Digital communication apparatus

Abstract: A digital communication apparatus includes a transmitter section, a receiver section, and a controller. In the transmitter section, an error detection encoder for adding a check bit to a signal to be transmitted is arranged on the input stage of an error correction encoder. The receiver section includes an error detection decoder arranged on the output stage of an error correction decoder, and a gate circuit for receiving outputs from a unique word detector and the error detection decoder, and for outputting a signal informing that a unique word is detected and no bit error is present. The controller controls the operation timing of the transmitter and receiver sections. The controller also controls the unique word detector such that its synchronized state is kept as long as an error is not detected even if a unique word is not detected.

Description: The present invention relates to a digital communication apparatus and, more particularly, to a digital communication apparatus having a reception function for detecting, validating, and monitoring reception information. In a digital communication system wherein a large number of stations use an identical carrier wave to interchange information, each station receives a unique word transmitted from a predetermined station at a predetermined period to set its own transmission/reception timing, and transmits information signals with the unique word at the set transmission timing, or receives information signals with a unique word at the set reception timing. In this case, in order to improve reliability of information transfer, the information signals except for the unique word are encoded into data having optimal redundancy with respect to the unique word. Such a digital communication apparatus is disclosed in JP-A-260-227542 MIYAUCHI and NOSAKA, "Digital Satellite Communication", pp. 76 77 published in Japan, and Dr. Kamilo, et al., "Digital Communication", pp. 375 - 377 published in the U.S.A. However, various troubles may occur in this system because of degradation in quality of communication channels or the influence of thermal noise in the system. For example, even if a received unique word is correct, it cannot be detected (to be referred to as mis-detection hereinafter). A bit pattern generated from a bit error included in a reception signal is erroneously detected as a bit pattern of a correct unique word (to be referred to as false detection). Although a unique word is correctly detected, an error caused in a reception signal other than the unique word exceeds an error correction performance (i.e., degradation in error correction performance). In such cases, various problems are posed in the above-described digital communication apparatus as follows. In the conventional apparatus, if miss-detection of a unique word is caused, a reception signal is not fetched or not regarded as valid even if it is fetched because a transmission/reception timing cannot be set. In this apparatus, a synchronization operation is repeated. For this reason, this mis-detection of a unique word tends to continuously occur, and hence necessary information cannot be obtained. Since a unique word detection timing is often normal even in the mis-detection period, a demand has arisen for an improvement in this respect. In such a digital communication apparatus, an identification code of the own station included in a reception signal is detected, thereby fetching necessary information of the station such as device control information or state monitoring information. However, in false detection, since a unique word is detected at a timing different from a correct timing, a detection timing of a station identification code differs from the correct timing, so that a bit pattern located at the wrong timing position may be recognized as the station identification code. In the case of degradation in error correction performance, the precision of a reception information signal is low. However, since a unique word is correctly detected, the reception information signal can be fetched as long as a station identification code is confirmed. That is, in the conventional digital communication apparatus, when a false detection or degradation in error correction performance is caused, an erroneous reception information signal is fetched. EP-A-195 421 discloses a synchronization circuit for digital communication in which the synchronization judgement is based on synchronization words and on the correctability of the data words. EP-A-196 647 establishes frame synchronization at the receiving end without inserting a frame synchronization word into the transmitted data. It is an object of the present invention to provide a digital communication apparatus, which can process a reception information signal as valid when its content is correct even if a mis-detection of a unique word is caused, and does not process a reception information signal as valid if a false detection or degradation in error correction performance is caused. This object is solved with the features of the claims. An embodiment of the present invention will be described with reference to the accompanying drawings, in which Fig. 1 is a block diagram showing an arrangement according to an embodiment of the present invention. Referring to Fig. 1, the digital communication apparatus comprises a transmitter 1, a receiver 2, and a synchronization/timing controller 3 for controlling the operations of the transmitter 1 and the receiver 2. The transmitter 1 includes an error detection encoder 11 and an error correction encoder 12. The error detection encoder 11 adds an error detection check bit for detecting a code error to an information signal S IN to be transmitted, which is input through a line 4, in accordance with a control signal S₃₁ from the controller 3 so as to generate an information signal S₁₁ constituted by, e.g., a cyclic code. An error detector using a cyclic code is described in detail in YAMAMOTO, "Data Communication", pp. 82 - 107. The error correction encoder 12 receives the information signal S₁₁ to which the error detection check bit is added by the error detection encoder 11. The error correction encoder 12 performs an error correction encoding process in accordance with a control signal S₃₂ from the controller 3 so as to add redundancy based on a predetermined rule to the information signal S₁₁, and transmits the resultant signal to a channel 5 as a transmission signal S T . The receiver 2 includes a gate circuit 21, a unique word detector 22, an error detection decoder 23, and an error correction decoder 24. A reception signal S R input from another station through a transmission path 10 is input to the error correction decoder 24 and the unique word detector 22. The error correction decoder 24 performs an error correction decoding process of the reception signal S R in accordance with a control signal S₃₅ from the controller 3 to restore a reception information signal S₂₄ corresponding to the information signal S₁₁ from the source station. Then, the error correction decoder 24 transmits the reception information signal S₂₄ to one input of a data gate circuit 25 and also outputs it to the error detection decoder 23. The error detection decoder 23 compares a check bit obtained upon checking of an error in the reception information signal S₂₄ with the error detection check bit added in the transmitter side to determine coincidence/non-coincidence therebetween, and outputs a code error detection information signal S₂₃ to the controller 3, one input terminal of the gate circuit 21, and to the other input of the data gate circuit 25 only when an error is not present. With this processing, an error is detected in units of bits. An output S₂₁ of the gate circuit 21 allows to gate the reception data, i.e., the output S₂₄ from the error correction decoder 24 if synchronization is obtained and an error is not detected. The unique word detector 22 detects a unique word in the reception signal S R in the following manner. A unique word is constituted by a given bit pattern. However, the bit pattern of a unique word may be changed into various forms because of degradation in quality of channels or thermal noise in the system. As a result, a unique word constituted by a correct bit pattern may not be obtained. For this reason, an allowable range is set with respect to the number of bit errors caused in a bit pattern in a unique word. Each bit of a reception signal is compared with a corresponding bit of a correct unique word. When the number of non-coincident bits of a bit pattern falls within the allowable range, it is determined that a unique word is received. When the unique word detector 22 detects a unique word, it transmits a signal S₃₃ representing the timing of the unique word detection and a signal S₂₂ representing the detected unique word to the controller 3 and the other input terminal of the gate circuit 21, respectively. The gate circuit 21 transmits a reception signal detection validation information signal S₂₁ informing that a correct unique word is detected and a code error is not present to the device of the next stage when conditions of the both inputs are satisfied. The controller 3 receives the unique word detection timing signal S₃₃ from the unique word detector 22, and generates timing signals S₃₁ and S₃₂ to be respectively supplied to the error detection encoder 11 and the error correction encoder 12 of the transmitter 1. The controller 3 is designed to continuously transmit unique word detection timing information signals S₃₄ to the unique word detector 22 in response to the code error detection information signal S₂₃ from the error detection decoder 23 even if the unique word detection timing signal S₃₃ from the unique word detector 22 is not input to the controller 3. Even if a mis-detection of a unique word is caused and the unique word detection timing signal S₃₃ cannot be transmitted, the unique word detector 22 keeps the apparatus in a synchronized state as long as the unique word detection timing information signals S₃₄ are continuously supplied. In addition, the controller 3 supplies a timing signal S₃₆ for performing error detection decoding to the error detection decoder 23. Additionally, the conventional apparatus does not include the error detection encoder 11, the error detection decoder 23, and the gate circuit 21. An operation of the circuit shown in Fig. 1 will be described below. In the transmitter 1, the error detection encoder 11 adds a check bit to the information signal S IN to be transmitted. Then, the resultant signal is subjected to error correction encoding in the error correction encoder 12 and is transmitted to the channel 5 as the transmission signal S T . In the receiver 2 for receiving signals transmitted from another station, the unique word detector 22 detects a unique word in the reception signal S R . In addition, the error correction decoder 24 performs error correction decoding of the encoded signal in the reception signal S R . Upon reception of a demodulated signal from the error correction decoder 24, the error detection decoder 23 determines the presence/absence of an error in the demodulated signal on the basis of a check bit included in the demodulated signal. According to this error detection scheme, most errors except for certain conditions can be detected in units of bits by utilizing, e.g., a cyclic code. The gate circuit, i.e., informing signal generator 21 generates the signal S₂₁ for informing that detection of a reception signal is confirmed when the unique word detector 22 detects a unique word and the error detection decoder 23 determines the absence of an error. With this operation, the problems of false detection of a unique word and of degradation in error correction performance are simultaneously solved. Furthermore, the synchronization/timing controller 3 basically controls the transmission/reception timing of the apparatus of the present invention in response to the unique word detection, and also controls the unique word detector to allow continuous detection of unique words. That is, the transmission/reception timing cannot be controlled during the mis-detection period of a unique word. In this case, however, the detection timing position of a unique word tends to coincide with that in the detection period. For this reason, according to the present invention, in addition to the basic functions, the synchronization/timing controller 3 is provided with means for controlling the unique word detector 22 so as to be kept in the synchronized state when the error detection decoder 23 determines the absence of an error even if a unique word is not detected at the unique word detection timing described above. As a result, even if a mis-detection of a unique word is caused, the synchronized state is held as long as the reception information signal is correct, and hence the reception information signal can be processed as valid. Therefore, necessary information can be reliably obtained. As has been described, according to the digital communication apparatus of the present invention, since the scheme using both the error correction and error detection functions is employed, a false detection of a unique word or degradation in error correction performance can be detected, thereby preventing a reception information signal having an error from being processed as valid. In addition, in the mis-detection period of a unique word, since the synchronized state can be held as long as an error is not present in a reception information signal, necessary information can be reliably obtained.

Claims: A digital communication apparatus comprising: a transmitter means (1) comprising an error coding circuit comprising an error correction encoder (12) for performing error correction encoding of the information signal to generate an encoded signal as a transmission signal ; and a receiver means (2) comprising; an error decoding circuit comprising an error correction decoder (24) for performing error correction decoding of an encoded signal in a reception signal and outputting a resultant signal as an output from said receiver means, a unique word detector (22) for detecting a unique word in the reception signal in accordance with a unique word detection timing signal, a gate circuit (21) for receiving a signal representing a detected unique word from said unique word detector and a signal representing absence of an error from said error decoding circuit and generating a signal externally informing that detection of a reception signal is confirmed ; and a controller means (3) for controlling a transmission/reception timing of said transmitter (1) and receiver means (2) in response to the signal representing detection of a unique word from said unique word detector (22), and for outputting said unique word detection timing signal when either said unique word detector (22) continuously detects unique words or when a unique word is not detected at the unique word detection timing but a signal representing absence of an error is supplied from said error decoding circuit, characterised in that: the error coding circuit comprises additionally error detection encoder means (11) for adding an error detection check bit to the information signal to be transmitted, the output signal of said error detection encoder means being inputted to said error correction means, and the receiver comprises additionally an error detection decoder (23) for receiving the decoded signal from said error correction decoder and determining a presence/absence of an error in the decoded signal on the basis of the error detection check bit included therein.

Inventors: SONETAKA NORIYOSHI, SONETAKA, NORIYOSHI, SONETAKA, NORIYOSHI C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1988
Main classification: H04L7
IPC codes: H04L7, H04B7, H04L1