Pulse or digital communications – Testing – Data rate
Reexamination Certificate
1997-08-08
2001-01-16
Le, Amanda T. (Department: 2734)
Pulse or digital communications
Testing
Data rate
C375S341000, C375S343000, C714S738000, C714S799000, C370S252000
Reexamination Certificate
active
06175590
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a method and apparatus for determining data rate in a variable rate communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM”, and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention, and incorporated by reference herein.
CDMA systems often employ a variable rate vocoder to encode data so that the data rate can be varied from one data frame to another. An exemplary embodiment of a variable rate vocoder is described in U.S. Pat. No. 5,414,796, entitled “VARIABLE RATE VOCODER,” assigned to the assignee of the present invention and incorporated by reference herein. The use of a variable rate communications channel reduces mutual interference by minimizing unnecessary transmissions when there is no useful speech to be transmitted. Algorithms are utilized within the vocoder for generating a varying number of information bits in each frame in accordance with variations in speech activity. For example, a vocoder with a rate set of four may produce 20 millisecond data frames containing 20, 40, 80, or 160 bits, depending on the activity of the speaker. It is desired to transmit each data frame in a fixed amount of time by varying the transmission rate of communications. Additional details on the formatting of the vocoder data into data frames are described in U.S. Pat. No. 5,511,073, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention, and incorporated by reference herein.
One technique for the receiver to determine the data rate of a received data frame is described in copending U.S. Pat. No. 5,566,206 entitled “METHOD AND APPARATUS FOR DETERMINING DATA RATE OF TRANSMITTED VARIABLE RATE DATA IN A COMMUNICATIONS RECEIVER,” filed Apr. 26, 1994, now U.S. Pat. No. 5,566,206, issued Oct. 15, 1996 assigned to the assignee of the present invention and incorporated by reference herein. Another technique is described in copending U.S. Pat. No. 5,710,784 entitled “MULTIRATE SERIAL VITERBI DECODER FOR CODE DIVISION MULTIPLE ACCESS SYSTEM APPLICATIONS,” issued Jan. 20,1998 now abandoned, assigned to the assignee of the present invention, and incorporated by reference herein. Yet another technique is described in U.S. Pat. No. 5,751,725 entitled “METHOD AND APPARATUS FOR DETERMINING THE RATE DATA IN A VARIABLE RATE COMMUNICATION SYSTEM”, issued May 12, 1998 now U.S. Pat. No. 5,751,725, assigned to the assignee of the present invention and incorporated by reference herein. According to these techniques, each received data frame is decoded at each of the possible rates. Error metrics, which describe the quality of the decoded symbols for each frame decoded at each rate, are provided to a processor. The error metrics may include cyclic redundancy check (CRC) results, Yamamoto quality metrics, and symbol error rates. These error metrics are well-known in communications systems. The processor analyzes the error metrics and determines the most probable rate at which the incoming symbols were transmitted.
SUMMARY OF THE INVENTION
The present invention provides a novel and improved method and apparatus for determining the data rate of received data in a variable rate communication system. The present invention is employed in a communications system having a transmission system and a receiving system, where the receiving system determines at which of a plurality of data rates individual frames in a signal has been transmitted by the transmission system. For example, if the transmission system employs four data rates, the receiving system decodes each frame of the received signal based on the four rates to produce four normalized correlation metrics, four cyclic redundancy check (CRC) bits, and zero or more Yamamoto quality metrics. In the present invention, the highest normalized correlation metric is first selected and the CRC bit for this data rate is checked. If the CRC checks, this data rate is indicated as the received data rate. Otherwise, the next highest normalized correlation metric is selected and the process continues. If no CRC check, an erasure is indicated.
In the typical situation, only the data rate corresponding to the highest normalized correlation metric is considered. This frame can be accepted or erased, depending on the CRC check and/or the Yamamoto quality metric. In some applications, CRC encoding may not be performed on all data rates. When this occurs, the Yamamoto quality metric can be used in place of the CRC check, other metrics can be used, or the data rate determination process can be made dependent only on the normalized correlation metrics.
It is an object of the present invention to provide a reliable determination of the received data rate. In the present invention, a normalized correlation metric is calculated for each possible data rate from a correlation value and a correlation constant which have been calculated for that data rate. The correlation value is determined from a correlation of the demodulated soft frame symbols and the re-encoded frame. The use of soft symbols enhances the quality of the normalized correlation metrics over other metrics of the prior art, such as the symbol error rate (SER) which only utilizes the sign bits of the demodulated soft frame symbols. The correlation constants can be calculated using theoretical values, simulated, or measured empirically to provided robust performance. In addition, the use of normalized correlation metrics works well over a wide range of input energy-per-bit-to-total-noise ratio E
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It is another object of the present invention to minimize erroneous data rate indication which results in frame error. In some communication systems, such as the CDMA communication system, a frame error is more catastrophic than an erasure. Thus, the present invention can be optimized to minimize the frame error rate, at the expense of slightly higher erasure rate, by comparing the normalized correlation metrics against a correlation threshold. Normalized correlation metrics which fall belo
Kalousek Pavel
Le Amanda T.
Qualcomm Inc.
Rouse Thomas R.
Wadsworth Philip
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