Optical Connections Magazine Autumn 2018

ANDREW COLE OPTIMAL TESTING

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OPTIMAL TESTING OF TODAY’S MOBILE FRONT HAUL NETWORKS

I n a mobile network, a base data trac so that it can be transmitted over the air interface in an RF signal and vice versa for mobile to media data. Mobile backhaul is a term used to describe the connection between the BTS/BBU and the fixed network, supported by fibre-optic technologies like SDH/Sonet, Synchronous Ethernet and OTN, where bandwidth and synchronicity can be guaranteed. Fronthaul, on the other hand, is the term used to describe the process of sending mobile RF data trac as a digitised signal between the BTS/BBU and the radio equipment for “over the air” transmission to and from the mobile device. Fronthaul technology is evolving to support an ever- increasing need for bandwidth as well as ecient and robust communication, while at the same time reducing power consumption: a major cost factor in mobile installations. In older mobile front haul networks, which rely on transmission over coaxial cables between a BTS and a passive antenna, bandwidths can be extremely limited. The BTS has to be close to the radio equipment - generally at the base of the tower - which means that power and environmental control have to be provided at each and every cell site. The figure to the right shows how coaxial and power cables link the BTS to a masthead amplifier, which is then connected to a passive antenna. Losses over the coaxial cable limit the cable length, and high power levels need to be transmitted between the BTS and the masthead amplifier to overcome signal degradation. NEW SERVICES Mobile operators have to meet the growing need to provide capacity and coverage to support the increasing variety of devices communicating over their networks. Some services, such as eCall - a feature soon to be made mandatory to all automotive manufacturers, where a vehicle can detect that it has been in a trac incident and automatically alert the transceiver station, or a baseband unit, acts as the interface between the global fixed network and a wireless access network. Its job is to convert media data to mobile

A test tool such as Anritsu’s Network Master, which works in the physical layer and in CPRI operation, can help to develop and troubleshoot the latest C-RAN systems. By Andrew Cole, Anritsu.

ANDREW COLE

necessary emergency services - will rely on guaranteed coverage and network availability. This requirement is only made possible by a reduction in the cell size coupled with the use of complex modulation schemes to maximise the eciency of the available bandwidth. Fortunately, technology is evolving: both in the communication techniques used between the antenna and the mobile device and in the front haul area, where these enhancements have to be considered along with cost and environmental considerations.

This process involves two stages of evolution, based on the use of fibre as a much more ecient carrier than costly and troublesome coaxial cable for high- quality low-loss data transmission: ● First, the use of fibre between the BBU and a remote radio head, followed by a link to a passive antenna via a short length of coaxial cable. ● Secondly, a more recent solution involving fibre and power being provided directly to radio equipment incorporating an active antenna, totally eliminating the need to use coaxial cable. Correctly installed optical fibres support low-loss, reliable and robust connectivity over far greater distances than coaxial installations, so that there is no need to site a BTS close to the radio equipment. A BBU can be sited some distance – maybe up to 40km - away from the radio equipment, so that BBUs can be housed together in a centralised location. These newer networks employ a distributed antenna system (DAS) – also referred to as a centralised radio access network (C-RAN) or a cloud radio access network. Transmission over the air provides a line-of-sight communications link. Challenges can result from the external environment, where the local geography or buildings might block the transmission path, or in buildings, where wall dividers and other infrastructure features can cause obstructions. With the advent of 4G and (currently in discussion) 5G networks, where huge antenna arrays to support technologies such as massive MIMO (multiple-input/multiple-output) might be used, these front haul networks will become vast and ever more complicated. It is generally agreed that the preferred method to address these challenges

Passive Aantenna

Coax

Masthead Amplifier

Coax & Power

Base Transmitter Station (BTS)

The BTS connects to a masthead amplifier via coaxial and power cable, which is then connected to a passive antenna.

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| ISSUE 10 | Q3 2017

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