From the course: NB-IoT (Narrowband IoT) Engineering: Deployment, Configuration, Call Flow Analysis

LTE frame structure and physical layer overview

From the course: NB-IoT (Narrowband IoT) Engineering: Deployment, Configuration, Call Flow Analysis

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LTE frame structure and physical layer overview

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As NB-IoT for in-band and guard band is a part of a LTE network, so it is important to understand LTE downlink frame structure in detail. We have a radio frame which is of 10 millisecond and each radio frame has 10 different subframes, each of 1 millisecond. Each subframe contains two slots of 0.5 millisecond. Now this particular slot would have certain resources in frequency and time domain. In frequency we have different subcarriers. In time domain we have OFDM symbols from 0 to 6. We have 12 different subcarriers each of 15 kilohertz which makes one resource block of 180 kilohertz. Every slot in this particular grid is termed as resource element. If we have 12 subcarrier 7 of FTM symbols we would have total 84 resource elements which makes one resource block. This is the Air interface resource allocation process which is used in LTE as well as in NB-IoT. Now let's understand the allocation of resources within this time and frequency grid. We have the frequency domain on the y-axis and time domain on the x-axis. We have this grid we are considering 1.4 megahertz of spectrum band in case of LTE. We have certain resource allocation for different type of information which is termed as channels or signals in case of LTE. All the information in the air interface is mapped on these physical resource blocks via different channels and signals. Starting our channel placement we have the synchronization signal, we have PSS which is primary synchronization signal and SSS which is secondary synchronization signal. They are used for initial timing and frequency alignment and also used for cell identity group detection. It occupies the central part of the total resource allocation in the grid on the left side. Then we have the broadcast channel which carries essential master information block. It carries the The important information which is required by the UE to access to the network. It's spread across multiple different symbols and resource blocks for robustness. We then have the control region structure where we have the critical control plane where the physical control format indicator channel comes into picture. It is present in first OFDM symbol indicates how many control symbols will follow. And we have different allocation for control channels ranging from 1 to 3. So we have this PCFICH for allocation of channels for control channel, which we are going to talk about. Then we have the PHICH, which is Physical Hark Indicator Channel. It carries the Hybrid Automatic Repeat Request Acknowledgement, which is the retransmission used for the transfer of data from point A to point B. And if it is not transmitted or received properly, then there is a retransmission being done by HARC, which is this physical hybrid automatic repeat request channel. Then we have the PDCCH, which is the downlink control channel, which carries the scheduling assignments. It uses the remaining control resources after the allocation of PCFICH and PHICH. And then whatever is left over would be given to PDSCH, which is the shared channel, which carries the actual data information. It is dynamically allocated to different users depending upon their needs and demands. While our main channels handle data and control part, LTE employs several critical reference signals that work behind the scenes and these signals enable key functions like channel estimation, mobility management, and positioning. Starting with the cell specific reference signals. These are the workhorses of LTE. They are everywhere and used by all the devices for channel estimation, for data demodulation. It also has the channel quality measurement and used for cell selection and handover decisions also. We then have the PRS, which is positioning reference signal, which is used for location services in case of emergency calls. It is also used for commercial location-based services and used for network optimization and planning. We then have the CSIRS, which is used for advanced MIMO. It is a channel state information reference signals. In nutshell, if we summarize, we have different signals and channels starting with the synchronization signals, which enable initial cell detection. We then have broadcast channel, which provides essential system information. We then have control channels, which are PCFI CH, PHI CH, PDC CH, which manage the signaling and communication part. Then we have the shared channels, which carry the actual user data. The time domain structure ensures efficient resource allocation in LTE. And we'll see that how these different channels and signals are defined in case of NB-IoT on the similar lines as that of LTE.

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