JP2007129385A - Cdma receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CDMA receiver capable of performing the postprocessing of a reception signal effectively. <P>SOLUTION: The CDMA receiver has a diffusion code output means 1, a path row generation means 2 for generating a path row for channel estimation, a postprocessing means 3, and a demodulation means 4. The postprocessing means 3 compares a relation in a level between the level ratio of the path of the maximum level to that of a second one and a first threshold for determining the number of paths to be selected, based on the averaged result of level for for a plurality of slots for each path row or each path in the path row; performs selection and removal from a path having an upper level successively in the path row by the number of determined paths for determining a second threshold by detecting an averaged level to the remaining paths; compares a relation in the level between each path in the path row from which the selected path is removed and the second threshold; and acquires an effective path row, where the path having a level equal to or smaller than the second threshold is removed from the path row. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a CDMA receiver that receives a signal by a CDMA (Code Division Multiple Access) system, and more particularly to a CDMA receiver that effectively performs post-processing of a received signal.

As one of the IMT-2000 standard systems, there are a TD-CDMA system and a TD-SCDMA (TD-Synchronous CDMA) system in which a TD (Time Division) system is applied to the CDMA system.
Here, a TD-SCDMA system will be described as an example.
In the TD-SCDMA system, the uplink frequency and the downlink frequency in the base station apparatus are common, and at a chip rate of 1.28 Mcps, one frame is divided into two subframes and further divided into seven time slots. Communication is performed by time division multiplexing of the signal and the downlink signal in units of time slots.

A CDMA receiver adopting the TD-SCDMA system has a configuration similar to that shown in FIG. 1 referred to in an embodiment of the present invention described later, for example. Here, for convenience of explanation, description will be made using the same reference numerals as those shown in FIG. In addition, one time slot in the TD-SCDMA system has a configuration as shown in FIG. 2 referred to in an embodiment of the present invention described later, for example.
In such a CDMA receiver, after the channel estimation unit 2 detects a channel estimation path sequence, which is a signal having a peak in the reception timing of the multipath reflection of the propagation path, the post-processing unit 3 detects noise and other users. Peaks other than the original peak appearing in the channel estimation path sequence due to the interference of the spreading code are removed, thereby improving the reception performance of the data demodulator 4 in the subsequent stage.

FIG. 6 is a flowchart showing an example of the post-processing process.
First, n paths having large amplitudes are selected as preliminary selection peak paths from the L channel estimation path sequences output from the channel estimation unit 2 (step S21). Here, normally, sample data of about 16 chips (for example, corresponding to 12.5 μs and 3.7 km) is output as L at an interval of one chip. Moreover, n is about 1-2 normally.
Next, the average power per path is calculated from the sum of the power values of the remaining (L−n) path trains excluding the selected n preliminary selected peak paths (step S22).
Next, a value that is α times the average power per path is set to be used as a threshold value (step S23). Here, α is usually about 2 to 8.
Then, for the (L−n) number of path strings, the power value is sequentially compared with a threshold value (step S24), and a path whose power value is equal to or less than the threshold value is cleared to 0 (step S25).

  By applying such post-processing, the influence of the path caused by noise and other user interference other than multipath caused by reflection on the wall surface of the reception propagation path, etc. is removed, so that the data demodulation characteristics at the subsequent stage can be improved. An improvement effect is obtained.

However, in the post-processing in the CDMA receiver as described above, since the number n for selecting the preselected peak path is fixedly set, there is a problem that the effect of improving the reception characteristics may be reduced. .
Here, with reference to the post-processing characteristic comparison example shown in FIGS. 7 and 8, such a problem will be described in detail.
FIG. 7 shows the difference in performance given to the reception characteristics by the difference in the number n for selecting the preselected peak path when the reception propagation path is a one-path model.
FIG. 8 shows the difference in performance that the difference in the number n of selecting the preselected peak path gives to the reception characteristics when the reception propagation path is a four-path model.
FIGS. 7 and 8 show a case where the number of simultaneously received users is 8, the horizontal axis indicates SIR (Signal to Interference Ratio) [dB] after despreading, and the vertical axis indicates the error rate (BER). ).

As shown in FIG. 7, in the 1-pass model, the characteristics are better in the case of n = 1 than in the case of n = 2, whereas in the 4-pass model, n is 1 The characteristics when = 2 are superior to those when n = 1. In this way, the optimum n varies depending on the characteristics of the propagation path, so fixing n is not always a good idea.
The present invention has been made to solve such a conventional problem, and an object of the present invention is to provide a CDMA receiver capable of effectively performing post-processing of a received signal.

In order to achieve the above object, the CDMA receiver according to the present invention processes a received signal spread using a spreading code (spreading code on the transmission side) by the CDMA system with the following configuration.
That is, the spreading code output means outputs the spreading code. A path sequence generation means despreads the received signal using the output spreading code to generate a path sequence for channel estimation. The post processing means performs a predetermined post processing process to obtain an effective path string. Demodulating means demodulates the received signal based on the acquired effective path sequence.
As the predetermined post-processing processing, the post-processing means may perform a maximum based on the generated path sequence (or a result of averaging levels for a plurality of slots for each path of the generated path sequence). The level path and the second level path are detected, the level ratio between the maximum level path and the second level path is compared with the first threshold value, and the comparison is performed. Determine the number of paths to be selected based on the result, detect the level averaged for the remaining paths by selecting and removing the paths having higher levels in the path sequence in order from the determined number of paths, A second threshold value is determined based on the detected averaged level, and the level of each path in the path sequence excluding the selected path is compared with the second threshold value. Te to obtain a valid path string paths having a level removed from the path column below the second threshold value (or less than).

  Accordingly, post-processing is performed in a manner in which the number of paths to be selected is determined based on the level ratio between the maximum level path and the second level path for the path sequence for channel estimation. The post-processing of the received signal can be effectively performed according to the state of the propagation path such as the number of multipaths in the propagation path.

Here, various methods may be used as the CDMA method.
Also, as the spreading code on the transmitting side and the spreading code on the receiving side, those corresponding to each other are used, for example, the same code is used. As the spreading code, for example, a unique code is used for each user (for example, a mobile terminal device).
As the spreading code output means, for example, a means for generating a spreading code or a means for storing the spreading code in a memory can be used.

  Further, as a path sequence for channel estimation, for example, a sequence of correlation results between a received signal and a spread code can be used. Specifically, a plurality of timings (a plurality of timings) arranged in time in the received signal can be used. What calculated | required the correlation result about each of (timing corresponding to a path | pass) can be used. Usually, a peak with such a high correlation result level is more likely to be a multipath signal transmitted from the transmission side, while a peak with a low correlation result level is more likely to be noise or the like. There is a high possibility.

In addition, when detecting the path of the maximum level and the path of the second level, as an example, it is preferable to use the result of averaging the levels for a plurality of slots for each path of the generated path sequence. As another example, it is also possible to use a generated path sequence (not averaged).
Also, various values may be used as the threshold value (first threshold value) regarding the level ratio between the maximum level path and the second level path depending on the use situation or the like. As an example, when the value of (maximum level path / second level path) is large, the number of paths to be selected is reduced, and when the value is small, the number of paths to be selected is increased.

Various values may be used as a threshold value (second threshold value) for determining a path to be removed in order to acquire an effective path string. For example, a value transmitted from the transmission side may be used. For the multipath of the signal, a value that is removed from the effective path string is used for a path such as noise while remaining in the effective path string.
Various modes may be used as a mode for demodulating the received signal based on the effective path sequence. For example, reception is performed at the position (timing) of the path included (remaining) in the effective path sequence. Retrieving data sent from the transmission side (data before being spread on the transmission side) by correlating the signal with the spreading code, or performing RAKE combining when there are multiple multipaths it can.

  As described above, according to the CDMA receiver according to the present invention, when post-processing is performed on the received signal, the path of the maximum level is determined based on the path sequence for channel estimation or the average of the path sequence. The level ratio of the second level path and the threshold value (first threshold value) are compared, and the number of paths to be selected (for example, a preselected peak path) is determined based on the comparison result. As a result, post-processing of the received signal can be performed effectively.

Embodiments according to the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration example of a receiving unit of a CDMA receiver that employs a TD-SCDMA system according to an embodiment of the present invention. The CDMA receiver of this example is provided in a CDMA receiver of a digital radio transmission system.
The CDMA receiver of this example includes a spreading code generation unit 1, a channel estimation unit 2, a post processing unit 3, and a data demodulation unit 4.

FIG. 2 shows a configuration example of one time slot in the TD-SCDMA system.
One time slot consists of a 144-chip midmble at the center, a 352-chip Data Symbol (data symbol) at each of the preceding and succeeding stages, and a 16-chip Guard Period (guard period) at the last stage. ).
In the digital wireless transmission system of the present example, a plurality of time slot signals as described above are continuously transmitted from a CDMA transmitter (not shown) having a CDMA transmitter to a CDMA receiver having the CDMA receiver of this example. Wirelessly transmitted.

The operation example of the reception process performed by the CDMA receiver of this example is shown.
A TD-SCDMA signal transmitted wirelessly from a CDMA transmitter (not shown) is received via a propagation path, and the received signal is input to the channel estimation unit 2 and the data demodulation unit 4. This received signal is a signal spread using each spreading code by the CDMA transmitter, and is a signal in a time slot as shown in FIG.
The spreading code generator 1 generates each spreading code used in the channel estimator 2 and outputs it to the channel estimator 2. Here, in the TD-SCDMA system, the spreading code corresponds to a medium code, and a spreading code length of 16 chips is used. Each spreading code is, for example, the same as each transmission spreading code used on the transmitting side, and corresponds to each receiving spreading code used on the receiving side. Further, for example, a spreading code corresponding to each user is used, and a received signal is processed for each user.
The channel estimation unit 2 performs propagation path state estimation processing by performing correlation processing between each spreading code input from the spreading code generation unit 1 and a midamble code of the received signal, and post-processes the channel estimation path sequence Output to part 3.

Here, the channel estimation path sequence is a signal having a peak at the reception timing of multipath reflection in the propagation path, but noise and other users' spread codes cause interference and peaks other than the original peak timing. Need to remove this. This removal is performed by the post-processing unit 3.
Further, in this example, the number L of channel estimation path sequences is one with an interval of 1 chip and a length of 16 chips. As control for updating the estimated range, for example, a path search unit (not shown) having a function of searching for a path is always assigned. Ideally, in the channel estimation path sequence, it is preferable to adjust so that the path with the least delay (preceding wave) among the multipaths is positioned at the head of 16 chips. There may also be a margin before.

The post-processing unit 3 performs post-processing for clearing the position in the channel estimation path sequence other than the original peak to 0 for the channel estimation path sequence input from the channel estimation unit 2, and the channel after the processing The estimated path sequence (effective path sequence) is output to the data demodulator 4. By such post processing, the reception performance of the data demodulator 4 at the subsequent stage can be improved.
The data demodulating unit 4 demodulates the input received signal by removing the interference of other user codes based on the post-processed channel estimation path sequence input from the post-processing unit 3, and the demodulation obtained thereby Output data.
In data demodulation, generally, RAKE reception, joint detection technology, or the like is used. The joint detection technique is a method for improving reception characteristics by removing interference between transmission spreading codes by utilizing the fact that transmission spreading codes are known. In the TD-SCDMA system, it is assumed that joint detection is used. The system is built.

With reference to FIG. 3, an example of the procedure of the post-processing performed by the post-processing unit 3 of this example is shown.
FIG. 3 is a flowchart showing an example of the post-processing process of this example.
In the post-processing unit 3 of this example, the average power of the channel estimation path from the past is calculated for a plurality of slots for each received spreading code (step S1). Specifically, the average power is calculated by (Equation 1) in order to observe the reception propagation path fluctuation of the same reception spreading code (the same user) by a plurality of slots by exponential weighted averaging or the like.
(Equation 1)

Av (i, t, j) = Av (i, t, j−1) × λ
+ D (i, t) × (1-λ)
・ ・ (Formula 1)

  Here, Av (i, t, j) is an average power value at time j of the t-th timing path of the channel estimation path received from user i, and D (i, t) is user i in the slot. The power value of the t-th timing path of the received channel estimation path, and λ is an exponential weighting coefficient for averaging, and takes a value in the range of 0.5 to 0.95, for example. Here, the power value of a path such as D (i, t) is calculated by taking a correlation at the midamble portion of the slot by, for example, a correlation method.

Next, for each user i, the maximum Av (i, t, j) = Max (i) in t = 1 to L is obtained as the maximum power path, and the second maximum Av ( i, t, j) = Next (i) is obtained as the second power path (step S2).
Next, the power ratio A (i) = Max (i) / Next (i) is calculated for each user i (step S3).
Next, the power ratio A (i) is compared with the threshold value β for each user i (step S4). Here, the threshold value β takes a value in the range of 2 to 4, for example.

If the power ratio A (i) is smaller than the threshold value β, the channel estimation path selection number (the number of preliminary selection peak paths) p is z, and the channel estimation path (for example, the current one and not averaged) ), The upper p (= z) paths with higher power are selected (step S5). Here, z is normally about 2, and in this example, z = 2.
Then, for each user i, the upper p (= z) paths with higher power are removed from L channel estimation paths (for example, current ones that are not averaged) (L−p). The average power Nav of each path is calculated (step S6).

On the other hand, when the power ratio A (i) is equal to or greater than the threshold value β, the channel estimation path (number of preselected peak paths) p is set to m and the channel estimation path (for example, the current one is averaged). The upper p (= m) paths with higher power are selected (step S7). Here, m is usually about 1, and in this example, m = 1.
Then, for each user i, the upper p (= m) paths with higher power are removed from L channel estimation paths (for example, current ones that are not averaged) (L−p). The average power Nav of each path is calculated (step S8).

Next, threshold value γ = average power Nav × α is obtained for each user i (step S9). Here, (alpha) takes the value of the range of 2-4, for example.
Next, for each user i, for each of the (Lp) channel estimation paths (for example, current ones that have not been averaged) excluding the upper p higher power paths. The power value is compared with the threshold value γ (step S10), and the path having the power value equal to or smaller than the threshold value γ is removed (cleared to 0) (step S11).

In post-processing as in this example, the channel estimation path selection number p can be appropriately determined by grasping the reception propagation path status of each user in advance.
For example, in a situation where the number of multipaths of the reception propagation path is small as in the one-path model shown in FIG. 7, the power ratio A (i) is equal to or higher than the threshold value β (or the possibility is high), and p = Since m (1 in this example) is adopted, a good error rate characteristic can be obtained as in the case of n = 1 shown in FIG. On the other hand, for example, in the situation where the number of multipaths of the reception propagation path is large as in the 4-path model shown in FIG. 8, the power ratio A (i) is smaller than the threshold value β (or the possibility is high). , P = z (2 in this example) is employed, so that a good error rate characteristic can be obtained as in the case of n = 2 shown in FIG.
Thus, by appropriately determining the channel estimation path selection number p according to the state of the reception propagation path, it is possible to obtain appropriate reception performance according to the state of the reception propagation path.

With reference to FIG.4 and FIG.5, the specific example of the effect acquired by the post processing of this example is shown.
FIG. 4 shows a case where the reception propagation path is a one-path model.
As shown in FIG. 4 (1), when one path is preliminarily selected and removed from the average power calculation, only the original multipath is removed and the threshold value is calculated from the average power such as remaining noise. Therefore, all the paths other than the original multipath 1 path can be cleared to 0 (or the possibility is high), and good characteristics can be obtained.
On the other hand, as shown in FIG. 4 (2), when two paths are preselected and removed from the average power calculation in advance, not only the original multipath but also peaks such as noise are removed, and the remaining noise is removed. Since the threshold value is obtained from the average power, etc., the peak of the preselected noise and the like as well as the original multipath remains without being cleared to zero (or the possibility is high), and the characteristic is deteriorated. It will occur.

FIG. 5 shows a case where the reception propagation path is a 4-path model.
As shown in FIG. 5 (1), when one path is preliminarily selected and removed from the average power calculation, only one path of the original multipath is removed, and the other three paths multipath and When the threshold value is obtained from the average power of the remaining noise and the like, the average power is pushed up, so that the threshold value is pushed up, and there is a possibility that the original multipath path is cleared to zero. Deterioration of characteristics will occur.
On the other hand, as shown in FIG. 5B, when two paths are preliminarily selected and removed from the average power calculation, the original multipath two paths are removed and the other two paths multipath are removed. Since the threshold value is obtained from the average power of the remaining noise and the like, the degree to which the average power is boosted is small, so the degree to which the threshold is pushed is small, and the original multipath 4-path Remains (or is highly likely), and good characteristics can be obtained.

As described above, in the CDMA receiver of this example, the spreading code generating unit 1 that generates a spreading code, and the channel estimating unit 2 that generates a channel estimation path sequence by performing despreading processing on the spreading code and the received signal. A post-processing unit 3 that selects a path with a large power from the channel estimation path string as a peak path, clears other path paths with a small power to 0 and extracts a valid path string, and the valid path string and the received signal The data demodulator 4 is provided for demodulating the received data using the.
Then, the post-processing unit 3 averages the power for each path of the channel estimation path sequence over a plurality of slots and outputs an average channel estimation path sequence, and the maximum power path and the second power path among them are output. A function to obtain, a function of comparing the power ratio of the maximum power path and the second power path with a threshold value β and switching the number of peak paths to be preliminarily selected based on the result, and a path sequence excluding the preselected peak path A function for obtaining the average power by averaging the power of each channel, and the power value of each channel estimation path excluding the preselected peak path in the channel estimation path sequence with a value α times the average power as a threshold value γ Compared with the threshold value γ, it has a function of clearing a path value having a power value equal to or lower than the threshold value γ to zero.

As described above, in the CDMA receiver of this example, in the digital wireless communication based on the TD-SCDMA system, as the path selection variable type post-processing using the propagation path estimation, the largest path and the second largest in the path estimation range. The number of paths to be preliminarily selected is determined based on the path power ratio.
Therefore, in the CDMA receiver of this example, it is possible to improve the error rate characteristic of the received signal and improve the reception characteristic as a whole. Further, for example, if the configuration is such that transmission power control is performed, the effect of reducing interference power can also be expected, and the effect of increasing the channel capacity as a system is great.

In the CDMA receiver of this example, a spreading code output unit is configured by the function of the spreading code generation unit 1, and a path sequence generation unit is configured by the function of the channel estimation unit 2. Post-processing means is configured by the function, and demodulating means is configured by the function of the data demodulating unit 4.
Further, the post-processing processing performed in the CDMA receiver of this example can be applied to all CDMA systems, for example, RAKE used in W (Wideband) -CDMA system with a large spreading gain. Compared with a method that performs maximum ratio combining such as reception, it is particularly effective for application to a TD-SCDMA system or a TD-CDMA system that has a low spreading factor and a large amount of interference from other user codes.

Here, the configuration of the receiver or the like according to the present invention is not necessarily limited to the configuration described above, and various configurations may be used. The present invention can also be provided as, for example, a method or method for executing the processing according to the present invention, a program for realizing such a method or method, or a recording medium for recording the program. It is also possible to provide various devices and systems.
The application field of the present invention is not necessarily limited to the above-described fields, and the present invention can be applied to various fields.
Further, as various processes performed in the receiver or the like according to the present invention, for example, control is performed by executing a control program stored in a ROM (Read Only Memory) in a hardware resource including a processor, a memory, and the like. For example, each functional unit for executing the processing may be configured as an independent hardware circuit.
The present invention can also be understood as a computer-readable recording medium such as a floppy (registered trademark) disk or a CD (Compact Disc) -ROM storing the control program, and the program (itself). The processing according to the present invention can be performed by inputting the program from the recording medium to the computer and causing the processor to execute the program.

It is a figure which shows the structural example of the CDMA receiver based on one Example of this invention. It is a figure which shows the structural example of 1 time slot in a TD-SCDMA system. It is a figure which shows an example of the procedure of the process of the post processing which concerns on one Example of this invention. It is a figure for demonstrating the effect in case a receiving propagation path is a 1 path | pass model. It is a figure for demonstrating the effect in case a receiving propagation path is a 4-path model. It is a figure which shows an example of the procedure of a process of post-processing. It is a figure which shows the comparative example of the characteristic of the post-processing in 1 pass model. It is a figure which shows the comparative example of the characteristic of post-processing in a 4-pass model.

Explanation of symbols

  1 .... spreading code generator, 2 .... channel estimator, 3 .... post processing, 4 .... data demodulator,

Claims (1)

In a CDMA receiver that processes a received signal spread using a spreading code by the CDMA method,
Spreading code output means for outputting a spreading code;
Path sequence generation means for despreading the received signal using the output spreading code to generate a path sequence for channel estimation;
A maximum level path and a second level path are detected based on a level averaged for a plurality of slots for each path of the generated path sequence or the generated path sequence, and the maximum level path is detected. The level ratio between the path and the second level path and the first threshold value are compared to determine the number of paths to be selected based on the comparison result, and only the determined number of paths In the path string, a path having a higher level is selected and removed in order, and an averaged level is detected for the remaining paths, and a second threshold is determined based on the detected averaged level. By comparing the level of each path in the path sequence excluding the selected path with the second threshold value, a path having a level less than or less than the second threshold value is selected as the path. Yes removed from column And post-processing means for obtaining the path column,
Demodulating means for demodulating the received signal based on the acquired effective path sequence;
A CDMA receiver comprising:

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