Microwave relay station having a common standbby channel for signals of different types of modulation

Abstract: A multichannel microwave relay station has first and second channels for respectively transmitting signals of different types of modulation, such as QAM and FM modula­ ted signals. These different types of signals rely on a common standby transmission channel that is selectively capable of transmitting the different types of signals. The type of signal currently carried by the standby channel is automatically identified by a detector circuit (6), such as one that can auto­ matically distinguish differences in the frequency spectrums of the different types of signals. A selecting circuit (3) re­ sponds to the output of the detector circuit (6) by selecting between first and second transmitting means (5) in the common channel, thereby dedicating it to the identified type of signal.

Description: This invention relates to microwave communication multichannel systems in which a standby channel transmission path is used to increase the reliability of the transmission paths of a group of channels, and more particularly to a transmission path switching system which makes common use of a standby transmission path which can be adjusted to accommodate signals which may have different kinds of modulation. Hereafter, such a system is called a "common standby channel switching system". Nowadays in microwave communication lines the two most common types of modulation used are frequency modulation (FM) and quadrature amplitude modulation (QAM), and in multichannel lines it is not unusual for signals of both kinds of modulation, which are very different, to be simultaneously transmitted. Therefore, it is increasingly necessary, both to reduce cost and to make efficient use of available bandwidth, that a standby channel provided for such lines be one that can be commonly used by signals of both types of modulation. However, for signals of different types of modulation to be able to commonly use the same standby transmission path, it must be transparent to both types of signals. The background of the invention and the invention itself, including its various features and the manner of obtaining them, will become more apparent and will be best understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a system block diagram of an embodiment of the invention in which a relay station has an automatic common standby channel stitching system for signals of different types of modulation; Fig. 2 is a system block diagram showing in greater detail an example of the detector block appearing in Fig. 1; Fig. 3A shows the spectrum of a received FM modulated signal and its relationship to the detected output voltages of the three demodulators shown in Fig. 2; Fig. 3B shows the spectrum of a received QAM modulated signal and its relationship to the detected output voltages of the three demodulators shown in Fig. 2; and Fig. 4 shows a system block diagram of a relay station having a common standby channel switching system for signals of different types of modulation, controlled by remote signals. A technique will first be discussed from which the present invention starts. Fig. 4 shows a relay station for a multichannel system having a common standby channel switching system for signals of different types of modulation. A microwave signal picked up by a receiving antenna 50 is input to a demultiplexer 41, such as a frequency demultiplexer. Demultiplexer 41 distributes the signal to receivers 42-1, 42-2, and 42-3. The output of receiver 42-1 is input to a regular QAM channel, and the output of receiver 42-3 is input to a regular FM channel; these are both shown as currently in use. The output of receiver 42-2 is input to a common standby channel, whose signal can be switched as needed between alternate transmission paths by switching circuits 43 and 46. Switching circuits 43 and 46 are controlled by a signal-switching control circuit 45. The regular QAM channel and the upper branch of the common standby channel respectively have QAM demodulators 44-1, 44-2, whereas the regular FM channel and the lower branch of the common standby channel respectively have a direct path for FM signals. Each channel leads to a respective transmitter 47-1, 47-2, and 47-3 whose output is connected as input to a multiplexer 48 which feeds a transmitting antenna 52. The FM signals in the relay station's FM channels have the convenient property that as long as the received FM signal quality remains above a predetermined minimum acceptable value, they can be fed directly to the next transmitter, such as transmitter 47-3. That is, they can be retransmitted without being demodulated. But in the case of the relayed QAM signals, to prevent them from becoming too weak they must be demodulated and the modulating waves reconstituted before they can be remodulated for transmission. So the QAM transmission paths used for QAM channels each use a QAM demodulator 44-1, 44-2. Therefore, the signal-switching control circuit 45 is provided in the common standby channel switching system to control the necessary switching of the differently modulated signals into appropriate alternate paths. Moreover, in this system, one of the terminal stations in the relay needs to keep this control circuit 45 informed about which type of modulation is being used on the standby channel, and this is done by adding such information to the main signal before sending it. In other words, when a relay station employs such a common standby channel switching system, it must additionally include a device capable of generating a signal-switching control signal in response to switching information embedded in the main signal received from a remote terminal station. This has the disadvantage or increasing the complexity, as well as cost, of the relay station unit. US-A-4 272 788 relates to a receiver for television sound multiplex broadcasting and FM stereo multiplex broadcasting. This known receiver particularly comprises a first detector circuit adapted as to detect the presence or absence of a 31,5 kHz subcarrier signal or a 19 kHz pilot signal included in a received composite signal. A second detector circuit is adapted to detect a modulation frequency of a control channel signal, which is included in said composite signal. The second detector circuit comprises a detector for detecting a modulation frequency of 922,5 Hz and a detector for detecting a modulation frequency of 982,5 Hz. Dependent on the reception of the before mentioned control signals a certain operation mode is used. In the case of monophonic FM broadcasting, the pilot signal of 19 kHz is not included in the composite signal. Whereas during the reception of FM stereo multiplex broadcasting the 19 kHz pilot signal is present. In the TV sound broadcasting reception the composite signal comprises a main channel signal, a subchannel signal and a control channel signal. In the case of the stereo broadcasting the stereo control signal of 982,5 Hz is detected whereas in the TV dual sound broadcasting a control signal of 922,5 Hz is detected. In the case of monophonic TV sound broadcasting no control signal is present. By means of a switch either the TV reception circuit or the FM reception circuit supplies the signal to the receiver. Accordingly, an object of this invention is to provide a new and improved relay station employing a common standby channel switching system and a respective method that eliminates the above disadvantages. A more particular object is to provide a relay station that can generate a control signal for the switching circuits 43 and 46 within the relay station itself without receiving switching information from a remote terminal station. This object is achieved by the features of the claims. The microwave relay station according to the present invention comprises first and second channels for respectively transmitting two different types of signals, such as QAM and FM modulated signals. These have a common standby transmission channel that is selectively capable of transmitting the two different types of signals. The type of signal currently carried by the standby channel is identified by a detector circuit, such as one that can automatically distinguish differences of the amplitudes at different frequencies of the different types of signals. A selecting circuit responds to the output of the detector circuit by selecting between first and second transmitting means in the common channel, thereby dedicating it to the identified type of signal. As shown in Fig. 1, a microwave station constructed in accordance with the teachings of this invention has a receiving antenna 50 which provides input to a demultiplexer 1 which distributes the signal to receivers 2-1, 2-2, and 2-3. The outputs of receivers 2-1 and 2-3 are respectively input to regular QAM and FM channels; both shown as currently in use. The output of receiver 2-2 is input to a common standby channel, whose signal can be switched as needed between alternate transmission paths by switching circuits 3-1 and 3-2, controlled by a detector circuit 6. The regular QAM channel and the upper branch of the common standby channel respectively have QAM demodulators 4-1 and 4-2, and the regular FM channel and the lower branch of the common standby channel respectively have a direct path for FM signals. Each channel leads to a respective transmitter 5-1, 5-2, and 5-3 whose output is connected as input to a multiplexer 7 which feeds a transmitting antenna 52. The signal from demultiplexer 1 for the common standby channel traverses receiver 2-2 and is sent to both switching circuit 3-1 and detector circuit 6. Detector circuit 6 then judges the type of modulation of the input signal and sends a corresponding control signal to activate switching circuits 3-1 and 3-2 to adjust the standby channel for QAM or FM modulated signals. Figure 2 shows an embodiment of detector circuit 6, while Figs. 3A and 3B respectively show the relationship of the FM or QAM spectrum of the received signal in the common standby channel to the detected output voltages V₋, V₀, and V+ of demodulators 64, 65 and 66. As can be understood from Figs. 3A and 3B, the spectrum of the signal in the common standby channel is clearly different depending on the type of modulation. In order to detect these spectrum differences, the strength of the signal at least 3 sample points f₋, f₀, and f+ on the frequency axis is detected by demodulators 64, 65 and 66, which together respectively output a set of detected voltages V₋, V₀, and V+ . A judging circuit 67 (Fig. 2) determines whether the type of modulation is FM or QAM based on the relative differences within this set output voltages. Then judging circuit 67 generates a corresponding control signal to control switching circuits 3-1 and 3-2. For example, when the detected output voltage V₀ associated with the center frequency f₀ is substantially higher than the amplitudes of the voltages V+ and V₋ respectively associated with the high and low frequencies, the judging circuit 67 can generate a control signal to make switching circuits 3-1 and 3-2 configure the common standby channel for FM. But should the detected output voltage V₀ not be substantially higher than the amplitudes of the voltages V+ and V₋, the judging circuit 67 can generate a control signal to make switching circuits 3-1 and 3-2 configure the common standby channel for QAM. For this reason, Fig. 2 shows that the signal from receiver 2-2 is input to a set of bandpass filters 61, 62 and 63 for respectively letting through just a narrow frequency band at each of the needed sample points (f₋, f₀, f+) on the frequency spectrum. Then demodulators 64, 65 and 66 convert the signal energy at the sample points into the corresponding detected output voltages V₋, V₀, and V+ . Those skilled in the art will readily appreciate that the present system can be easily adapted to work with other types of modulation. For example, phase modulation could be used in place of QAM modulation. As explained above, the present invention is particularly useful for a multichannel relay station employing a common standby channel switching system which must appropriately switch the standby transmission path to match the modulation type of a signal input to the standby channel. The invention provides the relay station with a relatively simple detector circuit which can generate and output the necessary signal-switching information from the received signal in the standby channel, thereby activating the switching circuits to adjust the standby channel for the type of modulation. This simplifies the construction of the relay station and lowers its cost.

Claims: A relay station having channels (2-1, 2-3) dedicated to receiving and retransmitting FM and QAM signals respectively, at least one standby channel (2-2) for transmitting either said FM or said QAM signals, detection means (6) associated with said standby channel (2-2) for measuring amplitudes at high, low, and center frequencies in said standby channel, and switching means (3-1, 3-2) responsive to said detection means (6) for switching said standby channel (2-2) to transmit FM signals responsive to a detection of amplitudes wherein the amplitude at said center frequency is substantially higher than the amplitudes at said high and low frequencies and for switching said standby channel (2-2) to transmit QAM signals responsive to a detection of amplitudes wherein the amplitude at said center frequency is not substantially higher than the amplitudes at said high and low frequencies. The relay station of claim 1 wherein said detection means (6) comprises three bandpass filters (61, 62, 63) coupled to receive signals appearing in said standby channel and respectively tuned to said high, low, and center frequencies, three demodulators (64, 65, 66) respectively coupled to the output of individually associated ones of said filters; and judging means (67) coupled to the output of said demodulators for determining the relative amplitudes of said high, low, and center frequencies. The relay station of claim 2 comprising means responsive to said judging means (67) for selectively operating said switching means (3-1, 3-2). The relay station of any of claims 1 to 3 comprising receiver means (50) for receiving and transmitter means (52) for retransmitting multiplexed multi-channel microwave signals; means (1) responsive to said receiver means for demultiplexing said multi-channels and applying the resulting signals to inputs of said channels; means (7) responsive to outputs of said channels for multiplexing said multi-channels for retransmission; means in dedicated FM channels for applying separated FM signals directly to said multiplexing means; and means in dedicated QAM channels for applying separated QAM signals to means for demodulating, reconstituting, and remodulating before applying them to said multiplexing means. The relay station of claim 4 comprising first means in said standby channel for selectively applying separated FM signals directly to said multiplexing means (7); second means (4-2) in said standby channel for demodulating, reconstituting and remodulating the separated QAM signals and applying them to said multiplexing means (7); and means (3-1, 3-2) responsive to said detection means (6) for selecting between said first and second means. A method of receiving and transmitting FM and QAM signals through a relay station, said method comprising the steps of: dedicating at least first and second channels for respectively transmitting FM and QAM signals; providing a standby transmission channel having first and second means for respectively transmitting either said FM or said QAM signals; detecting the type of said signals by measuring amplitudes at high, low and center frequencies appearing in said standby channel; selecting FM transmitting means responsive to said determined amplitudes when the amplitude at the center frequency is substantially higher than the amplitudes at the high and low frequencies, and selecting QAM transmitting means when the amplitude of the center frequency is not substantially higher than the amplitude of the high and low frequencies. The method of claim 6 wherein said detecting step comprises the steps of separating signals in said standby channel into high, low, and center frequencies, determining amplitudes of the signals at said high, low and center frequencies, and controlling said selecting step responsive to said determined amplitudes.

Inventors: TAKAHASHI KUNIHIKO C O NEC COR, TAKAHASHI, KUNIHIKO C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1986
Main classification: H04L1
IPC codes: H04L27, H04L1, H04B1