Introduction
A physical layer forms the backbone of wireless technology. The New Radio physical layer has a flexible and scalable design that can support diverse use cases which have extreme requirements. It has a wide range of frequencies and different deployment options which are available. The key technology components of the NR physical layer are the modulation schemes, waveforms, frame structure, reference signals, multi-antenna transmission, and channel coding, etc. LTE supports various modulation schemes like QPSK (Quadrature Phase shift keying), 16 QAM (Quadrature Amplitude Modulation), 64 QAM, and 256 QAM modulation formats. All of these modulation formats will be supported by NR technology. NR will cover a wide range of use cases. 1024 QAM may also be a part of the NR specifications. 5G wireless access is being developed with the three major use cases which are eMBB (enhanced Mobile Broadband), URLLC (Ultra-reliable low-latency communications), and mMTC (massive Machine type communications). eMBB focuses on across-the-board enhancements to the data rate, latency, user density, capacity as well as coverage of the mobile broadband access. The use of CP_OFDM waveform with scalable numerology like sub-carrier spacing, the cyclic prefix in both the UL and DL directions. The use of the same waveform in both directions simplifies the design, especially wireless backhauling and device-to-device communications. There is also support for the DFT-spread OFDM in the UL for the coverage-limited scenarios. A scalable OFDM numerology is required in this so that it can enable various different services on a wide range of frequencies and different deployment options. The NR physical layer has a flexible and scalable design to support diverse use cases with extreme requirements, as well as a wide range of frequencies and deployment options. The key components of the NR physical layer are modulation schemes, waveform, frame structure, reference signals, multi-antenna transmission, and channel coding. NR frame structure supports TDD and FDD transmissions and operation in both licensed and unlicensed spectrum. It enables very low latency, fast HARQ acknowledgments, dynamic TDD, coexistence with LTE, and transmissions of variable length. NR has an ultra-lean design that minimizes always-on transmissions to enhance network energy efficiency and ensure forward compatibility. DMRS is used to estimate the radio channel for demodulation. It is UE-specific, can be beamformed, confined in a scheduled resource, and transmitted only when necessary.
The Sub-carrier spacing in this is scalable, which is 15*2n kHz, where n is an integer and 15kHz is the subcarrier spacing used in the LTE technology. 2n is the scaling factor that ensures that the slots and symbols of different numerologies are aligned in the time domain and is important to efficiently enable the TDD (Time Division Duplex) networks. The choice of the parameter ‘n’ depends on the various factors including the type of deployment, carrier frequency, service requirements (latency, reliability, and throughput), hardware impairments, etc.
New Radio supports a spectrum that has a varied frequency range and the spectrum is categorized as a low band (below 1 GHz), mid-band ( 1-6 GHz), and high band (Above 24 GHz). The high band is also named as mmWave band. It uses two frequency ranges FR1 and FR2. FR1 includes 6 GHz frequency bands and below. FR2 supports bands in the mm-wave range which includes 24.25 – 52.6 GHz. The mmWave bands are helpful to enable 5G UWB (Ultra-wideband).5G NR supports five types of sub-carrier spacing of 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz in the FR 1 (Frequency range 1) and FR2 (Frequency range 2).
5G NR supported technologies
Different technologies that make it possible to have NR are Scalable OFDM numerology, which has a flexible slot-based framework, advanced channel coding techniques, multi-edge LDPC and CRC aided polar, massive MIMO Reciprocity based MU-MIMO ( Multi-user multiple inputs multiple outputs), beamforming and beam tracking techniques. The scalable OFDM-based 5G NR air interface has scalable numerology, frequency localization, lower power consumption, and asynchronous multiple access.
- Optimized OFDM: The specific version of OFDM used in the 5G NR downlink is cyclic prefix OFDM and DFT-S OFDM. CP-OFDM is used as the access technology for 5G NR, it is similar to the access technology used in LTE however CP-OFDM features variable subcarrier spacing termed numerology. It can utilize 15 kHz, 30 kHz, 60 kHz, and 120 kHz, etc subcarrier separation. When the SC spacing is changed, the cyclic prefix duration per symbol also changes. DFT-S OFDM is a discrete Fourier transform spread OFDM is a single carrier-like transmission scheme that is combined with OFDM. It is commonly known as SC-OFDM ( Single carrier OFDM). The transmission scheme of SC-FDMA is very similar to OFDMA. For each user, the sequence of bits transmitted is mapped to a complex constellation of symbols. Then different transmitters are assigned different Fourier coefficients.
- 5G MU-MIMO: MU-MIMO is a multi-user multiple inputs multiple outputs. In MU-MIMO, the base station sends multiple data streams, one per UE, using the same time-frequency resources. Hence, it increases the total cell throughput i.e the cell capacity. It enables the UEs to operate without the need for knowledge of the channel or additional processing to obtain the data streams. MU-MIMO in the downlink significantly improves the capacity of the gNB antennas. It can scale with the minimum of the gNB antennas which can achieve higher capacity gains.
- Spectrum sharing techniques: 5G spectrum sharing is a critical benefit for 5G technology. It is valuable for a wide range of deployments like licensed spectrum aggregation, enhanced local broadband, and 5G private networks.
- Small cells: A Small cell network is a group of low-power transmitting base stations that use mmWaves to increase the overall network capacity. The 5G small cell network operates by coordinating a group of different small cells to share the load and increase the capacity of the system.
NR will cover a wide range of use cases. 1024 QAM may also be a part of the NR specifications. There are two main components in the 5G NR network which are UE and the gNB. The connection from gNB to UE is known as downlink. It uses PBCH (Physical Broadcast channel), PDSCH (Physical Downlink Shared Channel), and PUCCH (Physical Uplink control channel). These channels are used for carrying different data and controlling information.
5G NR supports two frequency ranges (FR1 and FR2). FR1 is sub 6GHz and FR2 is a millimeter-wave range, 24.25 GHz – 52.6 GHz. NR uses flexible subcarrier spacing which is derived from basic 15 kHz subcarrier spacing used in LTE. Subcarrier spacing of 15/30 kHz is supported for below 6GHz 5G NR whereas 60/120/240 kHz is supported for mmWave bands. The maximum bandwidth of 100 MHz is supported in sub-6 GHz whereas 400 MHz is supported in mmWave frequency ranges.
