OFDMA: Advantages and Disadvantages

This page explores the advantages and disadvantages of OFDMA (Orthogonal Frequency Division Multiple Access). It also covers the benefits and drawbacks of OFDMA.

What is OFDMA?

Introduction: OFDMA stands for Orthogonal Frequency Division Multiple Access. It is a multi-user version of OFDM (Orthogonal Frequency Division Multiplexing).

In OFDM, the entire symbol (with all the subcarriers) is assigned to a single subscriber station or base station for the duration of the connection. In OFDMA, however, the total subcarriers are divided into groups called subchannels. These subchannels are then assigned to individual subscribers or users based on their resource requirements. Hence, it is considered an access technique for efficient resource utilization (specifically, frequency).

Figure 1: Types of OFDMA

Figure 1 depicts different types of OFDMA based on how subcarriers are distributed within an OFDM symbol. These include Distributed OFDMA and Localized OFDMA.

Figure 2: OFDMA Frame Structure

Figure 2 illustrates the frame structure of Mobile WiMAX as per the IEEE 802.16e OFDMA PHY layer. As shown, a variable number of subchannels and symbols are assigned to individual downlink and uplink bursts for downlink and uplink transmissions. The downlink transmission occurs from the Base Station (BS) to Subscriber Stations (SSs), while the uplink transmission is from SSs to the BS.

Since OFDMA is a variant of OFDM, the advantages and disadvantages of OFDM also apply to OFDMA. The following sections highlight the unique advantages and disadvantages specific to OFDMA.

Benefits or Advantages of OFDMA

Here are the benefits or advantages of OFDMA:

• Efficient Frequency Allocation: Unlike OFDM, which allocates all subcarriers of an OFDM symbol to one user, OFDMA allocates subsets of subcarriers to different users. This makes more efficient use of frequency allocations compared to FDMA and OFDM.
• Multi-User Diversity: OFDMA offers multi-user diversity because it allows different users to transmit over different parts of the frequency spectrum (traffic channels). As a result, different users experience different channel qualities. A deeply faded channel for User #1 might still be useful for other users.
• Simplified Receiver Design: OFDMA simplifies the receiver design by eliminating intra-cell interference, which avoids the need for multi-user detection (as in CDMA). Only FFT (Fast Fourier Transform) processing is required.
• Improved BER Performance: Fading environments lead to better Bit Error Rate (BER) performance with OFDMA.

Drawbacks or Disadvantages of OFDMA

The following are the disadvantages of OFDMA:

• Complex Permutation/Depermutation: The rules for permuting and depermuting subcarriers for allocation and deallocation to subchannels are complex. This increases the complexity of transmitter and receiver algorithms for data processing and extraction, unlike OFDM.
• High PAPR: OFDMA exhibits a higher PAPR (Peak-to-Average Power Ratio). Large amplitude variations lead to an increase in in-band noise. Furthermore, it increases the BER when the OFDMA signal passes through an RF amplifier with nonlinearities.
• Stringent Equalization Requirements: OFDMA requires very tight time, frequency, and channel equalizations between users. This is achieved through preambles, pilot signals, and other signal processing techniques.
• Complex Co-Channel Interference: Co-channel interference management is more complex compared to CDMA.