SK Telecom is the #1 mobile operator in Korea, with sales of KRW 16.6 trillion (USD 15.3 billion) in 2013, and with 50.1% of a mobile mobile subscription market share in 2Q 2014. It launched LTE service back in July 2011, and now more than half of its subscribers are LTE service subscribers, with 55.8% of LTE penetration as of 2Q 2014.

Due to LTE subscription growth, more advanced device features, and high-capacity contents, LTE networks are experiencing an unprecedented surge in traffic. To accommodate the flooded traffic, SK Telecom adopted LTE-A (Carrier Aggregation, CA) in 2013, and Wideband LTE-A (Wideband CA) in 2014 for improved network capacity.

As another effort to increase network capacity, the company made LTE/LTE-A macro cells a lot smaller, as small as hundreds of meters long, resulting in an increased number of cell sites. To save costs of building and operating the increased number of cell sites, it has built C-RAN (Advanced-Smart Cloud Access Network, A-SCAN, as called by SK Telecom) through BBU concentration since January 2012.

In 2014, SK Telecom began to introduce small cells (low-power small RRHs) in selected areas. As with macro cells, small RRHs have the same C-RAN architecture where they are connected to concentrated BBU pools through CPRI interfaces. SK Telecom calls it "Unified RAN (Cloud and Heterogeneous)".

To prevent performance degradation at cell edges caused by introduction of small cells, SK Telecom developed HetNet architecture (known as SUPER Cell) where macro cells cooperate with small cells. The company, aiming to commercialize 5G networks in 2020, plans to commercialize SUPER Cell first in 2016, as a transitional phase to 5G networks.

Figure 1. SK Telecom’s Network Evolution Strategies

We analyzed SK Telecom's network evolution strategies using the following three axes: 1) Carrier Aggregation (CA), 2) Inter-Cell Coordination, and 3) RAN Architecture in the Figure 1. Here, the CA axis shows how speeds have been and can be increased (n times) by expanding total frequency bandwidth aggregated. The Inter-Cell Coordination axis displays the company's strategy to achieve higher speeds at cell edges by improving frequency efficiency. Finally, the RAN Architecture axis shows SK Telecom's plan to switch to an architecture that would yield better LTE-A performance at reduced costs of building and operating RAN. Figure 2 is SK Telecom's evolved LTE-A network, as illustrated according to the evolution strategies shown in Figure1.

Figure 2. SK Telecom’s LTE-A Evolution Network 

1. CA Evolution Strategies
CA is a technology that combines up to five frequencies in different bands to be used as one wideband frequency. It allows for expanded radio transmission bandwidth, which would naturally boost transmission speeds as much as the bandwidth is expanded. So, for example, if bandwidth is increased n times, then so is the transmission speed. Table 1 shows the LTE frequencies that SK Telecom has as of September 2014, totaling 40 MHz (DL only) across three frequency bands, which operate as Frequency Division Duplexing (FDD).

SK Telecom commercialized CA in June 2013 for the first time in the world, and then Wideband CA a year later in June 2014. 

It is now offering a maximum speed of 225 Mbps through the total 30 MHz bandwidth. As of May 2014, out of the total 15 million LTE subscribers, 3.5 million (23%) subscribers are using CA-enabled devices. Let's see where SK Telecom's CA is heading.

1.1 Combining More Bands: 3-band CA
3-band CA combines three frequency bands, instead of the current two, for wider-band transmission. Currently, SK Telecom has three LTE frequency bands, and is offering 2-band CA of 20 MHz or 30 MHz by combining two of the bands at once. This is because, although LTE-A standards technically support combining of up to five frequency bands, RF chips in  CA-enabled mobile devices available now can support combining of two bands only.  

3-band LTE devices are on the way and will be arriving in the market soon - sometime in early 2015 or by late 2014 at the latest. So, SK Telecom is planning to commercialize 3-band CA that combines all of its three frequency bands, just in time. The commercialization of 3-band CA is expected to increase transmission bandwidth to 40 MHz and data transmission rate to 300 Mbps. SK Telecom is also planning to combine three 20 MHz bands to further expand transmission bandwidth up to 60 MHz, and boost data transmission rate to 450 Mbps.

1.2 Femto Cell with CA
SK Telecom commercialized LTE Femto cell for the first time in the world in June 2012, to provide indoor users with more stable communication quality, and now is attempting to apply CA technology to Femto cell as well. The company completed a technical demonstration of LTE-A Femto cell in MWC 2014, proving it is capable to support 2-band CA. It will be conducting trial tests in a commercial network in late 2014 for final commercialization of the technology in 2015.

1.3 Combining Heterogeneous Networks: LTE-Wi-Fi CA
In July 2014, SK Telecom performed a technical demonstration of heterogeneous CA that combines LTE and Wi-Fi bands by using multipath TCP (MPTCP), an IETF standard. MPTCP is designed to combine more than one TCP flow (or MPTCP subflow) to make a single MPTCP connection, and send data through it. This technology is applied to a device and application server. In the demonstration, an MPTCP proxy server was used instead of an application server (Figure 3).    

Figure 3. LTE - Wi-Fi CA using Multipath TCP (MPTCP)

This technology will allow SK Telecom to combine i) its LTE bands that are currently featuring 2-band CA and ii) 802.11ac-based Giga Wi-Fi bands, together offering up to 1 Gbps or so. 

The detailed commercialization timeline is to be determined in accordance with the company's plan for future development of MPTCP device and server.

1.4 Combining Heterogeneous LTE Technologies: FDD-TDD CA
This method enables operators to expand transmission bandwidth by combining two different types of LTE technologies: FDD-LTE and TDD-LTE. In a demonstration performed in Mobile Asia Expo in June 2014, SK Telecom successfully demonstrated FDD-TDD CA using ten 20 MHz bandwidths and 8x8 MIMO antenna showing 3.8 Gbps throughout.