Revisiting the Spread Spectrum Sliding Correlator: Why Filtering Matters

© 2009 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.DOI: 10.1109/TWC.2009.081388A 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 lowpass 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

RFID Backscattering in Long-Range Scenarios

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

Extended Indoor/Outdoor Location of Cellular Handsets Based on Received Signal Strength at Greenville, SC

This report documents an intensive, two-month measurement campaign exploring the performance of cellular handset location systems based on received signal strength (RSS). Building upon the success of the original Georgia Tech campus E911 location experiments, the new results in this report demonstrate the feasibility of RSS-based location in a 1920 MHz GSM network where the majority of calls originate indoors. Since most cell phone calls nowadays are believed to originate indoors (exactly where most proposed position location solutions fail), the ability to locate in-building E911 calls is a huge public safety problem. The results from this experiment are quite promising, however. The highest level of accuracy achieved by our location algorithm across the entire network was 51% of all test points having location error less than 100m and 79% having location error less than 300m. Although these numbers are slightly below the respective 67% and 95% safety targets set by the FCC, the Greenville trials represent a worst-case scenario for an RSS location system: a largely rural area with low density of base stations and a majority of indoor callers. The engineers from Georgia Tech’s Propagation Group compiled test measurements that included indoor data from 1 four-story hotel, 2 high-rise office buildings, 1 eight-story residential complex, 1 five-floor parking garage, 15 stand-alone restaurants, 23 small downtown shops, 1 grocery store, 2 department stores, and a variety of retail and shopping center structures. All of this data had to be painstakingly logged into georeferenced maps by hand since the GPS unit failed to acquire an indoor position 90% of the time. In addition to the extensive indoor measurements, the field engineers also collected numerous tracks of outdoor pedestrian test data and outdoor drive-test data. The end result is an extensive indoor/outdoor testbed for position location experiments that covers 63 square-kilometers of urban, suburban, and rural areas and contains nearly 90,000 measurement records. In arriving at the performance statistics, a number of position location innovations were made and documented along the way. These include: a new search-area limiting algorithm based on novel pieces of information in a network measurement report (NMR) (Section 6.1.3) an improved algorithm for estimating a handset location from a sequence of multiple NMRs (Section 6.1.4) new indoor penetration loss statistics for 1920 MHz (Section 4.1) demonstration that an RSS location system can still function after a major frequency plan change in the middle of a data collection (Section 5.1). Overall, the results of this experiment reveal interesting behavior of RSS-based position location that confirm the technology as an accurate, cost-effective way to improve public safety

Gains for RF tags using multiple antennas

© 2008 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Backscatter radio systems, including high frequency radio frequency identification (RFID), operate in the dyadic backscatter channel - a two-way pinhole channel that has deeper small-scale fades than that of a conventional one-way channel. This paper shows that pinhole diversity is available in a rich scattering environment caused by modulating backscatter with multiple RF tag antennas - no diversity combining at the reader, channel knowledge, or signaling scheme change is required. Pinhole diversity, along with increased RF tag scattering aperture, can cause up to a 10 dB reduction in the power required to maintain a constant bit-error-rate for an RF tag with two antennas. Through examples, it is shown that this gain results in increased backscatter radio system communication reliability and up to a 78% increase in RF tag operating range

Multipath Fading Measurements at 5.8 GHz for Backscatter Tags With Multiple Antennas

© 2010 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.DOI: 10.1109/TAP.2010.2071355Multipath fading can be heavy for ultra-high frequency (UHF) and microwave backscatter radio systems used in applications such as radio frequency identification (RFID). This paper presents measurements of fading on the modulated signal backscattered from a transponder for backscatter radio systems that use multiple antennas at the interrogator and transponder. Measurements were performed at 5.8 GHz and estimates of the backscatter channel envelope distributions and fade margins were calculated. Results show that multipath fading can be reduced using multiple transponder antennas, bistatic interrogators with widely separated transmitter and receiver antennas, and conventional diversity combining at the interrogator receiver. The measured envelope distribution estimates are compared to previously derived distributions and show good agreement

Link envelope correlation in the backscatter channel

© 2007 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.High-frequency backscatter radio systems operate in the dyadic backscatter channel, a pinhole channel whose envelope probability density function and bit-error-rate performance are strongly affected by link envelope correlation – the envelope correlation between the forward and backscatter links of the dyadic backscatter channel. This paper shows that link envelope correlation is most detrimental for backscatter radio systems using co-located reader transmitter and receiver antennas and a single RF transponder antenna. It is shown that using separate reader antennas and multiple RF transponder antennas will decrease link envelope correlation effects and a near maximum bit-error-rate can be achieved with link envelope correlation less than 0.6

Indoor/Outdoor Location of Cellular Handsets Based on Received Signal Strength

This report documents the results of a ground-breaking set of experiments for mobile handset location within the commercial cellular telephone network. With particular emphasis on the US emergency 911 (E911) location problem, we demonstrate the viability of Received Signal Strength (RSS) techniques to meet the safety requirements set forth by the Federal Communications Commission (FCC) in a semi-urban environment. Furthermore, we conclusively show that RSS location techniques are also accurate for indoor users – a characteristic unique among all currently proposed E911 technologies. Our measurement campaign and test results indicate RSS-based techniques can approach or even surpass the FCC guidelines of 100m accuracy 67% of the time and 300m accuracy 95% for a network with a majority of indoor users. Since most cellular phone calls are now placed from indoor environments, this result has enormous implications for the E911 rollout and public safety. The RSS location technique is a relatively new and controversial method for radiolocation within the cellular network. The principle idea is to solve for users’ xy-coordinates by studying signal strength measurements of nearby cellular sectors made by their handsets. All digital handsets measure the signal strength of neighboring control channels, and report the results back to the serving base station in the form of a network measurement report (NMR). All digital cellular air interfaces include the ability to report NMRs, largely for the purpose of performing mobile-assisted hand-offs (MAHOs). Once this NMR has been received at the base station and routed to the central switching office, its set of signal powers is matched to those in a well-calibrated database of RF maps. The closest match between measured and predicted signals likely occurs at a point near the groundtruth location within the database. This technique is similar to the scheme used to locate WLAN modems in a much smaller-scale location problem [Che02]. The technique has been proposed for use in the cellular network by [Wei03]. To perform this study in radiolocation, we turned the Georgia Tech campus into the world’s first indoor/outdoor cellular location laboratory. The ensuing location tests were performed on an 850 MHz IS-136 cellular network in mid-town Atlanta. The Georgia Tech campus approximates a typical semi-urban or dense suburban area with streets, moderate green space, and many 4-5 story academic and office buildings. Although the potential population density of cellular users is high, there are no skyscrapers or canyons that would be associated with dense urban deployments. A database of RF coverage maps for all nearby serving sectors was created from a combination of propagation modeling and varying degrees of indoor and outdoor measurement calibration using a Comarco IS136 scanner with baseband decoding. Real, pedestrian-style handset measurements were taken with an Ericsson handset connected to an Ericsson TEMs data collection unit. The results in this study show that RSS location techniques can satisfy the FCC E911 requirements for outdoor handsets in semi-urban environments. This result is shown in Section 6.2.6. When a majority of the test handset data originates from indoor locations (as it would in real life), the performance degrades somewhat. For example, the error distance between a location estimate and a handset’s groundtruth position drops from 100m or less 66% of the time to 100m or less 56% of the time (see the indoor analysis in Section 6.2.3). However, this report demonstrates a variety of ways to recover the lost accuracy by modifying the location algorithms, adding indoor calibration measurements, modeling indoor propagation using satellite photogrammetry, and using sequential handset measurements. The most accurate location algorithm is documented in Section 6.2.4; using a sequence of 10 linearly-averaged handset measurements and RF maps calibrated with both outdoor and indoor measurements, the error distance for this case is 100m or less 78% of the time and 300m or less 98% of the time. This upper limit of performance is well above the FCC E911 requirements. This measurement campaign lasted for 4 months (January through April) in the beginning of 2004. All data points were tagged with absolute longitude and latitude coordinates taken from a Global Positioning System (GPS) radio; however, due to the limitations of GPS, many outdoor coordinates and all indoor coordinates had to be painstakingly estimated from geo-referenced maps of campus and manually entered into the database. This is one source of error in our measurements. There are other unique sources of error in our measurements that may make our results somewhat pessimistic. For example, there was a seasonal change in the middle of our data collections where leaves grew back on the campus trees, changing the propagation characteristics by several dB. Also, one of the large buildings within our test area was demolished in the middle of our campaign. We also used a fairly simple location algorithm since we were concentrating on the more complicated question of indoor feasibility. There are many other algorithms that have been proposed which could improve the performance [Aso00][Lai01][PB00]. Several recommendations emerge from this study. Our experimental results suggest that RSS-based techniques may be resilient enough for deployment as a standalone position location technology for satisfying the FCC’s E911 requirements in most populated areas. There are still several questions about this technology that need to be addressed. First and foremost, it is unclear how much cost and effort that is required to maintain the performance in cellular networks that, to one degree or another, are always undergoing buildout, optimization, or modification. Ultimately the ideal solution for the US E911 problem will be a hybrid combination of handset-based Global Positioning System (GPS) technology and an RSS-based location system. These two technologies seem to complement each other so well. GPS works in rural, open-sky environments where all network-based location solution tends to degrade due to the low density of base stations. Conversely, GPS fails whenever satellite links become obstructed. This can happen in any environment, but is particularly accute in urban and indoor areas – precisely the places that RSS radiolocation works best. If public safety is the primary concern, then this long-term tandem of location technologies seems to be most sensible. At Georgia Tech, we are continuing to pursue research in the field of RSS-based position location. Several areas of proposed research are: How well do RSS-based location technologies perform in a wide variety of in-building environments (residences, skyscrapers, retail establishments, etc.)? How do we improve state-of-the-art propagation modeling to build accurate RSS databases in regions devoid of measurement calibration? How can the RSS databases be efficiently calibrated and maintained? There is much work left to be done in development of this late-coming location technology, but initial results are quite promising

Antenna Performance in a Corona Plasma

The formation of corona discharges at high potentials was studied. Glow coronas were found to be generated when the electric field magnitude is in excess of 3MV/m in general for air at atmospheric pressure. The number density of electrons in the glow corona can only be approximated, but the results obtained from the Boltzmann transport equation promise to be accurate enough to give meaningful wave attenuation results, so long as the number density is below 1019 m-3 electrons. Above this density, the attenuation increases dramatically and approximations for density may no longer be valid. Wave propagation and antenna parameters in a corona plasma can be computed through simulation using dielectric slabs of complex impedance to simulate a plasma sheath