A robust low complexity algorithm for carrier-to-interference ratio estimation in GSM

The adaptive multi-rate (AMR) feature of GSM can offer significant capacity and speech quality improvements provided the speech coder rate can be precisely adapted to the changing channel conditions. To enable this link adaptation requires an estimation of a suitable link quality measure. This paper details a robust, low complexity algorithm for estimating a link independent measure-carrier-to-interference ratio (C/I)-in GSM over a wide dynamic range. The complexity of the algorithm is reduced by exploiting the structure of GMSK modulation. An algorithm for diversity combining the link quality estimates is also presented. Performance results are presented for TU, RA, and HT multipath propagation conditions for mobile speeds of 3, 50, and 100 km/h. It is shown that the algorithm meets the GSM 05.09 link adaptation specification with sufficient margin and with a reasonably low computational complexity.