Sara Ribeiro, Paulo Montezuma, Daniel Marques, and Rui Dinis
Multilevel constellations, Phase imbalances, Higher tolerance to phase mismatches
Strict power and bandwidth constrains in wireless communications systems can be achieved by an energy efficient amplification combined with powerful equalizer to reduce interference impact. Energy efficiency on power amplification of 2M-QAM (Quadrature Amplitude Modulations) constellations can be achieved by an amplification structure where M BPSK (Bi-Phase Shift Keying) sub-constellations are separately amplified, since each the QAM constellation can be represented as a sum of BPSK components. This amplification with M amplifiers in parallel imposes some restrictions to the amplifiers since phases of the different amplifiers should be matched between them. When phase imbalances exist symbols associated to each amplification branch suffer different phase rotations that can distort the resulting constellation after the combiner. Therefore, it becomes crucial to design the receiver to estimate phase mismatches of the transmitter. For that we propose a phase imbalance estimate algorithm that despite the low complexity shows good tolerance against phase imbalances at the transmitter. Simulation results show that the good performance is attainable by the proposed receiver and estimate algorithm without significant increase in system and computational complexity. Moreover, this can be used to relax the tolerance mar- gin in hardware implementation of the amplification stage at the transmitter.
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