University of Southern California WiDeS - Wireless Devices and Systems Group The USC Andrew and Erna Viterbi School of Engineering USC

ULTRA-WIDEBAND COMMUNICATIONS AND RANGING


BACKGROUND


Ultrawideband (UWB) communications systems are commonly defined as systems with large absolute and/or large relative bandwidth. Such a large bandwidth offers specific advantages with respect to signal robustness, information transfer speed, and/or implementation simplicity, but leads also to fundamental differences from conventional, narrowband, systems. The past years have seen a confluence of technological and political/economic circumstances that enabled practical use of UWB systems; consequently, interest in UWB has grown dramatically.

The attractiveness of UWB systems stems mainly from the fact that they can be used as an overlay to existing systems. In other words, they do not require new spectrum, but can be operated in parallel to existing, legacy, systems. This can be understood from the following simple picture (Fig. 1): the transmit power of any system can be (approximately) expressed as the product of power spectral density (PSD) and bandwidth. A large (absolute) bandwidth thus enables a system with reasonable transmit power (to exhibit an extremely low power spectral density. A victim (narrowband) receiver will sees only the noise power within its own system bandwidth, which is only a fraction of the UWB bandwidth. Due to this small interference, frequency regulators all over the world have allowed intentional UWB emissions in the microwave regime, subject to certain restrictions for the emission power spectrum.

The large absolute bandwidth allows a transmission of extremely high data (> 100 Mbit/s), though the transmission can be achieved only over relatively short distances (< 10 m) because only very low power is available for each bit. Alternatively, low-data-rate communication (e.g., < 1 Mbit/s) is possible over much larger distances by exploiting the large spreading factor. Besides enabling large data rates or spreading factors, a large absolute bandwidth has also a number of other important benefits:
  • enables very fine resolution for ranging, which is useful for radar and geolocation applications.
  • it introduces a high degree of frequency diversity and decreases the fading depth of resolvable multipath components. This results in a robustness to small-scale fading that is significantly better than that of conventional narrowband systems.

At the same time, a large absolute bandwidth makes it more difficult to build effective transceivers. Accurate timing, as well as high sampling rate and subsequent high computational effort for the processing of the samples, are usually required. As a consequence, many papers deal with the design of efficient and/or simplified transceiver structures that allow to exploit the benefits of high absolute bandwidth at reduced computational cost.

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CURRENT RESEARCH

 

Ultrawideband Channel Measurements and Modeling


Ultrawideband propagation channels differ from narrowband propagation channels in several key aspects:
  • each multipath component (MPC) suffers from distortion - in contrast to narrowband systems, where MPCs are only attenuated and delayed.
  • the pathloss, shadowing, and angular spread becomes a function of the frequency at which it is considered
  • the amplitude fading statistics of resolvable delay bins are not necessarily Rayleigh
  • impulse responses can become "sparse", i.e., resolvable MPCs are separated (in the delay domain) by delay regions that do not contain any significant energy contribution
  • impulse responses can become "sparse", i.e., resolvable MPCs are separated (in the delay domain) by delay regions that do not contain any significant energy contribution
Thus, UWB channels require new measurement campaigns, and new modeling approaches. 

Current research topics, and topics planned for the near future, include measurement and modeling of
  • UWB channels in "infostation scenarios", e.g., at gas stations, drive-in restaurants, etc. It is envisioned that drivers can download large amounts of content (e.g., movies) while stopping at the infostation for a few minutes. In the context of modeling UWB channels in these environments, we are also refining high-resolution algorithms for scatterer identification. 
  • Angularly resolved UWB measurements: information about the angles from which the MPCs are arriving at the receiver (or departing from at the transmitter) is essential for UWB MIMO systems, as well as advanced geolocation methods and time-reversal systems.
  • Statistics of the "first arriving component": traditionally, channel models concentrate on accurately describing the strongest multipath components, because those determine the signal-to-noise ratio, and thus the performance, of communications systems. However, for ranging applications, the first MPC is the critical quantity, and in UWB it is not necessarily strong. Accurate measurements and models of the statistics of the first component is thus critical for assessing the capabilities of UWB ranging and geolocation systems.
  • Body-area networks: the propagation of UWB radiation from one part of the body to another is critical for so-called "body-area networks", which play an important role in telemedicine and other applications.
 
 

Localization

Object location estimation has recently received intensive interests for a large variety of applications. For example, localization of people in smoke-filled buildings can be life-saving, positioning techniques also provide useful location information for search-and-rescue, and security applications such as localization of intruders.

A variety of localization techniques have been proposed in the literature, which differ by the type of information and system parameters that are used. The three most important kinds utilize the received signal strength (RSS), angle of arrival (AOA), and signal propagation time, respectively. RSS algorithms use the received signal power for object positioning; their accuracies are limited by the fading of wireless signals. AOA algorithms require either directional antennas or receiver antenna arrays. Signal-propagation-time based algorithms estimate the object location using the time it takes the signal to travel from the transmitter to the target and from there to the receivers. The signal-propagation-time based algorithms are perfect candidate for Ultra-wide (UWB) band localization, since UWB signals achieve very accurate signal range estimate (sub-millimeter accuracy).

                                                Fig. 1  System Model of TOA based Localization using UWB signals

In the localization, we have following assumptions:

  1. The target reflects signals transmitted from transmitter.
  2. The transmitter is synchronized with multiple receivers.
  3. The signal travel range/time estimates are Gaussian distributed.
  4. If there are multiple targets, the signals from different targets are separated.

In the system model of Fig. 1, the receivers may only be synchronized with each other, instead of synchronized with the transmitter. In this situation, the time-difference of arrival (TDOA) based algorithm would be utilized instead of TOA based algorithm.

Our research focuses on following aspects of localization:

  • Individual target localization: The time of arrival (TOA) is a perfect candidate for UWB based localization, for the high resolution of time measurement of UWB signals. The system model of TOA based localization is shown in Fig. 1. We propose a low complexity in [1], two step localization algorithm, which is able to achieve the Cramer Rao Lower Bound (CRLB).
  • Indirect path (IP) detection: The IP means the signal propagation path, where the signal is reflected by non-target objects. The IP is a particularly challenge for precise localization, since it adds bias to localization-oriented parameters, such as direction of departure (DOD), direction of arrival (DOA) and time of arrival (TOA). In [2], we propose a novel algorithm, based on the distance from DOD/DOA intersection to TOA ellipse.

 

Network Localization


Position information is of critical importance for situational awareness. While GPS serves the purpose in open areas, there are many tactical scenarios, like war-fighters operating under dense foliage (or) in urban environment with high building, where GPS completely fails. Localization in such a scenarios can be achieved by information exchange between the devices. UWB communications is a popular choice in such situations. It is evident that interference from other UWB radios is unavoidable in these scenarios. While the Time-hopping impulse radio (TH-IR) mitigates the interference to certain extent, it cannot avoid it completely.

Our current focus is on extracting the ranging beliefs from the received waveforms. These ranging belief's can than be fed to a cooperative localization algorithms to determine the precise location of the nodes. The classical way to combat the interference and to estimate the range is by averaging the received waveforms and setting a threshold on the extracted MPC's to distinguish noise and interference MPC's from the TOA (Time of Arrival). In practise this approach suffers from the following: (1) The interference, even after averaging can have significant contribution and can arrive before the desired signal there by corrupting the ranging beliefs. (2) Finding the optimal threshold is non-trivial. It depends on the statistics like SNR and SIR, which are difficult to get. We get around this problem to some extent, by exploiting the lack of synchronization between signal and interference (the TH-IR waveforms for different transmissions are non-coherent). We are able to achieve ranging performance that is not sensitive to SIR. Moreover, the algorithm does not require the knowledge of SIR.

 
 
Fig. 1. Statistics of TOA estimation error

 

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PUBLICATIONS


Journal Papers


J. Shen, A. F. Molisch, "Indirect Path Detection Based on Wireless Propagation Measurements," submitted for Journal Publication, 2012.

N. Michelusi, U. Mitra, A. F. Molisch, and M. Zorzi, “UWB Sparse/Diffuse Channels, Part II: Estimator Analysis and Practical Channels”, IEEE Trans. Signal Process., in press.

N. Michelusi, U. Mitra, A. F. Molisch, and M. Zorzi, “UWB Sparse/Diffuse Channels, Part I: Channel Models and Bayesian Estimators”, IEEE Trans. Signal Process., in press

K. Haneda, A. Richter, and A. F. Molisch, “Modeling the Frequency Dependence of Ultrawideband Spatio-Temporal Indoor Radio Channels”, IEEE Trans. Antennas Propagation, 60, 2940 – 2950 (2012).

J. Shen A. F. Molisch and J. Salmi, "Accurate Passive Location Estimation Using TOA Measurements," IEEE Trans. on Wireless Communications, 2012.

J. Salmi and A. F. Molisch, “Propagation parameter estimation, modeling and measurements for ultrawideband MIMO radar,” IEEE Transactions on Antennas and Propagation, vol. 59, no. 11, pp. 4257–4267, Nov. 2011.    

T. Santos, F. Tufvesson, and A. F. Molisch, “Modeling the UWB Outdoor Channel – Model Specification and Validation”, IEEE Trans. Wireless Comm., 9, 1987-1997 (2010).

T. Santos, P. Almers, J. Karedal, F. Tufvesson, and A. F. Molisch, “Modeling the UWB Outdoor Channel - Measurements and Parameter Extraction Method”, IEEE Trans. Wireless Comm., 9, 282-290 (2010).

A. F. Molisch, “Ultrawideband Communications”, (invited) Radio Science Bulletin, 329, 31-42 (2009).

A. F. Molisch, “Ultrawideband propagation channels”,  Proc. IEEE, special issue on UWB, 97, 353-371, (2009).

J. Zhang, P. Orlik, Z. Sahinoglu, A. F. Molisch, and P. Kinney, “Low-Rate and Precision-Ranging UWB Systems”,  Proc. IEEE, special issue on UWB, 97, 313-331, 2009
 
 

Conference Papers


J. Shen and A. F. Molisch, "Indirect Path Detection of Passive Localization Based on Wireless Propagation Measurements," in IEEE ICUWB, 2012.

S. Sangodoyin, J. Salmi, S. Niranjayan and A. F. Molisch, “Real-time ultrawideband MIMO Channel Sounding,”  in Proc . 6th  EuCAP, May. 2012, pp. 2303-2307

X. S. Yang, J. Salmi, A. F. Molisch, S. G. Qiu, O. Sangodoyin, and B.-Z. Wang, “Trapezoidal Monopole Antenna and Array for UWB-MIMO Applications,” ICMMT, 2012.J. Shen, A. F. Molisch, "Passive Location Estimation Using TOA Measurements," in IEEE ICUWB, Sep. 2011. (Best Student Paper Award in ICUWB 2011)

J. Shen and A. F. Molisch, "Discerning Direct and Indirect Paths: Principle and Application in Passive Target Positioning Systems," in IEEE GLOBECOM, Dec., 2011.

N. Michelusi, U. Mitra, A. F. Molisch, and M. Zorzi, “Hybrid sparse/diffuse channels: A new model and estimators for wideband channels”, 49th Annual Allerton Conf. 2011.

J. Salmi, S. Sangodoyin,  and A. F. Molisch, “High Resolution Parameter Estimation for Ultra-Wideband MIMO Radar”, Asilomar 2010.

A. F. Molisch, “MIMO-UWB propagation channels”, EuCAP 2010, 2010.

H. Liu, A. F. Molisch, D. Goeckel, and P. Orlik, “Hybrid Coherent and Frequency-Shifted-Reference Ultrawideband Radio”, PHYCOM, 2, 265-273, 2009