5G Backhaul Dipping Toes in W-Band Frequencies

Ericsson, along with Deutsche Telekom and Cosmote, has demonstrated the viability of the W-band (92 – 114 GHz) for multi-gigabit wireless backhaul for 5G and beyond. The trial successfully established a 1.5 km, W-band wireless hop that achieved data throughput of 5.7 Gbps over 1.5 km and exceeded 10 Gbps over 1 km hops.

This trial indicates positive progress on the required backhaul evolution to accommodate growing 5G traffic and enable higher capacity use cases. While fiber will continue to be the de facto solution for modern networks, wireless backhaul can be a great alternative for deployments where fixed wireline would have to overcome commercial and logistical challenges (i.e., rural areas). That being said, a couple of factors—spectrum and backhaul technology—would need to be addressed/utilized to facilitate the increased adoption of fixed wireless backhaul in future networks.

From a backhaul spectrum perspective, the search for more spectrum has prompted operators, equipment vendors, and regulators to explore frequencies in the higher bands outside of the traditional 6 to 42 GHz frequency bands. The demand for wider channels and larger capacities have led regulators to open backhaul spectrum in the V-bands (57 – 71 GHz) and E-bands (71 – 86 GHz). The E-band has large bandwidth (10 GHz) capabilities, allowing transmission of high-speed data over short distances (2 km to 3 km), while the V-band is unlicensed and offers operators cost-efficient spectrum (albeit with higher interference risks).

The W-band (92 – 114 GHz) and D-band (130 – 175 GHz), on the other hand, offer around 15 GHz and 30 GHz of available spectrum respectively. The combination of wider channels and spectrum efficient methods can build on the capabilities of E-band for even higher capacity backhauling for 5G, and eventually 6G networks. The W- and D-bands are two ideal propagation windows with low atmospheric gas attenuation. D-band path loss is only 6 dB worse than E-band with minimal dB attenuation due to rain. The long-term technical targets of W- and D-band are providing capacities of up to 100 Gbps (4x25 Gbps MIMO) in 5 GHz channels in and extending link length of up to 2 km (a length that is comparable to E-band).

From a technology perspective, operators can leverage on the following set of technologies to enhance capacity, availability, and overcome the signal propagation limitations of backhaul links using the W- and D-bands:

• Band and Carrier Aggregation: Band and Carrier Aggregation (BCA) for backhaul involves bonding multiple channels across different frequency bands to build higher capacity point to point (PTP) connections. BCA for backhaul comes in many variations, with different frequency pairings catering to different deployment scenarios. Combining a lower band microwave link with a licensed/lightly licensed millimeter wave link (i.e., channel(s) within the W-band) to increase capacity. This pairing also assures reliable link availability; BCA links would circumvent atmospheric attenuation (rain and oxygen attenuation) and would thus be able to provide more resilient, higher capacity coverage over longer distances.
• Integrated Access Backhaul: Integrated Access Backhaul (IAB) involves utilizing access spectrum to backhaul data traffic to the core network. IAB has higher equipment cost efficiencies, as both access and backhaul would share the same radio hardware unit and have similar operation/maintenance systems. IAB can also help lower equipment costs, as cell sites do not require separate transmitter-antennas for backhaul links.
• Cross Polarization: Second Polarization, or XPIC, can double the spectral efficiency by propagating two signals in horizontal and vertical signals over the same channel, increasing channel reusability. XPIC also cancels interference brought about by atmospheric attenuation (e.g., raindrops, which cause polarization rotation as they fall, skewing a signal’s polarization out of alignment and causing interference with other polarized signals).

Pricing is another important aspect that greatly influences the trajectory of backhaul evolution in future networks. Unfortunately, pricing formulas regulators use to set backhaul spectrum costs often fail to adapt effectively to the much wider channels that are available in higher frequency bands. In practice, costs sometimes scale linearly—causing extremely costly prices for wider channel sizes in the W- and D-bands (e.g., 2 GHz instead of the usual 56 to 224 MHz in lower frequency bands). Formulas need to take into account improved geographical spectral efficiency, higher frequency reuse, and the larger spectrum availability/requirements—especially in higher frequency bands. Modern backhaul pricing formulas must therefore have variables that account for the sizable data demands of 5G. More nuanced pricing formulas that take energy consumption, technology, and area of operations into consideration can mitigate high prices from larger bandwidth purchases and can give operators more autonomy in their spectrum budgeting. By including variables that 1) incentivize the aforementioned backhaul technologies (BCA, IAB to overcome lower signal ranges of the E-, W- and D-bands), and 2) can mitigate the backhaul spectrum pricing, operators will be able to simultaneously reduce their backhaul spectrum costs and maximize the efficacy of using wider channels of spectrum in the higher millimeter wave frequencies.