Keysight Technologies NB-IoT System Modeling: Simple Doesn’t Mean Easy

This paper presents a method for modeling and evaluating a new NB-IoT (Narrowband Internet of Things) system in a combined multi-domain simulation environment. Due to the complex analog and digital components of the communication system, accurate modeling is crucial to understanding system behavior. Realistic modeling examples are used with a new simulation method for an in-depth study of the RF transceivers, advanced modem technology and non-ideal hardware. Author: Sangkyo Shin Keysight Technologies, Inc. Before Cloud Device things Must be simulated 02 | Keysight | NB-IoT System Modeling: Simple Doesn’t Mean Easy White Paper Understanding the NB-IoT Standard Developing a new standard Once the need for a standard has been established, a group of experts will form a technical committee to discuss and negotiate a preliminary proposal. As soon as this draft has been developed, it is dispersed to a broader group of members for comments and eventually approval. All elements of the proposal are negotiated, including scope, key definitions and content. Increasingly, simulation software is being used to study new technologies to speed the development of standards and cut hardware implementation costs. The standardization of Narrowband-IoT, a new cellular-based narrowband technology targeted for Internet of Things, began in 2014 as a 3GPP study item. The first version was released in June 2016, as a part of Release 13 of the global 3GPP standard. This standard aims to address: – Improved indoor coverage – Increased support for a massive number of low-throughput devices – Low delay sensitivity – Low device power consumption – Ultra-low device cost – An optimized network architecture on top of LTE air interface and network The NB-IoT specifications are expected to continue to evolve beyond Release 13, with support for multicasting and positioning towards the new 5G NB-IoT standards. 03 | Keysight | NB-IoT System Modeling: Simple Doesn’t Mean Easy White Paper The narrowband downlink physical resource block has 12 subcarriers with 15 kHz spacing, offering a 180 kHz transmission bandwidth. It only supports a QPSK modulation scheme. To facilitate low-complexity decoding for downlink transmission in devices, the use of turbo codes was abandoned in favor of a tail biting convolutional coding scheme. Figure 1. Example of a NB-IoT UL transmitting signal with 15 kHz subcarrier spacing, NPUSCH format; 1,12 subcarriers and a spectrum mask (blue). PHY specification The 3GPP TS 36.211 Release 13, V13.2.0 (2016-06) provides a physical channel and modulation specification for narrowband IoT. The new device category, Cat-NB1, supports tens of kbps speed and a 200 kHz channel bandwidth. Prior to this release, the eMTC (enhanced Machine Type Communications) data rate supported a variable rate up to 1 Mbps, with a bandwidth of 1.4 MHz with Category M1 (Cat-M1). The Narrowband Physical Uplink Shared Channel provides two subcarrier spacing options: 15 kHz and 3.75 kHz. The additional option of using 3.75 kHz provides deeper coverage to reach challenging locations, such as deep inside buildings, where there is limited signal strength. The data subcarriers are modulated using binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) with a phase rotation of π/2 and π /4 respectively. Selection of the number of subcarriers for a resource unit can be 1, 3, 6 or 12 to support both single tone and multi-tone transmission. 04 | Keysight | NB-IoT System Modeling: Simple Doesn’t Mean Easy White Paper Radio transmission and reception In order to efficiently employ the spectrum resource, NB-IoT was designed with three different operation modes: stand-alone, in-band and guard-band. Stand-alone aims to replace GSM carriers with the NB-IoT carriers, while In-band operation utilizes resource blocks within a normal LTE carrier. Guard-band operation uses the LTE carrier’s guard-band. For an LTE service provider, the in-band option provides the most efficient NB-IoT deployment. For example, if there is no IoT traffic, a Physical Resource Block(PRB), available for an NB-IoT carrier, may be used instead for other purposes; as the NB-IoT is fully integrated within the existing LTE infrastructure. This allows the base station scheduler to multiplex LTE and NB-IoT traffic in the same spectrum. Figure 2. An in-band operation uplink coexistence analysis with a victim LTE (10MHz) and an aggressor NB-IoT device. Two spectrum traces (left) intentionally separated and overlapped in the same plot. Total 1000 LTE sub frames transmitted to throughput analysis. Figure 3. A guard-band operation uplink coexistence analysis with a victim LTE (10MHz) and an aggressor NB-IoT device. Two spectrum traces (left) intentionally separated and overlapped in the same plot. A throughput analysis (right) of a total of 1000 LTE transmitted subframes. 05 | Keysight | NB-IoT System Modeling: Simple Doesn’t Mean Easy White Paper The simulation of NB-IoT and LTE coexistence in different operating scenarios is a popular for companies deeply engaged in 3GPP standardization. The examples shown in Figure 2 and 3 are the result of the in-band and guard-band operation modes in a scenario where the LTE system is a victim and the NB-IoT is an aggressor. Considering that the downlink subcarrier of NB-IoT is orthogonal with LTE PRB, and both are transmitting from the same base station, their coexistence was evaluated only for the uplink case. The simulation environments were established using Keysight’s SystemVue communication physical layer simulation software and its LTE-A reference library. Simulation results across companies may differ due to variations in their modeling approaches. Differences may be caused by occurences such as power leaks, modulation, and filtering. However, the basic conclusion of the 3GPP TR 36.802 V13.0.0 simulation example shown above is that the NB-IoT can coexist with LTE. The following points were observed: – Throughput degradation is less than 5%. – NB-IoT creates some interference on the first adjacent LTE PRB, while the interference on the other PRBs is insignificant or acceptable. – Coexistence in the guard band performs slightly better compared to in-band operation. 06 | Keysight | NB-IoT System Modeling: Simple Doesn’t Mean Easy White Paper Hardware Considerations