Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 10M

Unique word is again a sequence of bits that follows the carrier and clock recovery sequence of bits in the preamble. In the reference burst, this bit sequence allows the Earth station to locate the position of the received TDMA frame. The unique word bit sequence in the traffic burst provides a timing reference on the occurrence of the traffic burst and also provides a timing marker to allow the Earth stations to extract their part of the traffic burst. The timing marker allows the identification of the start and finish of a message in the burst and helps to correct decoding. Therefore unique word should have a high probability of detection.

For instance, when the unique word of a traffic burst is missed, the entire traffic burst is lost. To achieve this, the unique word is a sequence of 1’s and 0’s selected to exhibit good correlation properties to enhance probability of detection.

Figure shows a type of digital correlator circuit that can be used to detect the unique word bit sequence. Here, the unique word has N bits and is correlated with a stored pattern of itself. The received data is input to one of the N-bit shift registers, as shown in the figure, in synchronization with the data clock rate. The other N-bit shift register has a stored pattern of the unique word. Each stage of the shift register feeds a 2-bit adder, whose output is a ‘0’ if the bits are in agreement and a ‘1’ if they are in disagreement. The outputs of N adders are summed up.

The output of the summer is a step function that depends upon the number of agreements or disagreements between the received unique word bit pattern and the stored pattern. The output of summer then feeds a threshold detector that specifies the acceptable number of disagreements. If the number of mismatches is less than or equal to the preset threshold value, the unique word is considered to have been detected. The declaration of the detection of the unique word occurs at the time instant of reception of the last bit or symbol of the unique word. If the unique word belongs to the reference burst, it marks the TDMA receive frame timing. In the case of traffic burst, it marks the receive traffic burst timing.