Simple, low-power, reliable backscatter communication systems must overcome the severe, double-fading nature of the backscatter channel. In this article, we show experimentally that when a conventional transponder is replaced by a retrodirective transponder in backscatter systems, those deep fading nulls are reduced by as much as two orders-of-magnitude. We consider three multipath scenarios all of which show a pronounced link-reliability improvement with a retrodirective transponder compared to its standard non-retrodirective counterpart. The measurements also reveal that by comparing the best case scenario of the two transponder designs under 0.001 outage probability, the retrodirective transponder requires only a 13-dB fade margin while the conventional tag requires a 34-dB fade margin—a significant improvement that would be a key enabler for next-generation backscatter systems.

Some promising technologies such as Internet-of-things (IoT) communicate by means of backscattered power to minimize power consumption. In backscatter-based communications, the fading of a signal envelope is, in general, worse than that of standard one-way communications. To concisely study fading in backscatter channels, we-in this paper-summarize the theory of spatial fading in backscatter channels in addition to derive an analytical expression for the Rician K-factor in the case of monostatic backscatter configurations. Furthermore, we rely on the bit-error-rate (BER) to establish an upper- and lower-bound limit for partially correlated backscatter channels. BER analyses reveal that at low signal-to-noise ratio (SNR), all backscatter configurations result in a similar BER if the underlying channels undergo either Rayleigh or Weibull fading.

The round-trip nature of backscatter communications subjects the radio link to unusually severe small-scale fading. Over fading channels, the received signal envelope in backscatter-based systems such as radio-frequency identification (RFID) systems fades worse than a standard one-way communication. In this paper, we study spatial fading of the received signal envelope in an important class of backscatter systems using retrodirective transponders that allow microwave and mm-wave communications at extraordinarily low-power consumption by an RF tag. Our comparative study shows that the use of retrodirective RF tags changes the backscatter channel statistics from being doubly faded to singly faded, resembling a conventional, one-way radio channel. Surprisingly, fading in retrodirective backscatter channels is highly varied and sensitive to the fine structure of the diffuse multipath waves in the radio environment in ways that fading in conventional radio links are not.

This paper presents a 5.8-GHz RFID tag that, by exploiting the quantum tunneling effect, significantly increases the range of backscatter radio links. We present an electronically simple Tunneling RFID Tag characterized by return gains as high as 35 dB with link sensitivity as low as -81 dBm. Without relevant increase in power consumption, the tunneling tag enables a host of new wireless sensors and Internet of Things applications that require both the long range of conventional wireless links and the low power consumption of semi-passive RFID devices. Selected measurements demonstrate a reader-to-tag separation distance 10 times higher than the maximum range of ideal semi-passive tags. Moreover, the collected experimental results allowed to outline a mathematical model demonstrating how the long-range RFID tag prototype can achieve distances unusual for this technology.

Short ranges and limited field-of-views in semi-passive radio frequency identification (RFID) tags are the most prominent obstacles that limit the number of RFID applications relying on backscatter modulation to exchange data between a reader and a tag. We propose a retrodirective array structure that, if equipped on a tag, can increase the field-of-view and the coverage area of RFID systems by making the tag insensitive to its orientation with respect to one or more RFID readers. In this article, we derive and experimentally validate the conditions under which a rat-race coupler is retrodirective. The performance of the fabricated passive retrodirective structure is evaluated through the retrodirective ideality factor (RIF) amounting to a value of 1.003, which is close to the ideal RIF of one. The article ends with a discussion on how the proposed design can improve current RFID systems from a communication perspective.

This paper proposes a new fine-scale (millimeter level) radio localization and tracking scheme-Hybrid Inertial Microwave Reflectometry-for radio frequency identification and other wireless systems. The scheme combines the backscattered radio frequency signal properties, such as received signal strength and received signal phase, along with reflected inertial data from a tag-mounted accelerometer sensor. Experimental results yield a mean error of 2 mm in ranging accuracy, with a 90% confidence interval of ±1 mm, while tracking a tag moving at a maximum speed of 1.4 m/s.

Using simulated data with varying Rician K-factor, averaging block length (N), and sample size (P), this letter proposes and validates two new moment-based, unbiased estimators for block-averaged Rician data and presents a model for their standard errors. One estimator, K̑1,2, has standard error that varies linearly with K and requires iterative calculation. The other estimator K̑2,4, has standard error that can be orders of magnitude larger than the estimates, but has a closed-form solution. K̑2,4can, however, be recommended when (N ≤ 16, P ≥ 100) if real-time constraints require it. K̑1,2is recommended otherwise.

This paper investigates the minimum number of antenna elements of an adaptive array antenna (AAA) required to suppress unnecessary radiation to a single location in an indoor ubiquitous communication environment. In the investigation the relationship between the number of antenna elements of an AAA and its digitally-created antenna null width is demonstrated using antenna directivity control technique that has been proposed by the authors. Also, the angular spread around a terminal to a single location in an indoor communication environment is demonstrated with propagation measurements. Based on the computer simulation results and the angular spread obtained by propagation measurements, the authors specify that at least 6 antenna elements are required to suppress radiation to a single location in an indoor communication environment up to -60 dB.

A wireless channel sounder based upon the conventional spread spectrum sliding correlator implementation uses unfiltered pseudo-random noise (PN) at both the transmitter and receiver to generate a time-dilated copy of the channel's impulse response. However, in addition to this desired impulse response, the sliding correlator also produces a noise-like, wideband distortion signal that decreases the measurement system's dynamic range. Careful selection of the sliding correlator's low-pass filter can significantly reduce this distortion, but no amount of filtering will remove it completely. In contrast, using filtered PNs at both the transmitter and receiver enables one to remove this distortion in entirety and realize a measurement system whose dynamic range closely approximates the theoretical ideal for spread spectrum systems.

The sliding correlator technique remains one of the most versatile and effective methods for sounding the radio propagation channel in next-generation wireless systems. Despite their utility, there has never been a comprehensive set of metrics and rules for the design of a sliding correlator channel sounder. This paper presents quantitative guidelines for balancing the many system parameters to achieve optimal levels of temporal resolution, dynamic range, processing gain, and Doppler resolution. The design procedure presented at the end of the paper will allow researchers to probe the new radioscapes that result as wireless systems are pushed to higher carrier frequencies, wider bandwidths, multiple antennas, and ubiquitous operation.

As the deployment of outdoor WLAN networks continues to accelerate, it has become clear that network planning is an essential step in the deployment process. This paper outlines a study of the established 802.11g outdoor WiFi network deployed at the Georgia Institute of Technology. Analysis of measured data has shown that predictive planning for network coverage is possible without the need for overly complicated modeling techniques such as ray tracing. The proposed direct-ray, single path loss exponent, Seidel-Rappaport propagation model performs with accuracy comparable to other published models and is offered as a balance between model complexity and prediction accuracy.

This paper presents new envelope probability density functions (PDFs) that describe small-scale, local area fading experienced by narrow-band wireless receivers. The paper also develops novel PDFs that describe the local area fading of two specular multipath components in the presence of other diffusely propagating waves. These PDFs are studied in the context of classical fading PDFs such as the Rayleigh, Rician, and other distributions.

This paper contains measured data and empirical models for 5.85-GHz radio propagation path loss in and around residential areas for the newly allocated U.S. National Information Infrastructure (NII) band. Three homes and two stands of trees were studied for outdoor path loss, tree loss, and house penetration loss in a narrow-band measurement campaign that included 270 local area path loss measurements and over 276000 instantaneous power measurements. Outdoor transmitters at a height of 5.5 m were placed at distances between 30 and 210 m from the homes, to simulate typical neighborhood base stations mounted atop utility poles. All path loss data are presented graphically and coupled with site-specific information. We develop measurement-based path loss models for propagation prediction. The measurements and models may aid the development of futuristic outdoor-to-indoor residential communication systems for wireless Internet access, wireless cable distribution, and wireless local loops.