At first glance, it may look like a simple time-frequency grid. But behind that grid there is a very powerful idea: radio resources can be divided into small building blocks and dynamically assigned depending on the service, deployment scenario, and channel conditions.
OFDM builds the grid and OFDMA shares it.
OFDM creates the time-frequency structure using subcarriers and symbols, while OFDMA allows multiple users to be scheduled across that same structure.
In 5G NR, the smallest physical resource is the Resource Element:
1 subcarrier in frequency Γ 1 OFDM symbol in time
From there, the system builds Resource Blocks, slots, mini-slots, and flexible scheduling structures.
One of the key differences between LTE and 5G NR is flexible numerology.
Instead of relying on a single subcarrier spacing, NR supports different spacings such as 15, 30, 60, and 120 kHz for regular data transmission, with 240 kHz used for synchronization signals.
The importance of flexible numerology
5G NR was designed for very different scenarios:
Wide-area coverage at lower frequencies
High-capacity deployments
mmWave operation
Low-latency services
Massive MIMO and beam-based transmission
A smaller subcarrier spacing provides longer OFDM symbols and can be useful for larger cells and delay spread conditions.
A larger subcarrier spacing shortens the OFDM symbol duration and is better suited for higher frequencies, wider bandwidths, and lower-latency operation.
So, when we look at an OFDMA resource grid, we are not only seeing βusers assigned to blocks.β
We are seeing the structure that allows 5G NR to adapt the air interface to different deployment needs.
That flexibility is one of the reasons OFDM remains so important in modern cellular systems.
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