System Level Modeling and Optimization of the LTE Downlink

4. Extensions to the L2S Model4. Extensions to the L2S ModelThis chapter details extensions to include Hybrid Automatic Repeat reQuest(HARQ) and channel estimation errors, both aiming at more realistic modelingcapabilities of LTE systems and extending the L2S model described in Chapter 3beyond its basic capabilities.These two extensions apply to different parts of the L2S model. The HARQ modelapplies to the link performance model, described in Section 3.1.2, while the channelestimation errors applies to the link quality model only, which is described inSection 3.1.1.4.1. Hybrid ARQHARQ is part of the LTE MAC layer and provides retransmission capabilities aimedat improving link reliability. It consists of the retransmission of erroneously-receivedTBs and a joint decoding of the received retransmissions. It is implemented in therate matching module, which also adjusts the rate matching target code rate, asmentioned in Section 2.2.3. The rate matching module is capable of, for a giventarget code rate, generating up to four versions of a TB. The different TB versions,indicated in the LTE standard by a redundancy version index, denoted as rv idx(rv 0 , . . . , rv 3 ), and when possible composed of a different subset of the original turboencodedbits, are combined at bit level and jointly decoded once received.4.1.1. LTE HARQAfter turbo encoding (see Section 2.2.3), the rate-1/3-encoded bits, consisting ofsystematic and parity bits, are placed in a circular buffer, from which the bits for51