19 research outputs found
Edge Cache-assisted Secure Low-Latency Millimeter Wave Transmission
In this paper, we consider an edge cache-assisted millimeter wave cloud radio
access network (C-RAN). Each remote radio head (RRH) in the C-RAN has a local
cache, which can pre-fetch and store the files requested by the actuators.
Multiple RRHs form a cluster to cooperatively serve the actuators, which
acquire their required files either from the local caches or from the central
processor via multicast fronthaul links. For such a scenario, we formulate a
beamforming design problem to minimize the secure transmission delay under
transmit power constraint of each RRH. Due to the difficulty of directly
solving the formulated problem, we divide it into two independent ones:
{\textit{i)}} minimizing the fronthaul transmission delay by jointly optimizing
the transmit and receive beamforming; {\textit{ii)}} minimizing the maximum
access transmission delay by jointly designing cooperative beamforming among
RRHs. An alternatively iterative algorithm is proposed to solve the first
optimization problem. For the latter, we first design the analog beamforming
based on the channel state information of the actuators. Then, with the aid of
successive convex approximation and -procedure techniques, a semidefinite
program (SDP) is formulated, and an iterative algorithm is proposed through SDP
relaxation. Finally, simulation results are provided to verify the performance
of the proposed schemes.Comment: IEEE_IoT, Accep
Wideband Beamforming for STAR-RIS-assisted THz Communications with Three-Side Beam Split
In this paper, we consider the simultaneously transmitting and reflecting
reconfigurable intelligent surface (STAR-RIS)-assisted THz communications with
three-side beam split. Except for the beam split at the base station (BS), we
analyze the double-side beam split at the STAR-RIS for the first time. To
relieve the double-side beam split effect, we propose a time delayer (TD)-based
fully-connected structure at the STAR-RIS. As a further advance, a low-hardware
complexity and low-power consumption sub-connected structure is developed,
where multiple STAR-RIS elements share one TD. Meanwhile, considering the
practical scenario, we investigate a multi-STAR-RIS and multi-user
communication system, and a sum rate maximization problem is formulated by
jointly optimizing the hybrid analog/digital beamforming, time delays at the BS
as well as the double-layer phase-shift coefficients, time delays and amplitude
coefficients at the STAR-RISs. Based on this, we first allocate users for each
STAR-RIS, and then derive the analog beamforming, time delays at the BS, and
the double-layer phase-shift coefficients, time delays at each STAR-RIS. Next,
we develop an alternative optimization algorithm to calculate the digital
beamforming at the BS and amplitude coefficients at the STAR-RISs. Finally, the
numerical results verify the effectiveness of the proposed schemes
Beamforming Design for the Distributed RISs-aided THz Communications with Double-Layer True Time Delays
In this paper, we investigate the reconfigurable intelligent surface
(RIS)-aided terahertz (THz) communication system with the sparse radio
frequency chains antenna structure at the base station (BS). To overcome the
beam split of the BS, different from the conventional single-layer
true-time-delay (TTD) scheme, we propose a double-layer TTD scheme that can
effectively reduce the number of large-range delay devices, which involve
additional insertion loss and amplification circuitry. Next, we analyze the
system performance under the proposed double-layer TTD scheme. To relieve the
beam split of the RIS, we consider multiple distributed RISs to replace an
ultra-large size RIS. Based on this, we formulate an achievable rate
maximization problem for the distributed RISs-aided THz communications via
jointly optimizing the hybrid analog/digital beamforming, time delays of the
double-layer TTD network and reflection coefficients of RISs. Considering the
practical hardware limitation, the finite-resolution phase shift, time delay
and reflection phase are constrained. To solve the formulated problem, we first
design an analog beamforming scheme including optimizing phase shift and time
delay based on the RISs' locations. Then, an alternatively optimization
algorithm is proposed to obtain the digital beamforming and reflection
coefficients based on the minimum mean square error and coordinate update
techniques. Finally, simulation results show the effectiveness of the proposed
scheme
Joint Beamforming Design for the STAR-RIS-Enabled ISAC Systems with Multiple Targets and Multiple Users
In this paper, the sensing beam pattern gain under simultaneously
transmitting and reflecting reconfigurable intelligent surfaces
(STAR-RIS)-enabled integrated sensing and communications (ISAC) systems is
investigated, in which multiple targets and multiple users exist. However,
multiple targets detection introduces new challenges, since the STAR-RIS cannot
directly send sensing beams and detect targets, the dual-functional base
station (DFBS) is required to analyze the echoes of the targets. While the
echoes reflected by different targets through STAR-RIS come from the same
direction for the DFBS, making it impossible to distinguish them. To address
the issue, we first introduce the signature sequence (SS) modulation scheme to
the ISAC system, and thus, the DFBS can detect different targets by the
SS-modulated sensing beams. Next, via the joint beamforming design of DFBS and
STAR-RIS, we develop a maxmin sensing beam pattern gain problem, and meanwhile,
considering the communication quality requirements, the interference
limitations of other targets and users, the passive nature constraint of
STAR-RIS, and the total transmit power limitation. Then, to tackle the complex
non-convex problem, we propose an alternating optimization method to divide it
into two quadratic semidefinite program subproblems and decouple the coupled
variables. Drawing on mathematical transformation, semidefinite programming, as
well as semidefinite relaxation techniques, these two subproblems are
iteratively sloved until convergence, and the ultimate solutions are obtained.
Finally, simulation results are conducted to validate the benefits and
efficiency of our proposed scheme
Beamforming Analysis and Design for Wideband THz Reconfigurable Intelligent Surface Communications
Reconfigurable intelligent surface (RIS)-aided terahertz (THz) communications
have been regarded as a promising candidate for future 6G networks because of
its ultra-wide bandwidth and ultra-low power consumption. However, there exists
the beam split problem, especially when the base station (BS) or RIS owns the
large-scale antennas, which may lead to serious array gain loss. Therefore, in
this paper, we investigate the beam split and beamforming design problems in
the THz RIS communications. Specifically, we first analyze the beam split
effect caused by different RIS sizes, shapes and deployments. On this basis, we
apply the fully connected time delayer phase shifter hybrid beamforming
architecture at the BS and deploy distributed RISs to cooperatively mitigate
the beam split effect. We aim to maximize the achievable sum rate by jointly
optimizing the hybrid analog/digital beamforming, time delays at the BS and
reflection coefficients at the RISs. To solve the formulated problem, we first
design the analog beamforming and time delays based on different RISs physical
directions, and then it is transformed into an optimization problem by jointly
optimizing the digital beamforming and reflection coefficients. Next, we
propose an alternatively iterative optimization algorithm to deal with it.
Specifically, for given the reflection coefficients, we propose an iterative
algorithm based on the minimum mean square error technique to obtain the
digital beamforming. After, we apply LDR and MCQT methods to transform the
original problem to a QCQP, which can be solved by ADMM technique to obtain the
reflection coefficients. Finally, the digital beamforming and reflection
coefficients are obtained via repeating the above processes until convergence.
Simulation results verify that the proposed scheme can effectively alleviate
the beam split effect and improve the system capacity
Energy-Efficient Hybrid Precoding Design for Integrated Multicast-Unicast Millimeter Wave Communications with SWIPT
In this paper, we investigate the energy-efficient hybrid precoding design
for integrated multicast-unicast millimeter wave (mmWave) system, where the
simultaneous wireless information and power transform is considered at
receivers. We adopt two sparse radio frequency chain antenna structures at the
base station (BS), i.e., fully-connected and subarray structures, and design
the codebook-based analog precoding according to the different structures.
Then, we formulate a joint digital multicast, unicast precoding and power
splitting ratio optimization problem to maximize the energy efficiency of the
system, while the maximum transmit power at the BS and minimum harvested energy
at receivers are considered. Due to its difficulty to directly solve the
formulated problem, we equivalently transform the fractional objective function
into a subtractive form one and propose a two-loop iterative algorithm to solve
it. For the outer loop, the classic Bi-section iterative algorithm is applied.
For the inner loop, we transform the formulated problem into a convex one by
successive convex approximation techniques and propose an iterative algorithm
to solve it. Meanwhile, to reduce the complexity of the inner loop, we develop
a zero forcing (ZF) technique-based low complexity iterative algorithm.
Specifically, the ZF technique is applied to cancel the inter-unicast
interference and the first order Taylor approximation is used for the
convexification of the non-convex constraints in the original problem. Finally,
simulation results are provided to compare the performance of the proposed
algorithms under different schemes.Comment: IEEE_TVT, Accep