The original digital stream (digital baseband) that you intend to transmit needs to be converted into a form suitable for transmission on a radio. That is ultimately a DSP processor that "downconverts" RF signals (high frequency) into baseband (low frequency) signals first using a Local oscillator and then estimates the distortion that channel (in most cases, air) has introduced and then tries to undo that distortion. The baseband processor encodes the original digital stream into a form suitable for transmission. The digital stream must be manipulated into an analog constellation of amplitude, phases that represent the digital signal. This signal is called the analog baseband. Everyone's baseband is largely the same. To multiplex a bunch of basebands over radio frequencies, that baseband needs to be modulated and multiplexed into some kind of a channel, either a frequency (frequency domain multiplexing), a timeslot (time domain multiplexing) or a sequence (code division multiplexing), or some combination of those things. So, the baseband processor does all the heavy lifting of signal conversion at the unmodulated frequencies (the baseband) before being fed into a modulator/multiplexer that will place the baseband signal on to a relevant transmission channel.
The Baseband Processor performs all of the functions required for an IBS/IDR Framing Unit, a Reed-Solomon Codec, and an E1/T1 Drop and Insert System. In addition, the Baseband Processing Section provides for transmit clock selection and rate adaptation as well as a rate adapter and Plesiochronous/Doppler (PD) Buffer in the receive direction. A multiplexer is also provided for the SCT Clock Source for Loop Timing Applications. The transmit and receive paths may be configured independently under processor control.
Universal Satellite Modem Functional Block Diagram
Tx Baseband Processing
The Tx Data and Clock enters the Baseband Processor, passes through a Rate Adapting FIFO and enters the Framer/Drop Processor. In Closed-Net Mode, the data passes through the framer unaltered. In IDR, IBS, and D&I Modes, the framer adds the appropriate framing and ESC as defined in IESS-308 and 309. In D&I Mode, the framer acquires the terrestrial framing structure, E1 or T1, and synchronizes the Drop Processor. The Drop Processor extracts the desired time slots from the terrestrial data stream and feeds these channels back to the framer. The framer then places the ‘dropped’ terrestrial time slots into the desired satellite channel slots. The data is then sent to the Reed-Solomon Encoder.
The Reed-Solomon Encoder, encodes the data into Reed-Solomon Blocks. The blocks are then interleaved and synchronized to the frame pattern as defined by the selected specification (IESS- 308, IESS-309, DVB, etc.). After Reed-Solomon Encoding, the composite data and clock are applied to the BB Loopback Circuit.
Rx Baseband Processing
The Receive Processor performs the inverse function of the Tx Processor. Data received from the satellite passes through the BB Loopback Circuit to the Reed-Solomon Decoder to the Deframer. The Deframer acquires the IBS/IDR/DVB frame, synchronizes the Reed-Solomon Decoder and extracts the received data and overhead from the frame structure, placing the data into the PD Buffer, sending the overhead data to the UIM. In Closed-Net Mode, the data is extracted from the buffer and is sent to the UIM. Backward Alarm indications are sent to the M&C Subsystem. In Drop and Insert Mode, the Insert Processor synchronizes to the incoming terrestrial T1/E1 Data Stream, extracts satellite channels from the PD Buffer, and then inserts them into the desired terrestrial time slots in the T1/E1 Data Stream.