Simplifying FTTA Network Deployment, Maintenance, and TroubleshootingBaseband unit (BBU) emulation using CellAdvisor™ or T-BERD®/MTS

Application Note

Fiber to the antenna (FTTA) deployments are causing unprecedented challenges in the deployment, management, and troubleshooting of wireless networks. By some measures, carriers are experiencing up to 20% repeat tower climbs to fix problems. These re-deployments cause excessive delays and often result in $2,500+ in dispatch costs. This application note discusses FTTA challenges and how a structured test approach, using CPRI with BBU emulation, can help reduce repeat tower climbs, improve time to market, and reduce OpEx.

In today’s FTTA networks, problems occur in various locations: from the fiber infrastructure, to faulty equipment, to the coax/antenna sub-system and finally, the RF environment. The fundamental challenge to carriers is visibility, and identifying these problems typically involves the dispatch of a specialized, third-party tower crew to climb the tower and begin the troubleshooting process. Service providers are looking for efficient ways to deploy cell sites and reduce mean time to repair (MTTR).

FTTA Deployment Challenges FTTA architectures involve a BBU at the base of the tower connected via fiber to a remote radio head (RRH) at the top. At the top of the tower, the RRU connects to antennas through coax.

Figure 1. Typical cell-site deployment step

2 Simplifying FTTA Network Deployment, Maintenance, and Troubleshooting

A typical cell-site FTTA deployment workflow is as follows:

1. The tower contractor installs fiber, coax, RRH, and antennas at the top of the tower. Cable tests (fiber inspection, fiber continuity, coax sweep, and passive intermodulation (PIM) are performed prior to build completion.

2. Once tests are successfully completed, the tower crew leaves the site. The next phase involves the installation of the BBU and commissioning of the cell site. This usually happens later.

3. During or after the commissioning, problems are uncovered that could not have been caught during the first phase of installation.

These problems typically manifest in the following areas:

y Coax infrastructure — swapped cable

y Antennas — faulty equipment

y SFP selection and performance — incorrect or improper SFPs

y Equipment performance — radio defects, bad line cards, incorrect provisioning, wrong software versions

y RF environment — external interferers, uplink noise issues

y PIM — external or internal sources

CellAdvisor and T-BERD/MTS Use CasesHaving a Viavi Solutions® test instrument with BBU emulation lets technicians achieve more test coverage during the initial build process, identifying and isolating these issues in one site visit to avoid repeat dispatches. This test approach also isolates problems more effectively and helps to identify problem sources: fiber, coax, antenna, radio equipment, SFP, or RF.

And, with CellAdvisor or T-BERD/MTS-5800, all this testing can be conducted from the base of the tower—avoiding the safety issues of tower climbs.

Key use cases offered by CellAdvisor and T-BERD/MTS with BBU emulation include:

y CPRI Layer 2 RRH initialization

y SFP identification

y RRH configuration

y Antenna tilt (RET) and VSWR

y PIM testing

y Interference identification

y Digital DAS coverage and signal quality testing

y RRH performance verification

y Over-the-air LTE verification (CellAdvisor only)

Figure 2. Typical FTTA challenges

Troubleshooting these issues usually involves the expense of dispatching a tower crew.

3 Simplifying FTTA Network Deployment, Maintenance, and Troubleshooting

CPRI Layer 2 RRH InitializationVerify provisioning and RRH initialization at the top of a tower.

y Applications — initialize an RRH and measure basic CPRI parameters such as initialization startup sequence; C&M channel type (fast vs. HDLC); protocol ID; line rate (freq Rx)

y Key benefits — technicians verify basic RRH CPRI port operation, provisioning, and initialization from the base of the tower

Figure 3. Verifying provisioning and RRH initialization

RRH Polling/SFP IdentificationQuery performance and configuration information from an SFP in the RRH at the top of a tower.

y Applications — troubleshoot problems by retrieving RRH information via the CPRI link under test

y Information retrieved:

− SFP information — verify the correct SFP is installed through its serial and model number

− CPRI port ID — verify fiber is routed to the proper port, especially when multiple RRHs are chained together

− Port mode — verify the correct master/slave settings of the CPRI port under test

Figure 4. Querying performance and configuration information from an SFP in the RRH

4 Simplifying FTTA Network Deployment, Maintenance, and Troubleshooting

Antenna Tilt (RET) and VSWRVerify proper antenna installation, isolate RF noise, and verify VSWR results.

y Applications — Retrieve antenna tilt settings from the CPRI link under test; modify antenna tilt while measuring uplink spectrum for noise and interference; retrieve VSWR readings measured by the RRH (CellAdvisor only)

y Key benefits — verification and accurate modifications of antenna electrical tilt; isolation/mitigation of RF noise or interference; determination of the health of the antenna and feed-line system by measuring VSWR

Figure 6. Verify antenna information

RRH Polling/ConfigurationQuery performance and configuration information from the RRH at the top of tower.

y Applications — troubleshoot problems by retrieving RRH information via the CPRI link under test

y Information retrieved:

− RRH serial number — retrieve RRH information, useful for inventory, warranty information and validation of correct equipment

− RRH description attributes — validate RRH configuration and vendor information

− Carrier configuration — retrieve the technology, uplink/downlink frequency, downlink power and noise results from the RRH

− Software version — verify the correct software is installed on the RRH

Figure 5. Querying configuration information from the RRH

5 Simplifying FTTA Network Deployment, Maintenance, and Troubleshooting

PIM TestingTest PIM at the top of a tower using max RF TX power (OCNS).

y Application — generating an LTE signal using a CellAdvisor at the base of the tower enables an RRH to transmit at max power (OCNS); measuring the RFoCPRI uplink helps detect PIM conditions for the whole system

y Key benefit — cell site technicians detect PIM conditions prior to BBU installation

Figure 7. Testing PIM at the top of a tower

Interference IdentificationIdentify external (macro) vs internal (cell site) interferers.

y Application — CellAdvisor’s Dual Spectrogram feature helps identify interference issues—with external interference, anomalies are seen on both antenna paths, whereas an internal interference anomaly is only seen on one antenna path

y Key benefit — cell-site technicians and workgroups isolate and fix problems quickly and cost effectively

Figure 8. Detecting PIM conditions

6 Simplifying FTTA Network Deployment, Maintenance, and Troubleshooting

Digital DAS Coverage and Signal-Quality Testing DAS commissioning requires validating the downlink path. In a typical DAS deployment, an RF signal generator at the head end measures RF signal quality over the airlink at the venue. However, for digital DAS, an RFoFiber™ signal is required to perform signal quality tests. The RFoFiber Downlink Signal Generator function lets a CellAdvisor generate a continuous wave or a wide-band signal compatible with e-TM specifications.

y Application — generating an RFoCPRI™ LTE signal to the RRH—this LTE standards-based RF signal generation from the RRH lets an RF engineer make RF accurate power and signal quality (EVM) measurements at the venue

y Key benefit — RF transmit link have the expected coverage (power level) and signal quality (EVM), especially useful in DAS applications

Figure 9. DAS coverage and signal-quality testing

RRH Performance VerificationCell technicians often install the RRH at the top of the tower and only then realize an operational failure. This causes additional work and slows down the deployment process. Without having the ability to properly test the RRH, the RRH can be wrongly identified as the cause of a cell-site performance issue and is returned. The factory finds the RRH to be perfectly operational, and the service provider ends up paying extra charges.

y Application — CellAdvisor generates and receives an LTE signal from the digital RFoCPRI interface through the RRH to the RF coax interface(s)

y Key benefit — technicians can verify RRH operation and performance prior to tower-top mounting; validating RRH also helps reduce unnecessary returns of RRHs to the factory.

Figure 10. Tx and Rx testing from the ground

7 Simplifying FTTA Network Deployment, Maintenance, and Troubleshooting

Over-the-Air LTE VerificationEnsuring that the FTTA infrastructure, RRH, antennas, and coax cables are all installed correctly and validating end-to-end cell-site deployment before the BBU installation are key requirements for any service provider’s operations team. It helps reduce time-to-market and reduces OpEx. Using two CellAdvisors and their BBU emulation features, this testing is easily done from the ground.

y Application — CellAdvisors at the base of the tower generate a digital signal over the RFoCPRI interface through the FTTA infrastructure to the RRH to transmit an LTE signal over the RF downlink; another CellAdvisor measures RF signal quality (EVM), RSRP, and PCI

y Key benefit — end-to-end testing of the macro RF environment, FTTA infrastructure, and radio equipment from the ground

Figure 11. LTE verification from the ground

ConclusionTraditional models for cell-site installation and troubleshooting are not scalable to address the increasing coverage and capacity needs of end users. Part of the issue with old approaches is that not all system components are present during the installation and build processes. This fragmented workflow limits visibility into important issues that may be present. This costs service providers millions of dollars and considerable time in turning up, re-dispatching service teams, and troubleshooting.

A more thorough and complete test model is necessary to capture and resolve issues earlier in the cell-site deployment process. Adding CellAdvisor and T-BERD/MTS BBU emulation testing to close-out processes lowers deployment costs, speeds network builds, and reduces OpEx.

© 2016 Viavi Solutions Inc. Product specifications and descriptions in this document are subject to change without notice. bbu-an-fop-tm-ae30179611 001 0116

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