User's Manual

pulsAR radio Operator’s Manual

2-9

Radio

Filter

LNA

Filter

Local

Osc.

(undesired coupling)

freq

f2f1

freqIF

Figure 2.4– Co-located radio interference

The traditional approaches to reduce this self-interference include:

• Separate the antennas of the two radios further apart.

• Use different antenna polarization.

• Lower the transmit power of the interfering radio.

These approaches are limited and, at most, may allow you to co-locate three of four radios. The Afar

SPAN technology implements a synchronization scheme that completely eliminates this self-

interference allowing you to co-locate a much larger number of radios. This is explained in the

following sections.

2.4.3 SPAN Network synchronization

The PulsAR can be operated in a fixed TDD mode, where the complete cycle is divided into fixed

length outbound and inbound phases. You can specify this cycle split to be 50/50 or asymmetric.

When you co-locate multiple devices you must choose a fixed split and it must be the same for all the

co-located radios. The radios will then synchronize their cycle periods so that all co-located radios

transmit at the same time and then receive at the same time. This avoids the situation depicted in

Figure 2.4altogether. With a synchronized site you can then deploy upwards of 24 radios at the same

location.

The key to the synchronized SPAN network is the generation and distribution of the synchronization

information or heartbeat. At any site where there is more than one co-located radio, the various

radios detect each other, and automatically negotiate which should become the source of the

heartbeat. If that device later is turned off or fails, another device will take its place without user

intervention.