The system of air-ground communications is
one of the most fundamental elements of air traffic control in the National
Airspace System (NAS). The air-ground (A/G) communications for air traffic
management use a licensed Very High Frequency (VHF) band to support all phases
of flight between the pilots and air traffic controllers, including the control
of ground movements on the airport surface, the arrival/departure of aircraft
to and from the terminal area, and throughout the en-route environment.
For supporting these mission critical
functions in the NAS, the current air-ground voice communications systems are
still using the more than 50-year-old analog voice transmission technology in
the band. This technology uses the Amplitude Modulation (AM) with Double
Side-Band (DSB) technique and divides the aeronautical air-ground VHF (117.975
MHz -137 MHz) band with 25 kHz bandwidth per channel. With the increasing
demand for more voice channels, an obvious approach to increase the capacity is
to maintain the analog voice technology, but decrease the guard band of the
voice channel and divide the channel into 3 sub-channels; i.e., 8.33 kHz per
channel. Europe is already using this 8.33 kHz scheme to increase its voice
channels.
Currently, there are several proposed
approaches to solve the projected saturated spectrum in the future. The first
approach is to allocate a new spectrum for aeronautical A/G communications. The
use of the 5 GHz spectrum is still in the study phase and would require
coordination with the current users of that spectrum. The second approach would
use new digital communication technologies to overlay the communication
capacity on the existing A/G communication infrastructure. The concept is to
use broadband technologies over the spectrum to provide sufficient capacity for
aeronautical VHF communications. The third approach applies strict policy to
limit the use of voice and data applications on the limited spectrum by dividing
the aeronautical A/G communications into two categories: DataComm and Airborne
SWIM. DataComm would provide command and control exchanges in the protected
spectrum and the Airborne SWIM would provide advisory data in a commercial
spectrum. Each of these approaches could provide certain relief of capacity
issues, but would also introduce additional impacts to the current and future
aeronautical A/G communications systems.
More importantly, however, none of these
approaches address the current practice of static spectrum allocation, which
this author believes is a major bottleneck for effective use of the limited
spectrum. As pointed out in, the relatively low utilization of the licensed
spectrum is largely due to inefficient fixed frequency allocations rather than
any physical shortage of spectrum. For the air traffic control band (108-138
MHz), the utilization is less than 5%. Although this claim may not be true for
all airspace sectors, it does show a general under-utilization of the spectrum.
This is due to the fact that the assignment of communications channels is
static and based on geographical areas and organizational structures. The
channels pennanently assigned to the particular geographical areas and
organizations prevent other users from using the channels when the channels are
idle. Also, this static allocation of the spectrum constrains the reassignment
of channels and could create a long transition period for moving the existing
analog system to a new digital system.
The emerging Cognitive Radio (CR) technology
provides opportunities for us to address the static allocations of spectrum
issue and offer a more flexible transition approach for updating the legacy A/G
radio system.The emerging CR technology provides sensing, awareness of, and
adapting to the surrounding environment, and allows the radio to adapt to the
environment accordingly. Built on software-defined radio (SDR) technology, CR
is able to employ these features with the cognitive engine and the aid of
several sensors. The cognitive engine carries out these tasks by obtaining all
available infonnation from sources such as sensors, protocol layers, a policy
engine, and its own hardware, then interprets, reasons, and makes the optimum
decision to adapt. Integrated with ground radio stations and centralized
management systems, the CR can dynamically use the available channels based on
its actual location, environment condition, and therefore, maximize the use of
the limited spectrum.
This is organized as follows: A basic cognitive
radio technology is introduced and an implementation scheme is discussed on how
the cognitive radio can be used to provide the dynamic spectrum access. Next,
the use of the cognitive radio for aeronautical A/G communications is explored
and several advantages for using the cognitive radio technology for A/G
communications are pointed out. Finally, a summary and potential future studies
of this emerging technology for NextGen A/G communications are provided.
This explores the use of cognitive radio for
NextGen Aeronautical A/G communications. The technique offers a new approach to
address the needs of the NextGen Aeronautical A/G communications and can
optimize the use of the limited aeronautical spectrum. A CR network is proposed
for A/G communications to solve current A/G communications problems resulting
from the limited available spectrum and the inefficient usage of spectrum. The
CR network will provide dynamic spectrum awareness and assignment in A/G
communications.
The concept of using cognitive radio for
aeronautical A/G communications also raises many interesting questions for
future research and development. To ensure efficient spectrum-aware
communications and the safety of A/G communications, more research is required
in several key areas .The techniques of spectrum sensing should be investigated
to address the real-time wide band sensing capability in airspace. These can
include the techniques of primary transmitter and receiver detection and
interference management. Another area is the spectrum decision. The spectrum
decision detennines the best channel among the available channels. Several
techniques, such as decision model and cooperation with reconfiguration should
be explored in addressing this challenge. Coordination of transmission among CR
users is another challenge worthy of research and should include the spectrum
allocation and spectrum access techniques. Spectrum hand-off or spectrum
mobility also needs to be investigated. These could include the spectrum mobility in
the time and space domains, protocols to ensure smooth and fast transition
leading to minimum performance degradation and safety during the hand-off.
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