## 3 Communication Issues and Solutions
### Communication-Oriented Issues and Solutions
For the bi-static configuration, the RFID reader uses two or more co-located or dislocated antennas for separate transmission and reception. It can be found that with proper antenna spacing/orientation, bi-static systems can achieve a larger reading range and a more uniform distribution of tag RSSI in its reading area compared to mono-static systems [86]. As seen in cellular technologies, multi-antenna techniques offer simple and effective solutions that improve the uplink rate or reliability [87]. As a result, multi-antenna techniques in RFID systems have come into the focus of research to overcome the drawback [88]. The most effective way to improve the DBC link reliability is to increase the number of tag antennas. However, this is not practical because the increase in cost and complication of tag antennas is not allowed in most cases. A RFID reader only needs two to four receiving antennas and one transmitting antenna to improve the reliability of uplink; additional receive antennas provide diminishing gains [75]. However, these multi-antenna techniques increase the design complexity of the system. Alternatively, the magnitude of the vector effective lengths associated with tag and reader antennas improves with an increase in their respective antenna gains, which improves both downlink and uplink ranges. Furthermore, it was found that the choice of amplitude-shift-keying (ASK) impedance modulation indices can maximize the operating range as a function of key system parameters notably the tag sensitivity and bit error rate (BER) at the reader [81]. The above part describes the sources of uncertainties because of the channel and potential solutions to improve the uplink reliability. A typical interference from wireless channel and transceiver itself is shown in Figure 5. On the other hand, the bottleneck of the passive antenna sensor system is limited by the resolution and sensitivity of the receiver onboard the reader. It was shown that the gold encoded messages were received with less error than the Miller-coded ones [89]. This is due to the orthogonality of the symbols as well as the characteristics of the Pseudorandom Noise (PN) codes, that make them less susceptible to environment influences from additive white Gaussian noise (AWGN). Nevertheless, backscatter communications must contend with received noise that is dynamic and colored (because of self-interference) rather than simply static and white (because of thermal noise). In fact, this is an intrinsic limitation of conventional modulated scatterer techniques [90], because colored noise comes from: (1) local-oscillator leakage through a direct down-conversion receiver's mixer; (2) transmit-receive antenna coupling (in a bi-static reader) or antenna mismatch effects (in a mono-static reader); and (3) the unmodulated carrier reflected from the environment back into the receiver. When compared to a conventional one-way digital radio link, a significant amount of colored phase noise about the RF carrier makes its way through a backscatter receiver's RF chain. Careful selection of the bit rate along with the number of _inter-bit transitions_ with regards to a reader's noise spectral characteristics was shown to maximize sensitivity while being mindful of power or energy consumption by the backscatter RF tag [91]. This strategy maximizes the sensitivity of the backscatter modulation while maintaining the tag's power requirements. The use of _inter-bit transitions_ to improve backscatter modulation is not new; for example, the EPC Global C1G2 protocol allows for variations of the \(n\) = 4, 8, and 16 Miller schemes to increase sensitivity [73]. In addition, using multiple, 45\({}^{\circ}\) slant antennas on the RF tag, in conjunction with cross-polarized reader transmitter and receiver antennas, was demonstrated to improve backscatter modulation by reducing the reader's self-interference [84].