5G Self Backhaul- Integrated Access and Backhaul

5G, Future Network Optimization, Interviews, LTE, New Radio, NR, Tech Fundas
Self-Backhauling Concept

Self-backhauling defined as when the access i.e. Base station – Mobile and the backhaul (Base Station to Base Station or Base Station to Core Network) share the same wireless channel. Below picture depict the same. Here the center base station (gNB #1) does have fiber backhaul where as the base station on left (gNB #2) and right (gNB #3) does use backhaul from center base stations (gNB #1). The gNB #1 uses the same spectrum or wireless channel to serve the mobiles in its coverage as well as to provide backhaul connectivity two other base station i.e.  gNB #2 and gNB #3.

Figure #1

The sharing of this wireless channel resource can be done in Time, Frequency and Space , same is depicted in following picture

Why Self-Backhauling

The increased density of access nodes needed to meet future performance objectives poses considerable challenges in
deployment and management (e.g., backhaul availability, backhaul capacity and scalability). The use of wireless
backhaul for such access nodes helps to address some of the challenges.

Wireless self-backhauling in the radio access network can enable simpler deployment and incremental rollout by
reducing reliance on the availability of wired backhaul at each access node location. Network planning and installation efforts can be reduced by leveraging plug and play type features — self-configuration, self-organizing, and self optimization.

Integrated Access and Backhaul in 5G New Radio

Due to the expected larger bandwidth available for NR compared to LTE (e.g. mmWave spectrum) along with the native deployment of massive MIMO or multi-beam systems in NR creates an opportunity to develop and deploy integrated access and backhaul links. This may allow easier deployment of a dense network of self-backhauled NR cells in a more integrated manner by building upon many of the control and data channels/procedures defined for providing access to UEs. An example illustration of a network with such integrated access and backhaul links is shown in Figure #1, where relay nodes (rTRPs) can multiplex access and backhaul links in time, frequency, or space (e.g. beam-based operation).

The operation of the different links may be on the same or different frequencies (also termed ‘in-band’ and ‘out-band’ relays). While efficient support of out-band relays is important for some NR deployment scenarios, it is critically important to understand the requirements of in-band operation which imply tighter interworking with the access links operating on the same frequency to accommodate duplex constraints and avoid/mitigate interference.

In addition, operating NR systems in mmWave spectrum presents some unique challenges including experiencing severe short-term blocking that cannot be readily mitigated by present RRC-based handover mechanisms due to the larger time-scales required for completion of the procedures compared to short-term blocking. Overcoming short-term blocking in mmWave systems may require fast L2-based switching between rTRPs, much like dynamic point selection, or modified L3-based solutions.

The above described need to mitigate short-term blocking for NR operation in mmWave spectrum along with the desire for easier deployment of self-backhauled NR cells creates a need for the development of an integrated framework that allows fast switching of access and backhaul links. Over-the-air (OTA) coordination between rTRPs can also be considered to mitigate interference and support end-to-end route selection and optimization.

The benefits of integrated access and backhaul (IAB) are crucial during network rollout and the initial network growth phase. To leverage these benefits, IAB needs to be available when NR rollout occurs. Consequently, postponing IAB-related work to a later stage may have adverse impact on the timely deployment of NR access.

5G NR Self Backhaul Requirements
  1. The 5G network shall enable operators to support wireless self-backhaul using NR and E-UTRA.
  2. The 5G network shall support flexible and efficient wireless self-backhaul for both indoor and outdoor scenarios.
  3. The 5G network shall support flexible partitioning of radio resources between access and backhaul functions.
  4. The 5G network shall support autonomous configuration of access and wireless self-backhaul functions.
    The 5G network shall support multi-hop wireless self-backhauling to enable flexible extension of range and coverage area.
  5. The 5G network shall support autonomous adaptation on wireless self-backhaul network topologies to minimize service disruptions.
  6. The 5G network shall support topologically redundant connectivity on the wireless self-backhaul to enhance reliability and capacity and reduce end-to-end latency.
Drivers for Self-Backhauling in 5G
  1. Small cells densification calling for cost-effective and low latency backhauling
    • Wireless backhaul as cost-effective alternative to fiber
    • Low latency backhaul for tighter inter-cell coordination towards interference management and reduced number of handovers
  2. 1000x traffic increase calling for Gbps backhaul capacity, which requires new spectrum (e.g. mmWave) that is also required for access
  3. Use cases are driving backhaul technology to be similar to access
    • Mobile backhaul for vehicles, trains etc.
    • NLOS street-level small cell backhaul
    • Access links extending to higher frequencies
  4. Goal to maximize efficiency through joint optimization/integration of access and backhaul resources
  5. Overall simplification by using same technology for access and backhaul
Advantage and Disadvantage of Self Backhauling

The Advantages are:

  1. Higher spectrum efficiency
    • Reuse of time, frequency and space resources between access and backhaul
  2. Higher cost efficiency
    • Sharing the same radio hardware unit
    • Sharing same O&M systems, simplifying system management
  3. Higher Performance
    • Lower latency (Receive and Forward simultaneously)
    • Dynamic optimization of resource across access and backhaul

The disadvantages are:

  1. A new type of interference (access backhaul interference) to mitigate
  2. Complex scheduling of the channel resources (across two domains, access and backhaul)
  3. Potential limitations on the end user experience (e.g. rate, latency) due to the sharing of resources between access and backhaul
  4. Regulatory framework for spectrum rules may not be in place

References:

  1. RP- 17148 Study on Integrated Access and Backhaul for NR
  2. Inter digital 5g-crosshaul presentation on Self-backhauling in 5G