Wednesday, August 03, 2011

Spectrum Sharing: Not really sharing, and not just spectrum

There was endless talk about spectrum sharing at ISART in Boulder last week. I’ve become increasingly confused about what those words mean, since wireless has been about more than one radio system is operating at the same time and place pretty much since the beginning.
For example, whitespace devices are said to share the UHF band with television, but the operating rules have been drawn up to ensure that whitespace devices never interfere with TV, i.e. never operate in the same place, channel and time. What’s “sharing” about that? The purpose of radio allocation from the start has been to avoid harmful interference between different radio operations, which has always been done by ensuring that two systems don’t operate in the same place, channel and time – such as two TV stations not interfering with each other.

It seems that the “new sharing” has three characteristics: (1) more boundaries (in geography, frequency and particularly time) than ever before; (2) the juxtaposition of different kinds of services that differ more from each other than they used to; and (3) sharing without central control. It’s a difference in degree, not in kind.

It’s not about sharing, since the goal is to avoid interference, i.e. to avoid sharing. It’s not about spectrum, i.e. radio frequencies, since non-interference is achieved not only by partitioning frequencies but also by dividing space, time, transmit power and the right to operate.

1. More, closer boundaries

The Prime Directive for spectrum management has been to minimize harmful interference. (I believe this is misguided; minimizing interference is important, but is simply a means to a more important end: Maximizing the value of radio operation. See How I Learned to Stop Worrying and Love Interference) This can be achieved by arranging for large buffer zones in space and frequency between radio operations, and removing time boundaries by giving a licensee 24x7 control of an allocation.

The increasing demand for radio operating rights – we now all want wireless, for every possible use, everywhere – means that the buffer zones have had to shrink. This leads to more risk of interference as the signals of different services begin to overlap; implementing sharp cut-offs in space and frequency are impossible in theory, and expensive in practice.

The time boundary comes into play when a secondary user is allowed to operate when the primary is silent. My impression is that this is what most people have in mind when they talk about sharing, and particularly when they use the term “dynamic” sharing – although the dynamism can, and does, also refer to hopping around in frequency and space.

In both these cases, “sharing” actually means “not sharing”: that is, the point of the rules and the technology is to keep competing systems from operating at the same time and place.

The addition of time boundaries makes things “dynamic”, another favorite word these days (Google Ngram): phrases such as dynamic spectrum access or DSA (cf. the name of the DySPAN conference), dynamic allocation of spectrum. Dynamic access – multiple devices coming and going – is nothing new; it’s the essence of cellular networks. Dynamic spectrum allocation – dynamic assignment to be more precise, since in the regulatory jargon allocation is determining what kind of services are allowed, and assignment determines which particular parties may operate – is new, since traditionally license assignments are effectively perpetual. (Unlicensed is a transitional case, since any type-compliant device may operate; the assignment is not made at all.)

Dynamic assignment of operating authorizations allows independent transmissions to be interleaved, and is the characteristic of the new spectrum sharing. Note, though, that the key to this flavor of DSA is sharing not spectrum (i.e. frequencies), but assignments.

Perhaps the only kind of sharing that counts as sharing, in my book, is when distinct systems radiate energy at the same time and frequency at the same time; the classic “underlay” case is UWB, where a very wide-band but very low power signal coexists with a number of relatively narrow-band but high power signals. This can also be done by having different systems using the same frequency, place and power level use different signal modulations that can be teased apart by smart receivers (i.e. orthogonal spreading codes).

2. Less like-with-like

Close-packed systems (in space and frequency) have been deployed for decades, but they have been like-with-like operation, e.g. a number of cellular operations in adjacent channels. This is relatively easy to manage, since the interference is symmetrical. The harder, and growing, problem arises when the near neighbors have very different technologies, spatial layouts and business models. The goal of sharing is still to avoid sharing (i.e. to avoid spill-overs from to the other), but it’s harder to manage.

3. Decentralized coordination

The debate about sharing is confounded by the introduction of systems where many devices coexist without coordinating with each other, i.e. unlicensed systems. These systems “share” because they may well trample on each other’s signals, but even here the goal – implemented through technology and standards, rather than regulatory rules – is to avoid sharing, e.g. by having systems listen for radio silence before transmitting, and using orthogonal modulation codes when they do transmit at the same time.

This confuses the debate because for some advocates, dynamic sharing is largely synonymous with more unlicensed operation, e.g. allowing unlicensed devices to be the secondary operators in the spatial, frequency or time gaps left by primaries. However, while the secondaries could be unlicensed, they obviously don’t need to be. In fact, coexistence between a primary and uncoordinated, unlicensed secondaries is the hardest problem to solve, both technically and institutionally; the challenges include adding horizontal coexistence (independent secondaries among themselves) to vertical coexistence (primary to secondary), and identifying bad actors among the unlicensed secondaries when coexistence breaks down. The problems, especially the engineering ones, are not insoluble; but why start with the hardest part?


I think my confusion about the meaning of "sharing" derives from two meanings of the verb "to share": (1) to divide and distribute in shares, to apportion; (2) to use, experience or occupy with others, to have in common (Merriam-Webster). Definition #1 is what spectrum allocation has been about since time immemorial, and is still the meaning that many ascribe to the term, while the very different Meaning #2 has come into the radio regulation debate in the baggage of Open Spectrum Access advocates, and is not universally accepted.

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