Consequences of Interference in co-located Frequency Hopping Systems

Authors:

  • Sara Linder
  • Karina Fors

Publish date: 2016-11-30

Report number: FOI-R--4318--SE

Pages: 31

Written in: Swedish

Keywords:

  • frequency hopping
  • communication systems
  • co-location
  • out-of-band emissions

Abstract

On military platforms, several different radio systems are co-located, in a limited area. These radio systems can interfere with each other. Interference can also originate from electric equipment on or near the platform, for example computers on the platform. For frequency hopping systems sharing the same frequency hopping band the risk of interference increases when there are few frequencies. Especially in the UHF-band (Ultra High Frequency) the number of frequencies is limited at the same time as the band is used by many military systems. The number of channels are more limited for a wideband system and hence the risk for interference between the systems is higher. In this report, the impact of interference from electronic equipment or an interfering transmitter on a frequency hopping system is analyzed. The interfering transmitter is either placed on the same platform or in the vicinity of the receiver. Since the distance between the antennas is short the transmitter's out-of-band emissions can affect the receiver. To analyze the performance of a system using error correcting coding over several hops a model is derived. The coding yields robustness against interference in one hop. The transmitter is modelled with different number of sidebands that can affect the receiver in addition to the direct collisions on a frequency. To avoid such collisions, orthogonal frequency hopping can be used where the transmitters are prevented from using the same frequency at the same time. Results show that different methods to solve interference problems in frequency hopping systems are effective in different situations. If the error probability is caused by direct collisions the use of orthogonal frequency hopping can be effective, while solving the situation with increased isolation between antennas on the same platform can be difficult. However, an increased isolation can be a better solution than orthogonal frequency hopping if the cause is the sidebands of the transmitter. Hence, to find a proper solution to the interference problem it is important to know what part of the transmitter spectra that causes it.