Configuring MIMO operation in PTP 650/700

PTP 650 and PTP 700 are equipped with two separate transmitter channels and two separate receiver channels. These channels are normally connected to the Horizontal and Vertical ports of a dual-polarized antenna. The antenna might be integrated in the ODU or it might be connected through the RF connectors of a connectorized ODU. The PTP 700 Connectorized+Integrated offers the best of both worlds.

The two channels are used to provide either polarization diversity or polarization multiplexing.

Polarization diversity means that the same data is transmitted in the H and V channels. The receiver combines the data received in the two channels, meaning that it has two chances to get a decent signal. Polarization diversity makes the link more resilient to multipath fading because fading is not normally highly correlated between the two polarizations. The link is operating with polarisation diversity when it is in one of the "single" modulation modes.

Polarization multiplexing means that different data is transmitted in the H and V channels. The receiver has to be able to receive data in the H channel without any help from the V channel and vice versa. The benefit of polarization multiplexing is that the over-the-air data rate is doubled. The link is operating with polarisation multiplexing when it is in one of the "dual" modulation modes.

The PTP 650 or PTP 700 receiver selects single or dual modulation modes automatically, preferring dual when the wireless propagation is reasonably free from fading, and resorting to single when conditions are more challenging. Transitions between single and dual modes occur only in the 16QAM 0.63 modulation mode. We can think of the modes as a 'Y' shape like this:

Mod Modes.png

Note that the receiver drives the choice of modulation mode, signalling to the transmitter of the remote unit to move to the appropriate modulation mode.

A consequence of the 'Y' arrangement is that any transition between the higher single and dual modulation modes involves dropping the modulation mode to 16QAM 0.63, resulting in a temporary loss of capacity.

Sometimes it's important to maintain a certain minimum capacity. For example, when the network traffic includes constant rate traffic like TDM circuit emulation or VoIP. If the capacity of the single payload mode is sufficient, it makes sense to disable the dual payload modes using the Dual Payload attribute in the Wireless Configuration page of the Installation Wizard. If you select this option, you won't have the benefit of the higher capacity available from the dual payload modes, but you'll never suffer the dip in capacity that accompanies a single to dual (or dual to single) transition.

Remember that the receiver drives modulation mode selection, so this control affects the mode transmitted by the remote unit and therefore the mode received by this unit. For a symmetric configuration, you need to make the same settings at both ends of the link.

Another control worth mentioning is Maximum Receive Modulation Mode, also in the Wireless Configuration page. This limits access to the higher modulation modes. Consider reducing the maximum mode using this control if the link experiences intermittent interference and you don't need the capacity of the highest modes.

Consider also the Link Mode Optimization control. This control provides a tradeoff between capacity and latency, and affects the thresholds for transitions between modulation modes. Choose the IP setting for maximum capacity. Choose the TDM control for use with traffic that needs reliable delivery such as TDM emulation and VoIP.

Finally, it's worth pointing out that the Maximum Receive Modulation Mode control is duplicated on the System Configuration page. Unlike the controls in the Installation Wizard, this one does not need a reboot.

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