WO2001005025A1 - Method and device for correcting a corrupted useful signal - Google Patents

Abstract

A method for correcting a useful signal in the receiver part (100; 200; 300; 400) of a message transmission system which has been corrupted by a disturbance in the neighboring channel producing a particularly rectangular disturbing pulse in the useful channel. The initial or final point of the neighboring channel disturbance is determined in a first step. Offset correction using information on the initial or final point occurs in a second step.

Description

description

Method and device for correcting a corrupted useful signal

The invention relates to a method for correcting a useful signal falsification in the receiving part of a message transmission system, in particular operating according to the TDD or TDMA method, and a device for carrying out this method.

In mobile radio systems, due to the limited resources of the frequency spectrum, multiple access or multiplexing methods are absolutely necessary in order to enable subscriber numbers in the order of magnitude from millions to hundreds of millions. The type of access of several participants must be organized and the receiving parts must be designed in such a way that there are no undue disruptions in the information transmission.

Frequency, time, and code division multiplexing are known as basic multiple access methods, and all three methods are practically used in cellular networks. In particular, time division multiplexing methods, which are practiced as TDD (Time Division Duplex) or TDMA (Time Division Multiple Access), have found widespread use in digital mobile radio networks, often using frequency division multiplexing methods

(FDMA = Frequency Division Multiple Access) are combined in such a way that several carrier frequencies are provided. According to the GSM standard, carrier frequencies with a spacing of 200 kHz from one another are provided, and a division into eight time slots is prescribed on each carrier frequency.

Time division multiplexing techniques are also increasingly being used for cordless telephones, a combination of FDMA and TDMA also being provided here. With the European DECT standard Up to ten frequency channels are used for cordless phones, each of which is divided into twelve time slots for uplink and downlink.

The receivers in TDD or TDMA systems can be constructed according to the direct conversion principle. In such receivers, adjacent channel interference as a result of quadratic components which arise in the analog part of the receiver, in particular in the receiving mixer, results in rectangular interference which, in the case of the usual digital modulation, brings about an increase in the bit error rate. In order to meet the high requirements regarding the bit error rate specified in the system protocols, for example the GSM system, very high demands have to be made on the transmission properties of the mixer, which cannot currently be met with established circuit technologies.

It has therefore been proposed to subsequently correct the disturbances mentioned by means of iterative, computational determinations of the change in the signal amplitude (offset) associated with them by means of a correction device connected downstream of the mixer. However, this is associated with a high computing effort and correspondingly increased power consumption and therefore reduces the possible maximum call duration, which is an essential performance parameter of every mobile phone.

The invention is therefore based on the object of specifying an improved, computational and energy-saving method of the generic type and an apparatus for carrying out this method.

This object is achieved in terms of its method aspect by a method with the features of claim 1 and in terms of its device aspect by a device with the features of claim 8. The invention includes the essential idea of first determining the time of the onset of a malfunction and then using this knowledge to carry out a correction process in a manner which is less complex and thus energy-saving with regard to the calculation processes. By using an analog correction method you can do almost no computation (in the narrower sense).

In a preferred embodiment, a differentiation of the overall signal and then a threshold value discrimination of the first derivative with a predetermined threshold value are first carried out to determine the time of the onset of the disturbance. This threshold value corresponds to the maximum possible steepness of the useful signal, in order to avoid false detections multiplied by an appropriately chosen safety factor. Both the formation of the first derivative of the total signal and its comparison with the threshold value mentioned represent routine operations that are carried out in integrated circuit technology or by means of suitable software - as part of the complex digital signal processing (DSP = digital signal processing) that is implemented anyway, and with less Power consumption can be realized.

Alternatively, however, it is also possible to provide a special detector element (for example a detector diode) or a circuit of a corresponding function (for example a so-called RSSI circuit) to determine the point in time at which a disturbance sets in, which uses in particular the frequency offset of the interferer with respect to the carrier signal ,

The actual correction process carried out using the information about the time of use of the disturbance can be designed as a calculation process based on digitized values of the signal amplitude or alternatively also as an analog "subtraction". In the former case, a (digital) averaging of the overall signal takes place in the period from the rising edge of the useful signal to the point in time of the fault on the one hand and in the period from the point in time of the fault to the falling edge of the useful signal on the other and then a subtraction of the two calculated mean values. It is also possible to determine the interfering signal energy when the interferer is detected and to subtract it from the total signal energy from the time of the interruption. In this latter case, too, a subtraction in the sense of a real calculation is possible after the corresponding energy values have been digitized.

In a device for carrying out the method according to the invention - in accordance with the characterization of the method according to the invention given above - detection means are provided for determining the point in time of the fault and correction means connected on the input side with these detection means for carrying out the offset correction. Execution options for the recording means have already been mentioned above. If a detector element or a detector circuit is used, this is followed in particular by a correction stage to take the detector characteristic curve into account for the output signal shape and / or a pulse shaper stage. The actual correction means can have an analog subtraction device in an analog design; in the case of a digital calculation of the offset or correction amount, a reference value memory and a digital subtraction stage and in particular an average value calculation stage upstream of these units are provided.

Advantages and expediencies of the invention result from the subclaims or the following description of preferred exemplary embodiments with reference to the figures. Of these show: 1a and 1b show examples of the time dependence of the useful signal and interference signal or an overall signal of a real mixer in a direct conversion receiver,

2 shows a schematic illustration of a first embodiment in the form of a block diagram,

3 shows a schematic illustration of a second embodiment in the form of a block diagram,

4 shows a schematic illustration of a third embodiment in the form of a block diagram and

Fig. 5 is a schematic representation of a fourth embodiment in the form of a block diagram.

FIG. 1 a shows separately the time profile of a useful signal with a carrier frequency fl and a level P1 and an interference signal with a carrier frequency f2 and a level P2, as would occur in an ideal receive mixer. With t _A is the start time (the rising edge) of a useful signal pulse and with t _E the end time (falling edge), while with to the end time the - in this example before

Rising edge of the useful signal pulse starting - interference pulse is designated.

A corresponding output signal of a real receive mixer as a function of time is shown in FIG. 1b, the same time axis as in FIG.

2 shows an arrangement 100 comprising a mixer 101, an amplifier stage 102 connected downstream, a low-pass filter 103 connected downstream, and a downstream CO col \) M P> P ¹

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Detector characteristic corrected) noise signal realized by the overall signal and the difference signal finally amplified.

The arrangement shown last has the particular advantage of increasing the dynamics, since the interference signal is already corrected in the baseband.

The implementation of the invention is not limited to the examples described here, but is also possible in a large number of modifications which the person skilled in the art will readily appreciate in view of the above explanations.

Claims

claims 1. A method for correcting a useful signal falsification in the receiving section (100; 200; 300; 400) of a message transmission system which operates, in particular according to the TDD or TDMA method, by an adjacent channel interference which generates an in particular rectangular interference pulse in the useful channel, characterized in that in a first Step the start or end time of the adjacent channel interference is determined and in a second step an offset correction is carried out using the information about the start or end time. 2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that in order to determine the start or end time of the adjacent channel interference, a differentiation (104) of the overall signal and in a second sub-step a threshold value discrimination (105) of the first derivative is carried out with a predetermined threshold value. 3. The method according to claim 1, which also means that the determination of the start or end time of the adjacent channel interference is carried out by means of a detector element or a detector circuit (204; 304; 403), in particular using the frequency offset of the interference. 4. The method according to any one of the preceding claims, that the offset correction comprises digital averaging of the overall signal up to or from the start or end time of the fault and a subsequent subtraction (306) of the calculated averages. 5. The method according to claim 2 or 3, characterized in that upon detection of the interferer, the interference signal energy is recorded and then subtracted from the total signal energy from the start time or until the end time of the interference. 6. The method of claim 5, d a d u r c h g e k e n n z e i c h n e t that the subtraction as a computational subtraction (306) after a Digitization of the signal energy values is carried out. 7. The method of claim 5, d a d u r c h g e k e n n z e i c h n e t that the subtraction in an analogous manner, in particular by means of a Operational amplifier arrangement (405a, 405b). 8. The device (100; 200; 300; 400) for performing the method according to one of the preceding claims, characterized by Detection means for determining the start or end time of the adjacent channel disturbance and correction means (106; 207; 306; 405a, 405b) connected on the input side to carry out the offset correction. 9. The device as claimed in claim 8, so that the detection means have a differentiating stage (104) and a threshold discriminator (105) connected downstream thereof. 10. The method according to claim 8, characterized in that the detection means have a detector element (204; 304; 403), in particular an RF detector diode, and a correction and / or pulse shaping stage (205; 307; 404) connected downstream thereof. 11. Device according to one of claims 8 to 10, that the correction means have an A / D converter (305a; 305b), a reference value memory and a digital subtraction stage (306, 308). 12. The apparatus of claim 11, d a d u r c h g e k e n n z e i c h n e t that the reference value memory and the digital subtraction stage is preceded by an averaging stage. 13. Device according to one of claims 8 to 10, that the correction means have an analog subtraction device, in particular an operational amplifier arrangement (405a, 405b).