1. Field of the Invention
The present invention relates to a transmission diversity apparatus.
2. Description of the Related Art
The conventional transmission diversity apparatus is structured as follows:
FIG. 1 is a block diagram showing one example of the conventional diversity apparatus.
Carrier frequency radio signals received from antennas #1 and #2 are converted to base band signals by receiving RF sections 1 and 2. Level detectors 3 and 4 detect field strength of each base band signal for a fixed period of time, and send the detected value to mean value calculators 5 and 6, respectively. The mean value calculators 5 and 6 calculate the mean value of the detected values. A corrector 7 obtains an output difference of the mean value calculators 5 and 6, and adjusts the characteristics of these level detectors 3 and 4 based on the obtained output difference.
A comparator 8 compares the receiving level of the level detector 3 with that of the level detector 4, determines which receiving level is higher, and outputs the result to a switching device 9. The switching device 9 switches the transmission antenna in accordance with the output of the comparator 8.
A transmitting signal is input from a terminal 10, modulated by a modulator 11, and converted to a carrier frequency radio signal by a transmitting RF section. Thereafter, the radio signal passes through a receiving/transmitting separator 13 and is transmitted from either one of antennas #1 and #2 switched by the switching device 9.
Thus, the convention transmission diversity apparatus selects the antenna having the highest signal receiving level from the plurality of antennas, thereby transmitting the signal. As a result, irregularity of the characteristics of the receiving level measuring system is automatically corrected, so that transmission diversity is effectively performed.
The conventional diversity apparatus, however, selects the transmission antenna based on only the levels of the receiving signals. The receiving signals include a desired and an interfering signal. Due to this, even in a case in which the signal level of an interference station is: high, there causes a disadvantage in which such an antenna may be selected as an appropriate transmitting antenna.