Efficient NB-IoT Uplink Design
The number of devices connected to the so called Internet of Things (IoT) is expected to grow considerably over the next few years. To better suit the needs of such machine-type communi- cations (MTC) the Third Generation Partnership Progect (3GPP) is introducing new cellular technologies. These new standards were designed to enable enrgy efficient operation of com- munication devices while reducing their cost and extending their coverage range. To meet the extremely demanding low enegergy requirements of IoT applications, the 3GPP specifications target devices with ten years batery life.
To enable a fast introduction, the new standards build on the existing LTE and GSM tech- nologies. On the GSM side the so called Extended Coverage GSM (EC-GSM) standard was released, while on the LTE side two new device categories were standardized : LTE Cat-M1 and LTE Cat-NB1, also called Narrowband Internet of Things (NB-IoT). Both new LTE categories reduce the communication bandwidth from 20 MHz (LTE Cat-1) to 1.4 MHz for Cat-M1 and to 200 kHz for Cat-NB1. The focus of this project will be set on the efficient implementation of an uplink NB-IoT transmitter on an FPGA testbed. In order to avoid hardware overhead a software implementation is preferred firstly, while the transfer of specific tasks to dedicated hardware can be considered secondly to unload the CPU if needed.
The physical layer (PHY) of a cellular modem can typically be divided into two subparts :
- Radio Frequency (RF) analog processing,
- Digital Baseband (DBB) processing.
The RF processing is usually done in a dedicated Radio Frequency Integraged Circuit (RFIC) while the DBB processing can be done in a CPU, a Digital Signal Processor (DSP), an Appli- cation Specific IC (ASIC) or a combination thereof. For fast prototyping an FPGA testbed was set up for the development of an LTE Cat-M1 and Cat-NB1 modem. The testbed is distributed over two separate boards as depiced in Figure 1 :
- Radio Frequency (RF) evaLTE board for RF analog processing with an IRIS405 chip from our partner company ACP and
- Kintex-7 KC705 FPGA board for DBB processing.
The hardware already developed on the FPGA includes different receiver blocks as well as an Open-RISC processor. The processor can be used to control the baseband blocks as well as to execute part of the signal processing. During this project the first work on the uplink part of the modem will be done. The FPGA and the RF chip communicate over a Radio Front End - Baseband Digital Parallel (RBDP) Interface. The processor con use the RBDP interface in the uplink direction by directly writing the IQ samples to be transmitted in a buffer and enabling the transmission using the corresponding control registers.
The uplink baseband processing can be divided into channel coding and modulation. In the case of NB-IoT uplink the channel coding includes Cyclic Redundancy Check (CRC) generation and attachment, turbo or convolutional coding and rate matching. For baseband modulation two different schemes are possible : Single Carrier Frequency Division Multiple Access (SC- FDMA) or single tone transmission. Both schemes are based on the separation of the available spectrum into orthogonal subcarriers, each of which is then used to transfer a stream of data. In the case of SC-FDMA multiple subcarriers are simultaneously used by the same user while for single tone transmissions each user only uses one subcarrier at a time. Two different subcarrier spacings are possible : 15 kHz and 3.75 kHz. The first one is the same as for other LTE standards and leads to a total of 12 subcarriers while the second one leads to a total of 48 subcarriers. Single tone transmissions can use both subcarrier spacings while SC-FDMA transmissions can only use the 15 kHZ subcarrier spacing.
The NB-IoT uplink is composed by two physical channels :
- Narrowband Physical Uplink Shared Channel (NPUSCH),
- Narrowband Random Access Channel (NPRACH).
The NPUSCH can use both SC-FDMA and single tone transmissions, while the NPRACH only uses single tone transmissions. An overview of the overall NPUSCH transmission chain for SC-FDMA signals is depicted in Figure 2. The goal of this project is an efficient implementation of the two NB-IoT uplink physical channels NPUSCH and NPRACH. To this end the student shall first get familiar with the NB-IoT standard and identify the different operations and parameters required by the PHY for the generation of the NPUSCH and NPRACH signals. Then a Matlab model will be implemented and integretad to the existing Matlab framework LTESim to provide reliable functional verification. Next the channel coding and the modulation of both NPUSCH and NPRACH shall be ported on the test bed. For the implementation a combination of hardware and software can be used. The software shall be written in C and ported to the testbed with the available software framework (compiler tool-chain, RFIC and DBB drivers, etc.) while the hardware building blocks shall be implemented in VHDL and integrated into the existing system.
 Nokia Networks. LTE-M Optimizing LTE for the Internet of Things. http://networks.nokia.com/sites/default/files/document/nokia_lte-m_-_optimizing_lte_for_the_internet_of_things_white_paper.pdf, 2015.