Ecoc 2015 - ID: 0643
Optical Wireless Communication for Backhaul and Access (1)
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V. Jungnickel , D. Schulz , J. Hilt , C. Alexakis , M. Schlosser , L. Grobe , (1) (1) (2) (3) A. Paraskevopoulos , R. Freund , B. Siessegger , G. Kleinpeter (1)
Fraunhofer Heinrich-Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany OSRAM GmbH, Corporate Technology CT, Marcel-Breuer-Strasse 6, 80807 Munich, Germany (3) BMW Group, Werkstatt Zukunft, Hanauer Str. 46, 80788 Munich, Germany Email:
[email protected] (2)
Abstract We highlight new applications for optical wireless communication (OWC) as a mobile backhaul for WiFi, LTE and 5G and as a new access technology in the Internet of Things (IoT) where it enables secure and reliable communications at low latency. Introduction High-speed optical wireless communication 1 (OWC) has many potential applications . While radio is more often used, because it bridges longer distances more easily, OWC offers several terahertz of bandwidth in addition to the radio spectrum which is overcrowded nowadays due to the increasing demand for new datahungry mobile services. For this reason, the idea of offloading a fraction of the wireless traffic into the unregulated and nearly unlimited optical spectrum becomes more popular, leading to so-called converged network solutions using optical and wireless links in a 2 complementary manner . In this paper, we highlight two recent new applications of OWC addressing the demand for converged network solutions, i.e. an economic wireless backhaul solution for small mobile radio cells and an optical wireless access technology for the Internet of Things (IoT). Optical Wireless for Small-cell Backhauling In a mobile network, base coverage is provided in a first phase by means of a homogeneous deployment of macro-cells. In a second phase, a higher capacity is reached through densification. Small cells are added to the network at those locations where the traffic density is high. With modern interference management schemes, such as coordinated multipoint, the spectrum can be fully reused and each small cell offers new wireless capacity for additional users that can be served in parallel to the users already 3 covered by the macro-cell in the same area . However, the deployment of small cells needs economic backhaul solutions. Mobile backhaul can be provided by a variety of technologies, i.e. via fixed copper or fiber-optical links besides wireless technologies such as micro-, mm-wave and free-space OWC links. Wireless links play a role during the initial roll-out until the fixed backhaul becomes available and sometimes as a permanent solution if digging is too costly.
Outdoor OWC links do have a risk of irregular outage. Availability can be temporarily reduced by bad weather conditions, i.e. fog, rain, snow and sunlight. Over longer link distances, OWC is not so useful for mobile backhaul applications. However, the impact of bad weather on availability is closely related to the use of longer link distances and fixed data rates. While fog is a critical parameter for inter-site distances (ISD) of 500 m to 1 km typical between macro-cells, it affects shorter link distances for small cells less (50-200 m between macro- and small cells). For mobile network operators to reconsider OWC as a backhaul solution for small cells, several questions arise which are addressed in this paper. Can OWC provide sufficiently high data rates over realistic ISD as well as low latency? What availability is reached in realistic outdoor scenarios? Can costs be cut down so that massive deployment becomes possible? Lab and Initial Outdoor Trials For measurements, proprietary optical 1,4 were used together with two frontends realtime baseband chipsets for 500 Mbit/s and 1 Gbit/s gross data rate, respectively. The transmitter has a single low-cost infrared LED with 1 mm² emitting area, an optical concentrator and a 3” lens in diameter with a focal length of f=100 mm yielding a total beamwidth of 4°. At 100 m distance, the LED illuminates an area of 7x7 m² almost homogeneously realizing a link being robust to small pointing errors without using a costly active tracking technique. At the receiver, a 3” f=85 mm lens and a silicon photodiode with an optical concentrator yielding 14 mm effective diameter were used. The total field-of-view of FOV=9° is larger than the beamwidth at the transmitter. Hence, pointing the transmitter correctly also aligns the co-located receiver. Both chipsets support adaptive modulation and coding, to adapt the throughput due to both, long link distance and bad weather conditions. At 20 m link distance, a gross rate of 465 Mbit/s is reached with the 500 Mbit/s chipset.
Ecoc 2015 - ID: 0643
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Fig. 1: Gross data rate as a function of transmission range.
Fig. 2: Visibility range statistics in the outdoor trial.
The data rate is reduced to 280 and 120 Mbit/s in clear weather conditions, which is related to a negative link margin of roughly -5 and -8 dBopt at 5 50 and 100 m link distance, respectively . For both baseband solutions, data rate and latency were measured using the standard Ethernet RFC 2544 test in a back-to-back configuration. The 500 Mbit/s chipset reaches a net throughput of 270 Mbit/s, while 630 Mbit/s are offered by the1 Gbit/s chipset. By using only 95% of the physical (PHY) layer data rates at a given signal-to-noise ratio, congestion in the user queues can be avoided. In real Ethernet links, the same situation is realized by the flow control protocol. The 1 Gbit/s chipset offers below 2 ms one-way latency independent of the frame duration despite the use of adaptive modulation and coding and orthogonal frequency-division 6 multiplex (OFDM) . Obviously, the medium access (MAC) protocol has short frame duration. We have also measured energy consumption. By increasing the data rate, energy consumption starts at 6.9 W and increases linearly to 24.3 W at zero and maximum throughput, respectively. Therefore, we have implemented power control for the LED driver depending on the throughput. For initial outdoor trials, the OWC link was embedded in a weather-proof housing and deployed over a 100 m link distance between the rooftops of the Heinrich Hertz Institute (HHI) and the Institute for High Frequency Technology (HFT) on the campus of the Technical University (TU) in Berlin. In parallel, visibility range and precipitation were measured by using a Vaisala PWD12 weather station. Results measured during the winter term from Nov. 2014 to April 2015 are shown in Fig. 2. At any time, visibility was larger than 180 m, despite dense fog, snow and rainfall observed during this period. In 99,9% of all cases, visibility was above 200 m.
The measured rate is higher than 100, 57, 39 and 22 Mbit/s in 72, 90, 99 and 99,9 % of all 7 cases . As a benefit of both, short link distance and adaptive modulation and coding, we have observed no outage at any time, despite the negative link margins and weather conditions. Modeling results indicate that a combination of longer focal lengths at transmitter and receiver, an LED with higher radiance and a wider modulation bandwidth in the baseband signal processing are the main steps to reach higher data rates. Accordingly, an improved link design targeting 1 Gbit/s over 100 m in good weather conditions is currently implemented. Our recent results indicate that the throughput, latency, energy efficiency as well as availability requirements of mobile backhauling for HSPA and LTE small-cell deployments can be reached already now with commercially available components. Towards LTE-Advanced and 5G, lower latency (< 1 ms) and higher data rates (up to 10 Gbit/s) will be needed in the future. We believe that these future requirements can be met using laser diodes, due to the improved efficiency and higher modulation bandwidth, at little more cost compared to LED. The narrow laser line-width enables better optical filtering of sunlight and the use of avalanche photodiodes. Our technology roadmap suggests that OWC is a future-proof and reliable mobile backhauling solution at link distances below 200 m. Optical Wireless for the Internet of Things As a second new application, we propose to use OWC as a reliable wireless access solution in industrial automation scenarios. High-Tech products like cars become increasingly personalized for each customer. Accordingly, future car manufacturing cells shall become more flexible and reconfigurable. Getting rid of the cables in industrial communication is a major challenge towards this vision.
Ecoc 2015 - ID: 0643
Fig. 3: Possible use of MIMO for optical wireless communication in the Internet of Things.
Main requirements for this new application in the Internet of Things (IoT) are link robustness, low latency and support of large numbers of machine-type mobile users inside flexible manufacturing cells. WiFi is extensively used in manufacturing scenarios for the moment, as it is the only available technology meeting these requirements at least partially. However, WiFi is known as a technology with a high jamming risk. One challenge for optical as compared to radio links is robustness. Optical links often need the LOS while radio links essentially include a plurality of waves reflected from walls and obstacles in the room so that the LOS is not actually needed. Link robustness can thus be ideally supported also for mobile users. A further challenge is the requirement for low latency. One potential solution for both requirements might be an optical multiple-input multiple-output (MIMO) link, where multiple distributed LED spot lights at the ceiling serve jointly multiple mobile users in the room. Link robustness is provided inherently in this way. If, for instance, the LOS to one LED spot light is occasionally broken, there will be another free LOS to another spot light to which the LOS is free. MIMO allows also parallel data transmissions to multiple users. Space division multiplexing (SDM) is enabled by MIMO and it offers higher data rates, reduced packet duration and lower latency, accordingly. However, one spot light serves a limited coverage area only, depending on the directivity and the lens diameter at the receiver. The deployment costs of many such spot lights in a flexible manufacturing cell are obviously too high. Concentrating all spot lights in an array transmitter at one location at the ceiling and illuminating remotely the spots in the cell by using a grid-of-beams pointing downwards would encounter difficulties to meet the link robustness requirement. In this case, the LOS is obviously always needed. Spot diffusing is promising to achieve link robustness. One could point the grid of beams from a central location
(e.g. at the robot’s boom, see Fig. 3) upwards to the ceiling so that the light of each beam is diffusely reflected and multiple distributed spot 9 lights are generated remotely . Experiments show that non-directed OWC is possible several 4, 8 meters from the ceiling . At the receiver, a large photodiode can be used to serve multiple users by time-, frequency- or code-division multiplexing (TDM, FDM, CDM). Of course, SDM is more efficient. It can be easily realized with an imaging, camera-like receiver, see zoom in Fig. 3. The signal of each user is then detected on one or more pixels in 10 the array . LED arrays are widely used today. Individually addressable photodiodes were recently integrated into a low-cost CMOS 11 camera . The huge volumes of smartphone cameras inspired industry recently to reconsider the use of optical camera communications (OCC). Note that the IEEE has recently started a working group on OCC as well as high-speed OWC towards a new standard 802.15.7r1. Conclusions We have shown that high-speed optical wireless communication promises new applications for mobile backhauling of WiFi, LTE and 5G and for flexible manufacturing cells in the Internet of Things, where optical MIMO links are useful. Acknowledgements
The work was supported by the EU project SODALES (GA 318600) and the BMBF project OWICELLS.
References
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