Christoph Angerer

RFID Reader Receivers for Physical Layer Collision Recovery

Arbitration and scheduling of multiple tags in state-of-the-art Radio Frequency Identification (RFID) systems is accomplished on the medium access control layer. Currently, only answers of a single tag can be decoded in such a system. If multiple tags respond simultaneously, a collision occurs. In that case, conventional systems discard the physical layer information and a retransmission is executed. This work shows how to recover from such collisions on the physical layer and successfully read the data. The contributions of the paper are: 1) An analysis of the achievable throughput increase of a system, that can recover from collisions at a physical layer is given. 2) A model for a description of collisions on the physical layer is presented. 3) Based on this model, we propose a channel estimation method and two types of receiver structures for separating the signals of a collision of two tags: first, single antenna receivers that discriminate the sources of the two tags in the I/Q plane, and second, multiple antenna receivers which exploit the different spatial signatures of both tags. 4) The functionality of the proposed receiver structures is verified with measurement data of two colliding tags. Eventually, a performance analysis of the receivers is provided.

Slot-wise maximum likelihood estimation of the tag population size in FSA protocols

Framed Slotted Aloha (FSA) is a popular anticollision technique in state-of-the-art RF-ID systems, as in ISO/IEC CD 18000-6 for 900MHz or the EPCglobal HF Gen 2 draft for 13.56MHz. In many applications the number of tags entering and leaving the detection range of the reader is subject to a strong fluctuation and usually unknown. The current number of tags in the field is a crucial parameter to operate the FSA anti-collision in an optimal manner. Therefore, a lot of effort is spent on the estimation of this parameter and a range of different estimation techniques exist. The contributions of this paper are: 1) a closed formula for the probability of any observed event defined by the number of empty, singleton, and collision slots in the observed frame is developed and empirically verified. 2) This formula is then modified to compute the probability for partly observed frames as well which is of great interest as the referred standards allow for the in-frame adjustment of the frame size without quitting the interrogation round. 3) Then, a maximum likelihood estimator is formulated to yield the estimated number of tags on a slot-wise basis. 4) Its superior estimation performance is compared to the known best estimators over the complete parameter set. While its performance is strongly superior compared to Schoute¿s estimate, compared to Vogt¿s MSE estimator only marginally improvement is obtained1.

A comparative study of RFID schedulers in dense reader environments

In realistic deployments, several readers may be placed in the same area, forming a, so-called, dense reader environment. These scenarios are susceptible to suffer Reader Collision Problems, characterized by Reader-to-Tag and Reader-to-Reader interferences. Both affect network throughput, decreasing the overall number of tags identified per reader. This paper reviews the mechanisms proposed to mitigate the Reader Collision Problems. Besides, the constraints of these techniques are pointed out. The mechanisms have been evaluated to study the efficiency of the resources allocation.

Advanced synchronisation and decoding in RFID reader receivers

This paper focuses on one of the major challenges in RFID reader receivers, namely the data synchronisation and decoding at the RFID reader. According to the most widely used RFID standards, the data rate in RFID tag to reader communications is subject to variations within more than one decade, and may deviate from its nominal frequency up to 22%. This results in strong difficulties in synchronising and decoding this data at the receiver of the RFID reader. In order to achieve synchronisation and decoding, a sophisticated algorithm based on correlations is presented. The algorithm is optimised in terms of resource consumption to be processed on an FPGA or ASIC. Implementation details are presented as well as measurements, showing the performance of the receiver.

A UHF frontend for MIMO applications in RFID

The introduction of multi antenna applications in radio frequency identification (RFID) is expected to further improve the capability of RFID systems but requires new or extended simulation and experimental setups. This paper describes a developed analog 2×2 multiple input multiple output (MIMO) frontend for an RFID rapid prototyping system which allows for various realtime experiments to investigate MIMO techniques as beamforming, diversity combining, or localization at the reader. Finally, a measurement example with one transmitter and two receivers is presented for two different tag positions.

Single antenna physical layer collision recover receivers for RFID readers

Radio Frequency Identification (RFID) systems often are operated in environments with multiple RFID tags. In such an environment, a conventional RFID system resolves collisions of multiple tags on the medium access control layer, discarding the signals of the physical layer. This paper proposes a zero-forcing and an interference cancellation receiver architecture for an RFID reader, to recover from collisions of two tags on a physical layer and identify tags successfully even in case of a collision. The expected throughput increase is approximately 1.6 times the throughput of a conventional reader. We explore the signal properties of collisions and propose a model for the physical layer. Moreover, we present a method for estimating the signal constellation states in a collision. The entire structure, including channel estimation and both of the proposed receivers are verified with data generated during a measurement. Additionally, performance simulations of the two structures with different channels are shown.

Maximal ratio combining receivers for dual antenna RFID readers

Radio frequency identification (RFID) systems at ultra high frequencies operate in an environment exposed to fading. While state-of-the-art RFID readers utilise multiple receive antennas with antenna multiplexing in order to deal with the multipath propagation environment, this contribution proposes maximal ratio combining at RFID reader receivers. A dual receive antenna RFID reader is presented in this paper. The composition of the receive signal and the constellation in the I/Q plane on each antenna is analysed and discussed thoroughly. With that knowledge, we design a receiver estimating the channel coefficients and realising maximal ratio combining of the received signals, thus achieving the optimum combination of receive signals in terms of SNR maximisation. Underlying assumptions on the receive signals at the RFID reader for the design of the receiver have been cross-verified with measurements. Furthermore, the receiver has been implemented on an FPGA and functionally verified.

A digital receiver architecture for RFID readers

This paper presents a digital receiver architecture for an RFID reader. The main challenge in RFID reader design is the detection of the backscattered signals from the tags, which can be severely complicated due to the largely varying scale of possible receive powers. Furthermore noise, which power depends on the environment can degrade the detection performance. The detection of the signals of the tags is additionally impeded by the very strong self interference at the reader with the carrier it needs to send in order to supply the tags with energy. To fight these various disturbances, a new RFID receiver algorithm is proposed, that sets its decision threshold adaptively, depending on the strength of the input signal, the noise power at the receiver and the extent of the carrier interference. This is the first algorithm for signal detection in RFID, setting its threshold accordingly to the environmental conditions, and thus leading to near optimum performance. Details of the implementation of the digital receiver architecture on an FPGA are introduced. Bit error ratio measurements have been carried out to rate the receivers performance, which have never been shown before for RFID receivers. Presented measurement results substantiate the performance of the suggested algorithm.

Flexible evaluation of RFID system parameters using rapid prototyping

Todays RFID systems are dependent on a wide range of different parameters, that influence the overall performance. Such system parameters can for example be the selected data rate, encoding scheme, modulation setting, transmit power or different hardware configurations, like one or two antenna scenarios. Furthermore, it is often desired to optimise several performance goals, like read-out range, read-out quality, throughput, etc., which are often contradicting each other. In order to achieve a desired performance of an RFID system, it is essential to understand the influences of the individual parameters of interest and their interconnection. Due to the multitude, wide range and interdependencies of influencing factors, this however is a complex task. Simulations offer insights in these relations but rely on the correct modeling of the dependencies of- and between the parameters. With our established prototyping system for RFID, we are able to flexibly and accurately explore the influence and interconnection of such parameters in a wide range on a basis of real-time measurements. Results on the evaluation of read-out quality depending on the transmit power and the data rate are presented.

Evaluation and exploration of RFID systems by rapid prototyping

Today’s RFID systems exhibit relatively little functionality, while future systems and ubiquitous computing applications require an ample set of general purpose features, like wide communication ranges, high data rates, high reliability, and many more. In order to meet these high-performance goals, several challenges in state of the art RFID systems need to be managed: (1) the compatibility of RFID equipment, working according to different standards in various frequency domains, (2) the thorough understanding of the performance impact of physical layer system parameters, (3) the venture of novel wireless technologies in the context of RFID, and finally, (4) to deal with the increased complexity of high-performance RFID systems. Therefore, designers desire a highly configurable, flexible, and high-performance RFID environment to experimentally explore the underlying physical conditions and to evaluate novel RFID technologies and designs. This paper introduces the concept of rapid prototyping in RFID and provides a survey of system simulators, demonstrators, and rapid prototyping environments. A guideline for the setup for such a rapid prototyping system applicable for RFID is presented, and its implementation is demonstrated. Finally, some exemplary measurements carried out with this rapid prototyping system are presented.