Coded data generation or conversion – Analog to or from digital conversion – Nonlinear
Reexamination Certificate
1999-06-18
2001-07-10
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Nonlinear
C455S234100, C455S232100
Reexamination Certificate
active
06259391
ABSTRACT:
TECHNICAL FIELD
The present invention relates to data delivery on telecommunication links and, in particular, to an improved apparatus and method for adjusting an amplitude of a received analog discrete multi-tone signal by contaminated radio frequency interference to utilize a full input dynamic range of an analog-to-digital converter, thereby minimizing quantization noise of the digital signal.
BACKGROUND OF THE INVENTION
An emerging trend in the telecommunications industry is to provide data services deployed over existing telephone twisted pair copper wires utilizing a frequency spectrum above the voice frequency band. One such type of transceiver enabled to provide data services deployed over existing telephone wires is one employing Discrete Multi-Tone (DMT) techniques. The DMT encoding makes use of a wide bandwidth channel divided up into sub-channels and data is modulated onto the sub-channels using a modulation method called quadrature amplitude modulation. The frequency band of the DMT channel is dictated by physical properties of the twisted pair wires used in providing existing telephone services and the existing infrastructure of the Public Switched Telephone Network (PSTN). The DMT frequency band of downstream transmissions extends into the frequency range of amplitude modulated (AM) radio transmissions and is, therefore, susceptible to Radio Frequency Interference (RFI) from AM radio stations.
The telephone wiring acts as a receiving antenna and converts electromagnetic energy from AM radio transmissions into a common mode voltage in the wiring. When the telephone wiring and the receiver front-end of a data transceiver are perfectly balanced, this contaminating signal should not cause any problems because of a differential mode of signal detection employed by the receiver. However, when there are front-end or wiring imbalances, a common mode voltage is detected at the receiver. As a result, AM radio signals from nearby stations within the frequency range are added to the input DMT signal and contaminate it. AM radio stations can occupy frequencies ranging from 535 kHz up to 1605 kHz with 10 kHz channelization. The effect the high frequency stations have on the received signal at the DMT transceiver can be suppressed by employing analog filters in the front-end of transceivers for partial rate services, e.g. G.lite ADSC, which is well known in the art. In this way, radio frequency interference (RFI) from radio stations above about 650 kHz is suppressed leaving the AM carriers below 650 kHz as most damaging to the received DMT signal. Analog filters cannot be used to suppress the RFI from full-rate ADSC because the full-rate implementation utilizes about twice the bandwidth of the G.lite implementation.
In the G.lite implementation, a logical solution to the damaging effects of the remaining low frequency AM RFI signals is to employ sharper analog filters or filters with lower frequency ranges to provide a better suppression of the RFI. However, there is considerable cost associated with this solution, including an increase in complexity of the transceiver and a loss of useful sub-channels in the DMT bandwidth.
Digital filters are used to complement the filtering done by the analog filters and, these are employed in order to reduce the net effect of the RFI from low frequency AM radio transmissions after the contaminated DMT signal is digitized. In order for digital filtering to be effective, the received analog signal should be optimally digitized.
Another effect that AM RFI has on the front-end processing of the received DMT signal, is that the signal characteristics are no longer predetermined. This is due to the influence of geographical location on the mix of RFI frequencies and their relative power levels. The unpredictable nature of the characteristics of the received RFI contaminated DMT signal leads to an inability of prior art transceivers to optimally digitize the received signal resulting in a higher quantization error than necessary being introduced in the digitization process.
A logical solution to the inability to optimally digitize the received DMT signal is to employ analog-to-digital converters of a higher resolution. However, an analog-to-digital converter of a higher resolution adds cost and complexity to the DMT transceiver.
In order to deploy data services on existing telephone twisted pair wires, there exits a need for the development of new apparatus and methods for AM RFI suppression using digital filtering anid, in particular, for an apparatus and method of reducing quantization noise in the digitization process.
SUMMARY OF THE INVENTION
In accordance with the present invention, an apparatus for adjusting an amplitude of a received RFI contaminated analog DMT signal to utilize substantially all of an input dynamic range of an analog-to-digital converter is provided. The apparatus comprises a received signal monitor for extracting signal characteristics from a digitized copy of the received analog signal and an embedded processor module for calculating, according to a mathematical algorithm, an amplification factor to be applied by an analog gain controller to the received RFI contaminated analog DMT signal.
A method of adjusting an amplitude of a received RFI contaminated analog DMT signal is also provided. The method comprises the steps of determining a predetermined number of characteristics respecting an amplitude distribution of the analog signal by analyzing an output of an analog-to-digital converter (ADC) in a receiver circuit of the transceiver. The characteristics are used in a mathematical algorithm to determine a gain adjustment to be applied to the analog signal. The analog gain controller is then controlled to adjust the amplitude of the analog signal based on the gain adjustment in order to utilize a greater input range of an analog-to-digital converter. This reduces quantization noise in the digital signal output by the analog-to-digital converter.
The target attributes include a target threshold level corresponding to a target signal clipping ratio. Based on the target threshold level, the analog gain controller is set to amplify the received RFI contaminated analog DMT signal in order to utilize substantially all of an input dynamic range of the analog-to-digital converter, thereby reducing the quantization noise of the output digital signal. After an initialization phase, the transceiver continues to monitor the signal clipping ratio and readjusts the analog gain control as required to adapt the fluctuations in RFI.
REFERENCES:
patent: 5867528 (1999-02-01), Verbueken
patent: 5892472 (1999-04-01), Shu et al.
patent: 5946607 (1999-08-01), Shiino et al.
patent: 6038435 (2000-03-01), Zhang
Harris Gwendolyn Kate
Pakravan Mohammed Reza
Nortel Networks Limited
Swabey Ogilvy Renault
Tokar Michael
Tran Anh
Wood Max R.
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