CN1319293A - Method and device for estimating transmission quality of digital communication signal - Google Patents

Abstract

A method for evaluating the transmission quality of digital communication signals is described, in which for each bit transmitted from the transmitter to the receiver a measure of the reliability of its correct identification is determined and a low-pass filter is applied. The means (1) for implementing the method is connectable to an equalizer (3) and has a low-pass filter (5) which provides a signal representing an assessment of the quality of the transmission by smoothing the probability quantity.

Description

Method and device for evaluating transmission quality of digital communication signals

The invention relates to a method and a device for evaluating the transmission quality of a digital communication signal, in which a bit value is assigned to a bit transmitted by a transmitter to a receiver, and a measure of the correct reliability of the assignment is determined.

This method or device is used in particular to evaluate the transmission quality within the context of mobile radio systems and to adapt the transmission operating mode used to the transmission quality provided for use.

Following the GSM Enhanced Full Rate (EFR) voice codec standard in 1996, the current voice codec that implements this matching is standardized in ETSI SMG11 under the name Adaptive Multi-Rate (AMR) voice codec next generation. The main purpose of the AMR codec is to obtain a constant network quality of speech under different channel conditions and to ensure the best allocation of channel capacity. The codec works with half rate (HR) channels in good channel conditions and/or in heavily loaded cells. In the case of poor channel conditions, it can dynamically use GSM internal unit handover to switch to full rate (FR) channel mode or vice versa. In a channel mode (FR or HR), multiple encoding methods are provided for voice and channel encoding rates that also vary according to channel quality (rate adaptation). In this way, transmissions each with the best possible quality can be achieved by taking into account changing channel conditions.

A sufficiently accurate assessment of the channel quality plays a decisive role in the selection of the mode used for transmission (ie switching between channel modes FR and HR and/or between coding modes) and therefore also for the overall AMR scheme. Ideally, the voice quality experienced by the user should be used as the criterion for selecting the mode. It is therefore necessary to identify a metric that can objectively measure this prior subjective quality. Possibilities for citing this measure of channel quality are: RxLev and RxQual in bursts in GSM systems, DTX activation, frequency hopping activation, bit or burst channel state information CSI from the equalizer, channel decoder residuals Error rate, bad frame indicator (BFI), error concealment in channel codec or speech codec, etc.

The invention is based on the evaluation of the transmission quality on the basis of channel state information (CSI), which is provided in soft bits, for example, by equalizers of conventional mobile radio receivers. Such soft bits each correspond to bits of a communication signal transmitted by radio and include a given number of bits, such as 8 or 16. Soft bits can be understood as signed integers with a value between -2 ^i-1 and 2 ^i-1 -1, i=eg 8 or 16, and provide reliable information for identifying communication signal bits in the equalizer measure of sex. Thus, for example, the value -2 ^i-1 of the soft bit represents a reliable identification of the bit "-1" of the communication signal, and the value 2 ^i-1 -1 represents the reliable identification of the value "+1", where the value -1 is only assigned a logical 1, And +1 only assigns a logical 0. Values in between each correspond to a different reliable identification. The sign (MSB) of the soft bit contains the equalizer's decision whether the transmitted bit of the communication signal was +1 or -1. The soft bit value tells how reliable the decision is, that is to say the decision is a measure of the reliability of the correct allocation between MSB and transmitted bits.

These soft bits are typically used in the receiver to restore the transmitted communication signal as realistically as possible. The reliability measures contained therein are not suitable for evaluating the transmission quality of the channel. The reason is that the transmission quality of the mobile radio channel is limited by variations in the transmission quality caused by different causes. Thus, in an otherwise constant spatial environment, reflections, refractions and interferences generally cause, for example, so-called short-term fading, ie rapid changes in the received power within a few milliseconds. The shadowing effect of slowly changing geographical conditions caused by the movement of individual mobile radio users will result in long-term fading in which the average received power varies over a time interval of a few seconds. The impact of short-term fading on transmission quality can be reduced in a simple way by interleaving data blocks in time. A short-term deterioration of the received signal would be very detrimental to the recognition reliability of the equalizer, however, provided that this deterioration can be prevented by interleaving, it does not inevitably lead to a reduction in the quality of the transmitted voice and can therefore be disregarded in the evaluation it.

According to the invention, a simple possibility of achieving the object of a simple and rapid evaluation of the transmission quality is to low-pass filter the reliability values of the bit transmission sequence.

Such a reliability value is preferably obtained from the soft bits in such a way that the soft bit values are obtained as signed integers.

Furthermore, the reliability values of a given first number n of transmitted bits are preferably averaged before low-pass filtering, wherein the n bits with the lowest assignment reliability are selected from a given second number N of bits, and The average value is determined from the reliability values of these n bits. The reason for this measure is that when the transmission quality is poor, the equalizer usually also provides or allocates many bits with high reliability, so that when the reliability values of all transmitted bits are averaged, the average value obtained is only A fairly insensitive measure of transmission quality.

The relationship between the number n and N is preferably 5n<N<20n, preferably 10n≌N. A burst of a communication signal transmitted according to the AMR convention comprises N=114 bits. From these bits, n=10 least reliable bits are selected for averaging.

In the blockage region above a few Hz, low-pass filtering is preferably performed with incomplete suppression. Equal-systolic FIR filters are suitable for this, for example. Partial suppression reacts more quickly to intermittent but persistent changes in the transmission quality than when filtering with full suppression.

The low pass filtered signal is preferably compared with at least one threshold value in order to obtain a comparison result which is used as a control signal for switching between different transmission modes of the communication signal. To prevent rapid back-and-forth switching between transmission modes, a hysteresis can be purposefully introduced when switching between different transmission modes when the transmission quality varies within a threshold range. For this, 2 thresholds can be assigned to the 2 different transmission modes in such a way that when the lower value of the 2 thresholds is below, a switchover is made from the first of the 2 transmission modes to the second, when above the 2 The transition from the second to the first transmission mode occurs at a higher value of the threshold. If the different transmission modes have different data rates, it is also preferable to specify the number N of bits from which the always least reliable soft bits are selected, for each transmission mode, this number being proportional to its data rate. In this way it can be ensured that the speed of reaction to changes in the transmission quality is the same for the different transmission modes and is independent of the data rates of these transmission modes.

Further features and advantages of the invention emerge from the following description of an exemplary embodiment in conjunction with the drawings. In the picture:

Fig. 1 is a block diagram of a base station of a telecommunication system with mobile terminals, the base station having a device for evaluating transmission quality according to the invention;

FIG. 2 is a block diagram of a mobile terminal equipped with a device according to the invention and communicating with the base station in FIG. 1;

Fig. 3 shows the measurement process to the long-term attenuation in the communication signal process;

Figure 4 shows the results of the evaluation of the reception quality of the same communication signal when averaging the 10 bits with the lowest reliability value within a burst;

Fig. 5 shows the result when all bits of a burst are averaged;

Figure 6 shows the impulse response and frequency characteristics of the low-pass filter of the device of the present invention; and

Fig. 7 illustrates that the evaluation value of the transmission quality of the communication signal is converted into a control signal for switching between different transmission modes.

Figure 1 shows, in simplified block diagram form, part of a base station of a telecommunication system using a device 1 for evaluating the transmission quality of digital communication signals. The base station receives digital communication signals through antenna 2 . An equalizer 3 connected to the antenna 2 provides a soft bit of width, for example 8 bits, for each bit received from the antenna.

The signal at the output of the equalizer is fed to a processing circuit, not shown, for reconstructing the transmitted communication signal. Furthermore, the output of the equalizer is connected to the input of the CSI generator 4 of the evaluation device 1 . The CSI generator 4 evaluates the short-term attenuation of the transmission channel, wherein it determines the transmission quality of each burst of the communication signal. Depending on the transmission mode of the communication signal, the communication signal comprises a different number of bursts within each speech frame. In full-rate transmission, a speech frame contains 4 bursts, and in half-rate transmission it is 2 bursts.

For an equalizer with 8-bit resolution, the processing of each individual burst is implemented according to the following C program code.

C program code:

           
Word16 num_to_compute=10
    Word16 sort[128];

    /*initialization*/

    for(n=0; n<128; n++)

    sort[n]=0

    /*Count the bits with certain reliability*/

    for(n=0; n<114; n++)

    sort[abs(burst[n])]++;

    n=0; summe=0;

    while (1) {

    if(sort)[n]==0/* unreliable bits are n*/

    n++;

    else {

    if(sort)[n]<num_to_compute)[

    summe+=sort[n]*n; /* determine the number of bits that should still be calculated */

    n++;

    }

    else {

    summe+=num_to_compute*n;

    break;

    }

    }

        

The sign of each soft bit is always consistent with the guessed value of the received bit, and the value is a count value between 0 and 127, which contains a measure of the reliability of the sign decision. Here, a value of 0 is used to indicate a very unreliable decision, and 127 is used to indicate a very reliable decision.

For 2 ⁷ =128 possible different values of reliability information, a temporary data segment "sort" of size 128 is set up and initialized with 0. In the first cycle, the probability quantity that the soft bit symbol is consistent with the corresponding bit of the transmitted signal is obtained by calculating the value as a single soft bit "burst[n]"(0<n<114), and the measurement has a certain reliability value The number of bits in the burst, and corresponding to the value stored in the data segment "sort". Here, the index of the data segment represents the reliability, and the content of the data segment represents the number of bits within the burst with this reliability. So, for example, "sort[10]=12" means 12 bits with a reliability of 10. In the second cycle, starting from index 0 with the lowest reliability, the reliability values of at least 10 reliable bits are accumulated. The first average is obtained by dividing the obtained sum by the number of accumulated bits.

In addition, the CSI generator 4 performs the calculation of the second average value, wherein the minimum reliability value of the bursts of K bursts is added to the average value of 10 bits each time, and divided by K. The number K is equal to 2 in half-rate transmission and 4 in full-rate transmission. That is, it is equivalent to the number of bursts per frame, which means that it is proportional to the data rate of the transmission mode. Since the number of bursts considered depends on the transmission mode, the calculation of the second mean value provides an estimate of the transmission quality with a fixed, transmission-rate-independent repetition rate.

The output signal of the CSI generator 4 obtained by this averaging is approximately proportional to the short-term attenuation of the mobile radio channel through which the communication signal is transmitted. The resulting large variation of the output signal of the CSI generator 4 can be suppressed by means of a low-pass filter 5 . The reason for using a low-pass filter 5 instead of averaging over larger time intervals is that simple averaging over multiple frames would lead to unsatisfactory results, as short-term strong disturbances would also lead to large Amplitude reduces the estimated transmission quality, even if the reduction is only short-lived so that it can be compensated for by interleaving, but said reduction may also necessitate a switchover of the transmission mode. Thus, the unweighted average calculation shows that the low-pass filter is poor. Therefore, on the evaluation device 1, a low-pass filter 5 with the following specifications is connected to the output of the CSI generator 4:

- Filter type: FIR etc. systolic low-pass filter (constant blocking area)

- Filter order: 28

- Sampling rate: 50Hz

- Passband: 0.2Hz

- Blocking zone: 1.8Hz at 20db attenuation

Figure 6 shows the transfer function h(t) of such a filter in part A and the frequency characteristic 20 log(|H2(πf)|) in decibels as a function of frequency f(Hz) in part B . In principle, other possibilities of low-pass filtering are also conceivable, such as Butterworth filters, Tschebyscheff filters, IIR filters, etc., or weighted averaging, in which the weighting of the soft bits decreases with age.

FIG. 3 shows the measurement curves of long-term attenuation of actual communication signals over 2000 frames corresponding to a time interval (transmission rate of 50 frames per second) of 40 seconds. The signal-to-noise ratio C/(I+N) in decibels is plotted on the abscissa.

FIG. 4 shows an evaluation of the reception quality of a communication signal having the attenuation characteristic shown in FIG. 3 , which is provided by the low-pass filter 5 . The digital value of the output signal of the low-pass filter 5 is plotted on the abscissa, which value is between 0 and 127 (for 8-bit wide soft bits). It can be seen that the moment when the extreme value of the signal quality in Fig. 3 appears is very consistent with the evaluation at about 700, 1070 and 1490 frames in Fig. 4 . The magnitude of the evaluation bias in Figure 4 also coincides exactly with the process shown in Figure 3.

FIG. 5 shows a comparison of the results of an evaluation in which all 114 soft bits of the burst are considered instead of only the 10 bits with the lowest reliability value as in the case of FIG. 4 . Although the location of the extrema coincides exactly with that in Fig. 3, the magnitude of the deviation is reduced by about half. At frame 760, the evaluation shows a minimum with which none of the measured minimums of the decay curves in Figure 3 coincide. Therefore, the overall estimated reliability is smaller than that shown in Fig. 4.

It can be seen that the process of the measurement curve shown in Figure 3 can be well reproduced by selecting the n=10 bits with the lowest reliability value from the bursts of N=114 and averaging them. Obviously, other values of the number n of bits selected also allow better agreement of the evaluation with the quality process being measured, depending on the conditions of use of the equalizer 3, quality or other factors. It is assumed that the relationship 5n<N<20n needs to be satisfied in practically important cases.

The output signal of the low-pass filter 5 is connected to the input of a so-called metric generator 6 . The metric generator 6 is a modified comparator which compares the output signal of the filter with a plurality of threshold values and generates a 2-bit wide control signal from the result of the comparison. In FIG. 7 the horizontal lines A, B, C corresponding to the threshold values are shown by a curve corresponding to the curve shown in FIG. 3 . If the output signal _Lfilter of the low-pass filter 5 is greater than the threshold value B, the transmission quality is good and the control signal has a binary value of 10. The control signal has a binary value of 11 when having a good channel quality with B>L _filtering >A, a binary value of 01 when having a poor channel quality with A>L _filtering >C, and a very poor channel with L _filtering >10 quality, the value is 00. It can be seen that when the filter output _signal L exceeds a threshold value, only one bit of the control signal is changing; that is, the control signal is gray coded.

Thresholds A, B, C are freely selectable and all give the limits for switching transmission modes. It has the following meanings:

- Threshold A: below this threshold, the transmission mode with the highest speech rate is switched to the transmission mode with the medium speech rate,

- Threshold B: when the threshold is exceeded, the transmission mode with the medium speech rate is switched to the transmission mode with the highest speech rate, and

- Threshold C: switching from the transmission mode with the medium speech rate to the transmission mode with the lowest speech rate and vice versa.

As long as the selected threshold B is higher than the threshold A, it will cause hysteresis to the switching process, that is, the channel quality when switching from the medium rate to the highest rate must be better than that when switching from the highest rate to the medium rate. Thus, constant switching between these two transmission modes is prevented when the channel quality varies within the threshold A, B range.

The control signal is connected to a first input of the control unit 7 . The control unit 7 evaluates the control signal and implements a rate adaptation for the transmission of the mobile terminal to the base station (uplink). For this purpose, it transmits the required uplink rate (UL_REQ rate) to the mobile terminal together, ie together with the speech bits. In contrast, the mobile terminal transmits the transmitted uplink rate at the "UL rate" and transmits this control signal to the base station.

FIG. 2 shows a simplified block diagram of a mobile terminal which can operate with the base station of FIG. 1 . It comprises, like a base station, an equalizer 3 which, by means of a communication signal received via an antenna 2, supplies soft bits to an evaluation device 1 which, exactly like the base station of FIG. 1, comprises a CSI generator 4, a low-pass filter generator 5 and metric generator 6. The control signal generated by the metric generator 6 is transmitted via the antenna 8 to the control unit 7 of the base station, which, as described above, adapts the downlink transmission mode on the basis of the control signal provided by the mobile terminal.

The control signal received by the mobile terminal via the antenna 2 is evaluated by the control unit 7 in the same way as the signal provided by the metric generator 6 of the base station.

It is necessary to _filter and convert the signal L into a 2-bit wide control signal, because in order to control the matching of the downlink rate from the base station to the mobile terminal equipment, the control unit 7 always needs information about the downlink rate provided to it by the mobile terminal equipment. quality information. However, only a few bits are provided for transmitting this information. Therefore, transmitting only the most significant bits of the filter output signal Lfilter will result in coarse quantization. Conversely, transmitting a finely quantized or complete filter output signal would have to be divided into multiple frames, which would result in a significant increase in transition latency. Instead, the 2-bit-wide control signal of the metric generator 6 can be transmitted to the base station every speech frame, so that the base station can redetermine the transmission mode every speech frame.

Such a control signal analysis in the control unit 7 can be carried out in the same way for the uplink and downlink transmissions as follows: The values 3, 2, 1 or 0, which vary monotonically with the transmission quality, are respectively assigned to the doubled control signal values 10, 11, 01 and 00. The current value is added to the last 7 values (ie the evaluation results of the transmission quality of the last 8 frames), and the transmission mode for determining the speech transmission rate is selected according to the sum. This value is used for transmission in the downlink and is sent to the mobile terminal device in the uplink as an instruction to adjust the uplink rate.

Successive values can only be changed by one step, that is to say, for example, the value 3 can only be followed by the value 3 or 2 again. Accordingly, the transmission rate thus determined can only vary by one level between two frames. This can be used as a priori information in order to reduce transmission errors and thus the disturbing speech modulus errors.

Instead of the centralized determination of the transmission mode to be used for uplink and downlink by the base station control unit described here, a variant is also conceivable in which the decision for uplink and/or downlink is made by the mobile terminal itself. The transmission mode adopted by the link, and the corresponding adjustment command is sent to the base station.

Claims (24)

1. assess the method for the transmission quality of digital communication signal, wherein be transferred to bit value of Bit Allocation in Discrete of receiver for transmitter, and measure measuring of this distribution reliability, it is characterized in that measuring by the reliability value of bit sequence of transmission is carried out low-pass filtering of transmission quality obtains.

2. method according to claim 1 is characterized in that, measures the value of described reliability by bit and measures, and comprehensively becomes a unified data word (soft bit), and utilizes the Numerical Implementation low-pass filtering of this soft bit.

3. method according to claim 1 and 2 is characterized in that, the probable value to the transmitted bit of the given first number n before low-pass filtering is asked on average, and utilizes the mean value that is obtained to carry out low-pass filtering.

4. method according to claim 3 is characterized in that, selects n the bit that distributes correctness to have minimum probability from the bit of given second number N, and passes through the described mean value of probability calculation of this n bit.

5. method according to claim 4 is characterized in that, for each probable value of described probability is measured the number that is arranged in those bits this N bit, that have this value.

6. according to claim 4 or 5 described methods, it is characterized in that, 5n＜N＜20n, and be preferably 10n ≌ N.

7. according to claim 4,5 or 6 described methods, it is characterized in that a described N bit always is formed in the OU of transmission communication signal between the transmitter and receiver.

8. according to one of aforesaid right requirement described method, it is characterized in that having the low-pass filtering of imperfect inhibition at blocked-off region.

9. according to one of aforesaid right requirement described method, it is characterized in that, compare through measuring of low-pass filtering, so that the comparative result that acquisition is changed the different transmission mode of signal of communication with the control signal form with at least one threshold value (A, B, C).

10. method according to claim 9, it is characterized in that, 2 kinds of different transmission modes are so distributed 2 threshold values (A, B), when being lower than the low threshold value (A) of 2 threshold values with box lunch, first kind from 2 kinds of transmission modes is transformed into second kind, and when the higher thresholds that is higher than 2 threshold values (B), be transformed into first kind from second kind of transmission mode.

11., with regard to relating to claim 4, it is characterized in that described different transmission mode has different data rates according to claim 9 or 10 described methods, and regulation, for every kind of transmission mode, second number N is all proportional with data rate.

12. the device of the transmission quality of assessment digital communication signal, be used to be connected the output of the equalizer (3) of communication signal receiver, wherein, described device (1) receives the bit value of being distributed to these bits by equalizer (3) for the bit that transmitter transmitted from equalizer (3), and receive the measuring of distribution reliability of institute's transmitted bit, it is characterized in that, described device (1) comprises low pass (5), and this low pass (5) smoothly provides the signal of expression transmission quality assessment by the quick variation of measure of reliability to the bit sequence of transmission.

13. device according to claim 12 is characterized in that, it includes counting circuit (4) so that the mean value of the measure of reliability of the transmitted bit of calculating given number n.

14. device according to claim 13 is characterized in that, described counting circuit (4) has a kind of device, and being used for from quantity is that (bit of N＞n) is selected the described n bit that least reliability is measured that has to N.

15. device according to claim 14 is characterized in that, described measure of reliability is the digital value of i bit width, and the described device that is used to select (4) comprises 2 ⁱIndividual memory location is so that the frequency numerical value that probable value occurred that storage presents.

16. device according to claim 15 is characterized in that, a described N bit has constituted the OU of transmission communication signal between transmitter and receiver.

17., it is characterized in that described low pass filter (5) has incomplete inhibition at blocked-off region according to the described device of one of claim 12 to 16.

18., it is characterized in that described low pass filter (5) such as is at the Systolic FIR low pass filter according to the described device of one of claim 12 to 17.

19. according to the described device of one of claim 12 to 18, it is characterized in that metric generator (6), this metric generator (6) receives the output signal of low pass filter (5), this output signal and at least one threshold value (A, B, C) are compared, but also an output signal is provided, this output signal is determined an available transmission mode according to comparative result.

20. device according to claim 19, it is characterized in that, described metric generator (6) compares output signal and 2 threshold values (A, B) of low pass filter (5), and when being lower than the low threshold value (A) of 2 threshold values, control signal is become second kind of state from first kind of state exchange, when the higher thresholds that is higher than 2 threshold values (B), from second kind of state exchange to the first kind of state.

21. the mobile terminal device of mobile radio system is characterized in that, it comprises the described device of claim 12 to 20 (1), and this terminal equipment is provided for and will be transferred to the base station by the control signal that the expression transmission quality that device (1) provides is assessed.

22. terminal equipment according to claim 21 with regard to relating to claim 19 or 20, is characterized in that, the control signal that is transferred to the base station is the output signal of metric generator (6).

23. the base station of mobile radio system, it is characterized in that, it comprises the described device of claim 12 to 20 (1), and contain control unit (7), this control unit is determined the transmission mode that adopted during in transmission between base station and the mobile terminal device that distributed according to the control signal of expression transmission quality assessment.

24. base station according to claim 23 is characterized in that, described control unit (7) is provided for by the transmission mode of being determined by mobile terminal device institute control signals transmitted to be adopted.

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