Massive MIMO is often mentioned as a key 5G technology, but could it also be exploited in currently standardized LTE networks? The ZTE-Telefónica trials that were initiated in October 2016 shows that this is indeed possible. The press release from late last year describes the first results. For example, the trial showed improvements in network capacity and cell-edge data rates of up to six times, compared to traditional LTE.
The Massive MIMO blog has talked with Javier Lorca Hernando at Telefónica to get further details. The trials were carried out at the Telefónica headquarters in Madrid. A base station with 128 antenna ports was deployed at the rooftop of one of their buildings and the users were located in one floor of the central building, approximately 100 m from the base station. The users basically had cell-edge conditions, due to the metallized glass and multiple metallic constructions surrounding them.
The uplink and downlink data transmissions were carried out in the 2.6 GHz band. Typical Massive MIMO time-division duplex (TDD) operation was considered, where the uplink detection and downlink precoding is based on uplink pilots and channel reciprocity. The existing LTE sounding reference signals (SRSs) were used as uplink pilots. The reciprocity-based precoding was implemented by using LTE’s transmission mode 8 (TM8), which supports any type of precoding. Downlink pilots were used for link adaptation and demodulation purposes.
It is great to see that Massive MIMO can be also implemented in LTE systems. In this trial, the users were static and relatively few, but it will be exciting to see if the existing LTE reference signals will also enable Massive MIMO communications for a multitude of mobile users!
Update: ZTE has carried out similar experiments in cooperation with Smartfren in Indonesia. Additional field trials are mentioned in the comments to this post.