Data Bases • Custom Term Papers • Free Term Papers • Free Research Papers • Free Essays • Free Book Reports • Plagiarism? Links • Top 100 Term Paper Sites • Top 25 Essay Sites • Top 50 Essay Sites • Search 97,000 Papers @ DirectEssays.com • Search 101,000 Papers @ ExampleEssays.com • Search 90,000 Papers @ MegaEssays.com Free Essays • Term Paper Sites • Chuck III's Free Essays • Free College Essays • TermPaperSites.com • My Term Papers • Get Free Essays • Essay World • Planet Papers	Search Lots of Essays Back to Subjects - Technology satelliteradio satelliteradio While the transition of television to a digital technology with its improved picture and sound quality has been a much publicized and controversial process, television's venerable ancestor, radio, has stayed in the background. But this year, in the United States, radio broadcasting is making its own digital leap. Two start-ups are introducing a new type of radio broadcast--subscription-based digital audio sent from satellites. With satellite digital audio radio services (SDARS), as they're called, listeners will be able to tune in to the same radio stations anywhere in the United States. SDARS differs from so-called digital music services, in which direct broadcast satellite or cable system operators provide digitized and compressed audio over their networks, both because of its programming and because SDARS can be received in a moving car, where much of today's radio listening takes place; existing digital audio services cannot. (A different form of satellite digital radio, from WorldSpace Corp., Washington, D.C., is currently serving parts of Africa and Asia. It started service in 1999, and is less optimized for mobile use.) Meanwhile, the free, over-the-air terrestrial broadcasters are expected to choose digital audio broadcasting technologies for both the AM and FM bands by year-end.( Martin R. Davidoff 6-12) Just as was true for other media, the conversion to digital offers radio a wealth of benefits not available from current AM or FM analog systems. Radio is an audio service, so consumers naturally compare it to the most pervasive high-quality audio technology, the compact disk (CD). A good FM signal played by a high-quality, stationary receiver compares favorably to a CD's sound. But put the FM receiver in a moving vehicle (typical for radio listening), and channel imperfections experienced by FM quickly lower the sound quality. The problems include environmental noise and RF signal reflections that give rise to multipath fading--variations in RF signal level due to signal reflections. The improved robustness of digital radio promises to remedy this, delivering near-CD sound for most listening conditions. With AM radio, the transition will be even more marked. The new digital service will offer two-channel stereo sound and greatly increased frequency response, resulting in audio quality comparable to today's analog FM. In addition, digital will greatly enhance terrestrial radio robustness, which is the ability to withstand factors such as multipath fading, environmental noise, and impulse noise interference due to automobile ignitions or home appliances. Digital's robustness also holds up well against interference from nearby radio broadcast signals and terrain blockage for FM, as well as signal attenuation due to grounded conductive structures like reinforced-concrete highway overpasses for AM. With most radio listening done in a moving car, the signal degradation varies constantly with time, further complicating matters. To reduce or eliminate these problems, a variety of signal-processing techniques (described below) have been incorporated into digital radio systems. While enhanced quality and robustness and new services entice broadcasters, and should bode well for a smooth transition to digital, another, less tangible reason for making this transition involves consumer perception. Terrestrial radio is one of the few remaining analog communications services in an ever more digital world. With the advent of SDARS, radio in the United States faces a direct digital competitor, and even though SDARS will not provide a locally oriented service like traditional radio, the inevitable comparison of technologies will have "old-fashioned analog radio" coming up on the short end of the stick. It's taken 10 long years, but finally, later this year, two U.S. SDARS systems will begin commercial operation. These two digital radio systems [see table], operated by Sirius Satellite Radio Inc., based in New York City, and XM Satellite Radio Inc., in Washington, D.C., share a number of similarities--like approximately the same number of separate radio channels--as well as some interesting differences--for example, completely different orbital configurations for their satellites. But whether the for-pay SDARS offerings will lure listeners away from the free terrestrial service is an open question. On an advertiser-supported local station, many program formats with a limited audience could never be economically viable. SDARS' operators maintain that even narrowly specialized programming will be feasible over a service that reaches the entire nation and is supported almost entirely by subscriptions. (Sirius plans to be commercial-free; XM plans to include advertisements in its news/talk programs.) Reach is clearly the main strength of the satellite delivery medium. Radio markets (primarily rural), now under-served by radio services with only a handful of stations, will no doubt welcome the choices offered to them by satellite radio. Satellite radio is an idea nearly 10 years in the making. In 1992, the U.S. Federal Communications Commission (FCC) allocated a spectrum in the "S" band (2.3 GHz) for nationwide broadcasting of satellite-based Digital Audio Radio Service (DARS). Only four companies applied for a license to broadcast over that band. The FCC gave licenses to two of these companies in 1997. CD Radio (now Sirius Satellite Radio) and American Mobile Radio (now XM Satellite Radio) paid more than $80 million each to use space in the S-band for digital satellite transmission. At this time, there are three space-based radio broadcasters in various stages of development: Sirius Satellite Radio is now operational in the United States, but is still carrying out quality assurance tests. It is not yet available to the public. XM Satellite Radio launched commercial service in limited areas of the continental United States on September 25, 2001. (They were originally going to launch service September 12, but postponed the event because of the terrorist attacks on the United States.) WorldSpace is already broadcasting in Africa and Asia, and will begin broadcasting in South America sometime soon. Satellite radio companies are comparing the significance of their service to the impact that cable TV had on television 30 years ago. Listeners won't be able to pick up local stations using satellite radio services, but they will have access to hundreds of stations offering a variety of music genres. Each company has a different plan for its broadcasting system, but the systems do share similarities. Here are the key components of the three satellite radio systems: Taking a closer look, you will see slight variances in the three satellite radio companies' systems. In the next three sections, we will profile each of the companies offering satellite radio services. XM Radio uses two Boeing HS 702 satellites, appropriately dubbed "Rock" and "Roll," placed in parallel geostationary orbit, one at 85 degrees west longitude and the other at 115 degrees west longitude. Geostationary Earth orbit (GEO) is about 22,223 miles (35,764 km) above Earth, and is the type of orbit most commonly used for communications satellites. The first XM satellite, "Rock," was launched on March 18, 2001, with "Roll" following on May 8. XM Radio has a third HS-702 satellite on the ground ready to be launched in case one of the two orbiting satellites fails. http://www.washtech.com/specialreports/xmsr_explainer.htm) XM Radio's ground station transmits a signal to its two GEO satellites, which bounce the signals back down to radio receivers on the ground. The radio receivers are programmed to receive and unscramble the digital data signal, which contains up to 100 channels of digital audio. In addition to the encoded sound, the signal contains additional information about the broadcast. The song title, artist and genre of music are all displayed on the radio. In urban areas, where buildings can block out the satellite signal, XM's broadcasting system is supplemented by ground transmitters. (http://www.xmradio.com/newsroom/screen/pr_2001_12_12.htm) Each receiver contains a proprietary chipset. XM began delivering chipsets to its XM radio manufacturing partners in October 2000. The chipset consists of two custom integrated circuits designed by ST Microelectronics. XM has partnered with Pioneer, Alpine, Clarion, Delphi Delco, Sony and Motorola to manufacture XM car radios. Sharp is also working on a home version of the XM radio receiver. Each satellite radio receiver uses a small, car-phone-sized antenna to receive the XM signal. General Motors has invested about $100 million in XM, and Honda has also signed an agreement to use XM radios in its cars. GM began installing XM satellite radio receivers in selected models in early 2001. For $9.99 per month, subscribers can receive the XM signal. For that price, listeners get up to 100 channels of music, talk and news. Many of the channels have no commercials, with none of the channels having more than seven minutes of ads per hour. XM's content providers include USA Today, BBC, CNN/Sports Illustrated and The Weather Channel. The service bolsters that lineup with its own music channels. Unlike XM, Sirius does not use GEO satellites. Instead, its three SS/L-1300 satellites form an inclined elliptical satellite constellation. Sirius says the elliptical path of its satellite constellation ensures that each satellite spends about 16 hours a day over the continental United States, with at least one satellite over the country at all times. Sirius completed its three-satellite constellation on November 30, 2000. A fourth satellite will remain on the ground, ready to be launched if any of the three active satellites encounter transmission problems. (http://www.siriusradio.com/servlet/snav?/servlet/about/se_work.jsp) The Sirius system is similar to that of XM. Programs will be beamed to one of the three Sirius satellites, which will then transmit the signal to the ground, where your radio receiver will pick up one of the channels within the signal. Signals will also be beamed to ground repeaters for listeners in urban areas where the satellite signal can be interrupted. While XM offers both car and portable radios, Sirius is concentrating solely on the car radio market. The Sirius receiver will include two parts, the antenna module and the receiver module. The antenna module will pick up signals from the ground repeaters or the satellite, amplify the signal and filter out any interference. The signal will then be passed on to the receiver module. Inside the receiver module will be a chipset consisting of eight chips. The chipset will convert the signals from 2.3 gigahertz (GHz) to a lower intermediate frequency. Sirius will also offer an adapter that will allow conventional car radios to receive satellite signals. The adapter will cost about $199. So far, WorldSpace has been the leader in the satellite radio industry. It put two of its three satellites, AfriStar and AsiaStar, in geostationary orbit before either of the other two companies launched one. AfriStar and AsiaStar were launched in October 1998 and March 2000 respectively. AmeriStar is currently scheduled for launch in late 2001. Each satellite transmits three signal beams, carrying more than 40 channels of programming, to three overlapping coverage areas of about 5.4 million square miles (14 million square km) each. Each of the WorldSpace satellites' three beams can deliver over 50 channels of crystal clear audio and multimedia programming via the 1467-1492 megahertz (MHz) segment of the L-Band spectrum, which is allocated for digital audio broadcasting. Initially, the United States will not be part of WorldSpace's coverage area. However, the company has invested in XM Radio and has an agreement with XM to share any technological developments. WorldSpace is going beyond one nation and eyeing world domination of the radio market. That might be overstating the company's intent a bit, but WorldSpace does plan to reach the corners of our world that most radio stations cannot. There are millions of people living in WorldSpace's projected listening area who cannot pickup a signal from a conventional radio station. WorldSpace says it has a potential audience of about 4.6 billion listeners spanning five continents. WorldSpace broadcasters uplink their signal to one of the three satellites through a centralized hub site or an individual feeder link station located within the global uplink beam. The satellite then transmits the signal in one, two or all three beams on each satellite. Receivers on the ground then pick up the signal and provide CD-quality sound through a detachable antenna. WorldSpace contracted four consumer electronics companies to produce small portable receivers, including JVC, Matsu*censored*a (Panasonic), Hitachi and Sanyo. Receiver costs range between $250 and $550. Each receiver is capable of receiving data at a rate of 128 kilobits per second (Kbps). The receivers use the proprietary StarMan chipset, manufactured by ST Microelectronics, to receive digital signals from the satellites. Worldspace subscription rates are determined by local distributors. These distributors offer multiple service options, ranging from a low-cost basic service to a high-end specialty service. Programming for WorldSpace is delivered by Discover, CNN, BBC, Bloomberg News and MTV Asia, among others. Technically, the Sirius Satellite and XM Satellite systems are unique in a number of ways. This is so not just with respect to existing broadcasting systems and the many types of satellite systems (broadcast and otherwise) currently in use, but also with respect to one another. Because these systems are designed to serve primarily mobile users and are targeting a consumer market, the individual receivers need to cost less than any satellite receiver ever built. They cannot rely, for example, on high-gain antennas with costly antenna-tracking devices to keep the antenna pointed at the satellite, the setup used for other mobile satellite systems like Inmarsat, which serves maritime and other commercial markets. (http://www.wirelessnewsfactor.com/perl/story/16799.html#story-start) Another consequence of mobile reception, for automobiles in particular, is that the satellite signal will be intermittently blocked from view by buildings or trees as the vehicle moves. The magnitude of this problem is a function of the latitude of the receiver because this establishes the elevation angle of the satellite with respect to the horizon. The farther north one proceeds, the lower in the sky the satellite appears and the more susceptible it is to being blocked from view. For both systems, the antenna on the receiver is of the low-gain, nearly omni directional variety, its pattern shaped like a hemisphere. So obstructions notwithstanding, the satellite will be in view of the antenna regardless of the vehicle's position. In the XM system, two satellites in a conventional geostationary orbit, one at 85º W longitude and the other at 115 º W longitudes, are used. These locations afford optimum coverage of the United States. According to XM, these are the "most powerful satellites in the entertainment industry," helping to compensate for the relatively low gain of the vehicle's antenna. Three specific techniques, each representing a different kind of redundancy known as diversity, are used by the SDARS services to enhance performance. With its two satellites transmitting essentially identical signals from two, widely spaced locations (though in different frequency bands, so they won't interfere with one another), XM has implemented spatial diversity. This arrangement reduces the probability that the satellite signal will be completely blocked from the receiver. (Sirius uses spatial diversity as well, but in a different manner.) Under ideal circumstances, an XM receiver in a moving vehicle will "see" both satellites and be continuously receiving and processing the signals from both. Because the receiver is in motion, from time to time at least one of the satellites will be blocked by an obstacle; in this circumstance, the receiver will have to rely upon the signal from just the one (unblocked) satellite. Because the probability of signal blockage to a receiver in motion served by just a single satellite is so great, spatial diversity is a key attribute of this system. Without it, during these times of blockage the audio output of the receiver would simply disappear, situation listeners would hardly tolerate. The two other forms of diversity used by both systems are frequency diversity and time diversity. As mentioned for XM, each of its two satellites is transmitting the same signal but in different frequency bands. This is a form of frequency diversity and can help to combat problems associated with multipath fading, since sometimes multipath fades are frequency selective and limited to one band or the other. Finally, time diversity is implemented by introducing a time delay between the otherwise identical signals broadcast from each satellite and between the satellite and terrestrial repeater signals. (Bonsor, K 112-300) With time diversity, when a short signal blockage occurs, it will affect the two signals at different times (relative to the underlying modulation), allowing the receiver to eliminate the effect of the blockage by making use of the unblocked portions of each signal. One consequence of a time diversity system is the introduced delay. Typically on the order of 4-5 seconds, the delay makes it difficult, but not impossible, for broadcasters to use the broadcast signal as an in-studio monitor (a common practice in which the on-the-air and the control room staffs listen to the received broadcast signal to monitor its quality), or for sports fans at a sporting event to listen to a broadcast (since the audio will be lagging the action). Sirius has taken a more novel approach to spatial diversity by using three satellites in a highly elliptical orbit [see figure]. In contrast, other existing broadcasting and most communications satellites are in geostationary orbits--that is, they always appear to be above the same spot on the earth. As the Sirius satellites orbit the earth, they move about a specific longitude (100º) while moving across latitudes and, with respect to the northern hemisphere, rise and set approximately every 16 hours; thus two of the three satellites are visible to receivers in the United States at any given time (hence providing spatial diversity). Consequently, these satellites have an elevation angle of about 60º on average, higher than the typical 45º or so angle of the geostationary satellites used by XM, but are constantly in motion with respect to earth stations. (http://www.techtv.com/products/consumerelectronics/story/0,23008,3360750,00.html by Hahn Choi) With this higher elevation angle, the Sirius satellites are less likely to be blocked. The constant motion of the satellites is not a problem because the receiver antennas are nearly omni directional and, in fact, the motion of a vehicle speeding at 100 km/hour will have a much greater impact on reception than will the apparent motion of the satellites. In the case of XM, each of the satellites will be broadcasting the same signals, but in different frequency bands. There are some operational consequences of the moving Sirius satellites. Because only two frequency bands are set aside for the satellite signals, only two of the three satellites can transmit at any given time; otherwise, two of the satellites would have to transmit in the same frequency band and would interfere with each other. To avoid this problem, as the satellites rise and set, a "hand-off" must occur between their rising and setting--one satellite ceases transmission and the other initiates transmission. This is likely to cause a brief interruption (on the order of milliseconds) in signal reception at the receiver for the signal being handed off. As long as the signal from the third satellite is not blocked during this hand-off time, the listener will be unaware of the switch; this is likely the case since the third satellite, at the hand-off time, will be at its highest elevation angle and will therefore have its lowest probability of blockage. Repeaters for those really hard-to-reach spots Also affected by the motion of the Sirius satellites is the system's use of terrestrial repeaters. Both XM and Sirius will use a network of ground-based transmitters to rebroadcast the satellite signal into hard-to-reach areas such as tunnels or urban canyons, adding yet another leg to the spatial diversity of the system. These terrestrial repeaters use a modulation technique different from the one used by the satellites, being optimized for use by terrestrial transmitters--namely, coded orthogonal frequency division multiplexing, or COFDM, versus the satellites' QPSK, or quadrature phase shift keying. For XM, since the satellites are geostationary, a high-gain (that is, a directional) antenna can be used at the repeater site to receive the incoming satellite signal. This is necessary since the rebroadcast repeater signal, which is very close in frequency to the incoming satellite signal, yet at a much greater power level, could otherwise overload the input side of the repeater. If the Sirius repeaters were to operate in this fashion--that is, were to make use of the signal being broadcast from the satellite--then their receive antennas would have to track the satellites along their highly elliptical orbit in order to maintain isolation between transmit and receive circuits. This would be an expensive proposition. So instead, Sirius has elected to feed its repeater sites using commercially available capacity of geostationary communications satellites, which operate in a different frequency band (Ku-band), thus alleviating this problem. Clearly, the satellite SDARS and the terrestrial IBOC are poised for a battle for listener's ears in the not-too-distant future. The long-term future of radio is a bit more complicated, though. For IBOC, iBiquity is already planning all-digital IBOC, which eliminates the analog signal altogether. This version represents the final phase in the transition from analog to digital services in the AM and FM radio broadcast bands. It would differ from the hybrid IBOC version by increasing the power in the IBOC digital sidebands and by replacing the analog signal with additional, lower-power digital sidebands. Among other things, these would support a lower-quality, but more robust, version of the main channel audio, which the system would blend to under adverse conditions (instead of to the analog signal). The advantages of all-digital IBOC are its higher overall bit rate, better coverage, and reduced interference with adjacent channel signals. Since all-digital IBOC will produce additional interference in the analog portions of adjacent-channel hybrid IBOC signals, however, iBiquity is recommending that all-digital service not be initiated until the market penetration of IBOC receivers is at least 85 percent. Thus, a transition plan will be needed. iBiquity expects that every such receiver, starting from the first ones built, will be able to receive both hybrid and all-digital IBOC signals, so that consumers who upgrade during the hybrid service will not be left with obsolete equipment once all-digital service begins. http://www.rwonline.com/reference-room/satellite-radio/rw-xm-retail2.shtml Future radio services may also offer audio on demand. Digital radio receivers with memory allow for the downloading and storing of audio files that can be selected on demand, or for capturing specific news, traffic, and other programs that can be retrieved later, providing the audio equivalent of a VCR (or the newly popular hard-disk personal video recorders) for radio listeners. Command Audio, Redwood City, Calif., is pursuing this on-demand approach to digital radio, focusing on DAB systems, in particular Eureka-147 services in the UK. Serious competition from wireless services, both terrestrial and satellite, also looms. As digital cellular phone systems expand their capabilities, and wireless Internet networks are deployed, listeners will have ever greater options in both audio entertainment and data-based services while on the go. Internet radio, however, is off to a rocky start with the recent pullback by broadcasters of their streaming signals owing to royalty disputes. We all have our favorite radio stations that we preset into our car radios, flipping between them as we drive to and from work, on errands and around town. But when you travel too far away from the source station, the signal breaks up and fades into static. Most radio signals can only travel about 30 or 40 miles away from their source. On long trips that find you passing through different cities, you might have to change radio stations every hour or so as the signals fade in and out. And it's not much fun scanning through static trying to find something -- anything -- to listen to. Bibliography: Works cited http://www.washtech.com/specialreports/xmsr_explainer.html http://www.washtech.com/news/media/14821-1.html http://www.washingtonpost.com/wp-dyn/articles/A59714-2002Jan3.html http://www.rwonline.com/reference-room/satellite-radio/rw-xm-retail2.shtml http://www.xmradio.com/newsroom/screen/pr_2001_12_12.html http://www.siriusradio.com/servlet/snav?/servlet/about/se_work.jsp http://www.wirelessnewsfactor.com/perl/story/16799.html#story-start http://edtn.bitpipe.com/data/search?site=edtn&cp=bpres&st=1&qp=site_abbrev%3Aedtn&qt=how+a+sattelite+radio+works&Go=Go&cr=bpres&ct=trm#resources http://edtn.bitpipe.com/data/detail?id=997211509_309&type=RES&x=2006054045 www.businessreport.com/news/20_6/media/ Word Count: 3927
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