C-RAN Drives the Need for a Mobile Fronthaul Network and OTN is the Enabler

Author: Scott Wakelin

In my last post, I wrote about how operators are considering a C-RAN architecture enabled by an OTN-based fronthaul network to deploy costly new spectrum in a way that achieves the highest capacity and lowest cost.

The fundamental challenge to unlocking the advantages of C-RAN is the rollout of a cost-effective, fiber-efficient, and scalable fronthaul network that interconnects the radios at the cell sites with the centralized baseband units. Mobile network operators have a few options for how to do this.

Passive and Active WDM solutions are available, but they lack carrier-grade OAM, fault isolation and demarcation that should be a requirement in every mobile network operator’s Radio Access Network. They also require an overinvestment in higher cost WDM transceivers.

Interest is growing around how to use Ethernet for Fronthaul. While Ethernet is attractive from the sense that it enables the use of carrier Ethernet infrastructure, we’re likely several years away from standards and technology being in place to enable its widespread adoption in the RAN.

This leaves the ITU-T-standardized G.709 OTN (Optical Transport Network). The following table highlights the advantages and challenges for each of the fronthaul methods.

Fronthaul Advantages Disadvantages

As the table highlights, OTN based fronthaul offers many advantages to the mobile operator:

  1. It is the most fiber-efficient option, offering 4x the fiber efficiency of passive or active WDM.
  2. It features carrier-grade and standardized OAM.
  3. OTN-based equipment provides a natural demarcation point between the wireless operator and the wireline/fronthaul network operator.
  4. The combination of standardized OAM and demarcation enables important OpEx reducing characteristics like simplified fault isolation.
  5. It provides an opportunity to enable an SDN-controlled fronthaul network that:
    1. Gathers statistics about each connection and each link, including BER, as well as uplink and downlink delays;
    2. Remotely configures each node and into which OTN container each CPRI link should be mapped; and
    3. Enables load-based sizing of the fronthaul traffic. CPRI links from small cells or secondary carrier/sectors can be selectively enabled or disabled for fronthaul transport.
  6. An OTN-based fronthaul network can be shared by multiple operators, and it can transport multiple protocols including FE/GE simultaneously.
  7. OTN as a technology is well understood by carrier operations teams.

The combination of these advantages means that OTN represents the most cost-effective, fiber-efficient, and scalable “Fronthaul” network alternative.  The challenge, has been a lack of OTN-based fronthaul solutions which meet the strict 3GPP and CPRI latency and timing requirements. However, PMC recently announced an OTN fronthaul solution, based on their HyPHY 20Gflex OTN Processor, which enables low-latency multiplexing of CPRI Option 1-5 signals into 10G OTN-based optical links.  OTN fronthaul of CPRI clients with the HyPHY 20Gflex beats the frequency accuracy requirement of the CPRI standard by 75%, has zero impact on EVM performance, and achieves sub 3ns latency variation.   Carriers can now realize the benefits of OTN-based fronthaul without sacrificing network performance.

An operators’ Radio Access Network is arguably its most important asset. It is the superhighway that it uses to connect to its subscribers.  With the availability of carrier-grade, cost-efficient, and scalable OTN based fronthaul technology, why trust your most important asset to anything else?

This post was also published on Wireless Week.

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