system is that if RSRP difference between UE branches is above 6 dB noticeable dual-stream throughput degradation results. With practical integrated UE antennas power imbalance is difficult to control since the UE antenna patterns are not identical. In drive test measurements, external antennas with identical patterns may be used to reduce degradation and increase repeatability of results. On the other hand, as seen in Figure 12.43, using external LTE Key Performance Indicator Optimization 800 MHz vs 2.6 GHz condition number 800 MHz vs 2.6 GHz RSRP 800 MHz 800 MHz 2600 MHz 2600 MHz Condition number (dB) RSRP (dBm) Figure 12.44 Drive test measurement example of condition number of 800 MHz versus 2.6 GHz when both frequency layers are co-sited antennas with the same polarization can also result in higher receive correlation reducing the net throughput gain. Maximizing antenna spacing may alleviate this problem. In indoor installations, if the eNodeB antenna branches are different physical antennas their patterns should be directed to serve the same coverage area, otherwise unnecessary power imbalance will occur, again reducing the multi-stream throughput. For dual-stream MIMO, using a single cross-polarized indoor antenna has been found to be a good field-tested solution. Practical Throughput Gain of Some 3GPP Transmission Modes 3GPP has standardized several single-user spatial multiplexing schemes. The basic schemes include open-loop spatial multiplexing (3GPP TM3), closed-loop spatial multiplexing (3GPP TM4) and hybrid of beam forming and dual-stream spatial multiplexing (3GPP TM8). Addi- tionally, advanced transmission modes with up to eight streams have been standardized in latest releases. Figure 12.45 shows a field measurement example of throughput gain for basic 2x2 and 4x2 3GPP transmission schemes with stationary UE. For 2x2 schemes, there is not much difference between open- and closed-loop transmission schemes. Upgrading from two transmit antennas to four antennas produces clear link-level throughput gai