Regenerative RF bi-directional amplifier system

A regenerative RF bi-directional communications system is provided for establishing RF coverage within a RF block tunnel area. The system uses a plurality of cascaded amplifier stages for periodically regenerating signals which are transmitted and received along a series of radiating cable length which link base station transceivers to hand-held or like mobile communication units. An intermediate frequency distribution system is used so that the required level of amplification is achieved through the several cascaded amplifier stages at the level of low-power IF signals generated from the original RF signals in conjunction with the appropriate oscillator and pilot signals. The IF distribution system restricts the cascading effect occurring due to the plurality of cascaded amplifier stages on the relatively low-power IF signals, thereby producing a negligible amount of intermodulation.

FIELD OF THE INVENTION 
The present invention generally relates to radio frequency (RF) 
communication systems using regenerative amplifiers. More particularly, 
this invention relates to an improved RF communication system using 
cascaded regenerative amplifiers in which excessive intermodulation 
distortion is avoided. 
BACKGROUND OF THE INVENTION 
In implementing a variety of RF communication systems such as high 
frequency terrestrial communication systems, broadcast transmitting 
antenna systems, and particularly cellular and land mobile radio systems, 
a major obstacle is the provision of optimum RF coverage in difficult, 
confined areas such as tunnels, subways, depressed roadways, buildings, 
etc., where radiating RF energy is substantially blocked in both the 
transmitting and receiving directions. The problem is typically approached 
by using some form of directional antenna or radiating cable system for 
obtaining adequate coverage throughout the confined or blocked areas. 
Tunnels represent a particularly difficult environment for RF coverage 
because of the natural blockage presented to high frequency radio signals 
by the concrete, earth and steel disposed throughout the confines of the 
tunnels. In small tunnels, optimum RF coverage can be obtained by using 
directional antennas which are disposed at the tunnel portals and 
appropriately adjusted. A major disadvantage associated with the use of 
directional antennas, however, is that any large deviations in the tunnel 
geometry such as bends or turns can result in significant signal loss. In 
addition, such an approach is also highly prone to blockage from large 
vehicles such as trucks and trailers, and is impractical for use with 
extended enclosed areas such as long tunnels or subways. 
RF coverage for very long tunnels or subways has generally been provided by 
using distributed antenna systems using low-loss coaxial cable or 
fiberoptic arrangements for signal distribution, and a series of antennas 
which are fed by taps attached to the transmission line. The problem with 
this approach is that, since high frequency radio signals are completely 
confined to the tunnel due to the natural RF blocking properties of the 
tunnel composition, the radiation pattern of point source radiators, such 
as antennas, can cause reflections resulting in serious multi-path fading 
and signal attenuation and nulling. Further, it becomes necessary that the 
distributed antennas be carefully tuned in order to maintain the necessary 
isolation among the plurality of system antennas. 
A conventional approach to providing optimum signal distribution and 
coupling of RF energy within tunnels or subways without employing discrete 
antennas has been the use of radiating cables. As opposed to standard 
coaxial cables which transmit electrical signals to and from a generating 
station to some form of antenna from where the signals are radiated, 
radiating cables themselves function as continuous antennas--electrical or 
radio signals are transmitted directly from the cables rather than from an 
antenna. Such radiating or "leaky" coaxial cables form efficient and 
economic sources for transmitting radio frequency signals in a variety of 
applications such as 2-way mobile radio, radio paging and other localized 
broadcast services in applications involving extended underground 
installations. The radiating cable approach becomes indispensable in 
applications such as railways, subways, mines and tunnels, where 
conventional centralized VHF and UHF communication systems are not 
practical. 
In radiating cables, slots are provided in the typically corrugated 
metallic outer conductor of a coaxial cable to allow a controlled portion 
of the transmitted RF signals to radiate along the entire length of cable. 
Conversely, signals transmitted near the cable will couple into these 
slots and be carried along the cable back to the associated base station 
receiver. Because radiating cables can be designed for broadband 
operation, it is possible for a single length of radiating cable to 
simultaneously handle two or more communication systems. Since the cable 
can conveniently be routed wherever signal coverage is needed, radiating 
cables can be adapted for operation in areas of any form-factor, open or 
enclosed, which require localized coverage. 
Because of the above advantages, the radiating cable approach is 
increasingly being used in tunnel coverage systems where two-way RF 
communications between base station transceivers and portable or mobile 
communication units is essential. In these systems, signals transmitted to 
and from base stations to mobile communication units within a tunnel are 
distributed through a series of amplifiers over a plurality of radiating 
cable lengths. Because of the finite coaxial attenuation factor of the 
radiating cable, it becomes essential to use signal amplifiers at periodic 
intervals. The amplifiers are typically bi-directional in order to 
accommodate and amplify signals moving along the cable in both directions, 
and provide a fixed amount of gain for each RF carrier signal in each 
signal direction. Since most communication systems utilize multiple 
carriers for normal operations, it becomes necessary to use Class-A linear 
amplifiers capable of providing wide-band gain. 
Since ideal linear amplifiers cannot be realized in practice, two-way 
communication systems using the radiating cable approach are subject to 
signal distortion resulting from the finite amount of intermodulation 
distortion generated due to component non-linearities when amplification 
occurs at high power levels. Out-of-band intermodulation does not present 
a serious problem because it can usually be filtered out of the system. 
However, in-band intermodulation caused by intermodulation frequencies 
resulting from undesirable combinations of sinusoidal components of input 
frequencies can substantially affect system performance since it 
represents a noise source to a receiving unit which cannot be filtered. 
The intermodulation distortion problem is further compounded when a 
plurality of amplifiers are cascaded in order to achieve the high 
transmission signal levels necessary to combat coaxial loss resulting from 
use of radiating cable across long tunnels or subways. When a chain of 
cascaded amplifiers exists, the intermodulation produced by one stage is 
amplified by the gain factor of each succeeding stage, which in itself 
generates further intermodulation components. In effect, in-phase addition 
of frequency components at each stage of amplification in a cascaded 
amplifier system also realizes a cascading or compounding of the 
intermodulation components. The end result is serious degradation of the 
RF signals to an unmanageable level. 
Accordingly, in previously known systems, the implementation of radiating 
cable based two-way RF communication has severe restrictions regarding the 
practical number of amplifiers that may be used in any single chain. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide an improved RF 
bi-directional amplifier system exhibiting a substantially reduced amount 
of intermodulation distortion resulting from successive signal 
amplification. 
In this regard, it is a specific object of this invention to provide a 
regenerative bi-directional amplifier system of the type having a 
plurality of cascaded amplifier stages, each configured in such a manner 
that the overall intermodulation generated by the system is substantially 
independent of the number of amplifier stages. 
A related object is to provide a regenerative amplifier system of the above 
kind in which the in-phase addition of intermodulation components 
generated at each of the plurality of cascaded amplifier stages is 
substantially avoided. 
Yet another object is to provide such a regenerative amplifier system which 
is particularly adapted for providing RF coverage in tunnel or subway 
communication systems or the like using radiating cable for two-way 
communications between base stations and mobile communication units. 
Briefly, in accordance with the system of this invention, the above objects 
are accomplished by means of a RF communications system using a plurality 
of cascaded amplifier stages for periodically regenerating signals being 
transmitted and received along a series of lengths of radiating cable 
which link base station transceivers to hand-held or like mobile 
communication units. In accordance with an important aspect of this 
invention, an intermediate frequency (IF) distribution system is used so 
that the required level of amplification is achieved through the several 
cascaded amplifier stages, not directly at the RF signal level, but, 
instead, at the level of IF signals which are generated from the original 
RF signals in conjunction with appropriate local oscillator and pilot 
signals. As a result, the cascading effect occurring due to the plurality 
of cascaded amplifier stages is restricted to the relatively low power 
level IF signals, thereby producing a negligible amount of intermodulation 
components during the various amplification stages. 
More specifically, at the base station location, the RF carrier signals 
which are to be transmitted from the base station across the radiating 
cable segments to surrounding portable units (the "transmit" signals) are 
down-converted as a block to an IF level. The down conversion is realized 
by using a pilot signal in conjunction with a reference signal generated 
by a local oscillator. The IF signals so generated are subsequently summed 
with the pilot signal and applied to the radiating cable. 
At each amplifier location, the IF signals propagated across the preceding 
length of radiating cable are up-converted to the level of the original RF 
frequency signals through a mixing operation using a local oscillator 
signal derived from the reference pilot signal. Subsequently, the 
regenerated RF frequency signals are amplified, combined with the IF and 
pilot signals, and then applied to the succeeding length of radiating 
cable. Consequently, the transmit RF frequency signals are regenerated at 
each amplifier location from low level IF signals using a heterodyne 
process. It is significant that the RF signals are not cascaded through 
the various stages of the amplifier chain. Instead, the IF signals, which 
are at relatively low power levels, are cascaded through the various 
amplifier stages, thereby producing a negligible amount of intermodulation 
components and associated distortion. 
According to another aspect of this invention, the RF frequency signals 
which are to be received from the portable units and transmitted across 
the radiating cable lengths to the base stations (the "receive" signals) 
are not subjected to the above-described heterodyne process at all of the 
cascaded amplifier stages; instead, the receive signals are directly 
amplified at the RF frequency level at the cascaded amplifier stages. 
In the case of the transmit signals, it is essential to provide a 
substantially high power level at the receiver end because the signals at 
the output of each amplifier stage must have a power level sufficient to 
overcome a variety of signal attenuation factors including the coaxial 
loss of the cable, the coupling loss, the portable antenna loss, etc. The 
requisite high power levels, in turn, generate substantial intermodulation 
components when successively processed by the cascaded amplifier stages. 
In the case of the receive signals, however, the signal levels applied to 
the input of the amplifier stages are low enough that intermodulation is 
usually not a problem, so that the IF distribution system may be dispensed 
with. 
According to another feature of this invention, the cascaded amplifier 
stages comprising the two-way RF communications system of this invention, 
are continuously monitored for operations failure in both the transmit and 
receive paths using the pilot and the IF signals. Preferably, the 
monitoring function is performed by a centralized computer which is linked 
to the regenerative amplifier system by means of an appropriate interface, 
such as a SCADA (Supervisory Control and Data Acquisition) interface, 
having a parallel input/output port providing both status information and 
amplifier control. In addition, the gain control adjustment of the 
transmit and receive path signals can also be controlled by the central 
computer in order to counteract the effect of flow variations in gain 
levels resulting from exposure of the amplifiers to seasonal temperature 
variations and long-term aging of the amplifiers and the radiating cable. 
In accordance with yet another aspect of this invention, an IF distribution 
system is also used for the base station receive path in applications 
where the length of coaxial cable feeding signals to the radiating cable 
and from the base station lengths used in the RF blocked area is 
sufficiently long that high frequency RF carrier signals are unduly 
attenuated. Under such conditions, it becomes necessary to use several 
bi-directional amplifiers to compensate for the attenuation loss suffered 
by RF signals, particularly along the coaxial feeder line in the receive 
path from mobile or portable communication units to the base stations. 
The use of an IF-based system allows the transmission of received signals 
along the coaxial feeder line with adequate power levels without the use 
of a plurality of bi-directional amplifiers. The same pilot carrier signal 
that is used on the transmit side for reducing excessive intermodulation 
distortion at the cascaded amplifier stages is also used on the receive 
side to up-convert and down-convert the receive RF signals to and from 
specified IF frequencies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, there is shown a two-way communication system 10 
for providing RF coverage within an enclosed area such as a tunnel 12. 
According to this system of FIG. 1, a plurality of base stations 14 are 
linked together by means of a plurality of radiating cable lengths 16. The 
radiating cable 16 is typically of the slotted type wherein slots disposed 
on the metallic outer conductor of the cable permit a controlled portion 
of the electromagnetic signals being transmitted there through to radiate 
along the length of the cable. The radiating cable approach is 
particularly advantageous in two-way communication systems because 
electromagnetic signals transmitted near the cable are coupled into the 
radiation slots and carried along the length of the cable back to the 
associated base stations. Accordingly, radio signals transmitted by mobile 
or portable communication ("radio") units in the vicinity of the radiating 
cable can be picked up directly by the cable and relayed to the base 
stations. 
Lengths of conventional coaxial cable line 19 are used to feed signals to 
and from the base station 14 and the radiating cable lengths 16 within the 
tunnel area. Radiating cable is not used in these areas since the need for 
radiation of RF signals exists only within the tunnel area where the radio 
units are positioned. 
In a communication system of the above type using radiating cable, the loss 
in signal level inherently caused by the cable restricts the length of 
cable over which RF signals may be propagated without degradation of the 
transmitted signals below a required threshold level. As a result, it 
becomes necessary to boost or amplify the transmitted RF signals at 
periodic intervals over the enclosed area through which the cable is used 
to establish two-way communications. In practice, signals transmitted 
between the base stations 14 and radio units (not shown) are distributed 
through a series of amplifiers 18 linking the plurality of radiating cable 
lengths 16. As also shown in FIG. 1, the amplifiers 18 are typically 
bi-directional in order to accommodate and amplify signals propagated 
along the cable in both the transmit and receive directions. More 
specifically, each amplifier 18 includes an amplification stage providing 
a fixed amount of gain for the propagated RF signals in each signal 
direction. 
In order to accommodate the multiple RF carriers typically used in most 
communication systems, the amplifiers 18 usually need to be class-A linear 
amplifiers which are capable of providing wide-band gain. As described 
above, the inherent component non-linearities which exist even in "linear" 
amplifiers at high power levels result in substantial signal distortion 
due to the generation of a finite amount of intermodulation distortion. In 
a typical RF communication system having a plurality of bi-directional 
amplifiers cascaded together, each amplifier stage generates 
intermodulation components, and the intermodulation produced by one stage 
is amplified by the gain factor of each succeeding stage. The production 
of in-phase intermodulation frequency components at each stage of 
amplification compounds the intermodulation distortion to such an extent 
that serious degradation of the RF signals results. 
In order to combat the cascaded intermodulation effect, it is important to 
minimize the amount of intermodulation power generated by each of the 
linear amplifiers which, in turn, can be realized by reducing the total 
output power from the saturated level of the amplifier stages. The 
"back-off" power P.sub.Bo necessary to limit third order intermodulation 
components may be expressed as follows: 
EQU P.sub.Bo =B.sub.s /(N*P.sub.c) (1) 
where B.sub.s is the saturated output power of each amplifier stage, N is 
the total number of RF carriers to be processed by the amplifier, and 
P.sub.c is the power per carrier. 
In designing a cascaded amplifier system of the type shown in FIG. 1, an 
important system design parameter is the ratio of carrier power to the 
intermodulation power, commonly referred to as the 
carrier-to-intermodulation (C/I.sub.m) ratio. The ratio is expressed in 
dBs and represents the amount of desired signal power existing in the 
presence of undesired intermodulation power. Since the RF system 
configuration shown in FIG. 1 consists of a series of cascaded amplifiers, 
the total intermodulation contribution of each amplifier must be 
considered and is defined as follows: 
EQU C/I.sub.m (Amp.sub.1)=10*log.sub.10 (10.67*P.sub.bo.sup.2) (2) 
If N amplifier stages are cascaded together, the overall 
carrier-to-intermodulation ratio for the system is defined as follows: 
EQU C/I.sub.m (System)=C/I.sub.m (Amp.sub.1)-20*log.sub.10 (N) (3) 
It is apparent from the above relationship that the system C/I.sub.m 
performance of a cascaded amplifier system is degraded by a factor of 20 
log.sub.10 (N), which represents 6 dBs of degradation for each additional 
amplifier in a chain of cascaded amplifier stages. Accordingly, the 
practical number of amplifier stages that may be used in a chain of 
cascaded amplifiers for implementing a transmission cable-based RF 
coverage system for tunnels is severely restricted. 
The regenerative RF bi-directional amplifier system in accordance with the 
system of this invention eliminates the above-discussed cascaded RF 
amplifier intermodulation effect. 
In accordance with an important aspect of this invention, an intermediate 
frequency (IF) distribution system is used instead of amplifying the 
propagated signals directly at the original RF frequency levels. The 
result is to restrict the cascading effect occurring due to the plurality 
of cascaded amplifier stages to the relatively low power IF signals. 
Consequently, the amount of intermodulation generated during the various 
cascaded amplifier stages is restricted to a negligible level. 
More specifically, at the base station location, the RF carrier signals 
which are to be transmitted across the radiating cable are downconverted 
to a predefined IF level prior to transmission over the radiating cable 
lengths. An arrangement for accomplishing this result is shown in FIG. 2, 
where signals from transmitting units at the base station 20 are fed to a 
conventional mixer 22. The other input to the mixer 22 is a pilot signal 
generated by a pilot generator 24 in conjunction with an associated local 
oscillator 26. The frequency of the pilot tone is selected to be such that 
the input RF frequency signals to the mixer are effectively down-converted 
to the desired IF level. 
A narrow band of signals from the down-converted IF signals are filtered by 
using a band pass filter (BPF) 28 and are fed through an appropriate load 
30 to an amplifier 32. The signal generated by the pilot generator 24 is 
also fed to a summing unit 34 which receives its other input from the 
output of amplifier 32. The output of the summing unit 34 represents the 
combination of the filtered IF signals and the pilot signal, and is 
applied to the radiating cable 16 for transmission through the tunnel 
area. The pilot signal is sent over the radiating cable 16 for use by the 
subsequent amplifier stages. 
At each of the amplifier stages, the IF signals being propagated across the 
radiating cable lengths are up-converted to the original RF frequency 
through a mixing operation using a local oscillator signal derived from 
the reference pilot signals also transmitted over the radiating cable. A 
representative arrangement for accomplishing this is shown at FIG. 3, 
where the transmit RF signals are passed through a bi-directional signal 
splitter (or combination divider/summer) 40. In the transmit direction, 
the RF signals pass through a signal divider 42, one output of which leads 
to a band pass filter 44 which allows a selected band of frequencies to 
pass through to a mixer 46. 
The mixer 46 is fed with a local oscillator signal generated by a pilot 
reference recovery circuit 48, which extracts the pilot tone transmitted 
with the IF signals over the radiating cable. The mixing operation 
effectively up-converts the filtered IF signals back to the original RF 
frequency, a selected band of which is filtered by a band pass filter 50. 
The filtered RF signals pass through a variable attenuator 52 into a 
preamplifier 54 which provides the requisite RF gain before the signals 
are fed to the final power (summing) amplifier 56 of the amplifier stage. 
Subsequently, the amplified signals are passed through a bi-directional 
signal splitter 64 and then radiated by the radiating cable length linking 
the amplifier stage to the succeeding stage of amplification for reception 
by radio units within the coverage area in the vicinity of the cable. 
Signals from the divider 42 are also passed through a low pass filter 58 
which passes only the IF signals which need to be propagated downstream of 
the amplifier stage for subsequent regeneration of the RF frequencies. The 
filtered IF signals pass through a variable attenuator 60 and a 
preamplifier 62 which provides the requisite IF gain before being applied 
to the final power amplifier 56. The amplified IF signals are subsequently 
transmitted over the radiating cable along with the RF signals. 
Thus, the transmit RF frequencies are regenerated at each amplifier stage 
through a heterodyne process from low level IF signals. The IF signals are 
cascaded through the amplifier chain at relatively low power levels, 
thereby producing a negligible amount of intermodulation frequency 
components. It is significant that the regenerated RF signals are not 
cascaded through the amplifier chain. Instead, the signals undergo a 
single level of amplification at each amplifier stage prior to propagating 
through the associated radiating cable length to be radiated therefrom to 
nearby radio units. 
It should be noted that, in the transmit direction, the signals at the 
output of each amplifier stage must possess a power level sufficient to 
overcome the co-axial loss of the cable, the coupling loss, the loss 
associated with the receiving antenna, etc., and yet retain a sufficiently 
high signal level for the receiving units. As a result, relatively high 
power levels are required per carrier. 
The arrangement of FIG. 3, realizes the desired high signal carrier levels 
since the RF gain level at each amplifier stage can be appropriately 
adjusted. The desired signal power level is achieved without the 
intermodulation distortion typically associated with conventional 
amplifier systems. This is because the RF signals are not cascaded through 
the various amplifier stages and, instead, only the low power level IF 
signals are cascaded through the amplifier stages. 
For transmission along the receive path, i.e., for communications from 
radio units to the base stations, the signal levels applied to the input 
of each amplifier stage are sufficiently low that any associated 
intermodulation does not unduly distort the transmitted signal levels and, 
accordingly, can be ignored. Consequently, the receive signals can be 
amplified directly at the RF frequency levels without employing the 
above-described heterodyne process used for the transmit signals. In the 
arrangement of FIG. 3, the receive signals are directed through the 
bi-directional signal splitter 64 and then passed through a band pass 
filter 66 which passes only the desired RF transmit frequencies. The 
filtered RF signals pass through one or more signal amplifiers 68, each 
providing a preselected amount of RF gain. Next, the amplified signals 
pass through a high pass filter 70 before being routed through the signal 
splitter 40 to the radiating cable lengths and eventually to the base 
station. 
Referring now to FIG. 4, shown is a detailed diagram of a preferred 
embodiment of a bi-directional amplifier adapted for use with the IF 
signals generated by the arrangement of FIG. 2. The preferred arrangement 
of FIG. 4, is described in connection with a communication system using 
two separate bands of RF carrier frequencies centered at 80 MHz. and 160 
MHz., and a pilot tone of 5 MHz. It will be apparent that two separate IF 
bands, IF.sub.1 and IF.sub.2, corresponding to the two RF carrier bands to 
be used. The regeneration of RF signals at each amplifier stage is 
described herein with respect to these two bands of carrier frequencies 
for illustrative purposes only. It will be obvious to those skilled in the 
art that just one band or more than two bands of RF carrier frequencies 
may be used depending on the number of channels, and the transmission 
capacity of each such channel, required to accommodate the desired usage 
capacity of the system. 
As shown in FIG. 4, the IF signals in the transmit path are applied to a 
bi-directional coupler 70 where the transmit and receive signals are 
separated. Subsequently, the transmit signals are applied to a four-way 
power splitter 71 the outputs of which are applied to four signal paths 
which are described in detail below. 
The first path provides amplification gain for the IF bands. The composite 
transmit signals are passed through a low pass filter (LPF) 72 having a 
3-dB cut-off frequency of 40 MHz. so that all signals other than the 5 
MHz. pilot and the two IF bands IF.sub.1 and IF.sub.2 are rejected. The 
filtered signals are then applied to a programmable attenuator 73, the 
attenuation factor of which is controlled by a signal B.sub.1 applied to 
its control input. Subsequently, the filtered composite signals pass 
through a fixed gain stage 74. The output of the fixed gain stage 74 is 
applied to one input of the final power amplifier 75 of the amplifier 
stage. 
Preferably, the command signal B.sub.1 for controlling the programmable 
attenuator 73 is delivered from the control port on a computer interface, 
such as the SCADA interface, leading to a central computer adapted to 
control the operation of the overall RF coverage system. A preferred 
arrangement for programmably controlling various aspects of a RF 
communications system using such an interface is described in co-pending 
U.S. application entitled "DISTRIBUTED AMPLIFIER NETWORK MANAGEMENT 
SYSTEM" which is also owned by the assignee of the present application; 
the disclosure in that application is incorporated herein by reference. 
The second signal path from the power splitter 71 leads to a band pass 
filter 76 which extracts the 5 MHz. pilot signal included with the 
transmit IF signals The extracted pilot signal is then applied to the 
inputs of phase-locked oscillators (PLOs) 77 and 78 operating at 
frequencies adapted to upconvert the IF bands to the original RF levels 
through a pair of associated mixers 79, 80, respectively. More 
specifically, in order to obtain the original RF frequency levels of 80 
Mhz and 160 Mhz, the phase-locked oscillator 77 operates at a frequency of 
138 MHz. and the phase-locked oscillator 78 operates at a frequency of 
75.5 MHz. 
The third output path from the signal splitter 71 leads to a band pass 
filter 81 centered at a frequency of 30.5 MHz. and hexing an effective 
bandwidth of 12 MHz to realize the first IF band of frequencies (IF). This 
band of filtered frequencies is then mixed with the 138 MHz. signal from 
the PLO 77 at the mixer 79 so as to produce both sum and difference 
frequency component. The sum components are subsequently retained by 
passing the signals through a band pass filter 82 centered at 168.5 MHz. 
having a bandwidth of 12 MHz. to effectively regenerate the original RF 
signals. The resultant RF carrier signals are then applied to a 
programmable attenuator 83 which is controlled by a command signal 
B.sub.2, preferably through the control port of the SCADA interface. 
Subsequently, the attenuated RF signals are passed through a fixed gain 
stage 84 from which the signals pass through a coupler 85 to be applied to 
one input of the final power amplifier 75 for subsequent delivery to the 
radiating cable connected to the amplifier stage. 
The fourth output path of the 4-way splitter 71 delivers the IF transmit 
signals to a band pass filter 86 centered at 10.5 MHz. and having an 
effective bandwidth of 5 MHz., to realize the second IF band of 
frequencies (IF.sub.2). This band of frequencies is then mixed with the 
75.5 MHz. output of the PLO 78 at the mixer 80 to produce both sum and 
difference frequency components. A band pass filter 81A centered at 86 
MHz. and having a narrow bandwidth of 5 MHz. retains only the sum 
components from the signals at the output of mixer 80 so as to effectively 
regenerate the second RF carrier signals. The resultant RF signals are 
then applied to a programmable attenuator 87, the attenuation factor of 
which is controlled by a command signal B.sub.3 applied to its control 
port. As with the case of attenuators 73 and 83, the attenuation level of 
attenuator 87 is preferably controlled through the SCADA control port. The 
attenuated RF carrier signals are then passed through a fixed gain stage 
88 and finally applied through a signal coupler 89 to one input of the 
final power amplifier 75 for delivery to the radiating cable. 
With the above arrangement, the amplifier 75 acts upon the 5 MHz. pilot 
signal, the IF.sub.1 band, the IF.sub.2 band, the 86 MHz. band and the 
168.5 MHz. band. The amplified output of the amplifier 75 is applied 
through a signal coupler 90 to a 2-way power splitter 91 from which the 
signals are applied to the associated radiating cable connected thereto. 
Preferably, the power amplifier 75 is selected to be capable of providing 
up to 5 watts (37 dBm) of power for all signals propagating therethrough. 
The other input to the 2-way power splitter 91 receives the RF frequencies 
for the receive path, i.e., the receive signals received from radio units 
in the vicinity of the radiating cable length associated with the 
amplifier stage. Such a receive signal is applied to a suitable power 
divider (not shown) and then to a pair of band pass filters 92 and 93. The 
BPF 92 is centered at 164 MHz. and has an effective bandwidth of 15 MHz. 
The other BPF 93 is centered at 77 MHz. and has an effective bandwidth of 
5 MHz. The filters 92 and 93 provide the required selectivity for the 
frequencies in the receive path. In addition, these filters also prevent 
signals from the transmit path from looping back to the receive path. The 
outputs of the filters 92 and 93 are summed together at a summing unit 94 
and then applied to a fixed gain stage 95. 
In accordance with one aspect of this invention, the pilot carrier signal 
that is used in the transmit paths is also used in the receive path for 
monitoring amplifier failures. More specifically, the amplified transmit 
signals at the output of the power amplifier 75 are also directed through 
the signal coupler 90 to a narrow bandwidth band pass filter 96 having a 
center frequency of 5 MHz. The output of filter 96 is applied to one input 
of the amplifier 95 in the receive path. 
The output of BPF 96 is also applied to a power detector 97 and then to a 
comparator 98 having a fixed threshold level. The arrangement is such that 
the output of comparator 98 is a logical "1" when the 5 MHz. pilot signal 
is detected, thereby indicating the presence of an acceptable condition. 
If the pilot signal is not detected, which may occur due to a variety of 
reasons such as cable failure, etc., the output of comparator 98 will be a 
logical "0", thereby indicating the presence of a failure. This status 
condition is preferably interrogated through the status input port of the 
SCADA interface by means of a command signal A.sub.1. 
Under normal conditions, the output of the receive amplifier 95 contains 
the 5 MHz. pilot carrier signal and this condition is detected through a 
similar arrangement using a 5 MHz. band pass filter 99, a power detector 
100 and an associated fixed threshold comparator 101 capable of being 
interrogated using a signal A.sub.2. Prior to detection, the output of the 
receive amplifier 95 is processed through a programmable attenuator 102 
which has its attenuation level controlled by a command signal B.sub.4 
linked to the SCADA interface. If, for some reason, the receive amplifier 
95 fails, a failure condition is indicated by the presence of a logical 
"0" at the output port of the comparator 101. 
Certain of the transmit signal frequencies may fall within the frequency 
band covered by some of the receive signal frequencies. Since the 
bi-directional coupler 70 that couples the transmit path output signals 
with the receive path input signals typically provides only about 30 dB of 
isolation, it is important that leakage of the transmit path signals be 
restricted in order to avoid instability and oscillations. 
This separation of the transmit signals from the receive signals is 
achieved by means of a cancellation circuit comprising a high pass filter 
103 and a phase shifter 104. More specifically, the output transmit 
signals from the power amplifier 75 are directed through the coupler 90 to 
a high pass filter 103 having a cutoff frequency of 164 MHz. The filtered 
output is then applied to a phase shifter 104 which provides a 180.degree. 
phase shift for all frequencies at a level higher than 164 MHz. 
Subsequently, an attenuator 105 is used to adjust the output of phase 
shifter 104 to a level appropriate for providing cancellation at the 
output of the receive amplifier 95. 
After being processed by the programmable attenuator 102, the combined RF 
signals coming out of the receive amplifier 95 are filtered through a 40 
MHz. low pass filter 106 and then applied to the radiating cable in the 
receive direction through the bi-directional coupler 70. 
Preferably, all of the active devices in both the transmit and the receive 
paths are continuously monitored for failure on the basis of the pilot 
signal and the IF signal. At the IF signal level, for instance, the output 
of the fixed gain stage 84 is also fed through the coupler 85 to a power 
detector 107 and then to a fixed threshold comparator 108. The comparator 
output is interrogated using a signal A.sub.3 from a status port of the 
SCADA interface. 
Similarly, the IF output of amplifier 88 is coupled through the coupler 89 
to a power detector 109 and then to an associated fixed threshold 
comparator 110. The output of comparator 110 is interrogated using a 
signal A.sub.4 from the SCADA interface. Preferably, each amplifier stage 
is assigned a unique code "address" which is used by the central control 
port associated with the SCADA interface to poll each of the amplifier 
stages sequentially on a periodic basis. 
The above arrangement is also advantageous in that the gain control 
adjustment of the transmit and receive path signals is also easily 
controlled through the control port of the SCADA interface. The gain 
levels can, thus, be conveniently varied to account for the slow 
variations in gain levels resulting from exposure of the cascaded 
amplifier stages to seasonal temperature variations, as well as from 
long-term aging of the radiating cable and the amplifiers. 
Preferably, each of the amplifier stages is also provided with a completely 
independent redundant standby system in accordance with the 
above-described arrangement of FIG. 4. Such a standby system includes a 
redundant power supply so that the standby system can be powered at all 
times in order to ensure proper operation. Failure detection is 
accomplished in the standby system in a manner identical to that in the 
main amplifier, by using similar pilot detection circuits. The periodic 
interrogation of the transmit and receive amplifier outputs by the control 
computer through the SCADA control port allows convenient monitoring and 
detection of any failure of the amplifiers. 
Preferably, the SCADA and control status interface for the standby system 
is also kept independent of the main amplifier system and has its own 
address. A dummy load is used at the output of the transmit direction of 
the standby unit when the unit is not in operation. The SCADA interface 
incorporates a parallel input/output port providing both status 
information and amplifier control by means of gain level adjustments and 
switching of the redundant standby system. 
As described above in detail, the bi-directional regenerative amplifiers, 
in accordance with the system of this invention, are based on a heterodyne 
approach where the transmit carrier frequencies (80 MHz. and 160 MHz., 
according to a preferred embodiment), are up-converted from a 
corresponding pair of intermediate frequencies to eliminate the excessive 
modulation distortion caused by cascading a plurality of high-power linear 
amplifiers. Accordingly, a significant function required at the base 
station location is the generation of the requisite pair of IF frequency 
bands directly from the RF carriers for distribution over the radiating 
cable. 
As specifically described in connection with FIG. 1, the transmission line 
used for the propagation of RF signals within the enclosed or RF blocked 
area is preferably radiating cable. However, the link between the length 
of radiating cable and the base station in the area outside the blocked or 
tunneled area is typically formed of conventional co-axial cable since 
there is no need for radiation of propagating energy in this stretch (as 
well as other similar stretches) of the 2-way communication link. In 
addition, in certain areas of a tunnel communications system cable where 
the shape of the tunnel may necessitate disposition of lengths of 
transmission lines adjacent each other, it is essential to use 
non-radiating cable for at least one of the adjacent lengths to avoid 
radiation interference otherwise resulting from adjacently disposed 
radiating cable lengths. For instance, in FIG. 1, it is necessary to use a 
length of non-radiating co-axial cable 19A immediately adjacent the 
radiating cable length 16A. Where the length of co-axial line is 
relatively short (of the order of hundreds of meters), the feeder 
attenuation of the system RF carrier frequencies is not significant and 
can accommodate both the IF frequencies and the RF frequencies. However, 
where the co-axial line feeder becomes excessively long (on the order of 
thousands of meters), particularly, at the feeder end linking the base 
station to the radiating cable, the feeder attenuation becomes too severe, 
particularly for high frequency RF carriers. For instance, in the case of 
the illustrative carrier frequencies referred to in connection with FIG. 
4, the attenuation of the coaxial feeder becomes excessive for the 160 
MHz. carrier when the feeder length approaches or exceeds 2000 meters. 
Under these conditions, it becomes necessary to use a plurality of 
bi-directional amplifiers to compensate for the co-axial loss resulting in 
the feeder section between the base station and the initial length of 
radiating cable in the tunnel area. Such a use of several bi-directional 
amplifiers adds substantially to the installation costs as well as the 
maintenance costs of the overall RF coverage system. 
In accordance with another feature of this invention, the need for using 
such amplifiers is eliminated by use of an intermediate frequency 
distribution scheme for the base station transmit and receive path. In 
particular, the same pilot carrier signal that is used on the transmit 
path is also used on the receive path for the up-conversion to and 
down-conversion from the system IF frequencies. 
A preferred arrangement for implementing such an IF distribution scheme is 
illustrated in FIG. 5, where the transmit RF signals are generated by 
combining outputs provided by a plurality of RF transmitters. According to 
a preferred embodiment, one backup transmitter and eight operational 
transmitters are used to generate the 160 MHz. RF signals which are 
applied to a combiner 202 to produce the first composite RF signal at 160 
MHz. Similarly, one backup and two operational transmitters 203 are used 
to generate separate 80 MHz. RF signals which are combined by a combiner 
204 to realize the second composite RF signal. A plurality of transmitters 
and receivers is need to accommodate the plurality of channels required 
for handling the typically high volume of simultaneous communications 
required for most tunnel and subway communication systems. 
The two composite RF signals are then combined with the IF signals 
generated therefrom, and applied to a signal splitter 205 that separates 
the transmit signals from the receive signals. The output of splitter 205 
is then applied to the feeder cable for one of the transmit lines, 
designated as F.sub.1. The backup-transmitters for the 160 MHz. and 80 
MHz. RF carriers may be switched in to generate any of the required 
operational transmit frequencies. 
It should be noted that the illustrative arrangement of FIG. 5 is adapted 
to use the same base station services for handling RF communications 
between two separate tunnel systems (not shown) fed respectively by feeder 
lines F.sub.1 and F.sub.2. 
The feeder line F.sub.2 is adapted to lead directly to an amplification 
stage where the requisite RF signals are regenerated. Accordingly, the 
transmit signals for line F.sub.2 need only comprise the IF signals and 
the pilot signal. 
The feeder line F.sub.1, however, is adapted to feed RF signals, in 
addition to the IF signals and the pilot signal, to a radiating cable 
section so that the RF signals may be radiated before the signals are 
regenerated from the IF signals prior to being amplified at the first 
amplification stage. 
More specifically, in FIG. 5, a portion of the first RF transmit signal 
(the 160 MHz. RF signal) at the output of combiner 202 is fed through a 
directional coupler 206 to one input of a mixer 207. 
The other input of the mixer 207 is from a 138 MHz. local oscillator 208 
derived from a 5 MHz. pilot reference 209. The output of mixer 207 
comprises both sum and difference frequencies based upon the two input 
frequencies. A band pass filter (BPF) 210 centered at 30.5 MHz. and having 
an effective bandwidth of 12 MHz. is used to retain only the difference 
frequencies at the output of mixer 207. The output of the BPF 210 
constitutes the first set of frequencies IF.sub.1. 
Similarly, a portion of the second composite transmit signal (80 MHz. RF 
signal) at the output of combiner 204 is fed through a directional coupler 
211 to one input of a mixer 212. The other input of the mixer is a 75.5 
MHz. signal from a local oscillator 213 which operates in conjunction with 
the 5 MHz. pilot reference 209. Again, the output of mixer 212 comprises 
both sum and difference frequencies of the two inputs fed to the mixer. A 
band pass filter 214 centered at 10.5 MHz. and having an effective 
bandwidth of 5 MHz. is used to retain only difference frequencies at the 
output of the mixer 212. The output of BPF 214 constitutes the second set 
of IF frequencies IF.sub.2. 
The two sets of IF frequency bands, IF.sub.1 and IF.sub.2, are combined 
with the 5 MHz. pilot signal at combiner 215 and are fed through a signal 
splitter 216. From the signal splitter 216, the signals are combined at 
combiner 205 with the outputs of the combiners 206 and 211, which comprise 
the two composite RF carrier signals, for driving the first feeder line 
F.sub.1. The signal splitter 216 also makes available a composite signal 
comprising the two IF frequency bands and the 5 MHz. pilot carrier for 
driving the second feeder line F.sub.2, through another signal splitter 
217, after suitable amplification. 
It should be noted that the receive signals from the second feeder line 
F.sub.2 consist of the same pair of IF bands and 5 MHz. pilot reference 
signal as present on the transmit path. The bi-directional amplifier 
connected to this feeder is adapted to FIG. 4 generates the required IF 
band signals and is, therefore, set apart from the other line amplifiers. 
In practice, this amplifier is similar to the arrangement shown in FIG. 4 
but includes additional circuitry similar to the IF band generation 
arrangement of FIG. 3 for generating the appropriate IF bands adapted to 
the down-conversion of RF signals from radio units and the subsequent 
up-conversion to the original RF levels, as described above in detail. 
Preferably, the center frequencies of the IF bands are kept different from 
those in the transmit path so as to avoid interference problems. 
The receive signals from the F.sub.2 line are fed through a power splitter 
218 to a set of three band pass filters 219, 220, and 222. The first 
filter 219 is a narrow bandwidth band pass filter centered at 5 MHz. in 
order to extract the 5 MHz. pilot carrier from the receive signals. The 
second filter 220 is centered at 19 MHz. and has an effective bandwidth of 
5 MHz. for separating one IF band from the composite receive signal. The 
output of filter 220 is applied to one input of a mixer 223 which receives 
its other input from a 67 MHz. local signal derived by a local oscillator 
224 from a 5 MHz. pilot recovery circuit 225. The output of mixer 223 
includes sum and difference frequencies based on its two inputs, of which 
only the sum frequencies are retained for providing the 80 MHz. receiver 
frequencies. 
The third band pass filter 222 is centered at a frequency of 45 MHz. with 
an effective bandwidth of 12 MHz. so as to separate the remaining IF band 
from the composite receive signals. The output of filter 222 is applied to 
one input of a mixer 226 which has its other input supplied by a 123.5 
MHz. local signal derived by a local oscillator 227 from the 5 MHz. pilot 
recovery circuit 225. Again, the mixer output results in both sum and 
difference frequencies, of which only the sum frequencies are retained for 
providing the 160 MHz. receiver frequencies. 
In FIG. 5, the up-converted RF frequencies from the feeder line F.sub.2 are 
combined with the receiver frequencies from the feeder line F.sub.1 by a 
combiner 228 and then applied through a splitter 229 to a combiner 230 
which also receives an input from the output of mixer 226. The combined 
signals are fed to a pair of band pass filters 231 and 232 for separating 
the 160 MHz. RF band. The BPF 231 is centered at 160 MHz. and has an 
effective bandwidth of 2 MHz. in order to isolate the low end of the 160 
MHz. band. The other BPF 232 is centered at 167 MHz. with a 2 MHz. 
bandwidth for isolating the high end of the band. The pair of BPFs 231 and 
232, in effect, notches out the set of 164 MHz. transmit frequencies that 
couple through from the associated transmitters at the coupling point for 
the F.sub.1 feeder line. 
The receive signals from combiner 229 are further combined with the output 
signals from the mixer 223 at combiner 234. Subsequently, a band pass 
filter 235 centered at 77 MHz. and having an effective bandwidth of 5 MHz. 
is used to separate the 80 MHz. RF band from the composite receive signal. 
The separated receive signals at the output of filter 235 are applied 
through a multi-coupler 236 to each of three 80 MHz. receivers 237, 
including two operational receivers and one backup receiver. Similarly, 
the 160 MHz. filtered signals from the output of the filters 231 and 232 
are applied through a multi-coupler 238 to each of eight separate 160 MHz. 
receivers 239, including seven operational receivers and one backup 
receiver. As in the case of the backup transmitters, the 160 MHz. backup 
receiver and the 80 MHz. backup receiver may each be controllably switched 
in order to receive any of the operational receive frequencies. 
It should be noted that all of the audio outputs of the receivers can be 
interfaced with the serial diversity polling scheme used in conjunction 
with the SCADA interface for controlling the regenerative amplifier 
system, as discussed above. Each receiver can also provide up to two 
squelch thresholds (typically 20 dB and 40 db) which can be used to 
control the polling circuitry for selection of either the surface receiver 
(outside the tunnel area) or an underground receiver (inside the tunnel 
area). 
The above arrangement also makes possible the use of radio transmitters for 
master oscillator synchronization for simulcast operation with the surface 
communications system. In this case, transmit audio can be obtained from 
the existing distribution amplifiers that provide audio for the surface 
system. The backup transmitter and receiver units for both the 80 MHz. and 
160 MHz. RF bands can be controllably switched into operation if any of 
the operational units fail. All audio, control and RF connections can 
easily be switched to the appropriate channel with one simple operation. 
While the invention has been particularly shown and described with 
reference to various embodiments, it will be recognized by those skilled 
in the art that other modifications and changes may be made to the present 
invention described above without departing from the spirit and scope 
thereof.