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# Mesh Filters and Channel Frequency Assignments
The Gateway AP and Mesh AP channel frequencies have a specific relationship that must be considered when making frequency assignments in the Onyx Portal.
## Overview of Mesh Node Architecture
A Mesh Node uses a filter network to isolate the Mesh AP’s TX/RX frequency from the Mesh Node backhaul TX/RX frequency.
Mesh Node to Gateway Frequency Planning
In *Figure: Mesh Node to Gateway Frequency Planning*, the Onyx Gateway Node (AP) frequency is referred to as Fgw, and the Mesh Node backhaul frequency is referred to as Fmn. The Mesh Node backhaul frequency operates at the same frequency as the Gateway Node, so in all cases, Fmn = Fgw. To provide subscriber access, the Mesh Node is directly cabled to an outdoor AP (referred to as the Mesh AP). The Mesh AP is configured to transmit on a different frequency (Fmap) than the Mesh Node backhaul frequency (Fmn).
During the configuration of the SAS parameters in the Onyx Portal, the operator will configure the Gateway AP and Mesh AP for a series of **Preferred Frequencies** (from most to least).
{% hint style="info" %}
**NOTE:** When you request channels on the SAS, you should set Strict Preference to TRUE to ensure that the system does not accept a grant outside of the chosen preferences. The SAS currently supports 10 and 20 MHz bandwidth channels only. This should be considered when choosing the Preferred Frequencies.
{% endhint %}
The operator should take into consideration the desired bandwidth of 10 or 20 MHz to avoid selecting a frequency that will fall out of the “low or high” pass band ranges shown in *Table: Backhaul to Mesh Node-AP Frequency Assignments*. This process allows for SAS grant flexibility, as channel assignments can change.
{% hint style="info" %}
**NOTE:** *Table: Backhaul to Mesh Node-AP Frequency Assignments* example lists the Backhaul (Blue) and Mesh AP (Yellow) 10 MHz and 20 MHz channels that could be used when configuring the Gateway and Mesh AP frequencies. When the Backhaul falls into the “Low Band” the Mesh AP should be configured to transmit in the “High Band” and vice versa.
{% endhint %}
Backhaul to Mesh Node-AP Frequency Assignments
{% hint style="info" %}
**NOTE:** The Frequency Planning Example is a sample frequency plan. The Onyx Portal allows the selection of other center channels based on the selected bandwidth.
{% endhint %}
{% hint style="info" %}
**NOTE:** When the Gateway AP is configured for Carrier Aggregation (CA), the Preferred Frequencies chosen for both the primary (PCell) and secondary (SCell) cells must fall within the same CPE backhaul filter range (high or low).
{% endhint %}
## Frequency Planning Example
The following example in *Figure: Mesh Node Frequency Planning Example* illustrates the Mesh Node frequency selection concepts for one Mesh Node.
{% hint style="info" %}
**NOTE:** Antenna Isolation between the Mesh Node and Mesh AP antennas must be ≥ 50 dB. The Mesh AP should expand coverage radially away from the Gateway AP.
{% endhint %}
Mesh Node Frequency Planning Example
Given information for Gateway Node in the example in *Figure: Mesh Node Frequency Planning Example*:
* Operating BW = 20 MHz
* Fgw = 3575 MHz
1. The frequency passband for the Mesh Node can be determined; see the list of frequencies that could be used for the Mesh AP in *Table: Backhaul to Mesh Node-AP Frequency Assignments*.
2. The frequencies that could be used for the Mesh AP if the bandwidth of the channels is changed to BW = 10 MHz can also be determined; see the list of 10 MHz channels in *Table: Backhaul to Mesh Node-AP Frequency Assignments*, *column 3*.
In this example, Fmn = Fgw = 3575 MHz. Since the Gateway AP is operating with a BW=20 MHz, you can refer to *Table: Backhaul to Mesh Node-AP Frequency Assignments* and see that the Mesh Node backhaul will operate in the filter “low pass band” range of 3570-3590 MHz and the Mesh Node will operate in the “high pass band” of 3640 to 3700 MHz. The operator should then select Preferred Frequencies for the Mesh AP in the “high pass band”.
{% hint style="info" %}
**NOTE:** When you request channels on the SAS, you should set Strict Preference to TRUE to ensure that the system does not accept a grant outside of the chosen preferences.
{% endhint %}
Examples of frequencies that could be selected in the Onyx Portal for the Mesh AP are summarized in *Table: Preferred Frequencies Example Solution*.
Preferred Frequencies Example Solution
| Fgw = Fmn (MHz) (Backhaul Frequency, Low Band) | Fmap (MHz) 20 MHz (Mesh AP Center Channels, High Band) | Fmap (MHz) 10 MHz (Mesh AP Center Channels, High Band) | Fmap (MHz) 10 MHz (Mesh AP Center Channels, High Band) |
| ---------------------------------------------- | ------------------------------------------------------ | ------------------------------------------------------ | ------------------------------------------------------ |
| 3575 | 3650 | 3645 | 3675 |
| 3575 | 3670 | 3655 | 3685 |
| 3575 | 3690 | 3665 | 3695 |
Remember that this example looks at a single Mesh Node example. When more Mesh Nodes and APs are added, RF planning to avoid overlapping channels and SAS considerations will play an important role in frequency planning.
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