Abstract:
A communication system for communicating a modified geometric harmonic modulated (GHM) signal on a power line is described. The system includes a transmitter for transmitting a GHM signal having a plurality of carriers, a highest frequency carrier being about 630 Hz, and a receiver for receiving the transmitted GHM signal.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a power line communication system and more particularly, to a power line communication system for communicating commands and data on a power line. 
     Geometric harmonic modulated (GHM) signals for power line carrier applications consist of a set of carriers, each carrier being modulated by the same data. For example, eight carriers are evenly spaced in the frequency band between 5 kHz and 12 kHz yielding a GHM signal s(t) characterized by the relationship: 
     
       
         s i ( t )=sin(2π·5010 t )−sin(2π·5610 t )+sin(2π·6210 t )+sin(2π·6810 t )−sin(2π·7410 t )+sin(2π·8010 t )+sin(2π·8610 t )+sin(2π·9210 t ). 
       
     
     The eight carriers of the above characterized signal are positioned at selected frequencies of 5010 Hz, 5610 Hz, 6210 Hz, 6810 Hz, 7410 Hz, 8010 Hz, 8610 Hz, and 9210 Hz. The signs of the eight sinusoidal carriers are selected according to a Shapiro-Rudin sequence to keep the signaling crest factor low. 
     Typical GHM signals such as the one described above utilize frequencies above about 5 kHz to avoid the large spectral components of the 60 Hz power signal transmitted on most power lines. However, use of GHM signals at typical frequencies has attendant disadvantages. First, the efficiency of transmission of the GHM signal through distribution transformers drops off as the GHM signals increase in frequency. This is because the transformer acts like a low pass filter. In addition, typical GHM signals are severely attenuated by capacitor banks. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a communication system for communicating on a power line comprising a transmitter adapted to provide a GHM signal which is lower in frequency than the typical GHM signal such that the GHM signal is capable of being transmitted through distribution transformers and capacitor banks. The GHM signal is combined with a power signal for transmission with the power signal. The system further comprises a receiver configured to remove an estimate of the power signal from the combined signal and to provide the GHM signal at an output. The GHM receiver includes an estimator for generating an estimated power signal, a summer for subtracting the estimated power signal from the signal transmitted on the power line to segregate the transmitted GHM signal si(t), an analog to digital converter coupled to an output of the summer, and a demodulator coupled to an output of the analog to digital converter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a transmitter. 
     FIG. 2 is a simplified block diagram of a receiver. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an exemplary embodiment of a transmitter adapted to transmit GHM signals of the present invention. Generally, GHM allocates signaling energy into lobes, or tones, or different frequencies evenly spaced. GHM signal waveforms are true spread spectrum signals in that the signal bandwidth, i.e., the bandwidth from the lowest frequency tone to the highest, exceeds the information bandwidth conveyed by the GHM transmission. 
     In accordance with an embodiment of the present invention, transmitter  10  transmits, on the power line, a GHM signal si(t) characterized as: 
     
       
         s i ( t )=sin(2π·210 t )−sin(2π·270 t )+sin(2π·330 t )+sin(2π·390 t )−sin(2π·450 t )+sin(2π·510 t )+sin(2π·570 t )+sin(2π·630 t ). 
       
     
     The signal carriers have frequencies of 210 Hz, 270 Hz, 330 Hz, 390 Hz, 450 Hz, 510 Hz, 570 Hz, and 630 Hz, and each carrier frequency is situated in the interstices of multiples of 60 Hz. As compared to the signal carriers of typical GHM signal which have frequencies of 5010 Hz, 5610 Hz, 6210 Hz, 6810 Hz, 7410 Hz, 8010 Hz, 8610 Hz, and 9210 Hz, the carrier frequencies of the GHM signal described above are lower in frequency and therefore suitable for passing through both distribution transformers and capacitor banks. 
     Referring now particularly to FIG. 1, transmitter  10  is coupled to a power line by a coupler such as a transformer. In the exemplary embodiment, the number of tones, or carriers, to be transmitted is eight. The carriers are synthesized together to form a single signal, having eight carrier frequencies, by a calculation device  3 , which may be a general purpose computer (e.g., microprocessor) or a device which creates a waveform (e.g., digital signal processor). The synthesized signal generated by calculation device  3  is sampled by a sampler  5 , and sampler  5  stores the sampled signals in storage device  13  which is a non-volatile memory such as an electronically erasable and programmable read only memory. Synthesizing, sampling and storing the sampled signals are performed prior to transmission. 
     A transient protection device  11 , which limits power surges in order to protect equipment down line, is coupled to a power line, typically in parallel and across a distribution transformer. Device  11  applies the power line voltage to a timing controller  17  which, in turn, obtains timing information from the fundamental 60 Hz power line frequency. That is, timing controller  17  utilizes the 60 Hz fundamental power line frequency to maintain a real time clock. 
     At a preselected time, e.g., at a preprogrammed time selected by a system operator, a playback device  15 , controlled by timing controller  17  and coupled to storage device  13 , reads out the samples from storage device  13  at a rate of 60 Hz and synchronous with the power line voltage. A multiplier  19 , also coupled to timing controller  17 , multiplies the waveform created by playback device  15  by a single bit (i.e., one of bits b 1 , b 2 , b 3 ) during each single bit period. The bit sequence from playback device  15  is modified prior to transmission by multiplier  19  by a change in bit values to a series of one and negative ones, instead of ones and zeros. That is, when the waveform from playback device  15  is provided to multiplier  19 , a bit value of one will not change the waveform for a bit period, while a bit value of negative one will invert the waveform. More specifically, a data source supplies a series of bits a 1 , a 2 , a 3  . . . an to a level shifter which shifts the level of the supplied bits a 1 , a 2 , a 3  . . . an to b 1 , b 2 , b 3  . . . bn. 
     The resulting GHM modulated signal from multiplier  19  is summed with the power signal at a summer  18  and communicated over the power line to a receiver. Summer  18 , in one embodiment, is a current transformer for imposing the GHM signal on the power signal. 
     FIG. 2 is a block diagram of a receiver  50  for receiving the GHM signal described above. Receiver  50  is coupled to the power line by a coupler  52 , such as a transformer as is known in the art. The signal from the power line, which is a combination of both the modified GHM signal and the power signal, is supplied to a short term estimator  54 , which estimates the 60 Hz fundamental and first k-harmonics of the signal on the power line, where k is a selected integer (e.g., 5). The value for k is typically selected from empirical results such that sufficient bit accuracy is provided with minimal processing. The short term estimator is a phased lock loop. 
     The signal from the power line is also supplied to a summer  56 . Summer  56  subtracts the estimated power signal supplied by estimator  54  from the signal on the power line which contains both the 60 Hz power signal and the GHM signal. By removing the estimated fundamental and harmonics from the signal supplied directly from the power line, the GHM signal is provided from summer  56 . This technique allows reception of a GHM signal which has frequency components lower than a typical GHM signal. 
     Known techniques can be used to reduce the dynamic range of the 60 Hz component and also some of the other large components at multiples of 60 Hz. Examples of such known techniques include analog subtraction and phase locked and delta modulated loops. An example of one such technique is described in U.S. Pat. No. 5,550,880, entitled Motor Current Signal Processor Using Analog Subtraction of an Estimated Largest Sine Wave Component, and in U.S. Pat. No. 5,550,879, entitled Motor Current Signal Processor Including Phase Locked and Delta Modulated Loops, both of which are assigned to General Electric Company. An analog subtraction loop estimates not only the 60 Hz component but also other of the large components that are multiples of 60 Hz. The loop characteristics should be slowly changing parameters so as not to introduce excessive modulation interference in the GHM signals. 
     By reducing the frequency of the GHM signal so that the signal carriers are positioned interstitially between multiples of 60 Hz but near the 60 Hz fundamental, for example, below about 630 Hz, the GHM signal passes through capacitor banks as well as through distribution transformers. As will be readily understood by those of ordinary skill in the art, carrier frequencies up to 9690 Hz and beyond can be employed in conjunction with the present invention. However, the maximum benefit derived from utilizing the receiver of the present invention is inversely proportional to the frequency of the GHM signal. 
     Analog subtraction or phase locked and delta modulated loops are employed in combination with the modified GHM signal in accordance with another embodiment of the invention, to provide interference suppression. This facilitates efficient transmission of data using lower frequency GHM signals. 
     The output from summer  56  is supplied to analog to digital converter  58 , which converts the analog signal from summer  56  to a digital signal. The digital signal is then supplied to a GHM demodulator  60  which processes the digital signal to identify the components of the GHM signal, and the data being transmitted over the power line. Demodulator  60  includes, for example, a fast Fourier transform unit and a summer for summing the coefficients. A decision (i.e., either a logical zero or one is declared) is then made based on the sum. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.