JP3380435B2 - RAKE receiver - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a RAKE receiver used in a direct spread spectrum communication system and capable of efficiently receiving a multipath signal.

[0002]

In the spread spectrum communication system, a received signal is multiplied by a spreading code of a band which is several times to tens of thousands times larger than the band, and is transmitted as a wide band signal. The receiving side uses it for spreading on the transmitting side. It is a communication system that reproduces narrow band information by generating a despreading code synchronized with the code and multiplying it by the received signal.

In recent years, this spread spectrum communication system has been applied to multiple access for mobile radio, and CDMA (Code Division M
The ultiple access) method is drawing attention. Features of the spread spectrum communication system include noise resistance, interference resistance, and confidentiality.

In the case of a multipath transmission line, it is possible to separate the multipaths with the resolution of the spreading code length in the despreading process. Since a plurality of multipaths arrive via different paths, they have independent amplitudes and phases at the receiving point. In particular, in the case of a mobile transmission line, fading occurs because the transmission line characteristics fluctuate with movement. However, since each multipath changes independently, it is possible to obtain the path diversity effect by appropriately receiving the multipath.

This is called RAKE reception.

FIG. 6 shows an example of the configuration of a RAKE receiver according to the prior art. In FIG. 4, 10 is a despreader (1)-(n) corresponding to each multipath, 12 is a path searcher that detects the timing of multipath and gives timing to each despreader 10, and 16 is each path. Detectors (1)-(n) and 18 for detecting each of them are combiners for combining the outputs of the respective detectors.

In general, the path searcher 42 is composed of a sliding correlator capable of continuously sliding the despreading timing, a matched filter matched with a spreading code, or the like.

The operation of the RAKE receiver shown in FIG. 6 will be described. First, the path searcher 12 measures the multipath state of the transmission path. When the path searcher 12 detects a path, the path spreader 12 detects the path at the detected timing.
(1) -10 (n) are sequentially operated. That is, the despreaders 10 (1) -10 (n) receive different multipaths. Since each multipath is subject to phase rotation and amplitude fluctuations, the detector 16
(1) -16 (n) compensates for phase and amplitude.

Each of the despreaders 10 (1) -10 (n) and the detectors 16 (1) -16 (n) is also called a RAKE finger. The finger outputs are combined by the combiner 18. The combined output can be received efficiently because the signals that have passed through a plurality of paths are received and added, as compared with the case where only the received signals that have passed through one path are received, and a received signal with less deterioration due to fading can be obtained.

As described above, by performing RAKE reception, multipath can be received, and incoming power can be efficiently received. Further, a pass diversity effect can be obtained.

[0011]

In the configuration of the RAKE reception shown in FIG. 6 described above, FIG. 7 (a)
If the multipath is buried in noise, as shown in
When signals from these paths are combined, characteristic deterioration occurs.

FIG. 7A shows the relationship between the despreader output of the received signal from each path and the noise level, and how the output of the despreader is input to each detector.

In the above RAKE reception, a synchronous detection method is often used as a detection method. this is,
The synchronous detection system, when operated ideally, has a required signal power-to-noise power ratio (S / S) to obtain a certain average bit error rate.
This is because the N ratio) is the smallest.

However, this synchronous detection method is shown in FIG.
When synchronous detection is performed on a path in which noise is buried as shown in (3), the characteristic deteriorates more than the absolute phase estimation error.

Further, as shown in FIG. 7A, when the number of multipaths is larger than the number of RAKE fingers, the received power cannot be effectively combined. If the number of RAKE fingers is increased as a countermeasure, a path buried in noise will be synthesized.

Similar to FIG. 7A, FIG. 7B shows the relationship between the despreader output of the received signal from each path and the noise level, and the output of the despreader is input to each detector. It shows the situation. The output not connected to the detector indicates that there is no RAKE finger corresponding to that path.

It is an object of the present invention to obtain a reception with less reception deterioration due to noise by effectively utilizing the received power from the path even if there is a path buried in noise.

[0018]

[0019]

In order to achieve the above object, the present invention is directed to a RAKE receiver used in a direct spread spectrum communication system, in which a plurality of inverse signals corresponding to respective paths and despreading received signals of the paths are inversely spread. A spreader, a plurality of pre-detection combiners that combine at least two signals from the de-spreader, and a plurality of detectors that detect the output from the pre-detection combiner or the output from the de-spreader, A switch that can connect arbitrary outputs from a plurality of despreaders to the inputs of the plurality of pre-detection combiners, and a switch that can connect the outputs from the despreaders to the input of the detector, and a detector that combines the signals from the detector It is characterized by having a post-combiner and a path searcher that detects multipaths in a transmission path, assigns a despreader to the paths, and sets the connection state of the switches.

In the RAKE receiving device, the setting of the switch is performed dynamically according to the detection result of multipath.

In the RAKE receiving apparatus, the path having at least one large level and the path having at least 1 are included.
It can be combined with two low-level paths and combined before detection.

It is also possible to combine paths having a signal average level smaller than a certain threshold before a plurality of detections, and to combine signals having an average level larger than a threshold having a mean level after the detection.

By providing a first threshold level and a second threshold level, paths smaller than the smaller second threshold level are not synthesized, and a level between the first threshold level and the second threshold level is not synthesized. Are combined and combined before detection,
Paths above the level may be combined after detection.

It is advisable to combine the paths before detection so that the average level of each input synthesized by RAKE becomes equal.

The N paths may be combined after detection, and the remaining paths may be combined before detection, from the highest average level.

With this configuration, the RA with little deterioration
It becomes possible to perform KE synthesis.

[0027]

Embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a first embodiment of the RAKE receiving apparatus of the present invention.

In FIG. 1, 10 is n despreaders (1)-(n), 12 is a path searcher, 14 is pre-detector combiners (1)-(m), and 16 is m. Detectors (1)-(m) and 18 are post-detection combiners. In general,
The path searcher 12 is composed of a sliding correlator capable of continuously sliding the timing of despreading, a matched filter matched with a spreading code, or the like. In addition, it has the function of measuring the amplitude and delay time of each path. In addition, each despreader 10
It is also possible to set the timing for performing despreading for (1)-(n) to correspond to the path. Pre-detection synthesizer 1
4 (1) -14 (m) can be configured by a simple adder. The detector 16 also compensates for phase rotation caused by fading or frequency shift between transmission and reception for each path. As a result, proper synthesis is performed by RAKE synthesis. Further, the post-detection synthesizer 18 is R
In the AKE combiner, weighting is performed so that maximum ratio combining is achieved. A part of the despreaders 10 (1)-(n) may be directly connected to the detector 16.

The main difference from the conventional RAKE receiver shown in FIG. 4 is that the pre-detection combiner 14 is inserted between the despreader 10 and the detector 16. The pre-detection combiner 14 combines the outputs of a plurality of despreaders and inputs the combined outputs to the detector. In the example shown in FIG. 1, the despread signal is detected by two paths before being detected. Therefore, the number n of despreaders and the number m of detectors are

[0031]

## EQU1 ## m = n / 2 There is a relationship.

The operation of the RAKE receiving apparatus of the present invention shown in FIG. 1 will be described. First, the path searcher 12 measures the amplitude and delay time of the multipath from the received signal for each path. Next, pass searcher 12
For example, each of the despreaders 10 (1) -10 (n) is selected so that the number of the despreaders (n) is selected in order from the path having the largest amplitude and the selected respective paths are despread. By setting the timing of each despreader 10
(1) -10 (n) is assigned to each path.

The assignment of this despreader 10 to the path is
In the above example, a path with a large amplitude and a path with a small amplitude are assigned to the adjacent despreaders and input to the same pre-detection combiner.

The outputs of the despreaders 10 (1) -10 (n) are combined by the pre-detection combiner 14 (1) -14 (m) into two paths each having a large amplitude path and a small amplitude path. , And detected by the detectors 16 (1) -16 (m).
The detected outputs of the respective detectors 16 (1) -16 (m) are combined by the post-detection combiner 18. This allows RAK
E effect is obtained.

In this way, by combining the signals from the path with large amplitude and the path with small amplitude before detection,
Even if there is a path buried in noise, the power of multipath can be effectively used.

In the above example, the outputs of two paths are combined, but it is also possible to combine the outputs of three or more despreaders. In this case, for example, a combination of a path having a large amplitude and two or more paths having a small amplitude can be considered.

The number n of despreaders and how many despreader outputs are combined before detection are determined by measuring the multipath state of the propagation path to be used and the degree of despreading. It can be determined in advance depending on whether a container is available.

FIG. 2 shows the configuration of another embodiment of the RAKE receiving apparatus of the present invention.

In FIG. 2, 10 is a despreader (1)-(n) corresponding to each multipath, 22 is a path searcher,
23 is a switch, 14 is a pre-detection synthesizer (1)-(m),
Reference numeral 16 is a detector (1)-(m), and 18 is a post-detection combiner. The RAKE receiving device shown in FIG.
Is inserted between the despreader 20 and the pre-detection combiner 24. Switch 23 is
By controlling the searcher 22, the despreaders 10 (1) -10
The output of (n) can be input to any pre-detection combiner 14 (1) -14 (m) in any combination. Further, any output of the despreader 10 (1) -10 (n) can be directly connected to any detector 22 (1) -22 (n).

The operation of the RAKE receiver having the configuration shown in FIG. 2 will be described. The path searcher 22 detects and measures multipaths to determine the number of despreaders 10 to be used and the combination of pre-detection synthesis according to the number and amplitude of the multipaths, and the timing of the despreader 10 is determined. And the switch 23 is set.

An example of the number of despreaders 10 and the combination of pre-detection synthesis performed by the path searcher 22 will be described below.

(1) The number N of paths having an amplitude equal to or higher than a certain threshold level (for example, -6 dB) is obtained by comparing with the path having the largest amplitude among the multipaths.

(2) The despreader 10 with twice N is selected, and the despreader 10 is selected depending on the timing of the despreader 10.
To each path.

(3) N path with an amplitude of -6 dB or more and -6
The switch 23 is set so that the outputs of the despreader 10 from the N paths of dB or less are combined and input to the pre-detection combiner 14.

In this case, the pre-detection combiner 14 combines and inputs two paths, a path with a large amplitude and a path with a small amplitude, and combines them. After that, pre-detection synthesizer 1
The outputs of 4 are respectively detected by the detector 16, and the post-detection combiner 18 performs RAKE combining of N paths.

In this configuration, the switch 23 dynamically sets the outputs of the despreaders 10 (1) -10 (n) in any combination, and any pre-detection combiner 14 (1) -14 (m).
Can be entered. Therefore, even if the multipath state of the propagation path fluctuates over time, the combination of paths can be dynamically changed to reduce the combined power or combine paths that contain a lot of noise. Without this, optimum RAKE synthesis can be performed.

As a combination of path selection and combination, for example, (1) a path having a large average level of a signal and a path having a small average level are combined before detection (the example described above).

(2) A path whose average level of a signal is smaller than a certain threshold is combined before a plurality of detections. A signal whose average level is larger than a certain threshold is synthesized after detection.

(3) The threshold of the average level of the signal is provided in two stages, and the paths below the smaller second threshold level are not synthesized (indistinguishable from noise).
The paths that are levels between the first threshold level and the second threshold level are combined and combined before detection. The paths of the first level and above are combined with other paths after detection.

(4) Combination is performed so that the effect of RAKE combining is maximized, that is, the average levels of the inputs combined by RAKE are equal.

(5) The N paths are synthesized after detection from the higher average level. The remaining paths are combined before detection.

There are various combinations such as.

The combination of the above (4) will be described in detail with reference to FIG. As mentioned above, RAKE combining is most effective when the average level of each input to the RAKE combining is equal. This is similar to the case of space diversity using a plurality of antennas, and is similar to the case where the antenna gain and the reception average level are not equal (unequal gain), the diversity effect is reduced.

There are a plurality of combinations for equalizing the averages of the RAKE combining inputs, one example of which will be described with reference to FIG. First, the average level of each pass is calculated.
This is shown in FIG. 3 (a). The horizontal axis of FIG. 3A is the path, and the vertical axis is the average level. The obtained average levels of each path are sorted in descending order. This is shown in FIG. 3 (b). The paths sorted by this average level are
For example, the switch 23 is used to sequentially assign to the combiner 14 from (1). When (n) of the combiner 14 is allocated, from the path of the average level of n + 1, conversely, from the (n) of the combiner 14 to (1) of the combiner 14 are allocated in order of size. All paths are composed by the synthesizer 14
Repeat until assigned to. This is shown in FIG.
It is an arrow of (b). The assigned path is synthesizer 1
When combined in 4, nearly equal output levels are obtained.
This is detected and input to the RAKE combiner (post-detection combiner) 18. With this configuration, the RAKE combiner 18
Will result in almost equal inputs. with this,
The effect of RAKE synthesis is maximized.

FIG. 4 is a block diagram of a receiver incorporating the RAKE combining shown in FIG. In FIG.
The received signal from the antenna 41 becomes a baseband signal in the wireless section 42 and becomes a digital signal in the A / D converter 43. Then, it is input to the despreader 10 and also to the pass searcher 22. The signal input to the path searcher 22 is correlated using the correlator 52 and the code generator 53 while shifting the timing, and the correlation value determination unit 54 determines whether the correlation is obtained. If the correlation can be obtained, it means that the path has been found. Therefore, the timing control unit 51 sequentially assigns the timing to the code generators 45 (1) to 45 (n) of the inverse correlator 10. At this timing, despreading is performed by the code generator 45 and the correlator 44, whereby the path can be assigned to the path finger.

Now, in order to detect the average level of the signal for each path, the correlation value memory 55 provided for each path accumulates the correlation value of the path. By accumulating the correlation value for each path in the correlation value memory 55, the average level for each path is calculated. This allows the synthesis using the average level described above. After that, the signal processing unit 19
Then, the original transmitted signal is obtained by decoding.

When the present invention having the configuration shown in FIG. 1 is applied, 1 required to obtain the average bit error rate (BER) 10 ^-3 with respect to the normalized fading frequency (fDT).
Signal power to noise power density ratio per bit (E _b / N
The characteristic of ( ₀ ) is shown in the graph of FIG. This is because there are four multipaths, and in the conventional example, these four paths are RAKE-combined as they are. When the present invention is applied, four paths are classified into one with a large amplitude and one with a small amplitude. A combination of two output signals is RAKE-combined.

As can be seen from this graph, when the present invention is applied, about 0 to 0.5 dB when space diversity is not used, and about 0.3 to 0.5 dB when space diversity is used. An improvement of the signal power to noise power density ratio of is obtained.

[0059]

As described above, the RAKE of the present invention is used.
According to the receiving device, it is possible to effectively combine the powers of the received signals and suppress the characteristic deterioration due to noise.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a RAKE receiving apparatus of the present invention.

FIG. 2 is a block diagram showing another embodiment of the RAKE receiving apparatus of the present invention.

FIG. 3 is a diagram illustrating path selection / combination.

FIG. 4 is a detailed block diagram showing a RAKE receiver of the present invention.

FIG. 5 is a signal power-to-noise power density ratio (E _b ) per bit required to obtain an average bit error rate (BER) 10 ⁻³ with respect to a normalized fading frequency (fDT).
7 is a graph showing the characteristics of / N ₀ ).

FIG. 6 is a block diagram showing a configuration of a conventional RAKE receiving device.

FIG. 7 is a diagram illustrating a relationship between a multipath state, a noise level, and a RAKE finger.

[Explanation of symbols]

10 Despreader (1)-(n) 12 pass searcher 14 Combiner before detection (1)-(m) 16 Detector (1)-(m) 18 Post-detection combiner (RAKE combiner) 19 Signal processing unit 22 Pass Searcher 23 Switch 41 antenna 42 radio section 43 A / D converter 44 Correlator 45 code generator 51 Timing control unit 52 Correlator 53 code generator 54 Correlation value determination unit 55 Correlation value memory 56 Path Searcher Controller

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-7-298332 (JP, A) JP-A-8-213933 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04B 1/69-1/713 H04J 13/00-13/06

Claims (7)

(57) [Claims] 1. A RAKE receiver used in a direct spread spectrum communication system, comprising a plurality of despreaders for despreading a received signal of a path corresponding to each path, and at least a signal from said despreader. A plurality of pre-detection combiners for combining two or more, a plurality of detectors for detecting the output from the pre-detection combiner or the output from the despreader, and an arbitrary output from the plurality of despreaders A switch that can be connected to the inputs of the plurality of pre-detection combiners and that can connect the output from the despreader to the input of the detector, a post-detection combiner that combines the signals from the detector, A RAKE receiving device comprising: a path searcher that detects multipath, assigns the despreader to the path, and sets a connection state of the switch. 2. The RAKE receiving device according to claim 1 , wherein the setting of the switch is dynamically set according to a detection result of multipath. 3. The RAKE receiver according to claim 1 or 2 , wherein at least one high level path and at least one path are provided.
A RAKE receiving device characterized by combining paths with two small levels and combining them before detection. 4. The RAKE receiving apparatus according to claim 1, wherein a plurality of paths having an average signal level smaller than a threshold are combined before a plurality of detections, and a signal having an average level larger than a threshold is combined after detection. RAK characterized by
E receiver. 5. The RAKE receiver according to claim 1, wherein a first threshold level and a second threshold level are provided, and paths smaller than the smaller second threshold level are not combined, and the first threshold level and the second threshold level are not combined. A RAKE receiving apparatus characterized in that paths that are levels between a first threshold level and a second threshold level are combined and combined before detection, and paths having a first level or higher are combined after detection. 6. The RAKE receiver according to claim 1 , wherein the paths before detection are combined so that the average levels of the inputs combined by RAKE are equal. 7. The RAKE receiving device according to claim 1, wherein N paths are combined after detection and the remaining paths are combined before detection from the highest average level.
E receiver.