RSSI: Received Signal Strength Indication Explained

This page explains RSSI (Received Signal Strength Indication) and how it’s calculated in WLAN and WiMAX OFDM-based systems.

The term used to measure the received signal power/strength of the input signal to an RF receiver is called RSSI (Received Signal Strength Indication). It’s measured at different points in a wireless receiver:

• Before the IF amplifier in dual conversion receivers.
• At the baseband stage in zero IF receivers, after the ADC using I/Q signals.
• Before the ADC (measured as a DC voltage).

At the Baseband Stage, it’s often measured using the following equation:

$RSSI = I^2 + Q^2$

The calculation of RSSI can vary based on the number of OFDM symbols, depending on the IEEE standard (e.g., 802.11 or 802.16). It is often averaged over one OFDM symbol as needed.

RSSI in WLAN Devices (IEEE 802.11)

In WLAN devices following IEEE 802.11 standards, RSSI represents the measurement of RF signal energy received by the 802.11 OFDM Physical layer.

RSSI is represented by 8 bits, providing 256 levels of granularity. It is measured by the Station (STA) during the reception of the PLCP preamble, which is transmitted by the Access Point (AP).

RSSI in WiMAX Devices (IEEE 802.16)

When RSSI measurement is needed by the Base Station (BS) in a WiMAX system conforming to IEEE 802.16, the BS will request the Subscriber Station (SS) to provide the measurement.

The SS, upon request, measures RSSI using the OFDM downlink preamble. The measured mean and standard deviation of RSSI are then reported back to the BS via a REP-RSP message.

RSSI is measured in dBm and typically ranges from -40dBm (represented as 0x53) to -123dBm (represented as 0x00).