I/Q imbalance and nonlinearities of power amplifier (PA) are the main impairments of wireless transmitters degrading the spectral purity and error vector magnitude (EVM). In order to jointly migrate them, an asymmetrical complexity-reduced Volterra series model is proposed, namely joint-digital predistortion (joint-DPD). Our joint-DPD is inspired by the interaction between such two impairments, providing high degree of dynamic nonlinearities with large memory depths. Also, by understanding the structure of joint-DPD for OFDM signals, lower computational complexity can be achieved by pruning the redundant terms with an asymmetrical structure. The corresponding analysis offers the theoretical basis and its related complexity over different Volterra-seriesbased joint-DPDs. The performances are evaluated under two scenarios. Especially when a 3.4-dB back-off input power is applied, where severe PA nonlinearities are presented with the I/Q imbalance, the measured (left, right) adjacent channel power ratio is improved from (-19.9, -19.6 dBc) to (-37.3, -37.9 dBc), and the EVM is reduced from 22.1 to 3.4 %. For performance comparison, other Volterra-series-based DPDs such as memory polynomial, dynamic deviation reduction and generalized memory polynomial, are also extended to joint-DPD.