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Measuring C/I in TEMS Investigation

Abstract

With the new release of TEMS Investigation, the users are provided witha new measure of the radio network performance, namely the carrier tointerference ratio of the received radio signal, also referred to as C/I.This makes it possible to identify frequencies exposed to high levels ofradio interference

1 Background and motivation

Until now, operators have had a means of supervising their radio net-works downlink quality by using the speech quality index (SQI) presentin TEMS. Measurements of SQI help identify geographical areas withinadequate speech quality. However, if frequency hopping is applied insuch areas, it has traditionally been difficult for the users to determinewhich of the frequencies are disturbed and why the speech quality isunsatisfactory. To help resolve such ambiguities, TEMS users are nowprovided a means of measuring the average C/I for each of the frequen-cies used in an on-going call.

In the past, rough C/I measurements have been possible to perform insome cases by comparing signal power measurements for the servingcell and neighbouring cells which use the same traffic channels as usedin the serving cell (but different BCCHs). This type of measurement doesonly compare the BCCHs signal powers and does not regard whetherthe TCHs might use power control and/or DTX.

Utilization of prediction tools such as TEMS Cellplanner results in fre-quency plans that are supposed to be optimized in terms of C/I. Thesetools use detailed descriptions of the topology in combination withadvanced radio propagation models to produce the predicted C/I val-ues. However, features like DTX and power control yield high error mar-gins for the predictions. As a consequence, areas with low C/I levelsmight be missed.

TEMS Investigation makes it possible to obtain measurements of theactual C/I as experienced by the mobile station. Furthermore, TEMSInvestigation performs measurements of C/I for each of the frequenciesutilized for communication.

OVERVIEW

LU/EPL/T/RP Mats Nordlund 0920 - 20 20 78 1999-09-05 A EPL/T/R-99:150

LU/EPL/T/RP Gunnar Heikkil

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2 Measurement method

With continuous transmission from the communicating base station, theTDMA radio bursts enter the mobile stations air interface at a rate of 100bursts per half second. The average C/I is obtained by applyingadvanced signal processing algorithms to each of the received radiobursts.

Relating to the protocol stack, the measurement is performed withinlayer 1 - the physical layer. As shown in figure 1, the carrier and inter-ferer signals are summed before entering the receivers antenna. Afteranalog-to-digital conversion, the composite signal is represented in thedigital domain. The signal processing algorithms are applied to the dig-ital signal in order to measure the ratio between the power of the desiredsignal (the carrier) and the power of interfering signal.

In dedicated mode, TEMS Investigation presents the average C/I twicea second, which is equal to the ordinary measurement interval. If fre-quency hopping is employed, the average C/I for each frequency is pre-sented.

It should be noted that the measurement method does not regard whatI actually consists of. Normally, in interference limited scenarios, Ireflects the co-channel interference. However, in coverage limited (noiselimited) scenarios, C/I rather reflects the carrier to noise ratio, C/N.

3 Drive test example

To illustrate the usage of C/I measurements, the results from a test drivein a live network is shown in figure 2. The test drive lasts for 40 secondsand utilizes cyclic frequency hopping over four frequencies. Enhancedfull rate speech coding is used throughout the test drive. The top graphshows TEMS SQI and RxLev. The bottom graph shows the C/I measure-ments for the four frequencies used. As can be seen in figure 2, the SQI

Figure 1. Simplified illustration of where the measurement of C/I takes place.

C

I

RF receiver & A/D

synchronization

digital domain

data demodulation

error correction

C/I to speech codec, etc.

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curve dips sharply at the end of the test drive. RxLev alone does indicatethat the signal power level is about 50 dB above -110 dBm. Hence, it isconcluded that the dip in quality does not depend on low signal powerlevel, that is, the quality problem is related interference rather than cov-erage. An interesting observation is that RxLev in fact increases duringthe SQI dip. The reason is most likely due to increasing interferingpower.

By looking at the C/I curves it is seen that two of the four frequencies(shown bold) are exposed to C/I levels around 5 dB during the SQI dip.As can be seen in figure 2, with two out of the four frequencies below 10dB, the resulting speech quality is heavily deteriorated.

By taking corresponding positioning data into account, this informationmay then be regarded when optimizing the frequency plan of the spe-cific area.

4 Measurement performance

The C/I measurement range is from -5 dB to +25 dB. Outside this rangethe measurement method saturates. Average values lower than -5 dB areconsidered highly unlikely to encounter. In addition, when the numberof hopping frequencies is low, average C/I levels below the lower limitnormally result in a dropped call. Above the higher limit, the perceivedspeech quality is not further improved. Hence, the limited measurementrange is not a restriction.

Figure 2. Live measurements of SQI, RxLev and C/I

0 5 10 15 20 25 30 35 400

5

10

15

20

25

time [s]

C/I [dB]

0 5 10 15 20 25 30 35 40

0

20

SQI [dBQ]

time [s]0 5 10 15 20 25 30 35 40

40

50

60

RxLev [dB]

SQI

RxLev

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If downlink DTX is utilized, the number of transmitted bursts (from thebase station to the mobile station) may be lower than the maximum 100depending on the speech activity of the transmitting side. TEMS Inves-tigation performs measurements on those bursts actually sent from thebase station and disregards non-transmitted bursts.

The number of hopping frequencies determines the number of burstsused for each frequencys C/I measurement. For example, if four fre-quencies are used, on average 25 bursts per frequency are received each0.5s interval. The number of samples per frequency each 0.5s intervaldecreases with increasing number of hopping frequencies. The implica-tion is that the measurement accuracy is better for small number of hop-ping frequencies compared to high number of frequencies.

If the true C/I is within the range 0 dB to 15 dB, and four frequencies areused for transmission (without DTX interruptions all bursts usable),the measurement error is typically better than 1 dB.