5G Air Interface and Physical Layer Procedures – Part 2

The next New Radio (NR) concept that we will cover in this chapter is the initial access to a network and beam management procedures. These procedures are new to NR, and there is nothing similar in Long-Term Evolution (LTE), especially for initial access. As we will see during this chapter, both millimeter wave operation and reducing the number of signals that are always transmitted are the two main drivers for this initial access procedure design. Upon completion of this chapter, we will understand the concept of beam sweeping and especially its relationship with Millimeter Wave (mmWave) frequency. We will focus on how a device and the gNB can identify the best special beams to be used for transmission. We will also describe the beamforming’s effect on the initial access procedure. Upon completion of this chapter, we will understand some of the design decisions regarding initial access, as well as how decisions...

Initial access and beam management

mmWave operation is noteworthy. It opens frequency bands that were not accessible before – for example, Frequency Range 2 (FR2), as described in the first chapter. It brings more room for bigger bandwidth, which enables higher capacity and throughput. However, the transmission of mmWave is complicated. The path loss of mmWave due to free space propagation is much bigger than the typical LTE frequency. This culminates in limited cell coverage and cell size. To partially overcome this issue, directional beams are used on downlink transmission and uplink reception so that the path loss due to the free space propagation is partially compensated.

Directional transmissions enable new methods of multiplexing different users in the spatial domain, such as transmitting simultaneously to different directions instead of spatial multiplexing based on precoders, as with LTE. However, such directional transmissions mean that when connecting to the network...

Physical channels and signals for the initial access procedure

We have covered the general aspects of the initial access procedure. In this section, we will be discussing the details of how the SS blocks and the PRACH are defined. The channels that define SS block are the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS), and they are placed at the PBCH. These downlink signals are the only ones that are periodically transmitted in NR. Note that they can be turned off by the network thanks to NR’s ultra-lean design.

As we emphasized, coexistence and forward compatibility with LTE are crucial for NR. To achieve this, it is necessary to have as few blocked resources as possible. So, even though SS block transmission is periodic, having the option to turn off certain SS block locations or opportunities will help the network avoid interference with LTE or other featured services.

SS block

The SS block is composed of four consecutive OFDM symbols...

Summary

In this chapter, we learned that beam sweeping is required when operating at an mmWave frequency. The use of beam sweeping affects the synchronization signals and PRACH design, since the initial access procedure must account for the directionality of the transmissions.

We have seen how identifying this SS block transmission in each beam with different time indexes and linking the PRACH resources to these indexes helps in this procedure. This directional aspect of the transmission is new to NR.

Finally, we have also gone through some of the considerations about LTE-NR coexistence, especially how the SS block transmissions can utilize 30 kHz subcarrier spacing to avoid interference with the LTE cell-specific reference signals.

In the next chapter, we will detail the remaining downlink and uplink physical channels and signals. We will analyze one of the other different concepts of NR, which is called the PDCCH Control Resource Set (CORESET). We will also see different...