OFDM vs. CDMA: Key Differences in Wireless Communication

This page compares OFDM and CDMA technologies, highlighting the differences between them. Both OFDM and CDMA are wideband wireless digital communication systems commonly used for fixed and mobile applications.

OFDM (Orthogonal Frequency Division Multiplexing)

OFDM stands for Orthogonal Frequency Division Multiplexing. It’s a multi-carrier modulation technique used to modulate multiple low bit rate data streams onto multiple closely spaced carriers. It’s similar to FDM but utilizes the frequency spectrum more efficiently.

In OFDM, carriers are densely packed compared to FDM. OFDM’s spectrum efficiency stems from packing carriers orthogonally, which helps avoid Intercarrier Interference (ICI).

Figure 1 illustrates a typical OFDM system block diagram:

A generic OFDM transmitter typically consists of a FEC encoder (RS, CTC, Convolution encoder), an interleaver, data modulation/mapping, IFFT (Inverse Fast Fourier Transform), and cyclic prefix addition. A scrambler may or may not be present.

The OFDM receiver performs the reverse operations. After front-end synchronization to correct for time, frequency, and channel impairments, it removes the Cyclic Prefix, performs FFT, demodulates the data, deinterleaves, and performs FEC decoding.

CDMA (Code Division Multiple Access)

CDMA is a spread spectrum technique that doesn’t rely on time or frequency as resources. Instead, it assigns unique codes to users.

In CDMA, a low-bandwidth data message (voice or data) is multiplied by a high-bandwidth PN (Pseudo-Random) sequence, a process known as spreading.

Figure 2 depicts a CDMA system block diagram:

As shown, it incorporates modules similar to OFDM’s transmitter and receiver, except for IFFT, FFT, CP addition, and CP removal. It includes spreading and despreading modules, which are absent in OFDM. All users in a CDMA system share the same band and time space but are distinguished by different codes. The transmitted signal can be retrieved using the same PN code used by the transmitter.

Unlike OFDM, which uses IFFT and is a multicarrier technique, CDMA is a single-carrier technique.

OFDM vs. CDMA: Key Differences

The following points highlight the key differences and provide a comparison between OFDM and CDMA:

• Delay Spread Tolerance: OFDM generally performs better than CDMA and provides better tolerance to multiple delay spreads.
• Peak Power Clipping: OFDM exhibits superior peak power clipping performance compared to CDMA.
• Gaussian Noise Tolerance: OFDM demonstrates better tolerance to Gaussian Noise than CDMA.
• Subscriber Capacity (Single Cell): In a single-cell environment, an OFDM-based system can accommodate approximately 2 to 10 times more subscribers than CDMA.
• Subscriber Capacity (Multi-Cell): In multi-cellular applications, OFDM can accommodate 0.7 to 4 times more subscribers compared to CDMA.
• Multi-Cellular Performance: CDMA performs better in multi-cellular scenarios where a single frequency is allocated to all base stations.
• Multiple User Signal Strength Variation: In multiple-user scenarios, an OFDM receiver must handle significant variations in signal strength from multiple distributed users.
• Security: A major benefit of CDMA is more secure communication in high-noise environments due to PN codes. However, this is limited to a smaller number of users. OFDM can also provide secure communication through encryption and incorporating a scrambler (randomization) module.