MIMO Transmission for Single-fed ESPAR with Quantized Loads

Compact parasitic arrays in the form of electronically steerable parasitic antenna radiators (ESPARs) have emerged as a new antenna structure that achieves multipleinput- multiple-output (MIMO) transmission with a single RF chain. In this paper, we study the application of precoding on practical ESPARs, where the antennas are equipped with load impedances of quantized values. We analytically study the impact of the quantization on the system performance, where it is shown that while ideal ESPARs with ideal loads can achieve a similar performance to conventional MIMO, the performance of ESPARs will be degraded when only loads with quantized values are available. We further extend the performance analysis to imperfect channel state information (CSI). In order to alleviate the performance loss, we propose to approximate the ideal current vector by optimization, where a closed-form solution is further obtained. This enables the use of ESPARs in practice with quantized loads. Simulation results validate our analysis and show that a significant performance gain can be achieved with the proposed scheme over ESPARs with quantized loads. Finally, the tradeoff between performance and power consumption is shown to be favorable for the proposed ESPAR approaches compared to conventional MIMO, as evidenced by our energy efficiency results

Tunable Load MIMO with Quantized Loads

In this paper, we study the application of precoding schemes on practical electronically steerable parasitic array radiators (ESPARs), where quantized load impedances are considered for each antenna element. The presence of quantization in the loads results in a performance loss for practical ESPARs. To alleviate the performance loss, we propose to approximate the ideal current vector with convex optimization, where it is further shown that the optimality is achieved by optimizing the feeding voltages only. Specifically, we obtain the closed-form expression when single-fed ESPARs are assumed. Numerical results show that the proposed quantization-robust scheme can achieve a significant performance gain over ESPARs with quantized loads

Multiple-Antenna Systems: From Generic to Hardware-Informed Precoding Designs

5G-and-beyond communication systems are expected to be in a heterogeneous form of multiple-antenna cellular base stations (BSs) overlaid with small cells. The fully-digital BS structures can incur significant power consumption and hardware complexity. Moreover, the wireless BSs for small cells usually have strict size constraints, which incur additional hardware effects such as mutual coupling (MC). Consequently, the transmission techniques designed for future wireless communication systems should respect the hardware structures at the BSs. For this reason, in this thesis we extend generic downlink precoding to more advanced hardware-informed transmission techniques for a variety of BS structures. This thesis firstly extends the vector perturbation (VP) precoding to multiple-modulation scenarios, where existing VP-based techniques are sub-optimal. Subsequently, this thesis focuses on the downlink transmission designs for hardware effects in the form of MC, limited number of radio frequency (RF) chains, and low-precision digital-to-analog converters (DACs). For these scenarios, existing precoding techniques are either sub-optimal or not directly applicable due to the specific hardware constraints. In this context, this thesis first proposes analog-digital (AD) precoding methods for MC exploitation in compact single-user multiple-antenna systems with the concept of constructive interference, and further extends the idea of MC exploitation to multi-user scenarios with a joint optimisation on the precoding matrix and the mutual coupling effect. We further consider precoding for wireless BSs with a limited number of RF chains, in the form of compact parasitic antenna array as well as hybrid analog-digital structures designed for large-scale multiple-antenna systems. In addition, with a reformulation of the constructive interference, this thesis also considers the low-complexity precoding design for the use of low-resolution DACs for a massive-antenna array at the BSs. Analytical and numerical results reveal an improved performance of the proposed techniques compared to the state-of-the-art approaches, which validates the effectiveness of the introduced methods

Low-Cost Beam Steerable Antennas Using Parasitic Elements

Beam steerable antennas are considered as a possible solution for meeting challenges in military and civilian systems such as satellite communication networks, automotive collision avoidance radar, base stations and biomedical applications. Phased array antennas are a natural choice as the foundation for many steerable antenna platform due to its exibility and gain scalability. The implementation of a phased array requires a large number of electronic components, tending to drive the cost of phased arrays and limit their usage to military applications. The electrically steerable parasitic array radiator (ESPAR) has been introduced as an antenna which is capable of adaptively controlling its beam pattern using parasitic elements loaded with varactors. ESPAR has attracted the attention of researchers from the desire for electrically scanned beams with inexpensive fabrication and has found as a suitable candidate for communication systems applications, including advanced radars, cellular base stations and space communications. The ultimate goal of this research is to design and propose state of the art designs in the �eld of ESPAR that can satisfy the requirements of today's advanced communication systems, which should be cost-e�ective and can compete with other rival technologies. Considering the potentials of ESPAR, it can be proved that it is a good candidate for modern wireless communications. The thesis presents several contributions related to the design and analysis of ESPAR technology using dielectric resonator antenna (DRA) as the main radiator element. First, the thesis presents solutions to alleviate the problems associated in implementing a large ESPAR. The large array is useful in many applications since some required recon�gurable radiation characteristics may not be achievable with a single ESPAR element. The proposed structure consists of 240 perforated DRAs, whichare uniformly excited by a parallel-series feeding network. By employing the perforation technique, the need for aligning and bonding individual DRA is eliminated. The subarrays are placed in an interleaved arrangement to suppress the grating lobes. The proposed large ESPAR can incredibly reduce the number of phase shifter by 80% in comparison with the conventional phased array, which makes it inexpensive. Second, the thesis investigates potentials of ESPAR for massive multi-input multiple output (MIMO) communication. Massive MIMO technology has attracted tremendous interest due to its capabilities in enhancing the data transmission capacity, increasing the reliability, and reducing the multipath fading. However, in this technology for feeding each individual antenna, one radio frequency chain is required that can increase the power consumption and complexity of the structure. Moreover, to obtain decorrelated channels and to reduce mutual coupling, the antenna should be spaced suffciently far from each other that imposes increased physical dimensions. In contrast to the conventional MIMO structures, in ESPAR only one RF chain is needed and the small size constraint turns to be an advantage as the mutual coupling is exploited to form the desired signals. Furthermore, by controlling the tunable loads at each parasitic antenna element, different radiation patterns can be formed which can signi�cantly improve the performance of a MIMO antenna system operating in a changing environment. Thus, by using the advantages of ESPAR, a design approach to address the size and cost issues is proposed through this work. The proposed design is validated by simulation and measurement of a prototype, and results include the antenna and MIMO �gure of merits such as radiation patterns, efficiency, S-parameters, signal correlations, total active reection coeffcient (TARC), and channel capacity. These results have demonstrated that the proposed ESPAR design can be successfully implemented for a massive MIMO structure. Finally, the thesis presents an effective method to design a ESPAR with a circularly polarized (CP) beam-scanning feature. Circular polarization is an ideal polarization due to its advantages in signal propagation properties, which can address the di�culties associated with mobility, inclement weather conditions, and immunity to multi path distortion. In this work, the CP beam steering is achieved by adopting a sequential rotation approach for placing the parasitic antennas that are loaded with tunable varactors. The proposed CP-ESPAR technique eliminates the need of expensive phase shifters, which signi�cantly reduces cost and fabrication complexity. For performance evaluation, a prototype of the proposed antenna is designed, fabricated, and measured. It is observed that the proposed antenna has a monotonic CP beam scanning from { 22 to 22 operating at 10.5 GHz

Analog-Digital Beamforming in the MU-MISO Downlink by use of Tunable Antenna Loads

We investigate the performance of multi-user multiple-input-single-output (MU-MISO) downlink in the presence of the mutual coupling effect at the transmitter. Contrary to traditional approaches that aim at eliminating this effect, in this paper we propose a joint analog-digital (AD) beamforming scheme that exploits this effect to further improve the system performance. A jointly optimal AD beamformer is firstly obtained by iteratively maximizing the minimum received signal-to-interference-plus-noise ratio (SINR) in the digital domain, followed by an optimization on the load impedance of each antenna element in the analog domain. We further introduce a decoupled low-complexity approach, with which existing closed-form beamforming schemes can also be efficiently applied. For the consideration of hardware imperfections in practice, we study the case where the analog load values are quantized, and propose a sequential search scheme based on greedy algorithm to efficiently obtain the desired quantized load values. Moreover, we also investigate the imperfect channel state information (CSI) scenarios, where we prove the optimality for closed-form beamformers, and further propose the robust schemes for two typical CSI error models. Simulation results show that with the proposed schemes the mutual coupling effect can be exploited to further improve the performance for both perfect CSI and imperfect CSI

The electronically steerable parasitic array radiator antenna for wireless communications : signal processing and emerging techniques

Smart antenna technology is expected to play an important role in future wireless communication networks in order to use the spectrum efficiently, improve the quality of service, reduce the costs of establishing new wireless paradigms and reduce the energy consumption in wireless networks. Generally, smart antennas exploit multiple widely spaced active elements, which are connected to separate radio frequency (RF) chains. Therefore, they are only applicable to base stations (BSs) and access points, by contrast with modern compact wireless terminals with constraints on size, power and complexity. This dissertation considers an alternative smart antenna system the electronically steerable parasitic array radiator (ESPAR) which uses only a single RF chain, coupled with multiple parasitic elements. The ESPAR antenna is of significant interest because of its flexibility in beamforming by tuning a number of easy-to-implement reactance loads connected to parasitic elements; however, parasitic elements require no expensive RF circuits. This work concentrates on the study of the ESPAR antenna for compact transceivers in order to achieve some emerging techniques in wireless communications. The work begins by presenting the work principle and modeling of the ESPAR antenna and describes the reactance-domain signal processing that is suited to the single active antenna array, which are fundamental factors throughout this thesis. The major contribution in this chapter is the adaptive beamforming method based on the ESPAR antenna. In order to achieve fast convergent beamforming for the ESPAR antenna, a modified minimum variance distortionless response (MVDR) beamfomer is proposed. With reactance-domain signal processing, the ESPAR array obtains a correlation matrix of receive signals as the input to the MVDR optimization problem. To design a set of feasible reactance loads for a desired beampattern, the MVDR optimization problem is reformulated as a convex optimization problem constraining an optimized weight vector close to a feasible solution. Finally, the necessary reactance loads are optimized by iterating the convex problem and a simple projector. In addition, the generic algorithm-based beamforming method has also studied for the ESPAR antenna. Blind interference alignment (BIA) is a promising technique for providing an optimal degree of freedom in a multi-user, multiple-inputsingle-output broadcast channel, without the requirements of channel state information at the transmitters. Its key is antenna mode switching at the receive antenna. The ESPAR antenna is able to provide a practical solution to beampattern switching (one kind of antenna mode switching) for the implementation of BIA. In this chapter, three beamforming methods are proposed for providing the required number of beampatterns that are exploited across one super symbol for creating the channel fluctuation patterns seen by receivers. These manually created channel fluctuation patterns are jointly combined with the designed spacetime precoding in order to align the inter-user interference. Furthermore, the directional beampatterns designed in the ESPAR antenna are demonstrated to improve the performance of BIA by alleviating the noise amplification. The ESPAR antenna is studied as the solution to interference mitigation in small cell networks. Specifically, ESPARs analog beamforming presented in the previous chapter is exploited to suppress inter-cell interference for the system scenario, scheduling only one user to be served by each small BS at a single time. In addition, the ESPAR-based BIA is employed to mitigate both inter-cell and intracell interference for the system scenario, scheduling a small number of users to be simultaneously served by each small BS for a single time. In the cognitive radio (CR) paradigm, the ESPAR antenna is employed for spatial spectrum sensing in order to utilize the new angle dimension in the spectrum space besides the conventional frequency, time and space dimensions. The twostage spatial spectrum sensing method is proposed based on the ESPAR antenna being targeted at identifying white spectrum space, including the new angle dimension. At the first stage, the occupancy of a specific frequency band is detected by conventional spectrum-sensing methods, including energy detector and eigenvalue-based methods implemented with the switched-beam ESPAR antenna. With the presence of primary users, their directions are estimated at the second stage, by high-resolution angle-of-arrival (AoA) estimation algorithms. Specifically, the compressive sensing technology has been studied for AoA detection with the ESPAR antenna, which is demonstrated to provide high-resolution estimation results and even to outperform the reactance-domain multiple signal classification

Interference Exploitation 1-Bit Massive MIMO Precoding: A Partial Branch-and-Bound Solution With Near-Optimal Performance

In this paper, we focus on 1-bit precoding approaches for downlink massive multiple-input multiple-output (MIMO) systems, where we exploit the concept of constructive interference (CI). For both PSK and QAM signaling, we firstly formulate the optimization problem that maximizes the CI effect subject to the requirement of the 1-bit transmit signals. We then mathematically prove that, when employing the CI formulation and relaxing the 1-bit constraint, the majority of the transmit signals already satisfy the 1-bit formulation. Building upon this important observation, we propose a 1-bit precoding approach that further improves the performance of the conventional 1-bit CI precoding via a partial branch-and-bound (P-BB) process, where the BB procedure is performed only for the entries that do not comply with the 1-bit requirement. This operation allows a significant complexity reduction compared to the fully-BB (F-BB) process, and enables the BB framework to be applicable to the complex massive MIMO scenarios. We further develop an alternative 1-bit scheme through an ‘Ordered Partial Sequential Update’ (OPSU) process that allows an additional complexity reduction. Numerical results show that both proposed 1-bit precoding methods exhibit a significant signal-to-noise ratio (SNR) gain for the error rate performance, especially for higher-order modulations

Energy efficient MIMO SWIPT by hybrid analog-digital beamforming

In this paper, we investigate the simultaneous wireless information and power transfer (SWIPT) for MIMO systems with limited RF chains at the base station. We focus on the scenario where there is one information decoder with a targeted SINR and several separate energy receivers with energy harvesting thresholds. Based on the observation that the fully-digital beamformer consists of only the information beamformer, we propose an iterative hybrid analog-digital beamforming scheme, where we design the analog beamformer by minimizing the difference between the fully-digital beamformer and the hybrid beamformer, and the optimal solution can be obtained via a geometrical interpretation. Numerical results show that the proposed scheme can achieve a close-to-optimal performance with significant gains in the total power consumption over fully-digital SWIPT

Energy Efficient MIMO SWIPT by Hybrid Analog-Digital Beamforming

In this paper, we investigate the simultaneous wireless information and power transfer (SWIPT) for MIMO systems with limited RF chains at the base station. We focus on the scenario where there is one information decoder with a targeted SINR and several separate energy receivers with energy harvesting thresholds. Based on the observation that the fully-digital beamformer consists of only the information beamformer, we propose an iterative hybrid analog-digital beamforming scheme, where we design the analog beamformer by minimizing the difference between the fully-digital beamformer and the hybrid beamformer, and the optimal solution can be obtained via a geometrical interpretation. Numerical results show that the proposed scheme can achieve a close-to-optimal performance with significant gains in the total power consumption over fully-digital SWIPT