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ST70235A Datasheet(PDF) 11 Page - STMicroelectronics |
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ST70235A Datasheet(HTML) 11 Page - STMicroelectronics |
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11 / 28 page  ST70235A 11/28 The FFT module is a slave DSP engine controlled by the firmware running on an external controller. It works off line and communicates with other blocks through buffers controlled by the "Data Symbol Timing Unit". The DSP executes a program stored in a RAM area, which constitutes a flexible element that allows for future system enhancements. DPLL The Digital PLL module receives a metric for the phase error of the pilot tone. In general, the clock frequencies at the ends (transmitter and receiver) do not match exactly. The phase error is filtered and integrated by a low pass filter, yielding an estimation of the frequency offset. Various processes can use this estimate to deal with the frequency mismatch. In particular, small accumulated phase error can be compensated in the frequency domain by a rotation of the received code constellation (Rotor). Larger errors are compensated in the time domain by inserting or deleting clock cycles in the sample input sequence. Eventually that leads to achieve less than 2ppm between the two ends. Mapper/Demapper, Monitor, Trellis Coding, FEQ Update The Demapper converts the constellation points computed by the FFT to a block of bits. This means to identify a point in a 2D QAM constellation plane. The Demapper supports Trellis coded demodulation and provides a Viterbi maximum likelihood estimator. When the Trellis is active, the Demapper receives an indication for the most likely constellation subset to be used. In the transmit direction, the mapper receives a bit stream from the Trellis encoder and modulates the bit stream on a set of carriers (up to 256). It generates coordinates for 2n QAM constellation, where n < 15 for all carriers. The Mapper performs the inverse operation, mapping a block of bits into one constellation point (in a complex x+jy representation) which is passed to the IFFT block. The Trellis Encoder generates redundant bits to improve the robustness of the transmission, using a 4-Dimensional Trellis Coded Modulation scheme. This feature can be disabled.The Monitor computes error parameters for carriers specified in the Demapper process. Those parameters can be used for updates of adaptive filters coefficients, clock phase adjustments, error detection, etc. A series of values is constantly monitored, such as signal power, pilot phase deviations, symbol erasures generation, loss of frame, etc. Generic TC Layer Functions These functions relate to byte oriented data streams. They are completely described in ANSI T 1.4 13. Additions described in the Issue 2 of this specification are also supported. The data received from the demapper may be split into two paths, one dedicated to an interleaved data flow the other one for a fast data flow. No external RAM is needed for the interleaved path. The interleaving/deinterleaving is used to increase the error correcting capability of block codes for error bursts. After deinterleaving (if applicable), the data flow enters a Reed-Solomon error correcting code decoder, able to correct a number of bytes containing bit errors. The decoder also uses the information of previous receiving stages that may have detected the error bytes and have labelled them with an "erasure indication". Each time the RS decoder detects and corrects errors in a RS codeword, an RS correction event is generated. The occurrence of such events can be signalled to the management layer.After the RS decoder, the corrected byte stream is descrambled in the PMD (Physical Medium Dependent) descramblers. Two descramblers are used, for interleaved and non-interleaved data flows. These are defined in ANSI T1.413. After descrambling, the data flows enter the Deframer that extracts and processes bytes to support Physical layer related functions according to ANSI T1.413. The ADSL frames indeed contain physical layer-related information in addition to the data passed to the higher layers. In particular, the deframer extracts the EOC (Embedded Operations Channel), the AOC (ADSL Overhead Control) and the indicators bits and passes them to the appropriate processing unit (e.g. the transceiver controller). The deframer also performs a CRC check (Cyclic Redundancy Check) on the received frame and generates events in case of error detection.Event counters can be read by management processes. The outputs of the deframer are an interleaved and a fast data streams. These data streams can either carry ATM cells or another type of traffic. In the latter case, the ATM specific TC layer functional block, described hereafter, is bypassed and the data stream is directly presented at the input of the interface module. |
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