Wireless Fixed Access Network
5G Mobile Fronthaul
The CPRI standard recommends a wired connection between the centralized Base Band Units (BBU) and the Remote Radio Head (RRH). Currently, the CPRI standard specifies bit rates up to 12.16512 Gbit/s, but the advent of 5G will demand speeds of at least 100 Gbit/s per cell, and it is assumed that the technical barrier to increasing bit rates will be reached sometime soon. Consequently, a Radio over Ethernet (RoE) technology is under study, which encapsulates and transports the CPRI, etc. on Ethernet, which has 100 Gbit/s speed in practical use and 400 Gbit/s under standardization. The RoE technology itself has problems with frame jitter, synchronization between base stations, frequency and time synchronization. As a consequence, joint use of RoE, Synchronous Ethernet (SyncE), and IEEE1588v2 Precision Time Protocol (PTP) is being investigated and a new Next Generation Fronthaul Interface (NGFI) standard is being discussed as a replacement for CPRI.
The evolution of cell sites
Centralized Radio Access Networks
Cloud RAN is a specific concept that is rapidly being developed and prepared for commercial deployment. This aims to centralize the functions of the RAN network (e.g. eNodeB for LTE, NodeB & RNC for 3G) within the cloud servers, such that only the physical transceiver and antenna elements of the eNodeB need to be physically located at the cell site. This provides for a more cost efficient deployment, especially for “small cells” or local cell sites used to boost capacity or fill gaps in coverage. The Cloud RAN concept is taking advantage of technologies such as CPRI, that allow the baseband to TRX link of the basestation to be carried on dedicated high speed optical fiber links. This technology has already been developed for Remote Radio Head use (RRH), where the TRX/ Antenna is separated from the base station baseband by several meters (top and bottom of cell site mast) up to separation of hundreds of meters or of kilometers (e.g. for in building or shopping mall deployment, where a single baseband serves all TRX/antenna sites). So Cloud RAN is extending the same concept further such that all TRX/Antenna sites in a network region can be connected by a fiber ring to a centralized baseband server. Of course this technology currently relies on having dedicated fiber access to each cell site, and this can limit deployment in some scenarios. We will see later therefore that there is parallel research into providing a suitable wireless backhaul for Cloud RAN. The key challenge here is to have low enough latency and high availability across a wireless backhaul, because the baseband algorithms in the NodeB require latencies of very few milliseconds in order to meet the timing requirements of the scheduler and re-transmission functions in the basestation.
Installation and Maintenance Issues
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). The wideband receivers used in Base Stations for mobile broadband are more sensitive to PIM on the site (older GSM narrow band receivers less vulnerable). So PIM issues on the site (poor quality or corroded connectors, metal reflectors etc) will create uplink sensitivity problems that give poor quality in the cell. As the downlink is not affected, normal site survey and coverage checks may not detect this.
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.
CPRI/OBSAI Fiber Check
To answer the need for more throughput at lower cost, wireless network providers are moving 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).
Cellular tower showing typical RRH installation behind antennas
With this new flexible architecture came the necessity for an equally flexible interface between the BBU and RRH. 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.
- MT1000A – 10 GbE access network testing (WDM network test), and CPRI connectivity test.
- MT1000A – OTDR for fiber I&M
- MT1100A – 100 GbE access/core network testing.
- MT8220T – CPRI performance test in the field.
Base Station Analyzer
400 MHz - 6 GHz VNA frequency
150 kHz - 7.1 GHz SPA frequency