2 November 2016
Guest blog post at ORTENGA

This week Sivers IMA has a guest blog at ORTENGA. ORTENGA is a consulting firm which provides various Services related to radio communications applications such as 5G and IoT as well as technologies online and corporate training for educating engineers via Academy.

Sivers IMA’s view on mmWave fronthaul for 5G Cloud-RAN 

There is a race to release 5G solutions before the World Radio Communication Conference 2019 (WRC-19). Some of the main drivers are operators in the US, Japan and South Korea.

At WRC-19 there will be a final decision on the frequencies that will be used. However, FCC has already shared some of the new bands that will be available in the US:

  • 28GHz (27.5-28.35GHz)
  • 37GHz (37-38.6GHz)
  • 39GHz (38.6-40GHz)
  • 60 GHz (57-71GHz)

30 GHz-300 GHz has been defined as millimeter wave bands (mm-wave), hence all the new 5G frequency bands allocated by FCC can out of simplicity be defined as mm-wave. The first mm-wave frequencies that will be used for the cellular access scenario, seems to be 28 GHz which was recently confirmed by Qualcomm releasing information about their new Snapdragon X50 5G modem. Samples of the X50 modem will be available in the second half of 2017, with production in the first half of 2018, which means that the first mm-wave solution in handsets will be available in 2018. The Qualcomm X50 modem is promising speeds of up to 5 Gbps. IEEE have released 802.11ad for operation in the 60 GHz band, which support data speeds of up to 7 Gbps. In 2019, IEEE will also release the 802.11ay standard, which is an add-on to the 802.11ad standard, supporting data speeds of up to 20–40 Gbps. One of the most important 5G access performance requirements is latency, with latency requirements below 1 ms (sometimes <5 ms), to be compared with a latency in the range of 70-90ms as is the case in today´s LTE networks.

One proposed architecture for 5G base stations is Cloud-RAN (C-RAN), where the RF Transceiver is separated from the baseband unit (BBU) in a so called remote radio head (RRH).  The digital baseband signals are carried to the RRH via either mm-wave or optical fiber, which is referred to as fronthaul. Fronthaul for C-RAN is often using the Open Base Station Architecture Initiative (OBSAI) and the Common Public Radio Interface (CPRI).  The communication between the RRH module and the BBU requires very high bit rates and low latency (<100 µs). Today CPRI fronthaul may vary from approximately 0.6 Gbps up to approximately 9 Gbps depending on the type of LTE configuration that is supported. Conventionally, the CPRI specifications are designed for communication over fiber optic cables. However, CPRI based communication can also be used in wireless mm-wave fronthaul. For locations without fiber access, mm-wave is a very cost effective solution. However, CPRI has been shown to suffer from low transmission efficiency, poor scalability and limited flexibility, making it unsuitable for many of the future 5G fronthaul use cases. The latest CPRI Specification V7.0, defines line bit rates of up to 24 Gbps. With such data speeds already used for 4G+, even higher data speeds would be required for 5G.  Therefore, many players within the eco-system are working on alternative interfaces, e.g. the IEEE working group is working on the Next Generation Fronthaul Interface [1] as an alternative to CPRI.  The EU project 5G-Crosshaul [2] is focusing on integrated front and backhaul and proposes a unified multilayer data plane encompassing innovative high-capacity transmission technologies and novel deterministic latency switch architectures with the 5G-Crosshaul Packet Forwarding Element (XFE) and the unified 5G-Crosshaul Common Frame (XCF) format. The 5G-Crosshaul EU-project has shown that a packet based fronthaul interface is having less-stringent requirements compared to CPRI, where the packet based XCF will also support IEEE 802.11ad links (and eventually also 802.11ay is our guess). Another group within the CPRI Industry Initiative is working a new Specification called eCPRI, with 5G Front-haul support, this is expected to be released in August 2017. One of their focuses is to enable a ten-fold reduction of the required bandwidth

In June 2016 [3], the average CPRI connection speed is between 5-6 Gbps and as operators deploy LTE-Advanced features, the requirements on the CPRI links will keep on increasing. There are currently wireless mm-wave technologies that can support a data speed of up to 10 Gbps with low latency, which support CPRI V6.0 line data rates. These kinds of solutions will also support some of the LTE-Advanced uses cases and some of the initial 5G fronthaul line rates. For example, the Maxlinear (former Broadcom) BCM85100 Frequency Division Duplex (FDD) mm-wave modem uses two separate communications channels for simultaneous send and receive operation in the 60 GHz band, resulting in 10 Gbps data speed and latency below 100 µs (one way).  As Broadcom share in [4], the BCM85100 PHY layer Maximum latency is less than 40 µs (one way, with a mm-wave link distance at 2.5Km with 99,999% up time). This modem meet both the speed and latency requirement for wireless fronthaul. Using the Broadcom BCM85100 with the new Sivers IMA mm-wave transceiver [5], which supports 64QAM modulation with its built in VCO will be sufficient for the average CPRI connection speed of today. Using a 2 GHz bandwidth and 128 QAM, the full 10 Gbps speed is achieved. One weakness with the traditional point to point mm-wave solution is that beam steering and beam forming functionality is lacking, which is a quite important to have, for easy installation of the RRH with little time wasted on link alignment.

Which technology could support 5G fronthaul and still be cost effective? Let us list the main requirements:

  • Link speeds of 20-40 Gbps
  • <100 µs latency
  • Easy installation
  • Fronthaul Interface that does not require +100 Gbps

I believe by using the 60 GHz mm-wave band and 802.11ad/ay technology with some small modification as well as a smart packet based fronthaul Interface will be a very good solution for 5G fronthaul.

This then provides the following parts to offer cost effect fronthaul using mm-wave:

  • Speed >20 Gbps
  • Low latency, can be reach via using FDD “operation” (<100 µs) with 802.11ad/ay technology, with 14 GHz of available bandwidth in US.
  • 11ad/ay has support for beam steering and beam forming, making installations quick and easy.
  • Using the 5G-Crosshaul Packet Forwarding Element (XFE) and Common Frame (XCF) format with 11ad/ay, or possibly using the Next Generation Fronthaul Interface will solve the low transmission efficiency, poor scalability and limited flexibility which we today see for 5G CPRI.

Sivers IMA will be presenting at the IWPC conference 16-18 November “What Role Will mmWave Technologies Play in 5G?” in San Jose, CA USA.

Anders Storm
Sivers IMA – Your partner for advanced mm-wave and microwave solutions

Ref [1] IEEE, Next Generation Fronthaul Interface (NGFI) Working Group
Ref [2] http://5g-crosshaul.eu/wp-content/uploads/2015/05/D2.1-Detailed-analysis-of-the-technologies-to-be-integrated-in-the-XFE-based-on-previous-internal-reports-from-WP23.pdf
Ref [3]  https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/wp/wp-01265-the-emerging-need-for-fronthaul-compression.pdf
Ref [5]  http://www.google.com/patents/EP2582108A2?cl=en
Ref [6] http://siversima.com/wp-content/uploads/PB_TRX-1608-LT6275.pdf

You can also find the blog post here: https://ortenga.com/blog/