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A new multi-antenna or multiple-input-multiple-output (MIMO) configuration is introduced for RF modulated backscatter in a multipath environment. RF modulated backscatter is used by semipassive RF tags to transmit without a power amplifier, and is therefore appropriate for applications with extreme power constraints such as sensor communications, e...
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Citations
... The application of antenna diversity in the receiver side has been demonstrated [14,15]. The diversity gain is marginal because the forward channel is always Rayleigh, and the overall channel is the multiplication of Rayleigh and diversity combined channel. ...
An un-coded multi-transmitter query scheme is introduced for wireless backscatter communication systems in which M transmitters and N receivers are used for single-tag connectivity (M × 1 × N). The main idea is to harden the wireless communication channel with a tag device for high data rate readings. The proposed method is designed for multipath fading channels in which the backscatter channel is a multiplicative Rayleigh. A coherent transmit query scheme is used to increase the tag-reflected signals and simultaneously alter the fading statistics in the forward path by implementing a receiver feedback. Full-diversity performance and array gain is achieved using the receiver diversity without requiring any tag antenna diversity. Therefore, the tag device remains simple.
Mathematical expressions for the probability density function (PDF) of the backscatter channel are presented using closed-form equations. Bit error rate (BER) simulations for the binary phase shift keying (BPSK) modulation are computed numerically. Diversity gain of 10 dB is obtained by using a 2 × 1 × 1 scheme. The results show that the transmit diversity for single-tag usage performs the same as the tag antenna diversity, at the expense of a moderate transmitter complexity. The tag device remains intact as a requirement for the simplicity and size constraints. Also, the system realization becomes more feasible due to the available space on the transmitter side to accomplish the uncorrelated forward channel conditions. The feasibility study is demonstrated using software-defined radio (SDR) implementations.
... One way to reduce fading in the backscatter channel is through antenna diversity. Ingram et al. [5] brought up the idea of increasing the range and communication capacity using multiple radio frequency (RF)-tag and reader antennas. Mi et al. [6] also agree that the harvested power required to operate the RF tag chip can be increased by using multiple RF-tag antennas. ...
The authors employ the conditional moment generating function approach to analyse the performance of orthogonal space-time block codes over Nakagami- q (Hoyt) multiple-input multiple-output radio frequency identification backscattering channels. New exact and asymptotic symbol error rate expressions are derived for the case of two and four receiving antennas (N=2,4). The exact expressions are in the form of a sum of infinite series while the asymptotic ones are in the closed form. The diversity order that the system can achieve is found to be L , where L is the number of tag antennas, and the performance of this system is found to be more sensitive to the channel condition (the q parameter) of the forward link than that of the backscattering link. The theoretical results (exact and asymptotic) are verified through comparison with simulation results.
... In [90], this channel is investigated in the context of semipassive backscatter transmitters to achieve diversity and spatial multiplexing. The authors demonstrate that by using multiple antennas at both the backscatter transmitter and backscatter receiver, the communication range is significantly extended. ...
... The authors demonstrate that by using multiple antennas at both the backscatter transmitter and backscatter receiver, the communication range is significantly extended. The reason is that in the M × L × N backscatter channel, small-scale fading effects can be reduced [90], [91], thereby improving the performance of backscatter communications systems [92]. c) Link Budgets for Backscatter Channels: In a backscatter communications system, there are two major link budgets, i.e., the forward link and the backscatter link budgets, that affects performance of the system (Fig. 3). ...
Recently, ambient backscatter communications has been introduced as a cutting-edge technology which enables smart devices to communicate by utilizing ambient radio frequency (RF) signals without requiring active RF transmission. This technology is especially effective in addressing communication and energy efficiency problems for low-power communications systems such as sensor networks. It is expected to realize numerous Internet-of-Things (IoT) applications. Therefore, this paper aims to provide a contemporary and comprehensive literature review on fundamentals, applications, challenges, and research efforts/progress of ambient backscatter communications. In particular, we first present fundamentals of backscatter communications and briefly review bistatic backscatter communications systems. Then, the general architecture, advantages, and solutions to address existing issues and limitations of ambient backscatter communications systems are discussed. Additionally, emerging applications of ambient backscatter communications are highlighted. Finally, we outline some open issues and future research directions.
... In this paper, we propose a noise-injection precoding scheme to create additional interference at the eavesdropper 1 This signal model is first proposed in [10], wherein self-interference is not considered and only spatial domain is involved. The model in [10] is further generalized to the space-time coding model in [4], [13], [14] where temporal domain is also taken into account. In this paper, we focus on transmit optimization against eavesdropping only in spatial domain and thus adopt the signal model similar as that in [10]. 2 Since the reader receives its transmitted signals from both the backscatter and its self-interference channels at different time instants, we adopt x ′ in (1) to denote the signal received directly from the reader's self-interference channel and distinguish it from the signal received from the backscatter channel, i.e., x. ...
... The model in [10] is further generalized to the space-time coding model in [4], [13], [14] where temporal domain is also taken into account. In this paper, we focus on transmit optimization against eavesdropping only in spatial domain and thus adopt the signal model similar as that in [10]. 2 Since the reader receives its transmitted signals from both the backscatter and its self-interference channels at different time instants, we adopt x ′ in (1) to denote the signal received directly from the reader's self-interference channel and distinguish it from the signal received from the backscatter channel, i.e., x. A similar notation will be also used in (6). ...
... This signaling matrix takes several different forms depending upon the physical implementation of the modulation circuitry and RF tag antennas [11]. In this paper, we consider a common scenario employed in [4], [10] that the signaling matrix Q takes the form of a diagonal matrix, given by ...
Backscatter wireless communication is an emerging technique widely used in low-cost and low-power wireless systems, especially in passive radio frequency identification (RFID) systems. Recently, the requirement of high data rates, data reliability, and security drives the development of RFID systems, which motivates our investigation on the physical layer security of a multiple-input multiple-output (MIMO) RFID system. In this paper, we propose a noise-injection precoding strategy to safeguard the system security with the resource-constrained nature of the backscatter system taken into consideration. We first consider a multi-antenna RFID tag case and investigate the secrecy rate maximization (SRM) problem by jointly optimizing the energy supply power and the precoding matrix of the injected artificial noise at the RFID reader. We exploit the alternating optimization method and the sequential parametric convex approximation method, respectively, to tackle the non-convex SRM problem and show an interesting fact that the two methods are actually equivalent for our SRM problem with the convergence of a Karush-Kuhn-Tucker (KKT) point. To facilitate the practical implementation for resource-constrained RFID devices, we propose a fast algorithm based on projected gradient. We also consider a single-antenna RFID tag case and develop a low-complexity algorithm which yields the global optimal solution. Simulation results show the superiority of our proposed algorithms in terms of the secrecy rate and computational complexity.
... One solution is provided by the modulated back-scattering (MBS) systems, such as RFID, where the nodes (tags) modulate their information onto a carrier generated by a reader, and the reader decodes the modulated information. In the advanced forms of MBS systems, multiple-antenna techniques are applied to increase the capacity [1] and improve the reliability [2]. The communication range can be increased by allowing a carrier transmitter and a reader (receiver) to be physically separated, referred to as a bi-static MBS system [3]. ...
In ambient re-scatter communications, devices convey information by modulating and re-scattering the radio frequency signals impinging on their antennas. In this correspondence, we consider a system consisting of a modulated continuous carrier multiple-input multiple-output (MIMO) link (primary) and a multi-antenna modulated re-scatter (MRS) node (secondary), where the MRS node uses the signal generated by the primary transmitter. The receiver tries to decode both the original transmitted message and the information added by the MRS antennas. We show that the sum capacity of this system exceeds that, which the MIMO system could achieve alone. We also consider the impact of channel estimation errors under least squares channel estimation. The results suggest that the estimation error has negative impact on the capacity, however, its severity can be minimised by increasing the number of receiver antennas.
... In RFID backscatter systems, adopting multiple antennas at either the RFID reader or the RFID tag was investigated in [3], [4]. Furthermore, a multiple-input multiple-output (MIMO) scheme was studied in [5], where numerical results showed that employing multiple antenna techniques can extend the coverage of backscatter RFID systems and increase data capacity in the spatial multiplexing configuration. A spacetime coding scheme was proposed in [6] and it was shown that the reliability of the system can be significantly improved with multiple antennas. ...
... Besides using higher bandwidth, another way to increase the performance is to use multiple input, multiple output (MIMO) systems. For passive RFID tags the use of multiple antennas at the transmitter, tag and receiver have been analyzed with respect to the received power and bit error rate [8]- [11]. For ranging/positioning a MIMO-radar system can be employed using the geometric spread of the sensors and narrowband signals [12]. ...
This paper analyzes the achievable ranging and positioning performance for two design constraints in a radio frequency identification (RFID) system: (i) the bandwidth of the transmit signal and (ii) the use of multiple antennas at the readers. The ranging performance is developed for correlated and uncorrelated constituent channels by utilizing a geometry-based stochastic channel model for the downlink and the uplink. The ranging error bound is utilized to compute the precision gain for a ranging scenario with multiple collocated transmit and receive antennas. The position error bound is then split into a monostatic and bistatic component to analyze the positioning performance in a multiple input, multiple output (MIMO) RFID system. Simulation results indicate that the ranging variance is approximately halved when utilizing uncorrelated constituent channels in a monostatic setup. It is shown that both the bandwidth and the number of antennas decrease the error variance roughly quadratically.
... Although backscatter RFID enjoys low hard complexity and longer life expectancy, its physical channel experiences deeper fading than conventional one-way channels and leads to severe performance degradation. To mitigate such drawback, many efforts have been made [1]– [28], among which employing multiple antennas for both tags and readers appears to be one of the practical solutions. Such multiple-input multiple-output (MIMO) systems had a great success in mobile communications [29]–[33] and were also investigated and found promising in backscatter RFID [1]–[5], [7], [8], [12]–[14]. ...
... To mitigate such drawback, many efforts have been made [1]– [28], among which employing multiple antennas for both tags and readers appears to be one of the practical solutions. Such multiple-input multiple-output (MIMO) systems had a great success in mobile communications [29]–[33] and were also investigated and found promising in backscatter RFID [1]–[5], [7], [8], [12]–[14]. A general M × L × N MIMO backscatter RFID channel consists of M reader query antennas, L tag antennas, and N receiving antennas, which can be modeled as a two-way channel with forward sub-channels and backscattering subchannels [1] [35] [2], as shown in Fig. 1. ...
Because of the emerging field of Internet of Things (IoT), future backscatter
RFID is required to be more reliable and data intensive. Motivated by this,
orthogonal space-time block code (OSTBC), which is very successful in mobile
communications for its low complexity and high performance, has already been
investigated for backscatter RFID. On the other hand, a recently proposed
scheme called unitary query was shown to be able to considerably improve the
reliability of backscatter radio by exploiting query diversity. Therefore
incorporating the classical OSTBC (at the tag end) with the recently proposed
unitary query (at the query end) seems to be promising. However, in this paper,
we show that simple, direct employment of OSTBC together with unitary query
incurs a linear decoding problem and eventually leads to a severe performance
degradation. As a re-design of the recently proposed unitary query and the
classical OSTBC specifically for MIMO backscatter RFID, we present a
BUTQ-mOSTBC design pair idea by proposing the block-level unitary query (BUTQ)
at the query end and the corresponding modified OSTBC (mOSTBC) at the tag end.
The proposed BUTQ-mOSTBC can resolve the linear decoding problem, keep the
simplicity and high performance properties of the classical OSTBC, and achieve
the query diversity for the $M \times L \times N$ MIMO backscatter RFID
channel.
... All the above studies that use MIMO settings to exploit the diversity gain for the backscatter RFID are based on the uniform query, for which the query antennas send the same signal over all symbol times. Since [5], [6], where the M ×L×N backscatter RFID channel was formulated, there is no other query signaling methods have been considered. This is because the previous understanding is that, since spatial diversity can only be obtained by duplicating the information and transmitting it over multiple branches, while the query end is not the information source, designs of the query signal cannot bring spatial diversity in quasi-static channels. ...
... The backscatter RFID has three operational ends: the reader query end (i.e., the set of reader transmitting antennas), the tag end (i.e., the set of tag antennas), and the reader receiver end (i.e., the set of reader receiving antennas). These three ends can be mathematically modeled by an M × L × N dyadic backscatter channel which consists of M reader transmitter antennas, L RF tag antennas, and N reader receiver antennas [5], [6], [8], [13], [14], as shown inFig. 1. ...
... The uniform query is used as the query method for all previous studies of the M × N × L backscatter channel, and no further investigation has been made on the query signal design since the M × N × L backscatter channel has been formulated in [5], [6]. The reason that no other query signal design method has been considered probably from the understanding that the spatial diversity from the transmitter can only be made when transmitting duplicated information from different antennas, and since the query signals do not carry information, the spatial diversity can only be made from the tag antennas and the reader receiving antennas. ...
A MIMO backscatter RFID system consists of three operational ends: the query
end (with $M$ reader transmitting antennas), the tag end (with $L$ tag
antennas) and the receiving end (with $N$ reader receiving antennas). Such an
$M \times L \times N$ setting in RFID can bring spatial diversity and has been
studied for STC at the tag end. Current understanding of the query end is that
it is only an energy provider for the tag and query signal designs cannot
improve the performance. However, we propose a novel \textit{unitary query}
scheme, which creates time diversity \emph{within channel coherent time} and
can yield \emph{significant} performance improvements. To overcome the
difficulty of evaluating the performance when the unitary query is employed at
the query end and STC is employed at the tag end, we derive a new measure based
on the ranks of certain carefully constructed matrices. The measure implies
that the unitary query has superior performance. Simulations show that the
unitary query can bring $5-10$ dB gain in mid SNR regimes. In addition, the
unitary query can also improve the performance of single-antenna tags
significantly, allowing employing low complex and small-size single-antenna
tags for high performance. This improvement is unachievable for single-antenna
tags when the conventional uniform query is employed.
... This cascaded channel fades deeper than the Rayleigh channel and hence can reduce the data transmission reli- ability. To allow reliable data transmission, researchers [7,91011 investigated multiple-input multiple-output (MIMO) settings in the RFID backscattering channel and both bit error rate (BER) and reading range improvements were observed. The advantages of the MIMO RFID technology over the single-antenna RFID allow many potential applications in the areas of accurate tracking, identifications , since it can significantly increase the reliability and throughput. ...
In order to increase the reliability of data transmission, using multiple antennas in radio frequency identification (RFID) systems has been investigated by researchers, mainly through measurements and simulations. The multiple-input multiple-output (MIMO) RFID backscattering channel exhibits a special type of cascaded structure rather than that of other better-studied cascaded channels such as the keyhole fading and the double scattering fading. In this article, we analytically study the bit error rate performances of the MIMO RFID channel under two transmission schemes, the identical signaling transmission scheme and the orthogonal space–time coding scheme (OSTBC). We show that the diversity order of the MIMO RFID channel is min(N,L) under the identical transmission scheme, and the diversity order is L under the OSTBC scheme, where L is the number of tag antennas and N is the number of reader receiving antennas. A performance bottleneck is also observed in the MIMO RFID channel. Our results can provide useful guidance on designing an RFID system with multiple antennas.
