white paper The OFDM Advantage ADC's Homeworx™ cable telephony system permits the delivery of broadband services to end customers over a hybrid fiber coax (HFC) cable plant. Only a limited amount of spectrum is available in which to deliver a full portfolio of services. The greater the number of services that can be delivered within that bandwidth, the greater the revenue that can be generated. Thus a service provider should seek to make optimal use of his revenue-generating commodity that is the most scarce, bandwidth. The spectrum modulation scheme that is implemented by the Homeworx platform makes the most efficient use of bandwidth of any system on the market today. In addition, the Homeworx platform provides superior immunity to the type of interference that is most frequently seen in the hostile upstream path of an HFC plant. HFC transport systems typically divide, or multiplex, the signal by either frequency or time to permit simultaneous transmission of multiple signals. The Homeworx system uses a form of frequency division multiplexing — orthogonal frequency division multiplexing, or OFDM — to efficiently permit the transmission of many signals. The Homeworx system also uses 32-level quadrature amplitude modulation, or QAM-32, to pack the maximum number of signals into a given quantity of bandwidth. Many competing HFC systems use a combination of time division multiplexing (TDM) and quadrature phase shift keying (QPSK) to accomplish the same purpose. The combination of OFDM and QAM-32 makes more efficient use of bandwidth and is more resistant to interferers than its TDM/QPSK competitor. In addition, the Homeworx system is the only HFC telephony system with variable modulation order. This is called DC-OFDM (dual constellation OFDM). The system can switch from QAM-32 to QAM-4 while maintaining the same orthogonal multiplexing. This provides either a 2.5x bandwidth increase (QAM-32) or a 10dB SNR advantage (QAM-4) depending on the application. Though many commonly transmitted signals are fundamentally analog, or time-continuous in nature, a signal can also be represented in its discrete, or digital, form for the highest degree of noise immunity. Therefore, many transmission systems utilize digital modulation techniques. One digital modulation technique, quadrature amplitude modulation, takes advantage of the following mathematical property: separate input signals may be carried on different components of a single frequency carrier wave, and successfully resolved into the original inputs by the receiver. One version The spectrum modulation scheme that is implemented by the Homeworx platform makes the most efficient use of bandwidth of any system on the market today. In addition, the Homeworx platform provides superior immunity to the type of interference that is most frequently seen in the hostile upstream path of an HFC plant. The Homeworx system is the only HFC telephony system with variable modulation order, called DC-OFDM (dual constellation OFDM). of quadrature amplitude modulation (QAM) uses binary-level modulation of the single frequency carrier wave components, generating an output signal space, or constellation, with four message points. Each of these message points, or symbols, carries two bits of information. (Please see Figure l). This is known as QAM-4, or QPSK. By using two components of the carrier wave, QPSK is able to carry twice as much information in the same amount of bandwidth. Higher-level QAM is able to optimize bandwidth even further by modulating the carrier frequency to a higher number of discrete levels. For example, 32-level QAM, used by ADC, generates a signal space, or constellation, with 32 discrete symbols. (Please see Figure 2.) Instead of using binary-level modulation, QAM-32 employs modulation that uses six discrete levels of the frequency carrier wave components. Each symbol in QAM-32 carries five bits of information. While both QAM-4 and QAM- 32 improve bandwidth efficiency, the improvement that comes with QAM-32 is much more significant. Each of the competing modulation schemes must also implement a method of simultaneously transmitting these compressed digitized signals without interference. Time division multiplexing, or time division multiple access (TDM or TDMA) transmits many signals in a common 2 MHz of spectrum. The signals are transmitted alternately in time, so that the signals take turns occupying the full 2 MHz of spectrum but no one signal requires its own full 2 MHz channel. Approximately 24-32 different signals, or DS0s, typically share one 2 MHz channel. In frequency division multiplexing, or frequency division multiple access (FDM or FDMA), signals are modulated on varying carrier frequencies so that they can be transmitted at the same time. ADC's version of FDMA, orthogonal frequency division multiple access, or OFDMA, divides each DS0 payload byte into two parts, each of which are carried on a different tone (frequency) in the RF channel. 480 tones are carried in each 6MHz channel, or 240 DS0s. ADC's OFDM/QAM solution is capable of delivering 240 DS0s in each 6 MHz channel, a clear advantage compared to the 72-96 DS0s per 6 MHz channel delivered by TDM/QPSK systems. Furthermore, the OFDM modulation scheme more effectively combats the type of interference most frequently encountered in the upstream path of an HFC system. Figure 2. QAM-32 Constellation Figure 1. QPSK Constellation ADC's OFDM/QAM solution is capable of delivering 240 DS0s in each 6 MHz channel, a clear advantage compared to the 72-96 DS0s per 6 MHz channel delivered by TDM/QPSK systems. Analysis of the upstream channel of an HFC plant will show that the type of interference most commonly encountered is narrowband interference. Several different types of amateur- and citizen- band radio are allotted narrow slots of frequency that fall within the 5-42 MHz band that is used for upstream transmission. In addition, some household appliances also emit RF output in this frequency band. These signals appear in the HFC plant as narrowband interferers that could potentially be severe. In a TDMA system, a narrowband interferer that falls anywhere within the 2 MHz bandwidth carrying the 32 upstream signals may render the entire 2 MHz useless for transmission. In an OFDMA system, each tone occupies only a very narrow slice of spectrum. A narrowband interferer may disrupt a few tones, but the amount of bandwidth loss incurred by the presence of an interferer is minimized. Additionally, ADC's modulation scheme implements dynamic frequency allocation. In the event of an interferer, the Homeworx system can assign a DS0 to a new portion of the spectrum without dropping the connection. Note that no “special” 2 MHz bands must be assigned for frequency hopping, unlike TDMA systems which further waste the bandwidth of an already inefficient system. Thus, ADC's Homeworx OFDMA HFC system provides protection against narrowband interferers that is superior to that of TDMA competitors. Impulse noise is another type of interference that may be encountered in the upstream path of an HFC system. An impulse noise is very short in time, but occupies a wider bandwidth. Impulse noise may be caused by time varying discontinuities in the cable plant or ingress from motors or high power arcing. Since frequency-multiplexed signals are not divided in time, they have substantially longer symbol periods than time-multiplexed signals. In the case of low-level impulse noise, power is distributed throughout the entire spectrum; therefore, its effect on any single DS0 is greatly reduced in FDMA systems. The Homeworx system implements additional protection by the use of block error correction algorithms to compensate for bit errors resulting from high level impulse noise. No competing telephony system utilizes error correction, the most effective guard against impulse noise. ADC's system is more effective at combating impulse noise than TDMA systems by providing superior resistance to low-level impulse noise and by providing additional compensation for high level impulse noise. The broadband service delivery marketplace is likely to be a competitive one. Having the ability to deliver the highest quality of service while maximizing resources supports a favorable business equation for the service provider. The OFDMA/QAM-32 solution employed by the ADC Homeworx system delivers the most services in the least amount of bandwidth, providing unsurpassed spectral efficiency. Furthermore, the Homeworx system provides protection against interference that is far superior to that of any other platform on the market today. 3 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. 100084EC 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. ADC's Homeworx system is more effective against both narrowband interference and impulse noise than systems relying on TDMA. . business equation for the service provider. The OFDMA/QAM-32 solution employed by the ADC Homeworx system delivers the most services in the least amount of. resolved into the original inputs by the receiver. One version The spectrum modulation scheme that is implemented by the Homeworx platform makes the most efficient