Wireless localization plays a key role in enabling location-based services in commercial, healthcare, public safety, and military applications. Thus, an increasing number of localization techniques have been proposed based on motion or location dependent parameter (MLDP) including time-ofarrival (TOA), time-difference-of-arrival (TDOA), received signal strength (RSS), angle-of-arrival (AOA), angle-of-departure (AOD), Doppler-shift (DS), etc. Recently, the expansion, heterogeneity, and coexistence of wireless networks stimulate new research efforts in developing novel hybrid localization techniques, which utilize combinations of two or more MLDP types. However, the accuracy of these localization techniques highly depends on complex electromagnetic spectrum, e.g., multi-user interference, multipath effects, and non-line-of-sight (NLOS) propagation. The latter issue has been known as a major source of localization errors in wireless systems. Therefore, developing robust localizing algorithms for coping with NLOS propagation is of great importance. The aim of this paper is to survey the key characteristics and technical challenges of hybrid wireless localization techniques in NLOS environments, to provide comprehensive performance analysis and finally to identify possible new research directions and further improvements.

Keywords : Hybrid Localization Techniques, Non-Line-ofSight (NLOS) Identification, NLOS Mitigation, Wireless Localization, Positioning.