Method and system for detecting repetitive bit patterns

#The present invention relates to digital telecommunications. In particular, the present invention relates to a method and system for detecting a repetitive bit pattern from a continuous bit stream.

#In communication systems a transmitter sends data to a receiver, over a channel as a series of symbols. In practice the symbol set used in modem telecommunications is the binary digit set, composed of the bits ‘1’ and ‘0’. The transmitted symbols can be converted into characters, or other useable formats, by the receiver so long as the receiver starts decoding at the correct point in the communication data stream. This is an issue in many communication systems, but is especially prevalent in wireless broadcasts, where the receiver can come online at any point in the transmission.

#As a simple example of why this is an issue examine the transmission of a series of bits representing an unencoded ASCII transmission. If a receiver is expecting simple non-parity 8-bit ASCII transmissions and it receives the following bits as the first bits that it receives “001000010” it could interpret the received string in a number of ways. The receiver could look at the first eight bits and decode the string as ‘!’, or it could assume that the first bit was the end of another character and then decode the next 8 bits as ‘B’. Additionally the receiver could assume that the character starts at any arbitrary point and continually interpret the data stream incorrectly. A far worse situation can occur if the data being transmitted in the data stream is of a higher complexity and a more rigorously designed format than simple ASCII.

#Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

#FIG. 1 is an overview of a system of the present invention;

#FIG. 2 is a flow chart detailing a method of the present invention;

#Generally, the present invention provides a method and system for detecting repetitive symbol patterns that denote the flags upon which synchronization of a receiver to a transmitter are based. FIG. 1 illustrates the system in which the invention interacts. A data source 100 provides information to a transmitter 102. If the data is not already in a transmittable format the transmitter 102 processes it. The data is formatted by the transmitter 102 according to the rules set out in a standard that both the transmitter 102 and the receiver 106 have previously agreed upon. The transmitter 102 converts the data from the data source 100 into symbols for transmission. Typically the symbol set supported is the binary digit set of ‘0’ and ‘1’, wherein the symbols are conventionally referred to as bits. The symbols are transmitted as a datastream through the transmission medium 104, which is depicted here as a satellite based wireless system, though in practice it could be a conventional copper wire, fibre optic, non-satellite based wireless, or any other transmission medium. The transmission medium 104 allows the datastream of symbols to be transmitted to the receiver 106, which accepts the datastream through the receiver interface 108, and a demodulator 110. These components are known in the art and are not necessary in every embodiment of the present invention though they are shown as part of this exemplary embodiment. The synchronizer 112 receives the demodulated datastream, and provides at its output a synchronization signal that can be used by other components of the receiver 106 to determine where to start and stop the decoding of the signal. After the synchronization of the receiver 106 to the transmitter 102 the synchronizer 112 can serve to check that the synchronization is maintained. The receiver 106, after being synchronized, provides decoded data to a destination 114.

#The method by which the receiver 106 is synchronized to the transmitter 102 is referred to as word-based synchronization. The datastream is not necessarily an unformatted random stream of symbols, instead it can be a highly ordered data flow. Typically the datastream is divided into frames or words. Each word can be either a data word or a synchronization word. The synchronization words serve to allow the receiver 106 find the start of a data word so that the decoding of the signal can be done without synchronization induced errors. Upon confirming the presence of a synchronization word the synchronizer 112 informs other components of the receiver 106 so that decoding of the message can proceed.

#The process of detecting a synchronization word must account for the fact that erroneous synchronization words can occur in the middle of a data word as a result of the random, or pseudo-random, nature of data transmission. The process is illustrated in FIG. 2 and begins by receiving the datastream 116. The received datastream is then analyzed to identify a synchronization word candidate 118. The synchronization word found in this initial search is considered a candidate because at this point it is not possible to know if it is a valid synchronization word, or just a random occurrence. After identifying the synchronization word candidate more data is received 120. The content of this data is ignored by the synchronizer, but the symbols in the stream are counted. This process of ignoring the data is continued until the end of the data word, where it is expected that a subsequent synchronization word will occur 122. After detecting the location of a subsequent expected synchronization word a determination of whether or not a synchronization word is present is made 124. If no synchronization word is found the synchronizer 112 examines the data to find a new synchronization word candidate. If a synchronization word is found at the expected location the synchronizer repeats the process of receiving the datastream 120, and checking the location of the next expected synchronization word 124. The synchronizer 112 checks the stream for a series of consecutive synchronization words in the correct location. Since the datastream can be modeled as a pseudo-random sequence of symbols it is possible to calculate a probability of encountering a given number of false synchronization words consecutively. The number of synchronization words required to reach a desired reliability is calculable with sufficient stochastic information about the datastream. In a presently preferred embodiment the synchronizer 112 checks for seven consecutive synchronization word in their correct locations. As one skilled in the art will appreciate it is possible to vary this number without changing the scope of the present invention. If the predetermined number of synchronization words are found in 124, then the process terminates with the synchronizer 112 asserting that word-based synchronization of the receiver to the transmitter has been achieved 126.