6G mid-band spectrum technology explained

Research into 6G started in the 2018 time frame. It is projected that the first 3GPP 6G specification will be completed by 2028, to be followed by commercial 6G deployment and completion of the IMT-2030 specification by the end of 2029. Closer to hand, this coming May, the first 3GPP SA1 6G use cases and requirements workshop will be held. The goal is to gather the 6G use cases of the different global and regional research organizations for presentation to the 3GPP SA1 community. 

One of the latest accomplishments on the road to 6G, occurred at the recently concluded WRC-23 conference, held in Dubai, which, among other topics, addressed mid-band specturm and identified the upper 6 GHz band (6.425-7.125 GHz) for IMT (International Mobile Telecommunications) in Europe, Middle East and Africa, as well as in some countries in the Americas and Asia. The conference also agreed on a new IMT/6G study item for WRC-27 including new frequency bands in the 7-15 GHz range. This article discusses the WRC-23 outcomes in mid-bands[1], requirements on EIRP (equivalent isotropic radiated power), EMF (electromagnetic fields) exposure limits, and the impact on coverage and capacity. 
 

Mid-band spectrum overview

The figure below shows the WRC-23 outcomes which sets a favorable foundation for 6G success.

The following can be observed from the figure:

1. The identification of the 6.425–7.125 GHz (upper 6 GHz) band for IMT enables realization of 5G and 5G-Advanced and paves the way for 6G in many markets. Besides IMT identification in Europe, Middle East and Africa, this band is also identified in Mexico and Brazil, while it remains a WiFi-only band in the US and Canada. In Asia, only a few countries identify the full band for IMT, but the top 100 MHz is identified across the whole Asian region, with China, India and Japan planning to use it.

2. New spectrum ranges: 4.400–4.800 GHz, 7.125–8.400 GHz and 14.800–15.350 GHz are potentially available for 6G, subject to further study in the WRC-27 cycle.

There is also a US spectrum pipeline outside the WRC process concerning the lower 3 GHz band (3.100–3.450 GHz), which is being considered for shared use with military radar, and the 12.7 GHz band (12.700–13.250 GHz), which will be exclusively used for licensed mobile broadband.

In summary, there is up to 700 MHz of spectrum in the upper 6GHz band and potentially more than 2 GHz of spectrum for exclusive and/or shared use in the 7–15 GHz range which also includes 12.700-13.250 GHz band in US which is outside the WRC process.
 

Technical challenges and solutions in mid-band frequencies

It is well known that propagation and penetration loss increase with frequency. For example, the difference in free space pathloss from 3.5 GHz to 7 GHz is 6dB, whereas it is 11.4dB from 3.5 GHz to 13 GHz. Similarly, penetration loss increases by 4.5dB from 3.5 to 7 GHz, versus 6.6dB from 3.5 to 13 GHz (see 3GPP TR 38.901). To compensate these losses and keep using the same macro-grid as C-band, one has to increase the EIRP and use larger antenna arrays while keeping the EMF exposure within safe limits. One also has to address co-existence with incumbents like satellites in the upper 6 GHz band and various federal allocations like space research, fixed and mobile satellites, as well as meteorological and earth exploration satellites in the 7.125–8.400 GHz range. 

In the upper 6 GHz band, regulatory conditions for coexistence with satellites (WRC-23) are not overly restrictive but lead to achievable EIRP towards users — similar to the C-band — which results in reduced capacity and coverage compared to the 3.45 and 3.7 GHz bands. In the 12.700–13.250 GHz band, we have proposed to the FCC (US Federal Communications Commission) [2, 3] an EIRP limit of 85dBm/100 MHz with due consideration for EMF exposure limits. We understand that the same EIRP limit should be considered for the 7–15 GHz range to comply with the EMF exposure limits, while still achieving a decent cell edge and average user performance with large antenna arrays.

System performance

The table below compares cell edge spectral efficiency (bps/Hz) performance of a network operating at 7 GHz and 13 GHz versus 3.5 GHz for various inter-site distances and full buffer traffic. The benchmark system assumes 100 MHz of bandwidth at 3.5 GHz with 64TRX and 256 antenna elements. We assume extreme MIMO with 1024 antenna elements and 256TRX for both 7 GHz and 13 GHz using 200 MHz of bandwidth.

The table shows that a 7 GHz deployment can reuse the macro grid with 500m ISD (inter-site distance) and achieve comparable performance to the 3.5 GHz system with a maximum EIRP limit of 85dBm/100 MHz. A standalone 13 GHz system, however, shows inferior performance if using the 500m macro-grid using standard beamforming techniques. For example, the C-band benchmark system uses 100 MHz bandwidth at 3.5 GHz (4RX, OMNI) and TDD duty cycle of 60%, and achieves cell edge throughput around 16 Mbps (Cell Edge SE 0.26bps/Hz) with 350m ISD. By comparison, the cell edge throughput at 7 GHz using 200 MHz bandwidth is 63 Mbps (Cell Edge SE of 0.32 bps/Hz), while at 13 GHz, it is 19 Mbps (Cell Edge SE 0.16 bps/Hz). In other words, the wider bandwidth, higher EIRP and larger arrays provide higher (7 GHz) or comparable (13 GHz) absolute throughput at mid-bands relative to the C-band. Further, the mid-band performance numbers in the table can be further improved over 3.5 GHz (C-band) using advanced beamforming techniques. 

It may be noted that UL (uplink) coverage at mid-bands is also challenging. Indoor handheld devices with 8RX antennas at 7 GHz can have better coverage than C-band with 4RX, but insufficient coverage at 13 GHz. UL carrier aggregation of a sub-3 GHz carrier with mid-band is one solution, especially if combined with UL power control parameter tuning.

Summary

There is a potential of more than 2 GHz of spectrum for exclusive and/or shared use in the 7-15 GHz range and up to 700 MHz of spectrum in the upper 6 GHz band which also includes 12.7-13.25 GHz band in US which is outside the WRC process. The key for decent coverage and cell edge user performance is to ensure 85dBm maximum EIRP, accounting for EMF exposure limits. It may also be observed that higher EIRP and large antenna arrays are needed at midbands to have better / comparable performance to C band. These results are encouraging, and Nokia plans to engage with various regulatory bodies to make mid-bands an essential part of the evolution to 6G.

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