Overview - Digital Design of Signal Processing Systems: A Practical Approach - page 15

this state the systemalso detects the uniqueword (UW) in the transmitted burst and identifies the start
of data. If the UW in the received burst is not detected, the algorithm transits back into the burst
detection mode. When both the burst and the UW are detected, then the algorithm transits to the
estimation state. In this state the algorithm estimates amplitude, timing, frequency and phase errors
using the known header placed in the transmitted burst. The algorithm then transits to the
demodulation state. In this state the system executes all the timing, phase and frequency error-
correction loops. The output of the corrected signal is passed to the slicer. The slicer makes the soft
and hard decisions. For forward error correction (FEC), the system implements a Viterbi algorithm
to correct the bit errors in the slicer soft decision output, and generates the final bits [15]. The frame
and end of frame are identified. In a burst, the transmitter can transmit several frames. To identify the
end of the burst, the transmitter appends a particular sequence in the end of the last frame. If this
sequence is detected, the receiver transits back to the SoB state. The state diagram of the sequence of
operation in a satellite burst modem receiver is shown in Figure 1.12.
The algorithm is partitioned to be mapped on different components based on the nature of
computations required in implementing the sub-components in the algorithm. The following
mapping effectively implements the system.
. A DSP is used for mapping computationally intensive and non-regular algorithms in the receiver.
These algorithms primarily consist of the demodulator and carrier and timing recovery loops.
. ASICs are used for ADC, DAC, digital down-conversion (DDC) and digital up-conversion (DUC).
A direct digital frequency synthesis (DDFS) chip is used for cosine generation that mixes with the
baseband signal.
Figure 1.12 Sequence of operations in a satellite burst modem receiver