Distributed Antenna Systems grew from the need to provide wireless coverage and capacity to areas of highly concentrated users. More recently, as capacity and coverage demands expanded and some municipalities passed strict edicts against constructing towers, DAS systems got deployed along streets to provide service in the urban and suburban outdoors. Another solution to add capacity and coverage uses compact base stations which are getting large attention from both a cost and performance perspective. From a deployment perspective, they provide similar network architecture to DAS, which raises the question on how these two solutions compare. This application note will highlight the areas where each solution makes economic and technical sense.

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Application Note
Distributed Antenna Systems and Compact Base Stations:
When to Use Which?
By Frank Rayal, VP, Product Management & Marketing
Overview
Distributed Antenna Systems, or DAS, grew from the need to provide wireless coverage and capacity to
areas of highly concentrated users. This includes indoor settings like office buildings, convention centers,
airports and train stations, and outdoor settings like stadiums, campuses and plazas. More recently, as
capacity and coverage demands expanded and some municipalities passed strict edicts against
constructing towers, DAS systems got deployed along streets to provide service in the urban and
suburban outdoors. In all cases, DAS serves to distribute wireless services where needed and in the
process provide high capacity and excellent coverage. By ‘distribute,’ we mean serving a relatively small
area which limits interference and enables greater frequency reuse factor, consequently leading to
greater capacity. The ability to place antennas almost anywhere makes DAS systems perfect to reach
areas that are otherwise difficult to serve.
Another solution to add capacity and coverage uses compact base stations which are getting large
attention from both a cost and performance perspective. From a deployment perspective, they provide
similar network architecture to DAS, which raises the question on how these two solutions compare.
This application note will highlight the areas where each solution makes economic and technical sense.
In particular, we will address the concept of compact base station deployment with wireless backhaul
and highlight the benefit of BLiNQ’s solution in enabling a network architecture with low cost of
ownership.
DAS, RRH and Compact Base Stations
DAS has developed from the need to extend the service of legacy base stations. These base stations
consisted of a rack of equipment where baseband and radios are housed in the same chassis. DAS
systems include a RF‐to‐optical converter which digitizes radio signals and sends the data over a fiber
optical cable to a remote unit which in turn converts the optical signal into an RF signal as shown in
Figure 2. The data rate on the fiber cable is very high – on the order of Gbps. There are different
possibilities in deploying such a system where multiple remote units can be daisy‐chained and
conversion nodes used sparingly where capacity and coverage are required. DAS allows the operator to
concentrate baseband capacity in one location, such as a building basement, and use fiber cable laid to
different parts of the building. As higher capacity is required, the number of remote nodes per baseband
module is reduced (i.e. number of daisy‐chained nodes is reduced) while more baseband modules are
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added to address capacity requirements. Typical DAS systems start with 1:4 or 1:6 remote modules per
baseband module, and the ratio is reduced as higher capacity is required.
Antenna
Indoor Outdoor
Backhaul Optical Fiber Cable
Baseband RF‐Optical Optical‐RF
RF
Processing Converter Converter
Wireless Base Station Distributed Antenna System (DAS)
Figure 2 Block diagram for a Distributed antenna system.
In recent years, base station architecture evolved from a centralized to a split architecture where a
remote radio headend is connected to baseband via a fiber optical cable as shown in Figure 1. This
allows the radio itself to be placed close to the antenna where coverage is required. Baseband resources
can still be housed and ‘packed’ in a chassis which allows scalability of capacity. This base station
architecture provides similar usability to DAS with a cost reduction as RF/optical converters are
eliminated. The fiber cable connecting baseband with the RRH still runs very high data rate that can be 3
Gbps (as in OBSAI 3.01) or even higher in future generation base stations.
Antenna
Indoor Outdoor
Backhaul Remote
Baseband Optical Fiber Cable
Radio
Processing
OBSAI / CPRI Interface Head
Wireless Base Station
Figure 1 Block diagram for a split architecture base station.
Moving from a centralized to split architecture configuration represents an important transformation in
network operator’s deployment process as active electronics are deployed outdoors on pole or on tower
tops, an idea that was not acceptable earlier to maintain high reliability and enable redundancy in the
base station. Having broken through that barrier, it becomes natural to adopt deployment of zero‐
footprint, all‐outdoor base stations where the baseband processing is moved outdoor and integrated
with the radio into one mechanical package as shown in Figure 3. Each compact base station is
backhauled through a wireline (fiber included) or wireless connectivity.
To summarize the architectural perspective, DAS features a centralized base station architecture that
includes baseband and radio in one location which is then made decentralized by using an ‘applique’
optical/RF converters to ‘distribute’ the radio modules. Split base station architecture, using remote
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Antenna
Outdoor
Backhaul Remote
Baseband
Radio
Processing
Head
Compact Base Station
Figure 3 Compact base station block diagram.
radio headends, represents an evolution over DAS systems where the decentralized architecture of the
base station obviates the need for expensive optical/RF converters, and finally the compact base station
architecture is a complete decentralized baseband and radio architecture. Compact base station
therefore provides a capacity as high as a 1:1 baseband‐to‐remote ratio deployment of a DAS or RRH
system.
Aside from architecture, there are similarities and differences in how these systems are connected to
the core network. DAS systems concentrate baseband resources in one central location. Therefore,
backhaul capacity at one location needs to accommodate that capacity which typically means that fiber
backhaul or very high capacity microwave link is used (sufficient to accommodate multiple baseband, or
in other terms, base station instances). In legacy systems, backhaul through a high capacity leased line
may be used.
Split architecture base stations are backhauled in a very similar manner to DAS systems because they
also feature centralized baseband. However, the compact base station architecture offers a different
requirement for backhaul: the backhaul capacity is distributed and is on the order of the capacity of a
baseband unit. Therefore, if fiber is used to backhaul compact base stations, the capacity required is on
the order of Mbps and not Gbps as is the case in DAS and RRH systems. This opens the possibility to use
non‐line‐of‐sight wireless backhaul with compact base stations.The following figures show network
diagrams for DAS and compact base stations utilizing wireless backhaul installations.
DAS, RRH and compact base station solutions provide similar use case and benefits to network
operators. Therefore, it can be easy to see them as competing solutions. Yet, this is not the case since
each solution has a different cost depending on the deployment scenario. It becomes important to
identify a framework which helps us identify which solution is cost effective per the desired application
scenario. Architecture and backhaul configuration are two key elements in this framework: they indicate
which type of system to deploy as they impact the cost structure and place technical constraints on the
deployment scenarios. We will explore this framework in the next section focusing on outdoor
deployments.
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Figure 4 Network diagram for DAS installation.
Figure 5 Network diagram for compact base station with wireless backhaul.
Deployment Considerations
System architecture and backhaul are two key criteria to evaluate in the deployment of capacity or
coverage enhancing systems. Both impact the installation cost. For example, DAS and RRH installations
require fiber optical cable connection between baseband and every remote node. Compact base
stations on the other hand require backhaul which can be fiber or wireless. In case the backhaul is fiber,
the application becomes similar to that of DAS and RRH in terms of cost (especially when dominated by
capital and not operational expenditures). However, when it is possible to use wireless backhaul,
considerable cost savings can be achieved. So, when can wireless backhaul of compact base stations be
used?
BLiNQ’s non‐line‐of‐sight wireless backhaul solution is a point‐to‐multipoint system that allows backhaul
of up to four compact base stations using a single hub module. Each hub module operates in Time
Division Duplex (TDD) mode on a 10 MHz channel in sub‐6 GHz licensed spectrum band (e.g. 2.3‐2.4, 2.5‐
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2.7 or 3.3‐3.8 GHz). The amount of backhaul spectrum and compact base station density determines the
suitability of the wireless backhaul solution. If N is the number of available 10 MHz backhaul channels,
BLiNQ’s systems can be used to backhaul up to 4N compact base stations concentrated in a single
geographic area (e.g. a circle of 500 m in radius). This is because backhaul frequency reuse is required
for sufficient signal quality and it is not possible to achieve sufficient reuse factor while covering an
overlapping area. If the area is non‐overlapping, the same backhaul frequency can be used by leveraging
antenna directivity at the hub and BLiNQ’s interference mitigation techniques to achieve sufficient
separation in the frequency reuse plan of the backhaul network. Therefore, taking the example of an
outdoor stadium, it would only be possible to backhaul up to 4 compact base stations with one 10 MHz
channel. To backhaul a higher number of compact base stations, additional backhaul channels will be
required since the backhaul channels will overlap in coverage over the stadium resulting in interference
that degrades wireless backhaul performance, as shown in Figure 6. However, if we look at the example
of a campus or urban center deployment, as shown in Figure 7, where it is possible to reuse the same
backhaul channel by leveraging the hub antenna directivity and interference mitigation techniques,
wireless backhaul provides a highly cost effective solution to connect multiple compact base stations to
a central location (such as a macro cell where high capacity fiber or microwave backhaul is already
available) and thereafter to the core network.
Success in using wireless backhaul requires sufficient frequency isolation similar to frequency planning in
access systems (although less stringent in wireless backhaul due to use of directional antennas which
limit interference as well as the presence of fewer remote backhaul nodes than there are subscribers in
access systems). Adequate frequency reuse factor of the backhaul network becomes more challenging in
smaller size areas and very high concentration of compact base stations. While an outdoor stadium may
not be the ideal deployment scenario for non‐line‐of‐site backhaul system, especially if more than 4N
nodes are required, a street deployment to cover a neighborhood, a plaza, a pedestrian mall, a campus
or other such venue presents a very cost effective alternative to outdoor DAS systems. In such a
deployment scenario, enough separation can be achieved to reuse the backhaul frequency.
Figure 6Deployment of compact base stations with wireless backhaul in a stadium. Multiple backhaul
channels required to achieve sufficient isolation.
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Figure 7 Campus or urban deployment of compact base stations: isolation between adjacent sectors
allows cost effective wireless backhaul.
We should also note that the areas where wireless backhaul succeeds in providing a viable technical
solution and business case, DAS fails to provide the required economics and vice versa. This is because
fiber costs escalate with distance between the baseband and remote nodes in the case of DAS while
wireless backhaul reaches its limitations when there’s a large density of remote nodes in one
location.Table 1 below illustrates a simple guide on which technology is most suitable given the density
of remote nodes and length of fiber cable runs.Therefore, we view DAS/RRH and compact base station
deployments as complementary solutions where one provides a better business case than the other
depending on the deployment scenario.
Table 1Preferred technology for different deployment scenarios.
Low Density of Remote Nodes High Density of Remote Nodes
Long Fiber Run C‐BTS with Wireless Backhaul N/A
Short Fiber Run DAS or C‐BTS w. Wireless Backhaul DAS/RRH
Cost Drivers
Compact base stations provide an intrinsically lower capital cost solution than RRH‐based systems which
are an evolution of DAS. DAS systems are expensive because they require optical/RF converters at both
ends. Newer split architecture base stations inherently use fiber to connect the baseband with the RF
module resulting in lower cost. Since compact base stations combine baseband and RF into a single
module, the cost of fiber cable, optical fiber transceivers and the electronics associated with the OBASI
or CPRI interface is eliminated. Moreover, the baseband module chassis, traffic aggregation modules
and power supply modules are eliminated as well. This results in significant savings in equipment capital
expenditure of compact base stations over DAS/RRH systems.
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Admittedly, equipment cost is not generally the main cost driver, rather, it is the cost of fiber in the case
of DAS and the cost of spectrum for wireless backhaul that are the main cost drivers. Cost of fiber varies
depending on location (from one neighborhood to another in a city) while spectrum cost varies on a
country and region basis.Table 2shows typical cost of fiber in North America while it must be noted that
cost can exceed the ones indicated below in certain municipalities and dense urban centers such as
Manhattan and San Francisco. Spectrum costs have been about 2 euro‐cents per MHz‐PoP as per recent
auctions in Europe.
Table 2 Cost of Fiber.
Deployment Costs Aerial $4.5‐$11.5
(per meter; includes right of way Rural $10‐$30
and renovation construction Trenching Suburban $30‐$100
works) Urban $80‐$230
Fiber Cost
(per meter; includes cable, $5‐$12
connector, & testing)
Fiber Lease Cost (per month) Variable > ~16/Mbps
Table 3 Example of NLOS wireless backhaul spectrum pricing.
Country Operator Frequency Band Channel Size Price
Germany Vodafone 2.6 GHz 1x5 MHz € 9,051,000
Germany Clearwire 3.5 GHz 2x21 MHz € 20,000,000
UK UK Broadband 3.5 GHz 2x20 MHz £7,000,000
Netherlands WorldMax 3.5 GHz 20 MHz € 4,000,000
Austria WiMAX Telecom 3.5 GHz 2x28 MHz € 155,000
Greece Cosmotel 3.5 GHz 2x14 MHz € 20,475,000
Poland Clearwire 3.6 GHz 2x14 MHZ PLN 1,400,000
Canada Several Operators 3.5 GHz 2x25 MHz $11,240,615
System Costs Estimation
We assume that the network operator will deploy their own fiber in case of DAS & RRH deployments. In
this case, the annual operating expense for fiber is very low since the operator averts paying monthly
fees. This scenario generally leads to a better business case for DAS & RRH deployments since fiber lease
expenses can be very high, often ranging around $1,000 ‐ $1,500 in monthly fees, which is at least
$50,000 in operating cost over a 5 year period1. Hence, we focus on capital expenditure as,for the
assumptions made, the operating expenditure for each case would be very similar.
1
The present value of fiber optical cable leased at $1,000 per month with $1,500 initial setup fee is $51.7k
assuming 2% inflation rate and 12% discount rate.
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The cost for each solution is shown in Tables 4, 5 and 6. It is no surprise that the main cost driver is the
cost of fiberin the case of DAS & RRH deployments. Outdoor DAS is further burdened by the need for
additional optical converter modules which are ‘applique’ modules to existing base stations. RRH/base
station hotel concept deployment has similar economics to outdoor DAS, but makes use of evolved base
station technology to eliminate the RF/optical converter modules. Compact base station deployment
with wireless backhaul, when it is possible to implement, is the lowest cost alternative – by as much as a
factor of 4 in case of RRH deployment and a factor of 6 in case of outdoor DAS.
Table 4 Estimated capex costs for compact base station deployment with wireless backhaul.
Compact Base Station $2,500 Assume 1 W Micro BTS
Remote Backhaul Module $2,500 Representative cost – not actual BLiNQ product pricing
Backhaul Hub Module $2,500 Assume 1:1 (PTP) backhaul configuration. Cost is lower for
PMP
Spectrum / Link $1,000 Assume $20 m for 20 Year license and 1000 Links per
network
Total (C‐BTS) $8,500
Table 5 Estimated capex costs for remote radio headend deployment (base station hotel).
Remote Radio Headend $1,000 1W RRH
Base Station Baseband / $2,540 Per RRH ‐ BTS consists of 10‐sector chassis with following
Sector assumptions: $1,000 for chassis; $2,000 per baseband card;
$200 per power card; $2,000 for one control card.
Optical Fiber Cable $24,000 Assume $240/m for underground run of 100 m, includes
Construction materials, right‐of‐way and construction costs.
Total (RRH/BTS Hotel) $27,540
Table 6 Estimated capex costs for outdoor DAS.
BTS (per sector) $4,000 Assumed cost for a single sector of a standard base station
RF/Fiber Converters $5,000 Cost of converter at base station site.
Remote Radio $3,000 Cost includes optical‐to‐RF converter and the radio.
Optical Fiber Cable $24,000 Assume $240/m for underground run of 100 m, includes
Construction materials, right‐of‐way and construction costs.
Total (Outdoor DAS) $36,000
Conclusions
DAS, RRH and compact base stations provide solutions that distribute wireless capacity and coverage to
areas where service is needed. Traditional distributed antenna systems are ‘applique’ solutions used to
extend coverage and capacity of legacy base stations. They provide a viable business case for indoor
applications and highly concentrated outdoor structures like stadiums where very large subscribers are
located in one area such that very tight frequency reuse and high density of baseband resources are
needed to provide sufficient capacity. Enhancements of base station architecture allowed remote radios
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to be placed outdoors, collocated with the antenna on top of the tower, a building rooftop, or on a pole.
This further reduced the cost associated with legacy outdoor DAS systems by eliminating the RF/optical
converters of traditional DAS. Finally, compact base stations represent a further evolution where the
baseband and radio are collocated outdoors which presents an attractive cost reduction for the network
operator. Although compact base stations backhauled through wireline technologies, mainly fiber,
provide a similar business case as DAS/RRH deployment, they can offer significant cost savings when
NLOS wireless backhaul is used. However, there are limitations on the use of NLOS wireless backhaul
related to backhaul frequency reuse plan. Therefore, DAS/RRH and compact base stations can be viewed
as complementary technologies each succeeding in offering a competitive business case for a certain
deployment scenario. A framework based on density of nodes and length of fiber is introduced to assist
in determining the case where each solution is more competitive.
Acronyms
CPRI Common Public Radio Interface
DAS Distributed Antenna System
NLOS Non Line of Sight
OBSAI Open Base Station Architecture Initiative
PoP Per head of Population
RRH Remote Radio Headend
TDD Time Division Duplex
About BLiNQ Networks
BLiNQ Networks is a pioneer of backhaul self‐organizing network (B‐SON) solutions that fundamentally change the
way mobile operators deliver mobile broadband services. BLiNQ solutions provide the building blocks to cost‐
effectively and rapidly scale mobile data networks. The intelligent systems are designed to continuously adapt to
changing environments, maximize spectral efficiency, and are easy to configure, deploy, and maintain. For more
information, please visit www.blinqnetworks.com.
BLiNQ Networks Inc.400 March Road, Suite 240 Ottawa, ON Canada K2P 0E3  Main: +1 613.599.3388
Fax: +1 613.599.7228 Email: info@blinqnetworks.com www.blinqnetworks.com
© 2011 BLiNQ Networks Inc. 9