RADIO DATA TRANSMISSION SYSTEM

Abstract: In a mode switching control for controlling the tap weighing of a transversal automatic equalizer in a radio data transmission system using multivalued amplitude mo­ dulation, the improvement wherein a bit error rat (BER) is found by using pseudo-error pulses, and a threshold value (BER 1) for switching from the normal mode to the reset mode is set to be different from a threshold value (BER 2) for switching from the reset mode to the normal mode, so that the mode switching control can be effected finely and stably with a simple circuit.

Description: RADIO DATA TRANSMISSION SYSTEM TECHNICAL FIELD The present invention relates to a radio data transmission system which realizes multi-level amplitude modulation and more detail to a radio data transmission system comprising a transversal type automatic equalizer which is capable of changing the mode of weighting circuit control signal. BACKGROUND ART In a radio data transmission system, multi-path interference occurs resulting in the fading because the signal waves pass different routes depending on the conditions of air. This fading causes frequency characteristic of radio propagation path to change and thereby generates inter-symbol interference due to waveform distortion. Therefore, an automatic equalizer is necessary for eliminating such inter-symbol interference. As such an automatic equalizer, a transversal type automatic equalizer which controls signals within the time domain is often employed. This transversal type automatic equalizer realizes automatic control of equalizing function by tap control (weighting control) of the weighting circuit thereof. Therefore, efficiency of control is directly related to the control characteristic and it is essential to improve the tap control performance. In addition, it is recent trend that an integrated circuit is often employed for said transversal type automatic equalizer and it is also expected in line with such tendency to develop a circuit structure which helps reduction in size of circuit of tap control system. A transversal type automatic equalizer comprises a delay means with tap such as a shift register which sequentially delays input signals on a delay line, multipliers connected to respective taps, adders which add outputs of multipliers, discriminators which discriminate levels of output signals of adders and control means with weighted taps which detect error signal corresponding to inter-symbol interference, estimates a degree of inter-symbol interference by adding error signals to correlation circuits and controls multipliers. The discriminator discriminates level of added outputs, detects error for signal levels required at the time of discrimination, detects from which pulses at the time of discrimination such error is resulting with a correlation circuit and makes error to zero by controlling the weighting (coefficient of multiplier) for the pulses which give inter-symbol interference. The tap weighting control of the transversal type automatic equalizer is carried out in the following two modes; one is the normal mode for above controls and the other is the reset mode where equalization is not carried out by fixing the weighting of the main taps to the reference values and setting the weighting of the taps other than the main taps to zero. These modes are switched in accordance with the equalizing condition. The mode switching control has been conducted using an accummulated value of errorneous discrimination of frame signals inserted for each frame of transmission data, in place of the predetermined value. However, in the mode switching explained above, the signal which is enough for controlling the mode switching cannot be extracted if the accummulated pulses do not reach a considerably large value and accordingly it is impossible to execute fine control for mode switching and improvement in tap weighting control performance becomes insufficient. Moreover, such mode switching control is followed by a disadvantage that necessity of frame synchronization makes large the scale of tap weighting control circuit. Therefore, it is strategically considered to use a discriminator which discriminates levels of multivalue signals in more detail than the number of bits of the transmission data and outputs the digital signals in such a number of bits as is larger than the number of bits of data in order to always monitor the line condition with simplified additional circuit, to generate pseudo error signal using such extra bits and to execute tap weighting control depending on generating frequency of such pseudo error signals. However, the generating frequency of pseudo error signal is considerably influenced by the equalizing condition because the digital values which are discriminated more detail than the transmission data. Accordingly, it is probable that the generating frequency of pseudo error signal becomes low and it is misjudged that the line condition is rather improved and the reset mode is switched to the normal mode, when the transversal type automatic equalizer resulting in abnormal values of tap weighting due to deterioration of line condition is set to the reset mode attaining the no equalizing condition. In this case, since the line condition is actually not improved, the equalizer is returned again to the reset mode. Namely, the equalizer is placed to the unstable control conditions being switched between the normal mode and reset mode and thereby a problem that judgement of line quality frequently changes arises. A system corresponding to this is disclosed in JP-A-49-2416. Moreover, the transversal type automatic equalizer shows distortion resistance frequency characteristics (signature) which are largely different when fading gradually becomes large causing a bit error rate to increase, or when a bit error rate in the line OFF condi-ion or the equivalent gradually becomes smaller because fading gradually decreases, and shows hysteresis characteristic. Namely, even in the receiving condition of the same bit error rate, if the bit error rate changes to a large value from a small value, the automatic equalization is carried out but, on the contrary, when the bit error rate changes to a small value from a large value, the automatic equalization is not carried out in a certain range. In another known system (EP-A-0 105 503) the reset control is derived from a carrier detection circuit. Loss of the carrier, i.e. asynchronism causes reset of the adaptive equalizer. DISCLOSURE OF INVENTION With the aforementioned problems of the prior art, it is a first object of the present invention to realize fine and stable switching control for control mode of a transversal type automatic equalizer in the radio data transmission system with a simplified circuit and moreover it is a second object of the present invention to reduce hysteresis of distortion resistance frequency characteristic. According to the present invention, there is provided a radio data transmission system comprising, a transmitting apparatus comprising; converting means, which receive n-bit data, for converting the received data to multi-level transmission signal (n is a positive integer), modulating means for modulating carrier with said multi-level signal, and transmitting means for transmitting said modulated signal as a radio signal, a receiving apparatus comprising; means for receiving said radio signal, demodulating means for demodulating said received radio signal and for outputting multi-level signal, transversal filter for having the main tap and other taps, for receiving the weighting signal for controlling weighting of taps and for outputting after equalizing said demodulated multi-level signal, discriminating means for discriminating multi-level signal output from said transversal filter and for outputting the digital signal of n+m (m is a positive integer) bits, tap weighting control means for outputting said weighting signal using at least a part of digital signal among the n+m bits digital signal output from said discriminating means, and having at least one mode of operation, and control mode switching means for switching the control mode of said tap weighting control means in accordance with a pseudo error pulse rate generated using at least a part of lower m bits of n+m bits digital signal output from said discriminating means; characterized in that the control mode switching means are adapted to switch from a first control mode to a second control mode at a first threshold value of the error rate, and to switch from said second control mode to said first control mode at a second, different, threshold value of error rate. The second object is achieved by providing the MLE mode which mainly controls the main tap using the maximum level error (MLE) signal between the normal mode and reset mode. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the transmitting part of the system to which the present invention is adopted. Fig. 2 is a block diagram of the receiving part of the system to which the present invention is adopted. Fig. 3 is a block diagram of a transversal filter. Fig. 4 is a block diagram of a tap weighting control circuit in the first embodiment. Fig. 5 is a block diagram of a control mode switching circuit in the first embodiment. Fig. 6 shows relation between input levels and output bits in a discriminator. Fig. 7 is a graph representing relation between bit error rate and integral circuit output voltage. Fig. 8 shows mode switching between the normal mode and reset mode. Fig. 9 is a block diagram of control mode switching circuit in the second embodiment. Fig. 10 is a block diagram of tap weighting control circuit in the second embodiment. Fig. 11 shows mode switching among the normal mode, MLE mode and reset mode. Fig. 12 is a block diagram of a MLE control part. Fig. 13 shows waveforms of respectively signals in Fig. 12. Fig. 14 shows distortion resistance frequency characteristic. FIRST EMBODIMENT Fig. 1 and Fig. 2 respectively show an example of the system to which the present invention is adopted. Fig. 1 is a block diagram of the sending part, while Fig. 2 a block diagram of the receiving part. This system utilizes a 64-level quadrature amplitude modulation (64-QAM). In Fig. 1, the digital-analog converters 11-I and 11-Q are respectively responsive to the transmission data D I1 , D I2 , D I3 of 3 bits of the in-phase channel (I-ch) and the transmission data D Q1 , D Q2 , D Q3 of 3 bits of the quadrature channel (Q-ch) and provide outputs after conversion to 8-level signals. The low-pass-filters 12-I and 12-Q are respectively responsive to said 8-level signals and provides outputs of 8-level signals from which high frequency components are eliminated to the modulators 13-I and 13-Q. Meanwhile, a carrier generator 17 generates the carrier and outputs it to a 90° hybrid circuit 16, which branches said carrier to the two carriers in phase difference of 90° and outputs these carriers to the modulators 13-I and 13-Q. The modulators 13-I and 13-Q modulates said branched carriers with said filtered 8-level signals and provides the modulated outputs. The hybrid 14 combines the modulated signals sent from the modulators 13-I and 13-Q and provides an output. A radio transmitter 15 transmits the combined signal of said hybrid 14 as a radio signal. In Fig. 2, a radio receiver 21 receives said transmitted radio signal and provides an output to the hybrid 22. The hybrid 22 branches the received signal into two portions and provides outputs respectively to the demodulators 23-I and 23-Q. Meanwhile, a carrier recovery circuit 29 regenerates the carrier on the basis of the receiving signal and outputs the carrier to the 90° hybrid circuit 20, which branches said recovered carrier into two recovered carriers including phase difference of 90° and outputs these carriers to the demodulators 23-I and 23-Q. The demodulators 23-I and 23-Q respectively demodulate the receiving signals branched on the basis of the branched recovered carriers and provide the outputs of 8-level signals of I-ch and Q-ch. The low-pass-filters 24-I and 24-Q respectively eliminate noise of demodulated 8-level signals sent from the demodulators 23-I and 23-Q and provide the outputs. The transversal filter 25 removes inter-symbol inteference and inter-channel interference of demodulated 8-level signal sent from said low-pass-filters 24-I and 24-Q and provides outputs to the discriminators 26-I and 26-Q. The discriminators 26-I and 26-Q respectively discriminate the 8-level signals of I-ch and Q-ch sent from said transversal filter 25, respectvely output the transmission data D I ₁, D I2 , D I3 and D Q1 , D Q2 , D Q3 of 3 bits and output the signals for equalization control to the tap weighting control circuit 27 and control mode switching circuit 28. Said tap weighting control circuit 27 controls said transversal filter 25 for equalization on the basis of the signals sent from the discriminators 26-I and 26-Q. Namely, a transversal type automatic equalizer is composed of a transversal filter 25, discriminators 26-I, 26-Q, tap weighting control circuit 27 and control mode switching circuit 28. Fig. 3 shows detail structure of said transversal filter 25. In Fig. 3, the delay means 251-I and 251-Q are respectively composed of the delay elements(T) 2511-I 2514-I and 2511-Q―2514-Q, while the weighting means 252-1―252-4 are respectively composed of a variable gain amplifier 2520-I, multipliers 2521-I―2524-I, multipliers 2525-I―2529-I, multipliers 2525-Q―2529-Q, a variable gain amplifier 2520-Q and multipliers 2521-Q―2524-Q, respectively. C1₋₂―C1 +2′ ―C2₋₂―C2 +2′ C3₋₂―C3 +2′ C4₋₂―C4+₂ are weighting signals fo variable gain amplifiers and multipliers. The weighting means 252-1 and 252-4 do not have multiplier of main tap, and gain of the variable gain amplifiers 2520-I and 2520-Q directly set amplitudes of main tap. Namely the variable gain amplifiers 2520-I and 2520-Q equivalently correspond to the multipliers of main tap of the weighting means 252-1 and 252-4. The demodulated multi-level signals I in and Q in of the I-ch and Q-ch are applied to the variable gain amplifiers 2520-I, 2520-Q. The output signals are sequentially delayed by the delay circuits 2511-I 2514-I, 2511-Q―2514-Q, and the in-phase component and quadrature component are respectively sent to the adders 253-I, 253-Q through the multipliers 2521-I―2529I, 2521-Q―2529-Q and added therein. The added output signal of adders 253-I, 253-Q is then applied to the discriminators 26-I, 26-Q of Fig. 2. The discriminators 26-I, 26-Q discriminate the levels of said added output signals and these can output up to the lower two bits than the bits of transmission data. For example, when these discriminators are adopted to the 16-level QAM, the discrimination output of 4 bits can be obtained because the transmission data is 2 bits for each channel, while adopted to the 64-level QAM, the discrimination output of 5 bits can be obtained because the transmission data is 3 bits for each channel and when adopted to the 256-level QAM, the discrimination output of 6 bits can be obtained because the transmission data is 4 bits for each channel. In the case of this embodiment, since the discriminators are adopted to the 64-level QAM, the first through third bits D I1 -D I3 , D Q1 -D Q3 of the discrimation signals are applied to the processing circuits in the succeeding stages as shown in Fig. 2, the first bit is considered as the sign signals SI, SQ, while the fourth bit as the error signals EI, EQ, and these signals are applied to the tap weighting control circuit 27. The fourth and fifth bits D I4 , D I5 of I-ch are applied to the control mode switching circuit 28. Fig. 4 is a detail block diagram of the tap weighting control circuit 27. In Fig. 4, the error signals E I and E Q are respectively applied to the one input terminals of the exclusive OR gate (EX-OR) 2721-I―2725-I, 2731-Q―2735-Q and 2721-Q―2725-Q, 2731-I―2735-I of the correlation circuits 272-I, 273-Q and 272-Q, 273-I through the shift registers 271-I and 271-Q, each consisting of two flip-flops (FF). The sign signals S I and S Q are generally applied to the shift registers 275-I and 275-Q consisting of four flip-flop through the switches 274-I and 274-Q. These shift registers 275-I and 275-Q output the signals of the same delay time as that of each tap of the delay portions 251-I, 251-Q in the transversal filter 25 and are applied to the other input terminals of the EX-OR gates of the correlation circuits 272-I, 273-I, 273-Q, 272-Q. The outputs of the EX-OR gates 2721-I―2725-I, 2731-I―2735-I, 2731-Q―2735-Q, 2721-Q―2725-Q are respectively integrated by the integral circuits 2761-I―2765-I, 2771-I―2775-I, 277I-Q―2775-Q, 2761-Q―2765-Q and these are output as the weighting signals C1₋₂―C1+₂, C2₋₂―C2+₂, C3₋₂― C3+₂, C₄₋₂―C4+₂. Fig. 5 is a detail block diagram of the control mode switching circuit 28. In Fig. 5, the fourth bit D I4 and fifth bit D I5 of the discriminator 26-I shown in Fig. 2 are input to the exclusive NOR (EX-NOR) gate 2821. In general, the fourth bit D₄ and fifth bit D₅ of discriminator which discriminates, the 8-level signal further divide the range of eight levels L₀―L₇ for discrimination of input signal to the discriminator as shown in Fig. 6 into four ranges. EX-NOR of D4 and D₅ (indicated by D₄ ⊕ D₅) becomes "1" at the ranges of both ends among those divided and becomes "0" at the other ranges. When D₄ ⊕ D₅ is "1", it is considerably probable that it is discriminated at the next level and it can be used as the pseudo error pulse. The integrator 2822 integrates the pseudo error pulses and outputs voltage V corresponding to the generation frequency of pseudo error pulses. Fig. 7 shows a relation between the actual bit error rate BER and output voltage V of integrator. The comparator 2823 compares an output voltage V of integrator 2822 and voltage V₁ corresponding to the predetermined error rate BER1 and provides a comparison output. The comparator 2824 compares an output voltage V of integrator and the voltage V2 corresponding to the predetermined error rate BER2 and provides a comparison output. The control mode holding circuit 2825 holds the control mode and uses the error rate BER1 as a threshold value for swithing the control mode to the reset mode from normal mode as shown in Fig. 8 or the error rate BER2 as a threshold value for switching the control mode to the normal mode, based on the comparison results of comparators 2823 and 2824. In the reset mode, the reset signal rt is output. In Fig. 4, the reset signal rt is applied to the switches 274-I and 274-Q. In this case, the switches 274-I and 274-Q are in the condition indicated by the dotted line and output the alternate signals WF of "0" and "1". Since the alternate signal WF does not correlate with the input signal, outputs of respective integrators 2761-I ― 2765-I, 2771-I―2775-I, 2771-Q―2775-Q, 2761-Q―2765-Q all become zero or the value near to zero. Thereby, the gains of variable gain amplifiers 2520-I and 2520-Q become the reference gain, coefficients of multipliers 2521-I ― 2529-I, 2521-Q―2529-Q become zero and the demodulated multi-value signals I in , Q in of the I-ch and Q-ch are not equalized and amplified only with a constant gain and are input to the discriminators 26-I and 26-Q shown in Fig. 2. Namely, the tap weighting control circuit 27 and transversal filter 25 are reset. SECOND EMBODIMENT Fig. 9 is a block diagram of control mode switching circuit of the second embodiment of the present invention. Fig. 10 is a block diagram of the tap weigthing control circuit of the second embodiment of the present invention. In the second embodiment of the present invention, the maximum level error (MLE) mode is provided between the normal mode and reset mode. In Fig. 9, the EX-NOR gate 2821 and integrator 2822 ouptut voltages V corresponding to pseudo error pulse generation frequency as in the case of Fig. 5. This output voltage V is compared with the first, second and third values V1, V2, V3 which are preset by the comparators 2826, 2827. In this case, these values are set in the realtion, V2 < V1 < V3 and respectively correspond to the bit error rates 5 x 10⁻³ (BER1), 5 x 10⁻⁵ (BER2), 6 x 10⁻² (BER3). In the normal mode, the switching circuit 2828 selects V1 and outputs it to the comparator 2827. When the comparator 2827 detects that the bit error rate becomes worse than BER1 and issues an alarm signal am, the comparator 2828 selects V2 and outputs it to the comparator 2827. The timer 2829 outputs the reset signal rt for each redetermined period when the comparator 2826 detects that the bit error rate becomes worse than BER3. Fig. 11 is a diagram for mode switching. When the bit error rate is lower than BER2 (for example, 5 x 10⁻⁵) or less, the normal mode is selected. When the bit error rate, of normal mode becomes higher than BER1 (for example, 5 x 10⁻³)(> BER2), it is switched to the MLE mode. When the bit error rate is lower than said BER2 in this MLE mode, the MLE mode is switched to the normal mode, but if the bit error rate becomes higher than the BER3 (for example, 6 x 10⁻² ( > BER1), the MLE mode is switched to the reset mode. This reset mode is switched to the MLE mode when the bit error rate becomes smaller than BER3. Corresponding to the bit error rates BER1, BER3, said first to third values V1―V3 are set. In Fig. 10, the in-phase side circuit structure is indicated because the circuit structure is the same in both the in-phase side and quadrature side. In this figure, the like parts as those in Fig. 4 are given the like symbols. 278 is MLE controller, 2792 is selector, 2791 is selective controller, 2766, 2767, 2776 are flip-flop (FF's). CK is clock signal terminal, D is data terminal, Q1―Q5 are output terminals. The MLE control means 278 is responsive to the upper four bits D I1 ―D I4 among the discrimination output of 5 bits from the discriminator 26-I shown in Fig. 2 and outputs the signals m1 and m2 for the MLE control. The MLE control will be explained later in detail. The correlator 272-I-1 corresponds to a part in relation with the main tap among the correlator 272-I shown in Fig. 5, while the correlator 272-I-2 corresponds to the other portions. The selector controller 2791 controls the selector 2792 with the reset signal rt and alarm signal am. It switches the selector to the side of contacts 1a, 2a in the MLE mode where only the alarm signal am is applied or to the side of contacts 1b, 2b in the other modes. In the normal mode, the selector 2792 is in the connecting condition shown in the figure and the switch circuit 274-I is also in the connecting condition shown in the figure. Therefore, the sign signal SI and error signal EI are applied to the correlators 272-I-1, 272-I-2, the correlation output is applied to the integrators 2761-I, 2765-I through the flip-flop circuits 2766, 2767, and outputs of integrators 2761-I, 2765-I become the tap weighting signal C1, while output of integrators 2763-I becomes the tap weighting signal C1₀. On the other hand, the correlation output of the correlator 273-I to which the sign signal S1 and error signal EQ are applied is applied to the integrators 2771-I―2775-I through the flip-flop 2776 and becomes the tap weighting signal C2. As shown in Fig. 6, when the discriminator outputs D₁―D₄ are all "0" or all "1" for the discrimination of 8-level signal, it indicates that the input signal is in the outside of the outer most levels L₀, L₇ among the discrimination levels L₀―L₇ and therefore it is detected as the maximum level error. Fig. 12 is a block diagram of the MLE controller 278. Examples of the signals are indicated in the diagram for explaining operations of Fig. 13 using the same symbols. The clock signal CLK is applied to the shift registers 2783, 2786, clock terminal of flip-flop 2787 and the NAND gate 2789. The circuit consisting of the EX-OR gates 2781-1―2781-3 and NOR gate 2781-4 detects the maximum level error, and outputs the maximum level error signal MLE which becomes "1" when D I1 ―D I4 are all "0" and when D I1 ―D I4 are all "1" or becomes "0" in other cases. The maximum level error signal MLE is applied to the shift register 2783 through the inverter 2782 and is shifted in accordance with the clock signal CLK. In case the maximum level error signal MLE is indicated as MLE in Fig. 13, an output signal (a) of the specified number of stages of the shift registers 2783 is delayed as much as a number of stages corresponding to the delay of main tap at the transversal filter, while an output signal (b) of the next stage is further delayed by one clock signal. The output signal (a) is applied to the NAND gate 2785 through the inverter 2784 while the output signal (b) is directly applied to the NAND gate 2785. Therefore the output signal (c) of such inverter becomes equal to the signal (c) shown in Fig. 13, detecting the leading edge of the maximum level error signal MLE. The output signal (c) of this NAND gate 2785 is applied to the flip-flop (FF) 2787 and output signal of it is applied to the NAND gate 2789 through the inverter 2788. The output signal of this NAND gate 2789 becomes the signal m2 which is applied to the contact 2a of the selector 2792 (see Fig. 10). This signal m2 is applied to the clear terminal CL of the m-bit shift register 2786 in order to shift this shift register. The shift register 2786 shifts the output signal b of the shift register 2783 in accordance with the clock signal CLK, outputs the signal m1 and applies it to the contact 1a of the selector 2792 (refer to Fig. 10). For example, in the 64-level QAM, the maximum level error signal MLE once output for every 8 bits in average in each channel under the normal operating condition. Therefore, the mark ratio of the maximum level error signal MLE becomes 1/8. Accordingly, it is essential for setting the mark ratio under the normal operation to 1/2 that the signal which is equal to that when the maximum level error is detected four times is obtained when the maximum level error is once detected. Since the maximum level error signal MLE is input to the shift register 2783 through the inverter 2782 in Fig. 12, detection of maximum level error corresponds to "0". Therefore,when the leading edge of maximum level error signal MLE is detected, the m-bit shift register 2786 is cleared, allowing increase of "0". In case the maximum level error signal becomes "1" in regular 8 bits synchronization, the 3-bit shift register 2786 is desirably prepared, "0" is increased as many as 3 bits for each detection of the maximum level error signal and thereby "0" as many as 4 bits in total can be obtained. However, in actual, the maximum level error signal becomes "1" irregularly, it is probable that there are bits which are initially "0" when the shift register 2786 is cleared. Therefore, it is better that a number of bits of shift register 2786 is set larger than 3. According to the experiments, the optimum number of bits of shift regiter 2786 is 5 bits. The dotted line in the waveform m1 of Fig. 13 indicates the waveform in such a case that the shift register 2786 is not cleared. The signal m1 is applied to the data terminal D of flip-flop 2767 through the contact 1a of the selector 2792 (see Fig. 10) under the MLE mode, and the signal with mark rate of 1/2 is applied to the integrator 2765-I from the output terminal Q and thereby the integrator output becomes zero. Namely, in this case, the weighting signal C1₀ of main tap becomes zero and the gain of variable gain amplifier 2520-I (refer to Fig. 3) becomes the reference gain. When the maximum level error becomes large and the leading edge of maximum level error signal MLE is detected frequently, the shift register 2786 is more frequently cleared and therefore the mark rate of output signal m1 of the shift register 2786 becomes less than 1/2. Under the MLE mode, the signal m1 is applied to the flip-flop 2767 through the selector 2792. When output signal of such flip-flop is applied to the integrator 2765-I, the weighting signal of main tap becomes negative. Therefore, amplitude of input signal is suppressed by the control of variable gain amplifier 2520-I. Namely, the amplitude is controlled so that the maximum level error signal MLE is not output. The output signal m2 of NAND gate 2789 is applied, under the MLE mode, to the clock terminals CK of flip-flops 2766, 2776 through the selector 2792 and becomes the signal which shows less variation than the clock signal CLK. Therefore, the taps other than the main tap are less controlled and the main tap is mainly controlled in the MLE mode. In case the leading edge of the maximum level error signal MLE is detected frequently, the weighting signal C1₀ of main tap changes largely but the pulse of signal m2 is generated frequently. Therefore, control of taps other than the main tap follows the control of main tap. As explained above, the input signals are equalized by the ordinary tap coefficient control in the normal mode. When bit error rate increases due to the fading, the normal mode is switched to the MLE mode and the main tap coefficient control is mainly conducted. Moreover, when bit error rate increases further, the MLE mode is switched to the reset mode. Here, the selector 2792 is switched to the condition indicated in the figure. Moreover, the switch circuit 274-I is switched to apply the signal WF to the shift register 275-I. Therefore the alternate signal WF of "0" and "1" and the error signals EI, EQ (having the same period as the clock signal CLK) are applied to the correlators 272-I-1, 272-I-2, 273-I and an average of correlation values becomes zero. Accordingly, outputs of the integrators 2761-I―2765-I, 2761-Q― 2765-Q become zero. In this case, it is also possible to use the means which forces integration outputs of integrators 2761-I―2765-I, 2761-Q―2765-Q to become zero or the means which forces average value to become zero by switching in the input side of the integrators 2761-I―2765-I, 2761-Q―2765-Q. Zero output of integrators 2761-I―2765-I, 2761-Q―2765-Q corresponds to the case wherein the multiplier of Fig. 4 is separated and the gain of variable gain amplifier is fixed to the reference gain and therefore equalizing operation is not carried out. In this case, the reset signal rt is output when the timer 2729 shown in Fig. 9 is started. Accordingly, the equalizing operation is carried by setting the switch circuit 274-I to the side of sign signal SI from the side of signal WF for each specified period in order to check whether the bit error rate is improved or not. Namely, when the control mode is switched to the reset mode, the equalizing operation is carried out periodically in order to make it possible to easily shift to the MLE mode. Fig. 14 shows distortion resistance frequency characteristic of a transversal type automatic equalizer for 64 values QAM of the embodimemnt of the present invention where the roll-off rate is 35%. The notch frequency (MHz) is plotted on the X axis while the notch depth (dB) on the Y axis, wherein the hysteresis is little appearing when the fading changes to large value from small value (indicated by a solid line) or vice versa (indicated by a dotted line). Above embodiment is adapted to the 64-level QAM, but this can also be adapted to other multi-level QAM and moreover can also be adapted to automatic equalization for the multi-level amplitude modulated signals other than QAM signal. Moreover, it can be adapted to the case where a number of taps are further increased. In above first and second embodiments, the control mode switching circuit 28 generates pseudo error pulse using output of discriminator 26-I of I-ch, but it may be generated using an output of the discriminator 26-Q and the OR signal of the pseudo error pulse generated from output of discriminator 26-I and the pseudo error pulse generated from output of discriminator 26-Q may be input to the integrator 2822. The present invention provides the advantage as explained above that the control mode is switched in accordance with generation frequency of pseudo error pulse generated using the lower bits of transmission data sent from the discriminator, the threshold value for switching the normal mode to the reset mode or MLE mode is forced to be different from the threshold value for the reverse switching, thereby the fine mode switching control can be realized stably with simple circuit structure. Moreover, since the MLE mode is provided between the normal mode and reset mode, even in case the control mode returns to the normal mode after it is set to the reset mode, a certain degree of equalization is carried out by the MLE mode and thereby the reset mode can be quickly returned to the normal mode, resulting in less hysteresis in the distortion resistance frequency characteristic.

Claims: A radio data transmission system comprising, a transmitting apparatus comprising; converting means (11-I,11-Q), which receive n-bit data, for converting the received data to a multi-level transmission signal (n is a positive integer), modulating means (13-I,13-Q) for modulating a carrier with said multi-level signal, and transmitting means (15) for transmitting said modulated signal as a radio signal, a receiving apparatus comprising; means (21) for receiving said radio signal, demodulating means (23-I,23-Q) for demodulating said received radio signal and for outputting a multi-level signal, transversal filter (25) having a main tap and other taps, for receiving the weighting signal for controlling weighting of taps and for outputting after equalizing said demodulated multi-level signal, discriminating means (26-I,26-Q) for discriminating the multi-level signal output from said transversal filter and for outputting the digital signal of n+m (m is a positive integer) bits, tap weighting control means (27) for outputting said weighting signal using at least a part of the digital signal among the n+m bits digital signal output from said discriminating means, and having at least one mode of operation, and control mode switching means (28) for switching the control mode of said tap weighting control means in accordance with a pseudo error pulse rate generated using at least a part of the lower m bits of n+m bits digital signal output from said discriminating means; characterized in that the control mode switching means are adapted to switch from a first control mode to a second control mode at a first threshold value of the error rate, and to switch from said second control mode to said first control mode at a second, different, threshold value of error rate. A radio data transmission system according to claim 1, wherein said conversion means (11-I,11-Q) is composed of two conversion means for inphase channel (I) and quadrature channel (Q), said modulating means (13-I,13-Q) conducts multi-level quadrature amplitude modulation with two multi-level signals for inphase channel and quadrature channel sent from said two conversion means, said demodulating means (23-I,23-Q) demodulates said received signal and outputs two multi-level signals for inphase channel and quadrature channel, said transversal filter (25) also eliminates interference between two multi-level signals, and said discriminating means (26-I,26-Q) is composed of two discriminating means for inphase channel and quadrature channel. A radio data transmission system according to claim 2, wherein said modulating means comprises, means (17) which generates the carrier, means (16) which branches said carrier to two carriers for inphase channel and quadrature channel having phase difference of 90°, means (13-I) which modulates said inphase channel carrier with said inphase channel multi-level signal, means (13-Q) which modulates said quadrature channel carrier with said quadrature channel multi-level signal, means (14) which combines said modulated signal of inphase channel and modulated signal of quadrature channel, said demodulating means comprises, means (22) which branches said received signal into two received signals for inphase channel and quadrature channel, means (29) which recovers the carrier in relation with said received signal, means (20) which branches said recovered carrier into two recovered carriers for inphase channel and quadrature channel having phase difference of 90°, means (23-I) which demodulates the received signal of inphase channel from the recovered carrier for inphase channel and outputs demodulated multi-level signal of inphase channel, means (23-Q) which demodulates the received signal of quadrature channel from the recovered carrier for quadrature channel and outputs demodulated multi-level signal of quadrature channel. A radio data transmission system according to claim 2 or 3, wherein said transversal filter means comprises, delay means (251-I) for inphase channel and quadrature channel which are composed of a plurality of cascade-connected delay elements and provide the main tap and other taps, first weighting means (252-1) which comprises a variable gain amplifier which amplifies said demodulated multi-level signal of inphase channel with a gain corresponding to the first weighting signal of main tap, outputs such multi-level signal to said delay means of inphase channel and equivalently weights the main tap and a plurality of multipliers which multiply values corresponding to the first weighting signal of the other taps to the signals from the other taps and outputs the first weighted signal, second weighting means (252-2) which comprises a plurality of multipliers which multiply values corresponding to second weigthing signal of each tap to the signal sent from each tap of said delay means for inphase channel and outputs the second weighted signal, third weighting means (252-3) which comprises a plurality of multipliers which multiply values corresponding to third weighting signal of each tap to the signal sent from each tap of said delay means for quadrature channel and outputs the third weighted signal, fourth weighting means (252-4) which comprises a variable gain amplifier which amplifies said demodulated multi-level signal of quadrature channel with a gain corresponding to the fourth weighting signal of main tap, outputs such multi-level signal with a delay means for quadrature channel and equivalently weights the main tap and a plurality of multipliers which multiply values corresponding to the fourth weighting signal of other taps to the signals sent from the other taps and outputs the fourth weighted signal, an adder (253-I) for inphase channel which adds said first weighted signal (inphase channel) and said third weighted signal (quadrature channel) and outputs the equalized multi-level signal for inphase channel, and an adder (253-Q) for quadrature channel which adds said second weighted signal (quadrature channel) and said fourth weighted signal (inphase channel) and outputs the equalized multi-level signal for quadrature channel. A radio data transmission system according to claim 4, wherein said discriminating means (26-I, 26Q) for inphase channel and quadrature channel output respectively the data bits from the first to the n bits among said converted digital signal as the transmission data for inphase channel and quadrature channel, respectively the first bit as the sign signal for inphase channel and quadrature channel, and respectively the n+1 bit as the error signal for the inphase channel and quadrature channel, and at least the one discriminating means outputs the n+1 bit and n+2 bit for generating the pseudo error pulse. A radio data transmission system according to claim 5, wherein the control mode includes the normal mode and reset mode, said tap weighting control means (27) comprises, an inphase channel switch (274-I) which is responsive to the sign signal and the alternate signal of "0" and "1" for the inphase channel and outputs said inphase channel sign signal in the normal mode or said alternate signal in the reset mode in accordance with the signal which indicates the control mode, a quadrature channel switch (274-Q) which is responsive to said quadrature channel sign signal and said alternate signal and outputs said quadrature channel sign signal in the normal mode or said alternate signal in the reset mode in accordance with the signal which indicates said control mode, the inphase channel and quadrature channel sign/alternate signal delay means which are respectively responsive to the outputs of said inphase channel and quadrature channel switches and also respectively output the delay signals corresponding to the taps having the delay times same as that of respective taps of said transversal filter means, the inphase channel and quadrature channel error signal delay means (275-I,275-Q) which respectively output said inphase channel and quadrature channel error signals with the delay time same as that of main tap of said transversal filter means, first correlation means (272-I) which calculates exclusive OR of a delay signal corresponding to each tap from said inphase channel sign/alternate signal delay means and a delayed inphase error signal from said inphase channel error signal delay means and outputs a first exclusive OR signal corresponding to each tap, second correlation means (273-I) which calculates exclusive OR of a delay signal corresponding to each tap from said inphase channel sign/alternate signal delay means and a delayed quadrature channel error signal from said quadrature channel error signal delay means, and outputs a second exclusive OR signal corresponding to each tap, third correlation means (273-Q) which calculates exclusive OR of a delay signal corresponding to each tap from said quadrature channel sign/alternate signal delay means and a delayed inphase channel error signal from said inphase channel error signal delay means and outputs a third exclusive OR signal corresponding to each tap, fourth correlation means (272-Q) which calculates exclusive OR of a delay signal corresponding to each tap from said quadrature channel sign/alternate signal delay means and a delayed quadrature channel error signal from said quadrature channel error signal delay means and outputs a fourth exclusive OR signal corresponding to each tap, first integral means (276-I) which carries out integration of first exclusive OR signal corresponding to each tap and outputs as a first weighting signal corresponding to said each tap, second integral means (277-I) which carries out integration of second exclusive OR signal corresponding to each tap and outputs as a second weighting signal corresponding to said each tap, third integral means (277-Q) which carries out integration of third exclusive OR signal corersponding to each tap and outputs as a third weighting signal corresponding to said each tap, fourth integral means (276-Q) which carries out integraion of fourth exclusive OR signal corresponding to each tap and outputs as a fourth weighting signal corresponding to said each tap, said control mode switching means comprises, an exclusive NOR gate (2821) which calculates the exclusive NOR of the n+1 bit and n+2 bit from said discriminating means and generates pseudo error pulses, an integrator (2822) which carries out integration of said pseudo error pulses and outputs a voltage corresponding to generation frequency of said pseudo error pulses, a first comparator (2823) which compares a first reference voltage corresponding to first bit error rate with an output voltage of said integrator, a second comparator (2824) which compares a second reference voltage corresponding to second bit error rate which is lower than the first bit error rate with an output voltage of said integrator, and a control mode holding means (2825) which outputs the switching signal which indicates the normal mode under the normal mode based on the comparison result from said first and second comparators, switches the normal mode to the reset mode when an output voltage of said integrator becomes higher than the first reference voltage, outputs the switching signal which indicates the reset mode under the reset mode, and switches the reset mode to the normal mode when an output voltage of said integrator becomes lower than the second reference voltage. A radio data transmission system according to claim 5, wherein; the control mode is classified into the normal mode, maximum level error (MLE) mode and reset mode, said tap weighting control means comprises; an inphase channel switch (274-I) which is responsive to the sign signal and the alternate signal of "0" and "1" for the inphase channel and outputs said inphase channel sign signal in the normal mode or said alternate signal in the reset mode in accordance with the signal which indicates the control mode, a quadrature channel switch (274-Q) which is responsive to said quadrature channel sign signal and said alternate signal and outputs said quadrature channel sign signal in the normal mode or said alternate signal in the reset mode in accordance with the signal which indicates said control mode, the inphase channel and quadrature channel sign/alternate signal delay means which are respectively responsive to the outputs of said inphase channel and quadrature channel switches and also respectively output the delay signals corresponding to the taps having the delay times same as that of respective taps of said transversal filter, the inphase channel and quadrature channel error signal delay means (275) which respectively output said inphase channel and quadrature channel error signals with the delay time same as that of man tap of said transversal filter, first correlation means (272-I) which calculates exclusive OR of a delay signal corresponding to each tap from said inphase channel sign/alternate signal delay means and a delayed inphase error signal from said inphase channel error signal delay means and outputs a first exclusive OR signal corresponding to each tap, second correlation means (273-I) which calculates exclusive OR of a delay signal corresponding to each tap from said inphase channel sign/alternate signal delay means and a delayed quadrature channel error signal from said quadrature channel error signal delay means, and outputs a second exclusive OR signal corresponding to each tap, third correlation means (273-Q) which calculates exclusive OR of a delay signal corresponding to each tap from said quadrature channel sign/alternate signal delay means and a delayed inphase channel error signal from said inphase channel error signal delay means and outputs a third exclusive OR signal corresponding to each tap, fourth correlation means (272-Q) which calculates exclusive OR of a delay signal corresponding to each tap from said quadrature channel sign/alternate signal delay means and a delayed quadrature channel error signal from said quadrature channel error signal delay means and outputs a fourth exclusive OR signal corresponding to each tap, the inphase channel and quadrature channel MLE control means (278) comprises; the inphase channel and quadrature channel MLE detection means (2781) which respectively detect MLE from the first to n+1 bits sent from said inphase channel and quadrature channel discriminating means and output the MLE signals, the inphase channel and quadrature channel MLE signal leading edge detection means (2785) which respectively detect the leading edges of said inphase channel and quadrature channel MLE signal and output the inphase channel and quadrature channel leading edge detecting pulses, the inphase channel and quadrature channel MLE signal delay means (2783) which sequentially delay said inphase channel and quadrature channel MLE signals, set the inphase channel and quadrature channel MLE signals being held for delay to the MLE detecting condition when the leading edge pulses of said inphase channel and quadrature channel are generated and respectively output the delayed MLE signals of inphase channel and quadrature channel, the inphase channel and quadrature channel selection means (279) which are respectively responsive to the first and fourth exclusive OR signals corresponding to the main tap from first and fourth correlation means, said delayed MLE signals of inphase channel and quadrature channel, clock signal and said leading edge pulses of inphase and quadrature channel and selectively ouptut said delayed MLE signals of inphase channel and quadrature channel among the first to fourth exclusive OR signals corresponding to the main tap and said delayed MLE signals of inphase channel and quadrature channel when the alarm signal is generated but the reset signal is not generated in accordance with the alarm signal and reset signal and the leading edge pulses of inphase channel and quadrature channel among the clock signal and leading edge pulses of inphase channel and quadrature channel, or the first to fourth exclusive OR signals corresponding to main tap and clock signal in other cases, the first and fourth holding means (276) which are respectively composed of the first and fourth flip-flops corresponding to the main taps which are responsive to first and fourth exclusive OR signals corresponding to main taps from inphase channel and quadrature channel selection means or delayed MLE signals of inphase channel and quadrature channel at the data terminals and also responsive to the clock signals at the clock terminals and the first to fourth flip-flops corresponding to other taps which are responsive to the first and fourth exclusive OR signals corresponding to the other taps at the data terminal and are also responsive to the clock signals from the inphase channel and quadrature channel selection means or leading edge pulses of inphase channel and quadrature channel at the clock terminals, the second and third holding means (276,277) composed of the second and third flip-flops corresponding to each tap which are responsive to said second and third exclusive OR signal corresponding to each tap at the data terminal and are also responsive to the clock signal at the clock terminal, and the first to fourth integral means (276,277) which respectively carry out integral operations for the outputs of first to fourth holding means corresponding to each tap and output the first to fourth weighting signals corresponding to each tap, said control mode switching means comprises; an exclusive NOR gate (2821) which carries out the exclusive NOR operation of the n+1 bit and n+2 bit from said discriminating means and generates pseudo error pulses, an integrator (2822) which integrates pseudo error pulses and outputs a voltage corresponding to generation frequency of said pseudo error pulses, reference voltage switching means (2828) which selects and outputs a voltage corresponding to the first bit error rate and a voltage corresponding to the second bit error rate which is lower than said first bit error rate, third comparison means (2827) which compares an output voltage of said reference voltage switching means and an output voltage of said integral means and causes said integrator to generate an alarm signal and switches said reference voltage switching means to selectively output said second reference voltage when an output voltage of said integral means is higher, and also switches said reference voltage switching means to selectively output said first reference voltage when an output voltage of said integral means is lower, fourth comparison means (2826) which compares the third reference voltage corresponding to the third bit error rate with an output voltage of said integral means, and a timer (2829) which periodically outputs the reset signal when an output voltage of said integral means is higher, in accordance with an output of said fourth comparison means.

Inventors: AONO YOSHIHITO - - ASAHIMACHI, DAIDO YOSHIMASA - - WAKABADAI, MINOWA MORIHIKO KANAMORI, SAKANE TOSHIAKI - - SHIMOTAKAI, TAKENAKA SADAO SHINYOSHIDAMACH, AONO, YOSHIHITO 607, 1-16-15, ASAHIMACHI, DAIDO, YOSHIMASA 2-17-203, WAKABADAI, MINOWA, MORIHIKO 620, KANAMORI, SAKANE, TOSHIAKI 603, 2-5-5, SHIMOTAKAIDO, TAKENAKA, SADAO 411, 1879, SHINYOSHIDAMACHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1986
Main classification: H04B3
IPC codes: H04B7, H04L25