  This paper studies the performance of hierarchical modulation-based
multimedia transmission in cognitive radio (CR) systems with imperfect channel
sensing results under constraints on both transmit and interference power
levels. Unequal error protection (UEP) of data transmission using hierarchical
quadrature amplitude modulation (HQAM) is considered in which high priority
(HP) data is protected more than low priority (LP) data. In this setting,
closed-form bit error rate (BER) expressions for HP data and LP data are
derived in Nakagami-$m$ fading channels in the presence of sensing errors.
Subsequently, the optimal power control that minimizes weighted sum of average
BERs of HP bits and LP bits or its upper bound subject to peak/average transmit
power and average interference power constraints is derived and a
low-complexity power control algorithm is proposed. Power levels are determined
in three different scenarios, depending on the availability of perfect channel
side information (CSI) of the transmission and interference links, statistical
CSI of both links, or perfect CSI of the transmission link and imperfect CSI of
the interference link. The impact of imperfect channel sensing decisions on the
error rate performance of cognitive transmissions is also evaluated. In
addition, tradeoffs between the number of retransmissions, the severity of
fading, and peak signal-to-noise ratio (PSNR) quality are analyzed numerically.
Moreover, performance comparisons of multimedia transmission with conventional
quadrature amplitude modulation (QAM) and HQAM, and the proposed power control
strategies are carried out in terms of the received data quality and number of
retransmissions.
