  This paper investigates the adaptive subcarrier and bit allocation algorithm
for OFDMA systems. To minimize overall transmitted power, we propose a novel
adaptive subcarrier and bit allocation algorithm based on channel state
information (CSI) and quality state information (QSI). A suboptimal approach
that separately performs subcarrier allocation and bit loading is proposed. It
is shown that a near optimal solution is obtained by the proposed algorithm
which has low complexity compared to that of other conventional algorithm. We
will study the problem of finding an optimal sub-carrier and power allocation
strategy for downlink communication to multiple users in an OFDMA based
wireless system. Assuming knowledge of the instantaneous channel gains for all
users, we propose a multiuser OFDMA subcarrier, and bit allocation algorithm to
minimize the total transmit power. This is done by assigning each user a set of
subcarriers and by determining the number of bits and the transmit power level
for each subcarrier. The objective is to minimize the total transmitted power
over the entire network to satisfy the application layer and physical layer. We
formulate this problem as a constrained optimization problem and present
centralized algorithms. The simulation results will show that our approach
results in an efficient assignment of subcarriers and transmitter power levels
in terms of the energy required for transmitting each bit of information, to
address this need, we also present a bit loading algorithm for allocating
subcarriers and bits in order to satisfy the rate requirements of the links.
