Introduction
There have been numerous comments from concerned and interested amateurs about the "technology" used in amateur radio. They ask why we don't see a widespread use of high-speed "digital" modes that can do ASCII or voice or pictures. The Internet has it, why doesn't amateur radio? Well, I'd like to make a few comments and try to explain why we do what we do. Just to lend some validity to my comments, I currently work for Motorola in their Cellular Infrastructure Group, and in the past, I have worked in microwave products, RF systems research, and microwave antennas, looking at propagation in line-of-sight and scattered environments. My opinions aren't "gospel truth", but they are worth some thought and consideration.
HF - Communication Channel Bandwidth
At medium and high frequencies, from 1.8 to 30 MHz, propagation is usually via ionospheric reflection. The frequency bands are usually narrow (about 5%), and antennas are often flat for only 10% of their center frequency. Once a signal is bounced off the ionosphere, the received signal usually has a coherence bandwidth of only a few kilohertz. By that I mean that a "fade" is only a few kilohertz wide, maybe only 100 Hz wide. A few kilohertz away, there is no fade, at least not right then. A fade can be looked at as a "moving notch", a few kilohertz wide. Such a notch can be heard on AM shortwave stations, hitting first one sideband, then the carrier, then the other sideband, as it moves across the station.
There are a few ways to counter this problem. One way is to reduce the transmission bandwidth, like by running CW, so that the fade notch looks "flat" across the mode's bandwidth. Then AGC in the receiver can handle the level variation we observe. This is the method that most hams use. Another is to add diversity reception, either space or polarization diversity using two or more antennas to two receivers. The receiver with the better signal is selected in the simplest scheme, or the two signals could be weighted and combined together to give an improvement in availabilty. Along those lines, since a fade doesn't effect all frequencies simultaneously, frequency diversity could be used where only one antenna is needed but two frequencies are used to carry the information. This usually is not allowed for amateur purposes since two transmitters carrying the same information would be needed. Another form of diversity uses time ... the information is repeated if it is corrupted. Still another way might be to use "spread-spectrum" communications. But ...
Hold on ... there's more coming.
VHF and UHF - Line of Sight Propagation
At VHF and UHF, we could divide propagation into two categories: line of sight propagation, including some tropo-scatter and mobile multi-path scatter, and non-line of sight propagation, including auroral reflection, meteor trail reflection, and ionospheric reflection. These two propagation categories either have fade notches that are much wider than the information bandwidth (LOS case) or much narrower than the information bandwidth (non-LOS). The first allows us to treat fades as "flat" over the channel. AGC and diversity help here. If the fade is narrow or "selective", we are pointed at same countermeasures we use at HF. So the common weak-signal modes like CW and SSB, and maybe AM and NBFM, would be used for the non-LOS propagation at VHF as they are at HF. That leaves us with LOS propagation at VHF or UHF, at less than 60 miles (100 km).
VHF and UHF - the Wide-Area Coverage Repeater vs. the Cellular System
Let's consider an example: we would like to cover an area of 1000 square miles, that's a square about 30 miles on a side. We could cover it with one big repeater in the center about 110 feet up. We could also cover that area with 12 smaller repeaters about 40 feet up, if we can find some way of tying them together - maybe simulcast? If we use 20 khz channel spacing like on 222 MHz and about 1 MHz of repeater output band, we could have 48 channels in that MHz. If we use one wide-area coverage site, we can use all 48 channels, but the surrounding countryside has NO channels available ... there is no re-use possible. If we plan on re-using the channels elsewhere, and each repeater coverage area touches about six more, we can only put up FOUR channels at our site. We can have four users in one MHz in 1000 square miles. If we go with the smaller "cells", we could simulcast the transmit audio. The result would be just like the wide-area coverage "large cell" from before. If instead we have small coverage areas from our 12 repeaters, we can still have four users per MHz IN EACH CELL or about 48 users in 1000 square miles. Follow this line of reasoning and you have the "cellular concept" ... just use smaller cells to increase the total system's capacity in more densely populated areas. That's what used on your cell phones. They also have 25 MHz of output band, 10 khz per channel, and a "use factor" of maybe 0.03 to 0.09 (most phones are not in use at any given time). Together with trunking efficiency and antenna pattern sectorization, the cellular system can support thousands of phones in a 1000 square mile area. No magic ... just a different set of goals for what the system should do.
The VHF and UHF amateur repeaters are a "party line", often with multiple talkers on one channel. They are put up by clubs or groups of interested hams, and have no way of "handing-off" mobiles who drive out of range. The cellular system, on the other hand, has only one target phone, usually landline, with one cellular operator, and the mobiles would expect to be "handed-off" as they keep on driving through multiple cells. Also the average conversation on an amateur repeater is probably five to ten times longer than the typical two-minute telephone call.
I guess the key word to describe the amateur repeater is "range", while the key word to describe the cellular system is "capacity". All technologies that are considered for the cellular system are judged in "capacity". That's why cellular made the transition from AMPs to NAMPS and NADC (3x AMPS capacity), and then on to CDMA (6x AMPS capacity). But at what price?
DS-CDMA - The Big Lie?
Boy, that's a bit strong. But there are quite a few "misconceptions" about direct-sequence CDMA as used in IS-95 for the cellular system.
Statement 1. CDMA has a 20 times capacity improvement over AMPS in a three-sector system.
False. Motorola's CDMA system planner and citywide simulations show that IS-95 CDMA is about 6 times better than AMPS at voice-coder rate set 1. It is about 4 times better than AMPS using the better-sounding rate set 2. Of course, NADC-TDMA and NAMPS are about 3 times better than AMPS, and a variety of TDMA called E-TDMA is 4 times better than AMPS, or the same capacity as IS-95 CDMA. By the way, NADC TDMA uses a different, poorer-sounding voice coder at effectively rate set 1.
Statement 2. CDMA is inherently immune to jammers.
False. IS-95 CDMA is 1.2 MHz wide. The coding gain can reject interferers by 19 to 21 dB, and since 6 dB is needed for "error-free" transmission, we can figure on 15 dB of rejection of undesired signals. That's like having an IF filter that's only 15 dB down and 1.2 MHz wide. The "window" of susceptibility to interference is very wide. In fact, the only way CDMA works is if the base site can control ALL emisssions in the 1.2 MHz channel. No uncontrolled jammers, no harmonics, no "rogue" transmitters. If you can control all emissions in the band, you can get a factor of two in capacity. If not, you get less ... sometimes a lot less ... like zero. Ask about the trucking company interferer in St. Petersburg, Russia, or the abandoned cellular band in Japan that they couldn't get the "pirates" out of.
Statement 3. CDMA can co-exist with narrowband systems like AMPS, NAMPS, or NADC-TDMA.
False. Consider an NAMPS system. This is basically NBFM, 5 khz deviation like we are familiar with. The IF bandwidth is about 12 khz wide. Let's assume a CDMA transmitter puts out 20 watts in 1.2 MHz band. That's about 200 mw in a 12 khz band. With a 10 dB antenna at 40 feet and 3 dB of coax loss, we get an ERP of about 1 watt in our 12 kHz channel. That signal would be solid to the radio horizon (10 miles) and well beyond. That's one of the main reasons why DS-CDMA should NOT be allowed on the ham bands. The whole band is basically shot for any work using signal strengths under 2 uV. IS-95-style CDMA is 1.2 MHz wide and should be reserved for use at frequencies that can allow 1.2 MHz wide signals. That means NOT AT HF, NOT AT VHF, MAYBE AT UHF AND UP.
Ham Radio - Where are we going? Where Do We Want to Go?
Well ... Amateur radio is a BIG topic, and there are a whole lot of interests that are pursued on a whole lot of frequency bands. Let's look at a few.
HF
Well, the capability of these bands for ionospheric reflection means long distance communications over narrow channels. It looks like the casual contacts, DX chasing, and contesting on CW, SSB, SSTV, and RTTY-type communications are going to stay just as they are. Spread spectrum is just inappropriate for these narrow bands with negligible frequency reuse possible. The incompatibility with other modes makes it unuseable.
VHF - UHF
Well, the bands are wider, and frequency reuse is possible, but still the need to control all emissions in a wide channel rules out spread spectrum as a viable alternative to narrow modes. It looks like we will be sticking to CW, SSB, NBFM, and packet on NBFM for the forseeable future. Narrow-band repeaters with distinct input and output frequencies seem to be a good way to extend the coverage. Groups can still put repeaters up, and frequency coordination is really a system to "plan" frequency reuse. Digipeaters allow ASCII to be repeated on a single frequency. Schemes that include some voice-coding usually can be used to repeat voice on a single channel that's either twice as wide or has half the through-put. "You can't get something for nothing" or "there is no free lunch" to use a couple of old saws.
Microwaves and Optical
Here the bands are really wide and the antennas are really narrow and occupancy is very light. Run any mode, wide or narrow, on any antenna, and experiment. Who cares? People are glad to have ANY occupancy up here.
Conclusion
It seems that we are gradually going in a direction that fits our band and mode allocations. It figures, doesn't it? What works, works. What doesn't, dies out. What remains, remains for a long time. CW and SSB on HF ... and long-distance communication ... they work well. NBFM and repeaters on VHF and UHF ... and frequency reuse ... they work well. There have been and will be a lot of "cross-over technologies", like CW and SSB or even AM on VHF, or NBFM on HF, but the fact remains that communication at rates that human beings run at can be accomplished in a few kilohertz. That means that people will continue to tune the bands and chat. People will be able to provide public service communications or emergency communications. We should be concerned only that this unique method of person-to-person communication still can exist and has not been eliminated by government fiat or the spectrum sold to some business interests.