Codec Mode Selection and CQI Calculations

Atoll supports FR, HR, EFR, and AMR codec modes. A codec configuration contains codec mode adaptation thresholds and quality graphs for circuit quality indicators. Atoll has the following circuit quality indicators included by default:

• FER or Frame Erasure Rate: The number of frames in error divided by the total number of frames. These frames are usually discarded, in which case this can be called the Frame Erasure Rate.

• BER or Bit Error Rate: BER is a measurement of the raw bit error rate in reception before the decoding process begins. Any factor that impacts the decoding performance, such as frequency hopping, will impact the correlation between BER and FER, or the perceived end-user voice quality.

• MOS or Mean Opinion Score: Voice quality can be quantified using mean opinion score (MOS). MOS values can only be measured in a test laboratory environment. MOS values range from 1 (bad) to 5 (excellent). Different voice codecs have slightly different FER to MOS correlation since the smaller the voice codec bit rate is, the more sensitive it becomes to frame erasures.

The default codec configurations in Atoll include default FER, BER, and MOS quality graphs with respect to the carrier to interference ratio, and codec mode adaptation thresholds (calculated from the FER vs. C/I graphs for all codec modes at 5 % FER).

Figure 5.1FER vs. C/I Graphs

Figure 5.2BER vs. C/I Graphs

Figure 5.3MOS vs. C/I Graphs

5.4.1 Circuit Quality Indicator Calculations

Circuit quality indicator calculations include codec mode selection and CQI calculation. Codec modes may be selected using ideal link adaptation or without it. Once codec modes have been selected, CQI corresponding to these codec modes are determined from the look-up tables.

The following sections describe the two categories of calculations, i.e., with and without ideal link adaptations. Ideal link adaptation implies that the selected codec mode corresponds to the best value of the reference CQI under the given radio conditions. Without ideal link adaptation, the codec mode is selected based on the codec adaptation thresholds.

CQI calculations may be based on C/N or on C/(I+N). For calculating the noise, either the noise figure defined for the calculations or that of the selected terminal type is used.

Different codec configurations may be defined for transmitter and terminals. In this case, Atoll only selects the codec modes that are common in the two. If no terminal type is defined for the calculation, or if the terminal type does not have any codec configuration assigned to it, Atoll only uses the codec configuration of the transmitter. Similarly, if a transmitter does not have any codec configuration assigned to it, Atoll only uses the codec configuration of the terminal type. If both the transmitter and the terminal type do not have any codec configuration assigned to them, no codec mode selection and CQI calculation is carried out.

If more than one codec modes satisfy the C/N or C/I conditions, Atoll selects the higher priority codec mode.

In the following calculations, we assume that:

• is the signal level received from the selected TRX type (tt) or on all the TRXs of Txi on each pixel of the Txi coverage area,

• CM is the set of all available codec modes,

• are the values of adaptation thresholds for the codec modes available in the codec configuration,

The computed noise is compared to the codec configuration reference noise . If the values are the same, the defined graphs are used as is, otherwise the graphs are downshifted by the difference .

When the calculations are based on C/(I+N):

• Atoll calculates the carrier-to-interference ratio for all the TBC transmitters with codec configurations assigned, but takes into account all the TBC transmitters (with and without codec configurations) to evaluate the interference.

For more information on interference (I) calculation, see "Carrier-to-Interference Ratio Calculation" on page 138.

Ideal link adaptation for circuit quality indicator studies is defined at the codec configuration level. If the ideal link adaptation option is checked, Atoll will select the codec mode, for the transmitter under study, according to the codec quality graphs (CQI = f(C/N) and CQI = f(C/I)) related to the defined reference CQI, which may be different from the CQI being calculated.

Otherwise, Atoll will use the adaptation thresholds defined in the Adaptation Thresholds tab to determine the codec mode to be used in the studies.

5.4.2 CQI Calculation Without Ideal Link Adaptation

5.4.2.1 Calculations Based on C/N

Atoll selects the highest priority codec mode, cm, from among the codec modes available in the codec configuration:

For each TRX type, tt,

For , Atoll determines the CQI from the CQI=f(C/N) graph associated to the selected codec mode, cm.

5.4.2.2 Calculations Based on C/(I+N)

Atoll selects the highest priority codec mode, cm, from among the codec modes available in the codec configuration:

References:

The graphs are based on:

[1] T. Halonen, J. Romero, J. Melero; GSM, GPRS and EDGE performance – Evolution towards 3G/UMTS, John Wiley and Sons Ltd.

[2] J. Wigard, P. Mogensen; A simple mapping from C/I to FER and BER for a GSM type of air interface.

[3] 3GPP Specifications TR 26.975 V6.0.0; Performance characterization of the Adaptive Multi-Rate (AMR) speech codec (Release 6)

P_rec^TxiTRX

Adaptation Threshold

 _CM

N N_Ref

N–N_Ref

cm Highest Priority CM P_rec^TxiTRX

---N Adaptation Threshold_CM

 

 

 

 

=

P_rec^TxiTRX ---N

For each TRX type, tt,

For , Atoll determines the CQI from the CQI=f(C/I) graph associated to the selected codec mode, cm.

5.4.3 CQI Calculation With Ideal Link Adaptation

5.4.3.1 Calculations Based on C/N

Ideal link adaptation is used by a codec configuration according to a defined reference CQI (MOS by default).

Atoll calculates signal level received from Txi on each pixel of Txi coverage area and converts it into C/N values as described earlier. Then, Atoll filters all the codec modes that satisfy the C/N criterion (defined by the CQI = f(C/N) graphs for the reference CQI) and are common between the transmitter and the terminal type codec configuration.

The selected codec mode among these filtered codec modes will be,

For each TRX type, tt, , for MOS

Or, , for BER and FER

Where, cm is the codec mode with the highest priority among the set of codec modes CM for which the reference CQI gives the highest or the lowest value at the received C/N level, .

If more than one codec mode graphs give the same value for reference CQI, then Atoll selects the codec mode with the highest priority.

From the CQI = f(C/N) graph associated to the selected codec mode cm, Atoll evaluates the CQI for which the study was

performed corresponding to for the selected codec mode.

5.4.3.2 Calculations Based on C/(I+N)

Ideal link adaptation is used by a codec configuration according to a defined reference CQI (MOS by default).

Atoll calculates the C/I level received from the transmitter on each pixel of Txi coverage area, for each TRX and converts it into C/(I+N). Then, Atoll filters all the codec modes that satisfy the C/(I+N) criteria (defined by the CQI = f(C/I) graphs for the reference CQI) and are common between the transmitter and the terminal type codec configuration.

The selected codec mode among these filtered codec modes will be,

For each TRX type, tt, , for MOS

Or, , for BER and FER

Where, cm is the codec mode with the highest priority among the set of codec modes CM for which the reference CQI

gives the highest or the lowest value at the received C/(I+N) level, .

If more than one codec mode graphs give the same value for reference CQI, then Atoll selects the codec mode with the highest priority.

From the CQI = f(C/I) graph associated to the selected codec mode cm (indexed with the C/(I+N) values), Atoll evaluates

the CQI for which the study was performed corresponding to for the selected codec mode.

cm Highest Priority CM P_rec^TxiTRX

I+N

---Adaptation Threshold_CM

 

CQI_Ref Highest CQI=fC

N---- =P_rec^TxiTRX

CQI_Ref Lowest CQI=fC

N----=P_rec^TxiTRX

CQI_Ref Highest CQI=fC

---- =I P_rec^TxiTRX

CQI_Ref Lowest CQI=fC

----=I P_rec^TxiTRX

---5.4.4 Circuit Quality Indicators Coverage Predictions

The Circuit Quality Indicators coverage predictions show the areas BER, FER, and MOS values in the transmitter coverage areas.

For each TBC transmitter, Txi, Atoll calculates the selected parameter on each pixel inside the Txi calculation area. In other words, each pixel inside the Txi calculation area is considered a probe (non-interfering) receiver.

Coverage prediction parameters to be set are:

• The coverage conditions in order to determine the service area of each TBC transmitter,

• The interference and quality indicator conditions to meet for a pixel to be covered, and

• The display settings to select the displayed parameter and its shading levels.

The thermal noise (N = -121 dBm, by default) is used in the calculations if the coverage prediction is based on C/(I+N).

This value can be modified by the user.

5.4.4.1 Service Area Determination

Atoll uses parameters entered in the Condition tab of the coverage prediction properties dialogue to determine the areas where coverage will be displayed.

We can distinguish seven cases as below. Let us assume that:

• Each transmitter, Txi, belongs to a Hierarchical Cell Structure (HCS) layer, k, with a defined priority and a defined reception threshold.

• Each transmitter, Txi, has a codec configuration assigned.

• No max range is set.

5.4.4.1.1 All Servers

The service area of Txi corresponds to the pixels where:

5.4.4.1.2 Best Signal Level and a Margin

The service area of Txi corresponds to the pixels where:

And

Where M is the specified margin (dB). The Best function considers the highest value from a list of values.

• If M = 0 dB, Atoll considers pixels where the received signal level from Txi is the highest.

• If M = 2 dB, Atoll considers pixels where the received signal level from Txi is either the highest or within a 2 dB margin from the highest.

• If M = -2 dB, Atoll considers pixels where the received signal level from Txi is 2 dB higher than the signal levels from transmitters which are 2^nd best servers.

5.4.4.1.3 Second Best Signal Level and a Margin

The service area of Txi corresponds to the pixels where:

And

Where M is the specified margin (dB). The 2^nd Best function considers the second highest value from a list of values.

• If M = 0 dB, Atoll considers pixels where the received signal level from Txi is the second highest.

• If M = 2 dB, Atoll considers pixels where the received signal level from Txi is either the second highest or within a 2 dB margin from the second highest.

• If M = -2 dB, Atoll considers pixels where the received signal level from Txi is 2 dB higher than the signal levels from transmitters which are 3^rd best servers.

5.4.4.1.4 Best Signal Level per HCS Layer and a Margin

For each HCS layer, k, the service area of Txi corresponds to the pixels where:

And

SubcellReceptionThresholdP_rec^TxiBCCH

SubcellReceptionThresholdP_rec^TxiBCCH

P_rec^TxiBCCH Best

ji P_rec^TxjBCCH M–



SubcellReceptionThresholdP_rec^TxiBCCH

P_rec^TxiBCCH 2^ndBest

ji P_rec^TxjBCCH M–



SubcellReceptionThresholdP_rec^TxiBCCH

P_rec^TxiBCCH Best

ji P_rec^TxjBCCH M–



Where M is the specified margin (dB). The Best function considers the highest value from a list of values.

• If M = 0 dB, Atoll considers pixels where the received signal level from Txi is the highest.

• If M = 2 dB, Atoll considers pixels where the received signal level from Txi is either the highest or within a 2 dB margin from the highest.

• If M = -2 dB, Atoll considers pixels where the received signal level from Txi is 2 dB higher than the signal levels from transmitters which are 2^nd best servers.

5.4.4.1.5 Second Best Signal Level per HCS Layer and a Margin

For each HCS layer, k, the service area of Txi corresponds to the pixels where:

And

Where M is the specified margin (dB). The 2^nd Best function considers the second highest value from a list of values.

• If M = 0 dB, Atoll considers pixels where the received signal level from Txi is the second highest.

• If M = 2 dB, Atoll considers pixels where the received signal level from Txi is either the second highest or within a 2 dB margin from the second highest.

• If M = -2 dB, Atoll considers pixels where the received signal level from Txi is 2 dB higher than the signal levels from transmitters which are 3^rd best servers.

5.4.4.1.6 HCS Servers and a Margin

The service area of Txi corresponds to the pixels where:

And

And the received exceeds the reception threshold defined per HCS layer.

Where M is the specified margin (dB). The Best function considers the highest value from a list of values.

• If M = 0 dB, Atoll considers pixels where the received signal level from Txi is the highest.

• If M = 2 dB, Atoll considers pixels where the received signal level from Txi is either the highest or within a 2 dB margin from the highest.

• If M = -2 dB, Atoll considers pixels where the received signal level from Txi is 2 dB higher than the signal levels from transmitters which are 2^nd best servers.

5.4.4.1.7 Highest Priority HCS Server and a Margin

The service area of Txi corresponds to the pixels where:

And

And Txi belongs to the HCS layer with the highest priority. The highest priority is defined by the priority field (0: lowest).

And the received exceeds the reception threshold defined per HCS layer.

Where M is the specified margin (dB). The Best function considers the highest value from a list of values.

• If M = 0 dB, Atoll considers pixels where the received signal level from Txi is the highest.

• If M = 2 dB, Atoll considers pixels where the received signal level from Txi is either the highest or within a 2 dB margin from the highest.

• If M = -2 dB, Atoll considers pixels where the received signal level from Txi is 2 dB higher than the signal levels from transmitters which are 2^nd best servers.

SubcellReceptionThresholdP_rec^TxiBCCH

P_rec^TxiBCCH 2^ndBest

ji P_rec^TxjBCCH M–



SubcellReceptionThresholdP_rec^TxiBCCH

P_rec^TxiBCCH Best

ji P_rec^TxjBCCH M–



P_rec^TxiBCCH

Note:

• In the case two layers have the same priority, the traffic is served by the transmitter for which the difference between the received signal strength and the HCS threshold is the highest. The way the competition is managed between layers with the same priority can be modified. For more information, see the Administrator Manual.

SubcellReceptionThresholdP_rec^TxiBCCH

P_rec^TxiBCCH Best

ji P_rec^TxjBCCH M–



P_rec^TxiBCCH

5.4.4.2 Coverage Display

5.4.4.2.1 Coverage Resolution

The resolution of the coverage prediction does not depend on the resolutions of the path loss matrices or the geographic data and can be defined separately for each coverage prediction. Coverage predictions are generated using a bilinear interpolation method from multi-resolution path loss matrices (similar to the one used to calculate site altitudes, see "Path Loss Calculations" on page 77 for more information).

5.4.4.2.2 Display Types

It is possible to display the coverage predictions with colours depending on criteria such as:

BER

Only the pixels with a codec mode assigned are coloured. The pixel colour depends on the BER value. Coverage consists of several independent layers whose visibility in the map window can be managed. There are as many layers as transmitter coverage areas and BER display thresholds. Each layer shows the BER in the transmitter coverage area.

FER

Only the pixels with a codec mode assigned are coloured. The pixel colour depends on the FER value. Coverage consists of several independent layers whose visibility in the map window can be managed. There are as many layers as transmitter coverage areas and FER display thresholds. Each layer shows the FER in the transmitter coverage area.

MOS

Only the pixels with a codec mode assigned are coloured. The pixel colour depends on the MOS value. Coverage consists of several independent layers whose visibility in the map window can be managed. There are as many layers as transmitter coverage areas and MOS display thresholds. Each layer shows the MOS in the transmitter coverage area.

Max BER

Only the pixels with a codec mode assigned are coloured. The pixel colour depends on the highest BER value among the BER values for all the transmitters covering the pixel. Coverage consists of several independent layers whose visibility in the map window can be managed. There are as many layers as BER display thresholds. Each layer shows the BER value.

Max FER

Only the pixels with a codec mode assigned are coloured. The pixel colour depends on the highest FER value among the FER values for all the transmitters covering the pixel. Coverage consists of several independent layers whose visibility in the map window can be managed. There are as many layers as FER display thresholds. Each layer shows the FER value.

Max MOS

Only the pixels with a codec mode assigned are coloured. The pixel colour depends on the highest MOS value among the MOS values for all the transmitters covering the pixel. Coverage consists of several independent layers whose visibility in the map window can be managed. There are as many layers as MOS display thresholds. Each layer shows the MOS value.

In document Atoll 2.8.3 RF Technical Reference Guide E2 (Page 150-156)