One of the most important concerns in digital communications systems is minimizing distortion introduced by a communications channel, sometimes referred to as "channel effects." FIG. 1 is a block diagram 100 of a conventional digital communications system. Data is transmitted from a transmitter 102 to a receiver 104 over a communications channel ("channel") 106. Channel 106 may be any type of communication medium for transferring data between transmitter 102 and receiver 104. For example, channel 106 may be one or more network connections, wires, fiber-optic links or a wireless digital communications link.
Ideally, data is transmitted between transmitter 102 and receiver 104 over channel 106 without being distorted. That is, the data retrieved by receiver 104 from channel 106 is identical to the data placed onto channel 106 by transmitter. However, in practice, channel 106 introduces distortion that can corrupt data transmitted over channel 106. The distortion introduced by channel 106 can cause successive transmitted symbols to interfere with each other, otherwise known as inter-symbol interference (ISI). ISI can cause severe corruption of digital data, resulting in a very high bit error rate (BER). The corruption of data occurs while being transmitted across communications channel 106. In multi-carrier systems ISI must be removed before transformation of time domain data to frequency domain data, e.g., via a fast Fourier transform (FFT), which causes ISI to be spread across all frequency bins, producing significant signal to noise degradation. The common solution to this problem is to remove ISI from the sampled data using time domain equalization (TDEQ) before transforming the sampled data from the time domain data to the frequency domain. This allows the original data to be recovered from the communications channel.
FIG. 2 is a block diagram 200 illustrating an example implementation of transmitter 102 and receiver 104 of FIG. 1. Transmitter 102 typically includes an encoder 202, a digital-to-analog converter 204 and a transmit filter 206 and a line driver 207. Encoder 202 encodes the original digital data to generate encoded digital data. The encoded digital data is converted to analog encoded data by digital-to-analog converter 204. Transmit filter 206 removes unwanted components of the original data from the encoded analog data to generate filtered data. Line driver 207 amplifies the signal to transmit the signal across channel 106. The filtered data is transmitted over channel 106 to receiver 104.
Receiver 104 includes a differential amplifier 208, one or more receive filters 209, an analog-to-digital converter 210 and an equalizer 212. After data is received by receiver 104 from channel 106, receive filter 209 removes unwanted components from the encoded analog data received from transmitter 102 over channel 106. Analog-to-digital converter 210 converts the encoded analog data received from transmitter 102 to encoded digital data. Equalizer 212 processes the encoded digital data to remove ISI. The encoded digital data is further processed by converting the data to the frequency domain via a Fast Fourier Transform to recover the modulated tones. Residual frequency domain equalization is performed in the frequency domain before the data is transferred to a decoder 214 that recovers the original digital data. Decoder 214 may be separate from receiver 104 as illustrated, or may be incorporated into receiver 104.
Most conventional approaches for removing ISI have significant limitations that result in degraded performance. Many of the limitations in conventional approaches are attributable to the nature of ISI and the nature of other external noise sources. Conventional approaches address each problem as independent, i.e., ISI removal and noise mitigation. Thus, solving one problem can exacerbate the effects of another. For example, some TDEQ mechanisms do not account for noise sources such as thermal noise and crosstalk. In addition, the characteristics (transfer function) of a communications channel are not necessarily static and can change over time. As a result, conventional TDEQ mechanisms suffer from a number of drawbacks generally including not removing all of the channel-induced ISI or having stability problems. For example, decision feedback TDEQ mechanisms can provide improved performance, but are difficult to apply to discrete multi-tone (DMT) transmission system applications, which limits their use. Furthermore, decision feedback TDEQ mechanisms can suffer from error propagation problems, wherein the occurrence of one error leads to an increased probability of the occurrence of an error in the subsequent data symbols. As another example, transmission precoding is also used to compensate for known channel effects. However, transmission precoding requires an accurate characterization of the channel and a cooperative transmitter. Furthermore, transmission precoding cannot be implemented in all types of transmission systems. For example, transmission precoding cannot be implemented within the current asynchronous digital subscriber line (ADSL) standards ANSI T1.413 and ITU992.2.
Therefore, based on the need to reduce ISI in digital communications systems and the limitations in the prior approaches, an approach for reducing ISI in digital communications systems, and in particular DSL communications systems, that does not suffer from limitations inherent in conventional ISI removal approaches is highly desirable.