What is Asymmetrical Link Operation?

Introduction
Last year one of our microwave competitors introduced a new development for the point-to-point licensed microwave market – asymmetrical link operation. There are some very real challenges with the growth of mobile multimedia that are driving interest in this approach. However there are numerous harsh realities involved in introducing such a ‘radical’ technique into the relatively conservative licensed microwave industry. The myriad of Regulatory studies and approvals that will be needed to enable asymmetric operation to be deployed in existing bands means that it could be years, if ever, before asymmetric links can be deployed in most countries around the world.

Today’s Licensed Microwave Bands are Exclusively Symmetric
In current licensed microwave bands and all commercially available equipment today, transmission is symmetric – i.e., the same capacity and bandwidth in both directions. Frequency bands are arranged for frequency division duplex (FDD) operation, where two identical channels are used for Tx (‘go’) and Rx (‘return’). Asymmetric operation is usually reserved for unlicensed time division duplex (TDD) radios, which use a single channel for both go and return.

Spectrum Borrowing
The proposed Asymmetrical scheme is based upon a concept called ‘Spectrum Borrowing’, where frequency spectrum is taken from the upstream direction of a lower capacity link, and given to the downstream direction of an adjacent higher capacity link.

Before - Symmetrical Microwave Network

A second (but related) proposal has been also tabled to amend the standard channel options from the current 7, 14, 28, 56 MHz to an n*7MHz arrangement (i.e. 7, 14, 21, 28, 35, 42, 49, 56 MHz), which is required to support the asymmetric concept.

Asymmetrical Microwave Network, after Spectrum Borrowing

What is driving the need for this Asymmetry?
The underlying rationale is that in 3G and 4G mobile networks, a majority of the traffic over the network is increasingly web- and video- based, meaning more capacity is needed in the backhaul network in the direction towards the base station, and less in the opposite direction back to the core.

However, while this is true today, new emerging mobile applications such as video chat, video uploading, P2P sharing, and new cloud based services (eg: iCloud), have the potential to change the imbalance between upload and download demand over the longer term. This presents a challenge for the proposed asymmetric implementation, which is fixed in nature, not dynamic. This means that the link has no way to adapt to instantaneous uplink/downlink traffic demand, or to change over time as more uplink capacity is needed. Changing this ratio could prove to be very difficult once an asymmetric link is in place and has been operating for several years.

Regulatory Approval
Making substantial changes in the way that licensed microwave bands are used is not a simple process, since strict regulations and standards at the international and national level have been put in place to ensure that links deployed in these bands are assured to be virtually interference free.

A proposal has now been submitted to the Electronic Communications Committee (ECC), the Regulatory Body responsible for amending the channel plans for the existing frequency bands, a part of the European Conference of Postal and Telecommunications Administrations (CEPT), representing 48 countries throughout Europe and Russia. The ECC has agreed to set up a study group to examine the proposal, which is due to report their finding in February 2013.

If the ECC agrees to amend the channel plans to permit asymmetric operation, which may not happen before 2015, the national regulator in each CEPT country will then have to decide whether or not to adopt the recommendations. Further lobbying will also be necessary beyond the CEPT region, for example with the FCC in the USA, to successfully influence regulatory policy in favor of Asymmetrical operation.

A Long Road to (Possible) Adoption
In summary, asymmetrical operation may be a potentially useful technique to improve the efficiency of backhaul networks and frequency utilization. However, introduction of this technique will be extremely difficult within existing congested frequency bands, and will face significant and lengthy regulatory scrutiny and approval before we will see widespread adoption.

Stuart Little
Director of Marketing

Comments

    Fredrich Lamprecht October 21, 2011/ Reply

    Thought so too. Thank you for bringing some perpective to uninformed naivety in this regard.

    Jiri Linek October 21, 2011/ Reply

    Hi Stuart, greetings from Prague. I got your analyze by email and I have one question. Is in FCC rules somewhere defined that Licensed Microwave Bands are Exclusively Symmetric and not Asymmetric? Thanks Jiri

    aviatnetworks October 27, 2011/ Reply

    Hi Jiri. In the US Common Carrier bands (6, 11, 18 and 23 Ghz) there is no language in the FCC Part 101 Code of Federal Regulations that specifies that links must operate in symmetrical fashion, and not asymmetric, but it is implied as all channel plans are symmetrical – i.e.: equal go and return channel bandwidths.

    There are some bands, such as 24, 28, 31 and 38 Ghz, that are privately owned and operated, where asymmetrical operation could possibly be employed.

    Stuart Little

    kim@usb microscope camera October 25, 2011/ Reply

    It was well explained by the author it can be applied in linking operation of the system.


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