Special Report: 5G drives transport network innovation
Although significant commercial deployments are yet to come, momentum is building behind Integrated Access and Backhaul (IAB) as the answer to several challenges in rolling out 5G, especially network densification to harness the large amount of spectrum available at millimeter wave frequencies above 24 GHz.
The additional capacity enabled by mmWave will be needed in some scenarios, such as dense urban hotzones, to cater for projected mobile traffic growth over the next few years. But this will require densification in urban areas to overcome the limited range and greater impact of obstructions to the RF signal path.
Backhaul becomes a major cost and logistical constraint with densification, because it requires a significant amount of new fiber to be laid in the absence of a wireless xHaul alternative. In the USA, the cost of laying fiber is around $150 per meter in urban areas, with comparable, if sometimes somewhat lower costs, elsewhere. About 85% of this is associated with trenching and subsequent installation. There is also limited space for the extra backhaul terminating equipment and the need for automated operation to consider.
Standardized by 3GPP, IAB has emerged as an alternative without need for any extra equipment or cabling, combining backhaul with access traffic by in effect adding an extra layer to the cellular hierarchy to extend backhaul further out from the core over the air. This is achieved by dividing base stations into two categories – IAB nodes attached to the traditional fiber or microwave backhaul network as before; and IAB donors extending coverage by backhauling over the same spectrum as the RAN, sharing the spectral capacity.
The IAB nodes provide wireless connection for mobile users as well as backhaul traffic to and from the IAB donors, while being connected to the wired backhaul into the core. The IAB donors then provide onward wireless access to mobile users and transmit backhaul traffic to the IAB donors over that same shared spectrum. It is the IAB donors that extend the coverage and enable the network densification to increase capacity.
The idea of integrated access and backhaul emerged in the 4G era with LTE Relay, but this attracted little interest at the time, for two reasons. First, there was no tight integration between access and backhaul, so strict static partitioning of spectrum between the two was required, with no scope for dynamic reallocation to cater for varying levels of demands for each.
Second, and more significant, capacity was usually at a premium as access traffic proliferated with growing smartphone usage and especially video streaming, giving operators little scope for running backhaul in parallel over the same spectrum – though a few vendors, notably Airspan, did develop products that supported this option.
A similar constraint applies to 5G in midband and especially low-band spectrum, and so it is only really with mmWave that interest has arisen, as the extra capacity meets the challenge of lower range and need for densification. Another positive factor is that the shorter wavelengths in mmWave bands allow antenna size to be reduced so that more can be configured on base stations, enhancing the directional gain for signals transmitted for backhauling, and increasing link reliability, which is crucial.
Reliability is improved further by the multihop design as IAB nodes fan out into a larger number of IAB donors, bringing some resiliency with multiple paths between these two layers of the hierarchy.
IAB in 5G was enabled by 3GPP Release 16 specifications, which allow the total spectrum to be partitioned more dynamically than in 4G, with one section for access and one for backhaul. 5G NR should ensure that QoS requirements for users are met with this multihop topology, and just as importantly that the IAB operation is completely transparent to user equipment such as smartphones, with no additional features or standards having to be implemented at this client level.
There are some performance-related design considerations, especially relating to data rate, which is equal to the minimum of the backhaul and access link rates. Therefore, attention must be paid to balancing the two so that one does not become a performance bottleneck. This ideally requires a central or distributed controller optimizing the backhaul capacity according to dynamic access network demand.
Caching can also be deployed to smooth out variations in backhaul traffic that might otherwise impact QoS for users. The concept of wireless edge caching (WEC) has been added to IAB to moderate traffic variation by pre-fetching popular files such as on-demand video, controlled by suitably optimized algorithms. The principles of such optimization have already been established in content delivery networks (CDNs) for video transport in combination with edge caches to stage popular content closer to the users consuming it, reducing consumption of long-distance bandwidth. The same principle applies to WEC, except that the benefit is to increase access capacity rather than save trunk network bandwidth.
Although IAB has been touted chiefly for dense urban settings it has potential in at least two other scenarios. One is for some rural areas where terrain is relatively flat, when mmWave can be combined with line of sight over longer distances up to around 7 kilometers with plenty of capacity for both access and backhaul. In this case fiber would be non-viable economically, while IAB would avoid need for a separate microwave deployment in parallel with the mmWave network.
In one early example, China Unicom deployed a simplified rural coverage package RuralStar Pro supplied by Huawei in Maopo village, Guizhou province, during 2020 and 2021. Fiber would have been quite inappropriate for the area’s deep winding valleys, while the frequency of overnight fog ruled out microwave because the radiation at those frequencies is absorbed by water droplets in saturated air. The packages were also 80% faster to install than traditional equipment, according to Unicom.
The other scenario is for establishing mobile communications quickly on a temporary basis for construction sites, or as an interim stage, which was the focus of one of the earlier trials staged by Verizon and Ericsson in July 2020 of a battery-powered mmWave cell site that can be rapidly deployed for wireless 5G network backhaul, while awaiting permanent fiber or cabling to be installed.
This raised the possibility of IAB being employed to accelerate 5G roll-out without being bogged down by the planning constraints as well as installation delays that dogged many 4G deployments.
There is now the prospect of more IAB being rolled out at larger scale with 5G NR, beyond the various trials and small-scale local deployments.