1. Field of the Invention
This invention relates to an improved demodulator for use in communication systems employing a phase modulated carrier. More particularly, the invention relates to a demodulator having an improved apparatus for generating reference vectors, carrier detection signals, and bit synchronization parameters for use with receivers of power line communication systems.
2. Description of the Prior Art
In power line carrier communication systems, transmitters are used to transmit information to remote receivers. The information is transmitted via a carrier signal which is added to the normal power line frequency. A particular application of this technique can be used by electric utilities for meter reading and load management operations. In applications where the electric consumer's location is significantly remote from the carrier control unit, some means may be required for amplifying the communications signal at a location between the carrier control unit and the residence. This function can be accomplished by the use of a repeater which is placed at a location between the carrier control unit which generated the commands and a device located at the residence which responds to those commands. It should be understood that these commands can comprise meter reading commands, load shed commands or other information useful in automated distribution systems.
The repeater receives the command information from the power line carrier, amplifies and retransmits it to a load management terminal in the electric consumer's residence. In applications where the command from the carrier control unit demands a response from the residence, the repeater has the additional function of receiving the information which is transmitted from the load management terminal, amplifying it and retransmitting it to the carrier control unit. An example of this type of application occurs when an electric utility wishes to remotely read a watthour meter which is located at the consumer's home. The carrier control unit would initially transmit a meter reading command to the residence. This command would be modulated and added to the 60 Hz power line frequency. After traveling some distance on the power line, this signal would be received by a repeater, amplified and retransmitted to the residence. This intermediate amplification would compensate for signal loss due to the distance between the carrier control unit and the repeater and also due to significant electrical noise which may have affected the originally transmitted carrier signal. Another useful function of a repeater is to amplify carrier signals received from the residence prior to retransmitting it to the carrier control unit. This function is especially important since the load management terminal located at the consumer's residence has limited power capability and its transmissions are therefore at a significantly lower power than those which are transmitted from the carrier control unit. By using a repeater, these weaker transmissions from the consumer's residence can be amplified by the repeater in order to provide a much more powerful carrier signal to be received by the carrier control unit.
The use of power lines for communication purposes is well known. A common technique is to provide a carrier signal which is superimposed on the 60 Hz power line frequency and to modulate the carrier in such a way that messages may be transmitted and received by devices which are specifically designed to translate the modulated carrier signals into binary data or vice versa. One particular modulation technique utilizes coherent phase shift keyed digital modulation of the carrier signal.
Coherent phase shift keyed digital modulation techniques are known to be especially suitable for electric utility power line carrier communication systems. The basic task of such systems is to transmit information over the primary and secondary distribution conductors between a central utility location and customer locations. The information may consist of remote meter reading commands, metering data, load shed commands, load status information, and various other data useful in automated distribution systems.
The data is converted at the transmitting end to strings of binary data bits in a predetermined message format. The information, when converted to digital form, is referred to as baseband data.
In order to transmit the message from the source to the destination, the baseband data is modulated onto a carrier signal by causing the phase of the carrier to assume any of a plurality of predetermined relative phases according to the logic state of the applied baseband data. The modulated carrier signal is then coupled to the power line conductor and propagated to the destination.
A power line communication system employing coherent phase shift keyed modulation is described in U.S. Pat. No. 4,311,964 which issued on Jan. 19, 1982 to John R. Boykin and which is herein incorporated by reference. In the transmitter of U.S. Pat. No. 4,311,964, the bipolar data bits are phase encoded onto the carrier with identical bit intervals, or data symbol times, defining a predetermined data rate and are synchronized with the carrier signal so as to be integrally related to the carrier signal frequency.
The system described in the aforementioned U.S. Pat. No. 4,311,964 utilizes a phase reversal keying, such that the phase of the carrier is caused to assume either of two phase states separated by 180.degree., according to whether the baseband digital data modulated thereon is a logic 1 or a logic 0. In the receiving apparatus disclosed therein, the modulated carrier is hard limited to produce square wave carrier signals. The polarity of the hard limited carrier signals, divided into segments, is then sampled at a sampling pulse rate selected such that the ratio of the sampling rate to the carrier frequency is not an integer. The sampling process enables the demodulator to determine the relative position of the zero crossings of the square wave carrier signal. This information is used to generate a phase angle vector signal representative of the phase state of the present segment of the incoming carrier signal relative to a coordinate system generated internally within the demodulator.
The phase angle vector signal is used to generate a reference vector signal. The phase angle vector and reference vector signals are then applied to a phase detector to yield a signal output which indicates which of the allowable phase states has been assumed by a given segment of the incoming carrier signal. This phase information is then utilized to reconstruct the baseband digital data originally supplied to the transmitter. This prior art apparatus produces the reference vector signal by generating an intermediate vector signal at a frequency twice that of the incoming carrier signal. This intermediate vector signal, known as a double frequency vector, is then digitally integrated over an extended period of time to produce a reference vector signal. The reference vector and phase angle vector signals are then applied to a phase detector which produces a signal output indicating whether the incoming carrier signal has assumed the nominal phase or the reversed phase to in turn indicate whether the baseband digital data is a logic 1 or logic 0. Thus, if the phase angle vector signal derived from the incoming carrier is determined to be 45.degree. relative to the internally generated coordinate system, and is followed by a phase angle vector signal having a phase of 225.degree. (indicating a phase reversal, or change of the baseband digital data from a logic 1 to a logic 0) the phase of the intermediate vector signal generated at twice the frequency of the incoming carrier signal is 45.degree. for both of the aforementioned phase angle vector signals.
This system provides generally satisfactory service, but is rather complex. Furthermore, in some circumstances the system may not exhibit sufficient noise immunity and may become subject to unnecessary drop-out of the carrier detect signal.
In prior art digital demodulators such as described in the aforementioned U.S. Pat. No. 4,311,964 employing sampling techniques for systems with a data transmission rate much lower than the carrier frequency, bit framing is required, since each baseband data symbol, or bit, extends over several carrier segments. A data symbol extends over four segments in the apparatus of U.S. Pat. No. 4,311,964. It is necessary to determine at which carrier segment a data symbol begins. For example, is the carrier segment currently being processed actually the first segment of a new data symbol (which will also include the next three segments), is it the last segment of a data symbol also including the three preceding segments, or is it a middle segment of a data symbol which also extends over one or two preceding and succeeding segments. The process of bit framing, or synchronizing, was done only after the successful detection of a carrier signal by the receiver of U.S. Pat. No. 4,311,964. Carrier detection therefore was required to occur early enough to insure that successful bit framing would be accomplished during the transmission of the message preamble. Furthermore, the serial execution of the carrier detection and bit framing functions cause duplication of calculations which may allow significant error propagation due to the limited precison of 8-bit arithmetic used in some prior art microprocessor based systems. It would be desirable to provide demodulator apparatus and methods to enable more rapid bit framing and carrier detection.
In virtually any type of receiver which is intended to receive and translate a modulated carrier signal, it is necessary to determine the validity of the carrier signal in order to avoid attempted translations on false signals caused by electrical noise or interference. Some means is therefore required to distinguish true carrier signals from false signals which are induced by this electrical noise. One means for distinguishing true carrier signals from false signals is to form a magnitude which is representative of the validity of a carrier signal and compare that magnitude to a preselected threshold value. A significant benefit could be achieved if this threshold value could be changed when a receiver has a high confidence level in its expectation of an incoming carrier signal.