 corresponding to the selected group. The codebook of PU2RC is considered as a unitary matrix. One example is the Fourier basis-that is, Consider the case of two transmit antennas (M = 2) and two possible groups (G = 2). Here we will have the following precoder sets: C 2009 by Taylor & Francis Group, LLC MIMO OFDM Schemes for 3GPP LTE Consider now four transmit antennas (M = 4) and still only two groups (G=2). = Here we will have the following precoder sets: ***** j21/4 Throughput performance is carried out to verify the advantage of the PU2RC. Figure 5.6 compares performance results of the SU-MIMO, MU-MIMO, and com- bined schemes in a 4x4 MIMO system. For the sake of simplicity and because the channel is i.i.d. (independent and identically distributed) fading, no precoding is included in this simulation. As can be noted from the figure, the SU-MIMO scheme has a very large gain over the MU-MIMO scheme when there are few users in a cell; this is especially true when the signal-to-noise ratio (SNR) is high. The MU-MIMO scheme achieves multiuser diversity gain (MUDG) on a per-code-word basis, which accounts for the big performance gap over the SU-MIMO scheme. The combined scheme simply follows the envelope of the other two schemes. Here, large gains are achieved with the combined scheme; note how the crossover point between SU- and MU-MIMO changes with different SNRs. Figure 5.7 shows the results of the 2 x 2 MIMO case where we have correlated fading and precoding. Here, the MU-MIMO has more gain over the SU-MIMO because the combined effect of precoding and a correlated channel means that there is a big difference between the two stream qualities. Namely, the SU-MIMO UE derives very little benefit from having a second stream because the CQI on that stream is very low. The MU-MIMO, on the other hand, gains from this fact because the second stream interferes very little with the first stream, and it still gets the full MUDG. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technol