5G Fronthaul
This is the fourth in a series of blog posts on the Evolution of Fronthaul, links to other posts below.

As wireless operators announce their plans for the introduction of 5G services, ecosystem players are developing and introducing a new range of capabilities to pave the way for a smooth transition from 4G to 5G. While 5G is the ultimate goal, operators will not get there in one giant leap. As an interim step, operators will be forced to significantly increase the capacity of their 4G services with densification and introduction of LTE Advanced features such as CoMP. These developments are impacting the overall wireless architecture, including the fronthaul networks.

Fronthaul networks have been gaining visibility in the network community recently particularly with the rising demand for Central Radio Access Network (CRAN) deployments. Common Public Radio Interface (CPRI) is the mainstream transport protocol in current fronthaul networks. The latest CPRI specification adds new CPRI line rates up to 24.3 Gbit/s (CPRI Rate 10) which pumps more capacity to the LTE/LTE-A Remote Radio heads, achieves higher order MIMO, and allows multi-carrier configuration. Although it’s a simple interface, CPRI allows centralization and pooling of valuable Baseband resources.

While CPRI continues to be the dominant technology for CRAN and LTE-Advanced deployments, it is very bandwidth inefficient and cannot scale to meet the bandwidth-hungry services that 5G promises. Rather than simply throwing more bandwidth at the problem, ecosystem partners have been revisiting the split of functions between the baseband unit (BBU) and remote radio units (RRU) over the past few years and are converging towards a set of alternatives that can solve different use cases in the coming 5G world. The use cases include fixed wireless broadband, massive broadband, and ultra-low latency applications.

5G Sunctional Split Diagram

Massive broadband services are expected to take advantage of advanced mobility applications that require coordination of multiple radios. This capability requires a lower layer functional split option that leaves most of the functional elements (figure above) in a centralized location coordinating the radios. Options 6 and 7 are the ones currently under consideration for this use case. For the massive broadband use case, the CPRI organization published the first eCPRI specification in 2017. eCPRI recommends splitting the PHY functions and deploying Ethernet transport technology. While eCPRI is a major new milestone for the 5G world, it is not the end of the road. Further work for more powerful and interoperable fronthaul technologies are under way in organizations such as IEEE (IEEE 1914.1), xran and ORAN.

Read the other blog posts in this series:

If you would like to learn more, please read my white paper, The Evolution of Fronthaul Networks.

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