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BL stands for Bandwidth reduced Low complexity and CE stands for Coverage Enhancement. In release 13, you would see many statements about BL UE/CE UE, but I took me quite a while to find out what they stands for :) (I found it in 36.300).

In many Whitepaper or articles, you might have seen the term like MTC, LTE-M1. But in formal 3GPP Technical Specification, you would noticed that these terminology (e.g, MTC, LTE-M1) is not clearly defined. In 3GPP TS, the term BL/CE is usually used to indicate the implementation of LTE-M1.


Followings are topics / sections that I want to cover in this page.



Fundamental Features of LTE-M1 (BL/CE)


In short, LTE-M1 (BL/CE) is an design / implementation that is to meet the MTC criteria as shown below.




Followings are some of the major characteristics of LTE-M1.

  • LTE-M1 operate only in 1.4 Mhz (6 RB) bandwidth.
  • LTE-M1 operate in Half Duplex mode
  • LTE-M1 use the limited (reduced) max transmission power.
  • LTE-M1 would mainly operate with legacy LTE using wider system bandwidth (e.g, 10 Mhz, 20Mhz system bandwidth).
  • LTE-M1 divide the legacy LTE system bandwidth into multiple sections of 1.4 Mhz and use any one of those sections (theoretically, LTE-M1 can use different 1.4Mhz section within the system bandwidth at every subframe)
  • LTE-M1 does not use PCFICH, PHICH, PDCCH which is required to be spreaded across the whole system bandwidth of legacy LTE.
  • LTE-M1 use specially designed control channel called MPDCCH.
  • In LTE-M1, MPDCCH and the corresponding PDSCH (i.e, the PDSCH scheduled by the MPDCCH) is not in the same subframe. This is called 'Cross-subframe scheduling'.
  • LTE-M1 use specially designed DCI formats (6-0A,6-0B,6-1A,6-1B,6-2)
  • LTE-M1 can transmit PBCH, PRACH, M-PDCCH, PDSCH, PUCCH, PUSCH in repeating fashion. (This is to make these channels decodable even when the signal quality/power is very poor as in the harsh condition like basement. As a result, this kind of repeating transmission would make the effect of increasing cell radius and signal penetration)
  • LTE-M1 support only the limited number of transmission mode : TM 1,2,6,9 which can operate in single layer.
  • In LTE-M1, PDSCH scheduling (DCI) and transmission happens in different subframe (Cross Subframe Scheduling).
  • LTE-M1 utilize a new physical channel format that transmit SIB/RAR/Paging in repetitive mode without using control channel
    • All the resource allocation information (subframe, TBS, Subband Index) for SIB1 decoding is determined by a MIB parameter (No Control Channel is used for this)
    • All the resource allocation information (subframe, TBS, Subband Index) for SIB2 decoding is determined by several SIB1 parameters (No Control Channel is used for this)
  • LTE-M1 support the extended Paging (DRX) Cycle



Compatibility with legacy LTE


How is LTE M1 compatitle with legacy LTE ? Actually this can be a pretty fundamental question and help you a lot to understand LTE M1 from comparison, but I asked this question to myself with more practical reason. When I started to reading LTE M1 3GPP specification (I think it was around Aug 2016), I was eager to try something myself just to understand the specification itself. (I am not such a genious to understand the details just by reading the documents. There has been almost nothing that I got a detailed understanding without hands-on). Of course, at that time there was no LTE M1 device that I can try with and I didn't have any LTE-M1 capable test equipment either. However, Fortunately I had access to a couple of pretty good toys : a LTE network simulator with super detailed controllability and high performance vector signal analyzer with LTE analysis feature. So my idea was :

  • Just based on a couple of initial reading it seemed as if LTE-M1 is very similar to legacy LTE.
  • It is just using 1.4 Mhz which is also supported by legacy LTE.
  • It is using half-dulplex. Half-duplex was there from day 1 of the legacy LTE.
  • It is using a special control channel called M-PDCCH, but this is very similar (almost same) concept to E-PDCCH which is also supported by the legacy LTE

And with some other reason, I thought I might tweak my super flexible LTE network simulator to act like LTE-M1 eNB at physical layer at least. But the hard reality that I realized was 'it SOUND very similar to legacy LTE, but not same. Not even at the level of similarity where the legacy LTE can be tweaked to emulate a small feature set of LTE-M1'. In short, followings LTE-M1 feature is same as legacy LTE.

  • PSS (Primary Synchronization Signal) is common to both legacy LTE and LTE M1
  • SSS (Secondary Synchronization Signal) is common to both legacy LTE and LTE M1
  • CRS (Cell Specific Reference Signal) is common to both legacy LTE and LTE M1

It means if you have LTE-M1 device, you may test with the legacy LTE equipment to check if it can detect the cell and decode physical cell ID and check if it can come up with reasonal measurement of RSRP, RSRQ.

However, the similarity ends here. All other things are not compatible with the legacy LTE even if they sound similar. Even for MIB (PBCH), LTE-M1 uses different resource element mapping from legacy LTE (See LTE-M1 PBCH). SIB1 decoding is not compatible either. LTE-M1 SIB1 scheduling is not determined by DCI. It is determined by a single parameter contained in MIB and a set of pretty complex predefined table. On top of it, the physical location of SIB1 hops among multiple locations (i.e, across the multiple narrowband index). See LTE-M1 SIB1 (i.e, SIB1-BR) page for the details. From here (from RACH), the differences diverges even further. The scheduling method is completely different and almost every transmission of PDSCH, PUSCH is being done in very specially designed repeating fashion.


In short, my final conclusion was to give up the attempt to try LTE-M1 by tweaking the legacy LTE protocol stack and decided to decide until I get a touch on real LTE-M1 device and LTE-M1 equipment. Now (as of Mar 2017) I got access to LTE-M1 test equipment and waiting to get a touch to LTE-M1 UE :)



List of the detailed topics


Followings are list of the detailed topics. I am assuming that readers already have knowledge on how legacy LTE works and I will describe these topics with focus on the differences from the legacy LTE.