How do I design a multi-antenna MIMO SDR transceiver for research applications?

Designing a multi-antenna MIMO (Multiple-Input Multiple-Output) SDR transceiver for research applications requires a coherent, multi-channel transmit and receive system with precisely synchronized timing, frequency, and phase references across all antenna channels. The design involves: selecting the number of antennas (2x2, 4x4, 8x8, or massive MIMO with 16-128 antennas; the channel count determines the FPGA resources and data throughput requirements), ensuring channel coherence (all TX and RX channels must share a common reference clock and trigger for timing synchronization; the inter-channel phase must be calibrated and stable; see the synchronization techniques: shared LO distribution, common reference clock with individual PLLs, or a single multi-channel RF transceiver IC that inherently shares the LO), choosing the SDR hardware platform (for 2x2 to 4x4 MIMO: multi-channel SDR boards such as Ettus USRP N310 (4 TX / 4 RX), NI USRP-2974 (2x2), or Analog Devices ADRV9026 evaluation platforms; for 8x8 to 64x64 massive MIMO: multiple synchronized SDR boards connected via a timing backplane, or dedicated massive MIMO platforms like the Lund University LuMaMi testbed or the Rice University WARP platform), designing the antenna array (the antenna elements must be spaced at lambda/2 for spatial multiplexing; for 3.5 GHz (5G band n78): lambda/2 = 43 mm; the array must be calibrated for mutual coupling and element pattern variations), and implementing the MIMO baseband processing on the FPGA or CPU (channel estimation, precoding, spatial multiplexing, and detection algorithms such as zero-forcing, MMSE, or ML detection).