Robert Langwieser

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.

Vienna MIMO testbed

While the field of MIMO transmission has been explored over the past decade mainly theoretically, relatively few results exist on how these transmissions perform over realistic, imperfect channels. The reason for this is that measurement equipment is expensive, difficult to obtain, and often inflexible when a multitude of transmission parameters are of interest. This paper presents a flexible testbed developed to examine MIMO algorithms and channel models described in literature by transmitting data at through real, physical channels, supporting simultaneously four transmit and four receive antennas. Operation is performed directly from Matlab allowing for a cornucopia of real-world experiments with minimum effort. Examples measuring bit error rates on space-time block codes are provided in the paper.

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.

RFID reader with multi antenna physical layer collision recovery receivers

Radio Frequency Identification (RFID) is a wireless identification technology which often operates in environments with multiple RFID tags. Already introduced multiple antenna receivers can recover from a collision of up to M tags as long as M is less than the number of receive antennas and the channel is known at the receiver. This paper proposes a Zero-Forcing (ZF) and a Minimum Mean Square Error (MMSE) receiver which allows the separation of up to M=2NR tags, where NR is the number of receiving antennas on the reader. The proposed algorithms are verified through simulations.

Broadband suppression properties of active leaking carrier cancellers

In passive UHF RFID systems backscattering is used for tag to reader communication. This technique relies on a continuous wave signal being transmitted by the reader during the tags data transfer. In order to separate transmission and reception paths circulators, directional couplers or disjoint transmit and receive antennas can be used. Perfect isolation is not achievable with any of those approaches. So a leaking carrier is present at the receiver in any case. It is possible to reduce the interference from this signal by some kind of leaking carrier cancellation. Such a cancellation prevents receiver blocking and reduces the baseband hardwares requirements, depending on the receiver concept. Usually, the narrowband properties of carrier cancellers are studied. This is only sufficient for conventional RFID systems, if the transmitter noise can be neglected. Broadband RFID systems, as recently discussed in literature, also require broadband leaking carrier cancellers. In this paper the broadband suppression properties of carrier cancellers are first investigated theoretically. Further a hardware implementation is presented and characterized. Finally, measurement results are compared to the theoretical findings.

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 smart collision recovery receiver for RFIDs

In this work, we focus on framed slotted Aloha (FSA) and passive ultra high-frequency radio frequency identification multi-antenna systems with physical layer collision recovery. We modify the tags slightly by adding a so-called ‘postpreamble’ that facilitates channel estimation. Furthermore, we investigate the throughput performance of advanced receiver structures in collision scenarios. More specifically, we analyse the throughput of FSA systems with up to four receive antennas that can recover from a collision of up to eight tags on the physical layer and acknowledge all tags involved in that collision. Due to the higher collision recovery capabilities, the frame sizes can be significantly reduced, and thus, the throughput can be increased. We also derive analytically optimal frame sizes, given that a certain number of collisions can be resolved. We further study the constraints to the throughput due to the structure of our receiver and channel estimation for different collision scenarios. Furthermore, we propose a novel collision recovery method with two phases: first, a successive interference cancellation and, second, a projection of the constellation into the orthogonal subspace of the interference. Additionally, the inventory time, i.e. the number of slots necessary to successfully decode all tags in the reader range, is calculated and compared for different receiver types. A validation of our theoretical predictions is achieved by means of simulations. We show that by our proposed methods, we can realistically achieve more than ten times higher throughput or, equivalently, a reduction of the inventory time by more than 90%.

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.

Directional evaluation of receive power, Rician K-factor and RMS delay spread obtained from power measurements of 60 GHz indoor channels

To meet link budgets for millimetre wave wireless communications, antennas with high directivity are essential. This directional view of the channel is evaluated by the important small scale fading parameters receive power, Rician K-factor and the RMS delay spread. These parameters are all derived from frequency swept power measurements. The effect of transmit polarization is very visual in the measurement results. A maximum K-factor of almost 80 was observed. The RMS delay spread is approximately 4 ns at reflective reception.

Dual-band channel gain statistics for dual-antenna tyre pressure monitoring RFID tags

In this contribution we analyse the read probability enhancement using two simple dual-antenna techniques for passive RFID tags for Tyre Pressure Monitoring applications. Our analysis is based on real-world channel measurements carried out with a full vehicle body for different antenna and steering angle configurations. Two frequency ranges were analysed: European UHF band at about 866 MHz and 2.45GHz ISM band. Two antenna combining methods were investigated: Antenna selection and power combining. With the latter a read probability enhancement from 49% to 75% is achievable.