5G Air Interface and Physical Layer Procedures – Part 3

Regarding physical channels, we have discussed the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and Physical Broadcast Channel (PBCH) in the downlink, as well as the Physical Random Access Channel (PRACH) in the uplink, in the previous chapter. In this section, we will go through the rest of the New Radio (NR) physical channels and other aspects related to physical layer procedures. Briefly, we will cover the different channel coding schemes used for NR, and we will go through the processing chain of the downlink and uplink data channels. In this chapter, we will pay particular attention to how the channel structure is changed to allow for lower-latency use cases and how lower latency impacts the operation of Enhanced Mobile Broadband (eMBB) data traffic. Finally, we will discuss Bandwidth Parts (BWPs).

In this chapter, we’ll cover the following main topics:

• Downlink physical...

Downlink physical channels and signals

NR defines a similar set of physical channels to Long-Term Evolution (LTE). We already covered the PSS, SSS, and PBCH. In this section, we will cover the Physical Downlink Control Channel (PDCCH), the Physical Downlink Shared Channel (PDSCH), as well as the Channel State Information Reference Signal (CSI-RS). The Tracking Reference Signal (TRS) and Phase Tracking Reference Signal (PTRS) are new reference signals that have been introduced in NR – we will see what they are used for. Let’s learn about the PDCCH.

The PDCCH

In NR, the PDCCH carries the Downlink Control Information (DCI), like LTE. The modulation used for the channel is Quadrature Phase Shift Keying (QPSK) and the Radio Network Temporary Identifier (RNTI) masks the DCI Cyclic Redundancy Check (CRC) bits as it was done in LTE.

The PDCCH is divided into Control Channel Elements (CCEs) in NR. Each CCE is composed of six Resource Element Groups (REGs), where each...

Uplink physical channels and signals

Now, let’s have a look at the physical uplink channels and signals. In previous chapters, we covered the PRACH and Sounding Reference Signals (SRS). In this section, we will learn the physical uplink control and data channels, and signals such as the Physical Uplink Shared Channel (PUSCH), PUSCH Demodulation Reference Signal (DMRS), and Physical Uplink Control Channel (PUCCH). Let’s start with the PUSCH.

The PUSCH

The PUSCH carries data and also optionally Uplink Control Information (UCI). As we described before in previous chapters, the uplink waveform can be either Cyclic Prefix OFDM (CP-OFDM) or Discrete Fourier Transform-Spread-OFDM (DFT-s-OFDM). The waveform to be used is signaled to the device by the network. DFT-s-OFDM is only used for single-layer transmissions, and it is intended for power-limited scenarios. This waveform also supports intra-slot frequency hopping for more diversity.

As in the downlink case, PUSCH...

Channel coding schemes

Low-Density Parity Check (LDPC) code is used for eMBB physical data channels in which polar codes are employed for eMBB physical control channels, as well as for the PBCH. These channel coding schemes provide better throughput and latency performance compared to what we have in legacy LTE. Note that channel coding for other use cases such as massive Machine Type Communications (mMTC) and Ultra-Reliable Low Latency Communications (URLLC) are not part of Release 15’s scope. So, let’s start.

Transport channel coding chains

The following figure represents the basic coding scheme for the downlink shared channel. The diagram is very similar to what we have in LTE. A CRC is attached to the transport block, it is segmented onto the code blocks, and then, additional CRSes are attached to each code block:

Figure 6.14 – Transport channel coding for the downlink shared channel

Then, LDPC coding is performed for each...

BWPs

In this last section of this chapter, we will cover another concept introduced in NR, which is the BWPs. This concept is new in NR and very powerful for a few interesting and important use cases.

In this section, we will learn what BWPs are and how they are configured, activated, and deactivated. We will also describe the main use cases for BWPs to better understand their importance in the NR physical layer.

Defining a BWP

So, let’s start by defining what a BWP is. A BWP is a set of contiguous PRBs with nested bandwidths where the devices are only allowed to transmit or receive. Each BWP has its own configuration with a specific numerology to use and a set of reserved resources for which transmission is not allowed. Devices always operate using BWPs. This means that at least one initial BWP must be defined by the PBCH so that all devices can access it. This initial BWP will also contain the CORESET and PDSCH for the RMSI. However, a BWP does not need to contain...

Summary

In this chapter, we went through the downlink and uplink physical channels and signals for NR. Although certain aspects are taken from LTE, we still covered the similarities and differences for NR. We discussed channel coding schemes, LDPC code, and polar coding. We finished the chapter by evaluating the use cases of BWPs. As such, this chapter has fully covered the NR air interface and physical layer procedures.

Next, we’ll look at the packet core procedures in 5G.