Abstract:
A system and method for substantially reducing average transmit power by omitting the transmission of a majority of pulses by using a modified run length coding to reduce signal power. By lowering the average power, an opportunity is presented to either decrease the total power drawn by the transmitter (battery life) or raise the peak power of all symbols (Eb/No), thereby increasing range of a RF transmission system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of previously filed Provisional Patent Application Ser. No. 61/137,737. 
    
    
     FIELD OF THE INVENTION 
     This invention addresses the need to transport high bit-rate data over wired or wireless means using specially modulated radio frequency carrier waves. Specifically, this disclosure describes a new method of substantially reducing average transmit power by omitting transmission of a majority of pulses. 
     BACKGROUND OF THE INVENTION 
     Modulation is the fundamental process in any communication system. It is a process to impress a message (voice, image, data, etc.) on to a carrier wave for transmission. A band-limited range of frequencies that comprise the message (baseband) is translated to a higher range of frequencies. The band-limited message is preserved, i.e., every frequency in that message is scaled by a constant value. The three key parameters of a carrier wave are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. 
     There are various shapes and forms of modulators. For example conventional Amplitude Modulation uses a number of different techniques for modulating the amplitude of the carrier in accordance with the information signal. These techniques have been described in detail in “Modem Analog and Digital Communication Systems” by B. P. Lathi. Similarly conventional Frequency/Phase Modulation uses a number of different methods described in a number of textbooks. In all these techniques, carrier (which is a high frequency sinusoidal signal) characteristics (either amplitude, frequency, phase or combination of these) are changed in accordance with the data (or information signal). Thus there have been two major components of a modulated signal used in communication systems. One is the information-carrying signal and the other is the high frequency carrier. 
     Communication systems that have emerged in recent years include mono-pulse and Ultra-Wide Band communication systems. The problem with these systems is that all mono-pulse or Ultra-Wide Band communications systems form Power Spectrum Densities that tend to span very wide swaths of the radio spectrum. For instance the FCC has conditionally allowed limited power use of UWB from 3.2 GHz to 10 GHz. These systems must make use of very wide sections of radio spectrum because the transmit power in any narrow section of the spectrum is very low. Generally any 4 KHz section of the affected spectrum will contain no more than −42 dbm of UWB spectral power. Correlating receivers are used to “gather” such very wide spectral power and concentrate it into detectable pulses. In addition, UWB systems have somewhat of a “bad reputation” because they at least have the potential to cause interference. A heated discourse has gone on for years over the potential that UWB systems can cause interference to legacy spectrum users. 
     Tri-State Integer Cycle Modulation (TICM) and other Integer Cycle Modulation techniques, which have now become known by its commercial designation, xMax, were designed by the inventor of this application to help alleviate this massive and growing problem. Its signal characteristics are such that absolute minimal sideband energy is generated during modulation but that its power spectrum density is quite wide relative to the information rate applied. Also, a narrower section of the power spectrum output can be used to represent the same information. The technique of power saving coding disclosed herein is primarily applicable to these types of integer cycle and pulse modulation systems. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention disclosed in this application uses any integer cycle or impulse type modulation and more particularly is designed to work with a method of modulation named Tri-State Integer Cycle Modulation (TICM) which has been previously disclosed in U.S. Pat. No. 7,003,047 issued Feb. 21, 2006, filed by the inventor of this disclosure and is now know by its commercial designation, xMax. Pulse modulation is used in many forms and generally consists of a pulse of radio energy that can be as simple as On-Off Keying (OOK) to more complex systems like Pulse Position Modulation (PPM) and even more advanced systems such as xMax. The present invention outlines an improved method of substantially reducing average transmit power by omitting transmission of a majority of pulses. 
     For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in connection with the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be made to the accompanying drawings, in which: 
         FIG. 1  is a representation of a transmit pulse train; 
         FIG. 2  is a representation of the coding constellation; and, 
         FIG. 3  is a table showing the special coding. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention disclosed in this application is a modified form of run length coding, which is normally used to compress data as is well known by those skilled in the art. The invention of this disclosure is to use this modified run length coding to reduce signal power in RF transmission systems. By lowering the average power, an opportunity is presented to either:
     1. Decrease the total power drawn by the transmitter (battery life).   2. Raise the peak power of all symbols (Eb/No), thereby increasing range.   

     The amount of average transmitted power reduction will depend entirely upon the bit pattern to be transmitted. Coding systems can also be employed that actually encourage the formation of long strings of repeating data. This would further reduce the average power because more symbols are suppressed. 
     In the preferred embodiment of the invention the design rules are as follows:
         1. Modulation is bi-phase, the same as used in Binary Phase Shift Keying (BPSK), except as noted.   2. Non-repeating binary patterns use normal BPSK parameters, with standard phase thresholds of “0” degrees.   3. Repeating binary patterns are specially coded for power saving.
           a. Power saving coding is defined as:
               i. When a repeating binary pattern is anticipated, a 4-QAM (Quadrature Amplitude Modulation) based coding system will indicate starting and ending suppressed symbols.   ii. Subsequent repeating symbols are not transmitted at all.   iii. Bits between the suppressed string starting symbol and the ending symbol are not detected. They are assumed.   iv. There are four repeating binary patterns that are candidates for special coding. These are:
                   1. Repeating 1&#39;s   2. Repeating 0&#39;s   3. Repeating alternating 1&#39;s and 0&#39;s   4. Repeating alternating 0&#39;s and 1&#39;s   
                   v. Upon detection of the suppressed string ending symbol, the system returns to normal BPSK.   
               
               

     Thus the system of the preferred embodiment uses a hybrid BPSK/4-QAM modulation method. It is well known that 4-PSK and 4-QAM both require 3 db more power to deliver the same (Bit Error Rate) BER as BPSK (2PSK). Increasing the power by 3 db or more when the system enters the special coding mode (4PSK) will ensure a BER consistent with BPSK. 
     When transmitting in the normal mode, BPSK is used. BPSK is well understood and no discussion of that mode is warranted here as it is well known to those skilled in the art. 
     Transmission of a repeating binary pattern as shown in  FIG. 1  provides an opportunity to signal such impending pattern to the receiving end and to cease transmission altogether until the pattern is ended. At the receiving end, the reception of the repeating pattern is simply assumed until a specially coded symbol is received to signify the end of the pattern. One binary value is assigned to each symbol clock period, according to the repeating pattern that has been signaled. 
     The receiver will know to switch from BPSK mode to power saving coding when the symbol received contains a phase shift of 90 degrees PLUS an amplitude shift as shown in  FIG. 2 . It is the combination of both the phase shift AND the amplitude shift (QAM) that will indicate a special coding (suppressed bits) string has begun or ended. 
     When the correct QAM code arrives, the receiver will end the power saving coding mode and return to BPSK. 
     Four possible repeating patterns could be signaled to the receiver. They are: 
     a. Repeating 1&#39;s 
     b. Repeating 0&#39;s 
     c. Repeating 1&#39;s and 0&#39;s 
     d. Repeating 0&#39;s and 1&#39;s 
     Only reception of the correct ending signal will conclude the special coding. A table showing these coding patterns is shown in  FIG. 3 . 
     The opportunity then is available to raise the peak power of all bits in a ratio of the number of bits suppressed to the number of bits not suppressed. The peak power output can be controlled in response to the ratios presented to the coding system to maintain average power below the maximum dictated by law. Raising the Peak-to Average Power Ratio (PAPR) increases the Eb/No of each bit. This in turn improves the BER and increases the range of the system. 
     The objective of this invention is to create a coding system as good or better than BPSK. The un-coded symbols are BPSK, so they will have BPSK properties. The coded symbols are 4-QAM. It is well known that 4-QAM has the same BER performance as BPSK when the symbol power is 3 db higher, albeit at a higher power level. Therefore assuming that the BPSK coded symbols are as detectable as the coded symbols, which are transmitted at 3 db more power level than the BPSK symbols, all symbols are equally detectable and no change in the overall BER is expected. 
     Given that the BER curve is plotted with reference to Eb/No, and that this system uses symbols that are of two different modulation methods, the overall Eb/No value on the plot must take into account the average Eb/No of all symbols. Some of the symbols are not transmitted at all. Therefore, they have no bit energy at all. In reality, one must take the Eb/No of the specially coded symbols and divide that power amongst the symbols that have no power at all. This causes the total power of the starting and ending specially coded (4-QAM) symbols to be divided equally amongst themselves and all suppressed symbols. If there are two suppressed bits, then dividing the power of two bits with double power into two bits of no power will result in an Eb/No exactly equal to BPSK. 
     If there are more than two suppressed symbols, the power of the QAM coded symbols will be divided by more than two suppressed symbols. This results in an energy per bit that is actually less than the Eb/No of the BPSK coded symbols. When some of the symbols are actual BPSK and some of the symbols, which are as equally detectable as BPSK, contain less energy than the BPSK symbols, the overall performance exceeds BPSK. The amount by which BPSK performance is exceeded is entirely dependent upon the binary pattern of the data stream. 
     Since certain changes may be made in the power saving coding disclosed herein without departing from the scope of the invention herein involved, it is intended that all matter contained in the description thereof or shown in the accompanying figures shall be interpreted as illustrative and not in a limiting sense.