## 3 State-of-the-Art MIMO-SDR Systems
### MIMO Systems
#### 3.1.2 Hardware and Architecture Innovations
The computational performance was compared with an implementation based on TI TMS320C6416 DSP, which shows that the computation time using a GPU is 3-16 times faster than the DSP. The total computational time for every single WiMAX frame (of 5 ms duration) is 4.346 ms. However, [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [52] present another MIMO system, a \(4\times 4\) 3 GPP (3 rd Generation Partnership Project) TDD LTE-A (Long Term Evolution-Advanced), implemented over a GPU. Two NVidia's GeForce GTX Titan GPUs were used, one for the uplink data and the other one for the downlink data. Figure 6 illustrates the system architecture which includes four transmitting and receiving antennas at the base station site and two pieces of end-user equipment. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [53] propose a heterogeneous MIMO system with a maximum likelihood (ML) signal detector implemented over a GPU, namely a sphere decoding algorithm. A bit-interleave-coded modulation (BICM) [56] was adopted, while, [PERSON], [PERSON], [PERSON] and [PERSON] [54] chose a completely novel multi-pass trellis traversal (MTT) detection algorithm, implemented over an NVIDIA GPU, that performs similarly to already existing algorithms. [PERSON], [PERSON], [PERSON] and [PERSON] [55] use a GPU for accelerating the Least Squares (LS) channel estimation algorithm and the demodulation of the uplink OFDM single-user MIMO system. The implementation was realized in an ORBIT testbed [57], where the performance of the system was tested using different configurations depending on the number of antennas and the Fast Fourier Transform (FFT) length. The authorsconclude that the performance offered by a GPU is limited when increasing the number of antennas and the FFT length. Thus, in a following work they suggested parallelizing the implementation and using distributed algorithms and a distributed server. Another heterogeneous MIMO system was designed [27] to accelerate the logic treatment. This is the mm-wave testbed, which uses open computing language (OpenCL) to transfer data between the FPGA and GPU without involving the CPU. MIMO Antennas  
At this point, the researchers have been focused on designing a new antenna with a maximum gain and lower loss. There were some array antenna prototypes designed to work with base stations [58] and others for the end user's deployment [59]. The main innovation concerning this research axis is the concept of reconfigurable antennas [60, 61]. Reconfigurable antennas could change their operation frequency, in addition to their characteristics, to operate within multi-mode services and standards. Thus, they offer multiple advantages such as:  
* Operability in multi-band frequencies
* Polarization diversity
* Low size and cost with high performance. Figure 6: \(4\times 4\) 3 GPP TDD LTE-A MIMO system architecture. In fact, increasing the number of active elements used by the antennas allows more reconfigurable features (frequency, bandwidth,... ). To obtain this reconfigurable capability, it is necessary to deploy some micro-electronical components such as micro-electromechanical systems (MEMS), varactor, and PIN diodes. [PERSON], [PERSON] and [PERSON] [60], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [61] did an extensive review about reconfigurable antennas for 5G applications in general. They cited and analyzed several prototypes recommended for MIMO and cognitive radio systems [62, 63, 64, 65, 66].