AU1029900A - Method and device for estimating the transmission quality of a digital communication signal - Google Patents

Description

GR 98 P 8106 Foreign Version Description Method and device for estimating the transmission quality of a digital communication signal 5 The invention relates to a method and a device for estimating the transmission quality of a digital communication signal, in which, for bits transmitted by a transmitter, a bit value is allocated at the receiver and a measure is determined for the 10 reliability of the correctness of the allocation. A method or device of this type serves in particular to estimate transmission quality in a mobile radio system and to adapt a transmission mode which is used to the available transmission quality. 15 A speech coder/decoder (codec) which is intended to carry out an adaptation of this type is currently being standardized following the standardization of the GSM Enhanced Fullrate (EFR) speech codec in the year 1996 under the name of 20 Adaptive Multirate (AMR) speech codec as the next generation in ETSI SMG11. The main aims of the AMR codec are to achieve fixed-network quality for speech under different channel conditions and to ensure optimum distribution of channel capacity. The codec is 25 intended to operate under good channel conditions and/or in highly utilized cells in the Halfrate (HR) channel. Under poor channel conditions, it is intended to change over dynamically with the aid of the GSM intra-cell handover to the Fullrate (FR) channel and 30 vice versa. Within a channel mode (FR or HR), a plurality of code modes are available for different speech and channel coding rates, and are similarly intended to be varied according to channel quality (rate adaptation). Transmission with the respective 35 highest possible quality is thus intended to be achieved, taking into account changing channel conditions.

GR 98 P 8106 Foreign Version -2 A sufficiently accurate estimation of channel quality plays a decisive role in the selection of the modes used for transmission (i.e. on changeover between channel modes FR and HR and/or between code modes), and 5 therefore also in the overall AMR concept. Ideally, the speech quality perceived by a user should serve as a criterion for the selection of a mode. It is therefore necessary to define a metric which enables objective measurement of such a priori subjective quality. 10 Possibilities for derivation of such a channel quality metric are bursty RxLev, RxQual in the GSM system, DTX Activation, Frequency Hopping Activation, bit-by-bit or bursty channel state information (CSI) from the equalizer, Residual Error Rate of the channel decoder, 15 Bad Frame Indicator (BFI), Error Concealment in the channel or speech decoder, etc. The present invention is based on an estimation of transmission quality on the basis of channel state information (CSI), as supplied, for 20 example, in the form of soft bits by an equalizer of a conventional mobile radio receiver. Soft bits of this type correspond in each case to one bit of the communication signal transmitted by radio and comprise a given number of bits, for example 8 or 16. The soft 25 bit may comprise a signed integer with values between 21-1 and 2 i1 -1, i = for example 8 or 16, and provides a measure of the reliability with which a bit of the communication signal is identified in the equalizer. Thus, for example, a value -2'~1 of the soft bit 30 designates the reliable identification of a "-1" bit of the communication signal, while the value 2 -1l designates the reliable identification of the "+1" value, the value -1 being logically assigned to ONE and +1 being logically assigned to ZERO. Intermediate 35 values in each case correspond to variously reliable identifications. The sign (MSB) of the soft bit contains the decision of the equalizer as to whether a GR 98 P 8106 Foreign Version - 2a transmitted bit of the communication signal was +1 or 1. The amount of the soft bit indicates how reliable this decision was, i.e.

GR 98 P 8106 Foreign Version -3 it is a measure of the reliability that the allocation of the MSB to the transmitted bit is correct. These soft bits are conventionally used in the receiver to restore the transmitted communication 5 signal as faithfully as possible to the original. The reliability measures contained therein are not suitable for estimating the transmission quality of a channel. The reason for this is that the transmission quality of mobile radio channels is determined by fluctuations in 10 transmission quality due to different causes. Thus, for example, short-term fading, i.e. rapid changes in reception power within a few milliseconds, are generally caused by reflection, refraction and interference in an otherwise unchanged spatial 15 environment. Shadowing as a result of slow changes in the geographical environment, caused by the movement of individual mobile radio subscribers, produces long-term fading in which the mean reception power changes in time intervals of a few seconds. The effects of short 20 term fading on transmission quality can be reduced in a simple manner by temporal interleaving of data blocks. Short-term deteriorations in the reception signal have significant effects on the identification reliability of the equalizer, but, provided that they can be 25 contained by interleaving, do not yet necessarily result in a deterioration in the transmitted speech quality and should therefore be ignored in estimating the latter. A simple possibility for achieving the 30 objective of simple and fast estimation of transmission quality is, according to the present invention, low pass filtering of reliability values of a transmitted sequence of bits. These reliability values are preferably 35 derived from the soft bits by deriving the amount of the soft bit which is assumed to be a signed integer.

GR 98 P 8106 Foreign Version -4 It is furthermore preferable for the low-pass filtering to be preceded by averaging of the reliability values of a given first member n of transmitted bits, in which the n bits with the lowest 5 allocation reliability are selected from a given second number N of bits and the mean reliability value of these n bits is formed. The reason for this measure is that, even with poor transmission quality, the equalizer still frequently supplies or allocates a 10 large number of bits with very high reliability, so that, with averaging of the reliability values of all transmitted bits, the mean value obtained would represent only a very insensitive measure of the transmission quality. 15 The ratio of the members n, N is preferably 5n<N<20n, or preferably 10n~N. A burst of a communication signal transmitted according to the AMR convention comprises N=114 bits. The n=10 most unreliable bits are selected from these and used for 20 averaging. The low-pass filtering is preferably carried out with incomplete suppression in the stop band above a small number of Hz. An equiripple FIR filter, for example, is suitable for this purpose. The incomplete 25 suppression allows it to respond more quickly to abrupt long-term changes in transmission quality than it would in the case of filtering with complete suppression. The low-pass filtered signal is preferably compared with at least one threshold in order to obtain 30 a comparison result which is used as a control signal to switch over between different transmission modes of the communication signal. In order to prevent rapid switching backwards and forwards between transmission modes when the transmission quality fluctuates in a 35 borderline area, it is appropriate to introduce a hysteresis in the switchover between different transmission modes.

GR 98 P 8106 Foreign Version -5 To do this, two thresholds can be allocated to two different transmission modes in such a way that a switchover is effected from a first of the two transmission modes to the second if the lower of the 5 two thresholds is understepped, and a switchover from the second to the first transmission mode is effected if the higher of the two thresholds is exceeded. If the different transmission modes have different data rates, it is furthermore preferable for the number N of bits 10 from which the respective most unreliable soft bits are selected to be predefined for each transmission mode proportional to its data rate. This ensures that the speed of response to a change in transmission quality is the same for the different transmission modes, 15 regardless of their data rate. Further features and advantages of the invention are set out in the following description of embodiments with reference to the figures, in which: Figure 1 shows a block diagram of a base 20 station of a telecommunications system with mobile terminals, comprising a device for estimating transmission quality according to the present invention; Figure 2 shows a block diagram -of a mobile 25 terminal which is equipped with a device according to the invention and which communicates with the base station from Figure 1; Figure 3 shows a measured pattern of the long-term fading in the course of a communication 30 signal; Figure 4 shows the result of an estimation of the reception quality for the same communication signal with averaging over the ten bits with the lowest reliability within a burst; GR 98 P 8106 Foreign Version Figure 5 shows the result with averaging over all bits of a burst; Figure 6 shows the pulse response and frequency response of a low-pass filter of a device 5 according to the invention; and Figure 7 illustrates the conversion of an estimated value of the transmission quality of a communication signal into a control signal for switchover between different transmission types. 10 Figure 1 is a substantially schematic representation of a section of a base station for a telecommunications system which uses a device 1 for estimating the transmission quality of a digital communication signal. The base station receives the 15 digital communication signal via an antenna 2. An equalizer 3 connected to the antenna 2 supplies a soft bit which has a width of, for example, 8 bits, for each bit received by the antenna. The output signal of the equalizer is fed to 20 processing circuits (not shown in the figure) in order to reconstruct the transmitted communication signal. The output of the equalizer 3 is furthermore connected to an input of a CSI generator 4 of the estimating device 1. The CSI generator 4 estimates th-e short-term 25 fading of the transmission channel, determining the transmission quality of each individual burst of the communication signal. Depending on the transmission mode of the communication signal, the latter contains a different number of bursts for each speech frame. In 30 full-rate transmission, a speech frame comprises four bits, in half-rate transmission, two bits. The processing of each individual burst is carried out for an equalizer with an 8-bit resolution in accordance with the C program code set out below.

GR 98 P 8106 Foreign Version -7 C program code Word16 numto_compute=10 Word16 sort [128]; /* Initialization */ for (n=0;n<128;n++) sort [n] = 0 /* count bits with specific reliability */ for (n=0;n<114;n++) sort [ abs(burst)[n]) ] + +; n=0; sum=0; while (1) { if (sort) [n]==0 /* no bit with reliability n present */ n++; else if(sort) [n]<numtocompute sum += sort[n]*n; /* determine number of bits still to be calculated */ n++; } else { sum += numtocompute*n; break; } } } The sign of each soft bit always corresponds 5 to the presumed value of the received bit, and the amount is an integral value between 0 and 127, containing a measure of the reliability of the decision relating to the sign.

GR 98 P 8106 Foreign Version -8 An amount of 0 represents a very unreliable decision and 127 represents a very reliable decision. A temporary "sort" data field with a size of 7 128 is created for the 2 =128 possible different values 5 of the reliability information and is initialized with 0. In a first loop, a measure of the probability that the sign of the soft bit corresponds to the relevant bit of the transmitted communication signal is derived, initially by forming the amount, for the individual 10 soft bits "burst[n]", 0<n<114, and the number of bits within the burst with a specific reliability value is determined and stored according to this value in the "sort" field. Here, the field index represents the reliability and the field context represents the number 15 of bits with this reliability which are present in the burst. Thus, for example, "sort[10]=12" means that there are 12 bits with a reliability of 10. In a second loop, the reliability values of the 10 least reliable bits are added together, starting with the index 0 with 20 the lowest reliability. Division of the sum obtained by the number of bits added together provides a first mean value. The CSI generator 4 furthermore performs a second averaging in which the aforementioned mean 25 values over the 10 bits with the lowest reliability value of a burst for a number K of bursts are in each case added together and divided by K. The number K is equal to 2 for half-rate transmission and equal to 4 for full-rate transmission. It therefore corresponds to 30 the number of bursts per frame, i.e. it is proportional to the data rate of the transmission mode. Due to the dependency of the number of bursts taken into account on the transmission mode, estimated transmission quality values are provided by means of the second 35 averaging with a fixed repetition rate independent of the transmission rate.

GR 98 P 8106 Foreign Version -9 The output signal of the CSI generator 4 obtained by means of this averaging is approximately proportional to the short-term fading of the mobile radio channel on which the communication signal is 5 transmitted. The resulting substantial fluctuations in the output signal of the CSI generator 4 are suppressed with the aid of a low-pass filter 5. The reason for using the low-pass filter 5 instead of averaging over a substantial time interval is that simple averaging 10 beyond a plurality of frames would not produce a satisfactory result, since short-term substantial interference would continue to result in a considerable decrease in the estimated transmission quality, which could indicate that a transmission mode changeover was 15 necessary, even if the decrease lasts such a short time that it can be compensated by interleaving. Unweighted averaging therefore represents a poor low-pass filter. For this reason, the low-pass filter 5 is connected to the output of the CSA generator 4 in the estimating 20 device 1 with the following specifications: - Filter type: FIR equiripple low-pass filter (constant stop band) - Filter order: 28 - Sampling rate: 50 Hz 25 - Passband: 0.2 Hz - Stop band: 1.8 Hz at 20 db attenuation Figure 6 shows in Part A the transmission function h(t) of a filter of this type, and Part B shows the frequency response 20log(IH(2nf)| in decibels 30 as a function of the frequency f in Hz. Other possibilities for low-pass filtering are essentially also possible, for example Butterworth, Tschebyscheff, IIR filters, etc., or weighted averaging, the weight of a soft bit decreasing with increasing age. 35 Figure 3 shows an example of a measured pattern of the long-term fading of a real communication GR 98 P 8106 Foreign Version - 9a signal over 2000 frames, corresponding to a time span of 40 seconds (transmission GR 98 P 8106 Foreign Version - 10 rate 50 frames per second) . The signal-to-noise ratio C/ (I+N) in decibels is plotted on the X-axis. Figure 4 shows the estimation of the reception quality of the communication signal supplied 5 by the low-pass filter 5 with the fading behavior shown in Figure 3. The numerical values of the output signal of the low-pass filter 5, which may be between 0 and 127 (for soft bits with an 8-bit width) are plotted on the X-axis. As can be seen, the times of occurrence of 10 the signal quality extremes from Figure 3 and the estimation from Figure 4 correspond excellently at 700, 1070 and 1490. The amplitude of the deviations of the estimation from Figure 4 also corresponds closely to the pattern shown in Figure 3. 15 Figure 5 shows for comparison the result of an estimation in which all 114 soft bits of a burst have been taken into account, and not only the 10 with the lowest reliability value, as in the case of Figure 4. Although the position of the extremes still 20 corresponds closely to that of the extremes in Figure 3, the amplitude of the deviations is reduced to around half. With 760 frames, the estimation shows a minimum to which no minimum of the measured fading curve from Figure 3 corresponds. The reliability of the estimation 25 is therefore lower overall than in the case of Figure 4. As can be seen, the pattern of the measurement curve from Figure 3 can be accurately reproduced through selection and averaging of the n=10 30 bits with the lowest reliability value from a burst of N=114 bits. It is obvious that, depending on the conditions of use, the quality of the equalizer 3 or other factors, a different value for the number n of the selected bits can produce a closer correspondence 35 between the estimation and a measured quality pattern. It is assumed that, in the cases relevant in practice, a ratio of 5n<N<20n will be satisfied.

GR 98 P 8106 Foreign Version - 11 The output signal of the low-pass filter 5 is present at the output of a metric generator 6. This metric generator 6 is a further-developed comparator, which compares the filter output signal with a 5 plurality of thresholds and, depending on the comparison result, generates a control signal with a width of 2 bits. Horizontal lines A, B, C corresponding to the thresholds are shown in Figure 7 over a curve which corresponds to the curve from Figure 3. If the 10 output signal Lfilt of the low-pass filter 5 is greater than the threshold B, i.e. the transmission quality is therefore very high, the control signal has the binary value 10. In the case of high channel quality where B>Lfjt>A, it has the binary value 11, in the case of 15 poor channel quality where A>Lfjit>C, it has the value 01 and, in the case of very poor channel quality where Lfiit>10, it has the value 00. As can be seen, only one bit of the control signal changes in each case when the filter output signal Lfilt crosses one of the thresholds; 20 this means that the control signal is Gray-coded. The thresholds A, B, C can be freely selected and in each case specify the limits at which the transmission mode is to be switched over. They have the following meaning: 25 - Threshold A: Switchover from the transmission mode with the highest speech rate to a transmission mode with an average speech rate when the threshold is understepped, - Threshold B: Switchover from the transmission mode 30 with an average speech rate to the transmission mode with the highest speech rate when the threshold is exceeded; and, - Threshold C: Switchover from the average speech rate to the transmission mode with the lowest speech rate 35 and vice versa. If a higher value is selected for the threshold B than for the threshold A, a hysteresis is GR 98 P 8106 Foreign Version - lla introduced for the switchover process, i.e. the channel quality must be higher for the switchover from the average to the highest rate GR 98 P 8106 Foreign Version - 12 than for the switchover from the highest to the average rate. Continuous switchover between these two transmission modes is thereby prevented when the channel quality fluctuates in the area of the 5 thresholds A, B. The control signal is present on a first input of a control unit 7. The control unit 7 evaluates the control signal and effects the rate adaptation for the transmission from the mobile terminal to the base 10 station (uplink). For this purpose, it transmits a required uplink rate (ULREQRate) inband, i.e. together with the speech bits, to the mobile terminal. The mobile terminal on the other hand transmits the transmitted uplink rate as ULRATE and transmits the 15 control signal to the base station. Figure 2 shows a substantially schematic block diagram of the mobile terminal which can interwork with the base station from Figure 1. Like the base station, it comprises an equalizer 3 which, using 20 communication signals received via an antenna 2, supplies soft bits to an estimation device 1 which, as with the device from Figure 1, comprises a CSI generator 4, a low-pass filter 5 and a metric generator 6. The control signal generated by the metric generator 25 6 is transmitted via an antenna 8 to the control unit 7 of the base station which, as indicated above, adapts the downlink transmission mode depending on the control signal supplied by the mobile terminal. The control unit 7 evaluates the control 30 signal received by the mobile terminal via the antenna 2 in the same way as the signal supplied to the base station by the metric generator 6. The conversion of the signal Lfilt into a 2-bit control signal is necessary because the control unit 7, 35 in order to control the rate adaptation of the downlink from the base station to the mobile terminal, constantly requires information relating to the quality GR 98 P 8106 Foreign Version - 13 of the downlink, which must be supplied to it by the mobile terminal. However, very few bits are available to transmit this information. Transmission of only the most significant bits of the filter output signal Lfilt 5 would therefore produce too rough a quantization. Transmission of a more finely quantized or complete filter output signal on the other hand would have to be divided over a plurality of frames, which would, however, result in a significant increase in the 10 switchover delay. The 2-bit control signal of the metric generator 6 on the other hand can be transmitted in each speech frame to the base station so that the latter can redefine the transmission mode after each speech frame. 15 This evaluation of the control signal in the control unit 7 is carried out in the same manner for uplink and downlink transmission as follows: numerical values 3, 2, 1 and 0, which change uniformly with the transmission quality, are allocated to the control 20 signal values dual 10, 11, 01 and 00. The current numerical value and the last seven numerical values (i.e. the results of the transmission quality estimation for the last eight frames) are added together and, depending on the sum, a transmission mode 25 is selected which defines a speech transmission rate.

GR 98 P 8106 Foreign Version - 13 of the downlink, which must be supplied to it by the mobile terminal. However, very few bits are available to transmit this information. Transmission of only the most significant bits of the filter output signal Lfilt 5 would therefore produce too rough a quantization. Transmission of a more finely quantized or complete filter output signal on the other hand would have to be divided over a plurality of frames, which would, however, result in a significant increase in the 10 switchover delay. The 2-bit control signal of the metric generator 6 on the other hand can be transmitted in each speech frame to the base station so that the latter can redefine the transmission mode after each speech frame. 15 This evaluation of the control signal in the control unit 7 is carried out in the same manner for uplink and downlink transmission as follows: numerical values 3, 2, 1 and 0, which change uniformly with the transmission quality, are allocated to the control 20 signal values dual 10, 11, 01 and 00. The current numerical value and the last seven numerical values (i.e. the results of the transmission quality estimation for the last eight frames) are added together and, depending on the sum, a transmission mode 25 is selected which defines a speech transmission rate. This is used in the case of the downlink for transmission and, in the case of the uplink, is transmitted to the mobile terminal as a command to set an uplink rate. 30 Consecutive numerical values may in each case change by one step only, i.e., for example, a numerical value of 3 can be followed only by the numerical value 3 again or by 2. The transmission rate, which is defined in dependence thereon, can therefore also 35 change by one step only between two frames. This can be exploited as a priori information in order to minimize GR 98 P 8106 Foreign Version - 13a transmission errors and therefore high-interference speech module errors.

GR 98 P 8106 Foreign Version - 14 Instead of the decentralized decision described here in respect of the transmission modes to be used by the base station control unit for the uplink and downlink, a modification is also conceivable in 5 which the mobile terminal independently decides on the transmission mode to be used for the uplink and/or downlink and transmits corresponding setting commands to the base station.

Claims (24)

1. Method for estimating the transmission quality of a digital communication signal, in which, 5 for bits transmitted by a transmitter, a bit value is allocated at the receiver and a measure is determined for the reliability of the allocation, characterized in that a measure for the transmission quality is obtained by means of low-pass filtering of the reliability 10 values of a transmitted sequence of bits.

2. Method according to claim 1, characterized in that the value and the measure of its reliability are determined on a bit-by-bit basis and are combined into a uniform data word (soft bit) and the low-pass 15 filtering is carried out via the amounts of the soft bits.

3. Method according to claim 1 or 2, characterized in that the low-pass filtering is preceded by averaging of the probability values of a 20 given fixed number n of transmitted bits, and the low pass filtering is carried out via the mean values obtained.

4. Method according to claim 3, characterized in that the n bits with the lowest allocation- correctness 25 probability are selected from a given second number N of bits and the mean probability value of these n bits is formed.

5. Method according to claim 4, characterized in that, for each possible probability value, the number 30 of those bits among the N bits which have the value is determined.

6. Method according to claim 4 or 5, characterized in that 5n<N<20n and preferably 10n~N. GR 98 P 8106 Foreign Version - 16

7. Method according to claim 4, 5, or 6, characterized in that the N bits in each case form an organization unit of the communication signal transmitted between the transmitter and the receiver. 5

8. Method according to one of the previous claims, characterized in that the low-pass filtering is carried out with incomplete suppression in the stop band.

9. Method according to one of the previous 10 claims, characterized in that the low-pass-filtered measure is compared with at least one threshold (A, B, C) in order to obtain a comparison result which is used as a control signal for switchover between different transmission modes of the communication signal. 15

10. Method according to claim 9, characterized in that two thresholds (A, B) are allocated to two different transmission modes in such a way that switchover is effected from a first of the two transmission modes to the second if the lower of the 20 two thresholds (A) is understepped, and switchover is effected from the second to the first transmission mode if the higher (B) of the two thresholds is exceeded.

11. Method according to claim 9 or 10, and referred back to claim 4, characterized in that the 25 different transmission modes have different data rates, and the second number N is defined proportionally to the data rate for each transmission mode.

12. Device for estimating the transmission quality of a digital communication signal, for 30 connection to the output of an equalizer (3) of a receiver for the communication signal, wherein the device (1) receives from the equalizer (3), for bits transmitted by a transmitter, bit values GR 98 P 8106 Foreign Version - 17 allocated by the equalizer (3) and a measure of the reliability of the allocation of the transmitted bits, characterized in that the device (1) comprises a low pass filter (5) which, by smoothing rapid fluctuations 5 in the reliability measure of a transmitted sequence of bits, supplies a signal representative of the transmission quality estimation.

13. Device according to claim 12, characterized in that it comprises a computing circuit (4) for 10 calculating the mean value of the reliability measures of a given number n of transmitted bits.

14. Device according to claim 13, characterized in that the computing circuit (4) comprises means for selecting the n bits with the lowest reliability 15 measure from a set of N bits (N>n).

15. Device according to claim 14, characterized in that the reliability measure is a digital value with a width of i bits, and that the means for selection (4) comprise 2' memory spaces for storing numerical values 20 of the frequencies of occurrence of the displayable probability values.

16. Device according to claim 15, characterized in that the N bits form an organization unit of the communication signal transmitted between a- transmitter 25 and the receiver.

17. Device according to one of claims 12 to 16, characterized in that the low-pass filter (5) has incomplete suppression in the stop band.

18. Device according to one of claims 12 to 17, 30 characterized in that the low-pass filter (5) is an equiripple FIR low-pass filter. GR 98 P 8106 Foreign Version - 18

19. Device according to one of claims 12 to 18, characterized by a metric generator (6) which receives the output signal of the low-pass filter (5), compares it with at least one threshold (A, B, C) and supplies 5 an output signal which, depending on the result of the comparison, defines a transmission mode which is to be used.

20. Device according to claim 19, characterized in that the metric generator (6) compares the output 10 signal of the low-pass filter (5) with two thresholds (A, B) and the control signal switches from a first to a second condition if the lower (A) of the two thresholds is understepped, and from the second to the first condition if the higher (B) of the two thresholds 15 is exceeded.

21. Mobile terminal for a mobile radio system, and characterized in that it comprises a device (1) according to one of claims 12 to 20, and the terminal is set up in order to transmit a control signal, which 20 is supplied by the device (1) and is representative of the transmission quality estimation, to a base station.

22. Terminal according to claim 21 and referred back to claim 19 or 20, characterized in that the control signal transmitted to the base station is the 25 output signal of the metric generator (6).

23. Base station for a mobile radio system, characterized in that it comprises a device (1) according to one of claims 12 to 20 and a control unit (7) which defines the transmission mode used for 30 transmission between the base station and the allocated mobile terminals depending on a control signal representative of the transmission quality estimation. GR 98 P 8106 Foreign Version - 19

24. Base station according to claim 23, characterized in that the control unit (7) is set up to define the transmission mode used with the aid of a control signal transmitted by the mobile terminal.