Error Monitor Circuit

1. Field of the Invention

The present invention relates to an error monitoring apparatus provided in a system for transmitting and receiving data of plural bits as a multi-value signal and, more particularly, to a multi-value signal monitor circuit which utilizes an analog-to-digital converter for regenerating the transmitted data as well as error detection data.

2. Description of the Related Art

Various systems have been previously proposed for transmitting data of plural bits after converting it into a multi-value or multi-level signal. For example, in a digital radio transmission system utilizing quadrature amplitude modulation (QAM), I-channel data and Q-channel data are respectively converted to multi-value signals, two carriers having a phase difference of 90 degrees therebetween are amplitude modulated by the multi-value signals and combined for transmission, the received signal is quadrature-detected by a regenerated carrier which is regenerated from the received signal, the I-channel and Q-channel multi-value signals thus obtained are converted into digital signals from the analog signal to obtain the original data.

In such a transmission system, if the radio link begins to fail due to fading, etc., the error rate for the received data becomes high and if the measured error rate exceeds a specified value, the transmission line or path is switched to a spare transmission line or path or an equalizer is reset to prevent divergence of the equalizer.

One method of measuring an error rate for received data involves transmitting a particular error detection pattern and checking the accuracy of the received pattern on the receive side. However, this method has the following disadvantages:

Transmission capacity is reduced by the size of the particular error detection pattern.

A complicated circuit is required for detecting the error detection pattern.

The error rate is usually small and a long time is taken for a cycle of error rate measurement or a large time constant for integration is required for an accurate measurement and, as a result, error rate threshold crossing detection is delayed.

Another method for measuring the error rate of the received data includes using a plurality of comparators for discriminating quantization levels and other levels of the received signal, and logic circuits for logically calculating outputs of the comparators. The number of times the received signal is displaced from the quantization level at the time of sampling the received multi-value signal is measured as a pseudo error rate and is used as the error rate. However, this method requires many comparators and the necessary logic circuits become more complicated as the number of bits of data to be transmitted increases and the number of quantization levels increases.

Another method of obtaining an error rate for the received data includes performing an analog-to-digital conversion at a normal timing and also at a timing slightly shifted from the normal timing. The number of times the shifted timing sample value is different from that at the normal timing is measured as a pseudo error rate and used as the error rate. However, this method has the following disadvantages:

Two analog-to-digital converters are required for each channel, one for normal timing conversion and one for the shifted timing conversion, so that the pseudo error rate can be detected, thereby the size and cost of the circuit is increased.

Since a rate of deterioration around a quantization level after a shift from the correct sample timing is different depending on the causes of line failure (for example, multi-path-fading by a reflected wave, attenuation of the signal by rain and failure of transmitting device), accurate correspondence to the error rate cannot be obtained.

It is an object of the present invention to provide a multi-value signal supervisory circuit which measures quickly and accurately an error rate using a small scale additional circuit without reducing transmission capacity. It is another object of the present invention to provide for error rate detection without the need for error codes in a transmitted signal. An analog-to-digital converter converts the received signal into more bits than the number of bits of data transmitted. The extra or error detection bits indicate the position of the received signal with respect to an acceptable quantization level range. If the value of the extra bits falls outside the range a pseudo error detection is indicated by a detection circuit. A measuring circuit counts the number of pseudo error detections to produce a pseudo error detection rate.

These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

FIGS. lA and lB are block diagrams of a transmitting apparatus and a receiving apparatus for a 16-value QAM radio transmission system to which the present invention is applied;

FIGS. 2A-2C are waveform diagrams of a 4-value signal in which a high frequency component is not limited or filtered out, a 4-value signal in which the high frequency component is removed and a clock signal, respectively;

FIGS. 3A and 3B are waveform diagrams used for explaining the embodiments of the present invention; and

FIGS. 4A-4G are block diagrams of embodiments of the present invention.

In the transmitting side apparatus 10A of the 16-value QAM radio transmission system of FIG. lA, the digital-to-analog converters 101 and 102 respectively convert 2-bit data DI and DQ of an Inphase-channel (I-ch) and Quadrature-channel (Q-ch) into the 4-value signals shown in FIG. 2A. In FIG. 2A, V.sub.0 -V.sub.3 indicate the quantization levels for the transmitted signals. Low-pass-filters 103 and 104 limit or remove the high frequency component of the 4-value signals and shape the waveforms, as shown in FIG. 2B. A 90.degree. hybrid 105 divides the carrier output, from an oscillator 106 into two carriers having a phase difference of 90.degree. therebetween. The two carriers are applied to mixers 107 and 108, which respectively amplitude modulate the two carriers with the 4-value signals in which the higher frequency components have been limited. The two modulated signals are combined in the hybrid 109 and then transmitted by a transmitter 110 and an antenna 111. All the circuits and other elements 101-111 of FIG. 1A are known to those of ordinary skill in the art and details concerning their operation and construction have been omitted for simplicity.

In the receiving side apparatus 10B of FIG. lB, an incoming signal is received by an antenna 112 and a receiver 113. The received signal is divided into the I-ch and Q-ch signals by a hybrid 114. A 90.degree. hybrid 115 produces two carriers having phase a difference of 90.degree. therebetween from a carrier regenerated by a carrier regeneration circuit 116. The two carriers signals are applied to mixers 117 and 118 respectively, which respectively obtain the original 4-value signals by demodulating the I-ch and Q-ch received signals using the pair of regenerated carriers. The demodulated 4-value signals are similar to the modulated signals on the transmitting side shown in FIG. 2B when the line is normal. That is, if no noise or fading has occurred the demodulated 4-value signals are the same as the original 4-value signals modulated on the transmitting side 10A. Even if the transmission path starts to fail, the signal is equalized by an equalizer 119 as long as the failure is minor and, as a result, the signal is similar to that shown in FIG. 2B. A suitable carrier regeneration circuit and a suitable equalizer are described in the EPC application No. 84306977.4. The demodulated 4-value signals of I-ch and Q-ch are respectively converted by the analog-to-digital converters 120 and 121. The I-ch signal is converted into the 4 bits (b.sub.0 -b.sub.3), while Q-ch signal is converted into bits (b.sub.0 and b.sub.1) respectively at the timing of the rising edge of the clock of FIG. 2C. That is, the accuracy of the sample on the I-ch side is increased by two bits. This can be accomplished by merely using two extra output bits which are available on the A/D converter 120. The respective upper two bits (b.sub.0 and b.sub.1) are output as the decoded data (DI and DQ) and the lower two bits (b.sub.2 and b.sub.3) of I-ch are output to a pseudo error rate measuring circuit 122 as pseudo error detection bits. The pseudo error rate measuring circuit 122 indicates a pseudo error when the lower bits (b.sub.2 and b.sub.3) indicate a range different from the quantization levels (V.sub.0 -V.sub.3) and measures a frequency of the pseudo error as the pseudo error rate (err). The circuits and elements 112-121 of FIG. lB are known to those of ordinary skill in the art and details concurring their operation and construction have been eliminated for simplicity.

As shown in FIG. 3A, the total range associated with each of the quantization levels (V.sub.0 -V.sub.3) is classified into four subranges by the lower bits (b.sub.2 and b.sub.3). Of these subranges, the subranges indicated by the lower two bits (b.sub.2 and b.sub.3) having values of "00" and "11" are outside the quantization levels as indicated by .gamma. in the figure. When the two lower bits b.sub.2 and b.sub.3 have a value of "00" or "11" a pseudo error has occurred and when bits b.sub.2 and b.sub.3 have a value of "10" and "01" there is no pseudo error. The frequency of entering the pseudo error range is the pseudo error rate.

As shown in FIG. 4A, a pseudo error rate measuring circuit 122 comprises a pseudo error detection circuit 123 which receives the lower 2 bits (b.sub.2 and b.sub.3), produced by the analog-to-digital converter 120, and detects pseudo errors, and a measuring circuit 124 which receives the detected output produced by the pseudo error detection circuit 123 and measures the number of pseudo error detections. The pseudo error detection circuit 123 comprises a first AND circuit 125 which produces an AND of the 2 bits (b.sub.2 and b.sub.3), inverters 126 and 127 which respectively invert the 2 bits, a second AND circuit 128 which produces an AND of the outputs of the inverters 126 and 127 and an OR circuit 129 which produces an OR of the outputs of the AND circuits 125 and 128, and, as a result, outputs "1" when the lower 2 bits are "00" or "11". The measuring circuit 124 comprises a counter 130 which counts "1"s output by the pseudo error detection circuit 123, and a timer 131 which causes the counter 130 to output a count value periodically and also resets the counter 130. The value output by the counter 130 at the end of a constant period is the pseudo error rate. If the convergence to the quantization level deteriorates due to line or fading failure, the lower 2 bits of the analog-to-digital converter 120 increasingly become "00" or "11" and it is sufficient if the specified line switching processing is carried out when the count value exceeds the predetermined threshold value.

As shown in FIG. 4B, a pseudo error detection circuit 123 can be comprised of a single exclusive OR circuit 132. However, since the exclusive OR circuit 132 outputs "0" when the lower 2 bits (b.sub.2 and b.sub.3) are "00" or "11", the counter 130 of the measuring circuit 124 is required to count the number of times of the counter 130 input is "0" or a NOT circuit must be connected between the output of the exclusive OR circuit 132 and the counter 130.

As shown in FIG. 4C, the measuring circuit 124 may alternately comprise an integration circuit including a resistor R and capacitor C and the pseudo error rate can be output as an analog value. When a pseudo error detection is input as a high voltage level, the difference between integration output and a low voltage level is the pseudo error rate, while if the pseudo error detection is input as a low voltage level, the difference between integration output and a high voltage level is the pseudo error rate.

As shown in FIG. 3B, when the pseudo error detection decision is made using 3 bits which is more than the number of data bits, where the 3 bits b.sub.2, b.sub.3 and b.sub.4 are used for pseudo error detection, it is possible to set the pseudo error detection range narrower as indicated by .gamma..sub.1 or wider as indicated by .gamma..sub.2.

As shown in FIG. 4D, the pseudo error detection circuit which detects the range .gamma..sub.1 can be composed of an AND circuit 133 which produces an AND of the 3 bits (b.sub.2 -b.sub.4), a NOR circuit 134 which produces a NOR of the 3 bits and an OR circuit 135 which produces an OR of the outputs of the AND circuit 133 and NOR circuit 134. This error detection circuit 123 outputs "1" when the 3 bits are "b 000" or "111".

As shown in FIG. 4E, the pseudo error detection circuit 123 which detects the range of .gamma..sub.2 can comprise an inverter 136 which inverts the most significant bit (b.sub.2) among the 3 bits, an AND circuit 137 which produces an AND of the output of inverter 136 and the other 2 bits (b.sub.3 and b.sub.4), a NOR circuit which produces a NOR of the output of inverter 136 and the other 2 bits (b.sub.2 and b.sub.3) and a NOR circuit 139 which produces a NOR of the output of the AND circuit 137 and the output of the NOR circuit 138. This detection circuit 123 outputs "1" when the 3 bits are not "011" and "100".

If it is necessary to set the range of pseudo error detection smaller, additional bits can be connected from the A/D converter 130 to the detection circuit 123.

In FIG. lB, only the I-ch signal is monitored, however, if the Q-ch signal must also be monitored, the monitoring can be accomplished by providing an analog-to-digital converter and pseudo error rate measuring circuit for the Q-ch which are the same as those for the I-ch. When the I-ch and Q-ch are simultaneously monitored, the pseudo error detection circuit 123, as shown in FIG. 4F, comprises AND circuits 140 and 141 which produce an AND of 2 bits (b.sub.2 and b.sub.3) for detecting pseudo errors of the I-ch and Q-ch, NOR circuits 142 and 143 which produce a NOR of the respective 2 bits and an OR circuit 144 which produces an OR of the outputs of the AND circuits 140 and 141 and the outputs of the NOR circuits 142 and 143. Similar logical operations can be also be accomplished by the circuit 123 shown in FIG. 4G. The pseudo error detection circuit 123 of FIG. 4G comprises exclusive OR circuits 145 and 146 which produce an exclusive OR of 2 bits of the I-ch and Q-ch and a NAND circuit 147 which produces a NAND of the outputs of the exclusive 0R circuits 145 and 146. The circuit of FIG. 4G outputs "1" when 2 bits of I-ch are "00" or "11" or when 2 bits of Q-ch are "00" and "11". This output is input to the measuring circuit 124.

In these embodiments, the present invention is applied to the 16-value QAM radio transmission system but can also be adapted to any system where data is transmitted from the transmitting side after it is converted to a multi-value signal and the data is obtained in the receiving side through an analog-to-digital conversion.

As explained above, the present invention samples the data more accurately than the number of bits of data transmitted using a plurality of bits in the receiving side and detects pseudo errors from the plurality of bits. As a result a pseudo error can be detected with a simplified additional circuit and the circuit is not complicated even if the number of bits of data is increased. Moreover, an accurate error rate can be realized because data is monitored only at the normal timing of the data samples. Where the range of pseudo error detection is properly set, the frequency of generating pseudo errors will be adequate and the pseudo error rate can be detected quickly. In addition, the transmission capacity of the transmission path is not reduced because it is not necessary to transmit a particular error detection pattern used solely for error rate detection.

The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the circuit which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.