26.2. R x Q u a l a n d I t s L i m i t a t i o n sRxQual is obtained by transforming the bit error rate (BER) into a scale from 0 to 7 (see3GPP TS 45.008, section 8.2). In other words, RxQual is a very basic measure: it simplyreflects the average BER over a certain period of time (0.5 s). By contrast, a listener’sassessment of speech quality is a complex process which is influenced by many factors.Some of these, all of which RxQual fails to take into account, are the following:•The distribution of bit errors over time. For a given BER, if the BER fluctuates verymuch, the perceived quality is lower than if the BER remains rather constant most of thetime. Different channel conditions give rise to radically different BER distributions.However, since RxQual just measures the average BER, it cannot capture this. (In fact,the logarithmic scale of RxQual gives rise to the opposite effect: a high BER variancegives a better RxQual than a low variance does. This is completely misleading from aspeech quality point of view.)•Frame erasures. When entire speech frames are lost, this affects the perceived quality ina very negative way.•Handovers. Handovers always cause some frames to be lost, which generally gives riseto audible disturbances. This does not show at all in RxQual, however, since duringhandovers BER measurements are suppressed.•The choice of speech codec. The general quality level and the highest attainable qualityvary widely between speech codecs. Moreover, each codec has its own strengths andweaknesses as regards types of input and channel conditions.In short, RxQual fails to capture many phenomena that have a decisive influence on alistener’s judgment of speech quality. Using RxQual for optimization of speech quality inthe network thus leads to suboptimal results.Design of SQI SQI has been designed to take into consideration all the phenomena discussed in thepreceding section. This ensures that it will produce an unbiased prediction of the speechquality, independently of channel conditions and other circumstances. Somewhatroughly, the computation of SQI involves

• the bit error rate (BER)• the frame erasure rate (FER)• data on handover events• statistics on the distributions of each of these parameters. Furthermore, for each speech codec, SQI is computed by a separate algorithm which is tuned to the characteristics of that codec.Like RxQual, SQI is updated at 0.5 s intervals.