# Performance Analysis of MIMO System with Single RF Link Based on Switched Parasitic Antenna
## 3 Modeling of MIMO System in SystemVue
### Transmitter Modeling  
The transmitter module mainly realizes the process of mapping two binary symbols to a set of orthogonal basis by the switch parasitic antenna. It includes modulation and up-conversion of the first signal, XOR of the second signal and the first signal, and the switched parasitic element antenna. The internal structure of the modulation module is shown in Figure 3. The modulation includes baseband modulation and radio frequency modulation (IQ modulation), which are implemented by Digital-Mod and Mod components, respectively. Baseband modulation can be interpreted as \"zero frequency modulation\", and it is used to modulate the first input signal into digital baseband signals and to carry out oversampling and shaping filtering. The research in [32] was mainly based on the baseband spatial modulation and demodulation to validate the average bit error rate (BER) performance of the spatial modulation and spatial multiplexing modulation systems. The focus of [33] mainly lies on the hierarchical quadrature amplitude modulation to accomplish unequal error protection and enhance the received image quality and the results are intent to show the effect of the baseband modulation parameters on the performance of the proposed system. These two paper mainly focused on the baseband modulation and framing of MIMO Orthogonal Frequency Division Multiplexing (OFDM) system while the content of this paper is to dig into the system-level simulation to analyze the influence of the Single Pole Double Throw (SPDT) on the MIMO system performance. Therefore, considering the length of the paper, the baseband processing of the MIMO system is simplified and briefly discussed. Among them, oversampling is beneficial to improve the signal-to-noise ratio and reduce the BER. Shaping filter is mainly used to eliminate inter-symbol interference. It can be divided into rectangular  
Figure 3: Internal structure of modulation module. Figure 2: Diagram of single RF link MIMO system in SystemVue. filter, raised cosine filter, square root raised cosine filter, and Gauss filter. The difference between four kinds of filters depends on steep degree of the window function in rising and falling edges, which is closest to the ideal rectangular filter. But in practice, the raised cosine filter is commonly used for its low cost. The tail attenuation process is much quicker than the Nyquist filter (sinc function) and greatly reduces the inter symbol interference, with roll off factor of 0.35. The digital baseband signal is converted in digital analog converter after IQ modulation. The Mod can choose different modulation modes: amplitude and frequency modulation, amplitude and phase modulation and IQ modulation. In this system, IQ modulation is selected and the two signals are orthogonal to each other, with only a 90\({}^{\circ}\) difference in phase. They are transmitted with the same carrier frequency to effectively improve the spectrum utilization. An internal local oscillator signal is loaded on the component to move the real signal information to the specified frequency and sets it directly to the system's intermediate frequency of 220 MHz. The IQ modulated signal is the intermediate frequency analog signal. In order to improve the accuracy of this signal, it is necessary to load the signal to the high frequency signal. The schematic diagram of the RF transmitter module is shown in Figure 4.  
This module mainly realizes the signal conversion process. The modulated signal passes through the attenuator, Chebyshev band-pass filter, amplifier, and nonlinear adjustable attenuator to the signal source input mixer. The Chebyshev band pass filter is close to the IQ modulator to suppress off band stray signals at the transmitter, and the controllable attenuator is used to control the power gain of the RF link. The oscillator of the mixer is supplied directly by an oscillator with a resonant frequency of 2180 MHz, and the oscillator output signal will pass through an attenuator, an amplifier and a Butterworth low-pass filter in turn.