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Because Rx require at least -102dBm sensitivity, so need a sharp cut off shap filter to eliminate out-band or out-channel noise or interference.
In Tx path, because there always exists noise in free space, use SAW filter is needless.
Besides, use a LPF(usually included in FEM) is cheeper than a SAW filter and reduce more size.
Main thing is power: the SAW is an acoustic (vibrating) device and it would just cr@ck under high power loads. The other thing is that you have everything under control in a transmitter design. There can be a few spurs at the output that need suppression, but this is all the filtering you need. No need for sharp cutoffs.
I want to add that in GSM (half-duplex transmission) only a low pass filter is set after PA for harmonics suppresion. This could be realized by discrete components with low insertion loss (~0.5 dB). GSM RX-SAW has insertion loss > 2 dB.
The reason of RX BPF in GSM is not to suppress the Noise Power from the PA (or not only).
The main reason of RX BPF is to pass Blocking Test and to suppress the out-of-band frequencies (interference or image).
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