Speech Performance Viewer LTE 3G

Measuring Speech Quality across Mixed Networks

Aymeric TIBURCE How-to, In More Depth

Users of VoLTE phones expect wideband speech. Of course, if the other end of the call is PSTN, 2G or 3G narrowband then their experience will not meet expectations. The planned or unexpected tendency for mixed networks to deliver narrowband instead of wideband speech can be verified with MultiDSLA.

Using ITU-T Rec. P.863 POLQA enables speech quality measurements to be made on any network, from narrowband to super-wideband, with POLQA’s Super-wideband model. The score for narrowband speech will always be less than for wideband speech, given consistent impairments.

By way of demonstration, consider three transmission cases:

  • Narrowband to narrowband
  • Narrowband to wideband
  • Narrowband to super-wideband

Using the POLQA Super-wideband model, the score should be similar for each case. We can apply a 48k sample rate signal, filtered to the IRS or MIRS narrowband characteristics, capture it at 8k, 16k and 48k sample rates and evaluate it with POLQA SWB.

Input Measuring sample rate POLQA LQO SWB

48k IRS

8k

3.3

48k IRS

16k

3.4

48k IRS

48k

3.4

48k MIRS

8k

3.4

48k MIRS

16k

3.8

48k MIRS 48k

3.7

The lower 8k measuring sample rate scores reflect the lack of any signal above 4kHz. The MIRS filtered files do have more energy above 4kHz than those that have been IRS filtered.

 

If we apply the 16k SWB signal and use a measuring sample rate of 8k, 16k and 48k, we see the score is limited to 4.6

Input

Measuring sample rate

POLQA LQO SWB

16k SWB

8k

3.4

16k SWB

16k

4.6

16k SWB

48k

4.6

 

These results demonstrate how POLQA SWB measurement on a good quality signal path can differentiate between narrowband and wideband signals.

When evaluating end to end network performance we usually alternate speech direction so both ends should be transmitting and measuring at the same sample rate.

For end to end tests, the DSLA can apply a filter on the signal as it is being generated if the Equaliser option is included in MultiDSLA. So we can always use the same SWB 48k signal and apply the IRS filter for PSTN connection, or the MIRS filter for 2G or narrowband 3G networks. We can apply a wideband filter for wideband 3G or VoLTE networks.

For tests between the IP domain and a handset, MultiDSLA will automatically apply the SWB 16k version of the signal as an input to the SVN or VPP node if the codec requires a 16k sample rate signal.

An example of the effectiveness of this technique is shown below.

The 3G network supports wideband

3G 3G

 

The VoLTE network supports wideband

LTE LTE

 

But when the VoLTE network interconnects to the 3G network the result is narrowband (and a lot more delay!)

LTE 3G