nel at the transmit- ter-namely, open-loop schemes-and those needing information about the chan- nel, also called closed-loop schemes. Open-loop-based configurations have lower complexity and signaling overhead than closed-loop solutions, as well as lower per- formance. On the other hand, closed-loop-based schemes take advantage of the channel information at the transmitter by applying some signal processing prior to transmission in order to optimize data reception (e.g., weighting the transmitted signals by considering the channel conditions experienced at each antenna). As already mentioned in Section 3.1, LTE radio resources are organized into the two-dimensional time-frequency grid provided by OFDM. When transmitting through multiple antennas, the spatial dimension is added to the grid, as shown The term MIMO is also used in general to refer to any transmission scheme with multiple antennas at both the transmitter and the receiver sides. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Table 3.4 Different Multiantenna Schemes and Corresponding Benefits Tx Data Streams Multiantenna Scheme Benefits Link robustness Transmit diversity Diversity gain Coverage Coverage Classical beam forming Power or antenna gain Capacity Spectral efficiency Multiple Spatial multiplexing Capacity gain Data rates in Figure 3.18. The aforementioned multiantenna schemes can be applied inde- pendently to each OFDM subcarrier, which individually experiences a flat fading or non-time-dispersive channel. In LTE, multiantenna signal processing is applied independently to each frequency sub-band or group of consecutive OFDM subcar- riers, thus making it possible to have different transmission schemes for different frequency scheduled users. A multiantenna configuration is determined by the multiantenna scheme to be applied (e.g., spatial multiplexing) and other parameters such as the number of transmit and receive antennas. Different configurations may be applied for different Frequency Frequ