DE19907476C1 - Process for the control and dynamic adjustment of the directional characteristics of linear antenna arrays in the spatial separation of signals - Google Patents

Abstract

In order to be able to estimate the separability of the participants depending on their position and the requirements of the participants for the quality of the connection and thus to enable the control of the directional characteristic and dynamic allocation of frequencies, a method is specified in which the directivity (D) is a criterion for the separability of participants is selected and determined according to the equation DOLLAR F1, where H (e jw) generally denotes the directional characteristic of a linear antenna array, for the formation of which the weighting factors wl for the individual signals are calculated in accordance with wl = DOLLAR F2 and at the same time that coefficient bk = b m + 1 for the calculation of the weighting factors, which was determined for the maximum of the directional characteristic in the direction of the useful signal, is used for the calculation of the directivity according to D = 1 / b m + 1. If the comparison of the calculated directivity (D) with a necessary minimum value of the directivity (D min), which is adapted to the required quality of the connection, shows that D <D min, i. H. the associated interfering subscriber can no longer be separated from the others and is removed from the SDMA range by a frequency change. Subsequently, directional characteristics and directivity are determined for the existing users with the changed frequencies.

Description

The invention relates to a method for control and dynamic adaptation the directional characteristic of linear antenna arrays in spatial Separation of signals.

H. Boche gave a lecture on a method and an arrangement for generating predetermined directional characteristics of adaptive group antennas in wireless mobile radio systems on the occasion of the conference "Selected Problems of Modern Mobile Radio Systems" on December 11, 1998 at the Institute of Communication Technology at ETH Zurich (Switzerland) "Use of" smart antennas "in CDMA-based mobile radio systems" has been reported (CDMA - Code Division Multiple Access). This solution enables the creation of a directional characteristic with little expenditure on equipment and numerical means, in which both the direction of the global maximum (main beam) and the directions of the zeros can be specified. First, the useful and interference signals are received and the number of input signals is determined. After determining the angle of incidence of these signals, the information obtained so far is passed on to the beamformer (beamformer) in order to now separate the useful and interference signals according to the previously determined signal incidence angles, to amplify the useful signals and in parallel to limit the number of interference signals. Using digital signal processing, a coefficient matrix is first used to determine the antenna weights

to solve the directivity equation

if the conditions are met

set up. Then the system of equations

solved for determining the coefficients b _l , from which about

then the weighting factors w _{k are} determined, which are multiplied by the respectively associated signals of the antenna outputs, finally the weighted signals are added to an overall signal and the desired directional characteristic is thus formed. By using adaptive group antennas, such directional characteristics can be determined in future mobile radio systems, whereby useful signals from previously determined directions are received with an increased gain and, at the same time, interference signals from likewise previously determined directions are suppressed, which increases the quality of the received signal and thus the transmission capacity of the radio connection. A plurality of directional characteristics generated at the same time enables spatial separation of different useful signals (SDMA - Space Division Multiple Access), which enables the number of operable subscribers per radio cell to be multiplied. Due to the mobility of the participants and the rapid change in the properties of the radio channel over time, the directional characteristic formed must be continuously updated. It can always happen that two participants approach each other until they can no longer be separated using a directional characteristic. In such situations, the problem is to find criteria with which the separability of the participants can be assessed depending on their position and the requirements of the participants for the quality of the connection (Quality of Service - QoS). Based on such criteria, the directional characteristic and dynamic frequency assignment can be controlled. This means that the SDMA principle can be consistently used in this mobile radio system. So far, the decision about the separability of the participants has been made empirically.

In the magazine "Funkschau" 23/96, pp. 84-87 it is stated that a change of the frequency channel occurs when there is an overlap (Directivity D <Dmin) of two main lobes the transmission quality of the Radio channel very deteriorated. There is no concrete solution to this specified.

Therefore, it is an object of the invention to provide a method that Separability of the participants depending on their position and the Assesses participants' demands on the quality of the connection and thus the control of the directional characteristic and dynamic allocation of Frequencies.

According to the invention, a method for controlling and dynamically adapting the directional characteristic of linear antenna arrays in the spatial separation of signals is specified, in which the directivity D is selected and defined as a criterion for the separability of subscribers according to the equation

where H (e ^jω ) generally designates the directional characteristic of a linear antenna array, for the formation of which the weighting factors w _l for the individual signals according to

by forming a coefficient matrix A to solve the equation

if the conditions are met
H (e ^jω *) = 1 maximum of the directional characteristic in the direction of the useful signal
H (e ^{jω _l} ) = 0 zeroing the directional characteristic in the direction of the interference signals 1 ≦ l ≦ mm ≦ N - 1,
from the system of equations

are calculated, where A is the coefficient matrix with the elements A = {a _{k, l} } with 1 ≦ k, l ≦ m + 1 and

and the coefficient b _{m + 1} for the determination of the directivity (D) according to

D = 1 / b _{m + 1}

is used,
then the calculated directivity D with one of the required quality of the connection adapted to the necessary minimum value of the directivity of D _min is compared, wherein when D _<Dmin, the separability of the users is possible, and when D _<Dmin, the separability of the users is not possible and in the case of D <D _min the associated closest disruptive participant, who can no longer be separated from the others, is removed from the SDMA range by initiating a frequency change and then directional characteristics and directivity are determined for the now existing users with the changed frequencies.

The method according to the invention for controlling and dynamically adapting the directional characteristic of linear antenna arrays in the spatial separation of signals enables, given a required transmission quality, the assessment of the separability of the participants based on the directivity of the resulting directional characteristic and thus an efficient and simple control of an SDMA system. The use of the coefficient b _{m + 1} for the determination of the directivity does not require any additional complex calculations, but falls - so to speak as a "by-product" - with the adjustment of the directional characteristic, since at the same time the coefficient b _{m + 1} for the calculation of the weighting factors was determined for the maximum of the directional characteristic in the direction of the useful signal, is used for the calculation of the directivity according to D = 1 / b _{m + 1} . The directivity determined after a frequency change is always better than that previously determined.

The directivity has a direct influence on the bit error probability of the Radio link in such a way that the bit error probability becomes smaller, depending greater the directivity. Is a maximum permissible for a radio connection Bit error probability required, this results in a minimal necessary directivity. The directivity of the directional characteristic adjusts Quality characteristic for the current directional characteristic. Is located disturbing participant (zero point) too close to the participant to be selected (Main direction), this results in an insufficient directivity. Falls below this the required minimum value, these two participants can no longer be separated from each other. The consequence is a frequency change (Handover), the disturbing participant in question is removed from the SDMA area removed and thus this zero is eliminated, which is in one greater directivity. This can be used as a criterion for the directivity Separability of participants under different quality requirements be used.  

Separability of participants under different quality requirements be used.

In the following, the invention is illustrated using an exemplary embodiment Drawings explained in more detail. Show:

Fig. 1 is a block diagram of an adaptive antenna with Direktivitätssteuerung,

Fig. 2 flow chart for separating the users by means of the directivity criterion.

In the event that several participants around the antenna of a base station are arranged and all participants can send at the same time, without training a directional characteristic only a superposition of all transmission signals be received. Such situations typically occur with CDMA based mobile radio systems. An effective way to suppress this Overlays is the use of adaptive antennas that show the directions of everyone the incoming signals and determine with the help of a suitable Process form a directional characteristic that the desired signal with receives a maximum gain and at the same time through the interference signals Directional characteristics of zeros suppressed.

In Fig. 1 is a block diagram of an adaptive antenna is illustrated. First the useful and interference signals are received and their number and angle of incidence are determined. The task of the beam former is to determine the weighting factors w ₁ ,. , , Determine w _N for the individual signals so that the desired directional characteristic is formed. This is done in the following steps: From the determination of the angle of incidence of the signals, both the angle of incidence of the useful signal ω _* and the angle of incidence of the interference signals ω _l with 1 ≦ l ≦ N - 1 are known, where N corresponds to the number of antenna elements used. A beam pattern (directional characteristic) is generated which fulfills the following conditions:

• a) Maximum of the beam pattern in the direction of the useful signal
  H (e ^jω *) = 1
  
• b) zeroing the beam pattern in the direction of the interference signals
  
• c) the directivity should be maximum, ie
  m corresponds to the number of zeros to be generated. First, the basic function
  considered. With the zeros ω ₁ . , , ω _m and the main direction ω _* , which is set with ω _{m + 1} = ω *, the following general beam pattern is created
  

The aim is to determine the coefficients b _{l in} such a way that the boundary conditions with regard to ω _l and ω _{* are} met

This leads to a system of equations of order m + 1 in the form

A is the coefficient matrix with the elements A = {a _{k, l} } 1 ≦ k, l ≦ m + 1 with

The bit error probability of an asynchronous interference-limited CDMA system is determined according to (see IEEE Transactions on Vehicular Technology, Vol. 43, pp. 680-690, August 1994)

calculated, where M is the spreading factor of the CDMA code and K is the number of currently active subscribers (and thus K - 1 indicates the number of interferers) and E (x) is the error function that corresponds to

is defined. This bit error rate can be reduced by using intelligent antennas, it results in

where D is the directivity of the directional characteristic of the intelligent antenna. The directivity of a directional characteristic is included

Are defined. One can see that the directivity has a direct influence on the Bit error probability of the radio link in the form that the The greater the directivity, the smaller the probability of bit errors.

For the beamformer described in this exemplary embodiment, the directivity results directly from the solution of the system of equations (**)

D = 1 / b _{m + 1} .

The calculated value for the directivity must now be compared with a minimum value of the directivity D _min required for the currently required transmission quality. If the determined value D <D _min , the separability of the users is possible. However, if the determined value is D <D _min , the separability of the users is not possible; therefore the corresponding disturbing subscriber, who can no longer be separated from the others, is removed from the SDMA range by frequency changes. These steps can be repeated, so that with a negligibly small additional effort compared to the computing time for the formation of the directional characteristic, a very efficient control of the allocation of the frequencies can be realized, since the proposed solution enables the calculation of the directivity in real time, which - as already mentioned - directly affects the bit error rate. For this specific embodiment of a beam former, it is not necessary to determine the corresponding integral with great numerical effort in order to determine the directivity, but - as described above - only to determine the coefficient b _{m + 1} .

In FIG. 2, the flow diagram is shown for separating the user by means of Direktivitätskriterium. After the selection of the useful and interference signals, both the weighting of the signals and the calculation of the directivity take place in the beamformer. If the following comparison of the determined directivity D with the necessary minimum directivity D _min , adapted to the current application, determines that D <D _min , the separability of the users is not possible. In this case, the disturbing user closest to the main direction of the directional characteristic with the angle of incidence ω _f is removed by causing a frequency change and again with the input of the signals of the now reduced in number and the frequencies {ω _* , ω ₁ ,. , ., ω _K-1 } \ {ω _f } users to determine the directional characteristics and directivity for these conditions. This selection can be repeated until a result that is adapted to the quality requirements of the transmission is available.

Claims (2)

1. Method for controlling and dynamically adapting the directional characteristic of linear antenna arrays in the spatial separation of signals, in which
the directivity (D) is selected and defined as a criterion for the separability of participants according to the equation
where H (e ^jω ) generally designates the directional characteristic of a linear antenna array, for the formation of which the weighting factors w _l for the individual signals according to
by forming a coefficient matrix A to solve the equation
if the conditions are met
H (e ^jω *) = 1 maximum of the directional characteristic in the direction of the useful signal
H (e ^{jω _l} ) = 0 zeroing the directional characteristic in the direction of the interference signals 1 ≦ l ≦ mm ≦ N - 1,
from the system of equations
are calculated, where A is the coefficient matrix with the elements
A = {a _{k, l} } with 1 ≦ k, l ≦ m + 1 and
is
and the coefficient b _{m + 1} for the determination of the directivity (D) according to
D = 1 / b _{m + 1}
is used,
then the calculated directivity (D) with one of the required quality of the connection is not adapted to the necessary minimum value of the directivity (D _min) is compared, wherein when D _<Dmin, the separability of the users is possible, and when D _<Dmin, the separability of the users is possible, and in the case of D <D _min the associated interfering subscriber, who can no longer be separated from the others, is removed from the SDMA range by initiating a frequency change and then for the existing users with the changed frequencies, directional characteristics and directivity be determined. 2. The method according to claim 1, wherein the process steps are repeated.