Motorola GPS-100M manual Introduction, Link Time

Simple example of cross- interference when three links of different lengths are mounted on a mast and operating on the same or adjacent channels

Introduction

The Motorola wi4 Fixed Point-To-Point (PTP) 600 Series Wireless Ethernet Link, which consists of a pair of radios deployed one at each end of the link, operates on a single frequency channel in each direction using Time Division Duplex (TDD). In situations where a number of radios are installed on the same mast or where a large number of links are installed in a sizeable, dense network, it is possible that the performance or throughput of some of the links can be reduced. In some cases, a number of the links may not work at all. This is due to interference between the units, and the levels of interference can worsen when the links are operating on the same or adjacent channels.

The effect of this cross interference between units can be reduced by ensuring that the radios are in synchronism, meaning that transmit and receive frames of the units are synchronized so they do not interfere with each other. TDD Synchronization introduces a fixed TDD framing mode and allows frame timing in a PTP 600 link to be synchronized with an external reference – a Global Positioning System (GPS) unit.

Before TDD Synchronization

Link 1

Link 2

Link 3

Time

	Transmit	Receive
Interference Link 2 to Link 1			Interference Link 3 to Link 2
Interference Link 1 to Link 2			Interference Link 1 to Link 3
Interference Link 2 to Link 3			Interference Link 3 to Link 1

After TDD Synchronization

Link 1
Link 2
Link 3
Time
Transmit	Receive

The result is that TDD synchronization minimizes the interference between links and promotes optimal spectral re-use while greatly enhancing link performance. By timing and synchronizing transmit and receive signals, network operators can co-locate multiple radios on a rooftop or tower without interference.

GPS Synchronization Unit GUIDE