Narrowband Internet of Things (NB-IoT) is a narrowband system developed for cellular internet of things by 3GPP (3rd Generation Partnership Project). The system is based on existing LTE (Long Term Evolution) systems and addresses optimized network architecture and improved indoor coverage for massive numbers of radio network devices with following characteristics: low throughput (e.g., 2 kbps); low delay sensitivity (˜10 seconds); ultra-low device cost (below 5 dollars); and low device power consumption (battery life of 10 years).
It is envisioned that each cell (˜1 km2) in this system will serve thousands (˜50,000) of radio network devices such as sensors, meters, actuators, and the like. In order to be able to make use of existing spectrum for, e.g., GSM (Global System for Mobile Communications), a fairly narrow bandwidth has been adopted for NB-IoT technology. In particular, the bandwidth per carrier is one LTE Physical Resource Block (PRB), i.e., 12 subcarriers of 15 kHz each, or 180 kHz.
For Frequency Division Duplex (FDD) mode of NB-IoT (i.e., the transmitter and the receiver operate at different carrier frequencies), only half-duplex operation must be supported in the radio network device. In order to achieve improved coverage, data repetition is used as required, both in uplink (UL) and downlink (DL). The lower complexity of the radio network devices (e.g., only one transmission/receiver chain) means that some repetition might be needed also in normal coverage. Further, to alleviate radio network device complexity, the working assumption is to have cross-subframe scheduling. That is, a DL transmission is first scheduled on a Narrowband Physical DL Control Channel (NPDCCH) and then the first transmission of the actual data on the Narrowband Physical DL Shared Channel (NPDSCH) is carried out after the final transmission of the NPDCCH. Similarly, for UL data transmission, information about resources scheduled by the network and needed by the radio network device for UL transmission is first conveyed on the NPDCCH and then the first transmission of the actual data by the radio network device on the Narrowband Physical UL Shared Channel (NPUSCH) is carried out after the final transmission of the NPDCCH. In other words, for both cases above, there is no simultaneous reception of control channel and reception/transmission of data channel from the radio network device's perspective.
The NB-IoT radio frame length is the same as LTE, i.e., 10 ms and consists of 10 subframes. However, not all of the subframes are available for dedicated data communication in DL in an NB-IoT cell. The number of available subframes in the DL depends in part on which of three operation modes the NB-IoT is deployed in—Stand-alone, In-band, or Guard-band. For all operation modes, a radio network device must rate-match around numerous non-available subframes (or parts of subframe). These include NB-IoT primary and secondary synchronization channels (NPSS, and NSSS), where NPSS is transmitted in subframe 5 of every radio frame (NSSS transmission cycle is still to be defined in 3GPP). Non-available subframes also include the NB-IoT broadcast channel (NPBCH) containing the Master Information Block (MIB) occupying subframe 0 in every radio frame, and the NB-IoT system information blocks broadcast on NPDSCH (e.g., NSIB1 broadcast in the fourth subframe of every other radio frame). Still further non-available subframes include DL gaps when configured, and NB-IoT Reference Symbols (NRS). In addition, in the case of In-band operation mode, data cannot be transmitted where the LTE system, in which NB-IoT is deployed, transmits LTE reference symbols such as CRS (Cell-Specific Reference Signal) and PRS (Positioning Reference Signals), or in LTE MBSFN (Multicast-broadcast single-frequency network) subframes.
Due to the nature of NB-IoT with half-duplex communication, cross-subframe scheduling, low bandwidth, the available subframes, and the number of radio network devices to be served, it becomes evident that, as in all other wireless communication systems, NB-IoT will naturally benefit from utilizing more spectrum for efficient operation, especially if such spectrum is already available (e.g., in an in-band operation mode during low traffic hours when LTE carrier is not fully utilized). Therefore, in 3GPP Rel-13, NB-IoT multi-carrier operation has been adopted where the radio network devices operating in an NB-IoT anchor carrier are configured through higher layer signaling (Layer 3 RRC) to operate in an NB-IoT non-anchor carrier during connected mode operation. Because radio network devices do not need to search for non-anchor carriers, they are not constrained to be deployed on a 100 KHz raster, as the anchor carrier is. At the end of connected mode operation on a non-anchor carrier, the radio network device autonomously returns to the anchor carrier.
For 3GPP Rel-14, it has been proposed to extend this multi-carrier operation. According to one of the Rel-14 work item objectives, unlike Rel-13 operation, radio network devices shall be able to both monitor paging and perform Random Access on non-anchor carriers. Besides NB-IoT, numerous wireless communication networking standards provide for multi-carrier operation. In general, any time paging or Random Access is permitted on more than one carrier in a cell, radio network devices must be distributed among the available carriers in a deterministic manner. That is, both the network and each radio network device must agree as to which carrier each device will access. In NB-IoT and similar systems, such as enhanced Machine Type Communications (eMTC), where massive numbers of devices are anticipated, it is advantageous for the network and the radio network devices to independently come to the same conclusion of to which carrier each device is assigned, to avoid the massive signaling overhead that would result if each device had to be explicitly assigned to a carrier.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.