(A) beam-tapering phased array transmitter and an up-conversion mixer for 60 GHz full-band wireless communication

Abstract

This study describes a four-element beam-tapering phased array transmitter and 60 GHz upconversion mixer for 60 GHz full-band wireless communications. For applications such as uncompressed video data transmission requiring transfer rates up to 18 Gbps, four channels of 60 GHz frequency band are bundled to send a lot of data. We will design a 16QAM transmitter with a date-rate of up to 18 Gbps and a 1 $\times$ 4 phased array for a 3m transmission distance through 4-channel bonding and 16QAM modulation. When using this 4-channel bonding technique, there are problems such as gain flatness, multi-path problem, and squint effect. The phased array in this study should have wideband characteristics, and also try to reduce the side-lobe level to -20dB to solve the multi-path problem. To achieve this, the gain of each element was non-uniformly assigned and the ratio was set to 1/3: 1: 1: 1/3 (5dB in dB scale). Through EVM calculation, the signal loss due to squint was found to be less than 0.5dB in the 1 $\times$ 4 phased array, and the target values of RMS gain and phase error were set to less than 0.5dB and less than $9^\circ$, respectively. To achieve this error, we need a VGA with a phase compensation. The proposed VGA requires an additional 5dB gain control for beam-tapping while compensating for the gain error of the phase shifter. In addition, the proposed VGA is designed to help improve the linearity through the DS method. This VGA was integrated into a phased array using a commercial 65nm CMOS process, and the chip size of the whole array is 1.82mm $\times$ 1.58mm, consuming a total of 403.2mW. The RMS gain and phase error are less than 0.53dB and less than $8.8^\circ$ in the whole 60GHz band. Beam-forming and beam-tapping radiation patterns were measured by integrating with a 1 $\times$ 4 Teflon antenna. When the phase change was applied at $22.5^\circ$, $45^\circ$, $90^\circ$, and $135^\circ$, the emitted beam was formed into $6$ ~ $9 ^\circ$, $16$ ~ $17^\circ$, $22$ ~ $27^\circ$, and $35$ ~ $43^\circ$ depending on the frequency. The maximum squint is 8 degrees at both ends of the frequency band, and the gain difference is less than 1 dB, which is consistent with the EVM calculation. The beam-tapping was measured at the center frequency of 62 GHz, and the side-lobe level was improved by 2.2 to 5.2 dB when the phase change was applied to $0^\circ$, $45^\circ$, $90^\circ$, and $135^\circ$. This value is different from 8.7dB calculated by Array Factor. The reason for this is found to be due to the gain and phase mismatch of the feeding lines in the bond-wire and array antenna during the packaging process. An up-conversion mixer and a buffer amplifier are designed for a 60GHz 16QAM heterodyne transmitter. In converting an IF signal with a wide bandwidth of 9 GHz around a 20 GHz center frequency into an RF signal of 60 GHz, how flat a gain can be is the most important design direction. A Gilbert-cell type mixer without the Gm stage was adopted for a high linearity and a wideband characteristic. This is because the IF signal is applied directly toward the source of the switching stage to enable input matching in a very wide frequency band, and at the same time, lead to an improvement in linearity by securing a voltage headroom. The gain flatness of the fabricated mixer was measured below $\pm$ 0.6dB in the 57 ~ 66GHz frequency band, and OP1dB was measured below -1.7dBm. The 60 GHz buffer amplifier uses a 2-stage CS architecture, and the first stage is matched at 57 GHz and the second stage at 66 GHz for wide frequency characteristics. In the second stage, power matching through load-pull simulation was performed so that OP1dB reaches up to 6dBm. The gain flatness of this buffer amplifier is measured below 0.3dB at 57 ~ 66GHz. The 60 GHz mixer, buffer amplifier, 20 GHz modulator, and PLL were integrated to implement the layout of the 16QAM transmitter, and the performance was verified by full simulation. The conversion gain of all 16QAM transmitters was designed to be less than 2.2dB in the entire 60GHz frequency band, and 3-dB BW was found to be 56 ~ 68GHz. In this study, we implemented a 60 GHz phased array and a 16QAM transmitter with flat gain across the entire frequency band for wireless communication applications using 60 GHz 4-channel bonding. Also, in order to reduce the multi-path problem, the phased array uses a beam-tapering technique using non-uniform amplitude.