Prof. Ing. Andrej Luc, PhD. - Cpt. Ing. Milan Janicek, PhD,
VA Brno - MTIP Brno

Comparison of Frequency Hopping systems – jamming resistible, interception and security transmission

 

Definition of Subject

In this paper we will discuss and compare Frequency Hopping (FH) communication systems and stations, which are in our market. The paper will help to compare the FH stations, which are offering to Europe (NATO) countries.

We will discuss the fundamental principles of FH systems, which are important for jamming resistible, secure transmission and interception of these systems with respect to other communication systems, reconnaissance and jamming systems. We can remark that to compare the all properties of FH systems for any user is very complicated and it cannot be do in one paper.

Spread Spectrum (SS) signal consists of carrier, information and address signal [1]. SS signal is a standard signal modulated one more by address signal. FH signal does not contains address signal, for this reason it is pseudo spread system and this FH system can be jammed.

Frequently times, there are offered the station from firm THOMSSON, RACAL, ERICSSON, ROHDE&SCHWARZ and TRANSWORD.

Comparison of Frequency Hopping Systems

We will consider a Slow FH (SFH) systems only, fast FH systems are not used for tactical communication. But, we must discuss FH+DS system (see MRR200 ERICSON) and to compare this system with respect to pure FH systems.

A block diagram of SFH to the evaluation of fundamental parameters and to the comparison of FH systems is in Fig.1. From block diagram we can see the following stage can be evaluated :

  1. Error correction of code. Using of Reed-Solomon codes can be evaluated very positive.
  2. Ciphering (COMSEC) and transmission (TRANSEC) keys. Independent determination of COMSEC and TRANSEC is much better for a security of systems. When TRANSEC is determined by COMSEC, then it can happen the situation, at which the deciphering can be more easy.
  3. The number of COMSEC and TRANSEC keys is important too. It can be in the range 825 up to 848.

 

4. Types of PN address code and its length. The length of codes is usually several days.

5. Type of modulation is very important for jamming resistible and radio range. I/Q modulation can be recommended (MRR200 ERICSSON – GMSK, PR4G THOMSON – RC modulation).

6. Selection of modes : - FH, frequency adaptive mode (ALE, FCS), general and channel hailing, …

7. Synchronisation circuits – acquisition and tracking systems. This sensitive problem will be discuss latter.

8. User properties – form and configuration of radio nets, radio direction, alarms, priority of commander, …, emergency clearing.

9. Data transmission : - synchronous, asynchronous system, - static and triggering TDMA mode.

10. Interoperability – in fixed frequency with old radios in clear voice mode (STANAG 4203). Interoperability of FH system with interception and jamming systems will be discuss later.

Each above point represents separate paper. More detail we can analysed synchronisation and interoperability only.

 

 

Jam resistible of Synchronization Systems

Synchronisation system can be very sensitive to the jamming. Very important is a fine synchronisation at each starting of transmitting (push and talk). Fig.2 shows starting waveform of the transmitted signal of three real stations. The first waveform keeps the fix frequency longer time. During short time (several 10-th msec) interception system will be able to determine the hop frequency and the jammer can jam this FH system. The interception of second waveform on Fig.2 is a more difficult and the time for interception is short. The best starting waveform is third waveform on Fig.2.

Figure 2. The beginning of transmitted waveform of three various stations

 

Interoperability of FH Systems with Interception and Jamming Systems

After establishing of FH stations for all arms. The radio situation will be much more complicated. The interception receivers will receive a short frequency hops, but this receivers will not be able to distinguish own hops form the enemy hops. Also, these receivers will not know in radio space if there is same hops of enemy in our area (radio space).

Interoperability with the Interception Systems.

A reconnaissance can be successful only if the carrier frequency of our communication systems are known in each time (each milisecond).

Our FH signals and also the enemy FH signals can be in the difference or in the same frequency range and also the structure of hops can be the difference or the same. Clearly, the worse case for distinguish is if our and enemy FH signals are the same (when we use the same stations). Our interception receivers must be synchronised to our FH signals and also they must know the instantaneous hop frequency. This synchronisation can be reached by the following operations, Fig.3:

 

Interoperability with Jamming Systems

The received signal we can jam only if we are sure that this signal is enemy signals. We must know our hop frequencies and these frequencies will be determined as “time forbidden frequency”. This concept is equivalent to the concept “forbidden frequency”, which was use in standard continuos jamming systems. A complete jamming system consist of the interception, analyser and jamming system, as it shown in Fig.4.

The controller of jamming system must be synchronised by own radio nets and simultaneously it must be able to generate “time forbidden frequencies”. Also, the controller of jamming system must be also synchronised to the enemy FH signal so that interception receivers must be synchronised to the starting of each hops of enemy signal. The both synchronisation can be reached by the following operations, Fig.4:

Above measures show only fundamental steps, which must be done. Radio stations must enable above operations.

 

References

[1] VOSICKY, V., LUC, A.: Spread Spectrum Systems as Anti-Jamming and Hidden

Communication Systems. Coneferce EW, NDU Budapest 1999.

[2] JANICEK, M., LUC, A.: Recognition and Jamming Frequency Hopping Systems.

Communication Systems. Coneferce EW, NDU Budapest 1999.