This work presents a multi-band LO-generator that can support existing cellular bands below 6GHz and new millimeter-wave (mmW) bands for 5G, concurrently. Using a low-noise reference-frequency doubler, a fractional-N PLL generates GHz-range signals with low integrated phase noise (IPN). Then, injection-locked frequency multipliers increase these frequencies to mmW-bands without the degradation of IPN. The measured IPN and RMS jitter of the 29GHz signal were −31.4dBc and 206fs, respectively, which is the lowest among the state-of-the-art mmW-band fractional-N frequency synthesizers. This IPN is low enough to satisfy the EVM requirement of higher-order modulation for 5G systems. This frequency synthesizer also can generate various frequencies in multiple bands, including existing bands below 6GHz and new mmW band around 28GHz