put (MIMO) antenna. In these antennas, the The RF analog front-end amplifies and then converts interface boundary between the radio and the antenna is the radio carrier frequency of a signal of interest down to blurred by the wide bandwidth and large number of a low IF SO that the receive signal can be digitized by an complex interfaces between the beam-steering signal analog-to-digital converter (ADC), and then processed processing, the large number of parallel RF front-end by a DSP to perform the modem function. Similarly, the receivers, and the final modem signal processing. For transmitter consists of the modem producing a digital these complex antennas, the SDR Forum is developing representation of the signal to be transmitted, and then interface recommendations that will be able to anticipate a digital-to-analog converter (DAC) process produces the wide variety of multi-antenna techniques currently a baseband or IF representation of the signal. That signal used in modern transceivers. is then frequency shifted to the intended carrier fre- Another common external component is the RF quency, amplified to a power level appropriate to close power amplifier (PA). Typically, the external PA needs to the communication link over the intended range, and be told to transmit when the transceiver is in the transmit delivered to the antenna. If the radio must transmit and mode and to stop transmitting when the transceiver is in receive simultaneously, as in full duplex telephony, there the receive mode. A PA will also need to be able to sense will also be some filtering to keep the high-power its VSWR, delivered transmit power level, and its tem- transmit signal from interfering with the receiver's ability perature, so that the operator can be aware of any ab- to detect and demodulate the low-power receive signal. normal behavior or conditions. This is accomplished by complex filters usually using It is also common to have a low-noise amplifier (LNA) bulk acoustic wave or saw filters at frequencies below in conjunction 