Abstract:
A technique for enhancing the signal-to-noise performance of a digital communication link without affecting its power flux density. An information data stream has its original signaling rate selectively reduced prior to transmission, to enhance signal-to-noise performance. Then the reduced signaling rate information signal is combined with a pseudorandom data sequence at the original signaling rate, to provide a randomized data sequence to be transmitted at the original signaling rate, thereby maintaining power flux density levels below those permitted by regulatory limits.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to digital communication systems and, more particularly, to satellite communication systems in which information is transmitted from one point to another on or near the earth&#39;s surface, by way of one or more satellite-based space stations. The power flux density (PFD) of each uplink beam to a satellite and of each downlink beam from a satellite is limited to a prescribed maximum value by governmental regulation, specifically by the Federal Communications Commission (FCC) in the United States and by international and other agencies in other countries. The power flux density is defined as the measured power, in watts per square meter per hertz of spectral bandwidth. A related measure of radiated power density is the power spectral density (PSD), which is defined as the measured power per hertz of bandwidth.  
         [0002]     For most efficient operation, most communication systems operate at at or close to the maximum PFD limit. When operating in this condition, the system signal-to-noise ratio (SNR) cannot be increased either by decreasing the signaling rate or increasing the total transmitted power without exceeding the PFD limit.  
         [0003]     There is sometimes a need to compensate for attenuation by rain on the path from the satellite to the ground receiver. If the rain attenuation is uniform over the entire beam, the decreased SNR can be mitigated by reducing the signaling rate in proportion to the increase in attenuation, and the PFD at the surface of the earth will not exceed the PFD limit. If, however, the rain is localized so it affects only a portion of the beam, compensating for rain attenuation by decreasing the signaling rate will result in the PFD limit being exceeded in those regions unaffected by rain.  
         [0004]     It will be appreciated from the foregoing that there is need for a technique that will allow reducing the data rate to compensate for rain attenuation without affecting the power flux density of the beam. The present invention is directed to this end.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is concerned with a technique for reducing the signaling rate of a communication channel without a corresponding increase in the power flux density. In brief, the method of the invention comprises selecting a reduced signaling rate that is a fraction of an original signaling rate R; randomizing the reduced signaling rate signal to produce a signal at the original signaling rate R; and transmitting the randomized signal. The reduced signaling rate results in an enhanced signal-to-noise ratio per bit of information, and transmission at the full signaling rate ensures the power flux density will be unchanged.  
         [0006]     In terms of digital communication apparatus, the invention may defined as comprising means for reducing the rate of an information data stream to be transmitted from an original signaling rate R to a selected reduced rate; a pseudorandom noise source generating a stream of practically random data at the original signaling rate R; means for logically combining the reduced signaling rate information data stream and the data stream from the pseudorandom noise generator; and means for transmitting the logically combined data stream at the original signaling rate. Signal-to-noise performance is thereby enhanced to compensate for rain attenuation without increasing the power flux density.  
         [0007]     It will be appreciated from this brief summary that the invention provides a significant improvement over the prior art since enhanced signal-to-noise performance can be obtained by reducing the signaling rate, but without affecting the power flux density. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram depicting the technique of the invention for reducing the signaling rate without affecting the power flux density of the corresponding transmitted signal.  
         [0009]      FIG. 2  is a diagram showing the signal format for a signaling rate reduction by a ratio of 4:1 in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     As shown in the drawings for purposes of illustration, the present invention is concerned with a method, and corresponding apparatus, for reducing the information data rate transmitted through a communication channel, without affecting the power flux density of the transmitted signals. As discussed above, government regulations impose an upper limit on the power flux density (PFD) of radio-frequency signals, and these limits apply to transmission from space-to-Earth and from Earth-to-space. When there is a need to compensate for rain attenuation on satellite downlinks by decreasing the signaling rate, in general it will not be possible to do so without exceeding the PFD limit for some areas of the beam coverage. By way of further background, this problem is further discussed in the following paragraphs.  
         [0011]     In digital communication systems, an important performance characteristic, signal-to-noise ratio (SNR) is typically represented by the ratio E b /N 0 , where E b  is the energy per bit, or more specifically the total power of the transmitted signal divided by the bit rate, and N 0  is the noise power spectral density, or more specifically the noise power in each hertz of bandwidth. Thus the ratio E b /N 0  can be expressed as P T /(N 0 ·R), where P T  is the total power of the transmitted signal. If there is a need to increase the E b /N 0  ratio, this may only be done either by increasing the transmitted signal power or reducing the data rate R. The noise density N 0  is substantially constant, and the total power P T  cannot be increased without increasing the power flux density. Although reducing the data rate R will increase the E b /N 0  ratio, the power flux density (PFD) will also be correspondingly increased because reducing the signaling rate decreases the occupied bandwidth, and the PFD is measured on a power-per-bandwidth basis; hence reducing the data rate increases the PFD.  
         [0012]     A practical example of the need for increasing the SNR (or E b /N 0 ) ratio occurs when there is rain between a satellite transmitter and the ground. The rain will attenuate the downlink signal and decrease the received SNR. By way of example, suppose the rain attenuation were 10 dB (decibels). Without some change in the operation of the transmitter, this attenuation would significantly increase the bit error rates at the ground-based receiver. Ideally, it would be desirable to increase the SNR by either increasing the transmitter power or decreasing the signaling rate in proportion to the rain attenuation. Unfortunately, the diameter of the transmitted beam is typically significantly larger than a rain cell. For those locations on the ground for which the beam is attenuated by rain, the PFD will remain essentially unchanged; however, for those areas on the ground not experiencing rain attenuation on the path between the satellite and the receiver, the PFD will increase by 10 dB and violate the regulatory limit on PFD.  
         [0013]     In accordance with the present invention, the information data rate is decreased without any corresponding increase in the PFD. As shown in  FIG. 1 , digital data at its maximum rate (R) bits per second is subject to data rate control as indicated in box  10 , which reduces the information data rate, optionally, to a selected fraction, 1/n, of the full data rate R. Reduction of the data rate can be achieved by any desired means, such as using data buffers to store the data and then read a data stream out from the buffers at the desired lower rate, on line  12 . It will be understood that the data rate control  10  may also be operated to provide output on line  14  at the full data rate. The reduced rate data stream on line  12  is input to a randomizer  16 , which also receives as input, over line  18 , a randomly varying stream of bits at the full data rate. By way of example, the stream of bits on line  18  may be generated by a pseudorandom noise, or pseudo-noise (PN), source  20 . As is well known, a pseudorandom sequence is a sequence of bits generated using a mathematical algorithm, such that the elements of the sequence are practically independent and, for most purposes, the sequence may be considered to be random.  
         [0014]     By way of example, the randomizer  16  may be simply an exclusive OR (XOR) gate whereby successive elements of the rate-R PN sequence are logically combined with the information data bits on line  12 .  FIG. 2  shows this process diagrammatically. The information data stream at a rate of R/4 is shown at  24  and the PN code sequence at rate R is shown at  26 . The result of the randomizing step is an encoded sequence  28  at the full rate R, containing information data at the reduced R/4 rate. The latter data sequence is input to a transmitter  30 , where the sequence is used to modulate a carrier in a conventional manner, such as by binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK). The resulting signal is transmitted as indicated by the broken line  32  to a receiver  34 . If the original input data stream was transmitted at the full rate R, the receiver  34  will generate output signals at the full data rate on line  36 . When the data sequence is transmitted at the reduced information data rate R/4, additional data recovery processing is required, as indicated in block  38 . In this step of data recovery, a local PN source  40  is used to recover the original R/4 rate data stream from the received signals, as indicated on output line  42 . It will be readily understood that the local PN source  40  must be synchronized with the PN source  20  at the transmitter end of the communication channel. Also omitted from the illustration are conventional means for demodulating the received signals, including means for generating a local carrier signal at the receiving end, and means for synchronizing the local carrier signal with the received signals.  
         [0015]     The effect of transmitting at the reduced information data rate, such as R/4, is that the ratio E b /N 0  is increased by a factor of four because reduction in the information data rate increases the energy per bit, E b . However, the power flux density is unchanged because the randomized bit stream is still being transmitted at the full signaling rate. Randomizing the data stream has the effect of spreading the signal power over a broad spectrum. Data recovery at the receiver includes despreading the signal to recover the reduced data rate information signal.  
         [0016]     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of digital communication systems. In particular, the invention provides a technique for reducing the information data rate, and therefore increasing the signal-to-noise ratio associated with a transmitted beam, without affecting the power flux density of the beam. It will also be appreciated that, although a specific embodiment of the invention has been illustrated and described by way of example, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.