Massive MIMO for 5G explored by METIS
Massive multiple-input multiple-output systems is a quickly maturing technology, especially those involving an order of magnitude greater number of antenna elements than in the early releases of wireless standards. Indeed, ongoing work for 3GPP Release 13/14 of LTE and New Radio (NR) involves identifying technology enablers and the performance benefits of deploying up to 64 antenna ports and an even-greater number of antenna elements at base stations.
Scaling up MIMO to Massive MIMO
While deploying 64 transmit and 64 receive antennas is a significant increase in the number of antenna ports, compared to today's typical deployments, further research and system development work is required to fully realize the promises of scaling up MIMO to massive antenna arrays in practice. This has been the task for the European 5G METIS II project led by Ericsson.
Massive MIMO is a multi-user MIMO system that serves multiple users through spatial multiplexing. It can bring unprecedented gains in terms of spectral and energy efficiency and robustness to hardware failures and impairments. Higher frequency bands, like millimeter-wave, are appealing for these large-scale antenna systems, since the physical array size can be greatly reduced due to the decrease in wavelength.
Massive MIMO technology components
The technology components developed in the METIS project address two essential issues in massive MIMO:
- Channel-state information acquisition
- Scalable transceiver structures
Channel-state information acquisition in massive MIMO is challenging because of the many channel links that need to be estimated and the problem of pilot contamination. Likewise, the extremely large number of channel links represents one major impediment in transceiver design as it sharply increases the computational complexity, calls for robust designs against channel state information errors to achieve the desired gains, and increases the traffic data transport over the backhaul in coordinated systems.
The massive MIMO technology components in the research work in METIS targeted two major 5G goals defined in the project:
- The ability to deliver very large data rates to each user.
- The ability to deliver high quality of service to a dense population of users.
Results in the METIS Deliverable
As reported in the METIS deliverable, Final Performance Results and Consolidated View on the Most Promising Multi-Node/Multi-Antenna Transmission Technologies, large performance gains in aggregate throughput in the order of 10 times can be achieved using a 256 antenna array compared to a baseline LTE scenario with 8 antennas.
Interestingly, the second goal is rarely addressed, while it is becoming more and more relevant in view of the capability of massive MIMO to spatially multiplex a great number of users by leveraging on multi-user MIMO technology. The METIS solution for delivering high bit rates for dense populations utilizes the theory of coded random access. It has been shown that coded random access systems with a great number of antennas are near optimal in the sense that they achieve the throughput of a fully scheduled and interference free operation.
Furthermore, the METIS technology components target legacy bands below 6 GHz. This focus is justified by the allocation of frequency bands below 6 GHz by the latest International Telecommunication Union World Radio Conference WRC-15, while for higher frequency bands, no allocations for 5G have been made so far. While time-division duplexing is the widely-preferred solution for massive MIMO systems, as channel state information acquisition scales with the number of antennas at the base station, frequency-division duplexing remains an attractive solution for operators. Therefore, METIS developed technology components for both time-division and frequency division duplexing systems.
The METIS technology components — channel state information based precoding, pilot-data resource balancing, user grouping, limited multi-cell coordination and uplink transmission using coded random access — can be integrated or used separately in future massive MIMO systems (Illustration courtesy of the METIS project).
The ideas and concepts developed in METIS will help massive MIMO to live up to the promises and expectations on the capability of delivering very high data rates to each user in high user-density situations. In fact, the results of METIS suggest that massive MIMO is a useful technology component not only for creating high-capacity access links, but also for boosting the capacity of backhaul links in dense deployment scenarios.
Testing Massive MIMO in 5G New Radio
More details about the massive MIMO technology components developed in the METIS project can be found in a recent IEEE Communications Magazine article providing an Overview of Massive MIMO Technology Components in METIS.
Ericsson is already commercializing the world's first 5G NR radio for massive MIMO. For example, Ericsson and Sprint are working together to test 5G technologies such as Massive-MIMO for LTE using Ericsson's next-generation 5G-ready radio (AIR6468) with Sprint during the second half of 2017.