Antenna diversity switching with hysteresis.

BACKGROUND OF THE INVENTION

Field of the Invention

0001The present invention relates to a diversity antenna communication system to be mounted on and used in conjunction with a mobile body. More specifically, the invention relates to a diversity antenna communication system having two or more receiving or transmitting systems with different propagation paths from one another in which a received signal from one of the receiving systems having a higher reception level than another is selected for use to reduce influences of deep fading.

Description of Prior Art

0002In a signal communication between mobile bodies (mobile stations) which is carried out mainly with radiowaves modulated with digital data, a propagation path on the ground is a multipass type propagation path, and therefore the envelope and phase of a radio wave fluctuate randomly in accordance with the Rayleigh's distribution law and the uniform distribution law, respectively, (i.e., the radio wave suffers from fading). In order to reduce such fading, there is a diversity receiving antenna communication system in which the receiving system is changed over from one receiving antenna to another.

0003Conventional diversity receiving antenna communication systems are used primarily on a mobile body, and, for example, diversity systems using space, polarization, arrival angles, time and frequency have been utilized. Of those systems, a diversity receiving antenna communication system using an arrival angle is best suited for being mounted on a ground mobile station (such as a vehicle, or the like) because of its compactness and its ability to reduce fading in a mobile body which remarkably changes its traveling direction.

0004A diversity signal receiving apparatus employed in the above-described diversity antenna communication system is shown in Fig. 1. That is, two signal receiving antennas 3a and 3b provided for different signal propagation paths are connected to two distributors 4a and 4b, respectively. The first distributor 4a is connected to a signal receiver 6 and to a first level monitor circuit 12; whereas the second distributor 4b is connected to a second level monitor circuit 13 and to a dummy resistor 14 equivalent in impedance to the signal receiver 6. The two level monitor circuits 12 and 13 are connected to a level comparison circuit 8.

0005The distributors 4a and 4b operate to distribute signals received through the receiving antennas 3a and 3b to loads (the receiver 6 and the dummy resistor 14) and to the level monitor circuits 12 and 13, respectively. The level monitor circuits 12 and 13 monitor the levels of the signal received through the distributors 4a and 4b (hereinafter referred to as "reception levels", when applicable). The level comparison circuit 8 compares the reception levels monitored by the level monitor circuits 12 and 13 with each other for detection of the reversal of the reception levels, so that in the signal receiving system providing a higher reception level the distributor is connected to the receiver, and in the preceding signal receiving system the distributor is connected to the dummy resistor.

0006Fig. 2 is a diagram for explaining reception levels in the conventional diversity signal receiving apparatus, and its antenna switching timing. More specifically, Fig. 2(A) shows an electric field strength at a signal receiving point, Fig. 2(B) shows the antenna switching timing, and Fig. 2(C) shows the relationship between received data blocks and the antenna switching timing.

0007The two signal receiving antennas receive a radio wave from a signal transmitting station, to provide two reception levels (indicated by the thin solid line (a) and the broken line (b) in Fig. 2(A)). When the reception levels are reversed (as indicated by the broken lines in Fig. 2), the signal receiving antenna selected previously is switched over to the receiving antenna providing the highest reception level, which provides a diversity field strength (as indicated by the heavy solid line (c) in Fig. 2(A)).

0008Thus, the signal can be received through the signal receiving system providing the highest reception level at all times. Therefore, the effect of deep fading can be minimized.

0009However, when the communication system switches one receiving antenna to the other, the phases before and after the change-over action are not continuous because of the difference in propagation paths. Further, although data which are divided into blocks of data signals, such as a synchronizing word, a control word, a talking word, and so on are transmitted continuously, the continuity of data, and moreover, the data itself are lost at the time of the change-over action as shown in Fig. 2(C). Therefore, to provide the lost data, it is necessary to retransmit.

0010In addition, in the case where the difference in level between the input signals received by the signal receiving system is small, the frequency of switching the signal receiving systems is increased, and accordingly the probability that the data is lost by the switching of the signal receiving systems is increased.

0011This disadvantage will be described in detail with reference to Fig. 3. In the case where the difference in level between the input (indicated by the solid line in Fig. 3(A)) received by a signal receiving system A and that (indicated by the broken line in Fig. 3(A)) received by a signal receiving system B is small (hereinafter referred to as "having substantially the same level", when applicable), the frequency of reversal of the reception levels in increased. A reception level determining unit detects such a frequent level reversal to output signal receiving system switching signals (as shown in Figs. 3(B) and 3(C)). Thus, the signal receiving systems are frequently switched over to each other, with the result that the data is cut and lost during the switching operation.

0012Furthermore, there is a problem that data transmitted by a mobile body is not transmitted to the called station. In order to transmit signals from a mobile body, a non-directional mono-pole antenna is used. With the non-directional antenna, data is not transmitted at some times because the position of the mobile body is changed frequently and therefore the reception level transmitted from the called station changes frequently.

0013For a single communication, signals are transmitted and received between fixed stations through directional diversity antennas. This is to separate an aimed station from a plurality of stations, thereby to perform selective communications.

SUMMARY OF THE INVENTION

0014The object of the present invention is to provide a diversity receiving antenna communication system for preventing a possibility of data loss at the switching of signal receiving systems, and minimizing the failure that data is not transmitted to a called station.

0015The foregoing object of the invention has been achieved by the provision of the following systems and apparatuses: A diversity signal receiving apparatus, according to a first aspect of the invention, comprises: reception level setting means for setting the signal reception level of one of at least two signal receiving systems which is not used for actual signal reception to a lower level than the signal reception level of the signal receiving system which is used for the actual signal reception, to the extent that deep fading can be detected; signal receiving system detecting mans for comparing the signal reception levels of at least two signal receiving systems, the signal reception levels of which have been set by the reception level setting means and detecting the signal receiving system highest in signal reception level; and signal receiving system change-over means for switching the present signal receiving system over to the signal receiving system which is highest in signal reception level.

0016A diversity signal receiving apparatus, according to a second aspect of the invention, comprises: signal receiving system detecting means for comparing the signal reception levels of at least two signal receiving systems and detecting the alternation in highest signal reception level of the signal receiving systems; and signal receiving system change-over means for setting the upper threshold value and the lower threshold value for a signal reception level at the alternation in highest signal reception level of the signal receiving systems to the extent that the deep fading can be detected and switching the present signal receiving system over to the signal receiving system highest in signal reception level when the signal reception level of the present signal receiving system which is used for the actual signal reception becomes lower than the lower threshold value and the signal reception level of a signal receiving system which is highest in signal reception level exceeds the upper threshold value.

0017In a diversity antenna communication system, according to a third or fourth aspect of the invention, the signal receiving system highest in signal reception level which has been selected in the apparatus provided according to the first or second aspect of the invention is connected to a transmitter, so that signal transmission is carried out with the antenna in the signal receiving system highest in signal reception level.

0018In a diversity antenna communication system, according to a fifth aspect of the invention, a signal receiving system providing the lowest level in signal reception level of at least two signal receiving systems is interrupted, and the antennas in the remaining signal receiving systems are used for signal transmission and reception.

BRIEF DESCRIPTION OF THE DRAWINGS

0019<ul id="ul0001" list-style="none"><li>Fig. 1 is a block diagram showing the arrangement of a conventional diversity signal receiving apparatus;</li><li>Figs. 2 and 3 are diagrams illustrating the signal reception levels of the conventional diversity signal receiving apparatus as shown in Fig. 1 and the antenna switching timing thereof;</li><li>Fig. 4 is a block diagram showing one example of a diversity signal receiving apparatus, which is a first embodiment of the invention;</li><li>Fig. 5 is a diagram for explaining the operation of the diversity signal receiving apparatus shown in Fig. 4;</li><li>Fig. 6 is a block diagram showing the arrangement of another example of the diversity signal receiving apparatus according to a second embodiment of the invention;</li><li>Fig. 7 is a diagram for explaining the operation of the diversity signal receiving apparatus shown in Fig. 6;</li><li>Fig. 8 is a block diagram showing the arrangement of another example of the diversity signal receiving apparatus according to a third embodiment of the invention; and</li><li>Fig. 9 is a block diagram illustrating the diversity signal receiving apparatus of Fig. 8 which is equipped with a transmitter according to a fourth embodiment of the invention.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

0020A preferred embodiment of the diversity receiving antenna communication system according to the present invention will be described in detail with reference to the accompanying drawings.

0021Fig. 4 shows one example of a diversity signal receiving apparatus, which is a first embodiment of the invention. The diversity signal receiving apparatus has at least two signal receiving systems A and B different in propagation path; and a level comparison circuit 8 for detecting the signal receiving system which provides a higher reception level than the other, to output a signal receiving system change-over signal. In the signal receiving system A, a signal receiving antenna 3a is connected to a distributor 4a, which is connected to a level monitor circuit 12 and signal receiving system change-over means 5a, and the latter 5a is connected to a receiver 6a and a dummy resistor 14 whose impedance is lower than the equivalent resistance of the receiver 6a. In the signal receiving system B, similarly as in the above-described signal receiving system A, a signal receiving antenna 3b is connected to a distributor 4b, which is connected to a level monitor circuit 13 and a signal receiving system change-over means 5b, and the latter 5b is connected to a receiver 6b and a dummy resistor 15 whose impedance is lower than the equivalent resistance of the receiver 6b. In this embodiment, the level monitor circuits 12 and 13 are provided independently of the level comparison circuit 8; however, they may be made integral with the level comparison circuit.

0022Since the two signal receiving systems A and B are equal to each other in arrangement, only the signal receiving system A will be described. The signal receiving antenna 3a is a directional antenna, which receives a radio wave having its arrival angle in a predetermined range, and suppies it to the distributor 4a. For the directional antenna, there are available a variety of antennas such as a horn reflector type antenna and a corner reflector type antenna.

0023The distributor 4a operates to distribute the signal received through the antenna 3a to the level monitor circuit 12 and the signal receiving system change-over means 5a in a predetermined proportion. It is made up of a combination of transformers, or a combination of capacitors. However, the signal may be distributed in other simple methods.

0024The signal receiving system change-over means 5a comprises two paths 5a1 and 5a2 one of which is selected in response to the change-over signal provided by the level comparison circuit 8. Through the path 5a1 the received signal from the distributor 4a is applied to the receiver; whereas through the path 5a2 it is applied to the dummy resistor 14. The signal receiving system change-over means may be either a semiconductor switch element or relay; however, for high speed switching operation, it is preferable to use the semiconductor switch element.

0025The receiver 6a is to demodulate the received signal provided by the distributor 4a to obtain predetermined data (such as talking data, control data, etc.), and it is impedance-matched with the receiving antenna 3a before use.

0026The impedance of the dummy resistor 14 is lower than the equivalent resistance of the receiver 6a. The dummy resistor functions to attenuate the input signal by several decibels (dB) in comparison with that to the receiver 6a. However, an excessive attenuation of the input signal will adversely affect the detection of fading. Therefore, the impedance of the dummy resistor is suitably determined with the relationship between the received signal level and the fading.

0027The level monitor circuit 12 operates to detect the envelope of the received signal applied thereto through the distributor 4a, thereby to detect its reception level.

0028The level comparison circuit 8 compares both of the reception levels of the two signal receiving systems provided through the level monitor circuits 12 and 13, to determine the signal receiving system which is higher in reception level, and applies an instruction signal to the signal receiving system change-over means in the other (preceding) signal receiving system to switch the receiver over to the dummy resistor, and applies an instruction signal to the signal receiving system change-over means in the signal receiving system higher in reception level to switch the dummy resistor over to the receiver.

0029The operation of the diversity signal receiving apparatus will be described with reference to Fig. 5.

0030It is assumed that, with the antenna 3a connected to the receiver 6a and with the antenna 3b connected to the dummy resistor 15, the reception input (indicated by the solid line in Fig. 5 (A)) of the signal receiving system A is substantially equal to that (indicated by the broken line in Fig. 5 (A)) of the signal receiving system B.

0031Radio waves arriving along different propagation paths are received by the receiving antennas 3a and 3b, and are applied through the distributors 4a and 4b to the signal receiving system change-over means 5a and 5b and the level monitor circuits 12 and 13, respectively.

0032The reception signal applied to the change-over means 5a in the signal receiving system A is supplied through the path 5a1 to the receiver 6a, so that predetermined data are obtained through demodulation. On the other hand, the reception signal applied to the change-over means 5b in the signal receiving system B is supplied through the path 5b2 to the dummy resistor 15.

0033On the other hand, the reception signals supplied to the level monitor circuits 12 and 13 are processed for detection of their reception levels; that is, the reception levels of the two signal receiving systems are monitored by the level monitor circuits 12 and 13, respectively. In this case, because of the impedance mismatching of the dummy 15, the reception level of the signal receiving system B is lower by several decibels (dB) than the true reception level as indicated by the one-dot chain line in Fig. 5 (A).

0034The level comparison circuit 8 compares the reception levels of the two receiving systems provided by the level monitor circuits 12 and 13, thereby to select one of the signal receiving systems which is higher in reception level.

0035When the reception levels are not alternated, the signal reception is continued with the present signal receiving system A. When the reception levels are alternated, an instruction signal is applied to the signal receiving system change-over means 5a in the signal receiving system A to switch the receiver 6a over to the dummy resistor 14 (as shown in Fig. 5 (B)) while an instruction signal is applied to the signal receiving system change-over means 5b in the signal receiving system B to switch the dummy resistor 15 over to the receiver 6b (as shown in Fig. 5 (C)).

0036If, in this case, the reception levels are substantially equal to each other, since the reception level of the signal receiving system B in which the dummy resistor 15 is selected is set lower by several decibels (dB) than that of the signal receiving system A in which the receiver 6a is selected, the level comparison circuit 8 allows the signal reception by the signal receiving system unless the reception level of the signal receiving system exceeds that of the signal receiving system A by several decibels (dB). When the level of the input signal in the signal receiving system B increases so that the reception levels are alternated, the change-over signals are outputted. Therefore, in the case where the reception levels are substantially equal to each other, the frequency of switching the signal receiving systems can be decreased, and therefore the data loss can be prevented.

0037Fig. 6 is a block diagram showing the arrangement of another example of the diversity signal receiving apparatus shown in Fig. 4. The apparatus shown in Fig. 6 is different from that shown in Fig. 4 in that the impedances of the dummy resistors 14 and 15 are equal to those of the receivers 6a and 6b, respectively, and the level comparison circuit 8 is made up of an A/D (analog-to-digital) converter 84 and a CPU 85.

0038The A/D converter 84 converts the output signals of the level monitor circuits 12 and 13 which are obtained by envelope detection into digital signals.

0039The CPU 85 compares the output data of the A/D converter 84, to detect the alternation of the reception levels, and sets the upper threshold value and the lower threshold value with respect to the reception levels at the alternation thereof as shown in Fig. 7 (A), so that, when the reception level of the present signal receiving system becomes lower than the lower threshold value while the reception level of the other signal receiving system higher in reception level exceeds the upper threshold value, it issues an instruction signal to switch the present signal receiving system over to the other signal receiving system higher in reception level. The width between the upper and lower threshold values (hysteresis) is determined in advance; that is, it has several decibels (dB) across the reception level provided at the alternation.

0040The operation of the diversity signal receiving apparatus will be described with reference to Fig. 7.

0041Similarly as in the case of Fig. 5, it is assumed that the input signals of the signal receiving systems are substantially equal to each other in level as shown in Fig. 7 (A). In Fig. 7, characters C and D designate the above-described upper and lower threshold values, respectively.

0042The output envelope signal of the level monitor circuits 12 and 13 are applied to the A/D converter 84, where they are converted into digital signals. The digital signals are supplied to the CPU 85. The CPU 85 compares the digital signals to detect the alternation of the reception levels. Thereafter, the CPU sets the upper and lower threshold values on both sides of the reception level provided at the alternation time. When, under this condition, the reception level of the present signal receiving system A becomes lower than the lower threshold value while the reception level of the signal receiving system B higher in reception level exceeds the upper threshold value, the CPU 85 applies an instruction signal to the signal receiving system change-over means 5a in the signal receiving system A to switch the receiver 6a over to the dummy resistor 14 as shown in Fig. 7 (B), and applies an instruction signal to the signal receiving system change-over means 5b in the signal receiving system higher in reception level to switch the dummy resistor 15 over to the receiver 6b as shown in Fig. 7 (C).

0043That is, until the reception level of the present signal receiving system becomes lower than the lower threshold value and the reception level of the other signal receiving system exceeds the upper threshold value, the CPU does not switch the present signal receiving system over to the other; i.e., the present signal reception is continued. Therefore, when the reception levels are substantially the same, the frequent switching of the signal receiving systems over to each other is suppressed, and the data loss is prevented accordingly. In addition, in the diversity signal receiving apparatus, the dummy resistors 14 and 15 are matched in impedance with the receiving antennas 3a and 3b, respectively, and therefore no unwanted reflected waves are produced.

0044Fig. 8 is a block diagram showing the arrangement of a diversity signal receiving apparatus which is a third embodiment of the invention. The apparatus shown in Fig. 8 is different from that shown in Fig. 6 in that the distributors 4a and 4b are connected to a multiplexer 16, which is connected to one level monitor circuit 12, and the two signal receiving systems 5a and 5b are connected, commonly to one receiver 6 and one dummy resistor 14.

0045More specifically, the path 5a1 of the signal receiving system change-over means 5a is connected to the receiver 6 and the path 5a2 is connected to the dummy resistor 14; and the path 5b1 of the other signal receiving system changeover means 5b is connected to the receiver 6 and the path 5b2 is connected to the dummy resistor 14.

0046The diversity signal receiving apparatus thus organized operates as follows: The multiplexer 16 operates to switch the input signals applied thereto through the distributors 4a and 4b over to each other every predetermined period of time and supply them to the level monitor circuit 12, and the signal receiving system change-over means 5a and 5b. The level monitor circuit 12 subjects to envelope detection the input signals supplied thereto alternately every predetermined period of time, and applies it to the level comparison circuit 8. Similarly as in the case of Fig. 6, the level comparison circuit 8 detects the alternation of the reception levels, so that an instruction signal is applied to the signal receiving system change-over means in the present signal receiving system to switch the receiver 6 over to the dummy resistor 14, and an instruction signal is applied to the signal receiving system change-over means in the other signal receiving system higher in reception level to switch the dummy resistor over to the receiver.

0047Thus, the provision of the multiplexer 16 permits reduction of the numbers of level monitor circuits, receivers and dummy resistors; that is, the apparatus employs one level monitor circuit, one receiver, and one dummy resisitor for the same effect, with the result that the manufacturing cost is reduced as much.

0048Fig. 9 is a block diagram showing the diversity signal receiving apparatus of Fig. 8 which is provided with a transmission function according to a fourth embodiment of the invention. In the apparatus, the signal receiving system change-over means 5a and 5b are connected to a transmission and reception change-over means 9, which is connected to the receiver 6 and a transmitter 10, and the receiver 6 is connected to a transmission and reception change-over signal producing section 11 which produces a transmission and reception change-over signal to switch reception over to transmission is response to a data reception acknowledge signal or a data transmission request signal provided by the operator. The transmission and reception change-over signal thus produced is applied to the CPU 85 in the level comparison circuit 8.

0049In response to the transmission and reception change-over signal, the CPU 85 applies an instruction signal to the signal receiving system change-over means 5a and 5b to select the signal receiving system which is highest in reception level, and supplies a reception-to-transmission change-over instruction signal to the change-over means 9.

0050In the above-described diversity antenna communication system, the level comparison circuit 8 suppresses the frequency of switching the antennas over to each other when the reception levels are substantially equal to each other, with the result that, even in transmission of data with the transmitter, the frequency of switching the antennas over to each other is decreased, and the loss of transmission data can be prevented.

0051In the above-described embodiment, the level comparison circuit 8 detects the antenna providing the highest reception level on the basis of the reception levels which are monitored at all times. In transmission of data, the transmission and reception change-over means 9 operates to connect the transmitter to the antenna thus detected; that is, the transmission is carried out with the antenna which provides the highest reception level. In this case, the called station can receive the signal at a maximum level because of the antenna reversibility that the antenna providing the highest signal reception level at a transmission side can provide the highest signal reception level at a reception side. Thus, the mobile body communication can be improved in quality, and the difficulty that transmission data is not transmitted to the called station can be substantially eliminated.

0052In the communication system of the invention, unlike a conventional communication system using mono-pole antennas, the directional antennas are used with transmission and reception modes alternated, which eliminates the difficulty that a mobile body (mobile station) receives the radio wave which it has transmitted.

0053In the apparatus according to the first aspect of the invention, the monitor level setting means sets the reception level of the signal receiving system which is not used for actual signal reception to a lower level than the monitor level of the signal receiving system which is used for actual signal reception, and therefore when input signals are substantially equal in reception level, the signal receiving system detecting means detects the present signal receiving system as highest in reception level. Thus, in the case when signals substantially equal in reception level are received, the signal receiving system change-over means does not perform the switching of the signal receiving system, which reduces the possibility of data loss.

0054In the apparatus according to the second aspect of the invention, when the reception level of the present signal receiving system becomes lower than the lower threshold value while the reception level of the signal receiving system highest in reception level exceeds the upper threshold value, the present signal receiving system is switched over to the signal receiving system highest in reception level. Therefore, when input signals are substantially equal in reception level, the switching of the signal receiving systems is not carried out. In other words, when input signals are substantially equal in reception level, the signal receiving system change-over means does not operate, with the result that the possibility of data loss is reduced.

0055In the system according to the third and fourth aspect of the invention, the antenna in the signal receiving system highest in reception level is used as a transmitting antenna, so that the called station (or fixed station) can receive a radio wave with less fading, which reduces the probability that transmission data is not transmitted to the station.

0056In the system according to the fifth aspect of the invention a signal receiving system of the lowest level in signal reception level of at least two signal receiving systems is interrupted, and the antennas in the remaining signal receiving systems are used for signal communication. Thus, signals can be received with less fading, and can be transmitted over a wide range, with the result that, for instance in an emergency, signal communiation can be made with stations other than the predetermined stations.