This page covers Multicarrier CDMA tutorial which include CDMA and OFDM modulation/multiple access techniques.
Future mobile communication system requires high data rate and good quality of service to consumers. The higher data rate can be achieved with the use of multi carrier system such as OFDM. Multi carrier CDMA combines DS CDMA technique with OFDM. Hence multicarrier CDMA achieves higher data rate with optimum spectrum efficiency. Also Multicarrier CDMA is robust technique for multipath fading environment. This has made multicarrier CDMA very popular as future 4G technology.
CDMA is based on spread spectrum concept and is regarded as modulation & multiple access technique. In CDMA message signal is multiplied with pseudo noise sequence which spread the time domain message over the larger bandwidth. Hence make its power lower than the noise power. Hence it is difficult to demodulate/decode the message signal. Only at the receiver with appropriate PN sequence message signal can be decoded. Each Subscriber will have unique PN sequence.
In OFDM which is regarded as multicarrier modulation scheme, input data stream is divided among N parallel smaller data rate or narrow bandwidth channels. This is achieved using Serial to parallel converter. These parallel streams are multiplied with orthogonal carriers and summed up to achieve spectrum which efficient in bandwidth. Guard period is added to this OFDM spectrum to achieve orthogonality among each carrier and to eliminate ISI. Usually guard period is kept greater than delay spread of the channel. The simple multicarrier modulation is implemented using IFFT and FFT.
IFFT is used at transmitter and FFT is used at receiver.
Let us understand how multiple access technique is achieved with OFDM. Total numbers of subcarriers are divided among different subscribers. These set of subcarriers allocated to different subscribers is designated as sub channel. Hence each subscriber will have different sub channel for data/voice to be transmitted.
Figure mentions base station model of MC CDMA. As shown in the figure Data dm is modulated by modulation scheme chosen such as BPSK, QPSK or 16QAM. These modulated set of complex data is passed to Serial to parallel converter. The parallel data is fed to various PN codes C0 to Cn and these multiplied data is multiplied with orthogonal set of carriers' g0 to gn to achieve OFDM spectrum which forms one symbol. The same is done for all the subscriber stations at the base station to form Multicarrier CDMA signal spectrum. For Subscriber transmitter model zeros are inserted in the positions where data is not available at the input of IFFT block.