JP3462364B2 - RAKE receiver in direct spread CDMA transmission system - 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 for direct sequence CDMA (DS-CDMA) transmission which performs multiple access using spread spectrum in mobile communication.

[0002]

2. Description of the Related Art The DS-CDMA transmission system is a system in which an information data modulated signal is spread over a wide band signal by a spreading code having a spreading factor (= chip number / symbol) pg and is transmitted to different users. Is a method in which a plurality of communication parties perform communication using the same frequency band.

FIG. 5 shows the configuration of a receiver using a sliding correlator in a conventional DS-CDMA transmission system. In the receiver, after the spread modulation signal received by the antenna 101 is amplified by the low noise amplifier 103, the oscillator 105,
Frequency mixer 104 and band pass filter (BP
F) 106 is used for frequency conversion into an intermediate frequency (IF frequency) signal, and an automatic gain control amplifier (AGC amplifier) 1
Linear amplification is performed at 07. In the AGC amplifier 107,
The amplitude envelope of the received signal is detected by the envelope detector 108, and the amplitude variation is negatively fed back to the AGC amplifier 107 to compensate for the amplitude variation due to fading.

The signal linearly amplified by the AGC amplifier 107 is quadrature detected by the quadrature detector 109 into a baseband signal. Then, the baseband in-phase (I) and quadrature (Q) components are respectively converted into A / D converters 112 and 113.
Is converted into a digital value by. Spread code generator (1)
~ (N) 118, using the spreading code replica synchronized with the delay time of each multipath signal, R
The despreading process is performed by the sliding correlator 131 in the AKE combining pass finger 114. Data is demodulated by performing delay detection or synchronous detection on the despread signal.

Here, a description will be given of a system in which pilot symbols are inserted at regular intervals between information symbols in a received transmission frame, and absolute synchronous detection demodulation is performed using the pilot symbols. In land mobile communication, the amplitude and phase fluctuations of a received signal called fading are affected by the movement of relative positions of a base station and a mobile station. In order to perform the coherent detection demodulation, it is necessary for the receiver to estimate the complex envelope caused by this fading, that is, the amplitude and phase fluctuation (or channel). It is possible to obtain a reception fading complex envelope with pilot symbols inserted in a transmission information symbol at a constant period, and use this value to obtain a fading complex envelope at information symbol positions between pilot symbols. Fading complex envelope fluctuation (channel fluctuation) of each information symbol is compensated by using the value used for this pilot symbol. This is the channel estimator 132 and multiplier 13 in the RAKE combining path finger 114.
I'm going in 3.

The RAKE combiner 119 performs in-phase combining (RAK) on the plurality of multipath signals whose channel fluctuations have been compensated.
E combining) makes it possible to improve the signal power ratio with respect to an interference signal or thermal noise. A sliding correlator 115 called a search finger selects a multipath signal to be RAKE-combined. In the search finger, the average received signal power of the despread signals at L timings within the multipath search range is measured by the average received signal power measuring unit 116, and the multipath having an average large received signal power is detected by the path selection timing detecting unit. The selection is made at 117. For example, as shown in FIG. 5, when one sliding correlator 115 is used, a correlation value (despreading value) at one timing is obtained for each symbol, and a despread signal at this timing is obtained. The received signal power can be measured. Then, the timings of the spread codes are shifted one by one, and the power is measured for all L timings.

Now, in selecting the RAKE combining path, it is necessary to select a multipath signal having a large average signal power (after being subjected to a variation due to a distance variation between the base station and the mobile station and a variation due to shadowing). . This is subject to instantaneous fluctuations due to Rayleigh fading under land mobile communication environment. Therefore, in the estimation of the received signal power at one time,
Since the received signal power happens to drop due to the Rayleigh fading fluctuation for a certain multipath signal, the signal power is low, and the RAKE combining path may be missed. Therefore, it is necessary to measure the received signal power for a signal obtained by averaging the Rayleigh fading fluctuations in order to remove the influence of the instantaneous fluctuations in the received power.

Specifically, L within the multipath search range
The signal power measurement is repeated X times for the despread signals at the respective timings, the delay profile is generated by the average signal power, and the top N RAKE combining multipaths are selected. When one sliding correlator 115 is used as shown in FIG. 5, it takes L × X symbol time to generate the delay profile once. When f sliding correlators (search fingers) are used, it is necessary to generate (L
× X) / f symbol time is required. The timing of the spreading code replica used by the RAKE combining finger is updated every time the delay profile is generated. When the mobile station moves at a high speed with respect to the base station, this delay profile changes quickly.Therefore, in the multipath search using this sliding correlator, it may take time to follow the delay profile change. is there.

In order to perform a high-speed multipath search,
The multipath search range and the number of averagings may be reduced. However, narrowing the search range reduces the time diversity effect of RAKE combining, and reducing the number of signal power averagings reduces the RAK by the search finger.
It becomes impossible to accurately select the E composite multipath.

FIG. 6 shows a receiver configuration using a matched filter in the conventional DS-CDMA transmission system. In FIG. 6, the same components as those of the receiver in FIG. 5 are designated by the same reference numerals.

In FIG. 6, the received spread modulation signal is amplified by the low noise amplifier 103 and then frequency-converted to an IF frequency. Then, the AGC amplifier 107 compensates the amplitude fluctuation caused by the fading, and the quadrature detection is performed. The output baseband signal of the quadrature detector 109 is A / D
It is converted into a digital signal by the converters 112 and 113. The spread modulation signal converted into a digital value is despread by the matched filter 150 with the tap number pg using the output of the spread code replica generation unit 151, and separated into L timing signals. Here, when s is the number of oversamplings per chip, L = pg × s. L
Data demodulation is performed by selecting N multipaths from each timing and performing differential detection or synchronous detection.

In this receiver, a pilot symbol is inserted between information symbols in a transmission frame at a constant period, and absolute synchronous detection demodulation is performed using this pilot symbol. For each despread signal at the L timings, the channel is estimated by the channel estimator 132 using the pilot symbols, and the estimated value is used to compensate the channel fluctuation of each information symbol. On the other hand, the average received signal power at each of the L timings is measured by the average signal power measuring unit 153 to generate a delay profile, and the upper N RAKEs are generated.
The synthetic multipath is selected by using the path selection timing detection unit 154. At this time, the multipath is selected from the timing of large reception power, but the same multipath detected by oversampling is excluded and the next multipath is selected. In the configuration using the matched filter, despread signals at L timings are output for each symbol period. Therefore, it is not necessary to measure the power with the search finger using the sliding correlator as in the configuration of FIG. Further, multipath updating for RAKE combining can be performed at high speed.

By the way, as described above, when the mobile station moves at high speed with respect to the base station, the shape of the delay profile changes, and the number of multipaths also changes. However, since the receiver having the above-described configuration is configured to combine the upper N multipaths having a large received signal power, when the number of multipaths is greater than N, all of them are combined to generate interference components and thermal noise. The signal power ratio cannot be improved. Further, when the number of multipaths is less than N, the characteristics are deteriorated by synthesizing signals having only noise components and mutual interference components and multipaths having very small reception power.

[0014]

As described above, when the mobile station moves at high speed with respect to the base station, the delay profile changes rapidly and the shape also changes. Therefore, when the number of multipaths to be combined is fixed as in the conventional RAKE receiver, it is not possible to combine all multipaths having sufficient reception power. Alternatively, the signals of only the interference component and the noise component may be combined. As a result, the signal power ratio with respect to the interference component and thermal noise cannot be improved, and the characteristics deteriorate.

An object of the present invention is to provide a RAKE receiver which eliminates signals of only noise components and mutual interference components and combines as many multipaths as possible in RAKE reception based on a matched filter. To do.

[0016]

In order to achieve the above object of the present invention, RAKE in a direct sequence CDMA transmission system.
In the receiver, a matched filter that outputs a despread signal at a timing that is an integer multiple of the number of chips in one symbol, a demodulation unit that demodulates the despread signal at each timing from the matched filter, and a match The average signal power measuring unit that measures the average signal power of each despread signal at each timing from the filter, and the threshold that prevents the synthesis of noise and interference components from the output of the average signal power measuring unit. A and a threshold value calculating unit for determining a threshold value B for selecting a signal having sufficient signal power, and an average power from the average signal power measuring unit,
Outputs of a path selection timing detection unit that detects a signal having an average power larger than the threshold value A and the threshold value B from the threshold value control unit and determines timing of multipath to be combined, and outputs of the path selection timing detection unit The RAKE combining path selecting section for selecting a signal to be RAKE combined from the signal demodulated by the demodulating section, and the RAKE combining RAKE combining the demodulated signal selected by the RAKE combining path selecting section.
And a synthesizer.

Further, the threshold value calculation unit detects the minimum power value from the average power at all timings from the average signal power measurement unit, and the minimum power detection unit at all timings from the average signal power measurement unit. From the maximum power detection unit that detects the maximum power value from the average power, the minimum power value from the minimum power detection detection unit, and the maximum power value from the maximum power detection unit, the two threshold values A,
And a threshold control unit for obtaining B.

The threshold control unit multiplies the minimum power value by a constant G _A (G _A ≧ 1) and the maximum power value by a constant G _B (0 <G _B ≦ 1), and then 2 Two thresholds can be obtained.

In addition, the matched filter performs oversampling (oversampling number s per chip), and the path selection timing detection section
When detecting multipath timings from all timings in descending order of received signal power, the signals at ± k (k is a natural number) timings are excluded from the timing of the already detected paths and the next multipath The timing can also be detected sequentially.

As described above, in the RAKE receiver of the present invention, the average received power of the signal despread by the matched filter at all the timings is measured to determine two threshold values. Then, multipath is selected from the despread signals that satisfy the two threshold values and RAKE combining is performed.

By using this configuration, it is possible to exclude signals having only noise and interference components and to RAKE combine all multipath signals having a sufficiently large received power.
Therefore, even if the number of multipaths changes due to the variation of the delay profile, only effective paths can be combined.

In the configuration of the present invention, the RAK is particularly suitable for the high-speed chip rate, that is, the wide band DS-CDMA.
It is possible to improve the characteristics of the reception quality due to the time diversity effect of E.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the principle configuration of the present invention. In this figure, the same components as those in FIG. 6 are designated by the same reference numerals.

The baseband spread modulation signal subjected to the quadrature detection and A / D conversion is input to the matched filter 150 having the tap number pg. Matched filter 150
Uses the output of the spreading code replica generator 151 to despread the spread modulation signal. If s is the number of oversamplings per chip, the matched filter 150 outputs L (= p
The despread signals at the g × s) timings are output. The despread signals at the L timings are demodulated by the demodulation unit 152.

The average received power measuring section 153 measures the average received power at each of the L timings. The minimum power detection unit 201 and the maximum power detection unit 203 detect the minimum signal power and the maximum signal power at L timings. Threshold control unit A20
In step 2, the threshold value A is obtained using the detected minimum signal power. Further, the threshold control unit B204 obtains the threshold B using the detected maximum signal power. The threshold value A is set to prevent combining signals having only noise and interference components, and the threshold value B is set to combine signals having sufficient signal power. In this way, the two thresholds are determined in such a manner that all multipaths having sufficient signal power are combined and that the thermal noise component and the cross-correlation of the received signals of other users are not combined. This is to improve the characteristics. These thresholds are, for example, the minimum signal power detected by the minimum power detection unit 201 and the maximum power detection unit 20.
It can be obtained by multiplying the maximum signal power detected in 3 by the threshold gain. The path selection timing detection unit 205 compares the average signal power measuring unit outputs at L timings with the threshold value A and the threshold value B, and the average signal power value is the threshold value A or more and the threshold value B or more. The timing of multipath with a large signal power is detected from among the timings. At this time, the signals at ± k (k is a natural number) timings are excluded from the timing of the already selected multipath, and the timing of the next multipath is sequentially detected. The output of the demodulation unit 152 at the detected multipath timing is RAK.
The signals selected and selected by the E combination path selection unit 155 are combined by the RAKE combination unit 119.

The operation of threshold value determination and RAKE combining path selection in the present invention shown in FIG. 1 will be described. First,

[0028]

[Outer 1]

With respect to the threshold A and the threshold B, the following equations are determined.

[0030]

[Equation 1]

Here, G _A (G _A ≧ 1), G _B (0 <G
_B ≦ 1) is the threshold determination gain. next

[0032]

[Outside 2]

P timings satisfying the above conditions are detected as multipath candidates.

RAK is selected from the signals at these timings.
E Select the multipath to be combined. First, the signal at the timing with the largest received signal power is selected as the first multipath. Then, from the remaining candidates, multipaths having a large received power are sequentially selected. At this time, ± k timings are excluded from the timing of the already selected multipath, and the multipath having the next highest received power is selected. For example, if the q-th selected multipath timing is u _q , the candidates included in the timing of (u _q −k) ≦ l ≦ (u _q + k) are excluded from the targets of the next multipath selection. The reason why signals at timings before and after the selected multipath are excluded is to prevent the same multipath from being selected by oversampling. k is, for example, the number of oversampling is s,
Set as k = s / 2. In this way, the multipath selection is repeated, and all the multipaths are selected and RAK is performed.
E Perform synthesis.

FIG. 2 shows an example of multipath timing detection. Timing detection in the case where the timings of multipath candidates having reception power satisfying the threshold value are continuous as shown in the figure will be described. At this time, the number of oversamplings is s = 4, and the number of timings for excluding candidates from the already selected multipaths is k = s / 2 = 2.
It is assumed that the timing signal at the point p in FIG. 2 is selected as the multipath. At this time, the timing at which the received power is next highest is p + 1. However, ± p point
Four points of p−2, p−1, p + 1, p + 2 in the range of s are excluded from the candidates of the multipath to be selected next, and p + 4
The signal at the point is selected as the next multipath. Then, p out of 4 points within ± s with respect to p + 4 point
Three points +3, p + 5, and p + 6 are newly excluded from the candidates. In this way, signals at timings before and after the selected multipath are not selected.

FIG. 3 shows an example of the frame structure of a received signal received by the receiver of the present invention. This is a frame configuration in which N _p pilot symbols are inserted for every N _s information symbols. It is assumed that one slot is composed of N _p pilot symbols and N _s information symbols.

FIG. 4 shows an embodiment of the receiver configuration of the present invention. 4, the same components as those in FIGS. 1 and 6 are given the same reference numerals.

The spread modulation signal received is the low noise amplifier 10
After being amplified by 3, the oscillator 105 and the frequency mixer 1
The frequency is converted into an IF frequency by 04. And AG
An amplitude fluctuation caused by fading is compensated by the C amplifier 107 and the envelope detector 108, and quadrature detection is performed. The output baseband signal of the quadrature detector 109 is A /
The digital signals are converted by the D converters 112 and 113. The signal converted into a digital value is despread by the tap number pg matched filter 150. L (= pg × s) where s is the number of oversamplings per chip
The despread signal at each timing is output. The despread signals at the L timings are demodulated by the demodulation unit 152.

In this embodiment, for example, a method of performing absolute synchronous detection demodulation using pilot symbols in the frame structure of FIG. 3 is used. The channel estimation / compensation section estimates and compensates the channel fluctuation due to fading as follows. The matched filter output signal of the n-th slot at the l (1 ≦ l ≦ L) -th timing is averaged to obtain the average value at the l-th timing.

[0040]

[Outside 3]

Is calculated as follows.

[0042]

[Equation 2]

Where

[0044]

[Outside 4]

Is calculated by the following equation.

[0046]

[Equation 3]

In a two-slot pitot symbol

[0048]

[Outside 5]

Is estimated by the following equation.

[0050]

[Equation 4]

This estimated

[0052]

[Outside 6]

Compensate for

[0054]

[Equation 5]

Here, ^* indicates a complex conjugate. Also, the average signal power measuring unit 153 measures the received signal power at all timings and generates a delay profile. The average signal power measurement in each average signal power measurement unit 153 is performed as follows, for example. The matched filter output signal at the l-th timing in the n-th slot is averaged,

[0056]

[Outside 7]

## EQU3 ## is obtained as follows.

[0058]

[Equation 6]

Where

[0060]

[Outside 8]

At

[0062]

[Equation 7]

It is Furthermore, in order to average the fluctuation due to fading, the power of the instantaneous received power is averaged over the immediately preceding multiple slots,

[0064]

[Outside 9]

For example, it is obtained by the following two methods.

(1) Method of averaging the instantaneous received powers of a plurality of slots

[0067]

[Equation 8]

Here, R is the number of slots to be averaged.

(2) A method of sequentially averaging the instantaneous received powers of a plurality of slots

[0070]

[Equation 9]

Here, α is a forgetting factor. At this time R =
It becomes 1 / (1-α).

The minimum signal power and the maximum signal power are detected by the minimum power detection unit 201 and the maximum power detection unit 203 from the average received power of the L timings obtained as described above. The threshold controller A202 obtains the threshold A using the detected minimum signal power. Further, the threshold control unit B204 obtains the threshold B using the detected maximum signal power. In the path selection timing detection unit 205, first, the output of the average signal power measurement unit 153 at L timings is compared with the threshold A and the threshold B, and the average signal power is equal to or greater than the threshold A and the threshold is exceeded. The timing of B or higher is detected. Then, the timing of multipath is detected from the timing when the signal power is large. At this time, the signals at ± k (k is a natural number) timings are excluded from the timing of the already selected multipath, and the timing of the next multipath is sequentially detected. The output of the demodulation unit 152 at the detected multipath timing is the RAKE combining path selection unit 155.
And the selected signal is combined by the RAKE combining unit 119. The RAKE-combined signal has its errors randomized by the deinterleave circuit 120 and decoded by the Viterbi decoder 121. Then, the data determiner 12
2, the received data is obtained.

[0073]

As described above, in the RAKE receiver of the present invention, the average received power of signals at all timings despread by using the matched filter is measured, and two threshold values are calculated from the minimum value and the maximum value. To decide. Then, RAKE combining is performed by selecting a multipath from the despread signals whose received signal power is equal to or more than two thresholds. By using this configuration, signals with only noise and interference components are excluded, and
It is possible to RAKE combine all multipath signals having a sufficiently large received power. Therefore, even if the number of multipaths changes due to the variation of the delay profile, only effective paths can be combined. With this configuration, especially, the chip rate is high, that is, it is possible to improve the characteristics of the reception quality due to the time diversity effect by RAKE for wide band DS-CDMA.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of the present invention.

FIG. 2 is an explanatory diagram of path selection timing detection according to the present invention.

FIG. 3 is a diagram showing an example of a frame configuration of a received signal.

FIG. 4 is a block diagram showing an embodiment of a receiver of the present invention.

FIG. 5 is a DS-C using a conventional sliding correlator.
It is a block diagram which shows the structure of a DMA receiver.

FIG. 6 is a DS-CDM using a conventional matched filter.
It is a block diagram which shows the structure of A receiver.

[Explanation of symbols]

101 antenna 102 Band Pass Filter (BPF) 103 Low noise amplifier 104 frequency mixer 105 oscillator 106 Band Pass Filter (BPF) 107 Automatic gain control amplifier (AGC amplifier) 108 Envelope detector 109 Quadrature detector 110,111 Low pass filter 112,113 A / D converter 114 RAKE synthetic path finger 115 sliding correlator 116 Average signal measuring unit 117 Path selection timing detector 118 Spread Code Generator 119 RAKE combiner 120 Deinterleaver 121 Viterbi Decoder 122 data determiner 131 sliding correlator 132 channel estimator 133 multiplier 150 matched filters 151 Spread Code Replica Generation Unit 152 demodulator 153 Average signal power measurement unit 155 RAKE synthesis path selection unit 201 Minimum power detector 202 Threshold control unit A 203 Maximum power detector 204 Threshold controller B 205 Path selection timing detector

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References Japanese Patent Laid-Open No. 7-231278 (JP, A) Fukumoto Akira (2 others), Characteristics of two-step threshold selection type matched filter RAKE reception in DS-CDMA, Proceedings of IEICE Society Conference, August 13, 1997, 1, pp. 275, B-5-22 (58) Fields investigated (Int.Cl. ⁷ , DB name) H04J 13/00-13/06 H04B 1/69-1/713

Claims (4)

(57) [Claims] 1. RA in a direct sequence CDMA transmission system
In a KE receiver, a matched filter that outputs a despread signal of a path at a timing that is an integer multiple of the number of chips in one symbol, and demodulation that demodulates the despread signal at each timing from the matched filter Unit, an average signal power measuring unit that measures the average signal power of each of the despread signals at each timing from the matched filter, from the output of the average signal power measuring unit, the synthesis of noise and interference components A threshold value calculating section for determining a threshold value A for preventing and a threshold value B for selecting a signal having a sufficient signal power; and the threshold value from the average power from the average signal power measuring section. Path selection for detecting a signal having an average power larger than the threshold value A and the threshold value B from the control unit and determining timing of multipath to be combined The demodulation selected by the RAKE combining path selecting unit and the RAKE combining path selecting unit that selects a signal to be RAKE combined from the signal demodulated from the demodulating unit by the output of the imming detecting unit and the path selection timing detecting unit R for RAKE combining signals
An RAKE receiver comprising an AKE combiner. 2. The RAKE receiver according to claim 1, wherein the threshold value calculation unit includes a minimum power detection unit that detects a minimum power value from average power at all timings from the average signal power measurement unit, From the maximum power detection unit that detects the maximum power value from the average power at all timings from the average signal power measurement unit, the minimum power value from the minimum power detection detection unit, and the maximum power value from the maximum power detection unit , A threshold control section for determining the two thresholds A and B, respectively, and a RAKE receiver. 3. The RAKE receiver according to claim 2, wherein the threshold control unit sets a constant G _A (G _A ≧ G _A ) to a minimum power value.
1) is multiplied and the maximum power value is a constant G _B (0 <G _B ≦ 1)
R characterized in that two threshold values are obtained by multiplying by R
AKE receiver. 4. The RAK according to any one of claims 1 to 3.
In the E receiver, the matched filter performs oversampling (the number s of oversamplings per chip), and the path selection timing detection unit selects multipath timings from all timings in descending order of received signal power. A RAKE receiver which detects the timings of the next multipaths in order by excluding the signals at ± k (k is a natural number) timings from the timings of the paths already detected.