December 2010 Archives

Amateur Linking and Backhaul

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I have given a lot of thought to this. One of the issues with linking repeaters in the amateur world is that we're trying to do things that have already been done, years and years ago. Fortunately, all of that wisdom and experience is available at our fingertips, or out of the mouths of those old enough to remember and willing to speak. All the way back to Ma Bell's Long Lines microwave relay system, there have been various concepts introduced.

Long Lines was a channelized system which relied on translation (the block conversion of a group of frequencies to another group of frequencies) of existing channels for repeating or relaying and added a capability called "add/drop." Add/drop is a term still used in the telco world on circuits and certain types of gear. Long Lines was largely based on frequency-division multiplexing (FDM) of 300-4000Hz single-sideband (SSB) amplitude modulation (AM) signals. The resulting SSB signals were several tens or hundreds of kilohertz wide, depending on the number of channels the radio circuit was designed to carry. SSB is also subject to noise, and like most baseband or analog communications designs, requires equalization at multiple locations in the circuit equipment. Cable TV (CATV) experiences this as well. The increased attenuation at higher frequencies is referred to as "tilt". CATV amplifiers are designed with a "tilt" adjustment that allows for the higher frequencies to be amplified more than the lower frequencies to compensate for tilt losses. Later, Long Lines was transitioned in part to Digital Radio, using full-duplex DS3 radios costing $50,000 or more to connect cities or sites along the route.

Add/drop telephony refers to a piece of gear or circuit where channels are picked off and either not replaced, or replaced with other information. Synchronous and Asyncronous circuits alike may be configured for add/drop simply by adding equipment. In the sense of Long Lines, the Add/Drop functionality was not added by equipment; it was a function of that switching office itself. Microwave channels could be received on one dish or horn, translated to a different set of frequencies, and transmitted out another horn in a different direction. Likewise, a copper circuit could terminate at the Long Lines facility and be converted to a microwave channel and "Added" to the network. In the reverse sense, a microwave channel could terminate at the Long Lines Office and be converted (Dropped) to a copper circuit leaving the Central Office (CO). In T-series circuits, individual circuits may be added or dropped out of the larger group present. Modern equipment like the Adtran Atlas can do this on the DS0 (56K or 64K circuit) boundaries from T3s or DS3s. Other equipment, made by manufacturers such as ADC-Kentrox, may add/drop a single or multiple DS0 channels from a T-series (T1, T1c) circuit to allow combining virtual circuits over a physical copper loop from the telephone company.

The use of digital radios by Long Lines and another companies in the long-distance markets was supported by the marketing the systems allowed the company to do. Many systems which switched to PCM coding for voice immediately met with quieter, noise immune circuits. But the common plague of digital electronics still pervaded the space -- if the circuit was up and correctly configured, it would be error free. If the circuit was down or incorrectly configured, it may take months before the actual cause of the problem is located and remedied. Handling audio in the digital domain also freed technicians from the tedious labor associated with individually aligning circuits and amplifiers as well as equalization requirements for each stage of the equipment.

Several manufacturers make full-duplex digital T1 (24-channel) modems which allow telephone circuits to be extended many miles beyond the initial termination point. One well known radio station uses this method to allow the sales, management, and studio staff to have access to a nearby larger LATA (telephone calling zone) without paying long distance. This sort of connectivity, along with the ability to add and drop circuits at will, allows one to selectively "dial up" one or more channels of voice or data to a remote endpoint. In the data world, it is quite easy to inversely multiplex a larger, faster rate signal into multiple slower channel circuits to achieve higher throughput. This has been used since the infancy of the internet and the Apollo Space Program for transmitting data, telemetry, and video across analog circuits.

One of the larger issues and concerns surrounding amateur radio is the use of shared tower space. Tower space is often at a premium, and is very difficult to find for cheap or free. Furthermore, there is a complicated issue with respect to the Part 15 bands, primarily 902-928MHz, 2.4 - 2.45 MHz, and 5.3/5.8GHz. Many of these bands coincide with amateur radio bands, in which amateur radio operators are granted the immunity from other operators in the band who -- being unlicensed -- must tolerate interference from licensed users of the spectrum: amateur radio operators. Where this turns into a conflict of interest is that the Part 15 band user may want access to the tower and may mount commodity video, data or other equipment on the tower which may cause interference to users in that band who are licensed or otherwise permitted use of those frequencies. Moreover, the Part 15 user is a for-profit enterprise and is paying rent for the tower, while the amateur radio operators, who claim a higher priority to the frequencies desired by the Part 15 operator, are not paying for rent. The simplest solution in the eyes of the site manager is to tell the hams to pack it and leave -- money talks. Hams are strictly forbidden from engaging in for-profit enterprising using amateur radio, so as long as that rule stands, ham equipment on towers will cost a large amount of money to setup and maintain. The solution, therefore, is to avoid frequencies which are shared with other users of the site or that might cause interference to other paying users of the site. This basically confines a ham to bands which are solely the domain of radio amateurs, or shared with federal agencies. 222-225MHz (1.25m) is one these bands, as is 420-450MHz (70cm).

The FCC Rules, Part 97 limit the speed and amount of bandwidth that transmissions at or above certain frequencies may occupy. At the time of this writing, the following limits apply for 28MHz (10m) and up:

  • 10m: 1200 baud signaling rate, FSK may not exceed 1KHz
  • 6m: 19.6 kbaud signaling rate, 20KHz emission width
  • 2m: 19.6 kbaud signaling rate, 20KHz emission width
  • 1.25m: 56kbaud signaling rate, 100KHz emission width
  • 70cm: 56kbaud signalling rate, 100KHz emission width
What strikes my eye about this is the rate, specified as "symbol rate", particularly in bauds. A BPSK signal, operating at 9600 baud, transfers data at 9600 bits per second, at one bit per hertz of occupied bandwidth. QPSK on the other hand, transmits two data bits per symbol, doubling the capacity of the same 9600 Hz of occupied frequency. Switching over to QAM at higher densities allows for even more bits to be packed into a given space. Using QAM or QPSK requires linear amplifiers and different modulation methods to produce them. This adds complexity, however it extends the most precious resource we have -- limited bandwidth. By using 4/pi QAM on a 56Kbaud circuit occupying around 100KHz, 224Kbit/s of data may be transmitted using linear amplifiers. At these widths, duplexers may be used to fine-tune filters to keep noise out of the systems, as well as prevent possible interference from or to other stations on the same mountaintop. Additionally, being down in the amateur radio bands prevents a fight over who stays and who pays to use a given section of vertical real estate. Since the amateur radio operator isn't sharing frequencies with the unlicensed or commercial operator, this situation should not exist.

In short, we owe it to ourselves, as amateur radio operators, to explore dense data modulation techniques and apply them to amateur radio. We have had a thirst for bandwidth dating back decades; now we finally have commodity modems that make achieving those bandwidths possible using limited RF. Just look at Digital TV: 19 or 23Mbit/s in 6MHz. Twenty years ago, in 1990, that would have been astounding! And now we have that on ever single person's set top TV.

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This page is an archive of entries from December 2010 listed from newest to oldest.

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