One of the main characteristics of 5G technology is super-fast speeds, up to ten times more than existing 4G networks. 5G systems not only promise speeds unlike anything we have experienced with existing technology, but billions of new devices are expected to be connected to the internet and managed by wireless networks with the implementation of 5G IoT. 5G wireless systems need, therefore, to become more robust and efficient to be able to support these higher demands in connectivity and data rates, and provide a vast variety of new 5G applications.
In existing systems, there are several ways to increase overall network capacity. One way to deliver higher speeds is to expand the network’s physical resources by the use of extra spectrum. However, new frequency spectrum is expensive to acquire since it is usually awarded via dedicated spectrum auctions. These spectrum auctions are traditionally used as one of the primary means for any country to generate revenue and may result in extremely high costs for Mobile Operators. At the same time, the capacity needs have increased exponentially in the last several years and are expected to rise dramatically with 5G. Spectrum resources are, therefore, becoming heavily utilized and are expected to constitute a hard-to-find commodity in the near future.
Another way to increase network capacity is to deploy more sites to cover the same geographical areas. Using more base stations will, however, increase the site acquisition and installation costs together with the overall operating expenses of maintaining these new sites. A more cost-effective solution is to explore new ways to improve spectral efficiency using various spectrum efficient techniques. One approach is to install multi-antenna systems at each site since that would enable operators to offer extra capacity by using the same amounts of spectrum.
MIMO Technology
Multi-input multiple-output (MIMO) is a multi-antenna spectrum-efficient technique that has become, over the last years, the leading driver of next-generation antenna technology for 5G networks. By offering a significant improvement in spectral efficiency and used in conjunction with beamforming, MIMO, multi-user MIMO, and massive MIMO technologies aim to support the increased 5G throughput demands with the least possible spectrum resources and constitute the basis of 5G active antenna systems.
MIMO antenna technology is a technique that aims to improve communication reliability, throughput, and coverage. A MIMO system may transmit more than one signal over the same radio channel, increasing radio efficiency and overall throughput. By taking advantage of spatial separation, the antennas are spaced at specific distances and angles to compensate for self-interference. At the same time, the wireless system may establish a more robust radio communication mechanism to deal with fading and shadowing caused by multiple transmission paths and long distances. Furthermore, various streams of data can be transmitted at the same time, offering multiplexing gains and improving overall throughput. For the above reasons, MIMO has been in the spotlight of wireless technology in recent years and is part of various next-generation wireless projects and standards, including 5G NR recommendations.
Point-to-point MIMO
MIMO systems have been embedded in various wireless networks for years, mainly in their simplest form, point-to-point MIMO. In point-to-point MIMO, two systems with multiple antennas may communicate with each other, thus increasing the capacity on the air interface. However, a multi-antenna configuration means additional hardware both at the base station and the end-user side, making system complexity the main drawback for the development and expandability of point-to-point MIMO. In a typical mobile communication system, the end-user equipment cannot be provisioned with multiple antennas due to the small physical size and the low-cost requirements of terminal devices.
Multi-User MIMO (MU-MIMO)
An enhancement of point-to-point MIMO was single-user SU-MIMO, where the data rate is increased by transmitting data streams to a specific user. However, in MU-MIMO, multiple users are sharing the same time/frequency resources, while each base station is equipped with multiple antennas and serves many users simultaneously. This time each end-user is using a single-antenna device, and complex hardware is required only on the side of the base station. This way, the cost and complexity of the overall antenna system are significantly reduced, as low-cost single antennas can be used on the receiver side and more expensive complex hardware only at the base station side.
Furthermore, due to the variety in the distance, angle, and quality of the signal of multiple users, the performance of MU-MIMO systems is generally less affected by the transmission environment compared to point-to-point MIMO. This is achieved via selective beamforming and power control to cancel interference. As a result, MU-MIMO systems offer high reliability and throughput and have become an integral part of communications wireless systems, such as Wi-Fi, LTE, and next-generation 5G networks.
Massive MIMO
The Multi-User MIMO technology undoubtedly offers great advantages over the conventional point-to-point MIMO. It works with low-cost single antennas on the receiver side, taking advantage of the array gain offered by the existence of many antennas on the side of the transmitter, increasing the system SNR. At the same time, resource allocation is simplified because each active terminal uses all of the base station time and frequency resources. However, it is not an extensible technology since, in most current implementations, base stations are equipped with a limited number of antennas, and the spectral efficiency is relatively low.
Massive MIMO, on the other hand, is a form of Multi-user MIMO technology where the number of antennas at the base station and the number of users served may increase significantly. With a large number of antennas in the base station, the channel vectors between the users and the base station are per pair almost rectangular, and thus the linear transmission becomes exceptional. Therefore, a large throughput is achieved due to multiplexing gain, diversity gain, and array gain. In massive MIMO, this large number of antennas at the base station may serve hundreds of users with the same frequency resource, taking advantage of the latest beamforming techniques.
Beamforming and massive MIMO
Today, cellular antennas broadcast information at all user directions and all of those crossing signals can cause interference. With beamforming, the radiated energy is steered towards the intended directions. This way the base station may send a focused stream of data to each specific user while forming nulls in the direction of other users to reduce inter/intra-cell interference.
A massive MIMO base station receives the multiple user signals and keeps track of the direction and time of arrival. It then uses signal processing algorithms to locate precisely where each signal is coming from and chooses the best wireless path to communicate with each terminal device. This way, each station, by establishing a unique and consistent data communication channel formed with each specific user, can handle more incoming and outgoing data streams simultaneously, and without interference.
5G & massive MIMO technology
In massive MIMO, the more antennas the base station is equipped with, the more robust the operation is. Therefore, more antennas mean that more users can be served at the same time, resulting in a huge overall throughput, to cover every 5G system needs.
For example, a typical 5G deployment positioning in a dense urban environment with high buildings would be a 64TRX antenna system with vertical beamforming and 4 vertical sub-arrays. 32TRX systems can be used in suburban environments with medium-sized buildings and 16TRX antenna systems in rural settings of a typical low-size building environment.
In a massive MIMO system, expandability is also essential. A 5G operator may initially start with a relatively conservative approach of a 16TRX antenna system and expand to 32TRX or 64TRX upon system demands. Massive MIMO systems are expected to be developed even further over the following years. Base stations can be equipped with hundreds or more antennas while simultaneously serve an ever-increasing high number of multiple users.
Next-generation wireless systems should be able to provide high throughputs, be scalable by supporting simultaneous multi-user services, flexible and expandable. Massive MIMO technology meets the above requirements, and that is why it is becoming the dominant technology for next-generation 5G wireless networks.
