sal-15-36r2 distributed antenna system benefit from spectrum conditioning

7/27/2019 SAL-15-36r2 Distributed Antenna System Benefit From Spectrum Conditioning

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ISCO International, LLC SAL-15-36 Rev 1 Page 1Proprietary and Confidential

Distributed Antenna Systems Benefit from

Spectrum ConditioningIndoor and Outdoor Distributed Antenna Systems can Recover Lost Capacity to

Carry More Traffic, Improve Spectrum Utilization and Experience Improved

Performance with Spectrum Conditioning

ISCO International

January 2012

AbstractDistributed Antenna Systems (DAS) come in multiple flavors and configurations. At a high level

these systems can be viewed as a simple dichotomy, indoor or outdoor. The DAS system type

quickly becomes more complex after this first division. Regardless if the system is neutral host

or private, over the air donor or dedicated enode B/node B/BTS, fiber-fed special venue indoor

or outdoor system, this paper will explain why they all will benefit from the use of spectrum

conditioning. Spectrum conditioning is the application of digital signal processing to the air

interface, the physical layer, to identify and minimize unwanted adjacent channel RF, the

impact of near-far effect and the debilitating impact of co-channel interferers.

The air interface is becoming more polluted with random, unpredictable interference on a daily

basis, and DAS system operational qualities make them especially vulnerable. To overcome the

challenges of depleted available spectrum operators are packing more carriers into their

existing licensed spectrum, squeezing the guard bands between carriers to their limits. While

the idea of a DAS is to provide specific coverage and capacity to a defined area, there are more


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antennas to pick up the infinite sources of interference that can impede network performance.

Simply stated, by actively conditioning the physical layer of the Distributed Antenna System

operators can proactively confront spectral issues and achieve maximum spectrum utility,

assuring their customers the best quality of service consistently.

This paper will provide a brief overview and explanation of DAS system architectures, discuss

key design considerations, and use a specific example to explain how spectrum conditioning will

improve the reliability and performance of the network while making sure interference does

not reduce handset battery life and the capacity available to carry traffic.

The DAS Dichotomy: Branching into differentiated systems: indoor or

outdoor, neutral host or private operator, passive or active

Wireless network operators consider indoor coverage the final frontier for ubiquitous service.All are challenged to find the most cost-effective way to provide continuous service as callers

move indoors. There are many approaches to providing indoor coverage; overbuilt macro

networks, over the air repeaters, distributed antenna systems, microcells, picocells and

femtocells. DAS deployments bring a unique way to flexibly integrate specific indoor coverage

needs with the broader wireless network and are expected to continue to grow with major

deployments by integrators and wireless carriers.

Indoor DAS system types include active or passive, neutral host or private, over-the-air or

dedicated enodeB/nodeB/BTS equipment either local to the DAS deployment or in a BTS hotel.Figure 1 illustrates the complexity of DAS configurations and leads to the type of system used in

the Case Analysis section of this paper.

Indoor vs. Outdoor

o Indoor networks are typically deployed in high-traffic buildings such as airports

or convention centers, with remote antennas connected via fiber to a central

hub

o Outdoor networks similarly use fiber to connect the system of antennas back to

a central hub, but typically must cover a much larger geographical area


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Figure 1: Example Dichotomy of DAS Network Types

Neutral Host vs. Private Operator

o Neutral Host DAS networks provide coverage within their domain to all service

providers in the supported frequency bands, regardless of air interface oroperator

o Private Operator networks are implemented by a single operator to provide

service to their customers over their own network and do not support other

types of connections

Active vs. Passive

o Active networks use repeater amplifiers to re-broadcast the carried signals

through the DAS

o Passive networks are simpler, using only cabling, splitters and antennas to

distribute the signal to the antennas of the DAS

These DAS configurations vary in design but all bring multiple antennas into a specific area to

provide coverage that would not be practical with a macro solution.


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DAS networks have to take the bad with the good

Indoor DAS systems provide a high level of coverage and service to the great indoors without

suffering the building penetration losses that impact macro wireless networks. However, the

indoor DAS system has to contend with the interior environment which includes walls and

partitions, furniture and various types of interior materials as well as other path losses includingcables, splitters, and attenuators. These insertion losses and the associated noise figure impact

need to be taken into account during the design phase of the indoor system and lead to highly

complex, carefully-balance system design with significant challenges. There are zones of

operation within the indoor DAS network that are similar to macro networks but with unique

properties and problems of their own including handoff coordination, coverage, pilot pollution

and link balancing. The DAS system has to be carefully engineered for handoffs within the DAS

system as well as to and from the surrounding macro environment.

While indoor DAS systems typically enjoy a shorter distance between the access point orantenna and the UE or user handset, it is exactly because of the proximity of the users to the

access point that the near-far problem is exacerbated in the DAS environment. DAS system

users close to the access point are under power control from the DAS network while users that

are not served by the DAS system are under power control from a macro site but are still

physically as close to the DAS access point as the DAS users. All of the types of DAS systems are

susceptible to high-power adjacent RF, near-far effect and co-channel interference. The

challenges posed by any of these will degrade performance in numerous ways including a

reduction in carried traffic, increased dropped calls, lower data transfer rates, reduced

coverage areas and reduced handset battery life.

A Known Problem: Interference really occurs in DAS networks

In the ideal world there are no competing wireless service providers operating in adjacent

frequency bands. There is no near-far effect since all carriers use the same air interface and

power control behaves the same for all users being served by a single cell site. If there are

competitors they are all operating with the same air interface with serving cell sites the same

distance as yours with ample guard bands separating carriers. In the ideal world there is also

no occurrence of random co-channel interference to steal capacity available to carry traffic.

This, of course, does not describe the real world. In most installations operators will experience

all three culprits (adjacent channel, near-far and co-channel) that will impact the planned path

loss, propagation coverage models and reverse link channel power. Figure 2 shows RF

interference captured in an actual DAS network in three views: on the left is a view from a


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spectrum analyzer, and on the right is a waterfall report for the same location from ISCOs

interference mitigation equipment where the green and yellow represent interfering signals

being eliminated by the spectrum conditioning equipment. The third view shows another DAS

site with both consistent and high volume co-channel interference reported.

Figure 2: Actual spectral shots at DAS venues

These plots, and others from a variety of special event sites, show the presence of co-channel

interference that unnecessarily increases channel power, stealing capacity and reducing data

transfer rates. Also shown is adjacent RF impacting adjacent channel interference ratio and

further increasing the noise rise, disrupting link planning and reducing throughput and capacity.

The near-far effect from competing operators or adjacent GSM also exists. Specifically,

consider UMTS being serviced by an in-building DAS and adjacent GSM being serviced by an

external macro cell site. Even at its lowest level, the GSM handset transmit power will

desensitize the adjacent UMTS receiver, degrading performance. This is the real world and

Consistent

interference

High volume

interference


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deploying spectrum conditioning to counter these effects will restore the DAS to the originally

designed capabilities and deliver the desired subscriber experience.

DAS networks are susceptible to the worst case for GSM/UMTScoexistence

In a DAS network there are four interference cases that can impact the performance of this

coordinated UMTS GSM co-location deployment. These cases are related to transmit and

receive characteristics of the base transceiver stations and the UE equipment for both air

interfaces.

1. GSM uplink as victim, UMTS UE as interferer

2. GSM downlink as victim, UMTS node B as interferer

3. UMTS uplink as victim, GSM UE as interferer4. UMTS downlink as victim, GSM BTS as interferer

A sharing study by the Electronic Communications Committee; ECC Report 821

provides

comprehensive network simulation results on the compatibility of UMTS and GSM operating in

the same band. In the study the limiting scenario was determined to be case 3, UMTS uplink as

victim, GSM UE as interferer. This is the situation where the GSM handset in the adjacent band

is powered up and being serviced by a distant macro cell site overwhelming the near-in UMTS

DAS node B. Even in the most favorable conditions, the limited power control of the GSM UE

(typically +5 dBm minimum) results in significant impact on the adjacent UMTS network

operation.

The impact of adjacent channel interference on a UMTS system is typically parameterized by

the adjacent channel interference ratio (ACIR), the ratio of power from the desired signal to the

power from interference from adjacent channels. The ACIR itself is a composite quantity based

on the performance of the UMTS Node B and UE transceivers:

Equation 1: Relationship betweenACIR, adjacent channel selectivity

and adjacent channel power leakage


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where ACLR is the transmitters adjacent channel leakage ratio and ACS is the receivers

adjacent channel selectivity. The ACIR then represents a composite quantity that includes the

effects of both out of band power blocking and in-band power leakage, and the net loss in

capacity can be evaluated. To optimize performance, network operators should strive for a

higherACIR

. Equation 2 restates that relationship and clearly indicates thatACIR

will improve asboth ACS and ACLR increase.

Equation 2: Calculating the impact of adjacent RF on the node B sensitivity

Mathematically, is it straightforward to demonstrate that if either ACLR or ACS is arbitrarily

larger than the other, ACIR becomes dominated by the smaller quantity, thus limitingperformance to the weakest link. At the base station, the ACIR of the uplink can thus be

improved by improving the Node B receiver ACS, but only to the point at which the ACIR is

dominated by the UE ACLR. Table 1 is compiled from 3GPP technical standards TS25.104,

TS25.101 and TS45.005 and provides typical ACLR and ACS performance.

Parameter UTRA-FDD BTS GSM UE

ACLR (dB) NA 33

ACS (dB) 46.3 NA

Table 1: ACLR andACS for UTRA-FDD BS and UE

Given the constraints of ACLR of the GSM handset and ACS of the node B, spectrum

conditioning provides an opportunity to provide improved performance in two complementary

ways. First, additional filtering can by dynamically assigned to provide additional selectivity at

the band edge, improving upon the ACS of the node Bs receiver. Second, at the same time,

spectrum conditioning is applied to reduce the impact of GSM transmissions either adjacent or

co-channel with the UMTS band being used. Spectrum conditioning provides pure spectrum for

the UMTS channel in both cases, reducing the amount of RF power from interfering signals thatcan reach the Node B radio cards. In a DAS environment the situation is further exacerbated

because of the minimal distance between the GSM UE and the DAS antenna. Spectrum

conditioning readily addresses this difference.


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DAS brings the wireless networks antennas closer to sources of

Co-Channel Interference

The challenges for DAS networks do not end with adjacent channel interference. Distributing

antennas throughout a building or event site brings the system closer to potential sources of

interference such as interference from ID card readers, routers, wireless microphones, two-way

wireless communications systems, poorly designed RF amplifiers and other electronic

equipment, as well as environmental passive intermodulation (PIM). The many unpredictable

sources of co-channel interference can drastically impact network performance, reducing

capacity and disrupting the carefully planned DAS network coverage. These types of

interference cause the network to increase UE transmit power to overcome the interfering

signal, which can further cascade and impact adjacent DAS antenna zones. This phenomenon is

well studied and can be demonstrated to impact data capacities and zone coverage.

Spectrum conditioning provides a robust and dynamic response to these sources of

interference. Known, constant interference at a particular frequency can be specifically

blocked, enabling the network to operate at lower power levels. Dynamic interference that can

impact any network randomly can be effectively responded to by the automatic deployment of

dynamic notch filtering to selectively reject narrow-band interfering signals while leaving the

rest of the signal undisturbed. Together these spectrum conditioning capabilities combine to

provide resilience to both known and unknown sources of co-channel interference, preventing

degradations in system performance before the network is impacted.

Case Analysis: A Private Active UMTS DAS system with dedicated

node B and BTS

This case analysis will consider a typical UMTS operator that will need to continue supporting

GSM already deployed in the same band. The DAS system this example is based on is pictured

in Figure 3. In this example the DAS has a co-located dedicated node B plus a GSM base

transceiver station operating in the same band supplying the active DAS network. The active

DAS network includes both GSM BTS and UMTS node B, the DAS system main unit, fiber feeds,

remote units, coaxial cables and antenna access points. This is an example of coordinated

deployment; the GSM and UMTS access points are co-located with each other. The spectrumconditioning section is deployed as an adjunct to the radio channel cards in the UMTS node B.


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Figure 3: Indoor, UMTS node B plus GSM BTS on private active DAS with

Spectrum Conditioning

The insertion losses for both the uplink and the downlink are taken into account during the

design phase. System noise figure is a key component for both GSM and the wide band UMTS

air interface. The remote unit incorporates an LNA to keep the system noise figure within

reasonable bounds, and the forward link power amplifier sets the final downlink coverage area.

Spectrum conditioning of the UMTS uplink is the key to ensuring performance for this

installation. As discussed above, the critical combination limiting system coexistence is GSM

interference adjacent to the UMTS band and the use of spectrum conditioning on the UMTS

uplink provides substantial benefits to the ACIR, which results in improved capacity and

coverage when subject to near adjacent GSM signals. This deployment provided substantial

benefits to resistance to in-band co-channel interference as well, ensuring optimal performance

in the face of the random interferers that occur consistently.

Results: Improving Performance and Spectrum Utilization

Carrier separation is defined as the UMTS carrier center frequency to the first adjacent GSM

carrier center frequency as pictured in Figure 4.

Antenna

TX/RX

Fiber Feeds

RF Out

RF In

Remote

Units

Antenna

Access

Points

Main

Unit

Spectrum Conditioning

Coaxial

Cable

Feeds

GSM

BTS

Radio Channel

Card In

RX Out

Antenna

TX/RX

UMTS

Node B


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Figure 4: UMTS to GSM carrier separation

In todays world where spectrum is at a premium, spectrum conditioning affords greater utility

of the allocated radio resource by allowing GSM to UMTS carrier separation on the order of

2.4 MHz to 2.2 MHz. In a DAS environment being able to maintain channel power noise rise

and incident power to the radio channel card while reducing the separation is ideal for

increasing spectral efficiency at a concentrated customer event; in a macro environment

spectral efficiency is even more beneficial based on busy hour demands.

By providing significantly improved selectivity of these near-adjacent GSM channels, spectrum

conditioning reduces the incident RF power received at the radio channel card of the node B by

more than 18 dB affording four additional GSM channels, two on either side of the UMTS

carrier, increasing spectrum utilization, as shown in Figure 5.

Figure 5: Spectrum conditioning increases spectrum utility

GSM center to UMTS center reduced to 2.4 MHz

Another area for concern with respect to adjacent channel interference is uncoordinated

deployment. This is the case where the operator has UMTS on the DAS network and operates

GSM on a macro network. In this situation the adjacent channel interference can become

extreme. GSM has to cover the same inside area with macro sites outside the venue as the

UMTS DAS system covers. The GSM UE terminal transmit power will be on the high end of the


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range to complete the uplink back to the macro BTS. The DAS system noise floor will increase

as a result of the increased adjacent channel interference. This adjacent channel interference

increase will cause the DAS UMTS UE terminals to increase power to overcome the adjacent

channel interference (driving down the UE battery life), which in turn will further increase the

DAS noise floor. Spectrum conditioning eliminates this problem of adjacent GSM interferenceby reducing the power incident to the UMTS radio channel card.

Conclusion: Spectrum conditioning through RF Digital Signal

Processing can increase the capacity available to carry traffic, improve

spectrum utilization and protect dropped call and accessibility

performance of critical sites

Global mobile data traffic has increased by 160% in the past year growing more than 10 timesfaster than voice to 90 petabytes per month, or the equivalent of 23 million DVDs. This

requires wireless operators to squeeze as much capacity from the existing spectrum they own

as possible maximum utilization is a must. Up to now networks could afford and compensate

for the loss of capacity to guard bands, interference and unpredictable environments. Today

that is not the case. Idle capacity rarely exists and is certainly not available during busy hour

periods or at critical high-traffic or event sites.

Now, with Spectrum Conditioning through RF Digital Signal Processing, smaller guard bands and

offsets to increase spectrum available for carriers are possible. Mitigating co-channel

interference, whether random or self-induced such as GSM, can recover vital capacity being

unnecessarily wasted, resulting in improved data transfer rates. Proactively conditioning high

profile, high traffic sites, such as DAS environments, from high-power adjacent RF can maintain

capacity, performance and throughput, all while improving battery life for the end user by

keeping UE transmit power low.

For more information about how to add capacity, recover capacity or protect performance for

maximum spectrum utilization contact your ISCO International representative.


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About ISCO International:

ISCO International operates on the front lines of 3G and 4G communications by enhancing the integrity of a

mobile operators physical layer assets the cell site and acquired spectrum. ISCO understands that wireless

communications depend heavily on the users RF connection to the base station and the companys spectrum

conditioning product line ensures that this connection performs as expected even in the most hostile and

unpredictable environments. ISCOs new Proteusproduct, based on the latest PurePass digital signal processing

technology, adaptively identifies and corrects the physical layer impairments (PLI) that decrease a cell site's

coverage, capacity, data throughput and KPI performance. In sum, ISCO allows wireless carriers to get the most

out of their existing base stations and spectrum (possibly eliminating the need to build additional ones in certain

situations), reduce operating expense and deliver a consistently high quality of service. Please visit

www.iscointl.comfor more information.

More information about all ISCO wireless solutions can be obtained from the ISCO website atwww.iscointl.com.

1ECC Report 82, Compatibility Study for UMTS Operating Within the GSM 900 and GSM 1800 Frequency Bands, Roskilde, May

2006
http://www.iscointl.com/http://www.iscointl.com/http://www.iscointl.com/http://www.iscointl.com/http://www.iscointl.com/http://www.iscointl.com/http://www.iscointl.com/

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