Modem, sometimes we call baseband, and protocol stack can be mapped to Layer 1, Layer 2 and Layer 3 radio operation if this radio supports 3GPP standards. Layer 2 and Layer 3 are upper level protocols, we sometimes call it as protocol stack. It is responsible on radio operation resource control, link establishment, handover, etc... Layer 1 is signal processing layer, we may call it physical layer. The modulation, error-control coding, the signal timing synchronization, signal phase synchronization, low level transmit power control, low level receiver control are done within Layer 1. It is strongly related to radio control.All very time sensitive radio control operations done in ASIC blocks or fast digital signal processors within this layer.
Some high level radio control are done on RF software drivers, mostly run on a processor with protocol stack. The time sensitivity is about more than 100 microseconds. It programs RF system to system level specific use-cases, such as enabling/disabling receiver, programming the system to certain band and channel, monitoring RF system and applying adjustments.
Depending on the partitioning, the radio control functions to be done within Layer 1 and RF software drivers vary in large. A general trend towards high speed transmission, more functions becomes more time sensitive, and therefore many functions needs to be done either in ASIC hardware or in high speed DSP. In the past, most of the power control, maximum power control, receiver gain control were done in RF software drivers. In current systems these are part of Layer 1.
We have additional three important functions. One of them is clock management. It mainly controls the reference clock and all generated clocks distributed to multiple design blocks including processors, signal processing blocks. From the radio control aspects, it is related to reference clock supplied to radio, reference clock tuning, frequency synthesizer operation and RF clock generation.
Power management controls the power distribution to design blocks. For better power consumption management, power is supplied on the design blocks needed to be active in an operational mode. Radio part also has multiple design blocks and the power supplied or turned off to these blocks depending on the mode of operation. For example, if the radio is in idle mode listening the system time to time, the power management may keep the transmitter design blocks off, and enable the receiver design blocks time to time, whenever the radio listens the system.In today's radios, multiple power rails distributes the power supply in a tree structure with multiple branch point on a path. power management function is also distributed over this tree-topology. A radio can have different power rails for frequency synthesizers, power amplifiers, digital to analog, analog to digital signal converters, receiver amplifiers, ...
As the complexity of the operation increases, the diagnostic becomes inevitable. During system testing, or during the regular operation, radio control diagnostic collects the information on signal and events. The logging of these information during lab and field test provides tools for issue debugging. Even in the regular operation, at the end of end users, the diagnostic can still collect and stores the detected errors, and assist for debugging. Some of diagnostic information are received signal level, transmit power, transmitter enabled/disabled, receiver gain level, operation channel, operation band, operation mode.
The radio system have five main blocks: Receiver, transmitter, frequency synthesizers, RF front-end, and antennas. Antennas can be considered as a part of RF front-end or independent. In this sequel, we prefer to separate them from RF front-end.
Receiver and transmitter are self-descriptive. The transmitter converts the baseband signal at the output of Layer 1 to RF signal that can be radiated over antennas. The receiver transfer the perceived signal radiation from antenna to the baseband signal and provide to the input of Layer 1. The reason of conversion from baseband to RF, and RF to baseband is due to multiple reasons. Radiation over small size antennas, and propagation of the signal over the air requires higher frequencies. Excluding special radio transmission systems, most of the radio operation occurs more than 100kHz. FM radio channels is around 100MHz, cellular radio operation is mainly between 400MHz to 2.5GHz. HF transmission needs frequency range from 3MHz to 30MHz the signal to be propagated through the ionosphere.
Frequency synthesizers generates pure, actually approximately pure, sinusoidal RF signals. these signals are used to carry the baseband signal to RF frequency bands. An RF frequency band may be partitioned into many frequency channels. the tuning of the frequency synthesizers determined by the band of operation and channel number within the band of operation. Radio control configures the synthesizers accordingly.
RF front-end does not describe a definite object. It is between antenna and transceiver, or RFIC, where, transceiver may include the transmitter but no power amplifiers, receiver with or without low noise amplifiers, frequency synthesizers. Although some integration attempts is ongoing to add more RF front end into RFIC, economically and technically still there are many reasons to keep RF front-end outside of the integration today. Successfully Low Noise Amplifiers (LNA) have been integrated into RFIC but still in some designs and some interference in the operational environment, external LNA can be used. In the following sections, RF front-end will be explained in more details.
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