Spurs and linearity consideration for aliasing-free pulse-modulated polar transmitters under fewer harmonics in dual-phase operation

This paper presents an aliasing-free pulse modulated polar transmitter under fewer harmonics condition for the 4G long term evolution (LTE) femtocell base stations. Compared between two kinds of harmonic conditions K = 1 and 3, the fewer K implementation is capable of reducing the complexity and the number of modulation spurs resulted from the pulse modulator. However, the other outside modulation spurs will still be revealed after non-linear amplification. Based on this reason, the dual-phase digital pulse-width modulation and balanced configuration were adopted to suppress the undesired modulation spurs at the combination output port. A prototype transmitter system was constructed with commercially available components. Measuring using a 10-MHz bandwidth LTE 16-QAM signal at 836.5 MHz for application, it achieves 28% efficiency at 21.2-dBm power level. The condition of K = 3 also has more than 4.3 dB better adjacent channel leakage power ratio than the one of K = 1 under the same memoryless digital pre-distortion based on lookup tables.