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Spectrum availability

Dr John Naylon, Chief Technology Officer, CBNL

At first it doesn’t seem like modern transmission technology and accommodation booking would have many similarities!

In the abstract, however, they do share one key characteristic: the ability to dynamically reallocate resources according to demand.

In a PMP system the resources are units of time (of the order of microseconds) on a radio frequency carrier; in Airbnb the resources are units of time (in days this time) of occupancy of a room or apartment.

Why do we want to allocate resources dynamically in these two cases?

The answer is that we wish to increase the utilisation of the underlying asset: the RF carrier in the PMP case, and the room or apartment in the Airbnb case.

In a PMP system, if one link in a sector is instantaneously using less capacity than its “fair share”, then the system can reallocate those resources to another link that may have excess demand at that instant.

Likewise, if I am on holiday for two weeks, and so not using my apartment, then I may choose to rent it out while I am away.

In both cases, a resource that would have been idle - carrying no traffic, or sitting empty - is now utilised beneficially.

More importantly, this is not just a theoretical plus, but also translates into a financial benefit.

In the Airbnb case, the owner of the asset has extra income to pay for the purchase and maintenance of the asset. 

In the PMP case, overall spectrum requirements to carry a given volume of data across numerous links are reduced, and so is the financial cost of renting that spectrum from the regulator (we cover this reduction in much more detail here).

It’s this financial benefit that is driving the adoption of PMP, and also the uptake of platforms like Airbnb.

The same underlying characteristic is common to a number of other platforms and technologies; for instance Uber (like Airbnb dealing with physical resources), cloud computing and server virtualisation (like us dealing with intangible resources).

Incidentally, here at CBNL we often use Airbnb to meet our business travel needs, and we’ve stayed in some great and colourful places as a result!

Dr John Naylon, Chief Technology Officer, CBNL

The forthcoming Small Cell World Summit will again bring together leaders from across the industry to discuss the latest trends and technologies in this space, along with operator’s deployment strategies (a particular point of interest for almost all delegates).

Last year we focused on total cost of ownership for small cell backhaul (see the slides here) which coincided with the launch of our first VectaStar Metro product

With the World Radiocommunication Conferences (WRC) set to take place in 2015, we thought it was timely to shine a light on the opportunities for backhaul in various frequency bands and how future spectrum availability may affect operator’s backhaul choices.

High capacity and low total cost of ownership continue to dominate the list of requirements for small cell backhaul and spectrum can play a major role in this.

I anticipate the WRC may designate more low frequency spectrum for LTE RAN services next year.

If they do we’ll see even more traction for technologies operating in the 6-42Ghz band as backhaul is displaced out of the sub-6GHz space.

Backhaul products operating between 6-42Ghz will play a central role in creating the low TCO we demonstrated last year for our own small cell multipoint product, whilst at the same time having the ability to deliver the essential capacity requirements.

Of course there’s the question of how to maximise spectrum resources once they are acquired.

The bursty data profile of small cells (whether LTE or Wi-Fi) lends itself especially well to multipoint backhaul.

Multipoint can realise huge efficiency gains in the network by aggregating data from several small cells, saving equipment costs and reducing the capacity operators need to provision.

By utilising licensed frequency bands, multipoint also offers seamless quality of service between macro and small cell layers.

We firmly believe ‘backhaul is backhaul’ and if customer satisfaction (and retention) is to be achieved, the user should always see great availability, reliability and speed whether connecting via a small cell or a macro node.

I’ll be discussing this in more depth at the Small Cell World Summit when I join Deutsche Telekom on the “Opportunities for backhaul in various frequency bands” panel session – 14.40 - Wednesday 11th June.

We’ll also look at how spectrum availability is dictating backhaul choices across the globe and if there is a balance to be met with licensed and unlicensed strategies.

I hope to see you at the event – please read our events page for more information and to schedule a meeting with the CBNL team.

Dr John Naylon, Chief Technology Officer, CBNL

We recently talked about how to choose between PMP and PTP from a technical perspective, so now let’s have a look at this choice from a financial standpoint.

The following slides work through a simplified version of the TCO model we use with new customers, detailing all the assumptions we make.

We believe the total cost of ownership is by far the best way to make a rational choice between different types of equipment. 

After all, it is no use having very low cost equipment if the spectrum rental to operate it is extortionately expensive. 

Likewise free spectrum sounds fantastic, but if the equipment that operates in that band is itself very expensive the overall solution cost is not likely to be optimal.

As shown on slide 10, PMP begins to show very significant TCO savings as soon as the average number of sites per backhaul hub increases beyond one or two. 

In networks we deploy today, this average is usually 5 or 6 links per hub, and this number is steadily rising as networks continue to densify.

For our bigger customers, the savings realised run into millions of dollars per annum!

Dr John Naylon, Chief Technology Officer, CBNL

One of the most common questions we’re asked by new customers is,

“How do I decide when to deploy point-to-multipoint (PMP) and when to deploy point-to-point (PTP) for my backhaul?”

This is a great question, because the two technologies are very complementary to each other. 

An engineering perspective

From an engineering perspective, in making the choice between PMP and PTP for a given link, we are seeking to maximise efficiency and utilisation of the equipment and the RF channel while satisfying a set of requirements for throughput, latency and link availability. 

In economic terms, this translates into choosing the technology which gives the lowest total cost of ownership while satisfying those requirements.

Traffic characteristics

An excellent way to make the choice between PMP and PTP is to look at the characteristics of the data traffic we want to carry. 

I’m going to consider mobile broadband backhaul traffic here, because that’s what the majority of our customers use our technology for today. 

In a future post I will talk about small cell backhaul traffic.

As ever, the NGMN has some useful information we can use, in the whitepaper Guidelines for LTE Backhaul Traffic Estimation.

This paper describes (§2.2) the initially counter-intuitive result that the peak throughput for an eNodeB actually occurs, not during busy hour, but during quiet time. This is because:

“During busy times, there are many UEs being served by each cell. The UEs have a range of spectrum efficiencies, depending on the quality of their radio links. Since there are many UEs, it is unlikely that they will all be good or all be bad, so the cell average spectral efficiency (and hence cell throughput) will be somewhere in the middle.

During quiet times however, there may only be one UE served by the cell. The cell spectrum efficiency (and throughput) will depend entirely on that of the served UE, and there may be significant variations … the scenario under which the highest UE and cell throughputs occur [is]: One UE with a good link has the entire cell’s spectrum to itself. This is the condition which represents the ‘headline’ figures for peak data rate”.

Figure 4, reproduced here, illustrates this point:

Illustration of cell throughput during busy and quiet times

The paper goes on to give the following peak and mean traffic figures for a number of LTE configurations:

Figure 5: Mean and peak (95%-ile) user plane traffic per cell for different LTE configurations

Understanding the peak-to-mean ratio

What we can immediately see from this figure is that the peak-to-mean ratios of the traffic in the dominant, downlink, direction are very large, ranging from about 4:1 to almost 6:1. 

This agrees well with measurements we see from real networks. 

For example, the following from a very busy HSPA+ network show the peak-to-mean ratio of the backhaul traffic for each node B (on the y-axis) plotted against the peak backhaul demand for that node B on the x-axis.

Peak Backhaul Demand (Mbps)

When traffic has a high peak-to-mean ratio like this, we call it “bursty”, as opposed to “smooth” when the peak-to-mean ratio is close to 1. 

Data traffic in general, for example on LANs and residential internet access connections as well as mobile networks, is bursty; and this presents a difficulty in carrying it efficiently on PTP links, as shown here:

Bursty traffic is hard to carry effeciently

The problem here is that a PTP link with a single traffic source (a ‘tail link’ in the backhaul network) needs to be dimensioned to carry the peak traffic, but there is only a single source of offered load. 

Therefore the utilisation of the link (or efficiency) is equal to the mean offered load divided by the capacity, or in other words the reciprocal of the peak-to-mean ratio of the traffic. 

So if my traffic has a peak-to-mean ratio of 4:1, the maximum utilisation of a PTP link carrying that traffic is ¼, or 25%. 

In the chart above, you can visualise this as all the white space below the red line being wasted bandwidth, which is provisioned but unused.

It’s important to say that this is not a failing in PTP systems in any way – it is simply that the characteristics of the traffic are not well suited to the static bandwidth provisioning that PTP provides.

The advantage of a PMP system is that it can serve multiple sources of offered load simultaneously. 

The bandwidth of the shared RF channel is dynamically allocated to different sources as required. 

Conceptually, then, the peaks and troughs from different traffic sources ‘cancel out’ to some extent, as we illustrate in the following live network example showing eight nodeBs being backhauled by a single VectaStar Gigabit sector.

Multipoint backhaul packet switched not circuit switched

Here we are also relying on another property of the traffic, namely that peak demands for different nodeBs do not occur at exactly the same time. 

We discuss this at greater length in The Effect of System Architecture on Net Spectral Efficiency for Fixed Services.

Liberating spectrum to meet growing capacity demands

A useful analogy here is to think about a bank with deposit accounts.

Banks operate a fractional reserve system, meaning that they are only able to repay a defined fraction of the total of deposits at any given time.

This therefore relies on the observation that, statistically, not everybody goes to the bank and withdraws all their savings at the same time.

When this assumption breaks down, there is a ‘run on the bank’.

In a similar way, we rely on the observation that, statistically, not every node B requires its theoretical peak backhaul throughput at the same time.

When this assumption breaks down, things are a bit less dramatic however – we simply discard some low priority traffic.

This is perceived (if at all) by users as a temporary reduction in internet browsing speed.

Crucially, we can dimension the system in such a way as to set the probability that this occurs to a value of our choosing.

The advantage of fractional reserve banking is that it liberates dormant capital for further investment and lending.

Likewise, the more efficient use of RF channels in PMP systems liberates dormant electromagnetic spectrum (provisioned but unused, as in the example above) for use addressing the ever-growing capacity demands of modern mobile networks.

Conclusion

In conclusion, then, some brief rules of thumb for when to deploy PTP and when to deploy PMP are as follows:

Deploy PTP…
… when traffic is smooth (voice dominated)
… when traffic has already been aggregated
… in the middle mile of backhaul
… for long distance links
… when spectrum is uncongested or inexpensive
Deploy PMP…
… when traffic is bursty (data dominated)
… to create an on-air traffic aggregation
… for tail links (last mile)
… for dense deployments
… when spectrum is congested or expensive

 

Published 09 April 2013 in Backhaul, Small cells
Tags: Spectrum availability, Small cells, Economics

Dr John Naylon, Chief Technology Officer, CBNL

My colleague Julius Robson has previously written about the relative amounts of spectrum available in the sub-6GHz or ‘NLOS’ (non-line of sight), bands and the usual microwave and millimetre wave bands used for backhaul. 

In summary, above 6GHz there is more than twenty times as much backhaul spectrum available as below, with the obvious consequences for the relative cost of bandwidth above or below this division.

For this reason, we see LOS (>6GHz) microwave solutions as becoming the mainstay of small cell backhaul, just as they have been for macro backhaul for many years now. 

Of course there is a place for sub-6GHz solutions as we illustrate in the following chart - it’s just that the relative shortage of spectrum makes it uneconomical to use in the majority of cases.

So I was very interested to read Ericsson’s recent paper non-line-of-sight microwave backhaul for small cells which compares the throughput of 28GHz and 5.8GHz backhaul systems in various NLOS configurations. 

Their conclusion is that “contrary to common belief … microwave backhaul in bands above 20GHz will outperform sub-6GHz systems under most NLOS conditions”

The Ericsson results closely match our experience using VectaStar at 28GHz to backhaul a small cell network for O2 in London.

It seems then, that we can extend the use of high frequency microwave solutions even further into the adverse location part of the small cell distribution curve. 

This is great news for operators because multipoint microwave solutions offer compelling TCO benefits, resilience and solution maturity combined with capacity that far outstrips sub-6GHz systems.

We’ll return to this topic in a couple of weeks when we’ll see how multipoint microwave lets us exploit the statistical properties of backhaul traffic to maximise efficiency in our backhaul network.