The “Worst Uplink C/(I+N)” study

This study is named the “Worst Uplink C/(I+N)” because it shows the C/(I+N) ratio predicted for the uplink path using predicted interference levels which assume the interfering sector or mobile has a fully-loaded frame. In most real-world situations the Interference component of the ratio would not be this bad unless the network was extremely congested.

This worst-case approach is useful, though, because an effective channel plan and site configuration should prevent interference, even under heavily-loaded conditions. Showing an effective “worst-case” ratio allows you to determine where potential interference could arise and identifies the interference source. This article will describe the options available within the study and how to interpret the results.

Types of interference considered and study options

The study options contains a few settings particular to this study type:

Open

Include interfering remote/mobile units

When this option is checked, the study will consider the coverage footprint of co-channel and adjacent channel sectors that might contribute uplink interference for the grid cell in question. The study will then identify a single location where a mobile assigned to the interfering sector could contribute the highest level of interference in the desired base receiver. The interference level is included in the C/(I+N) ratio for each grid cell. The location and interference power level are reported in the WiMax_Uplink_Interference_Details report. Note that the location of worst interference could be a location where it’s very unlikely there would be much user activity (e.g.: on a rural hilltop); in those cases the impact on the C/(I+N) ratio will likely be exaggerated. But again, the aim of the study is to demonstrate that the channel plan in use will perform well even under extreme traffic loads or with users in odd locations.

Open

Include base-base interference (2nd txgroup)

This will include potential co-channel and adjacent channel interference from a set of sectors in the secondary Tx group. In a coordinated system the downlink and uplink signals are separated either in the time domain (TDD) or in the frequency domain (FDD), so there is no potential for self-interference between base station transmitters and receivers. However, if there are sectors on an uncoordinated network, such as a neighboring system using the same band, then there is the potential for the downlink signal from the uncoordinated sites to interfere with the uplink signal for the desired network. To account for this, you’ll need to set up a Secondary Tx group with the correct sector info including Tx power, antenna information, etc., to represent the potential interferers. These will be included in the C/(I+N) ratio predicted for each grid cell as well as in the Uplink Interference Details.

Setting up the Base sectors

To use this study, first set up the sectors in the Primary Tx group to use a compatible System Type (WiMax) and set up a Channel Plan Template file along with the rest of the System Details. Then assign channels to all sectors using the GUI, Spreadsheet Editor, or the Automatic Frequency Planning function.

Be sure to also set up the Receiver details on the sector, including the Equivalent Receiver Noise Bandwidth, which should be the same as the channel bandwidth listed in the CPT file.

Setting up the mobile/remote

Be sure to set up the parameters for the mobile remote unit, as its antenna gain, Tx Power, required service threshold, etc., will be taken into account. Note that if a directional antenna pattern is used on the mobile/remote unit, that antenna will be assumed to always be pointed directly back to the desired base sector. The typical use-case should use an omni mobile/remote antenna.

The Uplink Interference Details report

When the Worst Uplink C/(I+N) study runs, it also generates a text report which can be found by going to Utilities -> System Reports -> WiMax_Uplink_Interference_Details.txt.

This report will contain a section for each sector within the Primary Tx group that gives the sources of interference used for each sector and the interference power seen at the desired receiver. Note that for adjacent channel interference this value will be scaled by the Adjacent Channel Rejection set in the sector details.

In the above example SiteD1 is in the secondary group, representing an uncoordinated system that is co-channel to SiteC1 and adjacent-channel to SiteB1. The interference report for SiteB1 shows an interfering mobile/remote unit and gives the lat/long where a mobile assigned to SiteD1 would generate the most interference into the SiteB1 receiver; that interference value is scaled by the Adjacent Channel Rejection, so it’s relatively low. The report for SiteC1, however, shows direct co-channel interference from SiteD1’s transmitter, which has greatly reduced the C/(I+N) ratio for all of SiteC1’s coverage area.

By examining the WiMax_Uplink_Interference_Details.txt file after running the Worst Uplink C/(I+N) study, you can determine which potential sources of interference are degrading the performance of a given site and evaluate different options for mitigating that interference, such as down-tilting antennas or changing the channel assignments.