All wireless communication systems have traditionally employed a radio frequency front end (RF FE) (see Figure 3.1), located between the antenna and the baseband subsystem, the latter commonly implemented with digital signal processing technology. While ‘pure’ software radios anticipate analog-to-digital conversion at the antenna, dispensing with the need for this element of the radio, today's ‘pragmatic’ software defined radios (SDRs), still (and as will be seen later in this chapter, for the foreseeable future) require an RF FE and place stringent demands thereon. The requirement for more cost-effective and reconfigurable RF FEs is one of the major needs of the wireless industry.
The perspective of our discussion within this chapter is that of commercial SDR pioneers and practitioners, seeking economically viable solutions for the commercial wireless marketplace. As such, we describe an early multimode RF FE solution, the AN2/6 product family (a two-band six-mode design), discuss alternative architectural options, and outline constraints and possible directions for the future evolution of SDR RF front ends.
The basic functions of the RF FE are
Down-conversion is required for receivers. A receiver subsystem takes the weak signal from the antenna, converts [down-/up-conversion] the signal from the transmission radio frequency ...