60GHz (V-Band) is now becoming a popular frequency band in wireless world, with both short-range and wider area applications ahead for the tiny beams of this unlicensed millimeter radio technology.
The frequency — part of the V-Band frequencies — is considered among the millimeter radio (mmWave) bands. Millimeter wave radios operate using frequencies from 30GHz to 300GHz. Until recently, 60GHz has typically been used for military communications as well as some commercial applications.
Major technology vendors show growing interest in the technology and the associated patents. Qualcomm Inc. (Nasdaq: QCOM) bought Wilocity recently to combine 60GHz WiGig technology with WiFi. Google (Nasdaq: GOOG) bought Alpental, a startup that, according to one of its founders, is using 60GHz to develop a “hyper scalable mmWave networking solution for dense urban nextGen 5G & WiFi.”
Why 60GHz, and why now? Here are a few reasons the market is expanding:
A short-range wireless specification — using the Institute of Electrical and Electronics Engineers Inc. (IEEE) 802.11ad specification — that can link devices at up to 7 Gbit/s over a distance of up to 12 meters. That’s 10 times faster than the current 802.11n WiFi, though with less range. This makes the technology ideal for wirelessly delivering high-definition video in the home. The Wi-Fi Alliance is expecting WiGig-certified products to arrive in 2015. (See Wi-Fi Alliance, WiGig Align to Make WiFi Super Fast.)
Particularly for small cells, operators can use the 60GHz radios to connect small cells to a fiber hub. (See More Startups Target Small-Cell Backhaul.)
These are useful for providing extra capacity at events, ad-hoc networks, and private high-speed enterprise links. (See Pushing 60.)
Wireless video: Some startups have jumped the gun on the WiGig standard and plowed ahead with their own 60GHz video connectivity using the Sony-backed WirelessHD standard.
A global unlicensed band exists at 57-64GHz. It is largely uncongested compared to the 2.5GHz and 5GHz public bands currently used for WiFi. (See FCC to Enable Fast Streaming With New 60GHz Rules.)
There’s also a lot of it. “The 60 GHz band boasts a wide spectrum of up to 9GHz that is typically divided into channels of roughly 2GHz each,” Intel Corp. (Nasdaq: INTC)’s LL Yang wrote in an article on the prospects for the wide-area and short-range use of the technology. Spectrum availability is “unmatched” by any of the lower-frequency bands.
The spectrum is now open and approved for use across much of the world. This includes the US, Europe, and much of Asia, including China.
As we’ve already seen, 60GHz technology is expected to offer blazing wireless transmission speeds.
Issues with 60GHz
No technology is ever perfect, right?
Transmissions at 60GHz have less range for a given transmit power than 5GHz WiFi, because of path loss as the electromagnetic wave moves through the air, and 60GHz transmissions can struggle to penetrate walls. There is also a substantial RF oxygen absorption peak in the 60GHz band, which gets more pronounced at ranges beyond 100 meters, as Agilent notes in a paper on the technology. Using a high-gain adaptive antenna array can help make up for some of these issues with using 60GHz for wider area applications.
Some vendors have also argued that there are potential advantages for the technology over omnidirectional systems. “The combined effects of O2 absorption and narrow beam spread result in high security, high frequency re-use, and low interference for 60GHz links,” one vendor notes
5G radio access technology will be a key component of the Networked Society. It will address high traffic growth and increasing demand for high-bandwidth connectivity. It will also support massive numbers of connected devices and meet the real-time, high-reliability communication needs of mission-critical applications. 5G will provide wireless connectivity for a wide range of new applications and use cases, including wearables, smart homes, traffic safety/control, critical infrastructure, industry processes and very-high-speed media delivery. As a result, it will also accelerate the development of the Internet of Things. ITU Members including key industry players, industry forums, national and regional standards development organizations, regulators, network operators, equipment manufacturers as well as academia and research institutions together with Member States, gathered as the working group responsible for IMT systems, and completed a cycle of studies on the key performance requirements of 5G technologies for IMT-2020.
The Aim of 5G
The overall aim of 5G is to provide ubiquitous connectivity for any kind of device and any kind of application that may benefit from being connected. 5G networks will not be based on one specific radio-access technology. Rather, 5G is a portfolio of access and connectivity solutions addressing the demands and requirements of mobile communication beyond 2020.
The specification of 5G will include the development of a new flexible air interface, NX, which will be directed to extreme mobile broadband deployments. NX will also target high-bandwidth and high-traffic-usage scenarios, as well as new scenarios that involve mission-critical and realtime communications with extreme requirements in terms of latency and reliability.
In parallel, the development of Narrow-Band IoT (NB-IoT) in 3GPP is expected to support massive machine connectivity in wide area applications. NB-IoT will most likely be deployed in bands below 2GHz and will provide high capacity and deep coverage for enormous numbers of connected devices.
Ensuring interoperability with past generations of mobile communications has been a key principle of the ICT industry since the development of GSM and later wireless technologies within the 3GPP family of standards.
4G to 5G Evolution
In a similar manner, LTE will evolve in a way that recognizes its role in providing excellent coverage for mobile users, and 5G networks will incorporate LTE access (based on Orthogonal Frequency Division Multiplexing (OFDM)) along with new air interfaces in a transparent manner toward both the service layer and users. Around 2020, much of the available wireless coverage will continue to be provided by LTE, and it is important that operators with deployed 4G networks have the opportunity to transition some – or all – of their spectrum to newer wireless access technologies.
For operators with limited spectrum resources, the possibility of introducing 5G capabilities in an interoperable way – thereby allowing legacy devices to continue to be served on a compatible carrier – is highly beneficial and, in some cases, even vital. At the same time, the evolution of LTE to a point where it is a full member of the 5G family of air interfaces is essential, especially since initial deployment of new air interfaces may not operate in the same bands. The 5G network will enable dual-connectivity between LTE operating within bands below 6GHz and the NX air interface in bands within the range 6GHz to100GHz. NX should also allow for user-plane aggregation, i.e. joint delivery of data via LTE and NX component carriers. This paper explains the key requirements and capabilities of 5G, along with its technology components and spectrum needs.
In order to enable connectivity for a very wide range of applications with new characteristics and requirements, the capabilities of 5G wireless access must extend far beyond those of previous generations of mobile communication. These capabilities will include massive system capacity, very high data rates everywhere, very low latency, ultra-high reliability and availability, very low device cost and energy consumption, and energy-efficient networks.
MASSIVE SYSTEM CAPACITY
Traffic demands for mobile-communication systems are predicted to increase dramatically. To support this traffic in an affordable way, 5G networks must deliver data with much lower cost per bit compared with the networks of today. Furthermore, the increase in data consumption will result in an increased energy footprint from networks. 5G must therefore consume significantly lower energy per delivered bit than current cellular networks. The exponential increase in connected devices, such as the deployment of billions of wirelessly connected sensors, actuators and similar devices for massive machine connectivity, will place demands on the network to support new paradigms in device and connectivity management that do not compromise security. Each device will generate or consume very small amounts of data, to the extent that they will individually, or even jointly, have limited impact on the overall traffic volume. However, the sheer number of connected devices seriously challenges the ability of the network to provision signaling and manage connections.
VERY HIGH DATA RATES EVERYWHERE
Every generation of mobile communication has been associated with higher data rates compared with the previous generation. In the past, much of the focus has been on the peak data rate that can be supported by a wireless-access technology under ideal conditions. However, a more important capability is the data rate that can actually be provided under real-life conditions in different scenarios.
5G should support data rates exceeding 10Gbps in specific scenarios such as indoor and dense outdoor environments.
Data rates of several 100Mbps should generally be achievable in urban and suburban environments.
Data rates of at least 10Mbps should be accessible almost everywhere, including sparsely populated rural areas in both developed and developing countries.
VERY LOW LATENCY
Very low latency will be driven by the need to support new applications. Some envisioned 5G use cases, such as traffic safety and control of critical infrastructure and industry processes, may require much lower latency compared with what is possible with the mobile-communication systems of today. To support such latency-critical applications, 5G should allow for an application end-to-end latency of 1ms or less, although application-level framing requirements and codec limitations for media may lead to higher latencies in practice. Many services will distribute computational capacity and storage close to the air interface. This will create new capabilities for real-time communication and will allow ultra-high service reliability in a variety of scenarios, ranging from entertainment to industrial process control.
ULTRA-HIGH RELIABILITY AND AVAILABILITY
In addition to very low latency, 5G should also enable connectivity with ultra-high reliability and ultra-high availability. For critical services, such as control of critical infrastructure and traffic safety, connectivity with certain characteristics, such as a specific maximum latency, should not merely be ‘typically available.’ Rather, loss of connectivity and deviation from quality of service requirements must be extremely rare. For example, some industrial applications might need to guarantee successful packet delivery within 1 ms with a probability higher than 99.9999 percent.
VERY LOW DEVICE COST AND ENERGY CONSUMPTION
Low-cost, low-energy mobile devices have been a key market requirement since the early days of mobile communication. However, to enable the vision of billions of wirelessly connected sensors, actuators and similar devices, a further step has to be taken in terms of device cost and energy consumption. It should be possible for 5G devices to be available at very low cost and with a battery life of several years without recharging.
While device energy consumption has always been prioritized, energy efficiency on the network side has recently emerged as an additional KPI, for three main reasons:
Energy efficiency is an important component in reducing operational cost, as well as a driver for better dimensioned nodes, leading to lower total cost of ownership.
Energy efficiency enables off-grid network deployments that rely on medium-sized solar panels as power supplies, thereby enabling wireless connectivity to reach even the most remote areas.
Energy efficiency is essential to realizing operators’ ambition of providing wireless access in a sustainable and more resource-efficient way.
The importance of these factors will increase further in the 5G era, and energy efficiency will therefore be an important requirement in the design of 5G wireless access.
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Fiber Cuts – The Real Cost – How to solve using Gigabit Wireless
Often you can’t avoid fiber cuts: they happen on public land or under public streets, outside your control. The vast majority of corporate LAN connections, cable, Internet and LTE backhaul, is done over fiber optic cable. In one report CNN stated that about 99 percent of all international communications occur over undersea cabling. Alan Mauldin, research director at U.S.-based research firm Telegeography, noted that while some major cabling projects can come with high price tags, fiber optics is considered more robust and more cost-effective than common wireless alternatives like satellite.
But while fiber optic cabling is traditionally seen as the safer option, that may be a misconception. When installed correctly, fiber optics is the “perfect” media, transmitting Gigabits of data without interruption. However, any disruption to the fragile fiber causes data outages which take days or weeks to locate and repair. According to data from the Federal Communications Commission. about a quarter of all network outages that happened between 1993 and 2001 were from cables being cut. Regardless of how the fiber cut occurred, such outages can be particularly damaging.
How easy is it to repair a fiber cut?
Fiber is not a “self healing” media: skilled teams with specialist fiber-splicing and terminating equipment are required to repair a broken fiber connection. Most data communication engineers do not have this equipment or training on using them. fiber repair is a specialist business and getting trained people and splicing equipment to site costs time and money. Factoring the anticipated cost of a fiber repair into a budget for “downtime” and “unproductivity” for corporates – and missing SLA’s for uptime for Service Providers – is a serious issue, including business continuity planning. For rural areas, access to sites can be limited, with some locations limited by poor weather, and for islands sometimes only with infrequent access by sea or air.
By vandalism – This type of fiber cut outage has been worryingly common of late. According to CNN, there have been 11 separate incidents involving the cutting of fiber optic cable in the Bay Area since July 2015. The FBI noted that there have been more than 12 in the region since January, and that it’s been hard to stop in part because there is so much critical cabling in the area and because cables are typically clearly marked, The Wall Street Journal reported. Authorities noted that these incidents show no sign of slowing down either, as they don’t have a clear suspect(s) or motive at this time. The Journal also noted that some instances of fiber optic-related downtime are not due to vandalism, but rather someone trying to steal metal.
By accident – This is perhaps one of the most common causes of fiber cuts, but nevertheless they are just as damaging. In one example a 75-year-old woman in the country of Georgia was digging in a field when she accidentally severed a fiber optic cable, in an article in The Guardian. As a result of the mishap, close to 90 percent of Armenia and parts of Azerbaijan and Georgia were completely without Internet for five plus hours.
By force of nature – Tornadoes, hurricanes, earthquakes and other major natural disasters all have the potential to cut or entirely destroy fiber optic cabling. Other seemingly more benign forces of nature can also cripple connectivity, as Level 3 reported that 28 percent of all damages it sustained to its infrastructure in 2010 were caused by squirrels.
Calculating the impact of a fiber outage
In some of these fiber cut outage incidents, the fallout can be relatively minor. A cut that occurs in the middle of the night on a redundant line can be easy enough to deal with, with service providers sometimes able to reroute traffic in the interim. Unfortunately however, such incidents often lead to much bigger problems for end users. For example, a cut fiber optic cable in northern Arizona in April caused many thousands of people and businesses to go about 15 hours with telephone and Internet service. This meant many shops had to either close or resort to manual tracking, and that personal Internet usage grinded to a halt, The Associated Press reported. More importantly, 911 emergency communications were disrupted in the incident.
It’s not just a hassle for end users, as cut fiber can severely impact public health when emergency services like police departments, fire stations and EMTs can’t take and receive calls. Plus, such incidents are very costly for service providers, forced to repair expensive infrastructure. They can also lead to canceled service, as customers become irate at service providers for failing to provide reliable connectivity at all times.
What’s a solution to fiber cut outages?
One easy way to avoid the problems related to cut fiber is to not have fiber at all and instead pursue a wireless dark fiber alternative. For example, after a cable snafu caused residents of Washington state’s San Juan Islands to go without telephone, Internet and cell service for 10 days in 2013, CenturyLink installed a wireless mobile backhaul option there, according to The AP.
By opting for a solution like a Gigabit WirelessMicrowave, MMW, Free Space Optics or MIMO OFDM Radio, service providers gain a wireless alternative to cabling that is just as robust and fast as fiber. With the Gigabit Wireless link in place, cut fiber optic cabling is less disruptive to end users and ISPs.
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FSO (Free Space Optics, Laser, Optical Wireless) Guide
Free Space Optics (FSO) communications, also called Optical Wireless (OW) or Infrared Laser, refers to the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain optical communications. Like fibre, Free Space Optics (FSO) uses lasers to transmit data, but instead of enclosing the data stream in a glass fibre, it is transmitted through the air. Free Space Optics (FSO) works on the same basic principle as Infrared television remote controls, wireless keyboards or IRDA ports on laptops or cellular phones.
History of Free Space Optics (FSO)
The engineering maturity of Free Space Optics (FSO) is often underestimated, due to a misunderstanding of how long Free Space Optics (FSO) systems have been under development. Historically, Free Space Optics (FSO) or optical wireless communications was first demonstrated by Alexander Graham Bell in the late nineteenth century (prior to his demonstration of the telephone!). Bell’s Free Space Optics (FSO) experiment converted voice sounds into telephone signals and transmitted them between receivers through free air space along a beam of light for a distance of some 600 feet. Calling his experimental device the “photophone,” Bell considered this optical technology – and not the telephone – his pre-eminent invention because it did not require wires for transmission.
Although Bell’s photophone never became a commercial reality, it demonstrated the basic principle of optical communications. Essentially all of the engineering of today’s Free Space Optics (FSO) or free space optical communications systems was done over the past 40 years or so, mostly for defense applications. By addressing the principal engineering challenges of Free Space Optics (FSO), this aerospace/defence activity established a strong foundation upon which today’s commercial laser-based Free Space Optics (FSO) systems are based.
How Free Space Optics (FSO) Works
Free Space Optics (FSO) transmits invisible, eye-safe light beams from one “telescope” to another using low power infrared lasers in the terahertz spectrum. The beams of light in Free Space Optics (FSO) systems are transmitted by laser light focused on highly sensitive photon detector receivers. These receivers are telescopic lenses able to collect the photon stream and transmit digital data containing a mix of Internet messages, video images, radio signals or computer files. Commercially available systems offer capacities in the range of 100 Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.
Free Space Optics (FSO) systems can function over distances of several kilometres. As long as there is a clear line of sight between the source and the destination, and enough transmitter power, Free Space Optics (FSO) communication is possible.
FSO: Wireless Links at the Speed of Light
Unlike radio and microwave systems, Free Space Optics (FSO) is an optical technology and no spectrum licensing or frequency coordination with other users is required, interference from or to other systems or equipment is not a concern, and the point-to-point laser signal is extremely difficult to intercept, and therefore secure. Data rates comparable to optical fibre transmission can be carried by Free Space Optics (FSO) systems with very low error rates, while the extremely narrow laser beam widths ensure that there is almost no practical limit to the number of separate Free Space Optics (FSO) links that can be installed in a given location.
How Free Space Optics (FSO) benefits you
FSO is free from licensing and regulation which translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting Free Space Optics (FSO) equipment to operate in a very favourable environment. The only essential requirement for Free Space Optics (FSO) or optical wireless transmission is line of sight between the two ends of the link.
For Metro Area Network (MAN) providers the last mile or even feet can be the most daunting. Free Space Optics (FSO) networks can close this gap and allow new customers access to high-speed MAN’s. Providers also can take advantage of the reduced risk of installing an Free Space Optics (FSO) network which can later be redeployed.
The Market. Why FSO? Breaking the Bandwidth Bottleneck
Why FSO? The global telecommunications network has seen massive expansion over the last few years. First came the tremendous growth of the optical fiber long-haul, wide-area network (WAN), followed by a more recent emphasis on metropolitan area networks (MANs). Meanwhile, local area networks (LANs) and gigabit Ethernet ports are being deployed with a comparable growth rate. In order for this tremendous network capacity to be exploited, and for the users to be able to utilize the broad array of new services becoming available, network designers must provide a flexible and cost-effective means for the users to access the telecommunications network. Presently, however, most local loop network connections are limited to 1.5 Mbps (a T1 line). As a consequence, there is a strong need for a high-bandwidth bridge (the “last mile” or “first mile”) between the LANs and the MANs or WANs.
A recent New York Times article reported that more than 100 million miles of optical fibre was laid around the world in the last two years, as carriers reacted to the Internet phenomenon and end users’ insatiable demand for bandwidth. The sheer scale of connecting whole communities, cities and regions to that fiber optic cable or “backbone” is something not many players understood well. Despite the huge investment in trenching and optical cable, most of the fibre remains unlit, 80 to 90% of office, commercial and industrial buildings are not connected to fibre, and transport prices are dropping dramatically.
Free Space Optics (FSO) systems represent one of the most promising approaches for addressing the emerging broadband access market and its “last mile” bottleneck. Free Space Optics (FSO) systems offer many features, principal among them being low start-up and operational costs, rapid deployment, and high fiber-like bandwidths due to the optical nature of the technology.
Broadband Bandwidth Alternatives
Access technologies in general use today include telco-provisioned copper wire, wireless Internet access, broadband RF/microwave, coaxial cable and direct optical fiber connections (fiber to the building; fiber to the home). Telco/PTT telephone networks are still trapped in the old Time Division Multiplex (TDM) based network infrastructure that rations bandwidth to the customer in increments of 1.5 Mbps (T-1) or 2.024 Mbps (E-1). DSL penetration rates have been throttled by slow deployment and the pricing strategies of the PTTs. Cable modem access has had more success in residential markets, but suffers from security and capacity problems, and is generally conditional on the user subscribing to a package of cable TV channels. Wireless Internet access is still slow, and the tiny screen renders it of little appeal for web browsing.
Broadband RF/microwave systems have severe limitations and are losing favor. The radio spectrum is a scarce and expensive licensed commodity, sold or leased to the highest bidder, or on a first-come first-served basis, and all too often, simply unavailable due to congestion. As building owners have realized the value of their roof space, the price of roof rights has risen sharply. Furthermore, radio equipment is not inexpensive, the maximum data rates achievable with RF systems are low compared to optical fiber, and communications channels are insecure and subject to interference from and to other systems (a major constraint on the use of radio systems).
Free Space Optics (FSO) Advantages
Free space optical (FSO) systems offers a flexible networking solution that delivers on the promise of broadband. Only free space optics or Free Space Optics (FSO) provides the essential combination of qualities required to bring the traffic to the optical fiber backbone – virtually unlimited bandwidth, low cost, ease and speed of deployment. Freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) optical wireless transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting the equipment to operate in a very favorable environment. The only essential for Free Space Optics (FSO) is line of sight between the two ends of the link.
Security and Free Space Optics (FSO)
The common perception of wireless is that it offers less security than wireline connections. In fact, Free Space Optics (FSO) is far more secure than RF or other wireless-based transmission technologies for several reasons:
Free Space Optics (FSO) laser beams cannot be detected with spectrum analyzers or RF meters
Free Space Optics (FSO) laser transmissions are optical and travel along a line of sight path that cannot be intercepted easily. It requires a matching Free Space Optics (FSO) transceiver carefully aligned to complete the transmission. Interception is very difficult and extremely unlikely
The laser beams generated by Free Space Optics (FSO) systems are narrow and invisible, making them harder to find and even harder to intercept and crack
Data can be transmitted over an encrypted connection adding to the degree of security available in Free Space Optics (FSO) network transmissions.
Free Space Optics (FSO) Challenges
The advantages of free space optical wireless or Free Space Optics (FSO) do not come without some cost. When light is transmitted through optical fiber, transmission integrity is quite predictable – barring unforseen events such as backhoes or animal interference. When light is transmitted through the air, as with Free Space Optics (FSO) optical wireless systems, it must contend with a a complex and not always quantifiable subject – the atmosphere.
Attenuation, Fog and Free Space Optics (FSO)
Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared wavelengths that are employed in Free Space Optics (FSO) systems. Note that the effect of fog on Free Space Optics (FSO) optical wireless radiation is entirely analogous to the attenuation – and fades – suffered by RF wireless systems due to rainfall. Similar to the case of rain attenuation with RF wireless, fog attenuation is not a “show-stopper” for Free Space Optics (FSO) optical wireless, because the optical link can be engineered such that, for a large fraction of the time, an acceptable power will be received even in the presence of heavy fog. Free Space Optics (FSO) optical wireless-based communication systems can be enhanced to yield even greater availabilities.
Free Space Optics (FSO) and Physical Obstructions
Free Space Optics (FSO) products which have widely spaced redundant transmitters and large receive optics will all but eliminate interference concerns from objects such as birds. On a typical day, an object covering 98% of the receive aperture and all but 1 transmitter; will not cause an Free Space Optics (FSO) link to drop out. Thus birds are unlikely to have any impact on Free Space Optics (FSO) transmission.
Free Space Optics (FSO) Pointing Stability – Building Sway, Tower Movement
Only wide-beamwidth fixed pointed Free Space Optics (FSO) systems are capable of handling the vast majority of movement found in deployments on buildings. Narrow beam systems are unreliable, requiring manual re-alignment on a regular basis, due to building movement. ‘Wide beam’ means more than 5milliradians. Narrow systems (1-2mRad) are not reliable without a tracking system
The combination of effective beam divergence and a well matched receive Field-of-View (FOV) provide for an extremely robust fixed pointed Free Space Optics (FSO) system suitable for most deployments. Fixed-pointed Free Space Optics (FSO) systems are generally preferred over actively-tracked Free Space Optics (FSO) systems due to their lower cost.
Free Space Optics (FSO) and Scintillation
Performance of many Free Space Optics (FSO) optical wireless systems is adversely affected by scintillation on bright sunny days; the effects of which are typically reflected in BER statistics. Some optical wireless products have a unique combination of large aperture receiver, widely spaced transmitters, finely tuned receive filtering, and automatic gain control characteristics. In addition, certain optical wireless systems also apply a clock recovery phase-lock-loop time constant that all but eliminate the affects of atmospheric scintillation and jitter transference.
Solar Interference and Free Space Optics (FSO)
Solar interference in Free Space Optics (FSO) free space optical systems can be combated in two ways. Optical narrowband filter proceeding the receive detector used to filter all but the wavelength actually used for intersystem communications. To handle off-axis solar energy, sophisticated spatial filters have been implemented in CableFree systems, allowing them to operate unaffected by solar interference that is more than 1 degree off-axis.
Free Space Optics (FSO) Reliability
Employing an adaptive laser power (Automatic Transmit Power Control or ATPC) scheme to dynamically adjust the laser power in response to weather conditions will improve the reliability of Free Space Optics (FSO) optical wireless systems. In clear weather the transmit power is greatly reduced, enhancing the laser lifetime by operating the laser at very low-stress conditions. In severe weather, the laser power is increased as needed to maintain the optical link – then decreased again as the weather clears. A TEC controller that maintains the temperature of the laser transmitter diodes in the optimum region will maximize reliability and lifetime, consistent with power output allowing the FSO optical wireless system to operate more efficiently and reliably at higher power levels.
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What is OFDM? (Orthogonal Frequency Division Multiplexing)
OFDM: Orthogonal Frequency Division Multiplexing, is a form of signal modulation that divides a high data rate modulating stream placing them onto many slowly modulated narrowband close-spaced subcarriers, and in this way is less sensitive to frequency selective fading.
Orthogonal Frequency Division Multiplexing or OFDM is a modulation format that is being used for many of the latest wireless and telecommunications standards.
OFDM has been adopted in the Wi-Fi arena where the standards like 802.11a, 802.11n, 802.11ac and more. It has also been chosen for the cellular telecommunications standard LTE / LTE-A, and in addition to this it has been adopted by other standards such as WiMAX and many more.
Orthogonal frequency division multiplexing has also been adopted for a number of broadcast standards from DAB Digital Radio to the Digital Video Broadcast standards, DVB. It has also been adopted for other broadcast systems as well including Digital Radio Mondiale used for the long medium and short wave bands.
Although OFDM, orthogonal frequency division multiplexing is more complicated than earlier forms of signal format, it provides some distinct advantages in terms of data transmission, especially where high data rates are needed along with relatively wide bandwidths.
What is OFDM? – The concept
OFDM is a form of multicarrier modulation. An OFDM signal consists of a number of closely spaced modulated carriers. When modulation of any form – voice, data, etc. is applied to a carrier, then sidebands spread out either side. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.
Traditional view of receiving signals carrying modulation
To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC. The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero – in other words there is no interference contribution.
One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.
In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency. In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.
Data on OFDM
The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier. The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.
The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal.
OFDM advantages & disadvantages
OFDM has been used in many high data rate wireless systems because of the many advantages it provides.
Immunity to selective fading: One of the main advantages of OFDM is that is more resistant to frequency selective fading than single carrier systems because it divides the overall channel into multiple narrowband signals that are affected individually as flat fading sub-channels.
Resilience to interference: Interference appearing on a channel may be bandwidth limited and in this way will not affect all the sub-channels. This means that not all the data is lost.
Spectrum efficiency: Using close-spaced overlapping sub-carriers, a significant OFDM advantage is that it makes efficient use of the available spectrum.
Resilient to ISI: Another advantage of OFDM is that it is very resilient to inter-symbol and inter-frame interference. This results from the low data rate on each of the sub-channels.
Resilient to narrow-band effects: Using adequate channel coding and interleaving it is possible to recover symbols lost due to the frequency selectivity of the channel and narrow band interference. Not all the data is lost.
Simpler channel equalisation: One of the issues with CDMA systems was the complexity of the channel equalisation which had to be applied across the whole channel. An advantage of OFDM is that using multiple sub-channels, the channel equalization becomes much simpler.
Whilst OFDM has been widely used, there are still a few disadvantages to its use which need to be addressed when considering its use.
High peak to average power ratio: An OFDM signal has a noise like amplitude variation and has a relatively high large dynamic range, or peak to average power ratio. This impacts the RF amplifier efficiency as the amplifiers need to be linear and accommodate the large amplitude variations and these factors mean the amplifier cannot operate with a high efficiency level.
Sensitive to carrier offset and drift: Another disadvantage of OFDM is that is sensitive to carrier frequency offset and drift. Single carrier systems are less sensitive.
There are several other variants of OFDM for which the initials are seen in the technical literature. These follow the basic format for OFDM, but have additional attributes or variations:
COFDM: Coded Orthogonal frequency division multiplexing. A form of OFDM where error correction coding is incorporated into the signal.
Flash OFDM: This is a variant of OFDM that was developed by Flarion and it is a fast hopped form of OFDM. It uses multiple tones and fast hopping to spread signals over a given spectrum band.
OFDMA: Orthogonal frequency division multiple access. A scheme used to provide a multiple access capability for applications such as cellular telecommunications when using OFDM technologies.
VOFDM: Vector OFDM. This form of OFDM uses the concept of MIMO technology. It is being developed by CISCO Systems. MIMO stands for Multiple Input Multiple output and it uses multiple antennas to transmit and receive the signals so that multi-path effects can be utilised to enhance the signal reception and improve the transmission speeds that can be supported.
WOFDM: Wideband OFDM. The concept of this form of OFDM is that it uses a degree of spacing between the channels that is large enough that any frequency errors between transmitter and receiver do not affect the performance. It is particularly applicable to Wi-Fi systems.
Each of these forms of OFDM utilise the same basic concept of using close spaced orthogonal carriers each carrying low data rate signals. During the demodulation phase the data is then combined to provide the complete signal.
OFDM, orthogonal frequency division multiplexing has gained a significant presence in the wireless market place. The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of multi-path effects means that it is ideal for the high data applications that have become a major factor in today’s communications scene.
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