Data transmission apparatus with error computer and maximal likelihood calculator

Abstract: A data transmission apparatus with an improved error-rate characteristic after soft decision decoding by improving a decoder input such that, according to an embodiment, digital data such as voice data transmitted after being applied with coding and modulation are received, and are demodulated by a demodulator (16) as a working mode thereof. A square mean value of errors is obtained by an error calculator (17) on a demodulator output (X), and (1 - e -2x/σ2 ) / (1 + e -2x/σ2 ); is computed by a likelihood calculator (18) and adopted as a decoder input (Y).

Description: BACKGROUND OF THE INVENTION The present invention relates to a data transmission apparatus utilized in a digital mobile radio communication equipment and the like. Fig. 1 is a diagram showing a structure of a conventional data transmission apparatus. In Fig. 1, a reference numeral 1 represents a signal source for outputting transmission data such as voice data, 2 represents an encoder performing error correction coding on the above-mentioned transmission data, 3 represents a modulator for modulating the transmission data into signals for transmission, and 4 represents a transmitting antenna for radiating transmission signals. 5 represents a receiving antenna for receiving transmission signals, 6 represents a demodulator for demodulating received signals, 7 represents an amplitude limiter for limiting the amplitude of received signals and 8 represents a decoder for obtaining decoded data. Next, the operation of the above-mentioned conventional example will be described. In Fig. 1, the digital data outputted from the signal source 1 are code-converted by error correction coding such as convolutional coding in the encoder 2, applied with modulation such as frequency-shift keying (FSK) and phase-shift keying (PSK) by the modulator 3, and transmitted from the transmitting antenna 4. The transmitted signals are received by the receiving antenna 5 and demodulated by the demodulator 6. Here, a value at an identification point is outputted as it is from the demodulator 6 as a demodulator output X. In the amplitude limiter 7, in order to perform decoding using Euclidean distance such as Viterbi decoding (which means correction of errors produced in a transmission line by releasing coding coded in the encoder 2) with soft decision by the decoder 8, a demodulator output X is limited within a range from 1 to -1 as shown in Fig. 2 so as to be inputted to the decoder 8 as a decoder input Y. The output of the decoder 8 is processed as decoded data. In the above-mentioned conventional data transmission apparatus, it is possible to correct bit errors in digital signals by coding with the encoder and decoding with the decoder and decoding is performed in the decoder with soft decision. Thus, there is an advantage that error correction capacity is also higher as compared with a case of hard decision. In the above-mentioned data transmission apparatus, however, it is only made to limit the demodulator output X as the decoder input Y by the amplitude limiter 7 between 1 and -1. Accordingly, the error rate characteristic is improved when soft decision is made by the decoder 8 as compared with a case of hard decision, but the results are still not satisfactory. U.S. 4309772 describes a decoder which carries out soft decoding by giving a relative error probability to each detected bit according to a discriminate noise level. U.S. 4322848 describes a reliability weighting for an analog threshold detector. SUMMARY OF THE INVENTION It is an object of the present invention to provide a soft-decision capability. According to the present invention there is provided a data transmission apparatus comprising a transmitting side and a receiving side, said transmitting side including an encoder for carrying out error correcting coding of data to be transmitted and a modulator for modulating coded data so as to produce a signal suitable for transmission, and said receiving side including a demodulator for demodulating a received signal, an error calculator, a reliability calculator for calculating a reliability value from the output signals of the error calculator and the demodulator, and a decoder for carrying out soft decision decoding on the reliability value of the reliability calculator, characterised in that said error calculator is arranged to calculate for every one frame a mean square value σ 2 (sigma 2 ) of Gaussian Noise by using the output signal level of the demodulator and the sign thereof, and said reliability calculator is arranged to calculate for each output value of the demodulator at each identification point a reliability value in accordance with the following formula (1- e -2 x /σ 2 ) (1+ e -2 x /σ 2 ) . The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Fig. 1 is a schematic block diagram showing a structure of a conventional data transmission apparatus. Fig. 2 is a graph showing a method of calculation in an amplitude limiter in a conventional data transmission apparatus. Fig. 3 is a schematic block diagram of a data transmission apparatus according to the present invention. Fig. 4 is a graph showing a method of calculation in a likelihood calculator of the apparatus. Fig. 5 is a diagram showing signals in one frame in the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 is a diagram showing a structure of an embodiment of the present invention. In Fig. 3, a reference numeral 11 represents a signal source for outputting transmission data such as voice data, 12 represents an encoder performing error correction coding on the transmission data, 13 represents a modulator for modulating the transmission data into codes for transmission, and 14 represents a transmitting antenna radiating the transmission signals. Further, 15 represents a receiving antenna for receiving the transmitted signal, 16 represents a demodulator for demodulating the received signal, 17 represents an error calculator for calculating mean square of errors against the output of the demodulator 16, 18 represents a reliability calculator calculating for making the effect of soft decision decoding the maximum probability-wise, and 19 represents a decoder for obtaining decoded data. In the next place, the operation of the above-mentioned embodiment will be described. In the above-mentioned embodiment, the digital data outputted from the signal source 11 are code-converted by error correction coding such as convolutional coding in the encoder 12, applied with modulation such as FSK and PSK in the modulator 13, and transmitted from the transmitting antenna 14. The transmitted signal is received by the receiving antenna 15 and demodulated by the demodulator 16. Fig. 5 shows an example of the waveform of a signal in one frame. A waveform T shown with a broken line shows the output of the modulator 13, and a waveform S shown with a continuous line shows a signal when the signal shown with the waveform T is transmitted and inputted to the demodulator 16 through a transmission line. Here, values at identification points (A 1 , A 2 , ..., A 10 shown in Fig. 5) are outputted as they are from the demodulator 16 at each of these identification points as the demodulator output X, respectively. In the error calculator 17, the demodulator output is decided to be 1 in case it is larger than 0 and to be -1 in case it is smaller than 0, and mean square value σ 2 of the difference between the value (1 or -1) and the demodulator output X is calculated for every one frame. On the other hand, the reliability calculator 18 computes the reliability at each identification point against the demodulator output X on the basis of the mean square value σ 2 obtained by the error calculator 17, and outputs a decoder input Y as shown in fig. 4. In the decoder 19, soft decision decoding such as Viterbi decoding is performed using this decoder input Y, thereby to correct errors produced in the transmission line. Next, a method of computation in the reliability calculator 18 will be described while referring to Fig. 4 and Fig. 5. When transmission is performed without distortion, the demodulator output X maintains the waveform T as it is and always shows 1 or -1 because of digital transmission. However, in case the signal which should be at 1 practically shows Gaussian distribution with 1 as the center in the demodulation output X due to distortion, noise and the like in the transmission line, the probability that the demodulator output becomes x from 1 is expressed as: P 1 (x) = 1 2π • σ • e -(x-1) 2 /2σ 2 Further, the probability that the demodulator output X becomes x from -1 is expressed as: P -1 (x) = 1 2π • σ • e -(x+1) 2 /2σ2 When the probability at 1 to 0 that x is at 1 is made to correspond to the amplitude values 1 to -1 in case the demodulator output x is at x, the reliability is expressed as: P(x) = 2P 1 (x) P 1 (x) - P -1 (x) -1 = 1 - e -2x/σ 2 1 + e -2x/σ 2 This reliability shows how close to 1 or -1 the demodulator output x is, and this value becomes the decoder input Y at each identification point. Now, if the distance from such probability is set to the Euclidean distance when soft decision is made by the decoder 19, the most effective error correction in decoding is obtainable in probability terms. Further, if σ 2 changes when interleaving is performed in coding the output from the signal source, the decoder input Y is different even for the same demodulator output X, and the amplitude of that which has smaller σ 2 , i.e., that which has smaller noise or distortion becomes larger. Accordingly, it is possible to make the effect of soft decision of the decoder 19 the highest in probability terms thereby to improve the error rate characteristic by outputting the decoder input Y as shown in Fig. 4 by means of the error calculator 17 and the reliability calculator 18 with the demodulator output X. The modulator and the demodulator shown in the above embodiment may be formed, for example, by SL1640C (or SL1641C) and SL6639 respectively manufactured by GEC PLESSEY SEMICONDUCTORS, and functions of other block elements may be achieved by program controlled DSP (digital signal processor), for example.

Claims: A data transmission apparatus comprising a transmitting side and a receiving side, said transmitting side including an encoder (12) for carrying out error correction coding of data to be transmitted and a modulator (13) for modulating coded data so as to produce a signal suitable for transmission, and said receiving side including a demodulator (16) for demodulating a received signal, an error calculator (17), a reliability calculator (18) for calculating a reliability value from the output signals of the error calculator (17) and the demodulator (16), and a decoder for carrying out soft decision decoding on the reliability value of the reliability calculator, characterised in that said error calculator (17) is arranged to calculate for every one frame a mean square value σ 2 of Gaussian Noise by using the output signal level x of the demodulator and the sign thereof, and said reliability calculator is arranged to calculate for each output value of the demodulator at each identification point a reliability value in accordance with the following formula (1- e -2 x /σ 2 ) (1+ e -2 x /σ 2 ) . Apparatus according to claim 1, wherein convolutional coding is used in the error correction coding. Apparatus according to claim 1, wherein Viterbi decoding is used in the soft decision decoding.

Inventors: HONMA KOUICHI, TSUBAKI KAZUHISA, UESUGI MITSURU, HONMA, KOUICHI, TSUBAKI, KAZUHISA, UESUGI, MITSURU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1992
Main classification: H04L1
IPC codes: H03M13, H04L25, H04L1