Method and apparatus for controlling a peak envelope power of a PA

A method and apparatus is provided for controlling peak envelope power level in a power amplifier (PA) (10). The method includes the steps of measuring (101) a first peak envelope power value of an input of the PA during a first time period, comparing (102) the first peak envelope power value with a threshold value, and introducing a phase change (103) into a selected signal of a plurality of signals being amplified by the PA when the first peak envelope power value exceeds the threshold. The method further includes measuring (104) a second peak envelope power value during a second time period, comparing (105) the first and second peak envelope power values, and reversing (106) the phase change to the selected signal when the second peak envelope power value exceeds the first peak envelope power value.

FIELD OF THE INVENTION 
The field of the invention relates to power amplifiers (PAs) and in 
particular to a method and apparatus for controlling a peak envelope power 
of the PA. 
BACKGROUND OF THE INVENTION 
Cellular systems simultaneously handling a number of traffic channels 
through each base station are known. Such systems are typically assigned a 
number of channels (f.sub.1 -f.sub.n) in support of communications with 
mobile communication units through such local base stations. Each base 
station is, in turn, allotted a subset of the channels (f.sub.1 -f.sub.n). 
Of the subset of channels assigned to a base site at least one (and often 
more) is designated as a control channel for purposes of access control 
and channel set-up. 
Communication with a communication unit on a traffic channel within a 
service coverage area of the base site is often accomplished through an 
omnidirectional antenna centrally located within the service coverage 
area. A number of communications transactions may be simultaneously 
supported through the antenna with each individual communication supported 
by a transmitter (located at the base site) assigned to the traffic 
channel. Each transmitter includes a modulated transmit signal source 
within the transceiver and a radio frequency (RF) power amplifier). Each 
transmitter thereby provides signal generation, modulation and 
amplification. 
The simultaneous transmission of a number of traffic channel signals from 
the central antenna requires that the transmitter output of each active 
transceiver be combined before application to, and transmission from, the 
central antenna. In order to avoid interference-producing intermodulation 
products, signals must be combined after any non-linear steps within the 
amplification process. In addition, the combining topology must provide 
sufficient reverse isolation to insure that signals of parallel 
amplification branches will not be coupled into the output of other power 
amplifiers, again producing intermodulation products. 
Where each transceiver is equipped with its own power amplifier (PA), 
combining must occur after the PA where signal levels, as well as 
combining losses, are high. A cavity combiner, for combining such high 
level RF signals while providing the necessary isolation, is provided by 
U.S. Pat. No. 4,667,172 assigned to the assignee of the present invention. 
In other communication systems, transceivers are not equipped with 
individual PAs; instead, a common, multitone linear PA (LPA) is used for 
amplification after the RF signals have been combined at relatively low 
power levels at the output of the transceiver. The use of such common LPA 
for traffic channels in systems using a common antenna has resulted in 
considerable simplification of system topology, improvements in system 
efficiency, and reduction in system size. 
The use of an LPA, on the other hand, has certain disadvantages 
particularly where RF signals are placed on evenly spaced channel 
frequencies and phase locked to a common frequency source. In such a case, 
amplitude fluctuations resulting in signal clipping may occur where a peak 
envelope power of the composite signal exceeds the LPA's power rating. 
FIG. 1 demonstrates the effects of signal phasing in a simplified case 
involving three signals, A, B, and C, during a time period T. The three 
signals are shown in FIGS. 1-1, 1-2, and 1-3 respectively. The envelope of 
the summed composite signal (absolute value of A+B+C) is shown in FIG. 
1-4. As may be observed, an envelope peak occurs during the middle of the 
period (T/2), when all three signals are in phase. The magnitude of this 
peak can be reduced by reversing the phase of signal C (taking the 
absolute value of A+B-C), as shown in FIG. 1-5. 
Clipping may occur in an LPA when the peak envelope power of the composite 
input signal (squared envelope magnitude), multiplied by the gain of the 
LPA, exceeds the peak output power capability of the LPA. Peaks resulting 
from phase matches have been observed to last for periods of from one to 
ten seconds, or longer in some systems. Clipping of the RF signal results 
in the generation of intermodulation products on other RF channels and 
degradation of system performance. 
Clipping due to summation of in phase signals is most severe when the 
carriers of such signals are unmodulated (during speech pauses) or weakly 
modulated (during low energy portions of the speech waveform). Full 
modulation of the carriers produces random variations in the carrier 
phases which limits the duration of any clipping to time periods on the 
order of one millisecond. Where the carriers are unmodulated or weakly 
modulated, however, a repetitive clipping process may occur which is 
periodic at a rate equal to a frequency difference of contributing 
carriers. This repetitive clipping process is what gives rise to the 
generation of strong intermodulation products when the carriers are close 
in phase. 
Past efforts to control peak envelope power due to phase summations have 
included de-rating of LPAs or intentionally de-correlating frequency 
references. Derating accommodates phase peaks by requiring an inordinately 
large LPA. De-correlating frequency references is effective in that where 
peaks do occur the peaks are very short and, consequently, more easily 
tolerated. De-correlating carriers, on the other hand, creates problems in 
synchronization not only in receiving control information on other 
channels but also in handoff among base sites. 
The use of de-correlated (independent) frequency references is expensive 
and inefficient. The use of an inordinately large LPAs reduces the 
benefits inherent in signal combining at low power levels. A need exists 
for a more efficient method of peak envelope power control within a LPA. 
SUMMARY OF THE INVENTION 
A method and apparatus is provided for controlling a peak envelope power 
level in a power amplifier (PA). The method includes the steps of 
measuring a first peak envelope power value of an input of the PA during a 
first time period, comparing the first peak envelope power value with a 
threshold value, and introducing a phase change into a selected signal of 
a plurality of signals being amplified by the PA when the first peak 
envelope power value exceeds the threshold. The method further includes 
measuring a second peak envelope power value during a second time period, 
comparing the first and second peak envelope power values, and reversing 
the phase change to the selected signal when the second peak envelope 
power value exceeds the first peak envelope power value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The solution to the problem of controlling peak envelope power (PEP) of a 
LPA lies, conceptually, in the introduction of phase changes, either 
randomly or under the control of an algorithm, into some or all of the 
carriers amplified within the LPA. The phase changes are introduced one 
carrier at a time followed by a PEP measurement. Where a phase change 
increases PEP over the previous PEP power measurement the previous change 
is reversed. Where a current PEP is less than the previous measurement the 
phase change is allowed to remain and the next carrier selected for phase 
change. 
An absolute value of the PEP is taken as an indication of the need for PEP 
control. Where the PEP exceeds a threshold, phase changes continue one 
carrier at a time until the PEP is reduced to a point below the threshold. 
When the PEP again exceeds the threshold, the process is resumed. 
FIG. 4 is a flow chart of the process of PEP control in accordance with an 
embodiment of the invention. Reference will be made to FIG. 4 as 
appropriate in understanding the process of the invention. 
FIG. 2 is a block diagram of a transmitter section 10 of a cellular power 
amplifier of a cellular base station in accordance with the invention. 
Control information intended for a radiotelephone (not shown) is composed 
within a controller 11, modulated to a carrier frequency within a 
controller transmitter 12, combined with other signals within a transmit 
combiner 15, amplified within the LPA 17, and transmitted through an 
antenna 18. Traffic information, likewise, received by the controller 11, 
is modulated within traffic transmitters 13-14, combined in the transmit 
combiner 15, amplifier 17, and transmitted through the antenna 18. 
Traffic channel information originating from within a public switch 
telephone network (PSTN) or another base site (not shown) is routed to 
appropriate traffic transmitters 13-14 by controller 11. Control 
information originating within controller 11 is also routed to control 
transmitter 12 by controller 11. The low-level output signals of 
transmitters 12-14 are combined within combiner 15 through resistive 
combining techniques for subsequent amplification within the LPA 17. 
Within LPA 17 the combined signals are amplified to a level sufficient for 
transmission through the antenna 18. 
Combiner 15 PEP output levels are monitored by the controller 11 through 
the PEP detector 16. PEP levels measured by the PEP detector 16 are 
compared with a threshold value stored within the controller 11 for a 
determination of the need for PEP control. Upon determination for a need 
for PEP control the controller sequentially introduces phase changes into 
carriers, one at a time, until the PEP falls below the threshold. 
The controller 11 introduces phase changes into individual radio frequency 
signals produced within the transmitter section 10 through a phase control 
device located within each of the transmitters 12-14 or their 
corresponding signal paths in combiner 15. One example of such a phase 
control device 30 is shown in FIG. 3. This exemplary phase control device 
30 is comprised of a control 23, relay devices 24-25, and a one half 
wavelength conductor 22. The control 23, upon receiving the appropriate 
command from the controller 11, causes relay devices 24-25 to switch 
between two states. In the first state the relay devices remain in the 
quiescent state (shown in FIG. 3) in which the half wavelength conductor 
22 is not included within the RF circuit. In the second state, the 
controller 11 energizes the relay devices 24-25 through control 23, 
thereby inserting the half wavelength conductor 22 into the RF path. 
Coupling the half wavelength conductor 22 in the signal path introduces a 
phase change into a carrier signal equal to a value of .pi.. 
It is to be understood that the phase control device 30 of FIG. 3 is not 
the only type of phase shifter that could be used in this invention. One 
possible variant is a phase controller having more than two states, such 
as one that can produce phase shifts of .pi./2, .pi., and 3.pi./2. This 
can be produced by cascading the phase control device 30 shown in FIG. 3 
with a similar phase control device which contains a quarter wavelength 
conductor in place of the half wavelength conductor 22. Phase control 
could also be affected through the frequency synthesis circuits in 
transmitters 12-14, for example, by inserting a phase shift into the 
reference frequency signal driving a particular synthesizer. 
By way of example the transmitter section 10 is operating at full capacity 
with an RF signal being transmitted through each of the transmitters 
12-14. PEP levels are measured 101 by the PEP detector 16 and transferred 
to the controller 11. Within the controller 11 the PEP values are compared 
102 with a threshold value. When the PEP is below the threshold value no 
action is taken relative to PEP control. 
When controller 11 detects 102 that the PEP is above the threshold value, 
the controller 11 changes 103 the state of the phase control device 30 of 
a transmitter 12-14 identified within a register (carrier register) in a 
memory (not shown) of the controller 11. Changing the state of the phase 
control device 30 of one of transmitters 12-14 causes a phase change of a 
selected carrier passing through transmitter 12-14 identified by the 
register. 
After changing the state of the phase control device 30 the controller 
takes a second PEP measurement through the PEP detector 16. The second PEP 
measurement is compared with the first measurement. Where the second PEP 
value is smaller than the first PEP value the controller 11 selects 100 
another carrier (e.g., by incrementing the contents of the carrier 
register). Where the second measurement is larger than the first, the 
controller 11 reverses the phase change. After restoring the phase of the 
originally selected carrier to an original state, the controller 11 
selects another carrier and the process repeats, so long as the PEP value 
is above the threshold. 
Carrier selection for phase changes may be incremental (e.g., each carrier 
is processed in order) or random. Where a small number of carriers is used 
an incremental system provides a simple, easy to implement, process. Where 
larger number of carriers are used a random process may be indicated. 
Control of PEP through adjustment of phase beneficially provides a method 
of reducing PEP without having an effect on average power. Such an effect 
can be demonstrated in the simple case demonstrated in FIG. 1-5 where a 
phase change of .pi. to signal C would cause PEP to be reduced. Changing a 
phase of a carrier in a transmitter of a cellular system would have a much 
less significant effect because of larger number of carriers. The process 
of testing each carrier for its effect upon PEP, on the other hand, 
insures that only the carriers contributing to PEP will be affected. 
Comparison of PEP with a threshold value ensures that phase changes will 
only be introduced when a need exists. 
The many features and advantages of this invention are apparent from the 
detailed specification and thus it is intended by the appended claims to 
cover all such features and advantages of the system which fall within the 
true spirit and scope of the invention. Further, since numerous 
modifications and changes will readily occur to those skilled in the art 
(e.g., phase changes caused by diodes or reactive elements), it is not 
desired to limit the invention to the exact construction and operation 
illustrated and described, and accordingly all suitable modifications and 
equivalents may be resorted to, falling within the scope of the invention. 
It is, of course, to be understood that the present invention is, by no 
means, limited to the specific showing in the drawing, but also comprises 
any modification within the scope of the appended claims.