« PreviousContinue »
QUANTIZATION NOISE COMPENSATOR
APPARATUS AND METHOD
CROSS REFERENCE TO RELATED
This document claims priority to and the benefit of the filing date of copending and commonly assigned U.S. Provisional Patent Application entitled QUANTIZATION NOISE COMPENSATOR, assigned Ser. No. 60/057,046, 1Q and filed Aug. 22, 1997 and is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates generally to data 15 communications, and more particularly, to an apparatus and method for canceling noise in a control loop, primarily in communication devices, with the invention being applicable to any control system incorporating an integrator in a feedback loop. 20
BACKGROUND OF THE INVENTION
The field of data communications typically uses a device, such as a modem, to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables modems to communicate rapidly large amounts of data. Modems communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, encoding the information to reduce errors and improve throughput, and transmitting the information over a communication channel. The constellation can include both analog and digital information or often merely digital information.
At a receiver, the constellation is received, equalized and decoded in accordance with techniques that those skilled in 4Q the art will appreciate.
In the above mentioned communications system, a control device, i.e., one that is located at a telephone company central office, connects over the communication channel to a plurality of remote devices typically located at a customer 45 residential or business location. This topography is known as a multipoint communication environment because one control device is communicating with a plurality of remote devices over a single communication channel. The communication technique between the control modem and the 50 remote modems is generally half duplex in nature, meaning that only one device may transmit at any particular time. In order to establish a communications connection between the central office modem and any of the remote modems, a lengthy preamble, which allows the modems to synchronize 55 at the start of each message is required. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two modems at a time. In a multipoint environment, greater circuit efficiency go is possible because of the ability to connect multiple modems to the same communication channel.
In a 4 wire communications environment, the control device can transmit continuously so that outbound preambles (from central office to remote location) are 65 unnecessary, thus assuring remote device synchronization. In a 2 wire multipoint environment, as contemplated by the
present invention, when a remote device transmits, it disrupts the outbound control signal from the control device preventing all remote devices connected to the communication line from maintaining synchronization with the control device. It is desirable to allow all remote devices to continuously maintain synchronization with the control device in order to significantly reduce the training preamble necessary to establish a communication path between a remote device and the control device. A technique to accomplish this continuous synchronization is described in commonly assigned copending U.S. patent application Ser. No. 08/936, 415, titled "PILOT TONE SYSTEM AND METHOD TO ALLOW CONTINUOUS SYNCHRONIZATION IN MULTIPOINT NETWORKS" filed on Sep. 25, 1997, and is hereby incorporated by reference. Part of the system disclosed in the aforementioned application requires the use of highly accurate digital to analog (DAC) converters to supply an analog control signal to a voltage controlled crystal oscillator (VCXO). It would be desirable to provide the modem synchronization function described, at a reduced level of complexity and cost by simplifying the manner in which the analog control signal is developed and supplied to the VCXO.
SUMMARY OF THE INVENTION
The present invention provides an improvement to a half duplex multipoint communication environment by enabling a control device to maintain constant synchronization with a plurality of remote devices using a less complex, less expensive system. By employing the quantization noise compensation apparatus and method, the DAC converter, which supplies an analog control signal required to drive analog devices in phase locked loops (PLL) such as an integrator like a VCXO, can be replaced with a relatively simple quantization noise compensator, referred to hereinafter as a quantizer.
The present invention provides an apparatus for noise compensation, the apparatus comprising a first adder configured to receive a control signal and add the control signal to an accumulated quantization error signal resulting in a combined signal. Also included is a masker configured to convert the combined signal to a plurality of voltage levels. This plurality of voltage levels, which can be linear or nonlinear, is then supplied to a subtractor configured to subtract the plurality of levels from the control signal resulting in a quantization error signal. A second adder configured to add an accumulated quantization error signal with the quantization error signal resulting in a new accumulated quantization error signal is also included. The accumulated quantization error signal is stored in a memory device and used for compensating subsequent samples. The control signal supplied to the first adder can be a 16 bit or 32 bit signal. In the case of a multidimensional signal, the memory device can be n-dimensional.
The present invention also includes a method for noise compensation, the method comprising the steps of adding a control signal with an accumulated quantization error signal resulting in a combined signal. This combined signal is converted by a masking operation to a plurality of voltage levels. The plurality of voltage levels are subtracted from the control signal resulting in a quantization error signal. The accumulated quantization error signal is added to the quantization error signal resulting in a new accumulated quantization error signal, which is stored in a memory device. The memory device may be single or multidimensional depending on the application. Furthermore, the control signal supplied can be a 16 bit or a 32 bit signal.
In an alternative embodiment, the present invention provides an apparatus for correcting bias in a feedback loop, the apparatus comprising a subtractor configured to receive an error signal and subtract an accumulated quantization error signal therefrom, resulting in a combined signal. A Beta 5 scalar receives the combined signal and is configured to convert the combined signal to a bias free error signal. An adder configured to add the bias free error signal with the accumulated quantization error signal resulting in a new quantization error signal is also included. The accumulated 10 quantization error signal is stored in a memory device. Similarly to that described in the first embodiment, the error signal in this alternative embodiment can be 16 bits or 32 bits. Furthermore, in the case of a multidimensional signal, the memory device can be n-dimensional. 15
This alternative embodiment also includes a method for correcting bias in a feedback loop, the method comprising the steps of receiving an error signal in a subtractor. The subtractor is configured to subtract an accumulated quantization error signal from the error signal resulting in a 20 combined signal. This combined signal is converted to a bias free error signal, which is added to the accumulated quantization error signal resulting in a new quantization error signal. The new quantization error signal is then stored in a memory device and the bias free error signal is used to 25 correct bias in the feedback loop.
The invention has numerous advantages, a few of which are delineated hereafter, as merely examples.
An advantage of the present invention is that it eliminates 3Q the need for an accurate digital to analog converter for driving an analog device.
Another advantage of the present invention is that it reduces the cost of control circuitry by eliminating the DAC from phase locked loops that use an integrator. 35
Another advantage of the invention is that it can be used to remove the bias, or DC offset, from the error signal used to update adaptive equalizer coefficients used in communication devices.
Another advantage of the present invention is that it 40 eliminates the necessity for double precision arithmetic usually required for storing equalized coefficients, thus minimizing the amount of memory space required.
Another advantage of the present invention is that it is simple in design, reliable in operation, and its design lends 45 itself to economical mass production in modems.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention, as defined in the claims, can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each another, emphasis instead being placed on clearly illustrating the principles of the present invention. gQ
FIG. 1 is a schematic view illustrating a multipoint communications environment;
FIG. 2 is a block diagram of a communications device including the quantizer of the present invention;
FIG. 3 is a block diagram illustrating a transmitter and a 65 receiver containing one embodiment of the quantizer of the communications device of FIG. 2;
FIG. 4 is a schematic view illustrating the baud timing device of FIG. 3 including an embodiment of the quantizer of FIG. 2;
FIG. 5 is a schematic view illustrating a first embodiment of the quantizer of FIG. 4; and
FIG. 6 is a schematic view illustrating an alternative embodiment of the quantizer of FIG. 3.
DETAILED DESCRIPTION OF THE
The present invention can be implemented in software, hardware, or a combination thereof. In the preferred embodiment, the elements of the present invention are implemented in software that is stored in a memory and that configures and drives a suitable digital signal processor (DSP) situated in a communication device. However, the foregoing software can be stored on any computer-readable medium for transport or for use by or in connection with any suitable computer-related system or method. In the context of this document, a computer-readable medium is an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer-related system or method.
FIG. 1, shows a schematic view illustrating a multipoint communications environment 11 in which devices employing the quantization noise compensator of the present invention are used. Remote location 16 is connected to central office location 12 via communication channel 14. Located at central office location 12 is control device 13. Communication channel 14 is typically the copper wire pair that extends between a telephone company central office and a remote residential, business, or any other location served by local telephone service. Remote location 16 may contain a plurality of remote devices 18 connecting a plurality of user devices 17 to communication channel 14 via communication bus 19. Communication bus 19 is illustratively the copper wiring infrastructure used throughout a remote location to connect remote devices 18 to communication channel 14. By using control device 13 and remote devices 18 employing the concepts and features of one embodiment of the present invention, it is possible for control device 13 to transmit, and remote devices 18 to receive a pilot tone, using an improved quantization noise compensator apparatus and method, thereby allowing all remote devices 18 to maintain continuous synchronization with control device 13, and enabling control device 13 to maintain continuous synchronization with each of a plurality of remote devices 18.
Now referring to FIG. 2, shown is a schematic view illustrating remote device 18 of FIG. 1 including the quantizer of the present invention. Elements of the present invention reside both in control device 13 and remote devices 18. Typically, control device 13 will transmit a pilot tone signal to remote devices 18. Remote devices 18 contain the quantization noise compensator of the present invention which enables the remote devices to efficiently receive and lock timing to the pilot tone signal sent by control device 13.
Still referring to FIG. 2, remote device 18 contains conventional components as is known in the art of data communications. Digital Signal Processor (DSP) 21 controls the operation of the devices' transmitter 22 and receiver 23 through logical interface 24, and couples to line interface 26 to gain access to communications channel 14. Also included in receiver 23 of remote device 18 is quantizer 200/300 which enables remote device 18 to reliably and economically maintain synchronization with control device 13 by