Contradictions in Power Loading and Pilot Patterns in NC-OFDM

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					            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
                                                {boyan, neco}

                                                                           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 [1]. 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 [2], 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 [3]. 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 [4].
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 [5] 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 [6] [7]. 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.
                                    f Ts
[7] 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

                                                                                                           V. CONCLUSION
                                                                                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 [6] for NC-OFDM sub-   interfering with the PU are disabled. When considering the
channels in a CR environment, extracted from [6].                            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
                                                                             [1]   S. Coleri, M. Ergen, A. Puri and A. Bahai, "Channel estimation techniques
power requirement of pilot sub-channels is different due to                        based on pilot arrangement in OFDM systems," Broadcasting, IEEE
their channel estimation role.                                                     Transactions on, vol. 48, pp. 223-229, 2002.
                                                                             [2]   J. Mitola III and G. Q. Maguire Jr., "Cognitive radio: making software
                                                                                   radios more personal," Personal Communications, IEEE, vol. 6, pp. 13-
                    III. PROBLEM STATEMENT                                         18, 1999.
                                                                             [3]   I. Rashad, I. Budiarjo and H. Nikookar, "Efficient pilot pattern for OFDM-
   It can be noted then that the work done on pilot patterns                       based cognitive radio channel estimation - part 1," in Communications and
and power loading in CR environments is contradictory. The                         Vehicular Technology in the Benelux, 2007 14th IEEE Symposium on,
work done on pilot patterns claims that any pilot sub-                             2007, pp. 1-5.
                                                                             [4]   I. Budiarjo, I. Rashad and H. Nikookar, "Efficient pilot pattern for OFDM-
channels which happen to be in the PU’s frequency band need                        based cognitive radio channel estimation - part 2," in Communications and
to be disabled and the sub-channels closest to the PU’s band,                      Vehicular Technology in the Benelux, 2007 14th IEEE Symposium on,
but not inside it, need to be converted to pilot sub-channels so                   2007, pp. 1-5.
                                                                             [5]   Jiming Chen, Youxi Tang and Shaoqian Li, "Pilot power allocation for
as to ensure the closest approximation to an equispaced pilot                      OFDM systems," in Vehicular Technology Conference, 2003. VTC 2003-
pattern in order to maintain channel estimation optimality.                        Spring. the 57th IEEE Semiannual, 2003, pp. 1283-1287 vol.2.
                                                                             [6]   G. Bansal, M. J. Hossain and V. K. Bhargava, "Adaptive power loading for
   In order to achieve the best efficiency as well as the                          OFDM-based cognitive radio systems," in Communications, 2007. ICC
optimal coexistence between PUs and Secondary Users (SUs)                          '07. IEEE International Conference on, 2007, pp. 5137-5142.
in a CR environment, the design of the SU’s (using an NC-                    [7]   G. Bansal, M. J. Hossain and V. K. Bhargava, "Optimal and Suboptimal
                                                                                   Power Allocation Schemes for OFDM-based Cognitive Radio Systems,"
OFDM scheme) pilot pattern should be determined by the                             Wireless Communications, IEEE Transactions on, vol. 7, pp. 4710-4718,
solution of a multivariate optimisation problem where the                          2008.
performance objective is the reduction of the SU’s channel
                                                                             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.

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