A METHOD FOR DETERMINING CONNECTION QUALITY, AND A RECEIVER

Abstract: The invention relates to a method for determining connection quality in a receiver in a cellular radio system and to a receiver comprising means (25) for decoding a received signal in accordance with the Viterbi algorithm, said means (25) calculating estimates of the merit of the decisions made in addition to decoding the received symbols. To improve the bit error rate estimation accuracy over the connection between the transmitter and the receiver, especially with low bit numbers, the receiver of the invention comprises means (28, 29) for calculating the bit error rate over the connection on the basis of the calculated estimates.

Description: A method for determining connection quality, and a receiver The invention relates to a method for determin- ing connection quality in a receiver in a cellular radio system, in which method a received signal is decoded with a Viterbi decoder whose output provides, in addi¬ tion to the decoded symbols, estimates of the merit of the decisions made. The invention also relates to a receiver com¬ prising means for decoding a received signal in accord¬ ance with the Viterbi algorithm, said means calculating estimates of the merit of the decisions made in addition to decoding the received symbols. In a cellular radio system, the quality of the connection between a base station and a subscriber ter¬ minal varies continuously. This variation is due to dis¬ turbances in the radio path and to the attenuation of radio waves as a function of distance. For example, when a terminal moves further away from the base station, attenuation over the connection between the terminal and the base station increases. It is often attempted to compensate for this attenuation by gain control. Gain measurement as such is not a sufficient parameter for estimating connection quality, however. A unit universally employed to express connection quality in digital cellular radio systems is the bit error rate (BER), representing the number of incorrectly received bits compared to all received bits. In the GSM cellular radio system, for example, the base station and the ter¬ minal are continuously measuring the bit error rate at the air interface. The results of the measurement are transferred to a base station controller that makes a decision, according to the measurement result, on a handover to a channel offering better connection qual- ity, if necessary. The bit error rate measurement must be as accurate as possible to enable optimum utilization of the resources of the cellular radio system and min- imizing of the number of poor connections. In the GSM system, for instance, the performance of the bit error rate measurement has been specified in the GSM 05.08 specification, which is incorporated herein by refer¬ ence. In accordance with this specification, the meas- urement period for the bit error rate is 480 milli¬ seconds, which corresponds to 104 TDMA time slots with full rate channels and 52 TDMA time slots with half rate channels. It is previously known to estimate the channel bit error rate by 'pseudober' measurement. In this method, the bit error rate is estimated by encoding the channel decoding result again and calculating the number of corrected bits. This is not the best possible method, however, since there is no guarantee that the channel decoder always corrects an erroneous bit. The channel decoder may also change a correct bit into an incorrect bit. In addition, the accuracy of the method is very much dependent on the length of the bit sequence being treated. The bit sequence, i.e. the measurement period, should be long enough to enable correct detection of the bit error rate also when erroneous bits occur seldom in the channel. In GSM, this will present a problem par¬ ticularly in half rate channels wherein the number of bits is smaller than in full rate channels. Another known method for estimating the bit error rate is to calculate the bit error rate from the known training sequence in each traffic channel during the measurement period. Since in the GSM system there are 26 bits in the training sequence and over a half rate channel there are 52 frames during a measurement period in each traffic channel, there are only 52*26 known bits wherefrom the bit error rate could be cal¬ culated. This is too few to be able to produce a sufficiently accurate bit error rate estimate. It is previously known to utilize Viterbi de¬ coder soft decision outputs to improve the signal-to- noise ratio of the received signal, as accounted for in J. Hagenauer, P. Hoeher, A Viterbi algorithm with soft- decision outputs and its applications , IEEE GLOBECOM 1989, Dallas, Texas, November 1989, which is incor¬ porated herein by reference. It is an object of the present invention to implement a method enabling accurate estimation of the bit error rate even with low bit numbers, such as occur in a half rate channel. In the method of the invention, the estimation accuracy is not so much dependent on the length of the bit sequence as in the pseudober method. Furthermore, less computation is needed than heretofore. This is achieved with a method as set forth in the preamble, which is characterized in that the estim¬ ates calculated by the Viterbi decoder of the merit of the decisions made are converted into float value format, and that said estimates are utilized in deter¬ mining the connection quality. The receiver in accordance with the invention is characterized in that means (25) comprise means (54) for converting the decision variables representing soft decisions into float value format, and that the receiver comprises means for calculating the bit error rate over the connection on the basis of the calculated estimates. The basic idea of the invention is to utilize the soft decisions obtained from the Viterbi decoder in a novel way to calculate the bit error rate. In the method of the invention, the soft decisions are provided in the Viterbi decoder simultaneously with the bits to be detected such that in calculating the decision metrics in the trellis diagram, part of the decision variables representing the decisions are converted into float value format and stored in a memory unit, and that when the traceback step of the Viterbi algorithm is per¬ formed, the sum of the absolute values of stored decision variables in the correct path and the minimum value of the absolute values of the decision variables in the corresponding path are calculated. The method of the invention provides more accur¬ ate bit error rate calculation than the pseudober method. The method is particularly suitable for use in a half rate channel, as it does not require an equally high number of bits to give a reliable estimate of the bit error rate. Also, the method has less computational complexity, since in the method of the invention the requisite soft decisions can be generated in the Viterbi decoder in the same time that the actual decoding requires. The pseudober method requires extra cal- culation. In the following, a preferred embodiment of the invention will be explained in greater detail with reference to the examples in accordance with the accom¬ panying drawings, in which Figure 1 illustrates a cellular radio system in which the method of the invention can be used, Figure 2 illustrates an example of the con¬ struction of a receiver of the invention on block diagram level, Figure 3 illustrates an example of a trellis diagram employed in a Viterbi decoder, Figure 4 illustrates in greater detail the con¬ struction of a receiver of the invention by means of a block diagram, and Figure 5 illustrates an example of the format of a float value employed in the method of the invention. The method of the invention can be used in any digital cellular radio system in which bit error rate measurement is performed over the connection. In par- ticular, the most significant advantage of the invention is gained over connections with low bit numbers, such as the GSM system half rate channel. In the following, the invention will be described in the context of the GSM system. The construction of a cellular radio system, with parts essential to the invention, is as illustrated in Figure 1. The system comprises a base station 10 and a plurality of normally mobile subscriber terminals 11- 13 that are in two-way communication 14-16 with the base station. The base station switches the connections of the terminals to a base station controller 17, which switches them further to other parts of the system and to the fixed network. The base station controller 17 controls the operation of one or more base stations. In the GSM system, both the base station and the terminals continuously measure the bit error rate over the connec¬ tion between them and transmit the results to the base station controller. The solution in accordance with the invention utilizes the soft decisions obtained from the Viterbi decoder. The soft decisions provided by the channel decoder along with bit decisions, i.e. estimates of the merit of the decisions made, are averaged over a desired measurement period. The resultant averaged value is com- pared to predetermined threshold values. Each threshold value is set to correspond to a given bit error rate value in the channel, and estimate of the bit error rate is obtained as a result of the comparison. The threshold values can be set in type testing measurements on receivers, for instance, in which a known bit pattern is sent through different channel configurations at dif¬ ferent signal-to-noise ratios, which enables calculation of actual bit error rate values for different signal-to- noise ratios and channel types. At the same time, averaged soft decisions corresponding to the actual bit error rates are obtained. In the solution in accordance with the inven¬ tion, the soft decisions are generated in the Viterbi decoder simultaneously with the bit decisions. When the decision metrics of the trellis diagram are calculated, part of the decisions are converted from integer format into float value format and stored in a memory unit. In performing the 'traceback' step, i.e. tracing the state transitions in the trellis diagram in the reverse order, the sum of the absolute values of the stored soft decision variables in the correct path (SDVSUM) and the minimum value of the absolute values of the decision variables in the corresponding path (SDVMIN) are cal¬ culated. As described previously, said SDVSUM value is averaged over the measurement period and compared to the set threshold values to determine the bit error rate. The SDVMIN value enables detection of received faulty signal frames. In the following, the construction of the receiver implementing the method of the invention will be explained in more detail. Figure 2 illustrates an example of a receiver in accordance with the invention by means of a block diagram. The receiver comprises an antenna 20 through which a received signal is applied to radio-frequency parts 21 in which the signal is trans¬ lated to intermediate frequency. From the radio-fre¬ quency parts the signal is applied to converter means 22 in which the signal is converted from analog into digital form. Detector means 23 preferably providing the initial bit decisions using the Viterbi algorithm in the known manner are operationally connected to the con¬ verter means. The Viterbi detector has connected to it means 26 for estimating the signal-to-noise ratio, and the output of these means is applied to scaling means 24 in which the soft bit decisions from the output of the detector are scaled in accordance with the signal-to- noise ratio. This scaling will improve the channel de¬ coder performance. The scaled signal is further applied to a Viterbi channel decoder 25. The decoder 25 performs the final detection and decoding of the received signal in accordance with the Viterbi algorithm. The receiver of the invention util¬ izes the soft decisions obtained in the decoding, i.e. the terms SDVSUM and SDVMIN described above. In the re- ceiver of Figure 2, the decoded signal is further applied to a speech decoder 27. The calculated SDVSUM value is applied to calculation means 28 in which the received values are averaged over the measurement period, preferably 480 s in the case of GSM. The averaged estimate is applied to comparator means 29 in which the estimate obtained is compared to predetermined threshold values. As a result of the comparison, a good estimate 30 of the bit error rate is obtained, and this is further applied to the other parts of the receiver. Likewise, the decoder 25 calculates the term SDVMIN described above, and this is applied to means 31 in which faulty signal frames are detected. The following is a more detailed description of the operation of the Viterbi decoder and the method of the invention for generating soft decisions. Figure 3 illustrates a typical trellis diagram employed in con¬ nection with the Viterbi algorithm. The figure re¬ presents an eight-state trellis diagram. The solving of the Viterbi algorithm proceeds from the left to the right in the trellis diagram. Each point can be reached by two different routes, and at each point the better of the two routes arriving at said point is chosen, and this information is stored. Besides a mere hard bit decision (0 or 1 ), information on the merit of the decision ( 'soft decision' ) can also be stored, thus increasing the memory capacity required for each point in the trellis diagram. For a soft decision, a memory having a minimum of K = AM_SB * AM_ST * S0FT_WL bits is required, where AM_SB = number of bits on which a soft deci¬ sion is stored AM_ST = number of Viterbi states S0FT_WL = word length used in the soft deci- sion. As will be seen from the above formula, the memory capa¬ city required increases directly in accordance with the word length employed in the soft decisions. Furthermore, the time available for data processing is normally limited, and therefore the processing of the soft decisions must be performed with a minimum of cycle time. In the solution in accordance with the inven¬ tion, the Viterbi algorithm is realized with the aim of maximizing the decision variable obtained at each Viterbi node point, i.e., the better the decision, the higher the number representing the decision is. Furthermore, all fixed point values are presumed to be in two's complement form. Since the capacity of the memory storing the decisions is dependent on the word length of the value representing the soft decisions, in the approach of the invention the soft decisions obtained in the executing block are converted into float value format prior to being stored. For example 16-bit soft decisions can be compressed into a float value comprising eight bits, which will cut the memory requirement for the soft deci¬ sions by half. This is a significant advantage espe¬ cially if the Viterbi algorithm is realized by the ASIC technique. On the other hand, if the decisions were stored as fixed point values, having the word length of the float numbers, this would not provide as useful a value range as the present solution. Figure 4 illustrates a possible float number format. In the format, the first bit indicates the sign (Sgn. ), the next four bits the mantissa (Mant. ) starting from the most significant bit, and the last three bits the exponent (Exp.) starting from the most significant bit. With this notation, values (l-2*Sgn) * Mant * 2 Exp can be represented. To convert a 16-bit decision vari¬ able into a float number, the decision variable is first saturated to have 12 bits. This means that if the variable is outside the value range that can be repres¬ ented by 12 bits, the largest or smallest value that can be represented by 12 bits is scaled as the value of the variable. Thereafter the resultant 12-bit value is con¬ verted into the above float number format and stored. The traceback step proceeds from the right to the left in the trellis diagram. During this step, bit decisions regarding the received bits are made on the basis of the stored information. This is performed by tracing the state transitions in the trellis diagram in the reverse order, and choosing the correct state tran¬ sition at each node point in the path in accordance with the decision stored at the corresponding node point, and proceeding to the node point next to the state transition relating to the decision. A decision bit to be read from memory is at the same time a hard decision to be obtained from the Viterbi as regards the bit at that column. In the solution in accordance with the inven¬ tion, information on the merit of the decision in float number format is stored at each node point in addition to the mere decision. In the traceback step the hard decision can in the receiver of the invention be read from the Sgn bit, indicating whether the decision was negative or positive. Simultaneously with performing the traceback step, the sum of the absolute values SDVSUM of the soft decisions in the correct path and the minimum value SDVMIN of the absolute values of the soft decisions in the correct path are also calculated from the stored float value number. These two values are returned together with the obtained hard bit decisions for use in external further processing. To calculate the SDVSUM value, at each node point the float number read from memory is first con¬ verted into 16-bit format. The absolute value of the value obtained is added to a cumulative sum SDVSUM. The absolutized value obtained at each node point and read from memory is also compared to the current SDVMIN value, and if the value that is read is smaller, SDVMIN will receive the value of the number read. Prior to the initiation of the traceback step, SDVSUM has been initialized to 0 and SDVMIN to 7ff. In the manner described above, the SDVSUM and SDVMIN values repres¬ enting the soft decisions made can be read in the traceback step simultaneously with the hard bit deci¬ sions without using any extra cycle time. Figure 5 illustrates by means of a block diagram one possible embodiment for implementing a Viterbi de¬ coder. As described previously, the Viterbi algorithm is solved by proceeding from the left to the right in the trellis diagram. Each point can be reached by two different routes, and at each point the better of the routes arriving at said point is chosen, and this information is stored. Input data 50 is first entered into a Transition Branch Metrics Unit (BMU) 51. At each point, the BMU forms a value corresponding to each route by which that point is reached. A Cumulative Metrics Memory (CUM) unit 53 stores the values previously cal¬ culated for each point of the column. An Add-Compare-Select (ACS) unit 52 compares the variables calculated in the BMU with one another, sel¬ ects the value giving the better result, and adds said value to the sum obtained from the CUM unit; the resultant point-related value is stored in the CUM unit 53 for calculating the next column. The value obtained is stored for the traceback step in a Survivor Memory Unit (SMU) 57 as well, and in the solution in accordance with the invention the decoder comprises means 54 for converting the value into float value format as described above for saving storage space and for the soft decisions. The traceback step is performed in the memory unit 57 by travelling through the trellis diagram in the reverse direction, as described above, and the memory unit as such can be implemented in a manner known to those skilled in the art. At each point in the diagram, the value stored in the memory unit is read, and the Sgn bit directly gives a hard decision 61, which is applied to the other parts of the receiver. For soft decisions, the value that has been read is applied to means 58 in which the value is converted into 16-bit format, and the resultant value is applied to first calculation means 59 calculating the sum of the absolute values of the values obtained, SDVSUM, and to second calculation means 60 calculating the minimum value of the absolute values of the values obtained, SDVMIN. On the basis of these cal¬ culated results, the bit error rate over the connection can be estimated and faulty signal frames detected. Even though the invention has been explained in the above with reference to examples in accordance with the accompanying drawings, it is understood that the invention is not restricted to them but can be modified in a variety of ways within the scope of the inventive idea disclosed in the attached claims.

Claims: Claims: 1. A method for determining connection quality in a receiver in a cellular radio system, in which method a received signal is decoded with a Viterbi decoder whose output provides, in addition to the de¬ coded symbols, estimates of the merit of the decisions made, c h a r a c t e r i z e d in that the estimates calculated by the Viterbi decoder of the merit of the decisions made are converted into float value format, and that said estimates are utilized in determining the connection quality. 2. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the bit error rate over the connection can be determined and the possible incorrect signal frames detected on the basis of said estimates. 3. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the estimates calculated by the Viterbi decoder of the merit of the decisions made are averaged over a desired measurement period, and that the averaged values are compared to predetermined threshold values set to correspond to different bit error rate values. 4. A method as claimed in claim 1, c h a r - a c t e r i z e d in that the estimates of the merit of the decisions made are obtained in the Viterbi decoder in such a way that in calculating the decision metrics in the trellis diagram, part of the decision variables representing the decisions are converted into float value format and stored in a memory unit, and that when the traceback step of the Viterbi algorithm is per¬ formed, the sum of the absolute values of stored decision variables in the correct path and the minimum value of the absolute values of the decision variables in the corresponding path are calculated. 5. A method as claimed in claims 2 and 4, c h a r a c t e r i z e d in that the sum of the absolute values of the decision variables is averaged over a desired measurement period, and that the average value obtained is compared to a predetermined threshold value, as a result of which comparison the bit error rate over the connection is obtained. 6. A method as claimed in claims 2 and 4, c h a r a c t e r i z e d in that faulty signal frames are detected on the basis of the minimum value of the absolute values of the decision variables. 7. A receiver, comprising means (25) for decod¬ ing the received signal in accordance with the Viterbi algorithm, said means (25) calculating estimates of the merit of the decisions made in addition to decoding the received symbols, c h a r a c t e r i z e d in that the means (25) comprise means (54) for converting the decision variables representing the soft decisions into float value format, and that the receiver comprises means (28, 29) for calculating the bit error rate over the connection on the basis of the calculated estimates. 8. A receiver as claimed in claim 7, c h a r ¬ a c t e r i z e d in that the receiver comprises means (31) for detecting incorrect signal frames on the basis of said estimates. 9. A receiver as claimed in claim 7, c h a r ¬ a c t e r i z e d in that the receiver comprises means (28) for averaging the calculated estimates over a desired measurement period, and means (29) for comparing the averaged estimates to predetermined threshold values set to correspond to different bit error rate values. 10. A receiver as claimed in claim 7, c h a r ¬ a c t e r i z e d in that the means (25) for calculat¬ ing estimates of the merit of the decisions made com- prise means (57) for temporary storage of decision vari- ables representing the soft decisions and converted into float number format, and means (58) for reconverting the variables into integer format, and means (59) for calculating the sum of the stored values in the correct path, and means (60) for calculating the minimum value of the absolute values of the decision variables in the correct path.

Inventors: JYRKKAE KARI, PIIRAINEN OLLI, JYRKKAE, KARI, PIIRAINEN, OLLI
Applicants: JYRKKAE KARI, NOKIA TELECOMMUNICATIONS OY, PIIRAINEN OLLI, JYRKKAE, KARI, PIIRAINEN, OLLI
Application year: 1996
Main classification: H04L1
IPC codes: H03M13, H04B17, H04L1