Cross-Layer Rate and Power Adaptation Strategies for IR-HARQ Systems over Fading Channels with Memory: A SMDP-Based Approach

TitleCross-Layer Rate and Power Adaptation Strategies for IR-HARQ Systems over Fading Channels with Memory: A SMDP-Based Approach
Publication TypeJournal Article
Year of Publication2008
AuthorsKarmokar, A. K., D. V. Djonin, and V. K. Bhargava
JournalCommunications, IEEE Transactions on
Pagination1352 -1365
Date Publishedaug.
Keywordsautomatic repeat request, buffering delay, cross-layer rate, decoding, finite-size buffer, first-order Markov chain, hard-decision decoding, incremental-redundancy hybrid automatic repeat- request schemes, IR-HARQ systems, linear programming, linear programming approach, link layer, Markov processes, modulation order adaptation strategies, non-adaptive scheme, optimal policy, optimal transmission power, physical layer, power adaptation strategies, radio transmitters, Rayleigh channels, Rayleigh fading channels, semi-Markov decision process model, SMDP-based approach, soft-decision decoding, transmitter model, wireless standards

Incremental-redundancy hybrid automatic repeat- request (IR-HARQ) schemes are proposed in several wireless standards for increased throughput-efficiency and greater reliability. We investigate transmit power and modulation order adaptation strategies for the IR-HARQ schemes over correlated Rayleigh fading channels using semi-Markov decision process-based model. In order to jointly analyze physical layer and link layer, transmitter model incorporates a finite-size buffer that receives randomly varying traffic from a higher layer application. It is assumed that channel variations can be modeled with a first-order Markov chain. We show that the optimal transmission power and rate adaptation laws under buffering delay and packet overflow constraints can be obtained using the framework of semi-Markov decision process. We discuss three different adaptation models for the IR-HARQ schemes and compare their performances with the non-adaptive scheme. It is shown that unique optimal policy exists for each case and it can be computed using linear programming approach. This optimal policy is then applied for realistic channel fading and incoming traffic samples to evaluate its performance for both hard-decision and soft-decision decoding. Simulation results in general point out that substantial power savings can be achieved using adaptation and also if the transmission-delay requirements are relaxed.


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