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Capability of the TETRA Equalizer on Multi-path Interference

JW

I'm designing an indoor TETRA coverage making use of a Bi-Directional Amplifer (BDA). Due to the client's requirements, the designed coverage for the outdoor carpark is having both direct line-of-sight (LOS) from donor celllsite and its repeated signal from BDA. LOS and repeated signals are similar in strength. The BDA unit has a round-trip system delay of 15us while feeder propagation time is negligible. So, the time difference between LOS and repeated signal can assume 15us. This scenario can be regarded as multi-path interference.

Knowing that the User Equipment (UE) deploys the equalizers to handle the multi-path effect, I'm unable to find the equalizer capability requirement in the GSM Specification. For a common 2G UE of TERA or GSM, what's the minimum time spread on multi-path signals that the equalizer is able to handle?

  • +1

    I'm afraid I only worked on the later 3G stuff, so I'm a bit shaky on this, but as I understood it. GSM standards don't specify any specific receiver implementation, instead giving a test spec and a performance target, leaving it up to the chip makers to decide how to meet it, but the test waveform is well tailored for 16 state MLSE. In practice all makers do more or less the same, as it's never a good idea to start with a sub optimal designs. From memory the receiver test spec calls up fading with a maximum of 5uS delay, being realistic for reflections in an urban cellular setting - after all a uS is 300metres, so five of them is 1.5km and compares to cell sizes. Any reflection that far away is likely to be noticeably weaker, maybe even comparable to the random chatter from neighboring cells.


    This diagram may help you with navigating the standards numbering scheme, it is quite complicated, each team was in charge of a series of standards with a 2 digit start code, and then could create their own index, so 0501 is the first doc written by team 05 for example. There is a lot of it, literally a life's work for about 100 people for each group, and the styles are not fully aligned between groups, - and as above this is before the time I worked on 3G, so I'm not that familiar, but the structure is similar.I'd start with the handset test specs, and look the fading channel and data rates required to pass -I recall 5usec from discusssions, but I'm not sure what decoder performance is mandated with that.>

    all the standards are available via the 3GPP website, even though the early GSM stuff "GERAN"
    was originally developed under ETSI, it too is made available for free.
    If you want a more general intro to the type of equaliser, the following is a good reference in that it contains a thorough treatment of Viterbi equalization for phase modulation in general and GMSK in particular. The book is out of print but if you can get your hands on a used copy, it is easier than the original papers by Viterbi himself. Raymond Steele, Mobile Radio Communications, IEEE Press, 1992, ISBN 0-7803-1102-7.

    regards Mike

  • 0 in reply to mapj1

    Hello Mike,

    Yes, I also recall from a discussion long, long ago that GSM equalizer is able to overcome a time spread of 5us (10us round-trip) multi-path. BTW, I'll conduct a site acceptance for the new BDA. Let's check if there's some issue in the carpark area.

    Thank for your advice.

    Joseph

  • 0

    Tetra is different and has a maximum difference between direct and indirect paths of 14 Usec.   Getting enhancers to work means you have to in effect ensure that there is no direct path and only enhanced paths are available. (Tunnels are great for this).  However any car park work or other outdoor areas will struggle.  (Why do you want enhanced coverage outdoors if the direct path is ok already?    

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