Advantages of OFDMA | Disadvantages of OFDMA
This page covers advantages and disadvantages of OFDMA access technique. It mentions OFDMA advantages or benefits and OFDMA disadvantages or drawbacks. It provides links on various topics on OFDMA (Orthogonal Frequency Division Multiple Access) basics.
What is OFDMA?
OFDMA stands for Orthogonal Frequency Division Multiple Access. It is multi-user variant of OFDM technique. In OFDM technique entire symbol (with all the subcarriers) is assigned to the subscriber station or base station throughout the connection time. In OFDMA, total subcarriers are divided into group known as subchannels. These subchannels are assigned to individual subscribers or users based on resource requirements. Hence it is known as one of the access technique for resource utilization (here frequency).
The figure-1 depicts different types of OFDMA based on distribution of subcarriers in a OFDM symbol. They are distributed OFDMA and localized OFDMA.
The figure-2 depicts frame structure of mobile wimax as per IEEE 802.16e OFDMA PHY layer.
As shown different number of subchannels and symbols are
assigned to individual downlink and uplink bursts for downlink and uplink transmissions.
The downlink transmission is from Base Station (BS) to subscriber Stations (SSs) and
uplink transmission is from SSs to BS.
Refer OFDMA versus OFDM and OFDMA versus SC-FDMA>> for more information.
As OFDMA is variant of OFDM technique, the advantages and disadvantages of OFDM is also being applied to the OFDMA technique. Following are the some of unique advantages and disadvantages of OFDMA access technique.
Benefits or advantages of OFDMA
Following are the benefits or advantages of OFDMA:
➨Unlike OFDM which allocates all the subcarriers of the OFDM symbol to one user, OFDMA allocates subset of subcarriers to different users. Hence OFDMA makes efficient use of frequency allocations in comparison to FDMA and OFDM.
➨It provides multi-user diversity. This is due to the fact that OFDMA allows different users to transmit over different parts of the frequency spectrum known as traffic channel. Hence different users see different channel qualities. Hence deeep faded channel for user#1 may still be useful for the other users.
➨It makes receiver simple as it eliminates intra-cell interference which avoids multi-user detection of CDMA type. Here only FFT processing is needed.
➨Fading environment leads to better BER performance.
Drawbacks or disadvantages of OFDMA
Following are the disadvantages of OFDMA:
➨The permutation and depermuation rules of subcarriers for allocation and deallocation to subchannels are complex. This makes transmitter and receiver algorithms complex for data processing/extraction unlike OFDM.
➨OFDMA has higher PAPR (Peak to Average Power Ratio). Hence large amplitude variations lead to increase of in-band noise. Moreover it increases BER (Bit Error Rate) when OFDMA signal passes through the RF amplifier having nonlinearities.
➨OFDMA requires very tight time/frequency/channel equalizations between users. This is achieved with the help of preamble, pilot signals and other signal processing techniques.
➨Co-channel interference is more complex compare to CDMA technique.