In-building penetration loss

Background

Vilicom recently carried out a study to evaluate and refine the ways in which we calculate in-building penetration loss in the light of recent changes in construction methods and insulation materials. There is an industry-wide drive to embrace new materials in order to reduce carbon emissions and energy costs, and the effect of low-emissivity glass with metallic oxide coatings and foil-backed insulation on in-building coverage was of particular interest to us. The study involved over 1,600 measurements inside houses, apartments and offices in five Irish cities, with measurements taken at the front and back walls, both inside and out, as well as at the centre of the buildings.

The current understanding

The most common approach to designing a cellular network for indoor coverage is to define an outdoor reference signal level. This reference level takes account of losses estimated for the signal to penetrate to the inside of the building. Standard values exist for these losses and are used in many reference texts and equipment vendor link budgets. Some interesting modelling of indoor coverage can be seen in the work by Berg (1999) for both line-of-sight and non-line-of-sight conditions.

The link budget is used to determine the maximum allowed path loss to achieve the desired service level at the cell edge, and the network specifications are set to define the required cell-edge and cell-area coverage probability.

When considering indoor coverage, the cell edge is not a smooth curve at a given radius from the base station location. In point of fact, multiple cell edges exist at various points within the cell, typically inside buildings and behind terrain obstructions. Therefore, the assumptions made about the correct allowance for in-building penetration loss affect users both near to and far from the cell site location. Typically, link budgets assume 15dB penetration loss at 2100Mhz, as used by Holma and Toskala (2006) and in many vendor link budgets.

Our study’s results

Our study’s measurement results were normally distributed and some noteworthy findings were obtained. The median penetration losses found were slightly lower than those used in the standard link budgets, but the standard deviation was particularly interesting.

The standard deviation of the collected data was 6-7dB. To ensure 90% and 95% confidence in achieving indoor coverage, the penetration loss assumed in the link budget must account for the variance of penetration losses encountered in real buildings as shown in the figures above.

Recommendations

At Vilicom, our ultimate goal is to ensure that our customers are never left without indoor coverage, so we continue to challenge the accepted widespread assumptions about in-building penetration losses. It is no secret that in-building losses vary widely, and this should be carefully considered when setting the penetration loss in the radio link budget. Rather than using average values, the losses allowed should be adjusted to take account of the required confidence intervals.

The fresh insights from our study lead us to recommend that the standard link budget be adjusted to take account of the distribution patterns illustrated above. This will influence the radio design by increasing the required site count. Further study is also needed to examine the effects of combining this with the shadow fading margin.

Our ultimate goal is to ensure that our customers are never left without indoor coverage.

References

Holma, H., Toskala, T., 2004. HSDPA/HSUPA for UMTS. Wiley. p. 189
Berg, J.E., In: Damosso, E., Ed., 1999. Digital Mobile Radio Towards Future Generation Systems, European
Commission. pp. 167-174

Contact

For further information or to discuss further technical specs, please contact Gearoid Collins at gearoid.collins@vilicom.com

About Vilicom

Vilicom is an expert provider of wireless services with over seventeen years of experience in the analysis, design, test and implementation of wireless networks. Vilicom’s strengths lie in technology strategy consulting, the design of wireless networks, transmission network design, implementation of specialised coverage solutions and network benchmarking and testing. Vilicom has delivered its trademark end-to-end services in over 20 countries for network operators, network equipment vendors, industry regulators and investors. It delivers value for its customers by adopting a flexible, customer-focused approach, retaining cutting-edge expertise and maintaining its independence.