Communications: directive radio wave systems and devices (e.g. – Directive – Utilizing correlation techniques
Reexamination Certificate
2001-11-15
2003-08-12
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Utilizing correlation techniques
C342S367000, C455S562100
Reexamination Certificate
active
06606058
ABSTRACT:
This is the U.S. National Stage of International Application No. PCT/AT00/00072, which was filed on Mar. 24, 2000 in the German language.
The invention relates to a beamforming method for adaptive antenna arrays including several antenna elements in the downlink of frequency duplex systems, wherein antenna weights are determined for the antenna elements for downlink transmission on the basis of directional information of the uplink.
Furthermore, the invention relates to a beamforming device for adaptive antenna arrays including several antenna elements in the downlink of frequency duplex systems, comprising a signal processing unit used to determine antenna weights for the antenna elements for downlink transmission on the basis of directional information of the uplink.
It is known to electronically modify array antennas consisting of several individual antennas in respect to their directional characteristics in order to adaptively adapt the same to the respective channel situation in the optimum manner. Adaptive antennas initially were employed in radar technology, yet also their application in mobile communication systems has been investigated for quite some time. The use of adaptive antennas may lead to a reduction of the received interference by directed reception, a reduction of the generated interference by directed transmission and a reduction of the time dispersion of the mobile radio channel and hence a reduction of the intersymbol interference decisively codetermining the bit error rate.
These improvements may be used for a capacity gain, to increase the spectral efficiency, to reduce the necessary transmission power by the antenna array gain, to improve the transmission quality (reduced bit error rate), to increase the data rate and to extend the range of action.
Although not all advantages can be exploited at one and the same time, it is, nevertheless, feasible to achieve some of the above-mentioned improvements in each case. Thus, it would be absolutely essential to enable, by means of adaptive antennas, a more efficient utilization of the frequency spectrum available and, at the same time, an increase in the capacity and hence possible number of users in a cell at the same frequency band and the same number of base stations.
Mobile cellular wireless communication nets, in general, are limited in interference, i.e., the spatial reuse of one and the same radio channel, on the one hand, and the spectral efficiency, on the other hand, are limited by common channel interferers. A radio channel is defined by its frequency and/or its time slot (in the time multiplex—TDMA—time division multiple access) or its code (in the code multiplex—CDMA—code division multiple access). To supply more than one user by one and the same radio channel in TDMA and FDMA (frequency division multiple access) systems, methods based on the spatial divisibility and the direction-selective reception in the uplink (mobile station transmitting, base station receiving) as well as the direction-selective transmission of the user signals in the downlink (base station transmitting, mobile station receiving) have been proposed (socalled SDMA—space division multiple access system). The direction-selective transmission/reception in CDMA systems may also be used to increase the possible number of users on one frequency and hence raise the spectral efficiency and the capacity of a mobile cellular radio system. Thus, the possible number of users on a communication channel, that can be detected in the uplink by the base station through the linear adaptive antenna array and supplied in the downlink is increased with the interference remaining the same.
Three basic methods are known to divide the signals of the individual users by common channel interference suppression and detect the same: (1) Methods based on the knowledge of the spatial structure of the antenna array (socalled spatial reference methods), cf. R. Roy and R. Kailrath, “ESPRIT—Estimation of Signal Parameters via Rotational Invariance Techniques”, IEEE Trans. Acoust., Speech and Signal Processing, Vol. 37, July 1989, pp. 984-995; (2) methods based on the knowledge of a known signal sequence (socalled temporal reference methods), cf. in S. Ratnavel, A. Paulraj and A. B. Constantinides, “MMSE Space-Time Equalization for GSM Cellular Systems”, Proc. IEEE, Vehicular Technology Conference 1996, VTC 96, Atlanta, Ga., pp. 331-335; and (3) socalled “blind” methods using known structural signal properties for signal division and detection, cf. in A-J. van der Veen, S. Talwar, A. Paulraj “A Subspace Approach to Blind Space-Time Signal Processing for Wireless Communication Systems”, IEEE Transactions on Signal Processing, Vol. 45, No. 1, January 1997, pp.173-190.
Various methods based on different estimates of the mobile radio channel are used for the downlink. In principle, either the directions of incidence of the signals of the mobile stationd (cf., e.g., U.S. Pat. No. 5,515,378 A or EP-755 090 A) are used, or the spatial covariance matrix (spatial correlation matrix) is used for beam formation (cf. U.S. Pat. No. 5,634,199 A).
A difficult problem is set by the different carrier frequencies in frequency duplex systems (FDD systems). In FDD systems, the signals both in the uplink and in the downlink are transmitted at different frequencies, thereby ensuring the necessary division between transmitted and received data both at the mobile and base stations. Due to the frequency difference, the antenna directivity pattern will be different, if the same physical antenna array and the same antenna weights (amplitude and phase) are used at different frequencies. For this reason, it is not advisable to use the same antenna weights for transmission and reception at the base station of a mobile cellular communication system. The exclusive use of the direction of incidence estimated in the uplink does not have any problems with that frequency offset, yet restricts beam formation to a single discrete direction of incidence, what is in contradiction to the physical nature of the mobile radio channel and, therefore, results in a limited capacity gain by the adaptive antenna. The use of the spatial covariance matrix of the uplink, however, involves the drawback of a frequency offset.
Various approaches have already been described to compensate for that frequency duplex distance in the spatial covariance matrix. Thus, it has been proposed to estimate in the uplink the direction of incidence, the signal power and the pertinent angular spread of each user, cf. T. Trump and B. Ottersten, “Maximum Likelihood Estimation of Nominal Direction of Arrival and Angular Spread Using an Array of Sensors”, Signal Processing, Vol. 50, No. 1-2, April 1996, pp. 57-69. From that estimate for the uplink, an estimate of the spatial covariance matrix for the downlink is made, cf. also P. Zetterberg, “Mobile Cellular Communications with Base Station Antenna Arrays: Spectrum Efficiency, Algorithms and Propagation Models”, thesis, Royal Institute of Technology, Stockholm, Sweden, 1997. That method, however, will function only if each mobile station has but a single nominal direction of incidence in respect to the base station. Due to reflections on mountains in rural areas or large building complexes in urban areas, this condition is frequently not met, thus rendering this approach inapplicable.
Another prior art proposal aims to use in the base station for transmission and reception in a frequency duplex system, two different antenna arrays scaled with the applied wavelength; cf. G. G. Rayleigh, S. N. Diggavi, V. K. Jones and A. Paulraj, “A Blind Adaptive Transmit Antenna Algorithm for Wireless Communication”, Proceedings IEEE International Conference on Communications (ICC 95), IEEE 1995, pp. 1494-1499, or the corresponding WO 97/00543 A. There, the two “adapted” antenna arrays, however, have to be manufactured and calibrated in a highly precise manner and placed in exactly the same position. Moreover, a second antenna array is required, thus raising costs superproportionally.
According to U.
Bonek Ernst
Hugl Klaus
Issing Gregory C.
Nokia Networks Oy
Pillsbury & Winthrop LLP
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