Subject: [NEXTEL-1] Reinventing cell planning

From: [email-masked]
Date: Tue, 24 Jun 2003 23:25:26 -0400

Reinventing cell planning

Joachim Samuelsson, managing director of cell planning experts ComOpt, of Sweden, argues that Automatic Cell Planning is on 
the horizon.

Planning and optimising a cellular network has always been a very labour intensive and complicated matter, absorbing 
engineering time and resources. These engineering resources are scarce and expensive, and no operator wants to utilise them in 
anything but the most efficient manner. In traditional cell planning, the solutions that do emerge by way of the best efforts of 
talented planning engineers, are inevitably the result of dozens of imperfect attempts at optimisation, during which time quality 
and value for money are compromised. In a live network, with real subscribers, real balance sheets and real shareholders, this 
tends to concentrate the mind.

However automated different stages and levels of the data gathering, data analysis and even in recent years frequency planning 
have become, the vital network design phase has until now been a manual one. But there is no empirical reason why this should 
always be true. If frequency planning can be automated as it has been in recent years, why not the entire cell planning process?
There is certainly evidence from operators that Automatic Frequency Planning (AFP) is reaping significant benefits in cost 
savings and quality improvements. For six months in 1996, an AFP experiment at South Africa’s MTN network, encompassing 
the Johannesburg and Pretoria areas, showed some startling results. The dropped call rate (which was already at relatively low 
levels) decreased by a further 20%, to be followed in the year after the initial pilot phase by a further reduction while traffic 
more than doubled.

MTN’s National Planning Manager Hilton Goodhead, characterised the experience thus:
“The gains we see accruing from this breakthrough have massive implications for the planning community, our network as a 
whole, and our organisation’s competitive position.”

BellSouth, NZ, has seen its drop call rate in Auckland fall to its lowest ever
after implementing an automatically generated frequency plan. Stefan Engels, Engineering
Team Leader at Belgium’s Belgacom Mobile network is equally convinced of the power
of automation. “Frequency planning used to be a burden,” he says, “it took up to 6 weeks for a large scale retune (500-800 
transceivers). Today, we create plans in less than a week, with increased quality.”

This is impressive. But in relative terms, AFP is a straightforward challenge. It only really has to deal with one decision – 
frequency assignment – and with one central objective: achieving high quality. It does this by prioritising the fulfilment of 
conflicting planning rules and retuning to avoid interference. Very simply, AFP relies upon accurate modelling and optimising of 
the inevitable trade-offs in frequency planning.  Within this simple objective there is, of course, a degree of complexity. AFP 
considers several different kinds of planning preferences, and models many of the innovative techniques and features that 
modern cell planners apply to enhance capacity and quality, such as tightening the re-use through frequency hopping, 
discontinuous transmission and dynamic power control.

Furthermore, this AFP process must be underpinned by an optimisation algorithm which is extremely robust, converging reliably 
and rapidly to sound, good quality plan and to ensure that the optimised solution does not contain obvious mistakes requiring 
manual changes. But while we are proud to have broken ground in this area, we also recognise that is only a small part of the 
overall story.

Introducing Automatic Cell Planning

Automatic Cell Planning (ACP), on the other hand is something altogether more
varied, wide-ranging and even creative. Translate and expand the successful approach to
AFP to support the network design problem – delivering very clear guidance to site
positioning and configuration as dictated by the many variables in a network – and those gains
could grow exponentially. This is precisely what we will see happening for many networks
before the century is out.

The fundamental objective is, again, to reconcile conflicting elements, but this time they are more numerous and complex, and 
are far more strategic in the context of the operator’s business. These include judgements about best use of available 
provisioning budget; about the value of the high investment necessary to reach the awkward last 10 or 20 percent of a coverage 
region; about the need to meet undertakings on coverage given to licensing authorities; about achieving the right balance 
between coverage and interference potential by optimising BTS power, tilt and azimuth; about the need to deploy hierarchical 
cell structures involving micro and macro cells. It all adds up to making the right trade-off in the balance between cost, 
coverage, capacity and, of course, quality.

The automatic cell planning process requires a set of potential sites as input data, as well as the path loss from these potential 
sites to all points in the network. The path loss may be pre-calculated and derived from either field strength measurement or 
signal predictions. An initial design would be derived by configuring a subset of the potential sites. This initial design could be an 
existing design that requires expansion, or any Submitted to appear in Mobile Communications International, automatically 
generated starting point. By considering the precise site configuration –including decisions on the power, antenna models, tilt 
and azimuth of each sector – it is possible to predict the field strengths of each sector.

The field strengths of all sectors in a particular point in the network are used to model the sectors that would serve the traffic of 
that point. To model this it is necessary to take into account the specified service requirement and how traffic should be 
distributed given the field strengths of the sectors. If any points in the network are not covered by the configured sites, it might 
prompt the automatic cell planning tool to consider filling that coverage hole, for instance by adding a new site or possibly 
powering-up existing sites.  considering how traffic is distributed in each point of the network, and by projecting the traffic 
requirements described by the Erlang density map, it is possible to build traffic arrays, modelling the capacities of each sector. 
Using engineering rules describing the maximum number of transceivers desired in the sector, this could indicate the necessity of 
either adding more sites or possibly sectorising to relieve cells of congestion.  From a knowledge of the likely servers and the 
traffic density of each point of the network, ACP will calculate the interference potential by considering sectors that might 
interfere if they were reusing frequencies. To resolve a situation with potentially too much interference, the automatic cell 
planning process can, for instance add an additional site or possibly down tilt an interfering sector.

Networks today often employ hierarchical cell structures. It is important that the
behaviour of these networks is properly captured by the ACP tool in order to distribute traffic properly. This means, for 
example, that a microcell of lower signal strength than a macrocell might still pick up a larger proportion of the traffic, as the 
network’s handover algorithm would recognise that sufficient field strength is present.

Washington DC/ Baltimore

An ACP test involving more than 1400 potential sites in the Washington DC/Baltimore area of the USA, completed as recently 
as September 1998, produced some startling results. The study looked at how a network would grow from 50,000 subscribers 
to one million. As inputs, we used a detailed Erlang map identifying traffic densities.  The service requirement was modelled at 
–100 dBm in the entire area. Using 242 of the potential sites, and in only a six-hour optimisation run, the resultant design met 
98% of the traffic requirement. This was orders of magnitude faster and better than a manual plan could ever be.

As with MTN, BellSouth and Belgacom, where AFP alone was at work, the business
benefits of ACP are not difficult to imagine. Indeed, they are in many ways
similar to the business benefits of AFP – but more so. Vast areas of the planning process that
once took weeks or months will be manageable in days.

 A full 1400 site network plan could be run overnight, as opposed to the man-month it takes today. Planning happens faster and
involves fewer people, so human cell planning know-how can be used to focus on problems that really cannot be handled 
automatically. To use a slightly unfashionable computer industry term, we are talking here, about an expert system.

A good network design opens the way to better quality network performance.  Furthermore, the dramatic impact on quality 
witnessed from using AFP tools will, itself, be further extended by using ACP tools. In the past, AFP has been limited by 
quality of the network design that serves as the starting point for the frequency assignment process.

Using AFP in combination with ACP, significantly higher network quality can be realised. This implies fewer dropped calls and 
handover problems, and results in more satisfied subscribers and less churn.
There is a trade-off between cost and network performance. Automatic cell planning enables operators to meet the design 
objectives with fewer sites can lead to significant cost savings on infrastructure.
Nor should it ever be forgotten that time is critical to all mobile operators around the world. Competition is usually tough and 
roll-out time is one of the chief levers of competitive advantage. It is clear that a much faster network roll-out can be achieved 
by automating the cell planing process.

In a sense, the implications stretch out even further than networks as we know
them today. Whereas mere frequency planning has, by definition, no role to play in CDMA
(since CDMA systems assign individual codes to calls within just one frequency
spectrum), Automatic Cell Planning most certainly does. So it is important for the
industry as a whole, and its users, as we move towards third generation GSM systems and wide band CDMA, that the 
manifold possibilities of ACP are understood. If they are, the virtuous circle of higher quality, lower cost and faster growth is 
within reach.
http://www.comopt.com

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