OFDM vs CDMA-difference between OFDM and CDMA
This page provide comparison between OFDM vs CDMA and mentions difference between OFDM and CDMA technologies. Both OFDM and CDMA are wideband wireless digital communication systems widely used for fixed as well as mobile applications.
OFDM stands for Orthogonal Frequency Division Multiplexing. It is a multi-carrier modulation technique. It is used to modulate multiple low bit rate data streams onto multiple closely spaced carriers. It is similar to FDM but occupies less frequency spectrum compare to FDM for the same number of users. In OFDM carriers are densely packed compare to the FDM technique. Refer difference between FDM and OFDM to understand the comparison between FDM and OFDM. OFDM is spectrum efficient compare to FDM by way of packing carriers orthogonal to one another. This also helps in avoiding ICI (Intercarrier Interference).
Figure-1 depicts OFDM system block diagram. As shown generic OFDM transmitter consists of FEC encoder (RS,CTC,Convolution encoder), Interleaver, data modulation or mapping, IFFT and cyclic prefix addition. Scrambler may or may not exist. OFDM receiver consists of front end synchronization to correct for any time, frequency or channel impairments. This is followed by reverse modules of the one mentioned in the transmitter. It includes Cyclic Prefix removal, FFT, data demodulation, deinterleaver, FEC decoder.
CDMA is a spread spectrum which does not utilize time and frequency as resource but assigns codes. In CDMA low bandwidth data message is multiplied with large bandwidth PN sequence(Pseudo Random), referred as spreading. The low bandwidth message can be voice or data.
Figure-2 depicts CDMA system block diagram. As shown it has all the modules as mentioned in OFDM transmitter and receiver except IFFT, FFT, CP addition and CP removal. It has module spreading and despreading which does not exist in OFDM chain.
All the users in CDMA system use same band and time space but will have different codes . The transmitted signal can be retrieved back using the same PN code as used by the transmitter. As the IFFT module does not exist, CDMA is a single carrier technique unlike OFDM system.
Following are some of the factors which describes difference between OFDM and CDMA. This can be taken as
comparison between OFDM vs CDMA.
• OFDM performs better compare to CDMA and provides better tolerance with respect to multiple delay spread.
• Peak power clipping performance of OFDM is better compare to CDMA.
• Gaussian Noise tolerance of OFDM is better compare to CDMA.
• OFDM based system in single cell environment can allocate about 2 to 10 times more subscribers compare to CDMA.
• In multi cellular application OFDM can allocate 0.7 to 4 times more subscribers compare to CDMA.
• CDMA performs better in multi cellular case where in single frequency is allocated to all the base stations.
• In multiple user case, OFDM receiver had to tackle large variation of the signal strength emanating from multiple distributed users.
• The major benefit of CDMA is to provide more secure communication under high noise environment due to PN codes. But this is limited to less number of users. OFDM can also provide secured communication by way of using encryption and incorporating scrambler (randomization) module.
Refer hybrid OFDM techniques for difference between various hybrid OFDM techniques, which include MC/DS-CDMA, MC-CDMA and OFCDM.