LTE-5G-NR: Massive MIMO

What is a massive MIMO? MIMO stands for multiple-input, multiple-output antenna. MIMO increases the number of antennas on a radio. For example, a radio using 2T2R MIMO would have two antennas that are used for both transmit and receive. The same would go for 4T4R and 8T8R MIMO. MIMO improves spectral efficiency by creating multiple receive and transmit paths between the base station and the end device.

Massive MIMO does not have a specific meaning other than having more antennas than what is currently used in mobile networks. With massive MIMO comes beamforming, which allows for the focusing of the radio signal on areas of demand on specific user devices. It improves spectral efficiency mainly because the advanced beamforming helps to generate greater signal accuracy, including better call quality at the cell edge. Another technology trend emerging with massive MIMO is multi-user MIMO (MU-MIMO). MU-MIMO allows for the transceiver to talk to more than one receiver at a time.

TDD has been the first mover when it comes to massive MIMO, but FDD, despite having some challenges not found with TDD, will use massive MIMO as well. Beamforming requires a channel quality report. Because TDD uses the same frequency for both downlink and uplink (channel reciprocity), the uplink channel quality information can be used for the downlink as well. This makes beamforming easier to do with TDD than FDD. In the case of FDD, it is possible to use alternative techniques to obtain the feedback information necessary to implement beamforming but in a less efficient and accurate manner

The following provides more details on the benefits of massive MIMO:

• Improved spectral efficiency and network capacity for higher throughput. The system sends and receives multiple data signals over the same radio channel, which increases the spectral efficiency per cell and the number of users who can be served simultaneously. This raises the peak and average cell throughput more cost-effectively than other techniques, such as new spectrum or additional sites.

 • Stronger signal and reduction of interference for better coverage. Beamforming provides accurate and narrow beams through aiming of the signal, which reduces interference and improves signal quality, especially at the cell edge. Beamforming allows for expanded reach of the cell compared to traditional antennas. This is particularly true for higher frequencies where beamforming compensates for the higher path loss.

More compact, with easier and cost-effective installation The Samsung massive MIMO Access Unit (MAU) is an integrated solution with a radio unit and the active antennas of an LTE base station in a compact and lightweight form factor. It is designed to be installed easily and cost-effectively

Support for LTE and 5G New Radio (5G readiness) The smooth migration from 4G to 5G is pivotal. The Samsung massive MIMO Access Unit utilizes existing LTE resources to the highest possible degree, and is up gradable to 5G NR mainly through software. Thanks to a straightforward upgrade process, operators that plan to transition from 4G to 5G using sub-6GHz frequencies will be able to achieve fast time-to-market service launch in a cost-effective manner.

5G NR will not only be deployed in TDD bands, but also in mmWave and <3GHz Frequency Division Duplex (FDD) bands. For mmWave bands today, active antennas commonly use hybrid beamforming for cost and efficiency reasons. Beam selection and tracking procedures defined by the 3GPP address terminal mobility.

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Antenna Array

There are many ways of utilizing the multi-element antenna array. An antenna array can be partitioned into subarrays known as an array of subarrays (AOSA). The number of sub-arrays in the partition will determine the degrees of freedom in which antenna elements may interact with each other to provide the beamforming capabilities of the entire antenna array.

The spatial processing techniques for 5G NR below 6 GHz can be viewed as an enhancement on LTE Release 14 and defined with a single transmission mode like LTE TM10 with much more flexibility and enhanced supporting signals. The spatial processing in 5G NR as in LTE TMs is based on beamforming (BF), Single-User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO).

• Beamforming (BF) – concentrate energy to the target UE and reduces interference to other UE’s thereby improving coverage

• SU-MIMO – split the available SINR between different multiple data layers towards a target UE simultaneously where each layer is separately beamformed thereby improving peak user throughput and system capacity

• MU-MIMO – share the available SINR between multiple data layers towards multiple UEs simultaneously where each layer is separately beamformed thereby improving system capacity and user perceived throughput.

To see how an antenna array creates steerable high-gain beams, we start with an antenna array of a specific size, which is then divided into subarrays of different sizes. For illustrative purposes, we describe only one dimension. The same principles do, however, apply to both vertical and horizontal dimensions of the antenna.

The array gain is referred to as the gain achieved when all subarray signals are added constructively (in phase). The size of the array gain relative to the gain of one subarray depends on the number of subarrays. 

For example, Array gain= 10 log(M), for M=1 Array gain is 0 and for two subarrays give an array gain of 2 (therefore, 3 dB). By changing the phases of the subarray signals in a certain way, this gain can be achieved in any direction as shown in Figure -A.

Each subarray has a certain radiation pattern describing the gain in different directions. The gain and beam width depend on the size of the subarray and the properties of the individual antenna elements. There is a trade-off between subarray gain and beam width – the larger the subarray, the higher the gain and the narrower the beam width, as illustrated in Figure -B.

The total antenna gain is the product of the array gain and the subarray gain, as shown in Figure -C. The total number of elements determines the maximum gain and the subarray partitioning allows steering of high gain beams over the range of angles. Moreover, the subarray radiation pattern determines the envelope of the narrow beams (the dashed shape in Figure -C. This has an implication on how to choose antenna array structure in a real deployment scenario with specific coverage requirements.