Multiple-input/multiple-out (MIMO) technology is an established wireless communications technique for sending and receiving multiple data signals simultaneously over the same radio channel. MIMO techniques play a prominent role in Wi-Fi communications, as well as 3G, 4G, and 4G LTE networks. 

5G New Radio, however, takes it to the next level, introducing the concept of massive MIMO, which — as the name implies — involves the application of MIMO technology on a much larger scale for greater network coverage and capacity. Massive MIMO uses many more transmit and receive antennas to increase transmission gain and spectral efficiency. To achieve massive MIMO capacity gain, multiple UEs must generate downlink traffic simultaneously. Many variables impact the actual gain provided by massive MIMO.

While there is no specific minimum number of antennas required for the application of massive MIMO, the generally accepted threshold for a system is more than eight transmit and eight receive antennas. And the number can be much higher, extending to systems with tens or even hundreds of antennas. 

Massive MIMO — along with smart antenna techniques such as beamforming and beam steering — are among the key technologies for enabling the higher throughput and capacity gains promised by 5G. And they are essential for delivering the 100x data rates and the 1,000x capacity goals specified in the International Mobile Telecommunications-2020 (IMT-2020) vision. 

Since massive MIMO uses many more antennas than the number of UEs in the cell, the beam is much narrower, enabling the base station to deliver RF energy to the UE more precisely and efficiently. The antenna's phase and gain are controlled individually, with the channel information remaining with the base station, simplifying UE without adding multiple receiver antennas. Installation of a large number of base station antennas will increase the signal-to-noise ratio in the cell, which leads to higher cell site capacity and throughput. Since 5G massive MIMO implementation is on mmWave frequencies, the antennas required are small and easy to install and maintain. 

Infographic: Massive MIMO operation principle.

Still, for device designers, MIMO and beamforming at mmWave frequencies introduce many new challenges. 5G NR standards provide the physical-layer frame structure, new reference signal, and new transmission modes to support 5G enhanced mobile broadband (eMBB) data rates. Designers must understand the 3D beam patterns and ensure the beams can connect to the base station and deliver the desired performance, reliability, and user experience. Because massive MIMO, beamforming, and beam steering represent such significant changes in how 5G NR devices connect across sub-6 GHz and mmWave operating bands, validating the device quality of experience and performance on the network becomes even more critical. 

To implement MIMO and beamforming on 5G base stations, designers must carefully select hardware and software tools to simulate, design, and test highly complex systems containing tens or even hundreds of antenna elements.

Engineers will use active phased array antennas to implement MIMO and beamforming in base stations and devices. Not only are active antennas essential to overcome signal propagation issues such as higher path loss at mmWave frequencies, they also provide the ability to dynamically shape and steer beams to specific users. Active antennas offer more flexibility and improve the performance of 5G communications. 

But deploying active phased array antennas in commercial wireless communications represents a major change from the passive antennas used in previous generations. MIMO and beamforming technologies increase capacity and coverage in a cell. For 5G devices and base stations, multi-antenna techniques require support across multiple frequency bands — from sub-6 GHz to mmWave frequencies — and across many scenarios, including massive IoT connections and extreme data throughput.

Aerospace and defense radar and satellite communications have long used active phased array antennas, but these antenna arrays tend to be large and very expensive. Applying this technology to commercial wireless — where the antenna arrays will need to be much smaller and less costly — introduces many new challenges. There is a long list of 3GPP required tests for base stations, including radiated transmitter tests and radiated receiver tests. Depending on the base station configuration, some FR1 tests require radiated tests, and all FR2 tests require radiated tests.

Nearly all 5G MIMO testing requires over-the-air (OTA) testing. Early in development, OTA test solutions need to characterize the 3D beam performance across the range of the antenna, including aspects such as antenna gain, sidelobe, and null depth for the full range of 5G frequencies and bandwidths.