A practical guide for street-level connectivity
Service providers around the world are working to build and commercialize 5G networks where densification is a key aspect. By adding street-level sites operating in high-band (millimeter wave—mmW) or mid-band spectrum or by using Massive Multiple Input, Multiple Output (M-MIMO) radios, providers can provide gigabit speeds over the air. These sites, whether low power distributed radios, higher power urban macros or points in between are a growing segment. Crown Castle CEO Jay Brown stated during an investor call that they expect to deploy approximately 10,000 small-cell nodes in 2020 (RCR Wireless Mar 2, 2020). All face the key challenges of connectivity, power and space.
Densely populated urban areas with tall buildings need two radio coverage layers, one at street level and one at rooftop level. Street-level sites may be placed 15–30ft above ground and generally have short range (a few 100ft), especially those with mmW 5G radios. These sites can be placed on lampposts, utility poles, or even sides of buildings. For these deployments, space, power and connectivity are the same requirements as macro cells, but with significantly higher numbers and in more challenging deployment locations with varying market zoning and permitting requirements. This blog will focus on the connectivity options available.
Got fiber… But is it enough?
Traditional fiber connectivity for CPRI or Ethernet …
Because of space limitations, many street-level sites will be remotely connected to basebands in a Centralized RAN (C-RAN) configuration using Common Public Radio Interface (CPRI). In a C-RAN, the distance between remote radio and baseband will be restricted to CPRI fronthaul latency constraints of 75µs one-way. With fiber latency at 5µs/km, the distance from radio to baseband hub is limited to 15km (15km x 5µs/km = 75µs). As a result, these deployments will need to consider this 15km distance (latency) constraint when planning connectivity.
For street-level sites housing both radio and baseband, backhaul connectivity must be accommodated. This connectivity will have a less stringent latency requirement than fronthaul for C-RAN, but capacity needs could be significant with each radio + baseband requiring up to 10GB of backhaul.
Assuming sufficient fiber strands are available at the desired location, the provider would simply connect each radio with dark fiber for either fronthaul or backhaul. The quantity of dark fibers depends on the radio type, number of sectors, and bands deployed. Take the simple case of a three-sector deployment with a single band radio (non-MIMO or mmWave). Each radio has one optical transceiver requiring two fiber strands—one Transmit (TX), one Receive (Rx)—totaling six fiber strands for three-sector deployment. This quantity of fiber strands per location may be manageable until the provider decides to overlay NR with existing LTE or utilize millimeter wave (mmW) or multi-user multiple input, multiple output (M-MIMO) radios. Some mmWave and M-MIMO radio, require two to four optical transceivers per radio—yes, per radio! Doing the math, worse case fiber needs for a three-sector deployment is 24 fibers—3 radios x 4 transceivers/radio x 2 fibers/transceiver. Suddenly the provider needs 24 fibers to that street pole or side of the building or potentially more if overlaying with existing low-band radios. Does that quantity of fiber resource exist at each location or can it persist for each market requiring densification?
Easy button for 50 percent fiber reduction …
Enter deployment option—grey bi-directional transceivers (aka BiDi SFPs). These transceivers place two separate wavelengths on a single fiber, where one wavelength is transmit (Tx) and the second is receive (Rx). A typical BiDi SFP uses 1270nm and 1330nm for Tx and Rx wavelengths. The transceivers are a matched pair—meaning site A has Rx @ 1270nm and Tx @ 1330nm while site B has Rx @ 1330nm and Tx @ 1270nm. These same BiDi SFPs can be used for either fronthaul or backhaul. The maximum fiber distance of 15km should still be a consideration since most connections will be fronthaul CPRI based. By leveraging BiDi SFPs, the provider will cut their fiber consumption by 50 percent—based on two wavelengths sharing a single fiber. This is a very viable and economical solution approach for providers.
However, this could be a short-term solution based on the provider’s expansion plans for mmW or M-MIMO radios. Taking the above mmW radio example with 4 x optical transceiver per radio and three-sectors on a single pole, the providers would still need 12 fiber strands—3 radios x 4 transceivers/radio x 1 fiber/transceiver. While 50 percent fiber reduction sounds high, it may not sufficiently reduce the quantity of fiber strands needed to that specific location—pole, side of building, and so on—to support high-band mmW and M-MIMO radio expansions.
50 percent reduction is not enough… What’s next?
Providers may have fiber connectivity options available for street-level deployments, but typically not the fiber strand quantity demanded by the new high-capacity radios or supporting multiband overlays. Taking the above three-sector deployment example of mmW radios each requiring four optical transceivers, and two fiber strands per transceiver, twenty-four (24) fiber strands are needed to this single location. Leveraging dense wave division multiplexing (DWDM) technology, the quantity of fibers can be reduced by 24:1 and up to 28:1 when combining dense wave with course wave division multiplexing (DWDM + CWDM) solutions. These xWDM solutions should be optimized to support the latency constraints of 75µs for fronthaul CPRI transport while addressing the quantity of service connections needed for the radios. The xWDM solutions also support backhaul transport for radio + baseband sites. Note: The WDM technology requires placement of DWDM transceivers (aka DWDM SFPs) in the radio and baseband equipment along with optical filters at each end of the fiber connection (remote and hub) to combine or separate the numerous wavelengths or “colors” on a single fiber strand. See the below illustration that includes both CPRI and Ethernet transport over DWDM.
While WDM technology addresses fiber constraints, it creates a new challenge of placement of optical filters at both remote radio and baseband hub locations. Hub locations may be a central office environment or a shelter with telco racks. Placement of filters in this space must be optimized to conserve space and support typical 19in. rack-mount options.
Placement of filters at remote locations to support the radio tends to be more challenging. Most deployments will be outdoors and subject to municipal zoning and permitting requirements. A consistent “look and feel” of radio units and optical filter enclosures is key for achieving those municipal approvals and permits in a timely manner. Ideally, the building practices used for the optical filter enclosures should be consistent with the radio solutions. This approach optimizes operational impacts by leveraging the same installation training, the same installation method of procedures (MOPs), materials (brackets, nuts, bolts), tools, sparing, and the like, while minimizing overall installation time. Furthermore, a variety of filter enclosure options to address differing installation locations and environment requirements helps providers meet their aggressive deployment plans.
Examples of filter enclosure options include radio “backpack” enclosures—enclosures that match the radio unit for placement on a pole or within a shroud, and enclosures that are IP-68 compliant for placement inside poles or below grade in fiber handholds. As a result, service providers have a deployment toolbox of optical filters and filter enclosures to support a variety of transport needs and deployment environments while addressing different market zoning and permitting requirements.
No fiber… Now what?
While fiber is the choice for most mobile providers, it is not always available at the price point required or in the time frame needed. What is our connectivity option if no fiber? Welcome back into the transport toolbox our often-overlooked connectivity friend, microwave radio technology. Microwave is used extensively in all regions of the world to address both fronthaul and backhaul connectivity needs. In North America where fiber is more readily available, it is not used as widely but is becoming more attractive to providers as a viable and capable technology to augment fiber as they densify networks.
Fronthaul microwave solutions can support CPRI 3-7 transport with radio link speeds up to 10GB using E-band radios (70/80GHz). Radio links can be 2–3km per hop with latency less than 20µs. Dependent upon the radio type, the CPRI interface can be cascaded to other sector radios as well. As a result, the microwave fronthaul option provides providers a flexible, fast time-to-market alternative to constructing more fiber or extending fiber to a desired site.
For backhaul, numerous microwave technologies are available. Options include V-band (60GHz unlicensed) or E-band (70/80GHz lightly licensed). Selection of which technology to use comes down to capacity, size, and hop length. The 60GHz unlicensed radio can provide capacity of 1GB for 1–2km while 70/80GHz E-band can deliver 10-20GB for 2-3km. The microwave radios have options of integrated Ethernet switch ports to support multi-site and multi-band aggregation. By leveraging the integrated Ethernet switch ports, daisy-chaining of multiple sites can be supported. These connectivity options provide service providers flexibility for network deployments in an all-outdoor, zero-footprint node that helps them meet time-to-market (TTM) needs. For more information on microwave transport technology and connectivity options, download the Ericsson Microwave Outlook report.
Finally, another wireless backhaul option introduced in 3GPP Release 16 is Integrated Access and Backhaul (IAB). With IAB, the mobile spectrum is also used for backhaul, which is especially relevant for high-frequency bands where the bandwidth may be hundreds of megahertz. The backhaul and access may be on the same or different frequency bands, known as in-band and out-of-band. In-band IAB is of higher interest since it could provide backhaul without any additional equipment. However, it is also more challenging and requires tight interworking between access and backhaul to avoid interference, both within the radio nodes as well as across the radio network. As IAB comes to market, it will require careful planning to meet the targeted user experience at busy hours. Topologies with a limited number of aggregated relay nodes and few hops are expected to be most common.
Summarizing the Ericsson Transport difference
Transport designed by Radio for Radio. Transport products are part of Ericsson Radio System, and all releases are aligned. Transport products use the same form factors for easier permitting and network rollout. They have the same management system, and support the same operations, administration and management (OA&M) features, like auto-integration. This results in a lower cost for integration and a faster time-to-market for our customers.
Ericsson is committed to helping carriers take advantage of 5G with a transport network product portfolio developed in conjunction with our Radio portfolio, to support all 5G deployment scenarios. Ericsson’s strength is in building and supporting comprehensive solutions no matter the transport method, fiber or microwave. With Ericsson as a partner for 5G RAN and Transport, you can establish technology leadership in new markets, capture early adopters, and deliver enhanced mobile broadband, fixed wireless access, and private network solutions that make use of the new 5G spectrum.
To learn more about this topic and choices that minimize TCO, go to Ericsson Technology Review article.
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