Questions & Answers | ADSL Technology and AFE Application Notes | Glossary |
ADSL Technology and AFE Application Notes
2.1: What are the ADSL data rates and bandwidths?
2.2: What are the basic functions of an ADSL modem?
2.3: What are the characteristics of the KeyWave AFE's CO/RT configurations?
2.4: What is the relationship between data rates and DAC/ADC sampling rates?
2.5: How do analog and digital cancellation work?
2.6: What is the function of the KeyWave AFE's trim network?
2.7: What is the function of the hybrid circuit (line interface)?
2.8: What is the function of the AFE's power amp control block?
2.9: What are the rate and reach performance characteristics of the AFE?
2.1: What are the ADSL data rates and bandwidths?
ADSL data rates
Full rate standard:
G.dmt = ITU G.992.1 = ANSI T1.413, Iss.2
Downstream (DS): 64 kbps - 8.192 Mbps
Upstream (US): 16 kbps - 768 kbps
"Splitterless", low power standard:
G.lite = ITU G.992.2
DS: 64 kbps - 1.5 Mbps
US: 16 kbps - 368 kbps
Bandwidths
POTS: 300 Hz to 3.4 kHz
ISDN to 70 kHz (2B1Q) 140 kHz (4B3T)
G.dmt US: 25 kHz to 138 kHz (7 to 31 DMT tones)
G.dmt DS: 138 kHz to 1.1 MHz (FDM, 32 - 255 DMT tones)
25 kHz to 1.1 MHz (Echo Cancellation, 7 - 255 DMT tones)
G.lite US: 25 kHz to 138 kHz
G.lite DS: 138 kHz to 550 kHz
There are a total of 256 DMT tones, separated by 4.3125 kHz, each with a different centre frequency, fc. Since the POTS channel resides at the lower end of this spectrum, only tones 7 -255 are available. For full-rate ADSL, the downstream uses DMT tones 7 - 255 (echo cancellation) or 32 - 255 (FDM). The upstream channel uses tones 7 - 31. Bit-loading is adaptive and varies from 2 to 15 bits per tone (subchannel) depending on the relative noise of each carrier. When high noise levels are detected in a given subchannel, the DMT modem can shut down a particular subchannel altogether.
For the reduced bandwidth of G.lite, there are a total of 128 DMT tones. The downstream channel uses 7 - 128 tones and the upstream, 7 - 32. Bit-loading varies from 2 to 8 bits per symbol.
2.2: What are the basic functions of an ADSL modem?
The ADSL transmission signal is modulated onto discrete multi-tones (DMT). There are 256 independent parallel subchannels available in the 1.1 MHz ADSL bandwidth. Each subchannel is separated by approximately 4 kHz and has a distinct carrier frequency in the centre of this 4 kHz band. While DMT is the physical transmission level, framing and encoding (error correction) occurs at a higher level. The modulation technique used in each of the discrete multi-tone (DMT) channels is Quadrature Amplitude Modulation (QAM) where both the phase angle and the amplitude of the carrier band are modulated to represent the information being transmitted.
In a typical ADSL modem, the main sections are the Digital Interface (e.g. ATM), the Framer/FEC plus Encoder/Decoder, the DMT Modulator, and the AFE (Analog Front End). The Framer multiplexes serial data into frames, generates FEC (Forward Error Correction), and interleaves data. FEC and data interleaving corrects for burst errors. This allows DMT-based ADSL technology to be suitable for support of MPEG-2 and other digital video compression techniques. For the transmit signal, the Encoder encodes frames to produce the constellation data for the DMT Modulator. It assigns the maximum number of bits per tone (based on measured SNR of each carrier) and generates a QAM constellation where each point represents a digital value. Each constellation point is one of N complex numbers, x + iy, where x and y are the phase and amplitude components. The summation of bits in all carriers, multiplied by the frame rate (4kHz), represents the data rate. For the receive signal, the Decoder converts QAM symbols back into the data bitstream.
In the DMT Modulator, a frequency domain processor implements FFT/IFFT and associated processing. In the transmit path, the Inverse Fast Fourier Transform (IFFT) module accepts input as a vector of N QAM constellation points and duplicates each carrier with its conjugate counterpart so the 2N output samples are real. The 2N time domain samples have the last 2N/16 samples appended as a cyclic prefix, and are then delivered to the DAC. The set of time domain samples represents a summation of all the modulated subchannels, for the duration of one data frame. In the receive path, the first 2N/16 samples from the ADC (cyclic prefix) are removed. The FFT module transforms the carriers back to phase and amplitude information (N complex QAM symbols). Correction for attenuation of the signal amplitude and phase shifts (i.e. overall distortion) is implemented. If the QAM constellation is thought of as points in a grid where rows and columns represent phase and amplitude information respectively, then the grid effectively rotates reference to the constellation points to correct for these distortions.
The time-domain unit of the DMT Modulator implements digital filtering (interpolation and decimation) and echo cancellation. For echo cancellation, an inverted replica of the Tx signal that leaks into the receiver (AFE ADC) is generated. The latter effectively subtracts this near-end replica and the far-end signal can then be processed to remove any remaining line-induced noise sources.
The AFE converts the digital transmit bitstream to analog via dual DACs. In ATU-R configuration, the second DAC channel is used to transmit and loop back the echo replica. In the receive path the signal from the line is converted back to digital via the ADCs. The AFE interfaces to the line via a hybrid circuit.
2.3: What are the characteristics of the KeyWave AFE's CO/RT configurations?
ATU-C: Central Office:
ADSL channel characteristics are wideband Tx and narrowband Rx. Both DACs on the KeyWave AFE can drive a single full-rate ADSL channel at 8.832 MSa/s, or can drive two G.lite channels at 4.416 MSa/s. The ADCs only need to process narrowband upstream channel so 4.416 MSa/s is sufficient and reduces power consumption.
For the full-rate CO Tx channel, the DAC sampling rate of 8.8 MSa/s is oversampling the 1.1MHz bandwidth. A high rate of oversampling relaxes the requirement on the reconstruction (anti-imaging) filters on the transmit path. Two G.lite Tx channels can be supported where each DAC generates signals at 4.4 MSa/s for a downstream G.lite channel which has a bandwidth of 550 kHz. Likewise, the Rx paths (CO or RT) are configured such that the signal is also oversampled which, in turn, relaxes the requirement for the anti-aliasing filters.
ATU-R: Remote Terminal:
ADSL channel characteristics are narrowband Tx and wideband Rx. One of the DACs drives the upstream narrowband channel, while the other DAC generates the echo cancellation signal required when in full rate mode. The ADC's combine to sample at 8.832 MSa/s for full rate wideband downstream channel. If in G.lite mode, echo cancellation is not required and the second Rx and Tx channels can be used optionally to support an analog modem.
2.4: What is the relationship between data rates and DAC/ADC sampling rates?
Data rates for full-rate and G.lite are not directly related to the DAC/ADC sampling rates. The data rates are determined by the number of DMT channels in each mode and the respective bit-loading per subchannel based on the DMT QAM technology. The total bandwidth of each upstream or downstream channel (depending on ADSL mode and CO or CPE configuration) is the characteristic that determines both the selection of the DAC/ADC sampling rates and the filter cut-off frequencies.
2.5: How do analog and digital cancellation work?
The line driver for Central Office configuration requires high power (12 V) whereas the Remote Terminal line driver only requires about 5 V. For the reception of full rate at RT, echo cancellation is required. Echo cancellation is not employed at CO since interference by NEXT (near-end cross talk) is a more important limitation on Rx performance. (At CO, only a fraction of the Tx signal goes through the Rx filters while at RT, all Tx echo goes through the Rx filters).
Analog echo cancellation
The high power Tx signal leaks back into the Rx path through the hybrid circuit. A replica of the echo is generated digitally in the echo cancellation module of the ADSL modem. This echo cancellation signal is fed to the second Tx DAC channel which in turn is looped back into the receive chain of the AFE. Hence, there are three signals into receive path of the AFE:
- Far-end Rx signal from line (cable/wire/twisted pair)
- Near-end echo of high power Tx from line driver
- Replica of Tx echo cycled from EC DAC directly back into Rx path
Signals 2 and 3 cancel each other out. The weak Rx signal is amplified and fed to the ADC's for conversion. In summary, analog echo cancellation by the MB86626 allows higher gain before the ADC compared to using digital EC alone. This in turn gives lower receiver noise on long lines.
Digital echo cancellation
Subsequent digital EC handles incidental reflections and echoes from various points along the line coming in with the low level Rx signal by further processing with digital filters after the ADCs.
2.6: What is the function of the KeyWave AFE's trim network?
In order to accurately cancel the Tx near-end echo across the whole bandwidth, the effects of the line need to be accommodated. The Tx near-end echo, which gets reflected into the receive path along with the "wanted" receive signal, is not exactly the same as the original Tx output, but modified by the varying impedance of the line. The modem's echo cancellation module, which generates the Tx echo replica, can be designed to try to compensate for the effects of the line. This modified signal is then summed in the AFE's receive path for analog echo cancellation. Since this digital approximation cannot be perfect, the facility to try to provide more analog trimming is available with the AFE.
This is achieved with the trim network. The function of the trim network is to add or subtract small amounts of the Tx signal to the Rx signal to achieve impedance matching with line variations. In that way, the Tx line driver echo is adjusted before the analog/digital echo cancellation is applied. The resulting echo cancellation is more accurate which helps remove as much of the Tx echo as possible. At this point the weak far-end Rx signal is isolated and can be amplified for better resolution when processed by the ADC thus maximising the ADCs performance and achieving good SNR for the Rx signal.
2.7: What is the function of the hybrid circuit (line interface)?
The hybrid circuit serves as the interface to the two-wire copper telephone line. For the MB86626, advantages include avoiding the use of either a Rx path external amplifier or an active hybrid circuit. Except for the line driver, the remainder of the hybrid circuit required for the MB86626 is passive.
The hybrid circuit serves two main purposes:
- To isolate the AFE from the line (via line transformer)
- To separate Rx and Tx signals
Basically it is a high power differential amplifier in the Tx direction and a passive network in the Rx direction. High performance filtering and echo cancellation is configured in the AFE. The recommended line driver IC is dual differential and chosen to suit the maximum driver power required into the line. A differential driver configuration will reduce distortion (second order harmonics will be cancelled out).
2.8: What is the function of the AFE's power amp control block?
The PA Control block on the MB86626 AFE controls the maximum line driver power (by varying driver supply voltage). Line driver power requirement is a function of reach (transmission distance) required by system. The transmit power rail generally runs between +/- 10V. A plot of output power will show a continuous fluctuation with spikes. A continuous adjustment of supply voltage will help smooth out these spikes.
2.9: What are the rate and reach performance characteristics of the AFE?
DSL modems are characterised by data rate and reach performance in system applications. It is difficult to qualify the AFE by this criterion since performance is tied more to the data pump (digital) processing of the modem. However, low noise and good linearity are characteristics of the AFE that maximise this performance.
