Understanding OFDMA in Mobile Wireless Communication

OFDMA stands for Orthogonal Frequency Division Multiple Access. It’s a multiple access scheme allowing multiple users to communicate simultaneously over the same frequency band by dividing it into multiple orthogonal subcarriers.

In OFDMA, the frequency bins within a single IFFT (Inverse Fast Fourier Transform) are divided and allocated to different users. This technique is popular in next-generation wireless technologies like Mobile WiMAX, LTE, and LTE Advanced.

Here are some key concepts:

• OFDM (Orthogonal Frequency Division Multiplexing): OFDM is a modulation technique dividing the available frequency spectrum into multiple orthogonal subcarriers. Orthogonality minimizes interference between the subcarriers.

• OFDMA as an Extension of OFDM: OFDMA extends OFDM by adding a multiple access scheme, enabling multiple users to efficiently share the spectrum. Each user is assigned a set of subcarriers for a specific duration, creating a time-frequency resource block.

• Subcarrier Allocation: The frequency band is divided into subcarriers, allocated to different users. Allocation is dynamic, adapting to changing communication conditions, traffic, or user priorities.

• Flexibility: OFDMA offers flexibility in resource allocation, allowing the system to allocate different numbers of subcarriers and power levels based on user needs. This improves spectral efficiency and overall system performance.

• Efficient Spectrum Utilization: By using orthogonal subcarriers, OFDMA allows simultaneous transmission without interference, efficiently using the spectrum. This is crucial for high data rate applications in 4G LTE and 5G networks.

• Mitigating Frequency Selective Fading: OFDMA helps reduce the impact of frequency-selective fading, where some frequencies experience more attenuation than others. Spreading data across multiple subcarriers reduces the effect of fading on the overall signal.

Example Use Case: OFDMA in Mobile WiMAX

In Mobile WiMAX, resource allocation is based on slots. The definition of a “slot” depends on the zone type but consists of subcarriers and OFDMA symbols. One slot in PUSC (Partial Usage of Subchannels) comprises one subchannel and two OFDMA symbols.

One subchannel contains many subcarriers. For instance, in a 1024-point FFT, there are 30 subchannels, each made of 24 data subcarriers.

In LTE and LTE Advanced, resource allocation uses resource blocks. One resource block consists of 12 subcarriers and 7 OFDMA symbols.

In Mobile WiMAX, a terminal is a Mobile Subscriber Station, while in LTE/LTE Advanced, it’s called a UE (User Equipment).

Types of OFDMA: Centralized and Distributed

Based on resource management, there are two OFDMA types: centralized and distributed.

• Centralized OFDMA: A central entity (e.g., a base station) manages subcarrier allocation. It controls scheduling and resource allocation to optimize system performance.

• Decentralized (Distributed) OFDMA: Resource allocation is distributed among network nodes, allowing individual base stations more autonomy. This enhances scalability and adaptability, especially in dense or dynamic networks.

Types of OFDMA: Localized and Distributed (Sub-Carrier Allocation)

Based on subcarrier allocation, there are localized and distributed OFDMA. Resource allocation from a chunk of subcarriers of an IFFT size can be continuous or permuted per mobile subscriber user.

• Localized OFDMA: If subcarriers are contiguously assigned to each terminal, it’s called localized OFDMA. This is simpler to implement than distributed OFDMA.

• Distributed OFDMA: If distributed subcarriers are assigned to each terminal, it’s called distributed OFDMA. These are obtained using permutation techniques. Distributed OFDMA is more resistant to time-varying fading channels compared to localized OFDMA.

Conclusion

OFDMA is a fundamental technology in 4G LTE and 5G networks, providing high-speed, reliable, and simultaneous communication. Its efficient spectrum use and flexible resource allocation make it well-suited for modern wireless communication systems.