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NB-LTE

 

NB-LTE stands for Narrow Band LTE. For the high level description of NB-LTE and some comparative view of other technology, refer to LTE-M/Category M/M1/M2 page. Early July in 2016 (around Jul 10), the first version of 3GPP TS documents on NB IoT were posted and I started working on note based on these documents. It is very early stages for now and these notes will keep being updated and still some of high level description based on TR 45.820 (Ref [2]) remain in some notes. (I see pretty big differences between the official TS and TR 45.820 in terms of the details).

 

Followings are some of high level topics in this page

 

 

Followings are the list of the topics (pages) that I am working on now... usually my approach is to study on physical/MAC layers first and then RRC messages and then everything in between. I am writing these pages based on the assumption that readers are familiar with basic concepts used in legacy (normal) LTE. So I don't describe much on the basic concepts in these pages. If LTE-NB is your first experience of LTE and come across any concept/terminologies you are not familiar, I would recommend you to refer to pages for legacy (normal) LTE (Try "Search" menu at the top)

 

 

Questions before Study

 

When I first heard of a variation of LTE (NB-LTE) with using only 200 Khz (1 RB), I thought like this : "OK, I think I can understand the motivation is 200 Khz. It is same as basic channel bandwidth of GSM. So if we use this in LTE, we may be able to use GSM/GPRS spectrum for NB-LTE. Also, even the smallest bandwidth of the current LTE (1.4 Mhz) might be too much for such a super low throughput application like IoT. OK, so far so good. Then a question popped up in my mind.

  • How can we implement this bandwidth which is much smaller than the minimum bandwidth of the current LTE ?
  • Do we have redesign the whole system only for this ?
  • Do we need to invent new waveform, new slot/subframe structure  and come up with new scheduling/high layer signaling ?"

OK.. I think 200 Khz (more accurately 180 Khz) is to utilize 1RB of legacy LTE as it is. Also, it is not difficult think of how we utilize this single RB because data transmission and reception in the single RB is possible even in legacy LTE. However, there are somethings that cannot be reduced to single RB bandwidth in legacy LTE. So you may have following questions.

  • In legacy LTE, PSS and SSS is spreaded across 6 RB bandwidth (i.e, 1.4 Mhz). How should we redesign PSS, SSS to be fit in 1 RB bandwidth ?
  • In legacy LTE, PBCH is spreaded across 6 RB bandwidth (i.e, 1.4 Mhz). How should we redesign PBCH to be fit in 1 RB bandwidth ?
  • In legacy LTE, PCFICH and PHICH are designed to spread across the whole system bandwidth and the minimum of the whole system bandwidth is 1.4 Mhz. Are we still going to use PCFICH and PHICH in LTE-NB ? If yes, how should we redesign it to be fit into single RB bandwidth. If no, just forget about it.
  • In legacy LTE, PDCCH are designed to spread across the whole system bandwidth and the minimum of the whole system bandwidth is 1.4 Mhz. You can easily guess that it would not be possible to completely get rid of PDCCH. Are we going to revise the legacy LTE PDCCH to be fit into single RB bandwidth ? or Are we going to come up with completely new PDCCH structure ?

I would not give you answers to any of these questions in this page.  It is just my way of studying anything new. Coming up with my own question before jumping into textbook or specification documents.

 

 

Similarities and Differences

 

The first impression that I got while I was reading Release 13 LTE-NB specification is that it is almost a new design except a few very fundamental factors/parameters.

 

Followings are some of parameters that remains same in LTE-NB comparing to normal (legacy) LTE

  • Subcarrier Spacing = 15 Khz (except that UL can have an option for 3.75 Khz subcarrier spacing)
  • Subframe Length = 1 ms
  • Number of slots in a subframe = 2
  • Radio Frame Length = 10 ms
  • Number of Subframes in a Radio Frame = 10
  • Downlink Waveform = OFDMA (There is a subtle differences in waveform generation formula, but almost same)

 

Followings are some of parameters that are different from legacy LTE. I would just put down a high level descriptions of the difference here. For the details, I would recommend you to each separate pages that I listed at the top.

  • System Bandwith = 200 Khz fixed (this is obvious)
  • Uplink Waveform = SC-FDMA (Still named as SC-FDMA, but pretty big difference in terms of waveform generation formula)
  • PSS, SSS resource element mapping and transmission pattern
  • PBCH (MIB) resource element mapping and transmission pattern
  • DCI Format = only three types (N0, N1, N2) and parameters in the DCI shows pretty big difference
  • Repetitive Transmission = performs repetitive transmission for almost every channel (in LTE, only TTI Bundling is the intentional repetitive transmission.. all other transmittion is intended for single transmission.)

 

 

Waveform : Modulation/Subcarrier Spacing/Symbol Length/System Bandwidth

 

In short, there is no changes at all in Downlink and small changes in Uplink as follows.

 

Parameters

Downlink

Uplink

Modulation OFDMA (Same as Normal LTE) SC-FDMA (Same as Normal LTE)
Subcarrier Spacing 15 Khz (Same as Normal LTE) 15Khz or 3.75 Khz
Symbol Length Same as Normal LTE  
Slot Duration Same as Normal LTE  
System Bandwidth 200 Khz 200 Khz
Number of Subcarriers 12 ( 180 Khz in Total)  

 

 

LTE-NB Operation Mode / Why 200 Khz ?

 

As you know, LTE is originally designed to provide very high data rate. So it might sound ironic that we are trying to provide very low throughput service like IoT.  However, as far as I experienced.. in engineering there is no such a thing like ironic.. engineers are always to try to come up with a solution when there is need. Even though LTE is originally designed for high throughput, there has been continuous effort for low throughput (low cost) solution for the past couple of years. This is why you start seeing UE Category running backward. Most of initial LTE implementation was implemented as Category 3 and kept incrementing the numbers. However, we started seeing backward numbers like Category 1 and Category 0. However, all of exisiting LTE categories still keep the lowest boundary of LTE specification as follows :

  • Minimum System Bandwidth in frequey domain = 1.4 Mhz, 6 RB
  • Minimum Scheduling Unit in Time Domain = 1 ms TTI

However, the industry realized that there would be limitation to make LTE perfectaly suitable for IoT / MTC crieteria (super simple, untra cheap, extremly low energy consumption). Finally the industry tried to break at least one of the LTE boundary. It is to break frequency domain boundary and decide to make it narrower.

Then the question is how narrow it should be. Probably the most practical answer they found seems to be the bandwidth of 1RB because 1 RB is the minimum scheduling unit of current (legacy) LTE in frequency domain. They might have tried to pick narrower bandwidth (e.g, 6 subcarriers etc). However, in this case it would require too much redesign from the legacy LTE.

The industry decided to time domain boundary as it is, meaning that we can use the most of time domain design of the legacy LTE as it is.

 

With 1RB Bandwidth selected, we can utilize(deploy) this system in various situation as illustrated below.

 

 

Inband Mode : In this mode, we can deploy LTE NB within a legacy LTE if you just reserve only 1 RB anywhere in the legacy LTE band.

Guardband Mode : As you know, every LTE band has at least 100 Khz guardband on each end of the band. It means that you can easily have around 200 Khz space between any two non-contiguous band even if they are sitting in the closest possible locations. Guardband mode is to deply LTE-NB in the space that are reserved for the guardband of legacy LTE band.

Standalone Mod  : In previous two modes, LTE-NB coexists with an existing legacy LTE band. But we can think of another case where it is deployed completely independent from any legacy LTE. In an extreme case, we may deploy it in completely free space in the spectrum. However, in practical sense this mode of operation is intended to reserve (free up) a GSM channel and deploy LTE-NB in that space.

 

 

Reference

 

[1] 3GPP RP-151621

[2] 3GPP TR 45.820 V13.1.0 (2015-11) : Cellular system support for ultra-low complexity and low throughput Internet of Things (CIoT) (Release 13)

[3] NB-IoT concept demo by u-blox at Mobile World Congress 2016 (YouTube)

[4] 5G DEMO NB IOT v2 small (YouTube)

[5] Diversifying Cellular for Massive IoT - Live Demo: NB-IoT extended coverage (YouTube)

[6] NarrowBand IOT demonstration at MWC 2016 (YouTube)

[7] Internet of Things Conference: Vodafone's plans for NB-IoT (YouTube)

[8] RAN approved REL-13 NB_IOT CRs (RAN#72)

[9] 3GPP TS 36.211 V13.2.0 (2016-06) E-UTRA Physical channels and modulation

[10] 3GPP TS 36.331 V13.2.0 (2016-06) : E-UTRA RRC Protocol Specification