Transcript
Application Notes What’s at the Antenna Counts The Value of Distributed Amplifiers in DAS Solutions
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Active DAS
Facing increasing demands for higher data rates from their subscribers, mobile operators are becoming more interested in distributed antenna systems (DAS) to deliver indoor coverage. This paper examines the different types of DAS and highlights the advantages of using a DAS that incorporates distributed amplifiers.
The most prominent differentiators of Active DAS are distributed amplifiers at the antenna points. Because amplification occurs at the antenna points, an active DAS delivers strong and consistent wireless performance from each and every antenna, no matter how far an antenna is located from the base station. Amplification at the antenna is performed in both transmit and receive directions such that not only is high output power provided uniformly at each antenna but also the lowest possible receiver noise figure is provided uniformly at each antenna. A low uplink noise figure is critical for high uplink user data and control plane throughput and allows the mobiles to transmit at lower power levels, which also reduces uplink interference throughout the network. This combination of high transmit power and low receive noise figure results in a high performance DAS architecture that is independent of the span of the system. In the largest airports or multi-facility deployments such as major hotels on the Las Vegas Strip, some active DAS deployments extend for miles. Since every antenna has predictable signal strength and coverage, it is far easier to plan the antenna placement in an active DAS as opposed to a passive DAS. An active DAS is also uniquely able to support full end-to-end system monitoring and management from the RF source to the antenna points, including vital alarms such as cable disconnect and antenna disconnect indicators.
Types of DAS There are generally three types of DAS: passive, active, and hybrid. Passive and hybrid systems use thick and costly coaxial cable (1/2“ to 1.5“ in diameter) to distribute the wireless signals to antenna end points from centrally located high power amplifiers. In contrast, active DAS systems use lower power amplifiers located at the antenna end points, fed by flexible, low-cost cable to achieve a more uniform and consistent coverage area with reduced impact on the building infrastructure.
Passive DAS In a purely passive DAS, a main distribution unit, co-located with a base station, drives the wireless base station signals over the coaxial cable network to antennas placed at the ends of the cables. The signals experience significant attenuation as they propagate through the coaxial cable network, especially at higher frequencies such as those used in the 1900MHz PCS band, the 2100MHz AWS band and the 2600MHz band. It is not uncommon for the signals to be 10% of their original strength at the antenna points in a passive DAS as the use of splitters and couplers necessary to combine and separate signals in the cable distribution network result in much higher losses than would be observed from just the cables themselves. Lower conducted power at the antenna input ports results in lower radiated power and consequently lower power received at the mobile device. This results in a loss of signal to noise ratio (simply stated as the quality of the signal being received) and ultimately performance as the supported data rate is reduced.
Remote Unit
Expansion Unit
While the lower radiated power at the antennas is bad enough, a passive DAS also results in uneven power at the antenna points throughout the entire cable network as the different lengths of cables and different combinations of splitters and combiners result in varying amounts of signal attenuation at each antenna point. This non-uniformity of power at the antennas makes it difficult to design, maintain and even upgrade passive DAS systems. More importantly, this non-uniformity of power results in different coverage areas at each antenna point which often results in more holes, or dead spots, in the coverage area of the passive DAS network.
Host Unit
Figure 1: Active DAS
CommScope ’s active DAS products employ the same network architecture as Ethernet LAN and use the same standard building cabling to achieve the lowest total installed system cost as compared to competing DAS products. In CommScope ’s implementation of active DAS, a main distribution unit is colocated with the RF source, such as a base station or repeater, in a building’s equipment room or nearby cell site. The main distribution unit is analogous to a network server in a LAN network that is located in a building‘s computer network room. The main distribution unit distributes signals from the RF source over optical fiber to intermediate distribution units typically referred to as expansion units that are often located in telecom closets on different floors of a building. In similar fashion, the Ethernet signals from a LAN are distributed from servers to switches that are also typically located in telecom closets on each floor of a building. The expansion units in a CommScope active DAS
Passive systems do not offer end-to-end monitoring and management. The signal is simply being pushed out over copper cabling, so service providers and building owners never know if a particular cable or antenna has failed until users start complaining. Finally, while passive systems may be assumed to be simple and the most economical DAS option, passive systems are more difficult and expensive to install, because their heavy rigid cabling requires special expertise and often special cable raceways or hangers. Since the cabling is not as flexible, it is also more difficult to deploy in tight spaces.
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then distribute the wireless signals using standard building cabling to the remote access units (RAU) located at the specific antenna points in a building. In an analogous fashion, Ethernet signals are distributed from the switches in the telecom closets to client devices located throughout the building, including Wi-Fi access points. This multilevel architecture of main distribution units, expansion units and RAUs is referred to as a double-star architecture and is a very efficient and flexible method of deploying networked devices such as those used in active DAS or a LAN network.
Antenna Points
With their double-star architecture, active DAS can be expanded indefinitely through deployment of additional expansion units and RAUs. The distributed architecture of an active system scales easily and can be upgraded to support new services as they come on line. This leaves the most expensive and non-recoverable investment of the system – the cabling and antenna plant—untouched. Active systems usually support SNMP alarms as well, so a company’s IT staff can monitor the status of all remote antennas in the network using the same network management tools used for the LAN.
Remote Unit with Amplifiers
Hybrid DAS
Host Unit
Hybrid DAS combine attributes of both passive and active systems. Similar to an active DAS, hybrid DAS employs remote amplifiers however, these amplifiers are not located at the antenna points but at intermediate locations. The amplifiers are then used to drive thick coaxial cabling from these intermediate locations to the antenna points in similar fashion to passive systems. Hence, a hybrid DAS is subject to the same problems and pitfalls as a passive system, although not at the same scale if the system is small in terms of coverage area.
Data Center Figure 2: Hybrid DAS
cable costs twice as much and is more difficult to manage during installation due to its thicker diameter and higher degree of stiffness. (See Table 2.) Frequency in GHz
Hybrid DAS typically use fiber optic cabling to carry the signal from a main distribution unit, that is collocated with the base station, up a building riser to a remote unit which contains the amplifiers used to drive the thick coaxial cabling horizontally across each floor of the building. Hybrid systems partially alleviate the problem with signal loss and variable signal strength at each antenna, because there is far less signal loss in the vertical portion of the system as compared to the purely passive DAS. However, hybrid DAS has the same signal loss issues in the horizontal cable runs to individual antennas as a passive system. Overall, output at any given antenna will be higher than in a passive DAS, but there will still be wide variability in signal strength and coverage at each antenna, depending on its distance from the remote amplifier portion of the system. And these solutions are still vulnerable to the RF interference issues as signals combine in their native RF waveforms.
Coax Type
0.15
0.2
0.5
1
1.5
2
3
LDF 1/4"
1.52
1.77
2.86
4.16
5.19
6.10
7.67
LDF 3/8"
1.29
1.49
2.42
3.52
4.40
5.17
6.52
LDF 1/2"
0.82
0.95
1.53
2.22
2.77
3.25
4.09
VXL 7/8"
0.49
0.57
0.93
1.36
1.71
2.01
2.54
Table 1: Signal attenuation (in dB) in coaxial cabling over a 100-foot run.
As shown in Table 1, half-inch cable attenuates at the approximate rate of 3.25 dB per 100 feet when running AWS frequencies. If you have to run an AWS signal over 100 feet of half-inch coax, you only have 50 percent of the signal left by the time the cable reaches the antenna. The solution is to either run a thicker cable or run more antennas, which involves adding more splitters, which can also attenuate the signal.
In the same way, hybrid systems only partially solve the management and cost challenges of DAS deployment: they offer management between the main distribution unit and the remote units at each floor, but not between those remotes and individual antennas. And while building owners save partly on the cost of deployment on the fiber portion of a hybrid DAS, they encounter the same cost and disruption issues when installing heavy coaxial cabling on each floor.
In an active DAS, knowing that there is uniform signal strength at each antenna, the engineer can easily create a consistent high quality ‘best server’ coverage area to deliver a very clean signal that enables high data rates. The quality of the receive signal is tied to the device’s data rate, and it is much easier to provide a high quality signal with an active system. In contrast, a passive or hybrid system is likely to have holes in its coverage area, and subsequently user devices switch from one serving base station to another hunting for a useable signal, creating more frequent handoff scenarios that reduce data rates.
There are several key factors that differentiate the types of DAS Performance In a passive or hybrid system, signal strength diminishes the farther you are from the base station. Even in hybrid systems, cable runs can go 200-250 feet and produce loss (See Table 1.) These losses can be offset by using lower-loss cable –7/8 inch diameter cable, for example—but lower-loss
In high-rise buildings, it is difficult to use a passive or hybrid DAS because of the need to create dominant coverage areas at the building’s edge to overcome interference from the outside macro networks. Pilot pollution is a
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common interference issue in high-rise buildings where the mobile device can sense multiple signals from various cell sites surrounding the location, often making it appear there is adequate service available. But the user often cannot even place a call because there is no dominant signal to support the transmission. The physics of a deployment are such that it’s difficult to get the strongest signal from a passive or hybrid system out at the end of a cable run at the building’s edge, as the telecom closets and equipment rooms are almost always located towards the center portion or in lower levels of the buildings as opposed to the edges. Thus, there is a farther distance between the remote unit of a hybrid DAS and the antenna point.
used in a building riser and then 1/2-inch cable is used from the riser out to the antennas in some passive systems. Coaxial cable also requires skilled technicians with special tools for properly terminating and installing the cables, which increases the total installed system cost. With the distributed amplifiers in an active DAS, the antennas have wellknown and established performance specifications, so the engineer can optimally place the antennas where they are needed. During installation, the team needs only to ensure that they are within the cable distance specifications to preserve uniform signal quality at each antenna. In a CommScope DAS, RG6 CATV cable has a limitation of 459 feet and RG11 CATV cable has a limitation of 656 feet.
To overcome these performance issues, hybrid and passive systems often incorporate in-line amplifiers, sometimes called signal boosters. These in-line amplifiers add to the overall cost of the system, and degrade the quality of the signal as every booster contributes additional noise to the signal. One of the most compelling reasons for using active DAS is for high-speed data since placing the amplifiers as close as possible to the mobile user results in the highest quality airlink. From a comparison perspective, Wi-Fi systems are predicated on the use of distributed amplifiers and MIMO antennas, and CommScope’s system is closer to this architecture than any other system.
Upgradability With the necessity of higher signal quality in high speed data systems such as LTE and HSPA, and especially with the addition of MIMO (multiple input, multiple output) which requires multiple antennas for both transmit and receive, passive and hybrid DAS systems that were originally designed for voice services become difficult to upgrade as increasing the density of antennas requires rebalancing of the entire coaxial cable distribution network due to the variability of signals with the different lengths of cables. While higher power remote amplifiers can be used, this does not improve the receive capabilities of the system which would result in degraded uplink performance. For these same reasons, passive and hybrid DAS systems are also difficult to upgrade to higher frequencies (such as 2100MHz AWS services) if the DAS was originally designed for operation at lower frequencies. The larger attenuation of signals at higher frequencies results in lower transmission power and higher receive noise figures at the antenna points.
Image 1: Active DAS Hosts with Tranceiver Card Upgrade Space
MIMO Support
CommScope active systems, in contrast, do not require cable infrastructure upgrades. It is only necessary to upgrade the transceiver cards at the main distribution unit and expansion unit and add the frequency appropriate RAU to accommodate new frequencies.
Wireless carriers are adopting MIMO technology to keep up with rising demands for more data throughput, and the advantage of the CommScope active DAS is that it delivers true MIMO at all antenna locations without requiring an additional cabling infrastructure. In a hybrid or passive environment, the designer wanting to deploy MIMO has no choice but to lay a second set of cables. The cabling is the largest cost factor in a passive or hybrid system, so adding a second layer of cabling makes it expensive for the carrier to deploy MIMO with a passive or hybrid DAS.
Design and Deployment Hybrid and passive systems are difficult and time-consuming to deploy because of the tedious and iterative adjustments required to balance cable losses against the need for specific power levels at each antenna. It is difficult to get the right ERP (effective radiating power) at each antenna point, and adjusting one part of the system impacts other parts of the system. In a passive environment, the designer has a fixed output from a central location and tries to balance that power out across the multiple antennas in the passive distribution network. However, a paper design isn’t a deployed design. If installers come across issues during installation where the route becomes circuitous and they have to add 50 feet of cable, the system could be 3 dB lower on power at an antenna relative to what had been planned.
In contrast, the CommScope active DAS keeps the parallel co-frequency MIMO streams separate by frequency converting each stream independently to a different intermediate frequency or digitizing each stream and digitally multiplexing them onto the same cabling infrastructure. Because CommScope DAS systems use the same cabling infrastructure for supporting MIMO signals, the DAS can be initially deployed for SISO operation and then subsequently enhanced for MIMO operation when the need for higher capacity arises. It is a plug and play upgrade—add a card to the main distribution unit and expansion unit and add a second antenna at each location—the cable plant remains the same.
In addition, it is difficult to work with coax cable—care must be taken to preserve the integrity of the cable to avoid kinks. 7/8-inch cable is often
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MIMO Antenna Pair
Image 2 MIMO Antenna Pairing
Cost
CommScope‘s Active DAS can be less expensive and is less disruptive to deploy because their standard cabling is inexpensive, and the job can be handled by IT cabling contractors or electricians rather than specialized technicians. CommScope‘s installer training programs also ensure that best practice deployment methods are applied. Standard cabling can be run across suspended ceilings and in tight spaces like conduit just as easily as LAN cabling. In many cases, an active system can use existing, unused fiber that runs up a multi-story building’s utility riser to link a main distribution unit with expansion units, and then use low cost CATV cabling to connect each expansion unit to the RAUs. While multiple sets of electronics may be required to support all service providers (depending on the service providers’ requirements), the cost of cable runs is a larger factor in the overall price of a system in all but the smallest facilities.
Passive systems are perceived to be economical, but plenum-rated coaxial cables get very expensive very quickly. In fact, the total cost for a deployed solution is about the same for passive, hybrid, and active systems by the time you compare the electronics and media. The actual electronics of an active DAS system may be more expensive, but the cable is much cheaper. RG6 CATV cable is about 33 cents a foot, whereas half-inch coax is about $2.50 a foot. Installation is also more expensive for the coax cable as shown in Table 2.
Material and Installation Cost
1/2” Coax
7/8” Coax
1-5/8" Coax
RG6 or RG11
5.00/foot
8.00/foot
20.00/ foot
1.46/foot
Conclusion CommScope‘s active DAS products offers several key advantages over passive and hybrid systems. Through the use of distributed amplifiers at every antenna, CommScope‘s DAS produces a uniform, high-quality signal at each antenna for higher data throughput and easier system design. In the end, CommScope‘s DAS products are easier and less costly to install and produce superior performance for today’s and tomorrow’s cellular service needs.
Table 2: Relative costs to deploy active and passive system cabling.
The total installed cost of a CommScope DAS is roughly 60 percent for electronics and 40 percent for cable and labor. The total installed cost for a passive system is roughly 30 percent electronics and 70 percent cable and labor. Cabling is a non-recoverable expense, whereas electronics can be easily removed and reused elsewhere. This occurs with temporary deployments like sporting events, concerts, festivals, political conventions, etc., where the electronics are used to drive the system during the event and then harvested for use elsewhere after the event is over.
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www.commscope.com Visit our website or contact your local CommScope representative for more information. © 2015 CommScope, Inc. All rights reserved. All trademarks identified by ® or ™ are registered trademarks or trademarks, respectively, of CommScope, Inc. This document is for planning purposes only and is not intended to modify or supplement any specifications or warranties relating to CommScope products or services. AN-311494.1-AE (12/15)
