GNSS CODE AND CARRIER TRACKING IN THE PRESENCE OF MULTIPATH

It is well known that multipath represents a major error source in differential GNSS positioning. Errors produced are unique to each antenna position and so are uncorrelated between the reference station and the mobile receiver. Reduction techniques, such as narrow correlator spacing¹ and the multipath estimating delay lock loop (MEDLL)² have improved the situation. However, reflected signals emanating close to the antenna are not reduced by either technique. Carrier multipath is a particular problem because short delay multipath signals cause maximum carrier errors. This study describes the analysis that has been carried out into code and carrier tracking in the presence of multipath. Code tracking in this situation produces a non-zero mean error,³ and the source of this bias has been investigated. Code and carrier multipath errors of the global orbiting navigation satellite system (GLONASS) have been compared with those of the global positioning system (GPS). The effects of pre-correlation filtering on code and carrier multipath errors have been determined using data from a measured correlation function. The multipath fading bandwidth is also discussed, and fast and slow fading effects are presented. A comparison has been made of the performance of typical coherent and non-coherent code discriminators in the presence of multipath. The performance of the GPS and GLONASS P code signals have been determined with respect to code and carrier multipath errors. © 1997 by John Wiley & Sons, Ltd.     

New Phase Adjustment and Channel Estimation Methods for RF Combining Applicable in Mobile Terminals

Antenna diversity is able toalleviate multipath fading in a wireless communications system.Traditionally, multiple down-conversion chains have to be used inorder to implement antenna receive diversity, making it difficultfor implementation in mobile terminals, where the physical size,power consumption and implementation cost are three major limitingfactors. RF combining, which combines the received signals at RFlevel, is well applicable for mobile terminals as it requires onlyone down-conversion chain. This paper investigates RF combiningtechniques for improving downlink performance by applying atwo-element array at the mobile terminal. A new phase adjustmentalgorithm is suggested in order to combine the RF signals whileminimizing noise effect. Also, a new channel estimation methodbased on the suggested phase adjustment algorithm is proposed, andfurther applied to systems with transmit diversity. Computersimulations have shown that with the new channel estimation methodtogether with the new phase adjustment algorithm, the RF combiningtechniques can improve the BER performance considerably,especially for fast fading environment.     

A Novel Dual-Band Printed Diversity Antenna for Mobile Terminals

A novel dual-band printed diversity antenna is proposed and studied. The antenna, which consists of two back-to- back monopoles with symmetric configuration, is printed on a printed circuit board. The effects of some important parameters of the proposed antenna are deeply studied and the design methodology is given. A prototype of the proposed antenna operating at UMTS (1920-2170 MHz) and 2.4-GHz WLAN (2400-2484 MHz) bands is provided to demonstrate the usability of the methodology in dual-band diversity antenna for mobile terminals. In the above two bands, the isolations of the prototype are larger than 13 dB and 16 dB, respectively. The measured radiation patterns of the two monopoles in general cover complementary space regions. The diversity performance is also evaluated by calculating the envelope correlation coefficient, the mean effective gains of the antenna elements and the diversity gain. It is proved that the proposed antenna can provide spatial and pattern diversity to combat multipath fading.     

Miniature Interferometer Terminals for Earth Surveying: Ambiguity And Multipath with Global Positioning System

With the recent launching of several satellites of the global positioning system (GPS), a variety of schemes based on radio interferometry have been proposed for the accurate determination of relative positions of receiving terminals on the ground. Provided that the integer-cycle ambiguities of the interferometric phase observations can be correctly resolved, the baseline vector extending from the antenna of one terminal to that of another should be determinable with uncertainty much smaller than the 19-cm wavelength of the GPS transmissions. We propose a method of ambiguity resolution that is suitable for observations made with antennas of low directive gain. Such antennas are compact, but the feasibility of their use has been questioned because observations with them are susceptible to multipath interference. For short-baseline interferometric observations of GPS our method yields correct ambiguity resolution despite severe multipath interference and significant sky blockage, even when instability of the frequency standards governing the separate receiving terminals limits the time span of coherent integration to five minutes.     

Mitigation of Multipath Effects Based on a Robust Fractional Order Bidirectional Least Mean Square (FOBLMS) Beamforming Algorithm for GPS Receivers

Consideration on positioning and location services among the public has been increasing in the recent years with their applications in most of the anticipating milieus such as automobile navigation system etc. This insists for a development of high recitation global navigation satellite system such as global positioning system (GPS). Multipath effects, interference, signal jamming etc. are the major sources of error influencing the performance of the GPS receiver. Literature presents many of the multipath mitigation techniques. Among them, adaptive processing technology based beamforming algorithms appears a viable solution for multipath mitigation. The least mean square (LMS) beamforming algorithms were sensitive to dynamic environments thus affecting the accuracy of GPS. In this paper, an adaptive beamforming algorithm called fractional order bidirectional least mean square (FOBLMS) algorithm is proposed to mitigate the multipath effects and to conceal the jammer signal in a GPS receiver. The FOBLMS is an integration of the fractional calculus and bidirectional least mean square algorithm. The effectiveness of the proposed algorithm is validated using the bit error rate and experimentation gain results over the existing beamforming algorithms. Experimental results demonstrated that the performance of the proposed beamforming algorithm is better than LMS algorithm with maximal relative antenna gain of 28.92 dB, 32.84 dB for two and four element antenna arrays at ??60° and 10°, direction of arrivals respectively. The outcome of this work would be useful for developing a robust technique for multipath mitigation in GPS receivers.

     

Frequency domain detectors for ultra-wideband communications in short-range systems

In this paper, we propose an original detection scheme for high rate short-range impulse radio ultra-wideband systems. The proposed receiver relies on both the introduction of the cyclic prefix at the transmitter and the use of a frequency domain multiuser detector at the receiver. Zero forcing (ZF) and minimum mean square error (MMSE) detection strategies have been investigated and compared with the classical RAKE, considering a scenario where several mobile terminals communicate with a base station in an indoor environment characterized by severe multipath propagation. The results show that the MMSE receiver achieves the best performance, irrespective of the number of active terminals, both in the uplink and in the downlink communications. Hence, the proposed approach is well suited in indoor wireless environments where the multipath propagation tends to increase the effects of both the inter-path and the inter-user interference.     

Mobile antenna gain in the multipath environment at 900 MHz

A program has been completed that investigated the effect of the urban multipath environment on mobile antennas at 900 MHz. The program involved the construction of several mobile antennas, careful measurement of these antennas on an antenna range to determine their characteristics in a controlled environment, and measurement of their characteristics in the multipath environment of cities. Measured results were compared to computed results and pattern range results were compared to field data to permit generalized conclusions to be reached. The results of the second phase of the program, measurements made in the mobile multipath environment, are presented. Analysis of this data shows that gain antennas have significantly less gain in areas where multipath propagation predominates and that the increased aperture of the gain antennas provides no significant smoothing of the received signals in the multipath environment.     

Improved method for evaluating antenna performance in mobile environment with consideration of polarization and phase

The paper presents an improved method for evaluating antenna performance in a multipath propagation environment. It demonstrates, theoretically and experimentally, that by using polarization and phase terms of the incident field and the antenna system in the evaluation of the antenna effective gain (AEG), the AEG can be evaluated more efficiently as compared with the conventional method. First, a theoretical expression of the received power is given by using the open-circuit voltage at the antenna terminals, in which both the polarization and the phase of the incident field and antenna gain patterns are explicitly used. Secondly, a newly developed system for the measurement of field parameters, such as polarization and phase as well as amplitude, is introduced. Thirdly, incident field parameters measured in urban and suburban areas of Tokyo are described. It was confirmed that major incident waves were counted in up to five or six directions at a mobile terminal and their polarizations and phases were different depending on the direction of incidence. Also, measurement of the antenna gain patterns of the latest personal digital cellular handset at a talk position finds the dependence of the AEG on polarization and phase, which were significantly varied by the body effect. Finally, the AEG is evaluated based on the method introduced here, and its usefulness is compared with that of the conventional method.     

Mobile radio window random-access algorithm with diversity

We consider a mobile radio random-access algorithm equipped with a choice of three distinct antenna diversity plans (two-fold, maximum selection, or ideal selection diversity) to improve the performance. The algorithm is stable with high throughput and low delays. The mobile environment is regulated by the presence of multipath fading, shadowing, and path loss. The capture model is a threshold model based on the signal-to-interference ratio. The maximum throughput and optimal window size of the algorithm are analytically computed. The delay performance is evaluated through simulations, and significant improvement is demonstrated over slotted ALOHA under various mobile scenarios     

Effects of antenna directivity and polarization on indoor multipathpropagation characteristics at 60 GHz

In millimeter-wave indoor communications systems, the radiation patterns and polarizations of the antennas at base stations and remote terminals have a significant influence on channel characteristics. The work reported in this paper investigated the effects of the radiation patterns of the antennas at remote terminals on multipath propagation characteristics. These effects were investigated by indoor propagation measurements at 60 GHz conducted in a modern office room and by ray-tracing simulations based on geometrical optics. Multipath channel characteristics are compared in terms of impulse responses and their root-mean-square (rms) delay spreads for an omnidirectional antenna and for three directive antennas with different beam widths. From the results of measurements and ray-tracing simulations, the use of a directive antenna at the remote terminal is demonstrated to be an effective method of reducing the effects of multipath propagation. Further reduction in the multipath effects is found to be achieved by the use of circular polarization instead of linear polarization with the directive antennas     

The effects of the human body on UWB signal propagation in an indoor environment

Ultra-wideband (UWB) communication systems are investigated for their ability to operate in dense multipath environments. While a great deal of time and effort has been spent characterizing both the indoor and outdoor UWB channels, the effects of human body interaction with a close proximity UWB antenna remains unexplored. Measurements of a commercially available UWB antenna performance in an anechoic chamber and in various indoor multipath environments were conducted. Comparisons of these measurement results indicate that while the human body creates a deep (23.6 dB) in a light multipath environment, this is drastically reduced (6.8 dB) in a dense multipath environment.     

Experiment on coherent adaptive antenna array for wideband DS-CDMAmobile radio

A four-element pilot symbol-assisted coherent adaptive antenna array diversity receiver for 4.096 Mchip/s wideband direct sequence code division multiple access mobile radio is implemented and its performance in a multipath fading environment is evaluated by a laboratory experiment using hardware fading simulators. The receiver comprises an adaptive antenna array using the normalised least mean square algorithm and Rake combiner. It is demonstrated that, for the three-user case, the required average signal-to-interference ratio obtaining average BER of 10^-3 can be reduced by ~8 dB compared to four-branch antenna diversity     

BER performance of CMA adaptive array for high-speed GMSK mobilecommunication-a description of measurements in central Tokyo

This paper describes the performance of an adaptive array as a countermeasure to multipath fading for a 256 kbps Gaussian-filtered minimum shift keying (GMSK) mobile communication system operating in the 1.5 GHz band. An adaptive array having four antenna elements is implemented using the digital beam forming concept. The constant modulus algorithm (CMA) is employed for the adaptation process to ease the implementation. Measurements in central Tokyo of the bit error rate (BER) performance and an array pattern arising in the multipath environment are presented. Analysis of the array pattern confirms that the array succeeds in directing nulls to the delayed signals. BER performance shows an improvement in E_b/N₀, compared with that of a single antenna system, of 17.5 to 22 dB at a BER of 1.0×10^-2 in a frequency-selective fading channel     

Modified meander line monopole antenna for broadband operation

A modified meander line monopole antenna is presented for broadband operation. The impedance and radiation characteristics of the antenna are investigated. The proposed antenna achieved a bandwidth of approximately 845 MHz with S₁₁< -10 dB, which covered the GPS, DCS, PCS, UMTS and WiBro bands. The electrical characteristics of the proposed antenna appear to be attractive for mobile terminals and repeater applications.     

Mobile antenna gain at 900 MHz

A program has been initiated to investigate the effect of the urban multipath environment on mobile antennas at 900 MHz. The program involves the constructuion of several mobile antennas, careful measurement of these antennas on an antenna range to determine their characteristics in a controlled environment, then measurement of their characteristics in the multipath environment of cities. Measured results are compared to computed results to permit generalized conclusions to be reached. This paper presents the results of the first phase of the program; measurements made in the controlled pattern range environment.     

Time division adaptive retransmission for reducing signal impairments in portable radiotelephones

Multiple-antenna receiving diversity was shown previously to be effective in mitigating the effects of random angular orientation and multipath radio propagation for portable radiotelephones. It is shown that time-division adaptive retransmission used with appropriate antenna configurations can also mitigate these effects. The retransmission configurations require fewer antennas than the receiving diversity configurations for a given improvement in relative signal-to-noise ratio (S/N). Cumulative distributions of S/N were determined for adaptive retransmission and diversity using random orientation and multipath propagation models. Distributions of S/N for systems with two antennas at the portable set and two appropriately polarized antennas at the portable radiotelephone terminal (PORT) are similar to distributions for two-branch selection diversity in the fixed-orientation mobile radio environment. Systems with one portable antenna and two PORT antennas have distributions with slopes similiar to two-branch mobile radio distributions but the distributions for the portables range from 3 to 7 dB worse.     

一种DS-CDMA无线反向链路空间分集方案研究

本文利用自适应天线阵列分离出多径信号,利用RAKE消除多径衰落。导频位符号辅助(PSA)信道估计用来对RAKE的输入端进行加权,天线阵列的加权系数的自适应更新是采用RLS算法而得。结果表明,该方案具有很强的抗多径干扰和多址干扰的能力。     

Mobile small aperture satellite terminals for military communications

The United States Army is currently developing a satellite-based network-centric waveform. Mobile terminals that are small and compact are highly desirable for the military. This article gives an overview of the technical challenges and performance issues when mobile small aperture satellite terminals are connected to a network- centric communication system. The specific issues addressed are regulatory limits on off-axis emissions, performance of small aperture terminals in the presence of adjacent satellite interference, and the effects of motion induced antenna pointing errors on the satellite communication system.     

Design concept of antennas for small mobile terminals and the future perspective

With the progress and rapid increase in mobile terminals, the design of antennas for small mobile terminals is acquiring great importance. In view of this situation, the design concept of antenna systems for small mobile terminals and its future perspective are discussed, referring to the trends in modern mobile communications and the demands for the antenna system. The design concept of antenna systems has changed along with the progress of mobile systems. In the conventional design, the conducting material of the equipment case, existing near the antenna element, is included as a part of the radiator in the antenna system, without regard to whether or not it has been considered. In the latest mobile phones, the design concept has been advanced to aim at improving the antenna's performance, and the treatment of the case, etc., has become a part of this. Some particular considerations for the design are to relieve the degradation of the antenna's performance due to the human-body effect, to overcome the SAR problems, and yet to make the size small. Furthermore, it is now time to proceed to the more-advanced concept, in which implementation of adaptive control and software is considered, even in small mobile terminals. Typical trends in modern communications systems are discussed, and the demands of these trends-which are summarized as down-sizing, function, and intelligence-are taken up for future antenna structures. Small-antenna technology and the application of the integration technology to antenna systems are essential for realizing such future structures.     

Space diversity in presence of discrete multipath fading channel

Most analytical studies of the performance of space diversity systems on fading channels assume a very rich multipath environment. In certain wireless applications, however, the number of significant multipath components can be small. In this letter, we consider a multipath channel in which the signal propagates from the transmitter to the receiver via L discrete paths which are uniformly distributed about the transmitter and receiver. For this channel, we study the effects of the number of multipath components and antenna array size on the error probability and outage capacity of space diversity systems. We observe that performance is significantly influenced by the presence of a channel with few multipath components.