Robust scrambling for NB-IoT broadcast channels

Narrowband Internet of Things (NB-IoT) is a new cellular technology designed to provide wide-area coverage for IoT. It was introduced in 3GPP Release 13 and is expected that many commercial NB-IoT networks will be launched in 2017. Here, on the Ericsson Research blog, we shall provide a brief explanation for the rationale of a change request (CR) approved in 3GPP RAN meeting #75 in March 2017 for the enhancement of the robustness of NB-IoT broadcast channels when it comes to inter-cell interference – when NB-IoT is deployed inside a frame-synchronized LTE network. The approved CRs are mandatory and non-backward compatible. Introducing such CRs is possible considering that NB-IoT deployments had not yet occurred on a widespread basis. The approved CR preserves robust NB-IoT performance when deployed in an LTE network that also supports features such as Multicast-Broadcast Single-Frequency Network (MBSFN) or Positioning Reference Signal (PRS), which require frame-synchronization across cells to ensure good performance in practice.

Robust

NB-IoT is a new cellular technology designed to provide wide-area coverage for the Internet of Things (IoT). It is designed to meet an array of demanding requirements of different IoT use cases, including ubiquitous coverage, reaching deep indoors and basements, longevity of battery life, ultra-low device complexity and cost, and capacity of supporting many IoT devices per cell. NB-IoT is also designed to cater for the different migration needs of mobile operators. Its small system bandwidth requirement (200 kHz) and three operation modes, standalone, LTE in-band, and LTE guard-band, offer great deployment flexibility.

The core specifications of NB-IoT were completed in June 2016 and since then product development and network trials have rapidly progressed forward. It is expected that many commercial NB-IoT networks will be launched in 2017.

Field tests have always been crucial for successful introduction of a new cellular technology. 3GPP has well-established processes for addressing performance issues identified through field tests. A change request (CR) can be proposed to address a specific issue found, and, if agreed, the specifications will be revised accordingly. During NB-IoT field tests, it was realized that there were deficiencies in inter-cell interference migration in the original design of NB-IoT Physical Broadcast Channel (NPBCH) scrambling in frame-synchronized networks. In 3GPP RAN1 meeting #88, February 2017, a CR was brought up to address such deficiencies. This CR was eventually approved in 3GPP RAN meeting #75, March 2017.

NPBCH is used for signaling NB-IoT Master Information Block (MIB-NB). It uses a 640-ms transmission time interval (TTI), and within a TTI NPBCH uses subframe 0 in every 10 ms frame. This is illustrated in Figure 1. Therefore, within a NPBCH TTI, there are 64 NPBCH subframes.

NPBCH

Figure 1: NPBCH transmission during a 640-ms transmission time interval.

The 64 NPBCH subframes within a TTI are organized according to Figure 2. A NPBCH codeword is divided into 8 self-decodable subblocks, and each subblock is transmitted in one NPBCH subframe using 100 QPSK symbols. The same subblock is repeated 8 times in 8 consecutive NPBCH subframes. The original scrambling mask design is to have all the 8 repetitions of a code subblock share the same scrambling mask. This design results in the transmitted symbols in one subframe to be identical to those in another subframe carrying the same code subblock. Such a property can be used by a NB-IoT device to perform residual frequency offset estimation without knowing the actual modulation values of the NPBCH symbols in a subframe. Devices in poor coverage can combine the repetitions of a code subblock to boost the signal-to-noise ratio (SNR). The combining can be done before descrambling.

NPBCH repetition

Figure 2: NPBCH repetition pattern and original scrambling mask design.

The CR was triggered by the frame-synchronized cell deployment case, where the processing gain over interference is lost. Certain features in LTE, e.g. Multicast-Broadcast Single-Frequency Network (MBSFN) and Positioning Reference Signal (PRS), require frame-synchronization across cells to ensure good performance in practice. For example, all cells in a network need to transmit the same MBSFN signal in the specific subframe, as illustrated in Figure 3. If NB-IoT is deployed inside such an LTE carrier, then NB-IoT will also be frame synchronized in all cells. As a result, combining the repetitions of a code subblock does not achieve a processing gain over interference due to NPBCH transmission from a frame-synchronized neighboring cell as illustrated in Figure 4. (It still achieves a processing gain over noise.) Although the scrambling masks are cell dependent, the fact that an interfering cell also uses the same mask across all 8 repetitions of a NPBCH code subblock takes away the processing gain over interference. In this case, combining the repetitions of a code subblock of the desired NPBCH signal also results in coherent combining an interfering signal, giving rise to no processing gain.

MBSFN

Figure 3: NB-IoT deployment inside an LTE carrier enabling MBSFN subframes.

NPBCH synchronized

Figure 4: NPBCH interference from a frame synchronized neighboring cell.

To preserve the option of NB-IoT deployment inside an LTE carrier enabling the MBSFN feature, or other features that benefit from frame synchronization, and ensure robustness toward inter-cell interference, 3GPP in RAN1 meeting #88, February 2017, agreed to change the scrambling mask design for NPBCH based on a way-forward proposal, R1-1703814, brought up by Huawei, Hisilicon, Neul, Qualcomm, and Ericsson. The agreement was to vary the scrambling masks cross the 8 repetitions of a code subblock, as illustrated in Figure 5. With this revision, the scrambling masks applied in a neighboring cell vary differently than those in the desired NPBCH signal over the repetitions of a code subblock; and therefore, a processing gain over a frame-synchronized interfering NPBCH signal is achieved. A device can still take advantage of the repetitions of a code subblock for residual frequency estimation provided it undoes the varying scrambling first. This CR was eventually approved in 3GPP RAN meeting #75, March 2017. This CR is mandatory and non-backward compatible. Introducing such a CR is possible considering that NB-IoT deployments had not yet occurred on a widespread basis.

A similar revision was also introduced to Narrowband Downlink Shared Channel (NPDSCH) carrying system information.

NPBCH scrambling

Figure 5: New robust NPBCH scrambling according to the CRs agreed in 3GPP RAN1 meeting #88 and approved in 3GPP RAN meeting #75, March 2017.

In summary, as a result of NB-IoT field trial evaluations a robustness issue due to inter-cell interference was identified for frame-synchronized networks. A solution based on updated scrambling masks for the NB-IoT broadcast channel was approved through a Change Request approved in 3GPP RAN meeting #75, March 2017. With these mandatory changes the 3GPP specification now provides enhanced robustness for NB-IoT when deployed in an LTE network that also provides services which require frame-synchronization between cells, e.g. LTE Broadcast and LTE Positioning.


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