WO2002019542A2

WO2002019542A2 - Method and device for pre-equalizing radio channels

Info

Publication number: WO2002019542A2
Application number: PCT/DE2001/003145
Authority: WO
Inventors: Frank Kowalewski
Original assignee: Siemens Aktiengesellschaft
Priority date: 2000-08-28
Filing date: 2001-08-17
Publication date: 2002-03-07
Also published as: WO2002019542A3; DE10042203A1

Abstract

In order to transmit data/messages between at least one base station (BS1) and at least one mobile station (MS1) of a radio communications system, the signal-to-noise ratio (SN1) of at least one test signal, which is transmitted from the respective mobile station (MS1) to the assigned base station (BS1), is determined inside the base station in the currently available radio channel (FK) between the base station (BS1) and the respectively assigned mobile station (MS1). Based on this measured signal-to-noise ratio (SN1) in the current radio channel (FK), at least one equalizing parameter (μopt1) for pre-equalizing the radio channel during the transmission of signals from the base station to the mobile station is selected from a multitude of equalizing parameters (μ) provided. This equalizing parameter is selected in such a manner that the detection error rate (BER) with this measured signal-to-noise ratio (SN1) is minimal. The multitude of equalizing parameters (μ) is assigned to different signal-to-noise ratios (SN1 with SNk) and to different detection error rates (BER), and is established in at least one pretest and provided for evaluation.

Description

Method and device for pre-equalization of radio channels

Code Division Multiple Access (CDMA) enables multiple data streams to be transmitted simultaneously over a common frequency band. The symbols of the data streams to be transmitted are modulated with so-called spreading codes. The data streams transmitted simultaneously with different codes generally interfere. mutual: multipath propagation leads to the superimposition of successively transmitted data symbols (inter symbol interference = ISI).

CDMA coding and multipath propagation are the cause of multiple access interference (MAI). ISI and MAI can be eliminated, namely:

- in the receiver through joint detection (JD),

- in the transmitter by means of joint pre-equalization (JP).

From an article by A. Klein, G.K. Kaleh and P.W. Baier: "Zero Forcing and Minimum Mean Square Error Equalization for

Multiuser Detection in Code-Division Muliple-Access Channels ", IEEE Trans. Vehic. Tech., Vol. 45 (1996), 276-287, oint detection methods are already known, the so-called inter-symbol interferences (ISI) between data symbols of a user and multiple access interference (MAI), ie interference from other users, in a receiver of radio data. All interference with radio channel transmission at the receiver is thus largely taken into account. When using such methods in mobile telephone systems or mobile radio systems, the individual mobile stations are very complex because this procedure places high technical demands on the receiver.

According to BR Vojic and WM Jang: "Transmitter Precoding in Synchronous Multiuser Communications", IEEE Trans. Comm, Vol. 46 (1998), pp. 1346-1355, there is only a pre-equalization algorithm that provides the desired one for one-path channels Transmission power taken into account. This algorithm delivers less error-prone detection results than other algorithms, but in practice it is not usable with multipath propagation - as is the rule in cellular mobile radio networks.

The object of the invention is to transmit spread-coded signals in such a pre-equalized manner that interfering interferences are largely avoided when these signals are received in the respective receiver. In particular, both intersymbol and multi-user interference are to be eliminated.

This object is solved by the features of claim 1.

The method according to the invention and the device according to the invention has the advantage that all interference that can occur due to radio transmission are taken into account at the transmitter. The recipient of the data can therefore be interpreted particularly simply.

This is particularly advantageous for the transmission of data from one base station to a plurality of mobile stations. A method or a device can then be used for the retransmission (uplink = from the respective mobile station to the assigned base station), which largely takes into account all interference on the part of the receiver, so that the individual mobile stations of a mobile telephone system can be designed in a particularly simple manner. The method according to the invention and the device according to the invention can also be used for data transmission from mobile stations to base stations. The transmission quality or the channel impulse response is measured particularly simply in the base station and can be distributed from there if necessary.

Other developments of the invention are given in the subclaims.

The invention and its developments are explained in more detail below with reference to drawings.

Show it: 1 shows a schematic representation of a radio cell of a mobile radio system, FIG. 2 shows a conventional mobile radio system according to the prior art,

3 shows the data transmission according to the invention from a base station to a mobile station, FIG. 4 shows the data transmission according to the invention from a mobile station to a base station, FIG. 5 shows a pre-equalizer for a base station, which carries out a pre-equalization of the signals to be transmitted according to the invention, FIG -TDD operation with pre-equalization between a base station and a mobile station to be operated,

7 shows the time distribution of a burst signal for channel estimation and pre-equalization according to the method according to the invention, and FIG. 8 shows a table with simulated detection error rates, which were obtained in a preliminary experiment depending on the signal / noise ratio on the air interface of the mobile radio system and an equalization parameter, and can be used to optimize the radio channel equalization according to the invention.

Elements with the same function and mode of operation are provided with the same reference numerals in FIGS. 1 with 8.

1 shows an example of one of the radio cells of a cellular mobile radio system, in particular mobile radio telephones, with a base station BS1 and a plurality of mobile stations MSI, MS2, MS3. In this radio system, data is preferably exchanged only between the base station BS1 and the mobile stations. The exchange of data between the base station BS1 and the respective mobile station takes place by radio transmission. The radio transmission from the base station BSl to a mobile station such as MSI is referred to as a downlink, the data transmission from a mobile station such as MSI to the base station BSl is referred to as uplink. In such a mobile radio system with a multiplicity of radio cells, it is expediently determined how the signals are modulated for the different mobile stations so that they can be separately detected in the receivers of the different mobile stations. The system according to FIG. 1 is preferably a so-called CDMA system (Code division multiple access), as is the case, for example, in the UMTS standard (Universal mobile telecommunications system). In this a common frequency band is available for data transmission, the individual data channels between the respective base station such as BS1 and the respective mobile station such as MSI differing in terms of a code with which the signal for the corresponding mobile station is spread. This spreading, each with a specifically assigned code, means that each signal that is to be exchanged between the base station BS1 and a specific mobile station, such as, for example, MSI, is distributed over the entire available spectrum of frequencies. Each individual information bit to be transmitted is broken down into a large number of small “chips”. As a result, the energy of a bit is distributed over the entire predetermined frequency spectrum that is available to the CDMA system.

A conventional radio system is explained in more detail in FIG. 2 using a downlink transmission. FIG. 2 shows a base station BS1 and a mobile station MSI, each of which has an antenna ATBL, ATM1. The two stations exchange data here via a downlink radio channel FK. The base station BS1 has a modulator MODB1, which prepares the data streams from data sources DQ for transmission via the radio channel FK. For this, the modulator MODB1 still requires code information which is provided by a code generator CGB1. Examples are shown in FIG. 2 three arrows from the code generator CGB1 to the modulator MODB1 are shown, which represent three different data streams or three different code information. In the real system, a much larger number of data streams and code information are processed simultaneously.

The modulator MOD generates a transmission signal from the data streams and the code information, which is sent to all mobile stations in the radio cell of the base station BS1. Only the receiving mobile station MSI is shown as an example in FIG. In the case of a single mobile station with a single data stream, an associated code information would be required in the base station BS1. However, the base station BS1 generally sends several radio channels FK to several mobile stations, e.g. MSI, MS2, MS3, the respective data of which are modulated with different codes, in particular multiplied. For reasons of simplicity, the other mobile stations are not shown in FIG. 2.

When transmitting over the radio channel FK, there is now a

Variety of disorders. A first interference is referred to as ISI (intersymbol interference) and therefore results in the fact that a transmitted radio signal can reach the receiver via several different paths, the arrival times at the receiver being slightly different. It is therefore a disturbance that arises in the radio channel concerned by the fact that signals emitted earlier in time interfere with currently received signals (hence: inter-symbol interference). Another disruption is caused by the fact that several data streams are transmitted at the same time, which differ only in terms of the code. This interference occurs when the base station such as BS1 is in radio contact with several mobile stations such as MSI, MS2, MS3 at the same time, which is the rule in modern mobile radio systems such as UMTS. It is therefore a disturbance that emanates from the signals of different users and which is therefore also referred to as MAI (multiple access interference).

FIG. 2 shows the receiving part of a mobile station MSI, which is intended to receive downlink data via the radio channel FK. For this purpose, a demodulator DMODML is provided, which processes the radio signals received via the antenna ATM1 of the mobile station MSI. The demodulator DMODMl processes the received signals in order to generate a data stream for a data user DEC. If the transmitted data e.g. Represent voice information, the user DECum is a voice decoder, while other data is, for example, a computer or fax machine. As a rule, mobile stations have only a single data user DEC and thus only a single data stream. In the case of completely undisturbed transmission over the radio channel FK, the demodulator DMODMl only needed to know the code information of the data to be detected for the user DEC for demodulation. However, due to the disturbances described above, this is not sufficient. In order to take ISI into account, a channel estimator CE is required, which contains information about the transmission properties, i.e. the channel impulse response of the radio channel FK for the relevant mobile station, e.g. MSI provides. To compensate for MAI, the mobile station MSI expediently knows all the codes used in the base station. For this purpose, a code generator CGM1 is provided which, in addition to the code information of the data to be detected here, provides code information about all codes used in the system. This method is also referred to as "joint detection". The mobile stations, which are designed in this way to receive data from the respective base station, are relatively complex.

The method according to the invention and the device according to the invention will now be explained in more detail with reference to FIG. 3, in which the downlink transmission from a base station such as BS1 to a mobile station such as MSI is also shown. In FIG. 3, the base station BS1 also has a modulator MODB1, which generates the transmission signals for an antenna ATBl. The modulator MODB1 receives several data streams from data sources DQ, which are spread with code information from a code generator CGB1. In addition, a channel estimator CEBL is provided, which provides information about the transmission properties of all radio channels FK. The modulator MODBL generates a transmission signal that takes into account both the ISI and the MAI. The transmission signal is designed such that each of the mobile stations receives a largely interference-free signal when it is received (as far as this is possible). Both the interference caused by the simultaneous use of several codes and the interference caused by the transmission properties of the individual radio channels are taken into account. The receiver of the data, namely the mobile station MSI, is then correspondingly simple in FIG. 3. This has a demodulator DMODM1, which receives the signal from the antenna ATM1 of the mobile station. The code information for the relevant data stream is also made available to this demodulator DMODM1 by a code generator CGM1, from which the demodulator DMODM1 then generates the data stream for the data user. The mobile stations here are therefore particularly simple.

It was shown in FIG. 3 that all downtimes in the radio channel in the transmitting station, ie in downlink transmission in the base station, are advantageously taken into account in the downlink transmission. The downlink part of the MSI mobile station can therefore have a particularly simple structure. In order to keep the mobile station MSI simple for the uplink path, that is to say for sending data from the mobile station MSI to the base station BS1, the method corresponding to FIG. 2 could be used for this transmission, taking into account the ISI and MAI in the receiving station, that is, here in the base station. This makes a system possible in which the mobile stations are simply constructed, since the ISI and MAI are only taken into account in the base station. In a corresponding TDD system (time division duplex), especially in UMTS, it is also very easy to obtain the channel transmission properties from the channel estimator in the base station by evaluating the properties of the respective transmission channels by evaluating the received uplink data in the base station can be determined. Furthermore, the channel impulse response or channel quality can also be transmitted from the mobile station to the base station by means of a data telegram.

The method according to the invention can also be used to send data from the mobile station MSI to the base station BS1. This is shown in Figure 4. The mobile station MSI is shown here in the uplink, ie with the modulator MODML, which processes a data stream from a data source DQ. In order to take into account the transmission properties of all radio channels FK and codes used in the system, a code generator CGM1 is provided, which transfers the code information of all codes used in the system to the demodulator MODMl, and a channel estimator CEM1, which supplies the transmission properties of all radio channels. The information about the transmission properties of all channels could be made available to the mobile station MSI by the base station BS1. In the modulator MODM1, the interference caused by multi-path transmission of the radio channel FK and by simultaneous transmission of several data streams are taken into account when generating the radio signal. The radio signal is sent to the base station BS1 via the antenna ATM1 and the radio link FK. The base station BSl not only receives the data of the mobile station MSI shown in FIG. 4, but also also the radio signals of other mobile stations not shown in FIG. 4 in the radio cell of the base station BSl. The demodulator DMODB1 of the base station BS1 accordingly receives all code information from the code generator CGBl and decodes several data streams for several data users DNBL. However, it is no longer necessary to provide a channel estimator for the decoding.

The method with which the transmission properties of all radio links (ISI) and the codes of all radio links (MAI) are taken into account is described below in particular using mathematical formulas. These formulas can either be implemented by means of a corresponding program or corresponding hardware modules that implement these formulas.

The core of the invention is in particular a special algorithm for pre-equalization. Instead of forming a matrix A according to B.R. Vojic and W.M. Jang: "Transmitter Precoding in Synchronous Multiuser Communications",

IEEE Trans. Comm, Vol.46 (1998), pp. 1346-1355, the pre-equalization is carried out by means of a matrix M _λ = A ^H - (A - A ^H + Ä - E, E = standard matrix. This solution implicitly takes into account the desired transmission power. The algorithm is explained by an embodiment according to 5 with 8. The invention has the advantage over the pseudo inverse method of providing less error-prone detection results. Compared with the known algorithm according to BR Vojic and WM Jang: "Transmitter Precoding in Synchronous Multiuser Communications", IEEE Trans. Comm,

Vol.46 (1998), pp. 1346-1355, the method according to the invention has the particular advantage that it can also be used for multi-way channels. This is a prerequisite for sensible practical use in mobile radio systems. The algorithm presented here also differs in the case of one-way channels from that specified in BR Vojic and WM Jang: "Transmitter Precoding in Synchronous Multiuser Communications", IEEE Trans. Comm, Vol.46 (1998), pp. 1346-1355 Principle because it uses more degrees of freedom for pre-equalization, which enables better pre-equalization with lower detection error rates. The inventive step of the proposed method according to the invention consists in using the matrix M _λ = A "- (AA ^H + Ä-E) ^'1 instead of a pseudo inverse A ^fc of A.

Auεführungsbeispiel

UMTS TDD mode:

• CDMA system

Burst-wise transmission, burst contains reference signal for channel estimation (FIG. 7)

• TDD operation

Channel estimation in the reverse link, pre-equalization in the forward link

FIG. 5 shows the transmitting and receiving device for channel estimation in the reverse link and for sending the pre-equalized signals. Figure 6 shows the timing of the process.

The algorithm for calculating the pre-equalized transmission signals is described below. The description is in the baseband, that is, discrete. The data is transferred in blocks. Let d ^w = (d ^(K , ..., d ^w _M ), k = 1, ..., K be the vector of the M data symbols of the £ th user to be transmitted.

Codes c ^w = (c ^w ) \, ...... ,, Cc _n ^w . Q _Q ), k = 1, ..., K and the matrices

c ^{(k) T} = transposed vector α

the multiple access modulation can be written as Cd ¹

The signals are linearly pre-equalized after the modulation. The equalization is described by the matrix P:

P-Cd ¹

The pre-equalized signals are summed up to the transmission signal t:

t ^τ = D - P- Cd ^τ with

This sum signal is then transmitted to the receivers via multipath channels. With the impulse responses h ^(k) = (h ₁ ^k) , ..., h _w ^k) ), the additive noise n ^m = (n, ..., n ^(k) _M -Q ₊ W-0, k = l, ..., K of the different user transmission channels and the convolution matrices

the ^ th receiver of the system receives the signal

The matched filter receiver (= l-finker rake receiver) for the k-th user code c ^(k)

demodulates the received signal

M

-R (*) "„ (*) '

R> (*) = conjugate transposed matrix R (*)

With the summaries

n = (n ^w , ..., n ^(K) )

and the multiplication matrix D ^τ are obtained as the total vector of all demodulated signals:

d = R -HD ^τ -DPCd + R ^H -n The quadratic Euclidean deviation of the detected from the transmitted data is thus

d -d ² = \\ R ^H -H -D ^τ -M -d ^τ + R ^H -n ^τ -d ^{τ f} with

M = D-P-C

The mean value of this deviation over many bursts is given the secondary condition of given transmitter energy

Iπl = const. minimal for

M _λ = A ^H - AA "+ λ-Eγ, λ suitable A = R" -H -D ^T

This result can be derived using the Lagrangian method for optimization under secondary conditions.

The solution for M describes the improved algorithm. For λ-> 0 this solution is identical with the pseudo inverse solution M = A '. The algorithm can be optimized by a suitable choice of λ. FIG. 7 shows simulation results for the dependency of the detection error rate on λ, which were obtained in at least one preliminary test. The new algorithm (λopt) enables the detection error rate to be significantly improved compared to the pseudo inverse algorithm (λ = 0). The figure also shows that the optimal λ depends on the signal to interference ratio SN1 with SNk.

These reference measurements according to FIG. 7 are expediently stored and made available to the channel estimator in the base station for pre-equalization in downlink transmission. To summarize, data / message transmission between at least one base station, such as BS1 and at least one mobile station, such as MSI, of a radio communication system FK in the base station BSl pre-equalized the signals to be transmitted. For this purpose, the signal-to-noise ratio (SNl) of at least one test signal such as TS1 in FIG. 6, which is sent from the respective mobile station such as MSI to the assigned base station such as BSl, in the currently available radio channel between the base station and the respective one assigned mobile station in the base station. On the basis of this measured signal-to-noise ratio in the current radio channel, at least one equalization parameter such as λoptl in FIG. 8 for the pre-equalization of the radio channel when transmitting signals from the base station to the mobile station is selected from a large number of equalization parameters λ such that the detection Error rate BER becomes minimal with this measured signal / noise ratio. The equalization parameters λ are different signal

/ S / N ratios (SNl with SNk) and detection error rates BER have been assigned and determined in at least one preliminary test and made available for evaluation.

Claims

claims

1. A method for data / message transmission between at least one base station (BSl) and at least one mobile station (MSI) of a radio communication system, with the respective base station (BSl) pre-equalizing the signals to be transmitted, characterized in that the signal - / Noise ratio (SNl) of at least one test signal, which is sent from the respective mobile station (MSI) to the assigned base station (BSl), in the currently available radio channel (FK) between the base station (BSl) and the respectively assigned mobile station (MSI) in the base station determines that based on this measured signal-to-noise ratio (SNl) in the current radio channel (FK) at least one equalization parameter (λoptl) for the pre-equalization of the radio channel when transmitting signals from the base station to the mobile station from a large number of equalization parameters ( λ) is selected such that the detection error rate (BER) at this measured signal-to-noise ratio (SNl) is minimal, and that the plurality of equalization parameters (λ) are assigned to different signal-to-noise ratios (SNl with SNk) and detection error rates (BER) and are determined in at least one preliminary test and made available for evaluation has been.

2. The method according to claim 1, characterized in that for the transmission of data between a base station (BSl) and several mobile stations (MSI) via radio channels (FK), the data of different mobile stations are spread with different codes.

3rd Method according to one of the preceding claims, characterized in that that the signals to be transmitted are pre-equalized in a modulator, and that the transmission properties of all radio channels (FK) and all different codes are taken into account in the pre-equalization.

4. The method according to any one of the preceding claims, characterized in that data is transmitted from a base station (BSl) to a plurality of mobile stations (MSI).

5. The method according to any one of the preceding claims, characterized in that data is transmitted from a plurality of mobile stations (MSI) to a base station (BSl).

6. The method according to any one of the preceding claims, characterized in that the transmission properties of the radio channels (FK) from data transmissions from the mobile stations to the base station (BSl) are determined by the base stations.

7. Device for carrying out the method according to one of the preceding claims.

8, Device according to claim 7, characterized in that for the transmission of data via at least one radio channel at least one base station and several mobile stations are provided which communicate via radio channels (FK), the data of different mobile stations being spread with different codes become.

9. Device according to one of claims 7 or 8, characterized in that a modulator (MODBl), a code generator (CGBl), and a channel estimator (CEB1) are provided, and that the modulator (MODBl) predistortion based on the information of the code generator (CGBl) and the channel estimator (CEBl).

10. The device according to one of claims 7 to 9, characterized in that data is transmitted from a base station (BSll) to a plurality of mobile stations (MSI).

11. The device according to one of claims 7 to 10, characterized in that data are transmitted from a plurality of mobile stations (MSI) to a base station (BSl).