uling is tightly integrated with link adaptation and HARQ processes. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology User 1 Instantaneous SNR (dB) User 2 Resource allocation Figure 3.17 Example of channel-aware scheduling for two users. 3.3 Multiple Antennas 3.3.1 Introduction The use of multiple antennas at the transmitter or the receiver side in combination with signal processing may significantly improve the system performance. Multiple antenna technologies are necessary to achieve the initial LTE requirements in terms of coverage, QoS, and targeted data rates (3GPP 25.913). Some of the LTE key tar- gets, such as the number of supported users per cell or user peak data rates, cannot be fulfilled without considering multiple antenna schemes. Thus, rather than being a later addition to the standard, multiple antenna techniques are part of the LTE standardization. The radio link is typically affected by the well-known multipath phenomenon (Goldsmith 2005), which produces constructive and destructive interferences (or sig- nal fading) at the receiver. By having multiple antennas at the transmitter or the receiver side, multiple radio "channels" or paths are established (i.e., one radio path between each transmit and receive antenna). Different paths will ideally experience uncorrelated fading whether the distance between antenna elements is relative* or by applying different polarization directions to the transmit antennas. These different Distance between antenna elements to achieve uncorrelated fading depends on both the carrier frequency of the radio communication and the geometry of the deployment scenario. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer paths may be exploited in different ways in order to obtain spatial diversity from the ideally uncorrelated fading or to transmit multiple streams simultaneously. The basic principle of multiantenna diversity is to provide the receiver with multiple copies of the transmitted signal in order