WO2000038342A2

WO2000038342A2 - Receiving method and receiver

Info

Publication number: WO2000038342A2
Application number: PCT/FI1999/001066
Authority: WO
Inventors: Pentti Moilanen
Original assignee: Nokia Networks Oy
Current assignee: Nokia Oyj
Priority date: 1998-12-22
Filing date: 1999-12-21
Publication date: 2000-06-29
Anticipated expiration: 2001-06-22
Also published as: WO2000038342A3; FI982778A0; AU3046700A

Abstract

The invention relates to a receiving method and a receiver using the method. An adapted filter (208) has a sample sequence of predefined length of a received signal. The sample sequence is decorrelated in the adapted filter (208) with a spreading code received from a code generator (210) to form a decorrelation symbol. The sample sequence is processed stepwise onward by sample and the sample sequence is decorrelated at each step. The decorrelated symbols are combined in a combiner (222) for signal detection preferably utilizing an impulse response profile of an impulse block (314).

Description

RECEIVING METHOD AND RECEIVER

FIELD OF THE INVENTION

The invention relates to digital radio systems and, more closely, to CDMA radio systems in which the signal to be transmitted is spreadmg-coded The solution of the invention concerns the reception of a spreadmg-coded signal in particular

BACKGROUND OF THE INVENTION

In CDMA (Code Division Multiple Access) a narrow-band data signal of a user is modulated with a spreading code which has a wider band than the data signal to a relatively wide band Known CDMA systems use band- widths of over 1 MHz In WCDMA radio systems (Wide-band CDMA), the bandwidth is considerably wider The spreading code is usually formed of a long pseudo-random bit sequence The bit rate of the spreading code is much higher than that of the data signal, and to distinguish spreading code bits from data bits and data symbols, they are called chips Each data symbol of a user is multiplied by the chips of a spreading code This way, a narrow-band data signal is spread to the frequency band used by the spreading code Each user has its own spreading code A spreading code can have a length of one or more data bits Many users transmit simultaneously on the same frequency band and the data signals are distinguished from each other in the receivers on the basis of a pseudo-random spreading code

CDMA radio systems use rake fingers to enable time diversity, whereby multipatb-propagated signals are received A rake receiver comprises one or more rake fingers Each finger is an independent receiving unit whose task is to despread the spreading coding and demodulate one received signal component Each rake finger can be directed to synchronize with a signal component propagated along a different path In prior art, a search finger finds the most important signal propagation paths with a delay profile obtained by means of impulse response measurements, and each rake finger begins, by means of the delay profile, to despread the spreading coding of the signal of its own propagation path with its own adapted filter In a conventional CDMA receiver, the signals of the receiver fingers are combined to improve the quality of the signal

However, a large number of rake fingers cause problems The same received signal is processed in parallel in several rake fingers wasting a lot of resources.

BRIEF DESCRIPTION OF THE INVENTION

It is thus an object of the invention to implement a receiving method and a receiver implementing the method in such a manner that the problems described above are eliminated. This is achieved with a receiving method in which a digital spreading-coded signal is received and the received signal is sampled. Further, in the method, a sample sequence of predefined length is selected from the sampled signal and then processed stepwise onward by sample; when a new sample is introduced to the sample sequence, the sam- pie sequence of predefined length is decorrelated with a spreading code to form a decorrelation symbol; the decorrelated symbols are combined for signal detection.

The invention also relates to a receiver in a digital radio system in which radio system the transmitted signals are spreading-coded, the receiver of the radio system being adapted to form samples of the received signals. A system of the invention is characterized in that the receiver comprises at least one code means for generating at least one spreading code for decorrelation, a filter adapted to decorrelate with the spreading code of at least one code means a sample sequence of predefined length from the samples of a re- ceived signal and to shift the sample sequence on by one sample for a subsequent new decorrelation, and a combiner to combine the decorrelated symbols.

The invention is based on the idea that several decorrelating rake fingers are not used, but the decorrelation of a received signal is performed using one adapted filter.

The method and receiver of the invention provide several advantages. The solution of the invention simplifies the structure, operation and control of the receiver and saves resources which are usually needed for several integrated rake receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail in connection with preferred embodiments, with reference to the attached drawings, in which

Figure 1 shows a prior art rake receiver, Figure 2 shows a time diversity receiver of the invention, Figure 3A shows a time diversity receiver in which the symbols to be combined are selected,

Figure 3B shows the delay profile of an impulse response descriptor, Figure 4 shows a time diversity receiver which uses several spreading codes, and

Figure 5 shows a time diversity receiver which uses several spreading codes having different phases.

DETAILED DESCRIPTION OF THE INVENTION The solution of the invention is suited for CDMA and WCDMA radio systems. The receiver is preferably a base station of the radio system.

Figure 1 shows a prior art rake receiver. The receiver, which is preferably a CDMA radio system base station, comprises an antenna 100, a radio frequency means 102, and an A/D converter 104 before a selection means 106. There may be one or more fingers comprising an antenna, radio frequency means and A/D converter. The radio frequency means 102 lowers the frequency of the received signal to baseband. The baseband signal is converted in the A/D converter 104 into a digital sample sequence. The selection means 106, which is used when there are more than one signal coming in from the antennas 100, selects the most suitable of the signals to be forwarded to despreading means 108 to 118 of rake fingers 120. The despread- ing means 108 to 116 are despreading means related to signal detection. The despreading means 118 is what is known as a search finger from which an impulse response measuring block 122 forms a delay profile using the output signal and the average of the impulse responses. An allocation means 124 uses the delay profile to allocate the despreading means 108 to 116 of each detection finger to despread the signal received with different delays. The de- spreading is performed by multiplying the sample sequence of the received signal by a spreading code. The despread signals are symbol sequences and the signals going out from different despreading means 108 to 116 are combined by symbol in a combining means 126 from which the signals propagate to other signal processing processes of the receiver.

Figure 2 shows the main features of the solution of the invention. The basic requirement for the invention is that at least two consecutively transmitted symbols, which are bits or bit combinations, are multiplied by vari- ous spreading codes A signal coming from an antenna 200 is lowered to baseband in a radio frequency means 202 and the baseband signal is digital- ized in an A/D converter 204 Selection means 206 are used if there are several antennas in use A digital sample string flows into an adapted filter 208 which is able to receive a sample string of known length The sample string in the adapted filter 208 is multiplied by a spreading code received from a spreading code generator 210 The spreading code fed into the adapted filter 208 by the spreading code generator 210 can be a part of a long spreading code which is longer than the adapted filter 208 has space for, or the spread- ing code can be short, in which case it fits into the adapted filter at one go The spreading code can have a length of one or more data bits or data symbols The multiplication represents the forming of a correlation After this, the sample string is shifted on with the FIFO principle (First In First Out) like in a shift register by one sample, whereby a new sample comes into a shift register 208 and the shifted sample string is multiplied again by the spreading code This is continued as long as the signal is received Each multiplication result is presented as a correlation symbol C(n) produced by the adapted filter The digitally correlation-like symbol vector C is calculated as a product of sequences X and Y as follows

_v C(n) = ∑x(ι)y(n _{+ 1}) , ι = l

where each C(n) corresponds to an element of the correlation row C When the sample y(n + 1 ) in vector Y corresponds to the delayed symbol x(ι) in vector X, the delay of the symbol can be determined The correlation symbols C(n) of the adapted filter propagate to an impulse response measuring block 212 which forms a delay profile by means of the impulse response The delay profile is usually formed on statistical grounds using an average, for instance By means of the delay profile, a symbol selection block 220 can be directed to select the symbols coming from the adapted filter 208, whose correlation with the spreading code is high The selected symbols are combined in a combiner 222 using prior art diversity combination methods The method of combining the symbols is not essential for the invention

Figure 3A shows in detail how the selection block 220 can be di- rected by means of the delay profile The blocks in the figure are otherwise the same as in Figure 2, but block 314 has been added. The timing of the symbols to be combined is selected by means of the delay profile in an allocation block 314 making the allocation decision. The allocation is performed on the basis of the amplitude in the delay profile of the impulse response in such a manner, for instance, that the amplitude must exceed a certain threshold value before the delay corresponding to the amplitude is selected. Each delay corresponds to a certain correlation symbol. The allocation information of the allocation block 314 controls the symbol selection in selection block 220 and the selected symbols are combined in the combiner 222 according to prior art. Figure 3B shows the delay profile of an impulse response descriptor. The amplitude A of a symbol is on the y axis and the time (delay) T is on the x axis. The descriptor has four amplitudes D1 , D2, D3 and D4 higher than other amplitudes and the correlation symbols corresponding to them are allocated for combination and the correlation symbols corresponding to other de- lay values are preferably rejected.

Figure 4 describes a form of operation according to the solution of the invention, in which a sample sequence in an adapted filter 408 is multiplied by several different spreading codes. In this case, too, an antenna 400 receives a radio frequency signal which propagates to a radio frequency means 402 for lowering the frequency. The baseband signal going out of the radio frequency means 402 is converted into digital format in an A D converter 404. The digital signal propagates either directly to the adapted filter or, if signals are received by several antennas, the digital signal propagates to selection means 406 which select samples of the digital signal for the adapted filter 408. In the adapted filter 408, the sample string is multiplied separately by each spreading code. The spreading codes are obtained from spreading code banks 412 to 418 and the spreading codes are selected one at a time for the adapted filter 408 using a selection switch 410. The spreading code is changed when the spreading code of the received signal changes. After multi- plication the output symbols of the adapted filter 408 move on to a number of impulse response measuring blocks 420 equalling the number of spreading codes. Each block in the set of impulse response blocks 420 forms a delay profile for each spreading code. By means of the delay profiles, an allocation block 422 makes an allocation decision on which symbols to combine. The allocation block 422 controls a symbol selection block 426. The selected symbols are combined in a combiner 428. Figure 5 describes a solution of the invention by which the delay search window of the adapted filter can be expanded. The solution in Figure 5 is very similar to that of Figure 4. The adapted filter 508 operates in such a manner that a spreading code sequence is kept constant while a received sample sequence propagates in the adapted filter 508. When the entire sample sequence changes into a new one, the code is hopped to a new spreading code. Spreading code hops are periodical and they are performed on a time span corresponding to the sample length of the adapted filter 508. However, when it is necessary to measure the impulse response on a time span longer than the sample length of the adapted filter 508, the following action can be taken in the solution of the invention. When the sample string in the adapted filter 508 has been multiplied by the spreading code, the phase of the spreading code is shifted and the same sample string is multiplied also by the phase-shifted spreading code. This can be implemented for instance so that spreading code banks 512 to 518 comprise the same spreading code with different phases. Switch 510 switches the spreading codes of different phases to the adapted filter 508 at required time instants. An impulse response block 520 then forms a delay profile from the impulse response estimate obtained by means of the correlation symbols and, by means of the delay profile, an allo- cation block 522 directs a symbol selection means 526 to select the symbols to be combined in a combiner 528.

The solutions of the invention can be implemented for instance with VLSI and/or ASIC circuits (very large scale integration, application-specific integrated circuit) or with programs utilizing microprocessor technology. Even though the invention has been explained in the above with reference to examples in accordance with the accompanying drawings, it is obvious that the invention is not restricted to them but can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.

Claims

1. A receiving method, in which a digital spreading-coded signal is received and the received signal is sampled, characterized in that in the method a sample sequence of predefined length is selected from the sampled signal and processed stepwise onward by sample, when a new sample is introduced to the sample sequence, the sample sequence of predefined length is decorrelated with the spreading code to form a decorrelation symbol, the decorrelated symbols are combined for signal detection.

2. A method as claimed in claim 1, characterized in that values representing an impulse response are formed of the decorrelation symbols and the decorrelated symbols are combined for signal detection on the basis of the values representing the impulse response.

3. A method as claimed in claim 1, characterized in that a delay profile of the impulse response is formed at least for a time span corresponding the predefined sample sequence, and decorrelated symbols of signal components propagated along different paths are selected for combination on the basis of the amplitudes of the delay profile of the impulse response.

4. A method as claimed in claim ^ characterized in that the sample sequence is decorrelated with more than one spreading code and the decorrelated symbols are combined by each spreading code.

5. A method as claimed in claim ^ characterized in that the sample sequence is decorrelated with the same spreading code using more than one phase offset value to combine the decorrelated symbols which are temporally farther away from each other than the predefined sample sequence.

6. A method as claimed in claim ^ characterized in that the spreading code is changed after a given sample has advanced from the be- ginning of the sample sequence of predefined length to the end.

7. A method as claimed in claim ^characterized in that the spreading code is changed when the spreading coding of the transmitted signal changes.

8. A receiver for a digital radio system in which radio system the transmitted signals are spreading-coded, the receiver of the radio system be- ing adapted to form samples of the signal it receives, characterized in that the receiver comprises at least one code means (210, 412 to 418) to generate at least one spreading code for decorrelation, a filter (208, 408) adapted to decorrelate with the spreading code of at least one code means (210, 412 to 418) a sample sequence of predefined length from the samples of a received signal and to shift the sample sequence on by one sample for a subsequent new decorrelation, and a combiner (222, 428) to combine the decorrelated symbols.

9. A receiver as claimed in claim 8, characterized in that the receiver comprises at least one impulse response means (212, 420) according to the spreading code, which is adapted to form an impulse response amplitude by means of the decorrelated symbol.

10. A receiver as claimed in claim 9, characterized in that the receiver comprises an allocation means (314, 422) adapted to allocate the decorrelation symbols to be selected on the basis of the impulse response amplitude, a selection means (220, 428) adapted to select on the basis of the allocation the decorrelation symbols for combination by each spreading code.

11. A receiver as claimed in claim 8, characterized in that the receiver comprises several code means (412 to 418) with differing spreading codes, and a selection switch (410) adapted to select a spreading code for the adapted filter (408), and a combiner (428) adapted to combine decorrelated symbols by each spreading code.

12. A receiver as claimed in claim 8, characterized in that the receiver is adapted to decorrelate the sample sequence with each spreading code using more than one phase offset value for the decorrelated symbols that are temporally farther from each other than the predefined sample sequence.

13. A receiver as claimed in claim 8, characterized in that the code means (210, 412 to 418) is adapted to change the spreading code after a given sample has advanced from the beginning of the sample sequence of predefined length to the end.

14. A receiver as claimed in claim 8, characterized in that the code means (208, 412 to 418) is adapted to change the spreading code when the spreading coding of the transmitted signal changes.