What will be the 6G?

Paper: 6G takes shape

Problems in 5G

5G utilization Roughly half the operators globally area still 5G Non Stand Alone (NSA) mode, meaning they rely on an LTE carrier and use 4G core networks.
mmWave Due to severe penetration losses, it is primarily an outdoor-to-outdoor technology, where roughly 80% of cellular data consumption today takes indoors.

6G Spectrum Pyramid

Under 1 GHz: IoT Band (narrow-band IoT)
1-2.5 GHz: 6G FDD Converge layer, where control information or high-priority data can particularly benefit from the robust propagation in these bands
3-4 GHz, 7-16 GHz: The 6G Capacity workshorse. The C band will continue to be na important workhorse for 6G.
mmWave: Capacity Hotspot, where it offers routes to localized high capacity links and will be come more widespread in 6G than far in 5G

A New Waveform for 6G Beyond OFDM/OFDMA

The dominant waveform in 6G will remain scalable OFDM.
OFDM sub-waveforms such as OTFS, can be envisioned for special cases, e.g., ultra-high mobility.

S. K. Mohammed, R. Hadani, A. Chockalingam, and R. Calderbank, “ OTFS—a mathematical foundation for communication and radar sensing in the delay-doppler domain,” IEEE BITS the Information Theory Magazine, vol. 2, no. 2, pp. 36–55, 2022
1. Lozano and S. Rangan, “Spectral versus energy efficiency in 6G: Impact of the receiver front-end,” IEEE BITS the Information Theory Magazine, vol. 3, no. 1, pp. 41–53, 2023.

A New Types of Coding, Modulation, and Duplexing

facilitating more cost/energy-efficient implementations in higher bands.
improving spectral effciency in the lower bands.

in 5G

Although the 5G LDPC code coupled with a systematic bit priority mapping (SBPM) interleaver has shown aboud 0.5 dB gain over the LTE Turbo code, the primary benefit of LDPC is its enabling a much higher area efficiency (Gbps/mm^2). The inherent parallelism of the LDPC decoder and fewer parity checks at a high coding rate lead to more than 5x gain in throughput at peak rate for the same hardware area compared to Turbo code. On the control channel, the introduction of a Polar code enabled higher control channel reliability at small block length compared to LTE tail-biting convolutional code (TBCC), while achieving reasonable encoding and decoding latency.

in 6G

On the coding side, 6G is likely to use upgraded LDPC and Polar codes that allow the same hardware units to decode both 5G and 6G packets, as opposed to new coding families such as rate-less spinal codes, staircase codes, and polarization adjusted convolutional (PAC) codes, which may offer modest performance gains in some scenarios but are hardware-incompatible with 5G

Constellation shaping

Constellation shaping is one area that could bring significant performance gains in 6G since the 5G LDPC code is already within 1 dB of the constrained capacity of the QAM constellation. There are two main approaches to achieving the shaping gain, probablistic amplitude shaping (PAS) [36] and geometric shaping, both of which could potentially lead to coded modulation performance beyond the constrained capacity. Geometric shaping, which directly modifies the constellation points to mimic a Gaussian distribution, has been adopted in DVB and ATSC standards for broadcasting. But the demapping, especially for MIMO, may be too complex for 6G. PAS modifies the distribution of the input bits to an FEC followed by a QAM modulator, where the probabilistic distribution of the constellation points approaches that of a Gaussian distribution. This is optimal from an information theory/entropy perspective. A prefix-code based PAS scheme has been adopted in WiFi-7, but the distribution matcher is likely to be redesigned in conjunction with a 6G LDPC code to meet the rate, block length, and performance requirements.