4.6. High-frequency paths in a GSM mobile station

The high-frequency paths in the mobile station include:

- high-frequency path of the transmitting part of the MS;

- the high-frequency path of the MS receiving part.

Transmitting high frequency path of the mobile station

The GMSK modulating signal is fed to the SMP mixer input (see Fig. 4.2), to the second input of which high-frequency oscillations are fed from the frequency synthesizer, which allows the GMSK signal spectrum to move up the frequency scale to (890 ... 915) MHz, while using the filter of the concentrated selection of the FSS (band-pass filter based on ceramic filters), the higher harmonics that appear when converting signals to the SMP are cut off, and then the high-frequency (HF) signal goes to the PA power amplifier unit, whose parameters (gain and duration) are sent from the CPU.

In the GSM system, the transmitter and receiver work non-simultaneously, while the transmission, i.e., turning on the high-frequency path of the transmitter, takes place only for 1/8 of the frame duration (that is, 4.615 ms / 8 = 0.577 ms).

This dramatically reduces the energy loss in the transmission mode (when the subscriber speaks) and increases the battery operation time.

In addition, depending on the position of the MS with respect to the BTS, the CPU makes a decision on the transmitter power level for stable MS-BTS communications.

The closer the MS to the BTS, the lower the gain of the RF signal in the PA, the less the power emitted by the antenna.

The RF signal amplified in the PA, then goes through the FSS ceramic filter to the antenna switch, which connects the antenna to the transmitter output, as noted above, for 1/8 of the frame duration.

Receiving high frequency path of the mobile station

The radio signal received by the antenna in the frequency range (935 ... 960) MHz from the BTS via the antenna switch in reception mode enters the input of the high-pass filter (ceramic surface acoustic wave filter - SAW) in order to reduce the influence of mirror and neighboring frequency channels.

Next, the RF signal from the filter is amplified by a low-noise LNA amplifier and fed to the input of the first frequency converter (CM1 mixer), to the second input of which an RF signal is fed from the frequency synthesizer.

The spectrum of the received RF signal is thus shifted down the frequency scale to the first intermediate frequency that the FSS is tuned to SAW and then to the amplifier of the first intermediate frequency UPCH1 (/ npi) -

After amplification, the signal of the first intermediate frequency again undergoes frequency conversion and the frequency spectrum in CM2 shifts down the frequency scale to the second intermediate frequency (/ prg) -

Further, the signal after filtering in the FSS surfactant is amplified in UPCH2 and is fed to the input of the GMSK demodulator.

Thus, the high-frequency transmission and reception paths operate depending on the control from the CPU, the switching center (according to the control signals) and provide a duplex mode of conversation.

The stream of digital signals from the output of the GMSK demodulator passes through an equalization procedure in the channel equalizer before entering decoding.