Abstract:
A system and method are disclosed for determining a level of quality of a communications medium having an indeterminate delay. The communications medium communicatively couples a transmitting station and a receiving station. The transmitting station transmits over the communications medium a measurement sequence including a synchronization sequence followed by a measurement sample. The transmitted measurement sample has a predetermined beginning point, which is identified by the transmitted synchronization sequence. The receiving station receives over the communications medium the measurement sequence including the synchronization sequence followed by the measurement sample. The receiving station determines from the received synchronization sequence the beginning point of the received measurement sample. The transmitted measurement sample and the received measurement sample are then compared according to respective beginning points to determine the level of quality of the communications medium.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a communications system such as a cellular radiotelephone communications system. More specifically, the present invention relates to a method and apparatus for providing automated assessment of the quality of service provided by such communications system. 
     BACKGROUND OF THE INVENTION 
     The structure and operation of a typical communications system such as a cellular radiotelephone communications system is known. For example, the structure and operation of a cellular radiotelephone system has been disclosed in publications such as the January 1979 issue of The Bell Systems Technical Journal, and Specification EIA IS-3B entitled “Cellular System Mobile Station—Land Station Compatibility Specification” July, 1984, Electronic Industries Association, both hereby incorporated by reference. 
     In connection with such cellular radiotelephone communications system in particular, it is known that audio quality analyzers can be employed to measure and report quality information of audio communication channels. Such quality information is used to ensure a high level of quality of cellular service over the service provider&#39;s coverage area. In newer digital cellular radiotelephone systems, however, such audio quality information is not adequate to assess the quality of service provided by such digital systems. 
     Specifically, the audio quality measurement techniques utilized by such audio quality analyzers use continuous audio tones to measure the quality of the audio communication channel. However, digital cellular radiotelephone systems employ digital voice coders which are designed to efficiently encode and decode human speech, and which actually distort continuous tones. As should be understood, such distorted continuous tones cause measurements of audio quality that are much lower than expected. Accordingly, such distortion renders continuous tone techniques useless when characterizing the quality of service provided by digital radiotelephone systems. 
     Several techniques to measure the quality of the speech transferred through digital radiotelephone coders have been developed. See, for example, S. Wang, A. Sekey, A. Gersho, “An Objective Measure for Predicting Subjective Quality of Speech Coders,” IEEE Journal on Selected Areas in Communications, vol. 10, no. 5, June 1992, pp. 819-829, hereby incorporated by reference. Briefly, such digital techniques employ samples of human speech to measure the quality of a digital communication channel. Doing so overcomes the distortion problems in digital cellular radiotelephone systems caused by the continuous tone techniques and is also appropriate for measuring the quality of service provided by analog cellular radiotelephone systems. However, and importantly, the human speech sample techniques require exact timing and have been designed for use in a laboratory environment where such exact timing can be closely controlled. 
     As should be evident, though, a cellular radiotelephone service provider needs to ascertain whether high quality analog and digital service is being provided to all areas where the cellular radiotelephone system provides service, not just in the laboratory environment. Accordingly, a need exists for a method and apparatus for providing automated, in field, geographically located measurements of cellular radiotelephone audio quality employing human speech samples to either verify high quality cellular coverage is present in each area or to identify problem areas which need correction. Moreover, a need exists for such a method and apparatus wherein the measurement of cellular radiotelephone system quality is repeatable between days, weeks, months and even years of testing for statistical accuracy. 
     SUMMARY OF THE INVENTION 
     The aforementioned needs are satisfied by a system and method for determining a level of quality of a communications medium having an indeterminate delay. The communications medium communicatively couples a transmitting station and a receiving station. The transmitting station transmits over the communications medium a measurement sequence including a synchronization sequence followed by a measurement sample. The transmitted measurement sample has a predetermined beginning point, which is identified by the transmitted synchronization sequence. 
     The receiving station receives over the communications medium the measurement sequence including the synchronization sequence followed by the measurement sample. The receiving station determines from the received synchronization sequence the beginning point of the received measurement sample. The transmitted measurement sample and the received measurement sample are then compared according to respective beginning points to determine the level of quality of the communications medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. As should be understood, however, the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
     FIG. 1 is a block diagram illustrating an automated quality assessment system for a cellular radiotelephone system in accordance with one embodiment of the present invention; 
     FIG. 2 is a timing diagram not to scale showing a measurement synchronization sequence employed by the assessment system of FIG. 1; 
     FIG. 3 is an enhanced view of a portion of the measurement synchronization sequence of FIG. 2; 
     FIG. 4 is a timing diagram corresponding to FIG. 3, and shows the portion of the measurement synchronization sequence of FIG. 3 after distortion reduction; 
     FIG. 5 is an enhanced view of a sub-portion of the measurement synchronization sequence of FIG. 3; 
     FIG. 6 is a timing diagram and shows correlation output based on the distortion reduced measurement synchronization sequence of FIG. 4 as correlation input; 
     FIG. 7 is a block diagram illustrating the correlation process performed by the assessment system of FIG. 1; and 
     FIG. 8 is a flow diagram showing the steps performed by the assessment system of FIG. 1 during a retry. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Certain terminology may be used in the following description for convenience only and is not considered to be limiting. The words “left”, “right”, “upper”, and “lower” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” are further directions toward and away from, respectively, the geometric center of the referenced object. The terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import. 
     Referring to the drawings in detail, wherein like numerals are used to indicate like elements throughout, there is shown in FIG. 1 an automated quality assessment system (AQAS)  100  for a communications system such as a cellular radiotelephone system, digital or analog. Of course, such assessment system  100  may be used for other communications systems without departing from the spirit and scope of the present invention. As but one example, such assessment system  100  may be used with a land-line based system, digital or analog. The assessment system  100  is similar to and an enhancement of the system disclosed in U.S. Pat. No. 5,490,204, issued to Gulledge (the inventor of the present disclosure) on Feb. 6, 1996, hereby incorporated by reference. 
     As seen in FIG. 1, the assessment system  100  includes a mobile quality measurement sub-system MQM  1 , a fixed quality measurement sub-system FQM  3 , and an office quality analysis sub-system OQA  4 . As should be understood, the MQM  1  contains, monitors and controls one or more cellular radiotelephone mobile stations. Likewise, the FQM  3  provides service as one or more fixed telephone stations and is attached to a public switched telephone network PSTN  2 C, which connects to a mobile telephone switching office MTSO  2 B. Cellular radiotelephone calls are placed between MQM  1  and FQM  3  through a cellular radiotelephone system infrastructure  2  which includes the MTSO  2 B and one or more cell sites  2 A. 
     In operation, the system  100  causes data to be collected by the MQM  1  and the FQM  3  specific to the progress and audio quality obtained by each cellular radiotelephone call placed. At the conclusion of a test, the collected data is transferred from the FQM  3  and the MQM  1  to the OQA  4  through respective data paths  5 ,  6 . The data paths  5 ,  6  may be any data paths without departing from the spirit and scope of the present invention. For example, each data path  5 ,  6 , can be a hand-carried storage device such as a floppy disk or the like, a data line, a phone line, a network interface or any other type of appropriate data path. In one embodiment of the present invention, data collected by the FQM  3  is returned to the MQM  1  and transferred to the OQA  4  through data path  6 . 
     Preferably, the OQA  4  matches the data collected by the MQM  1  and the FQM  3  based on the time of day of the collection to produce statistical tables and graphs  4 A that represent the quality of the cellular service provided during the test. Many sets of test data collected throughout the coverage area of a cellular radiotelephone system may be combined to form a comprehensive view of the quality of service provided by the cellular radiotelephone system being assessed. 
     Preferably, the MQM  1  and FQM  3  each contain a Digital Signal Processor (DSP)  17  (FIG. 7) or other computer or processor, as well as appropriate memory and other supporting peripherals. The DSP  17  or other computer or processor, as well as the memory and other supporting peripherals, may each be any appropriate device without departing from the spirit and scope of the present invention, as long as each element has sufficient capabilities for performing the functions required in a timely manner. In the present invention, after each cellular radiotelephone call is placed or received, the DSP  17  in each sub-system  1 ,  3  sends a human speech sample to determine the audio quality of the communication channel provided by the cellular radio telephone infrastructure  2 . The human speech sample as received is then compared with a reference human speech sample which is stored locally. The comparison results in a measure of distortion known as Bark Spectral Distortion (BSD), and is more fully disclosed in the aforementioned S. Wang, A. Sekey, A. Gersho, “An Objective Measure for Predicting Subjective Quality of Speech Coders”. 
     BSD comparison measurement requires close time alignment between the beginning of the distorted human voice sample that has been transmitted and the beginning of the reference human voice sample. In a laboratory environment, such comparison measurement is a relatively simple matter to accomplish because the exact time when the sample is transmitted and received is known and easily controlled. Within an operating cellular radiotelephone system, however, there exist many dynamic factors that can cause delays between when the speech sample is transmitted and received, making the comparison difficult if not impossible. Importantly, timing errors greater that one millisecond can cause large negative errors in the measurement of the quality of the communication channel, thus producing erroneously poor voice quality measurements. 
     In the present invention, then, the laboratory-based measurement technique utilizing human speech samples is enhanced with embedded timing information to provide timing accuracy better than one millisecond. In particular, in the present invention, a synchronization sequence is transmitted before the human speech sample. Such synchronization sequence allows the receiving station to accurately determine the beginning of the received human speech sample and also accommodates for the varying delays within the communication channels of an operating cellular radiotelephone network, thus allowing accurate BSD measurements. 
     Measurement Sequence 
     BSD measurements are made according to a measurement sequence. Referring to FIG. 1, in a cellular radiotelephone system, measurement sequences are initialized after a cellular radiotelephone call is made from the MQM  1  to the FQM  3 . It should be understood, though, that a call may also be placed from the FQM  3  to MQM  1 , from a first MQM  1  to a second MQM  1 , or from a first FQM  3  to a second FQM  3 , all without departing from the spirit and scope of the present invention. 
     Assuming now that the call is made from the MQM  1  to the FQM  3 , the MQM  1  begins the first measurement sequence operating as a receiving station with the FQM  3  operating as a transmitting station. During the second sequence, the MQM  1  operates as a transmitting station and the FQM  3  operates as a receiving station. The measurement sequences continue in this alternating fashion for the duration of the cellular phone call. The duration of each call, and the periodicity of calls, is specified by the user of the AQAS assessment system  100 . 
     Referring now to FIG. 2, the measurement sequence is shown. As seen, the measurement sequence consists of two main parts: the synchronization sequence (elements  10 ,  11 , and  12 ) and the human speech sample (element  13 ). In particular, in one embodiment of the present invention, the synchronization sequence includes, in serial fashion, a 400-millisecond 1000-hertz tone (element  10 ), followed by an 8-millisecond 1000-hertz tone containing a single 180-degree phase shift (element  11 ), followed by a 200-millisecond 1000-hertz tone (element  12 ). The synchronization sequence  10 ,  11 ,  12 , is then followed by a human speech sample  13 , which in one embodiment of the present invention is 5600 milliseconds in duration. Together elements  10 ,  11 ,  12 , and  13  comprise the measurement sequence and are sent from the transmitting station to the receiving station. Of course, it will be recognized that different time values and frequency values may be employed in the measurement sequence  10 ,  11 ,  12 ,  13  without departing from the spirit and scope of the present invention. Likewise, it will be recognized that different sequence components and combinations of sequence components may also be employed in the measurement sequence  10 ,  11 ,  12 ,  13  without departing from the spirit and scope of the present invention. 
     Synchronization Sequence 
     Assuming that the measurement sequence  10 ,  11 ,  12 ,  13  is as described above and shown in FIG. 2, it is to be understood that the synchronization sub-sequence of the measurement sequence takes place in connection with the 8-millisecond 1000-hertz tone containing a single 180-degree phase shift (element  11 ). More specifically, and referring now to FIG. 3., a portion of the synchronization sequence of FIG. 2 is shown. Such portion includes the last 12 cycles of the 400-millisecond 1000-hertz tone  10 , the entire 8-millisecond 1000-hertz tone containing a single 180-degree phase shift  11 , and the first 12 cycles of the 200-millisecond 1000-hertz tone  12 . As seen, the transition between each tone  10 ,  11 ,  12  is smooth and continuous. 
     The 400-millisecond 1000-hertz tone  10  and the 200-millisecond 1000-hertz tone  12  are 180 degrees out of phase with each other due to the 180-degree phase shift within the 8-millisecond 1000-hertz tone containing a single 180-degree phase shift  11 . The 400 millisecond duration of the 400-millisecond 1000-hertz tone  10  allows a digital voice coder utilized in a typical digital cellular radiotelephone system to settle into a steady state, thereby reducing distortion of the 8-millisecond 1000-hertz tone containing a single 180-degree phase shift  11 . The entire synchronization sequence  10 ,  11 ,  12  is transmitted from the transmitting station to the receiving station as a preamble to the human speech sample  13 . The start of the human speech sample  13  is exactly 200 milliseconds from the end of the 8 milliseconds of 1000-hertz tone containing a single 180-degree phase shift  11 . 
     Distortion Reduction 
     As the receiving station receives the synchronization sequence  10 ,  11 ,  12  from the transmitting station, the sinusoidal waveform embodied therein is clipped into a square waveform, as can be seen in FIG. 4, which corresponds temporally to FIG. 3, with element numbers  10 ,  11 , and  12  corresponding to element numbers  10 A,  11 A, and  12 A, respectively. The clipping process, as is known, reduces the distortion of the sinusoidal waveform introduced by the aforementioned digital voice coder. Importantly, the clipping process does not affect performance in an analog cellular radiotelephone system where distortion of a sinusoidal waveform is not a problem. 
     Correlation 
     A receiving station called by a transmitting station begins a measurement sequence by listening for a 1000-hertz tone from the transmitting station. When 50 milliseconds of continuous 1000-hertz tone is detected, the receiving station begins a correlation process whereby such receiving station listens to the synchronization sub-sequence of the measurement sequence and attempts to detect a portion of the synchronization sub-sequence that corresponds to a correlation pattern  14  (FIG.  5 ). As should now be understood, and as seen in FIG. 5, the correlation pattern  14  is substantially identical to the 8-millisecond 1000-hertz tone containing a single 180-degree phase shift  11  shown in FIGS. 2-4. 
     In particular, in one embodiment of the present invention, the DSP  17  of the receiving station (either MQM  1  or FQM  3  of FIG. 1) samples the synchronization sequence at 8000 hertz as such sequence is received by the receiving station. Referring now to FIG. 7, during the correlation process, the DSP  17  of the receiving station places each sample in a sixty-four-element input array  18  that has been initialized to zero before the process begins. The DSP  17  also has the correlation pattern  14  of FIG.  5 . digitally stored in a sixty-four-element pattern array  19 . As each sample of the synchronization sequence is received and placed in the input array  18 , a correlation block  20  in the DSP  17  multiplies each element of the input array  18  with the corresponding element in the pattern array  19 , squares the result of each multiplication, and then adds together all  64  terms to result in the correlation output. Put mathematically, the correlation block  20  performs the following function on each sample:        CorrelationOutput   =       ∑     i   =   1     64                       (       InputArray   i     *     PatternArray   i       )     2                              
     Referring now to FIG. 6, the portion of the synchronization sequence  10 A,  11 A,  12 A shown in FIG. 4 along with the correlation output  15  that is computed by the correlation block  20  based on such synchronization sequence  10 A,  11 A,  12 A. As seen in FIG. 6, the correlation output  15  peaks at the point in the synchronization sequence where the digital pattern in the input array  18  closely matches the digital pattern in the pattern array  19 . Accordingly, the peak of such correlation output  15  is used as a timing mark to indicate to the receiving station that the human speech sample follows exactly 200 milliseconds later in time. 
     Correlation Peak Detection 
     Extracting the exact time of the correlation output  15  peak is performed by the peak detection block  21  of the DSP  17  (FIG.  7 ), and requires that the correlation process is running before the start of the 8-millisecond 1000-hertz tone containing a single 180-degree phase shift  11 . In one embodiment of the present invention, and as was discussed above, the DSP  17  of the receiving station starts the correlation process after 50 milliseconds of continuous 1000 tone hertz tone has been detected. The correlation process may then be ended 400 milliseconds later, although greater or lesser durations may be employed without departing from the spirit and scope of the present invention. 
     Preferably, during the 400-millisecond duration that the correlation process is running, the peak detector  21  of the DSP  17  (FIG. 7) remembers the largest value produced by the correlation block  20  and the exact time when such largest value was produced. At the completion of the correlation process, then, the difference in time between when the correlation process ended and when the largest correlation value was produced is subtracted from 200 milliseconds. The resulting difference is the exact time needed to wait before the start time of the human speech sample, and the DSP  17  may then synchronize the receiving station to such start time. In one embodiment of the present invention, the receiving station sets a count-down timer (not shown) to count down the time until such start time, and begins BSD measurements upon the expiration of such timer. Of course, it will be recognized that other particular synchronization methods based on a detected peak from a correlation process may be employed without departing from the spirit and scope of the present invention. 
     Preferably, the maximum output from the correlation block  20  must exceed a threshold for the peak detector  21  to consider the correlation process successful. For example, if 1.0 is a perfect correlation value, a value of about 0.3-0.4 may be employed as the threshold for successful correlation. In one embodiment of the present invention, a value of about 0.375 is employed. Of course, the threshold can be higher or lower without departing from the spirit and scope of the present invention, although it is to be understood that a higher threshold may be too restrictive, and a lower threshold may not be restrictive enough. 
     It is to be noted that the relatively simple single phase shift correlation pattern  14 /tone  11  disclosed herein results in a relatively low yet acceptable typical correlation value (i.e., greater than the threshold) in the present invention. A higher typical correlation value could instead be obtained if the correlation pattern  14 /tone  11  was more complicated. For example, such a more complicated correlation pattern  14 /tone  11  could be a ‘chirp’, i.e., a tone that transitions from a low frequency to a high frequency relatively quickly. Nevertheless, the relatively simple single phase shift correlation pattern  14 /tone  11  used in the present invention is still preferred, since such relatively simple single phase shift correlation pattern  14 /tone  11  passes through the various digital coders employed in a typical digital radiotelephone system with little distortion. Conversely, a chirp pattern passed through the various digital coders is highly distorted. 
     Fault Tolerance 
     Of course, unsuccessful correlation can be expected in any network, including an operating digital radiotelephone network. Temporary impairments within the communication channel provided by the network can and do cause the correlation process to fail. A retry mechanism is therefore utilized to overcome a failure to achieve correlation success. In one embodiment of the present invention, and as shown in FIG. 8, the retry mechanism comprises the following: 
     After the 400-millisecond duration that the correlation process is run, if the peak detection block fails to detect a correlation value greater than the minimum correlation threshold, the receiving station emits an 1800-hertz handshake tone, and the transmitting station listens for the 1800-hertz handshake tone (step  1 ). Once the 1800-hertz handshake tone is detected, the transmitting station then re-transmits the entire synchronization sequence  10 ,  11 ,  12  of FIG. 2, and the receiving station, upon detecting the 1000-hertz tone which begins the synchronization sequence, stops transmitting the 1800-hertz tone (step  2 ). Thereafter, the receiving station clips the 1000-hertz tone and runs the correlation process via the DSP  17 , the arrays  18 ,  19 , the correlation block  20 , and the peak detection block  21  of FIG. 7 for 400 milliseconds (as before) (step  3 ). Once more, the peak detector block  21  tests for successful correlation (step  4 ). If the correlation is not successful the receiving station transitions to step  1 . Assuming, though, that the correlation is now successful, the receiving station becomes synchronized to the start time of the human speech sample as transmitted by the transmitting station (step  5 ). The retry mechanism as discussed herein is repeated until successful correlation is achieved. Alternatively, a fixed number of retry attempts are tried. 
     During step  5 , the transmitting station listens for an 1800-hertz tone from the receiving station while transmitting the human voice sample, just as such transmitting station does in step  1 . In the case that such 1800-hertz tone is detected by the transmitting station while the human speech sample is being transmitted, then, the transmitting station transitions to step  2  to handle the correlation failure by the receiving station. Assuming, however, that no correlation failure occurs, and that the transmitted human voice sample has been received successfully, the receiving station and the transmitting station then exchange operating roles. 
     As should now be understood, the 1800-hertz retry tone may also be employed as an initialization tone by the receiving station to indicate to the transmitting station that such receiving station is ready to receive. Accordingly, when the receiving station and the transmitting station exchange operating roles, both stations may transition (in their new roles) to step  1 . 
     CONCLUSION 
     The programming necessary to effectuate the processes performed in connection with the present invention is relatively rudimentary and should be apparent to the relevant programming public. Accordingly, such programming is not attached hereto. Any particular programming, then, may be employed to effectuate the present invention without departing from the spirit and scope thereof. 
     In the foregoing description, it can be seen that the present invention comprises a new and useful system and method for time synchronization of human speech samples in a quality assessment system for a communications system. Importantly, the system and method take into account the fact that timing variations can be expected in an operating cellular network, based on differing communications paths and differing transmission-reception delays. By attaching timing synchronization information directly with the human speech sample, the present invention allows the production of accurate BSD measurements despite the differing delays. It should be appreciated that changes could be made to the embodiments described above without departing from the inventive concepts thereof. As but a few examples, each array  18 ,  19  may have a number of elements other than 64, and the handshake tone may be at a frequency other than 1800 hertz. It should be understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.