Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme suitable for high speed wireless links. In this paper, a new CMOS variable envelope detector-based polar transmitter for OFDM systems is designed, which is based on 90 nm CMOS technology. This 90 nm CMOS transmitter works in multilevel mode in such a way that it may switch up or switch down the transmitted voltage level according to Channel State Information (CSI). A digital envelope modulator, as a part of polar transmitter architecture for the 802.11 a/g WLAN OFDM standard is used. Assuming the knowledge of the instantaneous channel gain estimation for all users, the paper proposes an OFDM dynamic subcarrier allocation scheme to maintain optimum SNR level of the subchannels which are in deep fade. Simulation results show that system performance improves in terms of BER and SNR.

Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology for use in next generation wireless communication systems. OFDM provides high data rate and good spectral efficiency. Moreover, OFDM modulation technique has the ability to combat Intersymbol Interference (ISI) over multipath fading channels. OFDM-based systems utilize the limited RF bandwidth and transmit power in wide band transmission efficiently.

In multiuser OFDM-based systems, the entire spectrum is allocated to many users based on some algorithm. One subcarrier is allotted to each user. These subcarriers experience different fades and transmit different numbers of bits. However, the subcarriers which appear in deep fade to one user may not be in deep fade for other users. Due to variation in their physical location, speed, surrounding and channel characteristics, different users may experience different level of fading conditions at all the subcarriers. The distance from the base station is considered as an important parameter for fading of signal strength (Theodore, 2005), so the transmit-power level is assigned according to the distance from the base station. There are many algorithms to allocate resources in an OFDM system to improve system performance.

Telecommunications Journal, Orthogonal Frequency Division Multiplexing, OFDM, Reference Boundary, RB, Foot Print, Dynamic Adaptive Subcarrier Allocation, Fading Channels, Channel State Information, CSI, Intersymbol Interference, ISI, Integer Programming, IP, Voltage Level Shifter, VLS, PPhase Locked Loop, PLL.