On the Performance of Packet Aggregation in IEEE 802.11ac MU-MIMO WLANs

Multi-user spatial multiplexing combined with packet aggregation can significantly increase the performance of Wireless Local Area Networks (WLANs). In this letter, we present and evaluate a simple technique to perform packet aggregation in IEEE 802.11ac MU-MIMO (Multi-user Multiple Input Multiple Output) WLANs. Results show that in non-saturation conditions both the number of active stations (STAs) and the queue size have a significant impact on the system performance. If the number of stations is excessively high, the heterogeneity of destinations in the packets contained in the queue makes it difficult to take full advantage of packet aggregation. This effect can be alleviated by increasing the queue size, which increases the chances to schedule a large number of packets at each transmission, hence improving the system throughput at the cost of a higher delay

Towards a Realistic Assessment of Multiple Antenna HCNs: Residual Additive Transceiver Hardware Impairments and Channel Aging

Given the critical dependence of broadcast channels by the accuracy of channel state information at the transmitter (CSIT), we develop a general downlink model with zero-forcing (ZF) precoding, applied in realistic heterogeneous cellular systems with multiple antenna base stations (BSs). Specifically, we take into consideration imperfect CSIT due to pilot contamination, channel aging due to users relative movement, and unavoidable residual additive transceiver hardware impairments (RATHIs). Assuming that the BSs are Poisson distributed, the main contributions focus on the derivations of the upper bound of the coverage probability and the achievable user rate for this general model. We show that both the coverage probability and the user rate are dependent on the imperfect CSIT and RATHIs. More concretely, we quantify the resultant performance loss of the network due to these effects. We depict that the uplink RATHIs have equal impact, but the downlink transmit BS distortion has a greater impact than the receive hardware impairment of the user. Thus, the transmit BS hardware should be of better quality than user's receive hardware. Furthermore, we characterise both the coverage probability and user rate in terms of the time variation of the channel. It is shown that both of them decrease with increasing user mobility, but after a specific value of the normalised Doppler shift, they increase again. Actually, the time variation, following the Jakes autocorrelation function, mirrors this effect on coverage probability and user rate. Finally, we consider space division multiple access (SDMA), single user beamforming (SU-BF), and baseline single-input single-output (SISO) transmission. A comparison among these schemes reveals that the coverage by means of SU-BF outperforms SDMA in terms of coverage.Comment: accepted in IEEE TV

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

Evaluation of multi-user multiple-input multiple-output digital beamforming algorithms in B5G/6G low Earth orbit satellite systems

Satellite communication systems will be a key component of 5G and 6G networks to achieve the goal of providing unlimited and ubiquitous communications and deploying smart and sustainable networks. To meet the ever-increasing demand for higher throughput in 5G and beyond, aggressive frequency reuse schemes (i.e., full frequency reuse), combined with digital beamforming techniques to cope with the massive co-channel interference, are recognized as a key solution. Aimed at (i) eliminating the joint optimization problem among the beamforming vectors of all users, (ii) splitting it into distinct ones, and (iii) finding a closed-form solution, we propose a beamforming algorithm based on maximizing the users' signal-to-leakage-and-noise ratio served by a low Earth orbit satellite. We investigate and assess the performance of several beamforming algorithms, including both those based on channel state information at the transmitter, that is, minimum mean square error and zero forcing, and those only requiring the users' locations, that is, switchable multi-beam. Through a detailed numerical analysis, we provide a thorough comparison of the performance in terms of per-user achievable spectral efficiency of the aforementioned beamforming schemes, and we show that the proposed signal to-leakage-plus-noise ratio beamforming technique is able to outperform both minimum mean square error and multi-beam schemes in the presented satellite communication scenario