SISO (Single Input Single Output): what it is, how it works and what it does

SISO (Single Input Single Output) designates a radio-frequency link architecture in which the transmitter has one transmit antenna and the receiver has one receive antenna. In standard wireless networking notation this is expressed as 1x1.

Operating principle

In a SISO link the signal is generated at a single transmitter, propagates through the radio channel, and is captured by a single receiver. There is no spatial parallelisation of data streams and no combining of multiple antennas to improve the signal-to-noise ratio. Link performance therefore depends on transmit power, receiver sensitivity, and channel conditions (attenuation, interference, fading).

Relationship to MIMO

SISO is the simplest case within the family of multi-antenna techniques. When a WiFi device incorporates multiple transmit and/or receive antennas to spatially multiplex several data streams, it is described as MIMO (Multiple Input Multiple Output). The presence of the label 1x1 SISO in a technical datasheet is equivalent to declaring that the device does not implement MIMO: the attribute wifi_mimo takes the value false.

1x1 notation

The prefix 1x1 indicates, respectively, the number of antennas on the transmit side and the number of antennas on the receive side. Other combinations (2x2, 2x3, 4x4, etc.) correspond to MIMO configurations with greater spatial capacity.

Applications and scope

• Basic or low-cost WiFi devices that prioritise simplicity and power consumption over data throughput.
• Short-range point-to-point links where channel attenuation is predictable and spatial diversity is not required.
• Embedded or IoT components where physical space or design budget limits the antenna count to one.

Limitations

• No spatial gain: antenna diversity is not exploited to combat multipath fading or to increase data rate through spatial multiplexing.
• Channel sensitivity: a single propagation path makes the link more vulnerable to constructive or destructive interference.
• Lower maximum capacity: compared with an equivalent MIMO link, the theoretical data rate and robustness in multi-reflection environments are reduced.

Interpretation in product specifications

When a technical specification for an adapter, router, or WiFi module states 1x1 SISO, the buyer should understand that the device operates with a single antenna in each direction. This does not imply a defect but a design choice: the manufacturer has opted for the simplest configuration, generally associated with lower manufacturing cost and lower power consumption. The absence of MIMO is explicitly confirmed when the wifi_mimo attribute is set to false.

In summary, 1x1 SISO is the standard designation for a single-antenna WiFi link without spatial multiplexing, and it represents the baseline of the capacity scale that culminates in higher-order MIMO configurations.