## 3 State-of-the-Art MIMO-SDR Systems
### MIMO Systems
#### 3.1.1 Educational Platforms and Testbeds  
Massive MIMO Systems  
One of the first fully validated and functional massive MIMO testbeds is proposed in the context of a collaboration between the University of Bristol and Lund University, associated with National Instruments (NI) [18]. The MIMO-OFDM system is designated for the evaluation of spectral efficiency [2]. It is operable in the frequency band of 1.2-6 GHz, with a bandwidth of 20 MHz and a sampling rate of 30.72 MS/s. In addition, it supports 128 antennas and maximum of 10 users per time/frequency slot. The duration of a time slot is 0.5 ms, and the number of used subcarriers is 1200, with a 145.6 bits/s/Hz spectral efficiency [2,18]. In the same context, [PERSON], et al. [19] have suggested another hardware testbed with a base station of 128 antennas serving 22 users, with a spectral efficiency value of 145.6 bis/s/Hz. A receiving patch antenna array placed at 24.8 m away from the transmitting antenna array of 128 elements was arranged in front of the base stations. The time-division duplex (TDD) massive MIMO system operates at a carrier frequency of 3.51 GHz with a 256-quadrature amplitude modulation (QAM) scheme. The channel estimation was performed in a cyclic manner every 5 ms, and a zero-forcing (ZF) or Minimum Mean Square Error (MMSE) was used to assure a reliable data transmission. [PERSON], [PERSON], [PERSON] and [PERSON] [19] conducted some experiments to assess the system behavior for a high number of users (>22), spatially multiplexed within a single base station. The authors realized that within the 23 rd user addition the constellation diagram deteriorated, whatever the position of this user was. This behavior was justified by the CSI estimation accuracy. In addition, another massive MIMO-SDR system with time-division multiple access (TDMA) was proposed [20], at the Tennessee Technological University, that supports up to 70 nodes and 30 \(\times\) 30 antennas (i.e., 30 Universal Software Radio Peripherals (USRPs)). The system capacity and transmission rates were measured using the Vandermonde channel estimation model [21,22]. The authors proved that, by slightly adjusting the range of the arrival angles and the base station antenna distance, the model became more adequate for the outdoor environment with high loss propagation conditions. They remarked that the channel capacity measurement grew linearly with the deployment of more antennas in the base station. LuMaMi [23], the famous Lund University massive MIMO OFDM testbed, is able to handle up to 100 antennas (i.e., 50 USRPs) with a bandwidth of 20 MHz. The MIMO coding and decoding processes were implemented in 50 FPGA devices of the Xilinx Kintex-7 type. This system's main features are:* It allows the amount of data supported in both the uplink and the downlink to be 384 Gbits/s. * The synchronization is performed with an external signal derived from 8 Octo-Clock devices (7 Octo-Clock devices commanded with a master Octo-Clock). However, some phase tight distortion appears during the transmission tests performed between the base station radio frequency channels; this is due to the receiver channels [24]. * The system could be extended up to 128 antennas. * A planar T-shaped antenna array with 160 dual polarized elements was used. Moreover, five USRP-RIO-type 2953 Rs are deployed to emulate the receiving user equipment with a GPS reference signal connection capability. Another famous massive MIMO LTE system testbed is the OpenAirInterface [25] testbed, which is relatively smaller than the ones already discussed. The platform applies a time-division duplex (TDD) reciprocity channel calibration to obtain accurate CSI channel information. It supports up to 64 antennas with a bandwidth of 5 MHz, a center frequency of 2.6 GHz, and a sampling rate of 7.68 MS/s. In addition, the number of used subcarriers is 300, the duration of one time slot is 0.5 ms, and the maximum number of simultaneous users is four. [PERSON], et al.