455 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
Interference and X Networks with Noisy Cooperation and Feedback
The Gaussian -user interference and X channels are
investigated with no instantaneous channel state information (CSI) at
transmitters. First, it is assumed that the CSI is fed back to all nodes after
a finite delay (delayed CSIT), and furthermore, the transmitters operate in
full-duplex mode, i.e., they can transmit and receive simultaneously.
Achievable results are obtained on the degrees of freedom (DoF) of these
channels under the above assumption. It is observed that, in contrast with no
CSIT and full CSIT models, when CSIT is delayed, the achievable DoFs for both
channels with full-duplex transmitter cooperation are greater than the best
available achievable results on their DoF without transmitter cooperation. Our
results are the first to show that the full-duplex transmitter cooperation can
potentially improve the channel DoF with delayed CSIT. Then, -user
interference and X channels are considered with output feedback,
wherein the channel output of each receiver is causally fed back to its
corresponding transmitter. Our achievable results with output feedback
demonstrate strict DoF improvements over those with the full-duplex delayed
CSIT when in the -user interference channel and in the X channel. Next, the combination of delayed CSIT and output feedback, known
as Shannon feedback, is studied and strictly higher DoFs compared to the output
feedback model are achieved in the -user interference channel when K=5 or
, and in the X channel when . Although being strictly
greater than 1 and increasing with size of the networks, the achievable DoFs in
all the models studied in this paper approach limiting values not greater than
2.Comment: 53 pages, 15 figures; Submitted to IEEE Transactions on Information
Theory, May 2012. To be presented in part in ISIT 2012, Cambridge, MA, US
Generalized discrete Fourier transform with non-linear phase : theory and design
Constant modulus transforms like discrete Fourier transform (DFT), Walsh transform, and Gold codes have been successfully used over several decades in various engineering applications, including discrete multi-tone (DMT), orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) communications systems. Among these popular transforms, DFT is a linear phase transform and widely used in multicarrier communications due to its performance and fast algorithms. In this thesis, a theoretical framework for Generalized DFT (GDFT) with nonlinear phase exploiting the phase space is developed. It is shown that GDFT offers sizable correlation improvements over DFT, Walsh, and Gold codes. Brute force search algorithm is employed to obtain orthogonal GDFT code sets with improved correlations. Design examples and simulation results on several channel types presented in the thesis show that the proposed GDFT codes, with better auto and cross-correlation properties than DFT, lead to better bit-error-rate performance in all multi-carrier and multi-user communications scenarios investigated. It is also highlighted how known constant modulus code families such as Walsh, Walsh-like and other codes are special solutions of the GDFT framework. In addition to theoretical framework, practical design methods with computationally efficient implementations of GDFT as enhancements to DFT are presented in the thesis. The main advantage of the proposed method is its ability to design a wide selection of constant modulus orthogonal code sets based on the desired performance metrics mimicking the engineering .specs of interest.
Orthogonal Frequency Division Multiplexing (OFDM) is a leading candidate to be adopted for high speed 4G wireless communications standards due to its high spectral efficiency, strong resistance to multipath fading and ease of implementation with Fast Fourier Transform (FFT) algorithms. However, the main disadvantage of an OFDM based communications technique is of its high PAPR at the RF stage of a transmitter. PAPR dominates the power (battery) efficiency of the radio transceiver. Among the PAPR reduction methods proposed in the literature, Selected Mapping (SLM) method has been successfully used in OFDM communications. In this thesis, an SLM method employing GDFT with closed form phase functions rather than fixed DFT for PAPR reduction is introduced. The performance improvements of GDFT based SLM PAPR reduction for various OFDM communications scenarios including the WiMAX standard based system are evaluated by simulations. Moreover, an efficient implementation of GDFT based SLM method reducing computational cost of multiple transform operations is forwarded. Performance simulation results show that power efficiency of non-linear RF amplifier in an OFDM system employing proposed method significantly improved
Hardware Architecture of a QAM Receiver for Short-Range Optical Communications
[EN] Short-reach optical fiber communications systems aim to achieve high throughput, in the order of tens of Gbps. The implementation of these high-speed systems requires parallel processing, which makes low-complexity designs of their subsystems a key to the successful large-scale deployment of this technology. Half-Cycle Nyquist Subcarrier Modulation (HC-SCM) was originally suggested for these systems with the goal of using as much bandwidth as possible and, therefore, achieving high communication rates. Recently, Oversampled Subcarrier Modulation (OVS-SCM) was proposed as an alternative more computational efficient than HC-SCM and also with a better spectral efficiency. This paper proposes a hardware-efficient architecture for an OVS-SCM receiver, which takes into account the inherent parallel processing of these systems. This receiver takes 16 samples in parallel from a 5 GSa/s analog-to-digital converter with a 3.2 GHz 3 dB bandwidth. Design solutions for the frame detection block, the mixer, the resampler, the fractional interpolator, the matched filter and the timing estimator are presented. Our results show that, compared to the HC-SCM receiver, this proposal reduces the computational load of the downconverter stages by 90%. FPGA implementation results are given to demonstrate that our proposal can be implemented in state-of-the-art devices.This work was supported in part by MCIN/AEI/10.13039/501100011033 under Grants RTI2018-101658-B100 and PID2021-126514OB-I00, and in part by the European Union through "ERDF Away of making Europe."Valls Coquillat, J.; Torres Carot, V.; Pérez Pascual, MA.; Almenar Terre, V. (2023). Hardware Architecture of a QAM Receiver for Short-Range Optical Communications. Journal of Lightwave Technology. 41(2):451-461. https://doi.org/10.1109/JLT.2022.321735745146141
Hiding Symbols and Functions: New Metrics and Constructions for Information-Theoretic Security
We present information-theoretic definitions and results for analyzing
symmetric-key encryption schemes beyond the perfect secrecy regime, i.e. when
perfect secrecy is not attained. We adopt two lines of analysis, one based on
lossless source coding, and another akin to rate-distortion theory. We start by
presenting a new information-theoretic metric for security, called symbol
secrecy, and derive associated fundamental bounds. We then introduce
list-source codes (LSCs), which are a general framework for mapping a key
length (entropy) to a list size that an eavesdropper has to resolve in order to
recover a secret message. We provide explicit constructions of LSCs, and
demonstrate that, when the source is uniformly distributed, the highest level
of symbol secrecy for a fixed key length can be achieved through a construction
based on minimum-distance separable (MDS) codes. Using an analysis related to
rate-distortion theory, we then show how symbol secrecy can be used to
determine the probability that an eavesdropper correctly reconstructs functions
of the original plaintext. We illustrate how these bounds can be applied to
characterize security properties of symmetric-key encryption schemes, and, in
particular, extend security claims based on symbol secrecy to a functional
setting.Comment: Submitted to IEEE Transactions on Information Theor
Software Defined Radio Implementation of Carrier and Timing Synchronization for Distributed Arrays
The communication range of wireless networks can be greatly improved by using
distributed beamforming from a set of independent radio nodes. One of the key
challenges in establishing a beamformed communication link from separate radios
is achieving carrier frequency and sample timing synchronization. This paper
describes an implementation that addresses both carrier frequency and sample
timing synchronization simultaneously using RF signaling between designated
master and slave nodes. By using a pilot signal transmitted by the master node,
each slave estimates and tracks the frequency and timing offset and digitally
compensates for them. A real-time implementation of the proposed system was
developed in GNU Radio and tested with Ettus USRP N210 software defined radios.
The measurements show that the distributed array can reach a residual frequency
error of 5 Hz and a residual timing offset of 1/16 the sample duration for 70
percent of the time. This performance enables distributed beamforming for range
extension applications.Comment: Submitted to 2019 IEEE Aerospace Conferenc
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