n especially at low to medium SNR LTE Small Cell Optimization 2x2 TM3 2x2 TM4 4x2 TM4 UE - measured SINR(dB) Figure 12.45 Example of field measurement of 2Tx and 4Tx downlink throughput, stationary UE in different SINR conditions. TD-LTE 10 MHz, frame configuration 1 regime. In the example measurement, the total transmit power with four transmit antennas is 3 dB higher SO the result includes both power and diversity gain on the PDSCH channel. 2 The SINR shown in the horizontal axis is the value measured by the UE. As shown later in this section, even more important benefit from four eNodeB antennas is the possibility to employ four-way maximum ratio (MRC) or interference rejection (IRC) diversity combining in the uplink; indeed it is usually the uplink link budget that is the limiting factor in practical networks. At high-SNR regime the 4Tx transmission schemes actually have slightly reduced peak throughput since the reference signal (RS) overhead from 4Tx is about 5% higher. While the performance difference between open-loop and closed-loop transmission modes for two transmit antennas is fairly small, the gain of four eNodeB antennas over the two antenna eNodeB configuration has been demonstrated in a number of simulations and field measurements. The main reason is that for four transmit antennas the larger number (relative to two antennas) of precoding matrices in the closed-loop mode offers more granular antenna weighting which in turn results in higher gain than the open-loop transmit mode 3 at low UE speeds. This difference is even further pronounced at high antenna correlation (e.g. closely spaced eNodeB antennas and narrow angular spread) and low-SNR regime where at a given location the throughput of four antenna closed-loop system can be twice the throughput of a two transmit antenna system. On the other hand, large variation in the evaluated throughput gains has been noticed between different measurements, depending on the radio channel conditions, UE model capability, UE speeds and SO on. Performance of