ETD system

Electronic theses and dissertations repository


Tesi etd-04022009-150201

Thesis type
Tesi di laurea specialistica
Adaptive coded modulation with retransmission for partial channel state information
Corso di studi
Relatore Dott. Pfletschinger, Stephan
Relatore Prof. Luise, Marco
Relatore Prof. Giannetti, Filippo
Parole chiave
  • link adaptation
  • channel prediction
  • HARQ
Data inizio appello
Data di rilascio
Riassunto analitico
A promising way of improving the spectral efficiency of radio interfaces beyond<br>3G is by performing fast link adaptation. Since the channel is fluctuating due to<br>fading and shadowing effects, the transmission parameters of a wireless communication<br>system have to be adjusted according to the current channel state in order<br>to make best use of the available resources. In real-world systems, anyway, only<br>partial channel information is available at the transmitter. This has a strong impact<br>on achievable performances since, without exact knowledge of the fading state, it<br>is no longer possible to accurately adapt coding and modulation. The main objective<br>of this thesis is to design a link adaptation scheme which is robust despite<br>of this uncertainty about the channel. To this end, in a preliminary phase it is at<br>first quantified the performance loss with respect to the ideal case of complete information<br>and consequently it is studied how it can be minimized with the use of<br>channel prediction. Since a severe degradation can still be observed in some fading<br>conditions, an additional mechanism for fast link-level retransmissions based<br>on soft-combining of information is suggested. The retransmission protocol is used<br>not just to recover from incorrectly received codewords, but the link adaptation<br>strategy is defined anew so as to enhance throughput with the new degree of freedom<br>represented by delay. With this perspective, we get closed-form expressions<br>of the interesting parameters for the case of block fading and Chase combining, whilst a remarkable improvement of the outage probability approximation of the<br>word error rate for the case of incremental redundancy and/or selective channels is<br>also presented. With this analytical framework an optimization of the adaptive system<br>for any criterion and constraints is thus feasible at low computational cost without<br>necessarily resorting to time-consuming link-level simulations. In the examples<br>provided, encouraging results are obtained since it is observed that the achieved<br>throughput comes relatively close to the throughput of an ergodic fading channel at<br>the expense of few retransmissions, even when no channel state information apart<br>from the long-term average SNR is available at the transmitter.