ber of OFDM subcarriers (ranging from 128 to 2,048) is determined by the transmission band- width, whereas the number of OFDM symbols per slot (seven or six) depends on the cyclic prefix length (normal or extended). Hereinafter, we will assume a radio frame type 1 with normal cyclic prefix (i.e., seven OFDM symbols per slot). In cases of multiple antenna schemes, a different resource grid is used for each transmit antenna. The minimum resource unit allocable by the scheduler to a user is delimited by a physical resource block (PRB). Each PRB corresponds to 12 Frame (10 ms) Subframe #0 ms) Subframe #1 Subframe #9 (0.5 ms) Figure 3.1 Radio frame structure type 1. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Frequency Resource element (RE) Physical resource block (PRB) Figure 3.2 Subframe structure. consecutive subcarriers for one slot (i.e., a PRB contains a total of 7 x 12 resource elements (REs)* in the time-frequency domain, as shown in Figure 3.2). 3.1.2 Reference Signals In LTE downlink, special reference signals are used to facilitate downlink channel estimation procedures. In the time domain, reference signals are transmitted dur- ing the first and third last OFDM symbols of each slot. In the frequency domain, reference signals are spread over every six subcarriers. Therefore, an efficient chan- nel estimation procedure may apply a two-dimensional time-frequency interpola- tion to provide an accurate estimation of the channel frequency response within the slot time interval. When a downlink multiantenna scheme is applied, one reference signal is trans- mitted from each antenna in such a way that the mobile terminal is able to estimate the channel quality corresponding to each path. In this case, reference signals cor- responding to each antenna are transmitted on different subcarriers SO that they do not interfere with each other. In addition, resource elements used for transmitting reference signals on a specific antenna are not reused on other antennas f