A radio frequency pulse width modulation (RF-PWM) scheme based on phase-shift control and mapping PWM (MPWM) is described. Pulse coding is the key to improving the coding efficiency, flexibility, and configurability of the all-digital transmitter (ADTx). To solve the problem that the real-time performance of the system is limited by the time resolution, the phase-shift control principle is adopted to constrain the output pulse state. It decomposes the original signal into two phase-modulated constant envelope signals, and directly converts the two-level pulse waveform by MPWM. Finally, the fast generation of the three-level digital RF modulated signal is completed by vector synthesis. In this way, the rear power amplifier can be directly driven, and the difficulty of physical implementation is greatly reduced. Different from the traditional mapping strategy of traversal search, the proposed scheme does not require complicated error calculation and comparison. Simulation and offline experiments show that the proposed scheme has better comprehensive performance than other mapping schemes. For 16QAM modulated signals at a 300 MHz carrier, the proposed scheme can achieve nearly 70% coding efficiency (CE), less than −50 dBc, and 1% adjacent channel power ratio (ACPR) and error vector magnitude (EVM).
To relax the requirement of the filter and reduce the control complexity
of the SMPA, this paper proposes a 5-level RF-PWM method for all-digital
transmitters with 3rd and 5th harmonic elimination. The method is
achieved by changing the threshold signal to control the pulse width of
the 3-level sub-pulses. Finally, the feasibility of the method is
verified by simulation. For the 16QAM signal with a carrier frequency of
200MHz, the proposed method can achieve -46.24dBc and -54.05dBc
respectively when the coding efficiency reaches 77.51%.
To reduce the loss caused by power imbalance between switched-mode power
amplifier (SMPA) units of all-digital transmitters (ADTx), this paper
proposes an all-digital transmitter structure with power equalization
and harmonic elimination. Based on the third and fifth harmonic
cancellation, this structure achieves output power equalization of SMPA
by controlling the width of 3-level sub-pulses to be the same. Moreover,
the equalization of each switch transistor is realized by outphasing
method. Finally, the feasibility of the method is verified by simulation
results. For different input signals with a carrier frequency of 200
MHz, the proposed structure achieves the same output power of each SMPA
unit.
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