PHY-MAC Crosslayer Adaptive Design In OFDM System

In the mobile communication system, transmission environment and channel characteristics are complex. To overcome time-varying characteristics of wireless channel, we can use various methods to adjust to channel. Currently, adaptive research focuses on the physical layer, there are many shortcomings, including heavy signaling traffic flow, errors on channel estimation and sensitive feedback delay. But the cross-layer adaptive technology can improve spectrum efficiency with reducing delay and packet loss rate. How to combine adaptive method on physical layer and data link layer , utilize the data link layer transmission mechanism and use reliable statistical information for adaptive strategy as possible as we can, is of great significance.Orthogonal Frequency Division Multiplexing (OFDM) technology and Multiple Input and Multiple Output (MIMO) technology can eliminate the inter-symbol interference in the frequency-selective channel, and provide higher transmission rates and system capacity in the limited spectrum. As hot spots of the next generation of mobile communications, they have been widely used in 3GPP and 802.1x.After discussing the characteristics of wireless channel and adaptive modulation technology in OFDM system, this paper Verifies the OFDM system using adaptive modulation have significantly improved performance. Then raises two algorithms based on AM and truncated-ARQ method between PHY and MAC layer, which are able to guarantee less packet loss rate on condition of constant throughput in OFDM system. The difference between them is that one of it can provide better performance, while the other have less complexity. Simulation shows that the system can provide better performance at different channel state in the PHY and MAC layer, compared with traditional network structure.Finally, we do research about the possibility if we can apply cross-layer algorithm in OFDM to the MIMO systems. adaptive modulation based on the SVD decomposition can decompose MIMO channel to parallel singular value channel using the pre-encoder and decoder (singular value Represents the channel gain). The conclusion that SVD decomposition can change MIMO systems to SISO system makes the SISO cross-layer adaptive algorithm can be transplanted to MIMO-OFDM system. Finally, the simulation results indicate that MIMO can provide further gains to cross-layer system.