Severe Multipath Environments Research Articles

On the Performance of UWB-IR With Interleaved Coding Modulation and Polarity Randomization on AWGN and Multipath Fading Channels

One of the main challenges in designing high-data-rate ultrawideband impulse radio (UWB-IR) systems is their sensitivity to intersymbol and interpulse interferences, which may significantly degrade the performance. This paper deals with new methods of protection against errors for UWB-IR systems operating in severe multipath interference-prone environments. The proposed and investigated techniques include interleaved coding modulation and polarity randomization applied in combination with superorthogonal convolutional coding or frame repetition. The performance of high-data-rate (125 Mb/s) coded UWB-IR systems is evaluated on the additive white Gaussian noise and realistic UWB channels that have recently been proposed by the IEEE. Our UWB-IR system incorporates a differential autocorrelation receiver, allowing for a simple hardware architecture. Through theoretical analysis of the bit error rate (BER) and the use of Monte Carlo simulations, we demonstrate that the application of the proposed methods may lead to significant performance improvements.

Field Test Results of the E-VSB System in Korea

This paper presents and analyses field test results of the Enhanced-VSB (E-VSB) system proposed by Zenith/ATI and adopted as an Advanced Television Standard Committee (ATSC) standard. The possibility of successful reception at outdoor and indoor test points according to types of antennas, noise margin, and ease of reception are measured to analyse performance improvement of the E-VSB system. Verification of backward compatibility between the E-VSB and conventional 8-VSB systems, selection of test points, and configurations of the transmitter and the test vehicle are also presented. The field test results show that the E-VSB system does not only outperform the 8-VSB system at severe multipath environment such as indoor reception, but also is fully backward compatible with it

A Transmit MIMO Scheme With Frequency Domain Pre-Equalization for Wireless Frequency Selective Channels

In this paper, we introduce a transmit multiple-input multiple-output (MIMO) scheme with frequency domain pre-equalization for a multipath or frequency selective channel. In this scheme, MIMO processing in the frequency domain is performed at the transmitter or base station so that the receiver or mobile station only requires limited processing. This scheme provides high data rates and also inherits from the frequency domain equalization the property of relatively low complexity in severe multipath environments. The MIMO transmit processing is derived by minimizing the minimum mean square errors (MMSE), and expressions for the signal-to-interference-plus-noise ratio and error probability based on the Gaussian approximation of the interference term are provided. Some important associated issues, such as channel errors and computational complexity, are also investigated. Numerical simulations are also provided and these demonstrate the improved performance of our proposed scheme compared to other transmit MIMO schemes. In particular, they show that the proposed system can attain multipath or frequency diversity of the channel.

Differential GPS dynamic location experiments at sea

Differential GPS on board naval vessels has been tested in severe multipath archipelago environment against a commercial laser tracker system. The average location difference was 3 meters and two periodic error components were found. Their tune constants were 1.8 and 28 seconds and the peak amplitudes 1.5 and 1.1 meters, respectively. In addition to these, a clear dependency of location error standard deviation figures on the GPS antenna mounting distance from the sea level was observed.

Tight integration of digital map and in-vehicle positioning unit for car navigation in urban areas

Now GPS has been widely used for land, sea and air navigation. However, due to signal blockage and severe multipath environments in urban areas, such as in Hong Kong, GPS alone can not satisfy most land vehicle navigation requirements. Dead Reckoning (DR) systems have been widely used to bridge the gaps of GPS and to smooth GPS position errors. However, the DR drift errors increase with time rapidly and frequent calibration is required. Under the normal situation, GPS is sufficient to provide the calibration to the DR unit. However, GPS may not be available in urban areas for more than 20 min, and the DR position errors can reach hundreds of meters during the period. As land vehicles have to be on roads, digital map can be used to constrain the locations of vehicles, known as map-matching. One of the main problems for map-matching techniques is mis-matching, that may be caused by the positioning sensor errors and the complexity of city road network. In this paper, a newly developed model to tightly integrate digital map and in-vehicle positioning unit for car navigation is introduced. With this method, it improves the position accuracy by constraining the vehicle location on the roads. Moreover it provides the close-loop controls for the DR drift errors by feeding back the coordinates of the feature points of the road network and road bearings to the DR unit and therefore the navigation system can be used for longer period when GPS is not available. Extensive tests have been carried out in Hong Kong. It demonstrates that this close-loop approach is much better on the reliability of map-matching, as the positioning sensor errors are constantly calibrated by the digital map.

A Simplified Map-Matching Algorithm for In-Vehicle Navigation Unit

GPS has been widely used for land, sea and air navigation. However, due to signal blockage and severe multipath environments in urban areas, GPS alone cannot satisfy most land vehicle navigation requirements. Dead Reckoning (DR) systems have been widely used to bridge the gaps of GPS and to smooth GPS position errors. However, the DR drift errors increase with time rapidly and frequent calibrations are required. As land vehicles have to be on roads, digital map can be used to constrain the locations of vehicles, known as map-matching, which is an efficient way to improve the performance of positioning systems while working under urban environment. In this paper, a simplified map-matching algorithm is proposed for an in-vehicle navigation unit, as the processing power of in-vehicle processor is limited. Extensive field tests have been conducted in Hong Kong and Macau. The results reveal this map-matching algorithm can improve the performance of the vehicle navigation unit significantly.

A suboptimal receiver for joint spatial-temporal filtering and multiuser detection in mobile radio communications

This paper proposes a suboptimal receiver for joint spatial-temporal filtering and multiuser detection in mobile radio communications using single carrier signaling. The proposed receiver is a reasonable approximation of the maximum likelihood (ML) based optimal receiver described in the present paper. A cascaded connection of an adaptive array antenna and an ML multiuser sequence estimator is the basis of the proposed receiver. The major advantages of the proposed receiver over conventional adaptive array antennas are: (1) delayed path components of desired signals can be effectively combined; (2) interference signals exceeding the degree of freedom; and (3) those having the same incident angle as that of desired path components can both be suppressed. The proposed receiver does not require prohibitively large computational complexity. Results of computer simulations presented in this paper show that the proposed receiver exhibits excellent performance even in severe multipath fading environments.

Mobile ranging using low-accuracy clocks

Our position-location technique provides location information within milliseconds and is integrated in a handheld direct-sequence spread-spectrum (DSSS) communications system. We use a standard DSSS waveform, with a state of-the-art chipping rate to provide a position location capability to an accuracy of less than 1 m in a severe multipath environment. We use a two-way time-of-arrival (TOA) measurement technique with 1-ppm clocks that eliminates the need to synchronize master and reference radio clocks. Several techniques improve TOA and, therefore, range accuracy. A loop back calibrates internal system delay. Frequency diversity orthogonalizes multipath with respect to direct path, and leading edge curve fitting of the direct path reduces the effect of multipath. Applications include location of urban war fighters, firefighters, police, and medical personnel and resources.

Transmitter-based multiuser interference rejection for the down-link of a wireless CDMA system in a multipath environment

In wireless code division multiple access (CDMA) communications systems, there has been interest in processing the transmitted down-link signal in order to shift signal processing to the transmitter where power and computational resources are plentiful, thus simplifying receiver operation and reducing the power it requires. Multiuser interference (MUI) and multipath effects observed by the receiver are anticipated and suppressed at the transmitter; channel equalization and multiuser detection are therefore not required. This paper introduces two methods that are able to combat both degradations, yet allow the receiver to remain as simple as a single user receiver for a perfect channel. For mild multipath channels, the performance of the algorithms is excellent, within a few decibels of the single user ideal channel case, at the cost of additional computation at the base station at which complete knowledge of the channels and the receiver codewords is required. One method, the decorrelating prefilter, is most flexible and applicable to existing systems yet less powerful than other previously published methods. The second, the jointly optimized sequences algorithm, has a performance on average superior to published methods. In addition to theoretical analysis and simulation of the algorithms' potential, these algorithms have also been implemented and tested on a software radio testbed and experimental data are shown. The jointly optimized sequences performed particularly well even in severe multipath and multiuser interference environments.

Improvement of GPS Relative Positioning Accuracy by Using SNR

Interference and multipath transmission (multipath) are the main error sources of global positioning system (GPS) measurement in engineering surveying. This paper describes an algorithm that mitigates specular multipath as well as interference in GPS differential carrier phase measurement by using signal-to-noise ratio (SNR).The focus is on multipath because it accounts for most of the total error budget in differential carrier phase measurement. The algorithm is demonstrated on static platforms in severe multipath environments. The accuracy is tested with predefined displacements on an X-Y-Z stage, and the experimental results are compared to sidereal day-to-day repeatability. The data sets used for investigation are six-epoch measurements, and the accuracy of the six-epoch solution is shown.

Adaptive SNR-based carrier phase multipath mitigation technique

An improved technique that mitigates specular multipath in Global Positioning System (GPS) differential carrier phase measurements is described. It adaptively estimates the spectral parameters (frequency, amplitude, phase offset) of multipath in the associated signal-to-noise ratio (SNR), and then constructs a profile of the multipath error in the carrier phase. A multipath correction is subsequently made by subtracting the profile from the actual phase measurement data. The technique is demonstrated on ground based experimental data, as well as flight data from the atmospheric research satellite CRISTA-SPAS. Ground experiments were conducted on static platforms in severe multipath environments. Multipath was deliberately introduced by either strategic placement of reflectors or electronic injection. This allowed for some control over the strength and frequency of the multipath. Averaging the results from 43 ground and 18 flight data sets, the differential carrier phase multipath was reduced by 47%. The complete results for both ground and flight tests are presented and are accompanied by discussions of individual cases.

Digital outside-broadcasting-link using OFDM modulation scheme

A study on digital broadcasting systems has been conducted for the multimedia era. Regarding program production, a digital outside-broadcasting-link (OBL), in particular, from mobile source is required to gather digital video and audio sources in severe multipath environments. The orthogonal frequency division multiplexing (OFDM) modulation scheme is robust under multipath fading conditions. A digital OFDM modem for OBL in the 800 MHz band has been developed to enable stable transmission of digital sources from moving vehicles. In the experimental OFDM-OBL, the compressed digital video and audio data are distributed to 572 carriers in the 9 MHz bandwidth, using differential quadrature phase shift keying (DQPSK) modulation for each carrier. In computer simulation and indoor/outdoor experiments, it is confirmed that digital OFDM-OBL can stably transmit signals, even under severe multipath fading conditions, while pictures transmitted with conventional FM-OBL are seriously degraded. This paper describes the features of the OFDM modem for the digital OBL which transmits program sources from moving vehicles, and also discusses aspects of gathering digital program sources.

Universal digital portable radio communications

Providing voice and data communications to people away from their wireline telephones has become a major communication frontier. This frontier is being penetrated by evolving approaches to portable communications, e.g., cordless telephones, mobile radiotelephone, and radio paging. However, these approaches have many limitations; none can provide universal portable communications services. This paper discusses limitations of the evolving approaches and considers objectives and approaches for providing more universal digital portable communications as an integrated part of telephone exchange networks. These more universal communications could be accomplished by using demand-assigned radio links for the last thousand feet or so of telephone loops and sharing the remainder of the fixed distribution facilities. Fixed radio ports as integrated parts of telephone distribution networks could be placed throughout service regions. Efficient use of the radio spectrum could be insured by the planned reuse of radio frequencies throughout the regions. The severe multipath radio propagation environment within and around buildings that strongly influences the design of portable communications systems is described in the paper. System configurations and radio link techniques, that can provide reliable communications in the multipath environment, are discussed. Radio system calculations are illustrated for radio ports with 30-ft-high antennas in residential areas. The calculations indicate that radio link availability would be greater than 99 percent for 2000-ft port separations and 5-mW portable transmitters. Reuse of frequencies would require dividing the allocated frequency band into segments for use at adjacent ports. Calculations suggest that link availability in the cochannel interference environment would be greater than 99 percent, if 25 to 35 segments were used in residential areas.

System Considerations for the New DME/P International Standard

The International Civil Aviation Organization (ICAO) has completed technical standards for the precision distance measuring equipment (DME/P). The DME/P is an integral element of the microwave landing system (MLS) and will provide the precision ranging function to complement the already standardized azimuth and elevation guidance functions of the system. In 1978 an ICAO meeting concluded that the DME/P should be integrated into the existing standards for the conventional DME (DME/N) as a compatible service. The motivation for this was economic. It was reasoned that a single L-band airborne unit could satisfy the need for both existing enroute and the new precision approach and landing services, thereby avoiding unnecessary duplication of on-board ranging equipment. Furthermore, this approach would permit existing conventional airborne equipment to obtain service from the new DME/P ground facilities at least during the initial stages of MLS implementation. These compatibility requirements were later incorporated into the DME/P statement of operational requirements which was accepted at an ICAO meeting in April 1981. Further, they have been a primary focus in the DME/P standardization effort which has been actively pursued since late 1980. The conceptual design of the DME/P that evolved from the ICAO process is discussed here. The comptability issues are highlighted, and it is shown how they are accommodated in the system synthesis which also treats the obvious need for a guidance function of high integrity and robust performance in the severe multipath environment encountered in approach and landing operations.

Vehicle location system experiment

A vehicle location system concept which utilizes narrow-band-radio trilateration has been developed for use in command and control systems designed to dispatch mobile vehicles. A comprehensive system experiment was carried out in Schenectady, N.Y., designed to evaluate the system's performance in a severe radio multipath environment, and to evaluate the improvement in performanee realized by system design options developed to improve performance degradation of multipath, that is, redundant receiving stations and space diversity antennas. Experimental results from the system experiment show that although high instrumental precision can be realized by narrow-band-radio trilateration techniques, a severe multipath environment, such as the Schenectady area produces by surrounding hills and valleys, degrades the instrumental precision. The results also indicate that four receiving stations each having four space diversity antennas will improve performance yielding a total vehicle location error radius of 900 ft, which is standard deviation for the Schenectady area.