ICTEAM > ELEN > ISPGroup > ISPS > Calendar > ABS151015

Title: "Blind Interference Alignment for Cellular Networks"

Note: Joint ISP and DIGICOM seminar

Speaker: Máximo Morales (DIGICOM, ICTEAM, UCL)

Location: "Shannon" Seminar Room, Place du Levant 3, Maxwell Building, 1st floor

Date / Time (duration): Thursday 15/10/2015, 14h00 (~45')

Abstract: As more dense and heterogeneous cellular networks are required to satisfy the user demands on mobile communications interference becomes to be the principal limiting factor. Several schemes such as Linear Zero Forcing Beamforming (LZFB) or Interference Alignment (IA) have been proposed as means of achieving enormous data rates. These transmission schemes are based on exploiting the Channel State Information at the Transmitter (CSIT) to achieve the optimal Degrees of Freedom (DoF), also known as multiplexing gain. It is interesting to remark that satisfying this requirement in cellular environments involves to waste resources for channel feedback and backhaul links among the BSs. In consequence, the increase on the rates achieved by these schemes results futile because of the costs of providing CSIT.

Recently, Blind Interference Alignment (BIA) have been proposed as means of achieving a growth in DoF regarding the amount of users without the need for CSIT. For the Nt transmit antennas and K-user Multiple-Input Single-Output Broadcast Channel (MISO BC) it is demonstrated that {$\frac{N_t K}{N_t + K - 1}$} DoF are achievable by using BIA, which coincides with the information theoretical optimal-DoF in absence of CSIT. In this work, we derive the information theoretic DoF region for homogenous cellular networks with partial connectivity without CSIT. After that, we propose a network BIA (nBIA) scheme based on exploiting the network topology that achieves the optimal-DoF. Nevertheless, nBIA penalizes the achievable DoF in heterogeneous cellular networks because of the multiple-tier topology. We derive the DoF region for the two-tier cellular network by using the definition of linear DoF (lDoF). It is shown that the upper-tier attains the optimal DoF given by an isolated MISO BC while the lower-tier achieves non-zero DoF. A cognitive BIA (cogBIA) scheme able to obtain the optimal DoF in macro-femto cellular networks is presented in this work. Remarkably, in this work we demonstrate, by using information theory, that knowledge of the network topology achieves greater DoF and sum-rates than cooperation among the transmitter when CSIT is not available.

Although previous BIA schemes are DoF-optimal, i.e. when {${\rm SNR} \to \infty$}, it does not mean to be sum-rate optimal at finite SNR. The performance of BIA in practical channels is analyzed in this work. It is shown that transmission at limited SNR and coherence time are the main hurdles to overcome for practical implementations of BIA. We propose the use of BIA together with orthogonal approaches (pBIA). 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 coherent time. The optimal resource division for orthogonal transmission is obtained after solving a very specific optimization problem. In this sense, motivated by the flexibility of the network parameters expected for the future generation of mobile communications, obtain the optimal resource division for pBIA is still an open issue.

## Biography:

Máximo Morales Céspedes (S’10-M’15) was born in Valdepeñas, C. Real, Spain, in 1986. He received the B.Sc., M.Sc., and Ph.D. degrees from the Universidad Carlos III de Madrid, Spain, in 2010, 2012, and 2015, respectively, all in electrical engineering, with a specialization in Multimedia and Communications. In 2012 he was finalist of the IEEE Region 8 Student Paper Contest. From 2010 to 2014, he was with the Department of Signal Theory and Communications, Universidad de Carlos III de Madrid, Spain. He is currently working as a postdoctoral fellow with the Institute of Information and Communication Technologies, Electronics and Applied Mathematics (ICTEAM), UC Louvain. His research interests are interference management, hardware implementations, MIMO techniques and signal processing applied to wireless communications.

Last updated October 09, 2015, at 09:21 AM

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