by
ATDI Ltd.
www.atdi.co.uk

ATDI have now completed a number of TETRA network designs for emergency or "blue light" services for both operators and integrators. All "blue light" networks have similar themes beginning with the definition of requirements and ending with verification that the network performs as designed. This short article discusses the design steps necessary.

A radio network must be specified in terms of the services and facilities that the user will enjoy. Given the variable nature of signal propagation such specifications must deal in percentage chance. The concept of 100% certainty of communication is making way for understanding that the higher the percentage the higher the cost. As reality prevails the first element of any TETRA design is the documenting of the user requirement. In its simplest form this is the use of a series of coloured felt pens on a map with each colour representing some performance parameter. On a planning tool such specification is captured using vector points, lines and text. The zones of interest in "blue light" services will include administrative areas, urban areas and points of special significance. The activity to capture the requirements is sizeable but the time invested always bears fruit later in the project.

Coverage alone is probably the least well understood area and despite many books and articles on the subject the myths still abound. To begin the design and to adequately specify and meet the requirements, the designer must consider three aspects: availability, certainty and coverage. Availability is the percentage of locations over a short sector of ground of about 50-100 wavelengths that benefit from a usable signal. Availability is determined in mobile networks by the relationship between the short sector median signal level and the receiver threshold modified by the coding. User behaviour then sets whether the dynamic threshold or the static threshold with margin is to be used to achieve the typical minimum requirement of 90% availability.

Certainty is the chance that the predicted median operating field strength will in fact be achieved on the ground and is a function of the error in the propagation model (an average error of 0dB with a standard deviation of error of better than 4dB would be typical for a good algorithm). Where covergae of 90% is not important, it is adequate to predict to 50% certainty. Where a specific point must be covered to a 90% certainty some 1.25 times standard deviation or around 5dB must be added as a margin. Finally, coverage is the percentage of short sectors across a defined area benefiting from at least the specified availability. Specifying in terms of availability, certainty and coverage is key to every "blue light" design.

A key input to planning is the statement of demand. This may again be a pen-on-map specification of the demand in Erlangs per square kilometre in each area of town. It may alternatively be a stated data demand from randomly spread mobiles representing the user's traffic at a given time of day. Either method of modelling is valid. The data demand is then balanced with the resources (base stations, carriers and channels) to achieve a tolerable lost call ratio at each base station. Clearly this balancing act will require the modelling of a series of scenarios representing both 'normal' operation operation and that during major incidents. The work then continues to frequency planning and assignment. Like the impossibility of 100% coverage, users must endure some interference. Hopefully this will be for small percentages of time and locations. Given a target maximum interference the number of two-frequency carriers can be determined (or more likely the interference is predicted given a fixed number of channels) with considertion given to the needs of DMO (direct mode) and air-ground-air working.>

Finally but integral to the core planning, the effects of incoming and exported interference from other networks both at home and abroad must be assessed using the methods in the various memoranda of understanding.

Quality is designed in to the network and not tested on installation. How then do we produce a quality network? Taking the definition that a quality network is one that meets requirements then we can measure that quality - "state what you will do, do it and then confirm that you have done it". There are two elements: validation and verification. Validation is the confirmation that the design is a valid representation of the requirement and is done by audit and peer review on a network plannng and modelling tool such as ATDI's ICS Telecom. Verification is the confirmation that the installation deployed is as designed and this is done by statistically sampling both the signal level and perceived speech quality on the ground and inferring compliance.