Many wireless communication systems require the knowledge of the receiver signal-to-noise (and interference) ratio (SIR). SIR estimates are typically used in closed-loop power control, handoff, adaptive coding and modulation, MAP decoding for Turbo codes, etc.
Numerous methods exist for measuring the interference power associated with a received signal. One approach uses the total received power (which is typically received signal strength indication (RSSI)) at the automatic gain control (AGC) output of the wireless communications device.
Another approach uses re-modulation of a de-spread signal using either perfect knowledge of the transmitted sequence or tentative decisions in order to subtract signal components and measure non-orthogonal interference.
Still other approaches use estimations in which partial derivatives are set to zero to arrive at a maximum likelihood (ML) estimation, or estimates of the signal-to-noise variance are calculated, second and fourth order moments of the received signal are used to solve for S (signal power) and I (interference power), or a signal-to-noise variance ratio is determined using a moments method combined with usage of delayed received symbols.