[Shishuo Chip Products] Technical Tips | New opportunities and challenges for narrowband IoT RF front-ends, with examples of Qorvo solutions

Many people believe that 2019 will be the first year of 5G in China. With the opening of 5G, new applications and new demands, especially for smartphones, have stimulated the emergence of many new terms in the RF front-end market, such as MIMO, HPUE, NSA, SA, PAMiD, etc. The RF front-end needs to be more integrated to support more complex frequency bands and communication standards.

While the mobile phone market is pursuing faster and stronger devices, there is another market, namely the narrowband Internet of Things (Cat-M / NB-IoT), which meets market demand in another dimension, such as lower power consumption, longer standby time and lower cost.

Figure 1: 5G mission and tasks

5G not only means high-speed data connection, but also supports massive IoT applications and low-latency and high-reliability scenarios. Today we will talk about the narrowband interconnection part of massive connections.

China's three major operators have been increasing the deployment of Cat-M/NB-IoT networks. So far, China Mobile has 520,000 base stations, China Unicom has 330,000, and China Telecom has 410,000 (according to public information on the Internet). Behind the huge investment, there is a huge profit drive. Cat-M/NB-IoT is a wide area network access standard based on cellular networks. Telecom operators have the network services and number resources of NB-IoT and look forward to playing an important role in the world of the Internet of Everything.

The main application scenarios of NB-IoT are listed below:

• Smart Security

• Smart infrastructure: smart street lights, smart manhole covers, smart charging, smart parking.

• Smart Meter

• Intelligent monitoring

We have observed rapid growth in the following areas:

• E-Bike Monitoring and Management

• Smart smoke sensor

• Smart meters (water meters/gas meters/electricity meters)

In addition, there are some new applications based on NB-IoT that have also aroused great interest in the market.

• Smart parking service: It integrates the cloud service big data platform, collects on-site traffic and parking space information, and enables convenient unattended parking through electronic payment via mobile phone.

• Smart wearable market: Thanks to low power consumption, NB-IoT terminals can achieve ultra-long standby time. Through the operator's wide area network connection, positioning data and health data can be automatically uploaded to the personal account of the enterprise cloud, breaking away from the constraints of traditional local area networks or the need to connect to mobile phones to synchronize data. This is very suitable for unattended or out-of-home positioning services for the elderly and children. The administrator demarcates the electronic safety zone, and when the smart wearable device leaves the safety zone, the alarm information will be automatically transmitted to the cloud and the administrator.

Figure 2: IoT = Internet of things

The Internet of Everything based on cellular networks is a promising new technology. It has the characteristics of long distance, low cost and support for various communication bandwidths on a secure platform. This new platform can be built on the existing LTE network to meet the needs of new markets and applications.

From the perspective of RF front-end suppliers, we see some new market demands.

• New vertical markets: Based on the existing cellular network requirements, new low-cost and low-power solutions will be used in new market applications.

• Multiple connectivity standards will coexist, and product forms will range from simple low-power and single-band wireless units to complex LTE and 5G New Radio global cellular solutions.

• Diverse application scenarios and requirements. The complex and diverse end-user markets and applications will lead to diversified demands and products, including indoor applications and some outdoor scenarios with extreme temperatures and high reliability requirements.

Figure 3: Various communication standards and multiple application scenarios

The diverse end markets, applications and usage environments require optimized combinations of RF technologies to support them. It is predicted that in 2021, among the “10 billion” IoT connected devices, 70% will be low-power home/office applications.

As a leading RF solutions provider, Qorvo is able to provide optimized RF solutions from home to high-performance global wide-area cellular networks.

Figure 4: Various applications in low and medium speed scenarios

Cat-M/NB is a standard accepted by 3GPP, which meets the needs of new markets and applications by utilizing the existing 4G network. By reducing the complexity of the system, products applied to Cat-M/NB can meet the needs of low-cost and low-power mass M2M.

Figure 5: Global Cat-M/NB frequency band distribution

The frequencies used for narrowband IoT communications around the world are distributed in the medium and low frequency range. RF front-end designers need to choose the most optimized design solution in terms of broadband matching, harmonic suppression, ultra-low power consumption and low cost.

Table 1: Comparison of Cat-M/NB/LoRa technology features

We take the NB-IoT uplink signal as an example to illustrate the physical layer structure.

• Frequency domain: supports two bandwidths, 3.75KHz and 15KHz. The narrow bandwidth of 3.75KHz can provide a wider coverage. Supports two modes, single tone and multi-tone. Singletone is suitable for low-speed applications.

• Time domain: Different frequency bandwidths correspond to different time domain frame structures.

From the above, we can see that although NB-IoT requires the same uplink power as LTE (power class 3), the peak-to-average signal is relatively low. In addition, NB-IoT works in half-duplex mode, avoiding the use of FDD duplexers, and the insertion loss of the PA backend is small. These factors can make the NB-IoT PA more inclined to nonlinear design, while adopting a smaller die design, thereby achieving the purpose of saving costs and improving efficiency.

Figure 6: Key design directions for the RF front end

The above example shows some key requirements from the perspective of product design. The higher the score, the more important the project is to the final application scenario.

• Ultra-wideband: Taking low frequency as an example, NB-IoTPA needs to work at 663MHz~915MHz, and the available bandwidth is 252MHz.

• Low voltage: Need to support 1.8V to 4.3V operating voltage to meet different battery environment requirements.

• High efficiency: It has different power modes to optimize the efficiency under different power and voltage. At the same time, the headroom design takes into account the maximum output power requirements of Cat-M/NB.

• Extreme temperature: Meet the operating temperature range of -30/-40~+85 degree C.

• Small size: A typical NB module is 26.5mm x 22.5mm x 2.3mm. This is about one-seventh the size of a business card. The size of the RF front end will be an important consideration.

• Low cost: NB modules will gradually replace 2G modules on the market, and their sales prices will be closer to those of 2G modules. Price competition and cost considerations in the RF front end are unavoidable.

We use Qorvo's well-received RF3628 as an example to illustrate the above considerations.

Figure 7: RF3628 product description

The test signal uses 1.4MHz, QPSK, 6RB modulation, and we scanned the various performance indicators of B5 respectively.

Figure 8: B5 E-UTRA ACLR1 vs Pout/temperature/Vcc

Figure 9: B5 PAE vs Pout/temperature/Vcc

In the link budget of the RF front end, assuming a 2dB PA output loss, we can see the sweet point of the RF3628 high power mode.

• Vcc=2.85V, low voltage operation.

• PA output power is 25dBm and antenna port power is 23dBm.

• The ACPR is -37dBm with a 4dB margin, while the PA efficiency is close to 40%.

As a leading global provider of RF solutions, Qorvo provides a variety of solutions to meet the needs of different customers and different scenarios.

Figure 10: RF3628 and QM12154

RF3628 with QM12154/RF1648B is a common combination in the market, mainly used for single-mode Cat-M/NB modules. If you want to consider adding support for GSM, you can also choose QM52015 or RF5212A chips.

The separated PA+switch solution occupies a larger PCB area than the integrated chip solution, but it also has the following advantages:

• Can be flexibly configured between single-band and dual-band.

• To meet the needs of special customers or operators, an external filter can be added between the PA and the switch. Please refer to the figure below for possible interference issues between different frequency bands.

Figure 11: GPS frequency band and B3 uplink frequency band

Figure 12: Harmonics in the low frequency band affect the reception performance in the mid- and high-frequency bands

While integrating PA, switches and filters, the QM55001 also supports low-voltage battery operation, thereby reducing the need for DC boost chips while achieving ultra-small size and meeting RF performance.

Figure 13: Demand for boost chips for different types of batteries

Figure 14: QM55001 functional block diagram

Figure 15: Cat-M/NB-IoT PA and Switch selection matrix