50 research outputs found
Elements of Cellular Blind Interference Alignment --- Aligned Frequency Reuse, Wireless Index Coding and Interference Diversity
We explore degrees of freedom (DoF) characterizations of partially connected
wireless networks, especially cellular networks, with no channel state
information at the transmitters. Specifically, we introduce three fundamental
elements --- aligned frequency reuse, wireless index coding and interference
diversity --- through a series of examples, focusing first on infinite regular
arrays, then on finite clusters with arbitrary connectivity and message sets,
and finally on heterogeneous settings with asymmetric multiple antenna
configurations. Aligned frequency reuse refers to the optimality of orthogonal
resource allocations in many cases, but according to unconventional reuse
patterns that are guided by interference alignment principles. Wireless index
coding highlights both the intimate connection between the index coding problem
and cellular blind interference alignment, as well as the added complexity
inherent to wireless settings. Interference diversity refers to the observation
that in a wireless network each receiver experiences a different set of
interferers, and depending on the actions of its own set of interferers, the
interference-free signal space at each receiver fluctuates differently from
other receivers, creating opportunities for robust applications of blind
interference alignment principles
Degrees of freedom of wireless interference network
Wireless communication systems are different from the wired systems mainly in three aspects: fading, broadcast, and superposition. Wireless communication networks, and multi-user communication networks in general, have not been well understood from the information-theoretic perspective: the capacity limits of many multi-user networks are not known. For example, the capacity region of a two-user single-antenna interference channel is still not known, though recent result can bound the region up to a constant value. Characterizing the capacity limits of multi-user multiple-input multiple-output (MIMO) interference network is usually even more difficult than the single antenna setup.
To alleviate the difficulty in studying such networks, the concept of degrees of freedom (DoF) has been adopted, which captures the first order behavior of the capacities or capacity regions. One important technique developed recently for quantifying the DoF of multi-user networks is the so-called interference alignment. The purpose of interference alignment is to design the transmit signals structurally so that the interference signals from multiple interferers are aligned to reduce the signal dimensions occupied by interference.
In this thesis, we mainly study two problems related to DoF and interference alignment: 1) DoF region of MIMO full interference channel (FIC) and Z interference channel (ZIC) with reconfigurable antennas, and 2) the DoF region of an interference network with general message demands.
For the first problem, we derive the outer bound on the DoF region and show that it is achievable via time-sharing or beamforming except for one special case. As to this particular special case, we develop a systematic way of constructing the DoF-achieving nulling and beamforming matrices. Our results reveal the potential benefit of using the reconfigurable antenna in MIMO FIC and ZIC. In addition, the achievability scheme has an interesting space-frequency interpretation.
For the second problem, we derive the DoF region of a single antenna interference network with general message demands, which includes the multiple unicasts and multiple multicasts as special cases. We perform interference alignment using multiple base vectors and align the interference at each receiver to its largest interferer. Furthermore, we show that the DoF region is determined by a subset of receivers, and the DoF region can be achieved by considering a smaller number of interference alignment constraints so as to reduce the number of time expansion.
Finally, as a related research topic, we also include a result on the average throughput of a MIMO interference channel with single-user detector at receivers and without channel state information at transmitters. We present a piecewise linear approximation of the channel throughput under weak, moderate and strong interference regimes. Based on that we determine the optimal number of streams that a transmitter should use for different interference levels
Retrospective Interference Alignment
We explore similarities and differences in recent works on blind interference
alignment under different models such as staggered block fading model and the
delayed CSIT model. In particular we explore the possibility of achieving
interference alignment with delayed CSIT when the transmitters are distributed.
Our main contribution is an interference alignment scheme, called retrospective
interference alignment in this work, that is specialized to settings with
distributed transmitters. With this scheme we show that the 2 user X channel
with only delayed channel state information at the transmitters can achieve 8/7
DoF, while the interference channel with 3 users is able to achieve 9/8 DoF. We
also consider another setting where delayed channel output feedback is
available to transmitters. In this setting the X channel and the 3 user
interference channel are shown to achieve 4/3 and 6/5 DoF, respectively
Blind interference alignment for cellular networks
MenciĂłn Internacional en el tĂtulo de doctorManaging the interference is the main challenge in cellular networks. Multiple-Input
Multiple-Output (MIMO) schemes have emerged as a means of achieving high-capacity
in wireless communications. The most efficient MIMO techniques are based on managing
the interference instead of avoiding it by employing orthogonal resource allocation
schemes. These transmission schemes require the knowledge of the Channel State Information at the Transmitter (CSIT) to achieve the optimal Degrees of Freedom (DoF),
also known as multiplexing gain. Providing an accurate CSIT in cellular environments
involves high-capacity backhaul links and accurate synchronization, which imply the use
of a large amount of network resources. Recently, a Blind Interference Alignment (BIA)
scheme was devised as a means of achieving a growth in DoF regarding the amount of
users served without the need for CSIT in the Multiple-Input Single-Output (MISO)
Broadcast Channel (BC). It is demonstrated that BIA achieves the optimal DoF in
the BC without CSIT. However, the implementation of BIA in cellular networks is not
straightforward. This dissertation investigates the DoF and the corresponding sum-rate
of cellular networks in absence of CSIT and their achievability by using BIA schemes.
First, this dissertation derives the DoF-region of homogenous cellular networks with
partial connectivity. Assuming that all the Base Stations (BSs) cooperate in order to
transmit to all users in the network, we proposed an extension of the BIA scheme for the
MISO BC where the set of BSs transmits as in a network MIMO. It is shown that the
cooperation between BSs results futile because of the lack of full connectivity in cellular
networks. After that, this dissertation presents several transmission schemes based on
the network topology. By differentiating between users that can treat this interference
optimally as noise and those who need to manage the interference from neighbouring
BSs, a network BIA scheme is devised to achieve the optimal DoF in homogeneous
cellular networks. Second, the use of BIA schemes is analyzed for heterogeneous cellular networks. It is demonstrated that the previous BIA schemes based on the network topology result nonoptimal in DoF because of the particular features of the heterogenous cellular networks. More specifically, assuming a macro-femto network, cooperation between both tiers leads to a penalty for macro users while femto users do not exploit the particular topology of this kind of network. In this dissertation, the optimal linear DoF (lDoF) in a two-tier network are derived subject to optimality in DoF for the upper tier. It is demonstrated
that, without CSIT or any cooperation between tiers, the lower tier can achieve nonzero
DoF while the upper tier attains the optimal DoF by transmitting independently of
the lower tier deployment. After that, a cognitive BIA scheme that achieves this outer
bound is devised for macro-femto cellular networks.
The third part of this dissertation is focused on the implementation of BIA in practical
scenarios. It is shown that transmission at limited SNR and coherence time are the
main hurdles to overcome for practical implementations of BIA. With aim of managing
both constraints, the use of BIA together with orthogonal approaches is proposed in this
work. An improvement on the inherent noise increase of BIA and the required coherence
time is achieved at expenses of losing DoF. Therefore, there exists a trade-off between
multiplexing gain, sum-rate at finite SNR and coherence time in practical scenarios. The
optimal resource allocation for orthogonal transmission is obtained after solving a very
specific optimization problem. To complete the characterization of the performance of
BIA in realistic scenarios a experimental evaluation based on a hardware implementation
is presented at the end of this work. It is shown that BIA outperforms the sum-rate
of schemes based on CSIT such as LZFB because of the hardware impairments and the
costs of providing CSIT in a realist implementation.Programa Oficial de Doctorado en Multimedia y ComunicacionesPresidente: Luc Vandendorpe.- Secretario: MarĂa Julia Fernández-Getino GarcĂa.- Vocal: Ignacio SantamarĂa Caballer
Experimental Evaluation of Blind Interference Alignment
The proceeding at: 2015 Vehicular Technology Conference (VTC Spring) took place 11-14 May in Glasgow, Ireland.An experimental evaluation of Blind Interference Alignment (BIA) over a hardware platform is presented in this work. In contrast to other transmission techniques such as Linear Zero Forcing Beamforming (LZFB) or Interference Alignment (IA), BIA achieves a growth in Degrees of Freedom (DoF) without channel state information at the transmitter (CSIT). A real implementation based on Orthogonal Frequency Division Multiplexing (OFDM) and LTE parameters is implement on a testbed made up of a transmitter equipped with two antennas and two users equipped with a reconfigurable antenna each. Furthermore, a full CSIT technique such as LZFB is also implemented for comparison purposes. First, the theoretic achievable rates are obtained for both techniques. After that, the bit error rate of both schemes is evaluated regarding the achieved sum-thorughput.This work has been partially funded by research projects COMONSENS (CSD2008-00010), and GRE3N (TEC2011-29006-C03-02)