Pulse or digital communications – Pulse position – frequency – or spacing modulation
Reexamination Certificate
2002-01-31
2002-09-03
Vo, Don N. (Department: 2631)
Pulse or digital communications
Pulse position, frequency, or spacing modulation
C332S112000
Reexamination Certificate
active
06445737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of endeavor of the present invention is an apparatus and method for the transmission of digital data utilizing a carrier having the characteristics of a sideband with a very narrow bandwidth so as to reduce the bandwidth of the principal energy bearing portion of the transmitted radio frequency spectrum.
2. Description of the Prior Art
Single sideband transmission of information without a carrier is well known. For the transmission of digital data utilizing a single sideband, some form of baseband data encoding is generally required to reduce the bandwidth. The encoded data is then applied to a single sideband modulating device, which suppresses the carrier and removes one of the sidebands by a filtering or phasing method.
U.S. Pat. Nos. 4,742,532, 5,185,765 and 5,930,303, issued to the present inventor are representative of the prior art. A ‘PCT’ application published as WO 99/23754 (U.S. Ser. No. 98/23140) is an international filing of the '303 patent. U.S. patent application Ser. No. 09/612,520 is directed to a method related to the '303 patent.
The method described in the first and second of the above patents results in some frequency spread in the transmitted spectrum. The latter patent describes a method known as VMSK that results in a single frequency spectral line with phase changes so slight that they are not visible on spectrum analyzers. The VMSK method results in a very high spectral efficiency that enables very high data rates to be transmitted in a very narrow bandwidth. It was also found that it is not necessary to restore the suppressed carrier to detect the signal.
Since the modulation and filtering produce a spectrum of a single frequency, special very narrow band filters are required to pass the narrowest bandwidth possible to remove undesirable spectral components in the transmitter and to reduce the noise bandwidth in the receiver. In the '303 patent, phase reversals of the carrier are utilized together with a balanced modulator to remove the carrier. The phase reversal periods are made as nearly even as possible in order to reduce undesirable spectral components. When only one sideband is transmitted, after passing through the narrow bandwidth filter, the observed signal on the oscilloscope does not show the phase reversals at nearly equal time periods. Instead, the signal shows a complete phase rotation through 360 degrees at the transition periods of the phase reversing signals. The duration of this phase rotation is three to four cycles of the sideband frequency, which normally has 60 to 90 cycles per bit period. For the remainder of the bit period, the sideband frequency is constant in frequency and phase.
All of the useful modulation is contained in the brief phase rotation period. For the remainder of the bit period there is a constant signal that can be used to establish a reference. In the prior art, the data clock and the RF frequency are not necessarily numerically related so that there is a phase crawl or difference among the various crossings periods. This results in an inconsistent detected output, which varies in rise time and amplitude.
There is a need for a modulation method that will synthesize the sideband of the prior art by itself, with brief phase reversals at timed intervals representing digital ones or zeros. There is a need for a modulation method of this kind, and that does not require the use of a carrier similar to what has been done; yet without the customary single sideband processing with its drawbacks. That is, phase crawl or difference among the various crossings periods needs to be reduced or eliminated. This would fill an end goal or need for a method that results in a consistent detected pattern in rise time and amplitude.
Several complementary objects of the present invention involve elements in a system that comprises the invention. Hence, one object of the invention is to provide a detection means that will detect the simulated sideband of the invention without the restoration of a carrier separated in frequency from the sideband by ½ the data rate. Another object of the invention is to provide a signal processing means that will result in no apparent loss of signal power. Still another object of the invention is to provide a signal that can withstand the degradation caused by multipath interference and fading.
The present invention fills these needs as can be seen in the further description below.
BRIEF SUMMARY OF THE INVENTION
The wireless digital transmission and receiving method of the present invention combines phase reversal keying with pulse position modulation. The invention implements pulses that are of extremely short duration to indicate ones and zeros. These pulses can be as narrow as one cycle of the carrier frequency. As such, they often appear as missing cycles or pulses in a sequence of carrier cycles. The method creates a broad sinx/x spectrum with a principal power peak at the modulated frequency and numerous weaker peaks of varying frequencies and peak levels. The time duration of these smaller peaks is such that they have negligible mean power levels. The weaker peaks also do not cause measurable interference with other communications devices. Also, the smaller peaks can have amplitudes far below the noise level of the system. Hence, the smaller peaks are not an important component of the signal.
It is well known to those skilled in the art that all modulation energy is concentrated in the sidebands. It can be shown that the spectrum of the present invention represents a sideband only and not a carrier with two sidebands. It can also be shown that this sideband is comprised of a single frequency that does not change in frequency or phase for most of the bit period. The modulation occurs as a phase reversal of one cycle, or as the absence of one cycle in a series of cycles. The resulting “missing pulses” or “missing cycles” are detected to indicate digital ones and zeros in a signal of fixed frequency. It is possible to detect these “missing pulses” or “missing cycles” at preset time periods.
The method is extremely resistant to multipath interference, since the weaker path is a signal at the same single frequency as the stronger path, but of different phase. The phasors of the two paths add to produce large detected outputs for the principal path and weak outputs at a different time for the reflected path. Thus, this weak response can be time gated out of the resulting signal.
In a practical first embodiment of the invention, a circuit is used to create very narrow pulses utilizing pulse position modulation to distinguish ones and zeros. The narrow pulses are used in a phase reversing device, such as an XOR gate or balanced mixer, to cause the reversal of one cycle out of a stream of many cycles. The resulting spectrum has a principal peak at the modulated frequency and many minor peaks spread at bit rate intervals. The minor peaks have power levels proportional to the time duration of the pulses of each phase. The mean power of the minor peaks is low. Therefore, it is not normally required to provide bandpass filtering at the transmitter.
In a second embodiment, an AND gate is used to remove one cycle from the modulated frequency so that “missing cycle” modulation results. The result is the same as that for phase reversal.
In all embodiments, the timing of the narrow pulse and the start of the cycles of the modulated frequency can be made to coincide with each other so that the detected output has a uniform rise time and amplitude.
The present invention may be used in conjunction with any number of elements in a system. For example, in a system, the receiving apparatus is made to correspond with the method and device of the present invention. The receiving apparatus is comprised of a special very narrow bandwidth filter, having almost zero group delay, in combination with a limiter, synchronous detector, and pulse position decoder. The synchronous detector is locked in frequency and phase to the pulse modulated
Dawes Daniel L.
Myers Dawes & Andras LLP
Vo Don N.
LandOfFree
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