C-band Deployment and Testing
Mid-band is in the 1 GHz to 6 GHz spectrum and C-band is in the 3.7 GHz to 4 GHz spectrum. C-band has the potential to be a key element of 5G because the frequency can cover wide distances and has expected peak data rates between 600 Mbps and 900 Mbps.
C-band (3700 – 3980 MHz) borders the top end of another mid-band frequency – Citizens Broadband Radio Service (CBRS) (3550 – 3700 MHz). Because they immediately flank each other, there is a definite risk of adjacent band effects if the deployments of the two are not done properly.
How Mid-band Spectrum is Used
Both C-band and CBRS utilize time division duplex (TDD) technology. Despite their similarities, they have different use cases.
Many believe CBRS is best suited for private LTE and private 5G networks. One reason is that CBRS channels are not universally accessible by all network operators. Additionally, transmitted power is limited, restricting its useful range for mobile applications.
The Federal Communications Commission (FCC) has divided the CBRS spectrum into three tiers with designated priority levels for users:
- Incumbent – Satellite ground stations and the U.S. military; priority
- Priority Access License (PAL) – Mobile operators awarded licenses in the 2020 FCC auction; secondary
- General Authorized Access (GAA) – GAA channels are typically used for unlicensed spectrum and defer to the incumbents and PAL licensees
The wide frequency range of C-band (figure 1) makes it a prime spectrum choice for service providers. C-band has been divided into 14 unpaired 20-MHz license blocks. Frequencies will become available in two blocks as satellite companies vacate the spectrum.
Figure 1: C-band spectrum allocation.
C-band is considered well suited for 5G because it delivers a balance between coverage and bandwidth. Part of Frequency Range 1 (FR1), is included in 3GPP release 15. Three bands – n77, n78, and n79 – were identified for 5G operation in the C-band.
C-band Leverages Cellular Technologies
C-band takes advantage of proven, effective cellular technologies, including carrier aggregation (CA) and massive MIMO, and has 100 MHz of bandwidth. It also performs well in non-line-of-sight conditions and indoor penetrations because of its available spectrum and propagation characteristics. For all these reasons, it is particularly beneficial for Enhanced Mobile Broadband (eMBB) use cases.
Massive MIMO and beamforming will play an important role in 5G radio networks. Even minor distortions in the signal direction can affect the key performance indicators (KPIs). Failure to install per planned specification can lead to interference that degrades 5G network performance. To maximize mid-band frequencies, such as C-band, and massive MIMO beamforming antenna performance, precise antenna alignment and line-of-sight surveys are critical during installation.
TDD Impact on C-band Deployment
TDD creates some definite challenges for C-band deployment. TDD gains spectral efficiency by allocating precise time slots for uplink (UL) and downlink (DL) signals over the same frequency. Because of these factors, timing and synchronization requirements are extremely strict for TDD deployed over C-band to prevent crosstalk.
Base stations are closely synchronized to a common phase clock reference for all TDD systems. Frame and slot formats between adjacent networks must also be synchronized to prevent inter-cell interference.
Because CBRS and C-band are immediately adjacent to each other and there is no guard band in-between them, deploying and operating systems utilizing the spectrums in close proximity requires exact processes and testing. If not, then network performance will suffer.
For those reasons, how mobile carriers implement C-band will play a significant role in system performance. The result is that vendor-specific features, such as filtering, scheduling, data adaption, and sub-carrier/slot assignment, become paramount in a successful deployment.
Satellites Must Be Considered
During network planning, the impact 5G base stations will have on satellite earth stations must be factored in. Poor preparation may lead to saturation of the low noise block (LNB) downconverter of the satellite antenna system. Interference can be caused, especially if the 5G base stations are close to the satellite earth station.
C-band base stations have stringent out-of-band emissions (OOBE) requirements due to this fact. Each base station has an interference limit relative to any satellite earth station, expressed as a maximum power flux density (PFD) of -124 dBW/sqm in a 1 MHz bandwidth. The limit is usually translated into the maximum interference power the C-band radio may feed to the antenna. Factors that influence the translation are the gains and orientations of the C-band and the earth station antennas. The distance between the antennas has an impact, as well.
One approach is to install an interference mitigation filter (IMF) between each radio port and the antenna. The IMF is often integrated with the radio’s output filter. Contractors and mobile operators have to consider different filters for phase 1 and phase 2 re-farming in C-band.
Overcoming eCPRI Synch Challenges
eCPRI enables efficient and flexible radio data transmission over a packet-based fronthaul transport network. Its use of Ethernet for transport brings many benefits to C-band. For one, it is backward compatible, enabling greater convergence of access networks, and statistical multiplexing. Aggregate bit-rate requirement is lowered for these reasons.
Unlike CPRI, however, eCPRI is not a synchronous technology. GPS, precision time protocol (PTP), synchronous Ethernet, or similar options can be used to overcome this factor. eCPRI tests must be performed to measure throughput, delay, and packet jitter to ensure C-band systems meet specifications.
Heavy Antenna Integrated Radio Units
One potentially overlooked factor with C-band deployment is the Antenna Integrated Radio Unit. The combined radio and antenna weigh as much as 230 pounds, which is much heavier than conventional radio units. The excessive weight makes them impractical in many deployments. In those cases, the radio and antenna are split and then re-connected using a coax cable.
C-band Testing Considerations
C-band deployment must include spectrum clearance, interference detection, and interference hunting. Persistence spectrum analysis and monitoring will be required once C-band systems are powered up to identify interference during the transition from TDD UL and DL frames.
Spectrum clearance is the first step. A spectrum analyzer with a directional antenna can help in certain test scenarios to identify the presence of unnecessary signals that may not have been cleared. After networks have been launched, interference detection and interference hunting should be performed to identify interference-related performance issues.
Timing and interference are key measurements for C-band deployment and network operation. Tests include:
Time error (TE) and absolute TE – Because C-band utilizes TDD, timing measurements are essential. The time difference between two points or clocks is TE. The time difference between a device and Primary Reference Time Clocks (PRTC) is absolute TE. It is measured using PreciseTimeBasic (PTB) and must be 1.1 us maximum to comply with ITU-T.
Interference – As with every spectrum, interference is a concern. C-band and CBRS UE can interfere with each other. It’s also possible for unsynchronized or partially synchronized systems to experience interference between devices and base stations. A third scenario is interference can occur between CBRS base stations or C-band sites.
Line Sweep – Traditional line sweeps are an important measurement for C-band deployments due to the splitting and re-connecting of the Antenna Integrated Radio Units. Additionally, C-band and recently auctioned spectrum from Auction 110 (3.45 GHz to 3.55 GHz) will be deployed in parallel, with the radios and antennas separate in many installations. For this reason, the methods of procedure (MOP) for these installs will be updated to state line sweeps up to 4.2 GHz.
5G Network Time Synching – Co-located TDD systems will need to synchronize DL and UL transmissions. Additionally, co-located towers will support multiple mobile operators. Effective network time synching testing capabilities are necessary to prevent interference.
Even if the timing is perfectly synchronized, the slot format must be aligned among operators or there will be a weak signal transmission. Figure 2 shows the potential issue. Line 42 and line 43 will result in a site failing inspection due to DL and UL slots overlapping.
In practice, network operators are agreeing that the use of common slot formats eliminates DL or UL interference.
Figure 2: Potential interference issues (Lines 42 and 43) caused by improper timing synchronization.
Passive Intermodulation (PIM) – Traditional TDD-based systems do not usually suffer from passive intermodulation. C-band is different, though. The reason? C-band systems are being deployed at existing cell sites with 800 MHz PCS and other bands that might cause PIM (figure 3).
Figure 3: Potential PIM issues in C-band.
Training and Certification
Contractors and mobile operators will have greater confidence in testing C-band if technicians are properly trained. Anritsu offers instructor-led courses and e-learning classes on all the key measurements to help prepare field technicians for the task at hand.
Mobile operators often require proof of Anritsu certification in awarding a contract, and verification that a certified representative is on-site during the deployment and installation. Therefore, scheduling certified training courses is essential for success and payment.
Anritsu offers SmartPay™, a test equipment acquisition program designed to give contractors purchasing power and options. This financial flexibility helps offset the investment in personnel and test solutions to efficiently meet operator timelines and contract conditions associated with C-band.
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