 to combat instantaneous fading on the radio channel. Spatial diversity may be achieved by having multiple anten- nas at either the transmitter or the receiver side. A large group of so-called receive diversity schemes (Hutter et al. 2000, 707) takes advantage of the multiple receive antennas in order to improve the effective SNR. However, receive diversity is out of the scope of this chapter and we will focus on transmit diversity schemes, where multiple antennas are applied at the transmitter (Kaiser 2000, 1824; Goldsmith 2005). In general, multiantenna diversity plays a significant role in extending cover- age and enhancing link robustness. Additionally, multiple antennas may be employed to shape the overall antenna radiation pattern in a way that maximizes the transmitter or receiver beam in a desired direction (e.g., target receiver or transmitter). These schemes are referred to as classical beam forming or smart antenna systems and may be used to increase the received signal strength or to suppress dominant interfering signals. Therefore, the use of beam-forming techniques improves the SINR (signal-to-interference-plus- noise ratio), which results in coverage and, eventually, a capacity enhancement. In both transmit diversity and beam forming, one data stream is transmitted through multiple antennas in order to improve the SNR or SINR conditions at the receiver. However, data rates are mainly bandwidth limited when SNR reaches a certain level. In these cases, spatial multiplexing (SM) techniques allow transmit- ting multiple data streams simultaneously by having multiple antennas at both the transmitter and the receiver sides. Spatial multiplexing schemes, sometimes referred to as MIMO* (multiple input, multiple output), result in a significant increase of spectral efficiency and data rates. The different types of multiantenna schemes and their corresponding achiev- able gains are summarized in Table 3.4. Also, it is possible to distinguish between those configurations that do not need knowledge of the chan