912 research outputs found
AirSync: Enabling Distributed Multiuser MIMO with Full Spatial Multiplexing
The enormous success of advanced wireless devices is pushing the demand for
higher wireless data rates. Denser spectrum reuse through the deployment of
more access points per square mile has the potential to successfully meet the
increasing demand for more bandwidth. In theory, the best approach to density
increase is via distributed multiuser MIMO, where several access points are
connected to a central server and operate as a large distributed multi-antenna
access point, ensuring that all transmitted signal power serves the purpose of
data transmission, rather than creating "interference." In practice, while
enterprise networks offer a natural setup in which distributed MIMO might be
possible, there are serious implementation difficulties, the primary one being
the need to eliminate phase and timing offsets between the jointly coordinated
access points.
In this paper we propose AirSync, a novel scheme which provides not only time
but also phase synchronization, thus enabling distributed MIMO with full
spatial multiplexing gains. AirSync locks the phase of all access points using
a common reference broadcasted over the air in conjunction with a Kalman filter
which closely tracks the phase drift. We have implemented AirSync as a digital
circuit in the FPGA of the WARP radio platform. Our experimental testbed,
comprised of two access points and two clients, shows that AirSync is able to
achieve phase synchronization within a few degrees, and allows the system to
nearly achieve the theoretical optimal multiplexing gain. We also discuss MAC
and higher layer aspects of a practical deployment. To the best of our
knowledge, AirSync offers the first ever realization of the full multiuser MIMO
gain, namely the ability to increase the number of wireless clients linearly
with the number of jointly coordinated access points, without reducing the per
client rate.Comment: Submitted to Transactions on Networkin
Distributed 3D-Beam Reforming for Hovering-Tolerant UAVs Communication over Coexistence: A Deep-Q Learning for Intelligent Space-Air-Ground Integrated Networks
In this paper, we present a novel distributed UAVs beam reforming approach to
dynamically form and reform a space-selective beam path in addressing the
coexistence with satellite and terrestrial communications. Despite the unique
advantage to support wider coverage in UAV-enabled cellular communications, the
challenges reside in the array responses' sensitivity to random rotational
motion and the hovering nature of the UAVs. A model-free reinforcement learning
(RL) based unified UAV beam selection and tracking approach is presented to
effectively realize the dynamic distributed and collaborative beamforming. The
combined impact of the UAVs' hovering and rotational motions is considered
while addressing the impairment due to the interference from the orbiting
satellites and neighboring networks. The main objectives of this work are
two-fold: first, to acquire the channel awareness to uncover its impairments;
second, to overcome the beam distortion to meet the quality of service (QoS)
requirements. To overcome the impact of the interference and to maximize the
beamforming gain, we define and apply a new optimal UAV selection algorithm
based on the brute force criteria. Results demonstrate that the detrimental
effects of the channel fading and the interference from the orbiting satellites
and neighboring networks can be overcome using the proposed approach.
Subsequently, an RL algorithm based on Deep Q-Network (DQN) is developed for
real-time beam tracking. By augmenting the system with the impairments due to
hovering and rotational motion, we show that the proposed DQN algorithm can
reform the beam in real-time with negligible error. It is demonstrated that the
proposed DQN algorithm attains an exceptional performance improvement. We show
that it requires a few iterations only for fine-tuning its parameters without
observing any plateaus irrespective of the hovering tolerance
Impact and modeling of phase noise in mmW beamforming systems
Abstract. Due to the exponential growth of wireless communication, mobile communication applications require more bandwidth available in higher operating frequencies. High centre frequency makes the systems sensitive for phase variations caused by the phase noise (PN) of the imperfect local oscillators (LOs) used in wireless transceivers. Moreover, wide bandwidth also makes the faster phase variations of the phase noise spectra have an impact on the overall system performance by reducing effective signal-to-noise-ratio. These fast variations seen in the high offset frequencies in the phase noise spectra are typically ignored in the communication systems because the traditional system bandwidths are in order of megahertz, or in maximum few gigahertz. In mmW frequencies, i.e., at 30–300 GHz, the transceivers are typically using multiple antenna elements to achieve the required link range by highly directional beams. Often so-called phased arrays are used to implement the multi-antenna transceiver, where the beamforming is mostly performed in the analog domain by digitally controllable mmW phase shifters. For generating multiple beams from the same transceivers, more than one phased array is typically used in the same platform. The phased arrays often share a single LO, for multiple antenna elements. A typical LO generation architecture is containing a base clock, phased-locked loop (PLL), and some frequency multipliers to achieve the target mmW operating frequency. In multi-array systems, the LO signal can be divided into phased arrays in multiple domains, i.e., the arrays can have an independent clock, and a shared clock, but independent PLLs, shared PLL, or even the final mmW LO can be shared. In different architectures, the phase noise has different behavior, and it can have an impact for example on the beamforming accuracy. This thesis focuses on the effects of phase noise on milimeter-wave (mmW) beamforming systems to study different LO routing architectures. We mainly focus on LO architecture with multiple phased arrays that intend to make a common beamformer and their impact on overall system-level phase noise performance. The specific focus is given to the behavior of the wideband phase noise. The phase noise is modeled by using baseband equivalent models where a gaussian phase noise source is filtered by a filter modeling the equivalent phase noise spectra. The parameterization of the model is based on commercial LO phase noise spectra. The behavior is studied in different LO schemes in single-beam and multi-beam scenarios by using simple examples. The simulations are mostly carried out by using continuous-wave signals, but also the single-carrier modulated QAM waveform is demonstrated. The simulations are performed in MATLAB
Survey of Spectrum Sharing for Inter-Technology Coexistence
Increasing capacity demands in emerging wireless technologies are expected to
be met by network densification and spectrum bands open to multiple
technologies. These will, in turn, increase the level of interference and also
result in more complex inter-technology interactions, which will need to be
managed through spectrum sharing mechanisms. Consequently, novel spectrum
sharing mechanisms should be designed to allow spectrum access for multiple
technologies, while efficiently utilizing the spectrum resources overall.
Importantly, it is not trivial to design such efficient mechanisms, not only
due to technical aspects, but also due to regulatory and business model
constraints. In this survey we address spectrum sharing mechanisms for wireless
inter-technology coexistence by means of a technology circle that incorporates
in a unified, system-level view the technical and non-technical aspects. We
thus systematically explore the spectrum sharing design space consisting of
parameters at different layers. Using this framework, we present a literature
review on inter-technology coexistence with a focus on wireless technologies
with equal spectrum access rights, i.e. (i) primary/primary, (ii)
secondary/secondary, and (iii) technologies operating in a spectrum commons.
Moreover, we reflect on our literature review to identify possible spectrum
sharing design solutions and performance evaluation approaches useful for
future coexistence cases. Finally, we discuss spectrum sharing design
challenges and suggest future research directions
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