Performance Analysis of Long Term Evolution (LTE) Physical Layer by Hamid Mousavi & Iraj S Amiri

Author:Hamid Mousavi & Iraj S Amiri [Mousavi, Hamid]
Language: eng
Format: azw3
Publisher: UNKNOWN
Published: 2018-12-17T16:00:00+00:00

Figure 3.4 Physical channel processing (3GPP, 2011d) The signal processing applied to LTE physical channels in downlink direction is accomplished by going through the steps depicted in Figure 3.4. First of all, the coded bits in every codeword are scrambled and conveyed to physical channels. Scrambling spreads the redundant data evenly among the data to protect the data from the frequency selective fading in the channel, and increases the signal robustness. Then in order to create complex modulated symbols, the scrambled bits are modulated. The next step is to map these complex-valued modulated symbols into one or several transferring layers. Once the layer mapping is done, the process of precoding of modulated symbols on each layer will be started and the output will be conveyed to the antenna ports. Precoding is the process of rearranging data stream with the aim of transmitting multiple streams simultaneously in MIMO systems. This scheme is used in LTE to support transmit diversity and spatial multiplexing. It divides the transport blocks into a number of subcarriers and then conveys each subcarrier complex stream to each transmit antenna. The precoded symbols for each antenna port are then mapped into resource elements, and eventually, generation of time-domain OFDM signal for each antenna port takes place.

At the receiver however, in order to detect the transmitted signal and discover the original data, a reverse procedure of transmitter is performed. In addition, the receiver has to estimate the channel conditions and the instant SNR, send the CQI feedbacks to the transmitter and at the same time generate the soft bit information, which is the input for the turbo decoder. The receiver is also required to carry out the HARQ procedure, and in case of any transmission error, it has to send requests for up to 3 retransmissions of redundancy version to the transmitter.

3.4 Channel Coding and Rate Matching

3.4.1 Channel Coding The channel coding used in LTE physical layer Transport Channels is turbo coding, whose performance is close to the Shannon limit. The turbo encoder used in LTE consists of two convolutional concatenated encoders that are connected to each other by an interleaver. The block diagram of LTE turbo coder is shown in Figure 3.5, and the code rate of this turbo coder is 1/3 and two generated polynomials are G0= [1011] and G1= [1101].

Figure 3.5 Block diagram of LTE turbo coder (3GPP, 2011d)

3.4.2 Interleaving From two candidates of interleaver for LTE, namely Quadratic Permutation Polynomial (QPP) and Almost Regular Permutation (ARP), QPP was selected to improve the maximum throughput of the system. QPP is able to achieve high data rates, while it requires less memory and provides more parallelism factors. Assuming that the block size information is indicated by K, the output index is shown as i (0 ≤ i ≤ K-1), and the input index is π (i). The QPP polynomial is defined as:

π (i) = (f1i + f2i2) mod K (3.1) Where f1and f2are coefficients of the interleaver depending on block size K. f1 is a prime number while f2 is a even number.

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