This is the block of Time Division Multiplexing (TDM).
It is used to supply the signals of the four users to be transmitted, the frame synchronism signal and any data sequence instead of the voice signal of the iser 1 (MS1).
Five TS are used:
• TS0: time slot of synchronism of the sent frame
• TS1: time slot assigned to the CODEC1 in transmission • TS2: time slot assigned to the CODEC2 in transmission • TS3: time slot assigned to the CODEC3 in transmission • TS4: time slot assigned to the CODEC4 in transmission
CODEC4
The CODEC is the block of voice coding: it carries out the input/output
voice COding/DECoding.
It uses a PCM coding with μ−law compression, Analog/Digital sampling of 13bit - 8kHz and conversion from 13 to 8 bits.
The signal enters the TP6: a tone of 500 Hz or an auxiliary signal entering from TP5 can be selected via SW4.
The signal converted into PCM digital (PCM4) is supplied to the Time Division block.
The transmission Time Slot (TS) is selected automatically: the TSn is assigned: n is equal to the called user. For instance, if MS4 calls MS1, the TS1 will be assigned and the PCM signal will be inserted into this TS.
The received signal RX-MPX (TP27) is the complete frame of all the TS.
The TS4 that is fixed for MS4, is extracted; therefore, every time it is connected with another MS, the signal sent from the remote MS is inserted into TS4.
The 8-bit PCM signal extracted undergoes a process opposite to that occurred in transmission and after the Digital/Analog conversion it is sent to the output TP7.
The TIMING, that is all the selection signals of the transmission and reception TS are supplied by the Microcontroller and they are available for measurements on the following test points:
• TP14: Transmit Clock
• TP16: sync signal of TS0 (frame synchronism)
• TP17: sync signal of TS1; it is used by the CODEC of the general user to synchronize with the transmission frame in order to send its own data item to the user 1 (MS1)
• TP18: sync signal of TS2; it is used by the CODEC of the general user to synchronize with the transmission frame in order to send its own data item to the user 2 (MS2)
• TP19: sync signal of TS3; it is used by the CODEC of the general user to synchronize with the transmission frame in order to send its own data item to the user 3 (MS3)
• TP20: sync signal of TS4; it is used by the CODEC of the general user to synchronize with the transmission frame in order to send its own data item to the user 4 (MS4)
• TP21: sync signal of TS1; it is used by the CODEC1 of the user 1 (MS1) to synchronize in reception with the received frame
• TP22: sync signal of TS2; it is used by the CODEC2 of the user 2 (MS2) to synchronize in reception with the received frame
• TP23: sync signal of TS3; it is used by the CODEC3 of the user 3 (MS3) to synchronize in reception with the received frame
• TP24: sync signal of TS4; it is used by the CODEC4 of the user 4 (MS4) to synchronize in reception with the received frame.
CODEC1-2-3
The CODECs of the other three users MS1-2-3 work like the CODEC4. The signals in transmission and in reception undergo the same processing.
The TS assigned to the reception are different and no external signal can be inserted.
• MS1: the available tone is of 2 kHz and the received frame signal RX-MPX is the same of the other CODECs
• MS2: the available tone is of 1,6 kHz and the received frame signal RX-MPX is the same of the other CODECs
• MS3: the available tone is of 1 kHz and the received frame signal RX-MPX is the same of the other CODECs
CHANNEL CODING
The TDM frame coming from the TIME DIVISION block is NRZ digital.
A channel coding must be carried out to adapt the digital signal to the transmission medium: Radio Frequency, in this case.
Moreover the channel coding improves the ability of the receiver in synchronizing with the received signal.
A HDB3 coding is used: this introduces a violation at the fourth bit, if four subsequent zeroes are transmitted.
LPF
Gaussian low-pass filter that “smooths down“ the digital signal bifore sending to the modulator.
The filtered signal is available on TP25 at the modulator input.
RF MOD
RF modulator.
It uses the frequency modulation for the signal having to be sent.
The frequency of the selected RF channels is output by a VCO (Voltage Controlled Oscillator) block and is controlled by a PLL (Phase Locked Loop) circuit.
As this is a multi-channel FDM system, the PLL circuit enables to select four different RF channels via the CHANNEL SELECTOR.
RF AMP
Output RF amplifier.
It includes an Automatic Power Control (APC) that enables to control the output level and ensures a protection from malfunction.
The level of the output signal is extracted by a directional coupler and then it is measured.
The Microcontroller acts on the APC, in mobile phones, to set the desired power according to the parameters of quality and intensity of the received signal.