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Crossed Feeder Analysis The term crossed feeder is used to desc ribe the problem that arises when the feeders for two or more sec tors in a site are inadvertently c onnec ted inc orrec tly. This problem is most likely to oc c ur during the initial setup of a site or subsequently during periodic site maintenanc e. To illustrate the problem, c onsider a new c ell site that has three sec tors, A, B, and C:
The c rossed feeder problem would arise if the feeder for sec tor A is c onnec ted to sec tor C and vic e versa. When this happens, the sec tors typic ally c ontinue to provide good c overage. However, the network parameters for the two sec tors are also reversed. For example, the parameters storing the sec tors' BCCH (in a GSM network), SC (UMTS), or PN (CDMA) and lists of adjac ent sec tors would be swapped. This typic ally leads to an inc reased rate of dropped c alls and it is not always easy to understand why this is happening. For example, the network operators' operations and support systems (OSS) do not normally identify or distinguish these types of anomalies. You c an use the Analyzer Map to help diagnose this problem. For example, suppose we run a drive test through the area served by our fic tional c ell site and (assuming it is a GSM site) then display the ServBCCH attribute on the Map while the c ell sec tors are c olored by BCCH. This is what we might see:
Notic e that where the drive test passed in front of sec tor C, we c an see that the serving BCCH matc hed Sec tor A's BCCH, bec ause they are both c olored brown. Similarly where the drive test passed in front of sec tor A, we c an see that the serving BCCH matc hes Sec tor C's BCCH, bec ause they are both c olored pink. This is the c lassic pattern that you would expec t to find when the feeders have been c rossed. There are other problems that sometimes produc e a similar pattern on the Map. For example: Sometimes a tall building or other struc ture that is c lose to a c ell sec tor c an c ause exc essive reflec tion that will lead to signific ant c overage behind the c ell sec tor. Sometimes exc essive mec hanic al downtilt c an also lead to c overage behind the c ell sec tor, due to the bac k lobes tilting upwards. However, these problems c an generally be distinguished from the c rossed feeder problem, bec ause they produc e a "messier" pattern on the Map than that shown above. Spotlight and Troubleshooter both inc lude analysis that is designed to make it easy to identify GSM and UMTS c rossed feeder problems. This analysis is based on the assumption that in a well-c onfigured site, at least half of the power radiates within the beamwidth of eac h sec tor. This is sometimes referred to as the 3-dB half-power beamwidth. In other words, if a manufac turer's definition of a partic ular antenna's beamwidth is 65°, then the half-power or 3-dB beamwidth is 65°. Spotlight. This analysis is available for GSM and UMTS in the Radio Network Explorer. The KPI names as they appear in the projec t template are shown in the following table. Technology KPI Name GSM
Site Configuration Issues
UMTS
Outside Beamwidth
For both tec hnologies the analysis appears on the Cell Coverage page in the Radio Network Explorer. For GSM, selec t the Handset - Best Server option in the Analysis drop-down. For UMTS, selec t the Scanner - Best Server analysis. For both tec hnologies, the c ell sec tor table in the top panel has c olumns that show the perc entage of samples that are outside the beamwidth and outside 180°. Clic king an ic on in one of the c olumn headings c olors the c ell sec tor wedges on the map by the c orresponding perc entages. This and simply looking at
the values in the table, make it easy to spot c ell sec tors that have a high perc entage of samples outside the beamwidth or outside 180°. Look espec ially for sec tors within the same site that exhibit a high perc entage. When you have identified a sec tor that might be affec ted, selec ted its row in the table while the KPI ic on is selec ted. Spotlight then displays a binned attribute on the map that shows the signal level for the data points for whic h the selec ted c ell sec tor was the serving sec tor and highlights the area around the c ell that is within the beamwidth. This shows at a glanc e how the distribution of the data points relate to the beamwidth. When all or most of the points are outside the beamwidth and within the beamwidth of one of the other sec tors in the site and that sec tor shows a similar pattern, it would suggest that the problem might be due to a c rossed feeder. When the pattern is messier, it would suggest that the problem might be c aused by some other issue, suc h as exc essive mec hanic al downtilt. Troubleshooter. This analysis is available when you c hoose the Standard Analysis load option within Troubleshooter. The Troubleshooter Dashboard page shows the number of sec tors for whic h there might be a problem, if any. However, you would then need to use the Investigation page to eliminate other sourc es of exc essive c overage outside the beamwidth. The KPI names are the same as those shown above for Spotlight. When you use the link to investigate the KPIs, Troubleshooter lists eac h oc c urrenc e of the problem, along with a Map on whic h the BCCH (GSM) or SC (UMTS) is displayed and is used to c olor the sec tors. You would need to work through the list and for eac h oc c urrenc e, examine the pattern around the affec ted sec tor, in order to determine whether the problem is c aused by the c rossed feeder problem or some other issue, suc h as exc essive mec hanic al downtilt: The c rossed feeder problem typic ally exhibits a pattern where the c olors are reversed as shown in the example above. Other problems typic ally exhibit a messier pattern. Send feedbac k on this topic
