U.S. patent application number 16/784207 was filed with the patent office on 2021-05-27 for near-capacity iterative detection of co-channel interference for a high-efficiency multibeam satellite system.
This patent application is currently assigned to Hughes Network Systems, LLC. The applicant listed for this patent is Bassel F. BEIDAS, Rohit Iyer SESHADRI. Invention is credited to Bassel F. BEIDAS, Rohit Iyer SESHADRI.
Application Number | 20210159923 16/784207 |
Document ID | / |
Family ID | 1000004651551 |
Filed Date | 2021-05-27 |
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United States Patent
Application |
20210159923 |
Kind Code |
A1 |
BEIDAS; Bassel F. ; et
al. |
May 27, 2021 |
Near-Capacity Iterative Detection of Co-Channel Interference for A
High-Efficiency Multibeam Satellite System
Abstract
A communications apparatus to receive a composite signal
including a desired signal and interferer signals, where the
desired signal may include desired symbols and the interferer
signals may include interferer symbols. The system may include N
frameworks, each framework may include a detector to partition the
desired symbols and the interferer symbols based on an interference
severity into a dominant group and a non-dominant group, and to
generate A Posteriori Probabilities (APP) of the desired symbols
and the interferer symbols. The detector of each of the N
frameworks generates the APP based on a feedback of a priori
probabilities from each of the N frameworks.
Inventors: |
BEIDAS; Bassel F.;
(Germantown, MD) ; SESHADRI; Rohit Iyer;
(Germantown, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BEIDAS; Bassel F.
SESHADRI; Rohit Iyer |
Germantown
Germantown |
MD
MD |
US
US |
|
|
Assignee: |
Hughes Network Systems, LLC
Germantown
MD
|
Family ID: |
1000004651551 |
Appl. No.: |
16/784207 |
Filed: |
February 6, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62939928 |
Nov 25, 2019 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 17/345 20150115;
H03M 13/3933 20130101; H03M 13/3927 20130101; H03M 13/3911
20130101; H04B 7/18513 20130101 |
International
Class: |
H03M 13/39 20060101
H03M013/39; H04B 7/185 20060101 H04B007/185; H04B 17/345 20060101
H04B017/345 |
Claims
1. A communications apparatus comprising: a composite signal
comprising a desired signal and interferer signals, wherein the
desired signal comprises desired symbols and the interferer signals
comprise interferer symbols; and N frameworks, each framework
comprising a detector to partition the desired symbols and the
interferer symbols based on an interference severity into a
dominant group and a non-dominant group, and to generate A
Posteriori Probabilities (APP) of the desired symbols and the
interferer symbols, wherein the detector of each of the N
frameworks generates the APP based on a feedback of a priori
probabilities from each of the N frameworks.
2. The communications apparatus of claim 1, wherein the detector of
each of the N frameworks transforms the APP to a first
Log-Likelihood Ratios (LLRs) using a bit-to-symbol mapping rule,
and each of N frameworks further comprises a deinterleaver to
deinterleave the first LLRs into a decoder input, a Forward Error
Correcting (FEC) Decoder to decode the decoder input and to
generate a second LLRs from a decoded decoder input, and an
interleaver to interleave the second LLRs, wherein the second LLRs
are treated as a priori probabilities for a respective N framework
after the interleaver of the respective N framework, and the second
LLRs of each of the N frameworks represent either the desired
symbols or the interferer symbols.
3. The communications apparatus of claim 2, wherein the desired
signal is more robust than each of the interferer signals, and the
second LLRs from the FEC decoder of a first framework of the N
frameworks represent the desired symbols.
4. The communications apparatus of claim 2, wherein at least one of
the interferer signals is more robust than the desired signal, and
the second LLRs from the FEC decoder of a framework other than a
first framework of the N frameworks represent the desired
symbols.
5. The communications apparatus of claim 4, wherein the desired
symbols are recovered by Simultaneous Decoding (SD) or by
Simultaneous Non-unique Decoding (SND).
6. The communications apparatus of claim 1, wherein the detector is
selected from a Soft-In Soft-Out (SISO) detector, a
Divide-And-Conquer (DAC) detector or a SISO DAC detector.
7. The communications apparatus of claim 1, wherein the
communications apparatus is disposed in an SISO Iterative Divide
and Conquer (IDAC) receiver and the detector is a SISO DAC
detector.
8. The communications apparatus of claim 1, wherein the dominant
group comprises an Optimal-Bayesian (OB) group via a probability
mass function (pmf).
9. The communications apparatus of claim 1, wherein the
non-dominant group comprises a Noise-Floor (NF) group incorporated
via a power of each member and a Subtractive-Cancellation (SC)
group incorporated via first- and second-order moments derived from
the a priori probabilities, and the dominant group comprises an
Optimal-Bayesian (OB) group incorporated via a probability mass
function (pmf).
10. The communications apparatus of claim 9, wherein an output x [
k ] = .gamma. n d a n d , k + h _ I , n d ( OB ) [ k ] a _ I , n d
( OB ) [ k ] + h _ I , n d ( SC ) [ k ] a _ I , n d ( SC ) [ k ] +
h _ I , n d ( NF ) [ k ] a _ I , n d ( NF ) [ k ] + n [ k ] ,
##EQU00016## where a.sub.i,nd [k] partitioned into three groups,
a.sub.I,n.sub.d.sup.(NF)[k], a.sub.I,n.sub.d.sup.(SC)[k],
a.sub.I,n.sub.d.sup.(OB)[k], and h.sub.I,n.sub.d.sup.(NF)[k],
h.sub.I,n.sub.d.sup.(SC)[k], h.sub.I,n.sub.d.sup.(OB)[k] represents
spatial and temporal CCI channel coefficients corresponding to the
NF group, the SC group and the OB group, respectively.
11. The communications apparatus of claim 10, wherein the detector
is P DAC ( a n d , k | x [ k ] ) = a _ I , n d ( OB ) [ k ] p DAC (
x [ k ] | a _ n d , k , a _ I , n d ( OB ) [ k ] , a _ I , n d ( SC
) [ k ] , a _ I , n d ( NF ) [ k ] ) ##EQU00017## mathematically
expressed as P(a.sub.I,n.sub.d.sup.(OB)[k])P(a.sub.n.sub.d.sub.,k),
where p.sub.DAC( ) is p DAC ( x [ k ] | a n d , k , a _ I , n d (
OB ) [ k ] , a _ I , n d ( SC ) [ k ] , a _ I , n d ( NF ) [ k ] )
= exp { - ( x [ k ] - I ^ n d ( SC ) [ k ] ) - .gamma. n d a n d ,
k - h _ I , n d ( OB ) [ k ] a _ I , n d ( OB ) [ k ] 2 .sigma. n 2
+ c I , n d ( SC ) [ k ] + c I , n d ( NF ) [ k ] } ##EQU00018## a
likelihood function associated with observing x[k], P( ) is the a
priori probabilities corresponding to the second LLRs representing
the desired symbols,
I.sub.n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]{a.sub.I,n.sub.d.sup-
.(SC)[k]},
c.sub.I,n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]Cov{a.su-
b.I,n.sub.d.sup.(SC)[k]}(h.sub.I,n.sub.d.sup.(SC)[k]).sup.H, and
c.sub.I,n.sub.d.sup.(NF)[k]=h.sub.I,n.sub.d.sup.(NF)[k](h.sub.I,n.sub.d.s-
up.(NF)[k]).sup.H.
12. The communications apparatus of claim 1, wherein a count of the
N frameworks is selected from one (1), two (2) or three (3).
13. The communications apparatus of claim 1, wherein the desired
signal and the interferer signals comprise DVB-S2X standard
compliant signals.
14. A computer implemented method comprising: providing a composite
signal comprising a desired signal and interferer signals, wherein
the desired signal comprises desired symbols and the interferer
signals comprise interferer symbols; and iteratively computing, N
frameworks, each framework comprising partitioning the desired
symbols and the interferer symbols based on an interference
severity into a dominant group and a non-dominant group, and
generating A Posteriori Probabilities (APP) of the desired symbols
and the interferer symbols, wherein the detector of each of the N
frameworks generates the APP based on a feedback of a priori
probabilities from each of the N frameworks.
15. The method of claim 14, wherein the detector of each of the N
frameworks transforms the APP to a first Log-Likelihood Ratios
(LLRs) using a bit-to-symbol mapping rule, and the iteratively
computing further comprises: deinterleaving the first LLRs into a
decoder input, decoding the decoder input, generating a second LLRs
from a decoded decoder input and interleaving the second LLRs,
wherein the second LLRs are treated as a priori probabilities for a
respective N framework after the interleaver of the respective N
framework, and the second LLRs of each of the N frameworks
represent either the desired symbols or the interferer symbols.
16. The method of claim 15, wherein the desired signal is more
robust than each of the interferer signals, and the second LLRs
from the decoding of a first framework of the N frameworks
represent the desired symbols.
17. The method of claim 15, wherein at least one of the interferer
signals is more robust than the desired signal, and the second LLRs
from the decoding of a framework other than a first framework of
the N frameworks represent the desired symbols.
18. The method of claim 14, wherein the non-dominant group
comprises a Noise-Floor (NF) group incorporated via a power of each
member and a Subtractive-Cancellation (SC) group incorporated via
first- and second-order moments derived from the a priori
probabilities, and the dominant group comprises an Optimal-Bayesian
(OB) group incorporated via a probability mass function (pmf).
19. The method of claim 18, wherein an output x[k] is, x [ k ] =
.gamma. n d a n d , k + h _ I , n d ( OB ) [ k ] a _ I , n d ( OB )
[ k ] + h _ I , n d ( SC ) [ k ] a _ I , n d ( SC ) [ k ] + h _ I ,
n d ( NF ) [ k ] a _ I , n d ( NF ) [ k ] + n [ k ] , ##EQU00019##
where a.sub.i,nd [k] is partitioned into three groups,
a.sub.I,n.sub.d.sup.(NF)[k], a.sub.I,n.sub.d.sup.(SC)[k],
a.sub.I,n.sub.d.sup.(OB)[k], and h.sub.I,n.sub.d.sup.(NF)[k],
h.sub.I,n.sub.d.sup.(SC)[k], h.sub.I,n.sub.d.sup.(OB)[k],
represents spatial and temporal CCI channel coefficients
corresponding to the NF group, the SC group and the OB group,
respectively.
20. The method of claim 19, wherein the detector is mathematically
expressed as P DAC ( a n d , k | x [ k ] ) = a _ I , n d ( OB ) [ k
] p DAC ( x [ k ] | a _ n d , k , a _ I , n d ( OB ) [ k ] , a _ I
, n d ( SC ) [ k ] , a _ I , n d ( NF ) [ k ] ) P ( a _ I , n d (
OB ) [ k ] ) P ( a n d , k ) , ##EQU00020## where pDAC( ) is p DAC
( x [ k ] | a n d , k , a _ I , n d ( OB ) [ k ] , a _ I , n d ( SC
) [ k ] , a _ I , n d ( NF ) [ k ] ) = exp { - ( x [ k ] - I ^ n d
( SC ) [ k ] ) - .gamma. n d a n d , k - h _ I , n d ( OB ) [ k ] a
_ I , n d ( OB ) [ k ] 2 .sigma. n 2 + c I , n d ( SC ) [ k ] + c I
, n d ( NF ) [ k ] } ##EQU00021## a likelihood function associated
with observing x[k], P( ) is the a priori probabilities
corresponding to the second LLRs representing the desired symbols,
I.sub.n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]{a.sub.I,n.-
sub.d.sup.(SC)[k]},
c.sub.I,n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]Cov{a.sub.I,n.sub.-
d.sup.(SC)[k]}(h.sub.I,n.sub.d.sup.(SC)[k]).sup.H, and
c.sub.I,n.sub.d.sup.(NF)[k]=h.sub.I,n.sub.d.sup.(NF)[k](h.sub.I,n.sub.d.s-
up.(NF)[k]).sup.H.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY
REFERENCE
[0001] The present application claims the benefit under 35 U.S.C.
119(e) of U.S. Provisional Application Ser. No. 62/939,928, filed
Nov. 25, 2019, which is incorporated herein by reference in its
entirety.
FIELD
[0002] A receiver to improve spectral efficiency in a multibeam
satellite system with Co Channel Interference (CCI), in particular,
systems that employ aggressive frequency reuse. The receiver
successfully mitigates CCI by compensating for memory effects
compounding the CCI which are unavoidable in a multibeam satellite
system. The receiver may be deployed at a user terminal. In the
present teachings, an exemplary receiver is presented for satellite
applications; however, the receiver of the present teachings may be
applied in other radio communications systems, such as cellular
(e.g., 4G, LTE, or 5G) and WiFi.
BACKGROUND
[0003] In the prior art, precoding and beamforming techniques are
limited by the accuracy of Channel State Information (CSI)
information available at the gateway. Additionally, precoding
techniques are also limited by the lack of feeder-link bandwidth.
Compensating for CCI at a receiver, for example, a user terminal,
overcomes the above limitations.
[0004] A multibeam satellite system employs frequency reuse in
which different beams share the same frequency resource or
bandwidth. Beams sharing bandwidth are known as co-channel beams.
The closer the two co-channel beams are located geographically, the
greater is the co-channel interference (CCI). CCI can have
devastating impact on the performance, hence prior art systems
maintain CCI at acceptable levels by typically employing a reuse
factor of 4, illustrated in FIG. 1, where the total system
bandwidth B is divided into two parts on each antenna polarization,
such that each beam has bandwidth of B/2.
[0005] FIG. 1 illustrates a prior art beam laydown pattern with a
4-reuse plan for a multi-beam satellite, according to various
embodiments.
[0006] FIG. 1 illustrates a reuse plan combining signal
polarization and non-overlapping frequency spectrums to create a
4-reuse plan 100. In the example, the four colors (hashing in FIG.
1) correspond to four different frequency/polarization allocations.
An available frequency spectrum is divided into two frequency
sub-bands F1 and F2 and each sub-band is assigned/mapped to a
different color, and two orthogonal polarizations are
assigned/mapped colors to provide the two other colors in the
4-color reuse plan. A service or coverage area 114 may be divided
into cells. Multi-beam satellites typically illuminate multiple
hexagonal cells within a service area 114. In exemplary
embodiments, a cell 102 may be illuminated by a far field beam
pattern in band F1 using a right hand circular polarization (RHCP),
a cell 104 in band F1 may be illuminated by a far field beam
pattern in band F1 using a left hand circular polarization (LHCP),
a cell 106 may be illuminated by a far field beam pattern in F2
using a RHCP and a cell 108 which may be illuminated by a far field
beam pattern in F2 using a LHCP A multi-beam satellite implementing
a reuse plan may arrange the two polarizations in separate
alternate rows, for example, a LHCP row 110 and a RHCP row 112. It
is possible to tessellate a desired coverage area, such as, earth's
surface, using an Nc color reuse tessellate where Nc is any
positive natural number. A one-color and two-color is also known.
Moreover, a 3 or 7-color reuse plan, each of which use both
polarizations, is also known.
[0007] The very high capacity required by the next generation of
broadband satellites makes it necessary to adopt more aggressive
reuse factors such as reuse 2 and reuse 1 in which each beam can
make use of the entire bandwidth B. Under such deployments, users
at the edge of the co-channel cells experience severe CCI which
adversely affects their data rate and quality of service.
[0008] The CCI mitigation techniques applied at the satellite
gateway such as precoding and beamforming are well known. The
effectiveness of precoding and beamforming depends on the accuracy
of the Channel State Information (CSI) (for example, magnitude,
phase, and delay) available at the gateway. CSI depends on
effective estimation and frequent reporting from user terminals to
the gateway. Furthermore, transmitter-based techniques used, for
example, at the gateway, are also limited by the feeder-link
bandwidth since effective precoding requires that signals
transmitted to co-frequency beams come from the same gateway.
[0009] The multiuser detection framework of prior art receivers
assumes that CCI is memoryless, namely, that the current receive
filter sampled output depends only on the current symbol from CCI.
However, memory effects in CCI are inevitable since the desired
signal and interfering signals arrive asynchronously at the user
terminal. Even though the co-channel beams may be formed and
emitted by the satellite simultaneously, they propagate through
different paths causing unavoidable differential delays. Further,
the beams could be conveying signals originating from multiple
gateways, interconnected via terrestrial links. Other sources that
generate memory effects include different symbol rates required by
co-channel beams and/or using pulse shaping with different rolloff
values. Prior art receivers based on optimal realizations handle
memory effects with a complexity that is exponential with the
number of interferers and the memory span of each CCI source,
making such a receiver severely unaffordable. Other receivers
suffer from 2 dB of degradation with only moderate timing offset of
15% of the symbol period. Additionally, the memory effects can be
made worse due to different symbol rates and/or different roll-off
factors employed by the different co-channel beams. These critical
limitations motivate the development of low-complexity, innovative
solutions that can compensate for CCI with memory at the user
terminal and are described in detail in this disclosure.
SUMMARY
[0010] This Summary is provided to introduce a selection of
concepts in a simplified form that is further described below in
the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended to be used to limit the scope of the claimed
subject matter.
[0011] In the present teachings, receiver based CCI mitigation
alleviates the need for frequent reporting of CSI by the user
terminal to the different gateways. Additionally, the
receiver-based mitigation overcomes feeder-link bandwidth
limitation since it does not require the interfering signals being
processed at the user terminal to be generated by the same
gateway.
[0012] The present teachings disclose several signal processing
innovations absent in the prior art multiuser detection techniques.
An innovation is the ability to mitigate memory effects in CCI at a
user terminal receiver, having a modular structure, without an
exponential increase in the receiver complexity. The receiver
mitigates interference terms with computational power commensurate
with the intensity level of interference experienced at the user
terminal. Extensive performance evaluation of the receiver using
state-of-the art MODCODs selected from the DVB-S2X standard
demonstrates effectiveness under severe CCI with memory. The
performance associated with the receiver is near capacity
approaching the limits predicted by information theory.
[0013] A system of one or more computers can be configured to
perform particular operations or actions by virtue of having
software, firmware, hardware, or a combination of them installed on
the system that in operation causes or cause the system to perform
the actions. One or more computer programs can be configured to
perform operations or actions by virtue of including instructions
that, when executed by data processing apparatus, cause the
apparatus to perform the actions. One general aspect of a
communications apparatus includes processing a composite signal may
include a desired signal and interferer signals, where the desired
signal may include desired symbols and the interferer signals may
include interferer symbols; and N frameworks, each framework may
include a detector to partition the desired symbols and the
interferer symbols based on an interference severity into a
dominant group and a non-dominant group, and to generate A
Posteriori Probabilities (APP) of the desired symbols and the
interferer symbols. The detector of each of the N frameworks
generates the APP based on a feedback of a priori probabilities
from each of the N frameworks. Other embodiments of this aspect
include corresponding computer systems, apparatus, and computer
programs recorded on one or more computer storage devices, each
configured to perform the actions of the methods.
[0014] Implementations may include one or more of the following
features. The communications apparatus where the detector of each
of the N frameworks transforms the APP to a First Log-Likelihood
Ratios (LLRs) using a bit-to-symbol mapping rule, and each of N
frameworks further may include a deinterleaver to deinterleave the
first LLRs into a decoder input, a Forward Error Correcting (FEC)
decoder to decode the decoder input and to generate a second LLRs
from a decoded decoder input, and an interleaver to interleave the
second LLRs, where the second LLR are treated as a priori
probabilities for a respective N framework after the interleaver of
the respective N framework, and the second LLRs of each of the N
frameworks represent either the desired symbols or the interferer
symbols.
[0015] In some embodiments, the desired signal is more robust than
each of the interferer signals, and the second LLRs from the FEC
decoder of a first framework of the N frameworks represent the
desired symbols. In some embodiments, at least one of the
interferer signals is more robust than the desired signal, and the
second LLRs from the FEC decoder of a framework other than a first
framework of the N frameworks represent the desired symbols. In
some embodiments, the desired symbols are recovered by Simultaneous
Decoding (SD) or by Simultaneous Non-Unique Decoding (SND). In some
embodiments, the detector is selected from a Soft-In Soft-Out
(SISO) detector, a Divide-And-Conquer (DAC) detector or a SISO DAC
detector. In some embodiments, the communications apparatus is
disposed in an SISO Iterative Divide and Conquer (IDAC) receiver
and the detector is a SISO DAC detector. In some embodiments, the
dominant group may include an Optimal-Bayesian (OB) group via a
probability mass function (pmf). The non-dominant group may include
a Noise-Floor (NF) group incorporated via a power of each member
and a Subtractive-Cancellation (SC) group incorporated via first-
and second-order moments derived from the a priori probabilities,
and the dominant group may include an Optimal-Bayesian (OB) group
incorporated via a probability mass function (pmf).
[0016] In some embodiments, an output
x [ k ] = .gamma. n d a n d , k + h _ I , n d ( OB ) [ k ] a _ I ,
n d ( OB ) [ k ] + h _ I , n d ( SC ) [ k ] a _ I , n d ( SC ) [ k
] + h _ I , n d ( NF ) [ k ] a _ I , n d ( NF ) [ k ] + n [ k ] ,
##EQU00001##
[0017] where a.sub.i,nd [k] is partitioned into three groups,
a.sub.I,n.sub.d.sup.(NF)[k], a.sub.I,n.sub.d.sup.(SC)[k],
a.sub.I,n.sub.d.sup.(OB)[k], and h.sub.I,n.sub.d.sup.(NF)[k],
h.sub.I,n.sub.d.sup.(SC)[k], h.sub.I,n.sub.d.sup.(OB)[k],
represents spatial and temporal CCI channel coefficients
corresponding to the NF group, the SC group and the OB group,
respectively. In some embodiments, the detector is mathematically
expressed as
P D A C ( a n d , k | x [ k ] ) = a _ I , n d ( OB ) [ k ] p DAC (
x [ k ] | a n d , k , a I , n d ( OB ) [ k ] , a I , n d ( SC ) [ k
] , a I , n d ( NF ) [ k ] ) P ( a I , n d ( OB ) [ k ] ) P ( a n d
, k ) , , ##EQU00002##
[0018] where p.sub.DAC( ) is
p D A C ( x [ k ] | a n d , k , a I , n d ( OB ) [ k ] , a I , n d
( SC ) [ k ] , a I , n d ( NF ) [ k ] ) = exp { - ( x [ k ] - I ^ n
d ( SC ) [ k ] ) - .gamma. n d a n d , k - h _ I , n d ( OB ) [ k ]
a _ I , n d ( OB ) [ k ] 2 .sigma. n 2 + c I , n d ( SC ) [ k ] + c
I , n d ( NF ) [ k ] } ##EQU00003##
[0019] a likelihood function associated with observing x[k], P( )
is the a priori probabilities corresponding to the second LLRs
representing the desired symbols,
I.sub.n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]{a.sub.I,n.sub.d.sup-
.(SC)[k]},
c.sub.I,n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]ov{a.sub-
.I,n.sub.d.sup.(SC)[k]}(h.sub.I,n.sub.d.sup.(SC)[k]).sup.H, and
c.sub.I,n.sub.d.sup.(NF)[k]=h.sub.I,n.sub.d.sup.(NF)[k](h.sub.I,n.sub.d.s-
up.(NF)[k]).sup.H. In some embodiments, a count of the N frameworks
is selected from one (1), two (2) or three (3). In some
embodiments, the desired signal and the interferer signals may
include DVB-S2X standard compliant signals. Implementations of the
described techniques may include hardware, a method or process, or
computer software on a computer-accessible medium.
[0020] One general aspect includes a computer implemented method.
The method includes providing a composite signal may include a
desired signal and interferer signals, where the desired signal may
include desired symbols and the interferer signals may include
interferer symbols; and iteratively computing, N frameworks, each
framework may include partitioning the desired symbols and the
interferer symbols based on an interference severity into a
dominant group and a non-dominant group, and generating a
posteriori probabilities (APP) of the desired symbols and the
interferer symbols. The method also includes where the detector of
each of the N frameworks generates the APP based on a feedback of a
priori probabilities from each of the N frameworks. Other
embodiments of this aspect include corresponding computer systems,
apparatus, and computer programs recorded on one or more computer
storage devices, each configured to perform the actions of the
methods.
[0021] Additional features will be set forth in the description
that follows, and in part will be apparent from the description, or
may be learned by practice of what is described.
DRAWINGS
[0022] In order to describe the manner in which the above-recited
and other advantages and features may be obtained, a more
particular description is provided below and will be rendered by
reference to specific embodiments thereof which are illustrated in
the appended drawings. Understanding that these drawings depict
only typical embodiments and are not, therefore, to be limiting of
its scope, implementations will be described and explained with
additional specificity and detail with the accompanying
drawings.
[0023] FIG. 1 illustrates a prior art beam laydown pattern with a
4-color reuse plan for a multi-beam satellite, according to various
embodiments.
[0024] FIG. 2 illustrates two users serviced by two co-channel
beams showing CCI between beams according to various
embodiments.
[0025] FIG. 3 illustrates a multi-spot beam satellite system model
according to various embodiments.
[0026] FIG. 4 illustrates a block diagram of a soft-in-soft-out
(SISO) IDAC receiver according to various embodiments, shown here
for two dominant interferers.
[0027] FIG. 5 illustrates a coded PER of DVB-S2X 16APSK with code
rate 28/45 when interfered by QPSK with rate R.sub.2 at different
levels of timing offset (TO), according to various embodiments.
[0028] FIG. 6 illustrates an information-theoretic achievable-rate
region for 16APSK when interfered by QPSK at C/I=0 dB and
E.sub.s/N.sub.0=7.83 dB), according to various embodiments.
[0029] Throughout the drawings and the detailed description, unless
otherwise described, the same drawing reference numerals will be
understood to refer to the same elements, features, and structures.
The relative size and depiction of these elements may be
exaggerated for clarity, illustration, and convenience.
DETAILED DESCRIPTION
[0030] The present teachings may be a system, a method, and/or a
computer program product at any possible technical detail level of
integration. The computer program product may include a computer
readable storage medium (or media) having computer readable program
instructions thereon for causing a processor to carry out aspects
of the present invention.
[0031] The computer readable storage medium can be a tangible
device that can retain and store instructions for use by an
instruction execution device. The computer readable storage medium
may be, for example, but is not limited to, an electronic storage
device, a magnetic storage device, an optical storage device, an
electromagnetic storage device, a semiconductor storage device, or
any suitable combination of the foregoing. A non-exhaustive list of
more specific examples of the computer readable storage medium
includes the following: a portable computer diskette, a hard disk,
a random access memory (RAM), a read-only memory (ROM), an erasable
programmable read-only memory (EPROM or Flash memory), a static
random access memory (SRAM), a portable compact disc read-only
memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a
floppy disk, a mechanically encoded device such as punch-cards or
raised structures in a groove having instructions recorded thereon,
and any suitable combination of the foregoing. A computer readable
storage medium, as used herein, is not to be construed as being
transitory signals per se, such as radio waves or other freely
propagating electromagnetic waves, electromagnetic waves
propagating through a waveguide or other transmission media (e.g.,
light pulses passing through a fiber-optic cable), or electrical
signals transmitted through a wire.
[0032] Computer readable program instructions described herein can
be downloaded to respective computing/processing devices from a
computer readable storage medium or to an external computer or
external storage device via a network, for example, the Internet, a
local area network, a wide area network and/or a wireless network.
The network may comprise copper transmission cables, optical
transmission fibers, wireless transmission, routers, firewalls,
switches, gateway computers and/or edge servers. A network adapter
card or network interface in each computing/processing device
receives computer readable program instructions from the network
and forwards the computer readable program instructions for storage
in a computer readable storage medium within the respective
computing/processing device.
[0033] Computer readable program instructions for carrying out
operations of the present invention may be assembler instructions,
instruction-set-architecture (ISA) instructions, machine
instructions, machine dependent instructions, microcode, firmware
instructions, state-setting data, or either source code or object
code written in any combination of one or more programming
languages, including an object oriented programming language such
as SMALLTALK, C++ or the like, and conventional procedural
programming languages, such as the "C" programming language or
similar programming languages. The computer readable program
instructions may execute entirely on the user's computer, partly on
the user's computer, as a stand-alone software package, partly on
the user's computer and partly on a remote computer or entirely on
the remote computer or server. In the latter scenario, the remote
computer may be connected to the user's computer through any type
of network, including a local area network (LAN) or a wide area
network (WAN), or the connection may be made to an external
computer (for example, through the Internet using an Internet
Service Provider). In some embodiments, electronic circuitry
including, for example, programmable logic circuitry,
field-programmable gate arrays (FPGA), or programmable logic arrays
(PLA) may execute the computer readable program instructions by
utilizing state information of the computer readable program
instructions to personalize the electronic circuitry, in order to
perform aspects of the present invention.
[0034] Aspects of the present invention are described herein with
reference to flowchart illustrations and/or block diagrams of
methods, apparatus (systems), and computer program products
according to embodiments of the invention. It will be understood
that each block of the flowchart illustrations and/or block
diagrams, and combinations of blocks in the flowchart illustrations
and/or block diagrams, can be implemented by computer readable
program instructions.
[0035] These computer readable program instructions may be provided
to a processor of a general purpose computer, special purpose
computer, or other programmable data processing apparatus to
produce a machine, such that the instructions, which execute via
the processor of the computer or other programmable data processing
apparatus, create means for implementing the functions/acts
specified in the flowchart and/or block diagram block or blocks.
These computer readable program instructions may also be stored in
a computer readable storage medium that can direct a computer, a
programmable data processing apparatus, and/or other devices to
function in a particular manner, such that the computer readable
storage medium having instructions stored therein comprises an
article of manufacture including instructions which implement
aspects of the function/act specified in the flowchart and/or block
diagram block or blocks.
[0036] The computer readable program instructions may also be
loaded onto a computer, other programmable data processing
apparatus, or other device to cause a series of operational steps
to be performed on the computer, other programmable apparatus or
other device to produce a computer implemented process, such that
the instructions which execute on the computer, other programmable
apparatus, or other device implement the functions/acts specified
in the flowchart and/or block diagram block or blocks.
[0037] The flowchart and block diagrams in the Figures illustrate
the architecture, functionality, and operation of possible
implementations of systems, methods, and computer program products
according to various embodiments of the present invention. In this
regard, each block in the flowchart or block diagrams may represent
a module, segment, or portion of instructions, which comprises one
or more executable instructions for implementing the specified
logical function(s). In some alternative implementations, the
functions noted in the block may occur out of the order noted in
the figures. For example, two blocks shown in succession may, in
fact, be executed substantially concurrently, or the blocks may
sometimes be executed in the reverse order, depending upon the
functionality involved. It will also be noted that each block of
the block diagrams and/or flowchart illustration, and combinations
of blocks in the block diagrams and/or flowchart illustration, can
be implemented by special purpose hardware-based systems that
perform the specified functions or acts or carry out combinations
of special purpose hardware and computer instructions.
[0038] Reference in the specification to "one embodiment" or "an
embodiment" of the present invention, as well as other variations
thereof, means that a feature, structure, characteristic, and so
forth described in connection with the embodiment is included in at
least one embodiment of the present invention. Thus, the
appearances of the phrase "in one embodiment" or "in an
embodiment", as well any other variations, appearing in various
places throughout the specification are not necessarily all
referring to the same embodiment.
[0039] The ever-increasing demand for higher throughput and
ubiquitous connectivity implies extracting higher spectral
efficiencies and improved energy efficiencies from modern satellite
broadband systems. Spectrum scarcity imposes fundamental
limitations on the data rates and necessitates the most efficient
use of available frequency resources.
[0040] In the prior art, precoding and beamforming techniques are
limited by the accuracy of Channel State Information (CSI)
available at the gateway. Additionally, precoding techniques are
also limited by the lack of feeder-link bandwidth. Compensating for
CCI at a receiver, for example, a user terminal, overcomes the
above limitations. In the present teachings, a user terminal
mitigates CCI in a multiuser detection framework by jointly
processing a desired signal together with a dominant source. In
some embodiments, the dominant source can comprise one, two or more
interferers.
[0041] Extensive computer simulations, along with accompanying
information-theoretic results, demonstrate the superlative
performance of the present teachings when tackling severe cases of
CCI in multibeam satellite systems employing aggressive frequency
reuse, even without imposing the restrictive requirement of
synchronous reception.
System Model
[0042] FIG. 2 illustrates two users serviced by two co-channel
beams showing CCI between beams according to various
embodiments.
[0043] FIG. 2 illustrates a satellite communications system 200
including a user terminal (UT) 202 communicating with a satellite
208 via a beam 204. When the UT 202 is located near an edge 212 or
214 of their coverage area 210, a co-channel beam 206 (here
intended for coverage area 220), in a reuse 2 deployment will
interfere with the beam 204. In this case, the UT 202 will receive
its desired/intended signal (beam 204) in the presence of
interference arising from interferer signals/co-channel beams 206,
207 intended for a UT disposed in the coverage areas 220, 222
respectively. The ratio of the beam 204 at UT 202 (C) to the
interfering signal/co-channel beams 206, 207 (I) is known as the
carrier-to-interference ratio (C/I) and can be as high as 0 dB, or
of equal strength. In some deployments, one or more of the
interferer signals 206, 207 may be more robust than the desired
signal 204. An interference severity may be based on the C/I
ratio.
[0044] It is quite likely that the desired signal and the
interferer signals can come from different gateways (not shown),
employ different coded-modulation (MODCOD) formats, have different
symbol rates and pulse shaping filters with different roll-off
factors. The desired and interfering signals can arrive at the user
terminal in an asynchronous manner. Even though the co-channel
beams may be formed and emitted by the satellite simultaneously,
they propagate through different paths causing unavoidable
differential delays. Further, the beams could be conveying signals
originating from multiple gateways, interconnected via terrestrial
links. Other sources that generate memory effects include different
symbol rates required by co-channel beams and/or using pulse
shaping with different rolloff values. CCI mitigation techniques
are required to maintain reasonable performance for such users.
[0045] Here co-channel beam 206 may also be known as a dominant
source. When a UT, such as UT 202, is disposed near two edges of
its coverage area (i.e., coverage area 210), the UT 202 may receive
two co-channel beams (second co-channel beam not shown) and
portions of both may qualify as dominant sources with respect to
the desired signal. For example, a current sample of the co-channel
beam may be considered a dominant source, while a previous sample
of the co-channel beam may be considered a non-dominant source. In
some embodiments, an intended signal of a UT may be interfered with
by one or more dominant sources.
[0046] The interfering signal/co-channel beams 206, 207 may include
interfering symbols. Interfering symbols of the beam 206 targeting
the coverage area 220 immediately adjacent to the coverage area 210
where the UT 202 is disposed may be partitioned into a dominant
group of interfering symbols. Interfering symbols of the beam 207
targeting the coverage area 222 not immediately adjacent to the
coverage area 210 may be partitioned into a non-dominant group of
interfering symbols. Interfering symbols of a beam (not shown)
targeting the coverage area 224 that is immediately adjacent to the
coverage area 210 may also be partitioned into the non-dominant
group of interfering symbols as the UT 202 is not disposed adjacent
to a common edge of the coverage areas 210, 225.
System Model
[0047] FIG. 3 illustrates a multi-spot beam satellite system model
according to various embodiments.
[0048] A multi-spot beam satellite system 300 is illustrated in
FIG. 3. FIG. 3 focuses on a forward link from a satellite gateway
to a user terminal. Information bits 316 intended for a particular
user located in a specific beam n, are FEC-encoded 314,
bit-interleaved 312 and mapped onto an M-ary complex constellation
310 at a gateway 322 to form a symbol sequence of length Ns and
symbol rate T.sub.s,n.sup.-1 such that, {a.sub.n,i; i=0, 1, . . . ,
Ns-1}. The uplink signal after pulse shaping has a baseband
representation given by
s n ( t ) = i = 0 N s - 1 a n , i p n ( t - iT sn ) . ( 1 )
##EQU00004##
Uplink signals 318 intended for the different spot beams are
processed by their respective transponders 306 and transmitted to
intended beams 302 by the satellite feed and antenna mechanism
320.
[0049] When aggressive frequency reuse factors, such as reuse 1 and
reuse 2 are employed to boost a system's capacity, user terminals,
especially those at the beam edge, experience a high level of CCI
due to the sharing of common time-frequency resources. In such
cases, the terminal receives the intended transmission and
transmissions from N.sub.B-1 co-channel beams such that the
received signal at a user terminal can be modelled using (1) as
r ( t ) = n = 1 N B i .gamma. n a n , i p n ( t - ( i + n ) T s , n
) e j ( 2 .pi. b jn t + .theta. n ) + n ( t ) ( 2 )
##EQU00005##
In (2), .gamma..sub.n is a complex-valued channel gain that is a
function of the antenna gain from the nth beam's feed in the
direction of the user terminal under consideration. Here, {
.sub.n,.delta..sub.j.sub.n,.theta..sub.n} represents the normalized
differences in arrival times, carrier frequencies and carrier
phases between the N.sub.B co-frequency beams at the receiver. In
this example, downlink noise n(t) is assumed as an Additive White
Gaussian Noise (AWGN) with a single-sided Power Spectral Density
(PSD) level of N.sub.0 (Watt/Hz). The uplink noise may be assumed
to be negligible relative to the downlink noise, a situation
achieved through proper satellite link parameters including the
size of the transmit antenna. Finally, it is assumed that any
on-board High-Power Amplifiers (HPAs) have small nonlinear impact
at the user terminals.
Analytical Characterization of CCI
[0050] Without limitation, per statistical decision theory, a set
of sufficient statistics can be generated at the output of a filter
matched to the desired signal, labelled as n.sub.d, then sampled at
the symbol rate T.sub.s,nd, or x((k+.epsilon..sub.nd) T.sub.s,nd),
where
x ( t ) = .intg. - .infin. .infin. e - j.phi. .gamma. n d r (
.alpha. ) e - j ( 2 .pi..delta. f n d .alpha. + .theta. n d ) p n d
* ( .alpha. - t ) d.alpha. ( 3 ) ##EQU00006##
and .PHI..sub..gamma.n.sub.d is the phase associated with the
complex-valued channel gain .gamma..sub.n.sub.d. To characterize
the effective CCI channel response, substitute (2) into (3) to
yield
x ( t ) = n = 1 N B i = - .infin. .infin. e - j .phi. .gamma. n d
.gamma. n a n , i .eta. n , n d ( ( i + n ) T s , n , t ) + n ( t )
where e - j .phi. .gamma. n d . ( 4 ) ##EQU00007##
is the spatial contribution, while
.eta..sub.n,n.sub.d(t.sub.1,t.sub.2) represents the time-varying
impulse response due to the temporal contribution of CCI. In
(4),
.eta. n , n d ( t 1 , t 2 ) = e - j ( 2 ( .delta. f n d - .delta. f
n ) t 2 + ( .theta. n d - .theta. n ) ) .GAMMA. n , n d ( t 2 - t 1
; .delta. f n d - .delta. f n ) ( 5 ) and .GAMMA. n , n d ( .delta.
t ; .delta. f ) = .intg. - .infin. .infin. p n d * ( .alpha. ) p n
( .alpha. + .delta. t ) e - j2 .delta. f .alpha. d.alpha. ( 6 )
##EQU00008## and
.GAMMA..sub.n,n.sub.d(.delta..sub.t;.delta..sub.f)=.intg..sub.-.infin..s-
up..infin.p.sub.n.sub.d*(.alpha.)p.sub.n(.alpha.+.delta..sub.t)e.sup.-j2.p-
i..delta..sup.f.sup..alpha.d.alpha. (6)
[0051] Focusing on the pulse shaping that satisfies the Nyquist
criterion of zero Inter-Symbol Interference (ISI), such as
bandwidth-efficient Root-Raised Cosine (RRC) pulses, used in the
widely adopted DVB-S2X satellite standard. Thus, sampling the
matched filter output in (4) at the correct sampling instant
yields
x ( ( k + n d ) T s , n d ) = .gamma. n d a n d , k + n = 1 n
.noteq. n d N B l = - L n L n e - j .phi. .gamma. n d .gamma. n a n
, k - l .eta. n , n d ( ( k - l + n ) T s , n , ( k + n d ) T s , n
d ) + n ( ( k + n d ) T s , n d ) ( 7 ) ##EQU00009##
[0052] where L.sub.n denotes the memory-span associated with CCI.
From (7), it can be inferred that the desired symbols at the
matched filter output are affected by CCI coming from N.sub.B-1
beams, in addition to Gaussian noise. It can also be inferred that
CCI has memory due to the interfering signals combining
asynchronously. The memory effects can be made worse due to
different symbol rates and/or different roll-off factors employed
by the different co-channel beams.
Iterative Divide-and-Conquer Detection
Iterative Soft-In-Soft-Out Receiver
[0053] FIG. 4 illustrates a block diagram of a Soft-In-Soft-Out
(SISO) Iterative Soft-In-Soft-Out (IDAC) receiver according to
various embodiments.
[0054] A Soft-In-Soft-Out (SISO) Iterative Soft-In-Soft-Out (IDAC)
receiver 400 implements a plurality of Soft-In-Soft-Out (SISO)
frameworks 402, 402' and 402''. The count of SISO frameworks may
vary. For brevity, the framework 402 is further elaborated below.
However, each of the frameworks 402', 402'' function in a manner
similar to the framework 402.
[0055] The framework 402 performs joint detection with a SISO DAC
detector 404 along with decoding with a FEC decoder 408 (after
deinterleaving with a deinterleaver 406) is applied in an iterative
fashion to recover the information bits intended for a user
terminal. An outer or global iteration begins by processing a
composite signal 420 including a desired signal and interfering
signal. The outer or global iteration processes the interfering
signal by employing the most robust signal in the joint CCI SISO
DAC detector 404, while assuming equally likely a priori
information 422, 424, 426 for the interferer signal, the desired
signal and any additional interfering signal being processed
jointly. The Iterative Divide-And-Conquer Detection (IDAC),
detailed above, provides soft-information about CCI in the form of
the symbol A Posteriori Probabilities (APPS) 432, 434, 436 which
are transformed to bit Log-Likelihood Ratios (LLRs) using the
bit-to-symbol mapping rule employed at the transmitter.
[0056] These bit LLRs are in turn deinterleaved by the
deinterleaver 406 and input to the FEC decoder 408 as a decoding
input 428. The FEC decoder 408 subsequently generates LLR
soft-estimates of the interfering signal's information bits. These
are converted to extrinsic information by subtracting the LLRs at
the input to the FEC decoder 408. The receiver 400 immediately uses
this extrinsic information as a posteriori probability 432, 434,
436 for processing the composite signal 420 employing the next most
robust signal, thereby incorporating the latest information from
the previous signal's FEC decoder, during the same outer iteration,
leading to faster convergence.
[0057] At the completion of an outer iteration, the receiver has
the APP 432, 434, 436 estimates for all the signals being jointly
processed and can use them during the next outer iteration as a
priori information 422, 424, 426. As such the APP of a first
iteration is the a priori information for the next iteration. Each
framework may use the APP in succession or in parallel. For
example, the receiver 400 uses the APP generated by the framework
402 in succession for the framework 402', and the APP generated by
the framework 402' is used in succession for the framework 402''.
In a parallel implementation (not shown) of the receiver, the APP
of an iteration is not used by the plurality of frameworks within
the iteration.
[0058] The above framework can be applied to systems employing
either Simultaneous Decoding (SD) or Simultaneous Non-unique
Decoding (SND) methods. In SD, after a certain maximum number of
global iterations, the hard decisions provided by two or more
decoders is multiplexed by multiplexor 442 to form an estimate of
the desired symbols 444 that may be converted to the user's
information bits, for example, in a splitter 440. In SND, after a
certain maximum number of global iterations, the hard decisions
provided by only a single FEC decoder provides an estimate of the
desired symbols 444 that may be converted to the user's information
bits, for example, in the splitter 440. In SND, the multiplexor 442
may be eliminated.
[0059] Stacked Construction of CCI With Memory
[0060] Using the analysis above, a useful formulation is disclosed
for IDAC detection. It is based on a stacked construction that
models the spatial and temporal contributions of the CCI in
multibeam satellite systems on the forward link, namely from the
gateway to the user terminals. The kth time-instant of the MF
output x[k] of (7), received in the n.sub.dth beam, is described
as
x[k]=|.gamma..sub.n.sub.d|a.sub.n.sub.d.sub.,k+h.sub.I,n.sub.d[k]a.sub.I-
,n.sub.d[k]+n[k] (8)
where h.sub.I,n.sub.d[k] is a stacked row-vector containing the
channel coefficients associated with each of the neighboring
co-channel interfering beams and a.sub.i,nd [k] is a stacked
column-vector that models the corresponding symbols serviced by the
interfering beams, defined as
h _ I , n d [ k ] = [ h _ 1 [ k ; L 1 ] , , h _ n d - 1 [ k ; L n d
- 1 ] , h _ n d + 1 [ k ; L n d + 1 ] , , h _ N B [ k ; L N B ] ] (
9 ) and a I , n d [ k ] = [ a _ 1 [ k ; L 1 ] a _ n d - 1 [ k ; L n
d - 1 ] a _ n d + 1 [ k ; L n d + 1 ] a _ N B [ k ; L N B ] ] . (
10 ) ##EQU00010##
[0061] respectively. The individual row-vector h.sub.i,n [k] in (9)
is in turn composed of a vector containing the spatial and temporal
coefficients belonging to the nth beam with single-sided memory of
L.sub.n (see for example, 432, 434, 436 in FIG. 4) symbols,
outlined in (7), and expressed as
h _ n [ k ; L n ] = e - j .phi. .gamma. n d .gamma. n [ .eta. n , n
d ( ( k + L n + n ) T s , n , ( k + n d ) T s , n d ) .eta. n , n d
( ( k + L n - 1 + n ) T s , n , ( k + n d ) T s , n d ) .eta. n , n
d ( ( k + L n + n ) T s , n , ( k + n d ) T s , n d ) ] ( 11 )
##EQU00011##
In (10), a.sub.n[k, L.sub.n] is a column-vector containing the
individual interfering symbols from the nth beam or
a _ n [ k ; L n ] = [ a n , k - L n a n , k - L n + 1 a n , k + L n
] ( 12 ) ##EQU00012##
SISO Divide-and-Conquer Detector
[0062] The receiver implements a SISO Divide-And-Conquer (DAC)
detector of CCI which partitions the interfering symbols into three
smaller groups depending on the intensity of their interference
levels. These smaller groups use different methods of contributing
to the APP computation. The first is the Noise-Floor (NF) group
whose elements are incorporated only through their powers. The
second is the Subtractive-Cancellation (SC) group which is
incorporated in the SISO DAC APP module via first- and second-order
moments, derived from a priori probabilities.
[0063] The third group is based on the Optimal-Bayesian (OB) method
contributing to the SISO DAC APP module using the a priori
probability mass function (pmf) of the interfering symbols from
within the OB group only. To start the kth time-instant of the MF
output x[k] of (8) is equivalently expressed as
x k = .gamma. n d a n d , k + h _ I , n d ( OB ) [ k ] a _ I , n d
( OB ) [ k ] + h _ I , n d ( SC ) [ k ] a _ I , n d ( SC ) [ k ] +
h _ I , n d ( NF ) [ k ] a _ I , n d ( NF ) [ k ] + n [ k ] ( 13 )
##EQU00013##
where the elements of a.sub.i,nd [k] is partitioned into three
groups, a.sub.I,n.sub.d.sup.(NF)[k], a.sub.I,n.sub.d.sup.(SC)[k],
a.sub.I,n.sub.d.sup.(OB)[k], and h.sub.I,n.sub.d.sup.(NF)[k],
h.sub.I,n.sub.d.sup.(SC)[k], h.sub.I,n.sub.d.sup.(OB)[k] are the
corresponding spatial and temporal CCI channel coefficients
extracted from h.sub.i,nd[k]. Based on the partitioned expression
(13), the proposed SISO DAC APP module,
P.sub.DAC(a.sub.n.sub.d.sub.,k|x[k]), is mathematically expressed
as
P DAC ( a n d , k | x [ k ] ) = a _ I , n d ( OB ) [ k ] p DAC ( x
[ k ] | a n d , k , a _ I , n d ( OB ) [ k ] , a _ I , n d ( SC ) [
k ] , a _ I , n d ( NF ) [ k ] ) P ( a _ I , n d ( OB ) [ k ] ) P (
a n d , k ) , ( 14 ) ##EQU00014##
where p.sub.DAC( ) is the likelihood function associated with
observing x[k] given the desired and interfering symbols and P( )
is the a priori pmf corresponding to the symbols computed based on
the individual FEC decoders. In (14), the likelihood function
p.sub.DAC( ) assumes that x[k] is a random variable that retains a
Gaussian density expression or
p DAC ( x [ k ] | a n d , k , a _ I , n d ( OB ) [ k ] , a _ I , n
d ( SC ) [ k ] , a _ I , n d ( NF ) [ k ] ) = exp { - ( x [ k ] - I
^ n d ( SC ) [ k ] ) - .gamma. n d a n d , k - h _ I , n d ( OB ) [
k ] a _ I , n d ( OB ) [ k ] 2 .sigma. n 2 + c I , n d ( SC ) [ k ]
+ c I , n d ( NF ) [ k ] } ( 15 ) ##EQU00015##
where I.sub.n.sub.d.sup.(SC)[k] is the soft CCI estimate arising
from the SC group that is subtracted and is computed as
I.sub.n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]{a.sub.I,n.sub.d.su-
p.(SC)[k]} (16)
Also, the likelihood expression in (15) contains the variances from
the SC and NF interfering groups, c.sub.I,n.sub.d.sup.(SC)[k] and
c.sub.I,n.sub.d.sup.(NF)[k], respectively, obtained by
c.sub.I,n.sub.d.sup.(SC)[k]=h.sub.I,n.sub.d.sup.(SC)[k]Cov{a.sub.I,n.sub-
.d.sup.(SC)[k]}(h.sub.I,n.sub.d.sup.(SC)[k]).sup.H (17)
and
c.sub.I,n.sub.d.sup.(NF)[k]=h.sub.I,n.sub.d.sup.(NF)[k](h.sub.I,n.sub.d.-
sup.(NF)[k]).sup.H (18)
Performance Evaluation
[0064] Extensive performance evaluations demonstrate the
effectiveness of the IDAC receiver. The simulation setup implements
the system model described previously and employs Low-Density
Parity Check (LDPC) codes for FEC as well as the modulation formats
defined in the DVB-S2X standard. RRC filters with a rolloff of 0.05
are considered at the transmitters for pulse-shaping and at their
corresponding receivers for matched filtering. It is assumed that
the satellite transponders are operated in a single-carrier per-HPA
mode with the operating point causing small nonlinear distortion at
the user terminals. It is also assumed that a multibeam system is
employing aggressive frequency reuse, and as such the channel gains
from a particular beam in the direction of a user terminal located
in a neighboring co-channel beam are severe. This results in CCI at
the user terminal which can be as high as C/I=0 dB, i.e., of equal
strength, as considered in the performance evaluations. In a
multibeam system, a user's receiver may experience a substantial
amount of CCI with memory due to the desired signal and
interference arriving at the user terminal asynchronously with some
relative delay.
[0065] FIG. 5 illustrates a coded PER of DVB-S2X 16APSK with code
rate 28/45 when interfered by QPSK with rate R.sub.2 at different
levels of timing offset (TO) according to various embodiments.
[0066] FIG. 5 starts with an examination of the Packet Error Rate
(PER) performance in the presence of CCI and AWGN. FIG. 5 documents
PER versus E.sub.s/N.sub.0 for a desired user employing 16APSK with
the rate 28/45 LDPC code. In this scenario, 16APSK symbols intended
for the desired user arrive at the user terminal in the presence of
a strong co-channel interferer at C/I=0 dB that employs QPSK MODCOD
with rates of 1/4 or 2/5. A significant amount of distortion
experienced at the decoder input will cause the LDPC decoding to
fail without CCI mitigation. FIG. 5 also illustrates the inability
of state-of-the-art (SOA) memoryless CCI mitigation techniques
popular in the literature to handle asynchronous reception. As an
example, when the offset between the desired signal and the
interferer is 10% of T.sub.s, performance with the memoryless
solution degrades by more than 1 dB and a delay of 15% relative to
T.sub.s causes the receiver to become ineffective. These
limitations motivate the need for the innovative, low-complexity
and modular IDAC framework disclosed in the present teachings.
[0067] As is evident in FIG. 5, the receiver of the present
teachings does not suffer from performance degradation even at
delays as high as 25% of Ts and approaches no-CCI performance.
Further, the present teachings are just as effective when the
interfering beam carries QPSK with the weaker code rate of 2/5. The
low-complexity of the present teachings is particularly noteworthy
in light of the significant CCI memory span and APSK modulation
cardinality. The impressive performance offered by the present
teachings allows user terminals located at the edges of
co-frequency beams to benefit from high spectral efficiencies
offered by APSK MODCODs.
[0068] FIG. 6 illustrates an information-theoretic achievable-rate
region for 16APSK when interfered by QPSK at C/I=0 dB and
E.sub.s/N.sub.0=7.83 dB), according to various embodiments.
[0069] FIG. 6 illustrates a map out of the information-theoretic
rate regions supported at a user terminal when 16APSK and QPSK
symbols are transmitted on co-channel beams. These report mutual
information between the transmitted modulated symbols and the
received symbols at E.sub.s/N.sub.0=7.83 dB. By adopting that with
the SND method, only 16APSK symbols carry information for the
targeted user which the user terminal can recover by jointly
processing the desired signal and the interfering signal. The rate
region has several points of interest that are discussed here; the
point labelled A denotes the maximum rate R.sub.1 at which 16APSK
symbols can be reliably received at the user terminal when there is
no CCI and is approximately 2.65 bits-per-symbol.
[0070] Conversely, the point B denotes the maximum rate R.sub.2 at
which QPSK symbols can be reliably received at the same user
terminal, without any CCI and is 1.95 bits-per-symbol. Points C and
D are the maximum rates possible when CCI is unmitigated and
treated as noise at the receiver, these rates are R.sub.1=0.87
bits-per-symbol for 16APSK and R.sub.2=0.86 bits-per-symbol for
QPSK, respectively. It is clear from these results that CCI, if
left unmitigated can impose a significant penalty on the spectral
efficiency. Information theory also indicates that as long as
0<R.sub.2<0.86, it is possible to receive 16APSK at its
maximum rate at the user terminal in the presence of CCI by jointly
processing both signals. In particular, QPSK can be recovered first
and can subsequently assist in recovering the 16APSK symbols. As an
example, the curve marked in green diamond indicates that 16APSK
can be signaled at its maximum rate of 2.65 bits-per-symbol when
the interfering QPSK signal has rates R.sub.2=2.times.{1/4, 13/45,
1/3, 2/5} by jointly processing both signals at the user
terminal.
[0071] It is also interesting to visualize how the rates promised
by information-theory compare with performance achievable with the
IDAC receiver employing finite-length DVB-S2X LDPC codes and
operating under the realistic assumption of CCI with memory.
Towards this end, extensive PER performance simulations were
conducted by transmitting progressively more spectrally efficient
QPSK MODCODs on the interfering signal by increasing its code rate
to 13/45, 1/3 and 2/5. As done previously, C/I=0 dB and timing
offset of 25% of Ts were assumed. Results indicate that. similar to
FIG. 5, there is no noticeable performance loss relative to a
no-CCI scenario. This implies that the IDAC receiver does not
penalize the interfering user to employ its most spectrally
efficient MODCOD. Furthermore, the achievable rate for 16APSK of
2.5 bits-per-symbol, when using 28/45 code, is quite close to the
information-theoretic maximum of 2.65 bits-per-symbol. This is also
illustrated in FIG. 6, where the E.sub.s/N.sub.0 for the coded
simulations corresponds to a PER of 10.sup.-3. Hence, it is easy to
infer from these results that the IDAC receiver offers a
capacity-approaching solution to handling severe CCI at the user
terminal, even when confronted with symbol asynchronism.
[0072] Having described preferred embodiments of a system and
method (which are intended to be illustrative and not limiting), it
is noted that modifications and variations can be made by persons
skilled in the art considering the above teachings. It is therefore
to be understood that changes may be made in the embodiments
disclosed which are within the scope of the invention as outlined
by the appended claims. Having thus described aspects of the
invention, with the details and particularity required by the
patent laws, what is claimed and desired protected by Letters
Patent is set forth in the appended claims.
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