More Details on What’s In Front of the Baseband – Part 2

In the previous post I had a couple of links to interesting information on what is between an antenna and the baseband radio chip in today's smartphones. As a quick follow-up here's my cheat-sheet which lists the components which is best used with this diagram over at Anandtech:

Downlink (Receive, RX) Path:

RF Front End Components

  • Antennas (e.g. 2 required for diversity or LTE MIMO)
  • Optional:  Antenna Tuner – Matches the (changing) impedance of the antenna with the transceiver. Impedance scenarios can be loaded dynamically when the environment (hand, grip, body location) changes.
  • Antenna switch – Pipes the signal to/from the antenna to band specific filters and power amplifiers for the band currently used.
  • Duplexers – Sit in each path after the antenna switch to combine/split the uplink and downlink signal that is transmitted/received from the antenna(s). Also, the duplexers include band specific filters.

Transceiver – includes the following sub-components:

  • Low noise Power Amplifiers (PA), several present, each covering different band(s). 
  • Down-converter: converts the frequency of the signal to a baseband frequency (to remove the carrier frequency (700 MHz to 5 GHz). Have a look at the Superhetrodyne receiver article over at Wikipedia for more information.
  • Another Power Amplifier stage
  • (Latest development from a Qualcomm 9×45 point of view: No companion chip necessary anymore for LTE carrier aggregation!)

After all of these steps the analog signal is ready to be given to the baseband modem split into in-phase and quadrature components. It's still analogue so the first task of the baseband chip (e.g. the 9×15 / 25 / 35 / 45 Qualcomm MDMs) convert the signal from analog to digital and it then starts it's magic to decode the information (which is outside the scope of this cheat-sheet).

Uplink (Transmit, TX) Path:

  • Baseband: The digital baseband chips's last stage is a digital to analog converter to deliver a weak analog signal to the transmitter chain of the transceiver chip.
  • Transciever:
  • Narrow-band amplifiers
  • Up-converter: Raises the signal to the transmit frequency
  • Driver amplifier raises the signal before it leaves the transceiver
  • Power Amplifiers: A switch inside the transceiver chip forwards the output signal to one of several power amplifiers, each dedicated to a specific frequency range. Several power amplifiers are typically included in a single chip outside of the transceiver chip. Optional: An envelope tracker chip can control the power amplifiers to reduce power requirements. The envelope tracker gets information of how much power to apply directly from the modem (i.e. the component before the transceiver!)
  • Duplexer: Mixes transmit and receive signals as they use the same antenna
  • Antenna switch
  • Antenna