Abstract
Link adaptation (LA) process is a core feature for the downlink of 3GPP long-term evolution (LTE) and LTE-advanced (LTE-A). Through a channel quality indicator (CQI), the receiver suggests to the base station (BS) an appropriate modulation and coding scheme (MCS) according to the current channel conditions. In order to overcome any non-ideality in this process, the outer loop link adaptation (OLLA) algorithm is used to adaptively modify the mapping from signal-to-noise ratio (SNR) to CQI. OLLA basically modifies the measured SNR by an offset, according to whether data packets are received correctly or not, in order to adjust the average block error rate (aBLER) to a target. Although the OLLA technique has been extensively used, there exists a lack of analysis in the literature about its dynamics and convergence conditions. In this paper, a deep analysis of this algorithm has been carried out in order to cover this gap. From this analysis, we propose a new approach to the OLLA, the enhanced OLLA (eOLLA), which is able to adaptively modify its step size as well as to update its offset according to the reception conditions even if no data packets have been received. Thus, for LTE- and LTE-A-realistic scenarios, simulation results show that the proposed eOLLA outperforms the traditional OLLA, achieving a performance gain of up to a 15 % in terms of throughput.
Highlights
The adaptive modulation and coding (AMC) process carried out in the link adaptation (LA) is a crucial part of current wireless communication systems
3.1 Proposed enhanced OLLA (eOLLA) we propose a modification of the traditional OLLA algorithm, the enhanced outer loop link adaptation, which is able to adapt the size of the steps according to the convergence status of the system
Notice that the same results are achieved by the eOLLA independently of the traffic load, since the eOLLA only needs the estimated instantaneous BLER (iBLER) to update the offset
Summary
The adaptive modulation and coding (AMC) process carried out in the link adaptation (LA) is a crucial part of current wireless communication systems This technique allows to increase the data rate that can be reliably transmitted [1] and has been adopted as a core feature in cellular standards such as long-term evolution (LTE) and LTE-advanced (LTE-A) [2]. OLLA modifies the SNR thresholds by an offset [4, 5] which can be positive (making the MCS selection more robust) or negative (when the CQI selection was too strict) This offset is continuously updated based on the reliability of the received data blocks so that the average BLER is kept as close as possible to a predefined target
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