The Transmit Path
Let us first consider the design of the transmitter. The 3GPP standard requires that wideband noise and spurious emissions into downlink frequency ranges of other licensed bands must be below a level of -50dBm.
This target is known as the spurious emission band UE (user equipment) co-existence. From the active uplink band, all downlink ranges of networks operating in the same geographical region must be protected. For example, when transmitting in band 1, the frequency ranges relating to 28 other operating bands must be protected. Given the high number of possible band combinations, this can be achieved only if the transmitter output is kept free of wideband noise and spurious emissions at any offset frequency.
Required transmit band protection level is -50dBm
Narrowband public safety band, noise requirement is below -57dBm.
For some transmit bands, the protected regions are very close. In the US band 13, for example, the downlink channel of band 14 (used for wideband public safety applications) is a mere 10 MHz below the transmit band. Since the required protection level is -50dBm, some operators target a level below -60dBm for extra margin. Even closer in, at just 2 MHz below band 13, a frequency interval known as the narrowband public safety band, noise must be kept below -57dBm measured in 6.25 kHz. Figure 4 shows that both of these limits are well below the general spectral mask requirement for both 10 MHz and 1.4 MHz channels. Sequans’ solution meets this requirement without any filtering in the transmit path. This has been achieved by keeping phase noise and general thermal noise low. One of the most challenging aspects has been to keep the spurious content of the local oscillator clock low. The local oscillator clock is used for up-conversion of the signal from baseband frequencies onto the carrier. Any sidebands present in this carrier signal create images around the transmit signal. The -50dBm antenna limit corresponds to a limit of spurious content of less than -73dBc for a 23dBm transmitter. Sidebands can be created, for example, by the reference clock because the phase-locked loop generating the local oscillator signal continuously compares the phase of the carrier wave against the phase of the reference clock.
Figure 4: Emission Limits
Emission limits demanded by operators (blue) can be much harsher than the general LTE mask limits (teal), here shown for band 13.
The power amplifiers create RF harmonics that still need to be filtered, especially when these harmonics fall into frequency ranges of radios that may be within close proximity to the cellular IoT device. This includes emissions near the GPS carrier (e.g., the second harmonic of band 13 uplink) or the 2.4 GHz ISM band. For these cases in particular, low-pass filters are required for the power amplifiers. However, these filters can be implemented either in low cost LTCC (low temperature co-fired ceramic) process or as discrete L-C low pass structures.
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