Adaptive front end for discrete multitone modem

Details Adaptive front end for discrete multitone modem Adaptive front end for discrete multitone modem

1998-12-31

2004-02-17

Kizou, Hassan (Department: 2662)

Pulse or digital communications

Transceivers

Modems

C370S248000, C370S252000, C370S292000, C375S221000, C375S223000, C375S231000, C375S232000, C375S254000, C375S296000, C375S302000, C375S322000, C375S346000

Reexamination Certificate

active

06693957

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a discrete multitone modem. More particularly, the present invention relates to an adaptive front end for discrete multitone modems which adapts to subscriber loop conditions to provide reliable and efficient communications through the loop and which can reduce computational complexity in the digital filters of the modem and the power dissipated in the modem.
BACKGROUND OF THE INVENTION
Much interest has been expressed recently in discrete multitone (DMT) modems to increase bandwidth with various communication schemes, especially those digital subscriber line schemes such as ADSL, HDSL, etc. which are commonly referred to as xDSL systems.
Some DMT modems allocated the available bandwidth by frequency division multiplexing (FDM), where non-overlapping channels are assigned within the available bandwidth for the downstream and upstream data streams. DMT modems use a plurality of modulated carrier frequencies and each carrier can be encoded to transmit multiple bits, usually through Quadrature Amplitude Modulation encoding. Each of these carriers is commonly referred to as a sub-channel and, as twisted pair copper subscriber loops generally exhibit variation in gain and phase with frequency, each sub-channel can be arranged to carry a different number of bits appropriate for its frequency on the particular subscriber line. In the recent ANSI standard for ADSL DMT modems, up to 256 sub-channels are employed and the carriers for each sub-channel are spaced at 4,3125 kHz.
By assigning different numbers of bits to different sub-channels, each sub-channel can operate at a near optimal bit rate for the bandwidth available in the subscriber loop. Problematic frequencies or frequency ranges can have lower numbers of bits assigned to their sub-channels while frequencies with good gain and/or phase response can have higher numbers of bits assigned to their frequencies to keep the probability of a bit error constant across the sub-channels.
DMT modems are typically constructed with one or more digital signal processors (DSP) to implement the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT), time domain equalizer, digital filters and the other functional blocks of a DMT modem.
One problem with DMT modems is that the echo suppression requirements are subscriber loop dependent. Typically the duplexing filters implemented in an FDM DMT modem are preset to deal with worst case scenarios. In the worst case echo scenario, the loop has a large amount of attenuation and the echo component of the transmitted signal can be the dominant noise source in the loop. Thus, high order filters with carefully optimized cut off frequencies are employed to achieve acceptable performance.
However, in a better case scenario, the loop can have a relatively low amount of attenuation but the performance of the system will continue to be limited by the preset (fixed) high order filters. Further, implementing such high order filters for echo suppression or other purposes requires a relatively large amount of processing power from the DSP, thus necessitating a high clock speed for the DSP so that the DSP dissipates a relatively large amount of power.
Others have proposed that echo cancellation techniques be used to address the duplexing problems. These techniques inherently adapt to the different conditions described above. However these techniques, which require a very large amount of signal processing, result in a significant increase in power dissipation within the digital circuits and possibly the analog circuits too. The filtered approach to duplexing also suppresses non-linear echo components that are not suppressed by linear echo cancellation. Known techniques for non-linear echo cancellation are highly computationally complex when applied to DMT modems. It should be noted that echo cancelled implementations of ADSL modems, in which there is only a partial overlap between the upstream and downstream spectra, also employ duplexing filters. These modems also suffer from the above-described disadvantages.
Another problem with xDSL modems is that the loop may not be well balanced. This is especially true when connecting to the unbalanced telephone wiring at the premises of the subscriber as is proposed in standards such as ITU G.992.2. This unbalanced wiring can allow the radiation (egress) of electromagnetic interference (EMI) into the subscriber facilities, possibly adversely affecting electronic devices at the subscriber premises. The present inventors believe that EMI egress from xDSL services will be a significant problem as xDSL systems are widely implemented. This is particularly true as pertains to xDSL modems that utilize frequencies that overlap the AM radio broadcast band. In these and other cases EMI can be suppressed by filtering out the responsible portion of the signal at the transmitter. To achieve this result with fixed filters would result in reduced transmission performance on loops that did not require the additional filtering.
Further, DMT modems can be susceptible to receipt (ingress) of radio frequency interference (RFI) onto the loop which results in reduced transmission performance. Such RFI, principally from AM radio broadcasts, may be within the frequencies employed for xDSL systems (generally referred to as “in band RFI”) or it may be above those frequencies (generally referred to as “out of band RFI”). The impact of RFI on the transmission performance (capacity and/or bit error rate) of xDSL modems can be severe.
Out of band RFI can be a problem as artifacts of such interference can be both aliased and leaked into the frequencies employed in the xDSL system. Aliasing is a result of the sampling process employed to transform the signals between the analog and digital domains and can affect all xDSL modems. Leaking affects only DMT modems and leaking is said to occur when energy from outside the frequency band of a given sub-channel causes interference into that sub-channel. Leaking results from the rectangular time-domain window used to extract the input samples for a particular symbol which are input to the FFT demodulator. The rectangular window causes each sub-channel to exhibit side-lobes in the frequency domain with a Sine function characteristic centered about said channel. Any interfering signal, such as RFI, which is present within a particular side-lobe will result in interference into the associated sub-channel. Since each sub-channel has similar side-lobe responses, but with varying center frequencies, interference at a particular frequency may leak into a large number of sub-channels.
Accordingly, it is desirable to mitigate the effects of RFI in order to maintain good transmission performance. Out of band RFI is effectively mitigated by the application of high order filters in the receiver. In particular, as pertains to the DMT modems described in the ANSI T1.413 and the ITU G.992.1 and G.992.2 standards, it is the downstream receiver low pass filter which must be of a high order in order to effectively suppress out of band RFI. In band RFI is a greater challenge as many approaches at suppression of in band RFI will also result in loss of the desired in band signal.
It is therefore desired to have a DMT modem that does not have fixed high order filters for echo suppression and/or RFI mitigation and/or EMI suppression. Rather it is desired that the filters for each of these purposes are able to be adapted appropriately to the loop conditions in a particular application.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel adaptive front end for a DMT modem and a novel method of operating such a DMT modem that obviates or mitigates at least one of the above-identified disadvantages of the prior art.
According to a first aspect of the present invention, there is provided a method of operating a discrete multitone (DMT) modem with an adaptive front end and having transmitting and receiving channels. The adaptive front end has a programmable filter. A subscriber loop is co

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