When working with NR channels, understanding Channel Bandwidth (CBW) and Transmission Bandwidth Configuration is key to optimizing network performance.
Channel Bandwidth (CBW) can be thought of as the sum of the maximum transmission bandwidth plus guard bands on both sides. Here’s a simple formula to explain it:
NumOfSubcarrier= Number of subcarriers per Resource Block
SCS = Subcarrier Spacing
GuardBand = Guard band on each side
Why do we need Guard Bands?
Guard bands are small frequency that prevent interference between adjacent channels, ensuring clear and reliable communication. Without them, signals from different channels could overlap, leading to reduced performance and potential communication errors.
𝗘𝘅𝗮𝗺𝗽𝗹𝗲 : When using a 5 MHz CBW, the choice of Subcarrier Spacing (SCS)—whether 15 kHz or 30 kHz, doesn’t directly impact coverage or capacity but affects how the network handles delay spread:
-15 kHz SCS = 25 RBs
The lower SCS is often deployed in lower frequency bands where delay spread is a concern. Its longer symbol duration helps manage longer propagation delays, ensuring reliable communication in environments with significant multipath effects. Despite the lower data rates, this SCS is ideal for large cells where delay spread could cause inter-symbol interference.
-30 kHz SCS = 11 RBs
This provides a balanced approach, offering a shorter symbol duration and thus slightly reduced sensitivity to delay spread. It’s well-suited for environments with moderate delay spread, offering similar throughput to 15 kHz SCS when using the same channel bandwidth, but with lower latency.
Surely, in freq domain ∆F = 1/ SCS …
15 kHz SCS (25 RBs):
Wider Coverage: 15 kHz SCS is ideal for larger coverage areas. It operates at a lower frequency, which means the signal can travel further and penetrate obstacles like buildings more effectively. This makes it well-suited for rural areas or regions with fewer base stations.
30 kHz SCS (11 RBs):
Moderate Coverage: 30 kHz SCS provides a balance between coverage and capacity. It doesn’t offer the same extensive coverage as 15 kHz, but it still supports reasonable coverage with better data rates. This makes it suitable for suburban areas where both coverage and capacity are important.
Why Does This Happen?
The choice of SCS affects the size of each Resource Block (RB) and the duration of each symbol in the transmission.
Lower SCS(e.g., 15 kHz) means more RBs can fit into the same channel bandwidth, each carrying less data but over a longer duration, which helps the signal maintain integrity over longer distances.
Higher SCS(e.g., 60 kHz) reduces the number of RBs, with each RB carrying more data but over a shorter duration. This requires a stronger signal and thus more frequent base stations to maintain the same level of coverage.
Remember in LTE we use ECP with 6 Symbols for big cells
Adding: In LTE radio & Possibly NR the beloved guard band is allowed to be used up for NBIoT (Guardband NBIoT) on either side. It must be that improved RF performance is possible.
In NR, the current trend is to add Guardband NBIoT as component carrier rather than implement it through shared iFFT bins.
Penetration (loss) is determined by carrier frequency, so is propagation (free path loss). This is SCS agnostic.
If we keep the same Cell BW then 15KHz and 30KHz SCS will provide same Throughput (cell capacity).
Choice to deploy smaller SCS for lower bands is due to propagation delay (delay spread), it’s not coverage or capacity related.
