MIMO Techniques in RFID and Backscatter Communication Systems Considering Antenna Mutual Coupling

The principle of backscatter modulation, in which data transmission is achieved by intentionally altering the scattering behavior of a transponder, is commonly used in Radio Frequency Identification (RFID) applications, such as tracking and inventory management. However, backscatter communication is increasingly being adopted for novel applications as a technology for the wireless exchange of data (e.g., sensor or telemetry information), particularly in the domain of the Internet of Things. It represents an energy- and hardware-efficient alternative to other standards such as Bluetooth Low Energy or ZigBee. This work focuses on the overall goal of improving the range, reliability, and spectral efficiency of backscatter-modulation-based systems. To achieve this, the effects of multipath propagation are leveraged through Multiple-Input Multiple-Output transmission (MIMO) relying on multi-antenna backscatter transponders. In contrast to conventional MIMO communication systems, multiple antennas in backscatter communication systems can be used at three locations: as transmitting and receiving antennas at the reader and for backscatter modulation at the transponder. The use of multiple backscatter antennas at the transponder is of particular research interest, as the modulation method and the resulting dyadic channel fundamentally differ from the principles of traditional wireless systems. The signaling matrix of the transponder describes its backscatter behavior, serves as a link between the forward and backward channels, and modulates the information to be transmitted by the transponder onto the signal sent by the reader. While many publications on multi-antenna RFID systems primarily focus on the hardware and signal processing of the reader in combination with conventional, commercial transponders, this work concentrates on developing concepts for MIMO transmission based on novel multi-antenna transponders with freely adjustable and fully populated signaling matrices, also taking antenna mutual coupling into account. To date, literature has predominantly examined multi-antenna transponders with diagonal signaling matrices. This dissertation begins by reviewing the state of the art in research and technology, followed by an outline of the main objectives and the structure of the work. To investigate the particularly critical issue of antenna mutual coupling in backscatter-modulation-based systems, this work initially focuses on the simulation of MIMO backscatter systems. A scattering-parameter-based model is developed, upon which a system simulator for arbitrary antenna configurations is implemented. The transmission paths between the antennas and the modeling of the antenna element coupling are realized within a network model through cascading and embedding of scattering parameter blocks. For these investigations, suitable models for the forward and backward channels, along with signal processing algorithms for detecting the data backscattered by the transponder, are implemented. Using the simulation environment implemented in MATLAB, the effects of antenna coupling on backscatter modulation with multiple antennas are analyzed and illustrated for MIMO backscatter systems with various antenna configurations. The analysis is conducted relying on symbol error ratios and capacity. It is demonstrated that antenna mutual coupling at the transponder in backscatter communication systems plays a fundamentally different role compared to mutual coupling between the elements of an antenna array on the transmitting or receiving side in classical MIMO communication systems. This difference arises from the fact that the modulation of the backscattered signals is significantly influenced by the interaction between the load impedances constantly switched for data modulation and the altered antenna properties caused by mutual coupling. Since knowledge of the transmission channel is particularly crucial for the transmission quality in MIMO communication systems, this work subsequently focuses on developing measurement methods for characterizing the forward and backward channels in MIMO backscatter systems. The challenge lies in the fact that, due to the simple hardware structure of a backscatter transponder, the channel between the reader and the transponder cannot be directly determined. To address this, an analogy to calibration methods in multiport network analysis is established based on the scattering parameter modeling. A method for measurements at a reduced number of measurement ports is further developed for the purpose of backscatter channel characterization and validated through measurements. It is demonstrated that by interconnecting the antennas in the transponder, i.e., utilizing the non-diagonal elements of the signaling matrix, the uncertainties in the channel characterization can be significantly reduced. Subsequently, MIMO backscatter systems are analyzed based on time-varying scattering parameters. The investigations rely on a measurement setup utilizing a 4-port network analyzer as the reader, which functions as a multi-channel signal analyzer by measuring the time-varying scattering parameters followed by signal post-processing. Using developed 4-QAM backscatter modulators in conjunction with up to two backscatter antennas, both the channel under various realizations using a stepper motor rail and the data transmission via different MIMO transmission schemes are examined. It is demonstrated that methods from network analysis can also be applied to detect the transmitted payload data. Another focus of this work is the design of multi-antenna transponders to explore novel MIMO backscatter transmission techniques. For the first time, a transponder with a fully populated and freely adjustable signaling matrix is implemented. This enables backscatter modulation of the received reader signal with an arbitrarily configurable constellation diagram for each element of the signaling matrix, allowing for the internal transmission between the two antennas as well, while being simultaneously modulated in magnitude and phase. Based on this transponder, the novel concept of backscatter beamforming is introduced and experimentally validated.