P R O D U C T F o c u sTEST AND MEASUREMENT

Measurement system givesdrive testing a new lease on lifeTim Brooks of PCTEL explains why anew network-measurement systemimproves interference management.

Drive testing has always been a crucial part ofwireless network planning and optimization.Regularly scheduled drive tests provide aneffective means of tracking and maintainine

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network performance as the network changesdue to growth in traffic, addition of cells andchanges in traffic patterns.

Drive testing involves the measurement ofsignal propagation information as a functionof location. This data is used throughout thenetwork life cycle to predict coverage fromproposed new cell sites, shorten networkturn-up time, optimize network performanceand quality, and manage interference. Drive-test figures are the most reliable way of deter-mining radio-frequency (RF) propagationand are also used to assist in network trouble-shooting. Data is also used by automatic fre-quency planning, automatic cell planningand other network optimization systems.

Recently, drive-test data has been comple-mented by mobile-switch statistics. These arecollected by the mobile switching centrebased on measurements made by any mobilephone during normal network operation.These measurements, which include signalstrength and carrier-to-interference ratio(C/T), are comprehensive and closely relatedto the end-user experience. Some believe thatswitch statistics will make drive-testingredundant, but recent developments haveestablished that this is not the case.

The CLARIFY GSM Interference Man-agement system from US-based PCTELgreatly improves drive-test performance. Thesystem can determine C/l values down to-15 dB, which is far below the interferencelevel. It automatically compiles an exact andmeasured matrix of the propagation for everysector in the network. This information canbe used to determine the total channel powerat a location and the power of the individualcontributing base stations.

The key benefit of drive testing is that itmeasures the actual propagation of signalsfrom each cell site. This is almost always dif-

interferenceV braken down into

13 component sources

CLARIFY provides a breakdown of interference into component sources. This can be used to determine if the signal from

one cell is causing interference in another. Here, fourspurs of interference are identified.

ferent from what is predicted or expectedbecause of reflections and clutter. Duringnetwork deployment and roll-out, the focusis on achieving coverage. At this stage it isimportant for engineers to predict RF propa-gation in order to determine the coverage ofnew cells. Cells are then combined to createan effective coverage area.

Managing interferenceToday, networks have established coveragepatterns, a high density of cells and significanttraffic. Interference management and con-tainment have therefore become equally, ifnot more, important than coverage planning.Interference management requires an under-standing of how signals propagate beyondthe intended coverage area of a cell. This isbecause a strong carrier signal in one cellbecomes an interference signal in other cells.

The engineering challenge is to makechanges within the network to reduce inter-ference by understanding the origin of theinterfering signals. This daunting task iscomplicated by the fact that interference usu-ally comes from multiple sources that are dif-ficult to separate.

The traditional approach is the key-updrive test, which measures the propagationfrom each cell site. This is done by assigninga unique channel to each cell and then dri-ving the nearby area to measure propagationand signal strength. Although relatively effec-tive in gathering the necessary information,this approach is costly and time-consumingbecause an individual drive is required foreach cell site. Also, limitations on the amountof driving done per cell often restrict infor-mation gathering to the immediate vicinity.This is unlikely to detect signal propagationand interference in unexpected places.

CLARIFY has two features that address theinterference management challenge. First, byusing highly sensitive radio scanners com-bined with post-processing and digital-signal-processing analysis of the measured signal, thesystem can detect and decode signals at-15 dB, below interference and noise levels.

Second, by using techniques that identifythe unique nature of signals from each cellsite, CLARIFY can separate interfering sig-nals into components and identify whereeach component comes from. Propagationand interference patterns for many cell sites

w i r e l e s s e u r o p e wireless.iop.org O c t o b e r / N o v e m b e r 2004