For the rest of the Q3 2015 issue of Korea Communication Review magazine please click here

In its second demonstration of LTE in Unlicensed spectrum ("LTE-U", LAA in 3GPP term) conducted last month - 7 months after its first demonstration where it showed 300 Mbps through CA of 20 MHz at 2.6 GHz and another 20 MHz at 5.8 GHz, LG U+ successfully achieved 600 Mbps through carrier aggregation (CA) of 20 MHz at 2.6 GHz and 60 MHz (3 x 20 MHz) at 5.8 GHz.  

With LTE-U not even standardized yet, Korean operators are already in fierce competition in the new technology. LG U+ has been apparently leading the race so far, tightly followed by its competitors SK Telecom and KT, who also showcased 450 Mbps LTE-U, with Ericsson/Qualcomm in February, and with Samsung/Qualcomm in March, respectively. LG U+, with two successful demonstrations, said it is currently working on commercialization of the technology.
 
Then why are all the operators so obsessed with LTE-U demonstration, especially LG U+?  
 
First, let's see what is going on in the Korean communication market now. Korean LTE market is already pretty mature. As of 2015 Q1, it has i) 66% of LTE subscription, ii) 120 PB of LTE data usage, which accounts for 96% of its total mobile data usage, and iii) 3.4 GB of monthly average data usage per LTE subscriber.
 
The operators all have 40 MHz of LTE bandwidth (DL), and support up to 300 Mbps through 3-band CA (as of 2015 Q1). So far, they all somehow have managed to secure additional LTE frequencies gradually (10→20→30→40 MHz), offering faster speeds through CA. But, they are about to face a harder battle. At the end of the year, an auction for 700 MHz, 2.1GHz and 2.6GHz bands is scheduled. Especially, to get a wideband (20MHz) frequency, the 3 operators will have to compete not only with each other, but also with local over-the-air broadcasters requesting UHD channels, a possible 4th mobile operator, etc. Besides, the Korean government's policies on frequency allocation and the astronomical frequency costs are imposing a big burden on the operators.
 
LTE-U is a radio access technology that is designed to use 5 GHz unlicensed band, mostly used for WiFi so far, for LTE purpose as well. So, the technology will allow the band to serve pretty well as a supplementary band (of more than dozens of MHz) for LTE, although not as a dedicated, primary band like licensed bands do. For the operators who are under pressure to deal with issues like growing traffic, frequency shortage, and high costs of frequency, LTE-U certainly sounded like an attractive alternative. 
 
The output power is limited to low power in the 5 GHz unlicensed band. So small cells are most likely to be the first place where commercialized LTE-U will be employed. Also, indoor/outdoor hotspots or in-building solutions are expected to be the first target of the new technology deployment.

We had a chance to discuss with a member of LG U+ Access Network Development Team who conducted the LTE-U demonstration last month. Below we will briefly review what we learned from him about their LTE-U demonstration, target areas, and commercialization plan, one at a time.
 
■ LTE-U demonstration
LTE-U demonstration network: Figure 1 is a diagram that illustrates LTE-U demonstration by LG U+. Figure 2 is a picture of the real demonstration site prepared by the company.
 

Figure 1. LTE-U demonstration by LG U+ (Source: LG U+)
 
The LTE-U demonstration network consists of:

• UE : 1 test LTE-U UE 
• RAN
  • 1 commercial BBU operated in 2.6 GHz LTE network
  • 2 LTE-U small (multi-band) RRHs supporting multi-band, LTE 2.6 GHz and WiFi 5.8 GHz 
  • 1 HUB that connects the BBU and the small RRHs  
• Core Network: Commercial EPC

LTE-U combines LTE and WiFi bands, with an anchor in reliable licensed, LTE band, providing wider transmission bandwidth. In the demonstration, a total of 4 carriers - one from LTE 2.6 GHz band and 3 from WiFi 5.8 GHz band - were aggregated. Each carrier was 20 MHz wide, and so the UE used a total of 80 MHz, achieving download speeds of up to 600 Mbps, which was two times faster than the current LTE-A.
 
Optic fiber cables were used for connection between the BBU and HUB, and UTP cables were used between the HUB and small RRHs. Data sent to the UE travels as follows:  

1. The data travels through CPRI interface from BBU to HUB 
2. Then it travels through gigabit Ethernet (GE) interface, i.e. CPRI over Ethernet, from HUB to small RRHs (CPRI is terminated at the small RRHs).  
3. At the small RRHs, LTE baseband signals are converted into LTE/WiFi RF signals. The primary LTE carrier and 3 supplementary WiFi carriers are combined and sent to the UE through carrier aggregation. 

An LTE-U Small RRH has a modular structure, and consists of two types of modules: 2.6 GHz module (LTE module) and WiFi 5.8 GHz module (LTE-U module). In the figure above, Small RRH 1 has one LTE and one LTE-U modules while Small RRH 2 has 2 LTE-U modules. In the demonstration, each module in the small RRH used their own UTP cables to connect HUB. But once LTE-U is commercialized, connection will be made from each small RRH (not from each module) to HUB via a single dedicated UTP cable. Currently, LTE-U Small RRHs can support 2-band, but will be supporting 3-band next year. 
 

Figure 2. LG U+'s LTE-U demonstration room 
 
In the right shows an example of LTE-U deployment an in-building environment. It looks pretty much like Macro C-RAN applied in the same environment.
 
Here, LTE-U is similar to Macro C-RAN in that BBU and RRH are separated, but has a little bit modified architecture of C-RAN in that the BBU and small RRHs are connected via a HUB.
 
CPRI interface in Macro network between BBU and RRHs via optic cabling is, now in LTE-U, further divided into:
i) CPRI interface between BBU and HUB (optic cabling), and
ii) GE interface (CPRI over Ethernet, UTP) between HUB and small RRHs
 
LTE-U can be compared to DAS in that they both offer distributed access points for users in a building. The biggest difference between the two will be that DAS uses antennas distributed over a building as those access points, but LTE-U uses small RRHs, instead.   
 
■ Then, why LG U+ is so eager to launch LTE-U fast?
With the last demonstration done successfully, LG U+ said now their next plan is to aggregate up to 4 carriers (80 MHz) alone in 5.8 GHz unlicensed band, and combine them with one 20 MHz licensed carrier, to achieve speeds of up to 750 Mbps (i.e. through CA of 20 MHz at 2.6 GHz and 4 20 MHz (80 MHz) at 5.8 GHz). As LTE-A allows for CA of up to 5 20 MHz carriers, 750 Mbps is the maximum capacity LTE-A can support in 2x2 MIMO environment. So, why LG U+ is moving so fast, actually the fastest?
 
1. World's highest LTE subscription rate, but not enough frequency  
Unlike its competitors, LG U+'s network has evolved from CDMA directly to LTE, skipping WCDMA 3G network. And it already has 79% of LTE subscription rate, the highest among the big 3. Given the fact that the other two already have 3G and WiBro (Mobile WiMAX) frequencies that are convertible into LTE frequency, LG U+ needs additional LTE frequency more than anyone else.    

2. Excellent hotspot solution
After about 9 months of working on construction of a nationwide LTE network since its LTE service launch in July 2011, LG U+ finally built the world's first nationwide LTE network in March 2012. Shortly after, it also commercialized the world's first VoLTE service, providing voice service 100% through the LTE network. Voice service is a real-time service. Due to such nature, the company's hotspots have been built mostly in small cells, rather than in WiFi.
 
Solutions for improving data offloading and capacity in hotspots have been sought after due to traffic growth. And the LG U+ personnel said LTE-U small cell could serve as an excellent alternative hotspot/in-building solution. While others are busy working on WiFi, upgrading hotspot WiFi into giga-class WiFi, LG U+ decided to stick to working on LTE-U, accelerating expansion of LTE-U hotspot sites, and thereby ensuring more reliable and secure communication than WiFi in the hotspots. 
 
■ LTE-U service commercialization plan
For successful commercialization of LTE-U, co-existence between WiFi and LTE-U has to be secured first, and then the following two issues:
 
1. Korean government's policies on 5GHz band must be finalized. Co-existence policies must be announced first. Then based on them, the current technology standards on 5 GHz band must be revised accordingly. 3GPP is considering to adopt Listen-Before-Talk (LBT), a LTE-U channel access procedure, as an effort to ensure fair share of 5 GHz band with WiFi.
 
2. LTE-U enabled devices must be available. Qualcomm plans to release LTE-U enabled mobile chipsets without LBT feature this year, and ones with LBT feature next year.
 
LG U+'s plan now is to commercialize LTE-U service at the same time when LTE-U enabled devices hit the market. According to the LG U+ personnel, by next year i) the Ministry of Science, ICT and Future Planning (MSIP) will finish finalization of 5 GHz policy, and ii) LTE-U enabled devices with LBT feature will be available in the market. So, the new service will probably be released sometime next year as well.   
 
It looks like LG U+ wanted to make the most of LTE-U in indoor/outdoor hotspots. The new C-RAN based small cell architecture showcased last time is expected to play a large part as the key hotspot/in-building solution of the company. With LTE-U standardization due in about a year, if everything goes will, LG U+ should be able to launch the service next year as scheduled. Well, we will just wait and see.