dividual user throughput; right: sum throughput. channel conditions. TDMA shows a lower throughput because frequency resources are exclusively assigned to one UE per OFDM symbol. For instance, if a UE has a zero-supportable rate in a time slot, TDMA still allocates the spectrum to the UE. This makes inefficient use of the spectrum. In cases of OFDMA schedul- ing, resources are assigned opportunistically to users. This shows that a lot can be gained from multiuser diversity in LTE systems. 11.4.4.1 XPD Antennas for Multiuser Scheduling To understand the results of Figure 11.33 more clearly, we decouple the gains of multiuser scheduling and XPD antennas in Figure 11.34. We see that the XPD antennas increase the probability of dual-stream selection from 76 to 92% with TDMA scheduling. With OFDMA scheduling, this probability of selection increases to 98%. Further, the probability of loading 64 QAM on the dual streams also improves markedly from 24 to 70% with XPD antennas. 11.4.4.2 Impact of More Users on System Performance It is known that the MIMO channel-hardening effect reduces multiuser scheduling gains. This leads to two questions: Are XPD antennas in fact beneficial if the number of users in the system is large? How large can the number of users be before this effect occurs? © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology DS QPSK Co-polarized Cross-polarized DS 16-QAM 2% 8% DS 64-QAM No allocation Scheduler Input: Multi-User <1% 8% 2% 2% Scheduler Output: Polarization gain Figure 11.34 Gains of XPD antennas and multiuser scheduling gains. For this further analysis, off-line simulations were conducted by increasing the number of users scheduled per OFDM time slot. The number of users is increased from 2 to 10. The users are taken to be distributed along the measurement track. The CDFs for the sum rates for two users are shown in Figure 11.35. The two users are equidistantly placed on the measurement track and move along the same PFS, co-pol., 2 UEs TDMA, XPD, 2 