Tài liệu Digital RF Transport pptx

Digital RF Transport The Advantage of Digital RF Transport for Distributing Wireless Coverage and Capacity WHITE PAPER Introduction Every business needs an edge. Many industries—from consumer electronics to household appliances to automobiles—have successfully gained an edge with digital components and production methods supported by digital devices. The drive to digital is apparent in wireless voice and data communications, too, as networks migrate toward IP-based architectures, software defined radio, and other technologies. The digital edge isn’t just convenient. The digital edge translates into bottom line benefits for wireless service providers. For wireless service providers, the combination of limited spectrum and deployment of spectrum-hungry data services already points to digital solutions. Add in impending consolidations and capex restrictions, and wireless service providers need an edge that adds flexibility, improves quality of service, and reduces capex/operating costs. That edge is found in digital methods of transporting RF signals. What is digital transport of RF? Transporting RF signals over fiber cables provides a highly cost effective solution for distributing wireless coverage and capacity. Used for both indoor and outdoor, low power and high power applications, RF transport solutions link remote antennas and cell sites to base transceiver stations. Analog and digital RF transport systems feature significant differences that impact network performance. Analog RF transport systems may transport digitally modulated signals, such as TDMA or CDMA. Or analog RF transport systems may include a data path for carrying alarm information. In either case, the method of transporting RF signals is analog, where the RF signal is preserved as an analog waveform throughout the transport. Also, the network components required to support analog RF transport—including lasers, splitters/combiners, converters, and repeaters—are all analog devices. The Advantage of Distributing Wireless Coverage and Capacity Digital RF Transport The Advantage of Digital RF Transport Page 3 Unlike analog RF transport systems, digital RF transport solutions involve digitization of RF signals. To digitize RF signals, continuous samples or snapshots of the RF spectrum are taken and numerical values are assigned to each sample, converting a continuous voltage RF signal to a list of discrete values. It is this list of discrete 1s and 0s that becomes the digitized RF signal. ADC’s patented RF digitization technology converts an entire section of the RF spectrum to a digital bit stream, transporting the digital bit stream over fiber, and reconstructing the RF signal at full bandwidth at the other end of the link, shown in Figure 1. Taking a snapshot of the RF spectrum 71 million times per second, this technology can fully digitize a 35 MHz section of spectrum, which is within the Nyquist rate of 71 Mbps 1 . Thirty-five MHz of spectrum exceeds the spectrum holdings of any wireless service provider in 800 MHz, 1900 MHz or UMTS networks. With 14 bit samples taken at 71 Mbps, the resulting digitized representation of the RF signal is over 1 Gbps with dynamic range performance that can practically exceed 70 dB over all conditions. The relationship between dynamic range, loss, and noise in RF transport Dynamic range is like bandwidth—more is better. Expressed in decibels (dB), dynamic range is the difference between the strongest signal and signal just above the noise floor. Comparing the dynamic range of analog and digital RF transport systems is like comparing the dynamic range of vinyl LP records and compact discs. The LP replicates sounds from a musical performance using analog techniques; the compact disc, digital techniques. The dynamic range of a LP record is about 50 dB. For a compact disc, dynamic range is about 90 dB. With a wider and more accurate dynamic range, the compact disc captures sounds on the high and low end of a musical performance—sounds that do not appear on the LP record version of the same musical performance. The full depth of cannon booms and sparkle of percussionist’s triangle are fully replicated on the compact disc. These sounds are lost on the LP. With analog- formatted music, there is an incomplete replication of the original musical performance. In wireless communications, poor dynamic range is most apparent in data communications. When just several data bits of a RF signal are clipped due to a narrow dynamic range, entire blocks of information become corrupt or lost. Clearly, wide dynamic range is critical as more data services consume the wireless spectrum. What limits dynamic range in an analog RF transport system? Loss and noise. Every splitter, combiner, and connector adds loss. And RF transport systems generate a variety of noises. Every amplifier or transport medium adds noise. There’s also laser shot noise, laser RIN, and thermal noise in optical devices. There’s intermodulation, dispersion, and reflection. There’s ambient RF noise in coax and UTP cables. There is even interferometric intensity noise generated by optical transmission on fiber of longer distances. Figure 2 compares the impact of loss and noise in analog and digital RF transport systems. It is loss and noise that make digital and analog RF transport systems so different. In all analog RF transport systems, loss and noise generated in the transport are directly added to the RF signal. In analog systems, noise is cumulative and cannot be removed from the signal; once RF to Digital Digital to RF Optical TX Optical RX RF In RF Out fiber Figure 1. Digitization of RF signals Source of Loss of Dynamic Range in Loss of Dynamic Range in Loss/Noise Analog RF Transport Systems Digital RF Transport Systems Optical combiner 3.0 dB loss per 2:1 split None loss in transport path Optical transport loss 2 dB loss per 1 dB attenuation None Optical connector loss 2 dB loss per 1 dB attenuation None Optical transport noise 2 dB loss per 1 dB noise None RF noise 1 dB loss per 1 dB noise 1 dB loss per 1 dB noise Figure 2. Comparing loss of dynamic range in analog and digital RF transport systems the noise is there, it is there for good. Unlike analog systems, noise and loss have virtually no impact on digital RF transport because the discrete values of the digitized RF signal and the transport medium are fully independent. In digital terms, a noisy 1 is still a 1 and a noisy 0 is still a 0. In digital RF transport over fiber, as long as the digital bit stream is transported over allowed optical transport requirements— which are less stringent than analog optical transport requirements—RF signals are regenerated with identical performance as the original RF signal digitized to create the digital bit stream. With digital RF transport, as long as the signal can be detected, the digitized RF signal is transported error free, as depicted in Figure 3. To illustrate, suppose a RF signal shows the following characteristics: the weakest signal is –110 dBm, strongest signal is –40 dBm. Before transmission over an RF transport system, the dynamic range of the signal is therefore 70 dB. In a digital transport system over fiber cable, the dynamic range of the digitally transported RF signal remains constant at 70 dB—irrespective of optical loss and noise generated in transport. By contrast, in an analog RF transport system—where losses and noise are permanently added to the RF signal—adding 25 dB of optical loss reduces dynamic range by 50 dB, resulting in a dynamic range of 30 dB. This is far below the level of acceptable network operation, between 60 dB and 80 dB. See Appendix A, comparing dynamic range, for a more detailed example. The Advantage of Digital RF Transport Page 4 Figure 3. Even as optical loss grows, digital RF transport maintains dynamic range of the signal. The cumulative effects of loss quickly degrade analog RF transport. Loss of dynamic range inhibits the information carrying and call handling capability of analog RF transport systems. Digital RF transport systems—immune to degradation caused by noise in the transport that affects analog RF transport systems—offer a wider dynamic range and greater performance for wireless communications. Greater dynamic range allows strong and weak signals to be transported simultaneously, which translates into improved quality of service. The relationship between noise, higher speeds, and signal strength in RF transport In wireless communications, there is a clear relationship between signal level and bit rate—increased data rates require increased signal. Figure 4 below shows that for every doubling of the data rate, an approximately 3 dB increase in signal level is required. For example, as a network migrates from GSM (200 kHz) at 115 kbps to EDGE at 384 kbps, the increased data rate requires about an 8 dB increase in signal. The signal level corresponds to the signal to noise ratio (SNR). SNR is the ratio of the power of the signal to the power of the noise in the transport. To maximize network efficiency, the goal is to operate RF signals at the lowest possible SNR, the point where the signal is only a little bit stronger than the noise. In RF transport, there is an inverse relationship between signal and noise. As noise is added to the system, less signal output is achieved; conversely, less noise enables a greater signal. Because digital RF transport is immune to transport noise, a higher SNR is possible than with analog RF transport systems. With a better SNR, digital RF transport delivers significantly better signal quality that translates into higher data rate capabilities. With stronger RF output power on the forward path— base transceiver station broadcasting out to the mobile user—wireless devices can be farther away from the cell site, effectively expanding coverage and increasing capacity by enabling additional radios. With an inherently higher SNR, digital RF transport maximizes use of valuable spectrum and offsets shrinking coverage areas caused by higher speed services. Dollar for dollar, higher SNR enables better utilization of network equipment by improving signal quality and expanding coverage areas. Improved performance of digital RF transport on the reverse path enables higher power of the forward path, resulting in a balanced path, something sacred to network designers. Because lower noise and stronger signal— increased SNR—translate into wider dynamic range, both the more distant, weak signals and strong signals are detected on the return path of a digital RF transport system. Without a balanced path, strong signals swamp the system, blocking weak signals, frustrating users, and corrupting data. With higher forward path output and greater sensitivity to both weak and strong signals on the uplink, digital RF transport—with higher SNR and wider dynamic range—ensures enhanced coverage and a balanced path. Higher noise limits signal strength in analog RF transport. Because digital RF transport is immune to optical transport noise, greater signal quality is achieved—enabling higher data rates, better use of spectrum, and wider coverage areas on the forward and return paths. With higher SNR, digital RF transport improves QoS by maximizing coverage and preserves capital by maximizing valuable spectrum and network equipment. The Advantage of Digital RF Transport -125.00 -120.00 -115.00 -110.00 -105.00 -100.00 -95.00 -90.00 10 100 1,000 10,000 Data Rate (kbps) Sensitivity (dBm) 1xEV-DO/DV IS-95B 1xRTT TDMA/EDGE GSM/EDGE UMTS TDMA/EDGE GSM/EDGE IS-95B 1xRTT 1xEV-DO/DV UMTS Figure 4. Every doubling of the data rate requires approximately 3 dB increase in signal. Page 5 Digital delivers distance The foolproof method for ridding an analog RF transport system of noise is to reduce cable distance. For analog RF transport in coarse wave division multiplexing (CWDM) applications, fiber cable distance can be limited to only 2 to 4 miles, which may mean only a mile of straight line distance. Devices such as over the air repeaters are used in niche applications to increase distance but have significant limitations. For distance, analog RF transport systems just can’t measure up to digital RF transport systems, effectively imposing limits on network design and system performance. Analog RF transport systems do have an array of methods to compensate for noise and increase distance. Automatic Gain Control devices are essential for analog RF transport systems, even though there is a limit on the total number of loss/gain blocks in the system. There are devices that reduce noise by improving linearity and reducing dispersion. Analog lasers, which have a shorter lifetime than digital lasers, can be replaced more frequently to keep unwanted RF signal noise in check. Yet all of these work-around measures add complexity and cost to the system. And even with these measures, noise from splitters, connectors, converters, cables and other sources still limit the performance of analog RF transport systems. With digital RF transport, distance is virtually unlimited. Fiber cable lengths of 12 miles (19.3 km) or more are possible with no degradation of dynamic range, no degradation of service, and with less investment in network equipment. Figure 5 shows how digital RF transport maintains dynamic range over greater distances. Digital RF transport maintains dynamic range over longer distances, allowing network planners more flexibility in design. This enables a centralized radio capacity network architecture, whereby antennas are installed remotely from base stations. The distance limitations of analog RF transport systems are due to the basic weakness in all analog RF transport systems—the cumulative effects of noise and loss reduce dynamic range and signal strength. The Advantage of Digital RF Transport Page 6 Figure 5. With 1.5 dB of optical loss per mile and 6 dB CWDM loss, digital transport of RF signals maintains dynamic range for over 12 miles. At this rate of optical loss, analog RF transport can only maintain dynamic range for less than 4 miles. Digital RF transport stands for flexibility The relative noise immunity of digital RF transport systems delivers wider dynamic range, higher SNR, increased signal output, and virtually unlimited distance—all of which translates into improved quality of service in terms of coverage, clarity, and speed. Yet the key benefit of digital RF transport lies in a factor cherished by planners, engineers, operations personnel, and CFOs alike— flexibility. Digital transport of RF signals is transparent to modulation techniques. ADC’s patented digital RF transport technology supports AMPS, TDMA, CDMA and GSM. And this same technology works as networks migrate from IS- 95 to EU-DO. In addition, digital RF transport is not designed for any particular handset, any particular base transceiver station equipment, or any specific architecture. Digital RF transport works with singlemode fiber, multimode fiber, millimeter wave connections, and free space optics. Analog systems, by contrast, are limited to coax or singlemode fiber cables to minimize noise and signal reflections. Choosing digital RF transport is truly a choice for today and the future. As wireless data rates increase and coverage areas shrink, digital RF transport offers greater flexibility. For example, base station hotels tap existing and unused fiber cables in CATV and metro rings to distribute wireless coverage and capacity, minimizing capex as well as reducing annual fiber lease costs. By using technologies such as dense wavelength division multiplexing (DWDM), CWDM, or wavelength division multiplexing (WDM), base station hotels connect remote antennas to base stations, mining more bandwidth from existing spectrum and infrastructure. Digital bit streams are particularly suited for the task because the digital RF signal and the optics are totally independent. This is not true for analog RF transport systems where leakage from one channel to another degrades performance of the RF signal as well as signals in adjacent channels. In fact, network planners will find that many CATV and metro area network rings are not set up to handle the effects of analog RF signal transport on the fiber plant. Digital RF transport offers improved flexibility in design for indoor applications, too. “Home run” fiber cable runs are often required in analog RF transport systems, significantly increasing installation costs and inhibiting expansion capabilities. Digital RF transport systems split and add signals digitally, so the architecture of fiber cable runs can be optimized for each application, minimizing installation costs and time. Conclusion Digital transport of RF signals offers wireless service providers an edge that adds flexibility, improves QoS, and reduces capex/opex costs. Flexibility in network deployment means networks can evolve as needs change, adding and distributing new radio capacity or moving excess capacity where needed. Improved QoS from digital RF transport reduces churn. And most importantly, digital RF transport significantly reduces capex and opex when deploying networks with centralized radio capacity through base station hotels. Digital RF transport gives an edge to a wireless service provider to compete today and tomorrow. 1 The Nyquist theorem states that the maximum bandwidth that can be accurately represented is less than one-half of the sampling rate. So, to achieve a full 35 MHz bandwidth, sampling must be at least twice as fast—70 MHz, or more. 2 Blocking Dynamic Range (BDR) is the difference between the noise floor and the 1 dB compression point. It is typically used to describe receiver (not transmitter) performance, and describes the effect an off channel strong signal will have on a weak signal. When a strong signal reaches the 1 dB compression point, it will “block” or “desensitize” the weak signal by 1 dB. For this spec to have meaning, the channel bandwidth must be defined. Example: if a system has a 10 dB noise figure (a 30 kHz noise floor of -119 dBm) and a level limiting threshold 0f -40 dBm (which compresses gain by 1 dB at -39 dBm), then the blocking dynamic range would be 119-39 = 80 dB. This definition can be interpreted as limited to noise, not spur limited signals. The Advantage of Digital RF Transport Page 7 Appendix A - Comparing Dynamic Range of RF Transport Systems Digital RF transport systems are immune to the noise and loss that degrades the performance of analog transport systems. In the above example, the shaded area shows that acceptable system operation requires dynamic range between 60 dB and 80 dB. The analog RF transport system starts off strong with a dynamic range of 80 dB. However, as the analog RF signal traverses various cables, modules, patch cords, and splices, noise and loss quickly degrade dynamic range to less than 40 dB. On the other hand, digital RF transport maintains dynamic range of 70 dB all the way from remote antenna to the base transceiver station. 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 1239772 8/04 Revision © 2002, 2004 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 Appendix A – Comparing Dynamic Range of RF Transport Systems Patch cord (0.5 dB opt) 1 dB Splic e (1 dB opt) 2 d B R F cabl e RF cable los s & NF 7 dB Remote Unit BT S Host Unit antenn a CWDM Module (3 dB opt) 6 dB 8 0 7 0 6 0 5 0 4 0 D Y N A M I C R A N G E (dB) Analog System CWDM Module (3 dB opt) 6 dB Patch cord (0.5 dB opt) 1 dB Cabl e (2 dB opt) 4 d B Cabl e (2 dB opt) 4 d B Cabl e (5 dB opt) 10 d B Digital System Acceptable Operation ––––– Digital RF Transport _____ Analog RF transport . Coverage and Capacity Digital RF Transport The Advantage of Digital RF Transport Page 3 Unlike analog RF transport systems, digital RF transport solutions. Because digital RF transport is immune to transport noise, a higher SNR is possible than with analog RF transport systems. With a better SNR, digital RF transport