What is OFDMA in Mobile wireless communication

OFDMA stands for Orthogonal Frequency Division Multiple Access. It is a multiple access scheme that enables multiple users to communicate simultaneously over the same frequency band by dividing it into multiple orthogonal subcarriers. In OFDMA technique, number of frequency bins in a single IFFT is divided and allocated to different mobiles(users).

It is the most popular access technique adopted in next generation wireless technologies such as Mobile WiMAX, LTE, LTE Advanced etc. Following are the key basic concepts in OFDMA.
• OFDM is a modulation technique that divides the available frequency spectrum into multiple orthogonal subcarriers. Orthogonality means that each subcarrier is mathematically orthogonal to the others, minimizing interference between them.
• OFDMA extends the concept of OFDM by adding a multiple access scheme, allowing multiple users to share the available spectrum efficiently. Each user is assigned a set of subcarriers for a specific duration of time, forming a time-frequency resource block.
• Subcarrier Allocation: The available frequency band is divided into subcarriers, and these subcarriers are allocated to different users or devices. The allocation can be dynamic, adapting to changing communication conditions, traffic demands, or user priorities.
• OFDMA provides flexibility in resource allocation, allowing the system to allocate different numbers of subcarriers and transmission power levels to different users based on their communication requirements. This flexibility contributes to improved spectral efficiency and overall system performance.
• Efficient Spectrum Utilization: By using orthogonal subcarriers, OFDMA enables multiple users to transmit simultaneously without causing interference, leading to efficient use of the available spectrum. This is particularly important for high data rate applications in wireless communication, such as 4G LTE and 5G networks.
• Mitigating Frequency Selective Fading: OFDMA helps mitigate the effects of frequency-selective fading, where certain frequency components experience more attenuation than others due to the characteristics of the transmission medium. Since OFDMA spreads the data across multiple subcarriers, the impact of fading on the entire signal is reduced.

Example use case : OFDMA in Mobile WiMAX

In Mobile WiMAX resource allocation is based on number of slots. Slot definition varies based on zone types, but slot is composed of number of subcarriers and number of OFDMA symbols. One slot in PUSC is made of 1 subchannel and 2 OFDMA symbols. Here 1 subchannel consists of many subcarriers.

For example in a 1024 point FFT, there are 30 subchannels and each subchannel is made of 24 data subcarriers. In LTE and LTE Advanced resource allocation is based on number of resource blocks. One Resource block consists of 12 subcarriers and 7 OFDMA symbols. In Mobile WiMAX terminal is referred as Mobile Subscriber Station and in LTE/LTE Advanced terminal is referred as UE.

Types of OFDMA | Centralized and Distributed

Based on resource management there are two types of OFDMA viz. centralized and distributed OFDMA.
➨Centralized OFDMA: Here a central entity (e.g., a base station or access point) manages and coordinates the allocation of subcarriers to different users within the network. The central entity controls the scheduling and resource allocation to optimize overall system performance.
➨Decentralized (Distributed) OFDMA: In this OFDMA type, the responsibility for resource allocation is distributed among the network nodes, allowing more autonomy for individual base stations or access points. This approach can enhance scalability and adaptability, particularly in scenarios with dense networks or dynamic topologies.

Based on allocation of sub-carriers, there are further two types of OFDMA viz. localized and distributed. As mentioned and depicted in the figure above, resource allocation out of chunk of sucbcarriers of a IFFT size can be either continuous or permuted per mobile subscriber user.
➨ If the subcarriers are contiguously assigned to each terminal then it is called localized OFDMA. Localized OFDMA is simpler to implement compare to distributed one.
➨If the distributed subcarriers are assigned to each terminal then it is called distributed OFDMA. This distributed subcarriers are obtained using permutation technique in physical layer of OFDMA. Distributed OFDMA has advantage over localized OFDMA as it is more resistant to time varying fading channel.

Conclusion : OFDMA is a fundamental technology in 4G LTE and 5G networks, providing the foundation for high-speed, reliable and simultaneous communication among multiple devices in a wireless environment. Its efficient use of the frequency spectrum and flexibility in resource allocation make it well-suited for the diverse requirements of modern wireless communication systems.

Similar posts on OFDM and OFDMA

Refer following links which mentions OFCDM, OFDM and OFDMA Physical layers(PHY) as per IEEE 802.11a(WLAN), 802.16d(Fixed WiMAX) and IEEE 802.11ad. It mentions difference between OFDM and other terms which include OFDM vs DMT, OFDM vs FBMC, SC-FDMA vs OFDMA, OFDM vs CDMA etc.

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RF and Wireless Terminologies