Optimising your DAS for a solution that fits: Part Two

In our last blog entry we outlined how the optimisation of an in-building distributed antenna system (DAS) is a function of three main things: coverage, capacity and infrastructure. Good coverage brings efficient signal propagation. Plenty of capacity is required to achieve maximum throughput. And with the increased capacity demand created by new generation mobile technologies, a future-proof infrastructure is required early on in the installation phase.

In this series of three blogs, Moti Shalev, Director of Product Management at Axell Wireless, summarises a recently produced whitepaper, covering how combining high- and low-power remotes on the same DAS can provide flexible network designs which right-size the solution to the requirement, and reduce costs.

In this second blog in the series, we’ll take a look at function number two: capacity.

Growth in DAS sophistication
With increasing indoor usage come increasing demands on the mobile network. Requirements relate not just to the world of business – high-rise, multi-tenanted buildings and complex campus scenarios – but to a range of venue and location types from university and government/public institution campuses, to tunnels, to underground transportation networks and sports stadia. A stadium capable of holding 90,000 visitors, for example, can generate peak traffic equivalent to a city with 500,000 inhabitants; of these two, the stadium would be by far the biggest challenge since most of the visitors would be using their devices at the same time: an exceptionally high level of demand for network service in a concentrated timeframe.

Keep the noise down
In general terms, the overall optimal DAS solution consists of high power remotes to cover high-rise buildings and lower power remote units for smaller buildings or coverage sectors such as may be a further requirement in a stadium. Whatever configuration the layout of the remote unit architecture takes – in response to the in-building or venue requirements – the key consideration is to ensure that the base station itself is not affected by high noise. DAS installations feed base stations. If they have a high noise figure it can interfere, and cause capacity issues, with the base station. The converse is true; attainment of the lowest possible noise figure ensures maximum data throughput.

The lower the noise figure is on a remote it follows that the lower the overall noise figure for the system as a whole. Best-in-class noise figure on a high power remote is as low as 3dB typical (maximum gain) and on a low power remote 10dB typical (maximum gain). Indeed, at this point in time it appears that 3dB is the lowest noise figure in the industry, equivalent to the noise figure at the base station. Such a figure provides optimum use from the base station and a faster experience on users’ mobile devices.

Capacity is king
So considering the above scenarios, it’s clear that capacity on a network is vital for an increased number of users in one place to utilise their devices in the way they want; streaming video, sharing pictures and using social media sites, for example, with a seamless, uninterrupted connection.

The Shannon-Hartley Channel Capacity Formula is a widely used methodology for assessing capacity optimisation by measuring the relationship between throughput and noise. The formula reads: C=B log 2(1+S/N), where:

• C = Capacity bits/s/Hz
• B = Bandwidth
• S/N = Signal to Noise ratio. This is related to the S/N ~ 1/R n , n= path loss exponent
• S/N ~ 1/NF

With no change in such variables as power, bandwidth, and frequency, the operator needs to come closer to the BTS for higher data rates and maintain the noise figure as low as possible (as discussed in our last blog)Figure (i) below shows, on the basis of this formula, how higher signal to noise ratio (S/N) is required to achieve maximum capacity in higher order modulation (HOM) schemes such as QPSK and QAM.

Figure (i) – Shannon-Hartley Channel Capacity Formula

Take a look at the full whitepaper which includes more technical detail, and make sure you look out for the next blog in the series, covering the third and final main function of an optimised DAS: Infrastructure.

 

Moti Shalev Moti Shalev
Product Marketing Manager, Axell WirelessMoti Shalev joined Axell Wireless in 2009 as Product Marketing Manager with the responsibility for defining the product management strategy. Moti has a B.S.C EE in Electrical Engineering and over 20 years of worldwide experience in the telecommunications industry.

 

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