57 research outputs found
Robust Integrated Sensing and Communication Beamforming for Dual-functional Radar and Communications: Method and Insights
This work presents a novel robust beamforming design dedicated for
dual-functional radar and communication (DFRC) base stations (BSs) in the
context of integrated sensing and communications (ISAC). The architecture is
intended for circumstances with imperfect channel state information (CSI). Our
suggested approach demonstrates several tradeoffs for joint radar-communication
deployment. Due to the DFRC nature of the design, the beamformer can
simultaneously point towards an intended target, while optimizing communication
quality of service. We unveil several insights regarding closed form
expressions, as well as optimality of the proposed beamformer. Lastly,
simulation results demonstrate the effectiveness of the proposed ISAC
beamformer.Comment: arXiv admin note: text overlap with arXiv:2207.0492
Extended GFDM Framework: OTFS and GFDM Comparison
Orthogonal time frequency space modulation (OTFS) has been recently proposed
to achieve time and frequency diversity, especially in linear time-variant
(LTV) channels with large Doppler frequencies. The idea is based on the
precoding of the data symbols using symplectic finite Fourier transform (SFFT)
then transmitting them by mean of orthogonal frequency division multiplexing
(OFDM) waveform. Consequently, the demodulator and channel equalization can be
coupled in one processing step. As a distinguished feature, the demodulated
data symbols have roughly equal gain independent of the channel selectivity. On
the other hand, generalized frequency division multiplexing (GFDM) modulation
also employs the spreading over the time and frequency domains using circular
filtering. Accordingly, the data symbols are implicitly precoded in a similar
way as applying SFFT in OTFS. In this paper, we present an extended
representation of GFDM which shows that OTFS can be processed as a GFDM signal
with simple permutation. Nevertheless, this permutation is the key factor
behind the outstanding performance of OTFS in LTV channels, as demonstrated in
this work. Furthermore, the representation of OTFS in the GFDM framework
provides an efficient implementation, that has been intensively investigated
for GFDM, and facilitates the understanding of the OTFS distinct features.Comment: Accepted in IEEE Global Communications Conference 9-13 December 2018
Abu Dhabi, UA
Mutual Information Based Pilot Design for ISAC
The following paper presents a novel orthogonal pilot design dedicated for
dual-functional radar and communication (DFRC) systems performing multi-user
communications and target detection. After careful characterization of both
sensing and communication metrics based on mutual information (MI), we propose
a multi-objective optimization problem (MOOP) tailored for pilot design,
dedicated for simultaneously maximizing both sensing and communication MIs.
Moreover, the MOOP is further simplified to a single-objective optimization
problem, which characterizes trade-offs between sensing and communication
performances. Due to the non-convex nature of the optimization problem, we
propose to solve it via the projected gradient descent method on the Stiefel
manifold. Closed-form gradient expressions are derived, which enable execution
of the projected gradient descent algorithm. Furthermore, we prove convergence
to a fixed orthogonal pilot matrix. Finally, we demonstrate the capabilities
and superiority of the proposed pilot design, and corroborate relevant
trade-offs between sensing MI and communication MI. In particular, significant
signal-to-noise ratio (SNR) gains for communication are reported, while
re-using the same pilots for target detection with significant gains in terms
of probability of detection for fixed false-alarm probability. Other
interesting findings are reported through simulations, such as an
\textit{information overlap} phenomenon, whereby the fruitful ISAC integration
can be fully exploited
Sparse-DFT and WHT Precoding with Iterative Detection for Highly Frequency-Selective Channels
Various precoders have been recently studied by the wireless community to
combat the channel fading effects. Two prominent precoders are implemented with
the discrete Fourier transform (DFT) and Walsh-Hadamard transform (WHT). The
WHT precoder is implemented with less complexity since it does not need complex
multiplications. Also, spreading can be applied sparsely to decrease the
transceiver complexity, leading to sparse DFT (SDFT) and sparse Walsh-Hadamard
(SWH). Another relevant topic is the design of iterative receivers that deal
with inter-symbol-interference (ISI). In particular, many detectors based on
expectation propagation (EP) have been proposed recently for channels with high
levels of ISI. An alternative is the maximum a-posterior (MAP) detector,
although it leads to unfeasible high complexity in many cases. In this paper,
we provide a relatively low-complexity \textcolor{black}{computation} of the
MAP detector for the SWH. We also propose two \textcolor{black}{feasible
methods} based on the Log-MAP and Max-Log-MAP. Additionally, the DFT, SDFT and
SWH precoders are compared using an EP-based receiver with one-tap FD
equalization. Lastly, SWH-Max-Log-MAP is compared to the (S)DFT with EP-based
receiver in terms of performance and complexity. The results show that the
proposed SWH-Max-Log-MAP has a better performance and complexity trade-off for
QPSK and 16-QAM under highly selective channels, but has unfeasible complexity
for higher QAM orders
L'optimalité de l'OFDM en termes de performance en PAPR
National audienceLes systèmes multi-porteuses, et en particulier leur structure de modulation, connaissent un intérêt croissant. Suivant la forme d'onde considérée, plusieurs mesures de performance peuvent changer comme le PAPR (Peak-to-Average Power Ratio), l'efficacité spectrale, le taux d'erreur ou la complexité d'implémentation. Dans ce papier, nous montrons que, sous certaines contraintes sur les formes d'ondes, l'OFDM (Orthogonal Frequency Division Multiplexing) est une structure de modulation optimale en termes de PAPR
On Hybrid Radar Fusion for Integrated Sensing and Communication
The following paper introduces a novel integrated sensing and communication
(ISAC) scenario termed hybrid radar fusion. In this setting, the
dual-functional radar and communications (DFRC) base station (BS) acts as a
mono-static radar in the downlink (DL), for sensing purposes, while performing
its DL communication tasks. Meanwhile, the communication users act as
distributed bi-static radar nodes in the uplink (UL) following a
frequency-division duplex protocol. The DFRC BS fuses the information available
at different DL and UL resource bands to estimate the angles-of-arrival (AoAs)
of the multiple targets existing in the scene. In this work, we derive the
maximum likelihood (ML) criterion for the hybrid radar fusion problem at hand.
Additionally, we design efficient estimators; the first algorithm is based on
an alternating optimization approach to solve the ML criterion, while the
second one designs an optimization framework that leads to an alternating
subspace approach to estimate AoAs for both the target and users. Finally, we
demonstrate the superior performance of both algorithms in different scenarios,
and the gains offered by these proposed methods through numerical simulations
Integrated Sensing and Communication for Large Networks using Joint Detection and a Dynamic Transmission Strategy
A large network employing integrated sensing and communication (ISAC) where a
single transmit signal by the base station (BS) serves both the radar and
communication modes is studied. We consider bistatic detection at a passive
radar and monostatic detection at the transmitting BS. The radar-mode
performance is significantly more vulnerable than the communication-mode due to
the double path-loss in the signal component while interferers have direct
links. To combat this, we propose: 1) a novel dynamic transmission strategy
(DTS), 2) joint monostatic and bistation detection via cooperation at the BS.
We analyze the performance of monostatic, bistatic and joint detection. We show
that bistatic detection with dense deployment of low-cost passive radars offers
robustness in detection for farther off targets. Significant improvements in
radar-performance can be attained with joint detection in certain scenarios,
while using one strategy is beneficial in others. Our results highlight that
with DTS we are able to significantly improve quality of radar detection at the
cost of quantity. Further, DTS causes some performance deterioration to the
communication-mode; however, the gains attained for the radar-mode are much
higher. We show that joint detection and DTS together can significantly improve
radar performance from a traditional radar-network
Closed-form Approximations of the PAPR Distribution for Multi-Carrier Modulation Systems
International audienceThe theoretical analysis of the Peak-to-Average Power Ratio (PAPR) distribution for an Orthogonal Frequency Division Multiplexing (OFDM) system, depends on the particular waveform considered in the modulation system. In this paper, we generalize this analysis by considering the Generalized Waveforms for Multi-Carrier (GWMC) modulation system based on any family of modulation functions, and we derive a general approximate expression for the Cumulative Distribution Function (CDF) of its continuous and discrete time PAPR. These equations allow us to directly find the expressions of the PAPR distribution for any particular family of modulation functions, and they can be applied to control the PAPR performance by choosing the appropriate functions
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