Alamouti

The Alamouti scheme is a space-time block code for wireless communications that uses two transmit antennas to achieve transmit diversity. It was proposed by Siavash Alamouti in 1998 and published in the IEEE Journal on Selected Areas in Communications. The design is the first practical orthogonal space-time block code and remains foundational in multiple-input multiple-output (MIMO) communications.

Operation: Over two consecutive time slots, two symbols s1 and s2 are transmitted according to the code: time slot 1: antenna 1 transmits s1, antenna 2 transmits s2; time slot 2: antenna 1 transmits −conj(s2), antenna 2 transmits conj(s1). The resulting 2×2 code matrix is orthogonal, allowing the receiver to separate s1 and s2 with simple linear processing. The scheme achieves a rate of one symbol per channel use with two transmit antennas and provides a diversity order of 2 under Rayleigh fading.

Extensions and impact: The orthogonal design extends to multiple receive antennas, yielding a total diversity gain of 2R for R receive antennas. The Alamouti code is robust to fading and works best when the channel remains quasi-static over the two time slots. In frequency-selective channels it is typically used with OFDM. The scheme has influenced the design of MIMO signaling in various wireless standards and products, including early 3G and Wi‑Fi implementations.

See also Space–time block coding, Orthogonal designs, MIMO, Diversity gain.