The White Space Networking Computer Science Essay

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The White Space Networking technology is the latest development in terms of wireless networking technologies. It utilizes the unused portions of the TV frequency spectrum known as white spaces. The White Space Coalition companies are conducting research in order to implement this technology and make it an effective and profitable entity. This new technology operates over UHF white spaces and differs from the conventional Wi-Fi technology in three aspects: spatial variation, temporal variation and fragmentation of the UHF spectrum. To deal with the issues like spectrum variation and fragmentation, WhiteFi makes use of an adaptive spectrum assignment algorithm. The main goal of the white space networking technology is to provide a low cost and long range wireless network to each and everyone. This technology aims at providing wireless broadband services to the rural areas as well as filling up the gaps of the local Wi-Fi technology in the urban areas to provide a blanket of wireless communication across the entire region.

The technology has a very bright future and a magnanimous scope of development.

2. INTRODUCTION

Wireless internet has become a powerful and dominant service that people use worldwide. People no more consider it as "just a service", but a necessity. Wi-Fi which is the current technology has its own advantages but we cannot ignore the fact that it is suited for only short-to-medium range and also the bandwidth provided is not adequate. As we are aware of the fact that Wi-Fi operates at 2.4GHz, it is best suited for providing high speed internet access over short distances [1]. With the advent of WhiteFi, it is possible to provide wireless broadband internet access. WhiteFi operates at lower frequencies which emerged as a result of the transition from analog to digital TV broadcasting. It uses UHF spectrum (512 MHz to 698 MHz) that was initially allotted to analog TV channels (21 to 51) and offers a longer range than Wi-Fi. WhiteFi offers wider coverage since signals transmitted over lower frequencies travel longer distances than signals transmitted at higher frequencies, which are best suited for shorter distances [1].

White spaces refer to the unused frequencies present between existing TV stations .TV stations traditionally have used lower frequencies for broadcasting purpose. Nowadays due to the conversion of analog broadcast to digital broadcast, more and more unused frequencies are becoming available for use.

Thus, the spaces between channels/stations which are not used for transmission purpose are known as White-Spaces. This space that exists between the radio bands or channels is used to separate the channels from interfering with each other's transmissions. It is used as guard bands in some cases, the absence of which, would cause destructive interference to the adjacent channels. In addition, there is radio spectrum which has never been used or is becoming available due to the transition to digital television.

WhiteFi, refers to a set of protocols that was designed by members of Networking Research Group at Microsoft Research to navigate through these white spaces and make use of them, keeping in mind the technical limitations that come along with the great advantages of having white spaces [3].

3. History

Today, wireless network is a basic necessity and internet is like an essential commodity. Low cost internet access is a requirement everywhere. Hence, the need for a wireless network, which operates over a longer range and is cost effective, has become a necessary condition to ensure economic growth and technological advancement.

In the 1930's vacuum tubes and analog signals were touted as the best and the most advanced technologies. If signals were transmitted by the transmitters, over close frequencies, there would be interference which resulted in noise.

As a result, the Federal Communication Commission (FCC) assigned costly broadcast licenses to regulate the electromagnetic spectrum for traditional radio and television. This reduced the interference problem poised when frequencies were transmitted close to each other, and hence maintained a noise free channel.

However, the technological rise in the 1990's put new demands on the spectrum that had been limited to the emergency services and the military users, as the growth of the wireless communication from the cell phones and computers increased exponentially [2].

Also the advent of digital communication reduced the interference and improved the efficiency of the broadcasting.

There was a possibility and necessity to look for ways to open up the spectrum and FCC depicted that it might be possible to open the spectrum used by analog broadcasters in the television upper high frequency.

It resulted in the conversion of the TV stations as well as the TV sets to handle new digital signals. It also opened up the remaining unused spectrum to new digital services call "white spaces".

The white space technology allows more users and traffic on the same spectrum.

Researchers from Microsoft working together with researchers from Harvard University developed an awe-inspiring technology that uses white spaces spectrum for providing wireless broadband internet access [3]. Initially, FCC (Federal Communication Commission) was against the use of white spaces since it feared that if this spectrum was used, it would interfere with the existing users including the TV broadcasters. But eventually, FCC gave approval for the use of these empty spaces to those unlicensed devices that guaranteed they would not interfere with the assigned broadcasts.FCC then called for prototypes to test the working of these white space devices (WSDs). Microsoft was the first one to provide FCC with such a prototype [3]. Tests were conducted on these prototypes and one of these performed in an adroit manner, while the other one failed badly.

4. Scope

The white space networking technology is a fresh concept and is still considered to be a juvenile technology. But the mere fact that massive organizations such as Microsoft, Google, Dell, EarthLink, HP, Intel, Samsung, Motorola, Sprint and many other companies suggest that white space networking indeed has a bright future. The technology is in its developing stage and has provided immense opportunity for using the unused spectrum, which is unused TV broadcast channels, and has increased the range of the wireless communication networking [4].

The white space networking technology is raw and the FCC has given the green light to use the unused white spaces for wireless applications and devices as recently as last year in 2008.

5. The IEEE 802.22 Standard

IEEE 802.22 is a standard for Wireless Regional Area Networking abbreviated WRAN which utilizes "white spaces" in the TV frequency spectrum [5]. The focus of the IEEE 802.22 WRAN standard is to use cognitive radio techniques which enable the usage of the unused television spectrum without interfering with the medium.

The IEEE together with the FCC is working to find an approach for the discovery of the unused spectrum available. Each Base Station will be linked to a GPS receiver to know its position. This information about the position of the Base Station would then be sent over to the centralized server which would provide us with the information about available TV channels and guard bands in the BS (Base Station) area [5]. The base station decides for itself the channels that are available for communication instead of local spectrum sensing. There are basically two types of devices that work on TV White Space Band (TVWS): Fixed devices and Portable devices. Fixed devices communicate with the central database to identify other transmitters which are operating in the TVWS to avoid interference. Some other methods are also suggested by the IEEE and FCC, such as Dynamic Spectrum Sensing and Dynamic Power Control [5].

6. White Space Devices

These are the devices that use white-spaces. The basic purpose of these devices is to detect the presence of any existing user channel and make sure that it does not get disturbed. In order to avoid interference, it was suggested to provide these devices with GPS receivers and programming each WSD with a database containing the details about all other devices in its vicinity.FCC has approved the use of two kinds of WSDs:

1. Fixed Device: It is easy to install and requires professional help for installation. This device is installed at a fixed location, where no other channel is broadcasting so that the device does not interfere with nearby TV signals [6].

2. Portable Device: This device which will be set up at homes and organizations just like any other Wi-Fi router. The industry people want to cut down on the need of professionals for installation of such kind of devices to make it simpler. They have even come up with ideas such as to use spectrum sensing or "listen before talk" mechanisms, just like carrier sense protocols.

6.1 General Working

Since the FCC has imposed the restriction that the white space devices (WSDs) must make sure that they do not interfere with the existing television broadcast or wireless microphones, any WSD is supposed to switch frequency as and when it detects any signal from a television or microphone on the channel it is already using. To take care of this very issue that new devices could be introduced to the network any time, Microsoft designed a set of protocols which it named as "WhiteFi". This WhiteFi protocol is so designed that the WSDs analyze the spectrum parameters around them, and are able to detect or locate available frequencies which could be used. These WSDs make sure that interference is avoided at any cost since even a tiny bit of interference can induce major issues related to transmissions. The devices have to find a way to locate a free channel and they also need some mechanism that allows them to shift to some other frequency quickly, when something else pops up on the same frequency that they are using. A new channel can be located by using "chirps" sent out at regular intervals on a backup channel and the disconnect can occur within 3 seconds [3]. Thus, a backup frequency channel is always maintained so that in case interference is detected, WSDs can switch to the backup frequency immediately.

6.1.1 Microsoft prototype white space device

The prototype that Microsoft submitted to FCC works on channels 21-51 and it can effectively sense analog and digital TV signals as well as signals coming from wireless microphones. The system consists of two major subunits namely:

1) Wide band spectrum scanner, a network processor and a tunable UHF half-duplex transceiver which is controlled by network processor.

2) Window based laptop which uses Internet Explorer to establish command and control user interface via an Ethernet connection [8].

Figure 3.3 Microsoft Prototype WSD [8]

The sensor comprises of a broadband (521-698 MHz) computer-controlled frequency scanner and high-speed digitizers which incrementally scans the UHF TV channels from 21-51 in 6 MHz segments [8]. The information thus accumulated about the digitized time-domain is then forwarded to the network analyzer where a 2048 Fast Fourier Transform (FFT) is performed. Once this gets done, the signal features for analog TV, digital TV and wireless microphone waveforms are then compared one-by-one with the resulting FFT output to ascertain those channels which are occupied by digital TV and analog TV signals. The channels which are not used by these DTV or analog signals are subsequently analyzed to find if some potential narrowband signals like those of wireless microphones are present. The resulting channels which are finally determined not be occupied by either DTV or analog wireless microphone signals, are declared as white-spaces channels.

6.1.2 Motorola prototype white space device

Motorola came up its prototype which also operates on the same channels from 21-51 but it includes capabilities for geo-location along with spectrum sensing of digital TV signals. An important issue with this prototype is that it does not detect the presence analog TV signals or wireless microphone signals. The system comprises of two main subunits namely:

1) Cognitive Radio Rack.

2) A laptop computer which is connected via Ethernet [8]

The Rack comprises of UHF radio and two PRO-3500 carrier boards which are located together in a compact Peripheral Component Interconnect (PCI) chassis and the cognitive engine runs on the lower board [8]. This prototype relies mainly on its geo-location capability to find the occupied channels and uses its spectrum sensing capability to filter the results that were collected from the geo-location solution and then it prioritizes the channels which are available. The users of the prototype are supposed to manually input the geographic coordinates and the geo-location capability relies heavily on the FCC's TV database for a particular region.

Figure 3.4 Motorola Prototype WSD [8]

6.2 Testing

In spite of the fact that White-spaces are being considered as the next big thing in the domain of wireless broadband internet access, there are certain problems with the white space devices which cannot be neglected including the fact that they do not communicate with other devices. Spectrum sensing of TV signals can be done with great accuracy even if transmission causes problems. In order to make sure that the devices work properly, tests were carried out in which companies like Microsoft, Motorola, Philips , Adaptrum and Institute for Infocomm research submitted five devices .The devices performed differently and showed great variations in terms of detection speed; Motorola's box scanned the presence of each TV channel for an existing signal in 0.1sec whereas Adapt rum's device took as long as 185 sec to do the same.

All the devices did the main task correctly which was to detect the presence of TV signals. In fact, the devices did a commendable job in detecting digital TV signals over a single channel at low levels in the range of -116 dBm to -126 dBm [8].On the other hand when these devices were put to test to detect real world signals that are accompanied with problems like distortion, these devices did not perform that well. Since the real world communication is does not come with just plain signals that have no distortions, it was something that caused botheration to FCC. Another concern was that some devices showed false positives and false negatives which raised the probability of these WSDs transmitting on channels which are already occupied by TV signals thus causing local interference.

Figure 3.5 Motorola Prototype WSD detection sensitivity to RF captures [8]

Figure 3.6 Phillips Prototype WSD detection sensitivity to clean DTV Signal [8]

The only solution which appears to be likely is that the devices would consult the geo-location databases before broadcasting .These databases will help these white space devices to find out which TV channels are already occupied and which are free in the same region that these devices are operating. Geo-location databases along with the use of spectrum sensing techniques, these devices will be able to cause lesser interference even by less-sensitive devices. The geo-location database feature coupled with spectrum sensing was put to test using Motorola's device and it reported all the occupied channels correctly used by stations in the vicinity.

7. Characterization of White Spaces

As mentioned earlier, there are certain differences between the white spaces spectrum and the spectrum where Wi-Fi systems work. There are three major aspects which have a significant effect on the design and implementation of white space network.

7.1 Spatial Variation

UHF spectrum is greatly used by TV stations. Two main factors affecting the incumbent TV channels are the localization of the transmitters and the number of operational stations in a particular region [7]. It's interesting to know that wireless microphones which are nowadays used in classrooms, seminar halls and at huge stadiums can transmit over a distance of hundreds of meters. Thus, spatial variation is seen in white space spectrum.

Researchers conducted an experiment to find out the effects of spatial variation in which analysis of the UHF spectrum inside nine buildings were carried out over an area of 0.9 x 0.2 km., as shown in the figure below [7]:

Figure: An aerial view of the buildings where UHF spectrum analysis was carried out [7]

"Hamming distance, which is the number of channels available at one location but unavailable at another, was calculated across all pair wise buildings" [7]. The analysis revealed that the median number of channels which were found to be present at a particular location, but not at the other, was close to 7. This showed there was a considerable variation in the availability of the spectrum within close sites. Majority of the incumbent s were found to be TV channels, but some co-existing wireless microphones were also found. By carrying out this experiment, researchers were able to deduce that an access point (AP) say a router, should not randomly pick up a channel to operate on. It should take into consideration spectrum availability at the remote site (client side), as well as at its own end.

7.2 Spectrum Fragmentation

In contrast to the ISM bands which are conterminous in nature, white spaces are fragmented because of the presence of TV stations and wireless microphones. The fragment size varies and it can be as narrow as a single channel or it could be as broad as various channels combined together. The extent of fragmentation of the spectrum depends majorly upon TV stations density, which in turn depends upon the population density. Thus, rural areas have more UHF spectrum available for use than urban areas, which can be used for white space networking [7].

In order to quantify the spectrum fragmentation, the researchers performed the analysis of the TV station data collected from a website named TV Fool [7]. The data that was collected was used to approximate the UHF spectrum fragmentation in three cases:

Urban

Suburban

Rural

The histogram below shows the spectrum fragmentation after the digital TV transition.

Figure: Spectrum fragmentation after DTV transition [7]

From the above figure, it's evident that in all the cases there is at least one locale which has a continuous fragment of 4 channels. One can expect fragments containing as many as 16 channels. Due to the fragmentation, radios have to use variable channel widths or channel bonding.

7.3 Temporal Variation

White spaces are subjected to temporal variation due to the interference caused by wireless microphones which are used at various places like lecture rooms, stadiums etc. An experiment was carried out in two different settings:

Campus Setting

University Dormitory

This experiment focused upon using the prototype described earlier to find out about the incumbents. Wireless mics were detected to be operating at variable hours. As per FCC's ruling, white space devices must avoid interference with the mic transmissions. Thus, it becomes essential that access points (AP) as well as clients keep track of the presence of a wireless microphone over a channel [7]. These devices must also ensure that they move to a backup frequency if interference is detected. Each end must use a mechanism to inform the other when switching a channel is required. In practice, solving the problem of microphones to utilize the white spaces efficiently is not a simple issue.

Researchers have come up with an approach which uses a database that contains the pattern in which microphones use channels to transmit. The APs would then query this database to find out the channels that are being used at any given instant. This technique however, did not prove out to be really helpful because of the highly unpredictable nature of the microphone usage.

IEEE 802.22 working group came up with another approach which focuses upon using an explicit channel renegotiation protocol between clients and APs as and when they detect a wireless mic [5]. This approach is based on the assumption that the control messages will not cause any interference with the wireless microphones. In order to check the correctness of this assumption, researchers carried out an experiment in which a wireless microphone and a white space prototype were kept together in an anechoic chamber. Measurements of the audio quality of speech transmitted over the wireless microphone were taken in the presence and absence of the UHF transmissions. Reduction in the quality of the captured audio signals was found due to the data transmissions. Since interference cannot be neglected, thorough research was conducted to come up with a protocol that would aid in finding the presence of the wireless microphones without causing any interference.

8. Implementation of First White Space Network

Spectrum Bridge is credited with developing the first ever white space network and actually deploying it in Claudville, Virginia; a rural area lacking the broadband connectivity. The company said that the white space network serves as a link between the wired backhaul and the Wi-Fi hotspot networks which were deployed in Claudville's business and the school. The same network provides a last-mile broadband connectivity to the end users [9].

The network is controlled and managed by the company's intelligent TV white space database system and it does not interfere with the TV signals. The database system assigns frequencies to the network, which does not interfere with the TV signals in the area and which can adapt dynamically as per the requirements in order to avoid interference.

The wireless signal over the white space network frequencies travel about 1.5 miles away from the router, which means that the network can cover the same area with one-tenth or one-fifteenth of the nodes of the usual Wi-Fi [9].

Wi-Max is a long ranged wireless system which functions over a longer range than WiFi, but it has to operate in the licensed spectrum, which makes it more expensive to implement and hence the white space network is very cost effective.

9. Related Work

The work that has already been done in this field concentrated upon creating a single link over the white spaces [10]. It included the precise spectrum detection, recognition of the primary users and then vacating an occupied channel when a primary user is detected to be using the channel.

Complementing the WhiteFi technology, is the proposal by the IEEE 802.22 working group for having WRANs (Wireless Regional Area Networks) over the UHF white spaces [5]. All the methods that the WhiteFi uses, like spectrum detection and assignment, can also be applied to WRANs. IEEE802.22 draft also specifies that it supports variable widths.

Another technology that makes it possible for unlicensed devices to exist along with the licensed devices is called SWIFT [11]. This technology lets the user know about its presence, but unfortunately it's not feasible to use it since it pokes the incumbents to test their presence. This poking mechanism was turned down by FCC since it feared that the technology would cause interference.

10. Advantages of White Space Networking Technology

Currently, the United States finds itself facing a bottleneck situation as far as broadband services are concerned which can be overcome with the White Space technology.

The Wi-Fi exists in millions of homes in the US which can be connected in mesh networks without the need to pay to cable or phone companies.

The Wi-Max also provides wireless internet access over a longer range, but it uses a licensed spectrum and hence is expensive. The white space networking is cost effective and hence has a higher scope of future development because of its lower cost.

The white space networking utilizes the unused spectrum of the analog TV signals without creating interference allowing more users and more traffic on the same amount of spectrum.

The white space networking technology can be very useful in providing cost effective long range wireless services not only in the urban areas but also in the rural areas.

The unlicensed white spaces promise to open up a new dynamic market for new competitors and innovation of new ideas.

The White Spaces Coalition consists of eight large technological companies such as Microsoft, Google, Dell, HP, Intel, Samsung, Phillips and EarthLink as mentioned earlier and these companies plan to deliver a high speed broadband internet access via white spaces in the unused television frequencies between 54 to 698 MHz The coalition of these large companies and many other companies ensures a bright future for the white space networking technology and promises to propel the technological advancement in terms of a cost effective and long range wireless communication network.

11. Other Issues

However, there are some issues with the white space networking technology, which is quite understandable since the technology is still in its developing stages.

The technical issues of frequency and range still need to be worked upon.

The FCC has issued very strict regulations regarding the use of the unused spectrum, and hence there is a need to develop a more enhanced version of the current technology in order to use it more efficiently.

Since the technology has not yet developed, the research institutions must find profitable business models in order to lure people to invest in the R&D of the new technology.

The main challenge of the technology is that it should cover even the poor and the rural areas as well under its network.

More research is needed to be carried out in developing the technology in such a way that the interference with in the spectrum is minimized to a satisfactory level.

The National Association of Broadcasters (NAB) is against the use of white space technology, citing a major concern that the utilization of the unused bandwidth will interfere with the TV broadcast signals.

The NAB have also filed a lawsuit to shut down the FCC's utilization of the unused white spaces, alleging that the unlicensed personal devices operating in the spectrum alongside the TV broadcasts in the same band have proven to cause interference contradictory to the statement given by the FCC which earlier voted for allowing the unlicensed use of the white spaces.

Microsoft has designed the set of protocols known as WhiteFi which makes the best use of the spectrum and avoids the interference.

According to Shure's proposal to the FCC, a fixed and an adjacent channel approach could possibly provide the broadband internet access while maintaining the protections for the wireless mobile devices.

Google has also suggested that the interference can be overcome by combining spectrum sensing with two technologies: geo-locations for safeguarding broadcast TV and beacons-for preventing interference for wireless microphones.

Meanwhile, companies such as Sprint and Motorola have come up with an idea which revolves around introducing fixed wireless broadband services in the white spaces.

12. CONCLUSION

The white space networking technology is yet in the developing stage. However, since it attracts the interest of many huge corporations, its prominence is increasing day by day. Also the fast growing technological world requires wireless communication networking almost at every step. Wireless networking has become a common household necessity. However, due to the restrictions of the Wi­-Fi networking technology and the expensive nature of the Wi-Max networking technology, it has become imperative that a cost effective and long range wireless communication system is established.

The white space networking technology is a technology that provides low cost, long range and effective wireless network. The technology faces some serious issues regarding its efficiency as well as the interferences it might cause among the TV broadcasts in the spectrum in which it operates. However, the technology is still developing and measures are being taken to curb the interference caused when unused white spaces are utilized. The white space networking technology promises to meet the challenges faced by the current wireless networking system which has limited means to reach to the mass. The white space networking provides us with the hope that even the rural areas can be reached by the wireless network in an effective way, changing the lives of many a millions who are not able to savor the benefits reaped by people residing in the urban areas. The white space networking technology is a ground breaking technology which sticks firmly to the real goal of technological advancement, and that is, creating a better world.

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