2 Adaptive polarization division multiplexing technique using V

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Report ITU-R M.2175
(07/2010)
Simultaneous dual linear polarization
transmission technique using digital
cross-polarization cancellation
for MSS systems
M Series
Mobile, radiodetermination, amateur
and related satellites services
ii
Rep. ITU-R M.2175
Foreword
The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the
radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without
limit of frequency range on the basis of which Recommendations are adopted.
The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional
Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups.
Policy on Intellectual Property Right (IPR)
ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of
Resolution ITU-R 1. Forms to be used for the submission of patent statements and licensing declarations by patent
holders are available from http://www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the
Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found.
Series of ITU-R Reports
(Also available online at http://www.itu.int/publ/R-REP/en)
Series
BO
BR
BS
BT
F
M
P
RA
RS
S
SA
SF
SM
Title
Satellite delivery
Recording for production, archival and play-out; film for television
Broadcasting service (sound)
Broadcasting service (television)
Fixed service
Mobile, radiodetermination, amateur and related satellite services
Radiowave propagation
Radio astronomy
Remote sensing systems
Fixed-satellite service
Space applications and meteorology
Frequency sharing and coordination between fixed-satellite and fixed service systems
Spectrum management
Note: This ITU-R Report was approved in English by the Study Group under the procedure detailed
in Resolution ITU-R 1.
Electronic Publication
Geneva, 2010
 ITU 2010
All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU.
Rep. ITU-R M.2175
1
REPORT ITU-R M.2175
Simultaneous dual linear polarization transmission technique
using digital cross-polarization cancellation
for MSS systems*
(Question ITU-R 83-6/4)
(2010)
TABLE OF CONTENTS
Page
1
Introduction ....................................................................................................................
2
2
Adaptive polarization division multiplexing technique using V/H dual linear
polarization .....................................................................................................................
4
Channel model ................................................................................................................
6
3.1
Without multi-path components .........................................................................
7
3.2
With multi-path components ..............................................................................
7
4
Feasibility of APDM technique ......................................................................................
8
5
Considerations on interference among systems using APDM .......................................
11
6
Conclusion ......................................................................................................................
11
3
*
When submitting to the Radiocommunication Bureau a satellite network intended to be operated with the
technique described in this Report, administrations need to take into account that both orthogonal
polarisations have to be included in the submission, in order for them to be appropriately coordinated.
2
1
Rep. ITU-R M.2175
Introduction
Mobile-satellite communication services are now being offered all over the world. In order to share
the limited frequency bandwidth among many MSS systems, a perpetual requirement is to improve
the spectrum utilization efficiency. For this purpose, it is important to consider how to not only
share the same frequency bandwidth between different systems but also improve spectrum
utilization efficiency within an MSS system.
Figure 1 shows a typical MSS system that employs circular polarization. Circular polarization is
mainly adopted due to its polarization-tracking-free nature, which makes it suitable for mobile
services. In a typical system, right-hand circular polarization or left-hand circular polarization is
selected. If a single user earth station sends at the bit rate of R (bit/s) with bandwidth of W (Hz) and
user earth stations A and B make use of the satellite transponder, the total bit rate is 2R with the
bandwidth of 2 W in Fig. 1.
FIGURE 1
Spectrum utilization in a circular polarization system
without polarization tracking
Bandwidth = W (Hz)
Bit rate = R (bit/s)
Right-hand
circular
polarization
: R.H.C. pol.
W
R.H.C. pol.
Bandwidth = W (Hz)
Bit rate = R (bit/s)
Satellite
Polarization
tracking free
Polarization
tracking free
R.H.C. pol.
A
W
B
Freq.
Freq.
User earth
station A
User earth
station B
R.H.C. pol.
W
W
A
B
Freq.
System features
Base station
Polarization tracking free
Total bandwidth : 2W (Hz)
Total bit rate : 2R (bit/s)
A
B
Another MSS system utilizes V/H dual linear polarization. Figure 2 shows an example of spectrum
utilization in a V/H dual linear polarization system. Each user earth station communicates using
either V or H polarization. In Fig. 2, V polarization is assigned to user earth station A and H
polarization is assigned to user earth station B. Since user earth station A and user earth station B
share the same frequency bandwidth with different polarization, polarization tracking is required at
each user earth station so as to eliminate cross-polarization interference on the other user earth
station. In Fig. 2, if each user earth station sends at the bit rate of R with bandwidth of W, the total
bit rate, 2R, is achieved with the bandwidth of W.
Rep. ITU-R M.2175
3
FIGURE 2
Spectrum utilization in a dual V/H linear polarization system
with polarization tracking
Bandwidth = W (Hz)
Bit rate = R (bit/s)
Bandwidth = W (Hz)
Bit rate = R (bit/s)
Satellite
User A
V pol.
W
W
Precise
polarization
tracking
Precise
polarization
tracking
Freq.
B
A
Freq.
User B
H pol.
User earth
station A
Satellite
V pol.
User earth
station B
W
A
Freq.
B
Satellite
H pol.
System features
Base station
Precise polarization tracking
A
B
Total bandwidth : W (Hz)
Total bit rate : 2R (bit/s)
Compared to dual linear polarization, circular polarization excels in terms of its
polarization-tracking-free nature which yields simple user earth stations. However, linear
polarization offers double the spectrum utilization efficiency; 2R/W (bit/s/Hz) compared to
2R/2W (bit/s/Hz). This means that dual linear polarization makes better use of spectrum resources
than circular polarization.
It is true that dual linear polarization is attractive from the viewpoint of spectrum utilization
efficiency. However, in practice, accurate polarization tracking is difficult to realize, especially for
mobile user earth stations with low-profile antennas. Figure 3 shows the degradation in spectrum
utilization efficiency that occurs when the mobile user earth station experiences polarization
misalignment. As shown, to handle this misalignment, the spectrum utilization efficiency of both
user earth station A and user earth station B should be reduced to hold communication quality
steady in the face of cross-polarization interference between user earth stations. Figure 3 also shows
one solution to the problem of mutual interference: single linear polarization. This approach does
not employ polarization multiplexing and so avoids the mutual interference between user earth
stations. Its weakness is that it fails to increase the spectrum utilization efficiency.
4
Rep. ITU-R M.2175
FIGURE 3
Degradation in spectrum utilization efficiency due
to imprecise polarization tracking
Polarization miss-alignment
in each user earth station
Satellite
V pol.
User A
V pol.
With polarization
multiplexing
Satellite
V pol.
User B
H pol.
Without polarization
multiplexing
(Dual linear pol.)
(Single linear pol.)
Satellite
V pol.
Satellite
V pol.
W
Crosspolarization
interference
A
W
A
Freq.
Crosspolarization
interference
Freq.
B
Satellite
H pol.
System features
Base station
Satellite
H pol.
Base station
Imprecise polarization
tracking
A
B
Low rate FEC, etc
2
Total bandwidth : W (Hz)
Total bit rate : less than 2R (bit/s)
System features
Imprecise polarization
tracking
A
Total bandwidth : W (Hz)
Total bit rate : R (bit/s)
Adaptive polarization division multiplexing technique using V/H dual linear
polarization
The adaptive polarization division multiplexing (APDM) technique realizes a
polarization-tracking-free MSS system with dual V/H linear polarization transmission.
This technique offers improved spectrum utilization efficiency with a simple satellite tracking
antenna. Figure 4 shows the concept of APDM. Each signal is divided into two blocks and
conveyed independently using V or H linear polarization. The two signals are polarization
multiplexed at the antenna of each user earth station. In Fig. 4, user earth station A sends at the bit
rate of R with bandwidth of W. Therefore, user earth station A’s signal is divided into two
independent blocks, A1 and A2 (each W/2), and they are polarization multiplexed in the user earth
station A. The signal of user earth station B is divided into B1 and B2 (each W/2) similarly. Note
that the APDM station dispenses with polarization tracking. Therefore, the polarization states of
user earth station A and user earth station B are not aligned to those of the satellite as shown in
Fig. 4. Thus, cross-polarization interference occurs between A1 and A2, and between B1 and B2 in
the receiver. To counter this interference, a digital cross-polarization interference canceller is
implemented in the user earth station's receiver as shown in Fig. 5. Figure 6 shows an example
of the configurations of APDM transmitter and receiver. For realizing APDM, each transmitter
must have a V/H dual modulator/frequency converter with high-power amplifier. Each receiver, on
Rep. ITU-R M.2175
5
the other hand, needs to have a V/H dual low noise amplifier/frequency converter/demodulator with
interference canceller. In Fig. 6, since modulator, demodulator and interference canceller are
realized by digital circuits, they can be compactly implemented as integrated circuits. The major
differences from the earth stations without APDM are the additional RF components, shown by the
hatching in Fig. 6, that transmit/receive V/H dual polarized signals simultaneously.
This polarization division multiplexing with digital cross-polarization interference cancellation
realizes polarization-tracking-free MSS systems using V/H dual linear polarization and facilitates
broadband mobile-satellite communications services by achieving better spectrum utilization
efficiency.
FIGURE 4
Spectrum utilization in a dual V/H linear polarization
system with APDM technique
Satellite
Bandwidth = W (Hz)
Bit rate = R (bit/s)
User A
H pol.
A1 A2
Polarization
tracking free
Polarization
tracking free
User A
V pol.
User B
V pol.
A1 A2
A1
User B
H pol.
Freq.
W/2
W
User earth
station A
Bandwidth = W (Hz)
Bit rate = R (bit/s)
Satellite
V pol.
W/2
B1 B2
B1
Freq.
B2
W/2
User earth
station B
W/2
W
B
A
Freq.
A2
Freq.
A1
Satellite
H pol.
B1
B2
Freq.
Base station
with digital crosspolarization interference
canceller
A
B
System features
Polarization tracking free
Total bandwidth : W (Hz)
Total bit rate : 2R (bit/s)
6
Rep. ITU-R M.2175
FIGURE 5
APDM cross-polarization interference cancellation in the receiver
W/2
W
Sat V pol.
A1'
A1
Digital
cross-pol.
interference
canceller
Freq.
A2'
Cross
Sat H pol.
-pol.
A2'
W/2
Demod.
A2
Freq.
A1'
A
Freq.
Freq.
Freq.
Received V/H signals
Receiver
FIGURE 6
Configuration of APDM transmitter and receiver
Data
MOD
S/P
MOD
CONV.
CONV.
HPA
HPA
DEM
P/S
DEM
Digital
Interference
Canceller
Digital circuit
3
OMT
: Additional RF component
(a) Transmitter
Digital circuit
Data
CONV. : Frequency converter
LNA : Low Noise Amplifier
HPA : High Power Amplifier
OMT : Orthogonal Mode Transducer
MOD : Modulator
DEM : Demodulator
CONV.
CONV.
LNA
LNA
OMT
(b) Receiver
Channel model
Due to the polarization-tracking-free-nature of APDM, its transmission channel triggers mutual
coupling, V to V, H to H, V to H and H to V polarizations. To determine the basic properties of
these mutual couplings, we introduce θ which denotes the polarization rotation angle between the
transmitter (Tx) and receiver (Rx) as shown in Fig. 7. By using polarization rotation angle θ, XPI
(cross-polarization isolation) is defined as:
XPI (dB)  20 log
v
h
sin 
 20 log
 20 log
V
H
cos 
where v′, h′ and V′, H′ denote the amplitude of the cross polarization signal and that of the desired
polarization signal, respectively.
Rep. ITU-R M.2175
7
FIGURE 7
Polarization rotation between Tx and Rx
Tx V pol.
Rx V pol.
V'
v'

h'
H'
3.1
Tx H pol.
Rx H pol.
Without multi-path components
Figure 8 a) shows the channel model without multi-path components. This condition is typically
satisfied in bands above 6/4 GHz where most earth stations employ highly directional (pencil-beam)
antennas. In general, the channel model is formed as a combination of the propagation path
condition and the polarization rotation of the antenna. In other words, it is defined by a channel
matrix that is the product of the propagation matrix and the antenna matrix as shown in Fig. 8.
If each earth station uses a highly directional antenna, the multi-path component is negligible.
As a result, the channel model simply consists of the polarization rotation of the antenna. In
this model, cross-polarization cancellation can be carried out without performance degradation.
3.2
With multi-path components
Figure 8 b) shows the channel model with multi-path components. This model is typical in the
bands below 6/4 GHz where many of the earth stations employ omni-directional (or broad beam)
antennas. Differently from the channel model in § 3.1, the broad directionality of the
omni-directional antenna in each earth station means that multi-path signals as well as direct-path
signals are received. For example, a multi-path component that originates from the V polarization
signal is superposed on its own direct-path component. At the same time, another multi-path
component from the V polarization signal is mixed with the H polarization signal. The same
situation is true for the multi-path components from the H polarization signal. The resulting
propagation matrix is shown in Fig. 8 b), where A, B, C and D are environment-dependent
variables. These environment-dependent variables might affect the performance of
cross-polarization cancellation.
8
Rep. ITU-R M.2175
FIGURE 8
Channel models of dual V/H polarization signal transmission
Multi-path
component
V
+
Earth
station
Satellite
H
+
Propagation
matrix
1 0 
0 1 


V
V
+
Propagation
matrix
 sin  
cos  
V
Earth
station
H
H
A B
C D 


(a) Without multi-path component
4
+
Satellite
Antenna
matrix
cos 
 sin 

+
+
H
Antenna
matrix
cos 
 sin 

 sin  
cos  
(b) With multi-path component
Feasibility of APDM technique
To confirm the basic feasibility of cross-polarization interference cancellation technique, an APDM
modem module was developed and its performance was measured.
Figure 9 shows the constellation before/after cross-polarization interference cancellation. Various
types of interference canceller have been studied in the wireless communications field so far. In
Fig. 9, the minimum mean squared error (MMSE) algorithm is used for the canceller. As shown in
Fig. 9, cross-polarization interference can be removed by a digital cross-polarization interference
canceller.
FIGURE 9
Cross-polarization interference cancellation
Constellation after
interference cancellation
Constellation before
interference cancellation
W/2
A1'
A1
Freq.
A2'
Digital
cross-pol.
interference
canceller
Cross-pol.
A2'
A1'
Freq.
W/2
A2
Freq.
Freq.
Rep. ITU-R M.2175
9
To evaluate the overall performance of the APDM technique for polarization-tracking-free
broadband communications using Vertical/Horizontal dual linear polarization, several system level
satellite experiments were conducted in the 14/12 GHz bands. The experimental parameters are
listed in Table 1. In the satellite experiments, an APDM station and a base station were prepared.
The APDM station uses dual polarization simultaneously but does not track the polarization status
at all. Note that the polarization status of the base station is adjusted to match that of the satellite.
TABLE 1
Signal conditions
Access method
FDMA
Bit rate
1.28 Mbit/s, 5.12 Mbit/s
Modulation
QPSK
FEC
Turbo product code ( R = 0.66 )
First, in order to confirm the basic performance of dual polarization use, a dual polarized fixed
antenna and satellite modem were prepared for both an APDM station and base station; the bit error
ratio (BER) performance of a dual polarized signal was measured in satellite experiments. In the
experiments, the polarization angle of the APDM user antenna was rotated manually to emulate a
polarization-tracking-free environment with the fixed antenna.
Figure 10 shows the BER performance of the dual polarized signal while the polarization angle was
manually rotated. In Fig. 10, RTL denotes the link from the APDM user station to the base station,
and FWL the link from the base station to the APDM user station. As shown in Fig. 10, the
degradation in required Eb/N0 was about 0.5 dB with different polarization rotation angles, , and no
significant performance degradation was measured.
FIGURE 10
BER performance of dual polarized signal at the fixed antenna
with manual polarization rotation (θ)
1.0E-02
10-2
Computer simulation
RTL: θ = 0 deg.
RTL: θ = 30 deg.
RTL: θ = 45 deg.
Satellite
RTL: θ = 60 deg.
experiment
FWL: θ = 0 deg.
FWL: θ = 30 deg.
FWL: θ = 45 deg.
FWL: θ = 60 deg.
1.0E-03
10-3
BER
1.0E-04
10-4
1.0E-05
10-5
10-6
1.0E-06
10-7
1.0E-07
10-8
1.0E-08
2
3
4
5
6
Eb/N0 ( dB )
7
8
10
Rep. ITU-R M.2175
Next, to evaluate the performance of APDM in a more practical situation, a dual polarized
auto-tracking satellite directional antenna was developed. Its BER performance was measured while
rolling the dual polarized antenna mechanically by a ship motion simulator as if it were mounted on
a ship. Table 2 shows the motion parameters that were used by the ship motion simulator. In the
experiments, two kinds of ship motion were created. The parameters in Table 2 (a) simulate the
motion of a large ship (7,300 Gross Ton) on small waves and those in Table 2 (b) are for the motion
of a small ship (420 Gross Ton) on large waves.
TABLE 2
Simulated ship motion parameters
Motion (a): Large ship
Roll
2° in 13 s
Pitch
1° in 7 s
Yaw
30°
Motion (b): Small ship
Roll
5° in 6 s
Pitch
4° in 5 s
Yaw
5°
Figure 11 shows the BER performance that was measured while rolling the antenna by the ship
motion simulator. Figure 11 shows that no significant performance degradation was caused by ship
motion in the satellite experiments. This means that dual polarization use with APDM is feasible in
MSS systems. From these results, it is concluded that the APDM technique is a practical way of
implementing dual polarization use.
Rep. ITU-R M.2175
11
FIGURE 11
BER performance of dual polarized signal
with dynamic antenna motion
1.0E-02
10-2
Computer simulation
RTL: No motion
RTL: Motion (a)
Satellite
RTL: Motion (b)
experiment
FWL: No motion
FWL: Motion (a)
FWL: Motion (b)
1.0E-03
10-3
BER
1.0E-04
10-4
1.0E-05
10-5
1.0E-06
10-6
10-7
1.0E-07
10-8
1.0E-08
2
5
3
4
5
6
Eb/N0 ( dB )
7
8
Considerations on interference among systems using APDM
MSS systems using APDM are deployed via dual linear polarized satellite transponders that are
already being operated. Since APDM earth stations satisfy the same coordination conditions that are
imposed on systems operated in the same satellite transponder, no additional coordination is
needed.
Among the systems proposing to use APDM, it is assumed that the V/H dual polarization frequency
band is divided system by system and a guard band is prepared so as to mitigate interference among
systems. This approach is not unique and is a common practice used to avoid interference among
systems. Therefore, the use of APDM places no additional burden on the system.
6
Conclusion
To improve the spectrum utilization efficiency when cross-polarization isolation (XPI) is
insufficient in the mobile environment, this Report presents a technique of APDM with V/H dual
linear polarization. APDM does not induce cross-polarization interference between user earth
stations while keeping the spectrum utilization efficiency high through the use of dual polarization.
This technique is anticipated to be further developed and may lead to additions of this Report in the
future.
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