The Role of C-Band in 5G
Midband (or C-band) is considered the “waterfront property” of RF spectrum for 5G. It offers the best compromise between RF coverage and RF bandwidth. Per ITU’s minimum technical requirements to meet the IMT-2020 criteria for eMBB, at least 100 MHz of bandwidth will be required. For a true 5G experience, larger chunks of RF spectrum are absolutely needed. Most of these large spectrum assets are available either in the midband or in the millimeter wave.
In the past few years, industry regulators opened a significant amount spectrum in the midband and mmWave. On average, 380MHz of midband spectrum was made available for 5G deployments by regulators outside of US in leading 5G countries in Asia, Europe and Australia. The US, to catch up, first opened CBRS spectrum with Priority Access Licenses (PAL) licenses. Later in 2020 the US auctioned 280MHz of C-band spectrum.
One thing to note is that all 5G deployments above 3 GHz, including C-band (3.7 GHz to 3.98 GHz) range and mmWave, will be TDD deployment. TDD delivers a full-duplex communication channel over a half-duplex communication link. This means both the transmitter and receiver use the same frequency but transmit and receive traffic at different times by using synchronized time intervals. TDD turns out to be a more attractive option from a spectral efficiency point of view. It requires only unpaired spectrum for operation. This is beneficial considering the scarcity of frequency resources.
Further, physical layer features such as massive MIMO, beamforming, and precoding, which rely on channel state information (CSI) measurement in the uplink, are more robust due to channel reciprocity. While TDD brings spectral efficiency, it introduces a few challenges beyond the typical RF FDD deployment challenges in terms of interference management, and synchronization.
Satellite Earth Station Considerations
Another challenge is the presence incumbents in the 5G C-bands. In the US, C-band spectrum was used primarily by satellite Earth stations.
Before a 5G RAN is installed, spectrum clearance is the required first step in ensuring optimal performance of the future 5G network. Additionally, service providers may need to pay attention to the effect of 5G base stations on the satellite receive bands. Advanced 5G features such as massive MIMO and beam steering can deliver significant power. They also potentially can interfere and saturate the low noise block (LNB) downconverter of the satellite antenna system. This results in interference, especially if the 5G base stations are close to the satellite Earth station. Therefore, it is important during the planning phase to understand the impact of 5G base stations on the satellite Earth station.
Key Considerations for C-band Deployment:
C-band is the best RF spectrum for 5G deployments; yet, C-band 5G networks may require a different level of attention in terms of planning, deployment, and maintenance. Some key considerations for C-band deployment include:
- Ensuring spectrum is cleared and there are no interference issues between the new 5G services and the satellite earth stations
- Enhanced importance of antenna alignment for a much narrower RF footprint
- Stringent timing and synchronization requirements for TDD deployments
- Much more involved maintenance of 5G Advanced Antenna System (AAS) with higher-order MIMO and beams
There’s an expectation that C-band 5G deployment truly will deliver the coverage and capacity that 5G use cases need. However, deploying and maintaining 5G’s complex technology and network architecture will not be a trivial exercise. Time-to-market and network quality will depend on the rigor of test and measurement during the complete life cycle of the network.
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