SISO vs. MIMO Testing: A Basic Overview

This article explores the fundamentals of SISO (Single Input Single Output) and MIMO (Multiple Input Multiple Output) testing. We’ll cover the test setups used for each, as well as the common RF compliance tests performed on transmitters and receivers.

Understanding SISO and MIMO Systems

Figure-1: SISO vs MIMO System basics

As shown in Figure 1, SISO systems employ a single antenna for both transmitting and receiving signals. These systems rely on duplexing techniques like:

• TDD (Time Division Duplexing): Transmission and reception occur at different times using the same frequency.
• FDD (Frequency Division Duplexing): Simultaneous transmission and reception take place using two distinct frequencies.

In contrast, MIMO systems utilize multiple antennas for both transmission and reception. Figure 1 depicts a 2x2 MIMO configuration, but other configurations like 2x4, 4x4, and 8x8 are also common. Two main techniques are used in MIMO:

• STBC (Space-Time Block Coding): Multiple antennas transmit different data symbols (e.g., s1, s2) at a given time (t1), and then modified versions of these symbols (s1’, s2’) at a later time (t2). This approach enhances coverage distance due to its inherent redundancy.
• SM (Spatial Multiplexing) or SDM (Space Division Multiplexing): Each antenna transmits different sets of data symbols at different times (t1 and t2). SM/SDM is preferred for achieving higher data rates compared to STBC.

RF SISO Testing Basics

Figure-2: SISO test setup

Figure 2 illustrates a typical SISO test setup.

• Transmitter Testing: A single transmitter’s output is directly connected to a Vector Signal Analyzer (VSA), Spectrum Analyzer, or a Signal Network Analyzer (SNA/VNA).
• Receiver Testing: A signal generated by a Vector Signal Generator (VSG) or another RF signal generator is fed into the SISO receiver.

Common SISO RF Measurements

The following RF measurements are typically performed on SISO transmitters and receivers:

Transmitter Measurements:

• Gain and Transmit Power or spectrum measurement
• 1dB Compression Point or TOI (Third Order Intercept) point measurement
• Phase Noise
• Spurious and Harmonics measurement
• CCDF (Complementary Cumulative Distribution Function) measurement

Receiver Measurements:

• Noise Figure measurement
• Gain measurement
• BER/SNR measurement
• Maximum Allowed input power
• Minimum detectable RF power
• EVM (Error Vector Magnitude)

RF MIMO Testing: Transmitter

Figure-3: MIMO RF Transmitter test setup

Figure 3 shows the MIMO transmitter test setup, which can be done using different configurations: Parallel, Combined, and Switched. We’ll focus on a 2x2 antenna MIMO system.

• Parallel VSA: Both transmitters are simultaneously connected to two separate VSAs. This configuration is used for composite EVM and interleaved sub-carrier tests.
• Combined VSA: The outputs of both transmitters are combined using an RF combiner before being fed into a single VSA. This setup can provide combined EVM and STBC simulation results. However, SDM cannot be simulated.
• Switched VSA: The transmitters are sequentially switched as inputs to a single VSA. This setup allows for both STBC and SDM simulations.

MIMO Transmitter Measurements

The following measurements are performed during MIMO transmitter compliance testing:

• Spectrum Mask
• Spectral Flatness
• Peak Power
• Center Frequency Error
• Symbol Clock Frequency Error
• Center Frequency Leakage
• Error Vector Magnitude (EVM)

RF MIMO Testing: Receiver

Figure-4: MIMO RF Receiver test setup

Figure 4 shows the MIMO receiver test setup. The VSG consists of baseband signal generation software, an AWG (Arbitrary Waveform Generator), and an RF upconverter.

• Parallel VSG: Separate VSG signal outputs are provided as inputs to the receiver. Both SDM and STBC simulations are possible with this configuration. A fading channel simulator is often included to emulate real-world channel effects.
• Split VSG: This configuration is used for input sensitivity gain and keyhole effect testing. However, STBC and SDM simulations cannot be carried out here.
• Combined VSG: This setup is used for adjacent and non-adjacent channel testing, as well as broadband digital pre-distortion.

MIMO Receiver Measurements

The following receiver measurements are conducted during MIMO compliance testing:

• Minimum Input Level Sensitivity
• Maximum Input Level
• Adjacent Channel Rejection (ACR)
• Non-Adjacent Channel Rejection
• Clear Channel Assessment (CCA) Sensitivity

Conclusion

SISO testing is simpler and focuses on single path signal performance, while MIMO testing is more complex, requiring multi antenna validation to ensure enhanced throughput, signal integrity, and interference mitigation. With the increasing adoption of MIMO in 5G, Wi-Fi, and modern RF systems, advanced test methods such as OTA and channel emulation are essential to ensure real world performance.