This website uses cookies to enhance your browsing experience, analyze our traffic, and optimize our advertising efficacy as described in Quectel Cookies Policy. We also share information about your use of our site with our carefully-selected advertising and analytics third-parties to achieve the purposes set out in our Cookies Policy. To see what cookies we serve and set your preferences, please click the Find Out More link. By continuing your visit on our website, you consent to our use of cookies in accordance with Quectel Cookies Policy. Find Out More
Accept

NB-IoT Time Domain Resources

 

1 Introduction
NB-IoT is originated from LTE, so they have a lot in common, especially in the allocation of time domain resources in the downlink. However, they are regarded as different radio access technologies (RATs) because they use different types of frame structures and have different air interface features. Therefore, this article mainly introduces time domain resources of NB-IoT as well as those of LTE.
 
2 Downlink Time Domain Resources
The minimum time unit and maximum time unit of NB-IoT are TS and H-SFN, respectively. 1 TS approximately equals 32.5 ns and 1 H-SFN equals around 10.24 s. Other time units of NB-IoT include OFDM symbol, Symbol, slot, subframe (sf), and radio frame (rf).
 
 
 
 
The following describes the relationships between the units:
 
H-SFN indicates Hyper System Frame Number. 
 
One hyper system frame contains 1024 system frames, that is, 1 H-SFN = 1024 SFNs.
 
SFN indicates System Frame Number. System frame is also called radio frame. The term “radio frame” highlights the frame structure, while the term “system frame” highlights the sequence between radio frames. 
 
One system frame contains 10 subframes, that is, 1 rf = 10 sf.
 
One subframe contains two slots. Slots in different subframes in a radio frame are continuously numbered.
 
One slot contains seven symbols.
 
OFDM symbol is the useful symbol in the downlink. OFDM with a cyclic prefix (CP) forms a symbol. Each OFDM symbol is 2048 TS.
 
Among seven symbols of a slot, the CP of the first symbol is 160 TS, while the CP in the other six symbols is 144 TS. The reason is that a slot is 15360 TS in duration but 15360 cannot be exactly divided by 7.
 
3. Uplink Time Domain Resources
The time domain duration of resources occupied by NB-IoT is related to the corresponding frequency domain bandwidth. The uplink Subcarrier spacing of NB-IoT is 3.75 kHz or 15 kHz. The 15 kHz bandwidth supports single-tone and multi-tone modes. The multi-tone mode supports 3 tones, 6 tones, and 12 tones, which occupy different bandwidth resources and result in different time domain durations. 
 
For the 15 kHz subcarrier spacing, 12 tones of one PRB scheduled in the uplink correspond to 1 ms in the time domain. In terms of frequency domain, the subcarrier spacing of LTE and the downlink of NB-IoT is also fixed at 15 kHz, which will not be described here in detail. 
 
For the 3.75 kHz subcarrier spacing, one slot occupies 2 ms, which is quite different from that in the downlink in the layer from radio frames to SCFDMA symbols. The details are shown as below:
 
 
 
SFN indicates radio frame. It is the same as that in the downlink. 1 SFN = 5 slots = 10 ms.
 
 For the 3.75 kHz subcarrier bandwidth, a slot occupies 2 ms, that is, 1 slot = 2 ms.
 
In the uplink, 1 symbol = CP + SC-FDMA. Different from the downlink, there is a guard period (GP) in the front of the slot in the uplink in addition to the seven symbols. The guard period is 2,304 TS. 
 
SC-FDMA indicates an effective symbol which is 8,192 TS.
 
In summary, 1 slot = GP+ 7 symbols = GP+7 x (CP + SC-FDMA) = 2304+7 x (256 + 8192) = 61440 TS = 2 ms.
 
4. Random Access
The NB-IoT Physical Random Access Channel (NPRACH) always uses the single-tone transmission mode and 3.75 kHz subcarrier spacing. NPRACH supports two different kinds of CP lengths. The short CP lasts 66.67 μs, while the long CP lasts 266.67 μs. No matter whether a short CP or a long CP is used, an effective symbol is 266.67 μs. Five symbols and one CP form a symbol group. Each preamble consists of four symbol groups.
 
 
 
 
We can see that the short CP is a quarter of a long CP, while a long CP is the same as an effective symbol in length. So:
 
For a short CP, the length of a symbol group = (2048 + 8192 x 5) TS = 2048 x 21 TS = 21/15000S = 1.4 ms; the length of four symbol groups is 4 x 1.4 = 5.6 ms.
 
For a long CP, the length of a symbol group = (8192 + 8192 x 5) = 8192 x 6 TS = 2048 x 24 TS = 24/15 ms = 1.6 ms; the length of four symbol groups is 4 x 1.6 = 6.4 ms.
 
No matter whether a short CP or a long CP is used, the preamble occupies 8 ms and the remaining part is used as GT.
 
For NPRACH, the CP length decides its coverage radius in uplink. The length of a long CP is four times that of a short CP, so does its coverage radius. The corresponding distances are 40 km and 10 km, respectively. In actual networking, the coverage bottleneck is not the CP length but the path loss. Therefore, short CPs are usually used in live network to give more protection.
 
5. Different Features in LTE
NB-IoT and LTE basically have the same time domain resources. This section, however, describes their slight differences.
 
5.1 Hyperframe
Neither LTE-TDD nor LTE-FDD has the concept of hyperframe. Therefore, it does not have the highest level of time domain resource unit. In LTE, the system frame number is still 0-1023. The total length of all system frames equals the length of one hyperframe in NB-IoT.
 
5.2 Half-frame
There is a concept of half-frame in LTE-TDD. One system frame contains two half-frames, and each half-frame has a length of 5 ms. Therefore, the length of a system frame is still 10 ms, and that of a subframe is still 1 ms.
 
 
 
 
Here, h-sfn indicates half-frame. It is distinguished from H-SFN described above by using lowercase letters.  In fact, the concept of half-frame exists only in TDD while hyperframe exists only in NB-IoT which uses FDD. Therefore, they will never conflict even if they are not set apart by using uppercase and lowercase letters separately. Each half-frame contains 5 ms and each millisecond corresponds to one subframe.
 
For TDD, however, the second subframe of each half-frame is a special subframe if a 5 ms conversion interval is used. The slots contained by the special subframe are special slots. Normal subframes are the same between LTE FDD and NB-IoT with each subframe as 1 ms and each slot as 0.5 ms. A special subframe, however, contains three slots (DwPTS, GP and UpPTS) in 14 symbols, and its total time domain length is also 1 ms. However, the length of each slot can be configured. Three to 12 (inclusive) symbols can be configured for DwPTS, 1 to 10 (inclusive) symbols can be configured for GP, and 1 or 2 symbols can be configured for UpPTS.  The detailed configurations or the special subframe configuration of TDD, and the impact on the network will not be described here. 
 
 
5.3 Extended CP
There are two types of CPs in LTE. One is normal CP of 144 TS (approximately 4.6875 μs) and the other comparatively rare CP is extended CP of 512 TS (approximately 16.67 μs). An important function of the CP in LTE is to prevent inter-symbol interference, thus greatly affecting the cell radius. A short CP indicates smaller coverage while a long CP indicates wider coverage. A short CP is usually used in inland areas because wide coverage is impossible due to dense buildings that lead to great path loss. An extended CP can be used in the sea, desert and plain areas to broaden the coverage. For example, an extended CP is recommended in coastal cities such as Zhuhai and Qingdao, and those sparsely populated areas such as Tibet and Inner Mongolia.
 
 
 
 
When an extended CP is used, each slot has only six OFDM symbols. Every slot is 15,360 TS. As 15360/6 = 2560, each symbol is of the same length of 2,560 TS. The length of each CP is 2560 – 2048 (OFDM symbol) = 512 TS (approximately 16.67 μs).
 
5.4 Scheduling Bandwidth
LTE uses the same scheduling method for uplink and downlink. The smallest scheduling unit is PRB, that is, the bandwidth is always 180K or an integral multiple of 180K. Each subcarrier has a fixed bandwidth of 15 kHz (except for PRACH and MBSFN subframes). There are no concepts of single-tone and multi-tone. The transmission mode is similar to NB-IoT downlink and therefore is omitted here.
 
5.5 LTE Random Access
In LTE, preamble also consists of three parts which are CP, Tseq and GT. However, the level of symbol group does not exist. Therefore, preamble has only one CP. In NB-IoT, each group has one CP.
 
 
 
 
In LTE, preamble is of five different lengths which correspond to CP and Tseq of different lengths. The remaining length is used as GT. The five lengths are as follows:
 
 

Format

Tcp (Ts)

Tsep (Ts)

sf

0

3168

24576

1

1

21024

24576

2

2

6240

2*24576

2

3

21024

2 x 24576

3

4

448

4096

TDD only

 
 
Format 4 supports only LTE-TDD. The other four types support both TDD and FDD. CP indicates the longest distance from the UE to the base station, thus limiting the cell radius. GT limits the protection between two users. The impact of both CP and GT on cell radius should be considered in the networking practice.
 

By Jacobi Rao from the Software Department

 
 
Related news

Customer Service & Support

For any questions, please contact our sales, FAE and marketing team.

Contact Us