Techniques for cross-polarization cancellation in a space diversity radio system

The present invention relates to a technique for achieving cross-polarization interference (CPI) cancellation with or without equalization in a digital-radio, space-diversity communication system. More particularly, a present receiver includes a pair of spaced-apart diversity antennas, where each antenna is capable of receiving orthogonally polarized signals, e.g., Vertical and Horizontal polarizations, from a remote transmitter. The received orthogonally polarized signals from each antenna are separated and coupled to separate inputs of a separate cross-polarization canceler, where each canceler includes two straight-through and two cross-over paths including a separate complex gain multiplier disposed in each path. The correspondingly orthogonally polarized output signals from each of the cancelers are added to produce a first and a second orthogonally polarized overall canceler arrangement output signal. Power measurements are taken of the two overall output signals during predetermined periods of time to produce control signals in a Control and Dither means for appropriately adjusting the complex gain multipliers in each canceler and substantially cancel CPI while providing equalization where desired. Additionally, control signals can also be generated for transmission back to the remote transmitter for appropriately adjusting a canceler therein when CPI cancellation including a second order in frequency is desired.

TECHNICAL FIELD 
The present invention relates to a technique which effects cancellation of 
cross-polarization interference, with or without equalization, in a space 
diversity radio system. 
DESCRIPTION OF THE PRIOR ART 
The use of dual-polarization digital radio in microwave common carrier 
bands has been increasing to improve transmission efficiency. The major 
technical problems associated with dual-polarization operation in radio 
systems is (a) cross-polarization coupling occurs in both the radio 
equipment and the propagation medium; (b) the associated 
cross-polarization transfer functions are both dispersive 
(frequency-selective) and time varying; and (c) the crosspolarization 
transfer functions produce co-channel interference levels that can be too 
large for effective detection of QAM signals, e.g., 16-QAM, 64-QAM, or 
256-QAM, particularly when the desired co-polarization signal fades due to 
multipath. 
In recent years, many articles and patents have dealt with 
crosspolarization cancellation techniques, measurements over 
dual-polarization radio channels, and dual-polarization radio link 
analysis. In this regard see, for example, U.S. Pat. Nos. 4,283,795 issued 
to M. L. Steinberger on Aug. 11, 1981, and 4,577,330 issued to M. Kavehrad 
on Mar. 18, 1986; and the articles "Cofrequency Cross-Polarized Operation 
of a 91 Mbit/s Digital Radio" by S. Barber in IEEE Transactions on 
Communications, Vol. COM-32, No. 1, January 1984, at pages 87-91, and 
"Sweep Measurements Of Multipath Effects On Cross-Polarized RF-Channels 
Including Space Diversity" by M. Liniger in Globecom '84, Vol. 3, Atlanta, 
Ga., November 1984, at pages 45.7.1-45.7.5. 
The trend has also been to meet increased traffic demands by modifying 
existing single-polarization radio systems by adding dual-polarization to 
double the capacity on the same frequency band. The problem remaining in 
the prior art is to provide a simple way of eliminating cross-polarization 
interference when adding a second polarization to a channel in a radio 
system where space diversity antennas are used. 
SUMMARY OF THE INVENTION 
The foregoing problem in the prior art has been solved in accordance with 
the present invention which relates to a technique for providing 
crosspolarization interference (CPI) cancellation, and the ability to also 
provide equalization, in a space diversity radio system. More 
particularly, a separate simple cross-polarization canceler, which, by 
itself, cannot be effective in eliminating CPI, is associated with each 
antenna of a pair of space diversity antennas, where each antenna receives 
the first and second polarized signals from a remote transmitter. Each 
canceler receives the first and second polarized signals from the 
associated antenna at separate first and second inputs, respectively. Each 
canceler includes two straight-through and two cross-over paths, where 
each path includes a separate complex gain multiplier means which (a) is 
frequency independent and, hence, simple to build, and (b) selectively 
adjusts the gain and phase of a signal propagating in that path in 
response to receive control signals. The adjusted converging 
straight-through and cross-over path signals in each canceler are added to 
provide a separate polarized output signal with reduced CPI at each output 
of the canceler. The correspondingly polarized output signals from each 
canceler are then added, and power measurements made of each canceler 
output signal during predetermined periods of time. The resultant power 
measurements are used to generate control signals which appropriately 
adjust the complex gain multiplier means and provide a flat gain output 
signal with substantially no cross-polarization components. 
Other and further aspects of the present invention will become apparent 
during the course of the following description and by reference to the 
accompanying drawings.

DETAILED DESCRIPTION 
The present invention relates to method and apparatus for use with 
dually-polarized transmission signals in radio communication systems using 
space diversity antennas to substantially cancel cross-polarization 
interference (CPI) and also provide equalization where desired. More 
particularly, the present invention exploits the relatively mild 
dispersion of channel response functions (H-functions) over a digital 
radio channel bandwidth. In systems using space diversity reception, the 
mildly dispersive cross-polarization functions are suppressed by using 
non-dispersive G-functions, i.e., at most one adaptive gain per 
cross-polarization and cross-coupling branch of each diversity receiver. 
Probing intervals are used to derive control signals for adjusting the 
non-dispersive G-functions, and variations on the present invention can 
also add a degree of multipath equalization. 
Over the bandwidth of a microwave radio channel, each channel response 
function, H(.omega.), can be described by a low-order complex polynomial, 
i.e., 
EQU H.sub.ij (.omega.)=A.sub.ij +j.omega.B.sub.ij +(j.omega.).sup.2 C.sub.ij +. 
. . i=1,2; j=1,2 (1) 
where (a) the subscript i denotes the destination polarization component 
and j is the origination polarization component, and (b) all coefficients 
are complex. For purposes of description hereinafter, it will be assumed 
that the polarizations used are linear polarizations and that the Vertical 
polarization will be designated by a 1 while the Horizontal polarization 
will be designated by a 2 for either of the subscripts i or j. For 
example, H.sub.12 designates the cross-polarization response function for 
Horizontal(2)-into-Vertical(1) and H.sub.11 designates the inline, or 
co-polarization, response function for Vertical-into-Vertical. 
Additionally, the superscripts u and 1 will be used hereafter to denote 
the upper and lower space diversity antennas 10 and 11, respectively, 
shown in the Figures. Thus, in the diagram of FIG. 6, H.sub.11.sup.u 
(.omega.) i the V-into-V polarization channel response into the upper 
diversity antenna 10; H.sub.22.sup.l (.omega.) is the H-into-H 
polarization channel response into the lower diversity antenna 11; etc., 
where the latter response can be shown expanded, as in equation (1), by 
EQU H.sub.22.sup.l (.omega.)=A.sub.22.sup.l +j.omega.B.sub.22.sup.l 
+(j.omega.).sup.2 C.sub.22.sup.l +. . . (2) 
The cross-polarization interference (CPI) spectrum at the output 5 of the 
V-polarized canceler 12 of FIG. 6 can then be written as 
EQU V.sub.out (.omega.)=[A.sub.22 +j.omega.B.sub.22 +(j.omega.).sup.2 C.sub.22 
+. . . ]V.sub.in (.omega.)+[A.sub.12 +j.omega.B.sub.12 +(j.omega.).sup.2 
C.sub.12 +. . . ]H.sub.in (.omega.), (3) 
where A.sub.12, B.sub.12 and C.sub.12 are weighted sums over the complex 
gains .gamma..sub.1, .gamma..sub.2, .gamma..sub.3 and .gamma..sub.4. For 
example, from equation (1) it can be shown that 
EQU A.sub.12 =A.sub.12.sup.u .gamma..sub.1 +A.sub.22.sup.u .gamma..sub.2 
+A.sub.12.sup.1 .gamma..sub.3 +A.sub.22.sup.l .gamma..sub.4.(4) 
From the above description, it can be seen that A.sub.12, B.sub.12 and 
C.sub.12 can all be set to zero by appropriate choices of .gamma..sub.2 
/.gamma..sub.1, .gamma..sub.3 /.gamma..sub.1, and .gamma..sub.4 
/.gamma..sub.1, with .gamma..sub.1 being either fixed (e.g., set at unity) 
or adjusted to scale the desired cross-polarized signal appropriately. 
The foregoing discussion discloses the principle of the present technique, 
and FIGS. 1 and 2 illustrate an exemplary diagram of an arrangement and 
the format of the transmitted signals, respectively, for practicing the 
present technique. In FIG. 1, a first and second diversity antenna 10 and 
11 are each shown as comprising two separate antennas, one, e.g., antennas 
10.sub.1 and 11.sub.1, for receiving the Vertically (V) polarized signal 
components, and another, e.g., 10.sub.2 and 11.sub.2, for receiving the 
Horizontally (H) polarized components of the received signal. It is to be 
understood that such illustration is provided solely for purposes of 
explanation and not for purposes of limitation since the normal approach 
would be to use a separate single antenna 10 and 11, with the V and H 
polarized components from the received signal from each antenna being 
separated and directed along separate electrical or waveguide paths by any 
suitable means. The received V and H signal components at each antenna are 
understood to include the originally transmitted V and H signals, 
respectively, plus any cross-polarized signal components occurring during 
transmission and reception. 
The separated V and H output signal components from antenna 10 are received 
at separate input terminals 14 and 15, respectively, of canceler 12, while 
the separated V and H output signal components from antenna 11 are 
received at separate input terminals 14 and 15, respectively, of canceler 
13. Each canceler, e.g., canceler 12, propagates the received V and H 
signal components via a separate straight-through path 16 and 17, 
respectively, and a respective separate cross-over path 18 and 19. 
Disposed in each of paths 16 to 19 is a separate adjustable complex 
multiplier 20 to 23, respectively, to provide simple appropriate selective 
gain and phase shift adjustments for CPI cancellation of the V.sub.out 
lead 5 and H.sub.out lead 6 of the canceler arrangement of FIG. 1. Complex 
multipliers 20-23 can comprise any suitable device known in the art as, 
for example, the series CPM complex phase modulators from Olektron Corp., 
in Webster, Mass. In each of cancelers 12 and 13, the complex multiplier 
adjusted signals in paths 16 and 19 are added in an adder 24 to produce a 
resultant adjusted Vertically polarized output signal on lead 26. 
Similarly, in each of cancelers 12 and 13, the complex multiplier adjusted 
signals in paths 17 and 18 are added in an adder 25 to produce a resultant 
adjusted Horizontally polarized output signal on lead 27. The vertically 
polarized signals on leads 26 from cancelers 12 and 13 are added in an 
adder 28 to produce the V.sub.out signal from the present CPI cancellation 
arrangement on output lead 5. The horizontally polarized signals on leads 
27 from canceler 12 and 13 are also added in an adder 29 to produce the 
H.sub.out signal from the present CPI cancellation arrangement on output 
lead 6. 
In accordance with one embodiment of the prevent invention, the V.sub.out 
and H.sub.out signals on output leads 5 and 6, respectively, are partially 
coupled out via respective couplers 30 and 31 and are propagated via a 
switching means 32 to power measuring devices 33 and 34, respectively. The 
power measured by each of devices 33 and 34 during a particular time 
interval when switching means 32 is closed is converted into a 
corresponding signal which is transmitted to a control and dither means 
35. The control and dither means 35 functions to convert the input power 
measurement signals into appropriate control signals for transmission to 
the complex multipliers 20-23 of cancelers 12 and 13, and optionally back 
to a remote transmitter via antenna 36 to provide further CPI cancellation 
as will be explained hereinafter with regard to the arrangement of FIG. 3. 
The problem solved in accordance with the arrangement of FIG. 1 is that if 
only one of cancelers 12 or 13 is used, e.g., canceler 12, then such 
canceler can only cancel CPI if the H and V channels are flat, or are not 
changing, with frequency. In other words the amplitude of the received 
signal would have to be substantially flat over a predetermined frequency 
bandwidth. Such canceler 12 would not work well in a digital radio system 
where the channels change in frequency. Therefore, in accordance with the 
present invention, space diversity antennas 10 and 11 and a second 
canceler 13 are used to achieve cancellation of the flat part of the 
cross-polarization. A.sub.12, and the first order of the frequency, 
j.omega.B.sub.12, as represented by the first and second terms, 
respectively, in the second half of equation (3). PG,8 
To cancel the second order of the frequency, (j.omega.).sup.2 C.sub.12, 
shown by the third term in the second half of equation (3), a canceler 40 
is required to be disposed at the remote transmitter as shown in FIG. 3, 
plus the feedback path 41 provided by antenna 36 in FIG. 1. Canceler 40 
includes the same elements 14-25, and the functioning thereof, as 
explained for the corresponding elements of canceler 12 or 13. The output 
from canceler 40 at the remote transmitter is transmitted by an antenna 42 
which comprises section 42.sub.I, for transmitting the vertically 
polarized signals, and section 42.sub.2, for transmitting the horizontally 
polarized signals. The V and H polarized signals delivered to antenna 42 
have been appropriately adjusted by canceler 40 in response to feedback 
signals from control and dither means 35 at the receiver to cancel the 
second order of the frequency at the outputs 5 and 6 of the receiver 
canceler arrangement. 
Implementations of first and second embodiments of the present Space 
Diversity CPI technique are disclosed in FIGS. 1 and 2 and FIGS. 4 and 5, 
respectively. As shown in FIG. 2 for a first embodiment, the data streams 
for the Vertically and Horizontally polarized signals use quiet probing 
intervals 50 which alternate between the Vertically polarized and 
Horizontally polarized transmissions. At the Vertical polarization output 
lead 5 in FIG. 1, the V.sub.out signal from the canceler arrangement of 
FIG. 1 will contain only Horizontally polarized interference components 
during the Vertical polarized quiet probing intervals, and similarly the 
H.sub.out signal from the canceler arrangement on lead 6 will only contain 
Vertically polarized interference components during the Horizontal 
polarized quiet probing signals. Therefore, in two successive probing 
intervals 50, in alternating polarizations, average power measurements can 
be made on the Horizontal polarization interference signal, to be 
designated X, and the desired Vertically polarized signal, to be 
designated S, by power measuring device 33. 
Each of power measuring devices 33 and 34 operates in synchronism with each 
probing interval 50 in the V.sub.out and H.sub.out data streams because 
switching means 32 is synchronized to close during such quiet probing 
intervals. Control and Dither means 35 computes the ratio S/X and 
generates appropriate control signals to drive the gain in complex 
multipliers 20 and 23 in cancelers 12 and 13 to maximize this ratio. 
Techniques for dither control of circuit gains are well known in the art 
and any suitable technique can be used. Power measuring device 34 operates 
similar to that explained for power measuring device 33 in order to 
control the gain of complex multipliers 21 and 22 in cancelers 12 and 13 
and maximize the ratio for H.sub.out on output lead 6. 
The feedback path to antenna 36 in FIG. 1 can be used to facilitate and 
added degree of CPI control via adaptive cross-coupling in canceler 40 in 
the transmitter should in FIG. 3. Thus, if a small amount of V.sub.in at 
the transmitter is coupled into the Horizontally polarized transmission 
through complex multiplier 22 in canceler 40, and similarly a small amount 
of H.sub.in is coupled into the Vertically polarized transmission, then by 
proper control of these two added gains the net CPI responses at the 
outputs of FIG. 1 could be canceled to include the second order in 
frequency. More generally, transmitter cross-coupling adds another degree 
of control freedom to achieve CPI reduction. The control of the variable 
transmitter gains in canceler 40 can use the same technique as used for 
the receiver gains in cancelers 12 and 13, except that the control signals 
have to be communicated back to the transmitter over feedback path 41. It 
is to be understood that feedback path 41 could comprise a separate radio 
channel, or existing wire or data link facilities. 
Cross Polarization Inteference (CPI) and multipath equalization can also be 
achieved simultaneously using the basic approach discussed hereinbefore. 
Instead of canceling the cross-polarization responses to include the first 
order in frequency, or second order if transmitter cross-coupling is used 
in canceler 40, the variable gains complex multipliers 20-23 can be 
adjusted so that the copolarization responses are equalized to a first 
order in frequency, and the cross-polarization responses are canceled only 
to the flat response, or to include a first order if transmitter 
cross-coupling in canceler 40 is used. 
FIGS. 4 and 5 show an arrangement and technique similar to that of FIGS. 1 
and 2 for a second embodiment, but with two differences. A first 
difference is that the input to Control and Dither means 35 are digitized 
complex baseband samples taken once every frame period by an 
Analog-to-Digital (A/D) circuit 60. The second difference is that the 
alternating quiet probing intervals 50 of FIG. 2 are replaced by 
concurrent non-quiet probing intervals 61 of FIG. 5 including known data 
sequences in the Vertically and Horizontally polarized signals. With one 
of the polarizations, e.g., the Vertical polarization, these data 
sequences are the same and comprise the same polarity from one probing 
interval to another, and with the other polarization, e.g., the Horizontal 
polarization, these sequences are the same but alternate in polarity. As 
shown in FIG. 4, the V.sub.out and H.sub.out signals from adders 28 and 
29. respectively, are first provided as inputs to respective mixers 62 and 
63 where they are mixed with the output signal from a local oscillator 64 
to provide the respective baseband output signals V.sub.out and H.sub.out 
which are then sampled in A/D circuit 60. 
The sequence of complex samples taken from the baseband V.sub.out signal at 
the output of mixer 62 during the first probing interval 61 shown in FIG. 
5 can be called {V+h}, where {V} is the co-polarized sequence, including 
intersymbol interference (ISI), and {h} us the cross-polarized sequence 
found in the Vertically polarized signal. This composite sequence is 
digitized and stored in a memory means of Control and Dither means 35. In 
the next probing interval 61, the sampled sequence from V.sub.out is {V-h} 
because of the polarity inversion used in the Horizontally polarized 
transmission. A third sequence stored at the receiver in the Control and 
Dither means 35 is the known probing interval data sequence and designated 
{Y}. By summing the first two sequences {V+h} and {V-h}, dividing by 2, 
and subtracting the third, {Y}, Control and Dither means 35 obtains an 
estimate of ISI (ignoring thermal noise for simplicity) defined by: 
EQU [{V+h}+{V-h}]/2-{Y}={V}-{Y}=ISI (5) 
In Equation (5), the first term [{V+h}+{V-h}]/2 provides the Cross 
Polarization Interference residue {h}. Similar processing of H.sub.out 
during the probing intervals yields the same kind of information in the 
other polarization. The ISI and CPI sequences thus obtained can be used, 
via and suitable dither algorithm, by Control and Dither means 35 to adapt 
the variable gains in the receiver, and transmitter if appropriate. 
The criterion for the adaptation is to minimize the mean square sum of CPI 
and ISI (and thermal noise). Given the locally stored data sequence {Y}, 
the control technique automatically scales the gains, all of which must 
therefore be variable, so that the signal level is fixed. Thus, minimizing 
the mean square sum of CPI, ISI and noise is equivalent to maximizing the 
Signal-to-(CPI+ISI+Noise) ratio. 
FIG. 7 is a Table summarizing the maximum number of Quadrature Amplitude 
Modulation (QAM) levels that are expected to be supported using 
permutations of of the three transmitter/receiver structures listed at the 
right-hand side versus the two control strategies to (a) maximize the 
Signal-to-(CPI+ISI+Noise) ratio as in FIG. 4 with no additional 
equalization, and to maximize the IF Signal-to-(CPI+Noise) ratio as in 
FIG. 1 and add post-canceler equalization. As a practical matter, the best 
approach for, for example, 64-QAM is space diversity reception without 
transmitter cross-coupling, maximization of the IF Signal-to-(CPI+Noise) 
ratio, and post canceler equalization to provide good performance and 
simplicity. All other possibilities would be considerably more 
complicated, with uncertain benefits.