ThinkSmallCell Interview with Kim Chang-young, Senior Manager of Access Network Lab, SK Telecom

SK Telecom LogoSK Telecom is the largest network operator in South Korea and continues to retain a leading market share despite strong competition through constant innovation and investment. We asked Kim Chang-young, Senior Manager of Access Network Lab, to outline his views how they have used LTE technology and small cells to achieve this.

He foresees that LTE will be preferred by users over Wi-Fi, explains several techniques which helped speed deployment and optimise performance, and positions VoLTE as essential to deliver the best voice quality.

The size and scale of SK Telecom's LTE network is already very impressive. Could you share some statistics on the number of LTE users, macrocells, small cells and traffic throughput?

SK Telecom has over 10.6 million LTE subscribers as of the end of May, 2013. Since the launch in 2011, about 39% of our customers have migrated to LTE leading to growth of data traffic per user.

In June 2013, we launched the world's first LTE-Advanced service with a compatible smartphone (Samsung Galaxy S4 LTE-A), offering data rates up to 150Mbps using Carrier Aggregation of 10MHz bands from each of the 850MHz and 1800MHz bands.

At present, the number of small cells stands around 10,000 and more are being deployed in a gradual manner.

What proportion of SK Telecom's small cells are indoor vs outdoor?

SK Telecom is focusing on installing indoor femtocells in data traffic concentrated areas, mostly in buildings, so as to offload data traffic. Outdoor femtocells are being deployed in rural areas as a means to provide coverage in remote areas.

What proportion of data traffic is offloaded to Service Provider Wi-Fi? Do you envisage this will change, for example when HotSpot 2.0 is deployed.

Today, about 7 percent of data traffic is offloaded to our Wi-Fi, and we expect to see no great change in this proportion even after the deployment of Hotspot 2.0 since we are aggressively promoting the commercialization of next-generation mobile network technologies including LTE-Advanced. Hotspot 2.0 will allow us to manage our Wi-Fi network more efficiently.

You have encouraged the use of open standards between the macrocell and small cell vendors using the X.2 interface. Did you have to use different or extra signalling messages between these two vendor systems in addition to the standards?

We created our own Femto Gateway to support interworking between the macro cell and small cell layers. We applied the standard 3GPP X2 interface specification between the macrocell and Femto Gateway, while defining our own unique interface specification for Femto Gateway and Femtocell. By doing so, we were able to secure X2 interoperability between macrocell and small cell without using extra signaling messages.

Your LTE small cell architecture includes an LTE small cell gateway, although this isn't an essential part of the standards. Why did you include it and what benefits did it provide?

As mentioned above, our small cells can interwork with macro cells based on X.2 I/F specification. As there are a great number of small cells connected to a macrocell, connecting each small cell to the macrocell requires massive amount of resources. Thus, we created a gateway that can group the numerous small cells into one entity to manage resources in an efficient manner. In particular, it reduces MME signaling load and paging processing load during S1 interworking (Unlike X2, S1 uses MME). Moreover, maintenance/management of small cells becomes more convenient as we can upgrade features that support multiple vendors simply through the gateway node.

SK Telecom has developed its own SON server. Does this only configure the small cells or also adjust macrocell parameters? What are the main benefits and could you disclose any specific performance improvements?

As SK Telecom's SON system supports operational optimization of both small cells and macrocells, it is essential that it support interworking between small cells and macrocells. Also, our SON system is a 'Centralized SON' (C-SON) that supports interworking between macrocells and femtocells of many different vendors. Built upon our extensive know-how in network operation, the system calculates the optimal operating parameters by monitoring the whole access system within the network.

To cite a few specific performance improvements, we've witnessed a lower call drop rate through PCI/RSI/TAC* [*PCI: Physical Cell ID, RSI: Root Sequence Index, TAC: Tracking Area Code] auto settings, improved throughput in handover areas, decreased paging load, and enhanced Inter-RAT (Radio Access Technology) handover performance through WCDMA Neighbour List settings using the LTE Neighbour List as well as optimization features.

I believe you are rolling out small cells at a very high rate, perhaps as much as 100 units/day. How much automation of deployment processes have you achieved, and which areas/aspects do you believe to be most important?

The most important part of small cell deployment is to analyze information from the surrounding cells and then apply it to small cell's neighbor list and operational parameters. This process also applies to macrocells: These must also include information on the newly deployed small cells to its Neighbor List. And to automate this process, small cells need to have a sniffing feature, while both small cell and macrocell must have a UE based ANR (Automated Neighbor Relation) feature and a SON system in place to support information sharing between cells. By realizing all these features, we have achieved nearly 100 percent automation of the deployment process.

Wireless Small Cell Backhaul has been growing in importance in other regions. Is this relevant in Korea, or are almost all small cells hardwired into the network? Do you expect this to change in the future?

In Korea, most small cells are deployed in the urban areas to improve user throughput, expand capacity and offload data from macrocells. As these areas usually have a strong fixed infrastructure, wireless backhaul does not carry great importance in Korea. Also, it is also unpopular due to the issue of signal interference that stems from the fact that wireless backhaul utilizes the relatively low-cost Wi-Fi spectrum. However, as many trials are currently being conducted to enhance stability and quality of the wireless backhaul, we expect the use of wireless backhaul to increase in urban areas within a number of years.

VoLTE (Voice over LTE) is still a very new feature and not yet launched in many networks. What role do Small Cells play in the success of VoLTE, and would you expect that to change?

VoLTE is a not an option, but a necessity for LTE-only operators and must be supported by small cells, too. Although WDCMA network operators like SK Telecom can substitute VoLTE through CS Fallback, it will not support all LTE services such multi-call.

According to SK Telecom's MOS (Mean Opinion Score), VoLTE showed the highest voice quality among all voice-based services, meaning that it is a must-have service for carriers whose goal is to provide the best possible quality of service to customers. It is also an inevitable choice when considering migration to next-generation networks. Given that macrocells support VoLTE service, small cells must also do the same so as to maintain a certain level of quality in terms of mobility between both macrocell and small cell.

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