High-band mmWave – Plenty of Bandwidth and Flexibility to Meet the Requirements in the “X-Haul” Portion of a 5G Network Architecture.
The mobile evolution from 4G to 5G is about much more than just faster speeds. One way in which 5G differs significantly from 4G is in the network architecture concept. Key to this new architecture is a concept known as “x-haul,” which is a recently introduced term that refers to the network connectivity concepts comprising traditional backhaul and the newer “front” haul and “mid” haul topologies as well. All three will be used to interconnect cell sites to each other, to mobile core networks, and ultimately to data centers, where a great deal of subscriber and network data will be hosted. First, let’s take a quick look at the requirements in each of these three areas.
The arrival of 5G means much higher download and upload speeds for end users, so bandwidth requirements will rise significantly in all areas of the network. 5G features ultra-low latencies and will be deployed with a higher density of Base Stations – and that will mean many more macro and small cell base stations, particularly in urban areas. Another new feature is that of network slicing, that is, creating dedicated “virtual” portions of the network to serve a specific function – such as IoT or video content delivery.
All the “Hauls”
5G backhaul is similar to 4G backhaul, but will carry much more traffic due to the higher bandwidth and performance provided by 5G NRs (new radios). Backhaul latency requirements are far less stringent here as compared to front or mid-haul, so the focus is on capacity and distance. In addition as cell radii shrink and BTS density increases, the small RF footprint of mmWave backhaul links will ensure interference is not an issue. Capacity increases are seen in the new interfaces used for backhaul, namely 10GbE, 25GbE, 100 GbE and even 400 GbE at the extreme ends of the network. For this, fiber featuring DWDM optics is a preferred solution for the MNOs, but as has been seen over the years, access to fiber optic lines is not always where you need it or economical. In these situations, V- and E-band mmWave systems prove once again to be the perfect complement to fiber, given their multi-Gigabit capacities, extended kilometer reach and low microsecond latencies.
Mid-haul is a totally new concept exclusive to 5G and not used in 4G networks. It results from the 5G RAN evolving from the traditional 4G BBU (baseband unit) and radio head installed in a “split mount” configuration on a tower, or the more recent Remote Radio Head (RRH) architecture, in which the RRHs are connected via front haul to a BBU “warehouse” some distance away. The 5G RAN will now consist of a Distributed Unit (DU), Centralized Unit (CU), and Active Antenna Unit (AAU) architecture – that is, the BBU becomes a DU and a CU. For reasons related to 5G specs and engineering (network disaggregation) and on the path to a fully Virtual RAN (VRAN), the network functionality of a 4G BBU will be divided between the 5G CU and DU. Further, the DU and CU will likely be virtualized and in different locations of the network. Therefore, mid-haul is used to connect the CUs and DUs. So what distances and capacities are we talking about?
With virtualized units and a wide variety of MNO deployment strategies, it can be expected that mid-haul link distances will vary widely and capacity requirements will as well. For that reason, the finalization of the 3GPP standards are expected to support reaches of up to 100km with a latency of 5 milliseconds and a variety of Ethernet interface rates, such as 10GbE and 25GbE or 100GbE later – which means multi-Gigabit capacity requirements.
Although not as quite as “QoS sensitive” as the fronthaul approach, mid-haul connections will want to improve the end-user experience with faster attachment to the mobile network and faster data response between the end user equipment to the network. And as opposed to the relatively straightforward backhaul networks, mid-haul transport networks will feature more variety in terms of topologies, such as mesh, ring or hub and spoke.
For the mid-haul then, it is quite apparent that the flexible configuration and speedy deployment options offered by mmWave systems can play a critical role. They can be plugged in anywhere to fill in fiber gaps or act as a fiber substitute to meet the 10- 20 Gbps capacity and single digit millisecond latency promised with 5G networks.
As noted above, fronthaul existed in 4G networks when remote radio heads began to be connected to centralized baseband units deployed at some distance away (as opposed to BBUs deployed at the base of a tower and connected to the radio head on top). 5G front haul will differ greatly due to a much greater amount of network densification (e.g., many more small cell deployments) and the higher bandwidth requirements of the 5G new radios. More sites and more radios mean a lot more fronthaul capacity than used in 4G networks– 10 to 25 Gbps or even more per site, as opposed to 2 or 3 Gbps in 4G.
This type of demand is driving interest in optical technologies such as DWDM and the packet network aggregation technique known as Time-Sensitive Networking. A recent survey of MNOs by Heavy Reading, showed that these carriers planned to base 82% of their fronthaul networks on these three types of networks and given the bandwidth requirements, that comes as no surprise. That still leaves 18% of the market to be supported by wireless solutions and, according to Omdia, the fronthaul transmission market will almost triple by 2024, reaching over $2B in revenues. By that year, wireless fronthaul transmission will account for more than 20% of this market, compared to a mere 6% in 2019.
This is an attractive proposition for wireless equipment providers and here again, mmWave technology shines in comparison to microwave options, primarily because of the large amounts of channel capacity to support up to 10-20 Gbps links . Microwave can deliver up to 2Gbps using advanced modulations, X-Pic and other “tricks.” But these don’t come close to meeting the needs defined here. For that nearly 20% of the “Base Station connectivity” not served by some sort of fiber – mmWave solutions are the answer.