LED Backhaul Project Engineer Blog

Treatise Introduction "All-Indoor Optical Customer Premises Equipment for Fixed Wireless Access(FWA)"

Last Update: Jul 27th, 2021

Introduction

The provider of optical wireless communication technology to Sangikyo is a German public research institute called Fraunhofer HHI. The HHI is one of the institutes that researches communication technology, and there are more than 75 other institutes in Fraunhofer, including the IBMT for biomedical research and the EMB for marine biotechnology. Incidentally, HHI stands for Heinrich Hertz Institute, named after Dr. Hertz, who is well known for unit of frequency. In this issue, I would like to introduce a treatise published by a group researching optical wireless communication within the HHI. The title of the treatise is All-Indoor Optical Customer Premises Equipment for Fixed Wireless Access. In a nutshell, the treatise says, "Transmission through insulated glass is less lossy than wireless. One of the advantages of optical wireless communication is that it is possible to communicate from room to room through glass. The following is an excerpt from this treatise.

Introduction to the treatise

The introduction of FTTH (Fiber To The Home) will enable higher data rates and robustness, but the installation, especially the digging of the fiber to connect individual homes in the last few hundred meters, is costly and time consuming. Digging fiber to connect individual homes, especially in the last few hundred meters, is a costly and time-consuming process that varies from customer to customer. Especially in urban areas, fiber installation can be difficult at the cost and time fronts. There are also areas where fiber installation is not possible due to legal frameworks such as rights-of-way, or construction obstacles such as railroad tracks or rivers. An alternative to this is Fixed Wireless Access (FWA). This is a wireless link that provides the last hop to the customer. Instead of running fiber to every home, the fiber is only run to the last marshmallow, such as a street light in a residential area, and the last hop to the building is done using the so-called wireless-to-the-home (WTTH) concept to keep costs down. With its low-cost wireless technology, WTTH enables economical broadband expansion to meet current and future network demands.

For FWA applications, directional radio links in the millimeter-wave range (e.g. ︓60GHz) can be used. To avoid complex coordination between FWA and mobile access applications, LED-based optical wireless communication (OWC) can be used to mitigate the millimeter-wave spectrum and LED-based OWC, also known as LiFi, has been discussed frequently in recent years. While mobile access offers features such as mobility and multi-user support, LiFi can also be used as an alternative to 60 GHz in WTTH applications. In urban areas, the probability of getting a free line of sight (LOS) is considerably higher. The protocol required for LiFi in a point-to-point (P2P) topology is simplified; the OWC front-end is equipped with an additional lens to provide a directional LOS link and reduce optical loss. LiFi has been proven in long range P2P outdoor communications over 200m, where OWC links have achieved over 99.9% uptime due to closed-loop rate adaptation, despite fluctuations in SNR (signal-to-noise ratio) caused by attenuation and scattering in the atmosphere.

In order to use it as a low-cost FWA solution, all the CPE (customer premises equipment) needed to be installed indoors. Since the wireless link is completely blocked by insulated glass, the system would have to be separated into indoor and outdoor locations, connected by cable and deployed by the homeowner, which is undesirable. Carriers are interested in all-indoor CPE. In this treatise we demonstrate an advanced OWC prototype for FWA applications and report initial measurements in the intended deployment scenario. Data sets at several transmission distances are reported and the effect of metal coated insulating glass, increasingly used in low carbon footprint homes, on transmission performance with a commercial 60 GHz radio link is compared.

Performance of the OWC link (Optical Wireless Communicator) to be used

Using a commercially available DSP chipset compliant with G.9991 (G.hn), 200 MHz bandwidth DC-OFDM (Direct-Current Biased Orthogonal Frequency Division Multiplexing) modulation, up to 2 Gbit/ s is achieved.
The light source is a near-infrared LED with a central wavelength of 820nm and a die size of 0.75x0.75mm.
Large-area Si-PIN photodiode is used as the light receiving device.
The focal length of the lens is 208mm, the size of the transmitting lens is 50 square cm, and the size of the receiving lens is 150 square cm.
- The spot size (the size of the light that can be communicated) is 1.4m in diameter at 100m, so it is resistant to vibration without active tracking.
In the FWA scenario, there is little loss in the atmosphere due to the short distance.

fig.1 OWC link device

Experimental results

Measurement in OWC initial condition

Measurement conditions

Alignment of two OWC terminals with each other at any LOS (line-of-sight).
At each distance, the alignment was adjusted to obtain the highest possible SNR to maximize the data rate.
For comparison, the conventional type (author's note: our LED backhaul) was measured in the same way.

Measurement results

Figure 2: Initial measurements

Recorded 1500Mbps at 25m, 1400Mbps at 50m, and 1100Mbps at 100m
The difference between this type and the previous type is thought to be that the bandwidth has been extended from 100MHz to 200Mz, and the LEDs have become larger and brighter.

Comparison measurement with 60GHz wireless link through insulated glass.

Measurement conditions

Double metal-coated insulating glass was inserted into the LOS of the link, and the attenuation with and without the glass was compared (Figure 2).
Siklu's 60 GHz commercial radio (EtherHaul(TM)) was used as the 60 GHz wireless link for comparison.
The SNR of OWC is measured for each carrier, while that of 60 GHz radio is the average value of CINR (Channel to Interference and Noise Ratio).

Figure 3: Equipment configuration

Measurement results

Figure 4: Measurement results through insulating glass

In the case of the OWC, there is an SNR decrease of about 13 dB with the insulating glass compared to the case without.
- This is thought to be the result of the insulating glass partially reflecting the light.
- Although the SNR decreased, communication at 600Mbps was still possible.
In the case of the 60GHz radio, the CINR dropped to 0dB with the insulating glass.
- The link was completely broken, and a CINR of 0dB means "unmeasurable".
- The reduction of about 27dB is an apparent reduction, and the actual reduction in CINR may be much greater.
- The insulating glass is coated with a thin metal layer, which greatly attenuates radio waves** and cuts off communication**.

Conclusion

This treatise presents an optical wireless link that achieves Gbps data rates over a typical distance for fixed wireless access (FWA). It is also designed to be vibration resistant, and the module can be installed on a streetlight, for example, without the need for tracking. In contrast to 60 GHz links that require outdoor antennas, this optical link enables all-indoor CPE (customer premises equipment) and can be deployed by customers without impacting homeowners.

Sangiyo's comment

Fraunhofer HHI is a research institute in Berlin, Germany. (Incidentally, FTTH penetration in Japan is over 50%, and most DSL services are expected to be terminated by 2024. Germany is one of the slowest (among developed countries) in terms of high-speed fixed lines. In addition, the fact that many buildings are made of old wood or masonry, and the hurdles to wiring premises are high, is one of the factors that hinder the spread of the service. Fixed wireless access through glass, as described in this treatise, is a proposal based on such circumstances in Germany, and needs are expected in the future. In Japan, FTTH is not expected to be used indoors, but communication through glass is definitely one of the expected applications of optical wireless communication. For example, it is expected to be used for communication between buildings across a road or river, or for communication from indoors to outdoors, rather than from outdoors to indoors (i.e., the Internet line brought indoors is also deployed outdoors).