Comparing SISO and MIMO Radios¶
SISO (Single Input Single Output) radios have a single transceiver-antenna chain, while MIMO (Multiple Input Multiple Output) devices have multiple chains coordinated through the Digital Signal Processor (DSP). The MIMO devices supported by AREDN® have dual chains for both transmit and receive, and they support dual data streams [2x2:2].
Both SISO and MIMO radios use OFDM (Orthogonal Frequency Division Multiplexing), which inherently handles poor RF conditions such as multipath interference or fading. The rate selection algorithm in the wireless driver adapts to changing RF conditions so that the optimal MCS rate is always used. The selected MCS includes the appropriate modulation, forward error correction, and number of data streams.
By design SISO radios transmit all of their RF power on a single polarization. While it may seem like an advantage to have full power concentrated on a single polarization, there are specific limitations to SISO devices. A single chain device can only transmit one data stream at a time, and SISO devices do not have the ability to process and enhance multiple signals received simultaneously.
SISO radios are also limited in the data throughput they can achieve on their single chain. For example, a SISO device is limited to the 802.11n MCS7 protocol rate of 32.5 Mbps with Long Guard Interval (LGI), while a MIMO device using MCS15 can achieve up to 65 Mbps. In this regard SISO is at a definite disadvantage since it lacks sophisticated signal combining and the multiple simultaneous data streams that are possible with MIMO.
One of the advantages of MIMO radios is their ability to exploit multipath signals, achieving a better Signal to Noise Ratio (SNR) by combining multiple received transmissions. This is accomplished using 802.11n technologies such as Polarization Diversity and Maximal Ratio Combining.
On MIMO devices the total transmit power is split between its two polarizations, which means that MIMO radio signals have lower EIRP per polarization. It is possible that SISO radios on both ends of a link could have SNR values that match those of MIMO radios using 802.11n MCS0 to MCS7 on that same link. However, a MIMO device using MCS0 to MCS7 will transmit its data stream on both chains simultaneously, providing a distinct advantage on the receiving end where the MIMO radio uses MRC to enhance the signal. MRC is used when multiple antennas receive the same data stream, which applies only for MCS0 to MCS7. With MCS8 to MCS15 Spatial Multiplexing achieves multiple simultaneous data streams.
Given the same channel width and link characteristics, MIMO tends to out-perform SISO in both reliability and throughput. A good test to verify this would be to compare the performance of SISO vs. MIMO radios between the same endpoints. MIMO radios can attain double the throughput because they are capable of using twice the MCS rate. In the final analysis, the technology limitations of SISO will not allow it to match the throughput levels that are possible with MIMO.
SISO - MIMO Combinations¶
Today’s mesh networks are likely to contain a mixture of single and multiple chain devices, so it is important to understand how different combinations of devices might perform.
- SISO to SISO
- All transmit power is sent using a single polarization, but multipath signal combining does not occur. Only one data stream at a time can be sent at a rate that is limited by the protocol.
- SISO to MIMO
- All transmit power is sent using a single polarization, and the MIMO receiver will enhance reception by combining multipath signals using MRC. Only one data stream at a time can be sent at a rate that is limited by the protocol.
- MIMO to SISO
- The total transmit power is shared between MIMO chains, so the RF energy which is 90 degrees off-polarization from the receiving antenna may be lost. The SISO receiver cannot enhance multipath signals using MRC. Only one data stream at a time can be sent at a rate that is limited by the protocol.
- MIMO to MIMO
- The total output power is shared between MIMO chains, but the full power from both polarizations can be processed by the receiver so that nothing is lost. The MIMO receiver can enhance reception by combining multipath signals using MRC. Simultaneous data streams can be sent using spatial multiplexing, effectively doubling data throughput.
- Whenever possible try not to mix device types on radio links. As a general rule, use MIMO-to-MIMO for most types of radio links.
- If you have a marginal SISO-to-SISO link and you must replace one of the radios, either install another SISO radio or replace both ends with MIMO devices. A marginal but usable link between SISO devices may become unusable if one is replaced with a MIMO device.
Additional information on the operation of SISO and MIMO radios can be found in references such as this: MIMO for Dummies.