As is known, data communication equipment ("DCE") such as modems, for example, are able to transmit digital data over channels to remote DCE units. To accomplish this task, the sending DCE typically contains a constellation of transmitter signal points, with each transmitter signal point representing a digital code or value to be transmitted to the receiving DCE. The sending DCE thus modulates a carrier signal at the transmitter signal point corresponding to the digital code or value to be sent. The receiving DCE, in turn, contains a constellation of receiver signal points that is compatible with the transmitter signal points in the sending DCE. The receiving DCE thus demodulates the received carrier signal and identifies a corresponding receiver signal point from the receive constellation of receiver signal points. The receiving DCE then determines the digital code or value represented by the receiver signal point.
Presently, data communication equipment, such as modems, for example, are being used to communicate digital data between data terminal equipment (DTE), such as personal computers, workstations and the like, over telephone lines using a standard communication format or constellation of signalling points, such as the CCITT V.22 bis signalling format, for example. In transmitting and communicating digital data in a telephone network, the primary source of distortion of the communication signal is linear in nature and is handled effectively by equalization techniques implemented in the DCE's at both ends of the communication link. Most telephone lines do not induce severe enough non-linear signal distortion of the communication signal to warrant a separate processing subsystem in a modem to mitigate the non-linear effects on the standard signalling format.
Recently, it has been proposed to communicate digital data over a cellular telephone network using a standard communication signalling constellation, like the CCITT V.22 bis format, for example. However, the cellular network is recognized as introducing distortive effects on the communication signal substantially different from that of the telephone network because it is designed primarily to communicate voice only signals over voice band media in which commercial channel bandwidth is severely restricted, that is the cellular network is not designed for transmitting digital data.
In the transmission of voice signals in a cellular network, the audio signal input to a microphone is generally used in controlling the instantaneous FM radio signal frequency. A limiter circuit in the transmitter limits the audio voltage before it controls the radio signal to keep it from moving too far from its center frequency, thus reducing interchannel interference. This process is better known in the communication field as clipping and is a form of non-linear distortion of the communication signal. Accordingly, clipping is designed intentionally into the cellular transmitter of the cellular network to keep or limit radio frequency (RF) spillage over into an adjacent communication channel, that is, the limiter precludes moving the instantaneous frequency of the communication channel too far from that desired. Clipping causes time-varying group delay (amplitude induced group delay distortion) on the digital data transmission which results in the symbols thereof becoming distorted at the receiving end to the point where the reliability thereof is questionable. That is, a distorted symbol may be closer to a signal point of a receive constellation other than that intended and misconstrued by a data estimator of the receiving modem as applying to an unintended digital data code. These non-linear effects may cause poor throughput in digital data communication between modems by creating difficulties in making initial connection, and if retransmission capabilities exist may cause retransmissions of the same data due to poor quality transmission over a preceding communication period.
There are a number of possible solutions to the non-linear distortion problem, one being designing a signalling format which is impervious to non-linearities, but any new signalling format would be non-standard and the corresponding modem application would not be capable of communicating with everybody else. Another solution may be to accept the non-linearly distorted communication signal using a standard signalling format and undo the distortive effects at the receiving modem.
It has been proposed theoretically to combat non-linear effects as well as linear effects on communication signals through the use of Volterra series or kernel characterization of a general non-linear system. An example of this theoretical solution is described in the paper: D. D. Falconer, "Adaptive Equalization of Channel Non-Linearities in QAM Data Transmission Systems", Bell Systems Technical Journal, Volume 57, No. 7, September, 1978. In essence, these type systems condition the received signal to render it accurate for use in a fixed signalling format using a passband decision feedback equalizer to remove both linear and non-linear effects on the incoming communication signal based on a model of the passband channel. While offering an effective theoretical technique of solving the problem, the Volterra series method is not considered practical because it is extremely computationally intensive and would probably require an additional separate processor in a modem to handle the computational load associated therewith. In addition, the Volterra series requires an extremely long time to converge to a solution which is not suitable for conventional digital communication equipment.
Accordingly, what is needed is data communication equipment which can communicate digital data information at suitable throughput rates over a communication media under the effects of both linear and non-linear distortion. It is desirable that the digital communication equipment operate within a standard signalling format for digital data communication and undo the distortive effects of the communication signal using techniques which are substantially less computationally intensive and time consuming as those of the Volterra series.