.FLYINGHEAD INSIDE THE TECHNOLOGY
.TITLE Understanding the information rate of BPL and other last-mile pipes
.AUTHOR Glenn Elmore
.SUMMARY Throughout our research into BPL, we’ve talked about interference issues. In his in-depth interview, elsewhere in this issue, Glenn Elmore introduced the question of data rate across the various technologies. In this short, highly technical article, he shows how that data rate applies over a variety of "last-mile pipes".
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.H1 About this article
Throughout our research into BPL, we’ve talked about interference issues. In his in-depth interview, elsewhere in this issue, Glenn Elmore introduced the question of data rate across the various technologies. In this short, highly technical article, he shows how that data rate applies over a variety of "last-mile pipes".
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Shannon’s equation, proposed by Claude Elwood Shannon back in 1948, computes the upper-end of the capacity of a communications link (in bits/second). This is computed as a function of the link bandwidth and the signal-to-noise ratio (or the percentage of how much useful information passes through all the random interference on the line).
To compute information capacity, Shannon’s equation has been applied. If you’re an engineer, you’re going to be familiar with the equation shown in Figure A.
.FIG A Shannon can tell you just how fast your feed should be.
Looking at the above equation, C is the maximum information rate with an optimum coding technique, measured in bits/second. B is the bandwidth, measured in hertz. S is the encoded information (signal) power. N is the noise and interference power.
I’ve used Shannon’s equation to attempt to make a reasonable and fair comparison but even so the plot shown in Figure B should be viewed as a qualitative indicator rather than a precise one. The plot is a comparison of the estimated maximum information capacity versus distance for several last-mile propagation media.
.FIGPAIR B Different technologies result in different bandwidths.
The table in Figure C shows the parameters used for each medium in the above plot.
.FIGPAIR C Here’s what was used to measure maximum information rate.
I’ve estimated total information capacity by dividing the available spectrum into 100 segments, spreading the signal power evenly over the spectrum and performing a piece-wise integration of the information capacity using the parameters appropriate to the center of each individual segment.
Where possible, 0 dBm (1 milliwatt) is assumed for signal power. Fiber systems normally use less than this and BPL has been allowed more to better match existing practice. A spectrum width of 7 GHz is used except for conventional (HF) BPL and DSL lines.
BPL capacity has been calculated using the full spectrum studied by the OPERA (Open PLC European Research Alliance) report, which is about twice that used by the HomePlug standard. Spectrum has not been reduced to account for notching. Signal power is also about five times higher than that used for HomePlug devices. Although BPL chip sets which can operate at 200 Mbps exist, it still would not be possible for them to operate at full rate under these conditions.
It should be understood that the plot in Figure B shows a maximum theoretical capacity and that no allowance for imperfection or margin for variation has been included. Therefore any real system can only approach these results.
For example, common DSL modems use only a few MHz of spectrum but considerably more than 1 milliwatt of signal power. These differences along with modem imperfections and allowance for line variations may provide typical performance at 3000 feet (914 meters) of a few Mbps rather than something closer to the approximately ~30 Mbps, as shown on the plot.
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.H1 Product availability and resources
To read our full interview with Glenn Elmore, visit http://www.computingunplugged.com/issues/issue200608/00001829001.html.
For more information on OPERA, visit http://www.ist-opera.org.
For more information about Corridor Systems, visit http://www.corridor.biz.
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.BIO Glenn Elmore has over 30 years experience in communications and electronics design. During this time he has developed a world-class expertise in analog and microwave design, measurement, antennas and propagation. His 28 year tenure with HP/Agilent prior to founding Corridor Systems has included various engineering roles in the R&D Lab developing advanced RF and microwave products from 0.05-50GHz. In addition, Glenn has been an active leader in the amateur radio and communications communities, authoring over a dozen articles and achieving several firsts including a low-cost 1Mbps wide area fixed-wireless microwave packet network in 1989. Glenn holds patent #US Patent #4641086 for his work at HP as well as the multiple issued patents pending for the Corridor technology. For more information about Corridor Systems, visit http://www.corridor.biz.
