CPRI Technology: An economical medium to Improve Wireless Network Flexibility.



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Traditional cell sites consists of a ground-based radio connected to tower-mounted antennas by a long run of coaxial cable which has high loss and is vulnerable to interference. This has negative impacts on maintenance costs as equipment ages and also on system performance. To answer the need for more throughput at lower cost, wireless network providers have moved to using a remote radio head (RRH) where the radio equipment is connected to the baseband unit (BBU) by a fiber optic cable. This provides a new level of flexibility in how the cell site is deployed, including siting the RRH at the masthead (for low RF losses) or locating the BBU at a remote location (for improved operational efficiencies).

With this new flexible architecture came the necessity for an equally flexible interface between the BBU and RRH. Many high speed serial communication standards were available, but without modification none offered the high throughput, low latency and features required to support these cell site architectures. Therefore new interface standards were developed to support them. The two interfaces that lead this charge are the Open Base Station Architecture Initiative (OBSAI) and Common Public Radio Interface (CPRI). OBSAI is the more complex of the two to implement and CPRI technology is now used in the majority of new installations. CPRI pushes the complexity into the higher layers of the system so a connection between the BBU and the RRH can be established with minimal configuration. CPRI has become the more common standard, allowing manufacturers to tailor the interface to their own requirements. Also, the high throughput potential of later CPRI releases enables providers to futureproof their rollouts.

While the RRH based configuration has alleviated common problems with the traditional cell site, it creates a new requirement for test and measurement of these sites. The final interface exists at radio frequencies (RF), but testing at RF requires a tower climb. However, it is possible to perform a wide range of performance tests on the site without a tower climb by analyzing the data on the CPRI link instead. CPRI has support for many different topologies. For star configurations, a single BBU serves multiple RRHs and potentially multiple towers. This allows the operation of all the RRHs to be observed from that one BBU site. For chain and ring topologies, the operation of all the RRHs can be observed from a single CPRI link.

The move to RRH based cell sites has delivered flexibility, performance improvements and cost savings. However the increased number and density of wireless networks combined with increasing congestion of spectrum has greatly increased the vulnerability of such networks to RF interference and Passive Intermodulation (PIM). Being able to perform RF measurements through the CPRI link provides a powerful tool for hunting down interference and PIM without tower climbs and even without visiting the cell site where the BBU is sited remotely. Overview of the CPRI Protocol Setup A CPRI link consists of at least one radio equipment controller (REC) node and one radio equipment (RE) node. In a cell site, the BBU acts as the REC and the RRH is an RE. There are two main information flows: 1. Digitized downlink (DL) and uplink (UL) RF signals known as “antenna carriers”, which constitute the main CPRI payload. These are represented as streams of in-phase and quadrature (IQ) signal samples on the CPRI link. The primary task of the RRH is to convert these digital antenna carriers to and from the RF air interface. 2. Control and Management (C&M) flows. These are used to configure, control and monitor the RRHs and associated equipment. The CPRI specification concerns itself only with transport aspects for C&M, messaging protocols and syntax are vendor-specific,All information flows are multiplexed onto a digital serial communication line using appropriate layer 1 and layer 2 protocols. Layer 1 handles the physical connection between the REC and RE. Once the link is established at layer 1, the different information flows then have access to layer 2 via appropriate service access points (SAP). These include the control and management (C&M), synchronization, and in-phase and quadrature data (IQ) SAPs.

CPRI based RF testing is especially powerful in the uplink. Since user equipment (UE) has much lower transmit power compared to an RRH, interference has the biggest system impact in the uplink, figure 16 shows actual Spectrum of the Uplink via the CPRI link. The uplink is also susceptible to interference from passive intermodulation (PIM), figure 18 shows an actual PIM problem captured over CPRI. PIM detection and distance to PIM source can all be readily analyzed in the digital domain using CPRI IQ data. It is even possible under many circumstances to cancel PIM on the uplink using digital techniques. Interference hunting using CPRI has also been aided by the installation of optical taps at cell sites. An optical tap couples a percentage of the optical signal so an external device can monitor both the uplink and downlink CPRI link with no disruption of the link itself. RF interference can then be analyzed on a live system without locking or disabling the network. In chain or ring configurations, multiple RRHs can be investigated simultaneously. PIM detection can be performed by connecting one fiber containing the affected uplink data and another to potential PIM sources. Combined with fewer tower climbs, reduced maintenance time, and lower operating expenses, uplink testing over CPRI becomes an even more powerful tool.

Anritsu MT8220T BTS Master CPRI RF measurement option The Anritsu BTS Master has an option to enable CPRI RF measurements to be made at ground level. Specifically, the uplink LTE spectrum can be viewed in real-time on a live network to monitor for interferers. This provides a powerful test capability without the need to call a tower climbing crew. Many common causes of poor KPI indicators can be diagnosed, especially those resulting from accidental or illegal transmitters interfering with the uplink.