Abstract:
An error correcting decoder is disclosed which blocks correction of bits received during periods of relatively high signal intensity levels. A syndrome register and decision circuit provide error correcting bits for all bits which the sequence of input data determines to be in error. But only those data bits which occur during low levels of signal intensity are corrected.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a decoder for decoding a radio frequency carrier wave modulated with data encoded according to an error-correcting coding system. 
     In a wireless communication path wherein signals are subject to fading, code errors are concentrated in the period of lowered electric field intensity of the received signal. So long as the electric field intensity is high, code errors do not occur. However, the conventional error-correcting code decoders (Reference is made, for instance, to the article by R. T. Chien, et al., entitled &#34;Error Correction in a Radio-Based Data Communications System&#34;, published in the IEEE Transactions on Communications, pages 458-462, April 1975) have the disadvantage that due to an error bit having occurred during the period of the lowered field intensity, a bit received during the period of adequate electric field intensity tends to cause an erroneous correction. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a decoder that is free from the aforementioned disadvantage. 
     According to one feature of the present invention there is provided a novel decoder in which bits received during the period of adequate electric field intensity of the received wave are not corrected while only those received during the period of lowered field intensity are corrected thereby to avoid the erroneous code correction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole drawing is a block diagram showing one preferred embodiment of the decoder according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawing, reference numeral 10 designates a receiver (which may be constituted of receiver 400 described in U.S. Pat. No. 3,613,004); 11 and 12, buffer registers; 13, a syndrome calculator; 14, a decision circuit; 15, an adder; and 16, an AND gate. The receiver 10 comprises an RF amplifier 100, a mixer 101, a local oscillator 102, an IF amplifier 103, a limiter 104, a discriminator 105, an amplifier 106, and a noise intensity or electric field intensity detector 107 (including an amplifier 110 and a rectifier 111). An r.f. carrier wave modulated by error-correcting-coded data signal is applied to an input terminal 1. The modulated carrier wave is demodulated into a digital signal in the discriminator 105. The digital signal is fed via output 2 to the buffer register 11, which may, for example, be a shift register, and the syndrome calculator 13. Output 3 of a receiving electric field intensity detector circuit (which may be composed of a rectifier circuit and a voltage comparator) included in the receiver 10 emits an output signal &#34;0&#34; or &#34;1&#34; depending on whether the field intensity is higher or lower than a preset value. The electric field intensity detector output is fed to the buffer register 12 which has the same number of stages as the buffer register 11. The methods for detecting the received electric field intensity may include the generally known method of detecting a rectified output of a carrier wave or detecting a noise output obtained after the demodulation of an r.f. signal. In the former case, the detector circuit is connected, for example, to the output of the intermediate amplifier 440 appearing in FIG. 4 of U.S. Pat. No. 3,613,004. In the latter case, the detector circuit is equivalent to the noise amplifier 610 and the noise rectifier 620 in FIG. 4 of the same U.S. patent. 
     As is well known, when the output of the syndrome calculator 13 is judged by the decision circuit 14 as indicating an error in the received data the decision circuit 14 emits an output signal &#34;1&#34; at the moment when the erroneous part of the received data appears at the output of buffer register 11. At the same time, if the received bit judged to be erroneous by the decision circuit 14 is a bit received during the period of the lowered electric field intensity, then the output signal from the buffer register 12 is &#34;1&#34;, so that the output of the AND-gate 16 is turned to &#34;1&#34;, and thus the output signal from the buffer register 11 is inverted by the adder 15 (for instance, an Exclusive-OR gate or a Modulo-2 adder) to be subjected to error correction, and thereafter it is led to an output 4. Whereas, if the received bit judged to be erroneous by the decision circuit 14 is a bit received during the period of adequate field intensity, then the output signal from the buffer register 12 is &#34; 0&#34; at the moment when the decision circuit provided the output signal &#34;1&#34;, so that the output of the AND-gate 16 is turned to &#34;0&#34;. Thus, the output signal from the buffer register 11 is emitted from the output 4 without being inverted. Consequently, a correct bit received during the period of adequate field intensity if never corrected due to errors having occurred in the preceding and/or following bits. This feature is important especially in an r.f. communication path having abrupt electric field intensity variations. Thus, the present invention contributes to the lowering of the bit error rate. For details of the syndrome calculator 13 and the decision circuit 14, reference is made to the following literature: 
     W. W. Peterson: &#34;ERROR-CORRECTING CODES&#34;, M.I.T. Press March, 1962, pages 183-199.