Ferkans — Interactive Telecom Tutor

From LTE to NR — Evolution, Not Revolution

5G NR did not emerge in a vacuum. It deliberately evolved from LTE by generalising its parameters (scalable numerology), removing its limitations (always-on CRS, fixed TTI), and adding new capabilities (beam management, flexible slot formats, preemptive scheduling). Many LTE concepts survive in NR: the RB as the scheduling unit, HARQ with soft combining, CQI-based link adaptation, and the separation of control and data channels. Understanding the systematic differences between LTE and NR — and the engineering reasons behind each change — is the best way to appreciate both systems and anticipate the path to 6G.

Definition:
LTE vs. NR — Systematic Comparison

Feature	LTE (Rel-8/10)	NR (Rel-15/16)
SCS	15 kHz (fixed)	15/30/60/120/240 kHz (scalable)
FFT sizes	128--2048	128--4096
Max bandwidth	20 MHz (single CC)	400 MHz (FR2, single CC)
Slot duration	1 ms (fixed TTI)	0.0625--1 ms (depends on $\mu$ )
Mini-slot	No	Yes (2/4/7 symbols)
DL multiple access	OFDMA	OFDMA (CP-OFDM)
UL multiple access	SC-FDMA (mandatory)	CP-OFDM (default), SC-FDMA (opt.)
Channel coding (data)	Turbo codes	LDPC codes
Channel coding (ctrl)	Tail-biting convolutional	Polar codes
Reference signals	CRS (always-on)	DM-RS + CSI-RS (configurable)
Max MIMO layers (DL)	8 (Rel-10)	8 (Rel-15), 16 (Rel-17)
Beam management	No (cell-wide Tx)	P1/P2/P3 + beam failure recovery
Carrier aggregation	Up to 5 CC (100 MHz)	Up to 16 CC (6.4 GHz total)
Duplex	FDD / TDD	FDD / TDD / dynamic TDD
Frequency range	$< 6$ GHz	FR1 ( $< 7.125$ GHz) + FR2 (24--52.6 GHz)

The shift from turbo codes to LDPC is driven by the need for higher throughput decoding: LDPC enables $> 10$ Gbps hardware decoders, while turbo code decoders struggle beyond $\sim$ 1 Gbps due to their iterative, sequential nature. Polar codes for control channels provide good short-block performance with low decoding complexity.

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Definition:
Carrier Aggregation and Dual Connectivity

Carrier Aggregation (CA): Multiple component carriers (CCs) are combined at the physical layer to increase bandwidth.

LTE-A: up to 5 CCs $\times$ 20 MHz $= 100$ MHz.
NR: up to 16 CCs, maximum 6.4 GHz aggregate bandwidth.
All CCs are served by the same base station.
Combines intra-band (contiguous/non-contiguous) and inter-band CCs.

Dual Connectivity (DC): The UE simultaneously connects to two base stations:

EN-DC (E-UTRA--NR DC): LTE anchor + NR secondary (most common initial 5G deployment mode using Non-Standalone architecture).
NR-DC (NR--NR DC): Two NR base stations (e.g., sub-6 + mmWave).

DC provides:

Higher aggregate throughput (data split across two links).
Improved reliability (if one link fails, the other continues).
Smoother handover (make-before-break).

EN-DC was the key enabler of early 5G deployment (NSA mode): the LTE anchor handles signalling and provides coverage, while the NR secondary cell provides the high data rates. Standalone (SA) NR is now being deployed, removing the LTE dependency.

Theorem: 5G NR Peak Data Rate

The theoretical peak downlink data rate of NR is:

$R_{\text{peak}} = \sum_{j=1}^{J} v_j^{\text{layers}} \cdot Q_m^{(j)} \cdot f^{(j)} \cdot R_{\max} \cdot \frac{N_{\text{RB}}^{(j)} \cdot 12} {T_s^{(\mu_j)}} \cdot (1 - \text{OH}^{(j)})$

For a single 400 MHz FR2 carrier with $\mu = 3$ (120 kHz SCS), 8 MIMO layers, 256-QAM, code rate $948/1024$ :

$R_{\text{peak}} = 8 \times 8 \times 1 \times 0.926 \times \frac{264 \times 12}{125 \times 10^{-6}} \times (1 - 0.14)$

$\approx 8 \times 8 \times 0.926 \times 25{,}344{,}000 \times 0.86 \approx 12.9 \;\text{Gbps}$

With carrier aggregation (16 CCs at 400 MHz each): theoretical maximum $\approx 200$ Gbps.

Each doubling comes from a different dimension: 8 layers (spatial), 8 bits/symbol (256-QAM), 400 MHz bandwidth (frequency), and carrier aggregation (spectrum). The overhead reduction from configurable reference signals (14% vs. LTE's 25%) is a meaningful practical gain.

Proof

Resource counting for 400 MHz at 120 kHz SCS

Bandwidth: 400 MHz. Guard band: $\sim$ 10 MHz. Usable subcarriers: $390{,}000/120 = 3{,}250$ . RBs: $\lfloor 3250/12 \rfloor = 270$ (spec allows 264). Subcarriers: $264 \times 12 = 3{,}168$ .

Throughput per layer

Per slot (0.125 ms): $3{,}168 \times 14 = 44{,}352$ REs. After 14% overhead (DM-RS, PDCCH, CSI-RS): $38{,}143$ data REs. Bits per RE: $8 \times 0.926 = 7.41$ . Per layer per slot: $282{,}599$ bits. Per layer per second: $282{,}599 \times 8{,}000 = 2.26$ Gbps.

Total peak rate

8 layers: $2.26 \times 8 = 18.1$ Gbps (gross). Accounting for slot format overhead: $\sim$ 12.9 Gbps (net).

This exceeds the 20 Gbps IMT-2020 target when combined with CA or 2 CC $\times$ 400 MHz. $\blacksquare$

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LTE vs NR System Comparison

Compare LTE and NR across key performance metrics as a function of channel bandwidth. The plot shows peak data rate, spectral efficiency, latency, and resource utilisation for both systems. Adjust the NR bandwidth to see how NR scales beyond LTE's 20 MHz limit and observe the spectral efficiency advantage from reduced overhead and higher-order modulation.

Parameters

NR channel bandwidth (MHz)100

Example: Throughput Gain from LTE to NR Migration

An operator migrates a 3.5 GHz cell from LTE (20 MHz, 2x2 MIMO, Release 14) to NR (100 MHz, 64T64R massive MIMO, Release 16).

(a) Estimate the LTE cell average spectral efficiency (2.5 bps/Hz typical for 2x2 MIMO MU-MIMO). (b) Estimate the NR cell average spectral efficiency (12 bps/Hz typical for 64T64R MU-MIMO with 8 layers). (c) Compute the total throughput gain factor. (d) For a cell with 50 active users, estimate the average per-user throughput improvement.

Solution

LTE throughput

(a) LTE: $\eta_{\text{LTE}} = 2.5$ bps/Hz. Throughput: $2.5 \times 20 = 50$ Mbps (cell average DL).

NR throughput

(b) NR: $\eta_{\text{NR}} = 12$ bps/Hz (with MU-MIMO, 8 layer average). Throughput: $12 \times 100 = 1{,}200$ Mbps = 1.2 Gbps.

Gain factor

(c) Gain: $1200/50 = 24\times$ . Breakdown: $5\times$ from bandwidth ( $100/20$ ), $4.8\times$ from spectral efficiency ( $12/2.5$ ).

Per-user improvement

(d) LTE per-user: $50/50 = 1$ Mbps. NR per-user: $1200/50 = 24$ Mbps.

A $24\times$ improvement from the combined effect of wider bandwidth and massive MIMO spatial multiplexing. $\blacksquare$

Quick Check

What is the most significant single factor contributing to the throughput improvement of 5G NR over LTE in sub-6 GHz deployments?

Higher-order modulation (256-QAM in NR vs. 64-QAM in LTE)

Wider channel bandwidth combined with massive MIMO spatial multiplexing

LDPC codes replacing turbo codes

Flexible numerology allowing shorter TTIs

Correction:

Wider channel bandwidth combined with massive MIMO spatial multiplexing

NR supports up to 100 MHz per carrier in FR1 (vs. 20 MHz in LTE), a $5\times$ gain. Massive MIMO with 64T64R can serve 8+ layers of MU-MIMO, providing $4$ -- $5\times$ spectral efficiency gain. Together, these give $20$ -- $25\times$ throughput improvement — dwarfing gains from modulation or coding improvements.

Cellular Peak Rate Evolution

Generation	Standard	Year	Max BW	Max MIMO	Peak DL Rate
3G	HSPA+ (Rel-8)	2008	5 MHz	2x2	42 Mbps
4G	LTE (Rel-8)	2009	20 MHz	4x4	300 Mbps
4G+	LTE-A (Rel-10)	2011	100 MHz (5 CC)	8x8	3 Gbps
5G	NR (Rel-15)	2018	400 MHz	8 layers	13 Gbps
5G+	NR (Rel-17)	2022	400 MHz	16 layers	26 Gbps

Why This Matters: From NR to 6G — The Research Frontier

5G-Advanced (Release 18+) and 6G research extend the NR framework in directions covered by the specialised books in this library: RIS (Chapter 28) for programmable propagation, ISAC (Chapter 29) for joint communication and sensing using the same waveform and hardware, OTFS (OTFS book) for delay-Doppler domain modulation at extreme mobility, and AI/ML (Chapter 31) for learned CSI compression, beam prediction, and autoencoder-based transceivers. The ITA book provides the information-theoretic foundations needed to analyse fundamental limits of these emerging technologies.

Key Takeaway

NR achieves $\sim$ 25 $\times$ throughput over LTE in sub-6 GHz through two multiplicative factors: $5\times$ bandwidth (100 vs. 20 MHz) and $\sim$ $5\times$ spectral efficiency from massive MIMO. At mmWave, the bandwidth factor increases to $20\times$ (400 vs. 20 MHz), pushing peak rates beyond 10 Gbps per carrier. No single technology provides this gain — it is the product of wider spectrum, more antennas, better codes, and reduced overhead.

Carrier Aggregation (CA)

A technique that combines multiple component carriers at the PHY layer to increase total bandwidth. NR supports up to 16 CCs with aggregate bandwidth up to 6.4 GHz. CCs can be intra-band (contiguous or non-contiguous) or inter-band.

Dual Connectivity (DC)

A configuration where the UE simultaneously connects to two base stations (a master node and a secondary node). EN-DC uses an LTE anchor with NR secondary (NSA deployment). NR-DC uses two NR nodes (e.g., sub-6 GHz + mmWave) for throughput and reliability.

EN-DC (E-UTRA--NR Dual Connectivity)

The non-standalone (NSA) 5G deployment mode where an LTE eNB serves as the master node for control-plane signalling and an NR gNB provides additional data throughput. The first widely deployed 5G configuration (3GPP Option 3x).

System Comparison and Evolution