Generally, main transmitters and repeaters are disposed based on natural geographical features and service areas. Repeaters are set up in areas where signals from a main transmitter are received weak to resolve the problem of instable signal reception and widen the coverage of the main transmitter.
FIG. 1 shows a service using conventional repeaters, which repeat signals by using different frequencies.
In the service using conventional repeaters of FIG. 1, a main transmitter 101 transmits signals through a transmission frequency A, and repeaters 102 to 105 repeat the signals through frequencies B, C, D and E, which are different from the transmission frequency A. In other words, the conventional repeaters 102 to 105 shown in FIG. 1 resolves the instability in receiving signals transmitted from the main transmitter 101 and expands the service coverage of the main transmitter 101 by using different frequencies B, C, D and E.
Since the repeaters 102 to 105 use different frequencies to repeat the signals, it is inefficient in the respect of frequency usage because it requires many frequency resources.
FIG. 2 shows another service employing conventional repeaters 202 to 205, which repeat signals by using the same frequency as that of a main transmitter 201. In short, the main transmitter transmits signals through a transmission frequency A, and the repeaters 202 to 205, which will be referred to as on-channel repeaters, hereinafter, repeat the signals through the same frequency as the transmission frequency A. To provide the service, signals transmitted from the main transmitter 201 and the on-channel repeaters 202 to 205 should be the same and time delay between them should be small.
In other words, when the signals transmitted from the main transmitter and the on-channel repeaters which use the same frequency band are not the same, signals in the same frequency band become noise to each other and the noise signals become on-channel interference signals that are not removed by an equalizer or other devices in a receiver. Also, signals transmitted from the main transmitter and the on-channel repeaters have time delay that goes out of the multi-path signal removal ability of the equalizer at the receiver, the equalizer of the receiver cannot remove the delayed signals.
Therefore, to provide the service using the on-channel repeaters, the output signals of the on-channel repeaters should be the same as the output signals of the main transmitter and time delay between two output signals should be small.
The following explanation is provided to illustrate the drawbacks occurring when signals are repeated using the conventional on-channel repeater with reference to FIGS. 3 to 6.
FIG. 3 illustrates an exemplary view of a conventional RF amplification on-channel repeater.
As shown in the drawing, in the conventional RF amplification on-channel repeater, a RF broadcast signal is received from a main transmitter through a receiving antenna 301 and a RF receiver 302, and the received RF signal passes through a RF band pass filter 303 only at a desired signal band. The band passed RF signal is amplified through a high power amplifier 304 and transmitted through a transmitting antenna 305 of the on-channel repeater over an on-channel. The RF amplification on-channel repeater has a low system delay and a simple structure.
FIG. 4 describes an exemplary view of a conventional IF conversion on-channel repeater.
As shown in the drawing, in the conventional IF conversion on-channel repeater, a RF broadcast signal is received from a main transmitter through a receiving antenna 401 and a RF receiver 402. The received RF signal is converted into an IF signal through an IF down-converter 403, and the IF signal passes through the IF band pass filter 404 only at a desired signal band. The band passed IF signal is converted into a RF broadcast signal through the RF up-converter 405, and the RF broadcast signal is amplified through a high power amplifier 406 and then transmitted through a transmitting antenna 407. The IF conversion on-channel repeater also has a low system delay and a simple structure. Further, a selection characteristic of the band pass filter is superior to that of the RF amplification on-channel repeater of FIG. 3.
FIG. 5 illustrates an exemplary view of a conventional Surface Acoustic Wave (SAW) filter on-channel repeater.
As shown in the drawing, in the conventional SAW filter on-channel repeater, a RF broadcast signal is received from a main transmitter through a receiving antenna 501 and a RF receiver 502, and the received RF broadcast signal is converted into an IF signal through an IF down-converter 503. The IF signal passes through an SAW filter 504 only at a desired signal band, and the SAW filtered IF signal is converted into a RF broadcast signal through a RF up-converter 505. The RF broadcast signal is amplified through a high power amplifier 506 and then transmitted through a transmitting antenna 507. The SAW filter on-channel repeater has also a low system delay and a simple structure. Further, a selection characteristic of the SAW filter is superior to that of the IF conversion on-channel repeater of FIG. 4.
FIG. 6 illustrates an exemplary view of a conventional demodulation/modulation on-channel repeater. As shown in the drawing, in the conventional demodulation/modulation on-channel repeater, a RF broadcast signal is received from a main transmitter through a receiving antenna 601 and a RF receiver 602. The received RF broadcast signal is converted into an IF signal through an IF down-converter 603. The IF signal is converted into a baseband signal through a demodulator 604. The multi-path signals, noise generated due to the transmission channel between the main transmitter and the on-channel repeater, and feedback signals caused by low isolation are removed from the baseband signal in an equalizer and forward error correction (FEC) decoder 605. The output signal of the equalizer and FEC decoder 605 is error-correction encoded through FEC encoder 606. The FEC-encoded signal is converted into an IF band broadcasting signal through a modulator 607. The converted IF signal is converted into a RF broadcast signal through a RF up-converter 608. The RF broadcast signal is amplified through a high power amplifier 609 and transmitted through a transmitting antenna 610.
In the conventional on-channel repeater in FIGS. 3 to 5, the characteristics of the output signals of the on-channel repeaters are inferior to those of the input signals of the on-channel repeaters since the noise and multi-path signals generated due to the transmission channel between the main transmitter and the on-channel repeater, the feedback signal generated due to the low isolation of the transmitting/receiving antennas, and a system noise added from an on-channel repeater system, etc. cannot be removed. The above mentioned conventional on-channel repeaters shown in FIGS. 3 to 6 have drawbacks in that the transmission power of the on-channel repeater is limited due to the feedback signal generated due to the low isolation of the transmitting/receiving antennas.
The conventional on-channel repeater having a modulator and a demodulator in FIG. 6 improves the poor multi-path and noise elimination ability of the conventional on-channel repeaters in FIGS. 3 to 5 remarkably. However, since the conventional demodulation/modulation on-channel repeater in FIG. 6 includes a FEC decoder and a FEC encoder, the time delay in the on-channel repeater is increased from microseconds to milliseconds. Since the output signal generated due to ambiguity of a general standard trellis encoder of FIG. 6 is recognized as noise, the on-channel repeater does not remove the output signal generated due to ambiguity.
Accordingly, to solve the above-mentioned drawbacks, an on-channel repeater is required, which can output the same signal as the output signal of the main transmitter, has low time delay between two output signals, removes the noise and multi-path signals generated by the transmission channel between the main transmitter and the on-channel repeater so that the output signal of the on-channel repeater has a superior characteristic to that of the input signal, and removes the feedback signal generated by the low isolation between the transmitting/receiving antennas so that the transmission output power of the on-channel repeater can be increased.