PS20251 Datasheet, PDF (13/25 Pages) List of Unclassifed Manufacturers – Dual COFDM demodulator with PID filters and MRC diversity

PS20251
3.8 Fast Fourier Transform
The FFT module uses the trigger information from the timing synchronization module to set the start point
for an FFT. It then uses either a 2 K or 8 K FFT to transform the data from the time domain to the
frequency domain. An extremely hardware-efficient and highly accurate algorithm has been used for this
purpose.
3.9 Common Phase Error Correction
This module subtracts the common phase offset from all the carriers of the OFDM signal to minimize the
effect of the tuner phase noise on system performance.
3.10Channel Equalisation
This consists of two parts. The first part involves estimating the channel frequency response from pilot
information.
Efficient algorithms have been used to track time-varying channels with a minimum of hardware.
The second part involves applying a correction to the data carriers based on the estimated frequency
response of the channel. This module also generates dynamic channel state information (CSI) for every
carrier in every symbol.
3.11Impulse Filtering
PS20251 contains several mechanisms to reduce the impact of impulse noise on system performance.
3.12Transmission Parameter Signalling (TPS)
An OFDM frame consists of 68 symbols and a superframe is made up of four such frames. There is a set
of TPS carriers in every symbol and all these carry one bit of TPS. These bits, when combined, include
information about the transmission mode, guard ratio, constellation, hierarchy and code rate, as defined in
ETS 300 744. In addition, the first eight bits of the cell identifier are contained in even frames and the
second eight bits of the cell identifier are in odd frames. The TPS module extracts all the TPS data, and
presents these to the host processor in a structured manner.
3.13Diversity Combiner
This PS20251 combines the received diversity data with the channel-corrected data from its own OFDM
demodulation process. The resulting data is passed to the FEC logic to generate an MPEG transport
stream, refer to Figure 7 Diversity operation.
Two methods are available for diversity combining OFDM data carriers. The default method is Maximum
Ratio Combining, in which carriers are summed after being weighted by their SNR. The other method is
Switched Combining, in which the carrier with the higher SNR is chosen as output.
3.14 De-Mapper
This module generates soft decisions for demodulated bits using the channel-equalized in-phase and
quadrature components of the data carriers as well as per-carrier channel state information (CSI). The
de-mapping algorithm depends on the constellation (QPSK, 16 QAM or 64 QAM) and the hierarchy (α =
0, 1, 2 or 4). Soft decisions for both low- and high-priority data streams are generated.
3.15Symbol and Bit De-Interleaving
The OFDM transmitter interleaves the bits within each carrier and also the carriers within each symbol.
The de-interleaver modules consist largely of memory to invert these interleaving functions and present
the soft decisions to the FEC in the original order.
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