Adaptive coding and modulation using imperfect CSI in cognitive BIC-OFDM systems

Jeroen Van Hecke,Vincenzo Lottici,Luc Vandendorpe,Riccardo Andreotti,Marc Moeneclaey,Paolo Del Fiorentino,Filippo Giannetti

doi:10.1186/s13638-016-0739-5

Abstract

This work investigates adaptive coding and modulation (ACM) algorithms under the realistic assumption that the available channel state information (CSI) at the transmitter is imperfect due to estimation errors and/or feedback delays. First, we introduce an optimal performance metric for the secondary user (SU) bit-interleaved coded orthogonal frequency division multiplexing (BIC-OFDM) system, called the expected goodput (EGP). By using an accurate modeling approximation, we succeed in deriving a tractable and very accurate approximation for the EGP. This approximate EGP (AEGP) is then used for the derivation of several ACM algorithms which optimize the code rate and bit and energy allocation under a constraint on the interference caused to the PU network. In the numerical results, we show that the AEGP is far more accurate than previous attempts to model the GP in the presence of imperfect CSI. Further, we verify that, in spite of the imperfect nature of the available CSI, the derived ACM algorithms significantly increase the goodput of the SU network, compared to a non-adaptive selection of the transmission parameters.

Highlights

To meet the demand of high data rates and the increasing amount of traffic, the current and generation of wireless networks need spectrally efficient solutions such as multicarrier orthogonal frequency division multiplexing (OFDM) transmission, efficient channel coding techniques in the form of bit interleaved coded modulation (BICM) [1], and adaptive coding and modulation (ACM) [2]
We compare the performance of uniform bit and energy allocation (UB+UE), with our algorithm introduced in Section 4.3 which combines greedy bit allocation with uniform energy allocation (GB+UE)
In order to tackle this problem, we introduced an optimum performance metric called the expected goodput (EGP), which is the expectation of the goodput, conditioned on the imperfect channel state information (CSI)

Summary

Introduction

To meet the demand of high data rates and the increasing amount of traffic, the current and generation of wireless networks need spectrally efficient solutions such as multicarrier orthogonal frequency division multiplexing (OFDM) transmission, efficient channel coding techniques in the form of bit interleaved coded modulation (BICM) [1], and adaptive coding and modulation (ACM) [2]. The key idea behind the proposed method relies on optimizing the long-term average GP of the SU link, averaged over the realizations of both the actual channel and the available CSI at the SU transmitter, under the constraints of the total transmitted energy and the level of interference on the PU receivers This can be achieved by optimizing the expected GP (EGP) metric.. We successfully combine the practical assumption of imperfect CSI with the accurate model of the effective SNR, which results in the AEGP metric This AEGP metric, which takes care of the imperfect CSI, is proposed as the objective function of an OP to search for the optimal combination of the ACM parameters under the above mentioned transmit energy and interference constraints. The notation (X)i,j refers to the element on the ith row and jth column of the matrix X, while (x)i denotes the ith component of the vector x

Cognitive BIC-OFDM system model

Optimized energy and uniform bit allocation

Accuracy of AEGP

Findings

Conclusions