Noname manuscript No. (will be inserted by the editor) Packet Switching, AM Adjustment and Retry Mechanisms for Cross-Layer MIMO Link Design Erwin Anggadjaja · Ian McLoughlin Received: date / Accepted: date Abstract The effects of sub-channel packet routing and adaptive modulation (AM) are explored in this paper for MIMO linking system under differing chan- nel conditions. Simulations are derived from a single TCP protocol link and then extended into dual- and quad-link models. The aim is to exploit time-varying imbalances between logical sub-channels by different cross-layer routing and modulation strategies to im- prove overall goodput. Segmentation and reassembly (SAR) is employed with local error control in the form of ACK/NACK triggered retries with timeout. MIMO sub-channel characteristics are modelled in terms of frame-by-frame BER, channel delay, and data rate and used to explore the effect of channel BER imbalance. This paper demonstrates that even simple switching mechanisms can provide an overall performance im- provement in a common real-world situation where sub- channels exhibit slightly unequal error rates for du- rations of one or more transmission frames. A BER- directed adaptive modulation switching scheme is then developed and evaluated, again showing potential per- formance gain for mismatched sub-channels. Keywords Adaptive modulation · cross-layer design · MIMO sub-channels · Segmentation and Reassembly E. Anggadjaja School of Computer Engineering, Nanyang Technological Uni- versity, 50 Nanyang Ave, Singapore, 639798 Tel.: +65 9242-1268 Fax: +65 6790-1585 E-mail: er0001ja@ntu.edu.sg I. McLoughlin School of Information Science and Technology, University of Science and Technology of China, Hefei, Anhui, 230027 1 Introduction For future wireless systems, many challenges exist in fulfilling the demand for high data rates and good qual- ity of service (QoS) requirements (such as latency and error rate). These are in addition to the ever-present need to maximize throughput/goodput performance and to provide efficient and effective spectral utilisation. Although there are numerous methods worthy of in- vestigation to improve wireless network performance, as the title suggests, this paper assumes the use of TCP/IP networking – which implies a trade off between greater interoperability at a network level and the need to comply with such matters as the TCP packet struc- ture, resend/timeout policy, congestion control mea- sures and session establishment/disestablishment pro- cedures. The use of TCP limits the degrees of design freedom as well as reduces the amount of comparative research literature. However TCP/IP is the pre-eminent general purpose network connectivity standard in use today. Although it is known to suffer from performance issues when applied over wireless links, much work has been done to alleviate these issues. In terms of wireless communications, multiple-input multiple-output (MIMO) systems have become well- known for potentially delivering high data rate trans- mission without necessarily increasing bandwidth or trans- mit power. In fact, MIMO has become popular for its ability to provide higher spectral efficiencies and thus greater throughput or goodput to the end user. For this reason, it is also considered a foundational method for present and future wireless systems. This paper ex- ploits the multi-channel nature of MIMO at the data link layer. Another technique, adaptive modulation (AM) is emerging as a useful and practical method for manag-