## 3 Communication Issues and Solutions
### Communication-Oriented Issues and Solutions  
In a backscatter system, the power received by the tag or backscattered to the reader may drastically vary as a function of tag and reader positions--even when a line-of-sight (LOS) path exists in between. This variation, famous as _small-scale fading_, is caused by the constructive and destructive interference of waves scattered from objects in the propagation path. Passive RFID tags are traditionally assumed to be downlink limited since typical tag sensitivity (downlink) is considerably poorer than reader sensitivity (uplink), because of the stringent power limitations of tag chips. The above highlights an important facet of RFID systems that appears to have been underappreciated in the existing literature -- the fundamental _asymmetry_ of the uplink and downlink ranges at which information may be reliably communicated [81]. As a result, the small-scale fading effects are more severe than in classical one-way systems [82]. Hence, improving the downlinkrange for passive tags is a key design objective. With continuing advancements in integrated circuit (IC) technology, future passive tags that operate with reduced power may become uplink limited. In backscatter communication, the signal received at the reader arrives after traversing two independent paths. On the reader-to-tag downlink, the impinging signal at the tag antenna is the superposition of components from multiple scatterers in the vicinity of the tag. This incident signal is modulated by the tag chip and scattered back to the reader; en route, the backscattered signal encounters another set of scatterers close to the reader. Since the receiver observes the product of two independent small-scale fading effects, the net fading statistics differs from the standard Rayleigh fading, known as the _dyadic backscatter channel_ (DBC) model [82]. In general, the backscattered signal is subject to environmental _multipath_ en route to the reader that causes both frequency and time selective effects [75]. The uplink symbol rate is sufficiently low, such that we may ignore the impact of any frequency selectivity, i.e., we assume no inter-symbol interference. Typically the physical environment changes slowly over time, so the symbols experience _slow fading multipath_ conditions. The channel property will influence the stabilities of measurable parameters directly calculated from the received backscattered signal. For example, RSSI signatures are repeatable (and not merely random noise) when the environment remains unchanged. However, if a change in the environment happens, not all of the frequencies are equally impacted. Instead, a small change in the environment only results in a slight but noticeable change in the shape of the RSSI signature. Furthermore, if an object in the environment is incrementally moved, it will cause a ripple effect [83]. Based on these facts, the shape of the RSSI signature is dependent upon the multipath of the surrounding environments. For the above reasons, successful backscatter system design requires an understanding of the propagation mechanisms that affect both the power available to the RF tag and backscattered to the reader receiver. Meanwhile, accurate link-budget equations, along with a detailed description of the modulation factor, on-object gain penalties, path-blockage losses, polarization-mismatch losses, impedance-mismatch losses, and small-scale fading losses should be considered ahead [84]. The main performance metric of RFID systems is the reading range or coverage that is defined as the maximum distance between the reader and the tag at which the radiation field from the reader is strong enough to power up the tag and consequently, the backscattered signal from the tag reaches the reader with sufficient power (i.e., with power above the reader's sensitivity) [85]. For the mono-static configuration, a single antenna is employed to simultaneously transmit the CW signal to power the tag as well as receive the backscattered signal from the tag.