EP1234355B1 - Method for calibrating an electronically phase-controlled group antenna in radio-communications systems - Google Patents

Description

Method for calibrating an electronically phased array Group antenna in radio communication systems.

The invention relates to a method for calibrating a electronically phased array using a common reference point for all reference signals in radio communication systems and an arrangement therefor.

By using electronically phased array antennas, so-called intelligent antennas, in radio communication systems, such as digital mobile radio systems, can be a directional selectivity despite multipath propagation a mobile radio channel for radio communication be exploited advantageous.

Intelligent antennas form by means of corresponding phase-correct Control of the individual antenna elements of the antenna array a directional characteristic. The beam shaping can therefore used to receive a message from a base station to a subscriber station targeted in their direction too transfer. As a result, on the one hand the sensitivity to Interference in the current radio cell of the base station On the other hand, co-channel interference can be reduced be reduced in adjacent radio cells. In addition, the range of a base station that grows a certain Mobile station supplied with radio resources, at the same Transmission power essential. In addition, due to the spatial separation physical channels within a reused by a base station powered radio cell and the so-called antenna lobes of the directional diagram be moved adaptively during movement of subscriber stations.

In order to achieve a desired beam shaping, the original transmission signal via several antenna elements, usually with different but defined phase angles, sent. The corresponding phase angle becomes for each antenna element by a digital signal processing (DSP = Digital Signal Processing).

Generally occur in the adjustment of the phase angle in the analog domain between digital-to-analog converters and Antenna elements unpredictable phase errors and time delays on. As a result, the transmission signals are not sent with the desired phase angles and the beam shaping is corrupted or even impossible. To this unfavorable Property of the analog range of beamforming counteract is a so-called antenna calibration necessary. The antenna calibration eliminates the influence the entire analog signal chain to those described above Error.

EP 0 881 704 A2 describes a CDMA communication system with a calibration. In this case, so-called "processing sections" for baseband signal generation and "transmit radio circuits" for their processing are arranged in a transmitter. With the help of codes distinguishable signals from the "transmit radio circuits" carrier frequency on the one hand via "transmit terminals" for emission to antennas and on the other hand wired via cable to an addition device. A summed signal formed there passes via another cable and via another "transmit terminal" to a so-called "transmit radio circuit characteristic measuring section", which divides the sum signal into individual signals and evaluates them. On the basis of this evaluation, it is then possible to carry out a calibration of the signal branches which each comprise a "transmit radio circuit" and a "processing section". However, since the output signals formed by the "transmit radio circuits" reach the adder even before the antenna is emitted, the antennas and their supply cables are not taken into account in the calibration.
In addition, the cables connected to the adder must be tuned very accurately to each other with respect to their phase errors, which causes an increased effort in tuning.

To use beam shaping, first the direction of the base station to the mobile station. The Direction is determined by the evaluation of different phase angles the Emptangssignals to each antenna element of the antenna array detected. That is why an antenna calibration in the base station not just for the downlink to the subscriber station (downlink), but also for the uplink from the subscriber station to the base station (Uplink) necessary.

In a TD-SCDMA system (Time Division-Synchronous Code Division Multiple Access System) using intelligent Antennas will be an extra for antenna calibration Antenna used, a so-called reference antenna. About the reference antenna is in the case of an uplink calibration a reference signal to all antenna elements of the Antenna arrays sent. At the individual antenna elements becomes electromagnetic due to the finite propagation velocity Waves depending on the distance to the reference antenna a certain delay time and a certain phase position expected. The difference between the expected setpoint and the actually measured actual value is determined and as Correction factor saved. The correction factor will then involved in the normal signal processing process, whereby the antenna is calibrated.

For the downlink calibration, the reference antenna at a certain time receives a reference signal from an antenna element of the antenna array and the correction factor is determined. To counteract the distortion of the measurement result due to other antenna elements of the antenna array, they must not transmit any signal at this time. Subsequently, the reference antenna receives a reference signal from a second antenna element of the antenna array at a second time, and the correction factor for this second antenna element is determined, etc. Thus, for the calibration of n antenna elements of the antenna array, n time slots must be supported in support of a TDMA (Time Division Multiple) method Access) are expended.

The error in the delay time is often only one Fraction of a chip (chip = CDMA code element). To such a small delay time in the signal processing too is oversampling of the Reception and transmission signal necessary. By oversampling However, the data rate to be transmitted is clear greater.

The present invention, however, has the task Basically, a calibration of smart antennas like that perform that time required for both the calibration of the uplink as well as for the calibration the downlink is significantly shortened. It should be a Correction of an analog error without the need of Calculation of a correction factor for each individual Antenna element and without oversampling from it accompanying high data rates. The Transmission capacity of the physical channels is intended by the antenna calibration to be made only to a small extent be charged.

According to the invention, all antenna elements of a smart Antenna in the downlink calibrated in one step. For this purpose, the individual antenna elements of the antenna array simultaneously distinguishable reference signals sent and after reception on one for all antenna signals common reference point separated again.

An advantageous embodiment provides a separation of the reference signals using a CDMA method (CDMA = Code Division Multiple Access), which is on a separation of signals is based on individual spreading codes. In a further embodiment, the separation of the reference signals conventional spreading code techniques, such as correlation, used where the common reference point is on the respective reference code channel of the antenna elements synchronized and the reference signals back to their original bandwidth be reduced.

In this case, the reference signals after another Embodiment coded orthogonally, so the interference remain minimal despite simultaneous transmission.

The calibration factor may be from the result of the correlation be obtained in a digital signal processor.

Another advantageous embodiment of the invention is It is to use an optimized reference signal set, the an unbiased estimate of the calibration factor allowed.

The generation of such an optimized reference signal quantity and the estimated value can be calculated by methods in: Bernd Steiner, Paul Walter Baier: Low cost channel estimation in the uplink receiver of CDMA mobile radio systems ", frequency 47 (1993), pp. 292-298 are.

According to another embodiment, the correction of delay time, Phase error and / or amplitude of the transmission signals immediately within a digital UP conversion / down conversion be made, whereby no correction factor must be included and no oversampling of the Receive and transmit signal is necessary to delay error to eliminate.

This is done tuning (tuning) of the numerically controlled Oscillator (NCO) of the digital UP converter (DUC) and of the digital down converter (DDC).

In a further development of the invention is in a TDD system the calibration in the non-transmission delay time between the uplink and downlink time slots.

The downlink calibration can in a further embodiment too Start of delay time and uplink calibration on End of the delay time take place.

In another embodiment, as a common reference point for the reference signals from and to the antenna elements a Reference antenna used.

Fig. 1 schematisch ein Funk-Kommunikationssystem unter Verwendung von intelligenten Antennen,

Fig. 2 schematisch den Signalfluß bei einer Uplink-Synchronisation einer zu kalibrierenden intelligenten Antenne,

Fig. 3 schematisch den Signalfluß bei einer Downlink-Synchronisation einer zu kalibrierenden intelligenten Antenne, und

Fig. 4 schematisch die Signalisierung für eine Antennenkalibrierung in einem Verzögerungsintervall zwischen Uplink und Downlink im TDD-Modus.

The invention will be explained in more detail with reference to an embodiment. In the accompanying drawing shows:

1 schematically shows a radio communication system using intelligent antennas,

2 shows schematically the signal flow during an uplink synchronization of an intelligent antenna to be calibrated,

Fig. 3 shows schematically the signal flow in a downlink synchronization of an intelligent antenna to be calibrated, and

4 schematically shows the signaling for an antenna calibration in a delay interval between uplink and downlink in the TDD mode.

Fig. 1 shows a base station BS, which in the area of their supplied Radio cell Z with, by way of example, three mobile stations MS simultaneously connected. For an undisturbed Connection from and to the mobile stations MS is a channel separation according to a time division duplex method TDD (Time Division Duplex). For the separation of the compounds between the individual mobile stations MS can be exemplary the hybrid multiple access method TD-SCDMA (Time Division-Synchron Code Division Multiple Access), an expression of TD-CDMA (Time Division Code Division Multiple access). TD-CDMA is a combination the multiple access component TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access) and is determined by the degrees of freedom frequency, time slot and code characterized. TD-SCDMA differs from TD-CDMA by using a high-precision synchronization the received signals in the uplink. This will be the orthogonality the reception signals largely maintained, and thereby again, the detection properties are improved.

Prerequisite for a TD-SCDMA system or equivalent Radio communication system with intelligent antennas are antennas, with which a directional selectivity of a Base station BS emitted transmission signals leaves. Smart antennas can be used electronically generate pivotable, strongly focusing propagation diagrams. This intelligent antennas reduce the angle of incidence for environmental detours of the transmission signals the mobile stations, reducing the interference. From the same base station BS thus different antenna lobes, which pivoted in different directions are at the same time the same frequency channel within one Use cell Z In addition, it increases with the same transmission power the range of a base station BS.

In Fig. 1, the smart antenna detects the base station BS the directions from which the mobile stations transmit MS and forms in their direction corresponding antenna lobes.

In Fig. 2 is a schematic of the signal flow in an uplink calibration represented by a smart group antenna, consisting of several antenna elements AE1 to AEN and one Reference antenna AR for calibration. The arrows illustrate the different duration of a reference signal from a reference antenna AR to the antenna elements AE1 to AEN. The recorded by each antenna element AE1 to AEN and optionally amplified reference signals digitized parallel to each other in analog-to-digital converters A / D. The digitized values then become treated in parallel in a digital down-converter DDC. Out the measurement signals obtained in this way can, for example in a digital signal processor DSP correction factors determined and the correction values as control information to the digital down converter DDC of the individual antenna elements AE1 to AEN. In addition, will the reference signals from the signal processor DSP via a digital up-converter DUC and a digital-to-analog converter D / A sent to the reference antenna AR, this for calibration to the antenna elements AE1 to AEN sends etc.

In Fig. 3 is a schematic of the signal flow in a downlink calibration presented a smart group antenna. The antenna elements AE1 to AEN simultaneously transmit one each Reference signal to the reference antenna AR, these with different Reference signal transit time is received. The reference antenna AR possibly amplifies the reference signals and put them back in an analog-to-digital converter A / D digital signals around. Subsequently, the digitized Signals handled in a digital down converter DDC and the measurement signals obtained in this way the digital signal processor DSP supplied. In the signal processor DSP are off the measurement results, for example, correction factors determined and as control information to the digital UP converter DUC of the antenna elements AE1 to AEN given. In addition, will the digital UP converters DUC reference signals 1 to N for emission by the antenna elements AE1 to AEN fed.

The following is a calculation example for a TD-SCDMA system using a smart antenna with 8 antenna elements, a reference antenna and a length of the CDMA code elements (Chip) selected from 0.75μs.

The determination of the calibration factor is analogous to off the mobile radio technique known channel estimation method. It will the time delay and the phase angle of the received Reference signals determined. Because of the delay error in comparison to the delay setpoint is very low, are for For example, each antenna element has three measurements of channel impulse responses sufficient in the time available. This is the signal length for the calibration all antenna elements of a smart antenna in the downlink: (8 + 1) antenna elements * 3 measurements * 0.75μs chip length = 20.25μs.

The antenna calibration, that is the correction of the influence of the analog error on the entire signal chain the directional characteristic of the intelligent array antenna, is carried out directly by digital means. It is not Oversampling the receive and transmit signal necessary to eliminate delay errors.

In modern base stations will be digital UP conversion and Down-conversion used to address problems due to IQ phase errors and IQ amplitude offsets. The correction of Delay time and phase of the transmission signals can be direct by tuning the numerically controlled oscillator NCO (Numarical Controled Oscillators) of the digital UP converter (DUC) and the digital down converter (DDC), without a correction factor in digital signal processing must be included in the DSP.

Digital up-converter DUC and digital down-converter DDC enable also tuning the amplitude of the transmission signals, because a faulty amplitude is the radiation shaping also affected.

Due to the high data rates between the calibration instance and DUC / DDC is the disadvantage of additional control information signaling negligible to DUC and DDC small.

From Fig. 4 it can be seen that in a TDD system, such as TD-SCDMA, between uplink and downlink a delay time a certain length to meet maturity differences the signals and data to be transmitted provided is. Preferably, the calibration measurements are found in this delay time, because at this time no other signals can influence the measurements. The Downlink calibration is preferably at the beginning of the delay time and the uplink calibration performed at the end of this. In the same way, for example, a for Communication links provided time slot TS for the described calibration procedure are reserved.

The frequency of the antenna calibration is freely selectable and can be dynamically adapted to the transmission requirements. For example, a calibration in the downlink and uplink in each delay time between downlink and uplink TDMA frames or a calibration is done with a Of this multiple time interval made. Also, the Frequency of a downlink calibration from the frequency of one Uplink calibration deviate, for example, if the part the base station is determined that a mobile station only insignificantly or not at all during a communication connection, for example for voice transmission, moved for data transport or for a multimedia transmission.

Claims (12)

1. Method for calibrating an electronically phase-controlled group antenna having n antenna elements (AE1 to AEN) in a radio communications system, measuring signals related to a reference point being formed for error correction from at least one reference signal to be transmitted,
      characterized

   in that, in the case of an antenna calibration in the upward direction (UL), a reference signal transmitted by a shared reference antenna (AR) is received by every nth antenna element (AE1 to AEN) and passes in each case as the nth measuring signal (meas. sign.) to a device for error correction (DSP), which ascertains from every nth measuring signal (meas. sign.), having a signal transit time, error correction values which are assigned to the nth antenna element and are related to the reference point,

   in that, in the case of an antenna calibration in the downward direction (DL), every nth antenna element is used to transmit a reference signal which can be assigned to it and these n reference signals, which have been formed at the same time as one another, are received in superposed form at the reference antenna (AR) and pass as a shared measuring signal to the device for error correction, which ascertains from the shared measuring signal, which has signal components which have a respective signal transit time and can be assigned to the n antenna elements, in each case error correction values which are assigned to the nth antenna element and are related to the reference point.
2. Method according to Claim 1, characterized in that a coding and decoding of the reference signals is performed on the basis of a CDMA method.
3. Method according to Claim 2, characterized in that a correlation method is used for the synchronization of the reference point (AR) to the reference code channel of the antenna elements (AE1 to AEN).
4. Method according to one of the preceding claims, characterized in that the reference signals are orthogonally coded.
5. Method according to one of the preceding claims, characterized in that a correction of an analogue error in time delay, phase and/or amplitude is performed digitally.
6. Method according to Claim 5, characterized in that the correction is performed within a digital up-conversion or digital down-conversion.
7. Method according to Claim 5 or 6, characterized in that a calibration factor is obtained from the result of a correlation in a digital signal processor (DSP).
8. Method according to one of Claims 5 to 7, characterized in that an optimized amount of signals for an estimate of the calibration factor is used.
9. Method according to one of the preceding claims, characterized in that, in the case of time division duplex operation (TDD), the calibration is carried out within a delay time between the upward direction (UL) and the downward direction (DL).
10. Method according to Claim 9, characterized in that the reference signals for the calibration in the downward direction (DL) are sent at the beginning of the delay time.
11. Method according to Claim 9, characterized in that the reference signals for the calibration in the upward direction (UL) are sent at the end of the delay time.
12. Method according to one of Claims 1 to 8, characterized in that the reference signals for calibration in the upward direction (UL) and/or in the downward direction (DL) are sent in a respective time slot (TS).

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