MXPA98004001A - Method and apparatus for measurement of supervision audio tone signal resistance (sats) in a ra communications system - Google Patents

Abstract

The present invention relates to methods and systems for measuring the signal strength of supervisory audio tones (SATs) in radio communication systems. Exemplary embodiments describe how such measurements can be made using digital signal processing techniques, while reducing the amount of computing power, e.g., MIPs, used to accomplish this task. During each measurement interval a large number of SAT signal samples are received. Instead of processing a measurement interval of SAT samples received, exemplary modalities are based on a sliding average of samples that include both current and former block samples. In this way, the SAT signal resistance can be reported relatively frequently to the system but without the number of computations that would be required to process all the SAT signal samples received during a measurement interval.

Description

METHOD AND APPARATUS FOR MEASUREMENT OF SUPERVISION AUDIO TONE SIGNAL RESISTANCE (SATs) IN A RADIO COMMUNICATIONS SYSTEM RELATED APPLICATION The following patent application is related to the present application: Patent Application of E.U.A. No. 08 / 561,848, entitled "Signal Determination to Noise Utilizing Digital Signal Processing" to Ornar Ryde et al., Filed on the same day as the present application. The disclosure of this related patent application is expressly incorporated herein by reference.

BACKGROUND The present invention is generally directed to radiocommunication systems and, more particularly, to techniques for measuring a signal strength of a supervisory audio tone in said systems. In cellular mobile radio systems, it is desirable that a mobile station with a connection established in a radio channel must be able to maintain the established connection when moving from a cell served by a base station to another cell served by another radio station. base. The process by which a mobile station maintains an established connection when moving between cells in a cellular radio system is generally referred to as delivery. It is also highly desirable that a mobile station with a connection established in a radio channel be able to maintain the connection when moving within the same cell, even when the radio channel being used is subjected to increased interference. If the quality of the established connection falls below specified parameters, it is also desirable to automatically disconnect the connection in case the delivery or other signal processing fails to improve the quality of the connection. In general, radio communication is possible when the desired information radio carrier signals have sufficient signal strength in the receiver and are sufficiently strong in relation to the noise and radio interference signals in the receiver. Of course, the minimum signal strength depends on the particular characteristics of the system, v. gr. , the kind of modulation and receiver used. In order to determine whether an established connection should continue on a selected radio channel between a mobile station and a base station, the delivery and disconnection processes perform various measurements on the radio signals at the intended base and / or mobile stations . The first cellular mobile radio systems in public use were analogous systems used to transport speech or other analogous information. These systems comprised multiple radio channels for transmitting analogous information between the base and mobile stations transmitting analog modulated radio signals. In general, the signal measurements made during the delivery and disconnection processes in said systems were carried out by the base stations. One of these systems is known as the Nordic Mobile Telephone System NMT 450. Another analog cellular mobile radio system, of particular interest as background to the present invention, is the Advanced Mobile Phone Service (AMPS) mobile radio system used in the U.S.A. Recently, mobile cellular radio systems have been designed for public use. Digital cellular mobile radio systems provide digital radio channels to transmit digitized digital or analog information between base and mobile stations using digitally modulated radio signals. Digital cellular mobile radio systems can offer substantial advantages, v. gr. , greater system capacity per unitary bandwidth, over similar cellular mobile radio systems. To achieve these advantages there are different demands. In particular, channel supervision, delivery and disconnection processes need to be carried out quickly and be carried out frequently in relation to conventional analog systems. In contrast to the introduction of cellular mobile radio systems, only digital, such as the GSM system used in parts of Europe, in areas with existing analog cellular systems has been proposed to introduce digital cellular mobile radio systems that are designed to cooperate with existing analog cellular mobile radio systems. In this way, extended legacy customer bases will not suddenly find that their terminal equipment has become obsolete. System designers of these hybrid systems believe that the digital portion of the system can be introduced gradually and, over time, the number of digital channels can be gradually increased, while the number of analog channels is gradually decreased. In order to provide full compatibility, these systems must behave in a dual way with both analogous and digital standards that have been adopted, for example. the analog standards AMPS and TACS. In AMPS, a supervisory audio tone, abbreviated SAT, is transmitted on analog communication channels. More specifically, a base station transmits a SAT to a mobile station that receives the SAT and transposes the bonus back to the base station to close the loop. The reason for transmitting the SAT in AMPS is that, in a mobile radiocommunication network limited in interference, there must be some mechanism for the receiving entity (eg, the base station), to identify the transmitting entity (v.gr). ., a mobile station) or at least with high probability to exclude the exchange of transmitting entities without the need for continuous transmission of a transmitter identity. In this way, the base station expects to receive the same SAT it sent, that is, on the same frequency. If a different SAT is received through the base station, then it is perceived that the connection is inteferred and can be disconnected. In order to qualify as a valid SAT, a predetermined signal strength must be received by the base station. The AMPS standard specifies that the SAT does not need to be determined continuously, but must be determined at least every 250 ms. In this way, conventional analog base stations measured the resistance of SAT using analog hardware components and provided a report of SAT signal strength to the network periodically, for example the RBS 882 manufactured by Telefonaktiebolaget LM Ericsson reported the SAT signal strength every 20 ms. Although base stations and dual-mode mobile stations continue to support analog system functions, such as SAT detection, the ways in which these functions are sustained are continuously being improved to reduce cost and improve quality. With the increased power of digital signal processors (DSPs), system designers are interested in implementing many signal processing techniques previously implemented using analog hardware components such as DSP routines. The implementation of DSP has the added attraction of reducing the number of components, and therefore the size, of base stations and mobile stations. Of course, digital signal processing also has its limitations. A design change confronting system designers when trying to implement analog signal processing techniques such as DSP routines is that of the speed of execution of a routine versus the amount of DSP resources, v. gr. , the number of millions of instructions per second (MIPS), which are assigned to execute the routine. Since digital signal processing is not yet so cheap that the number of MIPS used for each routine is negligible, system designers need to develop innovative digital signal processing techniques that reduce the number of MIPs used so that the processor Digital signal can handle as many tasks as possible.

COMPENDIUM These and other disadvantages and limitations of conventional methods and techniques for measuring, for example, the SAT in radio communication systems are overcome in accordance with the present invention. Exemplary modalities describe how such measurements can be made using digital signal processing techniques, while reducing the amount of computational energy v, gr., MIPs, used to accomplish this task. During each measurement interval, a large number of SAT signal samples are processed by the base station. Instead of using all the SAT samples received during a measurement interval in a DSP measurement routine, exemplary embodiments of the present invention utilize a sliding average of samples that include both a block of signal samples received during the interval of measurement as blocks of previous samples. In this way, the SAT signal resistance can be reported relatively frequently to the system but without the number of computations that would be required to process all the SAT signal samples received during the measurement interval.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings, in which: Figure 1 is a block diagram which generally illustrates the measurement of SAT and the report in a radio communication system in accordance with an exemplary embodiment of the present invention; Figure 2 is a block diagram illustrating a digital fourier transformer applied to provide an indication of SAT signal strength; Figure 3 is a block diagram of an exemplary embodiment of the present invention, wherein the SAT signal strength is calculated using both the current and previous received samples; Figure 4 (a) is a graph illustrating a signal tone resistance of SAT through the air interface; Figure 4 (b) is a graph illustrating an exemplary measurement of the SAT tone of Figure 4 (a) using the routine of Figure 2; and Figure 4 (c) is a graph illustrating an exemplary measurement of the SAT tone of Figure 4 (A) using the routine of Figure 3.

DETAILED DESCRIPTION As described above, supervisory audio tone (SAT) is used to monitor connections between base stations and mobile stations for analog radio traffic channels. To describe how the SAT is generally used, Figure 1 illustrates exemplary functional units in a radio communication system through a general block diagram. The mobile station (MS) transmits an SAT to the radio base station (RBS) 12 during the time it is connected to the RBS 12 through an analog traffic channel. The SAT is detected and measured, in accordance with the present invention, by a digital processor (DP) or digital signal processor (DSP) 14 at the radio base station 12. The digital processor 14 reports the signal strength received by the SAT to another processor 16 (sometimes referred to as a regional processor (P)) in RES 12. The processor 16 interprets the reported signal strength of the SAT as indicating that the SAT is present (connected) or not present (disconnected), for example, by comparing the reported signal strength with some predetermined signal resistance threshold. This interpretation is then sent to the network through a central processor 18 (CP) of the mobile switching center 20 (MSC). Since many of the details of the base station equipment itself can not germinate to a discussion of the present invention, the applicants have omitted such details to avoid obscuring the invention. Readers interested in additional details of base stations including digital signal processors are generally directed to the U.S. Patent. No. 5,295,178 whose disclosure is incorporated herein by reference. Conventionally, analogous equipment was used to measure the signal strength of the SAT in the radio base stations. The SAT signal resistance measurement technique applied in these conventional systems can be recreated in a digital signal processing routine by performing a digital fourier transformation in the received SAT samples as illustrated in Figure 2. In the same, a current of incoming SAT samples is received at the entry node 22. The sample regime of the incoming stream will depend on various design considerations, but, for example, it can be 16 km / sec. Those skilled in the art will be familiar with the manner in which the digital fourier transformation is performed, however, a brief description of the functional blocks of Figure 2 below is provided to complete. The incoming sample stream is separated into its real (in phase) and imaginary (quadrature) components in mixers 24 and 26, respectively. The frequency selected for mixing in blocks 24 and 26 is that of the expected SAT to be returned by the mobile station. A number of different SAT frequencies can be used in a radio communication system to differentiate between transmissions from neighboring cells. for example, three frequencies of SAT, 5970, 6000 and 6030 Hz are used in AMPS. A predetermined number of sample components N is added in the adders 28 and 30, respectively.

The selection of the number of sample components N to be added for each measurement interval depends, among other considerations, on the resolution necessary to distinguish between different SAT frequencies that use the digital Fourier transformation. Conventionally, the SAT was measured continuously since the analogous equipment was used to perform this function and, consequently, the resolution of the measurement was not a significant problem. In a digital domain, however, the measurement resolution becomes interesting, using the AMPS standard as an example, a minimum resolution of 30 Hz is needed to distinguish between three different frequencies of SAT that are available. To provide better performance when distinguishing between SAT frequencies, a resolution of less than 30 Hz, e.g., 10 Hz is preferable. With increased resolution, however, comes the need for increased computation by the digital signal processor 14, that is, more MIPS are being used for this routine. It has been determined that providing a continuous measurement of SAT is not feasible due to the number of computations required in the DSP to implement said function continuously. In this way, the applicants have considered various alternatives. For example, providing a resolution of 1 Hz in the digital fourier transformation routine would require, in the previous example, where the input sample stream has a rate of 16 km / sec. that add-ons 28 and 30 add 16,000 signal components for each branch of digital fourier computing, that is, N = 1600. A resolution of 10 Hz has been considered by applicants to provide a desired balance between competing objectives of better detection performance and will use less DSP computation resource requirements, however, those skilled in the art will note that the inventive concepts described herein can be applied to any desired computational resolution. The outputs of the adders 28 and 30 apply to operators 32 and 34 squares, respectively, whose results are added to the adder 36 to complete the digital fourier transformation. This result can then be reported from the digital signal processor 14 to the regional processor 16 for threshold detection as described above. However, another difficulty arises when using the implementation of the digital fourier transformation as illustrated in Figure 2. When using the exemplary 10 Hz computational resolution to generate an output S? of the routine illustrated in Figure 2, the regional processor 16 receives a report only once every 100 ms. By way of contrast, conventional analog systems operating in accordance with AMPS provide reports of SAT signal strength every 20 ms. In order to be compatible with this existing standard it is desirable that the output rate of the routine of Figure 2 is increased to 20 ms, but without adding to the number of computations to be made during each measurement interval by the processor 14 of digital signal. Figure 3 illustrates an exemplary technique to achieve a shorter report or interval to bring up to date, In the same, those blocks that perform the same functional in the digital fourier transformation routine as described with respect to Figure 2, retain the same reference number and are not described again herein. In accordance with the Nyquist sampling theorem, since SAT tones are transmitted at approximately 6 kHz, at least 12 km / s are required to accurately digitize these tones. Sixteen km / s have been selected as an exemplary entry sampling regime since it is a convenient energy of two. In order to reduce the computational time needed to provide an up-to-date SAT power resistance to the system without changing the resolution of the computation, the exemplary embodiment of Figure 3 computes the digital fourier transformation using the number of M samples received during the turn-around period together with a number of previously received (and stored) N-M samples needed to provide the proper resolution to the computation. A numerical example will serve to better explain this concept. Assume that, as with the example of Figure 2, the input sample stream to node 22 of Figure 3 is again 16 km / s. Since an output is desired every 20 ms, 320 samples will be received during each start-up interval, ie. M = 320 In this way, for each n commissioning interval, the adders 40 and 42 each will operate on 320 sample components. If, for example, a computational resolution of 10 Hz is desired, however, 1600-320 = 1280 sample components are required for the digital fourier transformation. These additional sample components are provided, in accordance with this exemplary embodiment of the present invention, by adding the four previous outputs of the adder 40, ie, those produced in the intervals n-1, n-2, n-3 and n -4 for the current, at the current output in the n setting interval to the current in the adder 44. Similarly, for the imaginary components, the four previous outputs of the adder 42 are added to the current output of the adder 46. In order to retain the historical data necessary to provide this sliding average of samples to the digital fourier transformation, each output of the adders 40 and 42 is stored as indicated by blocks 48 and 50 respectively. Since only a certain number of historical outputs are needed, eg, four in the previous example. the memory devices (not shown) used to store these historical outputs, for example, may be circular memories that retain only the most recent outputs. This exemplary SAT measurement technique can also be described using the pseudo code as outlined below: START sum_i = 0; sum_q = 0; FOR k = 1 A 320 DO START SUM_i = sum_i + (in-frame (k) * cos (2 * PI * SAT_FREQ * cuental00 / 1600)); sum_q = sum_q + (in_frame (k) * sin (2 * PI * SAT_FREQ * cuental00 / 1600)); IF count 100 > = 1600 THEN count 100 = 0 OR count 100 = count 100 + 1; FINAL delta_sum_i (count20) = sum_i; delta_sum_q (count20 = = sum_q; YES count20 = 5 THEN count20 = 1 OR count20 = count20 + 1; sum_sat_i = 0; sum_sat_q = 0; FOR k = 1 A 5 DO START SUM_sat_i = sum_sat_i + delta_sum_i (k); sum_sat_q = sum_sat_q + delta_sum_q (k); FINAL sat-resistance = 10 * LOG10 (SQR (sum_sat_i) + SQR (sum_sat_q)); FINAL The graphs in Figures 4 (a) to 4 (c) are used to describe the results that can be achieved by the present invention In Figure 4 (a), a mobile station begins transmitting the ST at time T. In this manner, Figure 48a) illustrates the signal strength of the SAT over time as it could appear in the air interface. Figure 4 (b) illustrates the measurement and detection of the SAT signal resistance using the routine of Figure 2 in a "worst case" scenario. That is, suppose that the time t in which the mobile station begins transmitting the SAT is at the beginning of a measurement interval of the routine of Figure 2. Since the routine of Figure 2 takes 100 milliseconds to provide a setting In the worst case, as shown in Figure 4 (b), there will be a delay of 100 milliseconds between the transmission of the SAT by the mobile station and the detection of the SAT by the system. By way of comparison, Figure 4 (c) illustrates how the routine of Figure 3 handles the SAT tone of Figure 4 (a). In this case, since a sliding average is used, the worst-case scenario in Figure 4 (b) is always avoided. As each new block of 20 milliseconds of samples is added after time 5 in additions 44 and 46, the reported SAT signal strength increases gradually as illustrated in Figure 4 (c). As described above, SAT detection is based, among other things, on the SAT signal strength exceeding a predetermined threshold. In the exemplary chart of Figure 4 (c), the threshold A is adjusted to a level such that it takes approximately 50 milliseconds for the routine in Figure 3 to report an SAT signal strength that exceeds this threshold. In this way, a dramatic improvement in performance is achieved using the sliding average technique illustrated in Figure 3 in relation to the conventional digital fourier transformation illustrated in Figure 4 (b). The exemplary embodiments described above are intended to be illustrative in all respects, rather than restrictive, of the present invention. In this manner, the present invention is capable of many variations in the detailed implementation that can be derived from the description contained herein by a person skilled in the art. All of these variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.

Claims (16)

1. - A method for mediating a signal resistance of a supervisory audio tone using a digital signal processor, the method comprising the steps of: providing a computational resolution; provide a system update interval; determining a first number N of samples of the supervisory audio tone that are needed to provide the computational resolution based on an input sample rate; determining a second M number of samples of the monitoring audio tone that can be added during each set-up interval of the system by the digital signal processor, the second number of samples being less than the first number of samples; Add the second M number of samples during each system update interval? store the resulting sum; add a number of resulting sums previously stored to the resulting sum to create a compound sum; , - twenty-one - perform a digital fourier transformation on the composite sum; ? report the signal strength of the supervisory audio tone to the system, based on the result of the digital fourier transformation.

2. - The method of claim 1, further comprising the step of: determining whether to disconnect a connection between a mobile station and a base station, based on the reported signal strength.

3. The method of claim 1, wherein the step of adding a number of the previously stored resulting sums further comprises the step of: adding (N / M) -l resulting sums previously stored to the resulting sum, whereby the first number of samples n is used in the digital fourier transformation.

4. - The method of claim 1, wherein the computational resolution is 10 Hz.

5. - The method of claim 1, wherein the turn-on interval is 20 ms.

6. - A method for measuring a signal resistance comprising the steps of: adding samples of a signal taken through a current measurement range and a plurality of previous measurement intervals to output a composite sum; and using the composite sum in a digital fourier transformation computation to generate an indication of the signal strength.

7. - The method of claim 6, wherein the plurality of previous measurement ranges contains a number of samples that is sufficient to provide a predetermined resolution output from the digital fourier transformation computation.

8. - The method of claim 6, further comprising the step of: identifying a quality of a connection in a radio communication system based on the indication of the signal strength.

9. - A monitoring audio tone measurement system comprising: an input node for receiving a sample stream at a predetermined sample rate; a summing device for adding a number of samples of the received sample stream to generate a first output; a memory device for storing the first output; an adder, downstream of the summing device, for receiving the first output of the summing device and adding thereto a plurality of previous outputs of the summing device which have been stored in the memory device to generate a second output; and a square operator to square the second exit.

10. - The monitoring audio tone measurement system of claim 9, wherein the adder adds a number of previous outputs of the summing device to the number of samples summed by the summing device so that a sum of samples represented by the number of previous outputs plus the number of samples summed by the summing device is equal to a number of samples needed to provide a predetermined computational resolution.

11. - The monitoring audio tone measurement system of claim 10, wherein the predetermined computational resolution is 10 Hz.

12. - A base station comprising: an element for receiving a SAT; and a digital signal processor (DSP) to measure a signal resistance of the SAT, where the DSP operates in a block ds mu ^ s ra ^ s current gn conjunction with - 24 - previously received samples to provide a sliding average measurement of the signal strength of the SAT.

13. - The base station of the claim 12, wherein the digital signal processor performs a digital fourier transformation in the block of current samples added to previously received samples.

14. - The base station of the claim 13, where the block of current samples is smaller than the number of samples needed to satisfy the Nyquist theorem and the samples previously received to satisfy the theorem.

15. - The base station of claim 12, further comprising: a circular memory for storing each successive block of current samples as the samples previously received.

16. - The base station of claim 15, wherein the circular memory is of a size to store N blocks of M samples, wherein N x M is sufficient to satisfy the theorem. , - 25 - SUMMARY OF THE INVENTION Methods and systems for measuring the signal strength of supervisory audio tones (SATs) in radio communication systems are described. Exemplary embodiments describe how such measurements can be made using digital signal processing techniques, while reducing the amount of computing power, e.g., MIPs, used to accomplish this task. During each measurement interval a large number of SAT signal samples are received. Instead of processing a measurement interval of SAT samples received, exemplary modalities are based on a sliding average of samples that include both current and former block samples. In this way, the SAT signal resistance can be reported relatively frequently to the system but without the number of computations that would be required to process all the SAT signal samples received during a measurement interval.