Simulation and Testing of a MU-MIMO Beamforming System

Abstract

Multi-User Multiple-Input Multiple-Output (MU-MIMO) technology has become increasingly important in the field of wireless communication due to its ability to highly increase the capacity and efficiency of wireless networks [1]. Beamforming, as a technique used in MU-MIMO systems, improves network performance by improving signal quality and reducing interference. With the emergence of 5G and beyond, the complexity of Advanced Antenna Systems (AASs) that perform beamforming has increased considerably. Consequently, testing the AAS before
installation becomes vital to ensure the reliability and performance of the system. Meanwhile, the Butler matrix has gained significant attention as a passive device for efficient and cost-effective testing of the AAS and beamforming setup. Generally, the Butler matrix can be used in the Base Station (BS) of mobile networks to create beams towards the User Equipment (UEs).
In this thesis work, a method for beamforming tests based on channel reciprocity in MU-MIMO is studied. A beamforming setup in the laboratory using a Butler matrix to form beams is used before the BS is deployed in real-world scenarios. Based on the Sounding Reference Signal (SRS), which is received from the UEs, the BS estimates the channel for each UE separately and applies the appropriate weight matrix to determine beams towards the UEs.
The purpose of the study is to evaluate and validate the system for beamforming tests. Assessment is carried out in two parts. Taking everything into account, we first simulate a combination of directional signals using an Over The
Air (OTA) test method. This simulation involves four UE positions. This approach enables us to verify the accuracy of the beam patterns generated by the system. Furthermore, it identifies side lobes that might be present in the beam patterns. Through these simulations, we can mitigate and reduce these side lobes, enhancing the overall quality of the testing process.
Secondly, measurements were performed in the Downlink (DL) and Uplink (UL) modes. In the DL measurements, the Physical Downlink Shared Channel (PDSCH) power and throughput of the UEs were measured for the setup in the laboratory. Subsequently, on the basis of our observation of low PDSCH power or low throughput values, we explored the root causes using UL measurements. The UL measurements involved recording the SRS traces. Our approach for obtaining SRS data, testing methodologies, precise data selection from log traces, and subsequently beam mapping algorithm are described in detail. The study includes a comprehensive description of the methodology along with the corresponding results, ensuring a complete understanding of the process.

Popular Abstract

The history of wireless communication began with the discovery of electromagnetic waves. The wired telephone system was introduced in around 1870, which paved the way for the later transition from traditional landline phones to superfast wireless connections. Wireless technology enables devices to communicate with one another while on the move. To make connections possible, transmitting and receiving antennas are used to send and receive electromagnetic signals, respectively. Traditionally, a single antenna is used at both the transmitter and the receiver, which involves a simple antenna process.
The advent of new generations of mobile systems has opened up new possibilities for antenna design. The size and power of the antennas and the way they communicate with each other have changed with the emergence of 5G technology. Unlike previous generations, 5G uses small cell structures and smart antenna systems to simultaneously send signals directly to several users. The improved hardware in 5G communication created the possibility of operating multiple devices in different locations at a higher speed.

As a key property of 5G, we can point to the Multi-User Multiple-Input Multiple-Output (MU-MIMO), which employs multiple antennas to transmit and receive signals. This technology enables communication of several devices with a single base station simultaneously. To make things clearer, it allows many people to use their smartphones simultaneously without disruption. As an example, MU-MIMO gives this opportunity to the audience in sports stadiums to share their experiences online without any disconnections.

For better utilization of the shared medium and the frequency band in 5G systems, beamforming technology is used. The application of MIMO antennas in beamforming and wireless technologies makes communications more efficient. Beamforming directs wireless signals precisely where they are needed, which leads to fast and clear communication even in crowded areas.

In recent years, the number of mobile users has increased significantly. Thus, with increased demand for reliable communications, Advanced Antenna Systems (AASs) are essential when implementing the aforementioned technologies, such as MU-MIMO and beamforming. These smart systems enable the transmission of signals without interruption within environments with high traffic.

Before applying the AASs to practical and real applications, it is essential to test them. Testing these antennas in real scenarios is complicated. Therefore, in our thesis, we tested them in a laboratory setup in which the devices are connected by cables. In the laboratory, a Butler matrix is used to test the beamforming ability of the advanced antennas. The Butler matrix, a passive beamforming tool, helps us to assess the beamforming capability of these systems and how precisely they can guide signals in different directions.

One method of beamforming in 5G is called reciprocity-based beamforming. This beamforming method uses a special technique where devices send and receive signals along the same path and make communication clearer and more efficient. The Butler matrix is used for this process and ensures that these signals are sent and received in a way that minimizes confusion and that messages reach the intended place.

Before we start testing the advanced antennas with the Butler matrix, we need to double-check that the Butler matrix is doing its job correctly. It is like making sure that all the pieces of a device work perfectly together. So, we run dedicated tests. First, we measure the power of the signals that are transmitted from the base station to our devices. In this step, we check if the user receives enough power. We then measure special signals, transmitted from our devices to the base station. We make sure that the Butler matrix can take the signals from the advanced antennas and send them in the right direction with the expected power value. In this way, we can be sure that everything works smoothly when we use advanced and smart antennas in real deployments.

In this thesis project, our initial power measurements show that when we have one or two receivers, each of them receives enough power. However, with an increase in the number of receivers to four, our measurements reveal that some of the receivers cannot receive the expected power values. Consequently, we searched and identified the main cause of the problem.

• author: Mehdishishavan, Nazyar and Wang, Zonghan
• supervisor: Fredrik Tufvesson
• organization: Department of Electrical and Information Technology
• course: EITM02 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Multi-User Multiple-Input Multiple-Output (MU-MIMO), Beamforming, Sounding Reference Signal (SRS), Reciprocity-based, Butler matrix, Advanced Antenna System (AAS)
• report number: LU/LTH-EIT 2023-948
• language: English
• id: 9136395
• date added to LUP: 2023-09-22 14:44:19
• date last changed: 2023-09-22 14:44:19