The Receive Path
To remove SAW filters from the receive path, the receiver must be made much more tolerant of jammers.
3GPP specifies a number of blocking scenarios that are routinely tested in all bands. Of these, out-of-band blocking cases become much more challenging without SAW filters. In particular, the out-of-band blocking ranges 3 and 4, which test the resilience against blockers of -15dBm antenna level, is much harder to achieve without front-end filtering.
Large out-of-band blockers stress the receiver in three distinct ways. First, third order distortion compresses the gain of the receiver and distorts the wanted signal underneath a large blocker. Similarly, second order distortion creates noise that is added on top of the receive signal, and finally, phase noise or spurious components in the local oscillator signal fold out-of-band power into the baseband when down-converting the signal.
Sequans’ receiver implementation optimizes all three of these aspects. A very high compression point is achieved by running the LNA with low output impedance so that the incoming power is converted into a current rather than a voltage signal. The current is then down-converted using a passive mixer with excellent intrinsic linearity before the signal is gradually filtered and amplified. The approach also optimizes second order distortion, which is mainly due to geometrical mismatches between positive and negative branches of differential currents. We further calibrate second order distortion for each unit and, especially in low band, typically achieve IP2 levels in excess of 70dBm regardless of the frequency of the incoming jamming signal. Robustness against second order distortion is further improved by a patented approach to offset the local oscillator from the center of the receive band. Finally, as for the transmit mode, the local oscillator signal is kept exceptionally clean to avoid any reciprocal mixing between jamming signal and noise of the carrier signal. The effect of blocking, due to signals at multiples of the Rx carrier frequency, is mitigated by the use of simple low-pass filters in the antenna path.
Note that the receiver was dimensioned to meet more than the 3GPP blocking performance standards that test only the sinusoidal signal. Sinusoidal or continuous waveform signals have a low peak-to-average power ratio and do not desensitize the receiver in the presence of second-order distortion. Real-world jammers (for example digital TV signals), which are not tested routinely, are wideband, have a higher crest factor, and quickly impact sensitivity due to second order distortion. Sequans dimensioned its receiver to tolerate wideband jamming signals up to -15dBm with negligible desensitization. This level may be expected from a large digital TV transmitter with effective radiated power (EPR) of 30 kW at a distance of a few hundred meters. Even beyond this level the performance will degrade gracefully.
Sequans’ Monarch solution includes a unique combination of radio level signal processing and digital front-end processing that allows the elimination of SAW filters and therefore can be used anywhere in the world, for any operator in any region, with the same hardware.
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