DE19858951C2 - Method and arrangement for generating predetermined directional characteristics - Google Patents

Description

The invention relates to a method for generating predetermined directional characteristics of adaptive group antennas in wireless mobile radio systems, comprising the method steps of receiving the useful and interference signals by N antenna elements (x ₁ ... X _N ); Determining the number of incoming signals; Determination of the weighting factors for the individual signals of the antenna outputs in a beamformer; Multiplication of the weighting factors w _l at the output of the beam former with the associated signals of the antenna outputs; Adding these weighted signals to a total received signal in a summer; Formation of the desired directional characteristic, as well as an arrangement comprising antenna elements to which a baseband mixer for demodulating the received signals is connected, a beamformer for determining the antenna weights and a summing element for forming an overall reception channel.

With these directional characteristics, useful signals can be determined from beforehand Directions received with an increased gain and at the same time Interference signals are also suppressed from previously determined directions. This can increase the quality of the received signal and thus a larger one Transmission capacity of the radio link can be achieved. Furthermore, one spatial separation of useful signals (SDMA- [space division multiple access] space multiplex) by several generated at the same time Directional characteristics possible, which results in a multiplication of the number operable subscriber per radio cell results. Through permanent adjustment the directional characteristic to the current situation of the environment can depend on  Movement of the cellphone subscribers or movements of reflective ones or absorbent objects.

Typical areas of application are currently existing mobile radio systems (GSM 900 and GSM 1800 [global system for mobile comm. - worldwide Mobile radio system]), in which the number of operable participants per Radio cell can be increased. Furthermore, the use in future Mobile radio systems, such as UMTS (universal mobile telecomm. systems - universal mobile communications), to increase the number of participants and to Realization of higher data rates planned.

So far, the generation of a directional characteristic for given directions of the useful and interference signals has been solved by:

• a) the global maximum of the directional characteristic towards the strongest Useful signals is formed. Taking into account the interference signals not possible (see Barlett, M. S .: "Periodogram analysis and continous spectra ", Biometrica, vol. 37, pp. 1-16, 1950);
• b) a fixed number of, depending on the number of antenna elements Zeroing is specified. These zeros correspond to the directions of interference signals. The direction of the global maxima is not influenceable. Furthermore, the number of zeros cannot match the number the interference signals are adjusted. (see Capon, J .: "High resolution frequency-wave number spectrum analysis ", Proceedings of the IEEE, vol. 57, pp. 1408-1418, August 1969).

Furthermore, there is the disadvantage in both known solutions that the Adaptation to the respective situation of the environment is only possible to a limited extent, d. H. it will not give an optimal result regarding the quality of the Received signal reached.  

Two publications by Godara provide a comprehensive overview given the previously known methods, but this overview no new or compared to the first mentioned publications contains further developed solutions (see L. C. Godara: "Application of Antenna Arrays to Mobile Communications, Part I: Performance, Improvement, Feasibility and System Considerations, "Proceedings of the IEEE, vol. 85, no. 7, pp. 1029-1060, July 1997 and "Application of Antenna Arrays to Mobile Communications, Part II: Beam Forming and Direction of Arrival Considerations ", Proceedings of the IEEE, vol. 85, no. 8, pp. 1193-1245, August 1997).

In IEEE Transactions on Antennas and Propagation, Vol. 39, No. January 1 1991, pp. 21-28 is a method for beam shaping in adaptive Antenna arrays described that on a DoA (Direction of Arrival) estimate based. The classic approach of Minimizing the effects of the received interference signals and the Noise levels are run out and the Vienna solution is constructed, which is why Direction of the desired mobile station must be known. A complete The interference signals are not suppressed here.

The prior art from which the invention is based is described in EP 0 883 207. A method for generating directional characteristics is specified, comprising the method steps reception of the useful and interference signals by N antenna elements (x ₁ ... X _N ); Determining the number of incoming signals; Determination of the weighting factors for the individual signals in a beamformer; Multiplication of the weighting factors w _l at the output of the beam former with the associated signals of the antenna outputs; Adding these weighted signals to a total received signal in a summer; Training of the desired directional characteristic. For the determination of the antenna weights (weight factors) a training sequence (pilot signal) known for receivers and transmitters is required, which must be sent again and again at short intervals (as part of the synchronization signal).

The difference between the received signal and the pilot signal / training sequence known to the receiver, which ideally should be zero. This is achieved by the Antenna weights are controlled so that the error is small. At Minimal error, the antenna array is adapted to the desired signal and is received optimally. Between those for the pilot signal / the Training intervals provided time intervals are the unknown User data sent. During this time the adaptation mechanism switched off by switch, otherwise there will be incorrect adaptation would. During this time, the antenna weights are kept constant with the Assumption that the transmission properties of the Don't change cellular channel during the next time interval for that The pilot signal / the training sequence will be adjusted again. The The method described is based on an algorithm that minimizes of the error between the received signal and the pilot signal. In the The arrangement shown in EP 0 883 207 has antenna elements which are attached a baseband mixer for demodulating the received signals connects a beamformer to determine the antenna weights and a summing element to form an overall reception channel. Farther are means for performing the method just described Generation of the training sequence (the pilot signal) is provided as well Switches that control the adaptation mechanism while sending Switch off user data between the bursts.

Systems based on pilot signals always need a certain one Adaptation time until the error between the pilot signal (target) and the received signal (Is) becomes minimal. It then turns out to be disadvantageous that the Reception characteristics are very poor and a significant part (approx. 20%) the transmission capacity required for sending the pilot signal is.

The invention has for its object a method for generating predefined directional characteristics of adaptive group antennas in  wireless mobile radio systems and an arrangement for carrying out the Specify the method in which incoming interference signals are safely suppressed are achieved, a larger transmission capacity of the radio link and at the same time the quality of the received signal is improved.

The object is achieved according to the invention by a method of the type mentioned at the outset that, for the determination of the weighting factors for the individual signals, the angle of incidence of the useful and interference signals is first determined from a DoA estimate (DoA - [Direction of Arrival] direction of reception) by signal analysis then the information previously determined in the process steps of receiving the useful and interference signals and determining the angle of incidence of these signals is passed to a beamformer (beam former) and here a selection (separation) of useful signal and interference signals according to the previously determined signal angles of incidence and including the in your own communication system, the specifications for useful and interference signals are carried out, then the useful signal is amplified by a signal amplifier and, in parallel, the number of interference signals based on the power of the same (interference signals below a predetermined minimum value are eliminated len) is limited, then digital signal processing - formation of a coefficient matrix

to solve the equation

where H generally represents the directional characteristic,
Q is the directional characteristic for the angle of incidence θ = 0,
w = π.sinθ represents the spatial frequency ("direction") of the interference or useful signals,
if the conditions are met

and the system of equations by matrix inversion

by means of the Gaussian algorithm for determining the coefficients b _k , and finally the weighting factors w _l

be determined.

Determining the number of incoming signals, their power and their direction of incidence is advantageously carried out by an eigenvalue decomposition the covariance matrix of these signals.

For the selection of the incoming signals into useful and interference signals additional information from the signaling of one's own Communication system provided (time-variable determination of which Signals as interference signals and which is to be regarded as a useful signal).

With an antenna array with N antenna elements, a maximum of N - 1 Interference signals can be suppressed by forming a zero, if the power of an interference signal exceeds a specified value.

The signal processing is done by forming a coefficient matrix based on the condition

with ω _{m + 1} = ω _* .

In an arrangement for carrying out the production method predefined directional characteristics of adaptive group antennas wireless mobile devices of the type mentioned are According to the invention between baseband mixer and beamformer means for Determination of the angle of incidence of useful and interference signals from a DoA Arranged estimate and the beamformer determines according to claim 1 Antenna weights for the individual signals from information on the number of incoming signals and for DoA estimation (directional estimation).

The beamformer has a digital signal processor (DSP) for implementation of all processes running in the beamformer. For the summation to Formation of the total received signal is either known per se analog summing circuit or a digital signal processor (DSP) intended.

The invention enables with low apparatus and numerical Effort generating a directional pattern, in which both the direction of the global maxima as well as the directions of the zeros can be. Furthermore, the number of zeros to be specified is not fixed, rather a variable number between 0 and N - 1 Zeros are specified, where N stands for the number of used Antenna elements. This means that the Directional characteristics to the surrounding situation possible. At the same time greater transmission capacity of the radio link compared to from the Known prior art solutions achieved.

The invention is illustrated below using an exemplary embodiment associated drawing explained in more detail. Show it:  

Fig. 1 block diagram of an adaptive antenna;

Fig. 2 block diagram of a beamformer.

Fig. 1 shows the block diagram of an adaptive antenna. The received signals of the N antenna elements x ₁ . , , x _n are shifted into the baseband by means of a mixer, ie the carrier signal is removed (demodulation) and only the complex modulation signal is considered. These signals are multiplied by the weighting factors w _l formed in the beamformer and the resulting signals are then added to the total received signal. To determine the optimal weight factors, a direction estimate and then beam shaping (beam shaping) is necessary.

In order to be able to separate interference signals from the desired useful signals by means of a beam former, the directions of incidence θ _{1 of} all waves (signals) incident on the antenna array must be known.

If a plane wave s (t) sweeps over an array with equidistant N receiving elements with the element spacing d (linear array) at the angle θ, the signal reaches each individual element with a path difference Δs. This path difference has a time delay of at the speed of propagation c (speed of light)

result. For narrowband signals,

the signal can be considered constant as it sweeps across the array. Hence this time delay can be approximated as a complex phase factor:

where f _{0 represents} the carrier frequency.

Due to the equidistant arrangement of the elements, there is a fixed relationship of the phase factors depending on θ between the individual signals of the elements. The input signal of the lth element (wave incident on the lth element) can be with

can be specified. Taking advantage of these relationships can Analysis of the signals of all elements on the directions of incidence of the waves getting closed. Since neither the number of incoming waves nor whose directions of incidence are known, the number of waves must first determined and then determined their direction. Among real ones However, conditions occur due to noise and inaccuracies on the antenna elements, which is why here from a directional estimate the speech is. Known methods of direction estimation are "MUSIC" (Multiple Emitter Location and Spectral Estimation - localization of the Multiple radiation and spectral estimation) s. R. O. Schmidt: "A signal Subspace Approach to Multiple Emitter Location and Spectral Estimation ", ph. D. thesis, Stanford University, Stanford, CA, November 1981 and "ESPRIT" (Estimation of Signal Parameters via Rotational Invariance Techniques - Estimation of signal parameters via rotation invariance determination), see R. Roy, T. Kailath: "ESPRIT Estimation of Signal Parameters Via Rotational Invariance Techniques ", IEEE Transactions on Acoustics, Speech, and Signal Processing, vol. 37, No. 7, pp 984-995, July 1989).

These methods are based on an eigenvalue decomposition of the covariance matrix of the incident waves (signals) s _l (t). The number of incoming waves as well as their amplitude and direction of incidence are available as output variables for these processes. To form a suitable directional characteristic, additional information regarding the useful or interference signals is required. This information is provided by the control signaling of the own communication system as a definition (variable in time) of the interference signals and the useful signal. The information received about the angles of incidence and the useful and interference signals are fed to the beamformer as input signals.

In FIG. 2, a block diagram is shown of a beamformer. The division into the individual function blocks was made from an algorithmic point of view, which is why it is not possible to equate these blocks with specific technical functional units. A digital signal processor (DSP) is used to implement the beamformer, which implements all the functions listed in the block diagram. The useful signal and the interference signals are selected based on the input signals of the beamformer specified above. The “useful signal selection” block selects the angle of incidence of the useful signal and feeds this as a direction signal ω * to the block for generating the coefficient matrix. The "interference signal selection" block proceeds in an equivalent manner by selecting the angle of incidence of the interference signals and first feeding these as directional signals ω _k to a block for limiting the number of interference signals. The background to this limitation is the fact that with an antenna array consisting of N antenna elements, a maximum of N-1 interference signals can be suppressed by means of a zero point. In order to obtain the most precise directional characteristic possible and to minimize the numerical effort, the number of zeros should be limited to the necessary minimum. This means that zeros are only formed when the power of the interference signal exceeds a predetermined value. This value depends on the specific communication system and can be adapted as a free parameter to any system.

The selection of the useful or interference signals is carried out in the following Steps:  

From the previous determination of the direction of incidence of the signals, both the angles of incidence of the useful signal as direction signal ω * and the angles of incidence of the interference signals as direction signals ω _l with 1 ≦ k ≦ N-1 are known, where N corresponds to the number of antenna elements used. A beam pattern (directional characteristic) is formed which fulfills the following conditions:

• a) Maximum of the directional characteristic in the direction of the useful signal
  where H stands for the directional characteristic.
• b) zeroing of the directional characteristic in the direction of the interference signals
  
• c) the total performance should be minimal
  m corresponds to the number of zeros to be formed. First, the basic function
  considered, where Q represents the directional characteristic in the direction θ = 0. With the m zeros, the direction signals ω ₁ . , , ω _m , and the main direction signal ω *, which is set with ω _{m + 1} = ω _* , results in the following general directional characteristic:
  The aim is to determine the coefficients b _{k in} such a way that the boundary conditions with regard to ω _k and ω _{* are} met
  with ω _{m + 1} = ω _* . The coefficient matrix formed in the next block thus results in
  with the elements A = {a _{k, l} } 1 ≦ k, l ≦ m + 1, with
  

The system of equations results from the matrix inversion

which is solved with the Gaussian algorithm to determine the coefficients b _l (next block). This algorithm is a standard method for numerically solving systems of equations. As a result, it can be proven that | det A | <0. This means that there is always a clear solution to the system of equations that satisfies the boundary conditions.

The coefficients b _k result from the solution of the system of equations. The antenna weights w _l result from this:

This gives the directional characteristic to:

The weight factors w _l determined in this way form the output signals of the beam former. They are multiplied in a multiplier by the associated signals from the individual antenna elements. These weighted signals are converted into a total received signal in a summing element of the adaptive antenna. This summation can take place either analogously in a known summing circuit or digitally in a DSP.

Below is a concrete numerical example for determining the Antenna weights:

Since the invention relates to the shaping of the directional characteristic, the direction estimation is not dealt with in this example. The angles of incidence are assumed as follows:

From the signaling of the communication system, the information is provided that signal no. 2 is the useful signal to be selected and all other signals are interference signals. The following selection is made:

• - Main direction: ω _* = 0
• - Zeros: ω ₁ = -π / 2, ω ₂ = 3.π / 4, ω ₃ = π / 3

An array of N = 4 elements is to be used as the antenna. With this array it would be possible to form N - 1, ie 3 zeros. From this point of view, it is not necessary to limit the number of interference signals. Since signal No. 4 has a very small amplitude, no relevant disturbances are to be expected from this signal. In order to minimize the numerical effort and to increase the quality of the directional characteristic, signal no. 4 is not canceled with a zero. With ω _{m + 1} = ω _* the boundary conditions are:

The coefficient matrix is set up as follows:

The solution of the system of equations of the inverted matrix gives _k for b
b ₁ = 0.015 + 0.015i
b ₂ = -0.073 - 0.030i
b ₃ = 0.271447 + 0.0i

Using the equation

the antenna weights w _l result in:
w ₁ = 0.213 - 0.015i
w ₂ = 0.286 + 0.088i
w ₃ = 0.286 - 0.088i
w ₄ = 0.213 + 0.015i

These antenna weights (or weight factors) w ₁ -w ₄ form the output signals of the beamformer. In the adaptive antenna, they are multiplied in a multiplier by the associated signals of the individual antenna elements. From these weighted signals thus formed, a total received signal is finally formed by a summing element of the adaptive antenna.

Claims (6)

1. A method for generating predetermined directional characteristics of adaptive group antennas of wireless mobile radio systems, comprising the method steps of receiving the useful and interference signals by N antenna elements (x ₁ ... X _N ), determining the number of incoming signals; Determination of the weighting factors for the individual signals in a "beamformer" (beam former); Multiplication of the weighting factors w _l at the output of the beam former with the associated signals of the antenna outputs; Adding these weighted signals to a total received signal in a summer; Formation of the desired directional characteristic, characterized in that for the determination of the weighting factors for the individual signals, first the angles of incidence of the useful and interference signals are determined from a DoA estimate by signal analysis, then the reception of the useful and interference signals and determination beforehand in the method steps the angle of incidence of these signals, the information determined is directed into the beamformer "(beamformer) and a selection (separation) of the useful signal and interference signals (max. N - 1) according to the previously determined angle of incidence, taking into account the definitions of useful and Interference signals are carried out, then the useful signal is amplified by a signal amplifier and at the same time the number of interference signals is limited according to their output (interference signals below a predetermined minimum value are eliminated), then digital signal processing - Formation of a coefficient matrix
to solve the equation
where H generally represents the directional characteristic,
Q is the directional characteristic for the angle of incidence θ = 0,
ω = π.sinθ represents the spatial frequency ("direction") of the interference or useful signals,
if the conditions are met
done and
the system of equations through matrix inversion
is solved by means of the Gaussian algorithm for determining the coefficients b _k and finally the weighting factors w _l
be determined. 2. The method according to claim 1, characterized in that for determining the number of incoming signals (incoming waves), their power and their direction of incidence, an eigenvalue decomposition of the covariance matrix of these signals s _l (t) is carried out. 3. The method according to claim 1, characterized in that with an antenna array consisting of N antenna elements, maximum N - 1 Interference signals can be suppressed by means of a zero, only then Zeros are formed when the power of an interference signal exceeds the specified value. 4. Arrangement for performing the method for generating predetermined Directional characteristics of adaptive group antennas wireless Mobile radio systems, comprising antenna elements to which a baseband Mixer to demodulate the incoming signals, one Beamformer for determining the antenna weights and a summing element to form a total received signal characterized in that between baseband mixer and beamformer means for determining the Angle of incidence of useful and interference signals from a DoA estimate are arranged and the beamformer according to claim 1 Antenna weights for the individual signals from information on the number of incoming signals and for DoA estimation (directional estimation) certainly. 5. Arrangement according to claim 4, characterized in that the beamformer has a digital signal processor (DSP) for Realization of all processes in the beamformer. 6. Arrangement according to claim 4, characterized in that for the summation to form the total received signal one per se known analog summing circuit or a digital signal processor (DSP) is provided.