Embed Size (px)
Citation preview
Contents What exactly is a small cell?
Small cell technologies
Femtocell
Picocell
Microcell
Metrocell
DAS—the original small cell
Comparing key differences
Multicarrier support
Scalability
Quality of service
Cost
Conclusion
2
Introduction
The need to increase wireless coverage and capacity within an increasingly
crowded ecosystem has led to a variety of alternative solutions and new
challenges for the owners and operators of today’s mobile networks. Each new
solution forces the industry to reconsider the landscape and assess how it all fits
together.
One of the more recent developments has been the use of “small cells” in order to
provide coverage and capacity indoors and out. Whether deployed as standalone
networks or integrated with the macro layer to create heterogeneous networks,
small cell solutions are being touted for their ability to help operators achieve
higher radio density and increased capacity.
These heterogeneous networks also allow operators to achieve much better fill-in
coverage and, by using small cell nodes to off-load traffic from over-burdened
macro sites, they are realizing higher data throughput as well.
At the same time, the use of distributed antenna systems (DAS) has exploded as
facility owners and operators rush to satisfy the growing demand for seamless
high-speed indoor coverage.
3
Introduction
Today, DAS networks are being deployed in a wide variety of locations, including
universities, sports arenas, stadiums, hotels, casinos, corporate campuses, malls,
airports and subways.
Worldwide spending on DAS is expected to total $4.4 billion in 2014. By 2019, it is
projected to grow to more than $8 billion, a 14 percent compound annual growth
rate (CAGR).1
The surge in small cell and DAS deployments has led to comparisons between the
two solutions as network operators and owners attempt to determine the best
strategy for specific applications. Therein lies the problem. Vendors on both sides
have offered numerous white papers advocating for their specific technology. The
industry media has addressed the issue in a number of opinion, as well as fact-
based articles.
The question of DAS versus small cell is one of the more common issues raised
by those who are looking to improve capacity and coverage, both indoors and
outdoors.
Untangling the question requires a deeper look at both technologies.
4
What exactly is a small cell?5
At the heart of the DAS-versus-small-cell debate is the often confusing and
constantly evolving definition of a small cell network. According to ABI Research,
small cells can be characterized as low-powered radio access nodes that operate
in licensed and unlicensed spectrum that have a range of 10 meters to 1 or 2
kilometers.2 The term “small” refers to the physical footprint of the solution,
compared to a traditional macro cell.
On its website, the Small Cell Forum adds a bit more specificity to the definition,
stating that: “‘Small cells’ is an umbrella term for operator-controlled, low-powered
radio access nodes, including those that operate in licensed spectrum and
unlicensed carrier-grade Wi-Fi.”
While this definition adds a bit more clarity, it adds confusion as well. Does the
term “operator controlled” suggest that small cell is exclusively a single-operator
solution? Must it be owned by the mobile operator or can it be owned and
operated by a neutral host provider?
Still other industry resources and literature characterize small cells as low-
powered solutions that have a small physical footprint and are “typically deployed
piecemeal to provide coverage or enhance capacity in much smaller areas with a
single wireless communications technology for
a single wireless carrier.”
What exactly is a small cell?6
There is also confusion when identifying the types of technology solutions
that fall under the small cell rubric. The four types of small cell solutions
listed by the Small Cell Forum, femtocells, picocells, metrocells and
microcells are loosely defined by their general power output and the
coverage radius provided by each.
Others do not necessarily agree with these categories. According to Dr.
William Stallings, a well-known industry blogger, “An essential component
of the 4G strategy for satisfying demand is the use of picocells and
femtocells. Together, these are classified as small cells.”5 Stallings’
omission of microcells and metrocells is both curious and confusing.
Small cell technologies 7
Suffice it to say that there appears to be no concrete agreed-upon
definition for exactly what a small cell network is or the technologies it
includes. Therefore, the question, “DAS or small cell?” is really a
nonstarter. In order to accurately compare the available technologies, one
must make direct comparisons between DAS and all possible small cell
technologies. The following is a brief overview of each of the small cell
technologies.
Femtocell8
A femtocell is characterized as a very low-range, low-power base station,
able to be deployed in a home, home office or very small business. The
coverage range is usually less than 30 meters and the output power is
around 20 dBm.
Within the home or small office, the femtocell operates like a miniature
macrocell, providing consistent and reliable coverage for a limited number
of users. In most cases, the femtocell is owned or leased by the user who
operates and manages it. So, from the mobile operator’s perspective, it is
an unmanaged asset.
Femtocells operate in the same licensed frequency bands as macrocells
and support various mobile air interfaces, including UMTS and LTE. For
backhaul, the femtocell requires a connection—typically fixed line—to the
mobile network operator.
Femtocell9
When deployed to serve a limited number of pre-approved users, the
femtocell provides a good coverage solution. When deployed with any
density across a moderately-sized facility, however, interference can
quickly become an issue, requiring careful power control in order
to manage it. Quality of service can also become an issue as femtocells
utilize the network’s broadband connection, limiting the bandwidth
available for other broadband applications.
There is also the potential for conflicts with service-level agreements if the
provider of the broadband service differs from the mobile network
provider.
Picocell10
Picocells operate on the same principles as femtocells. A dedicated BTS
feeds the remote radio heads and antennas, creating a network of very
small individual cells. Whereas a femtocell is typically owned and
managed by the user, its slightly larger cousin, the picocell, is usually
owned, operated and managed by the mobile operator. These solutions
are primarily deployed indoors. Typically, a femtocell can serve only
somewhere between 4 and 16 simultaneous users, whereas a picocell
may be able to handle up to 100 users.
As with femtocells, if the building is small enough to be served by a single
picocell, these units are an ideal solution for coverage and capacity. In
larger environments, however, deploying multiple cells can create
interference problems. Like Wi-Fi access points, femtocells must
alternate channels to avoid co-channel interference, and doing this
requires carriers to use a lot of frequency in a small area given the
relatively small coverage footprint of the solution.
Picocell11
The technology also does not lend itself to supporting multiple carriers.
Picocells as well as femtocells are generally single-frequency devices, so
providing coverage for multiple mobile operators requires the network
operator to deploy a separate set of small cells for each frequency to be
covered. This scenario quickly becomes cost prohibitive and space
prohibitive.
Microcell12
Microcells are among the largest of the small cell solutions, operating at
an approximate power output of about 30 dBm and providing a coverage
radius of up to 500 meters. These solutions are most often used as part of
a heterogeneous network to enhance outdoor coverage in areas where
surrounding obstacles prohibit the use of macro cells. Occasionally they
are deployed indoors to add network capacity in areas with very dense
phone usage, such as train stations or shopping malls. The technology is
also used to increase capacity of cellular networks to offload usage during
peak hours.
Microcells appear to be a technology in search of a clear position.
Currently they are sandwiched between the smaller femtocells and
picocells (which are clearly targeted to smaller indoor deployments) and
the larger metrocells (which have found a niche in extending outdoor
coverage and capacity). As a result, the microcell market is experiencing
low growth. Revenues are expected to reach only $262 million by 2018.
this reflects a significant downgrade from 2013 expectations, which
forecasted sales would reach $3 billion by 2017.
Metrocell13
Metrocells are low-power single-sector-channel, independent small cells
that can support several hundred users. Combining a small independent
BTS and antenna, they are often deployed on lampposts or sides of
buildings to support mobile data demand in dense metropolitan areas.
Relative to a macro system, metrocells are easy, faster and less
expensive to deploy, making them an excellent choice for urban infill,
wireless backhaul, and offloading a portion of the macro network traffic.
Operating at about 5 watts of power, metrocells have a coverage radius
of about 600 to 1200 feet (approximately 183 to 366 meters). With the
exception of very large and open venues such as airports and stadiums,
metrocells are typically limited to use outdoors.
DAS14
Among the various definitions of small cells, DAS is rarely mentioned,
despite the fact that the technology has often been referred to as the
original small cell. In August 2014, an article in OSP Magazine observed:
“Long before small cells (femtocells, picocells, metrocells, and the
like) were invented, DAS was providing pinpoint mobile coverage and
capacity to buildings and outdoor areas. In many ways, DAS was the
original small cell.”
A typical DAS solution consists of a centrally located radio or headend
equipment, remote communications nodes, and a high-capacity transport
network—typically fiber—to connect the nodes to the central equipment.
The remote nodes are distributed in various IDF forms throughout the
facility or area. The antennas are distributed throughout the facility and
connect to the remote units via coaxial cable.
Comparing key differences 15
Despite the general similarities in power output, coverage area and size,
DAS and the various small cell solutions differ greatly in terms of how
they operate. DAS is a point-to-multipoint solution in which the DAS
headend shares and receives signals with all remote nodes
simultaneously. By simulcasting radio channels throughout the building, it
creates a single large cell, as opposed to the network of individual cells
typical of the various small cell solutions.
Centralized power management enables the DAS operator to change the
coverage and capacity characteristics of each node in order to respond to
changes in the RF environment.
Note: CDMA, GSM, and P25 technologies are also used in DAS
solutions.
Comparing key differences16
Solution Description Technology # Users Cell Radius
DAS Typically fed by a macro or micro base
station. High power, multifrequency,
multi-carrier.
UMTS
HSPA +
LTE
Up to 1800
users per
base station
Up to 3 miles
Wi-Fi A Wireless access port connects a
group of wireless devices to an adjacent
wired LAN.
802.11b
802.11g
802.11n
Up to 200
users per a
3-radio
access point
65 feet
Microcell Short-range base station used for
enhancing indoor and/or outdoor
coverage.
UMTS
HSPA +
32 to 200
usersUp to about
1 mile
Metrocell High-capacity, low power device that
fills in coverage holes within
buildings.
UMTS
HSPA +
16 to 32
users10,000 –
20,000 square
feet
Picocell Typically used for indoor applications
such as office buildings, airports, and
malls.
UMTS 32 users Up to 750 feet
Femtocell A small, low-power cellular base station
typically used for a home or small
business.
UMTS 4-6 users 40 feet
Comparing key differences17
As a result, DAS and small cells offer significant differences in terms of
functionality, interference issues, capacity, complexity and cost. It must be
stressed that these network architectures and technologies are not
interchangeable, and each is suitable only for the particular purposes and
environments it is designed to address. So it is important that network
owners and operators have a clear understanding of the strengths and
weaknesses of each.
Multicarrier support 18
One of the biggest differences between DAS and femtocells, picocells
and microcells is the ability to support multiple carriers. DAS systems can
be shared by multiple operators, each connecting their own base stations
to the shared RF distribution system. As a result, DAS allows carriers and
venue owners to take advantage of neutral host opportunities in which
the CapEx of the network can be shared by all participants, making it
more affordable. As a class of solutions, small cells typically do not
provide multicarrier support. Although multiband/multi-technology small
cells are in development—and may eventually support more than one
operator—today’s systems are highly limited in this regard.
Scalability 19
DAS was designed to scale in order to meet the growing needs of the
network. By adjusting the power to the antennas, a single BTS can serve
up to about 1,800 users and provide a coverage radius of several miles.
Picocells and femtocells were designed to deliver coverage and capacity
over a relatively small area, similar to a Wi-Fi access point (WAP). Adding
more coverage requires installing more nodes.
DAS solutions are also multi-frequency, able to handle 2G, 3G and 4G
commercial frequencies that operate in a range from 700 to 2500
megahertz (MHz), as well as public safety UHF and VHF frequency
bands (e.g., 150 and 450 MHz band channels). Femtocells
remain exclusively a single-frequency, single-carrier solution. Multi-
frequency picocells are not yet widely available but manufacturers expect
to ramp up production sometime in 2015.
Quality of service 20
As stated earlier, a DAS network functions by creating a single unified cell
with blanket coverage within its prescribed area. This eliminates multicell
interference along with the need to hand off from one cell to the next as
the user moves about. While potential exists for interference from nearby
macro networks, this is easily managed by adjusting the power at the
DAS headend or the power amplifiers, if they are in use.
Quality of service within the DAS network, therefore, is excellent. The
large capacity of a DAS enables it to be used in tightly packed venues
such as sports stadiums, where 50,000 users or more may be
downloading data, posting photos, etc. The system also provides the
ability to dynamically adjust to changes in capacity demands per area and
per carrier.
Quality of service21
Femtocells, picocells and microcells operate on a different principle.
These small cell solutions create a network of discrete cells, each with a
fixed and fairly limited capacity and coverage.
For defined and small “rifle shot” applications, small cells provide
excellent quality of service.
Their short range and ability to detect and adjust to other femtocells in the
area help to negate multicell interference.
This does not mean small cell solutions are immune to service issues.
When used for larger applications involving dozens of nodes, the potential
for interference increases significantly.
The sheer number of cells in use and the carrier’s inability to control their
position and use—as well as issues with the handoff between these ad
hoc cells and the overall network—create significant challenges in
spectrum and interference management. Small cells can also
experience interference problems when using low-band spectrum, and
diminished range when using high-band spectrum.
Cost22
In comparing DAS versus small cell solutions, cost is perhaps the most
curious characteristic of all. It also makes a convincing case for the basic
premise of this paper: making the best decision for a specific application
requires a solid understanding of the strengths and weaknesses of each
solution.
One argument against DAS has been high costs to deploy with RF
engineers. In fact, when deployed for smaller, low-density applications,
DAS may not be the best choice. The reason is simple: DAS is not
designed to excel in these types of scenarios. Femtocells, picocells and
microcells, however, are much better suited for these applications. In
order to support a few dozen users and a single carrier, small cells maybe
the better choice for these applications.
Cost23
As capacity and coverage requirements increase, the cost analysis
begins to tip in favor of DAS. For large deployments, it is much less
expensive to deploy a DAS for in-building coverage than to deploy
dozens or hundreds of picocells or femtocells. Operating expenses are
lower as well in a multi-carrier environment.
The challenge for the network operator or owner is determining precisely
where that tipping point is. In addition to cost, one must also consider the
importance of functional needs, interference control, scalability and the
other characteristics discussed here.
Conclusion24
If the wireless industry has learned anything over the past two decades,
it’s that—when it comes to coverage and capacity solutions—there is no
magic bullet, no one-size-fits-all solution. For most mid- to large-size
indoor scenarios, DAS has definite advantages over nearly all small cell
solutions. In small indoor applications with low capacity needs, small cells
provide a greater advantage over DAS.
In selecting the most appropriate solution for a given application, it is
important that network operators, mobile carriers and facility owners put
aside any preconceptions. The best solution—which, in some cases, may
actually call for both DAS and small cell—will be dictated by the details of
the project and objectives of the stakeholders.
As Tracy Ford of The DAS Forum noted in a 2013 article for BICSI,
“Taking a long-term approach to wireless coverage by working with all the
stakeholders in the ecosystem will ensure that the installation meets
current and future needs without having to re-address
coverage and capacity issues in the future.”
25
