Abstract:
A partial frame detector provides detection of a frame synchronization pattern in an environment of burst noise that prohibits detection by correlation. A frame synchronization pattern capable of sustaining a minimum of 17 bits clustered in error plus random errors while still providing frame detection is achieved by utilizing a 64 bit Barker derived frame synchronization pattern and an additional process included in a correlation algorithm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to frame detection and more particularly to detection of a frame synchronization pattern in an environment of burst noise that prohibits detection by correlation. 
     2. Description of the Prior Art 
     Typically, frame detection is performed by correlation. The captured incoming bit pattern is exclusive-ored with a known reference pattern. This results in the placement of ones at all locations in the captured incoming bit pattern where there are differences. Adding up the number of ones provides an indication of how closely the two bit patterns match or correlate. For the original bit pattern, an error of zero to one bit, provides an acceptable frame error rate. However, when Rayleigh fading is added to the channel (high speed rates) the burst errors appear in clusters of four bits or more. With a possible correlation threshold of one bit in error, a frame synchronization pattern can not be detected. Thus, an improved detector of a frame synchronization pattern is required. 
     It is an object of the present invention to provide a frame detector that has the ability to detect a frame synchronization pattern in an environment of burst noise that prohibits detection by correlation. 
     SUMMARY OF THE INVENTION 
     The partial frame detector of the present invention provides the ability to detect a frame synchronization pattern in an environment of burst noise that prohibits detection by correlation. The average types of errors that are found on a Rayleigh fading channel are burst errors from four bits to 17 bits plus background errors, based on the bit error rate. The burst errors are separated by strings of good bits that average from 21 bits to 84 bits also with background errors randomly inserting single bit errors in the strings. A frame synchronization pattern must be able to sustain at a minimum 17 bits clustered in error plus random errors and still provide frame detection. The solution to this situation, as described by the present invention, is the utilization of a 64 bit Barker derived frame synchronization pattern and an additional process included in the correlation algorithm. 
     The normal frame detection process will find frame synchronization if there are 13 errors in a 64 bit frame synchronization pattern. The partial frame detector of the present invention will detect frame synchronization if there are 19 clustered errors and four random errors (23 total errors) in a 64 bit frame synchronization pattern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a illustrates a first portion of a flow diagram of one embodiment of the present invention. 
     FIG. 1b illustrates a second portion of a flow diagram of the embodiment of the present invention referred to above in connection with FIG. 1a. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A DMX (trunked data radio) message typically contains modem synchronization, frame synchronization, addressing, text, and error correction information. The performance of recovery of the message in a low signal level, Rayleigh fading environment (8-20 dB SINAD), should be at least as high as voice. To improve message recovery performance at low signal levels, two parts of the message structure must be improved. First, the frame synchronization size and detection algorithm and second the error correction implementation, time diversity and the recommendation of correction code. 
     FIGS. 1a and 1b together illustrate a flow diagram of one embodiment of the partial frame detector of the present invention. The partial frame detection process begins by rotating the DMX message, exclusive-oring the message with a reference, and summing the errors. If the sum of the errors is less than or equal to a specified threshold, then frame synchronization has been found. If the sum of the errors is not less than or equal to the threshold, then the partial process is utilized. The partial process begins by establishing whether the sum of the errors is less than or equal to the upper bound. If the sum of the errors is not less than or equal to the upper bound, then no frame synchronization has been found. If the sum of the errors is less than or equal to the upper bound, then the message under test is divided into four 16 bit groups, as illustrated in Table 1. 
     
                       TABLE 1______________________________________16 bits  16 bits       16 bits 16 bits______________________________________First    Second        Third   Fourth______________________________________ 
    
     The four 16 bit groups are then rearranged by combining the first and second groups, second and third groups, third and fourth groups, first and third groups, second and fourth groups, and the first and fourth groups defining a set number of combinations including a last combination. Each of the combined bit groups is then analyzed for errors. Tag No. 1 establishes whether there was a last combination has been analyzed. If the last combination has been analyzed, then the software flow diagram skips to Tag No. 2. If a last combination has not been analyzed, then the partial process finds the errors in the combination next and sums the errors in the combination. If the combination sum of errors is less than or equal to a specified partial threshold, then a one is added to a partial counter. If the combination sum of errors is not less than or equal to a partial threshold, then the software flow diagram returns to Tag No. 1. Tag No. 2 establishes whether the partial counter is greater than or equal to the counter threshold. If it is, then frame synchronization is found. If it is not, then no frame synchronization has been found. 
     
                       TABLE 2______________________________________Upper Bound       25Threshold         13Partial Treshold   5Counter Threshold  3______________________________________ 
    
     The text message, including addressing information, needs to have the periodic burst errors converted into random errors. The randomization of periodic errors is accomplished by interleaving. The step size for interleaving must be large enough to randomize periodic errors for the entire length of the text message over the speeds in question. Numbers for interleaving greater than the shortest average interval (40 MPH-25 bits) and less than twice the shortest average interval (40 MPH-50 bits). Thus, interleave numbers from 26 to 49 should provide sufficient randomization of periodic errors. Interleave numbers tested for this embodiment of the invention where 33 bits and 41 bits. 
     The text message is buffered so that software can rotate the entire received bit pattern to find the frame synchronization pattern. Once the frame synchronization pattern has been found the text message bits can be deinterleaved. After deinterleaving the error correction can remove as many errors as the error correction code is capable. The maximum number of shifts before the frame synchronization pattern is not found can be limited. In this embodiment, a break out of the shift count of 140 is 64 for modem synchronization, 64 frame synchronization, and 10% of 128 bits. 
     The error correction for the address part of the text message needs a more robust error correction code than the text portion. The error correction is made up of two parts. The first part of the error correction is an inner code that is able to correct non-periodic random errors 6 bits or less in a 24 bit group. The inner error correction code needs to be easily implementable in software. The error correction code doubles the total number of address bits sent (new total 224 ). The intent is to make the recovery of addressing information better so that a mobile unit can more easily tell if a call was addressed to the unit even though the text is unusable. The new string of bits (112 original and 112 error correction) is appended to the front of the original text message bit string. The second, outer code, is implemented as part of the error correction for the text. 
     The text portion of the message is the actual information that is sent for display at the destination. The error correction for the text portion of the message is made up of a code that can correct non-periodic random errors 6 bits or less in a 24 bit group. The (outer) error correction code is used on all the bits from the text plus the addressing and inner error correction code bits. 
     The addition of 8 bits at the end of the message is to provide a buffer between the message and the termination of the data carrier. To provide time for last bits to clear various shift registers. The rate 1/2 error correction system is comprised of a two tap multiplexed convolutional code encoder. The rate 1/3 error correction system is comprised of a three tap multiplexed convolutional code encoder. The difference between the rate 1/2 and rate 1/3 is that the rate 1/2  reduces the baseline BER by approximately 2 and a rate 1/3 reduces the baseline BER by approximately 3. Table 3 shows the total bit size. 
     
                       TABLE 3______________________________________TOTAL BIT SIZEFunction   Original Rate 1/2  Rate 1/3______________________________________Modem Sync 200      64        64     BitsFrame Sync 15       64        64     BitsAddress    224      448       672    BitsText       512      512       768    BitsTail       0        8         8      BitsTotal Bits 951      1096      1576   BitsTotal Time 0.79     0.92      1.13   SecondsEffective Rate      466      400       325    BaudNumber of Bytes      119      137       197    BytesRate Reduction      0        14        30     %______________________________________ 
    
     The message does not need to be decoded in real time. However, this does require the use of double (triple, quadruple, etc.) buffering of incoming messages. A buffer area is needed for the deinterleaved message to be stored while error correction works on correcting any errors. A buffer area is needed for the error corrected message (address and text) to be stored prior to transfer to the display. Memory requirements for buffering of messages will be less than 2 kilobytes of static RAM (6116 static RAM 2K×8). 
     The error correction for the address bits is recommended to be a combination (concatenation) of several types of error correction. The inner code needs to be able to handle non-periodic short errors of 6 or less. The error correction for the text bits is recommended to be a combination (concatenation) of several types of error correction. The outer code needs to be able to handle non-periodic short burst errors of 6 bits or less. 
     A typical modem acquires synchronization within 8 bits of presentation of the data carrier. The modem utilized with this embodiment of the invention (Fujitsu MB87002) was set for slow synchronization acquisition of within 25 bits. The slow synchronization setting provides the longest possible delay for this device to obtain synchronization (worst case). By utilizing the present invention, the modem synchronization could be reduced from 200 bits to 64 bits while maintaining bit synchronization accuracy. 
     Table 4 shows the suggested DMX performance criteria based on test results (using the MB87002 Modem). 
     
                       TABLE 4______________________________________              FRAME        BIT ERRORSINAD   FADING     ACQUISITION  RATEdB      MPH        %            BER______________________________________ 8      10         80           4.38 × 10-112      10         89           2.50 × 10-120      10         99.9         6.70 × 10-120 + 20 10         99.9999      Less than 10-5______________________________________ 
    
     Table 5 shows the non-faded performance test results (using the MB87002 Modem). 
     
                       TABLE 5______________________________________       FRAME        BIT ERRORSINAD       ACQUISITION  RATEdB          %            BER______________________________________5           99.99        1.73 × 10-27           99.999       2.09 × 10-39           99.9999      1.10 × 10-4______________________________________ 
    
     It is not intended that this invention be limited to the software arrangement, or operational procedures shown disclosed. This invention includes all of the alterations and variations thereto as encompassed within the scope of the claims as follows.