1. Field of the Invention
The present invention relates to a method for demodulating and a demodulator for demodulating various signals by hard-decision techniques, such as phase shift keying (PSK) and quadrature amplitude modulation (QAM); as well as demodulating signals by soft-decision techniques, such as Viterbi decoding and trellis decoding. For hard-decision techniques, the decision regions are optimized at the receiver to reduce the bit error rate (BER); and for soft-decision techniques, the decision metrics are optimized at the receiver to reduce the bit error rate (BER). Optimizing the decision regions and metrics may obviate the need for non-linear predistortion at the transmitter.
2. Description of the Prior Art
Various modulation techniques are known for modulating a carrier signal with various types of information. Due to limited bandwidth allocations, modulation techniques have been developed to increase the amount of information that can be transmitted per frequency. One such technique is known as quadrature phase shift keying (QPSK). Such QPSK modulation techniques are known in the art and described in U.S. Pat. Nos. 5,440,259; 5,615,230; 5,440,268, 5,550,868; 5,598,441; 5,500,876 and 5,485,489, hereby incorporated by reference. In general, in such a modulation technique, the phase of both the real and quadrature components of the carrier are modulated to enable two bits, each having two stages, to be transmitted over a single frequency. As such, at each frequency, the carrier can be modulated into one of four different phase states, known as symbols, which form a constellation as generally shown in
In order to further increase the amount of information transmitted per frequency, other modulation techniques have been developed, such as quadrature amplitude modulation (QAM). Such QAM modulation techniques are relatively well-known in the art. Examples of such QAM modulation techniques are disclosed in U.S. Pat. Nos. 5,612,651; 5,343,499; 5,363,408; and 5,307,377; hereby incorporated by reference. Such QAM modulation techniques essentially involve amplitude and phase modulation of a QPSK signal to provide constellations of signals of 8, 16, 32 and 64 and more, for example, as illustrated in
Decoding of PSK and QAM modulated signals is also known in the art. In general, the PSK or QAM signal is received, demodulated, filtered and sampled. The sample is known as the decision variable. For example, the QPSK constellation illustrated in
Other coding and decoding techniques are known, such as trellis decoding and Viterbi decoding. Trellis coded demodulation is discussed in detail in U.S. Pat. No. 4,873,701, hereby incorporated by reference. Convolutional coding techniques are also known. Such convolutionally coded signals are known to be decoded by a procedure, known as Viterbi decoding. Viterbi decoding is discussed in “Error Bounds for Convolutional Codes and Asymptotically Optimum Decoding Algorithm” by A. J. Viterbi, IEEE Trans. Inf. Theory, IT-13 pp. 260–269, April 1967. Convolutional coding/Viterbi decoding techniques are also disclosed in “Error Coding Cookbook”, by C. Britton Rorabaugh, McGraw Hill copyright 1996, pp. 105–125, hereby incorporated by reference. Such techniques are known as soft-decision techniques. In such soft-decision techniques, a decision metric is typically computed as the distance between the received decision variable and a reference constellation.
Unfortunately, there are problems associated with the hard-decision techniques as well as the soft-decision techniques which lead to a degradation in the error rate (BER) performance of the system. Such problems are a result of modulator implementation imperfections, channel filtering, amplifier non-linearities and demodulator imperfections. These various problems result in the noise-free decision variables not being at their ideal locations and not equidistant from the nearest decision boundaries. As such, decisions that are made relative to the ideal decision regions are less than ideal leading to an increased BER. In soft-decision decoding techniques, the problems mentioned above result in the noise free constellation not being at an ideal location.
These problems are best understood with reference to
A non-ideal condition is illustrated in
Briefly, the present invention relates to an improved decoding technique useful for hard decision decoding, such as phase shift keying (PSK) and quadrature amplitude modulation (QAM), as well as soft-decision techniques, such as Viterbi decoding and trellis decoding. The system in accordance with the present invention provides adaptive decision regions for hard-decision decoding techniques and adaptive metrics for soft-decision detection techniques in which the decision boundaries and reference constellations, respectively are optimized in order to minimize the bit error rate (BER). In particular, the decision boundaries and metrics are optimized based on the locations of the received constellation points. By adaptively adjusting the decision boundaries and metrics, the BER can be greatly improved without using non-linear predistortion at the transmitter, thus reducing the hardware complexity and weight of the transmitter which provides additional benefits in applications, such as satellite communication systems, where the transmitter is located on the satellite.
These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein:
The present invention relates to a demodulator and a method for demodulating which improves the bit error rate (BER) for both hard-decision decoding techniques, such as quadrature phase shift keying (QPSK), 8PSK, 12/4 quadrature amplitude modulation (QAM), 16-QAM and 32-QAM as well as soft-decision decoding techniques, such as Viterbi decoding and trellis decoding. As discussed above, hard-decision detection techniques generally operate by determining the decision region in which the received signal is located. In soft-decision detection techniques, such as Viterbi decoding and trellis decoding, a decision metric which is the distance between the symbol and a reference constellation, is computed. In both hard-decision and soft-decision decoding techniques, various errors can result as a result of modulator imperfections, channel filtering, amplifier non-linearities as well as demodulator imperfections. In order to optimize the bit error rate (BER) for hard-decision decoding techniques, the system in accordance with the present invention provides an adaptive decision region in which the boundaries of the decision region are forced to be equidistant, for example, from the received signals in order to restore the maximum likelihood decision making ability of the demodulator. In soft-decision decoding techniques, such as Viterbi and trellis decoding, the decision metrics are adaptively adjusted to minimize the bit error rate and restore the maximum likelihood decision-making ability of the demodulator.
One benefit of the present invention is that is may obviate the need for non-linear predistortion techniques, known to be used to compensate for constellation bias errors. In such known compensation techniques, additional hardware is required at the transmitter which increases the complexity as well as the weight and power consumption of the transmitter. In various known satellite communication systems, the transmitters are located on the satellite. In such applications, weight as well as space is significantly limited. The present invention solves this problem by eliminating the need for additional hardware on the satellite while providing an improved BER by providing additional adaptive processing at the receiver, normally located on the ground.
Examples of the performance of the system is illustrated in
Various techniques can be used to optimize decision boundaries or a reference constellation. Examples of these techniques are illustrated in
The decision boundary may be optimized as illustrated in
Other optimization techniques are also known. The optimization of the decision boundaries or the reference constellation can be performed using any one or combination of techniques. Other optimization techniques include collecting a noiseless or high-SNR constellation in first determining the centroid of each constellation cluster. The optimum decision boundaries are determined by mapping each point in the I/O decision space to the nearest centroid point. Other optimization metrics include mean squared error and accumulated path error, both techniques are relatively well-known in the art.
Various techniques can be used to implement the adaptive decision regions and metrics in accordance with the present invention. These techniques include the techniques mentioned above, in which the centroids and decision boundaries are relocated as well as a technique in which the two dimensional I/Q space at the receiver is conformally remapped to an ideal decision space by way of a programmable lookup table. All such techniques are considered to be within the broad scope of the invention.
As best illustrated in
Exemplary block diagrams for the invention are illustrated in
Referring first to
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.
The U.S. Government has certain rights in this invention pursuant to the clause at FAR 52.227-12.
Number | Name | Date | Kind |
---|---|---|---|
4099121 | Fang | Jul 1978 | A |
4873701 | Tretter | Oct 1989 | A |
5307377 | Chouly et al. | Apr 1994 | A |
5343499 | Jasper et al. | Aug 1994 | A |
5363408 | Paik et al. | Nov 1994 | A |
5384810 | Amrany | Jan 1995 | A |
5440259 | Yokomura | Aug 1995 | A |
5440268 | Taga et al. | Aug 1995 | A |
5442655 | Dedic | Aug 1995 | A |
5485489 | Chiba | Jan 1996 | A |
5491698 | Patel et al. | Feb 1996 | A |
5500876 | Nagata | Mar 1996 | A |
5519356 | Greenberg | May 1996 | A |
5537439 | Choi | Jul 1996 | A |
5550868 | Boccuzzi | Aug 1996 | A |
5598441 | Kroeger et al. | Jan 1997 | A |
5604768 | Fulton | Feb 1997 | A |
5612651 | Chethik | Mar 1997 | A |
5615230 | Günther et al. | Mar 1997 | A |
5640417 | Barabash et al. | Jun 1997 | A |
5654986 | Lim | Aug 1997 | A |
5710793 | Greenberg | Jan 1998 | A |
5761216 | Sotome et al. | Jun 1998 | A |
5832039 | Rijns | Nov 1998 | A |
6487261 | Iwamatsu et al. | Nov 2002 | B2 |
Number | Date | Country |
---|---|---|
3239043 | Oct 1991 | JP |
5236039 | Sep 1993 | JP |
7288552 | Oct 1995 | JP |