Six into one will go
Technology already proven for 220MHz applications now focuses on the need to increase capacity and efficiency for VHF highband. The availability of linear modulation technology, already proven in 220MHz applications, and the requirements of the FCC refarming order (92-235), finalized in 1997, have converged to create new opportunities for VHF highband communications as well.
Technology in-waiting Although “linear modulation” (LM) has been in use for decades, in its contemporary context it refers to LM implemented with the merging of four additional technologies: reference vector equalization, Cartesian loop transmitter, feed-forward signal regeneration and digital signal processing. In this incarnation, very narrowband linear modulation (VNLM) was first demonstrated in 1992 and was type-accepted by the FCC in 1994, at which time it began to be applied to several 220MHz systems. The 220MHz-222MHz auction in 1998 has given impetus to extending use of that band beyond single-site applications.
Refarming revisited One of the goals of refarming was to get the best spectrum efficiency. There is limited existing spectrum, limited new spectrum and congestion in the bands in current use. There are also limitations to many of the current wireless technologies.
One solution is to increase the current spectrum capacity. Various ways are being promulgated to do that. The FCC could mandate a reduction in channel size. In a way, that is one effect of the type-acceptance process because increases in interference will actually drive system managers to reduce channel size. Because type-acceptance is questionable as a policy tool, some industry organizations are pushing for a more specific mandate that would encourage a transfer into more spectrum-efficient technologies.
Another solution is to increase the channel capacity by various other ways, either by using that channel for less time or by substituting data, a more efficient means of communicating information, for voice when feasible. A similar solution is to provide higher-speed data on the channels, and LM technology already provides 128QAM (quadrature amplitude modulation) trellis coding, yielding as much as 16.8kbps on 5kHz channels.
Another approach is to provide more functionality by trunking. Many systems are now moving toward trunking because of the increased efficiency that technology brings.
The refarming effort enables more users to simultaneously access the available spectrum by using bandwidth more efficiently. That’s the whole objective, and the FCC has mandated more spectrum-efficient technologies through its type-acceptance process.
Needs assessment What does this really mean to users? Users are saying, “We need more channels now, not in 10 years’ time.” More new users want access to spectrum, and existing users want more channels because they want to enhance the capabilities of their wireless systems. For example, public safety is demanding bandwidth to transmit large files containing photographs, fingerprints and video.
Users want better performance. Whenever there has been a reduction in channel bandwidth, there has been an attendant reduction in voice quality. The move toward purely digital systems also degrades overall voice quality.
Users want voice and data available in their products now. Most users do not want to undertake another change-out of equipment in seven years (2005) when the 6.25kHz-spacing requirements are in place.
Users are still looking for direction. We often read in the trade press that there still is not a “good direction” that users can go to for refarming because some of the proposed solutions have technical or practical drawbacks.
Applying LM technology LM is a practical solution to the strains driving refarming and the needs of users because it can exist within the crowded spectrum. LM does not need new, cleared spectrum to operate. It uses just 5kHz channel bandwidth, and it is a frequency-division multiple-access (FDMA) system, not TDMA or CDMA, alowing flexibility when planning a wide-area, multichannel or multisite system.
Read the fine print The capacity of a 30kHz FM channel can now be increased by up to six times. The rules are already in place.
A lot of people who have only glanced at the FCC rules say “No, no. At VHF highband, the channel spacing is 7.5kHz.” Re-read the rules more carefully. They say you can actually do anything you like, including 5kHz, which is important because it enables this technology. Users are asking for more channels now, and there have been ongoing demonstrations for the frequency coordinators of how LM technology can support management of crowded spectrum.
Run the numbers Differences exist between the technical performance of the prevailing FM technologies and LM, as shown in Table 1 below. The modulation type is linear, with a non-constant envelope system (as are most of the digital systems). LM uses just 5kHz of bandwidth and has a co-channel protection ratio for LM:LM that is 2dB better than a 12.5kHz FM system. At range edge, when you’re down at 12dB sinad and you’re comparing FM with LM, trials using random-word test procedures have established that voice intelligibility at low signal levels with LM remains highly intelligible and readable, even with a weak carrier.
How do we define a “useful” data rate? It is the rate at which information can actually be sent and recovered. The recoverability of data at 12.5kHz FM is considerably less than at 25kHz. LM provides a mean bit-error rate (BER) advantage because of how the system works and recovers the signal.
(Incidentally, 12.5kHz spacing is actually old technology. The United Kingdom adopted it 30 years ago. It is not only old, but in many respects it has become “time-expired.”)
The more RF a site generates, the greater the potential for interference. But that spiral can be broken. For FM, mean power, average power and peak power are all the same. (In fact, whether you’re transmitting information or not, you’re still transmitting peak power.) LM power output is not constant. The mean power is considerably less (1/3) than FM. Consequently, the amount of current that is drawn, and the amount of potential interference that is transmitted, is considerably less. The overall effect is a lessening of the level of RF that can cause interference.
Six out of one Co-channel interference ratios are the key to practical refarming. Figure 1 above illustrates that you can put LM into the FM environment with minimal interference. This is a 12.5kHz example, not a 30kHz example, that shows that with the LM signal just 3.75kHz away, it can cope with with an interfering FM signal 32dB higher, with the same co-channel interference ratio.
Looking at a VHF FM highband 30kHz bandwidth (Figure 2 on page 20), what you’re actually authorized to occupy is 20kHz; that is, the emission designator for that bandwidth is only 20kHz, and that is where the FM signal sits. The equipment bandwidth is 25kHz, but the occupied bandwidth is only 20kHz. This leaves space, the guard bands, at the upper and lower sidebands. Now, the 20kHz bandwidth will accommodate two LM channels in the adjacent guard bands with minimal interference into the FM, or for the FM into the LM. (We have spectrum analyzer documentation of this configuration. Combining equipment is not yet available to do this at the same site, but it is certainly achievable now within a mile or two radius.) Figure 2 also shows the 7.5kHz channel centers that have been designated, but not mandated. There is an advantage in not using those channel centers because it improves the interference ratios.
So how do we do refarming? At the first stage (Figure 3, above, top line), you can easily put three 5kHz channels on the 7.5kHz channel centers, but that leaves considerable wasted space. The second stage (Figure 3, middle line) we jokingly refer to as the “attorney’s view” that says “You actually have to use 25kHz equipment, therefore you can do whatever you want to, over 25kHz.” That would allow for five channels on 5kHz channel centers, but there is still wasted upper and lower space.
Six channels can be located in that same 30kHz as the rules are written. (Figure 3, lower line.)
Intek Global has already gotten a license issued for the stage-one configuration. A four-channel application is undergoing review at the FCC. A six-channel application is being processed, with the Industrial Telecommunications Association (ITA) as frequency coordinator. We’re testing the situation, orders are already in hand for the first six-channel systems.
Summary LM accomplishes the goals of users, regulators, frequency coordinators and manufacturers. VHF highband LM equipment is now type-accepted, and the first license has been issued. You do have a choice; six into one will go.
