
3 On-body channel measurements
4 Feasibility analysis
In the following, the different on-body RFID systems are analyzed by means of the outage probabilities found in the previous section.
As expected from theory and from previous measurement campaigns at 2.45 GHz , Figure 5 shows that on-body links with longer path lengths have higher outage probabilities in comparison to links with shorter distances (compare, e.g., the stomach-back link with the stomach-chest link). In addition, the link geometry and thus the channel gain are influenced by the movements of the body. The strength of this influence depends on the on-body link. An on-body link with a higher mobility experiences a wider range of outage probabilities than trunk-to-trunk links with lower mobility (compare, e.g., the stomach-wrist link with the stomach-chest link). These behaviors can be observed for all four antenna configurations in their forward and backward links (see Figures 6, 7 and 8).
In addition, the figures show that the outage probabilities of the 900 MHz antennas are lower than the probabilities of the 2.45 GHz antennas (compare, e.g., Figures 5 and 6). This difference is due to an increased energy absorption in human tissues at higher frequencies . Again, this behavior can be observed for both antenna types, the monopoles and the patch antennas.
Furthermore, the probability curves of all four figures show that the monopoles act indeed as best-case references.
4.1 State-of-the-art example
Subsequently, the outage probabilities for all four antenna configurations are explored individually for each system parameter of a state-of-the-art system example, i.e., PTX,Reader,τ,TChip,η, and TRX,Reader. In general, the maximum permitted outage probabilities are governed by the application. The outage probabilities should be close to zero for a system that monitors life parameters of patients in clinical care, while systems used in sports analysis can deal with higher outage probabilities. In the following, the systems allow a maximum outage probability of 10%.
In the state-of-the-art system, the on-body reader consists of an Impinj Indy reader chip with an external power amplifier . The reader provides a maximum transmit power of PTX,Reader=30 dBm and an RX sensitivity of TRX,Reader=−95 dBm. The on-body tags consist of passive Monza 5 tag chips . The passive chips provide a sensitivity of TChip=−17.8 dBm. The power transmission coefficient is assumed to be 100%, while the modulation efficiency is 20%. These system parameters lead to gain thresholds of FTh=−47 dB and BTh=−118 dB (see Equations (2) and (5)).
In conclusion, the author demonstrated the feasibility of backscatter UHF RFID systems on the human body at 900 MHz and 2.45 GHz. Until now, the investigation of on-body backscatter RFID systems has received less attention in the literature. A first feasibility analysis of an on-body backscatter RFID system was based on indoor backscatter measurements at 870 MHz, where the investigated RFID system consisted of an on-body reader with a patch antenna and on-body tags composed of custom-built wearable felt antennas. In this contribution, four different on-body RFID systems are investigated operating two different types of on-body antennas. Monopole antennas act as best-case references, while the less efficient patch antennas are used to give an insight into practical RFID system implementations.
In this article, the author presents a novel feasibility analysis based on on-body channel measurements in a realistic test environment. In particular, the channel transfer functions of the systems were measured versus different stationary and moving body postures and led to outage probabilities of the systems’ forward and backward links. These outage probabilities helped to easily identify limitations in the backscatter systems and to evaluate strategies to overcome these barriers for the realization of reliable on-body RFID systems. In comparison to backscatter measurements, the analysis based on channel measurements allows to examine each system parameter individually and thus gives a deeper insight in the wireless power transmission and communication of backscatter RFID systems.
It is worth pointing out that the presented analysis can be performed for any kind of backscatter RFID system. The analysis provides an initial overview of a backscatter system and ultimately allows to realize a reliable power transmission to the chips and a robust wireless communication between the reader and the tags.
This study was performed as part of the project “MAS—Nanoelectronics for Mobile Ambient Assisted Living-Systems” which is funded by “ENIAC Joint Undertaking” and Austria’s “Österreichische Forschungsförderungsgesellschaft”.
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