7.4. NR and NB-IoT coexistence

In 3GPP Release 15, a new radio access technology known as NR is introduced. NR offers performance advantages for both mobile broadband and ultra-reliable low-latency communication services, more deployment flexibility, and higher energy efficiency over LTE. NR also has superior scalability that makes it suitable for deployment in higher frequency bands such as millimeter wave bands where there is greater spectrum availability. Thanks to all these advantages, NR has attracted vast interest from existing and new operators. Network migration from LTE to NR has started in earnest in 2019 with US operators leading the way. Chapter 2.4 introduces NR, and the interested readers can refer to Ref. [28] for detailed description on NR.

Although NR is designed for enhancing the performance of mobile broadband and ultra-reliable low-latency communications services, it is not designed to be used for low-power wide-area IoT use cases. One main reason is that these use cases are already adequately addressed by existing 3GPP technologies such as LTE-M and NB-IoT. Thus, LTE-M and NB-IoT networks will continue to serve low-power wide-area IoT use cases even after the LTE-to-NR migration. In fact, as discussed in great details in Section 8.9, NB-IoT fully fulfills the International Telecommunications Union IMT-2020 and 3GPP Fifth Generation (5G) requirements on massive MTC, and therefore is a component of the 5G radio access technology. As such, it is important to ensure efficient and flexible coexistence between NB-IoT and other components of 5G radio access technology. In this section, we describe how these two important 5G component technologies, NR and NB-IoT, coexist.

NB-IoT is defined in LTE bands 1, 2, 3, 4, 5, 8, 11, 12, 13, 14, 17, 18, 19, 20, 21, 25, 26, 28, 31, 41, 66, 70, 71, 72, 73, 74 and 85 (see Section 7.2.2.1 for the frequency ranges of these bands). Many of these bands are also defined for NR [29]. Table 7.34 lists all the bands that are defined for both NR and NB-IoT, as of 3GPP Release 15. These bands thus can be used to deploy both NR and NB-IoT. It is worth mentioning that in all these bands, NR can use 15, 30, or 60 kHz subcarrier spacing, and an NR device operating in these bands is required to support the 15 and 30 kHz subcarrier spacings.

Table 7.34

Frequency bands that are defined for both NR and NB-IoT.

Band	Duplex mode	Uplink [MHz]	Downlink [MHz]	NR channel bandwidth for 15   kHz subcarrier spacing [MHz]	NR channel raster [kHz]
1	FDD	1920–1980	2110–2170	5, 10, 15, 20	100
2	FDD	1850–1910	1930–1990	5, 10, 15, 20	100
3	FDD	1710–1785	1805–1880	5, 10, 15, 20, 25, 30	100
5	FDD	824–849	869–894	5, 10, 15, 20	100
8	FDD	880–915	925–960	5, 10, 15, 20	100
12	FDD	699–716	729–746	5, 10, 15	100
20	FDD	832–862	791–821	5, 10, 15, 20	100
25	FDD	1850–1915	1930–1995	5, 10, 15, 20	100
28	FDD	703–748	758–803	5, 10, 15, 20	100
41	TDD	2496–2690	2496–2690	10, 15, 20, 40, 50	15 or 30
66	FDD	1710–1780	2110–2200	5, 10, 15, 20, 40	100
70	FDD	1695–1710	1995–2020	5, 10, 15, 20, 25	100
71	FDD	636–698	617–652	5, 10, 15, 20	100
74	FDD	1427–1470	1475–1518	5, 10, 15, 20	100

There are many options for deploying NR and NB-IoT in the same band. We will describe these options in the subsections below. But before getting to that, it is worthwhile describing certain important aspects first. Obviously, such deployment needs to satisfy the following objectives:

1. • NB-IoT legacy devices can operate without knowing any NR-specific information.
2. • NR devices can operate regardless whether there is an NB-IoT carrier deployed in the same band.
3. • There is minimal mutual interference between NR and NB-IoT so the impact on NR and NB-IoT performance is negligible when they are deployed in the same band.

To satisfy the first objective, one fundamental aspect is that an NB-IoT device can identify an NB-IoT cell during the initial cell selection process (see Section 7.3.1.1.) To achieve this, the anchor NB-IoT carrier needs to be placed according to 100   kHz channel raster (see Section 7.2.1.1), i.e. the center frequency needs to be at 100 N N R kHz, where N N R is an integer. As mentioned in Section 7.2.1.1, there is a raster offset in the case of in-band or guard-band deployment, and the raster offset is either 2.5   kHz, -2.5   kHz, 7.5   kHz, or -7.5   kHz. Furthermore, as NPSS, NSSS and NPBCH are the signals used by the device during cell search, they need to be preserved.

Similarly, to satisfy the second objective, the NR carrier needs to be placed according to NR channel raster shown in Table 7.34. In most cases, the NR channel raster is 100   kHz. There is an important difference in the location of channel raster between NR and NB-IoT. This is illustrated in Fig. 7.63. In the NB-IoT case, channel raster points to the center of the carrier, which is half-way (i.e. 7.5   kHz) between two subcarriers. NR channel raster however points to a subcarrier around the middle of the carrier. As shown, for an NR carrier with N resource blocks, there are 12   N subcarriers; and indexing all the subcarriers from 0 to 12N-1, the channel raster is mapped to subcarrier 6   N [29].

Finally, the third objective suggests that if the NR carrier is configured with 15   kHz subcarrier spacing, it is desirable to align the NR and NB-IoT subcarriers on the same subcarrier grids, with the frequencies between an NR subcarrier and an NB-IoT subcarrier differing by an integer multiple of 15   kHz. With this, if the NR and NB-IoT networks are synchronized, NR subcarriers and NB-IoT subcarriers are mutually orthogonal. If the NR carrier is configured with subcarrier spacing other than 15   kHz, a guard band is needed between NR and NB-IoT to ensure minimal inter-subcarrier interference.

Before going further, it helps to clarify certain important terminology. As described in Section 7.1.2.5, there are three operation modes defined for NB-IoT, stand-alone, in-band and guard-band operation modes. These terms were introduced in Release 13 to describe the deployment of an NB-IoT carrier in its relationship with an LTE carrier. Going forward though, when the LTE spectrum is refarmed for NR deployment while the NB-IoT carrier continues to be in service, the meaning of the NB-IoT operation mode loses its relationship with the LTE carrier, as the LTE carrier may no longer exist. Is the NB-IoT operation mode still relevant when there is no LTE carrier? The answer is yes, and interestingly the deployment flexibility offered by these three operation modes is a useful tool for NR and NB-IoT coexistence. For NB-IoT devices, the NB-IoT operation mode means:

Fig. 7.63 Locations of NR and NB-IoT channel rasters. (assume the NR carrier is configured with 15   kHz subcarrier spacing).

1. • Stand-alone operation mode: the NB-IoT carrier center is exactly at a 100   kHz channel raster. All REs in an NB-IoT subframe are available to an NB-IoT physical channel or signal.
2. • Guard-band operation mode: the NB-IoT carrier center is not exactly at a 100   kHz channel raster. The raster offset is -2.5   kHz, +2.5   kHz, -7.5   kHz, or +7.5   kHz. The exact raster offset is signaled in MIB-NB. All REs in an NB-IoT subframe are available to an NB-IoT physical channel or signal.
3. • In-band operation mode: the NB-IoT carrier center is not exactly at a 100   kHz channel raster. The raster offset is -2.5   kHz, +2.5   kHz, -7.5   kHz, or +7.5   kHz. The exact raster offset is signaled in MIB-NB. Not all REs in an NB-IoT subframe are available to an NB-IoT physical channel or signal. The device should expect some REs are taken by LTE CRS and PDCCH. The information about which REs taken by LTE is provided in MIB-NB and SIB1-NB. Although there is no LTE carrier, as far as the device is concerned, these resources are not available for NB-IoT.

The NB-IoT operation mode referred to in the following subsections can therefore be thought of as an indication to NB-IoT devices regarding channel raster location and resource element allocation in an NB-IoT subframe. It is not about the relationship between the NB-IoT carrier and the legacy LTE carrier which has been replaced by an NR carrier. It is also not about the relationship between the NB-IoT carrier and the new NR carrier that has replaced the LTE carrier.

7.4.1. NR and NB-IoT as adjacent carriers

The most straightforward option is to deploy NR and NB-IoT carriers as adjacent carriers as shown in Fig. 7.64. Depending on the subcarrier configuration of the NR carrier and whether there is tight synchronization between the two carriers, the guard band can be dimensioned accordingly to ensure minimal mutual interference. In the special case that NR and NB-IoT carriers are synchronized and the NR carrier is configured with 15   kHz subcarrier spacing, it is possible to achieve orthogonality between NR and NB-IoT without a guard band. As mentioned previously, the NR channel raster is mapped to a subcarrier. With a 100   kHz NR channel raster, the frequency of an NR subcarrier (in kHz) is:

Fig. 7.64 NR and NB-IoT deployed as adjacent carriers.

f N R ( i ) = 100 N N R + 15 ( i − 6 N ) ,

(7.23)

where N N R is an integer which specifies the location of the NR channel raster, i is the NR subcarrier index. As shown in Fig. 7.63, the channel raster, which is at the frequency 100 N N R , is mapped to subcarrier index 6 N . In comparison, NB-IoT channel raster is mapped to the midpoint between the middle two subcarriers, and thus the frequency (in kHz) of an NB-IoT subcarrier is:

f N B − I o T ( j ) = 100 N N B − I o T + 15 ( j − 5.5 ) ,

(7.24)

where N N B − I o T is an integer which specifies the location of the NB-IoT channel raster, j is the subcarrier index, j = 0,1 , … , 11 , and the channel raster is mapped to the midpoint between subcarriers 5 and 6, as illustrated in Fig. 7.63.

Eqs. (7.23) and (7.24) imply that perfect subcarrier alignment between NR and NB-IoT cannot be achieved as there is an additional offset of 7.5   kHz between the subcarrier grids of NR, Eq. (7.23), and those of NB-IoT, Eq. (7.24). This issue can be addressed by using the raster offset of NB-IoT. Let f o f f s e t be the raster offset of NB-IoT, the frequency of NB-IoT subcarrier j then becomes

f N B − I o T ( j ) = 100 N N B − I o T + 15 ( j − 5.5 ) + f o f f s e t .

(7.25)

It is easy to show that with f o f f s e t ∈ { ± 2.5 , ± 7.5 } kHz, the subcarrier grids of NR and NB-IoT can be aligned, i.e., there exist N N R and N N B − I o T such that the frequencies of f N R ( i ) and f N B − I o T ( j ) differ by an integer multiple of 15   kHz, hence ensuring subcarrier orthogonality between NR and NB-IoT.

Thus, if the NB-IoT carrier in Fig. 7.64 is configured as guard-band or in-band operation mode with a raster offset of -2.5   kHz, 2.5   kHz, -7.5   kHz or 7.5   kHz, with proper choices of N_NB-IoT , its subcarriers will fall on the same subcarrier grid as the NR carrier. In this case, it is possible to achieve orthogonality between the NR and NB-IoT subcarriers.

Fig. 7.65 NB-IoT deployed in the guard band of an NR carrier.

One major advantage of this approach is that the inter-carrier guard band shown in Fig. 7.64 might not be needed at all. Obviously, among the two NB-IoT operation modes (guard-band and in-band), the guard-band operation mode is preferred since in such a scenario all the REs in an NB-IoT subframe can be made available to NB-IoT physical channels or signals.

7.4.2. NB-IoT in the NR guard band

As discussed in Section 7.4.1, there exists a solution to maintain orthogonality between NR and NB-IoT subcarriers. In fact, this makes it possible to deploy an NB-IoT carrier in the guard band of an NR carrier as illustrated in Fig. 7.65. The guard band of an NR carrier is smaller compared to that of an LTE carrier. Nevertheless, it may still be possible to accommodate an NB-IoT carrier. Table 7.35 shows the minimum guard bands for NR carriers with 15   kHz subcarrier spacing and frequencies below 6   GHz. Observe that in all cases the guard band is large enough to accommodate an NB-IoT carrier.

One important aspect for this deployment option is the NR unwanted emission requirements, which in most scenarios will limit the power level of the NB-IoT carrier.

Like the adjacent carrier scenario discussed in Section 7.4.1, also in this case it is preferred to configure the NB-IoT carrier with the guard-band operation mode as all the REs on the NB-IoT carrier can be made available to NB-IoT devices.

7.4.3. NB-IoT deployed using NR resource blocks

Yet another deployment option is to deploy NB-IoT within an NR carrier using an NR resource block (RB) as shown in Fig. 7.66. Similar to the NR guard band option described in Section 7.4.2, it is necessary to use the raster offset offered in NB-IoT in-band and guard-band operation modes to align the NB-IoT subcarriers with the NR subcarriers for achieving subcarrier orthogonality.

Compared to the NR guard band option described in Section 7.4.2, this option allows a higher NB-IoT transmit power level since the NB-IoT carrier is away from the carrier edge and therefore its power level has less impact on the fulfilment of the NR unwanted emission requirements specified in Section 6.6 of Reference [29]. This option however requires sharing of radio resources between NR and NB-IoT. The sharing can be configured semi-statically. NR has a feature called reserved resources intended for ensuring forward compatibility. An NR PDSCH time-frequency resource can be declared as reserved and not made available to NR devices. NR reserved resource configuration can be done by using a frequency-domain bit map and a time-domain bit map as illustrated in Fig. 7.67. The frequency-domain bit map has granularity of resource block, with which each resource block can be individually indicated as reserved or not. The time-domain bit map has granularity of an OFDM symbol, with which each OFDM symbol can be individually indicated as reserved or not. When a resource element is indicated as reserved by both the frequency-domain and time-domain bit maps, it is not available to NR devices. Thus, the network can simply declare the resources used by NB-IoT carrier as reserved to enable the deployment of NB-IoT using one or more NR resource blocks. NR devices are required to rate match around the declared reserved REs.

Table 7.35

Minimum guard band for NR carriers with frequencies below 6   GHz. (15   kHz subcarrier spacing).

NR carrier bandwidth [MHz]	5	10	15	20	25	30	40	50
Guard band [kHz]	242.5	312.5	382.5	452.5	522.5	592.5	552.5	692.5

Fig. 7.66 NB-IoT deployed within an NR carrier by using an NR resource block.

Fig. 7.67 Use of NR reserved resource configuration for supporting NB-IoT deployed within an NR carrier using an NR resource block.

Due to the raster offset requirement of NB-IoT and the different mappings of NR and NB-IoT channel rasters illustrated in Fig. 7.63, when deployed within an NR carrier, the NB-IoT anchor carrier may overlap with one or two NR resource blocks, depending on whether there is resource block alignment between the two. When the NB-IoT carrier overlaps with two NR resource blocks, the frequency-domain bit map needs to be configured for reserving two NR resource blocks. A detailed analysis on resource block alignment can be found in Ref. [30], and it is shown that it is possible to achieve resource block alignment between an NB-IoT anchor carrier and an NR resource block.