PS20230 Datasheet, PDF (17/26 Pages) List of Unclassifed Manufacturers – COFDM demodulator with USB interface for PC-TV

PS20230
7.9 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.
7.10Channel 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.
7.11Impulse Filtering
PS20230 contains several mechanisms to reduce the impact of impulse noise on system performance.
7.12Transmission 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 processor in a structured manner.
7.13 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.
7.14Symbol 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.
7.15Viterbi Decoder
The Viterbi decoder accepts the soft decision data from the OFDM demodulator and outputs a decoded bit-
stream.
The decoder does the de-puncturing of the input data for all code rates other than 1/2. It then evaluates
the branch metrics and passes these to a 64-state path-metric updating unit, which in turn outputs a 64-
bit word to the survivor memory. The Viterbi decoded bits are obtained by tracing back the survivor
paths in this memory. A trace -back depth of 128 is used to minimize any loss in performance, especially
at high code rates. The decoder re-encodes the decoded bits and compares these with received data
(delayed) to compute bit errors at its input, on the assumption that the Viterbi output BER is significantly
lower than its input BER.
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