Comba Telecom White Paper: Distributed Antenna Systems - Key Considerations for Designing a High...

1 ©2014 Comba Telecom. All Rights Reserved

DISTRIBUTED ANTENNA SYSTEMS:

KEY CONSIDERATIONS FOR DESIGNING A

HIGH CAPACITY WIRELESS NETWORK

December 2014

A Comba Telecom White Paper


TABLE OF CONTENTS EXECUTIVE SUMMARY .................................................................................................... 3

WIRELESS QUALITY OF SERVICE EVOLUTION ....................................................................... 4

In the Past .................................................................................................................................... 4

Now .............................................................................................................................................. 5

RF CONSIDERATIONS FOR HIGH CAPACITY DESIGN ............................................................... 6

High Data Throughput Requires HOM and High Code Rate ........................................................ 6

High Data Throughput Requires Multiple antenna techniques ................................................... 7

IMPLICATIONS ON DAS DESIGN ....................................................................................... 8

Antenna Densification and Increased Directivity......................................................................... 8

Sectorization and Interference mitigation ................................................................................. 10

CONCLUSIONS ............................................................................................................ 11

ABOUT COMBA TELECOM ............................................................................................. 12


EXECUTIVE SUMMARY

With the evolution of mobile networks and internet connectivity on the go,

the average consumer is no longer satisfied with merely voice services but

consuming increasing amounts of data services and demanding higher data

speeds.

Designing networks to achieve high data rates for 3G and 4G no longer

depends only on good received signal levels, but also the following

considerations: -

Signal-to-interference-plus-noise ratio (SINR)

Higher Order Modulation (HOM)

Air interface code rate

Multiple antenna techniques with MIMO (Multiple Input multiple

output)

A clean RF environment, indicated by high SINR is required to achieve HOM,

high code rate and MIMO. SINR in turns depends on careful deployment of

Distributed Antenna Systems (DAS).

Sectorization, a typical technique to increase total network capacity, follows

the law of diminishing returns. As the number of sectors increase, inter

sector interference rises and reduces the effective data rates. Antenna

selection and optimal placement must be deployed to minimize inter sector

interference.

This paper will cover these techniques to achieve high data rate and its

implications on DAS design.


WIRELESS QUALITY OF SERVICE EVOLUTION

Operators are constantly facing a balancing act between improving QoS

(Quality of service) in their networks and ROI (Return on Investment) on their

CAPEX (Capital Expenditure) in improving their networks with the aim of

increasing revenue streams and minimizing customer churn.

Independent wireless consultancy Real Wireless formulated a model as

illustrated in Figure 1. The red lines are indications of the expectations of

service levels from mobile operators over time. The upper boundary

indicates the limit of service levels that consumers are willing to pay for, and

the lower boundary indicates the minimum service levels that consumers

expect from operators.

FIGURE 1: BALANCING QOS AGAINST CUSTOMER CHURN WITH INDOOR WIRLESS SERVICES (SOURCE: REALWIRELESS)

As such, operators strive to provide QoS (as indicated by the blue line)

within the two boundaries: providing QoS over the upper boundary may not

generate extra revenue since consumers will be unwilling to pay for it, QoS

under the lower boundary increases the risk of customers switching

operators due to dissatisfaction with services.

In order to understand the various options available to operators in

improving QoS, the past, present and future directions of voice and data

traffic trends must be examined first.

IN THE PAST

In the early days of wireless networks, operators around the world were

primarily focused on rolling out a regional and/or national network at a

macro level. Considerations for wireless network reception within buildings

were frequently secondary. As the mobile phone increased in popularity and

usage, customer complaints about poor signal reception in buildings began


to soar. Faced with increasing churn as dissatisfaction grew, operators built

repeater-driven in-building DAS (distributed antenna systems). Whilst the

solutions addressed the coverage issues, most were built using the “-85dBm

rule” without regards to uplink, downlink, balancing and the like. Hence, the

initial terminology of “in-building coverage systems” was highly appropriate

and descriptive of the solution.

NOW

With the improved network coverage extended to within buildings, usage of

mobile phones has steadily increased within indoor environments. In 2013,

it has been estimated that over 85% of all mobile calls originated indoors. It

is therefore feasible to assume that wireless data connections follow similar

patterns.

FIGURE 2: 85% OF ALL VOICE CALLS ORIGINATE FROM INDOORS (SOURCE: INFORMA)

The arrival of 3G and the smartphone heralded the exponential increase in

demand for wireless data. As such, operators are applying “band aid”

solutions and upgrading their networks to cope with the capacity crunch.

Indoor wireless system QoS no longer revolved around the coverage

requirement of “-85dBm rule” but meeting the high throughput demands.


RF CONSIDERATIONS FOR HIGH CAPACITY DESIGN

HIGH DATA THROUGHPUT REQUIRES HOM AND HIGH CODE

RATE

Higher order modulation allows higher peak data rates without increasing

bandwidth by increasing the number of bits carried per symbol. HSPA

evolution and LTE uses modulation schemes beyond quadrature phase shift

keying (QPSK) such as 16 QAM (quadrature amplitude modulation) and 64

QAM.

Coding is deployed to combat bit errors due to RF transmission corruption.

The higher the code rate, the higher the effective data throughput as fewer

bits are used for error correction.

The combination of modulation order and coding rate is implemented as

various MCS (modulation and coding schemes). High MCS need to operate in

a clean RF environment where the signal to noise ratio (SNR) is sufficiently

good. Simulations in an AWGN (additive white Gaussian noise) channel show

how spectral efficiency is increased by using higher MCS which is only

possible with increased SNR.

FIGURE 4: RELATIONSHIP OF SPECTRAL EFFICIENCY VERSUS SINR FOR DIFFERENT MCS (SOURCE:

AALBORG UNIVERSITET, AALBORG UNIVERSITET, “MIMO TECHNIQUES IN UTRA LONG TERM EVOLUTION” NA WEI, SEP 2007)


HIGH DATA THROUGHPUT REQUIRES MULTIPLE ANTENNA

TECHNIQUES

Multiple antenna techniques such as MIMO further increases the peak

possible throughput by using spatial multiplexing. For 2x2 MIMO shown in

Fig 5, two simultaneous spatial streams of data are sent over the same air

interface resources.

FIGURE 5: 2 X 2 MIMO DATA TRANSMISSION

The space-time difference enables the receiver to decode the paths

separately and effectively combine the data from the multiple streams. In a

single user condition, MIMO increases the peak throughput of the target UE

(user equipment). In a multi user condition, MIMO increases the total cell

throughput.

The conditions for MIMO to be effective are a rich scattering environment

and good SINR. RF scattering creates multi-paths which reduces the signal

correlation. Throughput gains of MIMO over SISO also depends on where we

are operating on the throughput graph. In Figure 6, 2x2 MIMO is

theoretically able to double the data throughput. However it is hard to

achieve the high SINR levels that enables doubling of the data throughput in

practical implementation. Typically, SINR peaks at 30dB with throughput

gains of about 1.5 times.

Deploying a 2x2 MIMO system would require significant CAPEX investment

with a doubling of infrastructure feeder cables and upgrade to MIMO

antennas.

FIGURE 6: PERCENTAGE OF THROUGPUT INCREASE OF 2X2 MIMO VERSUS SISO (SOURCE: 3G

AMERICAS)

Transmit Antennas Receiver

H11

H21 H12

H22


IMPLICATIONS ON DAS DESIGN Scaling ever higher data rates will rely on high investment in hardware. To

maximize ROI, we need to create signal dominance through increasing the

number of antenna, reducing the interference through using directive

antennas and innovative mounting techniques.

ANTENNA DENSIFICATION AND INCREASED DIRECTIVITY

The main interferer to indoor systems comes from the outdoor macro cells.

Therefore, indoor signal strength should be increased to overcome macro

interference especially at the edges of the building e.g. near the windows.

Increasing the density of indoor antennas has a direct benefit of improving

SINR.

Low In-building sites such as shopping centers typically experience less

interference from macro sites due to the penetration loss from surrounding

terrain and buildings. High rise buildings like office towers tend experience

more external interference from macro sites. Line of sight spillage from the

upper sidelobes of outdoor antennas combined with little blockage from

surrounding buildings results in multiple interferers and poor SINR.

A design approach is to measure the signal strength of the macro cells and

design the indoor DAS coverage to overpower the interferers. Signal

dominance can also be created by the use of panel antennas instead of omni

antennas. To ensure sufficient power of the indoor systems, active DAS can

be deployed. Active systems deploy a fiber optical repeater system that

overcomes the coaxial loss in deploying passive systems in high rise towers.

A case study of a high rise office above 60th

level in Fig 7, shows the existing

IBS design based on the “-85dBm” principle with 3 omni antennas. The signal

quality is poor with 3G pilot EcNo < -15dB despite relatively strong RSCP,

especially at the edges of the building.


FIGURE 7: HIGH RISE INTERFERENCE

In the redesign for 4G upgrade, the number of antenna points was increased

and directive panel antennas were used at the building corners instead of

omni antenna to overcome the outdoor interference.

FIGURE 8: HIGH RISE REDESIGN


SECTORIZATION AND INTERFERENCE MITIGATION

In high capacity areas such as stadiums, a large number of sectors are

deployed to cope with the traffic demands. However, there is a limit to how

much sectorization can help in increasing overall venue capacity.

A stadium case study illustrates the impact of increased sectorization on

throughput reduction. As the number of sectors increases from 8 to 16, the

simulated LTE throughput drops from 55Mbps to 39Mbps corresponding to a

drop in SINR from 17dB to 12dB because of more inter sector interference.

With higher sectorization, the CAPEX investment increases and eventually

outpaces the possible increase in throughput. The optimal ROI point where

CAPEX investment matches the increase in total venue capacity is at 11

sectors.

Number of sectors 8 11 14 16

Average SINR (dB) 17.0 16.2 12.6 12.1

Cell throughput (Mbps) 55.02 53.29 41.02 39.35

Total Capacity (Mbps) 440.16 586.19 574.28 629.6

% Capacity Increase - 33.18% 30.47% 43.04%

% Hardware increase - 37.50% 75.00% 100.00% FIGURE 9: SIMULATED PERFORMANCE FOR A STADIUM DESIGN (MIMO)

FIGURE 9: ANTENNA PLACEMENT FOR OPTIMAL LTE SINR PERFORMANCE

To reduce inter sector interference, antenna must be carefully selected and

their position, azimuth and tilt must be optimized. Narrow beamwidth

antennas with small sidelobes are selected to limit the coverage to the

intended areas. Antennas of different sectors are mounted almost back-to-

back to increase isolation between sectors.


CONCLUSIONS Signal-to-interference-noise ratio (SINR) is the single most critical

consideration in high capacity design. A good SINR can ensure higher order

modulation (HOM), good coding efficiency and MIMO effectiveness which is

the baseline for achieving high data rate service.

Techniques to increase SINR include increasing the number of antenna,

selecting the right antenna types to suit the coverage area, optimizing

antenna placements to increase dominance and control spillage. Optical

repeaters can overcome cable losses for deployment in large buildings or

high rise towers.

Sectorization can improve the total throughput of the defined planning area,

but tends to degrade individual sector throughput. Beyond a certain number

of sectors, interference may become too hard to control. Hence there is an

optimal point beyond which the additional CAPEX investment does not yield

an effective return on network capacity.


ABOUT COMBA TELECOM Comba Telecom is a leading supplier of infrastructure and wireless

enhancement solutions to mobile operators and enterprises to enhance and

extend their wireless communications networks. With over 50,000 system

deployments around the world including turnkey in-building systems,

urban/rural wireless systems, and transport wireless networks, Comba

Telecom’s end-to-end network solutions include consultation, network

design, optimization and commissioning.

Comba Telecom’s product portfolio includes DAS, small cells, tower mounted

systems, antennas, subsystems, passive accessories, Wi-Fi systems and digital

microwave links.

Listed on the Hong Kong Stock Exchange, Comba Telecom is headquartered

in Hong Kong and has operations throughout the Americas, Europe, Middle

East, Africa and Asia Pacific. To learn more, visit www.comba-telecom.com

and follow Comba Telecom on LinkedIn for regular updates.

www.comba-telecom.com [email protected]

© 2014 Comba Telecom. All rights reserved. Comba Telecom reserves the right to change, modify, transfer, or otherwise revise this

publication and the product specifications without notice. While Comba Telecom uses commercially reasonable efforts to ensure the

accuracy of the specifications contained in this document, Comba Telecom and its affiliated companies will assume no responsibility for

any errors or omissions. Nothing in this publication forms any part of any contract.

http://www.comba-telecom.com/

https://www.linkedin.com/company/comba-telecom

http://www.comba-telecom.com/

mailto:[email protected]

https://www.linkedin.com/company/comba-telecom

Comba Telecom White Paper: Distributed Antenna Systems - Key Considerations for Designing a High...

Technology

Transcript of Comba Telecom White Paper: Distributed Antenna Systems - Key Considerations for Designing a High...