distributed antenna systems and compact base stations: when to use which?

© 2011 BLiNQ Networks Inc. 1

ApplicationNote

DistributedAntennaSystemsandCompactBaseStations:WhentoUseWhich?

ByFrankRayal,VP,ProductManagement&Marketing

OverviewDistributed 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,RRHandCompactBaseStationsDAS 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


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.

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.

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

Baseband Processing

RF RF‐Optical Converter

Optical‐RF Converter

Antenna

Optical Fiber Cable

Wireless Base Station Distributed Antenna System (DAS)

Backhaul

Outdoor Indoor

Figure 2 Block diagram for a Distributed antenna system.

Baseband Processing

Remote Radio Head

Antenna

Optical Fiber Cable

Wireless Base Station

OBSAI / CPRI Interface

Backhaul Outdoor Indoor

Figure 1 Block diagram for a split architecture base station.


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.

Baseband Processing

Remote Radio Head

Antenna

Backhaul

Compact Base Station

Outdoor

Figure 3 Compact base station block diagram.


Figure 4 Network diagram for DAS installation.

Figure 5 Network diagram for compact base station with wireless backhaul.

DeploymentConsiderationsSystem 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‐


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.


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

CostDriversCompact 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.


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

(per meter; includes right of way

and renovation construction

works)

Aerial $4.5‐$11.5

Trenching

Rural $10‐$30

Suburban $30‐$100

Urban $80‐$230

Fiber Cost

(per meter; includes cable,

connector, & testing)

$5‐$12

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

SystemCostsEstimationWe 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.


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 / Sector

$2,540 Per RRH ‐ BTS consists of 10‐sector chassis with following assumptions: $1,000 for chassis; $2,000 per baseband card; $200 per power card; $2,000 for one control card.

Optical Fiber Cable Construction

$24,000 Assume $240/m for underground run of 100 m, includes 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 Construction

$24,000 Assume $240/m for underground run of 100 m, includes materials, right‐of‐way and construction costs.

Total (Outdoor DAS) $36,000

ConclusionsDAS, 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


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 InterfaceDAS Distributed Antenna System NLOS Non Line of Sight OBSAI Open Base Station Architecture InitiativePoP 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: [email protected] www.blinqnetworks.com

distributed antenna systems and compact base stations: when to use which?

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