Before the FEDERAL COMMUNICATIONS COMMISSION Washington DC 20554 In the Matter of: Proposed changes to the Amateur Service ) Rules (Part 97) to facilitate additional ) RM-8737 uses of certain portions of amateur spectrum ) by Amateur Spread Spectrum emitters )
Of particular relevance to this proceeding is that since 1991 I have been a Staff Engineer at Qualcomm Incorporated in San Diego, California. At Qualcomm I participate in the design, development and testing of an advanced Code Division Multiple Access (CDMA) spread spectrum system for digital cellular telephony that is now being deployed commercially worldwide. My work with this technology has been an invaluable educational experience with useful insights into the potential benefits and drawbacks of SS in the amateur service.
I write these comments on my own personal initiative as a radio amateur, not on behalf of Qualcomm. Qualcomm is in the commercial cellular telephone business, not the amateur radio equipment business. As far as I know, Qualcomm has no corporate position on this matter.
The indirect benefits would be even greater. Spread spectrum is now an important commercial radio technology, thanks to innovations such as the Global Positioning System (GPS), Part 15 wireless LANs and CDMA cellular telephones. It is essential that radio amateurs develop a hands-on understanding of SS technology not just to improve their communications abilities, but also to continue to satisfy the basis and purpose of the amateur service relating to technical experimentation, advancement and training as stated in Sections 97.1(b), (c) and (d).
So I am considerably distressed by the negative comments received so far in this action. They exhibit a remarkable degree of ignorance of the principles of spread spectrum and of basic communications theory, a strong fear of the unknown, and an unreasonable desire to maintain the status quo at all costs in a rapidly changing world.
To this list I add
FEC, nonbinary orthogonal modulation and wideband analog FM all resemble FH and DS in that they increase emission bandwidth beyond that occupied by an uncoded binary (or SSB-AM) signal at the same user data rate. The power spectral density is also decreased. But while FH and DS are usually "power neutral" -- the same transmitter power is generally required whether or not the signal is spread  -- FEC, nonbinary orthogonal modulation and wideband analog FM can all have the extremely desirable property of decreasing the total RF power required to support a given user data rate or audio S/N ratio. This necessarily comes at the expense of increased bandwidth, according to Shannon's famous formula that shows channel capacity to increase with both signal power and bandwidth.
I use this expanded definition of SS because the Commission's existing emission bandwidth limits keep amateurs from not only using spread spectrum as it is generally defined, but also from using wider bandwidths to reduce RF power requirements.
The frequency diversity [SS] system is intuitively ridiculous because it apparently "wastes" bandwidth rather indiscriminately. As we shall see, intuition is a poor guide in these matters. The feeling that we should always try to "conserve bandwidth" is no doubt caused by an environment in which it has been standard practice to share the RF spectrum on a frequency basis. Our emotions do not alter the fact that bandwidth is but one dimension of a multidimensional situation.
The other dimensions to which Costas alludes include time (e.g., duty cycle), RF power and geographical area. In particular, the amateur service has all but ignored the RF power dimension, giving little more than lip service to the requirement to run only the minimum power required to maintain communications. The FCC rules are also at fault to the extent that the bandwidth limits established for various band segments preclude the use of power-efficient wideband techniques.
It's sad to consider while reading many of the comments filed in opposition to this proposal that Costas wrote this paragraph over 36 years ago. Here are some typical "NIMBY" (not in my back yard) comments:
There are less crowded Amateur microwave bands, particularly the higher frequency bands, where space exists for a variety of SS emission types. [San Bernadino Microwave Society, at 9.]
The petitioner's pro-SS arguments in this matter only address technical and experimental concerns, and do not seriously consider the ill effects of the co-spectrum use of SS and existing narrowband systems in an already crowded spectrum. [SouthEastern Repeater Association at 3]
Such comments betray a complete ignorance of the potential of spread spectrum to substantially decrease congestion. Other comments, while grudgingly admitting some potential for improvement, misunderstand and deprecate the technology:
It is a fact that a digitally processed SS system utilizing "exclusive" spectrum can accommodate more traffic in the same bandwidth than can a FDMA system. This is mostly a result of the digital processing to compress (in time) the communications and the use of all the available spectrum space without having to leave "guardbands" between each user assignment. It is also dependent upon the communications user being willing to tolerate propagation delays which will increase as the system traffic increases. [Comments of SCRRBA at 6]. Costas shows that what a wideband system "spends" in excess bandwidth can be more than repaid by vastly increased interference resistance and by significantly reduced RF power requirements (meaning less interference to existing narrowband users.) Even with the limited analog technology of his day, Costas could show a net increase in the carrying capacity of a given frequency band. Subsequent developments in digital signal processing technology and error correcting codes have now made it possible for one commercially practical spread spectrum system (Qualcomm CDMA) to demonstrate, in carefully controlled field tests, capacity gains of 10-15x over existing narrowband analog FM cellular systems. Such dramatic gains are only possible in a wideband system. They are clearly of greatest value in our most congested bands!
A simple thought experiment will show how reducing power is the key to spectrum efficiency, and that limiting bandwidth is actually counterproductive. Assume a 1MHz band saturated with 1000 uniformly distributed users who, because shortsighted FCC rules preclude power efficient wideband modes, must run 1KW each to maintain communications with some narrowband scheme. The total transmitted power spectral density in the band is therefore 1000x1KW/1MHz = 1 watt/Hz. This represents the spectral density of interference as seen by a new user just arriving on the band.
Now assume that the rules are changed by an enlightened Commission to allow a power efficient wideband scheme that requires only 500W, spread over all or some random part of the 1 MHz band, to maintain communications against 1 watt/Hz of interference spectral density. If a few users switch to this mode, the overall interference level will go down by a small but nonzero amount. An individual narrowband user may see either an improvement or a degradation, depending on how close it is to a wideband station. 
But let's say the new mode really catches on, and all 1000 stations switch over to it. Now the interference spectral density is only 1000x500W/1MHz = .5 watt/Hz. Since the interference has gone down by 3dB, each station can now lower its power accordingly, to 250 watts. This lowers the total interference again, to .25 watt/Hz, and the stations can all reduce their powers again by another 3dB. And so on.
If interference from other stations were the only factor, this power "deescalation" could continue indefinitely until everyone ran virtually no power at all! Of course, at some point in a real system natural noise sources will emerge to stop the process.
Alternatively, let's keep our original 1000 wideband transmitters at 500W and add a second batch of 1000, each also operating at 500W. This would produce an interference spectral density of 2000*500/1MHz = 1W/Hz, which we know is the most our wideband scheme can tolerate at 500W. Now we have twice as many users sharing the band as in the narrowband case. All this because we removed the rules against "wasting" bandwidth!
The message is clear: if our objective is to promote spectral efficiency, limit power -- not bandwidth. The proposed rules achieve this by requiring automatic power control in exchange for decontrolling bandwidth. Having seen the inverse relationship between S/N ratio and band capacity, we can also understand the proposed limit on Eb/N0 ("digital S/N"). Since each decrease of 3dB in required Eb/N0 doubles the capacity of a shared band, amateurs should be encouraged to build and operate systems at the lowest possible Eb/N0 ratio. Because of the relatively generous 100W power limit, and because there is no limit on data rate, the Eb/N0 limit effectively says "take all you want, but eat all you take." Since the theoretical limit according to Shannon is -1.6 dB for infinite bandwidth, there is clearly a lot of room for improvement here.
Amateur satellite operation can also benefit substantially from wideband techniques. Power is a limited commodity even on expensive commercial satellites; on amateur satellites it is extremely scarce. The carrying capacity of these satellites could be increased considerably through the use of power-efficient wideband modulation and coding techniques. Furthermore, auxiliary applications of spread spectrum such as highly accurate tracking of spacecraft are clearly of considerable benefit.
One of the nice aspects of spread spectrum operation by satellite is that the near-far problem is often nonexistent. A satellite in high orbit is nearly equidistant from its users, so the range of user signal strengths it sees is scarcely affected by varying propagation losses.
I point out here that the amateur rules should not require any minimum processing gain or spreading bandwidths. Particularly in the relatively narrow weak signal band segments where the use of wideband techniques is more likely motivated by RF power savings than by minimizing power spectral density, requiring more bandwidth than necessary to achieve the desired power gains would be counterproductive. Any such standards should be promulgated by voluntary agreement among the users of each band segment.
To the extent that any of the comments contain actual numerical analyses, all are based on absolute-worst-case conditions. (See, for example, SCRRBA's comments at 5). They assume continuous use by the SS station of the maximum power of 100W, completely discounting the mitigating effects of duty cycle and automatic power control. They assume reckless and total disregard on the part of the SS operator for voluntary bandplans and interference complaints from nearby narrowband operations. Then they argue that because interference could occur under such extreme conditions, SS ought to be banned entirely or at most permitted under extremely restrictive rules.
Yet with such unreasonable assumptions, any operating mode could cause harmful interference. Indeed, most would be even worse! For example, it is legal under current rules to operate local FM simplex at 1.5KW on a satellite downlink band. (Never mind that such power levels are extremely rare. Those who oppose SS ignore my protests that average power levels will be substantially less than 100W, so I am entitled to make equally unreasonable assumptions here).
The average transmitted power spectral density of such a signal would be 1500W/20 KHz = 75 milliwatts/Hz, with many strong discrete spectral components. In contrast, a 100W SS signal spread over 1 MHz would be only 0.1 milliwatt/Hz. Whatever can be said about the interference potential of the latter signal is clearly even more applicable to the former. Yet FM is not banned from the satellite bands. Indeed, had such a ban been enacted it would have precluded the use of FM by satellites such as the Microsats.
The potential for interference is a fact of life in amateur radio. The FCC has long charged amateurs with the primary responsibility to work things out for themselves:
Each station licensee and each control operator must cooperate in selecting transmitting channels and in making the most effective use of the amateur service frequencies. No frequency will be assigned for the exclusive use of any station. [FCC Rules Part 97.101(b)]This rule would apply to SS operations with equal force. It is patently unfair to insist that "no SS interference potential whatsoever is tolerable" when no other mode is held to such an unattainable standard.
Although some, such as the Southeastern Repeater Association, raise the issue of interference to emergency communications, I consider this a red herring. This argument no longer carries much weight given the popularity of cellular telephones.  The nominal purpose of the amateur service is technical experimentation and self-training; it not a "safety of life" service like aeronautical or police radio where even occasional interference really cannot be tolerated.
But I do agree with the San Bernardino Microwave Society when they say that SS operations should adhere to local frequency coordination practices, assuming that these practices make reasonable accommodation for SS operations. For example, it would indeed be inappropriate to run a high speed SS metropolitan computer network in a band segment reserved for weak-signal DX. But this is an issue best handled by the voluntary bandplanning process, not the slow and inflexible FCC rulemaking process.
I further believe that the voluntary bandplanning process should isolate operation "classes" without regard to modulation mode or bandwidth. For example, a band plan might specify the following subbands:
Such band plans should completely alleviate the interference concerns of the weak signal and satellite groups while retaining the option to apply wideband techniques to their own needs as they see fit.
FM, packet and SS operations would coexist in the "local" subbands. One segment might be shared by FM, packet and SS repeater transmitters on hilltops while another would be shared by the receivers in these repeaters.
I note that current amateur band plans already look very much like what I have proposed here. This is not a coincidence! They have evolved in this manner to deal with the same "near-far" interference problem that is the source of so much concern with SS. That's because the near-far problem is not unique to SS, but exists to some degree with every operating mode because of the inability to build real receivers with perfect adjacent channel rejection.
I note that in many such interference incidents, one or more parties are running excessive power. This suggests the following principle:
Whenever harmful interference occurs between operations otherwise in accordance with voluntary bandplans that cannot be resolved by mutual agreement of the parties involved, the primary responsibility for resolving the interference shall rest with the station running the greater RF output power, without regard to emission bandwidth.
This elegant rule clearly encourages power efficiency, which we have seen is directly related to spectrum efficiency. Being "interference driven", it avoids arbitrary and inflexible restrictions that would apply even when no interference would otherwise be caused.
One's mind almost boggles at the thought of two warring repeater groups in a power de-escalation battle in order to claim priority on a channel! Beyond that, this principle would certainly encourage the use of power efficient modes, directional antennas, low power relays, and of course the minimum power actually required in each instance to maintain communications.
Respectfully submitted, Philip R. Karn, Jr., KA9Q
 Efficient SS systems need not "compress in
time". Indeed, if that's all they did, there would be no net gain in
capacity. They do compress the source material to remove redundancy,
e.g., through the use of low-bit-rate speech encoders. Propagation
delay is fixed; it does not increase with load (though the bit error
rate will as the channel approaches capacity). And while SS does
eliminate the need for inter-channel guard bands, this is generally
not a major issue. The big capacity gains in an efficient spread
spectrum system like Qualcomm CDMA come from the following:
This is the classic "near-far" problem; strictly speaking my analysis is valid only when every station is equidistant from all other stations. But later I will discuss practical band-planning approaches that minimize the near-far problem by segregating transmitters and receivers.
On several occasions I have used amateur FM repeaters to report emergencies. About a month ago I encountered a seriously injured woman lying in the street after her car was hit by a drunk driver. I immediately called for help on a local 2m repeater. Eventually, I obtained police assistance with the help of an inexperienced albeit well-intentioned fellow ham who called 911 on his home phone. I later learned that the woman was expected to make a full recovery. But I was so frustrated by the inefficiency and undependability of the whole process that I finally bought a portable cell phone that I now carry with me at all times.