Understanding Co-location of ePMP 1000 and PMP 100 Systems

ePMP 1000 Software Release 2.5 introduces the ability to configure the devices to operate with a 2.5ms frame duration.  This configuration allows ePMP 1000 and PMP 100 equipment to be deployed together with GPS synchronization allowing a migration path to ePMP's higher bandwidth offerings.  This article discusses the key topics for co-locating ePMP 1000 and PMP 100 equipment.  Additional information about migrating your network from PMP 100 to ePMP 1000 may be found here:

ePMP and PMP 100 Co-location and migration recommendations guide

Synchronization and Timing

When co-locating systems, either for migration from an older technology to a newer technology, or for a more permanent mixed deployment, it is important to properly configure system parameters in order to avoid interference.Screen Shot 2015-09-11 at 11.42.21 AM.png

Both PMP 100 and ePMP 1000 are TDD systems, which means that the same frequency resources are used both in the downlink (AP to SMs communication) and in the uplink (SMs to AP communication), but multiplexed in time. A TDD cycle, or frame, is the minimum amount of time used to communicate in both directions, and it also includes gaps for hardware turnaround and over the air propagation delays, as shown in Figure 1.

When multiple access points (APs) are deployed in the same geographical area, it is important that they all transmit and receive at the same time. If one AP transmits when another receives, the one that is receiving might not be able to correctly decode the signal coming from the SMs communicating with it, because of the interfering signal coming from the other AP.

In order to avoid this type of interference, three features are needed:

  • The TDD cycle, or frame, needs to start at the same time for all APs
  • The TDD cycle, or frame, needs to have the same length for all APs
  • The frame parameters need to be selected in each AP so that there is no overlap between one AP transmitting and another receiving. An example of these parameters is the duty cycle, i.e. the ratio of the time dedicated to communication in the downlink direction over the total frame time.
    Note that these parameters don’t need to be the same in all APs, but they need to be selected to avoid interference.

These features are needed regardless if the APs use the same technology or not.
Typically, when the APs use the same technology, it is sufficient to select the same configuration parameters to guarantee interference-free co-location. However, when the APs do not use the same technology, the parameters to select may be specific to the technology used, and care has to be taken during co-location and migration.

Frame Start

GPS synchronization is the way of guaranteeing that the frame start is the same for all APs. One important consideration is the sync source used for each AP.

If the APs are the same, and they use the same sync source, they all have the same frame start.
However, if the APs are not the same and/or they use different sync sources, the frame start times may be shifted. A shift in the frame start time may cause self-interference.

Figure 2 shows the synchronization options in the ePMP GUI, which can be found under Configuration > Radio.

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To co-locate and/or migrate an FSK system to an ePMP system,

  1. Select Co-location Mode = Enabled.
  2. In Synchronization Source, select the sync source used for the ePMP system.
  3. In Synchronization Source of Co-located System, select the sync source used for the FSK system.

An additional parameter that can be selected is the Synchronization Holdoff Time. This is the amount of time the AP will transmit after losing the sync pulse when it is not able to regain it.
In the example in Figure 2, the Synchronization Holdoff Time is set to 300secs, which means that the AP will continue to transmit for 300 seconds after losing the sync pulse. If it regains sync before the 300 seconds have elapsed, it will re-adjust the internal timing as needed and continue to transmit normally.

Frame Length

At this point, all APs are GPS synchronized, and the frame start has been corrected for any shift due to different sync sources. The next parameter to select is the frame length.

In order to avoid interference, it is necessary that all APs use the same frame length. This is the reason for introducing the option of a frame length of 2.5 ms in the ePMP product, to be able to co-locate and migrate an FSK network, which only supports a 2.5 ms frame in the 5 GHz band.

Figure 3 shows why it is not possible to co-locate APs supporting mismatched frame lengths.
Let us assume that AP1 uses a 5 ms frame, while AP2 uses a 2.5 ms frame.

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Figure 3 shows that in a 5 ms interval AP1 has one transmit time and one receive time, while AP2 has two transmit times and two receive times.

The interference that mostly affects the system performance is the one at the AP receiver.
For example, in the time indicated with the green arrow in Figure 3, AP1 transmits when AP2 receives. This may completely corrupt the reception of AP2’s uplink signal. Also, in the time indicated with the orange arrow in Figure 3, AP2 transmits when AP1 receives. This may completely corrupt the reception of AP1’s uplink signal.

Additional interference may be experienced at the SM. In the time indicated with the green arrow in Figure 3, the SM communicating with AP1 receives when the SM communicating with AP2 transmits. Similarly, in the time indicated with the orange arrow in Figure 3, the SM communicating with AP2 receives when the SM communicating with AP1 transmits. This source of interference is generally less critical in the overall system performance because the SMs’ antennas have a narrower beam and point at the corresponding AP. Therefore the signal received by other SMs is significantly more attenuated compared to the interfering signal received at the AP.

When co-locating or migrating an FSK system to an ePMP system, select the 2.5 ms frame option, which can be found under Configuration > Radio > Scheduler, as shown in Figure 4.

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Frame Parameters

At this point, all APs are synchronized, and the frame length is the same.

Next, the frame parameters have to be selected in order to avoid any overlap between one AP transmitting and another receiving.

As mentioned above, the parameters do not have to be the same in all APs, but they have to be coordinated in order to avoid interference. Figures 5 and 6 show one example of frames that do not interfere and one example of frames that do interfere.

In both Figures 5 and 6, the Downlink time and Uplink time of the two APs are not identical. However, in Figure 5, there is no overlap between one AP transmitting and another AP receiving, and the two APs can be co-located. In Figure 6, AP1is still transmitting when AP2 is already receiving. This creates interference at the AP2’s receiver and the APs cannot be co-located with these parameters.

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ePMP frame configuration parameters

In the ePMP system, the frame structure is calculated using by three parameters: Channel Bandwidth, Max range, and Downlink/Uplink Ratio.

The Channel Bandwidth and the Max Range can be selected under Configuration > Radio > Access Point Configuration, as shown in Figure 7.
The Downlink/Uplink Ratio can be selected under Configuration > Radio > Scheduler, as shown in Figure 8.

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The Downlink/Uplink Ratio options for the ePMP system are: 75/25, 50/50, 30/70 and Flexible.

The Flexible option should not be selected when co-locating with other systems. The reason is that in Flexible mode the time allocated to downlink and uplink transmissions is adjusted frame by frame according to the instantaneous traffic requirements. It is therefore not possible to guarantee that the AP will not overlap its transmit time to another AP’s receive time.

FSK frame configuration parameters

The FSK frame structure is determined using three parameters: Downlink Data (which is the duty cycle), Max Range, and number of Contention Slots. The Contention Slots are used in the FSK system for random access, for registration and bandwidth request.

All these parameters can be configured under Configuration > Radio, as shown in Figure 9.

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Note that the duty cycle (Downlink data) granularity in the FSK system is higher. It can be selected from 1% to 99% in 1% increments. For this reason, the duty cycle should first be selected in the ePMP system, and the FSK duty cycle adjusted in order to avoid interference.

ePMP-FSK co-location tool

The configuration parameters that affect the frame structure, need to be selected in both systems in order to avoid any overlap between transmit and receive times. Selecting the same duty cycle, channel bandwidth and max range does not necessarily guarantee that there will be no overlap. The reason is that the two systems use a different algorithm to calculate the frame structure, and also parameters that are related to only one technology, like Contention Slots in FSK, or FFT size and guard interval (GI) length in ePMP.

In order to help with the selection of system parameters, Cambium Networks offers an ePMP-FSK co-location tool, available at http://support.cambiumnetworks.com

Let us assume that an existing FSK system is deployed with the following parameters:

  • Max range: 10 miles
  • Downlink data: 75%
  • Contention slots: 4

The FSK AP in one sector of the existing FSK deployment is replaced with an ePMP AP. As the ePMP AP needs to replace the FSK AP, the same parameters are selected:

  • Max range: 10 miles
  • Downlink/Uplink ratio: 75/25
  • Bandwidth: 20 MHz

In order to verify that these parameters do not create interference between the two systems, the co-location tool is used, as shown in Figure 10.

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The co-location tool shows the parameters that are important for co-location, which are the end of the downlink time (DL end) and the beginning of the uplink time (UL start) for both systems.

For successful co-location, the two following conditions need to be met:

PMP100 DL end < ePMP UL start

ePMP DL end < PMP100 UL start

These conditions are checked in the co-location tool:

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The tool shows how the first condition is met, meaning the ePMP AP starts receiving after the FSK AP has finished transmitting. The second condition, however, is not met: the FSK AP starts receiving before the ePMP AP has finished transmitting. During this period of time, any uplink signal received from the FSK SMs will be corrupted by the downlink signal coming from the ePMP AP.
This example shows how selecting the same duty cycle, range and channel bandwidth does not necessarily guaranteed that there will be no interference between the systems. As the two systems use different technologies and additional parameters (such as the number of contention slots), co-location needs to be verified for each specific combination of parameters.

For successful co-location, one or more parameters need to be adjusted, until both conditions are met.
On the FSK side, all three parameters can be changed to align the FSK downlink and uplink times to the ePMP downlink and uplink times.
For example, the max range can be increased, leaving a longer gap between the downlink and the uplink time, and eliminating the overlap. However, increasing the max range beyond the distance that needs to be covered, increases the overhead of the system.
The easiest parameter to change in the FSK system is the Downlink data, as this input is offered with a granularity of 1%, and no additional overhead is introduced in the system. This is shown in Figure 12. This Figure shows the same example shown in Figure 10, with the only difference that the Downlink Data % in the FSK system is now changed to 77%.

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Figure 12 shows how both conditions are now valid, and these parameters are valid for co-location.

Note that in order to make a change in the FSK system, all APs in the same geographical area need to use the new selected parameters. In order to avoid this, the only parameters that can be changed on the ePMP side is the max range. As discussed above, a longer max range will leave a wider gap and eliminate the interference, at the expense of higher overhead.
Figure 13 shows how increasing the ePMP Max Range to 11 miles the two conditions are met. However, the wider gap subtracts air time to data transmission.

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