mercialized reference design, or from specialized design houses that combine components from multiple vendors as well as their own in-house expertise. Transmit The PA transmit chain has multiple sequential amplifiers, some of which can be chained inside a single component (for example, a predriver and a power stage). Besides the PA, we find an isolator, a Tx/Rx switch (as needed for Time Division Duplexing (TDD) systems) and filters. All but very low-power systems include a feedback path for Digital Predistortion required to meet 3GPP Adjacent Channel Leakage Ratio (ACLR) targets. The predriver is a low-power solution in a compact package with 3.3- or 5-Volt power supply. When operating at transmit powers above indoor (femto cell) levels, the power stage is typically fed from a higher voltage (28 V) supply and needs to be cooled appropriately. Receive The Rx/Tx switch routes the antenna signal to the Low Noise Amplifier (LNA) during Transmit mode. During Receive mode, the input to the LNA is connected to ground through a 50 Ohm resistor to make sure that the LNA does not pick up any reflected signals. The LNA can be single stage or dual stage, depending on external addition LNA stages and required receiver performance. Some modules include a band-pass filter as part of the design. Control The FEM control block includes Transmit/Receive control logic, PA bias control, current and temperature module, and other "low speed" logic. Interfacing to the control logic is done RF Front Module Antenna Port RF in/out Power Amplifier Pre-Driver Driver Final Stage Control Monitoring and Control PA Bias Enable/Disable Bias temperature compensation Dynamic drain voltage adjustment for low traffic state Current monitoring Observation port and DPD Rx LNA FIGURE 4-24 RF front end module (FEM). Chapter 4 Hardware architecture choices through I2C, Serial Peripheral Interface (SPI), General Purpose Input/Output (GPIO), and similar control interfaces. Key RF specifications include: The output power requirements map to the desired EIRP 