Bharat 6G Alliance Off To A Quick Start, Publishes First White Paper

July 31, 2023

Written by Alex Lawrence

Within weeks of being set up, India’s Bharat 6G Alliance has released a White Paper, “Meeting IMT-2030 Performance Targets: The Potential of OTFDM Waveform and Structural MIMO Technologies.”

It’s striking that the first whitepaper to be released is not a broad vision document or a statement of intent. The Indian government has already outlined its desired direction for 6G in the country, so in many ways it makes sense to avoid repetition. However, most organisations tend to put a stake in the ground; 6GWorld will be keen to see whether subsequent white papers are as focussed as this one.

While the document pays attention to these two candidate technologies for 6G radio, it is far from exclusively about that.

Technology Directions

While there has been speculation that the OFDM waveform used in 4G and 5G might carry over into future generations, it has limitations. One of the more prominent of these is a high Peak to Average Power Ratio (PAPR) and situations of energy inefficiency. In an environment where energy costs and sustainability are dual imperatives, this can be a problem.

The paper suggests that OTFDM – Orthogonal Time Frequency Division Multiplexing – can be used as a waveform to solve this for 6G. The paper notes that:

“OTFDM presents an innovative solution by time-multiplexing reference signals [RS] and data within a single OFDM symbol, thereby allowing one-shot transmission with flexible RS overhead and high-power efficiency.”

The paper goes on to explain how this works and shows some simulation results to back up the proposal, emphasising the performance at cell edges. It also highlights the very low latencies which can be achieved by combining the reference signals into the main OTFDM symbols.

Meanwhile, the paper also puts forward Structural MIMO, or S-MIMO, as a candidate to improve capacity in the radio. This is an evolution of Multi-User MIMO [MU-MIMO], which uses beamforming to effectively multiplex the spectrum being used by the antenna array.

The paper observes “Although a theoretical 16-fold increase in network capacity can be expected compared to a single-antenna system, practical limitations such as limited antenna directivity per port, interference from other sectors, and power division among scheduled layers restrict the number of users that can be paired and the achievable network capacity.”

Structural MIMO aims to overcome some of those limitations through three main innovations:

  • Highly directional beams associated with each antenna port/panel maximizes the coverage per port/panel
  • Structural arrangements of multiple antenna panels using a three-dimensional structure, enabling full coverage in azimuth and elevation.
  • Joint baseband processing of signals associated with multiple antenna panels, incorporating TDD reciprocity calibration, MU-MIMO precoding/beamforming in the DL, and joint processing of UL signals from multiple antenna panels.

One of the notable things about this proposal is the claim that, by setting up multiple antenna panels, coverage can be delivered not just on the ground but also across 180 degrees of elevation, enabling connections to drones or other flying objects. This would be quite a unique tie-in to the 3-dimensional networks proposed for 6G which link satellite, HAPS and drones to terrestrial services.

The paper includes details of a variety of S-MIMO tests using different numbers of antenna panels to test the performance of beam directionality and azimuth and the spectral efficiency. It concludes that, by using a combination of OTFDM and S-MIMO, it would be possible to serve scenarios such as hyper-low latency with better power efficiency and spectral efficiency at or beyond the 2-3 times required by IMT-2030.

It also notes that coverage can be improved – at 7GHz, coverage comparable to midband spectrum can be achieved, and that (with sufficient spectrum) peak data rates can also be greatly improved.

These are early experiments and tests, but certainly interesting. In practice there might be a tension between the improved power efficiency of OTFDM and the resources needed to run S-MIMO, so 6GWorld will follow the development of these concepts with interest to see whether they can cross the chasm from initial proof of concept to commercial usage. When use cases for future communications are so diverse, it can only be a benefit to have multiple ideas being put forward and perhaps adopted in parallel.

Added Extras

Beyond the core technologies examined in detail, the white paper outlines other technology areas where the Alliance sees scope for development in IMT-2030. These are largely predicated on the particular situation in India, but we saw that in the push for the adoption of 5Gi as part of 3GPP’s 5G standards the country positioned itself as a champion for other countries that don’t have the ARPUs or the urban populations of the countries where most telecoms systems are developed. Deliberately or coincidentally, there appears to be something similar at play here.

Topics covered include:

  • Improving coverage and capacity through relays. The white paper picks up on the Integrated Access and Backhaul [IAB] concept defined in Release 17, proposing a number of enhancements that might make it more effective for 6G.
  • Integrating satellite and terrestrial communications. This is already a topic of study but it is arguably more significant for countries without good pre-existing terrestrial infrastructure in place.
  • Machine Learning in the physical, MAC and higher layers. Not terribly surprising, perhaps, as network automation has been a focus of attention for quite some time. However, refreshing that it gets called ‘AI’ very little.

Overall the paper is worth reading, both for the specifics included but also for the very fact of its existence. While India has been a sleeping giant in previous generations and is arguably later in getting organised than some of the traditional leading countries, it suggests that the Bharat 6G Alliance is getting up and running quickly and may be more able to help India make more of an impact in the telecoms environment than ever before.

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