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SoftBank aims at 97% coverage for TD-LTE network , says CTO Yoshioki Chika

03 February 2012

SoftBank has already soft-launched its TD-LTE mobile broadband network, using a dense network of microcells, with a commercial launch planned before the end of February

Read more: SoftBank Huawei TD-LTE TDD mobile broadband 4G 3G


               
Yoshioki Chika: One of the big challenges of a mobile
broadband operator today is to get sites for base stations 
                    

                             
LTE microcells replace older PHS base stations on Tokyo rooftop 
                      
In a little over a year's time 99% of Japan's 127 million people will be within range of what is being hailed as the world's largest commercial TD-LTE network, a mobile broadband network that uses a dense network of tiny base stations.
The company behind the project is SoftBank, one of Japan's biggest telecommunications operators, and it believes that the architecture - with 150 base stations per square kilometre - will help it overcome the capacity problems that are threatening the viability of mobile broadband operators everywhere.
By using so many base stations so close together the available spectrum can be used again and again. The suitcase-sized base stations are linked in groups to create a super-base station, controlled centrally.
"We have already launched our system - on November 1 last year," says Yoshioki Chika, the chief technology officer of the project. So far it is for "friendly customers only", he adds, speaking to Global Telecoms Business in mid-January 2012. Initial deployment covered the three cities of Tokyo, Osaka and Fukuoka. "We are going to launch a commercial project next month."
Huawei, which is delivering the network backbone of the project to SoftBank, has nominated the project for a GSMA award and is awaiting the decision of the judges at this year's Mobile World Congress in Barcelona at the end of February.
TD-LTE is the TDD - time-division duplex - variant of LTE that is held by many to be more efficient at using spectrum than the more traditional FDD, frequency-division duplex.
LTE - long-term evolution - is already being adopted globally at a unified standard for mobile broadband services by operators from the GSM camp and others, such as Verizon Wireless, from the CDMA camp. But in a world where operators are trying to squeeze every bit of potential out of the limited spectrum that is available, many engineers see the TDD variety as more efficient.
Put simply, FDD uses different frequencies for uplink and downlink, with a guard band of completely unused spectrum in between to avoid interference. TDD uses the same frequency for both uplink and downlink, with a tiny time interval between the two transmissions. Control needs to be tighter, but if that can be achieved the wastage of time is less than the wastage of spectrum with FDD. 
                      
                      
Frequency efficiency 
                      
"This is a TDD system, not FDD," says Chika. "The frequency efficiency of TDD is much, much better than FDD. FDD allocates the same bandwidth to uplink and downlink, but TDD could flexibly allocate bandwidth. For mobile internet, TDD has much better frequency efficiency than FDD."
The biggest supporter of TD-LTE technology is China Mobile, the world's largest mobile operator, and at Mobile World Congress in 2011 it, along with SoftBank and three other companies - Bharti Airtel, Clearwire and Vodafone - announced the Global TD-LTE Initiative, with more than two dozen other operators. TD-LTE is believed by many to be a way for WiMax to integrate with the emerging global LTE technology family. And it should be possible to make integrated TDD-FDD microchips, so that terminals will work on both standards.
SoftBank is running its TD-LTE project as a separate division, called Wireless City Planning, which has emerged out of its October 2010 acquisition of a company called Willcom which ran an outmoded PHS phone network.
Willcom, then owned by private equity group Carlyle, reached four million subscribers in 2006 but by early 2010 - faced with competition from Japan's dynamic 3G mobile operators - it filed for bankruptcy with liabilities of over $2.6 billion. SoftBank saw the opportunity and took it over.
PHS - personal handy-phone system - was a DECT-like technology developed in Japan in the late 1980s before the arrival of mass cellular phone services. It never spread much beyond Japan and China, and is essentially obsolete.
But the advantage to SoftBank is that PHS technology uses low power - only about half a watt - and therefore each base station has a low range. In order to achieve coverage Willcom had built lots of base stations around Japanese cities. 
                      
                      
Microcell network 
                      
"One of the big challenges of a mobile broadband operator today is to get footprint, to get sites," says Chika. "Because SoftBank has already purchased Willcom, it already has a huge base of 160,000 microcell sites that are already up and running."
SoftBank is re-engineering these with its vendor partners for the Wireless City Planning project, says Chika. The company does not need to negotiate new rental deals with property owners: it simply has to book time to come along and re-equip the base stations.
"We can use all these sites for our 4G deployment," he says. "This company has the highest density microcell network in the world. It has 150 base stations per square kilometre. And it's up and running."
The Wireless City Planning Network operates with 30 megahertz of bandwidth on the 2.5 gigahertz band.
These small base stations are not really full-scale base stations, says Chika, who uses the term "cloud base stations" for this architecture. The equipment on rooftops is antennas, connected by fibre to a central hub, located in a local telephone exchange: SoftBank calls this hub a baseband unit hotel.
This architecture will work only if the radio antennas - the microcell base stations - are connected via fast fibre links to the hub. Because the microcell base stations are relatively simple, they need very high-speed connections into the core, of about 10 gigabits a second.
"But, here in Japan dark fibre is very cheap. It is only $40 per month for backhaul," he says. "We connect base station base band units to the local exchange carrier office. Therefore with the microcell [architecture], 100 sites work like one single base station." 
                      
                      
Radio interference 
                      
Cloud base station architecture is better than conventional cellular architecture at minimising radio interference, he adds. "Using cloud base stations, we can use interference cancellation technology better than before. It's like a very, very intelligent single base station."
In the normal architecture, every base station is independent, he points out. And that also means they use more power, because they have the full range of components and circuitry.
"Because of the architecture, the power consumption [of the cloud base station architecture] is a lot smaller for each site. We achieve half of the power consumption of each antenna site," he says.
That gives SoftBank significant cost advantages: "We can reduce size of the power generator on each site," he says, "and capex and opex are also much lower. Less power consumption, less components and less weight."
Each microcell unit is small, about 20 litres, "like a suitcase", he says. "Just one box and the antenna. That also makes it easier to get approval from the building owner." And the small size of the antennas means there is plenty of room to expand them in future.
"They are mainly on the top of buildings," he says. "These are multi-antenna systems." The system uses MIMO - multiple input, multiple output - but that needs multiple antennas. "Multi-antenna is the way to be. Our sites are eight-antenna systems, so we have a lot of space to expand in the future. Usually, cellular sites have two antennas per each sector. To give them four or eight antennas is very difficult."
The existing 160,000 base stations are being used for the commercial launch of the Wireless City Planning network, but eventually there will be more, though Chika won't say how many. We will use these sites for our 4G deployment. There is a huge, huge need because smartphones are going to be very popular. With microcells the most difficult challenge is to get the sites. But because these are existing sites, it is very easy for us to get approval from the building owners." 
                      
                      
Higher resilience 
                      
And the cloud base station architecture is "more robust", he adds, because it is more resilient - so many antennas close together in a tightly-managed cloud means the system can cope with the occasional failure.
A conventional base station that covers a square kilometre of a city centre will leave that area without coverage if it fails. If there are 150 microcells in a square kilometre, as there are with the Wireless City Planning system, the failure of one will have little impact.
SoftBank is working on an aggressive roll-out campaign. "We are planning to cover all big cities in Japan within this year," says Chika. "If we need we can quickly install more sites because we have a lot of sites. We have the technology to reduce interference and we can operate at very high speed."
He believes this architecture will be right for other urban operators such as AT&T and Verizon Wireless in the US. "Not only SoftBank but other operators such as AT&T and Verizon have capacity issues right now. The only way to solve it is microcells. It is essential to the future of New York, London, Paris, wherever there is a big city."
Huawei, though it has played a prominent part in the project, is not the only vendor. ZTE is supplying some base stations, says Chika, though he will not divulge the split between the two rival vendors. "The core network is provided by Ericsson, and the access network is from Alcatel-Lucent," he adds.
SoftBank will run its WCP network in parallel with its existing 3G network, which uses the GSM family's WCDMA standard. "We have a very nice 3G nationwide network. The capacity issue applies to urban network only," says Chika.
That means that SoftBank is using dual-mode 3G and 4G terminals - and also dual mode in that 3G uses FDD but the 4G LTE network uses TDD.
So the WCP network will be urban, and the 3G network will be for less dense areas. Ultimately though, SoftBank presentations hint that everything will eventually migrate to LTE, with a cocktail of frequency bands and cell sizes that are carefully coordinated - so that macro cells will use 700-900 and 1800-1900 megahertz, microcells will be on 2100, with picocells on 2600 and 3500 megahertz.
There are, believes Chika, two major challenges for SoftBank's Wireless City Planning project: to provide true mobile broadband. And to launch before China. If the commercial launch at the end of February goes well, he will be well on the way. GTB 
                      
Further reading from Global Telecoms Business: 
DirecTV launches TD-LTE in Brazil 14 Dec 2011
China Mobile to expand TD-LTE in 2012 24 Nov 2011
Sprint 'to introduce LTE' in 2012 29 Sep 2011
China Mobile to try TD-LTE in six cities 28 Mar 2011




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