maximum RBs for 60MHz with SCS = 30 kHz is defined to be 162 RBs, so 162 x 12 = 1944, 1944 < 2048. Then NFFT size for 60MHz bandwidth with SCS = 30 kHz should be 2048 instead of 4096.
Hi Elisa,
Maximum RBs for 60MHz with SCS = 30 kHz is defined to be 162 RBs, so 162 x 12 = 1944. Here 1944 > 85% of 2048, so we will map NFFT size for 60MHz bandwidth with SCS = 30 kHz should be 4096 instead of 2048. This same concept can be observed in NFFT size for 30MHz bandwidth with SCS = 30 kHz, where we take NFFT size as 2048 instead of 1024.
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why do we check if it’s 85% or not. Is there any specific reason.
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NFFT Size for 60 MHz Bandwidth with SCS = 30 kHz
To calculate the NFFT size, follow these steps:
- Determine occupied subcarriers: For a 60 MHz bandwidth, the number of resource blocks (RBs) can be calculated as:Number of RBs=Bandwidth (Hz)SCS (Hz)/12=60×10630×103/12=166.67\text{Number of RBs} = \frac{\text{Bandwidth (Hz)}}{\text{SCS (Hz)}} / 12 = \frac{60 \times 10^6}{30 \times 10^3} / 12 = 166.67Rounding this to the nearest integer, you get 166 RBs.Since each RB consists of 12 subcarriers, the total number of subcarriers is:166×12=1992 subcarriers.166 \times 12 = 1992 \text{ subcarriers}.
- Choose an NFFT size: The NFFT size must be a power of 2 and should accommodate the occupied subcarriers (1992). The smallest power of 2 greater than 1992 is 2048, so:NFFT=2048\text{NFFT} = 2048
Why Check for 85% Occupied Subcarriers?
The 85% threshold is commonly used for spectrum efficiency and ensuring that the FFT size is sufficient to accommodate the signal while minimizing computational resources. Here’s the reasoning:
- Efficient Use of FFT Size:
- The FFT size should be large enough to avoid aliasing but not excessively large, as it increases computational complexity. Typically, the occupied subcarriers should use around 85% of the total FFT size to balance efficiency and overhead.
- Avoid Aliasing:
- If the number of occupied subcarriers is too close to the FFT size (e.g., >90%), there is a higher risk of aliasing from adjacent channel interference or spectral leakage.
- Guard Bands:
- The remaining 15% (or more) allows for guard bands at the edges of the spectrum, ensuring that no critical signal components are lost and that the signal adheres to spectral masks defined in standards like 3GPP.
- Standard Compliance:
- Standards like 3GPP often define subcarrier usage and guard band requirements. Checking for 85% compliance ensures the design stays within these guidelines.
For example, with 1992 subcarriers used out of 2048 total FFT bins:
19922048≈97.3%\frac{1992}{2048} \approx 97.3%
This exceeds the typical 85% guideline, suggesting the FFT size might need to be increased (e.g., to 4096) to ensure compliance in some scenarios. However, in practical implementations, 2048 is often acceptable due to the low adjacent-channel interference in 3GPP-defined conditions.
