HFR's Mobile Fronthaul Solutions for LTE-A Era
Version July 2014

Table of Contents
1. Introduction
2. HFR's WDM Solutions for mobile fronthaul and backhaul
2.1 Passive WDM Solution
2.2 Active WDM Solution
3. Implementing Mobile Fronthaul/Backhaul with HFR's WDM solutions
3.1 Full Fronthaul Architecture
3.2 Hybrid BH/FH Architecture
3.3 Integrated BH/FH Architecture
4. Case Study: SK Telecom
About HFR

1. Introduction
The past few years have seen smartphones rapidly gain popularity and become one of the most loved daily essentials, especially with all of their ever-advancing multimedia processing features. Due to these advanced technologies behind mobile devices, the size of contents (video, music, picture, etc.) that users can enjoy on the devices are growing bigger and bigger every day (e.g. for videos, resolution SD (480p) → HD (720p) → now Full HD (1080p), and encoding rates, 500Kbps → 1Mbps → 2Mbps → now 4~8Mbps).

Because of this growth, data traffic in mobile operators' network is soaring, and will do even more so from now on. To handle soaring data traffic, operators have been making macro cells smaller, and this has apparently left the operators with more cells to build and operate. To save costs in building and operating cell sites, a new architecture called C-RAN was introduced. It is also known as “Centralized RAN” or “Cloud RAN”. With this C-RAN, operators can simply leave all RRHs in their cell sites, but move only BBUs to a centralized location at central offices or master cell sites.

C-RAN has drastically lowered the cell site cost (Capex/Opex), and has maximized the effects of CoMP and eICIC of LTE-A. This helps to improve not only the service quality, but also the LTE-A network performance. So, many operators have been actively employing C-RAN in their networks.

Figure 1. LTE RAN Trends: Migration to C-RAN and Fronthaul

Now that RRHs and BBUs are remotely separated in C-RAN, a new network was required in order to deliver a huge volume of baseband I/Q streams between the two across CPRI or OBSAI link. Previously, both RRHs (Remote Radio Heads) and BBUs (Base Band Units) were located in eNBs, and the transport network between them eNBs and EPC was called backhaul. Now in C-RAN, these new CPRI and OBSAI networks are called fronthaul.

The fronthaul network should be able to satisfy requirements under LTE layer protocol operation and under the CPRI specification. First of all, ultra-high transmission capacity (as high as 2.5 GMbps~10 Gbps) for delivering baseband I/Q data is required, and latency caused within equipment in the fronthaul network should be minimized to a few secs to maximize the distance between BBUs and RRHs.

In C-RAN, RRH traces clock and removes jitter from I/Q streams received from BBU to generate the clock (CPRI/sampling/carrier frequency. etc) to be used in RRH system. So, the RRH system performance varies depending on the quality of the recovered clock. That’s why jitter has to be minimized in the fronthaul network, and the CPRI specification defines the maximum frequency accuracy budget as 0.002 ppm. Also, to guarantee the time/phase synchronization required in LTE-A (eICIC, CoMP), the CPRI time/phase synchronization should be strictly ensured in the fronthaul as well. So, we can say ensuring low latency and synchronization between BBU and RRH are the most important and demanding jobs for the fronthaul.

There have been several ways to satisfy such demanding technical requirements. The best option would be using dark fiber. But the problem with this option is that it would only work for those who already have plenty of fiber, and others including most operators would have to lease it. And obviously this can cost a lot. For example, a network with LTE Carrier BW of 20MHz, 2x2 antenna, 3-sector, 2 Bands would require 6 RRHs in each cell site, which means 6 leased fibers in each cell site.

Figure 2. Passive WDM vs. Active WDM

The practical option is WDM. With WDM, just one or two fibers can cover tens of CPRI channels. So, fiber costs can be lowered, and high-volume transmission is possible. There are two types of WDM, passive and active (Figure 2). The best part of passive WDM is that it is inexpensive, and requires no power supply. Besides, little latency or jitter is caused, and so the distance between BBU and RRH can be maximized, without affecting LTE/LTE-A performance much. Active WDM is bidirectional (single fiber). So, dark fiber costs can be lowered. And by using Muxponder, the number of required λs can be minimized, which can further lower the fiber costs. What’s even better, operators can even monitor the quality of the fronthaul network by running a self loopback test on WDM units. But, one thing to note is that active WDM may cause latency and jitter, which should be kept under certain levels.

2. HFR's WDM Solutions for mobile fronthaul and backhaul
HFR provides both passive WDM and active WDM solutions. Passive WDM enables operators to build a high capacity of C-RAN fronthaul with less cost. HFR also provides active WDM solutions called flexiHaulTM. What Our flexiHaul solutions do is to fronthaul CPRI traffic and backhaul Ethernet traffic to a single aggregation network.

2.1 Passive WDM Solution
Passive WDM does not contain any active components like transponder, but instead is consisted of passive components such as Add/drop filter, splitter. So, it is inexpensive and requires no power supply. Due to the lack of active components, Passive WDM seldom causes any processing latency (excluding cable propagation delay) and jitter. Thus, it can maximize a cable distance between BBU and RRH, not affecting on the performance of LTE/LTE-A network. Consequently, operators can remove no need to perform interoperability tests with base station vendors.
Passive WDM multiplexes optical input signals over a single fiber through WDM MUX, not converting a wavelength of an optic signal. An optic transceiver (i.e., SFP/Small Form Factor Pluggable) to be plugged into the customer’s equipment like LTE BBU/RRH should be tuned to the unique optical wavelengths (unit: nm) for TX and RX port respectively, referring to the pre-assigned wavelength/channel table. HFR’s passive WDM solution fully supports various features and options such as CWDM or DWDM, single fiber or fiber pair, protected or unprotected. In C-RAN, a physical failure like “Fiber cut” is likely to occur because BBU and RRH are placed tens of Kms away from each other. Inherently, passive WDM is consisted of passive components, so it cannot perform any switching function in itself. In order to complement the limitation, HFR provides a small optical switch as an optional part. The optical switch is attached to the forehead of Passive WDM at RT(Passive WDM at remote sites) in external or built-in type, providing two input ports (West, East) connected to COT respectively. The optical switch determines a route from RT to COT among the east and west direction. In addition, passive WDM also utilizes OTDR which can monitor a signal quality of optic cable by allocating a additional wavelength (1,625nm) for OTDR use.

2.2 Active WDM Solution
Basically, active WDM is also based on the passive components used in passive WDM, but unlike passive WDM it has the active components like transponder, muxponder, etc. added to the passive components. So, active WDM is relatively expensive compared to assive WDM and requires a power supply for operation. Active WDM can convert a wavelength of optic signal by O-E-O conversion and then multiplex optic signals over a single fiber through WDM MUX. Unlike passive WDM, active WDM removes the need for the colored optic transceiver (i.e., SFP) tuned according to the pre-assigned wavelength/channel table for WDM transmission, utilizing a common colorless optical transceiver for customer’s equipment (i.e., LTE BBU/RRH).

HFR’s active WDM solution is flexiHaulTM. What Our flexiHaul series do is to aggregate (fronthauling) macro/micro/small RRH (CPRI) traffic, and aggregate (backhauling) legacy base station, compact base station (pico), and Wi-Fi traffic with this single aggregation network.

Figure 4. HFR's Active WDM solution (flexiHaulTM)

Our flexiHaul solution consists of the HSN series (HSN 8500/8300/8100/8110). HSN 8500 models are installed in BBU pool sites, and support 40 λs and 72 CPRI ports. These models support the three CPRI service cards, i) transponder card that supports three option 3/4/5/6 CPRI ports, ii) Muxponder card that supports four option 3/4 CPRI ports, and iii) Muxponder card that supports two option 3/4/5/6 CPRI ports. And all three CPRI cards have been deployed in SK Telecom’s commercial network.

Muxponder cards use one λ per card. So, fewer λs are required. And that allows HSN 8500 to aggregate RRHs at the maximum level. HSN8300/8100/8110 models are RTs installed at cell sites. You can find their specifications in figure 5.

The flexiHaulTM solution is a fronthaul solution using WDM, so has no capacity issue. One HSN 8500 RT can deliver CPRI traffic up to 180 Gbps. It has many excellent technical features we have developed to minimize latency and jitter which can affect LTE/LTE-A. So, for example, in a ring with COT and RTs, a fronthaul end-to-end latency excluding fiber latency can be kept under 1μsec, and jitter can be kept under a few nsecs. More than 3,500 flexiHaulTM units are currently running in many commercial LTE/LTE-A networks.
HFR’s flexiHaulTM solution offers extremely low latency and jitter. So, it can maximize the performance of LTE-Advanced features such as CoMP and eICIC, eventually improving the LTE-A service quality and network performance. These days operators are in fierce competition to attract customers. With HFR's solution, operators can prevent customer churn and attract new subscribers by providing better service quality than other competitors.

HFR's ring protection within 50 msecs feature ensures any link failure is recovered instantly to minimize LTE service interruption. Not only that, operators can monitor the quality of the fronthaul link through BER and CV (Code Violation) of CPRI data that is being monitored in real time.

HSN 8500 HSN 8300 HSN 8100 HSN 8110
Dimension (WxDxH [mm]) 483x435x220 483x435x134 483x435x44.6 Outdoor solution
WDM CWDM/DWDM CWDM/DWDM CWDM/DWDM CWDM
No. of services cards 20 8 2 Outdoor solution
Max CPRI Ports 72 30 7 3
CPRI Option 3/4/5/6 Option 3/4/5/6 Option 3/4/5/6 Option 3/4/5/6
OBSAI 3.072/6.144Gbps 3.072/6.144Gbps 3.072/6.144Gbps 3.072/6.144Gbps
Ethernet FE/GE FE/GE FE/GE -
G-PON OLT 4 ports 4 ports 4 ports -


Figure 5. Components of HFR's Active WDM solution (flexiHaulTM)

3. Implementing Mobile Fronthaul/Backhaul with HFR's WDM solutions
Every operator has their own RAN architectures/scenarios they want, depending on their needs and resources (available infrastructure, future roadmap, etc.). That’s what our flexiHaul solution is for. Because it supports many different RAN and fronthaul architectures. Table 1 depicts the fronthaul architectures presented by HFR. BBU pool is located at CO for the Full Fronthaul architecture, while it is distributed onto the master macro cell sites for Hybrid BH/FH architecture. In Integrated BH/FH architecture, HFR's WDM network aggregate both CPRI traffic form RRH and Ethernet traffic from 3G nodeB, small BS, or Wi-Fi AP.

Operators should determine a proper WDM technologies and network architecture, considering available dark fibers, a holding and planning frequency for LTE, cell sites and COs, network evolution strategy, TCO and so on.

Table 1. HFR's WDM solutions for various fronthaul architectures
Fronthaul Architecture Small Cell Macro Cell HFR solutions
Full Fronthaul Architecture Small RRH Macro RRH Active, Passive, Mixed
Hybrid BH/FH Architecture Small RRH Macro RRH +BBU Active, Passive
Integrated BH/FH Architecture
(D-RAN over C-RAN) Compact BS,
Wi-Fi Macro RRH Active

3.1 Full Fronthaul Architecture
In Full Fronthaul architecture, the BBUs are centralized at CO, RRHs (macro RRHs and small RRHs) at cell sites are connected to the BBU pool over CPRI interfaces. In this architecture, hundreds of RRHs are processed by a BBU pool, so the pooling effect is maximized. It is an optimal architecture to process LTE-A features such as CoMP and eICIC.
HFR's active and passive WDM solutions enables the operators to implement various full fronthaul networks - all passive WDM fronthaul, all active WDM fronthaul or mixed configuration with active and passive WDM [Figure 6].

Figure 6. Full Fronthaul Architecture mixed with active and passive WDM solutions

3.2 Hybrid BH/FH Architecture
In Hybrid BH/FH architecture, the BBUs are centralized at master macro sites, RRHs (macro RRHs and small RRHs) at cell sites are connected to the BBU pool over CPRI interfaces. In this architecture, tens of RRHs are processed by a BBU pool. This concept describes the fronthaul network that is built from macro RRHs and small RRHs extending the macro D-RAN. The existing backhaul network to macro site is still utilized, and new fronthaul network is built based-on the D-RAN macro site.
The fronthaul network can be deployed with either active WDM or passive WDM solutions. Figure 7 shows an example of fronthaul network built from HFR's active WDM solutions (HSN 8300/8100).

Figure 7. Hybrid BH/FH architecture (Active WDM case)

3.3 Integrated BH/FH Architecture
When a legacy operator builds an LTE network, there are already legacy 3G base stations in its cell sites. Our flexiHaul RT units (HSN 8300/8100) accommodate 3G BSs through the GE interface, and connect LTE RRHs through the CPRI interface. That way, they can accommodate the two access networks in a single network.

GE and GPON cards connect small cells (pico) or Wi-Fi APs. If no fiber is available in a small cell area, operators can connect small cells by accessing microwave devices through the GE interface of HSN series. Our flexiHaul series aggregate (fronthauling) macro/micro/small RRH (CPRI) traffic, and aggregate (backhauling) legacy base station, compact base station (pico), and Wi-Fi traffic in Iintegrated BH/FH architecture.

Figure 8. Integrated BH/FH architecture implemented with HFR's flexiHaul solution

4. Case Study - SK Telecom's fronthaul architecture & HFR's WDM solutions deployed
C-RAN was initially proposed by China’s CM. But, it was Korean operators (e.g., SK Telecom) who actually commercialized it. And a fronthaul network, which made C-RAN work, was also commercialized by Korean operators for the first time in the world. HFR have deployed the flexiHaul solution in SK Telecom’s network since 2012, helping SK Telecom to build its nation-wide fronthaul network, in 84 major cities. 80% of the fronthaul networks were built with active WDM, and 50% of the units deployed were our flexiHaul.

We have been stabilizing and optimizing systems in real commercial networks, and have accumulated technical know-how for many years. And those experiences and know-how are our biggest assets that can make us ready to work any time. Our solutions are not in the proof of concept (POC) step, but are fully proven, ready to use. That’s what really put us ahead of everyone else. We are the ONLY one who can achieve the best time-to-market with the least trial and error in building a fronthaul network.

Figure 9. The fronthaul architecture of SK Telecom and HFR's WDM solutions deployed in SK Telecom

About HFR (www.hfrnet.com)
HFR has been actively responding to the Cloud RAN market under LTE environment. We expect that our front-haul solution will become representative product in global equipment market. Also, HFR has been leading the high-speed internet equipment with the development for Giga Internet service area. Based on its competitive solutions in the wire and wireless communications fields, HFR is determined to become Korea’s leading network equipment company.
 Location and Contact Information
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TEL 82-31-712-7768 | FAX 82-31-712-7948 | E-MAIL resonant@hfrnet.com
 For more information, please visit us at http://www.hfrnet.com