## 19.5 OBFUSCATION TECHNIQUES FOR USER-PRIVACY

To guarantee user location privacy, we introduce three basic obfuscation techniques that modify a user location to reduce the associated relevance (henceforth the accuracy) until a given level.

### 19.5.1 Obfuscation by Enlarging the Radius

Obfuscating a location measurement area by increasing its radius (see Figure 19.3(a)) is the technique that most solutions exploit, either explicitly or implicitly by scaling a location to a coarser granularity (e.g., from few meters to hundred of meters, from a city block to the whole town, and so on). The obfuscation is a probabilistic effect due to a decrease in the corresponding joint probability density function (joint pdf), which can be expressed as ∀*r*, *r*′, *r* < *r*′ : *f*_{r}(*x, y*) > *f*_{r′}(*x, y*). The following proposition allows us to calculate the obfuscated area.

**Table 19.2** Examples of access control policies for a healthcare scenario

**Proposition 19.1.** *Given a location area of radius r with relevance *_{Init} and an obfuscated area of radius r′ *derived by enlarging the original radius, the relevance *_{Final} of the obfuscated area is calculated by reducing _{Init} of the ratio *f*_{r′}(*x, y*)/*f*_{r}(*x, y*) *of corresponding joint ...*