Cartesian multicarrier feedback

A system for transmission of combined, multi-carrier signals wherein combiner/filters, commonly called combiners, have been eliminated. A cartesian feedback loop linearizes the system and thereby suppresses carrier frequency intermodulation by feeding back a portion of the combined multi-carrier signal to each channel device.

BACKGROUND 
The present invention generally relates to radio transmitters and, more 
particularly, to radio transmitters used in base stations of cellular 
radio systems. 
In cellular radio systems, transmissions from a base station can comprise 
signals from many different channels which are combined prior to 
transmission by an antenna or antennas. These signals need to be 
distinctly spaced from one another in frequency so that they can be 
separated after reception without severe intermodulation. Conventionally, 
this has been achieved by using combiner/filters, commonly just called 
combiners, which comprise multiple tuned cavity devices that allow 
simultaneous transmission of signals from a plurality of transmitters at 
different but closely spaced frequencies by way of a single antenna. 
Typically, combiners include one tuned cavity for each frequency. Each of 
the tuned cavities is coupled to a separate transmitter and is also 
coupled to an antenna. Combiners, however, have always been troublesome 
because numerous external influences cause the tuned cavities to become 
detuned. For example, normal temperature changes cause variations in the 
critical dimensions of these tuned cavities. Detuning of the cavities 
results in a substantial increase in insertion losses, thereby decreasing 
the amount of transmitter power that reaches the antenna. These problems 
are particularly acute in cellular telecommunication systems. One solution 
for overcoming temperature-caused detuning is to manufacture the tuning 
cavities from Invar, an expensive metal which must be coated with copper 
to provide the necessary high surface conductivity required of tuned 
cavities used in high frequency transmission systems. 
Even this expensive solution, however, fails to prevent detuning due to 
other environmental factors such as variations in humidity and atmospheric 
pressure. Retuning the resonant frequencies of these cavities can also be 
accomplished manually or by computer control of tuning elements in the 
cavities, however, these solutions are also expensive and create other 
problems. Moreover, the combiner is physically bulky and takes up space in 
the base station which could be used for other purposes. 
SUMMARY 
The present invention provides, among other advantages and objects, for a 
multi-carrier transmission system in which the expensive and bulky 
combiners are eliminated. Further, separation characteristics between 
adjacent channels can be enhanced and a gain in output power can be 
achieved according to exemplary embodiments of the present invention. 
These advantages and objects are realized, for example, by one exemplary 
embodiment of the present invention wherein the various channel signals 
are summed, then amplified and a portion of the amplified output signal is 
fed back via a cartesian feedback loop to the I and Q reference input 
basebands. This feedback serves to suppress frequency intermodulation 
while maintaining channel separation. 
According to another exemplary embodiment, the intermediate frequency band 
is upconverted after the channel frequencies have been summed and the loop 
signal is downconverted prior to being fed back to the reference basebands 
.

DETAILED DESCRIPTION 
FIG. 1 illustrates a conventional transmission system for a radio 
telecommunication system which can, for example, be located in a base 
station of a cellular system. Each transmission branch shown in FIG. 1 
corresponds to a channel used for communications in the system. Similarly 
numbered elements in FIG. 1 operate in a similar manner. Although only 
three branches have been illustrated for simplicity, many branches will be 
provided in a typical system as reflected by the broken lines in FIG. 1. 
The operation of an exemplary branch is as follows. 
The information carrying I (inphase) and Q (quadrature) baseband drive 
signals are applied to the modulator 10 which, typically, upconverts the 
signals to a higher transmission frequency and sums the components. This 
composite signal is then amplified by the rf frequency power amplifier 12 
and filtered by combiner/filter 14 to ensure crisp frequency separation 
when combined with the other signals transmitted via antenna 16. A 
cartesian feedback loop 18 samples the power output from power amplifier 
12 and is used to compensate for the nonlinearities introduced by the 
power amplifier. Operation of the cartesian feedback loop is discussed in 
more detail below with respect to FIG. 2. 
FIG. 2 is a more detailed block diagram of one of the branches of the 
conventional system of FIG. 1 which illustrates how the cartesian feedback 
loop operates. Cartesian feedback loops for single-carrier environments 
are disclosed for example in "Linearization of RF Power Amplifiers Using 
Cartesian Feedback" authored by Mats Johansson, which is hereby 
incorporated by reference. Again, similarly numbered elements function as 
described in FIG. 1. 
A portion of the signal output from power amplifier 12 is synchronously 
demodulated into its components by means of the phase correction device 20 
and the frequency downconverters 22. The demodulated feedback signal 
components are then subtracted from the I and Q baseband signals in 
comparators 24. The resultant quadrature component signals are then 
upconverted at blocks 26 and summed at block 28 prior to being amplified 
and filtered at blocks 12 and 14, respectively. The resultant signal is 
then combined with those signals of other channels and transmitted via an 
antenna as discussed above with respect to FIG. 1. 
The cartesian feedback loop provided in this conventional system 
compensates for drifts in nonlinearities introduced by the power amplifier 
which are caused, for example, by temperature changes, DC power 
variations, load changes and component aging. However, the problems 
discussed previously that are associated with the conventional combiner 
are not solved by this conventional usage of cartesian feedback 
techniques. 
Thus, according to an exemplary embodiment of the present invention, 
illustrated in FIG. 3, a transmission system has been designed wherein the 
combiner has been eliminated. The operation of this system is as follows. 
In a manner similar to that used to illustrate the conventional system of 
FIG. 1, only three branches are shown in the illustrative block diagram of 
this exemplary embodiment of the present invention, however, those skilled 
in the art will readily appreciate that such a system can have as many 
branches as necessary to correspond to the number of channels used in the 
system. Again, the operation of a single branch will be described as 
operation of the other channel branches is similar thereto. 
The baseband quadrature components I and Q are input to the modulator 30 
wherein the components are upconverted to a predetermined rf transmission 
frequency assigned to the corresponding channel and summed thereafter. 
This signal is output on line 32 to the phase compensator at block 34 
where the phase of each channel signal is adjusted prior to summation. 
Although the phase compensator 34 has been illustrated in the exemplary 
embodiments as a separate element, the phase compensators could also be 
formed integrally with the modulators 30. The resultant signal is summed 
at block 36 with the same signals of the other channels. This composite 
signal is then amplified by the rf power amplifier 38 before being 
transmitted via antenna 40. A cartesian feedback loop 42 samples the 
combined, multi-carrier signal which is then demodulated and compared with 
the reference baseband components in each of the modulators 30 in the same 
manner in which the single carrier output signal was processed as 
described above with respect to FIG. 2. 
Thus, according to this exemplary embodiment, the forward transmission 
circuit need not be highly linear because linearity is provided by the 
cartesian feedback loop, which is readily accomplished since very little 
power needs to be amplified in the feedback loop. In this way, 
intermodulation between the various carrier frequencies is suppressed. For 
example, without the cartesian feedback loop, signals having two different 
carrier frequencies, f.sub.1 and f.sub.2, which were summed and input to a 
nonlinear rf power amplifier would output a signal having significant 
intermodulation. The output of the nonlinear amplifier would comprise, for 
example, frequencies f.sub.1, f.sub.2, 2f.sub.1 -f.sub.2, 2f.sub.2 
-f.sub.1, 3f.sub.1 -2f.sub.2, 3f.sub.2 -2f.sub.1, etc. 
With the provision of the cartesian feedback loop which provides feedback 
on every possible intermodulation frequency, however, the intermodulation 
is suppressed by the gain in the cartesian feedback loop. Thus, if the 
carrier frequencies have approximately the same separation over the total 
transmitted bandwidth there is no need for the provision of a combiner. 
FIG. 4 illustrates this feature of exemplary embodiments of the present 
invention by showing the bandwidth of the separated carrier frequencies as 
compared to the cartesian feedback loop bandwidth. The outer dotted line 
50 represents a bandpass filter which excludes frequencies outside those 
used by the base station. The frequency spectrum for each channel signal 
is shown as centered about its corresponding carrier frequency, for 
example the frequency spectrum 52 relative to frequency f.sub.1. The 
dotted lines 54 which surround each frequency spectrum denote the loop 
gain of the feedback loop. Note that the bandwidth 56 of the cartesian 
feedback loop is such that any intermodulation frequencies (e.g., 
frequencies between f.sub.1 and f.sub.2) would be suppressed by the loop 
gain. 
According to another embodiment, shown in FIG. 5 where the same reference 
numbers used in FIG. 4 are again used to identify similar features, the 
bandwidth of the feedback can be varied such that the loop bandwidths 
overlap at their edges. This can provide, for example, more freedom in 
varying the separation between carrier frequencies while still suppressing 
intermodulation. 
Another exemplary embodiment of the present invention is illustrated in 
FIG. 6 in which similar reference numerals are used to identify similar 
elements. This transmission system is identical to the system of FIG. 3 
except that the reference baseband components are not upconverted to the 
transmission frequency in the modulators 10 nor is the feedback signal 
downconverted from the transmission frequency in the modulators 10. 
Instead, a downconverter 43 is placed in the feedback loop after sampling 
of the output signal and an upconverter 44 is placed after the summation 
block 36 and before the power amplifier 38. Thus, modulators 30 upconvert 
to, and downconvert from, an intermediate frequency. This makes 
implementation of a 90 degree phase shift network in quadrature modulators 
more easily accomplished and also generally reduces interference. The 
exemplary waveforms of FIGS. 4 and 5 can also be achieved using this 
exemplary embodiment and the discussion set forth above is equally 
relevant thereto. 
Although the present invention has been described by way of the foregoing 
exemplary embodiments, it will be appreciated by those skilled in the art 
that the present invention can be embodied in other forms without 
departing from the spirit or essential character thereof. Thus, for 
example, other types of adaptive feedback techniques could be substituted 
for the cartesian feedback loop used in the exemplary embodiments 
described herein. Moreover, although the overall systems (e.g., base 
station) in which transmission systems according to the present invention 
can be used have not been described in detail, the present invention is 
intended to encompass the incorporation of the present invention therein. 
Thus, for example, the present invention readily lends itself to 
incorporation in any multicarrier transmission system, including FDMA 
systems and multi-carrier TDMA and CDMA systems. An exemplary system is 
disclosed in U.S. Pat. No. 5,140,627, entitled "Handoff Procedure that 
Minimizes Disturbance to DTMF Signalling in a Cellular Radio System", 
which is hereby incorporated by reference. 
The presently disclosed embodiments are therefore considered in all 
respects to be illustrative and not restrictive. The scope of the 
invention is indicated by the appended claims rather than the foregoing 
description, and all changes which come within the meaning and range of 
equivalents thereof are intended to be embraced therein.