Spectrally Efficient Backscatter Systems: A Hardware-Oriented Survey

The current wireless communication is experiencing rising data rate demands, extremely low latency, and efficient connections in a wide range of applications, which leads to a viable migration from the fifth-generation (5G) communication system to the sixth-generation (6G) system. The paper explores the potential of 6G networks to enhance the speed of data transfer through an analysis of Multi-Input Multi-Output (MIMO), Millimeter Wave (mmWave), and Artificial Intelligence (AI). Massive MIMO increases spectral efficiency with the use of massive arrays of antennas. Using high frequencies that have a large bandwidth, mmWave communication provides speedier communication. Even so, mmWave has to deal with issues due to path loss and blockage of the signal. These will be dealt with using beamforming and AI. In this research work, these limitations are overcome by integrating Deep Reinforcement Learning (DRL) using a Deep Q-Network (DQN), Tools for dynamic management of network resources, to better predict traffic and optimize real-time network slicing. When these technologies work together, the combination will significantly improve networks in 6G networks applications for autonomous systems, immersive virtual and augmented reality, holographic communication, etc. This is done in a hybrid setting where MATLAB/Simulink is used to analyze Massive MIMO and beamforming analysis, and the NS-3 tool is used for the mmWave module to evaluate end-to-end 6G network performance. The outcomes confirm that the proposed 6G framework has a 35 to 40 percent increase in spectral efficiency, and 25-30% decrease in end-to-end latency as compared to a 5G communication system. The results demonstrate that Massive MIMO, mmWave, and DQN-based optimization can be jointly applied to provide a robust architecture that can support new 6G applications, including autonomous vehicles, immersive extended reality, and holographic communications. The research presents the improvements in terms of performance, defines the barriers to the implementation, and suggests the possible solutions, which are a valuable input in the design of intelligent and future-wise wireless networks.

The uplink spectral efficiency (SE) of single and multi cell DS-CDMA systems using higher order modulation schemes is investigated in the presence of ergodic fading channels. Therefore, the asymptotic analysis for large CDMA systems with random spreading codes is used to obtain the SINR after the single user matched filter (MF) and LMMSE receiver. Based on these results, the SE is maximized for both receivers with coding at capacity for varied load. Furthermore, the concept of cut-off rate is applied to determine the SE for realistic coding and modulation. It is shown, that with an MF receiver switching to higher order modulation has no advantage. In contrast, for SNR above a certain threshold the SE of DS-CDMA systems can be improved with larger constellations when deploying an LMMSE receiver, which is shown using a number of M-QAM constellations. In general, the behavior of multi cell systems appears to be similar to single cell systems for the deployed linear cell-array model. However, it is demonstrated that depending on the interference level from the adjacent cells the SE can be degraded significantly.

To support ubiquitous computing for various applications (such as smart health, smart homes, and smart cities), the communication system requires to be ubiquitously available, ultra-low-power, high throughput, and low-latency. A passive communication system such as backscatter is desirable. However, existing backscatter systems cannot achieve all of the above requirements. In this paper, we present the first LTE backscatter (LScatter) system that leverages the continuous LTE ambient traffic for ubiquitous, high throughput and low latency backscatter communication. Our design is motivated by our observation that LTE ambient traffic is continuous (v.s. bursty and intermittent WiFi/LoRa traffic), which makes LTE ambient traffic a perfect signal source of a backscatter system. Our design addresses practical issues such as time synchronization, phase modulation, as well as phase offset elimination. We extensively evaluated our design using a testbed of backscatter hardware and USRPs in multiple real-world scenarios. Results show that our LScatter's performance is consistently orders of magnitude better than WiFi backscatter in all the above scenarios. For example, LScatter's throughput is 13.63Mbps, which is 368 times higher than the latest ambient WiFi backscatter system [54]. We also demonstrate the effectiveness of our system using two real-world applications.

Cascaded fading plays a vital role in backscatter communications but its effect on the transmission robustness of symbiotic backscatter system has not been well understood yet. In this paper, we investigate this effect under three symbiotic mechanisms, namely commensal, parasitic and competitive schemes. Our analysis indicates that: 1) For the commensal scheme, cascaded backscatter fading incurs an inferior outage performance of the backscatter system in comparison with the counterpart over single-hop backscatter fading; 2) For the parasitic scheme, cascaded backscatter fading leads to a superior outage performance of the primary system in comparison with the counterpart over single-hop backscatter fading; 3) For the backscatter system, regardless of single-hop or cascaded backscatter fading, the exact outage probability of the parasitic scheme overlaps with the asymptotic of the commensal scheme; 4) For the competitive scheme, as the decoding threshold of the primary system approaches zero, the asymptotic outage performance of the primary system in cascaded and single-hop backscatter fading tend to be the same; and 5) Unlike single-hop backscatter fading, as the decoding threshold of backscatter signal approaches zero, the outage probability of the competitive and parasitic schemes for the backscatter system in cascaded backscatter fading approaches that of the competitive scheme for the primary system.

To achieve good quality of service level such as throughput, power control is of great importance to passive quadrature amplitude modulation (QAM) multisensor backscatter communication systems. First, we established the RF energy harvesting model and gave the energy condition. In order to minimize the interference of subcarriers and increase the spectral efficiency, then, the colocated passive QAM backscatter communication signal model is presented and the nonlinear optimization problems of power control are solved for passive QAM backscatter communication systems. Solutions include maximum and minimum access interval, the maximum and minimum duty cycle, and the minimal RF-harvested energy under the energy condition for node operating. Using the solutions above, the maximum throughput of passive QAM backscatter communication systems is analyzed and numerical calculation is made finally. Numerical calculation shows that the maximal throughput decreases with the consumed power and the number of sensors, and the maximum throughput is decreased quickly with the increase of the number of sensors. Especially, for a given consumed power of sensor, it can be seen that the throughput decreases with the duty cycle and the number of sensors has little effect on the throughput.

Continuous phase modulation is a well understood modulation scheme which utilizes a constant envelope signal with inherent memory to create a bandwidth efficient waveform. The pulse shapes defined for CPM modulation influence the bandwidth, demodulation complexity, and power efficiency and can be spread across multiple symbols. This paper investigates a novel modulation scheme that is comprised of a family of pulse shapes which add parallel branches to the existing CPM trellis. The resulting modulation scheme maintains the power efficiency of the underlying modulation while increasing the data rate of the waveform. The spectral efficiency is not affected by the increase in data rate while the receiver complexity is reduced. The first part of the paper outlines the standard CPM modulation and a pair of complementary (non-orthogonal) phase pulse shapes is presented. The corresponding effect on bandwidth, power efficiency, and receiver complexity is analyzed and the advantages of this modulation technique are discussed. Then, a novel methodology for generating a family of optimal pulse shapes is discussed and various generation methods are compared. The spectral and power efficiencies of the resulting M-ary waveform are then compared to an equivalent, standard CPFSK modulation.

To meet the goal of ten-fold increase in spectral efficiency, multiuser multiple-input-multiple-output (MU-MIMO) techniques capable of achieving high spatial multiplexing gains are expected to be an essential component of fifth-generation (5G) radio access systems. This increase in multiplexing gain, made possible by equipping base stations with a large number of antennas, entails a proportional increase in channel state information (CSI) acquisition overhead. This article addresses the problem of reducing this CSI overhead by optimizing the amount of time-frequency resources allocated for channel training purposes while not affecting the quality of the associated channel estimate. First we show that in MU-MIMO, adapting pilot symbol density in the time-frequency grid should be performed both on a per resource block (RB) basis and on the basis of groups of users sharing similar channel conditions. Next, we propose a practical scheme that can perform grouping based per-RB pilot pattern adaptation. Finally, we evaluate using both analytical and numerical results the gain in spectral efficiency that can be achieved using this scheme as compared to conventional MU-MIMO systems that use fixed pilot patterns.

Envelope elimination and restoration (EER) based transmitters have attracted attention recently for two reasons: 1) The desire for increased spectral efficiency has mandated modulation standards that have large peak-to-average ratios (PAR). The increase in spectral efficiency is traded off against a decrease in average power-added-efficiency (PAE) when a linear power amplifier is used; however, when EER is used, it is decoupled somewhat from spectral efficiency. 2) Through the use of fast on-chip signal processing, the natural advantage of scaled CMOS is exploited by enabling switching power amplifier classes (e.g., class-E). This letter examines limitations associated with voltage supply switching glitches in class-G power amplifiers.

The achievement of the boundary values of spectral efficiency is associated with the development of methods for generating and receiving signals with a compact spectrum. The reason is a constant increase in the capacity of existing communication channels caused by an increase in the volume of transmitted information. Moreover, the allocated frequency bandwidths have natural limitations, and they are almost all reached. The effective way to approach the boundary values of spectral efficiency is the application of spectrally efficient signals, such as FTN (Faster-Than-Nyquist) signals. This article proposes to synthesize optimal FTN signals that are more compact in the spectrum than RRC (root raised cosine) pulses-based signals. The criterion of maximum energy concentration in the occupied frequency bandwidth and the constraint on the cross-correlation coefficient are used to solve the optimization problem. The contribution of this work is the optimization of FTN signal shape. The obtained optimal FTN signals provide a 24% increase in spectral efficiency compared to the RRC pulses-based signals. At the same time, the energy loss stays almost unchanged. To the best of authors’ knowledge, this is the first case when the frequency bandwidth reduction is achieved at practically no energy loss. The simulation modeling in channels with additive white Gaussian noise and fading channels is done with regard to the FTN-SC-FDE (Faster-Than-Nyquist - Single Carrier system with Frequency Domain Equalization) structure. The optimal FTN signals presented in this work can be used to increase the spectral efficiency of satellite broadcasting systems, such as DVB-S2/S2X.

This study focuses on the design of a MATLAB platform for non-orthogonal multiple access (NOMA) based systems with link-level and system-level analyses. Among the different potential candidates for 5G, NOMA is gaining considerable attention owing to the many-fold increase in spectral efficiency as compared to orthogonal multiple access (OMA). In this study, a NOMA simulator is presented for two and more than two users in a single cell for link-level analysis; whereas, for system-level analysis, seven cells and 19 cells scenarios were considered. Long-term evolution (LTE) was used as the baseline for the NOMA simulator, while bit error rate (BER), throughput and spectral efficiency are used as performance metrics to analyze the simulator performance. Moreover, we demonstrated the application of the NOMA simulator for different simulation scenarios through examples. In addition, the performance of multi-carrier NOMA (MC-NOMA) was evaluated in the presence of AWGN, impulse noise, and intercell interference. To circumvent channel impairments, channel coding with linear precoding is suggested to improve the BER performance of the system.

The advancement of mobile communication technology is driven by innovations or modifications in the latest Radio Access Techniques to meet user demands and enhance capacity. This paper focuses on improving spectral efficiency and average throughput using an OFDM-based multiple access technique. Specifically, we have chosen Overloaded Spread Spectrum OFDMA for this study. Our research findings indicate that implementing a 24% overload leads to an approximate 33% increase in spectral efficiency. This significant improvement demonstrates the potential of Overloaded Spread Spectrum OFDMA in optimizing the performance of mobile communication systems. By increasing spectral efficiency, we can support a larger number of users and higher data rates, which are crucial for the evolving demands of modern mobile networks. This study highlights the importance of adopting advanced access techniques to sustain the growth and efficiency of mobile communications

In this paper, 4 × 4 multiple-input multiple-output (MIMO) radio over 7-core fiber system based on sparse code multiple access (SCMA) and OFDM/OQAM techniques is proposed. No cyclic prefix (CP) is required by properly designing the prototype filters in OFDM/OQAM modulator, and non-orthogonally overlaid codewords by using SCMA is help to serve more users simultaneously under the condition of using equal number of time and frequency resources compared with OFDMA, resulting in the increase of spectral efficiency (SE) and system capacity. In our experiment, 11.04 Gb/s 4 × 4 MIMO SCMA-OFDM/OQAM signal is successfully transmitted over 20 km 7-core fiber and 0.4 m air distance in both uplink and downlink. As a comparison, 6.681 Gb/s traditional MIMO-OFDM signal with the same occupied bandwidth has been evaluated for both uplink and downlink transmission. The experimental results show that SE could be increased by 65.2% with no bit error rate (BER) performance degradation compared with the traditional MIMO-OFDM technique.

Owing to the rapid development of the ambient Internet of Things (IoT) industry, Backscatter Communication Systems (BCS) require higher receiver sensitivity for large-scale IoT scenarios than traditional Radio Frequency Identification (RFID) systems. This paper proposes a method of using Multiple-Input Multiple-Output (MIMO) technology to compensate for reflected signals in the 920 MHz backscatter communication systems. The researcher first created a MATLAB model to verify the theoretical feasibility and then built a One-Transmitter Two-Receiver (1T2R) BCS transceiver system compatible with the EPC global protocol for practical verification on LabVIEW. The ITIR and 1T2R tests were conducted using the National Instrument (NI) USRP-2952R device, the directional antennas, and the standard RF electronic tags. Through the process of superimposing and utilizing the phase shift between the two reflected carriers, it has been confirmed that the overall signal strength can be enhanced from 2.2 mV to 4.13 mV and 4.37 mV (nearly reaching 4.70 mV by using two Rx antennas in ideal conditions). After calculation, this advancement increases signal strength from −40.14 dBm (when using a single Rx antenna) to −34.18 dBm (when using two Rx antennas). The improvement in signal strength can provide a gain of 5.96 dB to the backscatter communication systems. Experimental results demonstrate that utilizing MIMO technology to the backscatter communication systems can effectively enhance the sensitivity of the receiver at 920 MHz.

Radio frequency identification (RFID) is an exciting, rapidly growing, multi-disciplinary field with emerging technologies and applications. Today RFID is a generic term for technologies that use radio frequencies to automatically identify people or objects. In a typical RFID system, digital information is transferred between a base station and movable objects, using wireless communication techniques. The base station, called reader, is usually a complex device connected to a host computer or network. The movable objects are simple, inexpensive devices called tags, which are attached to people or objects to identify. In far-field RFID, coupling mechanism between reader and tag is based on propagation of electromagnetic waves. Passive, far-field tags don't contain a power source, but rely on wireless power transmission instead; they are powered by rectification of radio signal sent by reader. These tags send data to reader by changing load connected to their antenna in a coded manner, a technique known as backscatter modulation. As of today (2009), passive RFID tags that are commercially available are typically optimized for operating at a large distance from reader. This inevitably comes at cost of a low data transfer rate. A new field in RFID is currently emerging, which focuses more on data rate than on operating range. The development of such high data rate RFID tags, communicating at several megabit per second (Mbps), opens way to a variety of promising applications in areas of ambient intelligence and the internet of things. A typical example is wireless memory tag, which consists in a passive, far-field RFID tag containing high capacity, high speed nonvolatile memory. Such tags enable to store multimedia content (video, music...) that can be downloaded by user in a few seconds. In near future, this will probably become a popular new distribution method for various digital products. Designing such tags involves a number of challenges. Because they communicate at high data rate, they consume a significant amount of power. Particular care must be taken in order to optimize efficiency of wireless power transmission, so that tags can reach a decent operating range. This thesis work discusses how various tag building blocks, such as antenna and rectifier, impact efficiency. The principle of wireless voltage regulation is introduced. It allows to further improve efficiency by lowering losses not only in tag, but also in reader. Various experimental results are provided, which show amongst other things that power emitted by reader can be reduced at least by a factor of four. Realizing passive, far-field tags that can transfer information at high data rate with backscatter modulation is not a trivial task. This work aims to determine how to maximize useful backscattered power, which conveys information. Because reader has a finite sensitivity, it needs to receive a minimum backscattered power to recover data, which limits tag operating range. This research project brings to light a fundamental limitation concerning reader sensitivity, and proposes a model to estimate maximum tag reading range. In order to be widely accepted by market, an important characteristic of these tags is that they must be very inexpensive. Hence, tag's integrated circuit (IC) can't be realized using high performance, but costly, fabrication processes. For this reason, this research project focuses on tag ICs realized in a mainstream CMOS process, which implies specific challenges. In particular, impact on architecture and power efficiency of rectifier is discussed. There are basically two approaches to design high data rate, passive, far-field RFID tags. The first one is to base design on a single frequency architecture, and to use a unique carrier frequency for both wireless power transmission and communication. The other approach is use a dedicated carrier frequency for each of these two links, which leads to dual frequency architecture. This thesis work presents a practical design example for each of these architectures, including various experimental results. Both tag ICs are realized using UMC CMOS 0.18 µm technology. When reader emits 3.3 W effective isotropically radiated power (EIRP) at 866.6 MHz, and 10 mW EIRP at 2.44 GHz, dual frequency tag can be read at 10 Mbps at an operating range of 33 cm. The tag's rectifier reaches a power efficiency of 36%. As of today, this tag outperforms commercial products and published works in area of high data rate, passive transponders.

A non-orthogonal multicarrier signal, termed spectrally efficient frequency division multiplexing (SEFDM), is investigated in this work. It improves spectral efficiency by compressing sub-carrier spacing below the symbol rate at the cost of self-created inter carrier interference (ICI). Sphere decoding (SD) is an efficient method to recover signals approaching maximum likelihood (ML) performance. However, the complexity of the SD approaches that of ML with the increase of system size. Studies in this work show that for a small number of sub-carriers, SEFDM signals with low order modulation formats outperform spectral efficiency equivalent OFDM signals modulated by higher order modulation symbols. A key achievement is that 16QAM SEFDM signal outperforms 64QAM OFDM signal at high Eb/N 0 of the same spectral efficiency. In order to maintain the performance benefit and reduce the complexity of SD for large size SEFDM signals, multi-sphere decoding for a multi-block architecture is applied. For a large number of sub-carriers, the multi-sphere architecture works well for SEFDM signals modulated by 4QAM symbols. Whilst for 16QAM symbols, the performance is related to the number of sub-carriers. This work offers an efficient detection solution for SEFDM signals and reveals challenges. It paves the way for future study of signal detection of large size interfered multicarrier signals.