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

Langattoman viestinnän kanavakapasiteetin optimointi keilanmuodostustekniikalla

Many modern devices rely on wireless Internet connection. This has increased the demand for high performance wireless networks. However, the traditional single-transmitting single-receiving antenna systems cannot provide the desired data rates and latencies in the future. The capacity of wireless communication systems can be increased by using more than one transmitting and receiving antennas. These multiple-input multiple-output (MIMO) systems allow the use of advanced signal precoding methods including beamforming. This method enables transmission of several concurrent data streams by controlling the electromagnetic wave interference. In this thesis, we review various beamforming techniques for single user and multi-user MIMO systems. In the single user case, the well-known eigen-beamforming method is derived, which achieves the optimal channel capacity. In the multi-user MIMO context, we focus on zero-forcing techniques that are based on a zero inter-user interference constraint. Finally, we define a new geometry-based MIMO channel model that is used for beamforming simulations. Results indicate that using beamforming can yield substantial increase in performance compared to the traditional communication systems.Monet nykyajan laitteet vaativat toimiakseen langattoman internet-yhteyden. Lisääntyvä älylaitteiden määrä on kasvattanut tarvetta korkean suorituskyvyn langattomille tietoverkoille. Perinteiset yhden lähetys- ja vastaanottoantennin viestintäsysteemit eivät kuitenkaan pysty tulevaisuudessa tuottamaan tarvittavia datansiirtonopeuksia ja viiveaikoja. Langattomassa viestinnässä kapasiteettia voidaan kasvattaa käyttämällä useita lähetys- ja vastaanottoantenneja. Nämä niin kutsutut MIMO-systeemit mahdollistavat edistyneiden koodaustekniikoiden käyttämisen. Yksi näistä tekniikoista on keilanmuodostus, jonka avulla voidaan lähettää useita datapaketteja samanaikaisesti. Menetelmä perustuu sähkömagneettisten aaltojen superpositioperiaatteen hyödyntämiseen. Tässä diplomityössä tarkastellaan useita keilanmuodostustekniikoita sekä yhden että monen käyttäjän MIMO-systeemeissä. Yhden käyttäjän tapauksessa johdetaan tunnettu menetelmä, joka saavuttaa optimaalisen kanavakapasiteetin. Usean käyttäjän MIMO-systeemien yhteydessä taas keskitytään tekniikoihin, joissa käyttäjien väliset häiriöt minimoidaan. Lopuksi määritellään uusi geometriaperusteinen kanavamalli, jota käytetään keilanmuodostussimulaatioissa. Tulokset osoittavat, että keilanmuodostusta käyttämällä voidaan parantaa merkittävästi langattomien viestintäsysteemien suorituskykyä

Improved Linear Precoding over Block Diagonalization in Multi-cell Cooperative Networks

In downlink multiuser multiple-input multiple-output (MIMO) systems, block diagonalization (BD) is a practical linear precoding scheme which achieves the same degrees of freedom (DoF) as the optimal linear/nonlinear precoding schemes. However, its sum-rate performance is rather poor in the practical SNR regime due to the transmit power boost problem. In this paper, we propose an improved linear precoding scheme over BD with a so-called "effective-SNR-enhancement" technique. The transmit covariance matrices are obtained by firstly solving a power minimization problem subject to the minimum rate constraint achieved by BD, and then properly scaling the solution to satisfy the power constraints. It is proved that such approach equivalently enhances the system SNR, and hence compensates the transmit power boost problem associated with BD. The power minimization problem is in general non-convex. We therefore propose an efficient algorithm that solves the problem heuristically. Simulation results show significant sum rate gains over the optimal BD and the existing minimum mean square error (MMSE) based precoding schemes.Comment: 21 pages, 4 figure