Friday 31 July 2009

LTE Band 13 mystery

Interesting observation from Hooman Razani in LTE university blog:

It was in a classroom, after I had opened Technical Specification 36.101 for handset radio transmission and reception that I suddenly noticed for the first time something odd about Band 13.

I said to the class without hesitation, Band 13 [UL = 777 MHz-787 MHz & DL = 746 MHz-756 MHz] has the uplink and downlink reversed! In contrast to all other FDD bands in the table (and many other bandplans elsewhere), in this band, the higher frequency is allocated to the mobile device and the lower frequency to the base station. The normal practice of FDD paired band allocation has to do with attenuation properties of the carrier frequency and the availability of the power. The mobile side has usually been allocated the “easier” lower-frequency portion of the band which experiences smaller amount of attenuation and therefore requires less power for closing the uplink. A reversal of this allocation strategy in band 13 is a sure sign that some other problem is lurking in the background, namely interference in the form of spurious emissions.

The 700MHz band is divided into four paired blocks, A, B, C and D (an unpaired E Block is also available). The A and B blocks are guard bands. The C blocks (LTE Band 13) in this upper 700MHz band were the main object of last year’s auction. 12MHz in the middle of this spectrum belongs to the Public Safety.

A – C – D – B – [Public Safety] – A – C – D - B

A look at the structure of the auctioned UHF band shows that upper portion of Band 13 is closer to the public safety band. In order to reduce the effects of interference to the public safety band and other bands due to intermodulation effects of this carrier, the higher more “difficult” frequency has been allocated to the lower powered handset. On the other hand we can expect a slight enhancement in indoor coverage for band 13, due to further lowering of the downlink frequency .The question to ask is if this maneuver is enough to suppress the spurious emissions caused by the handsets to nearby bands. Further details after initial deployment will shed light on the mysterious case of band 13.


Dr. WMS said...

There really is no mystery about band 13. The real mystery is why the author noticed that band 13 had UL and DL reversed, but did not notice that band 14 also has UL and DL reversed!

The main reason for reversing is to protect the relatively low power transmission of the LTE handsets from the high power transmission from the TV transmissions below the band. The European Digital dividend band will also have UL and DL reversed.

Zahid Ghadialy said...

Thanks Dr. This is a very broad area and I tried searching for an answer myself but with no luck. Really appreciated that you pinpoint the actual reason.