5G New Radio

The fifth generation of mobile communication, known as 5G New Radio (NR), is defined by the 3GPP standards body starting from Release 15. Unlike 4G LTE which focused primarily on broadband data, 5G NR is architected around three distinct use case families: enhanced Mobile Broadband (eMBB), Ultra Reliable Low Latency Communication (URLLC), and massive Machine Type Communication (mMTC). Understanding the key physical layer technologies of 5G NR is essential for both academic understanding and engineering practice.

Core Physical Layer: OFDM with Flexible Numerology

5G NR uses CP-OFDM (Cyclic Prefix Orthogonal Frequency Division Multiplexing) on the downlink and supports both CP-OFDM and DFT-s-OFDM (DFT spread OFDM) on the uplink. DFT-s-OFDM is preferred for uplink coverage-limited scenarios because of its lower Peak-to-Average Power Ratio (PAPR), which improves power amplifier efficiency at the device. The OFDM parameters are defined by a numerology index mu which scales the subcarrier spacing as delta_f = 2^mu * 15 kHz.

The flexible numerology is a key differentiator from LTE which had a fixed 15 kHz subcarrier spacing. 5G NR defines mu = 0 to 4, giving subcarrier spacings of 15, 30, 60, 120, and 240 kHz. Higher subcarrier spacing reduces OFDM symbol duration and cyclic prefix (CP) length proportionally, enabling shorter slot durations at higher frequencies where mmWave channels have smaller delay spreads. This also reduces latency because shorter slots mean faster scheduling.

Frame Structure and Slot Format

The 5G NR frame structure uses a 10 ms radio frame divided into 10 subframes of 1 ms each. Each subframe contains 2^mu slots. Each slot contains 14 OFDM symbols for normal CP. For mu = 1 (30 kHz spacing), a subframe has 2 slots, so the minimum scheduling unit is a 0.5 ms slot. For mu = 3 (120 kHz), a subframe has 8 slots of 0.125 ms, enabling very low latency communication for URLLC.

5G NR introduces mini-slot transmission where as few as 2, 4, or 7 symbols can form a transmission unit. This is used for low-latency URLLC transmissions that cannot wait for a full slot boundary. The self-contained slot concept allows downlink data, reference signals, and uplink ACK to all fit within a single slot, eliminating round-trip delays.

Frequency Bands and Channel Bandwidth

5G NR defines two frequency ranges. FR1 (sub-6 GHz) covers 410 MHz to 7.125 GHz and supports channel bandwidths up to 100 MHz per component carrier. FR2 (mmWave) covers 24.25 GHz to 52.6 GHz and supports bandwidths up to 400 MHz per component carrier. Up to 16 component carriers can be aggregated, theoretically reaching up to 6.4 GHz of total bandwidth in FR2. The wider bandwidths at mmWave enable the multi-Gbps peak rates advertised for 5G eMBB services.

At FR2 (mmWave), path loss is much higher than at sub-6 GHz bands. This necessitates the use of beamforming and beam management procedures. 5G NR defines beam sweeping, beam measurement, beam reporting, and beam failure recovery procedures as part of the beam management framework. The UE and gNB (base station) maintain beam pair links to ensure connectivity even as the device moves.

Key Channel Structure: SSB and PDSCH

The Synchronization Signal Block (SSB) is the 5G equivalent of the LTE PSS/SSS/PBCH. It occupies 4 OFDM symbols and 240 subcarriers (20 resource blocks) and contains the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and Physical Broadcast Channel (PBCH). The SSB is transmitted periodically (default 20 ms) in up to 64 different beam directions (for FR2 at 120 kHz numerology), forming the SSB burst set. A UE scans beams during initial access to find the strongest SSB beam.

Data is carried on the PDSCH (Physical Downlink Shared Channel) using resource blocks of 12 subcarriers each. 5G NR supports modulation orders up to 256-QAM (8 bits per symbol) with turbo-replaced LDPC (Low Density Parity Check) codes for data and Polar codes for control channels. LDPC codes offer better throughput at high code rates while Polar codes approach theoretical capacity at short block lengths, making them ideal for URLLC control information.

Mathematical Expression

The theoretical peak downlink data rate in 5G NR can be calculated from the number of component carriers, MIMO layers, modulation order, and coding rate. The simplified formula for peak data rate is:

R = (1/T_s) * N_cc * N_layers * Q_m * R_code * (1 - overhead_fraction) where T_s is the OFDM symbol duration, N_cc is number of component carriers, N_layers is the MIMO rank, Q_m is modulation bits per symbol, and R_code is the code rate.

Given:
Numerology mu = 1 (30 kHz subcarrier spacing)
Bandwidth = 100 MHz, N_cc = 1 component carrier
N_layers = 4 (MIMO rank 4)
Modulation = 256-QAM (Q_m = 8 bits/symbol)
Coding rate R = 948/1024 (highest 5G NR code rate)
Overhead (DMRS, PDCCH etc.) = 14%
ResourceBlocks in 100 MHz at 30 kHz = 66 RBs = 792 subcarriers

Why this formula applies:
Peak rate is product of symbols per second, bits per symbol, coding rate

Formula:
R = subcarriers * symbols_per_sec * Q_m * R_code * (1 - overhead)

Substitution:
Symbol duration at mu=1: T_sym = 1/(30000 * 14) per slot
Subcarriers per slot = 792
Bits per symbol = 8, Code rate = 0.9258, Overhead = 0.14

Calculation:
R ≈ 792 * 28000 * 8 * 0.9258 * (1-0.14) * 4
R ≈ 792 * 28000 * 8 * 0.9258 * 0.86 * 4
R ≈ 566 Mbps (approximate single-carrier 4-layer result)

Final Answer: Approximate peak rate ≈ 566 Mbps for 1 CC, 100 MHz, 4-layer 256-QAM

Exam Tip: 5G NR subcarrier spacing = 2^mu * 15 kHz for mu = 0,1,2,3,4. OFDM symbol duration and CP scale inversely. For GATE, remember LDPC for data channels, Polar codes for control channels, and that FR1 is sub-6 GHz while FR2 is mmWave. SSB occupies exactly 4 symbols x 240 subcarriers in the time-frequency grid.

Key Mechanism Points

• Flexible numerology: subcarrier spacing = 2^mu * 15 kHz. Higher mu reduces slot duration and supports lower latency at higher frequency bands.
• LDPC codes replace LTE turbo codes for data channels; Polar codes are used for control channels (PDCCH, PBCH) in 5G NR.
• SSB (Synchronization Signal Block) is transmitted in up to 64 beams for FR2, enabling beam-based initial access and mobility.
• FR1 supports up to 100 MHz bandwidth per carrier; FR2 (mmWave) supports up to 400 MHz per carrier with carrier aggregation up to 16 CCs.
• Mini-slot transmission (2/4/7 symbols) and self-contained slots are URLLC mechanisms to achieve 1 ms over-the-air latency.

Quick Revision

• 5G NR three pillars: eMBB (high data rate), URLLC (low latency reliable), mMTC (massive IoT devices).
• Numerology mu: SCS = 2^mu * 15 kHz; symbol duration = 1/(14 * 2^mu * 15000) seconds.
• FR1: 410 MHz to 7.125 GHz, max 100 MHz BW. FR2: 24.25 to 52.6 GHz (mmWave), max 400 MHz BW.
• Channel codes: LDPC for PDSCH/PUSCH data, Polar for PDCCH/PUCCH/PBCH control.
• Max modulation: 256-QAM (8 bits/symbol). Uplink uses DFT-s-OFDM for better PAPR.
• Exam trap: 5G NR does NOT use turbo codes (LTE uses turbo; 5G NR switched to LDPC and Polar).
• SSB occupies 4 symbols x 240 subcarriers (20 resource blocks). Contains PSS, SSS, and PBCH.