System Level Modeling and Optimization of the LTE Downlink

2. 3GPP Long Term EvolutionThe channel coding procedures are depicted in Figure 2.7, and describe for eachcodeword, the encoding of N TB bits into a Transport Block (TB) of size G bits[45].The channel coding procedures implement error-detecting capabilities by means ofone or several 24-bit CRCs and error correction with a turbo code [47]. Since theturbo coder interleaver has a maximum size of Z = 6 144 bits, the N TB bits aresegmented into C Code Blocks (CBs) of up to Z bits, each with an additional CBCRC. Each CB is coded by means of a rate one-third turbo encoder [ with two ]8-state constituent encoders with generator polynomial G (D) =1+D+D1, 3,1+D 2 +D 3identical to the one used in W-CDMA [48]. Per-CB rate matching is then appliedto adapt the overall resulting bits to the TB size of G bits. The rate matching blockis also tasked with generating different redundancy versions of the CB bits neededfor HARQ retransmission operation [49, 50] (see Section 4.1 for a more detaileddescription of the HARQ-related procedures).2.2.4. Channel Adaptive FeedbackLTE implements AMC, as well as closed-loop MIMO in order to adapt the transmissionrate to the instantaneous channel conditions reported by the feedback. Dependingon the transmission mode, LTE requires the calculation of up to three differentfeedback values at the receiver, which are explained in the subsections below.2.2.4.1. Channel Quality Indicator FeedbackThe Channel Quality Indicator (CQI) signals on a per-codeword basis the highest ofthe 15 Modulation and Coding Schemes (MCSs) specified in Table 2.5 that ensures,given measured actual channel conditions, a BLER lower or equal to 10 % [51, 52].Table 2.5.: Modulation scheme, Effective Code Rate (ECR) of the channel encoder, and data(coded) bits per modulated symbol for each of the LTE-defined CQIs.CQI Modulation ECR bits/symb CQI Modulation ECR bits/symb0 out of range 8 16-QAM 0.48 1.911 4-QAM 0.08 0.15 9 16-QAM 0.60 2.412 4-QAM 0.12 0.23 10 64-QAM 0.46 2.733 4-QAM 0.19 0.38 11 64-QAM 0.55 3.324 4-QAM 0.30 0.60 12 64-QAM 0.65 3.905 4-QAM 0.44 0.88 13 64-QAM 0.75 4.526 4-QAM 0.59 1.18 14 64-QAM 0.85 5.127 16-QAM 0.37 1.48 15 64-QAM 0.93 5.55The CQIs specify code rates between 0.08 and 0.92, and employ 4-QAM, 16-QAM,or 64-QAM modulation alphabets. This is translated into an effective number ofdata bits per modulated symbol ranging from 0.15 to 5.55, as listed in Table 2.5. As19