Tài liệu Modem Technology vs. HFC Ingress Noise doc

white paper Homeworx™ Introduction The subject of ingress on the coaxial transport (either accidental or intentional) is of the largest concern when considering the transportation of telephony services on a Hybrid Fiber Coax (HFC) system. With a well-maintained coax plant that is continually checked for compliance with FCC standards for cumulative leakage index, return path ingress should not present a problem. However, it must be assumed that no system is impervious to strong RF fields that are in close proximity to the telephony equipment. The most common weak points through which ingress enters the system are the customer drop, and faulty "F" connectors on the drop cable. Therefore, measures must be taken to ensure that there is a solution to all possible scenarios that might affect system operation. ADC understands very well the significance of ingress in cable plants. This is why ADC conducted ingress testing for more than two years before developing the Homeworx "virtual modem test system." Based on the results of subsequent trials, ADC added increased ingress protection to the system and solidified the choice of orthogonal frequency division multiplexing (OFDM) for the Homeworx multiplexing scheme and Quadrature Amplitude Modulation-32 (QAM-32) as the primary † modulation scheme. This approach has been validated by four years of field experience with the Homeworx system. CableLabs® in Boulder, Colorado, has also performed extensive research and study on this subject and has concluded that while the forward transmission path (50-750 MHz) is relatively free from the effects of ingress, the reverse path sub low spectrum on a cable system (5-42 MHz) is indeed an extremely hostile environment that suffers from the effects of narrowband signals such as RF broad- cast, amateur, and citizen band signal sources. Additionally, impulse noise also plays a significant role in affecting the digital transmission performance of a system. Any transmission scheme that is to operate in this environment will need to have a multi-tiered approach to combating the detrimental effects that these signal sources will have on telephony or data transport on hybrid fiber coax based communication systems. † The more recent Homeworx systems support QAM-4 (or Quadrature Phase Shift Keying [QPSK]) as an alternative modulation scheme to combat at least 9 dB higher plant noise but at 2.5 times lower channel capacity compared to QAM-32. This feature is known as Dual-Constellation OFDM scheme based on the fact that the user can choose either QAM-32 or QAM-4 modulation depending on the prevailing circumstances. Modem Technology vs. HFC Ingress Noise The reverse path sub low spectrum on a cable system is indeed an extremely hostile environment that suffers from the effects of narrowband signals. Any transmission scheme that is to operate in this environment will need to have a multi-tiered approach to combating the detrimental effects that these signal sources will have on telephony or data transport on HFC-based communication systems. Interference There are two types of ingress interference that are of interest in a broadband communication system, impulse noise and narrowband noise. These can be injected into the network disrupting video as well as telephony services. For example, all the telephony upstream information is carried in the 5-42 MHz range. If ingress from a subscriber's home enters the network, it has the potential of disrupting all other subscribers. The fact that the subscriber has a CATV tap inside their home opens the door for opportunities for mishap. One potential problem is a subscriber hooking up a TV antenna directly to his CATV cable. Hackers trying to create mischief could cause more serious problems. Any HFC system must consider both accidental and intentional ingress of narrowband interference and impulse noise when designing the system. The Homeworx cable telephony system is designed to handle both of these types of ingress. Narrowband Ingress Narrowband ingress is a narrowband RF signal that is coupled into the cable from an external source. An ingress signal which is located within a modem waveform can potentially take the entire band off-line. Narrowband interference comes from many sources. CB and Amateur (ham) radio are typical examples of narrowband interference; usually these are 5 kHz (sometimes 25 kHz) in bandwidth, and are located in several frequency bands that are also allocated for use in the upstream portion of the CATV RF spectrum. ADC addresses this problem by using OFDM (or Orthogonal Frequency Division Multiple Access — OFDMA), which allows channels to be moved to another portion of the spectrum with a granularity of 18 kHz (one DS0). Therefore, narrowband ingress may be avoided without losing large amounts of channel capacity. Some channels may be reserved for protection channels to allow calls in progress to be switched to a different channel in the event that ingress in the form of narrowband interference becomes present on the system. This call "hand-off" to another frequency, or channel, can be accomplished without the call being dropped. Impulse Noise Impulse noise is a time varying RF interference that is typically caused by such sources as lightning, automobile ignitions, faulty insulators on high voltage lines, and overloaded lasers which can produce clipping noise that resembles impulse noise. This type of ingress is much more difficult to deal with because of its random broadband nature. The ADC modulation technique has a unique way of dealing with this type of ingress by spreading the impulse energy over many symbols in frequency or over longer symbol length in time. Consider a Time Division Multiple Access (TDMA) system and an OFDM (or OFDMA) system. Assume the TDMA system has a symbol rate of 10 symbols/second. The OFDM system has a symbol rate of 1 symbol/second, and contains 10 OFDM tones, resulting in a net data rate of 10 symbols/second. Each signal has identical average power and therefore identical energy per symbol. The primary difference between the two systems are that the OFDM system has a much slower symbol rate, hence a narrow receiver bandwidth on each tone (a consequence of the FFT [Fast Fourier Transform] detection methodology). The receive bandwidth in the example TDMA system is 10x the receive bandwidth of the OFDM system. Consider a pulse that is 0.1 seconds in duration with a magnitude equal to the Root Mean Square (RMS) value of the TDMA symbol, and is also aligned with a TDMA symbol. The signal to noise ratio (SNR) under this circumstance would be 0dB. The pulse would pass through the TDMA receiver with little attenuation. Consider the same pulse within an OFDM symbol. The symbol length is 10x as long as the impulse, and the receive bandwidth is 10% of the equivalent TDMA system. Therefore, only 10% of the pulse power would pass through the receive filter and the resulting average SNR would be 10 dB. Consequently, the impulse noise susceptibility of the Homeworx system using OFDM/QAM-32 is reduced over a equivalent single channel TDMA system by: 10 x Log 10 ( Bwtone ) = 10 x Log 10 ( 9 kHz ) = -27 dB Note: 9 kHz rather than 18 kHz is used because a DS0 is actually composed of two 9 kHz tones. On the other hand, susceptibility for noise of the Homeworx system using OFDM/QAM-4 is reduced over an equivalent single channel TDMA system by: 10 x Log 10 ( 9 kHz ) = -23 dB Common Path Distortion (CPD) An OFDM system has one other advantage over a TDMA system: the tolerance for Common Path Distortion (CPD). CPD is a source of noise which is frequency dependent. Since OFDM uses multiple carriers, an OFDM system like Homeworx can abandon the bad frequency regions and select only the good ones for transport of telephony signals. Reducing Ingress Plant segmentation Reducing the size of the distribution area through plant segmentation will not only support increased system capacity to each subscriber, it will reduce the funneling effect that coaxial systems exhibit. Funneling occurs when all legs of the coaxial plant effectively create a funnel into which all forms of ingress are fed and appear at the combined input to the reverse path transmitter at the optical distribution node. The very nature of the reverse plant, in which all return signals share the same medium, does not guarantee a reliable reverse transmission path based on plant segmentation size only. There must be other means that can be used in conjunction with plant segmentation to minimize the effects of ingress. Impulse noise ingress is much more difficult to deal with because of its random broad- band nature. The ADC modulation technique deals with it by spreading the impulse energy over many symbols in frequency or over longer symbol length in time. Bwchannel 5 MHz 5 MHz/2.5 OFDM/QAM-4 Since QAM-4 with Forward Error Correction (FEC) has additional 3 dB of advantage against broadband white noise and retains the various OFDM advantages listed above, OFDM/QAM-4 coupled with FEC would be the most robust telephony transport scheme available over HFC. ADC is the only vendor supplying a cable telephony system that supports adaptable modulation — QAM-32 or QAM-4. Hence, by using ADC's Homeworx system, operators with questionable upstream plant can choose QAM-4 as the operating mode and once the plant improves or is improved and channel capacity becomes the driving force, they could switch to QAM-32 as the operating mode. Maintenance Field tests have determined that good building practice and regular maintenance of a coaxial-based communication system cannot be over emphasized. The system must be continually checked for precise gain alignment on both the forward and reverse paths. Also, a comprehensive and thorough plant hardening effort must be implemented. Frequent checks for ingress and leakage and immediate repair of the offending sections of the system must be an ongoing effort by plant maintenance personnel. With a properly maintained plant, all of the industry established objectives will be met by the system. OFDM Deployments As of 2000, ADC has deployed 150,000 subscriber lines with AT&T, helping to achieve AT&T's target of 500,000 subscribers. Additionally, ADC's installed base includes MSOs such as Comcast and Midcontinent, Alternative Service Providers/overbuilders, municipalities, and utilities. Homeworx Cable Telephony is currently installed in 15 markets throughout North America. Conclusion ADC has developed multiple lines of defense to deal with ingress on upstream telephony traffic: 1. Homeworx telephony uses a proprietary spectrum efficient OFDM modulation scheme. OFDM (Orthogonal Frequency Division Multiple Access, OFDMA) allows channels to be moved with a granularity of 18 kHz (one DS0). The system is able to detect when an interfering carrier is occupying a channel slot. It will then automatically search for and acquire an unused portion in the allocated spectrum to transfer the call to. Therefore, narrowband ingress may be avoided without losing large amounts of channel capacity. 2. The receive bandwidth and consequently the selectivity of an OFDM system is much narrower than that used with equivalent modulation techniques. This means that the system will allow much less impulse noise to pass through to the receiver, and therefore will create less chance of interference to the telephony carriers on the system. Recently some hardware manufacturers, software firms and users of OFDM technology have joined together to form a voluntary association called The OFDM Forum. One of the goals of this forum is to encourage the broad acceptance of a single compatible global OFDM standard on a worldwide basis. ADC had realized the immense potential of OFDM technology six years ago when it embarked upon the Homeworx project. And since 1996, telephony service providers in several major cities in US have successfully deployed ADC's Homeworx system which is the only OFDM-based HFC telephony system available in the market today. 5 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. 100327EC 03/01 Original © 2001 ADC Telecommunications, Inc. All Rights Reserved An Equal Opportunity Employer Web Site: www.adc.com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-946-3292 For a complete listing of ADC's global sales office locations, please refer to our web site. 3. Fixed frequency upstream ingress filters for the customer drop will prevent ingress from entering the system from the customer premise. 4. In the event that these methods of combating ingress do not sufficiently screen out the interference, the system also has Forward Error Correction capability. This will help to maintain the integrity of the data used in the telephony transport. 5. QAM-4 can provide additional noise immunity for questionable upstream plant; once the plant is improved and channel capacity becomes the driving force, QAM-32 may be implemented. 6. Further protection measures beyond those listed above are used employing the special characteristics of OFDM. These techniques remain proprietary to ADC. . QAM-4 modulation depending on the prevailing circumstances. Modem Technology vs. HFC Ingress Noise The reverse path sub low spectrum on a cable system is. types of ingress. Narrowband Ingress Narrowband ingress is a narrowband RF signal that is coupled into the cable from an external source. An ingress signal