A Cross Layer Adaptive Modulation And Coding Scheme For Realistic Ofdma Based Systems

2615 words - 10 pages

A Cross-Layer Adaptive Modulation and Coding Scheme for realistic OFDMA-based Systems

Abstract—In this letter, we investigate Adaptive Modulation and Coding schemes (AMC) in fast mobility environment with realistic conditions. We argue that feedback Channel State Information (CSI) are outdated and suffer estimation errors for most mobile channels especially fast users. Therefore, ignoring such conditions in AMC scheme design can be disastrous. We provide a brief discussion about these imperfections. Afterwards, we introduce a novel AMC scheme which can adapt to these conditions without much loss in throughput or losing control over Packet Error Rate (PER) by utilizing Automatic Repeated Request (ARQ) service to adjust Modulation and Coding Scheme (MCS) assignment.

Index Terms—Adaptive Modulation and Coding; Channel Estimation Errors; High Mobility environments; IEEE 802.16m; Mobile WiMAX; OFDMA systems.

INTRODUCTION
UNPRECEDENTED demand on wireless communications systems to access information sources and entertainment on the go has been a driving force behind plans to enhance currently deployed systems and implement completely new ones all over the world. Some of the recently implemented orthogonal frequency division multiple access (OFDMA) based systems are Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE) systems. IEEE 802.16 standard based WiMAX system is designed to provide wireless broadband access to stationary as well as mobile subscribers [1]. Many versions of IEEE 802.16 standard have been released to enhance systems capability and provide improvement in their performance. After the great success of IEEE 802.16e based Mobile WiMAX, a second version called WiMAX 2, based on recently approved IEEE 802.16m, was released and currently in deployment stage in many countries [2].
IEEE 802.16m standard works as an advanced air interface to meet the requirements of the International Telecommunication Union—Radiocommunication/International Mobile Telecommunications (ITUR/ IMT)-advanced for fourth-generation (4G) systems, as well as the next-generation mobile network operators [2]. One of the techniques included in these specifications is Adaptive Modulation and Coding (AMC). Basically, this technique can enhance overall system capacity by employing different modulation schemes of variant spectral efficiency to suit channel conditions for each transmission [3]. This is achieved by utilizing Channel State Information (CSI) feedback, which in most cases report back Signal-to-Noise (SNR) measurements at the mobile unit. These measurements are then mapped to the appropriate Modulation Coding Scheme (MCS) based on MCS-SNR fixed-thresholds table available at the transmitter.
Implementations complying with IEEE 802.16m are supposed to provide data rates up to 100 Mbps supporting mobility speeds up to 120 Km/h [1]. These requirements come with unavoidable expenses and difficulties in terms of...

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