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Application Notes / SparkWave AR 18/23G Active Repeater
Ensuring radio path clearance is a constant practical problem in microwave radio path engineering. Line-of-sight between two antennas is necessary but in many cases path clearance cannot be directly ensured. Several solutions to overcome this problem are well known: implementation of relay stations, passive repeaters or two back-to-back antennas.

  • A relay station is the most expensive solution. This is reasonable in the case of (radio-relay) RR network nodes if an add-drop possibility is expected.
  • The most commonly used solution is a passive repeater. The main disadvantage of a passive repeater is the high installation price and limited range of incident angle. If the incident angle increases, the necessary reflection surface increases as well. Large surfaces are inconvenient in both cost and ecological aspects.
  • The third possibility is two back-to-back connected antennas. This solution is applicable in the case of one of the terminal stations being close to the repeater (up to a few hundred meters).

This application note deals with the possibility of implementation of a lesser-known solution: the active repeater. The Iskra Transmission SparkWave AR 18/23G active repeater was found to be one of the most price effective and reliable solutions. Hundreds of devices have been installed since 1999 and they are still operating without any trouble.

A short description of the SparkWave AR 18/23G active repeater

The SparkWave 18/23AR active repeater is a proprietary solution of Iskra Transmission. It was primarily developed as an optional device for the family of SparkWave DRL 18/23GA (18 and 23 GHz frequency range) radio-relay systems, but it can also be used in combination with the equipment of other producers. It consists of a microwave module (antenna branching units and two wideband microwave amplifiers with automatic gain control) and two antennas for both directions. One of the two antennas can be mechanically bound with the AR module, while the second one is connected to the AR module by flexible wave-guide. The received signal from both sides is amplified by two amplifiers and transmitted in the opposite direction at the same frequency.

The low power consumption of the repeater (1.3 W) makes solar cells a possible power supply. The solar cells have a lifespan of more than 20 years. Batteries assure 30 days of autonomy even in the case of damage to the solar cells or continuous shade. Normal daylight without direct sunlight is sufficient to supply the device and charge the battery.

The advantages of an active repeater are:

  • The amplification of active repeater is independent of the incident angle (0 to 180°). High performance (directive) antennas enable enough isolation between both antennas for sthe stability of repeater operation. Both antennas can operate on the same polarization.
  • The signal amplification enables establishment of a long-haul transmission system - a chain of active repeaters can be used for this purpose.
  • A price-effective solution
  • Quick system set-up
  • Active repeaters can be much smaller in comparison to passive repeaters, so they are also suitable for ecologically sensitive locations.

Figure 1: Active Repeater layout

Questions arising during AR planning
  • transmission quality with regard to the position of the AR between two radio terminals

  • number of active repeaters connected in the chain
Position of the AR between two terminal stations
Let us assume 40 km radio path length is bridged with one active repeater AR. Simulated distance from the active repeater AR and the terminal station is 1 km, 5 km, 10 km and 20 km.

Other parameters:
  • High performance directive antennas were chosen. The antennas diameter (gain) is fitted to the hop length (between 0.8 and 0.3 m)

  • Transmitted power of SparkWave DRL 18GA: +16 dBm

  • Transmission capacity: 34 Mbit/s (B=24 Mbit/s)

  • AR output power +13 dBm (automatic gain control between 43 and 60dB)

  • During fading, maximum amplification of AR is 62 dB.

The subject of simulation is calculation of the flat fade margin (BER = 10-3) for different positions of AR.

During simulation, in addition to the signal amplification, thermal noise of the AR in channel bandwidth (B=24 MHz) must be taken into account. Both, the receiver noise and the transmitted AR noise will cause the receiver threshold degradation.

The simulation results are illustrated in the following diagram.



In case where AR is close to the terminal station, transmitted AR thermal noise at the SparkWave DRL 18GA input will dominate. Due to high level of the receiver input signal, the transmission quality remains nearly the same compared by the case of transmission without the AR. In case where AR is in the middle between terminal stations, thermal noise of SparkWave DRL 18GA will dominate and the thermal noise of the AR will be negligible. The transmission quality and availability will be increased.

The chain of active repeaters
Active repeaters can be connected in the chain and long haul connection can be established. The number of successive AR's is limited due to accumulated noise of the AR along the chain.

Example:
SparkWave DRL 18GA transmitted power 16 dBm
Frequency 18.7 GHz
Antenna gain (0.8 m) 41.2 dB
AR noise figure 5 dB
Channel bandwidth (34 Mbit/s 24 MHz
Hop lengths between AR 17 km
AR amplification 60 dB
CNR for BER=10-3 14 dB


As can be seen from the diagram, eight hops by AR will decrease FFM (flat fade margin) for 8 dB. The number of successive AR is dependent of required transmission quality and availability. Designer must optimize each case separately.

Summary
The implementation of Iskra Transmission SparkWave DRL 18/23AR active repeater was found to be one of the most competitive and reliable solutions. Many hundreds of devices have been installed since 1999 and they continue to operate without troubles. Long haul transmission systems can be implemented by using a chain of active repeaters. Low price, quickly set up, incident angle independence and ecological aspects are the main advantages of active repeater.
 
 
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