Abstract:
Reliable transmission of signals in a multi-channel system is achieved by transferring signal inputs from a defectively operating or inoperative transmitter to another transmitter which continues to transmit other channels. At least two transmitters are operated normally, each transmitting its own block of channels. A sensor detects when the output of one transmitter is improper or non-existent, and causes a change-over switch to operate. Operation of the switch adds the block of channels which had come from the one transmitter to the other (or one or more) of the transmitters, so that the remaining transmitter or transmitters now provide all the channels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to transmitter arrangements for multi-channel operation, and more particularly to arrangements in which switch-over to a back-up transmitter is desirable. 
     2. Description of the Prior Art 
     To prevent direct revenue loss, and loss of subscriber or listener loyalty, most commercial radio and television transmitter arrangements include some provision for continued broadcasting even though a portion of the regular transmitter has failed. Sometimes an entire duplicate transmitter and modulator are provided. However, such arrangements are quite expensive, so a common arrangement includes a back-up transmitter of less than half the power of the main transmitter. Antenna connections and. low level signal inputs usually must be changed from one transmitter to the other, and the back-up transmitter may require a significant warm-up time to provide stable operation. These make automatic switch over difficult and cause an undesirably long service interruption. 
     SUMMARY OF THE INVENTION 
     According to the invention, a multi-channel transmitting arrangement includes two transmitters, each of which in normal operation transmits a block of signals for approximately half the total bandwidth to a communications device such as a cable, waveguide or antenna; when the channels are similar, each transmits half the channels. In the event of failure of one of the transmitters, the signals for the block or channels it has been transmitting are provided to the other transmitter, which then transmits all the channels. Preferably the power transmitted per channel will remain substantially unchanged, although it may not be possible or practical to avoid an increase in distortion. Alternatively, the power level per channel may be reduced. 
     In normal operation, one block of channels may occupy the lower half of a band, and the other block occupies the upper half. An alternative arrangement may use two blocks with interleaved channels, where each block is spread over nearly the entire same band. 
     Because high level switching of antenna connections is not required, an arrangement according to the invention is relatively easy to automate, and no warm-up time is lost. 
     Whenever, in normal operation, each transmitter is operated below its maximum power output, for example to improve operating life or to reduce distortion, it is usually practicable that the back-up mode provide for transmission at the same power level per channel, so that service area coverage is not reduced. This is particularly applicable when adaptive power control is used to vary the transmitting power to accommodate time-varying changes in signal propagation over the service area, and the transmitters may be expected to be operating normally well below saturation. 
     In situations where signal attenuation in the service area fluctuates widely over the course of a year, transmitter operating cost considerations may make it desirable to operate the system such that, when minimum transmitter power will suffice, it is possible to turn off or to put one transmitter in a standby mode, and to use the switch to transfer all channels to the other transmitter. Where this mode is otherwise practicable, an important consideration will be the time required to activate, warm up or stabilize the transmitter which is not being actively used, so that high reliability of service can be maintained. 
     In a preferred embodiment, the transmitting arrangement handles a plurality of television or similar wide-band channels at frequencies above 10 to 12 GHz, such as a 1 GHz band between 27.5 and 28.5 GHz. For output powers of 120 watts per block, one traveling wave tube for each block of 25 FM channels provides sufficient power per channel to provide omnidirectional coverage out to a 3 mile radius. In normal operation, the lower frequency tube transmits channels 1-25 at 27.5 to 28.0 GHz, and the upper tube transmits channels 26-50 at 28.0 to 28.5 GHz. 
     In another embodiment, each transmitter includes a plurality of solid state amplifying or mixing/amplifying devices connected to one or a like plurality of antenna elements. Where each such device normally provides power for one small frequency block or respective channel of a large number of channels being transmitted, under standby operation each device would then transmit two such blocks or channels. 
     At some sacrifice in added complexity, the same inventive principle can be applied in situations where it is desired to have more than two amplifiers handling respective portions of the spectrum, and in the event of failure or poor performance by one, this portion can be shared among the others, or can be added in its entirety to the block transmitted by another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a cellular transmitting arrangement in accordance with the invention, and 
     FIG. 2 is a schematic diagram of a switching arrangement for use with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The system shown schematically in FIG. 1 includes two pluralities of signal sources  10  and  11 , which are respectively modulated and combined in modulators  20  and  21  to provide blocks of signals  22  and  23 . Each block of signals may include channels of different types, modulation characteristics and individual bandwidth, or could be a single broadband channel; it is not necessary that more than one signal source  10  or  11  be involved. In fact, the combined blocks  22  and  23  could represent one very wide band program material which, because of limitations on transmitter or receiver technology, or some other reason, is preferably transmitted in two portions over respective transmitters. 
     The signal blocks  22  and  23  are each input to a primary coupler  30  and a standby coupler  31 . These couplers are normally identical, and may be simple signal combining devices or active devices such as frequency converters. Each coupler has two outputs, one containing signals corresponding to block  22 , and the other containing signals corresponding to block  23 . The four outputs are input to a switch  40 , which would usually be automatically electrically controlled. The switch has two outputs, one providing signals to transmitter  50  and the other to transmitter  51 . Transmitter  50  has its output fed to antenna  52 , and transmitter  51  has its output fed to antenna  53 . Of course, where it is technologically feasible, the outputs of both transmitters can be fed to one antenna, so long as failure of one transmitter will not load the other because of coupling through the antenna or its feed. 
     A control  60  is connected to sense loss of output from either of the transmitters  50  and  51 . The sensing is shown as coming from the respective antennae  52  and  53 , but it will be clear that loss of transmitter output can be sensed at other locations within the transmitter and feed combination, or remote from the antenna. The control  60  has an output which is connected to cause operation of the switch  40 . 
     In normal operation, switch  40  is set so that the output signals from primary coupler  30  corresponding to block  22  are input to transmitter  50 , and the output signals from primary coupler  31  corresponding to block  23  are input to transmitter  51 . In the event of loss of output from one of the transmitters (or any other failure affecting output from one only of the transmitters), the switch  40  is set to couple signals from the standby coupler  31  to the transmitter whose output had been normal. 
     According to another feature of the invention, the control  60  can be connected to an adaptive power control circuit, so that the adaptive power control circuit provides the output loss signal to the control  60 , or can use the transmitting power control to affect a change in output power per channel automatically upon switching to the standby mode. The adaptive power control circuit may includes one or more sensors  70  provided at different locations within the transmitter service area, which transmit by wire (e.g., telephone lines, or telemetry signals such as “carrier current” over local power lines) or radio or other link to a unit  72  at the transmitter site. The unit includes a control portion  74  which provides power controlsignals; and a receiver portion  76  which receives the radio or other link signals from the sensor or sensors. The power level control is shown as being performed at the transmitters  50 ,  51 . However, depending upon the choice of equipment and modulation or frequency converting arrangement, it may be preferable to control transmitter power level by affecting circuits in the couplers  30 ,  31  or modulators  20 ,  21 . 
     The sensors  70  can be provided at one or more predetermined, fixed locations. Alternatively, some or all of the normal service receivers, such as customer premises equipments, may be equipped for two-way operation. In that case the receiving sections at those service receiving sites can be polled periodically to provide a report signal related to the signal strength being received at the individual site. Where propagation is affected heavily by rainfall, such a system provides a higher likelihood of identifying the effect of small intense rain cells. 
     The switching arrangement shown in FIG. 2 is especially adapted for transmission of a large number of FM television channels having a nominal bandwidth of 20 MHZ each. Of course, one or more channels can have wider or narrower bandwidth, and can carry digital or other signals at varying bit or burst rates. There is no need that the number of channels or their spacing be the same for the two blocks. 
     Frequency agile modulator blocks  120  and  121  each contain 25 individual modulators such as  120   a , and take input signals (not shown) which may be baseband signals or may themselves be higher frequency signals such as ISDN or T- 1  or E- 1  signals, and modulate them at different channel frequencies. The outputs of modulator blocks  120 ,  121  are blocks of signals  122 ,  123  between 50 and 550 MHZ, so that a total of 1 GHz of channel space is being provided without need for modulators which will operate at or above 1 GHz. Each of these blocks is input to a primary L-band upconverter  130  and a standby L-band upconverter  131 . 
     In this embodiment the two upconverters are identical. They have respective outputs of two signal blocks  132 ,  134  and  133 ,  135 . Blocks  132  and  133  contain signals between 2.1 GHz and 2.6 GHz, and blocks  134  and  135  contain signals between 2.6 GHz and 3.1 GHz. The blocks  133  and  135  are then combined in a coupler  137  to form a block between 2.1 GHz and 3.1 GHz, and provided to a common terminal of a single pole, double throw coaxial switch  147  which is part of a three independent section electrically controlled switch  140 . For normal operation, over a normally closed contact set whose common contact is connected to a mixer  154  of a transmitter  150 , a switch section  142  connects the block  132  to the input of the transmitter  150 ; similarly, over a normally closed contact set switch section  144  connects block  134  to a transmitter  151 . 
     In the event that transmitter  151  is sensed to have improper or no output, control  160  cause switch  142  to be operated to connect the input to mixer  156  to a normally open contact which in turn is connected to a normally closed contact of switch  147 , thereby providing the signals forming blocks  133  and  135  to the mixer  156 . At the same time, switch  144  is operated. The normally open contact of switch  144  is connected to a normally open contact of switch  147 , so that the input to transmitter  151  is interrupted. 
     In the event that transmitter  150  is sensed to have improper or no output, switch  144  is operated to connect the input to transmitter  151  to the normally open contact which in turn is connected to the normally open contact of switch  147 . Simultaneously, switch  147  is operated to connect signals from the coupler  137  to its normally open contact, thereby providing the signals forming blocks  133  and  135  to the transmitter  151 . At the same time, switch  142  is set to the normally open contact which is connected to the normally closed contact of switch  147 , so that the input to transmitter  150  is interrupted. 
     In this embodiment, transmitter  150  includes an oscillator  155  connected to the mixer  156 , to upconvert the L-band IF signals to the 27.5 to 28.5 GHz band. The 28 GHz band signals are then amplified in a traveling wave tube amplifier  158  which is connected to the antenna  152 . 
     The inventive arrangement is usable whether or not transmitter distortion cancellation techniques are included. Such techniques may, for example, include feed forward, or predistortion. 
     It will be noted that the preferred embodiment is optimized for transmission over a 1 GHz continuous band. If frequencies allocated for service are not in one contiguous band, different variations may be preferred. For example, if transmission is permitted from 27.5 to 28.35 GHZ and from 29.1 to 29.25 GHZ, and possibly also from 31 to 31.3 GHz, practical limitations on the equipment may require careful optimization. For example, it may be preferred, if only the two frequency bands below 30 GHz are utilized, to extend the IF band to a total of 1.75 GHz, and to leave a portion with no signal. This has the advantage that only one upconversion is required, but it greatly widens the pass band required for the IF. Alternatively, the modulators can be set to provide just sufficient empty band between those channels below 28.35 GHz and those above, so that sharp cut-off filters in the transmitter can feed the lower blocks to one upconverter (mixer and oscillator) and the upper block to another upconverter. The latter technique appears more suitable if the band above 30 GHz is also to be utilized 
     Many other variations using the invention will become clear to those of ordinary skill, upon reading this application. For example, the split band described above, where three somewhat separated blocks of frequency are used, may be optimized by use of more than two transmitting sections, and at some additional cost each, or selected ones, of the transmitting sections may have filters or other circuits permitting transmission, when operating in the back-up mode, at frequency bands separated from that normally transmitted. It will be clear that different bands or channels may not only have different bandwidths, but may contain markedly different signal types or be differently modulated. Although undesirable from the standpoint of complexity, back-up operation in which the added block is transmitted with a different polarization is also possible. Accordingly, the scope of the invention should be measured only by the appended claims.