A decade since its first launch, LTE has now become a fundamental network for mobile data services, capable of bearing 80% of mobile data traffic and serving 2.8 billion users. As commercial deployment of 5G networks approaches, the evolution of LTE networks has become a hot topic in the industry.
At the 3GPP PCG meeting#37, the Project Coordination Group (PCG) determined that the first set of 5G standards will be delivered in 3GPP Release 15, involving New Radio (NR) and LTE evolution. 5G NR and LTE will coexist for a long time and evolve in parallel, helping develop 5G era networks.
Over the next decade, most mobile services will still be carried on existing LTE networks, which have advantages in terms of coverage, terminals, subscribers, and the ecosystem. According to the GSMA, there will be an increase of 3 billion LTE subscribers by 2025. With subscribers' constant growing demand for data, LTE network traffic is expected to dramatically increase. LTE is in urgent need of continuous evolution to arise as a fundamental network in the 5G era.
Multiple-antenna technologies based on Transmission Mode 9 (TM9) have become a significant milestone in efforts of further LTE evolution. TM9-based multiple-antenna technologies can greatly expand network capacity by enhancing spatial multiplexing and increasing spectral efficiency without adding sites or spectrum resources. This manifests as the mainstream trend for operators to develop 5G-oriented fundamental networks.
In the transmission modes prior to 3GPP Release 10, only the cell-specific reference signal (CRS) can be used to measure the channel state information (CSI) and demodulate data. The reference signals (RSs) occupy a lot of time-frequency resources and are frequently transmitted, causing excessive RS overhead in multiple-antenna cells. When four antenna ports are used, RS overhead accounts for 14.3% of the total. According to these calculations, the proportion of RS overhead will increase to 23.8% if eight ports are used. Therefore, no more than four antenna ports are supported prior to 3GPP Release 10.
TM9 is a new transmission mode introduced in 3GPP Release 10. In TM9, UE-specific RSs are introduced, and CSI measurement and data demodulation are performed by different RSs. Efficient and accurate closed-loop feedback is used to enable dynamic beamforming using multiple antennas. Single-user multiple-input multiple-output (SU-MIMO) and multi-user MIMO (MU-MIMO) are achieved to help exponentially improve both user experience and cell capacity.
UE-specific RSs and multiple-antenna technologies are also key technologies for 5G networks, representing the technological evolution trend in wireless networks.
In addition to UE-specific RSs, the compatibility of legacy non-TM9 terminals is also considered in TM9. Non-TM9 terminals can work properly on TM9-enabled networks. However, they cannot benefit from enhanced cell capacity and user experience brought by multiple-antenna technologies, as dynamic beamforming cannot be enabled. TM9-capable terminals can perfectly match TM9-enabled networks, helping to maximize network capabilities and achieve better user experience.
On Massive MIMO networks, TM9 changes cell-specific wide beams into UE-specific narrow beams. Non-coherent narrow beams can be paired to enable MU-MIMO. With MU-MIMO, the data of multiple UEs are no longer alternately scheduled using time or frequency division but are instead simultaneously scheduled. This enables spectrum multiplexing and significantly increases network capacity. According to a series of field tests, Massive MIMO networks can provide three to five times the capacity of traditional 2T2R LTE networks.
Although MU-MIMO cannot provide benefits for a single user by UE paring, user experience can still be notably enhanced through SU-MIMO. TM9 enables UE-specific dynamic beamforming to ensure the alignment of the strongest beam signals with UEs. TM9 also increases multiplexing gains by using multiple ports. The average user-perceived throughput of a single TM9 UE is over 25% higher than that of a non-TM9 UE.
Based on the reciprocity between uplink and downlink channels, beamforming in TDD does not rely on TM9. However, enabling TM9 can improve user experience in TDD Massive MIMO cells.
Beamforming based on TM7 or TM8 only supports at most two streams. However, the single-user transmission capabilities in TM9 can support four streams and above. The single-user peak throughput in TM9 equals to almost double that in TM8 and four times that in TM7.
In addition, beamforming only takes effect on the primary component carrier (PCC) in TM7 or TM8 on networks with carrier aggregation (CA) enabled. However, beamforming does not take effect on secondary component carriers (SCCs) since uplink sounding reference signal (SRS) measurement is not performed. After TM9 is introduced, UEs can report the downlink channel measurement results of the SCCs to achieve beamforming on SCCs. In TM9, multi-user pairing is enabled on SCCs to further improve CA user experience and CA cell capacity.
Cell edge user (CEU) experience is relatively poor due to the interference caused by neighboring cells. JT enables neighboring cells to simultaneously transmit data to CEUs for the purposes of improving CEU experience.
In the transmission modes prior to 3GPP Release 10, CEUs cannot identify RSs of neighboring cells by using CRSs. In this case, downlink JT cannot be achieved. TM9 uses UE-specific demodulation reference signals (DMRSs), which are transmitted along with data streams. In TM9, CEUs can successfully demodulate and combine the data streams of neighboring cells, effectively turning interference into gains while improving CEU experience.
After several years of evolution, the TM9 industry has matured. Mainstream chipset vendors, such as Qualcomm, HiSilicon, and MediaTek, have released TM9-capable smartphone chipsets. Currently, there are more than 20 models of smartphones equipped with TM9-capable chipsets. Users can activate TM9 for their TM9-capable terminals by simply updating to the latest software released by terminal vendors. Since 2018, mainstream terminal vendors have started activating TM9 in the default configuration for their terminals. In less than six months, five mainstream vendors have activated TM9 for six new models of smartphones. The global penetration rate of TM9 terminals is expected to reach over 20% by the end of 2018.
Operators are actively pursuing further development of the TM9 terminal industry by conducting pilot deployments of TM9 on their commercial networks. TM9 has been enabled on nine commercial networks in countries and regions such as China, Japan, Hong Kong, Thailand, Turkey, Kuwait, and France. It is estimated that the number of TM9-enabled commercial networks will reach 30 by the end of 2018. In North America, a number of operators are fully prepared and have specified TM9-capable as a mandatory requirement for the procurement of terminals. This has helped to lay a foundation for Massive MIMO deployment and LTE evolution in the future ecosystem.
It is foreseeable that 2018 will witness large-scale commercial deployment of TM9. TM9 will also serve as a benchmark for terminal competitiveness. With the perpetual maturity of the TM9 industry, LTE network capabilities will overcome the inherent restrictions on antenna ports and make great leaps to enable LTE to evolve into the fundamental network in the 5G era.