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The 256-Channel ADSL demo, adsl_sim, models part of the asymmetric digital subscriber line (ADSL) technology for transmitting data and multimedia information over telephone lines. It illustrates a downstream path from the central office to the end user. It incorporates the discrete multitone (DMT) signaling modulation technique, which is the focus of the dmt_sim demo. This document highlights these aspects of the adsl_sim demo:
Alternatively, an animated tour of an ADSL model is at http://www.mathworks.com/products/dsp_comm/demos.shtml. Use this link if you are reading this in the MATLAB Help browser.
Structure of the Demo
The model generates random binary data frames, transmits them according to the ADSL specification, simulates a telephone line using an FIR filter of length 101 and the AWGN Channel block, tries to recover the information from the received data, and computes error statistics. The model uses frame-based processing, thereby processing many bits in each time step. The double connector lines between blocks indicate frame-based signals.
Because these processes involve many blocks, the demo uses subsystems to organize some groups of blocks, and it uses Goto/From block pairs and colored regions to make the block diagram visually neater.
Transmitting Data
The transmitter portion of the model, shaded in blue at the top of the model, contains two parallel paths. One path (the fast buffer) processes the first 776 bits of each 1552-bit data frame, while the other path (the interleaved buffer) processes the last 776 bits of each data frame. Each path appends eight cyclic redundancy check (CRC) bits to its 776-bit frame, scrambles the bits, and encodes them using a shortened Reed-Solomon code. The scrambling and encoding operations interpret the bits as integers between 0 and 127. In the second path but not the first, a Convolutional Interleaver block interleaves the encoded data. This interleaving operation increases the second path's resistance to burst errors but also its latency. Finally, the data from the two routes are concatenated and modulated. Data from the fast buffer is modulated to the low frequency subcarriers, while data from the interleaved buffer is modulated to the high frequency subcarriers, according the bit allocation vector b. This demo assumes that the bit allocation vector is known and uses the vector to calculate the channel. Type get_param('adsl_sim', 'preLoadFcn') to see the calculations involved. For more information about the DMT Modulator block in this demo, see Discrete Multitone Signaling Demo.
Processing Received Data
The receiver attempts to undo each operation that the receiver performed. Much of the receiver's design is straightforward; for example, to undo the actions of the Convolutional Interleaver block, use a Convolutional Deinterleaver block with the same mask parameters. The frequency domain equalizer in the DMT Demodulator subsystem mitigates the channel distortion.
Aligning Frames to Account for Delays
One subtle point in the receiver portion is the Integer Delay block that follows the Convolutional Deinterleaver block. This Integer Delay block delays the deinterleaved data by 800 samples. Because the delay between the original and restored sequences is 40 samples (5 shift registers times a maximum delay of 2*(5-1) samples among all shift registers), the extra 800-sample delay ensures that bits are properly aligned in the 840-bit frame.
Displaying Error Statistics
Two display icons show error statistics for comparisons between the transmitted and received data in the two paths (with and without interleaving). Two other display icons show error statistics based on the CRC bits, where any nonzero bit among the eight CRC bits indicates a frame error.
In each of the display icons, the error statistics consist of the bit error rate, the number of bit errors, and the total number of bits processed.
Selected Bibliography
[1] Bingham, John A. C. ADSL, VDSL, and Multicarrier Modulation. Wiley: New York, 2000.
[2] ITU-T Recommendation G.992.1 Asymmetric Digital Subscriber Line (ADSL) Transceivers. Geneva: Telecommunication Standardization Sector of International Telecommunication Union, 1999.
[3] Maxwell, Kim. "Asymmetric Digital Subscriber Line: Interim Technology for the Next Forty Years." IEEE Communications Magazine, October 1996. 100-106.
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