## 5 Future Trends and Perspectives
### Integration and Standardization  
The rapid evolution of large-area electronics printing technologies, e.g., inkjet printing, has enhanced the development of low-cost RFID-enabled sensors as well as accelerated their high granularity deployment in large scale structures. Tag and tag communication [149], grid issues [150; 151; 152], e.g., granularity and cross-talk, might be considered or utilized to enhance the resolution, coverage, and detection of the inter-tag defects. Integrating with UWB technology is a promising solution for next generation RFID systems to overcome most of the limitations of the current narrow bandwidth RFID technology such as: low-data rate, reduced area coverage, insufficient ranging resolution for accurate localization, sensitivity to interference, and scarce multiple-access capability [153; 154]. The maturation of passive low-cost RFID tag technology has made it a viable candidate for scenarios where short-range, low-rate links suffice. A recent innovative trend centres on re-engineering passive RFID tags towards WSN applications, i.e., to more data-intensive applications rather than tag identification applications [155; 156; 157]. The tasks involved in the integration of WSNs and RFIDs are to tackle issues of energy conservation, real-time performance, data cleaning and filtering, localization, anti-collision, and authentication [158]. The success of IoT depends on standardization, which provides interoperability, compatibility, reliability, and effective operations on a global scale [17]. There are already several standards such as the International Organization for Standardization (ISO) and Electronic Product Code (EPC) Global, which allow for the simultaneous interrogation of multiple tags with a low data-collision probability for a variety of environments and tag configurations [73]. The anticipated higher data rates for sensor nets will exacerbate tag collisions on the uplink with existing protocols; future RFID networks are thus likely to be uplink limited, based on this consideration. Compressive sensing (CS) can be applied to reduce the ID search space and thus read more tags in a shorter time [159]. This technology can also be utilized to reduce the stringent data rate requirement enabled by ubiquitous computing in the tag side [160]. On the other hand, as deployments scale to larger tag populations requiring in turn many more readers in a given area, the likelihood of reader collisions (inability by tags to decode reader commands) on the downlink will also increase (for a given frequency band or number of channels) [74]. As the number of users, data volume, and range of sensor systems grow, passive backscatter-based networks will require improved links and power efficiencies, thereby opening a new set of challenges for RFID system designers at all (circuit, device, communication link and network stack) levels [82]. Extending the chip's interface capabilities to a sensor is straightforward. An example of a passive sensor platform with power harvesting ability is the WISP [161]. In addition to the basic identification functions of conventional tags, WISP is equipped with sensors connected to a microcontroller unit, thus providing sensing and computing capabilities [162]. Moreover, it is powered and read by standard Gen2 readers [163]. Though extremely flexible and versatile, the WISP solution is, of course, more expensive than traditional passive RFID tags and has limitations in terms of read range, that is almost 3 m [164]. The integration of RFIDs and WSNs will increase their combined data reporting capabilities, e.g., context-aware services [165]. But the standardization activities in this area remain unclear since RFID and sensors have been traditionally covered by different standardization bodies [166].