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Distributed Antenna Systems Advantages of Digital Transport in the Design, Operation and Growth of Distributed Antenna Systems (DAS) WHITE PAPER Introduction The Digivance® product line with FLEX technology has many features and functions made possible by its use of a digital platform. This white paper describes the distinct advantages of Digivance when contrasted with legacy analog fiber repeaters. Topics covered include: Benefits of Digital Over Analog Fiber Backhauls; Benefits of Digitizing RF signals; Interference Protection; and Future-Proofing. Digivance NXD and CXD feature the innovative FLEX technology that offers the option of one, two, three or four 15 MHz bands in a single cabinet. This technology also provides simulcasting -- the option of spreading the coverage of a BTS sector across two or more transmit/receive locations. FLEX technology allows multiple simulcast arrangements for different bands or different RF technologies. In addition, this technology offers operators increased flexibility in meeting fluctuating capacity demands. By allowing service providers to change simulcast patterns through the use of remotely controlled digital cross connect switches, FLEX technology can reallocate capacity and reduce BTS costs. This technology can also be integrated with Software Defined Radio (SDR) to enable wireless networks of the future. Benefits of Digital Fiber The Digivance system uses digital fiber for connecting the hub to the remote units. Digital fiber has many advantages over analog including greater band-to- band isolation, consistent RF performance with automated delay management and integrated IP connectivity to all RANs, and accurate fiber link fault detection and isolation. • Band-to-Band Isolation The multiple RF bands transported between the hub and remote unit are digitally time-multiplexed onto the fiber backbone. Therefore, once the signals are in the digital domain, it is not possible for one RF band to affect another. When using analog, one band or even one signal can adversely affect performance on all other bands on the fiber. Isolation capabilities and performance benefits apply to both the RF and optical domains. Not only does digital transport/multiplexing isolate signals from different RF bands but it also provides complete isolation for signals being optically Advantages of Digital Transport in the of Distributed Antenna Systems (DAS) Design, Operation and Growth Advantages of Digital Transport Page 3 multiplexed (via CWDM) on the same fiber. For an analog transport system, isolation between bands/different wireless providers is limited by the optical filter performance of the CWDM equipment. It must also account for varying power levels provided by near/distant RANs. Analog optical isolation, especially for adjacent wavelengths, is significantly less than the RF isolation required between the RF bands being transported. • RF Performance A digital fiber system will have more consistent performance with respect to gain and noise figures for example, the uplink noise figure on the digital fiber will be the same at 1 mile of fiber or at 30 miles. An analog system will have increased losses at longer fiber distances and therefore, RF performance (e.g. dynamic range, gain, and noise figure) will vary across the analog network. As fiber distances increase, the performance advantage of digital over analog increases as well. • Delay Management Digivance provides Automated Delay Management features that provide a much simpler simulcast configuration and optimization process than is possible when using analog fiber. When simulcasting, signals are repeated at more than one remote node. This is an efficient way of spreading a BTS sector over a wider geographic area with fewer BTSes to cover larger areas. Additionally, ‘simulcast hand-offs’ occur between adjacent simulcasted remote nodes. These simulcasted hand-offs do not require a change in RF channel or timeslot and so they are less likely to drop calls that can occur during hard hand-offs or sector-to-sector hand-offs. Simulcast works because the radio equipment in mobile phone networks is designed to handle multi-path signals. Typical multi-path is spread by several microseconds and the radio equipment is optimized to handle this correctly. If the multi-path delays become much larger than this, then the radio receivers will be adversely impacted and performance is affected. For optimal simulcast performance, the RF delays to each remote must be accurate and closely managed. If the delays between simulcasted remotes are too far apart, when the mobile is in a simulcast overlap area region approximately half- way between two remotes, performance is affected. Field-testing has shown significant performance impact for non-optimized simulcast delays. Digivance has GPS receivers at all hubs and remotes. Using these GPS receivers, Digivance is able to measure the delay of all fiber runs with nanosecond accuracy (usable for E911 providers to perform location over a Digivance network). Since Digivance uses digital fiber, the system can automatically adjust the delays (via delay memory) to have all signals come out of every remote with the same delay. Therefore, a wireless service provider never has to worry about degraded performance when simulcasting due to non- optimized delay. As systems grow, the associated fiber plant connecting the hubs to the remote nodes will change in routing and length. In an analog environment, these growth-driven changes can require the operator to create new delay spools to balance the system’s performance and delay. However, in an analog system, as the length of the fiber increases, the dynamic range of the system is lower overall. With a digital system, delays can be equalized without introducing any fiber spools and the system can grow or change with no plant intervention. Likewise, if an operator wishes to change simulcast patterns with an analog system, fiber would once again have to be cut and re-spliced, with added or subtracted lengths, to equalize delay between the simulcast nodes. • Integrated Ethernet Connectivity Digivance provides Ethernet connectivity to all remotes. This allows a powerful operations and management tool for monitoring performance and alarms at the remote units. Additionally, software updates do not require site visits since they are easily and automatically done over the network connection. • Fiber Link Fault Detection and Isolation Digivance fault management software continually monitors the fiber interface health and immediately reports any degradation in performance. In the event of a failure, the fault management software identifies the failed module requiring replacement. Degradation in fiber connectivity (excluding an abrupt cut) is detected digitally before the effects are visible in the RF airlinks. Digital fiber error rates are well below 10-6 while the hosted airlink is designed for <10-2. Analog fiber systems, like the hosted RF waveforms, will detect fiber problems only when the effect is great enough to already degrade the airlink. Advantages of Digital Transport Page 4 Digitized Signals Digivance digitizes the RF signals by band for transport over the fiber backbone. Digitizing of the signals has the following benefits: high spur-free dynamic range, simple and reliable simulcast configurations, and digital switching. • Spur-Free Dynamic Range Using high performance 14-bit analog-to-digital and digital-to-analog converters, Digivance is able to achieve spur-free dynamic ranges of greater then 70 dB. Additionally, each band is independently digitized to insure that only signals in the selected band go into the digitizers. Because of the independence of the digitizers, the total dynamic range across all the bands is 90 dB. Likewise, with each frequency block maintaining its independence, any gain control used to eliminate strong signals will not affect any other frequency block. Analog repeaters claim 70 dB SFDR. However, in an analog fiber system, multiple bands are combined onto the same fiber, which can lower the overall dynamic range. Also, any gain control system employed to deal with the presence of strong signals in one band will necessarily affect all other bands. The alternative is to disable gain control and allow intermodulation products to randomly affect performance. • Simulcast Configurations The Digivance digital platform provides many simulcast configuration benefits, as it is an integral part of the platform. All simulcast configurations are done at the hub location using CAT-5 patch cables (similar to a computer network room). Analog systems require separate RF combiners, splitters and cabling to perform simulcasting. Reconfiguring the simulcast on an analog system would require different splitters and combiners based on the new simulcast configurations. Digivance downlink simulcast splitting is done digitally (an exact copy of the digitized signals are sent to all remotes). Therefore, there is no impact on the downlink signals. In an analog system, splitters are used that add loss to the downlink signals (lowers signal to noise). Digivance uplink signals are combined digitally for predictable simulcast combining. In an analog system, RF combiners are used that have more variability as to how the signals are combined. • Digital Switching Band independent simulcast/capacity re-allocation can be easily added to a digital transport system. Digivance provides the option of a fully software controlled digital cross-connect switch to allow wireless operators to easily reconfigure capacity distribution within their DAS network. Because of its digital nature there is no performance impact with the inclusion of the switch within the network. The addition of a switch within an analog DAS network will necessarily degrade dynamic range since the analog switches performance (IP3, noise figure, band to band isolation) cascades with that of the remote unit(s) and the host base station. The superior isolation of a digital transport also allows switching operations within each band/wireless operator to be completely independent and transparent with no performance compromises. Advantages of Digital Transport Page 5 Interference Protection The Digivance FLEX platform, using a NXD, allows co- location of up to eight wireless service providers over nine different frequency bands (SMR, Cell-A, Cell-B and PCS A-F). In order to support all these different bands and not have them interfere with one another, a great deal of attention was given to the isolation of the various bands (transmit-to-transmit, transmit-to-receive). This was successfully demonstrated and confirmed by independent third party testing. The same interference protection is used in all Digivance with FLEX platform products. Additionally, Digivance is designed to prevent maintenance actions in one band from affecting service in another. This is accomplished by partitioning of bands, individually digitizing each band and having all modules be hot swappable. In an analog repeater, if one band needs maintenance, then all bands serviced by that remote are affected. Future-Proofing As in most high-tech areas, digital is the future. Not only does digital operate more predictably and reliably, the cost of digital technology is often on a much steeper curve (Moore’s Law) than that of analog. For example, over the last few years we have seen 2 Gbps digital lasers give way to 10 Gbps and now 40 Gbps lasers. This increase in digital fiber capability allows more efficient use of fiber. And since fiber does not drop in cost as quickly as the technology being used on it, the improving digital technology has a larger impact on lowering the cost of digital DAS networks. Additionally, in the next few years, base stations and software radios will use a digital interface to RF modules. This will make it easy to move the digital DAS system to this new interface and recoup benefits such as cost savings and simplification. Analog systems will still require the old analog BTS interface with its added cost and complexities. Authors: John Sabat, Director of Engineering Research and Thomas Hebert, Manager of Systems Engineering ADC Telecommunications, Inc., P.O. Box 1101, Minneapolis, Minnesota USA 55440-1101 Specifications published here are current as of the date of publication of this document. Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice. At any time, you may verify product specifications by contacting our headquarters office in Minneapolis. ADC Telecommunications, Inc. views its patent portfolio as an important corporate asset and vigorously enforces its patents. Products or features contained herein may be covered by one or more U.S. or foreign patents. An Equal Opportunity Employer 1333471 9/05 Original © 2005 ADC Telecommunications, Inc. All Rights Reserved Web Site: www.adc.com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 • For a listing of ADC’s global sales office locations, please refer to our web site. WHITE PAPER . Distributed Antenna Systems Advantages of Digital Transport in the Design, Operation and Growth of Distributed Antenna Systems (DAS) WHITE. signals being optically Advantages of Digital Transport in the of Distributed Antenna Systems (DAS) Design, Operation and Growth Advantages of Digital