5,714 research outputs found
Signal Shaping for Generalized Spatial Modulation and Generalized Quadrature Spatial Modulation
This paper investigates generic signal shaping methods for
multiple-data-stream generalized spatial modulation (GenSM) and generalized
quadrature spatial modulation (GenQSM) based on the maximizing the minimum
Euclidean distance (MMED) criterion. Three cases with different channel state
information at the transmitter (CSIT) are considered, including no CSIT,
statistical CSIT and perfect CSIT. A unified optimization problem is formulated
to find the optimal transmit vector set under size, power and sparsity
constraints. We propose an optimization-based signal shaping (OBSS) approach by
solving the formulated problem directly and a codebook-based signal shaping
(CBSS) approach by finding sub-optimal solutions in discrete space. In the OBSS
approach, we reformulate the original problem to optimize the signal
constellations used for each transmit antenna combination (TAC). Both the size
and entry of all signal constellations are optimized. Specifically, we suggest
the use of a recursive design for size optimization. The entry optimization is
formulated as a non-convex large-scale quadratically constrained quadratic
programming (QCQP) problem and can be solved by existing optimization
techniques with rather high complexity. To reduce the complexity, we propose
the CBSS approach using a codebook generated by quadrature amplitude modulation
(QAM) symbols and a low-complexity selection algorithm to choose the optimal
transmit vector set. Simulation results show that the OBSS approach exhibits
the optimal performance in comparison with existing benchmarks. However, the
OBSS approach is impractical for large-size signal shaping and adaptive signal
shaping with instantaneous CSIT due to the demand of high computational
complexity. As a low-complexity approach, CBSS shows comparable performance and
can be easily implemented in large-size systems.Comment: Summited to IEEE TW
Peak to Average Power Ratio Reduction for Space-Time Codes That Achieve Diversity-Multiplexing Gain Tradeoff
Zheng and Tse have shown that over a quasi-static channel, there exists a
fundamental tradeoff, known as the diversity-multiplexing gain (D-MG) tradeoff.
In a realistic system, to avoid inefficiently operating the power amplifier,
one should consider the situation where constraints are imposed on the peak to
average power ratio (PAPR) of the transmitted signal. In this paper, the D-MG
tradeoff of multi-antenna systems with PAPR constraints is analyzed. For
Rayleigh fading channels, we show that the D-MG tradeoff remains unchanged with
any PAPR constraints larger than one. This result implies that, instead of
designing codes on a case-by-case basis, as done by most existing works, there
possibly exist general methodologies for designing space-time codes with low
PAPR that achieve the optimal D-MG tradeoff. As an example of such
methodologies, we propose a PAPR reduction method based on constellation
shaping that can be applied to existing optimal space-time codes without
affecting their optimality in the D-MG tradeoff. Unlike most PAPR reduction
methods, the proposed method does not introduce redundancy or require side
information being transmitted to the decoder. Two realizations of the proposed
method are considered. The first is similar to the method proposed by Kwok
except that we employ the Hermite Normal Form (HNF) decomposition instead of
the Smith Normal Form (SNF) to reduce complexity. The second takes the idea of
integer reversible mapping which avoids the difficulty in matrix decomposition
when the number of antennas becomes large. Sphere decoding is performed to
verify that the proposed PAPR reduction method does not affect the performance
of optimal space-time codes.Comment: submitted to IEEE Transactions on Signal Processin
One-Bit Sigma-Delta MIMO Precoding
Coarsely quantized MIMO signalling methods have gained popularity in the
recent developments of massive MIMO as they open up opportunities for massive
MIMO implementation using cheap and power-efficient radio-frequency front-ends.
This paper presents a new one-bit MIMO precoding approach using spatial
Sigma-Delta () modulation. In previous one-bit MIMO precoding
research, one mainly focuses on using optimization to tackle the difficult
binary signal optimization problem that arises from the precoding design. Our
approach attempts a different route. Assuming angular MIMO channels, we apply
modulation---a classical concept in analog-to-digital conversion
of temporal signals---in space. The resulting precoding approach
has two main advantages: First, we no longer need to deal with binary
optimization in precoding design. Particularly, the binary
signal restriction is replaced by peak signal amplitude constraints. Second,
the impact of the quantization error can be well controlled via modulator
design and under appropriate operating conditions. Through symbol error
probability analysis, we reveal that the very large number of antennas in
massive MIMO provides favorable operating conditions for
precoding. In addition, we develop a new modulation architecture
that is capable of adapting the channel to achieve nearly zero quantization
error for a targeted user. Furthermore, we consider multi-user
precoding using the zero-forcing and symbol-level precoding schemes. These two
precoding schemes perform considerably better than their direct
one-bit quantized counterparts, as simulation results show
The information and wave-theoretic limits of analog beamforming
The performance of broadband millimeter-wave (mmWave) RF architectures, is
generally determined by mathematical concepts such as the Shannon capacity.
These systems have also to obey physical laws such as the conservation of
energy and the propagation laws. Taking the physical and hardware limitations
into account is crucial for characterizing the actual performance of mmWave
systems under certain architecture such as analog beamforming. In this context,
we consider a broadband frequency dependent array model that explicitly
includes incremental time shifts instead of phase shifts between the individual
antennas and incorporates a physically defined radiated power. As a consequence
of this model, we present a novel joint approach for designing the optimal
waveform and beamforming vector for analog beamforming. Our results show that,
for sufficiently large array size, the achievable rate is mainly limited by the
fundamental trade-off between the analog beamforming gain and signal bandwidth.Comment: Presented at ITA, february 201
Constant Envelope Precoding for MIMO Systems
Constant envelope (CE) precoding is an appealing transmission technique,
which enables highly efficient power amplification, and is realizable with a
single radio frequency (RF) chain at the multi-antenna transmitter. In this
paper, we study the transceiver design for a point-to-point multiple-input
multiple-output (MIMO) system with CE precoding. Both single-stream
transmission (i.e., beamforming) and multi-stream transmission (i.e., spatial
multiplexing) are considered. For single-stream transmission, we optimize the
receive beamforming vector to minimize the symbol error rate (SER) for any
given channel realization and desired constellation at the combiner output. By
reformulating the problem as an equivalent quadratically constrained quadratic
program (QCQP), we propose an efficient semi-definite relaxation (SDR) based
algorithm to find an approximate solution. Next, for multi-stream transmission,
we propose a new scheme based on antenna grouping at the transmitter and
minimum mean squared error (MMSE) or zero-forcing (ZF) based beamforming at the
receiver. The transmit antenna grouping and receive beamforming vectors are
then jointly designed to minimize the maximum SER over all data streams.
Finally, the error-rate performance of single- versus multi-stream transmission
is compared via simulations under different setups.Comment: Submitted for possible journal publicatio
Antenna Current Optimization and Realizations for Far-Field Pattern Shaping
Far-field shaping of small antennas is a challenge and the realizations of
non-dipole radiation of small to intermediate sized antennas are difficult.
Here we examine the antenna bandwidth cost associated with such constraints,
and in certain cases we design antennas that approach the bounds. Far-field
shaping is in particular interesting for Internet-of-things (IoT) and Wi-Fi
applications since e.g. spatial filtering can mitigate package loss through a
reduction of mutual interference, and hence increase the power efficiency of
the devices. Even a rather careful far-field shaping of smaller antennas can be
associated with a steep reduction in the best available bandwidth. It is thus
important to develop constraints that a small antenna can support. We describe
a power front-to-back ratio, and a related, beam-shaping constraint that can be
used in optimization for the minimum Q-factor. We show that such a non-convex
Q-factor optimization can be solved with the semi-definite relaxation
technique. We furthermore show that certain of the above optimized non-standard
radiation patterns can be realized with a multi-position feeding strategy with
a moderate loss of Q-factor:
Compute-and-Forward Strategies for Cooperative Distributed Antenna Systems
We study a distributed antenna system where antenna terminals (ATs) are
connected to a Central Processor (CP) via digital error-free links of finite
capacity , and serve user terminals (UTs). We contribute to the
subject in the following ways: 1) for the uplink, we apply the "Compute and
Forward" (CoF) approach and examine the corresponding system optimization at
finite SNR; 2) For the downlink, we propose a novel precoding scheme nicknamed
"Reverse Compute and Forward" (RCoF); 3) In both cases, we present
low-complexity versions of CoF and RCoF based on standard scalar quantization
at the receivers, that lead to discrete-input discrete-output symmetric
memoryless channel models for which near-optimal performance can be achieved by
standard single-user linear coding; 4) For the case of large , we propose
a novel "Integer Forcing Beamforming" (IFB) scheme that generalizes the popular
zero-forcing beamforming and achieves sum rate performance close to the optimal
Gaussian Dirty-Paper Coding.
The proposed uplink and downlink system optimization focuses specifically on
the ATs and UTs selection problem. We present low-complexity ATs and UTs
selection schemes and demonstrate, through Monte Carlo simulation in a
realistic environment with fading and shadowing, that the proposed schemes
essentially eliminate the problem of rank deficiency of the system matrix and
greatly mitigate the non-integer penalty affecting CoF/RCoF at high SNR.
Comparison with other state-of-the art information theoretic schemes, such as
"Quantize reMap and Forward" for the uplink and "Compressed Dirty Paper Coding"
for the downlink, show competitive performance of the proposed approaches with
significantly lower complexity.Comment: Submitted to IEEE Transactions on Information Theor
Reverse Compute and Forward: A Low-Complexity Architecture for Downlink Distributed Antenna Systems
We consider a distributed antenna system where antenna terminals (ATs)
are connected to a Central Processor (CP) via digital error-free links of
finite capacity , and serve user terminals (UTs). This system model
has been widely investigated both for the uplink and the downlink, which are
instances of the general multiple-access relay and broadcast relay networks. In
this work we focus on the downlink, and propose a novel downlink precoding
scheme nicknamed "Reverse Quantized Compute and Forward" (RQCoF). For this
scheme we obtain achievable rates and compare with the state of the art
available in the literature. We also provide simulation results for a realistic
network with fading and pathloss with UTs, and show that channel-based
user selection produces large benefits and essentially removes the problem of
rank deficiency in the system matrix.Comment: 5 pages, 3 figures, submission to the 2012 IEEE International
Symposium on Information Theory (ISIT 2012
A Journey from Improper Gaussian Signaling to Asymmetric Signaling
The deviation of continuous and discrete complex random variables from the
traditional proper and symmetric assumption to a generalized improper and
asymmetric characterization (accounting correlation between a random entity and
its complex conjugate), respectively, introduces new design freedom and various
potential merits. As such, the theory of impropriety has vast applications in
medicine, geology, acoustics, optics, image and pattern recognition, computer
vision, and other numerous research fields with our main focus on the
communication systems. The journey begins from the design of improper Gaussian
signaling in the interference-limited communications and leads to a more
elaborate and practically feasible asymmetric discrete modulation design. Such
asymmetric shaping bridges the gap between theoretically and practically
achievable limits with sophisticated transceiver and detection schemes in both
coded/uncoded wireless/optical communication systems. Interestingly,
introducing asymmetry and adjusting the transmission parameters according to
some design criterion render optimal performance without affecting the
bandwidth or power requirements of the systems. This dual-flavored article
initially presents the tutorial base content covering the interplay of
reality/complexity, propriety/impropriety and circularity/noncircularity and
then surveys majority of the contributions in this enormous journey.Comment: IEEE COMST (Early Access
MIMO Beampattern and Waveform Design with Low Resolution DACs
Digital beamforming and waveform generation techniques in MIMO radar offer
enormous advantages in terms of flexibility and performance compared to
conventional radar systems based on analog implementations. To allow for such
fully digital design with an efficient hardware complexity, we consider the use
of low resolution digital-to-analog converters (DACs) while maintaining a
separate radio-frequency chain per antenna. A sum of squared residuals (SSR)
formulation for the beampattern and spectral shaping problem is solved based on
the Generalized Approximate Message Passing (GAMP) algorithm. Numerical results
demonstrate good performance in terms of spectral shaping as well as
cross-correlation properties of the different probing waveforms even with just
2-bit resolution per antenna
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