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5 | RF Communications RF communications encompasses the theory, techniques, and technologies that enable the transmission of information encoded as electromagnetic signals between RF communication devices or systems. The most common mediums for RF signal transfer are open-air channels, conductive paths in circuits, and transmission lines and waveguides. RF communication relies on standardized frequency allocations, often subdivided into channels, with signals that may be modulated using analog or digital techniques. One of the main benefits of RF communications technology is that radio signals can propagate through free space across long distances to multiple receivers while conveying substantial amounts of information. RF communications can be either point-to-point/peer-to- peer (P2P) or point-to-multipoint (P2MP or PMP), depending on the applications. They can even extend to vast mesh networks or interlinked radio communication networks, such as cellular communications and Wi-Fi. The common use of Bluetooth® connectivity is an example of P2P RF communications, while Wi-Fi is generally used as a P2MP, with a wireless router broadcasting to multiple devices. RF communications can support wide-area information broadcasts, such as AM/FM radio stations, or precise and narrow P2P links designed to provide an optimized communication channel between two points. To convey information with RF signals, they must be modified so RF transmitter circuits can perform the signal manipulation designed to meet operation criteria for specific frequency ranges and channel dynamics (modulation) while also being able to be successfully demodulated by the intended receiving circuits. Because many of the information types transferred using RF communications do not operate at the same frequencies as a desirable RF communications medium, these electrical signals are often upconverted to the desired RF spectrum during transmission and downconverted on the receiving side. This concept is called superheterodyne, where the signal that contains original information (i.e., the low-frequency (LF) baseband signal) is modulated to an intermediate frequency (IF) that is then frequency translated (using a mixer and local oscillator) to a higher-frequency RF signal, which serves as the carrier for further processing. If one or more upconversion and downconversion stages are used, the frequency stages between the baseband and RF are known as IF stages, and the system is referred to as a multiple (i.e., dual, triple) conversion superheterodyne system. Common examples of superheterodyne RF communications are AM/ FM, two-way, and citizens band (CB) radios, as well as many communication systems in the microwave spectrum and beyond. For RF communications within the frequency and performance range of modern analog-to-digital converters (ADCs) and digital-to- analog converters (DACs), it is possible to digitally synthesize and sample RF signals directly. This approach is known as full-digital, fully digital, all-digital, or direct- digital RF communications, where upconverters and downconverters are not required to shift RF to IF for synthesis or sampling, and discrete analog modulators and demodulators are unnecessary. Instead, digital signal processing generates and conditions the communication signals with ADCs and DACs, directly converting between digital and RF signals. The quality of the components and RF circuit design greatly impact the performance of RF communication systems. With both analog and RF systems, the frequency accuracy, noise, phase noise, and amplitude accuracy must meet certain thresholds for successful communication. In the case of digital communications, the digital data contained within the RF signals can be corrupted by the nonidealities of the RF circuit components, interconnect, and transmission medium. This is why many digital wireless communication systems use sophisticated signal processing techniques and error correction to ensure an optimized communication link. RF Sensing and Radar RF sensing is the use of RF spectrum radiation and the distinct interactions this electromagnetic radiation has with materials and interfaces. RF sensing can be either passive or active. With passive RF sensing, specialized RF circuits are designed to capture