Frame synchronization in a digital communications system

In a digital communications receiver, a frame-synchronization system comprising:

• an input that receives a series of received data frames in a data stream received from a remote transmitter, wherein the data frames comprise a plurality of symbols, wherein the data frames are received at substantially regular intervals, and wherein each data frame in the series of data frames includes a SYNC field comprising a predetermined sequence of symbols;
• a symbol clock coupled to said input, wherein said symbol clock indicates a timing of the symbols in the data stream;
• timing logic coupled to said symbol clock, wherein said timing logic indicates a detection window in response to the timing of the symbols in the data stream, wherein the detection window is a predetermined narrow time interval during which the SYNC field is expected;
• a SYNC-field detector coupled to said input and to said timing logic, wherein said SYNC-field detector is operable to recognize an occurrence of the SYNC field in the data stream during the detection window;
• and
• a receiver frame clock coupled to said SYNC-field detector, with a period substantially equal to the interval at which the data frames are received, wherein said receiver frame clock is operable to set a frame timing in response to said SYNC-field detector recognizing the occurrence of the SYNC field.

In a digital communications receiver, a method for synchronizing a receiver frame clock with a series of received data frames in a data stream received from a remote transmitter, wherein the data frames comprise a plurality of symbols, wherein the data frames are received at substantially regular intervals, and wherein each data frame in the series of data frames includes a SYNC field comprising a predetermined sequence of symbols, the method comprising:

• recovering a symbol timing;
• acquiring a frame timing after said recovering a symbol timing, wherein said acquiring comprises:
• the receiver continuously scanning the data stream for the SYNC field,
• the receiver identifying the SYNC field in the data stream,
• the receiver setting the frame clock in response to said identifying the SYNC field, and
• returning to said recovering the symbol timing if the receiver does not identify the SYNC field in the data stream within a predetermined time limit;
• tracking the frame timing after said acquiring, wherein said tracking comprises the receiver using the frame clock to determine the start of the data frames, the receiver checking the frame clock with SYNC fields in subsequent data frames, the receiver incrementing a count for each correct match of the frame clock with the SYNC fields in subsequent data frames, and the receiver decrementing the count for each incorrect match of the frame clock with the SYNC fields in subsequent data frames;
• locking the frame timing after said tracking if the count exceeds a high threshold count, wherein said locking comprises the receiver using the frame clock to determine the start of the data frames;
• and
• returning to said acquiring after said tracking if the count is less than a low threshold count.

In a digital communications receiver, a frame-synchronization system comprising:

• an input that receives a series of received data frames in a data stream received from a remote transmitter, wherein the data frames comprise a plurality of symbols, wherein the data frames are received at substantially regular intervals, and wherein each data frame in the series of data frames includes a SYNC field comprising a predetermined sequence of symbols;
• a symbol clock coupled to said input, wherein said symbol clock is operable to determine a timing of the symbols in the data stream;
• a SYNC-field detector coupled to said input and to said symbol clock, wherein said SYNC-field detector is operable to recognize an occurrence of the SYNC field in the data stream;
• wherein said symbol clock determines a new timing of the symbols in the data stream if said SYNC-field detector does not recognize an occurrence of the SYNC field in the data stream within a predetermined time limit after said symbol clock determines the timing of the symbols;
• a receiver frame clock coupled to said SYNC-field detector, with a period substantially equal to the interval at which the data frames are received, wherein the receiver frame clock is operable to set a frame timing in response to said SYNC-field detector recognizing an initial occurrence of the SYNC field;
• a counter coupled to said SYNC-field detector and to said receiver frame clock, wherein said counter increments a count if said SYNC-field detector recognizes a SYNC field in an expected position in a data frame according to the frame timing, and wherein said counter decrements a count if said SYNC-field detector does not recognize a SYNC field in the expected position in a data frame according to the frame timing;
• wherein if the count reaches a high threshold value the receiver uses the frame clock to determine the start of the data frames, and wherein if the count reaches a low threshold value said SYNC-field detector monitors the data stream for a new initial occurrence of the SYNC field and resets the frame timing.