The Use of Multiple-Input and Multiple-Output
Technology As Well As OFDM in Cognitive Radio
Network Systems to Maximize Systems Availability
Allen L. Green. Jr.
TSYS Department of Computer Science
Columbus State University
Columbus, Georgia, USA
e-mail:green_allen@colstate.edu
Abstract:
Cognitive radio (CR) is an emerging technology
that promises a vast increase in the amount of available consumer
bandwidth by opening areas of the radio frequency spectrum that
were previously reserved for specific functions by the FCC. This
paper provides an overview of processes within CR networks and
proposes use of CR networks to help reduce the likelihood of
overlapping signals by diverting a portion of radio transmissions to
frequencies that would otherwise be unused. The use of a multiple
input – multiple output or MIMO system along with orthogonal
frequency division multiplexing will allow the cognitive network to
maintain continuity of operations while performing a spectrum
vacation or handover occurs.
Keywords:
Spectrum access discovery; Spectrum selection;
Spectrum handover; Cognitive radio; CR; Cognitive radio mobility
management architecture; CRMMA; Multiple input – multiple
output; MIMO; Orthogonal Frequency Division Multiplexing;
ODFM
I.
INTRODUCTION
At the current time, a vast majority of the usable radio
frequency range goes unused in many regions, due to
restrictions set by the Federal Communications Commission in
the United States or other authorities in various countries. This
system of assigning a dedicated range of frequencies to set
purposes is known as Fixed Spectrum Allocation (FSA). The
FSA system is static in nature and leaves a majority of the
available frequency spectrum unused.
Cognitive Radio(CR) [1] is seen as a possible solution
for the inefficient management of the frequency spectrum
currently set by FSA. CR is a new technology that combines
Software Defined Radio (SDR) and artificial intelligence to
maximize the amount of the frequency spectrum used without
affecting the areas of the frequency spectrum used by legacy
users. Legacy users are also known as primary users, while CR
users are also known as secondary users since the primary users
always have first priority when found within range of CR
nodes. This is done by setting an interference temperature limit.
An interference temperature limit is the amount of interference
generated by CR users that can be tolerated by legacy users in
order to operate satisfactorily[2,3].
In order for a CR system to operate, it must first
discover the unused portion of the frequency spectrum. This is
known as Spectrum Sensing. Spectrum Access Discovery
follows spectrum sensing by finding any radio access points
within the spectrum. The purpose behind Spectrum Selection is
to find the best possible frequency spectrum according to
quality of service (QoS) requirements and customer needs.
Spectrum Cohabitation is the ability to share the frequency
spectrum with other CR users sans conflict with the other users
or interfering with legacy users.[4]
If a legacy user is found on the used portion of the
frequency spectrum, or if signal quality degrades, then
Spectrum Vacation will occur. Spectrum Handover is intended
to occur when the signal quality is degraded. The purpose is to
establish a new connection with minimal disruption of
service.[4]
Frequency-division multiplexing is the process of
dividing a set frequency spectrum into smaller non-overlapping
frequency ranges for different signals or users on a medium.[5]
Multiple-Input and Multiple-Output is the use of
multiple antennas at both the transmitter and receiver to
improve communication performance. [6]
II.
RELATED WORK
The concept of cognitive radio was introduced in 1998
by Joseph Mitola. In 1999, he co-wrote “Cognitive Radio:
Making Software Radios More Personal” with Gerald Maguire.
His work was obviously very early efforts and mostly just set a
baseline for further research. This work does not address areas
of MIMO or other means to ensure the ability of the user to
maintain connection with the CN. [7]
FCC Notice of proposed rulemaking and order in ET
docket no. 03-322, December 2003 established a standard for
available frequency usage for cognitive radio networks. This
sets a formal definition of a CN and states the allowed
frequency ranges for CN systems. This does not address
possible technologies used.[1]
Zoran Damljanovic wrote “Mobility Management
Strategies in Heterogeneous Cognitive Radio Networks” in
2009. This paper summarizes the various facets of cognitive
radio network topologies. While being a good overview of
current research in CR networks, as well as defining how CR
network systems will implement a layered model similar to the
OSI model. Specific radio technology is not addressed. [4]
In 2001, Helmut Bolcskei, David Gesbert and
Arogyaswami J. Paulraj wrote the article “On the Capacity of
OFDM-Based Spatial Multiplexing Systems” which explained
the benefits of combining multiple-input and multiple-output
with orthogonal frequency division multiplexing (OFDM). This
work does not address the CR aspect of the proposed system.
[8]
III.
PROBLEM STATEMENT
As current radio technologies begin to saturate their
respective networks, and as newer technologies become more
bandwidth intensive, new technologies will be required to
satisfy the needs of users. The typical user will desire a system
that requires as little user intervention as possible, while still
maintaining near continuous operation. CR network
technology will be able to answer the needs of users a majority
of the time, but if a legacy user operating in the same
frequency range enters the operational radius of the CR
network, the network may have to immediately stop traffic and
perform a spectrum vacation. Spectrum vacation and spectrum
handover both require a certain amount of time for the CR
system to overcome and find a new operating frequency. By
combining CR network technology with other technology, the
problem of total loss of services due to a change in frequency
will be much less likely to occur.
IV.
Note in figure 1 how the spectrum is separated and
how the CR users (highlighted in yellow) are spread out. If a
primary user was to begin using the 2409 MHz channel, the
CR device would immediately perform a spectrum vacation on
that channel, and User 1 would be able to communicate with
the CR system to gain access to a new usable frequency. Since
User 1 is still able to transmit via the 2400MHz frequency
channel, the user will not experience a loss of service and will
not likely even realize that a change has occurred.
If a similar loss of service had occurred in a Single
Input and Single Output (SISO) CR network system, the user
would have to wait for a new frequency band to be offered by
the CR system before the user would be able to communicate.
PROPOSED SOLUTION
My proposal is for a system that uses CR network
technology to perform spectrum sensing to determine the
available frequencies in a given area. After the system is
aware of the available spectrum, it will use OFDM in
combination with MIMO in as broad a spectrum as possible to
minimize the likelihood that a primary user will enter the area
of the spectrum used by secondary users and interrupt more
than a single access channel used by any individual customer.
In addition to CR programming, the SDR must be
able to recognize when a single transmit or receive channel
has been given up. Once the frequency band is relinquished by
the secondary user, traffic transmitted on that channel must
cease, and if it is a receive channel for a user, the SDR must
route the traffic that would have been transmitted on that
channel to the other receive channel to avoid a loss of service.
V.
CONCLUSION
Combining the technologies of MIMO, OFDM, and
CR networks, as well as additional management software
programming in addition to SDR will create a CR system that
shall be able to ensure greater system reliability by performing
intelligent spectrum handover or spectrum vacation when
necessary on one frequency band while maintaining network
connectivity on separate bands. This will help to maximize the
use of CR network systems while minimizing the downtime of
the customer due to conflict with possible primary users
within the operational frequency range of the CR network or
due to a reduction in signal quality in one of the CR channels.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
F.C.C (FCC) “Notice of proposed rulemaking and order” in ET docket
no. 03-322, December 2003
Mitola, J., “The software radio architecture” in IEEE Communication
Magazine, 33(5), 1995, pp. 26-38
Haykin, S., “Cognitive radio: brain empowered wireless
communications” in IEEE Journal on Selected Areas in
Communications, 23(2), 2005, pp. 201-220
Damljanovic, Z., “Mobility management strategies in heterogeneous
cognitive radio networks” in Journal of Network and Systems
Management, Vol. 18 No. 1., 2010, pp. 4-22
White, C. “Data communications and computer networks” Thomson
Course Technology, 2007, pp. 140-143
Shuguang, C. “Energy-efficiency of MIMO and cooperative MIMO
techniques in sensor networks” in Journal on Selected Areas in
Communications, IEEE, 2004
Mitola, J., Maguire, G., “Cognitive radio: making software radios more
personal” in Personal Communications, IEEE, 1999, pp. 13-18
Bolcskei, H., Gesbert, D., Paulraj, A. J., “On the capacity of OFDMbased spatial multiplexing systems” in Transactions on
Communications,
IEEE,
2001
Fig. 1.: Possible frequency allocation using a combination of cognitive radio, MIMO, and OFDM.
2400 MHz
2401 MHz 2402 MHz 2403 MHz 2404 MHz
2405 MHz 2406 MHz 2407 MHz 2408 MHz 2409 MHz
User 1 TX 1 User 2 TX 1 User 3 TX 1 Primary User Primary User User 1 RX 1 User 2 RX 1 Primary User User 3 RX 1 User 1 TX 2
2410 MHz
User 2 TX 2
2411 MHz
User 3 TX 2
2412 MHz 2413 MHz
2414 MHz
User 1 RX 2 User 2 RX 2 User 3 RX 3