Theoretic Fundamentals, Regulatory issues, Physical Limitations, and the Future of Opportunistic Transmission. Vahid Tarokh Harvard University. Introduction. The Goal. The Goal = Providing Wireless Services. Example of Services. Information Services. Software Distribution.
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The Goal = Providing Wireless Services
Source cnn.com Feb 21, 2012
Source money.cnn.com Feb 21, 2012
Spectrum sharing with other (legacy or other novel) systems.
for all involved systems.
Secondary system has to control its emissions to prevent interference towards primary system.
All involved systems share the spectrum based on a set of rules (spectrum etiquette).
Little can be done
To avoid interference
Can these potentials
be actually realized?
The previous result gives some promise in potential of secondary users not having harmful effects on the primary user capacity.
However, even if we can address the issue of interference between primary and cognitive networks, we will still have potential interference between various cognitive networks operating on available white spaces.
In other words, if because of availability of free spectrum many secondary systems emerge, can these systems support any reasonable data rate?
From point of view of scaling laws (growth order), of ad hoc networks: “cognitive networks achieve throughput scaling of a homogeneous network”, [WDVCT]:
Randomly distributed n primary users, and m secondary users with m = nβwith β > 1
Specifically, the primary network achieves the sum throughput of order n0.5 and, for any δ, the secondary network achieves the sum throughput of order m0.5- δwith an arbitrarily small fraction of outage.
These results are only of theoretical interest.
It has been proved [VT] that under the assumption of a cap on the interference caused by secondary network to primary receivers
the secondary networks are single hop and
transmissions transmit either (i) with constant transmit power, and (ii) with transmit power scaled according to the distance to a designated primary transmitter, then
as the number of secondary networks N ∞ , the secondary receivers can achieve at least a non-vanishing throughput.
This shows that cognitive radios are at least scalable for single hop networks.
Another option is not to allow too many secondary networks.
The FCC proposes that some form of contention protocol be employed to reduce the interference between co-existing cognitive networks but does not specify such a protocol.
If the number of cognitive networks in a region grows large, this may not be very efficient and may produce capacity losses.
The general problem of allocation of available white spaces to various cognitive networks in order to optimize the capacity is a.
computationally hard (NP-hard) problem.
We will next study proposals for various secondary networks to co-exist.
For reducing interference the FCC proposed methods are based on
Transmit power limitations/power control
Geo-location enabled devices
Combinations of these methods
In this method of interference reduction, secondary users must be endowed by GPS (or similar geo-location systems) with at least 300m accuracy.
Primary users location is known to the secondary users (using a geographic database) and buffer regions around the primary users are specified where secondary user transmissions are not allowed in certain bands.
Geo-location and also FCC power limits are safe but conservative:
May make more sense to allow different power limits in various bands based on the location of the secondary user.
Secondary devices sense the channel and based on the activity level decide if it is busy or not.
FCC: -114 dBm power means the channel is busy.
Failure Causes Interference
It is obvious that -114 dBm is not the optimum threshold for detecting a busy channel.
If this threshold is not correctly selected it limits the efficiency of cognitive devices, thus
Optimum threshold for detection must be computed although:
Typically the underlying ambient noise std бis not known
The distribution of the primary signal is not known.
The busy channel threshold must be selected based on geographic region (and the underlying primary systems) at least for devices using geo-location and databases.
Similar conclusion can be made for beacon detection.
Here we have to be also careful about transmission strategy.
Computation of CCT (Clear channel threshold) for deciding on idle channels and associated detection strategies for both sensing and beacon based systems is a straightforward exercise in detection theory.
Given a peak power Ps and average transmit power Pav for the secondary user, what is the best secondary user transmission strategy that minimizes the interference to primary receivers?
The answer is non-trivial and is given by the following theorem.
What kind of services cognitive radios can enable?
The answer is already known in the horizontal sharing scenario.
For vertical sharing scenarios if there is an active primary user in the area, then quality of service may be an issue, unless secondary signals can be spatially separated from that of primary signals.
If multiple cognitive radios exist, then contention can effect their ability to provide quality of service.
Possibly some polling of dedicated spectrum (wireless or wired) with cognitive radio spectrum can be used to provide some quality of service by future service providers.