WO2000052871A1

WO2000052871A1 - Transceiver with adjustable coding gain

Info

Publication number: WO2000052871A1
Application number: PCT/US1999/025233
Authority: WO
Inventors: Terry L. Cole
Original assignee: Legerity, Inc.
Priority date: 1999-03-02
Filing date: 1999-10-27
Publication date: 2000-09-08

Abstract

A transceiver (100) includes a receive unit (110), a transmit unit (105), and control logic (170). The receive unit (110) is adapted to receive data at a receive rate. The transmit unit (105) is adapted to transmit data at a transmit rate. The control logic (170) is adapted to receive a first coding gain parameter. At least one of the receive and transmit units (110, 105) is adapted to implement a coding gain technique based on at least the first coding gain parameter and at least one of the receive rate and the transmit rate. A method for determining use of a coding gain technique in a transceiver (100) is provided. The transceiver (100) is capable of receiving data at a receive rate and transmitting data at a transmit rate. The method includes receiving a first coding gain parameter. The coding gain technique is implemented based on at least the first coding gain parameter and at least one of the receive rate and the transmit rate.

Description

TRANSCEIVER WITH ADJUSTABLE CODING GAIN

TECHNICAL FIELD

This invention relates generally to communication systems, and, more particularly, to a transceiver with an adjustable coding gain to control the complexity of the transceiver processing algorithms.

BACKGROUND ART In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. The Plain Old Telephone System (POTS), designed primarily for voice communication, provides an inadequate data transmission rate for many modern applications. To meet the demand for high-speed communications, designers have sought innovative and cost-effective solutions that take advantage of the existing network infrastructure. Several technological advancements have been proposed in the telecommunications industry that make use of the existing network of telephone wires. One of these technologies is the xDSL technology. DSL technology uses the existing network of telephone lines for broadband communications. An ordinary twisted pair equipped with DSL interfaces can transmit videos, television, and high-speed data.

DSL technologies leave the POTS service undisturbed. Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (kHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels from 100 kHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data. DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. Certain DSL systems provide a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of about 1.5 Megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS) transfer rate. One popular version of the DSL technology is the Asymmetrical Digital Subscriber Line (ADSL) technology. The ADSL standard is described in ANSI T1.413 Issue 2.

ADSL modems use two competing modulation schemes: discrete multi-tone (DMT) and carrierless amplitude/phase modulation (CAP). DMT is the standard adopted by the American National Standards Institute. The technology employed by DMT ADSL modems is termed discrete multi-tone. The standard defines 256 discrete tones. Each tone represents a carrier signal that can be modulated with a digital signal for transmitting data. The specific frequency for a given tone is 4.3125 kHz times the tone number. Tones 1-7 are reserved for voice band and guard band (i.e., tone 1 is the voice band and tones 2-7 are guard bands). Data is not transmitted near the voice band to allow for simultaneous voice and data transmission on a single line. The guard band helps isolate the voice band from the ADSL data bands. Typically, a splitter may be used to isolate any voice band signal from the data tones. Tones 8-32 are used to transmit data upstream (i.e., from the user), and tones 33-256 are used to transmit data downstream (i.e., to the user). Alternatively, all the data tones 8-256 may be used for downstream data, and upstream data present on tones 8-32 would be detected using echo cancellation. Because more tones are used for downstream communication than for upstream communication, the transfer is said to be asymmetric. Through a training procedure, the modems on both sides of the connection sense and analyze which tones are less affected by impairments in the telephone line. Each tone that is accepted is used to carry information. Accordingly, the maximum capacity is set by the quality of the telephone connection. The maximum data rate defined by the ADSL specification, assuming all tones are used, is about 8 MBPS downstream and about 640 KBPS upstream. In a typical ADSL system, a central office (CO) modem communicates with a customer premise (CP) modem. The CP modem is typically installed in a home or office. Techniques have been developed to increase the transfer rate and/or accuracy of the modem data exchange. These techniques typically trade a more complicated processing algorithm for such improvements. The increased complexity results in greater demands on the digital signal processor (DSP) used to implement the modem functionality. Exemplary techniques known in the art for providing such an improvement include constellation encoding with trellis coding, Reed-Solomon forward error correction coding, and spectrum overlap (i.e., transmit and receive signal spectrums overlap and are separated by echo cancellation). Collectively, these techniques to increase data rate and/or decrease error rate, and others like them, are hereinafter referred to as coding gain techniques. The specific algorithms used to implement these and other coding gain techniques are known to those of ordinary skill in the art, and they are not discussed in greater detail herein for clarity purposes. For example, the ADSL standard includes exemplary techniques for accomplishing coding gain. To support coding gain techniques, the DSP must be equipped to handle the processing load under worst case conditions (e.g., line length, noise, distortion, etc.). As defined by the ADSL standard, during the training period, the modem communicates whether or not it supports trellis coding or echo cancellation. Thereafter, the DSP must be able to support the coding gain techniques regardless of line conditions. For example, trellis coding is known to allow communication on longer lines. The complexity of trellis coding is proportional to the number of tones and the bit rate of the connection. If the developer of a modem wishes to includes trellis coding, the DSP must be capable of performing trellis coding on both short (high bit rate, high number of tones) and long (lower bit rate, less tones) lines. This significantly increases the cost of a modem equipped with trellis coding.

The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. DISCLOSURE OF INVENTION

One aspect of the present invention is seen in a transceiver including a receive unit, a transmit unit, and control logic. The receive unit is adapted to receive data at a receive rate. The transmit unit is adapted to transmit data at a transmit rate. The control logic is adapted to receive a first coding gain parameter. At least one of the receive and transmit units is adapted to implement a coding gain technique based on at least the first coding gain parameter and at least one of the receive rate and the transmit rate.

In another aspect of the present invention, a method is provided for determining use of a coding gain technique in a transceiver. The transceiver is capable of receiving data at a receive rate and transmitting data at a transmit rate. The method includes receiving a first coding gain parameter. The coding gain technique is implemented based on at least the first coding gain parameter and at least one of the receive rate and the transmit rate.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:

Figure 1 is a block diagram of a communications system in accordance with the present invention; Figure 2 is a simplified block diagram of a modem in accordance with the present invention; and

Figure 3 is a simplified functional block diagram of the modem of Figure 2. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

MODE(S) FOR CARRYING OUT THE INVENTION Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

Referring to Figure 1, a block diagram of a communications system 10 is provided. The communications system 10 includes a first modem 15 coupled to a second modem 20 through a connection 25. In the illustrated embodiment, the first modem 15 is located at a customer premise 30, and the second modem 20 is part of a central office 35. The connection 25 is an ordinary twisted pair connection, as is common in present-day telephone networks. However, other connection types (e.g., wireless, cellular, etc.) are contemplated, depending on the specific implementation. Also, it is contemplated that the second modem 20 may not be part of the central office 35, but rather the second modem 20 may be installed in a second customer premise (not shown). For purposes of illustration, the modems 15, 20 are described as they might be implemented under the ADSL standard. It is contemplated that the techniques described herein may be applied to other communication protocols, depending on the specific implementation.

In the illustrated embodiment, the second modem 20 acts as a gateway to a larger communications network (not shown), such as a local or wide area network, or the Internet. Typically, the first modem 15 establishes a connection to the communications network (not shown) through the second modem 20. During the process of establishing the connection, the first and second modems 15 and 20 complete a training process whereby an initial bit loading technique is employed to establish the throughput available for communication between the modems 15, 20. Based on the determination of the number of tones and bits per tone available, the modems 15, 20 negotiate coding gain options.

Although the present invention is described as it may be implemented in a modem, it is contemplated that, in light of this disclosure, the invention may be applied to any type of transceiver, including, but not limited to, a modem or some other wired or wireless communication device.

Referring to Figure 2, a simplified block diagram of a modem 100 is provided. The modem 100 may be the first modem 15 or the second modem 20. The modem 100 includes a digital interface 101 for interfacing with the system (not shown) (i.e., computer or communications system to which the modem 100 is connected). The digital interface 101 receives digital data for transmission by the modem 100 and provides digital data received by the modem 100. A processing unit 102 and memory 103 provide processing resources for implementing the algorithms (i.e., coding, modulating, demodulating, etc.) required to operate the modem 100. An analog interface 104 connects the modem 100 to an external connection, such as a phone line (not shown). Referring to Figure 3, a simplified functional block diagram of a modem 100 is provided. For clarity and ease of illustration, not all functional blocks are illustrated in detail because these items are known to those of ordinary skill in the art, and are further defined in well known modem standards. The actual functions are performed by circuitry and processing resources provided by the digital interface 101 , processing unit 102, memory 103, and analog interface 104 of Figure 2.

The modem 100 includes transmit, receive, and control functional blocks 105, 110, 115. The transmit block 105 includes an encoding unit 120 adapted to receive outgoing digital data over a data-out line 122. The encoding unit 120 performs functions such as cyclic redundancy checking (CRC), scrambling, forward error correction (Reed-Solomon coding), and interleaving. The complexity of the processing algorithm for the encoding unit 120 depends, at least in part, on the codeword length specified for the forward error correction function. As stated above, the actual algorithms used to implement these functions are known to those of ordinary skill in the art.

The data in binary form is grouped into sets referred to as frames. A plurality of frames (i.e., 68 in the illustrated embodiment) is referred to as a superframe. The transmit block 105 also includes a modulator 125 that receives the data frames from the encoding unit 120 and modulates a carrier or carriers with the data. The modulator 125 performs tone ordering, constellation encoding, gain scaling, and an inverse discrete Fourier transform (IDFT) function to provide time domain waveform samples. The set of time domain waveform samples corresponding to a frame of data is referred to as a symbol. A cyclic prefix (CP) unit 130 performs cyclic prefix insertion (i.e., a subset of the output samples from the modulator 125 is replicated and prepended to the existing output samples to provide an overlap and allow for better symbol alignment). A buffer 132 stores the samples received from the CP unit 130. A digital to analog (D/A) converter and filter 135 converts the samples from the CP unit 130 to an analog waveform suitable for transmission over the connection 25 through an external line interface 140. The complexity of the processing algorithm for the modulator 125 depends, at least in part, on the type of constellation encoding algorithm used (i.e., with or without trellis coding). The receive block 110 includes an analog to digital (A/D) converter and filter 145 that receives an analog waveform over the connection 25 and samples the analog waveform to generate a time domain digital signal. An alignment and equalizing unit 150 performs functions known in the art, such as symbol alignment and time domain equalization. In time domain equalization, because the tones are at different frequencies, certain frequencies travel faster than others, and as such, all the tones do not arrive at same time. The time domain equalization function of the alignment and equalizing unit 150 delays the faster tones to compensate for the propagation speed differences.

The alignment and equalizing unit 150 also performs gain control to increase the amplitude of the received signal.

A demodulator 155 receives the time domain samples from the alignment and equalizing unit 150 and converts the time domain data to frequency domain data. The demodulator 155 also performs an echo cancellation function to derive the received signal if the transmit and receive spectrums of the modem 100 and the interfacing modem (not shown) overlap. The demodulator 155 performs a slicing function to determine constellation points from the constellation encoded data, a demapping function to map the identified constellation point back to bits, and a decoding function (e.g., Viterbi decoding if trellis constellation coding is employed). The complexity of the processing algorithm for the demodulator 155 depends, at least in part, on whether trellis coding was used by the transmitter (not shown) of the interfacing modem (not shown) and whether echo cancellation is required. In the case where the modem operates using the ADSL protocol, the demodulator 155 also performs tone deordering to reassemble the bytes that were divided among the available tones. A decoding unit 160 in the receive block 110 performs forward error correction, CRC checking, and descrambling functions on the data_ received from the demodulator 155. The reconstructed data provided by the decoding unit 160 represents the sequential binary data that was sent by the interfacing modem (not shown). The reconstructed data is provided to a data-in line 165. Again, complexity of the processing algorithm for the decoding unit 160 depends, at least in part, on the codeword length specified for the forward error correction function.

The control block 115 includes control logic 170 and a configuration register 175. The configuration register stores data useful for configuring the modem 100. During the training period, the control logic 170 determines relevant attributes of the connecting channel and establishes transmission and processing characteristics suitable for that channel. During the exchange process, the transmit unit 105 shares with its corresponding far-end receive unit (not shown) certain transmission settings that it expects to encounter. For example, the transmit unit

105 communicates to its far-end receive unit (not shown) the number of bits and relative power levels to be used on each DMT tone. The transmit unit 105 also communicates a configuration message signal based on the information contained in the configuration register 175. The configuration message signal includes vendor identification information, transmit power level, trellis coding option, echo canceling option, Reed-Solomon codeword length, etc.

Before sending the configuration message, the control logic 170 determines the coding gain parameters that can be supported by the modem 100 and the interfacing modem (not shown). There are numerous methods by which the control logic 170 may determine the coding gain techniques to support, and an illustrative sample of some of these methods are provided. In light of this disclosure, other evaluation techniques to determine the coding gain techniques may be developed.

A first example illustrates how the control logic 170 may decide under what conditions to support trellis coding. Consider that the modem 100 cannot support worst case trellis coding, but has enough processing resources (e.g., DSP cycles, memory, power, etc.) to support trellis coding on a limited basis (i.e., long lines with less tones and bits). The control logic 170 stores a multi-bit trellis coding option parameter (TC Param) 180 in the configuration register 175. The value of the trellis coding option parameter 180 corresponds to the ability of the modem 100 to handle trellis coding under different environments. The specific bit value stored in the configuration register 175 for the trellis coding option parameter 180 is application dependent, and a variety of schemes are possible. A high trellis coding option parameter 180 indicates that the modem 100 is capable of handling trellis coding under near worst case conditions. Conversely, a low trellis coding option parameter 180 indicates that the modem 100 is only capable of supporting trellis coding under less strenuous operating conditions

(i.e., long lines).

If the connection between the modem 100 and the interfacing modem (not shown) does not support high transfer rates (i.e., defined by the number of tones and bits per tone), it is likely that the modem 100 will have unutilized DSP cycles. Under this condition, trellis coding may be supported without increasing the demands on the modem 100 by taking advantage of the unused cycles. The trellis coding option parameter 180 may be based on actual measurements of the line conditions, such as estimated line lengths based on attenuation or estimated line capacity based on noise measurement, etc. Alternatively, the trellis coding option parameter 180 may be based on a worst case condition. For example, the modem 100 may be capable of supporting trellis coding if the transfer rate is less than 256 kbps. Alternatively, the modem 100 may be capable of supporting trellis coding if the number of tones being used is less than 96, for example. In still another exemplary case, the trellis coding option parameter 180 may be based on an estimate of the actual processing resources available. Techniques for estimating processing resources (e.g., DSP cycles, memory, power, etc.) are known in the art.

Table 1 illustrates an exemplary relationship between the trellis coding option parameter 180 and the bandwidth at which trellis coding is supported. As stated above, the thresholds at which to support coding gain techniques may be based on available processing resources, worst case throughput, etc. The number of bits in the trellis coding option parameter 180 may be varied depending on the degree of control required. A similar table may be developed for other coding gain techniques.

Table 1 If at some time during the time the modem 100 is connected to an interfacing modem (not shown), the line conditions change to a point where the modem 100 can no longer support trellis coding, it is contemplated that the modem 100 may retrain with the interfacing modem (not shown) and specify a different trellis coding option parameter 180, including a trellis coding option parameter 180 that specifies no trellis coding.

A similar technique may be employed by the control logic 170 to determine the availability of echo cancellation or to specify the Reed-Solomon codeword length. The control logic 170 stores a multi-bit echo cancellation option parameter (EC Param) 185, and a Reed-Solomon option parameter (RS Param) 190 in the configuration register 175. The control logic 170 might choose some combination of coding gain techniques, depending on the specific characteristics of the connection. For example, the control logic 170 might elect to support trellis coding and not echo cancellation. It is contemplated that the modem 100 may negotiate with the interfacing modem (not shown) to determine the greatest common coding gain parameters supported. For example if the modem 100 can support trellis coding at a transfer rate of 1024 kbps, but the interfacing modem can only support trellis coding for connections less than 512 kbps, the lower of the two trellis coding option parameters 180 may be selected for use. Similarly, if the modem 100 can support a 32-bit Reed-Solomon codeword length, but the interfacing modem (not shown) can only support a 16-bit codeword length, the lesser codeword length is selected.

Determining the conditions under which to support coding gain techniques has numerous advantages. Using negotiated coding gain parameters, rather that fixed, on/off parameters increases the flexibility for upgrade paths. The modem 100 may operate with the highest common coding gain options, and if, for example, the interfacing modem (not shown) is upgraded, the modem 100 will adjust to the new operating environment and possibly increase its performance. Coding gain techniques can be implemented in products sooner because the threshold at which the modem 100 supports the techniques can be varied. Products can also be easily upgraded to increase the coding gain threshold from long loops, to medium loops, to all loops without requiring a new standard. For example, a modem 100 having a DSP (not shown) that cannot support the approximately 240 tone worst case processing load and at the same time support trellis coding can be configured to support trellis coding with, for example, 50 tones. As the power of the DSP (not shown) increases, the threshold can be raised to support trellis coding with additional tones. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

CLAIMS _

1. A transceiver (100), comprising a receive unit (1 10) adapted to receive data at a receive rate, and a transmit unit (105) adapted to transmit data at a transmit rate CHARACTERIZED IN THAT the transceiver ( 100) further comprises: control logic (170) adapted to receive a first coding gain parameter, wherein at least one of the receive and transmit units (110, 105) is adapted to implement a coding gain technique based on at least the first coding gain parameter and at least one of the receive rate and the transmit rate.

2. The transceiver (100) of claim 1, wherein the receive unit (110) is adapted to receive the first coding gain parameter and provide the coding gain parameter to the control logic.

3. The transceiver (100) of claim 1, wherein the coding gain technique comprises at least one of trellis coding, echo cancellation, and forward error correction.

4. The transceiver (100) of claim 1, wherein the receive unit (110) is adapted to receive an external coding gain parameter and provide the external coding gain parameter to the control logic, and the control logic (170) is adapted to implement the coding gain technique based on the first coding gain parameter and the external coding gain parameter.

5. The transceiver (100) of claim 1, wherein the first coding gain parameter is based on at least one of a maximum data rate, a maximum number of tones, and an amount of processing resources available to a processing unit (102) adapted to provide processing resources to the transmit unit (105) and the receive unit (110).

6. A method for determining use of a coding gain technique in a transceiver (100) capable of receiving data at a receive rate and transmitting data at a transmit rate, the method comprising: receiving a first coding gain parameter; and implementing the coding gain technique based on at least the first coding gain parameter and at least one of the receive rate and the transmit rate.

7. The method of claim 6, wherein implementing the coding gain technique includes processing at least one of data for transmission and received data using the coding gain technique.

8. The method of claim 6, wherein receiving the first coding gain parameter includes receiving an external coding gain parameter in the transceiver.

9. The method of claim 6, wherein implementing the coding gain technique includes implementing at least one of trellis coding, echo cancellation, and forward error correction.

10. The method of claim 6, further comprising receiving an external coding gain parameter, and wherein implementing the coding gain technique includes implementing the coding gain technique based on the first coding gain parameter and the external coding gain parameter.

11. The method of claim 6, further comprising determining the first coding gain parameter based on at least one of a maximum data rate, a maximum number of tones, and an amount of processing resources available to a processing unit (102) adapted to provide processing resources to the transceiver (100).