Pulse or digital communications – Transmitters – Quadrature amplitude modulation
Reexamination Certificate
1998-02-10
2002-06-04
Deppe, Betsy L. (Department: 2634)
Pulse or digital communications
Transmitters
Quadrature amplitude modulation
C375S223000, C375S308000
Reexamination Certificate
active
06400776
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally directed to high-speed data communication and more specifically to the area of high-speed modem design. It also concerns achieving high spectral efficiency in signaling systems.
BACKGROUND OF THE INVENTION
Due to present increased bandwith requirements there is a present need to simultaneously support voice, video and data applications at low BER (Bit Error Rates), using new modem designs operating for twisted pair wires at rates exceeding 10 Mbits/s at error rates better than 10
−6
. Using conventional Pulse Amplitude Modulation (PAM) techniques, each signal is represented and its amplitude level is determined by a transmitted symbol. For example, in 16-QAM (Quadrature Amplitude Modulation), typical symbol amplitudes of ±1 and ±3 are utilized in each quadrature channel. For digital communications systems, efficient use of bandwidth is crucial when dealing with time dispersive channels as is common with wireless systems. In these types of systems, whenever there is a distortion of the signals due to preceding or following pulses, normally referred to as pre-cursors and post-cursors, respectively, the amplitude of the desired pulse is affected due to superimposition of the overlapping pulses. This phenomenon is known as intersymbol interference and is an impediment to high-speed data transmission, especially in systems that are constrained by limited bandwidth.
One way to minimize the effects of intersymbol interference is to use an equalizer. Fixed equalizers are designed to be effectively operated between an upper and lower bound between which the channel is expected to deviate. Whenever these limits are exceeded, the equalizer ceases to operate effectively. Hence there has to be a certain degree of precision when channel equalization is employed, and fixed equalizers are implemented. There are adaptive equalizers (i.e. continuous) that track dynamic channel dispersion, and make continuous adjustments to compensate for such intersymbol interference. This provides some improvement in performance over the fixed equalizer.
Incorporation of the equalizer into some communication systems does not come without penalty. In wireless systems, for instance, insertion loss becomes a critical factor if the equalizer is present and the associated impairment does not occur. The main purpose of equalizer implementation is to enhance the information bearing capability of the communication system with the design objective of asymptotically approaching the capacity bounds of the transmission channel. Consequently, the use of the equalizer can be regarded as one instance of an array of possibilities that may be implemented to enhance the bit rate of a communication system design.
SUMMARY OF THE INVENTION
In accord with the invention, a method and apparatus is provided that increases the bits per baud beyond rates that are achievable with digital signaling systems at present without a necessity to increase the bandwidth of the channels. This is done by significantly reducing the effects of intersymbol and interchannel interference by a judicious choice of the signaling pulse shapes. In particular prolate pulses are used to extend channel capacity and reduce interference. By use of orthogonal axes that span the signal space combined with water filling techniques for efficient allocation of transmission energy based on the noise distribution, the information content can be increased without increase in bandwidth.
The signaling space is spectrally decomposed to support the simultaneous transmission of multiple signals each with differing information bearing content, and being orthogonal are non-interfering. Signals are constructed as complex sets and are generally represented with axial coordinates all orthogonal to one another within the complex plane. The real axis is termed the in-phase (I) component and the imaginary axis is termed the quadrature (Q) component. These components for a signal define a spanning vector in the signal space.
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AT&T Corp.
Deppe Betsy L.
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