Transmit Beamforming in Dense Networks-A Review

Communication technology has prospered in manifolds over the last decade. The scarcity of spectrum as well as the demand for higher data rates and increase in capacity has become a matter of concern. Newer technologies have evolved time and again, the latest of which is Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A) systems more commonly known as 4G technology. The striking feature of LTE/LTE-A is the deployment of smaller cells (femto cells) in the vicinity of a large macro cells resulting in a dense network. As a result the data rate as well as capacity has increased in manifolds but the detrimental factor is the issue of interference between the various cells. Beamforming provides a solution in removing the issues of interference in dense networks. This paper focuses on the interference scenario in LTE dense networks and gives an overview of different beamforming methods that can provide a solution to the interference problem. Further, a review of several such methods so far proposed in available literature has been presented in this paper.Keywords:LTE/LTE-A, Dense Network, Interference,Beamformin

Blind interference alignment for cellular networks

Mención Internacional en el título 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: María Julia Fernández-Getino García.- Vocal: Ignacio Santamaría Caballer

Cognitive Orthogonal Precoder for Two-tiered Networks Deployment

In this work, the problem of cross-tier interference in a two-tiered (macro-cell and cognitive small-cells) network, under the complete spectrum sharing paradigm, is studied. A new orthogonal precoder transmit scheme for the small base stations, called multi-user Vandermonde-subspace frequency division multiplexing (MU-VFDM), is proposed. MU-VFDM allows several cognitive small base stations to coexist with legacy macro-cell receivers, by nulling the small- to macro-cell cross-tier interference, without any cooperation between the two tiers. This cleverly designed cascaded precoder structure, not only cancels the cross-tier interference, but avoids the co-tier interference for the small-cell network. The achievable sum-rate of the small-cell network, satisfying the interference cancelation requirements, is evaluated for perfect and imperfect channel state information at the transmitter. Simulation results for the cascaded MU-VFDM precoder show a comparable performance to that of state-of-the-art dirty paper coding technique, for the case of a dense cellular layout. Finally, a comparison between MU-VFDM and a standard complete spectrum separation strategy is proposed. Promising gains in terms of achievable sum-rate are shown for the two-tiered network w.r.t. the traditional bandwidth management approach.Comment: 11 pages, 9 figures, accepted and to appear in IEEE Journal on Selected Areas in Communications: Cognitive Radio Series, 2013. Copyright transferred to IEE