Mid Sweden University

High data-rate wireless access systems are currently under discussion since the demand for wireless multimedia communication is rapidly increasing due to strong advances in wireless Internet services. Reliable high-speed data communications is one of the major challenges with regards to harsh conditions. With the necessity for high data rates, linear multi-level modulation schemes are becoming more and more important in wireless communication systems since they are bandwidth efficient. In this thesis protocols are designed and evaluated which improve system performance by combining ARQ-induced retransmissions through multipath channels in order to reduce the latency and improve the system throughput. The starting point is to show that the employment of simple packet combining schemes to wireless LANs such as IEEE 802.11, can achieve a considerable performance gain with the addition of only a small increase in complexity. A low-complexity method for enhancing and exploiting retransmission diversity by varying the bit-to-symbol mapping for each retransmission of a packet is evaluated. The selected mappings are chosen in order to maximize a bit log likelihood ratio (LLR) based metric. An ARQ scheme is also proposed, which combines retransmissions and bandwidth efficient multi-level modulation techniques with a change in the symbol mapping for every retransmission. The main idea behind this proposal is to take advantage of the extra dimension provided by a retransmission diversity scheme in improving the power efficiency if the used modulation without altering the diversity order of the system. Considering the overall scheme as one entity it is possible to obtain a transmission scheme able to perform very well in both additive Gaussian and fading channels without increase in receiver complexity. Finally, multiple antenna systems with ARQ functionality are evaluated. It has been established that using multiple antennas at both the transmitter and receiver along with intelligent signal design enables us to achieve better reliability as well as increase the transmission rate. In this thesis a space-time block coded hybrid ARQ scheme is considered which exploits both the spatial and time diversity of the MIMO channel. A bit-to-symbol mapping ARQ scheme suitable for multiple antenna systems is also considered.

Konvergensen mellan internet och mobil teknologi skapar stora möjligheter till nya innovativa internettjänster på marknaden. Dessa nya och i många fall avancerade tjänster kräver höga datahastigheter och mycket bandbredd. För att använda sig av höga datahastigheter över trådlös kommunikation behövs bl.a. felrättande koder samt avancerad modulationsteknik som är robust emot störningar och samtidigt bandbreddseffektiva. I denna avhandling designas och utvärderas olika ARQ-protokoll för att öka systemprestanda såsom ökad genomströmning av datapaket samt minska antalet omsändningar. Vi börjar med att visa att man med hjälp av paketkombinering i trådlösa nätverk som IEEE 802.11 kan uppnå en signifikant prestandaökning utan markant ökning av systemets komplexitet. Vidare så studeras olika omsändningsstrategier där man varierar bit-till-symbolmappningen i varje omsändning och på så sätt erhålla en diversitetsvinst. Denna metod visar sig vara bättre än att sända samma datapaket med likadan bit-till-symbolmappning i varje omsändning. Slutligen utvärderas ARQ-system som använder flera sändar- och mottagarantenner (MIMO-system).