Scheduler Algorithms for MU-MIMO

In multi-user multiple input multiple output (MU-MIMO), the complexity of the base-station scheduler has increased further compared to single-user multiple input multiple output (SU-MIMO). The scheduler must understand if several users can be spatially multiplexed in the same time-frequency resource. One way to spatially separate users is through beamforming with sufficiently many antennas. In this thesis work, two downlink beamforming algorithms for MU-MIMO are studied: The first algorithm implements precoding without considering inter-cell interference (ICI). The second one considers it and attempts to mitigate or null transmissions in the direction of user equipments (UEs) in other cells. The two algorithms are evaluated in SU-MIMO and MU-MIMO setups operating in time division duplex (TDD) mode and serving with single and dual-antenna terminals. Full-Buffer (FB) and file transfer protocol (FTP) data traffic profiles are studied. Additionally, various UE mobility patterns, UE transmit antenna topologies, sounding reference signal (SRS) periodicity configurations, and uniform linear array (ULA) topologies are considered. Simulations have been performed using a system level simulation framework developed by Ericsson AB. Another important part of this thesis work is the functional verification of this simulation framework, which at the time of writing is still undergoing development. Our simulation results show that in SU-MIMO, the second algorithm, which considers ICI, outperforms the first one for FB traffic profile and all UE speeds, but not for FTP traffic profile and medium (30 km/h) or high (60 km/h) UE speeds. In this case, the first algorithm, which does not consider ICI, can be used with advantage. In MU-MIMO, cell downlink throughput gains are observed for the second algorithm over the first one for low and medium system loads (number of users). For both algorithms, the cell throughput is observed to decrease with increasing UE speed and sounding periodicity.Scheduling in modern wireless standards, e.g., 3G, 4G and future 5G, can be defined as the task of allocating time and frequency resources by the base station (BS) to each user equipment (UE) that wants to engage in communication. Resources are allocated every transmission time interval (TTI), which is typically one millisecond. There exist both uplink (from the UEs to the BS) and downlink (from the BS to the UEs) resource schedulers implemented in the e-Node B, i.e., the base station (BS) in Long Term Evolution (LTE). The aim of this thesis work is to study how various communication techniques proposed for 5G can increase the overall system throughput of the downlink (DL) when a realistic resource scheduler is used. In particular, we consider: (i) Beamforming, (ii) Multi-user multiple input multiple output (MU-MIMO), and (iii) Inter-cell interference (ICI) mitigation. Beamforming can be achieved by deploying a large number of antenna elements at the BS with the aim of increasing the signal to interference noise ratio (SINR) towards the UE. Contrary to single-user multiple input multiple output (SU-MIMO), in MU-MIMO more than one UE are scheduled for transmissions in the same time-frequency resource; this is possible by judiciously pairing various UEs which are spatially sufficiently separated (according to some metric that we will define later). ICI mitigation can be achieved by means of proper precoding at BS where the precoder attempts to mitigate the interfering signal from BS towards UEs belonging to neighboring cells. In this thesis work, we investigate the performance of two scheduler algorithms for MU-MIMO, using SU-MIMO as baseline. The first algorithm does not consider ICI while the second one does. Dual layer beamforming (that is, two independent data streams are transmitted to each UE) and time division duplex (TDD) are assumed. In TDD mode the BS acquires the channel information from sounding reference signals (SRS) transmitted in the uplink (UL) and, by virtue of channel reciprocity, reuses the so-obtained channel information in the downlink. The performance evaluation of the two algorithms is based on the following parameters: UE Traffic profile, UE speed, SRS UL antenna configuration, SRS parameters, and BS antenna topology. - UE speed includes 3,30, and 60 km/h. - UE traffic profile includes full-buffer (FB) and file transfer protocol (FTP). With FB traffic profile, UEs send/receive data to/from the BS all the time, while this is not the case in the FTP traffic profile case. Some examples of FTP traffic profiles may include chatty, video, VoIP, web, etc. - SRS UL antenna configuration includes: (i) Two SRS, in which each UE sends two SRS to the BS from two antennas, (ii) one SRS with antenna selection, in which each UE alternately sends one SRS to the BS from each of two antennas, and (iii) one SRS without antenna selection, in which each UE sends one SRS to the BS from only one antenna. For two SRS UE case (note that in the downlink two layers, and hence two UE antennas, are always used). - SRS parameters include SRS bandwidth and SRS periodicity. In this thesis work, full-bandwidth SRS (20 MHz) with various SRS periodicities such as 5 ms, 10 ms, 20 ms are considered. - BS antenna topology includes 8 and 64 antenna elements at the BS. The main result of this thesis work is that in both SU-MIMO and MU-MIMO with FB traffic profile, it is better to use the second algorithm which considers ICI rather than the first one which does not. However, with FTP traffic profile, this is not always the case

A Distributed Approach to Interference Alignment in OFDM-based Two-tiered Networks

In this contribution, we consider a two-tiered network and focus on the coexistence between the two tiers at physical layer. We target our efforts on a long term evolution advanced (LTE-A) orthogonal frequency division multiple access (OFDMA) macro-cell sharing the spectrum with a randomly deployed second tier of small-cells. In such networks, high levels of co-channel interference between the macro and small base stations (MBS/SBS) may largely limit the potential spectral efficiency gains provided by the frequency reuse 1. To address this issue, we propose a novel cognitive interference alignment based scheme to protect the macro-cell from the cross-tier interference, while mitigating the co-tier interference in the second tier. Remarkably, only local channel state information (CSI) and autonomous operations are required in the second tier, resulting in a completely self-organizing approach for the SBSs. The optimal precoder that maximizes the spectral efficiency of the link between each SBS and its served user equipment is found by means of a distributed one-shot strategy. Numerical findings reveal non-negligible spectral efficiency enhancements with respect to traditional time division multiple access approaches at any signal to noise (SNR) regime. Additionally, the proposed technique exhibits significant robustness to channel estimation errors, achieving remarkable results for the imperfect CSI case and yielding consistent performance enhancements to the network.Comment: 15 pages, 10 figures, accepted and to appear in IEEE Transactions on Vehicular Technology Special Section: Self-Organizing Radio Networks, 2013. Authors' final version. Copyright transferred to IEE

Optimize Power Allocation Scheme to Maximize Sum Rate in CoMP with Limited Channel State Information

Extensive use of mobile applications throws many challenges in cellular systems like cell edge throughput, inter cell interference and spectral e�ciency. Many of these challenges have been resolved using Coordinated Multi-Point (CoMP), developed in the Third Generation Partnership Project for LTE-Advanced) to a great extent. CoMP cooperatively process signals from base sta- tions that are connected to various multiple terminals (user equipment (UEs)) at transmission and reception. This CoMP improves throughput, reduces or even removes inter-cell interference and increases spectral e�ciency in the downlink of multi-antenna coordinated multipoint systems. Many researchers addressed these issues assuming that BSs have the knowledge of the common control channels dedicated to all UEs and also about the full or partial channel state information (CSI) of all the links. From the CSI available at the BSs, multiuser interference can be managed at the BSs. To make this feasible, UEs are responsible for collecting downlink CSI. But, CSI measurement (instantaneous and/or statistical) is imperfect in nature because of the randomly varying nature of the channels at random times. These incorrect CSI values available at the BSs may, in turn, create multi-user interference. There are many techniques to suppress the multi-user interference, among which the feedback scheme is the one which is gaining a lot of attention. In feedback schemes, CSI information needs to be fed back to the base station from UEs in the uplink. It is obvious, the question arises on the type and amount of feedback need to be used. Research has been progressing in this front and some feedback techniques have been proposed. Three basic CoMP Feedback schemes are available. Explicit or statistical channel information feedback scheme in which channel information like channels's covariance matrix of the channel are shared between the transmitter and receiver. Next, implicit or statistical channel information feedback which contains information such as Channel quality indication or Precoding matrix indicator or Rank indicator. 1st applied to TDD LTE type structure and 2nd of feedback scheme can be applied in the FDD system. Finally, we have UE which tranmit the sounding reference signal (CSI). This type of feedback scheme is applied to exploit channel reciprocity and to reduce channel intercell interference and this can be applied in the TDD system. We have analyzed the scenario of LTE TDD based system. After this, optimization of power is also required because users at the cell edge required more attention than the user locating at the center of the cell. In my work, it shows estimated power gives exponential divercity for high SNR as low SNR too. In this method, a compression feedback method is analyzed to provide multi-cell spatial channel information. It improves the feedback e�ciency and throughput. The rows and columns of the channel matrix are compressed using Eigenmode of the user and codebook based scheme speci�ed in LTE speci�cation. The main drawback of this scheme is that spectral e�ciency is achieved with the cost of increased overheads for feedback and evolved NodeB (eNB). Other factor is complexity of eNodeB which is to be addressed in future work

Desenvolvimento em VHDL da camada física de um transmissor 4G

The LTE and LTE-Advanced technologies are standards to the fourth mobile generation, or 4G. The planned successor of this mobile generation is 5G, which will be based on 5G-New Radio (5G-NR) standard. The 5G technology is on an initial phase of deployment. One of its features that are essential in this initial phase is the support for 4G communications, because many of the mobile devices currently in use do not have support for 5G communications. This support is made possible if there is an implementation where 4G and 5G networks both coexist with each other. In the future, with the increasing usage of mobile devices with 5G support, there will be a gradual migration of 4G networks to 5G, releasing frequency spectrums currently reserved for 4G so that those can be occupied by 5G. The data transmissions in 4G require quite a lot of the processing capacity of all systems within the mobile network. For 5G, the data transmissions, in terms of traffic volume and speed, are larger than 4G transmissions, requiring new systems to be implemented, to allow the processing of larger quantities of data. Implementation in hardware of a 4G Uplink transmission chain, at the physical layer level PHY-Low, will allow the optimization of certain processes that a CPU could handle, reducing CPU usage and time spent on processing. The use of FPGAs makes this possible, as FPGAs can perform parallel tasks simultaneously and perform digital signal processing. The purpose of this dissertation is the modelling of a 4G LTE Uplink transmitter, at the physical layer level. Then, synthesizable VHDL code is generated from the modeled system, which can be eventually implemented in FPGAs. The modelling of the system is made in Simulink, a tool inside the MATLAB software, which allows for modelling, simulating and analyzing systems in a graphic environment and has applications in control systems and digital signal processing. The VHDL code is generated from HDL Coder, another tool in MATLAB software, generating synthesizable Verilog and VHDL code, from the MATLAB functions and Simulink models. The results obtained of processed data from the system are analyzed and validated, comparing the reference data generated from Wireless Waveform Generator toolbox in MATLAB.A tecnologia LTE e LTE-Advanced são standards da quarta geração de comunicações moveis atuais, ou 4G. Futuramente, o 5G marca a próxima geração de comunicações moveis, segundo o standard 5G-New Radio (5GNR). A tecnologia 5G encontra-se numa fase inicial de implementação, sendo que nessa fase uma das suas características fundamentais é o suporte para comunicações 4G, pois muitos dos dispositivos moveis usados atualmente não possuem suporte para comunicações 5G. Este suporte para 4G é tornado possível, se for feita uma implementação onde as redes 4G e 5G se encontrem em coexistência. No futuro, com o aumento do uso de dispositivos moveis com suporte para 5G, haverá uma migração gradual de redes 4G para 5G, libertando os espectros de frequências reservados atualmente para o 4G para serem ocupados pelo 5G. As transmissões de dados no 4G exigem bastante da capacidade de processamento de todos os sistemas da rede movel. Para o 5G, as transmissões de dados tem volumes de tráfego e velocidades maiores do que as transmissões de dados 4G, fazendo com que novos sistemas tenham de ser implementados para poder processar maiores quantidades de dados. A implementação em hardware da cadeia de transmissão 4G Uplink, ao nível da camada física PHY-Low, permitirá a otimização de certos processos que um CPU poderia lidar, diminuindo o uso do CPU e o tempo gasto em processamento. O uso de FPGAs torna isto possível, tendo em conta que podem realizar tarefas em paralelo, em modo simultâneo, e fazer processamento digital de sinal. O objetivo desta dissertação assenta na modelação de um transmissor 4G LTE Uplink, ao nível da camada física. Depois, é gerado código VHDL sintetizável a partir do sistema modelado, que eventualmente será implementada em FPGAs. A modelação do sistema é feito em Simulink, uma ferramenta no software do MATLAB, que permite modelar, simular e analisar sistemas num ambiente gráfico e tem aplicações para sistemas de controlo e processamento digital de sinal. O código VHDL é gerado a partir do HDL Coder, uma outra ferramenta no software do MATLAB, que gera Verilog e VHDL sintetizáveis, a partir de funções MATLAB e de modelos Simulink. Os resultados obtidos dos dados processados pelo sistema são analisados e validados, comparando com os dados de referência obtidos a partir da toolbox Wireless Waveform Generator do MATLAB.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Channel Dynamics and SNR Tracking in Millimeter Wave Cellular Systems

The millimeter wave (mmWave) frequencies are likely to play a significant role in fifth-generation (5G) cellular systems. A key challenge in developing systems in these bands is the potential for rapid channel dynamics: since mmWave signals are blocked by many materials, small changes in the position or orientation of the handset relative to objects in the environment can cause large swings in the channel quality. This paper addresses the issue of tracking the signal to noise ratio (SNR), which is an essential procedure for rate prediction, handover and radio link failure detection. A simple method for estimating the SNR from periodic synchronization signals is considered. The method is then evaluated using real experiments in common blockage scenarios combined with outdoor statistical models