4,009 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
Maximum-rate Transmission with Improved Diversity Gain for Interference Networks
Interference alignment (IA) was shown effective for interference management
to improve transmission rate in terms of the degree of freedom (DoF) gain. On
the other hand, orthogonal space-time block codes (STBCs) were widely used in
point-to-point multi-antenna channels to enhance transmission reliability in
terms of the diversity gain. In this paper, we connect these two ideas, i.e.,
IA and space-time block coding, to improve the designs of alignment precoders
for multi-user networks. Specifically, we consider the use of Alamouti codes
for IA because of its rate-one transmission and achievability of full diversity
in point-to-point systems. The Alamouti codes protect the desired link by
introducing orthogonality between the two symbols in one Alamouti codeword, and
create alignment at the interfering receiver. We show that the proposed
alignment methods can maintain the maximum DoF gain and improve the ergodic
mutual information in the long-term regime, while increasing the diversity gain
to 2 in the short-term regime. The presented examples of interference networks
have two antennas at each node and include the two-user X channel, the
interferring multi-access channel (IMAC), and the interferring broadcast
channel (IBC).Comment: submitted to IEEE Transactions on Information Theor
Exploiting Spatial Interference Alignment and Opportunistic Scheduling in the Downlink of Interference Limited Systems
In this paper we analyze the performance of single stream and multi-stream
spatial multiplexing (SM) systems employing opportunistic scheduling in the
presence of interference. In the proposed downlink framework, every active user
reports the post-processing signal-to-interference-plus-noise-power-ratio
(post-SINR) or the receiver specific mutual information (MI) to its own
transmitter using a feedback channel. The combination of scheduling and
multi-antenna receiver processing leads to substantial interference suppression
gain. Specifically, we show that opportunistic scheduling exploits spatial
interference alignment (SIA) property inherent to a multi-user system for
effective interference mitigation. We obtain bounds for the outage probability
and the sum outage capacity for single stream and multi stream SM employing
real or complex encoding for a symmetric interference channel model.
The techniques considered in this paper are optimal in different operating
regimes. We show that the sum outage capacity can be maximized by reducing the
SM rate to a value less than the maximum allowed value. The optimum SM rate
depends on the number of interferers and the number of available active users.
In particular, we show that the generalized multi-user SM (MU SM) method
employing real-valued encoding provides a performance that is either
comparable, or significantly higher than that of MU SM employing complex
encoding. A combination of analysis and simulation is used to describe the
trade-off between the multiplexing rate and sum outage capacity for different
antenna configurations
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