Abstract
Purpose of optimization guide is to help improve and/or debug existing EPMP network performance from RF perspective. EPMP AP and SM comes with default settings for all parameters. However to get good performance under different conditions, it is important to tweak default values to what will be suitable for deployment. This guide lists some important parameters which are usually confusing and important to change as per requirements.
By: Dhaval Chaudhari
Fixed Mode Vs. Flexible Mode of Operation:
- User can select fixed or flexible mode of operation on AP under Configuration >> Radio tab.
- For Fixed mode of operation, select either 75/25, 50/50 or 30/70. These three modes will allow radio to operate in fixed Downlink and Uplink ratio with or without GPS sync (Depends if GPS sync is enabled or not). In other words, this ratio represents the duration of transmit and receive cycle within TDD frame.
- In Flexible mode of operation, radio dynamically selects Downlink and Uplink resources every frame based on traffic demands. Flexible mode gives more throughput and lower latency. However radio operates in unsynchronized mode as explained later in the document (NO GPS Sync)
- If AP is in sector or in range of another AP, it is recommended to use GPS sync i.e. user should select one of three Downlink/Uplink Ratios and should be configured same on all AP’s in cluster or nearby AP’s to avoid overlapping RF transmissions resulting in system interference.
Synchronized vs. Unsynchronized mode of operation:
- This feature is supported on GPS synchronized units only. GPS sync can be enabled by navigating to Configuration >> Radio tab.
- When AP is deployed in Cluster or in vicinity of other EPMP AP’s it is a good practice to use GPS sync to avoid self-interference. To enable GPS sync select either GPS or CMM as a sync source as per availability. GPS synchronization is supported in fixed mode of operation (fixed TDD ratio) as explained above
- In Flexible mode of operation GPS synchronization is not supported and AP will switch to internal mode i.e. no GPS sync.
- When doing RF Planning for the deployment of collocated APs with channel reuse, it is recommended to use a channel reuse pattern as recommended in the ePMP ABAB frequency reuse deployment document (https://support.cambiumnetworks.com/files/epmp/).
PTP mode of operation:
- User can select PTP only mode from AP radio page (under Configuration >> Radio) as shown in screenshot below. For PTP link it is recommended to select this mode. For many countries regulations are different for PTP and PMP mode (E.g. Transmit power, EIRP).
- For PTP mode, it is recommended not to re-use frequencies that are being used on the nearby PMP clusters.
- If there are no other ePMP sectors in nearby area, it is recommended to use flexible mode. (Flexible mode provides low latency and more throughput)
Subscriber Module Target Received Level: (TRL)
- Each SM’s transmitter output power is automatically set by the AP. The AP monitors received power from each SM and adjusts each SM’s transmitter output power so that received power at the AP from the SM is not greater than what is configured in SM Target Received Power Level. These automatic power adjustments ensure that the SM is not transmitting excessive energy (raising system noise level) and that the SM is able to achieve an optimal modulation state (and maximum achievable throughput).
Nominally, target receive levels must be set lesser than -60 dBm in order to prevent interference from co-located co-channel sectors.
This parameter is located on access point under Configuration >> Radio tab
- In Fixed TDD ratio mode, since GPS synchronization is supported, minimizing self-interference is important. In this scenario this parameter should be set to the lowest value possible so as to keep the SM Transmit power to a minimum to lower the noise floor/self-interference but at the same time allow the SMs to transmit at the highest possible MCS. For Co-located AP’s reusing same channels this is important parameter and should be set same on all AP’s. Recommended and default value in such scenarios is -60dBm. (Low enough to prevent self-interference but high enough to allow SM that can reach such target RSSI to operate at MCS15).
- In flexible mode, self-interference is still a challenge as the ePMP radio is not using GPS synchronization. However, it is still good practice to keep the self-generated noise to a minimum noise floor so as not to cause interference with other sectors on the same tower or nearby towers. This value should be approximately -50dBm.
- Sensitivity for MCS15 is around -64dBm for 20MHz. Default value of TRL is -60dBm. This value may not be good enough in noisy/interference environment. Set this to -50dBm in case of lower throughput than what is expected at MCS15 OR wireless link is not operating at MCS15. Please note if lowering TRL value do not help set this back in range from -50dBm to -60dBm. Do not set this value to -40dBm it will cause all SM’s to transmit at higher power to meet this value increasing noise floor for all nearby sectors.
Max Range
- Max Range parameter can be configured from Configuration >> Radio tab
- Enter a number of miles or kilometers for the furthest distance from which an SM is allowed to register to this AP. Do not set the distance to any greater number of miles. A greater distance:
Does not increase the power of transmission from the AP.
Can reduce aggregate throughput.
- Regardless of this distance, the SM must meet the minimum requirements for an acceptable link. The AP will reject any SM network entry attempts from outside the configured maximum range. Default value is 3 miles.
- This parameter is set based on farthest SM being deployed. Calculate distance of farthest subscriber module using Link Planner or any other suitable tools. Set this value 1 to 2 miles higher than calculated number. Do not set this parameter to max value as it will waste time allocated for data transmission when it is not needed.
Network Entry RSSI Threshold:
- This parameter is located on Subscriber Module under Configuration >> Radio tab
- This parameter represents the minimum Received Signal Strength Indicator (RSSI) required at the SM for the SM to attempt registration with AP. Main purpose of this parameter is to prevent SM with poor link quality from registering to an AP.
- If all Subscriber Modules are expected to have good link budget of -60 to -70dBm and hence a good link quality, leave this value to default.
- If SM’s being deployed are at long distance from AP where link budget is close to -75dBm to -80dBm, Network Entry RSSI Threshold should be set to lower value. E.g. -85dBm.
- However, it is recommended not to deploy SM’s with low link quality (RSSI) as it may result in an unstable link. Customer should explore options of adding another AP to improve coverage of such Subscriber Modules.
Network Entry SNR Threshold:
- This parameter is located on Subscriber Module under Configuration >> Radio tab
- This parameter represents the minimum Signal to Noise Ratio (SNR) required at the SM for the SM to attempt registration with AP. Purpose of this parameter is to not allow SM’s to register that are not in the AP’s sector. Main purpose of this parameter is to prevent SM with low SNR from registering to an AP
- If all Subscriber Modules are expected to have good SNR, leave this value to default.
- If SM’s being deployed are at long distance from AP where SNR is low, Set Network Entry Threshold to lower value e.g. 10dB.
- However, it is recommended not to deploy SM’s with such a low SNR as it may result in an unstable link. Customer should explore options of adding another AP to improve coverage of such Subscriber Modules.
Management Packet Rate:
- This parameter can be configured from Configuration >> Radio tab
- EPMP AP allows user to set MCS (modulation) of management packets. Default is MCS1.
- User can set this value to MCS0 if deployment is in really noisy environment.
- If event log is shows GPF’s lost messages, user can switch MCS of management packets from default value to MCS0. MCS0 will be more tolerant to noisy environment than MCS1 and can prevent SM’s from losing session in noisy environments.
Transmit power and antenna gain:
- This parameter can be configured from Configuration >> Radio tab
- Antenna gain value should be set based on the antenna being used.
- Please note setting antenna gain to value higher than actual antenna gain does not mean increase in total output power. It may decrease radios actual transmit power to meet EIRP requirements for regulatory purposes.
- Set transmit power to max value allowed for country code.
Automatic Channel Selection (ACS):
- User can Enable/Disable ACS feature on AP from Configuration >> Radio tab
- Automatic channel selection feature is recommended to use when user cannot find any specific channel with low noise/interference from spectrum scan.
- When ACS is enabled, radio will scan entire band and look for channel with lowest interference level. ACS feature is very useful when all channels have high energy OR user do not want to run a manual spectrum analysis and select channel manually.
- It is recommended not to use ACS feature for co-located AP’s i.e. when AP is in a cluster with perfectly planned RF network (e.g. frequency re-use is enabled)