Yoshioki Chika: One of the big challenges of a mobile
broadband operator today is to get sites for base
LTE microcells replace older PHS base stations on Tokyo
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,
"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
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,
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.
"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
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
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
"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
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
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
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
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
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:
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