2013 Australasian Telecommunication Networks and Applications Conference (ATNAC) Cross-layer MIMO-Link Exploiting Packet Re-routing Mechanisms and Adaptive Modulation in Diverse Channel Condition Erwin Anggadjaja School of Computer Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore, 639798 erOOOlja@ntu.edu.sg Abstact-This paper considers a cross-layer adaptive modula tion switching concept for TCP-enabled MIMO wireless commu nication systems implementing a packet re-routing mechanism over physically decomposed sub-channels. We demonstrate that combining both adaptive modulation and packet re-routing mechanisms improve overall performance over MIMO sub channels that posses unbalanced or uneven time-varying error rate characteristics. A simulated wireless communications system employs and evaluates packet re-routing techniques over four sub-channel connections made available to the link layer. It then considers the effect of cross-layer directed link modulation adaptation switching for point-to-point wireless communications. We investigate the system in the light of goodput under various channel diferences and characteristics, including data rate and bit error rate (BER). For each developed scenario, a separate analysis and discussion is presented. Based on the investigation results, we demonstrate that even simple re-routing and adaptive modulation mechanisms may be benefcial for systems utilising multiple antennas. Index Terms-Adaptive modulation, channel diference, MIMO sub-channels, packet re-routing, switching methods. I. INTRODUCTION Achieving both high data rate and spectral ef fciency has long been considered to be the Holy Grail of future wireless communication systems, with MIMO technology often touted as the means to achieve this goal. Without necessarily increas ing bandwidth or transmit power at the antennas, MIMO can provide higher data rate transmission [1]. Early studies [2] [3] showed that MIMO can yield remarkable spectral ef fciency improvements over single-input single-output (SISO) systems, given favourable channel conditions. Multiple antenna arrays at both ends of the MIMO system are used to exploit multipath propagation over the wireless channel, increasing efective throughput and improving transmission reliability - an advan tage that can be used to increase the users data rate (formerly reported by Foschini et. al. [2] [4D. Besides MIMO, another method of mitigating against multi fading effects is to switch the transmission signal characteris tics according to instantaneous channel conditions. For exam ple, adapting signal parameters such as coding rate or transmit power, or switching between multiple transmit antennas. This Ian McLoughlin School of Information Science and Technology University of Science and Technology of China 443 Huangshan Rd, Hefei, Anhui, 230027 is collectively known as link adaptation, and adjusting the signalling method is known as adaptive modulation (AM) [5]. In general, this describes the ability to change code rate (or transmission power) in response to channel conditions. Such schemes tend to use a frame-based transmission system whereby the power and modulation of a transmitted signal are held constant over a fame interval, chosen according to past channel conditions, and then adapted on subsequent fame boundaries. Several studies have shown that these techniques can im prove unbalanced conditions between BER and spectral ef ciency [6] [7] [8] [9] [10] [11] [12]. These results indicate the fexibility of matching channel properties, i.e. coding, modu lation, and signal and protocol parameters, to the conditions of the transmission link. Or in other words, the signal modulation and coding format is altered to match instantaneous channel condition knowledge, often measured in terms of received signal quality. Thus these schemes posses a dynamic nature in which the signal and protocol parameters are adapted to suit the varying radio link conditions. II. CONTRIBUTION AND RELEVANT WORK As the main contribution of this paper, a MIMO-Iink model employing packet re-routing strategies, is enhanced with cross layer adaptive modulation (AM) capabilities and investigated in terms of user-experienced goodput for various channel conditions. The model utilises a payload-RDL (P-RDL) packet re-routing solution, which is based on the authors recent work [13]. Briefy described, the system adapts the handling of payload data over multiple sub-channels to suit local channel conditions, by making the erroneous payload retransmitted to diferent link (RDL). Payload data refers to small fragmented TCP packets, transmitted over air using a fast local-ARQ link employing a sliding window reconstruction mechanism (which is itself a MIMO extension of previous systems using a radio link protocol RLP concept [14D. These systems all rely on the fact that the multiple sub-channels implicit in a MIMO connection scheme, are made available as raw transmission pipes to the data link layer, which is able to exploit BER 978-1-4799-1081-6/13/$31.00 ©2013 IEEE 148