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Frequency Hopping in GSM Networks 179

used for indirectly adjusting cell parameters. The Dropped Call Ratio is an

counter available from the Operations and Maintenance (OMC) for off-line

processing of statistics. The Dropped Call Ratio has been traditionally used

in the performance monitoring and optimization of cellular systems. This

indicator is also closely linked to the Radio Link Time-out (RLT) which is

determined by the decoding failure rate of the SACCH frames. Although

widely used, the indicator only indirectly represents the performance of the

Traffic Channel (TCH). Therefore in certain frequency reuse scenarios, it

cannot always provide accurate indication of the TCH quality.

Both RXQUAL and FER can be measured simultaneously with Test

Mobile equipment and at the BTS with A-bis Call Trace measurement

facilities. These are special arrangements that are needed in the optimization

stages because the behavior of RXQUAL with Frequency Hopping is

different to non-hopped systems. One way to show this is to plot the system

reported Dropped Call Ratio against the number of events where the

RXQUAL exceeds a threshold level e.g. RXQUAL greater than 5 in a cell.

This gives an area-wide impression of the call quality, which involves many

mobiles and reflects the true behavior for the RXQUAL parameter: The cell

parameters in GSM are defined on a per cell basis and the RF optimization is

performed by adjusting the thresholds for these parameters in terms of the

reported parameters e.g. RXQUAL and RXLEV. The drive tests are useful

to build a detailed log of the behavior in known problem areas. The plot in

Figure 2 shows that the Dropped Call Ratio against the percentage of bad

quality of calls, defined as the events where RXQUAL exceeds 5. The

observed data confirms that the Dropped Call Ratio does not have a strong

dependence on bad quality defined by the RXQUAL threshold. This

behavior is due to the averaging effects of interference in Frequency

Hopping systems.

Interference Averaging

Carrier frequency hopping causes interference from close-in and far-off

mobiles to change with each hop. This means that a mobile continually

suffering severe interference in a non-hopped case would be expected to

experience lower interference due to the statistical averaging effect. The

significance of this effect expressed in a simplified way translates to:

• The average interference during a call is lower and the average call

quality is improved.

• The standard deviation of the interference is expected to become less, as

the extreme events are fewer per call. For the same C/I outage the

interference margin is reduced resulting in a lower C/I threshold.

180 Chapter 9

This lower C/I cannot be directly mapped into a planning threshold. A

determination of the quality threshold in terms of Frame Erasure Rate (FER)

is a prerequisite as it is directly related to voice quality. This means that

standard planning tools do not accurately reflect practical network quality

and the frequency plans produced cannot be depended upon to evaluate

capacity.

Voice Quality and FER

The quality gain is not directly related to the mean C/I. This is because a

certain mean C/I can result in different Frame Erasure Rates (FER) and

unlike the non-hopped case where there is a unique mapping between the

two parameters. The interference averaging causes the C/I distribution to

change in a way that short term C/I are individually related to each FER, and

the mean C/I can be identified with more than one FER distribution. This

relationship has been observed in detailed system simulations based on snap￾shot locations of mobiles over a large area and by assuming different traffic

intensity per mobile. A sample result from simulations based on a

homogeneous network of 50 sites covering an area of approximately 1500

square km, uniform offered traffic intensity of 25mE per mobile and

spectrum allocation of 36 carriers is shown in Figure 3. The effects of

downlink power control and Discontinuous Transmission (DTX) were

modeled in these simulations.

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