Performance Analysis of Dynamic Sub-Carrier Allocation Technique for Adaptive Modulation based MC-CDMA System

Multi carrier CDMA (MC-CDMA) combines the CDMA and OFDM technologies together and potentially offers the advantages of both the technologies. In frequency selective channel, the sub-carrier experiences different amplitude and phase. In frequency domain spreading, the frequency diversity is employed but the best of different sub-carrier conditions are not fully exploited. In MC-CDMA the performance of the system is improved by adaptively loading sub-carriers in accordance with the varying channel conditions. This paper is focused on selecting the best sub-carrier and transmitting the data through that sub carrier by adaptive bit loading algorithm.. Using a fixed modulation technique for the system, the system would have to be built for such a standard which would take care of the worstcase scenario of the channel to offer an acceptable bit error rate (BER). To achieve robust and spectrally efficient communication overmultipath fading channels, adaptive modulation is used which adapts the transmission scheme to the current channel characteristics. Taking advantage of the time-varying nature of wireless channels, the adaptive modulation scheme varies the transmission power, data rate (constellation size), coding and modulation schemes, or any combination of these parameters according to the state of the channel . If the channel can be estimated then the transmitter can adapt to the current conditions by varying the modulation type while maintaining a constant BER. This is typically done by making a channel estimate at the receiver and transmitting this estimate back to the transmitter. Thus, the adaptive technique will have a higher data throughput when the channel conditions are favorable and will reduce the throughput as the channel worsens. In other words, the principle of adaptive modulation consists of allocating many bits to carriers with a high SNR, whereas on carriers with low SNR only a few or no bits at all are transmitted.In this paper water filling algorithm is used to select the best sub-carrier,over the existing subcarriers. The adaptive control technologies, also known as the Intelligent Radio Transmission Technologies, for wireless communication include adaptive equalizer; transmit power control and adaptive modulation.