Background

Boldyn conducted a study to evaluate and refine how we calculate in-building penetration loss in light of recent changes in construction methods and insulation materials. With an industry-wide push to adopt new materials that reduce carbon emissions and energy costs, we focused particularly on the impact of low-emissivity glass with metallic oxide coatings and foil-backed insulation on in-building coverage.

The study encompassed over 1,600 measurements across houses, apartments and offices in five Irish cities. Measurements were 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 involves defining an outdoor reference signal level. This reference level accounts for estimated signal losses as it penetrates the building. Standard values for these losses are widely used in reference texts and equipment vendor link budgets. Notably, Berg's 1999 work provides insightful modelling of indoor coverage for both line-of-sight and non-line-of-sight conditions.

The link budget determines the maximum allowable path loss to achieve the desired service level at the cell edge, and network specifications set the required cell-edge and cell-area coverage probabilities. When considering indoor coverage, the cell edge is not a smooth curve at a fixed radius from the base station. Instead, multiple cell edges exist at various points within the cell, often inside buildings and behind terrain obstructions. Consequently, assumptions about in-building penetration loss impact users both near and far from the cell site. Typically, link budgets assume a 15dB penetration loss at 2100MHz, as noted 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% to 95% confidence in achieving indoor coverage, the penetration loss assumed in the link budget must account for the variability of penetration losses encountered in real buildings, as illustrated in the figures above.

Recommendations

At Boldyn, 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