System Level Modeling and Optimization of the LTE Downlink

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4. Extensions to the L2S Model4.1.3. Application to LTEThe LTE channel coding procedures (see Section 2.2.3) employ a turbo code of rater c = 1/3, followed by rate matching, which adjusts the output ECR (r eff ) to that ofany of the defined MCSs (between 0.08 and 0.93, as listed in Table 2.5). The channelcoding procedures for each of the (re)transmitted TBs is depicted in Figure 4.4.data bitsChannel code:rateRate matchingcoded bitsFigure 4.3: LTE rate matching procedure. The output of a turbo code of rate r c = 1/3 israte-matched in order to obtain the coded bits with the target rate r eff . Theretransmission index m, parametrizes exact bit subset of G.To allow for an ACMI representation of the combined TB, the HARQ-combined TBis modeled as resulting from the combination of an inner code of rate r m and anouter repetition code of rate 1/Nrep, m where r m is in the range 1/3 ≤ r m ≤ 1 and theretransmission index m in the range m = {0, 1, 2, 3}. In this model, the output fromthe inner code represent the IR bits, while the outer repetition code represents theCC bits. The process is depicted in Figure 4.4.data bitsChannel code:Repetition code:coded bitsFigure 4.4: The combined TB is modeled as a combination of an inner channel code with arate r m between 1/3 and 1 (IR bits) and an outer repetition code of rate N m rep(CC bits).If a capacity-approaching channel code with suitably long block length is used, itis well known that the BLER can be approximated by the MI outage probability[25, 112, 113]. In the case of a system with HARQ, equivalent expressions canbe derived by using ACMI. Under this assumption, the outage probability ε is theprobability P that I ∗ < D. Thus, for the case where the SNR γ is constant over theretransmissions, we obtain:[GIRε = Pn · I (n Nmrep · γ ) ]< D . (4.4)In order to extend the presented model for application to OFDM, MIESM is appliedto compress the SINR vector γ into an AWGN-equivalent effective SINR value γ eff ,which can then be plugged into Equation (4.4).54
4. Extensions to the L2S ModelTo accomplish this, the subcarrier SINR vectors γ 0,...,M of each (re)transmission arestacked into a vector γ of length (M + 1) · N SCsγ = vec (γ 0 , γ 1 , . . . γ M ) , (4.5)which is then compressed into an effective SNR value γ eff by means of MIESM:γ eff (γ) = I −1n()1 ∑I n (γ(M + 1) N i ) , (4.6)SCswhere, N SCs is the total number of subcarriers. Adapting Equation (4.4), the outageprobability ε can be calculated as:⎡⎢ε = P ⎣ G IRn· I (n Nmrep · γ eff (γ) )} {{ }γ AWGNi⎤⎥< D⎦ , (4.7)where γ AWGN is denoted as the AWGN-equivalent SINR of the combined TB includingthe repetition gain.In order to consider the non-ideal behaviour of the channel coding and the loss inperformance due to the rate matching process, AWGN BLER curves are employedinstead of the outage probability. Thus, ε is approximated as:ε ≈ BLER AWGN (r m , n, γ AWGN ) . (4.8)In LTE, the values for r m cannot simply be obtained from the final code rate appliedby the rate matching [45]. However, by using the implementation of the rate matcherin [98], the equivalent puncturing matrices applied to the mother code of rate r c =1/3 can be extracted and employed to obtain the outer turbo coding rate r m andthe inner repetition coding rate 1/Nrep m for each of the HARQ retransmission indexand MCS value pairs.For each MCS and retransmission index m, the obtained effective turbo code rates(r m ) and repetition rates (Nrep) m are shown in Figure 4.5. The r m code rates requiredfor each of the modulations defined for the LTE data channel are listed in Table 4.1.Model accuracy is evaluated by means of link level simulations with the Vienna LTEsimulator [98] for both AWGN and time-correlated ITU Pedestrian-B channels [114–116].For each of the 15 LTE MCSs, the BLER curves from the simulation and from theproposed model are compared at the 10% BLER point, which is known to lead tonear-optimal performance [113] and is thus also the target BLER for link adaptation.55
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DissertationSystem Level Modeling a
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I hereby certify that the work repo
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KurzfassungDie vorliegende Arbeit p
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ContentsContents1 Motivation and Sc
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Contentsix
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1. Motivation and Scope of Work1. M
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Page 23 and 24: BibliographyThe combined throughput
Page 25 and 26: 2. 3GPP Long Term Evolution2. 3GPP
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System Level Modeling and Optimization of the LTE Downlink

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