Method and apparatus for adjusting power delivered from a central power unit to a remote unit via a supply cable

A power delivery apparatus and method which calibrates a voltage delivered from a central unit to a remote unit to compensate for losses in the supply lines between the central unit and remote unit is disclosed. The impedance of the supply lines is determined utilizing a reference voltage conducted by a calibration line, a sweep tone transmitted down the supply line, or a time domain reflectometer technique. Based upon the determined impedance of the supply lines, the voltage delivered from the central unit can be calibrated accordingly. Alternatively, the impedance can be determined and the voltage calibrated based on the measured input voltage to the remote unit or the measured power level of a signal transmitted from the remote unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a method and apparatus for 
providing power to remotely located units using power supply lines or 
cables connected directly to central equipment serving the remote units. 
2. Description of the Related Art 
In many telecommunication systems, it is advantageous to power remotely 
located units, such as remote radio units, using power supply lines or 
cables connected to central equipment serving the remote units. The power 
from the central unit is typically converted by a power supply in the 
remote unit to provide the power necessary for the remote unit. This 
arrangement is generally more effective and less costly than using an 
external power supply for the remote units. In using central equipment to 
supply power to remotely located units, however, specific requirements 
must be met. Namely, the current and voltage supplied to the remote units 
must be strictly controlled. In addition, the power and heat dissipation 
of the conversion power supply in the remote units are limited and the 
electrical noise generated in the remote units must be kept extremely low. 
Further, the cost of the power supply in the remote unit for converting 
the power supplied from the central equipment must be kept low. 
A key factor limiting the effective control of the aforementioned 
requirements is the variable line length of the cables connecting the 
central equipment to the remote units. The distance from the power source 
in the central equipment to each remote unit connected to the central 
equipment may vary. As such, the length of the cables necessary for 
connection of the central unit to each remote unit will vary accordingly. 
Because of the variable length of cables that may be required for each 
remote unit, the impedance of the power delivery system differs from 
remote unit to remote unit. This limits the effective control of the 
current and voltage applied to each remote unit. 
With reference to FIG. 1, a typical power delivery arrangement exhibiting 
the aforementioned limitations is shown. An in-building wireless 
telecommunication system is an exemplary system in which power delivery 
systems may be used. A power source 14 is controlled by central unit 12 to 
provide power to power supply 22 of remote unit 20. In a wireless 
telecommunication system, remote unit 20 may be a remote radio unit for 
transmission of signals via antenna 23. In this arrangement, power is 
transmitted from power source 14 to power supply 22 via power transmission 
lines 16 and 18. Currently, power conversion in remote unit 20 is 
accomplished using either a linear power supply or a switching power 
supply within remote unit 20. Each of these power supplies has distinct 
disadvantages which limit the effective control of power supplied to 
remote units. With reference to FIG. 2, a linear power supply 28 is 
illustrated in which power is converted as it passes from the input 24 to 
the supply 26. The use of a linear power supply 28 at the remote unit 20 
may have a very inefficient power conversion, and may also be quite 
dependent on the voltage drop across the supply cables. This tends to 
limit the line length of the cables. In addition, there is often quite a 
bit of heat dissipation associated with such an approach. 
With reference to FIG. 3, a switching power supply 34 is shown in which 
power is converted as it passes from input 30 to supply 32. A switching 
power supply 34 is advantageous over a linear power supply 28 because it 
is more tolerant of large swings in input voltage. However, switching 
power supplies tend to be more expensive, and they produce considerable 
electrical noise. 
Thus, there exists a need for an inexpensive, efficient power delivery 
system that provides power to remotely located units from a centrally 
located power source which includes a calibration system that can 
effectively compensate for voltage drops caused by the impedance of the 
supply cables. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a power delivery system, method 
and apparatus are described and illustrated which do not exhibit the 
drawbacks associated with previous approaches. According to the present 
invention, a variable power source in the central unit is calibrated, 
based on the impedance of the supply lines within the power cable 
connecting the central unit to a remote unit, to deliver a voltage which 
compensates for the impedance of the supply lines to a linear power supply 
in the remote unit. Alternatively, the power source in the central unit 
can be calibrated to compensate for losses in the supply lines based on 
the measured voltage being input to the remote unit, or the measured 
output power of a transmitter located in the remote unit. The delivery of 
a voltage which compensates for the losses associated with the impedance 
of the supply lines to the linear power supply allows the linear power 
supply to be designed for small input voltage variations, thereby 
considerably decreasing the power conversion inefficiencies and reducing 
the power and heat dissipation required by the remote unit. 
According to a first preferred embodiment, the calibration is performed by 
determining the impedance of the supply lines utilizing a pre-determined 
reference voltage signal applied by the remote unit to calibration lines 
contained within the power cable connected between the remote unit and the 
central unit. The central unit measures the voltage drop in the reference 
voltage signal to determine the impedance in the calibration lines, which 
corresponds to the impedance of the supply lines, and adjusts the voltage 
applied to the supply cable accordingly. 
According to a second preferred embodiment, the calibration is performed by 
determining the impedance of the supply lines using an AC coupled sweep 
tone generated at the central unit which is transmitted down the supply 
lines to a known impedance at the remote unit and returned to the central 
unit. At the central unit, the impedance is determined based upon the loss 
measured in the sweep tone, and the voltage applied to the supply lines 
adjusted accordingly. 
According to a third preferred embodiment, the calibration is performed by 
determining the impedance of the supply lines using a Time Domain 
Reflectometer (TDR) measurement technique, and adjusting the voltage 
applied to the supply lines accordingly. 
According to a fourth preferred embodiment, the calibration is performed by 
measuring the voltage supplied at the remote unit and communicating the 
measured voltage back to the central unit via a telemetry link. The 
central unit can then adjust the voltage supplied to the remote unit 
accordingly. 
According to a fifth preferred embodiment, the calibration is performed by 
measuring the power level of a signal output by a signal transmitter in 
the remote unit and communicating the measured power level to the central 
unit via a telemetry link. The central unit can then adjust the voltage 
supplied to the remote unit accordingly. 
These and other features and advantages of the invention will become more 
apparent from the following detailed description of preferred embodiments 
of the invention which are provided in connection with the accompanying 
drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will be described as set forth in the preferred 
embodiments illustrated in FIGS. 4-8. Other embodiments may be utilized 
and structural or logical changes may be made without departing from the 
spirit or scope of the present invention. Like elements are referred to by 
like numerals in the drawings. 
According to the present invention, power is provided to a remote unit 
using a variable power source to produce optimum voltage based upon the 
measured impedance of the power supply lines. FIG. 4 shows a simplified 
block diagram of a first preferred embodiment of the invention 
incorporating the power delivery system of the present invention. 
Referring to FIG. 4, the power delivery apparatus 39 includes a central 
unit 36 and a remote unit 38 which are connected by a cable 59. Remote 
unit 38 may be, for example, a remote radio unit in a wireless 
telecommunication system for transmitting signals via antenna 23. Cable 59 
includes power transmission lines 48 and 50 and calibration lines 52 and 
54. Alternatively, calibration lines 52, 54 may be provided as a separate 
cable. The central unit 36 includes a voltage/impedance sensor 46, a 
control section 44, a digital-to-analog converter 42, and a variable power 
source 40. The remote unit 38 includes a power supply unit 56 and a 
calibration reference signal producing circuit 58. 
The control section 44 of the central unit 36 manages overall control of 
the central unit 36 including the voltage output of variable power source 
40. Control section 44 may include a CPU 47 (a central processor unit 
which may be a microprocessor, a digital signal processor, a 
micro-controller or other programmable logic device). The control section 
44 is connected to voltage/impedance sensor 46 via line 49. The control 
section 44 is connected to variable power source 40 through 
digital-to-analog converter 42 via lines 51 and 53. 
Operation of the present invention shown in FIG. 4 will now be described. 
During normal operation of the power delivery apparatus 39, power is 
supplied from the central unit 36 to the remote unit 38 via cable 59 on 
power transmission lines 48 and 50. During power transmission, a reference 
voltage is generated and applied to calibration lines 52 and 54 of cable 
59 by calibration signal producing circuit 58. The impedance sensor 46 
determines the impedance of the calibration lines 52 and 54 by measuring 
the drop in the reference voltage across the calibration lines 52, 54. 
Once determined, the impedance sensor 46 sends a signal representing the 
determined impedance to the control section 44 via line 49. 
Since both transmission lines 48, 50 and calibration lines 52, 54 are 
connected between the central unit 36 and the remote unit 38, the length 
of calibration lines 52, 54 is approximately the same as the length of the 
transmission lines 48, 50. Accordingly, the impedance of each will be 
approximately equal. Control section 44, in response to the signal 
received from sensor 46, will calibrate the voltage applied to the remote 
unit 38 by causing variable power source 40 to adjust the level of the 
voltage applied to transmission lines 48, 50 for transmission to remote 
unit 38. 
For example, if a high impedance for calibration lines 52, 54 is 
determined, which indicates a corresponding high impedance for 
transmission lines 48, 50, variable power source 40 may adjust, i.e., 
increase, the voltage applied to the power transmission lines 48, 50 to 
compensate for the losses, i.e., voltage drop, due to the high impedance 
of transmission lines 48, 50. In accordance with the present invention, by 
increasing the voltage output from variable power source 40 to compensate 
for losses due to the impedance of the transmission lines 48, 50, the 
resulting voltage input to power supply 56 of remote unit 38 will remain 
within the specified tolerance for the input voltage of power supply 56. 
If a low impedance for calibration lines 52, 54 is determined, which 
indicates a corresponding low impedance for transmission lines 48, 50, 
variable power source 40 may adjust, i.e., decrease, the voltage applied 
to the power transmission lines 48, 50 since there may be only a small 
drop in the voltage due to the low impedance of transmission lines 48, 50. 
In accordance with the present invention, by decreasing the voltage output 
from variable power source 40, the resulting voltage input to power supply 
56 of remote unit 38 will remain within the specified tolerance for the 
input voltage of power supply 56. 
FIG. 5 shows a simplified block diagram of a second preferred embodiment of 
the invention incorporating the power delivery system of the present 
invention. Referring to FIG. 5, a power delivery apparatus 75 is shown 
including a central unit 36 and a remote unit 38 which are connected by 
power transmission lines 48 and 50. Remote unit 38 may be, for example, a 
remote radio unit of a wireless telecommunication system for transmitting 
signals via antenna 23. The central unit 36 includes a control section 44, 
a digital-to-analog converter 42, a sweep tone generator 80, a sweep tone 
detect device 82, a hybrid transformer 84 and a variable power source 40. 
The remote unit 38 includes a power supply unit 56 and a reflective 
termination unit 86. 
The control section 44 of the central unit 36 manages overall control of 
the central unit 36 including the voltage output of variable power source 
40. Control section 44 includes a CPU 47. The control section 44 is 
connected to sweep tone measuring device 82 via line 90. 
Operation of the FIG. 5 embodiment will now be described. During normal 
operation of the power delivery apparatus 75, power is supplied from the 
central unit 36 to the remote unit 38 via power transmission lines 48, 50. 
Sweep tone generator 80 generates a sweep tone of a known magnitude and 
bandwidth. The generated sweep tone is coupled onto the transmission lines 
48, 50 by hybrid transformer 84 as is typically done in loop telephony. 
The AC coupled sweep tone is transmitted down the transmission lines 48, 
50 to the reflective termination unit 86 in remote unit 38. Termination 
unit 86 reflects the AC coupled sweep tone back to the central unit 36, 
where sweep tone detect device 82 measures the power of the reflected 
signal through hybrid transformer 84. The measured power of the return 
signal is input to control section 44 via line 90. Control section 44 then 
determines the impedance of transmission lines 48, 50 based upon the power 
loss measured in the sweep tone. Once the impedance of transmission lines 
48, 50 has been determined, the control section 44 prompts the variable 
power source 40 to adjust the voltage applied to the power transmission 
lines 48, 50 based on the determined impedance similarly to that as 
described above with reference to FIG. 4. 
FIG. 6A illustrates a power delivery system in accordance with a third 
preferred embodiment of the present invention. Referring to FIG. 6A, a 
power delivery apparatus 100 is shown including a central unit 36 and a 
remote unit 38 which are connected by power transmission lines 48 and 50. 
Remote unit 38 may be, for example, a remote radio unit of a wireless 
telecommunication system for transmitting signals via antenna 23. The 
central unit 36 includes a control section 44 with an input connected to 
transmission line 50 via line 114, a digital-to-analog converter 42, a 
variable power source 40, and a coupler 102, such as for example a 
tri-state buffer, having an input connected to control section 44 by line 
104 and an output connected to transmission line 50 by line 116. 
Alternatively, lines 114 and 116 could be connected to transmission line 
48 instead of transmission line 50, or connected to both transmission 
lines 48, 50. The remote unit 38 includes a power supply unit 56. 
The control section 44 of the central unit 36 manages overall control of 
the central unit 36 including the voltage output of variable power source 
40. Control section 44 includes a CPU 47. 
Operation of the FIG. 6A embodiment will now be described. During normal 
operation of the power delivery apparatus 100, power is supplied from the 
central unit 36 to the remote unit 38 via power transmission lines 48, 50. 
A short rise time pulse is sent by control section 44 through coupler 102 
to transmission line 50 via line 116. The pulse is also input back into 
control section 44 through line 114. The pulse will travel down 
transmission line 50 and reflect back to central unit 36 after 
encountering an impedance discontinuity, such as remote unit 38. 
FIG. 6B illustrates a diagram of the sending pulse and return pulse. The 
sending pulse is output at time t.sub.1 on line 116. The sending pulse is 
sent down transmission line 50 and is also seen as a return pulse by 
control section 44 via line 114. The sending pulse will be reflected back 
to central unit 36 at some time t.sub.2 and input to control section 44 
via line 114. Control section 44 can determine the time delay t.sub.d 
between the sending pulse and the return pulse and determine the length of 
the transmission line 50 based upon the determined delay. Once the length 
of transmission line 50 is determined, the impedance of transmission line 
50 can be determined by control section 44 based upon the length. Once the 
impedance has been determined, the control section 44 prompts the variable 
power source 40 to adjust the voltage applied to the power transmission 
lines 48, 50 similarly to that as described above with reference to FIG. 
4. 
FIG. 7A shows a simplified block diagram of a fourth preferred embodiment 
of the invention incorporating the power delivery system of the present 
invention. Referring to FIG. 7A, a power delivery apparatus 150 is shown 
including a central unit 36 and a remote unit 38 which are connected by 
power transmission lines 48 and 50. Remote unit 38 may be, for example, a 
remote radio unit of a wireless telecommunication system for transmitting 
signals via antenna 23. The central unit 36 includes a control section 44, 
a digital-to-analog converter 42, and a variable power source 40. The 
remote unit 38 includes a power supply unit 56 and a voltage sense unit 
152. 
The control section 44 of the central unit 36 manages overall control of 
the central unit 36 including the voltage output of variable power source 
40. Control section 44 includes a CPU 47. 
Operation of the FIG. 7A embodiment will now be described. During normal 
operation of the power delivery apparatus 150, power is supplied from the 
central unit 36 to the remote unit 38 via power transmission lines 48, 50. 
Voltage sense unit 152 measures, using lines 155,156, the voltage 
delivered to power supply 56 in remote unit 38. The measured voltage is 
communicated back to control section 44 of central unit 36 via a telemetry 
link between central unit 36 and remote unit 38. Control unit 44, based on 
the voltage measured by voltage sense 152 which represents the impedance 
of the transmission lines 48, 50, prompts the variable power source 40 to 
adjust the voltage applied to the power transmission lines 48, 50. 
Alternatively, as illustrated in FIG. 7B, a signal indicating the measured 
voltage can be communicated back to central unit 36 via transmission lines 
48, 50. Voltage sense 152 measures the voltage being input to power supply 
56 via lines 155, 156. A signal is output via lines 157, 158 back onto 
transmission lines 48, 50 and returned to central unit 36. A filter 154 
extracts the signal indicating the measured voltage and inputs a signal 
representing the impedance of transmission lines 48, 50 to control unit 
44, which prompts the variable power source 40 to adjust the voltage 
applied to the power transmission lines 48, 50. 
FIG. 8A shows a simplified block diagram of a fifth preferred embodiment of 
the invention incorporating the power delivery system of the present 
invention. Referring to FIG. 8A, a power delivery apparatus 160 is shown 
including a central unit 36 and a remote unit 38 which are connected by 
power transmission lines 48 and 50. Remote unit 38 may be, for example, a 
remote radio unit of a wireless telecommunication system for transmitting 
signals via antenna 23. The central unit 36 includes a control section 44, 
a digital-to-analog converter 42, a variable power source 40, and a signal 
source unit 168. The remote unit 38 includes a power supply unit 56, a 
signal transmitter 162 and a power sense unit 164. 
The control section 44 of the central unit 36 manages overall control of 
the central unit 36 including the voltage output of variable power source 
40. Control section 44 includes a CPU 47. 
Operation of the FIG. 8A embodiment will now be described. During normal 
operation of the power delivery apparatus 160, power is supplied from the 
central unit 36 to the remote unit 38 via power transmission lines 48, 50. 
Transmitter 162 in remote unit 38, in response to a signal from signal 
source unit 168 in central unit 36 via signal lines 170, 171, generates a 
transmit signal of a known power level to be sent by remote unit 38. This 
signal may be sent via antenna 23 of remote unit 38, for example, or could 
be transmitted by other signal transmission methods as are known in the 
art. Power sense unit 164 measures the power level of the signal present 
at the output of transmitter 162 in remote unit 38. Since the signal gain 
associated with the signal source unit 168 and transmitter 162 are known, 
the measured power level of the signal output from transmitter 162 is 
proportional to the impedance, which corresponds to the length, of signal 
lines 170, 171. Since both transmission lines 48, 50 and signal lines 170, 
171 are connected between the central unit 36 and the remote unit 38, the 
length of signal lines 170, 171 is approximately the same as the length of 
the transmission lines 48, 50. The measured power level of the output 
signal is communicated back to control section 44 via a telemetry link 
between central unit 36 and remote unit 38. Control unit 44, based on the 
power level of the signal measured by power sense 164, determines the 
length of signal lines 170, 171 and correspondingly the length of 
transmission lines 48, 50. Control unit 44, based on the determined length 
of transmission lines 48, 50, outputs a signal which represents the 
impedance of transmission lines 48, 50 and prompts the variable power 
source 40 to adjust the voltage applied to the power transmission lines 
48, 50. 
Alternatively, as illustrated in FIG. 8B, a signal indicating the measured 
output power can be communicated back to central unit 36 via transmission 
lines 48, 50. Power sense unit 164 measures the power present at the 
output of transmitter 162. A signal indicating the measured output power 
is output from power sense unit 164 onto transmission lines 48, 50 and 
returned to central unit 36. A filter 166 extracts the signal indicating 
the measured output power and inputs a signal to control unit 44. Control 
unit 44 outputs a signal representing the impedance of transmission lines 
48, 50 which prompts the variable power source 40 to adjust the voltage 
applied to the power transmission lines 48, 50 based on the output power 
measured by power sense unit 164. 
In accordance with the embodiments as described above, a variable power 
source in the central unit is set to deliver a voltage, which compensates 
for losses due to the impedance of the supply lines connecting the central 
unit to a remote unit, to a linear power supply in the remote unit. This 
compensation ensures that the level of the voltage signal received by the 
linear power supply will consistently be within a narrow range, and thus 
allows the linear power supply to be designed for small input voltage 
variations. The narrow input range for the linear power supply 
considerably decreases the power conversion inefficiencies and reduces the 
power and heat dissipation required by the remote unit. 
The above description and accompanying drawings are only illustrative of 
preferred embodiments that can achieve and provide the objects, features 
and advantages of the present invention. It is not intended that the 
invention be limited to the specific embodiments shown and described in 
detail herein. Accordingly, it should be understood that the invention is 
not to be considered as being limited by the foregoing description, but is 
only limited by the scope of the appended claims.