Exercise
9
Viterbi Decoding in ATU Receivers EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the Viterbi decoding of convolutionally-encoded data as implemented in ATU receivers. You will be able to demonstrate the error correction capability of the Viterbi decoder used in ATU receivers. You will also be able to demonstrate that together the Reed-Solomon error correction and the trellis-coded modulation (TCM) / Viterbi decoding provide ADSL applications with a robust error control scheme.
DISCUSSION OUTLINE
The Discussion of this Exercise covers the following points:
DISCUSSION
Introduction to Viterbi Decoding Error Correction Capability of the Viterbi Decoder in ATU Receivers Summary of the Error Correction Capability of ATU Receivers
Introduction to Viterbi Decoding The trellis-coded modulation (TCM) is a form of convolutional-coding FEC used in ATU transmitters to increase the immunity against occasional errors caused by noise during transmission. When TCM is enabled in an ATU transmitter, the data to be transmitted, i.e., the data extracted from the data frame buffer, is not used directly to modulate tones. Instead, the extracted data is trellis coded to obtain data symbols that are sent to a QAM mapper which modulates the tones. This is illustrated in the simplified diagram of Figure 108.
Figure 108.When TCM is enabled, the data to be transmitted is trellis coded to obtain the data symbols modulating the tones.
Conversely, a decoding circuit is required in ATU receivers to convert the received data symbols into the corresponding data, i.e., the original data transmitted. A Viterbi decoder is commonly used to convert the data symbols at the QAM slicer output into data. A Bit-to-Byte Converter rearranges the data bits from the Viterbi Decoder into data bytes (i.e., it concatenates and slices the groups of data bits recovered from the various pairs of tones to obtain data bytes) to recover the original data frames transmitted. This is illustrated in the simplified diagram of Figure 109.
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Exercise 9 – Viterbi Decoding in ATU Receivers Discussion
Figure 109.The Viterbi decoder in ATU receivers converts the received data symbols into the corresponding data bits.
The Viterbi decoder in ATU receivers not only recovers the original data from the received data symbols but can also detect and correct, within certain bounds, occasional errors in the recovered data that may be caused by noise during transmission. The error detection and correction capability of the Viterbi decoder is due to the convolutional coding applied in ATU transmitters to some of the data bits transmitted. The rest of this exercise discussion deals with the errorcorrection capability of the Viterbi decoder used in ATU receivers.
Error Correction Capability of the Viterbi Decoder in ATU Receivers Figure 110 shows a block diagram of the trellis encoder used in ATU transmitters. Convolutional coding is applied to bits u1 and u2 to produce redundant check bit u0. This redundant check bit plus bits u1, u2, and u3 are then used to generate bits v0, v1, w0, and w1, which determine the 2-D coset related to each tone in a pair. All other bits extracted from the data frame buffer (bits u4 to uz) passed through the trellis encoder unmodified. Consequently, only bits v0, v1, w0, and w1 are protected by convolutional coding. In other words, only the 2 LSB’s of the data symbol associated with each tone (bits v0 and v1 for the first tone in a pair or bits w0 and w1 for the second tone in a pair) are protected by convolutional coding. At reception, therefore, only errors on the 2 LSB’s of each data symbol can be corrected. All errors affecting the other bits in the received data symbols pass through the Viterbi decoder undetected.
Figure 110.Certain bits extracted from the data frame buffer are protected by convolutional coding.
When an error occurs during transmission, the point received on the constellation differs from the point transmitted, thereby causing the received data symbol to differ from the original data symbol transmitted. Error correction is successful only when the point received is brought back to the location of the original point transmitted, thereby allowing recognition of the correct data symbol, i.e., the one that was transmitted.
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Exercise 9 – Viterbi Decoding in ATU Receivers Discussion Figure 111 shows the 16-point constellation employed in ADSL applications. When point 1 + j1 (data symbol 0000) is transmitted, any transmission error moving the received point in such a way that the 2 MSB’s of the detected data symbol are not 00 cannot be corrected by the Viterbi decoder. This is because errors affecting bits other than the two LSB’s of the data symbol received cannot be detected by the Viterbi decoder. This makes error correction impossible for 12 of the 16 points in the constellation (these points are shown over a grey background in the constellation of Figure 111). This leaves only 3 points in the constellation for which error correction could be possible (these points are shown over a yellow background in the constellation of Figure 111). These 3 points have the following data symbols: 0001, 0010, and 0011. Notice that these 3 points are located in 2-D cosets (1, 2, and 3, respectively) other than the 2-D coset (0) of the original point transmitted.
Figure 111. Error correction capability of the Viterbi decoder.
Error correction by the Viterbi decoder is in fact possible for only two of the three points mentioned above. This is because the Viterbi decoder operation is based on a statistical technique which is commonly referred to as the maximum likelihood approach. In brief, this approach states that if noise causes an error to occur during transmission, this error is most likely to move the point received to the nearest point possible. Close inspection of the 16-point constellation above shows that among the 3 remaining points for which error correction could be possible, two points (points represented by data symbols 0001 and 0010) are nearer to the original point transmitted (0000) than the third point (point represented by data symbol 0011). In fact, the Hamming distance between the original point transmitted (point 0000) and either point 0001 and 0010 is 1, while the Hamming distance between the original point transmitted (point 0000) and point 0011 is 2. Therefore, an error moving the received point to either one of the 2 nearest points will be corrected while an error moving the received point to the farthest of the 3 points will be left uncorrected. This brief analysis of the error correction capability of the Viterbi decoder applies to any other point in any one of the constellations used in ADSL applications.
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If you are not already familiar with the maximum likelihood approach, you can refer to appendices B and C of this manual which deal with the decoding of convolutionally-encoded data and the Viterbi decoding algorithm, respectively.
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Exercise 9 – Viterbi Decoding in ATU Receivers Discussion The Viterbi decoder can correct multiple errors affecting the 2 LSB’s of several data symbols in the same data frame as long as it is able to recover the original path traced in the trellis diagram by the trellis encoder of the ATU transmitter. The number of such errors that can be corrected every data frame is variable and mainly depends on the nature of the errors and their location within the data frame. This makes the Viterbi decoder well suited to correct occasional errors caused by noise during transmission.
Summary of the Error Correction Capability of ATU Receivers Together, the two types of FEC implemented in ADSL applications, ReedSolomon error correction and TCM / Viterbi Decoding, provide a fairly robust error control scheme. The Viterbi decoder in ATU receivers corrects occasional minor errors caused by noise that affect the LSB’s of the received data symbols. On the other hand, the Reed-Solomon decoder corrects the errors not corrected by the Viterbi decoder, that is, more serious errors caused by noise bursts that are likely to affect any bits in the received data symbols. However, the maximum number of errors which can be corrected by the Reed-Solomon decoder every data frame is strictly limited. Fortunately, this number can be increased by enabling data interleaving. This number can be further increased by increasing the interleave depth. However, this is made at the expense of the system latency. Figure 112 shows the arrangement of the Viterbi decoder, bit-to-byte converter, deinterleaver, and Reed-Solomon decoder in ATU receivers. The Viterbi decoder first converts the received data symbols into the corresponding data bits and corrects occasional noise-caused errors that may affect these data bits. The bitto-byte converter rearranges the groups of data bits into bytes. When data interleaving is used at transmission, the deinterleaver rearranges the received data bytes to rebuild the original data frames transmitted. Each of these data frame contains the redundant check bytes added by the Reed-Solomon encoder at transmission. Finally, the Reed-Solomon decoder uses these redundant check bytes to correct errors remaining in each data frame.
Figure 112.Arrangement of the Viterbi decoder, bit-to-byte converter, deinterleaver, and Reed-Solomon decoder in an ATU receiver.
When errors remain uncorrected in certain data frames after Reed-Solomon decoding, it indicates that the number of errors in those data frames exceeded the maximum number of errors per data frame that the system can correct. In this situation, the number of bits allocated to certain tones should be decreased to allow constellations with lower numbers of points to be used. This increases the distance between points in the constellations used, and thereby, lowers the number of transmission errors occurring every data frame. In other words, when too many errors occur during data transmission, the ATU’s at both ends of the telephone line are forced to go into training mode to reevaluate the quality of the telephone line and readjust the number of bits allocated to each tone accordingly.
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Outline
PROCEDURE OUTLINE
The Procedure is divided into the following sections:
PROCEDURE
Equipment Setup and Connections Operation of the Viterbi Decoder Error Correction Capability of the Viterbi Decoder Error Correction Capability of ATU Receivers
Equipment Setup and Connections 1. Turn on the RTM Power Supply and the RTM and make sure the RTM power LED is lit.
2. Turn on the host computer. Make sure that the system has been installed and configured as described in the Communications Technologies Training System User Guide.
3.
Start the LVCT software. In the Application Selection dialog box, choose ADSL and click OK. This begins a new session with all settings set to their default values and with all faults deactivated. The System Diagram appears showing the ATU-R Transmitter and the ATU-C Receiver.
4. Make the Default external connections shown on the System Diagram tab of the ADSL application. For details of connections to the Reconfigurable Training Module, refer to the RTM Connections tab of the software.
Operation of the Viterbi Decoder 5. Display the block diagram of the ATU-R Transmitter by clicking the corresponding tab in the ADSL application. Use the Pan and Zoom commands to display the portion of the ATU-R Transmitter shown in Figure 113.
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure
Figure 113.Portion of the ATU-R Transmitter block diagram showing the data frame buffer and the Constellation Encoder.
Turn the Scrambler off by changing the Scrambler / Descrambler setting to Off in the ADSL Settings table. Enable TCM in the ADSL application by setting the Trellis Encoder / Viterbi Decoder parameter in the ADSL Settings table to On.
b
TCM can also be enabled by clicking the Trellis Encoder ON/OFF button in the ATU-R Transmitter or the Viterbi Decoder ON/OFF button in the ATU-C Receiver.
6. Select the Frame Step mode by clicking the Frame Step button ( ADSL application toolbar.
) in the
Click the Edit button in the Bit/Tone Table. This opens the Bit/Tone Table window. Set the number of bits allocated to all tones to 4 by entering 4 in the data field next to the Apply to All Tones button, click this button, and click the Apply button located at the bottom of the Bit/Tone Table window. Observe that the total number of bits allocated to all tones is equal to 96, which is equivalent to 12 bytes. However, since 16 bits are used by TCM, only 80 data bits (10 data bytes) can be extracted from the data frame buffer every data frame. Close the Bit/Tone Table window.
7. Click the switch located just before DP2 to open the path between the Byte Multiplexer and the Scrambler. Double click DP2 in the ATU-R Transmitter block diagram to open the corresponding data point window. This window displays the data at the Byte Multiplexer output (Scrambler input) for one data frame.
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Edit the 10 data bytes in the DP2 window so that they are identical to the arbitrary data sequence shown in Figure 114. Click the Record button in the DP2 window to record the new values of the 10 data bytes. This arbitrary data sequence will be transmitted at the next data frame.
Figure 114.Arbitrary data sequence used to verify the operation of the Viterbi Decoder in the ATU-C Receiver.
Double click DP5 in the ATU-R Transmitter block diagram to open the corresponding data point window. This window displays the data at the Interleaver output (input of the Bit Extractor and Trellis Encoder) for one data frame. Click the Frame Step button once to transmit one more data frame. The arbitrary data sequence you entered in the DP2 window should appear at DP5. Close the DP2 window. Double click DP7 in the ATU-R Transmitter block diagram to open the corresponding data point window. This window displays the data symbols at the output of the Bit Extractor and Trellis Encoder for one data frame.
8. Display the block diagram of the ATU-C Receiver by clicking the corresponding tab in the ADSL application. Use the Pan and Zoom commands to display the portion of the ATU-C Receiver shown in Figure 115, which includes the Constellation Decoder, Deinterleaver, and Reed-Solomon Decoder.
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure
Figure 115.Portion of the ATU-C Receiver block diagram showing the Constellation Decoder, Deinterleaver, and Reed-Solomon Decoder.
Double click DP7 in the ATU-C Receiver block diagram to open the corresponding data point window. This window displays the data at the Constellation Decoder output (output of the Viterbi Decoder and Bit-to-Byte Converter) for one data frame. Click the Frame Step button once to receive one more data frame. Observe that the data at DP7 of the ATU-C Receiver is identical to the arbitrary data sequence transmitted (data at DP5 of the ATU-R Transmitter), thereby confirming that no errors occurred during transmission. Double click DP5 in the ATU-C Receiver block diagram to open the corresponding data point window. This window displays the data symbols at the QAM Slicer output for one data frame. Compare the received data symbols (data symbols at DP5 of the ATU-C Receiver) to the transmitted data symbols (data symbols at DP7 of the ATU-R Transmitter). Observe that they are identical because no errors occurred during transmission.
9. Display the detailed block diagram of the Viterbi Decoder and Bit-to-Byte Converter by clicking the corresponding tab in the ADSL application. Double click DP19 in the detailed block diagram of the Viterbi Decoder and Bit-to-Byte Converter to open the corresponding data point window. This window displays the data at the Viterbi Decoder output for one data frame.
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Observe that the data at the Viterbi Decoder output differs from the data at its input, i.e., the data coming from the QAM Slicer output (DP5 of the ATU-C Receiver). Briefly explain why.
10. Display the detailed block diagram of the Bit Extractor and Trellis Encoder by clicking the corresponding tab in the ADSL application. Double click DP14 in the detailed block diagram of the Bit Extractor and Trellis Encoder to open the corresponding data point window. This window displays the data at the Bit Extractor output for one data frame. Compare the data bits at the Viterbi Decoder output to the data bits extracted for each pair of tones in the ATU-R Transmitter (data at DP14 of the ATU-R Transmitter). Are they identical?
Yes
No
11. Double click DP15 in the detailed block diagram of the Bit Extractor and Trellis Encoder to open the corresponding data point window. This window displays the sequence of states followed by the Convolutional Encoder of the Trellis Encoder during a data frame. Display the detailed block diagram of the Viterbi Decoder and Bit-to-Byte Converter. Double click DP16 in the detailed block diagram of the Viterbi Decoder and Bit-to-Byte Converter to open the corresponding data point window. This window displays the sequence of states followed by the Viterbi Decoder during a data frame. Compare the sequence of states followed by the Viterbi Decoder to that followed by the Convolutional Encoder of the Trellis Encoder. Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder? Why?
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure
Error Correction Capability of the Viterbi Decoder 12. Display the block diagram of the ATU-C Receiver. In the ATU-C Receiver block diagram, click the switch located just before DP5 to open the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. Observe the data at the QAM Slicer output (DP5 of the ATU-C Receiver). Notice that the data symbol for tone 17 is 0101. Edit the value of this data symbol so that it is equal to 1101 (see Figure 116), thereby introducing an error on the MSB of this data symbol. This error introduced manually is equivalent to an error that occurs during data transmission. Click the Record button in the DP5 window to record the new value of the edited data symbol.
Figure 116. Editing a data symbol at the QAM Slicer output to introduce a transmission error.
Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Compare the recovered data frame to the transmitted data frame (data at DP5 of the ATU-R Transmitter). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Set the value of the data symbol related to tone 17 back to its original value (0101) to remove the error. Click the Record button in the DP5 window of the ATU-C Receiver to record the original value of this data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure 13. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 17 so that it is equal to 0001, thereby introducing an error on the bit next to the MSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Compare the recovered data frame to the transmitted data frame (data at DP5 of the ATU-R Transmitter). Is the recovered data frame identical to the transmitted data frame?
Yes
No
From your observations, is the Viterbi Decoder able to correct a transmission error affecting bits other than the two LSB’s of a data symbol received? Briefly explain why.
Set the value of the data symbol related to tone 17 back to its original value (0101) to remove the error. Click the Record button in the DP5 window of the ATU-C Receiver to record the original value of this data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
14. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 17 so that it is equal to 0111, thereby introducing an error on the bit preceding the LSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Compare the recovered data frame to the transmitted data frame (data at DP5 of the ATU-R Transmitter). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Compare the sequence of states followed by the Viterbi Decoder (data at DP16 of the ATU-C Receiver) to that followed by the Convolutional Encoder of the Trellis Encoder (data at DP15 of the ATU-R Transmitter). Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
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No
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Set the value of the data symbol related to tone 17 back to its original value (0101) to remove the error. Click the Record button in the DP5 window of the ATU-C Receiver to record the original value of this data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
15. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 17 so that it is equal to 0100, thereby introducing an error on the LSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Compare the recovered data frame to the transmitted data frame (data at DP5 of the ATU-R Transmitter). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Compare the sequence of states followed by the Viterbi Decoder (data at DP16 of the ATU-C Receiver) to that followed by the Convolutional Encoder of the Trellis Encoder (data at DP15 of the ATU-R Transmitter). Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
Yes
No
From your observations, is the Viterbi Decoder able to correct an error affecting either one of the 2 LSB’s of a data symbol received? Briefly explain why.
Set the value of the data symbol related to tone 17 back to its original value (0101) to remove the error. Click the Record button in the DP5 window of the ATU-C Receiver to record the original value of this data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
16. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 17 so that it is equal to 0110, thereby introducing errors on the
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure 2 LSB’s of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Compare the recovered data frame to the transmitted data frame (data at DP5 of the ATU-R Transmitter). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Compare the sequence of states followed by the Viterbi Decoder (data at DP16 of the ATU-C Receiver) to that followed by the Convolutional Encoder of the Trellis Encoder (data at DP15 of the ATU-R Transmitter). Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
Yes
No
From your observations, is the Viterbi Decoder able to correct an error affecting the 2 LSB’s of a data symbol received? Briefly explain why.
Set the value of the data symbol related to tone 17 back to its original value (0101) to remove the error. Click the Record button in the DP5 window of the ATU-C Receiver to record the original value of this data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
17. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 17 so that it is equal to 0100 instead of 0101, thereby introducing an error on the LSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure 18. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 9 so that it is equal to 0111 instead of 0110, thereby introducing an error on the LSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
Yes
No
19. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 27 so that it is equal to 0110 instead of 0100, thereby introducing an error on the bit preceding the LSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
Yes
No
20. In the DP5 window of the ATU-C Receiver, edit the value of the data symbol related to tone 14 so that it is equal to 0100 instead of 0110, thereby introducing an error on the bit preceding the LSB of this data symbol. Click the Record button in the DP5 window to record the new value of the edited data symbol. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Is the path traced in the trellis diagram by the Viterbi Decoder identical to that traced by the Convolutional Encoder of the Trellis Encoder?
Yes
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure From the observations you have made in this step and the previous ones, is the Viterbi Decoder able to correct errors affecting either one of the 2 LSB’s of several data symbols in a data frame received? Briefly explain why.
Error Correction Capability of ATU Receivers 21. In the DP5 window of the ATU-C Receiver, set the values of the data symbols related to tones 9, 14, 17, and 27 back to their original values (the original values are indicated in the DP7 window of the ATU-R Transmitter) to remove all errors. Click the Record button in the DP5 window of the ATU-C Receiver to record the original values of these data symbols. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
22. Enable data interleaving in the ADSL application by setting Interleaver / Deinterleaver parameter in the ADSL Settings table to On.
the
Make sure that the Interleave Depth parameter in the ADSL Settings table is set to 2. Enable the Reed-Solomon error correction in the ADSL application by setting the Reed-Solomon Encoder / Decoder parameter in the ADSL Settings table to On. In the ATU-C Receiver block diagram, click the switch located just before DP5 to close the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. Click the Frame Step button three times to transmit three more data frames. Observe the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
a
The red LED in the Reed-Solomon Decoder should light up for the first two data frames received and go out when the third data frame is received.
23. In the ATU-C Receiver block diagram, click the switch located just before DP5 to open the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. In the DP5 window of the ATU-C Receiver, edit the values of the data symbols related to tones 14, 19, and 30 so as to introduce an error on the
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure LSB of each one of these data symbols. Click the Record button in the DP5 window to record the new values of the edited data symbols. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Has the Viterbi Decoder been able to correct the errors affecting the LSB of 3 data symbols in the data frame received?
Yes
No
24. In the DP5 window of the ATU-C Receiver, edit the values of the data symbols related to tones 14, 15, 17, and 18 so as to introduce an error on the MSB of each one of these data symbols. Click the Record button in the DP5 window to record the new values of the edited data symbols. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
25. Double click DP8 and DP9 in the ATU-C Receiver block diagram to open the corresponding data point windows. These windows display the data at the output of the Deinterleaver and Reed-Solomon Decoder for one data frame. Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
26. In the ATU-C Receiver block diagram, click the switch located just before DP5 to close the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. Click the Frame Step button once while observing the data at DP7, DP8, and DP9 of the ATU-C Receiver.
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Is the recovered data frame (data at DP7) identical to the transmitted data frame?
Yes
No
Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
27. Click the Frame Step button to receive one more data frame. The green LED should be lit in the Reed-Solomon Decoder to indicate that no errors have been detected in the last data frame. In the ATU-C Receiver block diagram, click the switch located just before DP5 to open the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. In the DP5 window of the ATU-C Receiver, edit the values of the data symbols related to tones 14, 19, and 30 so as to introduce an error on the LSB of each one of these data symbols. Click the Record button in the DP5 window to record the new values of the edited data symbols. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Has the Viterbi Decoder been able to correct the errors affecting the LSB of 3 data symbols in the data frame received?
Yes
No
28. In the DP5 window of the ATU-C Receiver, edit the values of the data symbols related to tones 14, 15, 17, 18, and 19 so as to introduce an error on the MSB of each one of these data symbols. Click the Record button in the DP5 window to record the new values of the edited data symbols. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
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No
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
29. In the ATU-C Receiver block diagram, click the switch located just before DP5 to close the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. Click the Frame Step button once while observing the data at DP7, DP8, and DP9 of the ATU-C Receiver. Is the recovered data frame (data at DP7) identical to the transmitted data frame?
Yes
No
Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
What could be done in this case to improve the situation? Explain briefly.
30. Set the Interleave Depth parameter in the ADSL Settings table to 4 to increase the interleave depth. Click the Frame Step button five times to transmit five more data frames. Observe the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). The recovered data frame should be identical to the transmitted data frame (data at DP5 of the ATU-R Transmitter).
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure
a
The red LED in the Reed-Solomon Decoder should remain lit for the first four data frames received and go out when the fifth data frame is received.
31. In the ATU-C Receiver block diagram, click the switch located just before DP5 to open the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. In the DP5 window of the ATU-C Receiver, edit the values of the data symbols related to tones 14, 19, and 30 so as to introduce an error on the LSB of each one of these data symbols. Click the Record button in the DP5 window to record the new values of the edited data symbols. In the DP5 window of the ATU-C Receiver, edit the values of the data symbols related to tones 14, 15, 17, 18, and 19 so as to introduce an error on the MSB of each one of these data symbols. Click the Record button in the DP5 window to record the new values of the edited data symbols. Click the Frame Step button once while observing the data frame recovered at the output of the Viterbi Decoder and Bit-to-Byte Converter (data at DP7 of the ATU-C Receiver). Is the recovered data frame identical to the transmitted data frame?
Yes
No
Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
32. In the ATU-C Receiver block diagram, click the switch located just before DP5 to close the path between the QAM Slicer and the Viterbi Decoder and Bit-to-Byte Converter. Click the Frame Step button once while observing the data at DP7, DP8, and DP9 of the ATU-C Receiver. Is the recovered data frame (data at DP7) identical to the transmitted data frame?
Yes
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No
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Exercise 9 – Viterbi Decoding in ATU Receivers Procedure Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
33. Click the Frame Step button once while observing the data at DP7, DP8, and DP9 of the ATU-C Receiver. Is the recovered data frame (data at DP7) identical to the transmitted data frame?
Yes
No
Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
34. Click the Frame Step button once while observing the data at DP7, DP8, and DP9 of the ATU-C Receiver. Is the recovered data frame (data at DP7) identical to the transmitted data frame?
Yes
No
Are there errors in the data frame at the Deinterleaver output? If so, has the Reed-Solomon Decoder been able to correct the error(s)? Explain briefly.
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Exercise 9 – Viterbi Decoding in ATU Receivers Conclusion Does increasing the interleave depth improve the error correction capability of the ATU-C Receiver? Explain briefly.
35. When you have finished using the system, exit the LVCT software and turn off the equipment.
CONCLUSION
In this exercise, you learned that a Viterbi decoder is commonly used in ATU receivers to convert the received data symbols into the corresponding data, i.e., the original data frames transmitted. You saw that the Viterbi decoder can also detect and correct occasional errors in the recovered data frames that may be caused by noise during transmission. You observed the error correction capability of the Viterbi Decoder in the ATU-C Receiver of our ADSL application. You also observed the overall correction capability of the ATU-C Receiver in our ADSL application.
REVIEW QUESTIONS
1. What are the two main functions of the Viterbi decoder in ATU receivers?
2. What is the error correction capability of the Viterbi decoder in ATU receivers? Explain briefly.
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Exercise 9 – Viterbi Decoding in ATU Receivers Review Questions 3. Briefly explain why an error affecting either one of the 2 LSB’s of a data symbol is corrected by the Viterbi Decoder whereas an error affecting both of these LSB’s is not corrected?
4. Complete the following sentence. - The Viterbi decoder of an ATU receiver can correct errors affecting either one of the 2 LSB’s of data symbols in a same data frame as long as…
5. Summarize the error correction capability of ATU receivers.
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