Connected World. Connected Experiences. Fronthaul Technologies in vran

Transcription

1 Connected World. Connected Experiences. Fronthaul Technologies in vran

2 Introduction The mobile communication industry is currently heading towards the fifth generation (5G) network. The new network strategy is to meet 5G objectives & challenges such as 1000x rise in traffic volumes, 1000x more connected devices, 100x improvement in speeds, near-zero latency demand and supported by battery life up to 10 years. The network segment that is closest to the subscriber is the radio access network. While end to end network architecture is relooked at, it is these network segment that needs to be transformed on priority. Virtualized Radio Access Network (vran), one of the promising evolution path for the next-generation mobile network, has attracted many service providers as it promises massive scalability and reduced cost of operations. The vran architecture separates the Base Band Unit (BBU) from the Remote Radio Head (RRH). vran is an attempt at decoupling the hardware and software platforms of wireless base stations. Instead of proprietary hardware, operators would deploy commercial servers in data centres to run base station functions and thereby leverage the cost structure of data centres in running wireless networks. The link which separates RRH and BBU plays a vital role in this architecture which is called as fronthaul. This paper discusses various fronthaul Technologies, their pros and cons and possible options that Telco s can adopt while embracing vran. RAN Evolution The main components that form a Base Station (BTS) site are Base Band Unit, Remote Radio Head and Antennae. In the current Distributed RAN (DRAN) architecture, the remote radio head (RRH), is separated from the baseband unit (BBU) and are connected to each other by fiber. The BBU communicates with the RRH using Common Public Radio Interface () protocol. An alternative standard to is Open Base Station Architecture Initiative (OBSAI). DRAN CENTRALIZED - RAN CLOUD RAN OR VIRTUALIZED RAN INTERNET INTERNET INTERNET GATEWAY GATEWAY GATEWAY EPC EPC EPC BBU Hotel BBU Hotel vbbu Pool Site 1 BBU Site n BBU Site 1 RRH 1,2,3 Site 2 RRH 3,4,5 Site n RRH 34,35,36 Site 1 RRH 1,2,3 Site 2 RRH 3,4,5 Site n RRH 34,35,36 On COTS servers on Cloud RRH 1 RRH 2 ETHERNET RRH n Networks started becoming denser and the need for network expansion continued. With it came the need of acquiring more sites and putting up more radio infrastructure on existing sites. This lead to significant rise in Capex and Opex. The DRAN architecture posed a major challenge here. To address this, the concept of Centralizing the BBU was evolved, wherein the BBUs were aggregated at a central location named BBU hotels giving rise to a Centralized RAN (C-RAN) architecture. One BBU could then be connected to more number of RRHs, making it more manageable to scale up centrally. The C-RAN architecture, however, needs a high speed, low latency connection between the centralized BBU to RRH. This can essentially be met via Fiber connectivity running or OBSAI protocol. While the results of C-RAN looked promising, it was not adopted on a large scale due to huge bandwidth requirement, scalability of BBUs at the BBU hotel and limitation on the number of RRHs that can be connected to one BBU, limiting peak performance. The C-RAN architecture was therefore further evolved to Virtualized RAN (vran). In this, the BBU functionality was achieved using a virtual network function (VNF) known as vbbu. The vbbu could now be deployed on COTS hardware. With this is was possible to connect more number of RRHs to one vbbu addressing the cost, scalability and performance issues faced by C-RAN. These new architectures needed one thing in common - a good reliable, high speed, low latency connectivity between BBU and RRH.

3 Definition of Fronthaul The links that interconnect the BBU Pool/BBU Hotel and the multiple RRHs is referred to as Fronthaul. Challenges of Fronthaul The challenges in considering the fronthaul are latency, Synchronization, Bandwidth and Performance. Latency Typical latency requirement is around 800uSec round trip (from RRH to BBU and back to RRH). In case of Centralized RAN, it is expected that around 75% of it will be utilized by BBU processor and rest (around 150 to 200uSec) will be left for the transmission path. Latency is one the main parameter while selecting the fronthaul media. Synchronization LTE-A has tight requirements of on both Frequency (16ppb/50ppb) and phase (+/-1.5 to 5usec ) synchronization. With a passive or dark fiber as a fronthaul this will not be an issue, however, any active fronthaul solution must be supported by good sync transfer. So a complete fronthaul solution must provide synchronization of frequency, phase between the baseband unit and one or more remote radio heads. Bandwidth and performance consideration As mentioned in the earlier sections, the expected increase in traffic volumes for 5G network will be 1,000x (downlink capacity >1Gpbs). This should be supported by the fronthaul along with good performance indicators (BER 10 (-12) and LTE Error Vector magnitude). So bandwidth scalability is a big challenge for the fronthaul

4 Fronthaul Options Fiber Fronthual Dark fiber is one of the fronthaul solution that has the best performance and is best suitable where there is sufficient fiber. The industry standard protocol which is designed for short distance can be used to communicate between the BBU and RRH. However, is bandwidth hungry. As an example, a 2X2 MIMO deployment over 20MHz bandwidth will need a fronthaul capacity of approximately 2Gbps. Add to that multiple such RRHs and the demand on the fronthaul will be enormous. Scalability becomes an issue. Techniques like Compressed reduces the capacity requirements to some extent, but not enough to meet the 5G network objectives. The fiber fronthaul solution is viable only in markets that are fiber rich. In other markets, the cost of deploying new fiber will be prohibitively high, limiting this option. Other fiber based options that help in reducing the cost are: 1. OTN (Optical Transport Network) 2. PON (Passive Optical Network) 3. over Ethernet Both OTN and PON use statistical multiplexing technique that help in aggregating the signals coming from multiple RRHs into one signal towards vbbu. over Ethernet uses existing Ethernet infrastructure available at the site and helps reduce the cost of laying fiber to some extent. However, Ethernet overheads on top of payload increases the bandwidth requirements further and has higher latency than dark fiber. The fiber fronthaul solution is viable only in markets that are fiber rich. In other markets, the cost of deploying new fiber will be prohibitively high, limiting this option. Wireless Recent introduction of millimeter wave (E band and V band) that supports larger bandwidth and higher throughputs has made wireless a viable option for Fronthaul. However, it can be used only in case of short distances (<200m) and provided there is a clear Line-of-sight between two points of connectivity. Ethernet & Functional split Networks are now migrating towards flat IP networks. Most of the transmission equipment at the radio sites have Ethernet interface. Considering this, vran vendors have come up with options that can use Ethernet for fronthaul. However, this alone is not enough. Techniques to reduce the bandwidth requirement on the fronthaul need to be implemented. Recent developments have made it possible to split the L1 and L2 functions between RRH and vbbu. Depending on the level of split that can be achieved it is possible to drastically reduce the bandwidth on fronthaul compared to. The figure below depicts the various available options for functionality split and its impact on bandwidth and latency. With appropriate Split function technique employed, the additional bandwidth that will be needed on the fronthaul will be a marginal 10-20% more than the current running capacity. Ethernet along with split functionality is the best possible alternative to fiber in terms of availability, cost and time to implement.

5 The following table highlights the advantages and challenges for each of the fronthaul options. Wireless Contributed by: Purushotham Mangalapelly Principal Consultant Transmission Hemant G Patil Competency Head, CRAN & 5G hpatil@techmahindra.com Conclusion Mobile Network Operators (MNOs) are exploring new architectures in the Radio Access Network as a means of increasing capacity while reducing costs for next generation networks. The vran architecture is a promising step in this regard. It is expected that vran gains will improve over time and will become quite viable in high-traffic metropolitan areas. Fronthaul technology is going to play a vital role in realizing the vran architecture. Multiple Fronthaul options are available. Each option has its own methods and challenges in meeting the latency, bandwidth and synchronization demands. The adoption of the right Fronthaul option will depend on MNO s existing access network infrastructure and the willingness to invest in the cost of transformation with a view to meet future network demands. It is highly likely that operators will experiment in smaller clusters or markets and continue to use existing backhaul as Fronthaul wherever feasible and deploy new fronthaul in a phased manner. Much flexibility will be required in the design of such networks to ensure seamless and cost-optimized migrations. Availability of millimeter wave spectrum will provide a viable option for short hop Fronthaul. Functional Split implementation on Ethernet technology will have a significant role to play as operators embrace all IP network and scale hybrid backhaul/fronthaul and all-fronthaul solutions.

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