DE19858951A1 - Method and arrangement for generating predetermined directional characteristics - Google Patents

Abstract

The invention relates to a method and an arrangement for generating predetermined directional characteristics of adaptive group antennas in wireless mobile radio systems. The invention is based on the object of specifying a method for generating predetermined directional characteristics of adaptive group antennas in wireless mobile radio systems and an arrangement for carrying out the method in which incoming interference signals are reliably suppressed, a greater transmission capacity of the radio connection is achieved and at the same time the quality of the received signal is improved . The object is achieved according to the invention by a method with the steps specified below: DOLLAR A 1. Receiving the useful and interference signals - determining the number of incoming signals, DOLLAR A 2. Determining the angle of incidence of the useful and interference signals by means of a directional estimate, DOLLAR A 3 Separation of useful and interference signals, DOLLAR A 4. Limitation of the number of interference signals, DOLLAR A 5. Determination of the weighting factors for the individual signals, DOLLAR A 6. Summary of the signals weighted with the weighting factors to form a total received signal.

Description

The invention relates to a method and an arrangement for generating predefined directional characteristics of adaptive group antennas in wireless mobile radio systems.

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 subscriber or movements of reflective 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).

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.

According to the invention, the object is achieved by a method having the following steps:

• 1. reception of the useful and interference signals by N antenna elements (x ₁ ... X _n ); Determining the number of incoming signals,
• 2. Determination of the angle of incidence of the useful and interference signals from a DOA Estimation (DOA - direction of arrival) by Signal analysis,
• 3. directing the information previously determined in process steps "1" and "2" into a beamformer (beam former),
  • 1. 3.1. Selection (separation) of the useful signal and interference signals according to the signal angles of incidence previously determined and taking into account the definitions of useful and interference signals made in one's own communication system,
  • 2. 3.2. Amplification of the useful signal by a signal amplifier,
  • 3. 3.3. parallel to this limitation of the number of interfering signals according to the power of the same (interfering signals below a predetermined minimum value are eliminated),
  • 4. 3.4. digital signal processing - formation of a coefficient matrix
    to solve the equation
    with a _{k, l} = Q e ^{j ( ω k- ω l)} ,
    in which
    H generally represents the directional characteristic,
    Q is the directional characteristic for the angle of incidence θ = 0,
    ω = π.sin θ represents the spatial frequency ("direction") of the interference or useful signals,
    if the conditions are met
    
  • 5.3.5. Solution of the system of equations by matrix inversion
    using the Gauss algorithm to determine the coefficients b _k ,
  • 6. 3.6. Determination of the weight factors w _l from
    
  • 7. 3.7. Multiplication of the weighting factors w _l as output signals of the beamformer with the associated signals of the antenna outputs,
• 4. Addition of these weighted signals to a total received signal adaptive antenna,
• 5. Training the desired directional characteristic.

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 _* .

The arrangement for carrying out the method for generating predetermined directional characteristics in adaptive group antennas has N reception elements (x ₁ ... X _n ) in each antenna, to which a baseband mixer for demodulating the received signals is connected. This is followed by a beamformer for determining the antenna weights. Finally, a summing element is provided for summing the individual signals coming from the beamformer and forming an overall received signal.

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 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.

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 equidistantly arranged 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:

s (t-τ) ≈ s (t) .e ^{-j.2 π f0. τ} ,

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 Ith element (wave incident on the Ith 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 it as a directional signal to the block for generating the coefficient matrix. The "interference signal selection" block proceeds in an equivalent manner by selecting the angles of incidence of the interference signals and first feeding them as direction 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 w _l with 1 ≦ k ≦ N -1 are known, where N corresponds to the number of antenna elements used. A beam splatter (directional characteristic) is formed which fulfills the following conditions:

• a) Maximum of the directional characteristic in the direction of the useful signal
  H (e ^{j ω *} ) = 1,
  where H stands for the directional characteristic.
• b) zeroing of the directional characteristic in the direction of the interference signals
  H (e ^{j ω k} ) = 0 for 1 ≦ k ≦ m; m ≦ N -1
• 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 A = 0. With the m zeros, the direction signals ω ₁ . . .ω _m , and the main direction signal, which is set with ω _{m + 1} = ω _* , the following general directional characteristic arises:
  

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 a _{k, l} = Q (e ^{j ( ω k- ω l)} )

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.

The following 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, i.e. 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 beam former. 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 (8)

1. Method for generating predefined directional characteristics of adaptive group antennas of wireless mobile radio systems in the following steps:

• 1. Receiving the useful and interference signals by N antenna elements (x ₁ ... X _n ), determining the number of incoming signals,
• 2. determination of the angle of incidence of the useful and interference signals from a DOA estimate by signal analysis,
• 3. management of the information previously determined in process steps "1" and "2" into a "beamformer" (beam former),
  • 1. 3.1. Selection (separation) of the useful signal and interference signals (max. N -1) according to the signal incidence angles previously determined, taking into account the definitions of useful and interference signals made in your own communication system,
  • 2. 3.2. Amplification of the useful signal by a signal amplifier,
  • 3. 3.3. at the same time limiting the number of interfering signals based on their output (interfering signals below a specified minimum value are eliminated),
  • 4. 3.4. digital signal processing - formation of a coefficient matrix
    to solve the equation
    with a _{k, l} = Q e ^{j ( ω k- ω l)} ,
    in which
    H generally represents the directional characteristic,
    Q is the directional characteristic for the angle of incidence θ = 0,
    ω = π.sin θ represents the spatial frequency ("direction") of the interference or useful signals,
    if the conditions are met
    
  • 5.3.5. Solution of the system of equations by matrix inversion
    using the Gaussian algorithm to determine the coefficients b _i
  • 6. 3.6. Determination of the weight factors w _l from
    
  • 7. 3.7. Multiplication of the weighting factors w _l as the output signal of the beam former with the associated signals of the antenna outputs,
• 4. addition of these weighted signals to a total received signal in a summer,
• 5. Training the desired directional characteristic.

2. The method according to claim 1, characterized in that for determining the number of incoming signals (incident 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 additional for the selection of the incoming signals in useful and interference signals Information from the signaling of your own communication system are provided in the form of time-variable determinations, which Signals as interference signals and which is to be regarded as a useful signal. 4. The method according to claim 1, characterized in that for an antenna array consisting of N antenna elements, a maximum of N-1 Interference signals are suppressed by means of a zero, but only then Zeros are formed when the power of an interference signal exceeds the specified value. 5. The method according to claim 1, characterized in that the coefficient matrix based on the condition
with ω _{m + 1} = ω _* . 6. Arrangement for performing the method for generating predetermined directional characteristics in adaptive group antennas according to claim 1, characterized in that each antenna has N receiving elements (x ₁ ... X _n ), which is followed by a baseband mixer for demodulating the received signals , which is followed by a beamformer for determining the antenna weights and that finally a summing element is provided to form an overall received signal. 7. Arrangement according to claim 6, characterized in that the beamformer has a digital signal processor (DSP) for Realization of all processes in the beamformer. 8. Arrangement according to claim 6, 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.