ing frequency stability of 1 ppm results in a frequency error of 2.6 kHz, which is 17% of the SC spacing. In early 2007, our group demonstrated feasibility of tight time and frequency synchronization by an SDMA demo on our LTE flex- ible HW platform. Two UEs were synchronized to one eNB over the air. For simultaneous transmission in UL, both UEs estimated the frequency offset of their LOs and prerotated their Tx signals by the opposite frequency offset, which has been called frequency advance and is easily possible using numerically controlled oscillators (NCOs) in the baseband unit. Residual phase offsets between both receive signals were estimated and corrected independently from each other at the NB [19] by a specific channel estima- tion algorithm. The demodulated QAM constellations for a two UE UL SDMA experiment can be found in Figure 7.14. With the topic of channel estimation, a much more severe and far-reaching issue is addressed compared to the topics discussed earlier. The channel matrix H consists of ixj channel elements, [H],==hii Each hij represents a radio channel from one Tx- to one Rx (receive)-AE. Due to multipath reflections and moving © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Figure 7.14 Separated user constellations (left) and channel coefficients from the two UEs to the two NB antennas after proper frequency synchronization. mobile users, H is time varying and frequency selective so, in principle, each SC k (k = 1 ..NK) and each symbol l (l = 1 N1) has another channel matrix Hk, leading to a very high dimension of N X N X NK X N complex chan- nel coefficients. Ideally, these would have to be estimated with high accuracy. Luckily, the real dimension D of the radio channel-sometimes also called subspace dimension [29]-is much smaller than the full dimension. Assuming a 2x2 MIMO system with NK = 128 SC-corresponding to a channel band- width (BW) of 1.25 MHz-and N L = 100 symbols, the channel dimen- sion D has been analyzed for an urban macr