## 3 LTE DL Physical Layer  
The LTE system operates using 10 ms radio frames, with each frame containing 10 subframes of 1 ms duration [20]. Each subframe is divided into two equal-length slots, with each slot comprising seven OFDM symbols, labeled symbol 0 to symbol 6, when a normal CP is used. The time-frequency representation of a DL subframe is depicted in Figure 2, where each small cell represents one subcarrier in one OFDM symbol period. Pilots, indicated by green cells, are inserted in the first and third last OFDM symbols of each slot with a frequency domain spacing of six subcarriers. There is no power boosting in pilot subcarriers. The basic unit for resource allocation is the physical resource block (PRB), which consists of 12 consecutive subcarriers in one slot. This paper considers an LTE system with a 10 MHz bandwidth/50 PRBs, comprising 600 subcarriers (excluding DC).  
In DL transmission, data are carried in the PDSCH in units of transport blocks. Each transport block is first segmented into code blocks if its size exceeds 6144 bits. Each code block is then encoded with a rate-1/3 symmetric turbo code, which is the parallel concatenation of two identical 8-state (1, 15/13) constituent codes. Finally, the coded sequence of each code block is processed by a rate-matching module, which adjusts the total number of coded bits in a transport block to match the number of bits supported by the assigned PRBs. In the following simulation experiments, a subframe, comprising 50 PRBs/600 subcarriers in both slots, is assigned to one transport block of length 1408 bits (transmitted using one codeword). Note that the resources available to the PDSCH do not include the pilots and the first three OFDM symbols in each subframe, as shown in Figure 2, since these symbols are occupied by the control channel. Finally, the coded bits are modulated using the QPSK scheme and then transmitted using OFDM with system parameters listed in Table 2. The resulting code rate is 0.11175, which is close to the lowest rate supported by the LTE standard.  
To conduct the experiments, we use MATLAB to implement both the transmitter and the receiver. A simple receiver diagram is shown in Figure 3. The incoming OFDM signal contains a cyclic prefix (CP), which needs to be removed. Then, a fast Fourier transform (FFT) is performed to obtain the subcarriers. To mitigate the impact of the wireless channel, we use perfect channel estimation and compensation in the experiments. This is followed by a resource element demapper, which constructs resource blocks. Next, a soft demapper is used. In the experiments, QPSK modulation is employed, making the demapping process straightforward. The subsequent step is derate matching, which includes debit selection and pruning, bit separation, and subblock deinterleaving. The final component is a turbo decoder, used for error correction decoding.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Channel bandwidth & 10 MHz \\ \hline Carrier frequency & 1.8 GHz \\ \hline Subcarrier spacing & 15 KHz \\ \hline Sampling frequency & 30.72 M \\ \hline FFT size & 2048 \\ \hline CP duration (NORMAL CP) & 160 samples for symbol 0 \\  & 144 samples for symbols 1–6 \\ \hline OFDM symbol duration & 66.6 \(\upmu\)s \\ \hline TX/RX antenna & SISO \\ \hline Modulation scheme for PDSCH & QPSK \\ \hline Code rate & 0.11175 \\ \hline \hline \end{tabular}
\end{table}
Table 2: System parameters.  
Figure 2: Time-frequency representation of a DL subframe.