7 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
Hausdorff dimension of exponential parameter rays and their endpoints, Nonlinearity 21
Abstract. We investigate the set I of parameters κ for which the singular value of z ↦ → e z + κ converges to ∞. The set I consists of uncountably many parameter rays, plus landing points of some of these rays [FRS]. We show that the parameter rays have Hausdorff dimension 1, while the ray endpoints in I alone have dimension 2. Analogous results were known for dynamical planes of exponential maps [K, SZ]; our result shows that this also holds in parameter space. 1
An Experimental Evaluation of Voice Quality Over the Datagram Congestion Control Protocol
Abstract — Most Internet telephony applications currently use either TCP or UDP to carry their voice-over-IP (VoIP) traffic. This choice can be problematic, because TCP is not well suited for interactive traffic and UDP is unresponsive to congestion. The IETF has recently standardized the new Datagram Congestion Control Protocol (DCCP). DCCP has been designed to carry media traffic and is congestion-controlled. This paper experimentally evaluates the voice quality that Internet telephony calls achieve over prototype implementations of basic DCCP and several DCCP variants, under different network conditions and with different codecs. It finds that the currently-specified DCCP variants perform less well than expected when compared to UDP and TCP. Based on an analysis of these results, the paper suggests several improvements to DCCP and experimentally validates that a prototype implementation of these modifications can significantly increase voice quality. I