Low complexity link abstraction for retransmission in LTE/LTE-advanced with IR-HARQ

Abstract

Recent wireless cellular systems, like 3GPP’s Long Term Evolution (LTE) and LTE-Advanced (LTE-A), benefit from many enhancements of wireless techniques. These include multicarrier modulation schemes, fast link adaptation, forward error correction with HARQ, powerful turbo codes and mutiple antenna techniques. System level simulators usually use link abstraction models for capturing the effects of these enhancements from link level simulators. This reduces both time and complexity of large system simulations. Also the link abstraction models are used in the mobile stations for an efficient feed- back of instantaneous channel quality indicator in real systems. Therefore, it is extremely important for a link abstraction model to not only have low complexity but also very good accuracy. However modeling the effects of incremental redundancy (IR)- HARQ in LTE-based systems is a challenging and interesting task. In LTE-based systems two types of gains can be achieved with IR-HARQ at the receiver. These are coding gain, which is achieved through the transmission of new parity bits in each of the retransmission, and an SNR gain, which is achieved due to repetition of coded bits in each of the retransmission. A good link abstraction model should be able to abstracts the effects IR- HARQ and other enhancements with as much low complexity as possible and reasonable accuracy. In this paper we propose a low complexity link abstraction which abstracts the performance of LTE-based link level simulators with multiple antennas, incremental redundancy (IR) HARQ and other enhancements. The complexity is reduced by the fact that our proposed scheme only uses three reference curves corresponding to the mother code rate for each type of QAM modulation. Then we show that how the reference curves for an arbitrary code rate can be generated by using these three reference curves and an offset. This offset is based on the effective code rate after combining the transmissions from different HARQ rounds. To strengthen the case for our proposed link abstraction model we show the results for LTE-based wireless system. The results show that our method provides reasonably accurate results with very low complexity and even no calibration of adjustment factors is performed. For all the simulations we used Eurecom’s OpenAirInterface platform which implements physical layer of LTE release 8 as per the standards of 3GPP.