Contradictions in Power Loading and Pilot Patterns in
NC-OFDM Cognitive Radio Systems
Boyan V. Soubachov, Neco Ventura
Department of Electrical Engineering, University of Cape Town,
Rondebosch, Cape Town, South Africa
such that every CR user is assigned a sub-channel. Should a
Abstract—Cognitive Radio (CR) is seen as the next paradigm sub-channel interfere with a Primary User (PU), the sub-
in radio communications and as a practical solution to the channel will be disabled by the CR so as not to interfere with
problem of spectrum scarcity. Non-Contiguous Orthogonal the PU and appropriate steps will be taken by the CR so that
Frequency-Division Multiplexing (NC-OFDM) is a proposed
implementation which allows the use of frequency bands to be the communications are resumed in a different, unused
placed arbitrarily close while maintaining the ability to disable frequency band.
those bands which are interfering with licensed users. This
implementation poses challenges such as the optimal pilot II. RELATED WORK
pattern when pilots used for channel estimation need to be
disabled to reduce interference as well as the Pilot-to-Data
In current, non-CR implementations of OFDM, a
Power Ratio (PDPR) needed to maintain channel estimation commonly used channel estimation method is pilot-aided,
accuracy. This work-in-progress paper discusses a theoretical hexagonal-pattern channel estimation due to its ability to
problem with regard to the pilot pattern and power loading of track fast fading channels compared to its block-type
pilot sub-channels when applied to a CR environment. estimator counterparts . Since the concept of the CR is to
disable any sub-channels which interfere with the frequency
Index Terms—Pilot Pattern, Power Loading, Orthogonal
band used by a PU , in some situations (provided the PU’s
Frequency Division Multiplexing, Pilot to Data Power Ratio.
spectrum band is wide enough or centred about specific
frequencies) a pilot sub-channel would need to be disabled.
I. INTRODUCTION The successful implementation of a CR using NC-OFDM
would therefore require the pilot pattern to be adjusted should
M ODERN wireless communications have become
characterised by the need for high link speeds. This
any sub-channels of the CR interfere with a PU.
There are several ways of dealing with the adjustment of
naturally translates to higher symbol rates, which also
pilot patterns in a CR environment should a PU appear. One
increase the effect and severity of fading effects imposed by a
proposed solution is to reposition the disabled (interfering)
pilot sub-channels adjacent to the PU’s frequency band. This
One of the most important innovations of recent years is
would mean converting the data sub-channel next to the PU
the successful design and implementation of Orthogonal
to a pilot sub-channel . This concept has also been defined
Frequency-Division Multiplexing (OFDM) modulation
as the optimal way to reposition the pilots in a CR
systems, allowing us to combat and almost eliminate channel
environment by imposing it on a two dimensional (time and
multipath fading effects by parallelising the single-carrier,
frequency) variation of the standard pilot pattern .
high symbol rate channel to many, low symbol rate channels.
It should be noted however, that due to the nature of comb-
The concept of OFDM has also been applied to channel
type pilot patterns, the pilot sub-channels may need to have
access methods, resulting in the creation of Orthogonal
more transmit power allocated to them compared to the data
Frequency-Division Multiple Access (OFDMA). OFDMA
sub-channels  in order to allow for optimal channel
allows the orthogonal sub-channels to be assigned to different
users. This allows different users to be placed arbitrarily close
Another topic which needs to be considered is the power
with no guard band in between them. OFDMA is therefore a
loading applied to NC-OFDM sub-channels in a CR
great candidate for CR communications, where several
environment. In order to maintain interference levels to a PU
proposed solutions use a version of OFDMA (NC-OFDM)
to a minimum, intelligent power loading needs to be applied
to individual sub-channels close to the PU. It has been
The authors would like to thank Telkom SA, the National Research
Foundation (NRF), Technology and Human Resources for Industry proposed that the optimal way to comply with one of the
Programme (THRIP), the Department of Trade and Industry (DTI), Nokia principles of cognitive radio and keep interference to the PU
Siemens Networks and TeleSciences, for supporting this research project.
below a certain threshold is to adjust the power dynamically It should also be noted that an important problem is the
in sub-channels close to the PU using modified water-filling quantisation of the SU to PU interference so as to keep the
methods  . These methods specify the decrease of a sub- interference to the PU below an acceptable threshold. This
channel’s power the closer it is to a PU based on the problem gives rise to the difficulty in solving the optimisation
interference it creates due to spectral roll-off modelled by the problem proposed in this paper, where the tolerable SU to PU
equation interference threshold has to be posed as an argument for the
2 optimal solution function which specifies the pilot pattern and
sin( f Ts )
i (f) P Ts
(1) PDPR for a CR system.
 which represents a squared sinc function scaled by Pi IV. RESEARCH GOALS
representing the power of the ith sub-carrier and Ts The aim of the research is to establish and mathematically
representing the symbol duration. link the hypothesised existence of the contradiction between
optimal power loading algorithms and optimal pilot pattern
algorithms for CR systems.
The research would also investigate how the optimality of
both the pilot pattern and the power loading schemes are
affected when they are applied in the contradictory manner
Cognitive radio is the next paradigm in wireless
communications. Many proposed implementations use NC-
OFDM as the proposed solution whereby sub-channels
Fig. 1. The adaptive power loading scheme proposed in  for NC-OFDM sub- interfering with the PU are disabled. When considering the
channels in a CR environment, extracted from . necessary concepts of channel estimation and power loading
required for NC-OFDM, there appears to be an inherent
The proposed power loading therefore implies that the sub-
contradiction between the solutions proposed for the pilot
channels closest to a PU should have their sub-carrier power
pattern algorithms and the power loading algorithms applied
level reduced the most while the sub-channels with the
in a CR environment.
farthest spectral distance from any PUs should be assigned
the most power to their sub-carriers. These methods,
however, are only optimal for data sub-channels since the
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Boyan V. Soubachov obtained his BEng Computer Engineering degree with
estimation Mean Squared Error (MSE) (or alternatively the distinction from the University of Pretoria in 2009. He is currently in the pursuit
detection MSE) with the constraint being the amount of of an MSc (Eng) degree at the Communications Research Group based at the
University of Cape Town’s Telkom Centre of Excellence. His focus is on
interference introduced to the PU by the SU. cognitive radio systems and multi-carrier modulation systems.