co-channelinterference

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Dynamic Telecommunications, Inc.

Dynamic Telecommunications, Inc. Version 052401 Page 1 of 512810 Wisteria Drive, Germantown, MD 20874 USAwww.DynamicTelecommunications.com

Co-Channel Interference

in

Wireless Networks

Effects and Methodsof Measurement

May 2000


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Dynamic Telecommunications, Inc. (DTI), a leading provider of wireless receivers used in

network optimizations and drive test applications, offers the following information on co-channel

interference as a means of both defining the issue and promoting a solution.

Information about DTI products can be found on the World Wide Web at www.dynatele.com.

What is Co-Channel Interference?

It is well known that one of the major limitations in cellular and PCS wireless telephone

networks is the so-called co-channel interference. In the case of TDMA networks, such as

GSM/GPRS or NADC (otherwise known as IS-136), the co-channel interference is mainly

caused by the spectrum allocated for the system being reused multiple times (frequency reuse).

The problem may be more or less severe, depending on the reuse factor, but in all cases, a signal

received by a handset will contain not only the desired forward channel from the current cell, but

also signals originating in more distant cells.

Figure 1-1 illustrates how two cells in an urban environment can interfere with each other,

especially when frequency planning has not been optimized. This figure shows the data obtained

during an actual drive test performed using DTIs SeeGull-DX scanning receiver. The cellular

system is the TDMA type. The upper trace represents the detected values of DVCC, whereas the

lower trace gives the corresponding signal levels, in dBm. Notice the gray area between the

cells where both values of DVCC are intermittently detected. This definitely decreases the

effective size of each cell and affects shadowed areas inside the cells, potentially causing

unnecessary hand-offs and even dropped calls. Note that the interference level never reaches low

values, which are characteristic of rural and suburban environments, due to the same ubiquitous

co-channel interference.

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Figure 1-1: Example of two cells interfering with each other
http://www.dynatele.com/


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Why is Co-Channel Interference Bad?

Co-channel interference, when not minimized, decreases the ratio of carrier to interferencepowers (C/I) at the periphery of cells, causing diminished system capacity, more frequent hand-

offs, and dropped calls.

How is Co-Channel Interference Minimized?

The deleterious effect of co-channel interference is minimized by optimizing the frequency

assignments of the base stations and their transmit powers and tilting the base-station antennae to

limit the spread of the signals in the system. In other instances, a more substantial re-design of

the cellular/PCS system may be needed, including changing the height of the cellular towers, as

well as adding more cells.

If interference from a distant cell causes a degradation of the ability of the handset to receive the

desired signal correctly, then identify the source of the co-channel interference and measure the

relative strength of interference relative to the desired signal.

How is Co-Channel Interference Measured?

Co-channel interference is measured by decoding the so-called color code contained in the

signals of every cell in the system, as well as by making measurements of the C/I ratio. When

used during a drive test in a moving vehicle, the test system determines the signal strength andcolor code (called in the case of GSM/GPRS, base station identification code, or BSIC) for the

desired signal, as well as the ratio of this signals strength to the total strength of all co-channel

interfering signals at regular, frequent intervals on the vehicle path. In addition, at each

measurement interval, the test system tries to decode the BSIC of one (dominating) co-channel

interfering signal. The idea here is that if all determinations were correct, they would provide the

identification of interfering signals for the entire area covered by the drive test.


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There are several reasons why the described method of co-channel interference measurement and

identification has a severely limited utility.

By definition, the interfering co-channel signals are below the power level of the desiredsignal and the decoding of the color code of such signals is a difficult task. In the absence of

any processing gain associated with the decoding of the color code, the only approach

available for this is some variation of the joint decoding of the desired signal and interfering

signal(s). There is a body of work describing such joint methods, but all share a common

feature; to be useful, they have to be extremely complex in terms of the accuracy of the used

channel models and exponentially increase in the number of required operations versus the

accuracy of the models and the number of signals decoded. This common feature necessitates

a truncation of the channel models in practical systems as well as limits, in practice, the

number of jointly demodulated signals by two. It is not surprising that such systems have

difficulty decoding color codes (BSIC in the case of GSM/GPRS), in most cases, especially

in the presence of multipath, and on board in a moving vehicle. They work more reliably in alab, although in a narrow range of relative power. The result is that the coverage of the

interference information during a drive test is intermittent. The processing time of test

instruments is long when they perform co-channel measurements and identification. Further,

it restricts the completeness of the coverage.

Color codes, including the BSIC in the GSM/GPRS case, provide limited identification

capability for establishing which distant cell is the source of interference because they are not

unique to base stations and are repeated periodically, even when decoded.

As previously mentioned, practical systems provide only information on one interfering co-

channel signal. In practice, it is desirable to identify multiple interfering signals and tomeasure their relative powers.

A key-up is another method of estimating the strength of co-channel interference. This is a

painful process for telephone companies. This method requires an interruption in the cellular

system operation. It is performed at night when all the frequency channels are shut down on all of

the base stations in the area, except for the channels that are selected for the measurement.

Individual path losses of every co-channel interferer to any cell can be measured.

Is There a Better Way?

Yes. DTI has developed a robust method of co-channel interference measurement based on

exploiting signal components, which provide processing gain, as well as on the use of a signal-

component database for signal identification. These and other efficient techniques, allow a

reliable measurement of interfering signals with decreased processing time wasted on failed

DVCC detections.


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What is DTIs Approach to Co-Channel Interference Measurements?

DTI achieves the goal1 of substituting the color code with a more reliable identification propertyof each co-channel component of the composite signal; namely, the time of arrival of a known

part of a signal.

Figure 1-2: Co-Channel Information in a Drive Test Collection System

The test instrument is able to keep track of each identified co-channel component during the

course of the drive test for the duration of its existence. It is able to decode the color code content

of each component, at least once. When detected, the found color code information can be back

annotated in the device database, which already contains all the determinations of the component,

albeit with no accompanying color code up to this point. In this manner, the color code can be

positively detected only once in the life of the component during the drive test. That limits the

number of time-consuming detections and allows them to be performed at the points in space and

time where the conditions are the most appropriate for such determinations. For example, when

the test vehicle is nearest to the base station that sends the component, or where the dominating

signal is temporarily shaded by an obstacle.

1Patent is pending.

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