Understanding LTE Model Internals and Interfaces

1581 Understanding LTE Model Internals and InterfacesPhysical Layer Packet Combining ofHARQ• All packets tagged as “valid” are forwarded to the MAC whether they are decodedcorrectly by the PHY or not• HARQ module is implemented in MAC for its extensive MAC functionality, although its physicallayer component is responsible for combining the packets• Type II incremental redundancy simulated• Logic: 2 types of gains: SNR gain – can be simply found by adding effective SNRs ofsuccessive packets. Coding gain – simulated as SNR gain by adding the SNR of the “extrabits” stuffed into the MPDU• Example: MPDU of size 128, corresponding burst has 4 allocation blocks. The maximumbit carrying capacity of the burst = 192. Thus 64 “extra bits” can be carried within the burst,which can provide an extra gain at the receiver• Packet receiving and processing functions:• lte_enb_as_mpdu_decode_with_harq() : At lte_enb_as.pr.m• lte_ue_as_mpdu_decode_with_harq(): At lte_ue_as.pr.m• Efficiency support:• Possible to characterize PHY by drop probability parameters• If first transmission, drop probability = p (configurable attribute)• If n th retransmission, drop probability = p *1/n k• k is a configurable parameter – idea is that the drop probability reduces exponentially witheach retransmission attemptCONFIDENTIAL ─ RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. © 2010 OPNET Technologies, Inc.74