A 65-nm CMOS $2\times2$ MIMO Multi-Band LTE RF Transceiver for Small Cell Base Stations

This paper presents a 680 MHz-6 GHz 2 x 2 multiple-input and multiple-output (MIMO) long-term evolution (LTE) RF transceiver in 65-nm CMOS for low-cost and multi-band capable femtocell base stations. The transceiver integrates two receivers (RXs), two transmitters (TXs), and two frequency synthesizers, for the 2x 2 MIMO operation to support both the frequency division duplex (FDD) and the time division duplex (TDD) modes. Each pair of an RX and a TX features eight single-ended low noise amplifiers (LNAs), and eight TX outputs that extensively share active and passive circuits with minimal performance degradation. In the measurement, each RX illustrates the noise figure (NF) from 2.9 to 5.2 dB, the input-referred third-order intercept point (IIP3) of more than -2 dBm, and the IIP2 of more than 48 dBm, over the entire frequency range at the maximum gain. Each TX achieved the adjacent channel leakage ratio (ACLR) that was less than -54 dBc at -5-dBm output power with -157-dBc/Hz phase noise at the RX band, while achieving an error-vector-magnitude (EVM) of less than 2.8%, over the entire frequency range. The transceiver, packaged in a flip-chip chip-scale package (fcCSP), is mounted on the board of the commercial femtocell of the LTE Band5, along with a commercial duplexer, power amplifier, and modem. The femtocell achieved -100-dBm reference sensitivity without the use of an external LNA. It also achieved -51-dBc TX ACLR and 1.68% TX EVM at 20-dBm output power in the LTE 10-MHz mode with the 2 x 2 MIMO configuration, without applying a digital pre-distortion (DPD) technique.