The present invention relates to transmission of digital information over analog medium connected to a digital network and more particularly to the PCM modems ("Pulse Code Modulation").
The world based on the Internet has seen tremendous growth in recent months. As more users begin browsing and downloading information from the World Wide Web, there has been a great desire to be able to increase the data rate. The desire is even greater for users accessing the Internet through an Internet service provider (ISP), since most of the users are linked up to the Net through a personal computer and a modem. Conventional analog modems, such as the V.34 modems, however, view the public switched telephone network ("PSTN") as an analog channel, even though the signals are digitized for communications throughout most of the network. As such, various effects and impairments due to quantization impose a limitation on the data rate of the channel to about 35 Kbps. This limit has been commonly known as Shannon's Limit. (See Shannon, C. E. and W. Weaver, The Mathematical Theory of Communication, University of Illinois Press, 1949).
There has been a recent development of a high-speed communications technology based on the PCM modems, where data rate of at least 56 Kbps is said to be actually attainable. The PCM modem technology is based on the simple realization that the public switched telephone networks (PSTN) is increasingly a digital network and not an analog network. Also, more and more central site modems are connected to the PSTN through digital connections, i.e. T1 in the U.S. and E1 in Europe, without utilizing a CODEC (coder/decoder). The conventional modem, however, still interprets this digital stream as the representation of the modem's analog signal. With the PCM modems, a much higher data rate can be achieved without the complicated task of re-wiring the user's site or modifying the telephone network.
Note that by "central site modems," it is referred to those modems installed at an ISP, or at a corporation to allow many simultaneous connections for remote LAN access. Also note that a CODEC is a device which sits between the digital portion of the network and the analog local loop for converting between analog and digital.
The recent 56 Kbps technology seeks to address an impaired section of the communications path of the PSTN digital network, where the impairment is due to the hybrid and the copper wire connection between the telephone central office and the user's home, usually referred to as the analog local loop.
Since recently, much has been described about the PCM modems and how they can and should facilitate downstream data communication at a much higher rate than present paradigm. For example, the PCM modem has been the subject of a recent Telecommunications Industry Association (TIA) Technical Committee TR-30 Standards meeting on Oct. 16-17, 1996. The submitted technical contributions include Mr. Guozhu Long's DC Suppresser for 56K Modems, Mr. David C. Rife's 56 Kbps Channels, Mr. Veda Krishnan's V.pcm Modem Standard, Mrs. Vedat Eyuboglu's PCM Modems: A Technical Overview, Mr. Richard Stuart's Proposal for a High Speed Network Access Modem, and Mr. Vladimir Parizhsky's U.S. Robotics' x2 Technology: Technical Brief. These contributions are hereby incorporated by reference.
Also, there have been recent publications on the overall data communication system based on the PCM modem. The first one is a 1995 presentation disclosed by Pierre A. Humblet and Markos G. Troulis at Institute Eurecom, entitled The Information Driveway, 1995, which purports to explain the basic concepts on the high speed modem. The second one is a PCT Patent Publication, dated Jun. 13, 1996, International Publication Number WO/9618261, by Brent Townshend, which discloses a High Speed Communications Systems for Analog Subscriber Connections. This Publication, on pages 17-19, discloses an overall high speed system based on the PCM modems, which also implements DC null elimination on the transmitter side. These are also hereby incorporated by reference, since they provide a fair reference to the basics of the high speed PCM modems and their environment.
Additionally, U.S. patent issued to Ender Ayanoglu of AT&T, U.S. Pat. No. 5,528,625 dated Jun. 18, 1996, entitled High Speed Quantization-Level-Sampling Modem with Equalization Arrangement, discloses a QLS modem for high-speed data communication. Another U.S. patent also issued to Ender Ayanoglu of AT&T, U.S. Pat. No. 5,394,437, dated Feb. 28, 1995, entitled High-Speed Modem Synchronized to a Remote CODEC, discloses a high-speed modem for data transmission over an analog medium in tandem with a digital network. These references are also hereby incorporated by reference.
FIG. 1 depicts a conceptual diagram of the high-speed communication path using the PCM modem technology. An ISP, or "central site", 100 is digitally connected to a telephone network 130 through its transmitter 110 and receiver 120. The network 130 is connected to a local loop 150 through a central office line card 140. The line card typically has a Pulse Code Modulation ("PCM") CODEC implemented within. The local loop 150 is connected to the user's PC at the user's site through the user's modem 160. As can be appreciated by those skilled in the art, the connection between the ISP modem's transmitter 110 to the telephone network 130 is a digital connection with a typical data rate of about 64 Kbps. Since the parameters of the telephone network 130 and line card 140 are dictated and set by telephone company's specification and operation, the central site transmitter 110 will need to transmit the digital data in certain way to fully exploit its digital connection to the network. However, dealing with the central site transmitter in this new paradigm has its obstacles.
In this type of data communication systems, such as the transmitter 110 in the central site, the transmit signal points are determined by physical constraints, and cannot be made part of the overall transmitter design. An example is when a signal is transmitted from within the digital part of the telephone network to the residential customer. That system will be the focus of the following description, although the methods described hereinafter will in many cases be applicable to other systems.
The transmitter 110 sends 64 Kbps of data into the network 130, which eventually gets translated to an analog signal in a digital-to-analog converter in a central office line-card CODEC 140. To send the maximum of 64 Kbps of data, all transmitted bits must be determined by the incoming data. Any modification of the outgoing sequence of bits resulting in correlation between the bits represents a redundancy in the signal, and will thus result in a lower data rate. Therefore, if the analog signal is to be controlled in any way, the data rate must be lowered. The main task confronting the designers, then, is to perform the desired control with the least amount of redundancy added.
As will be described in the present application, methods and apparatus for controlling the spectrum of the line codec's output signal are disclosed. In particular, methods of minimizing the energy of the signal at unwanted frequencies are described. These methods can be readily extended to other parts of the spectrum by those skilled in the art based on the teaching of the present invention.
Impairments Presented by the Communication Channel
There are other difficulties from the channel which affect the implementation of the PCM modem and the information transmission. The communication channel from the line card PCM CODEC to the user's modem can be characterized by 4 primary functional units: the codec's anti-aliasing filter, the line card circuit, the subscriber loop and the modem's input circuit.
First, the CODEC anti-aliasing filter is mostly flat, starting to attenuate the signal at about 3.5 kHz and increasing to 15-20 dB at 4 kHz.
Second, the line card circuit is in some ways the most challenging impairment. It is difficult to characterize, since there are many types in use, and it has been designed to transmit a voice signal rather than a data signal. The hybrid transformer circuit includes a null at 0 Hz, and may induce high levels of nonlinear distortion, especially on signals with low-frequency energy.
Third, the subscriber loop itself is generally a linear impairment, creating inter- symbol interference (ISI) and picking up largely Gaussian noise. Non-Gaussian noise sources are near-end crosstalk ("NEXT"), far-end crosstalk ("FEXT") and single frequency interference (SFI). The prime source of SFI is the electricity distribution network, adding a 50 or 60 Hz tone to the signal.
Finally, the modem input circuit consists of a line interface, including a hybrid transformer, and an analog-to-digital converter (ADC). The hybrid introduces a null in the spectrum at 0 Hz, and may also induce nonlinearities. However, since the signal level at the modem end is lower than at the central office, and the transformer can be part of the receiver design, nonlinear distortion in the modem input circuit will generally be minimal compared to that added by the central office line card. While the ADC will of course add quantization noise, which can be made negligible by design, many ADC's will add Gaussian-type noise, often with a "1/f" spectral characteristic, i.e. with highest energy at the low-frequency end of the spectrum. Also, a DC offset can be expected in the ADC, requiring some compensation by the receiver.
Given these impairments, it might seem difficult to take advantage of the low-end of the spectrum. The two transformer hybrids introduce nulls at 0 Hz, SFI is mostly around 50-60 Hz, and nonlinear distortion is increased if energy is transmitted at low frequencies (and will extend into higher frequencies). ADC noise also tends to be most prevalent at low frequencies.
Thus, it is desirable to simplify the receiver design by having the transmitter avoid sending energy at low frequencies. Without the low frequency energy, the receiver may elect to simply implement a high-pass filter to filter out noise there without the need to reconstruct the signal component removed.
In the following disclosure of the present invention, methods will be disclosed for controlling the sequence of bits transmitted to the line card CODEC to eliminate as much energy as possible at low frequencies. For illustration purposes, the following disclosure will assume a redundancy of 4 Kbps, where 4 Kbps out of the 64 Kbps are generated based on the other bits. Generally, a higher redundancy may improve the performance, and the methods disclosed herein can readily be modified for a different level of redundancy.