r is the main contributor to performance degradation for dual-stream transmission. This happens if the channel matrix is ill © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Figure 11.32 Downlink transmission scheduling for colocated terminals. conditioned. However, this drawback is overcome by single-stream transmit mode selection. Therefore, the observation in Figure 11.31 is that codebook-based pre- coding is close to optimal when performed in conjunction with transmit mode selection. 11.4.4 Multiuser Scheduling This section presents the results for OFDMA scheduling for two users. The throughput results are the real-time measured throughput from the measurement campaign. Two colocated user terminals are scheduled in the downlink, depicted in Figure 11.32. The implemented MAC architecture is the same as that described in the previous section. The two terminals move along the same measurement track, shown previously in Figure 11.25. The two users are scheduled according to the proportional fair algorithm. Results are compared to the OFDM-TDMA (time division multiple access) scheduler, for which the sum throughput is given by (11.3) where uk is the supportable throughput for the kth user at time instance t. The plots of the CDFs of individual user rates and sum rates are shown in Figure 11.33. It is seen that, on median, XPD antennas give a sum rate gain of approximately 16.5 Mb/s with OFDMA proportional fair scheduling (PFS). In the same plot, we also compare the performance of a round-robin TDMA scheduler. OFDMA scheduling appears to be a clearly better strategy. Surprisingly, multi- user scheduling gains are observed even when the users are colocated and in LOS © 2009 by Taylor & Francis Group, LLC Early Real-Time Experiments and Field Trial Measurements Individual User Rates, PFS Sum Rates Co-Polarized, TDMA XPD, TDMA Co-Polarized, PFS XPD, PFS Co-Polarized 20 40 60 80 100 120 140 160 Throughput (Mbps) Throughput (Mbps) Figure 11.33 System performance. Left: in