on in MIMO can be tackled as a combination of different single-input, single-output (SISO) estimations (Figure 3.35). Frequency CH #0 Antenna #0 Frequency CH #0 Antenna Antenna Antenna #3 Pilot subcarrier Null subcarrier Data subcarrier Figure 3.35 Channel estimation in DL-OFDMA from MIMO reference signal (4 x 4 case). C 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology 3.4.2.2 Channel Estimation in SC-FDMA UL In the case of LTE uplink, reference signals are transmitted with two different objec- tives. The first is to facilitate channel estimation at the eNodeB prior to demodula- tion (demodulation reference signal). The second, the sounding reference signal (SRS), helps to provide information of the uplink channel response in a range of frequencies greater than the allocated bandwidth for a particular UE. This allows the eNodeB to implement uplink frequency-dependent scheduling strategies. Both variants of the uplink reference signal are based on Zadoff-Chu sequences (3GPP 36.211). The demodulation reference signal is transmitted in the fourth SC-FDMA symbol of the slot (see Figure 3.5), and its size is equal to the uplink allocated band- width for that UE. Demodulation reference signal structure can be easily identified with a block type pilot arrangement scheme (see Figure 3.34), SO classical channel estimation mechanisms for this strategy may be used. Also, time interpolation is not necessary to perform because the allocated resources for the UE are likely to be frequency hopped by the eNodeB uplink scheduler. The SRS is transmitted within a bandwidth greater than that allocated for this UE. This way, the eNodeB can have out-of-band information of the channel frequency response for this individual user, which facilitates the implementation of channel-aware scheduling mechanisms in the uplink. The transmission of an SRS may interfere with the PUSCH of other users*; hence, a broadcast param- eter is defined to indicate the possible location of SRS subframes SO that 