Method for detecting multipath signals

Abstract

The invention relates to a method for detecting multipath signals comprising the following steps: a total impulse response (10), composed essentially of multipath signals, is cross-correlated with a reference signal and a maximum (12) of the result of said cross-correlation is determined and saved.

Description

[0001] The invention relates to a method for detection of multipath signals, in particular for operation of a RAKE receiver, as claimed in patent claim 1, and to a RAKE receiver as claimed in the precharacterizing clause of patent claim 5.

[0002] Signals in general propagate over two or more paths in one mobile radio channel (multipath propagation). Each individual received signal has a different propagation time, amplitude and Doppler shift, corresponding to its transmission path. The various received signals are superimposed at the antenna of a receiver either constructively or destructively, and thus making it harder to detect the symbols transmitted by means of the signals.

[0003] In particular, multipath propagation of a radio signal such as this results in channel impulse responses over an extended time, which lead to a delay spread and thus to a shifted time reference between the individual symbols in the transmission signal. In mobile radio systems which operate using the Wideband Code Division Multiplex (W-CDMA) method, this means that special receivers (RAKE receivers) are required for processing the multipath signals.

[0004] A RAKE receiver such as this allows a considerable gain as a result of the use of multipath signals which arrive with different propagation time delays at a receiving antenna of a mobile radio. This is done by the signal coming from the antenna being processed in the receiver in two or more paths, the so-called “fingers” of the RAKE receiver. Each of these fingers is set, in each case with an optimized phase angle of the pseudo-noise sequences which have been used for coding of a signal to be transmitted via the mobile radio channel, to in each case one component of the received multipath signals.

[0005] In an IS-95 mobile radio system, a RAKE receiver in a mobile station has at least three such fingers, and a base station has at least four such fingers. Furthermore, both the mobile station and the base station each have at least one “search finger”, which continuously looks for stronger multipath signals. As soon as the search finger finds a stronger multipath signal, the finger on the previously weakest multipath signal is optimally set to the new, stronger multipath signal. Typically, this makes it possible to demodulate up to three of the strongest multipath signals in the mobile station and up to four of the strongest multipath signals in the base station, with a time delay of at least 0.8 to 1 μs, and to combine them by means of maximum ratio combining.

[0006] In this case, it is particularly important to detect the time reference between the individual multipath signals exactly, that is to say to define the optimum “finger position”. A specially tuned filter, a so-called matched filter, thus uses a sequence or a pilot sequence which is known in the receiver to produce the overall impulse response or channel impulse response of a received signal, on the basis of which the position of the individual fingers can be determined. However, the overall impulse response does not always allow detection of all the relevant finger positions. Particularly when adjacent fingers differ, for example, only by a very short delay time of less than the chip duration, reliable detection of the position and of the number of the fingers is difficult.

[0007] Known methods for detection of the finger positions are, however, highly computation intensive since they have a complexity of O(N), where N is the length of the overall impulse response.

[0008] The object of the present invention is thus to propose a method for detection of multipath signals and a corresponding RAKE receiver whose data processing is less complex than the known detection methods, thus allowing fundamentally shorter adjustment times and a reduced processor capacity requirement.

[0009] This object is achieved by a method having the features of patent claim 1 and by a RAKE receiver having the features of patent claim 5. Preferred refinements of the method can be found in the dependent patent claims.

[0010] The idea on which the invention is based is to use the overall impulse response to determine peaks or maxima successively, by means of cross-correlation. Once a first maximum has been determined, the overall impulse response is reduced by a reference signal, which was cross-correlated with the overall impulse response in order to determine the first maximum. This means that maxima which indicate the possible position of fingers are determined step-by-step from the overall impulse response by means of the reference signal.

[0011] The method aspect of the invention accordingly relates to a method for detection of multipath signals, in particular for operation of a RAKE receiver, having the following steps:

[0012] a) cross-correlation of an overall impulse response with a reference signal,

[0013] b) determination and storage of a maximum of the result of the cross-correlation and of a proportionality factor α,

[0014] c) (if appropriate, repeated) reduction of the overall impulse response by the reference signal scaled by α,

[0015] d) carrying out steps a) to c) using the overall impulse response reduced by the scaled reference signal until a predetermined number of maxima have been obtained, or until the scaling factor α falls below a predetermined value.

[0016] This method allows signals contained in the overall impulse response to be identified even when they have only very short propagation time differences, that is to say they are very close to one another in time. Normally, short propagation time differences such as these between two signals mean that a pulse which is transmitted by the signals arrives at a receiver extended in time. However, this is highly disadvantageous for detection of pulses that are close to one another, for example in the case of chips in a signal transmitted by means of CDMA. Furthermore, the method according to the invention involves considerably fewer computation steps than the methods which are known from the prior art. While the first two iteration steps for determination or selection of the first two maxima have a complexity of O(N), all the further iteration steps now have only a complexity of O(E2), where E is the width of the reference signal and N is the length of the overall impulse response.

[0017] The method is preferably used in a RAKE receiver. As already mentioned, RAKE receivers are used in particular in code-division multiplex mobile radio systems. In principle, a RAKE receiver uses the propagation time differences between multipath signals by having two or more paths or fingers which are each set to the phase angle of one signal from the multipath signals. The signals which are optimized in the individual paths or fingers of the RAKE receiver are then combined to form an overall received signal. The method according to the invention allows the operation of a RAKE receiver to be improved considerably, in particular with regard to an overall impulse response for individual signal components which have only very short propagation time differences (approximately ¼ of a chip).

[0018] The reference signal which is used for the method is preferably a finger pattern which corresponds to a typical mobile radio channel. In this case, the expression a typical mobile radio channel means a mobile radio channel which has limited propagation time differences resulting from multipath propagation of transmitted signals, and which represents a good approximation of a large number of actual mobile radio channels.

[0019] The reference signal is preferably scaled such that a scaled reference signal essentially matches the cross-correlation maximum determined in step b) of the method. This considerably improves the efficiency of step c) of the method.

[0020] In one preferred embodiment of the invention, multiply and accumulate operations are essentially carried out while carrying out steps a) to d). These operations considerably improve the efficiency of the method in comparison to the methods which are known from the prior art, since a multiply and accumulate operation can be carried out with considerably less computation complexity than the operations of squaring, addition, subtraction and multiplication that are required for known methods.

[0021] Finally, the invention relates to a receiver for carrying out the method, which has two or more paths or fingers for processing of received multipath signals. According to the invention, the receiver has a cross-correlator for cross-correlation of an overall impulse response with a reference signal, a memory for the overall impulse response, and a scalar and subtractor in order to reduce the overall impulse response by the reference signal.

[0022] The cross-correlator, the memory and the scalar and subtractor are preferably in the form of program routines in a program for a signal processor for processing the overall impulse response, or are in the form of programmable or hard-wired logic, in particular in an ASIC. A signal processor has special functions and/or commands for signal processing and can thus carry out the abovementioned program routines very efficiently. Furthermore, a signal processor such as this can be operated at a very high clock frequency, in order to make it possible to process the high data rates that occur. Implementation in the form of logic has the advantage that this is very fast and can thus process even higher data rates.

[0023] Further advantages and application options of the invention will become evident from the following description of an exemplary embodiment in conjunction with the drawings, in which:

[0024] FIG. 1 uses signal profiles to show the processing of an overall impulse response of received multipath signals, on the basis of the method according to the invention,

[0025] FIG. 2 shows an example of the profile of a channel impulse response or overall impulse response of a mobile radio channel,

[0026] FIG. 3 shows the profile of a virtual channel impulse response or overall impulse response of the mobile radio channel, whose actual overall impulse response is illustrated in FIG. 2,

[0027] FIG. 4 shows a search pattern which is used as a reference signal,

[0028] FIG. 5 shows a further example of the profile of an overall impulse response for a mobile radio channel, which essentially has three individual pulses and a noise component,

[0029] FIG. 6 shows a flowchart of one exemplary embodiment of the method according to the invention, and

[0030] FIG. 7 shows a block diagram of one embodiment of the receiver according to the invention.

[0031] The basic procedure for the method according to the invention will be explained briefly in the following text using an exemplary embodiment and with reference to FIG. 1. Examples of the profiles of the overall impulse response and the search pattern and/or reference signal will then be explained with reference to FIGS. 2, 3, 4 and 5.

[0032] The diagram A in FIG. 1 shows the overall impulse response 10 of received multipath signals. The overall impulse response 10 essentially has three maxima 12, 14 and 16, which occur at different reception times. Using the method according to the invention, the overall impulse response 10 is cross-correlated with a reference signal (which is not illustrated). The result of the cross-correlation and the determination of the maximum and of the scaling factor result in the signal which is illustrated in the diagram B, which essentially has one maximum 18, which corresponds to the maximum 14 at the time τmax1 in the diagram A, likewise at the time τmax1.

[0033] In the next step, the overall impulse response 10 as illustrated in the diagram A is reduced by the scaled reference signal. The result of this subtraction is illustrated in the diagram C in FIG. 1. The signal 20 which is illustrated in the diagram C now has essentially two maxima 22 and 24. The maximum 22 at the time τmax2 will be determined from the signal 20 in a next step. The determination process is once again carried out by means of the cross-correlation process based on the method according to the invention. The result of the cross-correlation process is illustrated in the diagram D. This has a maximum 26 at the time τmax2, which corresponds to a second signal component in the overall impulse response.

[0034] The signal 20 which is illustrated in diagram C is once again reduced by the scaled reference signal, thus resulting in the signal 28 as illustrated in the diagram E, which essentially has only one maximum 30 at the time τmax3, this maximum corresponds to a third signal component in the overall impulse response 10. Finally, after cross-correlation of the signal 28 as illustrated in the diagram E by means of the reference signal and subsequent determination, the maximum 32 which is illustrated in the diagram F, is obtained at the time τmax 3.

[0035] In the next step, the signal 28 as illustrated in the diagram E is once again reduced by the scaled reference signal, thus resulting in the signal 34 as illustrated in the diagram G. This signal 34 now essentially comprises only noise components, so that the method according to the invention for detection of multipath signals ends at this point. Specifically, this means that the maxima 18, 26 and 32 which are illustrated in the diagrams B, D and F have been extracted on the basis of different propagation paths and propagation times in the mobile radio channel.

[0036] FIG. 2 shows an example of the profile of an overall impulse response 11 after passing through a matched filter in a RAKE receiver. The sample value i is plotted on the abscise, corresponding to a specific reception time. The overall impulse response comprises signal components from the various propagation paths in one mobile radio channel. By way of example, FIG. 5 shows how an overall impulse response 13 such as this is formed from superimposition of two or more individual pulses 15, 17, 19 and a noise component 21. The upper diagram in FIG. 5 shows the noise component 21 with “noise”. Furthermore, the figure shows three individual pulses 15, 17, 19 with finger 1, finger 2 and finger 3. The individual pulses each correspond to one propagation path and typically arrive at different times in a receiver. The overall impulse response 13, which is composed of the noise component 21 “noise” and the three individual pulses 15, 17, 19 finger 1 to finger 3, is shown in the lower diagram. The object of the receiver is now to use this overall impulse response to determine start times of a transmitted data block. These start points can be used for reception. The start points correspond to finger positions, that is to say in principle to the reception times of the individual fingers. A simple maximum search for determination of the finger positions in the overall impulse response is, however, highly inaccurate and leads to poor reception. In a corresponding manner to the procedure described with reference to FIG. 1, the overall impulse response is searched for a pattern which corresponds to the impulse response of a single propagation path in the mobile radio channel. As soon as this pattern has been found, it is subtracted from the overall impulse response. This procedure is repeated until no more significant maxima occur in the continually reduced overall impulse response.

[0037] FIG. 3 shows an example of the profile of a virtual overall impulse response. This virtual overall impulse response is composed of a search pattern or reference signal, whose magnitude can be scaled, and of an additional component γ. The search pattern is scaled using a scaling factor α. The component γ corresponds to a noise component.

[0038] FIG. 4 shows the search pattern contained in the virtual overall impulse response in FIG. 3. There is a main maximum in the search pattern at υmin, and a secondary maximum at υmax.

[0039] The flowchart which is illustrated in FIG. 6 shows the steps which are used according to the invention to extract the individual finger positions from an overall impulse response. In a first step S1, the impulse response received by a receiver from one mobile radio channel is subjected to matched filtering. A finger position counter is set to zero. In a second step S2, the overall impulse response is then cross-correlated with a reference function. The result of the cross-correlation is processed in a third step S3, which is used to find a maximum of the result of the cross-correlation and a proportionality factor α. This is followed by a question step A1, in which a check is carried out to determine whether the maximum is a finger. If it is found that the maximum is not a finger, the method is ended, since the “residual” overall impulse response now contains only noise components. Otherwise, the maximum is stored in a step S4 as a defined finger position, and the overall impulse response is reduced by the search pattern or reference signal that has been scaled using the proportionality factor α. The finger position counter is also implemented. The next question step A2 checks whether the value of the finger position counter has already exceeded a predetermined value. The predetermined value corresponds to a predetermined number of maxima or finger positions. This is because the method can be terminated after a specific number of maxima or finger positions. Typically, this number of maxima or finger positions corresponds to the “fingers” of a RAKE receiver. However, if the predetermined value has not yet been reached, the method is continued with the step S2.

[0040] The way in which the search pattern is identified in the overall impulse response will be explained briefly in the following text, using the least error squares method. Specifically, this means that the search pattern in the overall impulse response is in fact identified where the square of the error between the overall impulse response and a virtual overall impulse response (which has only one propagation path) is a minimum. This is essentially obtained by the cross-correlation. In other words, this means that there is a finger position at the point at which the cross-correlation of the overall impulse response with the search pattern or the reference signal is a maximum. For this case, the proportionality factor α has particular importance. This is because the overall impulse response is reduced by the search pattern or reference signal that has been scaled by α.

[0041] FIG. 7 shows the essential means in a receiver R for carrying out the method according to the invention.

[0042] In this sense, the receiver R comprises a cross-correlator 40, a matched filter 42 and a subtractor and scalar 44. A multiplexer 46 is also provided, whose output signal is supplied to an impulse response memory.

[0043] A data signal which has been transmitted via a mobile radio channel is supplied to the matched filter 42 and the receiver R. The matched filter 42 is used for equalization of the received data signal. At its output signal, the matched filter 42 produces the overall impulse response 54 of the mobile radio channel. The overall impulse response 54 is supplied to a first input of a multiplexer 46. The overall impulse response 54 can be supplied via the multiplexer 46 to an impulse response memory 48 which is used for temporary storage of the sample values of the overall impulse response.

[0044] The output signal from the impulse response memory 48 is supplied to the cross-correlator 40, and to the subtractor and scalar 44. The cross-correlator 40 cross-correlates the overall impulse response, as stored in the impulse response memory 48, with a reference signal 50 that has been supplied. The output signal from the cross-correlator 40 is a finger position signal 56 and is supplied to the subtractor and scalar 44, to which the reference signal 50 can also be supplied, via a switch. First of all, the subtractor and scalar 44 scales the reference signal 50 that has been supplied via the switch, corresponding to the finger position signal 56. It then reduces the supplied overall impulse response, as temporally stored in the impulse response memory 48, by the scaled reference signal 50. The subtraction output signal 52 is supplied as a second input signal to the multiplexer 46.

[0045] The RAKE receiver R operates as follows:

[0046] A signal which is received via an antenna (which is not illustrated) is subjected to the conventional signal preprocessing, and is supplied to the matched filter 42. The output signal from the matched filter 42 is the overall impulse response 54, and is temporally stored directly in the impulse response memory 48, via the multiplexer. To be more precise, the impulse response memory 48 stores sample values of the overall impulse response 54. The overall impulse response which has been temporally stored in the impulse response memory 48 is processed via the correlator 40 and the subtractor and scalar 44 using the method according to the invention.

[0047] After processing, the multiplexer 46 is switched to the second input, that is to say the subtraction output signal 52 is temporally stored in the impulse response memory 48 via the multiplexer 46. The overall impulse response which has been temporally stored in the impulse response memory 48 is now processed via a controller (which is not illustrated) using the method according to the invention until either the finger position signal 56 does not indicate any more new finger positions, or a predetermined number of finger positions have already been determined. This may be done, for example, simply by counting the finger positions which are signaled by the finger position signal 56.

[0048] List of reference symbols

[0049] 10 , 11, 13 Overall impulse response

[0050] 12 , 14, 16, 22, 24, 30 Maximum

[0051] 15 , 17, 19 Individual pulses

[0052] 18 , 26, 32 Detected maximum

[0053] 20 , 28 Multipath signals reduced by the reference signal

[0054] 21 Noise component

[0055] 34 Noise signals

[0056] 40 Cross-correlator

[0057] 42 Matched filter

[0058] 44 Subtractor and scalar

[0059] 46 Multiplexer/demultiplexer

[0060] 48 Impulse response memory

[0061] 50 Reference signal

[0062] 52 Subtraction output signal

[0063] 54 Overall impulse response

[0064] 56 Finger position signal

[0065] R RAKE receiver

Claims

1. A method for detection of multipath signals (10) which are received via a mobile radio channel, in particular for operation of a RAKE receiver having the following steps: a) cross-correlation of an overall impulse response (10) with a reference signal, b) determination and storage of a maximum (18) of the result of the cross-correlation and of a proportionality factor α, c) reduction of the overall impulse response (10) by the reference signal scaled by α, d) carrying out steps a) to c) using the overall impulse response (10, 20, 28) reduced by the scaled reference signal until a predetermined number of maxima (18, 26, 32) have been obtained, or until the scaling factor α falls below a predetermined value.

2. The method as claimed in claim 1, characterized in that the reference signal is a window-limited impulse response of one path of the mobile radio channel.

3. The method as claimed in claim 1 or 2, characterized in that the reference signal is scaled such that the scaled reference signal essentially matches the cross-correlation maximum determined in step b).

4. The method as claimed in one of the preceding claims, characterized in that multiply and accumulate operations are essentially carried out while carrying out steps a) to d).

5. A RAKE receiver for carrying out the method as claimed in one of the preceding claims, which has two or more paths or fingers for processing received multipath signals (10), characterized in that a cross-correlator (40) is provided for cross-correlation of the overall impulse response (10) with a reference signal (50), a memory (48) is provided for the overall impulse response, and a subtractor and scalar (44) are provided in order to decrease the overall impulse response by the scaled reference signal.

6. The RAKE receiver as claimed in claim 5, characterized in that control logic is also provided in order to control the cross-correlator (40) and the subtractor and scalar (44).

7. The RAKE receiver as claimed in claim 5 or 6, characterized in that the cross-correlator (40), the memory (48) and the subtractor and scalar (44) are in the form of program routines in a program for a signal processor for processing multipath signals, or are in the form of programmable or hard-wired logic, in particular in an ASIC.