Transmit diversity schemes rely on the use of multiple antennas at the transmitter side in order to obtain spatial diversity when transmitting one data stream. The basic principle is to provide the receiver with multiple copies of the transmitted signal as a means to combat instantaneous fading, resulting in a significant gain in instantaneous SNR. Therefore, transmit diversity is commonly applied when users experience bad channel conditions, with the objective of extending coverage (sup- port for larger cells) or enhancing QoS issues. In order to provide spatial diversity, it is necessary to have low mutual correlation between the channels associated with the different transmit antennas (i.e., relatively large antenna spacing). Under this assumption, both open-loop and closed-loop schemes are used to achieve spatial diversity in LTE. Open-loop schemes aim to exploit spatial diversity blindly by means of transmitting different versions of the signal through different transmit antennas. On the other hand, closed-loop schemes employ the information about the downlink channel at the transmitter to apply ade- quate antenna weights (i.e., phase and amplitude of the signals transmitted through different antennas are properly adjusted). In this case, multiple transmit antennas provide beam forming in addition to diversity. Hereinafter, the term beam forming* will be used to refer to closed-loop transmit diversity schemes. The difference should be noted between beam forming, with low mutual antenna correlation, and classical beam forming or smart antennas, where diversity cannot be provided due to the high mutual antenna correlation. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology In LTE, open-loop transmit diversity is achieved by means of cyclic delay diver- sity and space frequency block coding. The next two subsections provide more details about these diversity techniques. Additionally, closed-loop transmit diversity schemes, such as beam forming or transmit antenna s